Catalysts For Reduction Of NO Research Articles

Effect of morphology on the performance of MnOx catalysts for selective catalytic reduction of NO with NH3

Modulating the physicochemical properties of catalysts through the construction of special crystal structures or microscopic morphology provides new insights for developing high-performance low-temperature denitrification catalysts. This paper explores the effect of morphology structure on catalyst performance and identifies the dominant driving factors. The catalyst activity sequence was found to be MnOx spiny nanospheres (Mn-SNS) > MnOx nanowires (Mn-N) > MnOx nanospheres (Mn-NS) > MnOx nanoparticles (Mn-P). Among these, the Mn-SNS catalyst prepared by the hydrothermal method exhibited the best catalytic activity and the good resistance to water and sulfur. It achieved over 70% NOx conversion at 150 °C, and maintained 100% at 200–350 °C. In the presence of SO2 and H2O, the NOx conversion was maintained at 87%. The spherical structure consisting of nanospikes provides the Mn-SNS catalysts with a large specific surface area, more (Mn4++Mn3+), abundant acid sites, and stronger reducibility, thus exhibiting excellent performance through the "fast-SCR" pathway. It also inhibits oxide crystallization and provides more sites for the adsorption activation of NOx and NH3. The nanowire structures can provide a large specific surface area and a high concentration of Mn3+ and Mn4+ for MnOx catalyst. Characterization analyses revealed that the specific surface area, the ratio of (Mn4++Mn3+)/Mn, and the number of Lewis acid sites (L-NH3) are the most important factors affecting the catalytic activity. This present empirical analysis provides new ideas for designing high-performance catalysts with a large specific surface area.

Enhanced low-temperature activity of Mn-based catalysts for NH3 selective catalytic reduction of NO through CE-modulated redox and TiO2 shell construction

Manganese-based catalysts presented great challenges for the wide operating window in the selective catalytic reduction of NOx with NH3 (NH3-SCR). Aiming at enhancement of the SCR performance, a series of MnCeOx (MnyCe1) and MnxCe1@TiO2 catalysts were prepared. Impressively, the Mn1Ce1 catalyst exhibited excellent low-temperature activity, achieving over 80 % conversion of NO within the temperature range of 85–350 °C, but the N2 selectivity decreased with the increasing temperature. Upon coating a TiO2 shell, the temperature window with N2 selectivity exceeding 80 % was significantly widened to 85–350 °C since the weak redox ability limited the NH3 oxidation to undesired N2O. High surface area, together with abundant acid sites and chemical adsorbed oxygen that formed via the interaction between Mn4+/Mn3+, Ce4+/Ce3+ and Ti4+/Ti3+, contributed to the superior catalytic performance of MnxCe1@TiO2. In addition, through in situ DRIFTS analysis, both Eley-Rideal and Langmuir-Hinshelwood reaction routes were observed. This work would provide an efficient and low-cost strategy for preparing Mn-based catalysts with excellent low-temperature activity for NOx removal.

Promoted catalytic property of Cu/SSZ-13 by introducing a minority of Mn for NO removal from diesel engine exhaust

The Cu-exchanged SSZ-13 with the small-pore chabazite framework is considered as a highly efficient catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR). In order to further improve the catalytic property, a series of Mn ion-assisted Cu/SSZ-13 powder catalysts were prepared by co-exchange method and stepwise exchange method. It is found that the NH3-SCR activity, N2 selectivity, hydrothermal stability and sulfur resistance of Cu/SSZ-13 are promoted by introducing a minority of Mn (0.15%–0.23%(mass)) through co-exchange method. Characterization results revel that the Cu,Mn co-exchange enables the higher amounts of Cu2+ active sites, the abundant medium strong and strong acid, the optimized ratio of lewis acid to Brønsted acid, etc., which are required for a good NH3-SCR catalytic property over broad temperature range and under harsh working environment. Moreover, a monolithic catalyst was prepared by impregnating a cordierite ceramic support into the coating slurry containing the optimized CuMn/SSZ-13 powder. The diesel engine bench tests show that Cu,Mn co-exchange gives the monolith catalyst a better catalytic property than commercial catalysts. This work provides an important guidance for the rational design of secondary-ion-assisted zeolites applied in NH3-SCR.

