Low-temperature Selective Catalytic Reduction Research Articles

Insights into the enhanced activity and SO2 resistance of air oxidation treated Mn-Fe doped biochar catalyst in the low-temperature catalytic reduction of NOx with NH3

In this study, a sustainable two-stage air oxidation (TAO) was employed to prepare Fe modified MnOx-biochar catalyst (C-O2-MF-O2) instead of high temperature treatment and/or chemical activation for selective catalytic reduction of NO with NH3 (NH3-SCR). The collaborative effect of Fe modification and TAO on the physicochemical properties and catalytic performance of catalysts was systematically studied. Compared with counterpart catalysts treated with two-stage N2 (C-N2-MF-N2) or one-stage O2 (C-O2-MF-N2, C-N2-MF-O2), C-O2-MF-O2 showed obvious enhanced physicochemical properties (SBET, pore properties, high-valance metal ratio, chemisorption oxygen (Oα) content), superior catalytic performance, achieving 91.3% NO conversion with nearly 100% N2 selectivity at 150 °C, and much better H2O and SO2 resistance. It is worth noting that TAO facilitated the redox performance of MnOx after Fe modification to generate more dispersed high-valance metal (Mn4+ and Fe3+), thereby favoring the low-temperature SCR activity of the catalyst. Further, the in situ DRIFTS studies revealed TAO produced remarkable effects on the adsorption behaviors of NH3, NO and SO2, and the corresponding mechanisms of catalyst activity enhancement and SO2 resistance were proposed. Generally, TAO could be a favorable avenue to prepare catalysts with binary active metals for improving performance of NH3-SCR.

Enhanced low-temperature SCR performance of metal Oxide/Cu-SSZ-13 composite catalysts: The role of oxygen species in metal oxide

A composite catalyst composed of metal oxide and Cu-SSZ-13 components (OXZEO) presents a promising approach to solve the current challenge of improving the low-temperature efficiency of SCR catalysts. The diverse forms of oxygen present within the metal oxide components impact the formation of nitrate, which plays a critical role in the deNOx pathway, thereby influencing the overall SCR reaction in OXZEO. Despite this, the relationship between the various types of oxygen in metal oxide and their catalytic role in OXZEO catalysts remain not fully understood. To clarify this relationship, the activities of Cu-SSZ-13 combined with MnOx, CeOx, and ZrOx, respectively, were compared and analyzed. The low-temperature pathway (below 200 °C) for the three catalysts is primarily ascribed to the interplay between oxide-derived nitrate precursors and NH3 bound to Brønsted acids within the zeolite. However, MnOx/Cu-SSZ-13 demonstrated a stronger synergistic effect, achieving around 50 % higher NOx conversion than Cu-SSZ-13 alone at 200 °C, which was superior to the other two composite catalysts. Characterizations reveal that MnOx with surface oxygen vacancies can supply a significant quantity of chemisorbed oxygen, thus producing more reactive bridged nitrates and bolstering the reaction between nitrate and NH4+ in MnOx/Cu-SSZ-13. On the contrary, for CeOx/Cu-SSZ-13 and ZrOx/Cu-SSZ-13 catalysts, the lattice oxygen of the metal oxide encourages the formation of less active bidentate and monodentate nitrates, resulting in inferior low-temperature SCR performance.

Improving low-temperature NH3-SCR denitration activity and resistance to H2O and SO2 over Nb-modified NbaSn0.3CeOx catalysts by enhancing acidity to compensate for its weak oxidation ability

In this paper, a series of Nb-modified NbaSn0.3CeOx (a = 0.2, 0.4, 0.5) catalysts for denitrification were prepared using a facile co-precipitation method. In order to explore the various individual properties of the samples, the catalysts were characterized by XRD, Raman, N2 adsorption and desorption, TEM, NH3-TPD, H2-TPR, XPS and in-situ DRIFTS tests to understand the influences of the Nb introduction on the NH3-SCR reaction. Among these catalysts, the Nb0.4Sn0.3CeOx catalyst exhibit excellent NH3-SCR performance in the temperature range of 155–450 °C with more than 90 % NO conversion, 100 % N2 selectivity, and possess good resistance to H2O and SO2. Compared with the Sn0.3CeOx catalyst, we believe that the improvement of the low-temperature NH3-SCR performance of the Nb0.4Sn0.3CeOx catalysts is mainly due to the change in the amount of acid, special for the increase in B acid content, which compensates for the effect of the weaken oxidizing ability on the low-temperature activity. Beyond that, the good H2O and SO2 resistance at low temperature after the introduction of Nb into Nb0.4Sn0.3CeOx catalysts was analyzed by using in-situ DRIFTS characterization in detail, and the results indicate that the incorporation of Nb can inhibit the oxidation of SO2 to form sulfate on the catalysts, and prevent the competing adsorption of H2O and NOx, but the adsorption of NH3 is not affected. This work proves that the acidity regulation can improve low-temperature SCR activity by the synergistic effect between oxidation capacity and acidity.

