Journal of Catalysis 225 (2004) 498–509 www.elsevier.com/locate/jcat XPS study of oxidation of rhenium metal on γ -Al 2 O 3 support J. Okal, a,∗ W. Tylus, b and L. K¸ epi´ nski a a Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O.Box 1410, 50-950 Wroclaw, Poland b Institute of Inorganic Technology, Wroclaw University of Technology, 50-353 Wroclaw, Poland Received 2 March 2004; revised 29 April 2004; accepted 1 May 2004 Available online 1 June 2004 Abstract The oxidation of Re/γ -Al 2 O 3 catalysts, containing 1 and 10 wt% of rhenium, sintered in hydrogen was examined in the temperature range of 20–800 ◦ C. The structures of the catalysts were investigated by XPS spectroscopy, TEM, and O 2 uptake measurements. The low-loaded catalyst comprises metallic particles with sizes of 1–4 nm (d av = 2.1 nm), while the high-loaded catalyst comprises particles with sizes of 1–9 nm (d av = 4.9 nm). Even short exposure to air at room temperature causes complete oxidation of small clusters of metallic Re, while larger particles are covered with very thin ReO x skin (undetected by TEM). XPS shows that the high-loaded catalyst still contains 94.5% of metallic Re, while the low-loaded catalyst contains only 60.5%. The remaining part of the Re is oxidized to Re 4+ , Re 6+ , and Re 7+ species. Oxidation at 150 ◦ C causes enhanced formation of Re 4+ –Re 7+ species and the amount of metallic Re quickly decreases to 33 and 2% for high- and low-loaded catalysts, respectively. This indicates a high affinity of the highly dispersed Re to oxygen. At this temperature, the Re/Al atomic ratio increases 2–4 times, indicating a large spreading of the oxide species on the support surface. Simultaneously, the average size of Re particles decreased as determined by TEM. At 300 ◦ C, whole Re was oxidized mainly to Re 2 O 7 , though some amount to Re 4+ and Re 6+ species remained. The O 2 uptake measurements confirm oxidation of rhenium particles. For the high-loaded catalyst O 2 uptake attained a maximum level (O/Re = 3.3) already at 300 ◦ C, while for the low-loaded catalyst even at 500 ◦ C the uptake (O/Re = 2.98) is below the maximum level. XPS data showed, however, that at 500 ◦ C, oxidation of rhenium to Re 2 O 7 occurs for both catalysts. The Re/Al atomic ratio remains nearly constant after oxidation of both catalysts at 300–800 ◦ C, indicating that Re 7+ species are firmly bonded to alumina surfaces even at 800 ◦ C. A detailed mechanism of oxidation of Re particles with different sizes is proposed based on a quantitative analysis of the XPS, O 2 uptake, TEM, and previous Raman results.  2004 Elsevier Inc. All rights reserved. Keywords: Re/γ -Al 2 O 3 catalyst; Oxidation of Re; TEM; O 2 sorption; XPS spectroscopy 1. Introduction The practical importance of Pt–Re/Al 2 O 3 -reforming cat- alysts and recently also Re–Co/Al 2 O 3 catalysts [1,2] has created interest in catalytic chemistry on the Re-containing catalysts. Generally, Re addition promotes diesel and gaso- line production [3]. Supported rhenium catalysts have been tested for hydrocarbon conversion craking [4–6], hydrogena- tion of benzene [6,7], n-heptane reforming [8], and recently as promising catalysts for methane [9] and ethane [10] arom- atization and also for ammonia synthesis [11]. Catalytic hy- drocarbon reactions are always accompanied by the catalyst deactivation, which results from the sintering of the catalyst, * Corresponding author. Fax: + 48 71 441 029. E-mail address: jana@int.pan.wroc.pl (J. Okal). the presence of poisons in the reagents, and the deposition of carbonaceous species. Thermal treatment in oxygen or in air is usually used to remove the effects of catalyst de- activation, and it is also an important step in the activation or regeneration procedures of rhenium-containing catalysts. Regeneration of the spent (reduced) Re 2 O 7 /Al 2 O 3 catalyst by oxidation at elevated temperatures brings about the redis- persion of rhenium crystals and the formation of the well- dispersed oxide phase [12–14]. However, the interaction between supported rhenium and oxygen has not been well characterized and only few studies have reported on the oxidation behavior of the supported Re particles [13–19]. On the contrary, for the Pt–Re/Al 2 O 3 cat- alyst, the effects of oxidation–reduction or oxychlorination– reduction cycles on the structure and activity are often stud- ied [8,20–24]. It is well known that oxidation of the Pt– 0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2004.05.004