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Assessing the effect of O2 dithering on CH4 oxidation on Pd/Al2O3

Davide Ferri

Under periodic lean/rich operation generated by repeated O2 pulses (O2-dithering), the abatement of CH4 and NO is enhanced compared to static operation. However, it is not well understood how the dithering parameters (pulse amplitude and frequency) affect CH4 conversion, making it difficult to anticipate the impact of such pulses on the catalytic performance. Here, we show that the effect of O2-dithering on the activity of Pd/Al2O3 can be reliably assessed as a function of dithering parameters, temperature, and exhaust gas composition. Operando Xray absorption spectroscopy was used to support the conclusions based on considerations of the Pd oxidation state. Our work relies on explicit calculations of the O2 concentration present in the reactive feed, which is based on the amount of O2 pulsed to the system and the pollutants' conversion. Since NO contributes to the amount of oxygen available during reaction, its concentration and conversion need to be correctly evaluated.

ELSEVIER SCIENCE SA2023

Improving time-resolution and sensitivity of in situ X-ray photoelectron spectroscopy of a powder catalyst by modulated excitation

Filippo Buttignol, Davide Ferri, Oliver Kröcher

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful tool to characterize the surface structure of heterogeneous catalysts in situ. In order to improve the time resolution and the signal-to-noise (S/N) ratio of photoemission spectra, we collected consecutive APXP spectra during the periodic perturbation of a powder Pd/Al2O3 catalyst away from its equilibrium state according to the modulated excitation approach (ME). Averaging of the spectra along the alternate pulses of O-2 and CO improved the S/N ratio demonstrating that the time resolution of the measurement can be limited solely to the acquisition time of one spectrum. Through phase sensitive analysis of the averaged time-resolved spectra, the formation/consumption dynamics of three oxidic species, two metal species, adsorbed CO on Pd-0 as well as Pdn+ (n > 2) was followed along the gas switches. Pdn+ and 2-fold surface PdO species were recognised as most reactive to the gas switches. Our approach demonstrates that phase sensitive detection of time-resolved XPS data allows following the dynamics of reactive species at the solid-gas interface under different reaction environments with unprecedented precision.

ROYAL SOC CHEMISTRY2023