Mechanism and Kinetics Study on Low-Temperature NH3-SCR Over Manganese–Cerium Composite Oxide Catalysts

Insight into the effect of exposed crystal facets of anatase TiO2 on HCHO catalytic oxidation of Mn-Ce/TiO2

Indoor formaldehyde pollution seriously jeopardizes human health. The development of efficient and stable non-precious metal catalysts for low-temperature catalytic degradation of formaldehyde is a promising approach. In this study, TiO2 {001} and {101} supports were loaded with different ratios of Mn and Ce active components, and the effects of the ratios of the active components on the catalytic activity were investigated. The elemental oxidation states, redox capacities, active oxygen mobilities and acid site distributions of the catalysts were determined using characterization techniques such as XPS, H2-TPR, O2-TPD, and NH3-TPD. In situ infrared spectroscopy was utilized to reveal the differences in the two-step dehydrogenation reactions of dioxymethylene (DOM) in 5Mn1Ce/Ti-NS and 5Mn1Ce/Ti-NP. Density-functional theory was used to investigate the differences in the catalytic steps and maximum energy barriers of Mn-Ce/Ti-NS and Mn-Ce/Ti-NP for HCHO. The differences in catalytic activity due to the influence of the manganese and cerium active components on the {001} and {101} crystal faces of anatase titanium dioxide are comprehensively revealed. Exposure of the supported crystalline surfaces alters the catalytic activity centers and reaction pathways at the molecular level. This study provides experimental and theoretical guidance for the selection of exposed crystalline surfaces for loaded catalysts.

New mechanistic insight into catalytic decomposition of dioxins over MnOx-CeO2/TiO2 catalysts: A combined experimental and density functional theory study

Elucidation of the catalytic decomposition mechanism of dioxins is pivotal in developing highly efficient dioxin degradation catalysts. In order to accurately simulate the whole molecular structure of dioxins, two model compounds, o-dichlorobenzene (o-DCB) and furan, were employed to represent the chlorinated benzene ring and oxygenated central ring within a dioxin molecule, respectively. Experiments and Density Functional Theory (DFT) calculations were combined to investigate the adsorption as well as oxidation of o-DCB and furan over MnOx-CeO2/TiO2 catalyst (denoted as MnCe/Ti). The results indicate that competitive adsorption exists between furan and o-DCB. The former exhibits superior adsorption capacity on MnCe/Ti catalyst at 100 °C - 150 °C, for it can adsorb on both surface metal atom and surface oxygen vacancies (Ov) via its O-terminal; while the latter adsorbs primarily by anchoring its Cl atom to surface Ov. Regarding oxidation, furan can be completely oxidized at 150 °C - 300 °C with a high CO2 selectivity (above 80 %). However, o-DCB cannot be totally oxidized and the resulting intermediates cause the deactivation of catalyst. Interestingly, the pre-adsorption of furan on catalyst surface can facilitate the catalytic oxidation of o-DCB below 200 °C, possibly because the dissociated adsorption of furan may form additional reactive oxygen species on catalyst surface. Therefore, this work provides new insights into the catalytic decomposition mechanism of dioxins as well as the optimization strategies for developing dioxin-degradation catalysts with high efficiency at low temperature.

Revealing the crystal facet effect on N2O formation during the NH3-SCR over α-MnO2 catalysts

The detailed atomic-level mechanism of the effect induced by engineering the crystal facet of α-MnO₂ catalysts on N₂O formation during ammonia-selective catalytic reduction (NH₃-SCR) was ascertained by combining density functional theory (DFT) calculations and thermodynamics/kinetic analysis. The surface energies of α-MnO₂ with specific (100), (110), and (310) exposed planes were calculated, and the adsorptions of NH₃, NO, and O₂ on three surfaces were analyzed. The adsorption energies showed that NH₃ and NO molecules could be strongly adsorbed on the surface of the α-MnO₂ catalyst, while the adsorption of O₂ was weak. Moreover, the key steps in the oxidative dehydrogenation of NH₃ and the formation of NH₂NO as well as dissociation of NH₂ were studied to evaluate the catalytic ability of NH₃-SCR reaction and N₂ selectivity. The results revealed that the α-MnO₂ catalyst exposed with the (310) plane exhibited the best NH₃-SCR catalytic performance and highest N₂ selectivity, mainly due to its low energy barriers in NH₃ dehydrogenation and NH₂NO generation, and difficulty in NH₂ dissociation. This study deepens the comprehension of the facet-engineering of α-MnO₂ on inhibiting N₂O formation during the NH₃-SCR, and points out a strategy to improve their catalytic ability and N₂ selectivity for the low-temperature NH₃-SCR process.

