Insight into the Alkali Resistance Mechanism of FeMoTiOx Catalysts for NH3 Selective Catalytic Reduction of NO: Self-Defense Effects of MoOx for Alkali Capture

The deactivation of selective catalytic reduction (SCR) catalysts caused by alkali metal poisoning remains an insurmountable challenge. In this study, we examined the impact of Na poisoning on the performance of Fe and Mo co-doped TiO2 (FeaMobTiOx) catalysts in the SCR reaction and revealed the related alkali resistance mechanism. On the obtained Fe1Mo2.6TiOx catalyst, the synergistic catalytic effect of uniformly dispersed FeOx and MoOx species leads to remarkable catalytic activity, with over 90% NO conversion achieved in a wide temperature range of 210-410 °C. During the Na poisoning process, Na ions predominantly adsorb on the MoOx species, which exhibit stronger alkali resistance, effectively safeguarding the FeOx species. This preferential adsorption minimizes the negative effect of Na poisoning on Fe1Mo2.6TiOx. Moreover, Na poisoning has little influence on the Eley-Rideal reaction pathway involving adsorbed NHx reacting with gaseous NOx. After Na poisoning, the Lewis acid sites were deteriorated, while the abundant Brønsted acid sites ensured sufficient NHx adsorption. As a benefit from the self-defense effects of active MoOx species for alkali capture, FeaMobTiOx exhibits exceptional alkali resistance in the SCR reaction. This research provides valuable insights for the design of highly efficient and alkali-resistant SCR catalysts.

Iterative Approach of Experiment-Machine Learning for Efficient Optimization of Environmental Catalysts: An Example of NOx Selective Reduction Catalysts

An iterative approach between machine learning (ML) and laboratory experiments was developed to accelerate the design and synthesis of environmental catalysts (ECs) using selective catalytic reduction (SCR) of nitrogen oxides (NOx) as an example. The main steps in the approach include training a ML model using the relevant data collected from the literature, screening candidate catalysts from the trained model, experimentally synthesizing and characterizing the candidates, updating the ML model by incorporating the new experimental results, and screening promising catalysts again with the updated model. This process is iterated with a goal to obtain an optimized catalyst. Using the iterative approach in this study, a novel SCR NOx catalyst with low cost, high activity, and a wide range of application temperatures was found and successfully synthesized after four iterations. The approach is general enough that it can be readily extended for screening and optimizing the design of other environmental catalysts and has strong implications for the discovery of other environmental materials.

Ultrahighly Alkali-Tolerant NOx Reduction over Self-Adaptive CePO4/FePO4 Catalysts

Catalyst deactivation caused by alkali metal poisoning has long been a key bottleneck in the application of selective catalytic reduction of NOx with NH3 (NH3-SCR), limiting the service life of the catalyst and increasing the cost of environmental protection. Despite great efforts, continuous accumulation of alkali metal deposition makes the resistance capacity of 2 wt % K2O difficult to enhance via merely loading acid sites on the surface, resulting in rapid deactivation and frequent replacement of the NH3-SCR catalyst. To further improve the resistance of alkali metals, encapsulating alkali metals into the bulk phase could be a promising strategy. The bottleneck of 2 wt % K2O tolerance has been solved by virtue of ultrahigh potassium storage capacity in the amorphous FePO4 bulk phase. Amorphous FePO4 as a support of the NH3-SCR catalyst exhibited a self-adaptive alkali-tolerance mechanism, where potassium ions spontaneously migrated into the bulk phase of amorphous FePO4 and were anchored by PO43- with the generation of Fe2O3 at the NH3-SCR reaction temperature. This ingenious potassium storage mechanism could boost the K2O resistance capacity to 6 wt % while maintaining approximately 81% NOx conversion. Besides, amorphous FePO4 also exhibited excellent resistance to individual and coexistence of alkali (K2O and Na2O), alkali earth (CaO), and heavy metals (PbO and CdO), providing long durability for CePO4/FePO4 catalysts in flue gas with multipollutants. The cheap and accessible amorphous FePO4 paves the way for the development and implementation of poisoning-resistant NOx abatement.

Anti-Poisoning Mechanisms of Sb on Vanadia-Based Catalysts for NOx and Chlorobenzene Multi-Pollutant Control

Modulating vanadia-based metal oxides is one of the effective methods for designing difunctional catalysts for simultaneous control of NO_x and chlorobenzene (CB) from the emissions of industrial sources. Excessive NH₃ adsorption and polychlorinated species accumulation on the surface are the primary issues poisoning catalysts and reducing their lifetime. Herein, Sb is selected as an NH₃ adsorption alleviator and polychlorinated species preventor dopant on V₂O₅-WO₃/TiO₂. The catalyst exhibits an excellent performance for total NO_x and 90% CB conversions at 300-400 °C under a gas hourly space velocity (GHSV) of 60,000 mL g^-1 h^-1. The HCl and N₂ selectivities are maintained at 90 and 98%, respectively. The anti-poisoning ability could be attributed to the generated V-O-Sb chains on the surface: the band gap of vanadium is narrowed and the electron capability is strengthened. The above variation weakens the Lewis acid sites and blocks the electrophilic chlorination reactions of the catalyst surface (formation of polychlorinated species). In addition, oxygen vacancies on Sb-O-Ti also increase: the ring opening of benzoates is accelerated and NH₃ adsorption energy is weakened. The above variation lowers the energy barriers of C-Cl cleavage even under NH₃ pre-adsorption models and enhances NO_x reduction thermodynamically and kinetically.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation

Selective catalytic reduction (SCR) of NO_x with NH₃ is the most efficient technology for NO_x emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O₃ into the simulated stack gas, we find that O₃ can stoichiometrically oxidize NO to generate NO₂, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V₁-W₁ active sites supported on TiO₂(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O₃ allows SCR to proceed along a NH₄NO₃-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O₃ exclusively oxidizes NO even in the co-presence of SO₂ and H₂O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Revealing the Excellent Low-Temperature Activity of the Fe1-xCexOδ-S Catalyst for NH3-SCR: Improvement of the Lattice Oxygen Mobility

The development of selective catalytic reduction catalysts by NH₃(NH₃-SCR) with excellent low-temperature activity and a wide temperature window is highly demanded but is still very challenging for the elimination of NO_x emission from vehicle exhaust. Herein, a series of sulfated modified iron-cerium composite oxide Fe_1-xCe_xO_δ-S catalysts were synthesized. Among them, the Fe_0.79Ce_0.21O_δ-S catalyst achieved the highest NO_x conversion of more than 80% at temperatures of 175-375 °C under a gas hourly space velocity of 100000 h^-1. Sulfation formed a large amount of sulfate on the surface of the catalyst and provided rich Brønsted acid sites, thus enhancing its NH₃ adsorption capacity and improving the overall NO_x conversion efficiency. The introduction of Ce is the main determining factor in regulating the low-temperature activity of the catalyst by modulating its redox ability. Further investigation found that there is a strong interaction between Fe and Ce, which changed the electron density around the Fe ions in the Fe_0.79Ce_0.21O_δ-S catalyst. This weakened the strength of the Fe-O bond and improved the lattice oxygen mobility of the catalyst. During the reaction, the iron-cerium composite oxide catalyst showed higher surface lattice oxygen activity and a faster replenishment rate of bulk lattice oxygen. This significantly improved the adsorption and activation of NO_x species and the activation of NH₃ species on the catalyst surface, thus leading to the superior low-temperature activity of the catalyst.

