Transient In Situ Studies on Supported Catalysts: CO2 Methanation and Methane Oxidation

Doctoral thesis, 2021

This dissertation aims to increase the understanding of important steps in the catalytic CO2 hydrogenation and total methane oxidation reactions over supported noble metal catalysts. A general theme is the role of the metal oxide support for the catalytic reactions. In the case of methane oxidation, the reported detrimental effect of water is of primary interest.

A range of powder catalysts were prepared by incipient wetness impregnation including Rh/MO (MO = SiO2, Al2O3, CeO2) and Pd/MO (MO = Al2O3 and ZSM-5) for CO2 hydrogenation and methane oxidation, respectively. The catalysts were studied in situ using high-energy X-ray diffraction, ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy as to follow structural dynamics and with diffuse reflectance infrared Fourier transform spectroscopy to monitor surface species. Transient measurements were designed and obtained data were analysed with phase sensitive detection in order to distinguish inactive (spectator) species from active ones.

For CO2 hydrogenation on the rhodium catalysts, an initial step of dissociation of carbon dioxide into carbon monoxide (and oxygen) occurs on the rhodium phase, and is enabled by the presence of hydrogen. The irreducible supports (SiO2 and Al2O3) show a minor contribution to the catalytic mechanism whereas for the CeO2 based catalysts, several kinds of carbonyls (b-CO, h-CO, m-CO) and carbonates (b-CO3, p-CO3) are active species. While more experimental data is needed as to establish the complete pathway, the activity of the carbonyl species suggests that the reaction follows a carbon monoxide based pathway such as the carbide route.

As for the detrimental effect of water on the methane oxidation, two aspects are shown to be of critical importance. The first concerns the build up of a low but strongly inhibiting hydroxyl coverage on the PdO nanoparticles hampering their redox dynamics and seemingly shifting the operating mechanism from a Mars-van Krevelen to a Langmuir-Hinshelwood type of mechanism that proceeds slower. The second is the support hydrophilicity, which contributes to inhibition of important active sites on the rim of the developed PdO nanoparticles through the formation of surface hydroxyls.

The knowledge about the catalyst structure-function relationships obtained in this work may be used to guide future catalyst design and synthesis, and catalyst operation, as to provide more efficient catalytic processes.