Hydrothermally stable metal oxide-zeolite composite catalysts for low-temperature NOx reduction with improved N2 selectivity

Development of hydrothermally stable, low-temperature catalysts for controlling nitrogen oxides emissions from mobile sources remains an urgent challenge. We have prepared a metal oxide-zeolite composite catalyst by depositing Mn active species on a mixture support of CeO2/Al2O3 and ZSM-5. This composite catalyst is hydrothermally stable and shows improved low-temperature SCR activity and significantly reduced N2O formation than the corresponding metal oxide catalyst. Comparing with a Cu-CHA catalyst, the composite catalyst has a faster response to NH3 injection and less NH3 slip. Our characterization results reveal that such an oxide-zeolite composite catalyst contains more acidic sites and Mn3+ species as a result of oxide-zeolite interaction, and this interaction leads to the generation of more NH4+ species bound to the Brønsted acid sites and more reactive NOx species absorbed on the Mn sites. Herein, we report our mechanistic understanding of the oxide-zeolite composite catalyst and its molecular pathway for improving the low-temperature activity and N2 selectivity for NH3-SCR reaction. Practically, this work may provide an alternative methodology for low-temperature NOx control from diesel vehicles.

Cobalt-modified limonite catalyst for low temperature selective catalytic reduction of NOx in ore sintering flue gas by NH3

Natural limonite possesses characteristics of rich reserves and low sintering efficiency. In this study, the Co/Limonite composites were prepared through a one-step impregnation method by using limonite as a precursor and cobalt oxide as a modifier. The influence of cobalt doping amount and calcination temperature on the denitrification efficiency over the composite were investigated. The results showed that the Co/Limonite composite was the optimum over Co/Fe mass ratio of 0.06 and the calcination temperature of 350 °C, it exhibited an excellent low-temperature catalytic activity, stability and good resistance to H2O(g) and SO2. When NO initial concentration was 500 ppm, gas hourly space velocity (GHSV) was 36,000 h−1 and reaction temperature was 100 ℃, the denitrification efficiency reached 100%. Adding of 10 vol% H2O(g) or 50 ppm SO2, respectively, the denitrification efficiency was about 90% at reaction temperature was 150 ℃, while it could approach to 80% with H2O(g) and SO2 were added simultaneously. The micromorphology and physicochemical properties of the catalysts were analyzed by systematic characterization techniques. The Limonite could obtain more surface acid sites and larger adsorption capacity by cobalt modification. More Lewis acids and weak Brønsted acids could greatly contribute to improve the low temperatures denitrification catalytic activity.

Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning.

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2 poisoning. The stubborn sulfates make insufficient O2 activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2 poisoning through accessible O2 activation, in which O2 was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Ea reduction and was actively involved in the NH3 activation and NO oxidation process. Besides, ROS maintained high stability over the SO2 + H2O-deactivated γ-MnO2 catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.

Nitration-Promoted Vanadate Catalysts for Low-Temperature Selective Catalytic Reduction of NOX with NH3.

Vanadium-based catalysts have been commercially used in selective catalytic reduction (SCR), owing to their high catalytic activity and effectiveness across a wide temperature range; however, their catalytic efficiency decreases at lower temperatures under exposure to SOX. This decrease is largely due to ammonium sulfate generation on the catalyst surface. To overcome this limitation, we added ammonium nitrate to the V2O5-WO3/TiO2 catalyst, producing a V2O5-WO3/TiO2 catalyst with nitrate functional groups. With this approach, we found that it was possible to adjust the amount of these functional groups by varying the amount of ammonium nitrate. Overall, the resultant nitrate V2O5-WO3/TiO2 catalyst has large quantities of NO3- and chemisorbed oxygen, which improves the density of Brønsted and Lewis acid sites on the catalyst surface. Furthermore, the nitrated V2O5-WO3/TiO2 catalyst has a high NOX removal efficiency and N2 selectivity at low temperatures (i.e., 300 °C); this is because NO3- and chemisorbed oxygen, generated by nitrate treatment, facilitated the occurrence of a fast SCR reaction. The approach outlined in this study can be applied to a wide range of SCR catalysts, allowing for the development of more, low-temperature SCR catalysts.