Density Functional Study on Adsorption of NH3 and NOx on the γ-Fe2O3 (111) Surface

γ-Fe₂O₃ is considered to be a promising catalyst for the selective catalytic reduction (SCR) of nitrogen oxide (NO_x). In this study, first-principle calculations based on the density function theory (DFT) were utilized to explore the adsorption mechanism of NH₃, NO, and other molecules on γ-Fe₂O₃, which is identified as a crucial step in the SCR process to eliminate NO_x from coal-fired flue gas. The adsorption characteristics of reactants (NH₃ and NO_x) and products (N₂ and H₂O) at different active sites of the γ-Fe₂O₃ (111) surface were investigated. The results show that the NH₃ was preferably adsorbed on the octahedral Fe site, with the N atom bonding to the octahedral Fe site. Both octahedral and tetrahedral Fe atoms were likely involved in bonding with the N and O atoms during the NO adsorption. The NO tended to be adsorbed on the tetrahedral Fe site though the combination of the N atom and the Fe site. Meanwhile, the simultaneous bonding of N and O atoms with surface sites made the adsorption more stable than that of single atom bonding. The γ-Fe₂O₃ (111) surface exhibited a low adsorption energy for N₂ and H₂O, suggesting that they could be adsorbed onto the surface but were readily desorbed, thus facilitating the SCR reaction. This work is conducive to reveal the reaction mechanism of SCR on γ-Fe₂O₃ and contributes to the development of low-temperature iron-based SCR catalysts.

Kinetic Model of Urea-Related Deposit Reactions

The thermal analysis kinetic method was employed to solve the activation energies of the thermal decomposition reactions of urea and cyanuric acid, with the purpose of understanding the formation of deposits in the diesel engine SCR system. The deposit reaction kinetic model was established by optimizing the reaction paths and reaction kinetic parameters based on the thermal analysis test data of the key components in the deposit. The result shows that the established deposit reaction kinetic model can accurately describe the decomposition process of the key components in the deposit. Compared to the Ebrahimian model, the simulation precision of the established deposit reaction kinetic model is significantly improved above 600 K. The activation energies of the urea and cyanuric acid decomposition reactions are 84 kJ/mol and 152 kJ/mol, respectively, after model parameters identification. The identified activation energies were closest to those of the Friedman one-interval method indicating that the Friedman one-interval method is reasonable to solve the activation energies of deposit reactions.

Monolith or Powder: Improper Sample Pretreatment May Mislead the Understanding of Industrial V2O5-WO3/TiO2 Catalysts Operated in Stationary Resources

Although various characterizations are widely applied to commercial V₂O₅-WO₃/TiO₂ catalysts, the influence of the catalyst physical structure, i.e., monolith or powder, on the characterization results has not been investigated. Several important catalytic behaviors and phenomena were observed in this study using V₂O₅-WO₃/TiO₂ monolithic catalysts employed for over 5000 h in various stationary flue gases, and many of the results were only observable on monolithic catalysts, such as depth-dependent distribution of external elements, penetration of As₂O₃, and the formation of Tl₂O-TiO₂ p-n junctions. If the monolith is ground into powder states, it will alter or destroy the catalyst surface and remove important clues closely related to catalytic performance under working conditions. The redox and acidity properties of V₂O₅-WO₃/TiO₂ obtained from powder samples may be significantly different from their true state under working conditions, resulting in a misperception of catalyst performance. Therefore, a cautious pretreatment should be taken into careful consideration when analyzing commercial honeycomb V₂O₅-WO₃/TiO₂ catalysts.

The Effect of Iron Content on the Ammonia Selective Catalytic Reduction Reaction (NH3-SCR) Catalytic Performance of FeOx/SAPO-34

Iron-based catalysts are regarded as promising candidates for the ammonia selective catalytic reduction reaction (NH₃-SCR) which show good catalytic activity at medium and high temperatures, whereas SAPO-34 molecular sieves have a micro-pore structure and are ideal catalyst carriers. In this paper, four FeO_x/SAPO-34 molecular sieve catalysts with different iron contents (Fe = 1%, 2%, 3%, 4%) were prepared using an impregnation method. The effect of iron content on the surface properties and catalytic activity was investigated by a series of characterization techniques including XRD, SEM, BET, XPS, H₂-TPR and NH₃-TPD. Iron species in the FeO_x/SAPO-34 catalysts exist in the form of isolated iron ions or well-dispersed small crystals and iron oxide species clusters. With the addition of iron content, the integrity of CHA (chabazite) zeolite structure remained, but the crystallinity was affected. The FeO_x/SAPO-34 catalyst with 3% Fe loading showed a relatively flat surface with no large-diameter particles and strong oxidation-reduction ability. Meanwhile, more acidic sites are exposed, which accelerated the process of catalytic reaction. Thus, the FeO_x/SAPO-34 catalyst with 3% Fe showed the best NO conversion performance among the four catalysts prepared and maintained more than 90% NO conversion efficiency in a wide temperature range from 310 °C to 450 °C.

Compensation or Aggravation: Pb and SO2 Copoisoning Effects over Ceria-Based Catalysts for NOx Reduction

Severe catalyst deactivation caused by multiple poisons, including heavy metals and SO₂, remains an obstinate issue for the selective catalytic reduction (SCR) of NO_x by NH₃. The copoisoning effects of heavy metals and SO₂ are still unclear and irreconcilable. Herein, the unanticipated differential compensated or aggravated Pb and SO₂ copoisoning effects over ceria-based catalysts for NO_x reduction was originally unraveled. It was demonstrated that Pb and SO₂ exhibited a compensated copoisoning effect over the CeO₂/TiO₂ (CT) catalyst with sole active CeO₂ sites but an aggravated copoisoning effect over the CeO₂-WO₃/TiO₂ (CWT) catalyst with dual active CeO₂ sites and acidic WO₃ sites. Furthermore, it was uniquely revealed that Pb preferred bonding with CeO₂ among CT while further being combined with SO₂ to form PbSO₄ after copoisoning, which released the poisoned active CeO₂ sites and rendered the copoisoned CT catalyst a recovered reactivity. In comparison, Pb and SO₂ would poison acidic WO₃ sites and active CeO₂ sites, respectively, resulting in a seriously degraded reactivity of the copoisoned CWT catalyst. Therefore, this work thoroughly illustrates the internal mechanism of differential compensated or aggravated deactivation effects for Pb and SO₂ copoisoning over CT and CWT catalysts and provides effective solutions to design ceria-based SCR catalysts with remarkable copoisoning resistance for the coexistence of heavy metals and SO₂.