Effects of Flue Gas Impurities on the Performance of Rare Earth Denitration Catalysts

Selective catalytic reduction (SCR) is still the most widely used process for controlling NOx gas pollution. Specifically, commercial vanadium-based catalysts have problems such as narrow operating temperature range and environmental pollution. Researchers have developed a series of cerium-based catalysts with good oxygen storage performance and excellent redox performance of CeO2. However, the anti-poisoning performance of the catalyst is the key to its application. There are many kinds of impurities in the flue gas, which has a huge impact on the catalyst. The deposition of substances, the reduction of active sites, the reduction of specific surface area, and the reduction of chemically adsorbed oxygen will affect the denitration activity of the catalyst to varying degrees, and the poisoning mechanism of different impurities on the catalyst is also different. Therefore, this review divides the impurities contained in flue gas into different types such as alkali metals, alkaline earth metals, heavy metals, and non-metals, and summarizes the effects and deactivation mechanisms of various types of impurities on the activity of rare earth catalysts. Finally, we hope that this work can provide a valuable reference for the development and application of NH3-SCR catalysts for rare earth denitration in the field of NOx control.

Investigation of chlorine-poisoning mechanism of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts during o-DCBz catalytic decomposition: Experiment and first-principles calculation

The catalytic activity and stability of MnO_x/TiO₂ and MnO_x-CeO₂/TiO₂ catalysts for the oxidative degradation of 1,2-dichorobenzene (o-DCBz) at low temperatures (≤275 °C) were experimentally examined. The chlorine (Cl) poisoning mechanism of the catalysts was also clarified based on the catalyst characterization combined with theoretical calculations. Experimental results show that the MnO_x/TiO₂ catalyst is considerably deactivated during o-DCBz catalytic decomposition, mainly due to the chlorination of the catalytic active component. Ce addition and high temperature can effectively promote the resistance of MnO_x/TiO₂ catalyst to Cl poisoning. Density functional theory (DFT) calculations in the framework of first-principles reveal that Cl atom prefers to anchor on surface oxygen vacancy (OV) rather than on top site of Mn atom. The adsorption of Cl atom on surface OV hinders the dissociated adsorption of O₂ on surface OV and interrupts the regeneration of the surface reactive oxygen species. The adsorption of Cl atom on top site of Mn atom increases the formation energy of surface OV and damages the surface Lewis acid sites which act as the important adsorption sites for o-DCBz molecules. Ce addition causes Cl atom to adsorb preferentially onto the OV around Ce atom, which weakens the interaction between Cl atom and Mn atom. Consequently, the chlorination of the MnO_x species is prevented and the oxygen mobility of the catalyst is guaranteed to some extent.

Insight into the reaction mechanism over PMoA for low temperature NH3-SCR: A combined In-situ DRIFTs and DFT transition state calculations

Phosphomolybdic acid catalyst (PMoA/TiO₂) is a promising catalyst for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) due to its strong acidity and excellent redox property. This work presents the NH₃-SCR reaction mechanism by In-situ diffuse reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTs) and density functional theory (DFT). In-situ DRIFTs results indicated that the NH₃-SCR performance over PMoA/TiO₂ followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. The reaction pathway, intermediate, transition state and energy barrier over PMoA to complete NH₃-SCR reaction were calculated by DFT. The results showed that the catalytic cycle includes foundational reaction (NH₃ + NO reaction) and regenerative reaction (NH₃ + NO₂ reaction). NH₂, NH₂NO, HNNOH and HO₂NNH species were the key intermediates. In the foundational reactions, NO₂ played an important role in the removal of remaining H atoms. The NH₃ dissociation on Lewis acid site, the internal hydrogen transfer on Brønsted acid site and the formation of HO₂NNH species were the rate-controlling steps. The catalytic cycle of NH₃-SCR over PMoA consists of standard SCR and fast SCR.