Comparative study and screening of Single-Atom and homonuclear Dual-Atom catalysts for NO reduction via electrocatalysis

Electrocatalytic technology offers a promising solution to address environmental pollution and resource scarcity. The development of high-stability and selective catalysts is crucial in this regard. This paper focuses on single-atom catalysts (SACs) and homonuclear dual-metal catalysts (DACs) constructed using transition metal atoms as active centers. Catalytic activity and selectivity of these catalysts in the nitric oxide reduction reaction (NORR) process are systematically investigated through first-principles calculations, while exploring the influence of adjacent metal atoms on their characteristics. Using a four-step screening approach, we identify Ti2-N6 as a standout candidate catalyst with a low applied voltage (-0.24 V) for selective NO catalysis and significant inhibition of competitive responses. Homonuclear DACs inherit the advantages of SACs while disrupting the inherent linear relationship, resulting in reduced applied voltage requirements and enhanced catalytic performance. This work presents an efficient catalyst screening method and highlights the unique characteristics and advantages of homonuclear DACs over SACs.

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.

Stability challenges and prospects for the industrial application of non‐noble catalysts for selective catalytic reduction of NOx by CO (CO‐SCR)

AbstractThe selective catalytic reduction of NOx by CO (CO‐SCR) holds considerable promise as a simple and cost‐effective technology capable of concurrently removing CO and NOx from mobile and stationary sources. This is especially pertinent for industry sectors with high CO emissions, such as the iron and steel industry, making CO‐SCR an attractive solution with significant industrial applicability. However, despite the promising industrial potential of CO‐SCR technology, its widespread adoption is hindered by several factors, most notably the impacts of SO2, H2O, and O2 on catalyst performance and stability. Current research on these limiting factors is not exhaustive, posing challenges for the comprehensive industrial implementation of CO‐SCR technology. Therefore, this review paper focuses on the variables impeding the industrial utilization of CO‐SCR, summarizing existing bottlenecks and emphasizing the urgent need to address the inhibitory effects of O2 on the CO‐SCR reaction, a critical issue in contemporary research. This review aims to clearly articulate the key challenges of CO‐SCR technology and provide direction for future research.

Reactivity of Ni-modified MIL-100(Fe) catalysts for C3H6-SCR and reaction mechanism

Metal-organic frameworks (MOFs)-based carrier, MIL-100(Fe), was synthesized by hydrothermal method and Ni was supported on the MIL-100(Fe) by wet impregnation method to prepare the catalysts for selective catalytic reduction of NO with C3H6 (C3H6-SCR). The addition of Ni significantly improved the reactivity of C3H6-SCR. At 275 °C, the NO conversion of MIL-100(Fe) was 74.5%, whereas the NO conversion of 5.8%Ni/MIL-100(Fe) with 100% N2 selectivity was 100%. The in-situ DRIFTS prove that the interaction between NO2−/NO3− and CxHyOz substances generated intermediate isocyanates (R-NCO), and the reaction pathway was proposed based on the DRIFTS results to understand the mechanism.

Hollow Mn-doped CeO2@Co3O4 catalyst for NO reduction by CO

Selective catalytic reduction of NO using CO (CO-SCR) is a promising technology for the purification of NO in the flue gas. However, the presence of O2 significantly inhibits the activity of the CO-SCR catalyst. Herein, we synthesize a hollow Mn-doped CeO2@Co3O4 catalyst (Mn-CeO2@Co3O4), which achieved 82% NO conversion, 100% CO conversion, and 78% N2 selectivity under 5 vol% O2 at 200 °C, superior to those of CeO2@Co3O4 and Co3O4@CeO2 catalysts. The characterization results indicate that the strong electronic interaction at the heterointerfaces between Co3O4 and CeO2 plays a crucial role in promoting selective adsorption of NO. Furthermore, the Mn doping into the Co3O4 lattice of the catalyst facilitates the conversion of N2O intermediates, thereby enhancing the overall activity. This research provides a new strategy for the design and synthesis of CO-SCR catalysts.

Direct reduction of NO into N2 catalyzed by fullerene-supported rhodium clusters.

Catalytic conversion of NO has long been a focus of atmospheric pollution control and diesel vehicle exhaust treatment. Rhodium is one of the most effective metals for catalyzing NO reduction, and understanding the nature of the active sites and underlying mechanisms can help improve the design of Rh-based catalysts towards NO reduction. In this work, we investigated the detailed catalytic mechanisms for the direct reduction of NO to N2 by fullerene-supported rhodium clusters, C60Rh4+, with density functional theory calculations. We found that the presence of C60 facilitates the smooth reduction of NO into N2 and O2, as well as their subsequent desorption, recovering the catalyst C60Rh4+. Such a process fails to be completed by free Rh4+, emphasizing the critical importance of C60 support. We attribute the novel performance of C60Rh4+ to the electron sponge effect of C60, providing useful guidance for designing efficient catalysts for the direct reduction of NO.

