A review of the catalysts used in the reduction of NO by CO for gas purification

IrSn Bimetallic Clusters Confined in MFI Zeolites for CO Selective Catalytic Reduction of NOx in the Presence of Excess O2

We developed a simple strategy for preparing IrSn bimetallic clusters encapsulated in pure silicon zeolites via a one-pot hydrothermal synthesis by using diethylamine as a stabilizing agent. A series of investigations verified that metal species have been confined successfully in the inner of MFI zeolites. IrSn bimetallic cluster catalysts were efficient for the CO selective catalytic reduction of NOx in the presence of excess O2. Furthermore, the 13CO temperature-programmed surface reaction results demonstrated that NO2 and N2O could form when most of the CO was transformed into CO2 and that Sn modification could passivate CO oxidation on the IrSn bimetallic clusters, leading to more reductants that could be used for NOx reduction at high temperatures. Furthermore, SO2 can also influence the NOx conversion by inhibiting the oxidation of CO. This study provides a new strategy for preparing efficient environmental catalysts with a high dispersion of metal species.

Theoretical study of adsorption properties and CO oxidation reaction on surfaces of higher tungsten boride

Most modern catalysts are based on precious metals and rear-earth elements, making some of organic synthesis reactions economically insolvent. Density functional theory calculations are used here to describe several differently oriented surfaces of the higher tungsten boride WB5-x, together with their catalytic activity for the CO oxidation reaction. Based on our findings, WB5-x appears to be an efficient alternative catalyst for CO oxidation. Calculated surface energies allow the use of the Wulff construction to determine the equilibrium shape of WB5-x particles. It is found that the (010) and (101) facets terminated by boron and tungsten, respectively, are the most exposed surfaces for which the adsorption of different gaseous agents (CO, CO2, H2, N2, O2, NO, NO2, H2O, NH3, SO2) is evaluated to reveal promising prospects for applications. CO oxidation on B-rich (010) and W-rich (101) surfaces is further investigated by analyzing the charge redistribution during the adsorption of CO and O2 molecules. It is found that CO oxidation has relatively low energy barriers. The implications of the present results, the effects of WB5-x on CO oxidation and potential application in the automotive, chemical, and mining industries are discussed.

Catalytic reduction of nitrogen monoxide using iron-nickel oxygen carriers derived from electroplating sludge: Novel method for the collaborative emission decrease of polluting gases

The valorization of electroplating sludge (ES) for high added value presents greater economic and environmental benefits than conventional treatment methods such as thermal processing, solidification, and landfill. Inspired by the mechanism of chemical looping combustion (CLC), this study developed a novel cost-effective method for denitrification by preparing FeNi-OCs from ES to achieve the synergistic reduction of CO and NO emissions. The phase structure, micromorphology, and valence state changes of the FeNi-OC catalyst during the CO-catalyzed reduction of NO and the pathway for catalytic denitrification using FeNi-OCs were analyzed. Results showed that CO could reduce FeNi-OCs to FeNi, and the reduced FeNi was subsequently oxidized back to FeNi-OCs by NO, a process analogous to CLC. During experiments, the simultaneous consumption of CO and NO gases was observed at 350 °C. This phenomenon was highly pronounced at 600 °C, where the CO and NO concentrations decreased from initial values of 8550 and 470 ppm, respectively, to 6719 and 0 ppm, respectively, with conversion rates of 21.41 % and 100 %, respectively. Hence, synergistic emission reduction was achieved. Further experiments also indicated that the addition of 1.5 % ES during iron ore sintering could substantially reduce the CO and NO concentrations in the sintering flue gas from 1268.32 and 244.81 ppm, respectively, to 974.51 and 161.11 ppm, respectively.

Ultrathin MnO2 with strong lattice disorder for catalytic oxidation of volatile organic compounds

Catalytic oxidation proves the most promising technology for volatile organic compounds (VOCs) abatement. Lattice disorder plays a crucial role in the catalytic activity of catalysts due to the exposure of more active sites. Inspired by this, we successfully prepared a series of ε-MnO2 with different lattice disorder defects via several simple methods and applied them to the catalytic oxidation of two typical VOCs (toluene and acetone). Various characterizations and performance tests confirm that the ultrathin (1.4-1.8 nm) structure and strong lattice disorder can enhance the low temperature reduction and reactive oxygen species, so that MnO2-R exhibits excellent toluene and acetone oxidation activities. In-situ DRIFTS tests were carried out to detect reaction intermediates in the toluene and acetone oxidation process on the catalyst surface. Moreover, we propose a possible synergistic mechanism for toluene and acetone mixtures catalytic oxidation. This work reveals the important role of lattice disorder defects in the catalytic oxidation of VOCs on Mn-based catalysts, and deepens the insights of the reaction path in toluene and acetone catalytic oxidation.

CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeO_x catalysts (CuCeO_x/CuO) by calcination of a hydrotalcite precursor [Cu₆Ce₂(OH)₁₆]CO₃·nH₂O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH₃-TPD, XPS, Raman spectroscopy, and N₂ adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeO_x clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu²⁺ and reduced copper species (Cu⁺/Cu⁰) and (Ce⁴⁺/Ce³⁺). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeO_x/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

Noble Metal Nanoparticles Decorated Boron Nitride Nanotubes for Efficient and Selective Low-Temperature Catalytic Reduction of Nitric Oxide with Carbon Monoxide

Parallel to CO₂ emission, NO_x emission has become one of the menacing problems that seek a simple, durable, and high-efficiency deNO_x catalyst. Herein, we demonstrated simple syntheses of platinum group metal nanoparticle-decorated f-BNNT (PGM = Pd, Pt, and Rh, and f-BNNT stands for -OH-functionalized boron nitride nanotubes) as a catalyst for efficient and selective reduction of NO by CO at low-temperature conditions. PGM/f-BNNT with a low amount of noble metal nanoparticles (0.7-0.8 wt %) presents very efficient catalytic activity for NO reduction as well as CO oxidation during their removal process. The removal efficiencies of NO and CO with Pd/f-BNNT, Pt/f-BNNT, and Rh/f-BNNT catalysts were investigated under various temperatures, flow rates, and reaction times, respectively. For most cases, NO catalytic reduction with CO reaction was >99% at a temperature as low as ∼200 °C. The catalyst robustness and efficiency were also verified by presenting almost 100% conversion of NO using a Rh/f-BNNT catalyst, which was aged under humid air at 600 and 700 °C for 24 h, respectively. The synergic effect of the catalytic efficacy of the well-dispersed noble metal nanoparticles and the excellent surface properties of BNNT are reasons for the high selectivity and catalytic property at a low temperature. On the basis of this investigation, we demonstrated that the noble metal nanoparticle-decorated f-BNNT catalysts are possible to save expensive PGM catalysts, such as Pt, Pd, and Rd, as much as 100 times while presenting similar or better catalytic performance for simultaneous NO and CO removals.

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

In the field of nitrogen oxides (NO_x) abatement, developing selective catalytic reduction (SCR) catalysts that can operate stably in the practical conditions remains a big challenge because of the complexity and uncertainty of actual flue gas emissions. As water vapor is unavoidable in the actual flue gas, it is indispensable to explore its effect on the performance of SCR catalysts. Many studies have proved that the effects of H₂O on de-NO_x activity of SCR catalysts were indeed observed during SCR reactions operated under wet conditions. Whether the effect is promotive or inhibitory depends on the reaction conditions, catalyst types and reducing agents used in SCR reaction. This review focuses on the effect of H₂O on SCR catalysts and SCR reaction, including promoting effect, inhibiting effect, as well as the effecting mechanism. Besides, various strategies for developing a water-resistant SCR catalyst are also included. We hope that this work can give a more comprehensive insight into the effects of H₂O on SCR catalysts and help with the rational design of water-resistant SCR catalysts for further practical application in NO_x abatement field.

Designing Highly Efficient Cu2O-CuO Heterojunction CO Oxidation Catalysts: The Roles of the Support Type and Cu2O-CuO Interface Effect

