Article pubs.acs.org/JPCC
Deactivation of Ru Catalysts under Catalytic CO Oxidation by Formation of Bulk Ru Oxide Probed with Ambient Pressure XPS Kamran Qadir,† Sun Mi Kim,† Hyungtak Seo,‡ Bongjin S. Mun,§ Funda Aksoy Akgul,∥ Zhi Liu,⊥ and Jeong Young Park*,† †
Graduate School of EEWS (WCU), and NanoCentury KI, KAIST, and Center for Nanomaterials and Chemical Reactions, Institute of Basic Science, Daejeon 305-701, South Korea ‡ Department of Materials Science & Engineering, Ajou University, Suwon 443-749, Republic of Korea § Department of Physics and Photon Science, School of Physics and Chemistry, and Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea ∥ Physics Department, Nigde University, Nigde 51240, Turkey ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: The surface science approach of using model catalysts in conjunction with the development of in situ spectroscopic tools, such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS), offers a synergistic strategy for obtaining a substantially better understanding of deactivation phenomena. In this study, we investigated the nature of Ru oxides on a Ru polycrystalline film under oxidizing, reducing, and catalytic CO oxidation reaction conditions. Thus, bulk Ru oxide was easily formed on such Ru catalysts, the growth of which was dependent on reaction temperature. Once formed, such an oxide is irreversible and cannot be completely removed even under reducing conditions at elevated temperatures (200 °C). Our reaction studies showed substantial deactivation of the Ru film during catalytic CO oxidation, and its activity could be partially recovered after reduction pretreatment. Such continuous deactivation of a Ru film is correlated with irreversibly formed bulk Ru oxide, as shown by AP-XPS. Such in situ spectroscopic evidence of the transition of oxides to a catalytically inactive state can enable more effective design of catalysts with less deactivation. “sintering”. Other reasons for deactivation can be chemical transformation of active catalytic phases to nonactive or noncatalytic phases.3,7,8 Factors responsible for deactivation can be reversible or irreversible, and novel regeneration treatments need to be devised to restore a catalyst’s performance (i.e., catalytic activity). Enhanced catalyst stability against deactivation is as important as controlling activity and selectivity. Thoroughly understanding the mechanisms of deactivation is often challenging, and it is critical to investigate deactivation under technologically relevant conditions. In situ spectroscopy techniques, such as AP-XPS, open up the possibility to bridge these gaps and devise more rational catalyst design.9,10 The CO oxidation reaction has been of continuous interest in heterogeneous catalysis11−15 because of its significance in scientific research and industrial applications, such as CO removal from automobile exhaust streams16 and hydrogen purification in polymer electrolyte membrane fuel cells.17
I. INTRODUCTION Catalyst deactivation is of significant technical and economic concern in industrial chemical processes.1−5 In petrochemical and bulk chemical production, which occur mainly through heterogeneous catalysis, deactivation phenomena are of great interest.6 Therefore, there is a need to minimize deactivation phenomena by gaining a thorough understanding of factors affecting a catalyst’s performance. The knowledge gained can then be applied in tailoring catalyst design and processes. Catalysts undergo mechanical, physical, or chemical alterations that result in the loss of activity. Among all causes of deactivation, it clearly appears that in most cases (except mechanical alterations) chemical alterations play a major role. Loss of activity can be related to a decrease in the number of active sites, the activity of the sites itself (turnover frequency (TOF)), and accessibility of these sites to the reactant molecules. Catalysts can show loss of activity due to carbon, which may chemisorbor physically adsorbon metal surface sites and block the reactants’ access, or it can completely deactivate the metal sites by total encapsulation. Catalyst deactivation can also be due to the loss of catalytic surface area due to growth of a crystallite phase, often referred to as © 2013 American Chemical Society
Received: March 18, 2013 Revised: June 2, 2013 Published: June 3, 2013 13108
dx.doi.org/10.1021/jp402688a | J. Phys. Chem. C 2013, 117, 13108−13113
The Journal of Physical Chemistry C
Article
through the reaction line by a Metal Bellows recirculation pump. A gas chromatograph equipped with a thermal conductivity detector and a 6′ × 1/8″ SS molecular sieve 5A was used to separate the products for analysis. The measured reaction rates, reported as TOF, were measured in units of product molecules of CO2 produced per metal surface site per second of reaction time. The active metal sites on the Ru thin film were calculated by assuming that both the coverage and the exposed face are 1 and that there are typically 1014 to 1015 active metal sites on the surface in a 2D system that are available for catalysis.37 We also assumed that the reaction data were obtained in a kinetically controlled regime because the reaction process was relatively slow and the reaction was carried out in the low conversion regime ( 10(−3) Torr) to Liquid Interfaces. Phys. Chem. Chem. Phys. 2007, 9, 3500− 3513.
V. CONCLUSIONS In summary, we carried out AP-XPS studies on a ∼45 nm thick Ru polycrystalline film under oxidation, reduction, and catalytic CO oxidation reaction conditions. We observed that the Ru film showed progressive growth of Ru oxide under reaction conditions. At higher temperatures, there was significant Ru oxide formation. The Ru metallic film showed the formation of bulk Ru oxide that was stable and irreversible. Even at higher reducing temperatures of 200 or 260 °C under CO or H2, the Ru oxide still existed. The Ru film showed vast deactivation under catalytic CO oxidation. The catalytic activity degraded 13112
dx.doi.org/10.1021/jp402688a | J. Phys. Chem. C 2013, 117, 13108−13113
The Journal of Physical Chemistry C
Article
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