Mason Marcos M, Dixon David A
Department of Chemistry and Biochemistry, The University of Alabama, Shelby Hall, Tuscaloosa, Alabama 35487-0336, United States.
J Phys Chem A. 2022 Nov 24;126(46):8618-8632. doi: 10.1021/acs.jpca.2c05340. Epub 2022 Nov 9.
NO and NO, which are generated in combustion processes, binding to vanadium oxide clusters including TiO-supported catalyst models in the selective catalytic reduction (SCR) of NO has been studied by density functional theory and coupled cluster methods. NO binding on vanadium oxides is predicted to depend on several factors, including the excitation energy of the oxide, ionization energies of both the unbound oxide and the deoxygenated reduced oxide, and the strength of the molecular V-O bonds. NO chemisorption occurs either through covalent bond formation in a HONO-like pattern or through abstraction of a metal oxide oxygen leading to the formation of NO. Nitrate formation is more favorable than what was predicted for group IVB or group VIB oxides [except (CrO)] and is either the lowest energy binding mode or within a few kcal/mol of the lowest mode in all clusters, likely due to the stability of V in the +4 oxidation state. Physisorption on V oxides is very weak. V with 2 oxo groups have a lower excitation energy and a more sterically open geometry which results in strong chemisorption as predicted for group IVB oxides. Tetrahedrally coordinated vanadia with a single oxo group and 3 V-O single bonds are predicted to have significantly higher excitation energies and behave like group VIB oxides such that chemisorption is unlikely and weak physisorption dominates the interaction. In larger clusters, including SCR catalyst models, only tetrahedrally coordinated vanadia are present and NO binding is not expected to occur. NO adsorption is weaker overall than NO binding and occurs either as physisorption or as chemisorption through the formation of NO analogous to nitrate formation in NO binding. The ability of NO to bind reflects the patterns predicted for NO, such that NO is strongly bound vanadia with two V═O groups and only weakly physisorbed when there is a single V═O or none at all.
在燃烧过程中产生的一氧化氮（NO）和二氧化氮（NO₂），与包括负载在TiO上的催化剂模型在内的钒氧化物簇在选择性催化还原（SCR）NO中的结合，已通过密度泛函理论和耦合簇方法进行了研究。预计NO在钒氧化物上的结合取决于几个因素，包括氧化物的激发能、未结合氧化物和脱氧还原氧化物的电离能以及分子V - O键的强度。NO的化学吸附要么通过类似HONO的模式形成共价键，要么通过夺取金属氧化物中的氧导致形成NO₂发生。硝酸盐的形成比预测的IVB族或VIB族氧化物（除了（CrO））更有利，并且在所有簇中要么是最低能量的结合模式，要么与最低模式相差几千卡/摩尔，这可能是由于V在 +4氧化态下的稳定性。在钒氧化物上的物理吸附非常弱。具有两个氧代基团的V具有较低的激发能和更具空间开放性的几何结构，这导致如预测的IVB族氧化物那样的强化学吸附。预测具有单个氧代基团和3个V - O单键的四面体配位钒氧化物具有显著更高的激发能，并且表现得像VIB族氧化物，使得化学吸附不太可能发生，而弱物理吸附主导相互作用。在更大的簇中，包括SCR催化剂模型，仅存在四面体配位的钒氧化物，预计不会发生NO结合。总体而言，NO的吸附比NO的结合弱，并且要么以物理吸附的形式发生，要么通过形成类似于NO结合中硝酸盐形成的NO₂以化学吸附的形式发生。NO结合的能力反映了对NO₂预测的模式，即当有两个V═O基团时NO₂与钒氧化物强烈结合，而当只有一个V═O基团或根本没有V═O基团时仅弱物理吸附。
