In SituX-Ray Photoelectron Spectroscopy of Model Catalysts: At the Edge of the Gap

Abstract

We present high-pressure x-ray photoelectron spectroscopy (HP-XPS) and first-principles kinetic Monte Carlo study addressing the nature of the active surface in CO oxidation over Pd(100). Simultaneously measuring the chemical composition at the surface and in the near-surface gas phase, we reveal both O-covered pristine Pd(100) and a surface oxide as stable, highly active phases in the near-ambient regime accessible to HP-XPS. Surprisingly, no adsorbed CO can be detected during high CO(2) production rates, which can be explained by a combination of a remarkably short residence time of the CO molecule on the surface and mass-transfer limitations in the present setup.

Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)

“…The CO2 190 signal starts to increase at about 160°C and reaches a maximum at about 195°C. Further increase in 191 the temperature does not affect the CO2 signal, implying that the reaction becomes mass-transfer 192 limited at this point, as previously also reported for Pd(100) [6]. Upon reducing the temperature, the9 CO2 production is maintained until about 160°C where the production rapidly decreases and 194 eventually extinguishes.…”

“…Palladium is a preferred catalyst for a number of reactions, including the oxidation of hydrocarbons 33 [1,2] and CO [3,4], and the state of the active Pd surface during reaction has been extensively 34 discussed [5,6]. Surface oxides rather than surfaces covered by chemisorbed oxygen have been 35 observed as the most active towards CO oxidation under near ambient as well as more realistic 36 conditions (above ambient pressure) [5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Surface oxides rather than surfaces covered by chemisorbed oxygen have been 35 observed as the most active towards CO oxidation under near ambient as well as more realistic 36 conditions (above ambient pressure) [5][6][7][8][9][10][11][12][13][14][15][16]. However, some studies also indicate Pd surfaces covered 37 by atomic oxygen as highly active [8,17], and generally both will exhibit activity.…”

“…However, some studies also indicate Pd surfaces covered 37 by atomic oxygen as highly active [8,17], and generally both will exhibit activity. For Pd(100), the 38 presence of a (√5x√5)R27° surface oxide (henceforth denoted √5) is found to exist when the surface 39 is highly active towards CO oxidation [5,6,8,10,14,17,18], and this is consistent with the reaction 40 following a Mars-van Krevelen mechanism with gas-phase CO reacting with the surface oxide to 41 form CO2 [5][6][7]9,11,[13][14][15]19]. The presence of the surface oxide during high CO2 production is also 42 supported by kinetic Monte-Carlo simulations [20,21].…”

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…By utilizing energy tunable synchrotron X‐rays for APXPS, the active catalyst surfaces and the interfaces between metal and oxide support of catalysts can be explored including information on the reactivity using mass spectrometric analysis. Although the working gas pressure inside APXPS cells has mostly remained in the low mbar range,90, 91 recent developments in this field, however, show that operando XPS measurements under more realistic electrolysis conditions will become possible. The groups of Schlögl92 and Nilsson93 could, by means of XPS, unravel that during the electrochemical oxygen evolution reaction the degree of surface oxidation of platinum92 and iridium oxide93 electrodes change depending on the applied potential.…”