Cu docking-activated Nb incorporation in multivariate CuO-Nb2O5/CeO2 catalysts for selective reduction of NOx with NH3

Improving the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR) meanwhile maintaining N2 selectivity are challenging in the field of catalysis. Herein, we showcase the fabrication of the synergistic catalytic sites in the CuO-Nb2O5/CeO2 catalysts. Impressively, the CuO-Nb2O5/CeO2 is characteristic with abundant oxygen vacancies at the interfacial regions, which facilitate the redox reaction between Ce3+, Nb5+ and Cu2+ with the assistance of surface oxygen, exhibiting high NH3-SCR activity, N2 selectivity and wide temperature window at a gas hourly space velocity of 150,000 h−1. Contrast experiments, EXAFS analysis, in situ infrared and Raman spectroscopy, and theoretical calculations indicate that the introduction of proper copper ions in the CeO2 lattice induces the structure distortion of surface CeO2, improving more niobium ions incorporation into the CeO2. The embedded Nb synergizes Cu and Ce to evoke the generation of newly adsorptive sites for NOx and NH3, thus contributing to the NH3-SCR reaction.

Highly efficient MnOx catalysts supported on Mg-Al spinel for low temperature NH3-SCR

Selective catalytic reduction (SCR) has been extensively employed in industries for the removal of NOx while it is still a challenge to develop novel low-temperature catalysts. The redox and acid sites on the catalyst are often preconditions that cooperate in the SCR reaction. In this study, a series of MnOx supported on MgAl2O4 spinel catalysts (denoted as Mn/MAS) were successfully prepared by incipient wetness impregnation method for low-temperature SCR of NOx with ammonia. The effects of the calcination temperature of MAS support and Mn loading on catalyst structure and performance were examined. The Mn0.12/MAS-700 catalyst showed the best low-temperature denitrification performance, with more than 80% conversion of NO at 150–300 °C and higher than 60% N2 selectivity. Moreover, the denitrification efficiency remained at nearly 90% in the presence of H2O. The physicochemical properties of the catalysts were determined using various characterization methods, and the corresponding results found that the higher concentration of Mn4+ and surface chemisorption oxygen enhanced the low-temperature activity of the catalysts. The addition of Mn species increased the surface acidity of weak acid centers which promoted the catalyst activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) revealed that the Eley-Rideal mechanism was introduced mainly at 100 °C on the Mn0.12/MAS-700 catalyst. This work provides new inspiration into the effect of Mn species, and the synergistic role of active components Mn and MAS support in the SCR reaction.

The promotion of NH3-SCR performance and its mechanism on Sm modified birnessite

To develop novel NH3-SCR catalysts with low-temperature and high-efficency, a series of birnessite (δ-MnO2) catalysts with different Sm doping by methanol reduction are synthesized. The results showed that the introduction of Sm successfully inhibited the crystallization of MnOX, promoted the specific surface area, the contents of chemisorbed oxygen species and the formation of surface acid sites. It also increased the relative content of (Mn4+ + Mn3+)/Mn, which was beneficial to low-temperature SCR activity. The catalytic performance of xSm/Mn (δ-MnO2 based with xSm doping) catalysts was evaluated by NH3-SCR performances. Among them, 0.05 Sm/Mn showed 100% NOX conversion and high N2 selectivity compared to the other Sm addition catalysts within the operating low-temperature window (25–200 °C), at a high GHSV (gas hourly space velocity) of 60,000 h−1. The modified 0.05 Sm/Mn catalysts followed the L-H and E-R reaction mechanisms and were dominated by L-H, as revealed by in-situ DRIFTs analysis.

