title: Mechanistic Insight into Heterogeneous Oxidation Catalysts using Combined Operando and Transient Spectroscopies Supported by DFT
Creators: name: Schumacher, Leon
Publisher: name: UNSPECIFIED
Year: 2024

description: In this work, various supported (CeO₂, TiO₂, TiOₓ/CeO₂) vanadium oxide catalysts and a bulk oxide catalyst (Fe₂(MoO₄)₃) were investigated in order to understand their fundamental properties and to elucidate the reaction mechanisms during the oxidative dehydrogenation (ODH) of propane. The catalysts used in this work are supported on reducible oxides, which are characterized by the fact that they actively participate in the reaction by providing oxygen, thus significantly influencing the performance of the catalyst. This opens up new avenues towards a rational catalyst design to obtain new materials with improved catalytic performance. ODH reactions enable a route to a more environmentally friendly synthesis of basic chemicals through significant energy and resource savings, which can even be made carbon neutral by using the ubiquitous greenhouse gas CO₂ as the oxidizing agent. Therefore, the influence of using O₂ and CO₂ as oxidants was investigated. The aim of this work is to investigate the influence of different support materials, vanadia nuclearities, and oxidizing agents on the propane ODH reaction mechanism and to use this knowledge for the rational design of supported catalysts and to finally transfer it to the analytically more challenging behavior of bulk oxides. For the mechanistic analysis a combination of in situ and operando methods was used. Multi-wavelength Raman spectroscopy in combination with UV-Vis spectroscopy allows the investigation of the catalysts oxygen dynamics in the active phase and in the support. Transient modulation-excitation (ME-)IR spectroscopy was
used to identify actively participating adsorbate and vanadium oxide species as well as the rapid hydrogen transfer from propane to the catalyst surface. The latter is particularly important, as the initial C-H bond break is often described as the rate-limiting step. The combination of these spectroscopies forms the core of most of the mechanistic studies in this work, but depending on the system and the specific question, other methods were used. These include X-ray diffraction (XRD)
for monitoring crystal structures, X-ray photoelectron spectroscopy (XPS) to monitor oxidation states on the catalyst surface, 51V solid-state nuclear magnetic resonance spectroscopy (ssNMR) to enable the nuclear-dependent analysis of vanadium oxide and electrical impedance spectroscopy (EIS) for direct monitoring of mass transport phenomena in bulk oxides. In addition, density functional theory (DFT) calculations were used as a supporting method.
In a first step, basic properties of the VOₓ/CeO₂ system were investigated. As part of theoretical investigations of the material by DFT, vibrational Raman spectra were simulated as a function of the vanadia nuclearity and the loading-density dependent behavior was characterized. Furthermore, the reducibility by means of temperature-programmed reduction (TPR) in H₂ and the reoxidation behavior in CO₂, starting from a partially reduced state, was investigated. The reduction behavior at the surface and in the subsurface was strongly influenced by the nuclearity distribution of vanadia, which in turn affects the oxygen mobility in ceria. A similar nuclearity-dependent behavior occurs during reoxidation with CO₂, in which ceria is reoxidized via the formation of carbonate species. Vanadia species block these regeneration sites because they are anchored to ceria via surface oxygen. Short nuclearities can only be partially reoxidized due to their lesser thermodynamic stability compared to longer-chain nuclearities, which are fully reoxidized due to their increased stability.
Next, the propane ODH reaction mechanism over VOₓ/CeO₂ catalysts as a function of the vanadia loading was investigated using O₂ as the oxidizing agent. This is highly relevant to optimize the more sustainable synthesis of the basis chemical propylene compared to traditional production processes like stam cracking, making the propane-ODH process industrially more relevant. Here, surface oxygen of the ceria lattice was identified as the active site. Dimeric and oligomeric vanadia species act as the transfer site for hydrogen to surface oxygen of the ceria lattice and for the coordination of propane. Monomeric species do not actively participate in the reaction but are able to interact with and block oxygen vacancies of ceria. In comparison, the use of CO₂ as the oxidizing agent leads to a more complex reaction network, in which the dehydrogenation of propane plays a significant role, but total oxidation is still very pronounced due to the high oxygen mobility in ceria. The reactivity behavior also depends strongly on the pretreatment conditions.
