Wachs Israel E, Jehng Jih-Mirn, Ueda Wataru
Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA.
J Phys Chem B. 2005 Feb 17;109(6):2275-84. doi: 10.1021/jp048839e.
CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.
采用甲醇程序升温表面反应（TPSR）光谱法来确定体相混合金属氧化物催化剂活性表面位点的化学性质。发现模型负载金属氧化物和体相纯金属氧化物的甲醇 - TPSR光谱峰温度Tp对特定表面金属氧化物及其氧化态敏感。这些位点还原后，表面金属氧化物位点的催化活性降低，最活跃的表面位点是完全氧化的表面阳离子。发现表面V5 +位点比表面Mo6 +位点更具活性，而表面Mo6 +位点又比表面Nb5 +和Te4 +位点活性高得多。此外，形成的反应产物也反映了表面活性位点的化学性质。表面氧化还原位点能够释放氧气并生成H2CO，而表面酸性位点不能释放氧气，含有H +或氧空位，并生成CH3OCH3。表面V5 +、Mo6 +和Te4 +位点表现为氧化还原位点，表面Nb5 +位点是路易斯酸位点。该实验信息用于确定体相Mo - V - Te - Nb - Ox混合氧化物催化剂（Mo(0.6)V(1.5)Ox、Mo(1.0)V(0.5)Te(0.16)Ox、Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox）中不同表面阳离子的化学性质。体相Mo(0.6)V(1.5)Ox和Mo(1.0)V(0.5)Te(0.16)Ox混合氧化物的催化特性由表面V5 +氧化还原位点的催化性质主导。这些体相混合氧化物表面V5 +的富集与其高迁移率有关，V5 +在不同氧化物阳离子中具有最低的坦曼温度，以及与V = O键表面终止相关的较低表面自由能。四元体相Mo(1.0)V(0.3)Te(0.16)Nb(0.1)Ox混合氧化物同时具有表面氧化还原和酸性位点。表面氧化还原位点反映了表面V5 + 的特性，表面酸性位点反映了通常与负载型Mo6 +相关的性质。Nb5 +和Te4 +的主要作用似乎是作为更活跃的表面V和Mo位点的配体促进剂。由于甲醇氧化脱氢（ODH）和丙烷ODH的速率决定步骤相似，都涉及C - H键的断裂，所以它们的反应活性趋势相似。这种新颖 的甲醇 - TPSR光谱方法是一种通用方法，也已成功应用于其他体相混合金属氧化物体系，以确定活性表面位点的化学性质。
