Mamedov Konstantin, Shrestha Anukriti, Whitcomb Colby A, Paolucci Christopher, Davis Robert J
Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22903, United States.
J Phys Chem C Nanomater Interfaces. 2024 Aug 14;128(33):13864-13878. doi: 10.1021/acs.jpcc.4c03652. eCollection 2024 Aug 22.
Supported tungsten oxides are widely used in a variety of catalytic reactions. Depending on the support, the cluster size, oxidation state, reducibility and speciation of the tungsten oxides can widely differ. When promoted with a platinum group metal, the resulting spillover of hydrogen may facilitate the reduction of supported tungsten oxide species, depending on the support. High resolution scanning transmission electron microscopy imaging showed nanometer scale WO clusters were synthesized on SiO whereas highly dispersed species were formed on TiO. Results from H-temperature-programmed reduction showed the presence of Pd lowered the initial reduction temperature of SiO-supported WO species but interestingly did not affect that of TiO-supported WO . X-ray photoelectron and absorption spectroscopies showed the W atoms in SiO-supported WO species reduce from a +6 oxidation state to primarily +5 after thermal treatment in 5% H, while the fraction of W in the +5 oxidation state was relatively unaffected by reduction treatment of TiO-supported WO . The unusual behavior of TiO-supported WO was explained by quantum chemical calculations that reveal the lack of change in the oxidation state of W is attributed to charge delocalization on the surface atoms of the titania support, which does not occur on silica. Moreover, modeling results at <600 K in the presence of H suggest the formation of Brønsted acid sites, and the absence of Lewis acid sites, on larger aggregates of WO on silica and all cluster sizes on titania. These results provide experimental and theoretical insights into the nature of supported tungsten oxide clusters under conditions relevant to various catalytic reactions.
负载型氧化钨广泛应用于各种催化反应中。根据载体的不同，氧化钨的簇尺寸、氧化态、还原性和形态会有很大差异。当用铂族金属促进时，根据载体的不同，产生的氢溢流可能会促进负载型氧化钨物种的还原。高分辨率扫描透射电子显微镜成像显示，在SiO上合成了纳米级的WO簇，而在TiO上形成了高度分散的物种。H程序升温还原结果表明，Pd的存在降低了SiO负载的WO物种的初始还原温度，但有趣的是，对TiO负载的WO物种的初始还原温度没有影响。X射线光电子能谱和吸收光谱表明，SiO负载的WO物种中的W原子在5%H中热处理后从+6氧化态还原为主要的+5氧化态，而+5氧化态的W的比例相对不受TiO负载的WO物种还原处理的影响。TiO负载的WO物种的异常行为通过量子化学计算得到了解释，该计算表明W氧化态缺乏变化归因于二氧化钛载体表面原子上的电荷离域，而二氧化硅上不会发生这种情况。此外，在<600K且存在H的情况下的模拟结果表明，在二氧化硅上较大的WO聚集体上以及二氧化钛上所有簇尺寸上形成了布朗斯台德酸位点，而没有路易斯酸位点。这些结果为与各种催化反应相关条件下负载型氧化钨簇的性质提供了实验和理论见解。