Boosting the low-temperature activity of fast-synthesized Cu-SAPO-18 for NH3-SCR by hydrothermal treatment

Cu-SAPO-18 catalysts attracts increasing attention in the reaction of selective catalytic reduction (SCR) of nitrogen oxides (NOx), but the catalyst synthesis process is complicated and time-consuming. Also, it is urgent to enhance the low-temperature catalytic conversion of NOx for practical applications. Herein, we reported a fast-synthesis method to prepare the Cu-SAPO-18 catalysts in 32–80 h, and the catalysts showed an excellent catalytic performance of over 80 % NO conversion at 260–480 °C. The crystalline of the catalysts was improved in short crystallization time and high crystallization temperature, accelerating the NH3-SCR reaction. The low-temperature activity of all catalysts was improved after hydrothermal treatment, and the temperature range of over 80 % NO conversion was extended to 190–515 °C for the optimized catalyst, which was due to the increasing active Cu2+ species (ZCuOH and Z2Cu, where Z represents an anionic site on the zeolite framework). The redox cycle of Cu2+/Cu+ was accelerated and more Lewis acid sites on the catalyst enhanced the NO/NH3 adsorption and activation, promoting the low-temperature catalytic activity. Furthermore, after hydrothermal treatment, part of Z2Cu species converted to ZCuOH species, which exhibited stronger NO adsorption energies (−2.06 eV) than that of Z2Cu (−1.38 eV) based on theoretical calculation. Both fresh and aged catalysts showed remarkable SO2/H2O resistance. This work reveals the effects of hydrothermal treatment on the Z2Cu and ZCuOH species, which is helpful to prepare the fast-synthesized zeolite catalysts with excellent low-temperature SCR performance.

Promoting the catalytic activity and SO2 resistance of CeO2 by Ti-doping for low-temperature NH3-SCR: Increasing surface activity and constructing Ce3+ sites

CeO2-based oxides are promising catalysts for low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), but the poor SO2 poisoning resistance hampers its further application. Herein, we developed a Ti-doped CeO2 (Ti-CeO2) catalyst with high specific surface area by calcining TiSO4-doped Ce-MOF precursor for NO removal. On Ti-CeO2, abundant Ti3+ sites are generated by the thermally reducing carbon in the organic ligand during calcination, the electronic interaction between Ti3+ and Ce4+ leads to the generation of many more exposed Ce3+ sites, effectively preventing the sulphuration of surface Ce species to Ce2(SO4)3. Besides, the residual sulfate species provides plentiful Brønsted acid sites for NH3 adsorption. Therefore, both the catalytic performance and SO2 resistance of CeO2 are significantly enhanced after Ti-doping. Remarkably, above 80% NO conversion is obtained between 220 and 450 °C on Ti0.10-CeO2 (the molar ratio of Ti:Ce is 0.10 in the synthesis process), and the NO conversion maintains around 76.0% for 170 h in the presence of 100 ppm SO2 and 5 vol% H2O at 250 °C. The reaction on CeO2 and Ti-CeO2 follow the Langmuir-Hinshelwood (L-H) mechanism, but abundant Ce4+ in CeO2 having high oxidation ability leads to the generation of plentiful stable nitrate or nitrite to occupy the active sites. In contrast, the over oxidation of NO and SO2 adsorption are inhibited because of Ti-doping, resulting in superior catalytic performance in the SCR reaction.

Sm-MnOx/TiO2-{001} with preferentially exposed anatase {001} facet for selective catalytic reduction of NO with NH3

TiO2-{001} with preferentially exposed {001} supported SmMnOx was prepared for NOx reduction with NH3. The optimal TiMn30Sm10-{001} demonstrated remarkable low temperature catalytic activity (T50 % = 71 °C). Besides, TiMn30Sm10-{001} achieved 80 % NO conversion under a wide temperature window (140–280 °C) under gas hourly space velocity (GHSV) of 50000 h−1, along with excellent catalytic stability (> 85 % NO conversion for 24 h at 200 °C) in both dry and humid conditions. It also showed enhanced N2 selectivity with T80 % range from 60 to 160 °C. The outstanding catalytic performance of TiMn30Sm10-{001} was attributed to the combined efforts of high concentration of Mn3+, abundant surface adsorbed oxygen, and more acid sites due to the addition of SmOx and facets effect of TiO2-{001}. In situ infrared fourier transform spectroscopy analysis revealed that the NO reduction over TiMn30Sm10-{001} obeyed Langmuir-Hinshelwood mechanism. This work provides a rational strategy for designing facet-engineered catalysts for low-temperature SCR.

