Mechanism insights into CO oxidation over transition metal modified V2O5/TiO2 catalysts: A theoretical study.

The V2O5/TiO2 based selective catalytic reduction (SCR) catalysts possess not only promising capability on the denitrification of nitrogen oxides (NOx), but also certain effects on the oxidation of carbon monoxide (CO) in the flue gas. Modification of traditional SCR catalysts with certain transition metals can further improve their catalytic oxidation ability of CO. Therefore, it is of great significance to reveal the catalytic oxidation mechanism of CO for developing modified SCR catalysts to achieve the co-removal of CO and NOx. Theoretical calculations based on density functional theory (DFT) were performed to probe the comprehensive reaction mechanism of CO oxidation on M doped V2O5/TiO2 catalysts (M = Mo, Fe, and Co). The whole CO oxidation cycles include three stages, i.e., the first CO oxidation, the re-oxidation of the surface, and the second CO oxidation. The terminal oxygen and the surface oxygen formed by the adsorbed O2 all play vital roles in the whole CO oxidation cycles. The activation barriers of the rate-determining steps for CO oxidation on Fe-V2O5/TiO2 and Co-V2O5/TiO2 are much lower than that of Mo-V2O5/TiO2, which indicates Fe and Co dopants can apparently promote the CO oxidation activities of the modified SCR catalysts. Meanwhile, the electronic structure analysis confirms that Fe and Co dopants can cause electron distribution change with strong oxidation ability at the active oxygen sites.

Intrinsic mechanism insight of the interaction between lead species and the Vanadium-based catalysts based on First-principles investigation.

Lead (Pb) species trigger serious poisoning of selective catalytic reduction (SCR) catalysts. To improve the Pb resistance ability, revealing the impact mechanism of Pb species on the commercial SCR catalysts from a molecular level is of great significance. Herein, first-principles calculations were applied to unveil the Pb adsorption mechanism on the vanadium-based catalysts, the results were also compared with the previous experimental findings. The intrinsic interaction mechanism between Pb and catalyst components was interpreted by clarifying the change of the catalyst electronic structures (including charge transfer, bond formation situations, and active sites reactivities). It is found that the adsorption of Pb species belongs to chemisorption, evident electron transfer with the catalyst surface is inspected and intense charge transfer indicates strong adsorption. A remarkable interaction with the V = O active sites occurs and stable Pb-O bonds are formed, which significantly changes the electronic structures of the V = O sites and inhibits the NH3 adsorption, thus suppressing the SCR activity. Finally, thermodynamic analysis was applied to elucidate the temperature influence on Pb adsorption. It is found that Pb adsorption on catalysts cannot proceed spontaneously over 500 K. At higher temperatures the adsorption is inhibited and the Pb species become less stable, which partially explains why the Pb-poisoning effect at high temperatures is relatively moderate than that at low temperatures.

First-principles insights into the adsorption and interaction mechanism of selenium on selective catalytic reduction catalyst.

Selenium (Se) species can deposit in selective catalytic reduction (SCR) system during the denitrification process, which is harmful to the catalyst. To improve the Se-poisoning resistance of SCR catalysts, the influence mechanism of Se species on vanadium-titanium-based catalysts should be elucidated from an atomic scale. In this paper, theoretical calculations were conducted to reveal the adsorption and interaction mechanism of Se species on V2O5-WO3(MoO3)/TiO2 surface based on the first-principles. The impact of Se species on the electronic structure of the catalyst was investigated from electron transfer, bond formation, and VO site activity. The results show that the adsorption of elementary Se (Se0) belongs to chemisorption, while SeO2 can undergo both physisorption and chemisorption. For the chemisorption of Se species, obvious charge transfer with the catalyst is observed and Se-O bond is formed, which enhances the oxidation activity of the catalyst, triggers the reaction of Se0 and SeO2 with the catalyst components to generate SeVOx and SeW(Mo)Ox. The active sites are thereby damaged and the SCR performance is reduced. The above conclusions are mutually confirmed with the previous experimental research. By studying the correlation with the adsorption energies of Se species, descriptors manifesting the Se species adsorption were initially investigated to unveil the relationship between the electronic structure and the adsorption energy. Finally, the influence of temperature on Se adsorption was investigated. The adsorption can only proceed spontaneously below 500 K and is inhibited at high temperatures.

Migration and transformation of lead species over CaO surface in municipal solid waste incineration fly Ash: A DFT study.

Heavy metal pollutants generated from municipal solid waste (MSW) incineration are mainly concentrated in the fly ash, among which lead species have received considerable attention due to their high content and biotoxicity. CaO is an active component in fly ash to adsorb heavy metal species. In this study, based on density functional theory (DFT) calculations, the migration and transformation mechanisms of lead species over the CaO (100) surface were investigated by calculating the adsorption configurations, energies, and electronic structures, etc. The results indicate that the adsorption of lead species over the CaO (100) surface is dominated by chemisorption, and PbCl2 molecule exhibits a stronger affinity to the CaO surface than Pb0. The dissociation of HCl molecule on the CaO (100) surface facilitates the adsorption and chemical reactivity of lead species. The chlorination of Pb0 to PbCl2 is a two-stage route. In the first stage, two HCl molecules are exothermically adsorbed on the surface without an energy barrier, and Pb0 is directly bonded to the active Cl atom, which is controlled by the Eley-Rideal mechanism. In the second stage, PbCl intermediate bonds with another Cl atom over the surface to form the PbCl2 molecule, following the Langmuir-Hinshelwood mechanism, which is also the rate-determining step. Compared with the homogeneous chlorination, CaO catalyzes the heterogeneous process to greatly reduce the oxidation energy barrier and promotes the formation of PbCl2. Consequently, CaO is able to accelerate the lead enrichment in fly ash, which is favorable for lead species purification.