Catalysts Transform While Molecules React: An Atomic-Scale View

Abstract

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO2(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the V═O vanadyl group. Catalytic tests on the powder-supported VOx revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts.

Redox-driven atomic-scale changes in mixed catalysts: VOX/WOX/α-TiO2 (110)

“…[33][34][35][36] Recently we have demonstrated that XSW is a unique in situ tool to understand the atomic-scale behavior of oxide-supported planar model catalysts during various chemical reactions. 12,[15][16][17][37][38][39][40] In this work, we use XSW to observe chemical state-dependent changes in the V and W geometry and surface site occupancy. We also determine the fraction of atoms that are uncorrelated to the substrate lattice.…”

“…Previous studies have shown that monolayer (ML) and sub-ML vanadia supported on TiO 2 and other oxides can go through both structural and chemical state changes during reduction-oxidation (redox) reaction cycles. 2,10,37,38,41,42 In contrast, ML and sub-ML tungsten oxide lms supported on oxide substrates tend to be immobile and irreducible from the W 6+ state under similar catalytic conditions. 12,43-45 A notable exception is for sub-ML tungsten oxide on the reducible substrate a-Fe 2 O 3 (0001), for which a reduction to W 5+ and corresponding redox-reversible structural shis are observed.…”

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

“…In Figure 5a, the black curve (labeled 'N2, 300 °C') corresponds to the catalyst in a nitrogen flow at 300 °C before exposure to the reaction gas mixture. Features located at around 919 and 1036 cm −1 and at around 1849 and 2002 cm −1 have previously been assigned to V-O-V and V=O stretching modes of surface VOx species and their first overtones, respectively [2,50,89,90]. The feature at 1203 cm −1 has been ascribed to L acid sites on TiO2 [91].…”

“…The use of in situ electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) under reaction conditions has revealed the reducing/reoxidizing processes to be accompanied by changes of the vanadium valence and VO x surface structure [45][46][47][48]. Feng et al reported VO x with polymeric structure to be redox-active and provided an atomic view of the change of the VO x structure with redox state [49][50][51]. Consistently, density functional theory (DFT) results showed that formation of both oxidized and reduced VO x , i.e., V 5+ and V 4+ , kept the vanadyl bond intact for isolated VO x on TiO 2 , as verified by in situ EPR data for VO x /TiO 2 exposed to ammonia [52].…”

COx‐Resistant Oxidative Dehydrogenation of Cyclohexane Catalyzed by sp3@sp2Nanodiamonds towards Highly Selective Cyclohexene Production

“…Metal/metal oxide catalysts are commonly used catalysts for ODH of cyclohexane [1d,6c,e,8] . However, deep oxidation and dehydrogenation [9] often occur inevitably yielding undesired CO x and benzene as byproducts.…”