An effective strategy to enhance the sulfur resistance of Ce-based catalyst for low temperature SCR reaction

SO2 has a significant impact on SCR reaction, and how to suppress the effect of SO2 on Ce-based catalyst activity has been a goal pursued by the scientific community. In this work, the CeSmFTi catalyst was prepared by one-pot hydrothermal method to study the catalytic activity and SO2 resistance performance in NH3-SCR reaction. The experimental findings suggested that Ce5Sm1FTi94 catalyst had better catalytic performance, with the NO conversion exceeding 90% at 320 °C. After enduring SO2 test, the NO conversion could gradually recover to more than 80%. The results showed that the presence of CeF3 efficiently regulated the proportion of active state Ce3+ and accelerated the “fast SCR” reaction, which significantly enhanced the catalytic performance of the catalyst and inhibited the oxidation of SO2. Notably, doping a certain amount of Sm into Ce5FTi94 catalyst not only improved the surface acidity and redox performance, but also promoted the electron transfer and interaction between Ce and Sm species, thereby inhibiting the combination of SO2 and cerium species. Therefore, the formation of sulfate species on the catalyst was minimized, resulting in a better SO2 tolerance. Simultaneously, the in-situ DRIFTs analysis revealed that both L-H and E-R mechanisms were presented on Ce5Sm1FTi94 catalyst to carry out the NH3-SCR reaction process, with the L-H mechanism being dominant.

Water-tolerant and anti-dust CeCo-MnO2 membrane catalysts for low temperature selective catalytic reduction of nitrogen oxides

The present work successfully proposes a domain knowledge–guided Machine Learning (ML) strategy, which successes the development of a water-tolerant anti-dust catalyst for Low-Temperature (LT) Selective Catalytic Reduction (SCR) of nitrogen oxides (NOx) from catalyst discovery to industrial deployment. The discovered catalyst is able to convert 99 % of NOx at 150 °C in the standard waste gas, even in the waste gas containing 7 vol% of water vapors, the efficiency is still retained at 97 %. The superior LT activity and water-tolerance are attributed to abundant surface-active oxygen, Brønsted acid and microcellular structure. The SCR reaction mainly follows the Eley-Rideal (E-R) pathway driven by Brønsted acid and the Langmuir-Hinshelwood (L-H) pathway maintained by abundant reactive oxygen species in moisture waste gases. And then, catalyst is synthesized on a polyphenylene sulfide filter to render the membrane configuration in order to have the anti-dust ability. The final membrane catalyst has the capacity of converting 95 % NOx in the waste gas containing 7 vol% of moisture at 150 °C and the dedusting, and denitrification ability. The domain knowledge–guided Machine Learning (ML) strategy paves a wide avenue for the whole chain development from the data-driven discovery to the industrial deployment associated with mechanism exploration.

Redox on Mn-Ce interface and its effects on low temperature selective catalytic reduction for NOx removal

The objective of this work was to clearly elucidate the interactions on Mn-Ce interface and understand how they affect low temperature selective catalytic reduction (SCR) for NOx removal. In this work, three Mn-Ce catalysts in similar particle size and morphology with different Mn-Ce interface areas were prepared. MnO2@CeO2-F had the tightest and largest amount of Mn-Ce interfaces compared with MnO2@CeO2-P and MnO2-CeO2-M, and also exhibited the highest NO conversion degree at 100–150 °C as well as the strongest activity of Mn species and lowest generation of N2O. The TG-DTG, XPS and electrochemical impedance spectroscopy results proved the redox of Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ on the Mn-Ce interface, and MnO2@CeO2-F had the highest Mn4+, Ce3+, Oα concentrations. The results of the study on the redox reactions during the NH3-SCR process provided powerful evidence of the Mn4+/Mn3+ and Ce3+/Ce4+ recycles that are important for low temperature NH3-SCR process. Moreover, the Mn-Ce interface was directly proved to benefit the surface acid sites, redox ability, and activity of the adsorbed intermediates. Both the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms were observed in the SCR reaction with prepared Mn-Ce materials, where “fast SCR” can be observed with MnO2@CeO2-F. Moreover, the Mn-Ce interface also lowers the activation energy of the low-temperature SCR process, with MnO2@CeO2-F having the lowest activation energy. The results from this work offered powerful evidences for the interactions of Mn/Ce redox at the Mn-Ce interface and elucidated clear understanding of the effects of Mn-Ce redox on low-temperature SCR process, which is expected to promote the NOx removal process.

