Abstract
Performing fundamental operando catalysis studies under realistic conditions is a key to further develop and increase the efficiency of industrial catalysts. Operando X-ray photoelectron spectroscopy (XPS) experiments have been limited to pressures, and the relevance for industrial applications has been questioned. Herein, we report on the CO oxidation experiment on Pd(100) performed at a total pressure of 1 bar using XPS. We investigate the light-off regime and the surface chemical composition at the atomistic level in the highly active phase. Furthermore, the observed gas-phase photoemission peaks of CO2, CO, and O2 indicate that the kinetics of the reaction during the light-off regime can be followed operando, and by studying the reaction rate of the reaction, the activation energy is calculated. The reaction was preceded by an in situ oxidation study in 7% O2 in He and a total pressure of 70 mbar to confirm the surface sensitivity and assignment of the oxygen-induced photoemission peaks. However, oxygen-induced photoemission peaks were not observed during the reaction studies, but instead, a metallic Pd phase is present in the highly active regime under the conditions applied. The novel XPS setup utilizes hard X-rays to enable high-pressure studies, combined with a grazing incident angle to increase the surface sensitivity of the measurement. Our findings demonstrate the possibilities of achieving chemical information of the catalyst, operando, on an atomistic level, under industrially relevant conditions.
Highlights
Palladium is a well-known catalyst for CO oxidation
The metal bulk is still the most intense component, but two new components shifted by 0.4 eV and 1.3 eV are observed in the spectrum, which is consistent with the expected peak positions of two- and fourfold coordinated Pd atoms originating in the (√5 × √5)R27° surface oxide.[41]
We demonstrate that an oxidized Pd(100) surface can be recognized by a photoemission peak at 529.9 eV in O 1s, but in the CO oxidation experiment, no clear evidence of surface oxidation can be observed
Summary
Palladium is a well-known catalyst for CO oxidation. The reaction has been studied for several decades, both for industrial applications and to gain a fundamental understanding of the reaction. The large difference under applied conditions, often referred to as the pressure gap, has generated an ongoing debate whether the results achieved at low pressures are relevant for industrial conditions.[3,4] It has been recognized that if insights into the reaction mechanism are to be linked to surface structures, the catalyst characterization must be performed under reaction conditions, so-called operando studies. To fulfill the criteria of performing operando surface-sensitive experiments under realistic conditions, a significant effort has been made to develop experimental setups over the last decades.[5−7] A well-known model system for fundamental studies of catalytic reactions is CO oxidation using Pd(100) as a catalyst. For a broad partial pressure range aoffteCr OtheanldighOt-2o, fft.h1e6−s1u8rfTahcee oxide is reported to be observed thicker PdO oxide formation has been observed in the highly active phase when more
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