Step dynamics and oxide formation during CO oxidation over a vicinal Pd surface.

Mikhail Shipilin,Chu Zhang,Lindsay Richard Merte,Johan Gustafson,Edvin Lundgren

doi:10.1039/c5cp07488f

Mikhail Shipilin, Chu Zhang + Show 3 more

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https://doi.org/10.1039/c5cp07488f

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Abstract

In an attempt to bridge the material and pressure gaps - two major challenges for an atomic scale understanding of heterogeneous catalysis - we employed high-energy surface X-ray diffraction as a tool to study the Pd(553) surface in situ under changing reaction conditions during CO oxidation. The diffraction patterns recorded under CO rich reaction conditions are characteristic for the metallic state of the surface. In an environment with low excess of O2 over the reaction stoichiometry, the surface seems to accommodate oxygen atoms along the steps forming one or several subsequent adsorbate structures and rapidly transforms into a combination of (332), (111) and (331) facets likely providing the room for the formation of a surface oxide. For the case of large excess of O2, the diffraction data show the presence of a multilayer PdO with the [101] crystallographic direction parallel to the [111] and the [331] directions of the substrate. The reconstructions in O2 excess are to a large extent similar to those previously reported for pure O2 exposures by Westerström et al. [R. Westerström et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 155410].

Highlights

Many industrial and everyday processes surrounding us result in the production of poisonous substances
A Pd(553) single crystal was placed in an ultra-high vacuum (UHV) chamber combined with a gas flow reactor.[15]
Our data recorded under reaction conditions exhibit a similar change in diffraction pattern, which can mean that before refaceting the surface of our sample subsequently adopted some or all of the adsorbate oxygen structures found by Westerstrom et al In panel 4 of Fig. 4 the diffraction rods assigned to Pd(553) steps with adsorbed oxygen atoms are weaker and coexist with the appearing diffraction rods characteristic for (332) and (111)

Summary

Introduction

Many industrial and everyday processes surrounding us result in the production of poisonous substances. Oxidation of the Pd(553) surface in pure oxygen was thoroughly studied over a broad pressure range by Westerstrom et al using several in situ and ex situ experimental techniques and ab initio simulations.[14] The authors showed that with increasing pressure the surface undergoes several oxygen induced transformations They notice that studies of the structural changes in this catalytic system under real working conditions would be of great importance for the comprehension of heterogeneous catalysis. This idea motivated us to study the Pd(553) surface acting as a catalyst in the process of CO oxidation and to compare our results with the results obtained by Westerstrom et al for the surface exposed to pure oxygen. The results show that the surface structure drastically changes sequentially adopting several states upon the catalyst activation and providing a room for the thick PdO film growth when the O2 excess is significant

Experimental details

Results and discussion

Stationary conditions

Dynamic conditions

Conclusions