The reaction mechanism of selective catalytic reduction (SCR) of NO with NH3 on W-doped CeO2 catalysts was systematically investigated using density functional theory calculations corrected by on-site Coulomb interactions (DFT+U). A complete catalytic cycle was proposed, which consists of four steps, namely (i) Lewis acid site reaction, (ii) Brønsted acid site reaction, (iii) oxygen vacancy reaction, and (iv) catalyst regeneration. The calculated key intermediates in these four steps are in good agreement with previous experimental results, which indicates that our suggested catalytic cycle is rational. The catalytic nature of W-doped CeO2 catalysts for NH3-SCR reaction was discussed by analyzing the role of oxygen vacancy, the synergistic effect between surface acidity and reducibility, and the difference from NH3-SCR reaction on V2O5-based catalysts. Our results show that the oxygen vacancy on the surface which creates two Ce3+ cations plays a critical catalytic role in the NH3-SCR reaction, where adsorbed N2O22– species can be readily formed and then acts as a precursor for SCR reaction, opening a unique reaction pathway. The formation of adsorbed NO2 species on W-doped CeO2 facilitates the SCR reaction via Langmuir–Hinshelwood mechanism with a relative low energy barrier. This study provides atomic-scale insights into the catalytic cycle and the important role of oxygen vacancy in NH3-SCR reaction on W-doped CeO2 catalysts, which is of significance for the design of highly active ceria-based SCR catalysts.

The adsorption of ammonia at Brönsted and Lewis acid sites on three low-index (001), (010) and (100) surfaces of V2O5 catalyst was investigated using density functional theory (DFT) method. Three levels of surface relaxation periodic models including top single layer relaxation (S-model), moderately deeper relaxation (M-model) and full relaxation model (F-model) were applied to examine the effect of the surface relaxation on the binding structures and adsorption energies. The results of calculations showed that on the saturated basal plane V2O5 (001), ammonia adsorption at the Brönsted acid sites (VOH) is energetically more favorable. On unsaturated (010) and (100) surfaces, ammonia is adsorbed strongly on both Brönsted (VOH) and Lewis acid sites (V). Surface relaxations have no influence on ammonia adsorption on saturated (001) surface, while a strong dependence on the relaxation models is observed for NH3-adsorption energies on (010) and (100) surfaces, especially at the Lewis acid sites of both side planes. When complete relaxation considered (F-model), ammonia adsorption on the Lewis acid sites (V) dominates for side planes (010) and (100). In the presence of VOH as neighbor, the ammonia adsorption at V sites is however weakened significantly due to steric hindrance. Hydrogen bonds may play a role, although not determining one, in the respect of the adsorption of ammonia on (010) and (100) surfaces. Moderate relaxation and full relaxation are absolutely necessary for the description of both H and NH3 adsorption on unsaturated (100) and (010) surfaces, respectively.We have theoretically investigated ammonia adsorption on the V2O5 surfaces in the view of geometry structure and adsorption energy and showed differences in NH3 adsorption ability (i) between different surfaces like (001), (010) and (100) and (ii) between different Brönsted (VOH) and Lewis (V) acid sites. For a comparison, H-adsorption is also studied.Highlights► On the basal V2O5 (001), NH3 adsorbs preferentially at Brönsted acid sites. ► On side (010) and (100), NH3 adsorbs strongly at Brönsted and Lewis acid sites. ► Surface relaxations have no influence on NH3 adsorption on saturated (001) surface. ► NH3-adsorption energies on (010) and (100) surfaces depend on the relaxed models.

We have investigated the mechanism of M(CO)5 (M = Fe, Ru, Os) catalyzed water gas shift reaction (WGSR) by using density functional theory and ab initio calculations. Our calculation results indicate that the whole reaction cycle consists of six steps: 1 → 2 → 3 → 4 → 5 → 6 → 2. In this stepwise mechanism the metals Fe, Ru, and Os behave generally in a similar way. However, crucial differences appear in steps 3 → 4 → 5 which involve dihydride M(H)2(CO)3COOH– (4′) and/or dihydrogen complex MH2(CO)3COOH– (4). The stability of the dihydrogen complexes becomes weaker down the iron group. The dihydrogen complex 4_Fe is only 11.1 kJ/mol less stable than its dihydride 4′_Fe at the B3LYP/II(f)++//B3LYP/II(f) level. Due to very low energy barrier it is very easy to realize the transform from 4_Fe to 4′_Fe and vice versa, and thus for Fe there is no substantial difference to differentiate 4 and 4′ for the reaction cycle. The most possible key intermediate 4′_Ru is 38.2 kJ/mol more stable than 4_Ru. However, the barrier for the conversion 3_Ru → 4′_Ru is 23.8 kJ/mol higher than that for 3_Ru → 4_Ru. Additionally, 4′_Ru has to go through 4_Ru to complete dehydrogenation 4′_Ru → 5_Ru. The concerted mechanism 4′_Ru → 6_Ru, in which the CO group attacks ruthenium while H2 dissociates, can be excluded. In contrast to Fe and Ru, the dihydrogen complex of Os is too unstable to exist at the level of theory. Moreover, we predict Fe and Ru species are more favorable than Os species for the WGSR, because the energy barriers for the 4 → 5 processes of Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas 140.5 kJ/mol is calculated for the conversion 4′ → 5 of Os, which is significantly higher. In general, the calculations are in good agreement with available experimental data. We hope that our work will be beneficial to the development and design of the WGSR catalyst with high performance.

