The promoting/inhibiting effect of water vapor on the selective catalytic reduction of NOx

Hydrochars derived from real organic wastes as carbonaceous precursors of activated carbons for the removal of NO from contaminated gas streams

The improvement of air quality in densely-populated urban regions constitutes an environmental challenge of increasing concern. In this respect, the abatement of NO emissions, primarily emanating from combustion processes associated with motor-vehicles, along with industrial/domestic combustion systems, represents one of the main problems. Here, three hydrochars from diverse organic residues were used as activated carbon precursors for their evaluation in the NO removal in two potential application scenarios. Hydrochars were physically activated at 800 °C with pure-CO2 or diluted-O2. These materials were tested in a lab-scale biofilter at different conditions (NO concentration, temperature, relative humidity, NO-containing gas and carbon particle size) and in a larger-scale biofilter to evaluate the long-term NO removal capacity. Hydrochar-derived carbons present a relatively well-developed micro- and mesoporous structure, with BET areas of up to 421 m2/g, and a variety of oxygen surface functionalities (carboxylic, lactone, carbonyl and quinone groups), especially concerning CO2-activated carbons. These exhibited an excellent behaviour at low NO concentration (5 ppmv) between 25 and 75 °C with removal capacities of ≈97 % and > 82 %, respectively; and still good-performance (≈66 %) in a more concentrated gas (120 ppmv). Whilst, carbons obtained by diluted-O2 activation from the same hydrochars, evidenced a higher removal capacity loss at high NO concentration. The O2 presence in the gas stream was confirmed as a crucial factor in the NO elimination, since both co-adsorb on the carbon surface favouring NO oxidation to NO2. Besides, the humidity in the airstream diminished the NO removal capacity from 0.88 to 0.51 mgNO/gcarbon, but still remained at 0.54 mgNO/gcarbon, when the carbon (in pellet) was operated at larger-scale biofilter in 9-fold longer test under humid air. Therefore, this study highlights the potential of renewable carbons to serve as cost-effective component in urban biofilters, to mitigate NO emissions from exhaust gases in biomass boilers and urban semi-close areas.

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Engineering In-Co3O4/H-SSZ-39(OA) Catalyst for CH4-SCR of NOx: Mild Oxalic Acid (OA) Leaching and Co3O4 Modification

Zeolite-based catalysts efficiently catalyze the selective catalytic reduction of NOx with methane (CH4-SCR) for the environmentally friendly removal of nitrogen oxides, but suffer severe deactivation in high-temperature SO2- and H2O-containing flue gas. In this work, SSZ-39 zeolite (AEI topology) with high hydrothermal stability is reported for preparing CH4-SCR catalysts. Mild acid leaching with oxalic acid (OA) not only modulates the Si/Al ratio of commercial SSZ-39 to a suitable value, but also removes some extra-framework Al atoms, introducing a small number of mesopores into the zeolite that alleviate diffusion limitation. Additional Co3O4 modification during indium exchange further enhances the catalytic activity of the resulting In-Co3O4/H-SSZ-39(OA). The optimized sample exhibits remarkable performance in CH4-SCR under a gas hourly space velocity (GHSV) of 24,000 h-1 and in the presence of 5 vol% H2O. Even under harsh SO2- and H2O-containing high-temperature conditions, it shows satisfactory stability. Catalysts containing Co3O4 components demonstrate much higher CH4 conversion. The strong mutual interaction between Co3O4 and Brønsted acid sites, confirmed by the temperature-programmed desorption of NO (NO-TPD), enables more stable NxOy species to be retained in In-Co3O4/H-SSZ-39(OA) to supply further reactions at high temperatures.

Integrated catalytic systems for simultaneous NOx and PM reduction: a comprehensive evaluation of synergistic performance and combustion waste energy utilization

The global transition towards sustainable automotive vehicles has driven the demand for energy-efficient internal combustion engines with advanced aftertreatment systems capable of reducing nitrogen oxides (NOx) and particulate matter (PM) emissions. This comprehensive review explores the latest advancements in aftertreatment technologies, focusing on the synergistic integration of in-cylinder combustion strategies, such as low-temperature combustion (LTC), with post-combustion purification systems. Selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) are critically examined, highlighting novel catalyst formulations and system configurations that enhance low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, including thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve overall system efficiency. By analyzing the complex interactions between engine operating parameters, combustion kinetics, and emission formation, this study provides valuable insights into the optimization of integrated LTC-aftertreatment systems. Furthermore, the review emphasizes the importance of considering real-world driving conditions and transient operation in the development and evaluation of these technologies. The findings presented in this article lay the foundation for future research efforts aimed at overcoming the limitations of current aftertreatment systems and achieving superior emission reduction performance in advanced combustion engines, ultimately contributing to the development of sustainable and efficient automotive technologies.

Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Efficient photocatalytic NO removal with inhibited NO2 formation and catalyst loss over sponge-supported and functionalized g-C3N4

Though photocatalytic purification of NO has been widely studied, how to avoid secondary pollution during gas-solid reaction is still a challenge, especially in inhibiting the formation of toxic intermediates (NO2) and avoiding the blow away of powdery photocatalyst. Herein, we proposed a one-step solvothermal method to prepare melamine sponge (MS) supported and functionalized g-C3N4 (CN), which simultaneously realizes the inhibition of NO2 formation and catalyst loss. Sodium hydroxide, which plays a dual role, has been introduced during the preparation of supported photocatalyst. Specifically, sodium atom, as the modifier of performance, could facilitate the randomly distributed charge of pristine CN to be converged, which accelerates the adsorption/activation of reactants for efficient and deep NO oxidation. Hydroxyl group, as the binder between CN and MS, induces the interaction by forming hydrogen bonds, which contributes to the firm immobilization of powdery photocatalyst. The supported sample exhibits outstanding NO removal rate (58.90%) and extremely low NO2 generation rate (1.41%), and the mass loss rate of photocatalyst before and after reaction is less than 1%. The promotion mechanism of performance also has been elaborated. This work takes environmental risks as a prerequisite to propose a feasible strategy for perfecting the practical application of photocatalytic technology.

Overview of mechanisms of Fe-based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

With the increasingly stringent control of NOx emissions, NH3-SCR, one of the most effective de-NOx technologies for removing NOx, has been widely employed to eliminate NOx from automobile exhaust and industrial production. Researchers have favored iron-based catalysts for their low cost, high activity, and excellent de-NOx performance. This paper takes a new perspective to review the research progress of iron-based catalysts. The influence of the chemical form of single iron-based catalysts on their performance was investigated. In the section on composite iron-based catalysts, detailed reviews were conducted on the effects of synergistic interactions between iron and other elements on catalytic performance. Regarding loaded iron-based catalysts, the catalytic performance of iron-based catalysts on different carriers was systematically examined. In the section on iron-based catalysts with novel structures, the effects of the morphology and crystallinity of nanomaterials on catalytic performance were analyzed. Additionally, the reaction mechanism and poisoning mechanism of iron-based catalysts were elucidated. In conclusion, the paper delved into the prospects and future directions of iron-based catalysts, aiming to provide ideas for the development of iron-based catalysts with better application prospects. The comprehensive review underscores the significance of iron-based catalysts in the realm of de-NOx technologies, shedding light on their diverse forms and applications. The hope is that this paper will serve as a valuable resource, guiding future endeavors in the development of advanced iron-based catalysts.

Uncovering the centrality of mono-dentate SO32-/SO42- modifiers grafted on a metal vanadate in accelerating wet NOX reduction and poison pyrolysis

NOX rarely binds with labile oxygens of catalytic solids, whose Lewis acidic (LA) species possess higher binding strengths with NH3 (ENH3) and H2O than Brönsted acidic counterparts (BA--H+; -OH), oftentimes leading to elevate energy barrier (EBARRIER) and weaken H2O tolerance, respectively. These limit NH3-assisted wet NOX reduction via Langmuir-Hinshelwood-type or Eley-Rideal (ER)-type model on LA species, while leaving ER-type analogue on BA--H+ species proper to reduce wet NOX. Given hard-to-regulate strength/amount of -OH species and occasional association between ENH3 and EBARRIER, Ni1V2O6 (Ni1) was rationally chosen as a platform to isolate mono-dentate SO32-/SO42- species for use as BA--H+ bonds via protonation to increase collision frequency (k'APP,0) alongside with disclosure of advantages of SO32-/SO42--functionalized Ni1V2O6 (Ni1-S) over Ni1 in reducing wet NOX. Ni1-S outperformed Ni1 in achieving a larger BA--H+ quantity (k'APP,0↑), increasing H2O tolerance, and elevating oxygen mobility, thus promoting NOX reduction activity/consequences under SO2-excluding gases. V2O5-WO3 composite simulating a commercial catalyst could isolate mono-dentate SO32-/SO42- species and served as a control (V2O5-WO3-S) for comparison. Ni1-S was superior to V2O5-WO3-S in evading ammonium (bi-)sulfate (AS/ABS) poison accumulation and expediting AS/ABS pyrolysis efficiency, thereby improving AS/ABS resistance under SO2-including gases, while enhancing resistance against hydro-thermal aging.