Selective catalytic reduction of NO by NH3 using Mn-based catalysts supported by Ukrainian clinoptiolite and lightweight expanded clay aggregate

In this study, Mn-based multicomponent catalysts supported by two different carriers (lightweight expanded clay aggregate and the Ukrainian clinoptiolite) were prepared by electroless metal deposition method and tested for the selective catalytic reduction of NO with ammonia (NH₃-SCR de-NO). Prior to the activity test, all the catalysts prepared were characterized by inductively coupled plasma optical emission spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive X-ray mapping, X-ray photoelectron spectroscopy, H₂-TPR and NH₃-TPD techniques. The particular interest of the present study was focused on the investigation of the carrier's role in the NO catalytic reduction and the promoting effect provided by the incorporation of the small amount of Pt (0.1 wt.%) in the Mn-based catalytic layer. The results revealed that the carrier's role in the NO catalytic conversion can be considered as a factor determining the effectiveness of the conversion process. Ukrainian clinoptiolite was proved to be a more attractive carrier for the preparation of the effective SCR de-NO catalysts due to its intrinsic sorption capacity, surface acidity and the redox potential. The high NO conversion efficiency provided by the Mn-based clinoptiolite-supported catalysts can be explained by the synergistic effect between the carrier and the active species deposited. It was shown that both the Mn_97.6Cu_2.4/clinoptiolite and the Mn_97.5Co_2.5/clinoptiolite catalysts can be successfully applied as the low-temperature (100-300°C) catalysts for NH₃-SCR de-NO. When the NO removal efficiency varies in the range of 86-91%, the additional incorporation of Pt in the active layer in the amount of 0.1 wt.% can enhance the NO reduction by about 5% on average.

Boosting SO2-Resistant NOx Reduction by Modulating Electronic Interaction of Short-Range Fe-O Coordination over Fe2O3/TiO2 Catalysts

SO₂-resistant selective catalytic reduction (SCR) of NO_x remains a grand challenge for eliminating NO_x generated from stationary combustion processes. Herein, SO₂-resistant NO_x reduction has been boosted by modulating electronic interaction of short-range Fe-O coordination over Fe₂O₃/TiO₂ catalysts. We report a remarkable SO₂-tolerant Fe₂O₃/TiO₂ catalyst using sulfur-doped TiO₂ as the support. Via an array of spectroscopic and microscopic characterizations and DFT theoretical calculations, the active form of the dopant is demonstrated as SO₄^2- residing at subsurface TiO₆ locations. Sulfur doping exerts strong electronic perturbation to TiO₂, causing a net charge transfer from Fe₂O₃ to TiO₂ via increased short-range Fe-O coordination. This electronic effect simultaneously weakens charge transfer from Fe₂O₃ to SO₂ and enhances that from NO/NH₃ to Fe₂O₃, resulting in a remarkable "killing two birds with one stone" scenario, that is, improving NO/NH₃ adsorption that benefits SCR reaction and inhibiting SO₂ poisoning that benefits catalyst long-term stability.

CeO2 Nanoparticle-Loaded MnO2 Nanoflowers for Selective Catalytic Reduction of NOx with NH3 at Low Temperatures

CeO₂ nanoparticle-loaded MnO₂ nanoflowers, prepared by a hydrothermal method followed by an adsorption-calcination technique, were utilized for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. The effects of Ce/Mn ratio and thermal calcination temperature on the NH₃-SCR activity of the CeO₂-MnO₂ nanocomposites were studied comprehensively. The as-prepared CeO₂-MnO₂ catalysts show high NO_x reduction efficiency in the temperature range of 150-300 °C, with a complete NO_x conversion at 200 °C for the optimal sample. The excellent NH₃-SCR performance could be ascribed to high surface area, intimate contact, and strong synergistic interaction between CeO₂ nanoparticles and MnO₂ nanoflowers of the well-designed composite catalyst. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) characterizations evidence that the SCR reaction on the surface of the CeO₂-MnO₂ nanocomposites mainly follows the Langmuir-Hinshelwood (L-H) mechanism. Our work provides useful guidance for the development of composite oxide-based low temperature NH₃-SCR catalysts.

In-Use Passenger Vessel Emission Rates of Black Carbon and Nitrogen Oxides

This study quantified emission factors of black carbon (BC) and nitrogen oxides (NO_x) from 21 engines on in-use excursion vessels and ferries operating in California's San Francisco Bay, including EPA uncertified and Tier 1-4 engines and across engine operating modes. On average, ∼60 fuel-based emission factors per engine were measured using a novel combination of exhaust plume capture combined with GPS location and speed data that can be more readily deployed than common portable emissions measurement systems. BC and NO_x emission factors (g kg^-1) were lowest and least variable during fast cruising and highest during maneuvering and docked operation. Selective catalytic reduction (SCR) reduced NO_x emissions by ∼80% when functional. However, elevated NO_x emissions that exceeded corresponding exhaust standards were measured on most Tier 3 and Tier 4 engines sampled, which can be attributed to inactive SCR during frequent low engine load operation. In contrast, BC emissions exceeded the PM emission standard for only one engine, and SCR systems employed as a NO_x reduction technology also reduced emitted BC. Using these measured emission factors to compare commuting options, we show that the CO₂-equivalent emissions per passenger-kilometer are comparable when commuting by car and ferry, but BC and NO_x emissions can be several to more than ten times larger when commuting by ferry.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites

Selective catalytic reduction of NO_x in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NO_x reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO₂ pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NO_x into N₂. It was not deactivated during the NO_x reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NO_x reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NO_x from the complex flue gas in practical applications.

SO2-Tolerant Catalytic Reduction of NOx via Tailoring Electron Transfer between Surface Iron Sulfate and Subsurface Ceria

Currently, SO₂-induced catalyst deactivation from the sulfation of active sites turns to be an intractable issue for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. Herein, SO₂-tolerant NO_x reduction has been originally demonstrated via tailoring the electron transfer between surface iron sulfate and subsurface ceria. Engineered from the atomic layer deposition followed by the pre-sulfation method, the structure of surface iron sulfate and subsurface ceria was successfully constructed on CeO₂/TiO₂ catalysts, which delivered improved SO₂ resistance for NO_x reduction at 250 °C. It was demonstrated that the surface iron sulfate inhibited the sulfation of subsurface Ce species, while the electron transfer from the surface Fe species to the subsurface Ce species was well retained. Such an innovative structure of surface iron sulfate and subsurface ceria notably improved the reactivity of NH_x species, thus endowing the catalysts with a high NO_x reaction efficiency in the presence of SO₂. This work unraveled the specific structure effect of surface iron sulfate and subsurface ceria on SO₂-toleant NO_x reduction and supplied a new point to design SO₂-tolerant catalysts by modulating the unique electron transfer between surface sulfate species and subsurface oxides.

SO2 Poisoning of Cu-CHA deNO x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH₃-assisted selective catalytic reduction of NO (NH₃-SCR) for the abatement of NO _x emission from diesel vehicles. However, the presence of a small amount of SO₂ in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO₂ using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO₂ exposure, we have selectively prepared Cu^I and Cu^II species with different ligations, which are relevant for the NH₃-SCR reaction. The highest reactivity toward SO₂ was observed for Cu^II species coordinated to both NH₃ and extraframework oxygen, in particular for [Cu^II ₂(NH₃)₄O₂]²⁺ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO₂ uptake was significantly lower. These results explain why SO₂ mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [Cu^II ₂(NH₃)₄O₂]²⁺ is a crucial intermediate in the low-temperature NH₃-SCR catalytic cycle.

