Bridging the Pressure Gap in CO Oxidation

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 CO 2 , CO, and O 2 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% O 2 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.

Spiers Memorial Lecture: prospects for photoelectron spectroscopy

“…62 This has been used to measure CO oxidation on Pd(100) in the ‘light-off’ regime (when CO 2 begins to be produced) in pressures of up to 1 bar for the first time. 35 The measurements indicate that under the conditions of the experiment, the system is not in the mass-transfer-limited regime immediately after light-off, allowing the measurement to be performed in the kinetic regime. The variation with temperature of the intensity of the C 1s and O 1s signals from the working catalyst allowed the extraction of an apparent activation energy.…”

“…48,52,56 Current research on electrochemical systems is clearly benetting from a number of simultaneous technical developments such as those described above; particularly important future directions are the development of liquid-jet sources 22 and the rapid development of capability to perform XPS at pressures of 1 bar and above. 35,[62][63][64] These developments are discussed further in Section 2.7.…”

“…As noted above, there is currently a strong drive to close the pressure gap completely in NAP-XPS, and to make possible studies at atmospheric pressure and above 35,[62][63][64] (nally allowing the 'N' in the acronym to be dropped without any shade of exaggeration!) This is coupled with a strong drive to use harder X-rays, and to implement the technique in the regime of hard X-ray photoelectron spectroscopy (HAXPES).…”

“…The variation with temperature of the intensity of the C 1s and O 1s signals from the working catalyst allowed the extraction of an apparent activation energy. 35 In this arrangement, there is no separation between the sample and the main chamber, but effectively a 'virtual' cell is achieved by the localised high-pressure pocket. An alternative approach to closing the pressure gap is to further develop the in situ cells discussed in Section 2.3.…”

Concluding remarks: Photoelectron spectroscopy and the future of surface analysis

“…In reactive mixtures we have been able to follow CO oxidation at a total pressure of 1 bar. 10 Although hard X-rays, generating high kinetic energies of the photoelectrons, are being used, in order to maintain surface sensitivity it is essential to conduct experiments below the critical angle for total external reection (typically well below one degree). Such measurements allow the effective probing depth to be limited not by the scattering of electrons in the solid but rather by the penetration depth of the evanescent X-ray wave.…”