Promotion Effect of Iron on Activity and N2 Selectivity of Mn/Ce Catalysts in the NH3-SCR Reaction

Manganese oxides are known to have good low-temperature SCR performance, whereas improving their medium- and high-temperature catalytic performance and N2 selectivity remain a challenge. Herein, manganese-based ternary catalysts Fe–Mn/Ce, using CeO2 as the carrier and Fe as the second active component, were constructed to effectively solve this problem. Fe–Mn/Ce catalysts show excellent catalytic performance, with NOx conversions of more than 90% over a wide operating temperature range (164.5–419.4 °C) with high and N2 selectivity. Further investigation showed that, for the Fe–Mn/Ce catalyst with rich redox sites and surface acidity, higher amounts of Fe2+ and Mn4+ are the important factors promoting excellent SCR reactivity. Besides, the Fe modification can generate effective electron transfer to reduce the N2O generation pathway and increase the catalytic reaction sites, thus enhancing the overall catalytic performance.

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

Selective catalytic reduction (SCR) of NOx with NH3 is the most efficient technology for NOx 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.

Insight into the low-temperature DeNOx performance of copper-based oxides: Perspective from the valence state distribution

Understanding the effect of CuOx valence state specifically is significant for constructing high-performance Cu based oxide catalyst for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR). In this work, CuOx with different valence state including Cu2O, Cu2O-CuO and CuO particles are synthetized as model catalysts, which present same variation laws in catalytic performance at 180 ℃, 210 ℃ and 240 ℃. NH3-SCR catalytic performance of Cu2O-CuO heterojunction is the best, CuO exhibits good catalytic activity, while Cu2O possesses excellent N2 selectivity. Experimental results combined with density functional theory (DFT) calculations reveal that Cu2+ favor to form monodentate nitrate and possess strong ability of activation and dehydrogenation of NH3. The energy barrier for the form of NH2* is 79.5 kJ/mol, which is much lower than that for Cu+. While Cu+ is beneficial to NH3 and NOx adsorption and the form of bidentate nitrate, which have a positive impact on improving the selectivity of N2. Moreover, the collaboration of Cu2+ and Cu+ facilitates the redox capability and the form of surface adsorbed oxygen species, thus the reaction progress is further promoted. The SCR reaction catalyzed by three samples are dominated mainly by Langmuir-Hinshelwood (L-H) mechanism at 180 ℃. While at 240 ℃, the reaction over Cu2O follows Eley-Rideal (E-R) mechanism, during which the rate-limiting step is the internal hydrogen transfer procedure (NH2NO*→NHNOH*). Over CuO, the reaction follows L-H mechanism, the process of atomic isomerism (NHNOH*+ *→N2*+H2O*) is the rate-determining step. The energy barriers are 136 and 146 kJ/mol, respectively. This work may supply important theoretical support for the preparation of high-performance Cu-based oxide catalysts.

Cobalt-/pH-Modified V2O5-MoO3/TiO2 Catalyst with Enhanced Activity for the Low-Temperature Selective Catalytic Reduction Process

Currently, the elimination of gaseous pollutants—particularly nitrogen oxides—has emerged as a significant concern. Among various deNOx technologies, selective catalytic reduction (SCR) has gained prominence as the primary approach for NOx abatement, owing to its superior performance. In this study, novel low-temperature SCR catalysts were developed by regulating the pH value and doping cobalt based on a V2O5-MoO3/TiO2 (VMT) catalyst. The results show an increased SCR performance with 82.8% and 91.1% for catalysts after pH (=10) modification (VMT-10) and (1 wt%) Co/pH (=10) modification (1CoVMT-10), respectively. H2-TPR, NH3-TPD, XPS and DRIFTS confirmed that the pH regulation transformed polymerization V species into isolated V5+=O, thus leading to an increase in the number of acid sites, which enhanced the NH3 and NO2 adsorption capacity. Furthermore, the DRIFTS study indicated that the NH3-SCR reaction over 1CoVMT-10 followed the E–R and L–H mechanism.