Alkali and Heavy Metal Copoisoning Resistant Catalytic Reduction of NOx via Liberating Lewis Acid Sites

The catalyst deactivation caused by the coexistence of alkali and heavy metals remains an obstacle for selective catalytic reduction of NO_x with NH₃. Moreover, the copoisoning mechanism of alkali and heavy metals is still unclear. Herein, the copoisoning mechanism of K and Cd was revealed from the adsorption and variation of reaction intermediates at a molecular level through time-resolved in situ spectroscopy combined with theoretical calculations. The alkali metal K mainly decreased the adsorption of NH₃ on Lewis acid sites and altered the reaction more depending on the formation of the NH₄NO₃ intermediate, which is highly related to NO_x adsorption and activation. However, Cd further inhibited the generation of active nitrate intermediates and thus decreased the NO_x abatement about 60% on potassium-poisoned CeTiO_x catalysts. Physically mixing with acid additives for CeTiO_x catalysts could significantly liberate the active Lewis acid sites from the occupation of alkali metals and relieve the high dependence on NO_x adsorption and activation, thus recovering the NO_x removal rate to the initial state. This work revealed the copoisoning mechanism of K and Cd on Ce-based de-NO_x catalysts and developed a facile anti-poisoning strategy, which paves a way for the development of durable catalysts among alkali and heavy metal copoisoning resistant catalytic reduction of NO_x.

SO2- and H2O-Tolerant Catalytic Reduction of NOx at a Low Temperature via Engineering Polymeric VOx Species by CeO2

Selective catalytic reduction (SCR) of NO_x over V₂O₅-based oxide catalysts has been widely used, but it is still a challenge to efficiently reduce NO_x at low temperatures under SO₂ and H₂O co-existence. Herein, SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature has been originally demonstrated via engineering polymeric VO_x species by CeO₂. The polymeric VO_x species were tactfully engineered on Ce-V₂O₅ composite active sites via the surface occupation effect of Ce, and the obtained catalysts exhibited remarkable low-temperature activity and strong SO₂ and H₂O tolerance at 250 °C. The strong interaction between Ce and V species induced the electron transfer from V to Ce and tuned the SCR reaction via the E-R pathway between the NH₄⁺/NH₃ species and gaseous NO. In the presence of SO₂ and H₂O, the polymeric VO_x species had not been hardly influenced, while the formation of sulfate species on Ce sites not only promoted the adsorption of NH₄⁺ species and the reaction between gaseous NO and NH₄⁺ but also facilitated the decomposition of ammonium bisulfate through weakening the strong bond between HSO₄^- and NH₄⁺. This work provided a new strategy for SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature.

Self-Defense Effects of Ti-Modified Attapulgite for Alkali-Resistant NOx Catalytic Reduction

Nowadays, the serious deactivation of deNO_x catalysts caused by alkali metal poisoning was still a huge bottleneck in the practical application of selective catalytic reduction of NO_x with NH₃. Herein, alkali-resistant NO_x catalytic reduction over metal oxide catalysts using Ti-modified attapulgite (ATP) as supports has been originally demonstrated. The self-defense effects of Ti-modified ATP for alkali-resistant NO_x catalytic reduction have been clarified. Ti-modified ATP with self-defense ability was obtained by removing alkaline metal cation impurities in the natural ATP materials without destroying its initial layered-chain structure through the ion-exchange procedure, accompanied with an obvious enrichment of Brønsted acid and Lewis acid sites. The self-defense effects embodied that both ion-exchanged Ti octahedral centers and abundant Si-OH sites in the Ti-ion-exchange-modified ATP could effectively anchor alkali metals via coordinate bonding or ion-exchange process, which induced alkali metals to be immobilized by the Ti-ion-exchange-modified ATP carrier rather than impair active species. Under this special protection of self-defense effects, Ti-ion-exchange-modified ATP supported catalysts still retained plentiful acidic sites and superior redox ability even after alkali metal poisoning, giving rise to the maintenance of sufficient NH_x and NO_x adsorption and the subsequent efficient reaction, which in turn resulted in high NO_x catalytic reduction capacity of the catalyst. The strategy provided new inspiration for the development of novel and efficient selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) catalysts with high alkali resistance.

Dynamic Change of Active Sites of Supported Vanadia Catalysts for Selective Catalytic Reduction of Nitrogen Oxides

Selective catalytic reduction of NOx by ammonia (NH₃-SCR) on V₂O₅/TiO₂ catalysts is a widely used commercial technology in power plants and diesel vehicles due to its high elimination efficiency for NOx removal. However, the mechanistic aspects of the NH₃-SCR reaction, especially the active sites on the V₂O₅/TiO₂ catalysts, are still a puzzle. Herein, using combined operando spectroscopy and density functional theory calculations, we found that the reactivity of the Lewis acid site was significantly overestimated due to its conversion to the Brønsted acid site. Such interconversion makes it challenging to measure the intrinsic reactivity of different acid sites accurately. In contrast, the abundant V-OH Brønsted acid sites govern the overall NOx reduction rate in realistic exhaust containing water vapor. Moreover, the vanadia species cycle between V⁵⁺═O and V⁴⁺-OH during NOx reduction, and the re-oxidation of V⁴⁺ species to form V⁵⁺ is the rate-determining step.

Revealing the Synergistic Deactivation Mechanism of Hydrothermal Aging and SO2 Poisoning on Cu/SSZ-13 under SCR Condition

In real-world application, Cu/SSZ-13 simultaneously suffers severe deactivation from hydrothermal aging and SO₂ poisoning during the periodic regeneration of diesel particulate filter (DPF). Herein, we first investigated the synergistic deactivation mechanism of hydrothermal aging and SO₂ poisoning on Cu/SSZ-13 under SCR condition. Hydrothermal aging alone induces more severe degradation of selective catalytic reduction (SCR) performance than SO₂ poisoning alone, while the presence of SO₂ during hydrothermal aging causes further worse SCR performance compared with hydrothermal aging alone. Hydrothermal aging not only damages Si-OH-Al sites, particularly in four-membered ring (4MR) of the CHA cage, but also brings the conversion of ZCuOH, leading to the formation of inactive CuO/CuAlO_x species. By contrast, SO₂ poisoning alone is more prone to promote the transformation of ZCuOH to Z₂Cu. Synergistic deactivation of hydrothermal aging and SO₂ poisoning would exacerbate the damage of Si-OH-Al sites and then the formation of CuO/CuAlO_x species. These results are expected to assist the knowledge-based catalyst design for diesel aftertreatment applications.

SO2-Induced Alkali Resistance of FeVO4/TiO2 Catalysts for NOx Reduction

Selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) is an efficient NO_x abatement strategy, but deNO_x catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances such as K, SO₂, etc. in the flue gas. It is essential to understand the interaction among various poisons and their effects on NO_x abatement. Here, we unexpectedly identified the K migration behavior induced by SO₂ over K-poisoned FeVO₄/TiO₂ catalysts, which led to alkali-poisoning buffering and activity recovery. It has been demonstrated that the K would occupy both redox and acidic sites, which severely reduced the reactivity of FeVO₄/TiO₂ catalysts. After the sulfuration of the K-poisoned catalyst, SO₂ preferred to be combined with the surface K₂O, lengthened the K-O_Fe and K-O_V, and thus released the active sites poisoned by K₂O, thereby preserving an increase in the activity. As a result, for the K-poisoned catalyst, the conversion of NO_x increased from 21 to 97% at 270 °C after the sulfuration process. This work contributes to the understanding of the specific interaction between alkali metals and SO₂ on deNO_x catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NO_x reduction.

Analysis of NH3 -TPD Profiles for CuSSZ-13 SCR Catalyst of Controlled Al Distribution - Complexity Resolved by First Principles Thermodynamics of NH3 Desorption, IR and EPR Insight into Cu Speciation*

NH₃ temperature-programmed desorption (NH₃ -TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NO_x . Herein, we propose an interpretation of NH₃ -TPD results, which takes into account the temperature-induced dynamics of NH₃ interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH₃ adsorption on single Cu²⁺ , Cu⁺ , [CuOH]⁺ centers, dimeric [Cu-O-Cu]²⁺ , [Cu-O₂ ^2- -Cu]² species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH₃ desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH₃ -TPD experiment. Thus, a rigorous interpretation of the NH₃ -TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers.