In this work, a series of Cu₂O/S (S = α-MnO₂, CeO₂, ZSM-5, and Fe₂O₃) supported catalysts with a Cu₂O loading amount of 15% were prepared by the facile liquid-phase reduction deposition-precipitation strategy and investigated as CO oxidation catalysts. It was found that the Cu₂O/α-MnO₂ catalyst exhibits the best catalytic activity for CO oxidation. Additionally, a series of Cu₂O-CuO/α-MnO₂ heterojunctions with varied proportion of Cu⁺/Cu²⁺ were synthesized by further calcining the pristine Cu₂O/α-MnO₂ catalyst. The ratio of the Cu⁺/Cu²⁺ could be facilely regulated by controlling the calcination temperature. It is worth noting that the Cu₂O-CuO/α-MnO₂-260 catalyst displays the best catalytic performance. Moreover, the kinetic studies manifest that the apparent activation energy could be greatly reduced owing to the excellent redox property and the Cu₂O-CuO interface effect. Therefore, the Cu₂O-CuO heterojunction catalysts supported on α-MnO₂ nanotubes are believed to be the potential catalyst candidates for CO oxidation with advanced performance.

Understanding the Iron-Cobalt Synergies in ZSM-5: Enhanced Peroxymonosulfate Activation and Organic Pollutant Degradation

Iron- and cobalt-based heterogeneous catalysts are widely applied for activating peroxymonosulfate (PMS) to degrade organic pollutants. However, few studies have unveiled the clear synergistic mechanism of iron and cobalt in ZSM-5. In this paper, the synergistic mechanism of enhanced PMS activation was revealed by constructing iron and cobalt bimetal modified ZSM-5 zeolite catalysts (FeCo-ZSM-5). The tetracycline hydrochloride (TCH) degradation experiments showed that the catalytic activity of FeCo-ZSM-5-2:3 was much higher than those of Fe-ZSM-5 and Co-ZSM-5. In addition, the influences of catalyst dosage, PMS concentration, reaction temperature, initial pH, and coexisting ions on TCH removal were systematically investigated in this paper. Density functional theory calculations indicated that Co was the main active site for PMS adsorption, and Fe increased the area of Co's positive potential mapped to the electron cloud. The Fe-Co bimetallic doping increased the area of positive potential mapped to the electron cloud and benefited the adsorption of PMS on the catalyst surface, which revealed the synergistic mechanism of bimetals. Electron paramagnetic resonance spectra and quenching experiments showed that sulfate radicals, singlet oxygen, and hydroxyl radicals were involved in the degradation of TCH. Furthermore, liquid chromatography-mass spectrometry was conducted to propose possible degradation pathways. This work provides certain guiding significance in understanding the synergistic effect of heterogeneous catalysts for tetracycline wastewater treatment.

Tracking the redox reaction-induced reconstruction of NiAu nanoparticles via environmental scanning transmission electron microscopy

Atmosphere-related atom migration and phase reconstruction are an easy way for optimizing the catalytic activity of a bimetallic catalyst. Herein, the structure evolutions of NiAu nanoparticles under oxidative and reductive environments are investigated via combining identical location and in situ environmental scanning transmission electron microscopy. During oxidation, a NiO layer first forms and the redispersion of Ni and Au atoms yields a Ni@Au@NiO multi-shell structure at 350 °C. Further, Ni and Au segregate into an Au-NiO hybrid structure at 600 °C. During reduction, Au atoms disperse over the particle surface forming a NiAu alloy shell with scattered Au atoms/clusters. In situ observation further discloses that the reduction changes the local structural ordering from Ni₃Au to NiAu alloy. Very interestingly, the reduced NiAu exhibits promoted activity over oxidized ones for the CO-NO reaction. Density functional theory calculations further reveal the structure-property relationships of CO, NO, and O adsorbates on NiAu alloy surfaces. This study is beneficial for understanding the atmosphere-related evolution behaviors of bimetallic systems, thereby inspiring the catalytic surface optimization.

Enhanced activity and sulfur resistance of Cu- and Fe-modified activated carbon for the reduction of NO by CO from regeneration gas

The reduction of NO by CO was proposed to be applied for regeneration gas to remove NOx from industrial flue gas with activated carbon purification technology.

Effect of the Configuration of Copper Oxide-Ceria Catalysts in NO Reduction with CO: Superior Performance of a Copper-Ceria Solid Solution

Various copper-ceria-based composites have attracted attention as efficient catalysts for the reduction of NO with CO. In this comparative study, we have examined the catalytic potential of different configurations of copper oxide-ceria catalysts, including catalysts based on a copper-ceria solid solution, copper oxide particles supported on ceria, and ball-milled copper oxide-ceria. The structurally different interfaces between the constituents of these catalysts afforded very different catalytic performances. The solid solution catalyst outperformed the corresponding ceria-supported and ball-milled CuO-CeO₂ catalysts. The copper cations incorporated into the ceria lattice strongly improved the activity, N₂ selectivity, and water vapor tolerance compared to the other catalyst configurations. The experimental observations are supported by first-principles density functional theory (DFT) studies of the reaction pathway, which indicate that the incorporation of Cu cations into the ceria matrix lowers the energy required for activating the lattice oxygen, thereby enhancing the formation and healing of oxygen vacancies, and thus promoting NO reduction with CO.

