Oxidation of propylene over Pd(5 5 1): Temperature hysteresis induced by carbon deposition and oxygen adsorption
Graphical abstract
Introduction
Palladium is used in various catalytic applications because of its high intrinsic activity and its relatively low price compared with other noble metals. In particular, Pd-based catalysts are used for the purification of automotive exhaust gases that contain significant amounts of unburned fuel and other hydrocarbons formed by pyrolysis [1], [2], [3]. Many studies have been devoted to the combustion of hydrocarbons over Pd-based catalysts [4], [5], [6]. However, the mechanism for the oxidation of hydrocarbons over palladium is still not fully understood. The mechanism is complex and the catalyst activity and selectivity are influenced by variations in the process pressure, temperature, and the gas mixture composition. Moreover, under oxidizing conditions, Pd-based catalysts can exhibit an interesting dynamic behavior, including hysteresis phenomena, self-sustained rate oscillations, spatial pattern formation, and deterministic chaos; a fast deactivation of palladium is frequently observed as well [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. It is obvious that in all these cases a change in the chemical state of palladium determines the catalytic performance, causing the critical phenomena or catalyst deactivation. To develop more-effective Pd-based catalysts with a high activity in a wide temperature range, the reasons that cause these phenomena have to be understood.
The main objective of this work was to study the oxidation of propylene over palladium in a wide range of temperatures and pressures under which the temperature hysteresis is observed. It allowed us to clarify the mechanism for deactivation of palladium during the combustion of hydrocarbons and the influence of the chemical state of palladium on its activity and product distribution. Propylene was chosen as a model hydrocarbon because it is a light unsaturated hydrocarbon, which is typically detected in automotive exhaust gases at a considerable concentration. The reasons for the decrease or increase in activity of palladium were unraveled by X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction spectroscopy (TPRS). XPS is one of the powerful tools in the catalytic surface science to investigate both the surface composition and the nature of adsorbed species; while TPRS can provide direct information about catalytic properties of samples under study [19], [20], [21], [22]. A Pd(5 5 1) single crystal was used as a model catalyst. Because the stepped surface of Pd(5 5 1) consists of many regular defects (the surface contains three-atomic terraces of the (1 1 0) plane separated by the (1 1 1) steps), it is a good model for the surface of metal nanoparticles in supported catalysts. It is generally accepted that stepped surface of metal nanoparticles plays an important role in their catalytic activity [23].
Section snippets
Experimental
The TPRS measurements were performed in an ultra-high vacuum (UHV) surface analysis system at the Leiden Institute of Chemistry (Leiden University, Leiden, the Netherlands). The system was equipped with facilities for Ar+ ion sputtering, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and a shielded, differentially pumped, UTI-400 quadrupole mass-spectrometer (MS). The system was continuously pumped by turbomolecular pumps backed by mechanical pumps and the base
Results and discussion
Two sets of experiments were performed. The first set was a series of TPRS experiments with the mass-spectrometric analysis of the gas phase during heating and cooling of the Pd(5 5 1) single crystal with a constant rate between 120 and 1200 K. The second set was measurements of the C1s, O1s, and Pd3d core-level spectra of the palladium surface at 373, 523, 673, and 773 K during stepwise heating from room temperature and subsequent cooling in the same manner. The spectra were acquired at the set
Summary and conclusions
The presented data clearly indicate that the product distribution and activity depend in a different way on the temperature during heating and cooling the Pd(5 5 1) single crystal in oxygen/propylene mixtures. By comparing the TPRS and XPS data, we can suggest that this unusual kinetic behavior, referred to as temperature hysteresis, is induced by the concurrent accumulation of carbon and oxygen atoms on the palladium surface. At low temperatures, a high concentration of carbonaceous deposits
Acknowledgments
This work was partially supported by the Russian Foundation for Basic Research (Research project No. 12-03-00766-a) and the Ministry of Education and Science of the Russian Federation. The authors are also grateful to M. Hävecker, D. Teschner, and R. Blume as well as the staff of BESSY for their support during the beamtime. The authors would like to thank C.J. Weststrate and J.W. Bakker for helpful discussions and technical support.
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