Superior Oxidative Dehydrogenation Performance toward NH3 Determines the Excellent Low-Temperature NH3-SCR Activity of Mn-Based Catalysts

Mn-based oxides exhibit outstanding low-temperature activity for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) compared with other catalysts. However, the underlying principle responsible for the excellent low-temperature activity is not yet clear. Here, the atomic-level mechanism and activity-limiting factor in the NH₃-SCR process over Mn-, Fe-, and Ce-based oxide catalysts are elucidated by a combination of first-principles calculations and experimental measurements. We found that the superior oxidative dehydrogenation performance toward NH₃ of Mn-based catalysts reduces the energy barriers for the activation of NH₃ and the formation of the key intermediate NH₂NO, which is the rate-determining step in NH₃-SCR over these oxide catalysts. The findings of this study advance the understanding of the working principle of Mn-based SCR catalysts and provide a fundamental basis for the development of future generation SCR catalysts with excellent low-temperature activity.

Study on denitration and sulfur removal performance of Mn-Ce supported fly ash catalyst

Nitrogen oxides (NOx) are the main pollutants of air, which mainly come from the combustion of coal and fossil fuels. In this paper, with fly ash used as the catalyst carrier, the effects on the denitration and sulfur resistance of Mn-Ce loading sequence and molar ratio were studied. The catalyst was characterized and analyzed by XRD, XPS, SEM. The results show that when Mn-Ce bimetal is loaded at the same time, Mn ions enter the CeO₂ lattice to form a solid solution of Mn-O-Ce fluorite structure, which makes the catalyst has the best denitration and sulfur resistance. The catalyst denitration performance increases first and then decreases with the increase of Mn-Ce molar ratio. When Mn-Ce is 1:1, the denitration efficiency is higher, the total conversion rate of NO is the highest and the deactivation time is the longest, the catalyst is resistant to sulfur performance is also the best.

Titanium-silicon ferrierites and their delaminated forms modified with copper as effective catalysts for low-temperature NH3-SCR

Titanium-silicon ferrierites with different Si/Ti ratios and their delaminated forms were modified with copper by ion-exchange. The obtained samples were characterized with respect to their chemical composition (ICP-OES), structure (XRD), texture (N2 sorption), morphology (SEM), form and aggregation of titanium and copper species (UV-vis-DRS), reducibility of deposited copper species (H2-TPR) and surface acidity (NH3-TPD). The porous structure of the zeolitic samples strongly influenced the form and aggregation of deposited copper species. In the case of the three dimensional microporous structure of ferrierites (Ti-FER), copper was deposited mainly in the form of aggregated copper oxide species, in contrast to the open micro- and mesoporous structure of delaminated ferrierites (Ti-ITQ-6), where mainly copper in the form of monomeric cations was identified. It was shown that monomeric copper cations are more catalytically active in NO to NO2 oxidation than aggregated copper oxide species and, therefore, for the low-temperature conversion of nitrogen oxides the fast SCR reaction pathway is more effective for delaminated ferrierites modified with copper (Cu-Ti-ITQ-6) than for microporous three dimensional ferrierite catalysts (Cu-Ti-FER).

Characterization of WMnCeTiOx catalysts prepared by different methods for the selective reduction of NO with NH3

WMnCeTiOx(CP) catalysts showed excellent NH3-SCR activity and better SO2 tolerance.

Modification of composite catalytic material CumVnOx@CeO2 core-shell nanorods with tungsten for NH3-SCR

Novel composite material CumVnOx-NF@Ce-MOF nanorods with a core-shell structure were successfully fabricated by the in situ growth of Ce-MOF on electrospun copper vanadate precursor nanofibers. Following calcination at 500, 600 and 700 °C, Cu2V2O7@CeO2, Cu3(VO4)2@CeO2 and Cu11O2(VO4)6@CeO2, respectively, were obtained. The CeO2 shell not only protected the copper vanadate nanofibers from breaking apart during the calcination process, but also induced an interaction between Ce, Cu and V species, which resulted in an excellent redox capacity. This revealed its potential as a catalyst for the selective catalytic reduction of nitrogen oxides with NH3 (NH3-SCR). Further surface modulation was accomplished by WOx anchoring on the shell of CumVnOx@CeO2. According to a series of characterizations, the crystallinity of surface ceria on CumVnOy@CeO2-WOx was apparently reduced and the amount of acid on its surface was also significantly increased. In addition, different calcination temperatures also had nonnegligible effects on the amount of surface acid as well as the redox capacity of the composite catalytic material CumVnOy@CeO2-WOx. With the largest total quantity of acid sites as well as a suitable balance between acidity and reducing ability, the Cu3(VO4)2@CeO2-WOx calcined at 600 °C exhibited satisfactory catalytic performance in the NH3-SCR process, and the NO conversion could remain above 90% at 230-380 °C.