In SituNAP-XPS and Mass Spectrometry Study of the Oxidation of Propylene over Palladium

Abstract

The oxidation of propylene over a Pd(551) single crystal has been studied in the millibar pressure range using near-ambient pressure X-ray photoelectron spectroscopy and mass spectrometry. It has been shown that, irrespective of the O2/C3H6 molar ratio in the range 1–100, the total oxidation of propylene to CO2 and water and the partial oxidation of propylene to CO and H2 occur when the catalyst is heated above the light-off temperature; increasing the partial pressure of O2 leads to decreasing the catalytic activity. The selectivity toward CO2 is at least two times higher than the selectivity toward CO, indicating that the total oxidation is the main reaction route. The normal hysteresis with a light-off temperature higher than the extinction temperature is observed in the oxidation of propylene between 100 and 300 °C. According to NAP-XPS, the main reason for the hysteresis appearing is a competition between two surface processes: carbonization and oxidation of palladium. At low temperatures, the adsorption and following decomposition of propylene dominate, which results in accumulation of carbonaceous deposits blocking the palladium surface. Increasing the catalyst temperature leads to burning the carbonaceous deposits which initiates the following oxidation of propylene. The highest conversion of propylene is observed when both free surface sites and adsorbed oxygen atoms exist in a large amount on the catalyst surface. As the partial pressure of O2 increases, the catalyst surface gets covered by clusters of surface 2D palladium oxide, which is accompanied by a decrease in the catalytic activity. The mechanism of the oxidation of propylene over palladium is discussed.

Change in the Pd Oxidation State during Light-Off of Three-Way Catalysis Analyzed by in situ Diffuse Reflectance Spectroscopy

“…Nonetheless, data obtained using these techniques provide bulk oxidation states, in contrast to the surface sensitivity of DRS. Another surface-sensitive analysis is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), − which is still carried out in the millibar pressure range.…”

The Reasons for Nonlinear Phenomena in Oxidation of Methane over Nickel

“…Such an effect can be caused by the gradual oxidation of nickel with the formation of NiO clusters on the catalyst surface. Recently, a similar mechanism of catalyst deactivation in the oxidation of propylene on palladium was described by Kaichev et al [5]. It should be noted that nickel oxidation (V) is also exothermic.…”

“…Nonlinearity is determined by the interaction of species at the atomic-molecular level, as well as by the influence of temperature and mass and heat transfer processes on the rate of a chemical reaction. The most typical examples of nonlinear phenomena are temperature hysteresis [4,5], regular and damped self-sustained oscillations [6][7][8][9][10][11], chemical waves [12][13][14] and deterministic chaos [15]. In all of these cases, unusual dependences of the main parameter of a catalytic system-the reaction rate-on time and/or temperature are observed.…”

Enhanced Catalysis under 2D Silica: A CO Oxidation Study

“…Notably, a higher magnitude of CO 2 products (∼12 % increase) is observed in the confined catalytic reaction taking place under the 2D silica film. Note that a quantitative description of the relative quantities measured in the MS data is reliable due to an almost identical intensity of reactant CO and O 2 gas pressures at the start of each reaction and probing approximately the same areas we are measuring due to the geometric set‐up of the cone used in the MS/APXPS experimental set‐up [51–53] . However, to account for a slightly increased O 2 gas pressure over time in the case of silica‐covered Pd, the CO 2 activity resulted in a ∼12 % change due to the silica cover (Note S3, Figure S4).…”