We have revisited the water−gas shift reaction catalyzed by iron pentacarbonyl at the DFT-B3LYP level. The reaction mechanism proposed by Rozanska and Vuilleumier (Inorg. Chem. 2008, 47, 8635−8640) has been followed and revised. The results show that transition states TS4/5 and TS5/2_a actually connect other intermediates rather than those suggested by Rozanska and Vuilleumier. Furthermore, the entire reaction has been proven to proceed with processes 1 → 2 → 3 → 4 → 6 → 7 → 2. It is the first time that species 6 and 7 are reported as intermediates for this reaction mechanism.

This paper reviews the most important developments on the desulfurization mechanism of Fluid Catalytic Cracking (FCC) gasoline. First, the origin of sulfur compounds in FCC gasoline and the current developed desulfurization approaches and technologies are briefly introduced, and then the researches on desulfurization mechanism are summarized from experimental and theoretical perspectives. Further researches on the desulfurization mechanism will lay a foundation for optimizing desulfurization sorbents and technologies.

The structure and chemical shifts of the carbene complex (CO)4FeCF2 were investigated at DFT level. The CF2 ligand will occupy an equatorial position in a trigonal bipyramidal iron complex. The carbenic C atom is calculated to be much more deshielded with respect to the CO ligand at both BP86 and B3LYP level. Anisotropies of 13C chemical shifts of carbenic C can show us the certain direction where the attack of nucleophilic regents may take place. The 19F NMR chemical shift is predicted to be in the range of 160–180 ppm respect to the standard CCl3F scale by GIAO calculations.

•A novel mechanism for the SCR cycle of NO by NH3 on the V2O5 goes in module steps.•L-acid, single B-acid, double B-acid, and the defective sites are formed in turn.•At the first three kinds of active sites, NO and NH3 are converted into N2 and H2O.•The addition of O2 plays a role of resuming the defective surface to the V2O5.Selective catalytic reduction (SCR) of NO via NH3 and O2 over the V2O5 surface has been the focus of considerable research interest due to its role in mitigating air pollution. Our theoretical investigations at the periodic DFT level reveal that the Lewis acid active center could be a starting point in the dominant Eley–Rideal mechanism, while other active sites might either exist or be formed during the reaction process and play roles in competition. In this systematic study, an integrated catalytic cycle consisting of four module steps (i) NH3 + NO + V2O5 → N2 + H2O + HV2O5, (ii) NH3 + NO + HV2O5 → N2 + H2O + HHV2O5, (iii) NH3 + NO + HHV2O5 → N2 + 2H2O + HV2O4, and (iv) NH3 + NO + O2 + HV2O4 → N2 + 2H2O + V2O5 is proposed by using uniform theoretical model for the most possible processes involved. This suggested mechanism is easy to understand and agrees well with the experimental observations and results of other theoretical studies. More satisfactory, differences in the catalytic activity for diverse active sites can be explained not only by relative energies and barrier heights but also by geometries of the intermediates and transition states appeared in the cycle. For Step I, the formation of species HOVNH2 followed by H-migration of HO at Lewis acid site Va is decisive because of the very high activation energy (63.6 kcal/mol), while following transformations and the release of N2 and H2O are relatively easy. The most favorable path is however going through Vb site for Step I. The change from intermediate 1 to 2 must suffer a barrier of 52.7 kcal/mol, which is only 10.9 kcal/mol lower than that for Va. After the formation of one VOH Brønsted acid site, the transformation from intermediate 11 to 12 is the most difficult process for Step II (Ea = 38.6 kcal/mol). The most stable configuration for double VOH sites displays two potential pathways depending on the priority of removing H2O at the late stage of Step III. Our calculations indicate that Step IV favors to occur through HVt1 related pathway in which the oxygen vacancy and VOH sites are opposite to each other. This novel multi-step mechanism can provide us a deeper understanding of the SCR reaction over V2O5 surface, and we expect that the design of SCR catalyst could be improved on the basis of theoretical predictions related to these key sites and important processes.Graphical abstractFour elementary steps served as blocks can be flexibly combined to describe the SCR mechanism of NO via NH3 over V2O5 (0 0 1) surface in the presence of O2.Download high-res image (62KB)Download full-size image