Zabilska Anna, Clark Adam H, Moskowitz Benjamin M, Wachs Israel E, Kakiuchi Yuya, Copéret Christophe, Nachtegaal Maarten, Kröcher Oliver, Safonova Olga V
Paul Scherrer Institute, 5232 Villigen, Switzerland.
École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
JACS Au. 2022 Mar 14;2(3):762-776. doi: 10.1021/jacsau.2c00027. eCollection 2022 Mar 28.
Titania-supported vanadia (VO /TiO) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% VO/15% TiO/SiO catalyst (consisting of submonolayer VO species anchored onto a TiO monolayer, which in turn is supported on SiO) during the oxidative dehydrogenation of ethanol. The VO species in 5% VO/15% TiO/SiO show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO species supported on SiO with a similar density. time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% VO/15% TiO/SiO is kinetically coupled to the formation of a V intermediate, while the formation of V is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VO species (V, V, and V) in the 5% VO/15% TiO/SiO catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti/Ti couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiO is rather complex. TiO species might be involved in a fast electron transport between VO species and might affect the electronic structure of VO , thereby promoting their reducibility. This study demonstrates the high potential of element-specific X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.
二氧化钛负载的钒氧化物（VOₓ/TiO₂）催化剂在许多选择性氧化和还原过程中表现出出色的催化性能。尽管进行了大量研究，但参与各种催化过程的钒和钛的氧化还原转变的本质仍然难以检测，也难以与产物形成速率相关联。在这项工作中，我们研究了双层5%VOₓ/15%TiO₂/SiO₂催化剂（由锚定在TiO₂单层上的亚单层VOₓ物种组成，而TiO₂单层又负载在SiO₂上）在乙醇氧化脱氢过程中活性位点的氧化还原动力学。5%VOₓ/15%TiO₂/SiO₂中的VOₓ物种对乙醛具有高选择性，与负载在SiO₂上密度相似的VOₓ物种相比，乙醛形成速率约高40倍。时间分辨的V和Ti K边X射线吸收近边光谱，结合瞬态实验策略，定量表明5%VOₓ/15%TiO₂/SiO₂上乙醛的形成在动力学上与V中间体的形成相关联，而V的形成延迟且慢10 - 70倍。使用标准V K边XANES光谱和特别合成的分子晶体的广泛数据库，证实了5%VOₓ/15%TiO₂/SiO₂催化剂中VOₓ物种（V⁴⁺、V³⁺和V²⁺）各种氧化还原状态的低配位性质。还检测到Ti⁴⁺/Ti³⁺对的氧化还原活性要弱得多；然而，发现它在动力学上与乙醇氧化的速率决定步骤不相关联。因此，TiO₂的促进作用相当复杂。TiO₂物种可能参与VOₓ物种之间的快速电子传输，并可能影响VOₓ的电子结构，从而促进其还原性。这项研究证明了元素特异性X射线吸收光谱在揭示涉及各种金属氧化物氧化还原动力学的复杂催化机制方面的巨大潜力。
