The operando observation of catalytic reactions is known as a powerful approach toward the understanding of reaction mechanisms. Recently, near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) has been often used for such an approach to obtain information on chemical states of both catalysts and adsorbates under reaction conditions. We have built an endstation for NAP-XPS measurements at the Photon Factory and applied this technique to various mechanistic studies on high-pressure-induced surface phenomena and catalytic surface reactions under near realistic conditions.1 The presence of near ambient pressure gas causes changes in surface free energy under the equilibrium with the gas phase resulting in formation of high-density adsorption phases,2 site switching of adsorbates to energetically unfavorable sites under high-pressure conditions3 and gas-phase-induced surface segregation of metals and formation of surface complexes, which could influence on catalytic activities. We conducted systematic studies on active phases for catalytic CO oxidation under NAP conditions on several platinum group metal (PGM) surfaces. High-density oxygen phases were found in near surface regions under the most active conditions in common irrespective of element of PGMs, which suggests that weakly bound oxygen atoms in the high-density phases play a key role in the CO oxidation due to high availability.4 Furthermore it was found that availability of CO is crucial for the CO oxidation on an alloy surface.5 The NO reduction by CO on PGM surfaces was also studied using NAP-XPS and mass spectroscopy.6,7 Although the rate-determining step for this reaction has been considered to be NO dissociation, our results suggest that the activity is not determined by the NO dissociation ability but by availability of dissociated N and O. The availability of the surface adsorbates could be related to their binding energies.

A key factor for activity of catalytic reaction observed via operando measurements with near ambient pressure XPS

256th National Meeting (2018)

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