Thermogravimetric Experiment of Urea at Constant Temperatures

There are still many unsolved mysteries in the thermal decomposition process of urea. This paper studied the thermal decomposition process of urea at constant temperatures by the thermal gravimetric–mass spectrometry analysis method. The results show that there are three obvious stages of mass loss during the thermal decomposition process of urea, which is closely related to the temperature. When the temperature was below 160 °C, urea decomposition almost did not occur, and molten urea evaporated slowly. When the temperature was between 180 and 200 °C, the content of biuret, one of the by-products in the thermal decomposition of urea, reached a maximum. When the temperature was higher than 200 °C, the first stage of mass loss was completed quickly, and urea and biuret rapidly broke down. When the temperature was about 240 °C, there were rarely urea and biuret in residual substance; however, the content of cyanuric acid was still rising. When the temperature was higher than 280°C, there was a second stage of mass loss. In the second stage of mass loss, when the temperature was higher than 330 °C, mass decreased rapidly, which was mainly due to the decomposition of cyanuric acid. When the temperature was higher than 380 °C, the third stage of mass loss occurred. However, when the temperature was higher than 400 °C, and after continuous heating was applied for a sufficiently long time, the residual mass was reduced to almost zero eventually.

Ammonium Ion Enhanced V2O5-WO3/TiO2 Catalysts for Selective Catalytic Reduction with Ammonia

Selective catalytic reduction (SCR) is the most efficient NO_X removal technology, and the vanadium-based catalyst is mainly used in SCR technology. The vanadium-based catalyst showed higher NO_X removal performance in the high-temperature range but catalytic efficiency decreased at lower temperatures, following exposure to SO_X because of the generation of ammonium sulfate on the catalyst surface. To overcome these limitations, we coated an NH₄⁺ layer on a vanadium-based catalyst. After silane coating the V₂O₅-WO₃/TiO₂ catalyst by vapor evaporation, the silanized catalyst was heat treated under NH₃ gas. By decomposing the silane on the surface, an NH₄⁺ layer was formed on the catalyst surface through a substitution reaction. We observed high NO_X removal efficiency over a wide temperature range by coating an NH₄⁺ layer on a vanadium-based catalyst. This layer shows high proton conductivity, which leads to the reduction of vanadium oxides and tungsten oxide; additionally, the NO_X removal performance was improved over a wide temperature range. These findings provide a new mothed to develop SCR catalyst with high efficiency at a wide temperature range.

Alkali-Resistant Catalytic Reduction of NOx via Naturally Coupling Active and Poisoning Sites

Releasing the poisoning effect of alkali metals over catalysts is still an intractable issue for selective catalytic reduction (SCR) of NOx with ammonia. The presence of K in fly ash always dramatically suppressed catalytic activity by impairing acidity and redox properties, leading to severe reduction of lifetime for SCR catalysts. Herein, alkali-resistant NOx reduction over TiO2-supported Fe2(SO4)3 catalysts was originally demonstrated via naturally coupling active and poisoning sites. Notably, TiO2-supported Fe2(SO4)3 catalysts expressed admirable NOx conversion and K resistance within a quite broad temperature window of 200-500 °C. The catalysts with more conserved sulfate species revealed that sulfate groups preferred to migrate from the bulk phase to surface, thus effectively binding with K poisons to release the damage on iron active sites. Because of protection effects of migrated sulfates and closely coupling effects with Fe active sites, NH3 and NO adsorption amounts and rates were well maintained. In this way, Fe metal sites and sulfate species closely coupled together on a self-preserved TiO2-supported Fe2(SO4)3 catalyst played essential roles as highly active sites and unique poisoning sites. This work paves a new way to design SCR catalysts with superior alkali resistance that are more reliable in practical deNOx application.

Improved NOx Reduction over Phosphate-Modified Fe2O3/TiO2 Catalysts Via Tailoring Reaction Paths by In Situ Creating Alkali-Poisoning Sites

The deactivation issue arising from alkali poisoning over catalysts is still a challenge for the selective catalytic reduction of NO_x by NH₃. Herein, improved NO_x reduction in the presence of alkaline metals over phosphate-modified Fe₂O₃/TiO₂ catalysts has been originally demonstrated via tailoring the reaction paths by in situ creating alkali-poisoning sites. The introduction of phosphate results in the partial formation of iron phosphate species and makes the catalyst to mainly exhibit the characteristics of FePO₄, which is responsible for the widened temperature window and enhanced alkali resistance. The tetrahedral [FeO₄]/[PO₄] structures in iron phosphate act as the Brønsted acid sites to increase the catalyst surface acidity. In addition, the formation of an Fe-O-P structure enhances the redox ability and increases surface adsorbed oxygen. Furthermore, the created phosphate groups (PO₄^3-) serving as alkali-poisoning sites preferentially combine with potassium so that iron species on the active sites are protected. Therefore, the enhanced NH₃ species adsorption capacity, improved redox ability, and active nitrate species remaining in the phosphate-modified Fe₂O₃/TiO₂ catalyst ensure the de-NO_x activity after being poisoned by alkali metals through the Langmuir-Hinshelwood reaction pathway. Hopefully, this novel strategy could provide an inspiration to design novel catalysts to control NO_x emission with extraordinary resistance to alkaline metals.

Effect of Catalyst Crystallinity on V-Based Selective Catalytic Reduction with Ammonia

In this study, we synthesized V₂O₅-WO₃/TiO₂ catalysts with different crystallinities via one-sided and isotropic heating methods. We then investigated the effects of the catalysts’ crystallinity on their acidity, surface species, and catalytic performance through various analysis techniques and a fixed-bed reactor experiment. The isotropic heating method produced crystalline V₂O₅ and WO₃, increasing the availability of both Brønsted and Lewis acid sites, while the one-sided method produced amorphous V₂O₅ and WO₃. The crystalline structure of the two species significantly enhanced NO₂ formation, causing more rapid selective catalytic reduction (SCR) reactions and greater catalyst reducibility for NO_X decomposition. This improved NO_X removal efficiency and N₂ selectivity for a wider temperature range of 200 °C–450 °C. Additionally, the synthesized, crystalline catalysts exhibited good resistance to SO₂, which is common in industrial flue gases. Through the results reported herein, this study may contribute to future studies on SCR catalysts and other catalyst systems.

Methane Combustion Using Pd Deposited on CeOx-MnOx/La-Al2O3 Pellistors

Pd deposited on CeO_x-MnO_x/La-Al₂O₃ has been prepared as a sensitive material for methane (CH₄) detection. The effect of different amounts (1.25%, 2.5% and 5%) of Pd loading has been investigated. The as prepared materials were deposited on Pt microcoils using a drop-coating method, as a way of developing pellistors operated using a Wheatstone bridge configuration. By spanning the operating temperature range between 300 °C and 550 °C, we established the linearity region as well as the maximum sensitivity towards 4900 ppm of CH₄. By making use of the sigmoid dependence of the output voltage signal from the Wheatstone bridge, the gas surface reaction and diffusion phenomena have been decoupled. The pellistor with 5% Pd deposited on CeO_x-MnO_x/La-Al₂O₃ exhibited the highest selective-sensitivity in the benefit of CH₄ detection against threshold limits of carbon monoxide (CO), sulfur dioxide (SO₂) and hydrogen sulfide (H₂S). Accordingly, adjusting the percent of Pd makes the preparation strategies of pellistors good candidates towards CH₄ detection.

