title: Development and Application of Operando Spectroscopy for Vanadium Oxide Catalysts in ODH Reactions: A Comparison of O2 and CO2 as Oxidising Agents
Creators: name: Rogg, Simone Erika
Publisher: name: UNSPECIFIED
Year: 2020

description: The oxidative dehydrogenation (ODH) of propane is considered an attractive route for the production of propylene in order to meet its rising demand. Due to consecutive and parallel overoxidation reactions, low propylene yields were obtained so far. To this end, this work presents the development and application of various operando techniques with the purpose to elucidate the molecular structure-reactivity/selectivity relationship of supported vanadium oxide catalysts in ODH reactions. A new operando gas chromatograph/Fourier-transform infrared (GC/FTIR) setup was installed, which was combined with multiwavelength Raman (256.7, 385, and 515 nm) spectroscopy. It is, to my knowledge, the first of its kind. In order to prevent a local hot spot and laser induced sample damage, the fluidised bed mode was implemented, resulting in a continuous mixing of the powdery sample during the measurements. This overcomes laser-induced effects, which have been a drawback in the past, especially at excitation wavelengths in the ultraviolet (UV). In the preliminary stage of the operando experiments, the commercial in situ/operando cells were evaluated. Especially the temperature gradient along the sample bed of the Raman cell was addressed. The nominal temperature is controlled by the output of an integrated thermocouple, which, however, is not in contact with the sample. In order to establish a correlation between the set temperature and the sample temperature, a direct measurement by means of a thermocouple was performed. A self-made attachment, which fixes the external thermocouple in the sample bed of the Raman cell, proves to be useful in order to study the temperature dependency in the closed cell under flowing gas and fluidised bed conditions. The setup was subsequently tested in propane ODH with a silica supported vanadium oxide catalyst. The catalyst shows catalytic activity at temperatures &gt;300°C, and the selectivity drops with increasing conversion. For the operando experiments, a reaction temperature of 502°C was chosen. The conversion and selectivity data obtained at this temperature is in agreement with literature values acquired in a conventional reactor under similar experimental conditions. This qualifies the setup in conjunction with the Raman cell and fluidised bed method for the operando approach (i.e., the simultaneous recording of spectroscopic information and activity data), and verifies the temperature correction to be valid. The experimental approach includes the application of multiwavelength Raman, diffuse reflectance (DR) UV-vis and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. As no major structural arrangements are observable upon switching from oxidative to reaction conditions, the first C-H activation is determined to be the rate-determining step. In the absence of oxygen, the catalyst is highly reduced, as determined via UV-vis spectroscopy. Raman spectra, excited at 515 nm, are highly affected by the high self absorption in the visible range, not containing any information on the structure of reduced vanadium oxide species. In contrast, 256.7 nm Raman spectra reveal spectral features related to reduced vanadium oxide species. In addition, the catalyst was examined in propane and hydrogen reducing environments in dependency on the temperature. Whereas vanadyl (V=O), bridging V-O-V, and interface V-O-Si vibrations significantly decrease at high temperatures, SiO-H bonds become visible. The overall results give direct evidence for possible reaction pathways of the oxidative dehydrogenation of propane, as previously examined by density functional theory (DFT). Hydrogen abstraction from propane by terminal oxygen (V=O) results in the formation of a propyl radical, which subsequently reacts at the same or another vanadium oxide site. Under the assumption of fast reoxidation via molecular oxygen (O2), a fully oxidated (low reduced) vanadium oxide species is present under reaction conditions, as spectroscopically observed. In order to gain further mechanistic insights into ODH reactions, the selective oxidation with ethanol (as a substrate) towards acetaldehyde serves for comparison. From a spectroscopic point of view, the catalyst shows a significant dependency on the substrate. Ethanol is chemisorbed forming ethoxy species attached to vanadium (V), resulting in reduced vanadium oxide species prior to the rate-determining step, the C-H activation. In contrast, propane is weakly adsorbed, which explains the apparent absence of the binding of the molecule in the Raman spectra. The use of carbon dioxide (CO2) as an oxidising agent instead of molecular oxygen shows an enhanced selectivity towards propylene. The successful activation of CO2 in the ODH cycle is proven by the detection of carbon monoxide in the outlet gas stream. UV-vis and 256.7 nm Raman spectra reveal the presence of a highly reduced vanadium oxide catalyst under operando conditions. After the reaction, CO2 reoxidises reduced vanadium oxide species, observable via UV Raman spectroscopy (256.7 and 385 nm). Under the assumption that CO2 is able to recover the vanadyl oxygen over a V3+ species, isomerisation processes with hydrogen attached to terminal, bridging, and interface oxygen may occur in parallel. Hence, the ODH reaction with CO2 follows a V5+/V3+ redox cycle, whereas with O2 a V5+/V4+ redox couple is also possible. Several selectivity-determining factors are identified, which are related to the degree of reduction. Catalyst reduction results in a decrease of oligomerisation and a decline of available active oxygen species, which effectively suppresses overoxidation reactions. Ceria supported vanadium oxide catalysts show high catalytic activity in propane ODH already at low temperatures, which is attributed to the active support participation of cerium oxide (CeO2). The developed multiwavelength Raman study allows CeO2 bulk, surface, and vanadium oxide related modes to be selectively enhanced, dependent on the excitation wavelength. The approach is extended with FTIR gas phase analysis to allow for operando experiments. The active support contribution of CeO2 was studied by using ethanol ODH as a prototype reaction. The observed reduction of CeO2 points to the stabilisation of vanadium in its +5 oxidation state, in agreement with theoretical predictions. The significant decrease of interface bonds (V-O-Ce) is a direct indicator for a synergetic effect.