Unraveling the Active Vanadium Sites and Adsorbate Dynamics in VO_x/CeO₂ Oxidation Catalysts Using Transient IR Spectroscopy

Abstract

The oxidative dehydrogenation (ODH) of propane over supported vanadia catalysts is an attractive route toward propene (propylene) with the potential of industrial application and has been extensively studied over decades. Despite numerous mechanistic studies, the active vanadyl site of the reaction has not been elucidated. In this work, we unravel the ODH reaction mechanism, including the nuclearity-dependent vanadyl and surface dynamics, over ceriasupported vanadia (VO x /CeO 2 ) catalysts by applying (isotopic) modulation excitation IR spectroscopy supported by operando Raman and UV−vis spectroscopies. Based on our loading-dependent analysis, we were able to identify two different mechanisms leading to propylene, which are characterized by isopropyland acrylate-like intermediates. The modulation excitation IR approach also allows for the determination of the time evolution of the vanadia, hydroxyl, and adsorbate dynamics, underlining the intimate interplay between the surface vanadia species and the ceria support. Our results highlight the potential of transient IR spectroscopy to provide a detailed understanding of reaction mechanisms in oxidation catalysis and the dynamics of surface catalytic processes in general.

Understanding the Reduction Behavior of VOx/CeO2 on a Molecular Level: Combining Temperature-Programmed Reduction with Multiple In-Situ Spectroscopies and X-ray Diffraction

“…As a result, vanadia structures with higher nuclearities show larger blue-shifts, resulting in a fine structure on a ceria support. The above positions can be assigned to monomeric, dimeric, trimeric, tetrameric, and pentameric species, respectively. ,, In the following, the tetra- and pentameric species will be termed oligomeric vanadia species. In general, only very small changes in the vanadyl fine structure are detected below 230 °C.…”

“…These are often attributed to ceria’s ability to form oxygen vacancies, where a lattice oxygen atom is consumed in the reaction and an oxygen vacancy is created along with two Ce 3+ ions. The latter can then be regenerated using either oxygen from the gas-phase or subsurface oxygen, which is transported to the surface by diffusion. , Therefore, due to its redox properties, ceria is an excellent support material and has been employed in a variety of reactions, such as NO 2 storage, NH 3 selective catalytic reduction (SCR), CO oxidation, reverse water-gas shift reaction, and CO 2 hydrogenation as well as alcohol and alkane oxidative dehydrogenations (ODHs). The corresponding active phases are very diverse and can typically be either a metal (e.g., gold, copper, platinum) or an oxide (e.g., MnO x , VO x , MoO x ), depending on the application.…”

“…A catalyst commonly used in the oxidation of alcohols and short chain alkanes is ceria-supported vanadia (VO x /CeO 2 ). ,, The reducibility of CeO 2 in this system is often cited as the reason for the good catalytic performance in oxidation reactions and is accessible by H 2 temperature-programmed reduction (TPR) measurements. However, a detailed understanding of the catalyst reduction on a molecular level through spectroscopic means is still lacking. ,− Furthermore, the intimate interaction between vanadia and ceria has a significant influence on the reduction behavior of the catalyst as it has often been described in the literature that ceria keeps vanadia in its highest oxidation state and vanadia interacts with lattice oxygen as well as oxygen vacancies on the surface. ,,− Therefore, the reduction behavior is much more complicated and cannot be fully understood by using TPR only.…”

“…5,6 Therefore, due to its redox properties, ceria is an excellent support material and has been employed in a variety of reactions, such as NO 2 storage, 7 NH 3 selective catalytic reduction (SCR), 8 CO oxidation, 9 reverse water-gas shift reaction, 10 and CO 2 hydrogenation 11 as well as alcohol 12 and alkane 13 oxidative dehydrogenations (ODHs). The corresponding active phases are very diverse and can typically be either a metal (e.g., gold, 10 copper, 14 platinum 15 ) or an oxide (e.g., MnO x , 16 VO x , 13 MoO x 17 ), depending on the application. A catalyst commonly used in the oxidation of alcohols and short chain alkanes is ceria-supported vanadia (VO x / CeO 2 ).…”

Boosting electrocatalytic water oxidation of NiFe layered double hydroxide via the synergy of 3d–4f electron interaction and citrate intercalation

“…25,26 With the relatively high redox activity and low cost, the Ce element is desired to attract more attention in electrocatalysis. [27][28][29] Among the rare earth elements, Ce ions can involve binding to 3d transition metal counterparts easily due to the less localized properties of Ce 4f electrons. 30 Moreover, it has been evidenced that the binding between interlayered anions and edge-site Fe atoms is related to electrocatalytic water oxidation, 31,32 and selecting appropriate interlayered anion species may also be efficient to boost the synergy effect.…”