Influence of different loads on PCDD/F removal by SCR during municipal solid waste incineration

This study was conducted on a full-scale (500 t/d) municipal solid waste incinerator (MSWI), investigating the influence of different loads on the emission of polychlorinated dibenzodioxins and polychlorinated dibenzofurans (PCDD/Fs) and their removal by selective catalytic reduction (SCR) system. The total concentration of PCDD/Fs at the SCR inlet under 100% load was higher than that under 80% load. The changing loads caused different distribution characteristics of PCDD/Fs at the SCR inlet, and the dominant congeners changed from high-chlorinated PCDDs (80% load) to low-chlorinated PCDFs (100% load). Moreover, the increased load enhanced the removal efficiency of PCDD/Fs by SCR from 17.3% to 64.2%, which was influenced by the inlet PCDD/F distribution and the moisture content. The high-chlorinated PCDD/Fs with the more stable structure were more difficult to be deteriorated and the high moisture content can weaken the catalytic activity of SCR catalysts. Correlation analysis was used to study the relationship between major air pollutants and PCDD/F emissions. The results showed that HCl positively correlated with PCDD/F emission concentrations, while NOx and SO2 negatively correlated. The results of this study can provide a reference for MSWI to operate properly under variable loads.

Removal of NO at low concentrations from polluted air in semi-closed environments by activated biochars from renewables feedstocks

Efficient measures are urgently required in large cities for nitric oxide (NO) elimination from air in urban semi-closed environments (parking lots and tunnels), characterized by low NO concentrations (<10 ppmv) and temperatures. One of the most promising abatement alternatives is the NO oxidation to NO₂, which can be further easily captured in an alkali solution or over a porous solid. However, most of the research devoted to this topic is focused on the elimination of NO from fuel exhaust gases, with high NO concentrations (400-2000 ppmv). In this work, sustainable and low-cost activated biochars of different origin and having very different ash contents were employed in NO removal at very low concentrations. Thus, low ash content forestry (oak woodchips, OAK) and high ash content from agriculture (oilseed rape straw, OSR) biochars were subjected to physical activation with CO₂ at 900 °C (OAK550-A900CO₂ and OSR700-A900CO₂, respectively). The NO removal performance tests of such activated carbons were carried out at different experimental conditions: i.e., temperature, relative humidity (0-50 vol% RH), NO-containing gas (N₂ or air), amount of activated carbon, and NO concentration, to assess how the activated biochar properties influence their NO removal capacity. The sample OSR700-A900CO₂ contained a higher population of oxygen surface functionalities, which might play an important role in the NO removal efficiency in dry conditions since they could assist NO oxidation on carbon active sites when used above room temperature (50-75 °C). However, at room temperature (25 °C), the presence of narrow micropore size distribution at 6 Å became a more relevant property, since it facilitates an intimate contact between NO and O₂. Accordingly, the activated biochar from OAK was much more efficient, achieving complete removal of NO from air flow (dry or with 50 vol% RH) at 25 °C during 400 min of testing, making it an ideal candidate as biofilter for purifying air streams of semi-closed spaces contaminated with low concentrations of NO.

Research landscape and hotspots of selective catalytic reduction (SCR) for NOx removal: insights from a comprehensive bibliometric analysis

Selective catalytic reduction (SCR) has been one of the most efficient and widely used technologies to remove nitrogen oxides (NO_x). SCR research has developed rapidly in recent years, which can be reflected by the dramatic increase of related academic publications. Herein, based on the 10,627 documents from 2001 to 2020 in Web of Science, the global research landscape and hotspots in SCR are investigated based on a comprehensive bibliometric analysis. The results show that SCR research has developed positively; the annul number of articles increase sharply from 246 in 2001 to 1092 in 2020. People's Republic of China and Chinese Academy of Sciences are the most productive country and institution, respectively. The global collaboration is extensive and frequent, while People's Republic of China and USA have the most frequent research cooperation. Applied Catalysis B-Environmental is the leading publication source with 711 records. Five major research areas on SCR are identified and elaborated, including catalyst, reductant, deactivation, mechanism, and others. Zeolite is the most widely studied SCR catalyst, while copper, silver, platinum, and iron are the most popular metal elements in catalyst. Ammonia (NH₃) is dominated among various SCR reductants, while hydrocarbon reductant has gained more attention. Sulfur dioxide (SO₂) and vapor are the two most concerned factors leading to catalyst deactivation, and catalyst regeneration is also an important research topic. Density functional theory (DFT), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and kinetics are the most widely used methods to conduct mechanism study. The studies on "low temperature," "atomic-scale insight," "elemental mercury," "situ DIRFTS investigation," "arsenic poisoning," "SPOA-34," "Cu-CHA catalyst," "TiO₂ catalyst," and "Ce catalyst" have been the hotspots in recent years.