Phosphorus poisoning and regeneration of the Cu-LTA catalyst for the selective catalytic reduction of NOx with NH3

In this study, the phosphorus poisoning and regeneration of the Cu-LTA catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated.

Heteropoly acid-grafted iron oxide catalysts for efficient selective catalytic reduction of NOx with NH3

A series of heteropoly acid-grafted Fe2O3 catalysts (i.e., HPA/Fe2O3) for selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated.

Copper-exchanged SUZ-4 zeolite catalysts for selective catalytic reduction of NOx

The copper-exchanged SUZ-4 zeolite with optimal compositions exhibits comparable catalytic properties to commercial Cu-SSZ-13 for the selective catalytic reduction reaction.

Unexpected offsetting effect of Na and HCl deactivation over CeO2-MoO3 catalyst for NO reduction

This work mainly illustrated the synergistic effects of Na and HCl on the SCR performance of CeMo catalysts. Given that the fresh CeMo catalyst was treated with HCl or doped by Na, the weakened acidity and oxidizability would interfere with the adsorption and activation of NH3, thus resulting in a downward trend of the catalyst deNOx performance. Notably, for the HCl treated Na-poisoned catalyst, the treatment with HCl introduced additional acid sites onto the Na-poisoned catalyst surface and increased the availability of Ce4+ and chemisorbed oxygen. These behaviors exerted a certain promotional effect on the adsorption and activation of NH3. Apparently, the catalyst's deNOx performance slightly rebounded, suggesting the offsetting effects of HCl. Such knowledge would be vital to the stable operation in real coal-fired flue gas.

Effect of Promoters (Co, Cr, Fe, Zr) on the Performance of MnO2/CeO2 Catalysts for NO Reduction at Low Temperatures Using NH3-SCR

Effect of Promoters (Co, Cr, Fe, Zr) on the Performance of MnO2/CeO2 Catalysts for NO Reduction at Low Temperatures Using NH3-SCR

Low-temperature CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation for selective catalytic reduction of NO using NH3: A structure–activity relationship study

CeCoMnOx spinel-type catalysts for the selective catalytic reduction of NO using NH3 (NH3-SCR) are usually prepared by alkaline co-precipitation. In this paper, a series of CeCoMnOx spinel-type catalysts with different calcination temperatures were prepared by acidic oxalate co-precipitation. The physicochemical structures and NH3-SCR activities of the CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation and conventional ammonia co-precipitation were systematically compared. The results show that the CeCoMnOx spinel-type catalysts prepared by the oxalate precipitation method (CeCoMnOx-C) have larger specific surface area, more mesopores and surface active sites, stronger redox properties and adsorption activation properties than those prepared by the traditional ammonia co-precipitation method at 400 °C (CeCoMnOx-N-400), and thus CeCoMnOx-C have better low-temperature NH3-SCR performance. At the same calcination temperature of 400 °C, the NO conversion of CeCoMnOx-C-400 exceeds 89 % and approaches 100 % within the reaction temperature of 100–125 °C, which is 14.8 %-2.5 % higher than that of CeCoMnOx-N-400 at 100–125 °C. In addition, the enhanced redox and acid cycle matching mechanisms on the CeCoMnOx-C surface, as well as the enhanced monoadsorption Eley-Rideal (E-R) and double adsorption Langmuir-Hinshelwood (L-H) reaction mechanisms, are also derived from XPS and in situ DRIFTS characterization.

New Insights into the Effect of Ce Doping on the Catalytic Performance and Hydrothermal Stability of Cu-USY Zeolite Catalysts for the Selective Catalytic Reduction of NO with NH3

5Cu-USY and Ce-doped 5Cu8Ce-USY zeolite catalysts were prepared by the conventional impregnation method. The obtained catalysts were subjected to the hydrothermal ageing process. The catalytic performance of the selective catalytic reduction of NOx with NH3 (NH3-SCR) was evaluated on both fresh and aged catalysts. Physical/chemical characterizations such as X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) were performed, along with detailed in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) experiments including CO adsorption, NH3 adsorption, and NO + O2 reactions. Results showed that, for the 5Cu-USY catalyst, hydrothermal ageing treatment could somehow improve the low-temperature SCR activity, but it also led to significant formation of unfavorable byproducts NO2 and N2O. Such an activity change can be attributed to hydrothermal ageing inducing the migration of isolated Cu+ species in the sodalite cavities towards the super cages of the USY zeolites. The increased content of Cu+ species in the super cages was beneficial for the low-temperature activity improvement, but, at the same time, it also facilitated ammonia oxidation at high temperatures. Ce doping after hydrothermal ageing has a “double-edged sword” effect on the catalytic performance. First of all, Ce doping can inhibit Cu species migration by self-occupying the internal cage sites; thus, the catalytic performance of 5Cu8Ce-USY-700H remains stable after ageing. Secondly, Ce doping introduces a CuOx–CeO2 strong interaction, which facilitates lattice oxygen mobility by forming more oxygen vacancies so as to increase the concentration of surface active oxygen. These changes, on the one hand, could help to promote further oxidative decomposition of nitrate/nitrite intermediates and improve the catalytic performance. But, on the other hand, it also causes the byproduct generation to become more severe.