Vanadia reduces the prevalence of the total oxidation reaction and increases the overall activity of the catalyst. The surface is partially deactivated by the formation of stable carbonate species under reaction conditions.
To investigate the influence of the support material on the propane ODH reaction mechanism, the VOₓ/TiO₂ system was investigated in detail using TiO₂ P25 as the support material. Here, the V=O and V-O-V groups of vanadia act as the main active sites, while titania provides the transfer
site of hydrogen to the vanadium oxide via lattice oxygen and influences the vanadia nuclearity via the anatase/rutile ratio in the support. In comparison, the VOₓ/TiO₂ system is highly selective for the dehydrogenation of propane when using CO₂ as the oxidizing agent and exhibits catalytic activities comparable to CrOₓ catalysts. The only side reaction is propane dry reforming. However, the catalytic performance is not stable. The deactivation of the catalyst is caused by the complete
reduction and an insufficient regeneration rate of the catalyst by CO₂. The regeneration rate of the catalyst is increased by the presence of vanadia, which catalyzes the formation of rutile and actively participates in the redox cycle, increasing the regeneration rate of the catalyst and its
overall selectivity.
Based on the obtained mechanistic knowledge on propane ODH over VOₓ/CeO₂ and VOₓ/TiO₂ catalysts, a rational design attempt was made by synthesizing a TiOₓ/CeO₂ support system with variable TiOx loading using atomic layer deposition (ALD) and subsequent loading with vanadia.
This allows for the active participation of oxygen atoms of all three oxides, while concurrently, titania covers most of the unselective surface oxygen atoms of the ceria surface and coordinates the vanadia, which results in a higher selectivity than that observed for the two-component
oxides.
Finally, Fe₂(MoO₄)₃ was investigated as a commercial bulk oxide catalyst. Bulk oxides differ from supported catalysts in the fact that they are not a homogeneous material but gradually transition from the stoichiometric bulk structure to a modified surface structure. All of these parts can be involved in the reaction and take on individual roles. This makes such a system more complex than a supported one, since the reaction takes place not only at the catalyst surface, but relevant processes can also take place in the bulk. First, the influence of the synthesis conditions
on the structure, phase composition and Mo/Fe ratio of Fe₂(MoO₄)₃ by means of Raman and XP spectroscopy was investigated. Mechanistic studies during propane ODH by means of operando and transient spectroscopy reveal that lattice oxygen is of great importance for the reaction. Hydrogen can be transfered from the propane to the lattice by a layer of amorphous MoOₓ, which forms on the catalyst surface and increases its selectivity. Here, diffusion of the hydrogen to the subsurface of the catalyst can occur, leading to water and subsequent vacancy formation. The regeneration of the lattice takes place by diffusion from the bulk or by a phase transition to FeMoO₄. The oxygen mobility in the catalyst is highly relevant for the reactivity and strongly depends on the iron content.
This work demonstrates that the combination of operando and transient spectroscopy, supported by DFT, allows for a detailed description of the mechanism of ODH reactions over supported and bulk oxide catalysts. This is of great importance for the rational design of new catalyst materials, aiding in developing new processes, or to improve the current state of existing catalytic processes, thus driving the transformation in the chemical industry towards sustainability. In this context, the successful rational design of a support material shows that the propane ODH
selectivities can be enhanced (in comparison to other materials) by a detailed understanding of the reaction mechanism. Overall, this demonstrates the importance of combining complementary spectroscopic methods under reaction conditions in order to obtain a complete picture of the mode of operation of catalysts and to distinguish between surface, subsurface and bulk processes and their interplay.