Emission of Toxic HCN During NOx Removal by Ammonia SCR in the Exhaust of Lean-Burn Natural Gas Engines

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment. Natural gas (NG) engines, in particular at lean conditions, emit less CO2 in comparison to combustion engines operated with liquid fuels but NG engines still require emission control devices for NOx removal. Using state-of-the-art technologies for selective catalytic reduction (SCR) of NOx with NH3 , we evaluated the interplay of the reducing agent NH3 and formaldehyde, which is always present in the exhaust of NG engines. Our results show that a significant amount of highly toxic hydrogen cyanide (HCN) is formed. All catalysts tested partially convert formaldehyde to HCOOH and CO. Additionally, they form secondary emissions of HCN due to catalytic reactions of formaldehyde and its oxidation intermediates with NH3 . With the present components of the exhaust gas aftertreatment system the HCN emissions are not efficiently converted to non-polluting gases. The development of more advanced catalyst formulations with improved oxidation activity is mandatory to solve this novel critical issue.

Facile Fabrication of Ce/V-Modified Multi-Channel TiO2 Nanotubes and Their Enhanced Selective Catalytic Reduction Performance

To optimize one-dimensional (1D) TiO₂ nanofibers, tailor-made multi-channel TiO₂ nanotubes have been successfully fabricated by electrospinning technology. After loading with Ce and V, the CeVTi-tube catalyst exhibited a broad working temperature window and acceptable resistance to H₂ O and SO₂ for elimination of NO_x . The corresponding analysis revealed that the multi-channel structure provided more surface adsorbed oxygen species and that the wall of nanotubes anchored active components efficiently, which was beneficial to improve the stability as well as dispersion of the active components. Besides, a synergistic effect between Ce and V easily occurred at the CeVTi-tube catalyst, and its reducibility was significantly improved since the electron transformation between Ce and V was dramatically enhanced. Consequently, the tailor-made multi-channel CeVTi-tube catalyst exhibited satisfied de-NO_x efficiency at the temperature range of 220-460 °C. It seemed that the multi-channel TiO₂ nanotubes hold great potential as an excellent carrier for SCR catalysts.

The utilization of red mud waste as industrial honeycomb catalyst for selective catalytic reduction of NO

As a new way for the high-value utilization of red mud (RM) waste, we proposed an improved approach to prepare the RM-based sludge/powder via the sulfuric acid hydrothermal dissolution and NH₃ aqueous precipitation route and then the RM-based industrial-sized honeycomb (150 × 150 × 600 mm) was successfully produced by the extrusion moulding method in pilot scale. The synthesized RM-based powdery/honeycomb catalyst exhibited more than 80% deNO _x activity and good durability of H₂O and SO₂ above 350°C. But the decline of NO conversion was also observed above 350°C, which was confirmed to result from the increased oxygenation of NH₃ at high temperature. To improve the NO conversion at high temperature, NH₃ was shunted and injected into the catalyst bed at two different places (entrance and centre) to facilitate its uniform distribution, which relieved the oxidation of NH₃ and increased deNO _x efficiency with 98% NO conversion at 400°C. This work explored the industrial application feasibility for the RM-based honeycomb catalyst as well as the possible solution to decrease the oxygenation of NH₃ at high temperature, which presented a valuable reference for the further pilot tests of RM catalyst in industry.

Oxide Catalysts with Ultrastrong Resistance to SO2 Deactivation for Removing Nitric Oxide at Low Temperature

Nitrogen oxides are one of the major sources of air pollution. To remove these pollutants originating from combustion of fossil fuels remains challenging in steel, cement, and glass industries as the catalysts are severely deactivated by SO₂ during the low-temperature selective catalytic reduction (SCR) process. Here, a MnO_X /CeO₂ nanorod catalyst with outstanding resistance to SO₂ deactivation is reported, which is designed based on critical information obtained from in situ transmission electron microscopy (TEM) experiments under reaction conditions and theoretical calculations. The catalysts show almost no activity loss (apparent NO_X reaction rate kept unchanged at 1800 µmol g^-1 h^-1 ) for 1000 h test at 523 K in the presence of 200 ppm SO₂ . This unprecedented performance is achieved by establishing a dynamic equilibrium between sulfates formation and decomposition over the CeO₂ surface during the reactions and preventing the MnO_X cluster from the steric hindrance induced by SO₂ , which minimized the deactivation of the active sites of MnO_X /CeO₂ . This work presents the ultralong lifetime of catalysts in the presence of SO₂ , along with decent activity, marking a milestone in practical applications in low-temperature selective catalytic reduction (SCR) of NO_X .

Deposition of Selective Catalytic Reduction Coating on Wire-Mesh Structure by Atmospheric Plasma Spraying

A series of catalytic coatings consisting of MnOx-CeO2 and TiO2 support were prepared by atmospheric plasma spraying, which was aimed at the application of selective catalytic reduction (SCR) of NOx. The effect of the load of active component on the coating was firstly studied. The results showed that all the coating presented the highest catalytic activity at approximately 350 °C and the coating with the composition of 20MnOx/5CeO2/TiO2 (wt%) achieved the most powerful performance. The coating was then prepared on a wire-mesh structure substrate, which can be easily assembled as a gas filter. The results showed that the specific surface area was greatly increased resulting in the significant improvement of the catalytic activity of the coating. This strategy offered a promising possibility of removing NOx and particulate fliting simultaneously in industrial applications.

Spatially Confined Tuning the Interfacial Synergistic Catalysis in Mesochannels toward Selective Catalytic Reduction

Low-temperature selective catalytic reduction of nitrogen oxides (NO _x) with NH₃ (NH₃-SCR) has been identified as a promising strategy to mitigate the pollution of NO _x. The fine control of synergistic effect and the suppression of aggregation of the active component, however, are still the challenge because of the weak interaction between the active component and matrix. In this work, a series of Ce-promoted Mn-based heterogeneous catalysts supported on mesoporous silica (SBA-15) with different Mn contents were prepared by two separated impregnation processes. Low-temperature NH₃-SCR activity demonstrates that the Mn content in the catalyst has a great influence on the activity of the NH₃-SCR reaction. The 20% MnO _x-CeO _x/SBA-15 catalyst exhibited the best catalytic performance in a broad temperature window. Moreover, it exhibits enhanced resistance to SO₂ and H₂O and long-term durability during 72 h reaction. The highly dispersive active phase, the formation of solid solution, the high ratio of Ce³⁺, and the spatial confinement effect largely contribute to the outstanding activity and durability of the 20% MnO _x-CeO _x/SBA-15 catalyst. Finally, a monolithic catalyst fabricated by the 20% MnO _x-CeO _x/SBA-15 catalyst powder and cordierite substrate show promising industrial application.