Simple anion-modified layered double oxides use for controlling Cu valence states for low-temperature CO-SCR

The modification of transition metal catalysts for selective catalytic reduction of NO by CO has been studied extensively. However, the effect of anion on CO-SCR catalyst modification is rarely reported. In this study, four different LDHs precursors were synthesized by using urea hydrothermal method and introducing different inorganic anions into the reaction system to control the self-assembly growth during the synthesis of LDHs. It is then loaded with a mass ratio of 7.5 % copper by impregnation method. The LDO(N)/Cu7.5 catalyst using nitrate ions regulated LDH as support achieved a high NO conversion of 96 % at 150 °C, which outdistanced the LDO(S)/Cu7.5 with support regulated by sulfate ions. A series of characterizations demonstrated that CoFe-LDH modified with different anions could modulate the valence state of loaded copper species. Among them, the LDO(N)/Cu7.5 catalyst was able to produce more Cu+ as the active site for the catalytic reaction. In situ DRIFTS further revealed the reaction mechanism, following the E-R mechanism at ≤150 °C and the L-H mechanism at 200–350 °C. This work supplies a new idea for the valence modulation of copper and the modification of LDH materials for applications in CO-SCR.

Promoting effect of Cu and Mn doping on the Fe/ZSM-5 catalyst for selective catalytic reduction of NO with NH3

In this paper, a series of Fe-based catalysts with Zeolite Socony Mobil-5 (ZSM-5) as supports were synthesized using the sol–gel method to obtain the Fe/ZSM-5, Mn-Fe/ZSM-5, Cu-Fe/ZSM-5 and Cu-Mn-Fe/ZSM-5 catalysts. The synthesized catalysts were examined and characterized via catalytic activity testing, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption, scanning electron microscope (SEM), temperature-programmed desorption of NH3 (NH3-TPD), temperature-programmed reduction with H2 (H2-TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results showed that the Cu-Mn-Fe/ZSM-5 catalyst had the best catalytic performance compared with the other catalysts. In addition, Fe/ZSM-5 and Cu-Mn-Fe/ZSM-5 all exhibited excellent stability as well as good tolerance to SO2 & H2O under the experimental conditions. Further characterization results showed that with co-doping of Cu and Mn, the amount of Mn4+, Cu+, Fe3+ and surface chemisorbed oxygen (Oβ) on the surface of Fe/ZSM-5 increased, promoting the catalytic performance. The DRIFT studies revealed that the NH3-SCR reaction followed both the E-R and L-H mechanism on the surface of the Fe-based ZSM-5 catalyst. The Fe-based ZSM-5 catalyst is a good candidate for NH3-SCR both in high reactivity and outstanding stability.

Effect of different acid anions on highly efficient Ce-based catalysts for selective catalytic reduction of NO with NH3

Effect of different acid anions on highly efficient Ce-based catalysts for selective catalytic reduction of NO with NH3

Revisiting dispersion and reactivity of active sites via a restricted random distribution model over supported vanadia catalysts for NO reduction

The structure and dispersion of active sites substantially impact the performance of supported metal oxide catalysts, as exemplified by the V2O5/TiO2 catalysts used for NH3-SCR reaction. Due to the complexity of the active site dispersion on the catalyst surface, the intrinsic law of this structure–activity relationship remains controversial. Herein, we proposed a restricted random distribution model to describe the stochastic anchoring behavior of active sites on the TiO2 surface during the loading process of vanadia. The structures of monomeric, dimeric, and polymeric VOx sites were determined by DFT calculations and verified with spectroscopy and adsorption information experimentally. The calculated reaction pathways and energy profiles for these structures indicated that the dual sites configuration and terminal V(=O)2 bond are responsible for the high reactivity of aggregated VOx sites. Combining the reactivity of these various sites and the random distribution method, we built a general profile of the correlation between the V2O5/TiO2 catalyst surface and its reaction performance. The reactivity results obtained from experimental in situ DRIFTS and kinetic tests validate our theoretical model. The random distribution method links DFT calculations and experiments, coupling the structure and dispersion of the VOx active sites to the catalyst reactivity.