Synergistic Effects of Bimetallic AuPd and La2O3 in the Catalytic Reduction of NO with CO

Bimetallic AuPd nanoparticles supported on TiO2 are known to catalyze the reduction of NO with CO. Here, we investigated the effects of the addition of lanthanum oxide to a AuPd/TiO2 catalyst with a AuPd particle size of 2.1–2.2 nm. The addition of La2O3 enhanced the catalytic activity; for example, at 250 °C, there was 40.9% NO conversion and 49.3% N2-selectivity for AuPd/TiO2, and 100% NO conversion and 100% N2-selectivity for AuPd-La (1:1)/TiO2. The temperature requiring 100% NO conversion dropped from 400 °C to 200 °C by the simple post-impregnation of La2O3 onto AuPd/TiO2. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with modulation excitation spectroscopy (MES) demonstrated that CO adsorption occurs first on Au atoms and then, within 80 s, moves onto Pd atoms. This transformation between two adsorption sites was facilitated by the addition of La2O3.

Catalytic oxidation of CO on noble metal-based catalysts

Carbon monoxide (CO) catalytic oxidation has gained increasing interest in recent years due to its application prospects. The noble metal catalysts commonly exhibit outstanding CO catalytic oxidation activity. Therefore, this article reviewed the recent research on the application of noble metal catalysts in the catalytic oxidation of CO. The effects of catalyst support, dopant, and physicochemical properties on the catalytic activity for CO oxidation are summarized. The influence of the presence of water vapor and sulfur dioxide in the reaction atmosphere on the catalytic activity in CO oxidation is emphatically discussed. Moreover, this paper discussed several reaction mechanisms of CO catalytic oxidation on noble metal catalysts. Finally, the challenges of removing CO by catalytic oxidation in practical industrial flue gas are proposed.

Investigation of the NO reduction by CO reaction over oxidized and reduced NiOx/CeO2 catalysts

NO reduction by CO reaction was investigated by NiOx/CeO2 catalysts with different pretreatment conditions. Surface area, oxygen defect sites, and CeO2 crystallite size are closely related to the catalytic performance.

Optimizing reaction conditions and experimental studies of selective catalytic reduction of NO by CO over supported SBA-15 catalyst

Selective catalytic reduction of NO with CO (CO-SCR) was investigated based on optimizing the operating conditions by response surface methodology (RSM) and by appropriately choosing the supported SBA-15 catalysts. The effects of the CO-SCR reaction parameters such as NO:CO molar ratios and oxygen concentrations on the catalytic performance were determined by RSM to evaluate the NO conversion using a first-order polynomial model. The CuO/SBA-15 and Fe₂O₃/SBA-15 catalysts were synthesized by a hydrothermal method and characterized by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), N₂ adsorption-desorption (BET), scanning electron microscopy coupled to energy dispersive X-Ray spectroscopy (SEM-EDS), and Fourier transform infrared spectroscopy (FTIR) to investigate the physicochemical properties of the solids. The RSM showed a very good agreement between predicted values and experimental results with the Pareto analysis confirming the accuracy and reliability of the model. The optimized results indicated the maximum NO conversion at 500 °C with using the NO to CO molar ratio of 1:2 (500:1000 ppm) in the absence of oxygen. Under these conditions, CuO/SBA-15 catalyst achieved 99.7% of NO conversion, whereas Fe₂O₃/SBA-15 had 98.1% of the catalytic parameter. Catalytic tests in CO-SCR reaction were performed on both catalysts at optimum operating conditions with CuO/SBA-15 exhibiting better performance compared to that of Fe₂O₃/SBA-15. The results revealed that CuO/SBA-15 was a promising catalyst for CO-SCR of NO due to the well-dispersed CuO phase on SBA-15 surface that allows the solid being more tolerant to the presence of oxygen.