Operando XAS Study of Pt-Doped $CeO_2$ for the Nonoxidative Conversion of Methane

ACS catalysis 12(7), 3897 - 3908 (2022). doi:10.1021/acscatal.2c00092
The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO$_2$ catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtO$_x$ single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH$_4$. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO$_2$/Ce$_2$O$_3$ support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH$_4$ by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general.
Published by ACS, Washington, DC