Dual Promotional Effects of TiO2-Decorated Acid-Treated MnO x Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NO x

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NO _x) emission. Hence, improved selective catalytic reduction of NO _x with ammonia (NH₃-SCR) in the presence of alkali metals is a major issue for practical NO _x removal. In this work, we developed a novel TiO₂-decorated acid-treated MnO _x octahedral molecular sieve (OMS-5(H)@TiO₂) catalyst with improved alkali-resistant NO _x reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO₂ catalysts were clarified. It was found that the special structure of the acid-treated MnO _x octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNO _x activity at low temperature. Subsequently, the decoration by TiO₂ further improved the redox properties by accelerating the high ratio of Mn⁴⁺ and O_α on the surface of the highly active (OMS-5(H)@TiO₂) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO₂ further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNO _x reaction. Besides, a "fast SCR" process was observed to certify that the decoration of TiO₂ promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO₂ decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNO _x catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NO _x reduction efficiency in the presence of alkali metals.

Ammonia Distribution Measurement on a Hot Gas Test Bench Applying Tomographical Optical Methods

Measuring the distribution of gas concentration is a very common problem in a variety of technological fields. Depending on the detectability of the gas, as well as the technological progress of the sector, different methods are used. In this paper, we present a device and methods to detect the ammonia concentration distribution in the exhaust system of diesel engines in order to increase the performance of the exhaust aftertreatment system. The device has been designed for usage on a hot gas test bench simulating exhaust gas conditions. It consists of multiple optical beams measuring ammonia line concentrations by applying nondispersive absorption spectroscopy in the deep ultraviolet region. The detectors consist of photodiodes allowing high sampling rates up to 3 kHz while providing a high signal-to-noise ratio. A detection limit of only 1 ppm has been achieved despite the short path length of only eight centimeters. The obtained line concentrations form an inverse problem. The methodology of the tomographic techniques is described in detail in order to best solve the inverse problem and obtain the ammonia concentration distribution images for each time step.

MnO2 Nanowire-CeO2 Nanoparticle Composite Catalysts for the Selective Catalytic Reduction of NO x with NH3

MnO _x-based catalysts have been applied to the selective catalytic reduction of NO _x with ammonia (NH₃) owing to their high NO _x removal efficiency and catalytic stability. In general, the fabrication of a variety of nanomaterials in a complex structure requires complicated processes, including heat treatment and a series of cleaning steps. In addition, MnO₂ which has diverse polymorphs, exhibits different catalytic effects depending on its crystalline structure. Among them, synthesizing the ε-MnO₂ phase, which functions as a nanocatalyst, has been the most difficult and has hardly been reported. Here, we report the synthesis of heterostructured composite nanocatalysts consisting of ε-MnO₂ nanowires (NWs) and CeO₂ nanoparticles (NPs) by applying pulsed currents sequentially. This method drastically simplifies the overall process compared to the conventional techniques. Through X-ray diffraction and transmission electron microscopy, it was confirmed that 2-3 nm of CeO₂ NPs were formed on the surfaces of the ε-MnO₂ NWs. The de-NO _x efficiency of the nanocatalysts was analyzed in terms of content variation, specific surface area, and the elemental chemical state of the surface. A ceramic filter containing the nanocatalysts shows a high catalytic activity over the broad operating temperature range 100-400 °C. In the low-temperature region, ε-MnO₂ plays a major role in determining the catalytic property, which is consistent with the Brunauer-Emmett-Teller (BET), H₂ temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) results. On the other hand, in the high-temperature region, the efficiency increases gradually as the content of CeO₂ increases. The H₂ TPR, NH₃-temperature-programmed desorption, and XPS patterns reveal why the composite exhibits such superior characteristics in the temperature range mentioned above.

Enhanced Oxygen Vacancies in a Two-Dimensional MnAl-Layered Double Oxide Prepared via Flash Nanoprecipitation Offers High Selective Catalytic Reduction of NOx with NH₃

A two-dimensional MnAl-layered double oxide (LDO) was obtained by flash nanoprecipitation method (FNP) and used for the selective catalytic reduction of NOx with NH₃. The MnAl-LDO (FNP) catalyst formed a particle size of 114.9 nm. Further characterization exhibited rich oxygen vacancies and strong redox property to promote the catalytic activity at low temperature. The MnAl-LDO (FNP) catalyst performed excellent NO conversion above 80% at the temperature range of 100⁻400 °C, and N₂ selectivity above 90% below 200 °C, with a gas hourly space velocity (GHSV) of 60,000 h-1, and a NO concentration of 500 ppm. The maximum NO conversion is 100% at 200 °C; when the temperature in 150⁻250 °C, the NO conversion can also reach 95%. The remarkable low-temperature catalytic performance of the MnAl-LDO (FNP) catalyst presented potential applications for controlling NO emissions on the account of the presentation of oxygen vacancies.

Selective catalytic reduction of nitric oxide with carbon monoxide over alumina-pellet-supported catalysts in the presence of excess oxygen

Selective catalytic reduction of nitrogen oxides with carbon monoxide (CO-SCR) is a promising technology to remove NO_x and CO simultaneously from flue gas. The thermodynamic analyses of catalytic process were performed toward four kinds of active metal oxides (Cu_xO_y, Co_xO_y, Mn_xO_y and Ce_xO_y). According to the standard Gibbs free-energy changes calculated, Mn had better resistance to oxygen than Cu, Co and Ce, while Cu and Ce had better resistance to water vapor poisoning, as active metals. Then, a series of binary- and ternary-preformed catalysts with different metal ratios were prepared by the impregnation method using Al₂O₃ pellets as support and tested in excess oxygen (16 vol%) atmosphere with or without SO₂. The results of experiment were analyzed based on thermodynamic analyses. Results indicated that the NO conversions of Cu-Co/Al₂O₃ catalysts increased with the rise of reaction temperature; however, the tendency changed at 160°C for Cu-Mn/Al₂O₃. Besides, the NO conversions of Cu-Mn/Al₂O₃ were better than Cu-Co/Al₂O₃. The catalysts with the metal ratio of 1.5 had the best denitrification performance. Among various binary catalysts, Cu-Mn/Al₂O₃ with the metal ratio Cu:Mn of 1.5 showed promising activity for CO-SCR, giving nearly 90% NO conversion. Besides, the doping of Ce could inhibit the sulfur poisoning and promote the oxide of CO under experimental conditions.

Recovery Improvement for Large-Area Tungsten Diselenide Gas Sensors

Semiconducting two-dimensional transition-metal dichalcogenides are considered promising gas-sensing materials because of their large surface-to-volume ratio, excellent electrical conductivity, and susceptible surfaces. However, enhancement of the recovery performance has not yet been intensively explored. In this study, a large-area uniform WSe₂ is synthesized for use in a high-performance semiconductor gas sensor. At room temperature, the WSe₂ gas sensor shows a significantly high response (4140%) to NO₂ compared to the use of NH₃, CO₂, and acetone. This paper demonstrates improved recovery of the WSe₂ gas sensor's NO₂-sensing performance by utilizing external thermal energy. In addition, a novel strategy for improving the recovery of the WSe₂ gas sensor is realized by reacting NH₃ and adsorbed NO₂ on the surface of WSe₂: the NO₂ molecules are spontaneously desorbed, and the recovery time is dramatically decreased (85 min → 43 s). It is expected that the fast recovery of the WSe₂ gas sensor achieved here will be used to develop an environmental monitoring system platform.

A review of carbon-based and non-carbon-based catalyst supports for the selective catalytic reduction of nitric oxide

Various types of carbon-based and non-carbon-based catalyst supports for nitric oxide (NO) removal through selective catalytic reduction (SCR) with ammonia are examined in this review. A number of carbon-based materials, such as carbon nanotubes (CNTs), activated carbon (AC), and graphene (GR) and non-carbon-based materials, such as Zeolite Socony Mobil-5 (ZSM-5), TiO2, and Al2O3 supported materials, were identified as the most up-to-date and recently used catalysts for the removal of NO gas. The main focus of this review is the study of catalyst preparation methods, as this is highly correlated to the behaviour of NO removal. The general mechanisms involved in the system, the Langmuir-Hinshelwood or Eley-Riedeal mechanism, are also discussed. Characterisation analysis affecting the surface and chemical structure of the catalyst is also detailed in this work. Finally, a few major conclusions are drawn and future directions for work on the advancement of the SCR-NH3 catalyst are suggested.

Performance of selective catalytic reduction of NO with NH3 over natural manganese ore catalysts at low temperature

Natural manganese ore catalysts for selective catalytic reduction (SCR) of NO with NH₃ at low temperature in the presence and absence of SO₂ and H₂O were systematically investigated. The physical and chemical properties of catalysts were characterized by X-ray diffraction, Brunauer-Emmett-Teller (BET) specific surface area, NH₃ temperature-programmed desorption (NH₃-TPD) and NO-TPD methods. The results showed that natural manganese ore from Qingyang of Anhui Province had a good low-temperature activity and N₂ selectivity, and it could be a novel catalyst in terms of stability, good efficiency, good reusability and lower cost. The NO conversion exceeded 85% between 150°C and 300°C when the initial NO concentration was 1000 ppm. The activity was suppressed by adding H₂O (10%) or SO₂ (100 or 200 ppm), respectively, and its activity could recover while the SO₂ supply is cut off. The simultaneous addition of H₂O and SO₂ led to the increase of about 100% in SCR activity than bare addition of SO₂. The formation of the amorphous MnO_x, high concentration of lattice oxygen and surface-adsorbed oxygen groups and a lot of reducible species as well as adsorption of the reactants brought about excellent SCR performance and exhibited good SO₂ and H₂O resistance.

Non-thermal-plasma-activated de-NOx catalysis

The combination of non-thermal plasma (NTP) with catalyst systems as an alternative technology to remove NO_x emissions in the exhaust of lean-burn stationary and mobile sources is reviewed. Several factors, such as low exhaust gas temperatures (below 300°C), low selectivity to N₂ and the presence of impurities, make current thermally activated technologies inefficient. Various hybrid plasma-catalyst systems have been examined and shown to have a synergistic effect on de-NO_x efficiency when compared with NTP or catalyst-alone systems. The NTP is believed to form oxygenated species, such as aldehydes and nitrogen-containing organic species, and to convert NO to NO₂, which improves the reduction efficiency of N₂ during hydrocarbon-selective catalytic reduction reactions. The NTP has been used as a pretreatment to convert NO to its higher oxidation states such as NO₂ to improve NO_x reduction efficiency in the subsequent processes, e.g. NH₃-selective catalytic reduction. It has been applied to the lean phase of the NO_x storage to improve the adsorption capacity of the catalyst by conversion of NO to NO₂ Alternatively, a catalyst with high adsorption capacity is chosen and the NTP is applied to the rich phase to improve the reduction activity of the catalyst at low temperature.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Promotional Effects of Ti on a CeO2-MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3

In this study, Ti was doped to CeO₂-MoO₃ to promote the catalytic performance for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The preparation method for CeMo_0.5Ti_aO_x (a = 0, 1, 2, 5, 10) catalysts was a stepwise precipitation method. When Ti was doped, all of the Ce-Mo-Ti catalysts exhibited remarkably improved NO_x conversion and N₂ selectivity than the CeMo_0.5O_x without Ti. The CeMo_0.5Ti₅O_x with excellent activity in a broad temperature range was selected as an optimal catalyst to investigate the effects of Ti addition. The formation process analysis of the CeMo_0.5Ti₅O_x showed that, the Mo and Ti species first precipitated together from the mixed solution with the increase of pH, and then Ce species precipitated onto the Mo-Ti precipitates. The obtained catalyst exhibited remarkably facilitated NO_x and NH₃ adsorption, enhanced charge imbalance, promoted redox property, and improved surface acidity, which are all important reasons for the excellent catalytic performance of an NH₃-SCR catalyst.

Significant Promotion Effect of Mo Additive on a Novel Ce-Zr Mixed Oxide Catalyst for the Selective Catalytic Reduction of NO(x) with NH3

A novel Mo-promoted Ce-Zr mixed oxide catalyst prepared by a homogeneous precipitation method was used for the selective catalytic reduction (SCR) of NO(x) with NH3. The optimal catalyst showed high NH3-SCR activity, SO2/H2O durability, and thermal stability under test conditions. The addition of Mo inhibited growth of the CeO2 particle size, improved the redox ability, and increased the amount of surface acidity, especially the Lewis acidity, all of which were favorable for the excellent NH3-SCR performance. It is believed that the catalyst is promising for the removal of NO(x) from diesel engine exhaust.

Low temperature SCR of NO with catalysts prepared by modified ACF loading Mn and Ce: effects of modification method

Achievement of a higher NOx conversion ratio in selective catalytic reduction (SCR) at low temperature is challenging. In this work, pure activated carbon fibres (ACFs) were modified with different ratios of H2O (g), NaOH, CO2 and HNO3, respectively (named as modified ACF). The chemical and physical properties of modified ACFs were identified by Brunauer-Emmett-Teller, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy methods. The NOx conversion ratio of ACF was improved from 56.1% to 82.4% at 80°C after modification with 30% (mass ratio) NaOH. These modified ACFs were further loaded with the mixture of MnO2 and CeO2 in the form of metal salt solutions (named as Mn0.5Ce0.5O2/modified ACF). The NOx conversion ratio of 30% SHACF remained similar at 80°C but was increased from 60.0% to 98.5% at 360°C after loading with Mn and Ce, which showed the best performance in SCR of NOx at low temperature. It could be seen that ACF delivered higher performance in low temperature SCR after being modified with the aforementioned reactants and further loading with metals. Based on chemical and physical characterization and the performance of the catalysts, the reasons for different performances of these catalysts in low temperature SCR are discussed.

Chemiluminescence analyzer of NO x as a high-throughput screening tool in selective catalytic reduction of NO

A chemiluminescence-based analyzer of NO _x gas species has been applied for high-throughput screening of a library of catalytic materials. The applicability of the commercial NO _x analyzer as a rapid screening tool was evaluated using selective catalytic reduction of NO gas. A library of 60 binary alloys composed of Pt and Co, Zr, La, Ce, Fe or W on Al₂O₃ substrate was tested for the efficiency of NO _x removal using a home-built 64-channel parallel and sequential tubular reactor. The NO _x concentrations measured by the NO _x analyzer agreed well with the results obtained using micro gas chromatography for a reference catalyst consisting of 1 wt% Pt on γ-Al₂O₃. Most alloys showed high efficiency at 275 °C, which is typical of Pt-based catalysts for selective catalytic reduction of NO. The screening with NO _x analyzer allowed to select Pt-Ce_(X) (X=1-3) and Pt-Fe₍₂₎ as the optimal catalysts for NO _x removal: 73% NO _x conversion was achieved with the Pt-Fe₍₂₎ alloy, which was much better than the results for the reference catalyst and the other library alloys. This study demonstrates a sequential high-throughput method of practical evaluation of catalysts for the selective reduction of NO.