Journal of Catalysis 262 (2009) 102–110 Contents lists available at ScienceDirect Journal of Catalysis www.elsevier.com/locate/jcat In situ EXAFS and FTIR studies of the promotion behavior of Pt–Nb 2 O 5 /Al 2 O 3 catalysts during the preferential oxidation of CO S. Guerrero a , J.T. Miller b,c , A.J. Kropf c , E.E. Wolf a,∗ a Chemical Engineering Department, University of Notre Dame, Notre Dame, IN 46556, USA b BP Research Center, E-1F, 150 W. Warrenville Rd., Naperville, IL 60563, USA c Chemical Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, IL 60439, USA article info abstract Article history: Received 27 September 2008 Revised 8 December 2008 Accepted 9 December 2008 Available online 7 January 2009 Keywords: PROX reaction Pt supported catalyst Nb promoter The promotional effect of Nb to Pt/Al 2 O 3 supported catalysts during the preferential oxidation of CO (PROX) was studied using various spectroscopic techniques. Addition of small amounts of Nb (<5%) stabilizes 40% of the loaded platinum as Pt 2+ , which remains oxidized even after reduction treatments. This Nb-promoted catalyst is very active and selective for the PROX reaction. On Pt/Nb 2 O 5 and at high Nb loading for the Pt/Nb/Al 2 O 3 catalysts, the selectivity to CO 2 decreases and the selectivity for H 2 oxidation increases opposite to the selectivity observed at low Nb loadings. The increase CO 2 selectivity due to Nb promotion is ascribed to the inhibition of CO at low temperature which decreases hydrogen oxidation. Operando FTIR results indicate the presence of adsorbed CO as well as carbonates, bicarbonates and formates during the PROX reaction. An IR band at 968 cm −1 indicates the presence of Nb=O moieties at low Nb loadings. At higher Nb loadings, IR suggests the formation of three-dimensional Nb 2 O 5 aggregates. The surface of the Nb containing catalysts is complex containing reduced and oxidized Pt which is modified by NbO x species either surrounding the Pt crystallites or decorating them.  2008 Published by Elsevier Inc. 1. Introduction The increased demand for fossil fuels have led to a renewed interest in alternative fuels, among them, the use of proton ex- change membrane (PEM) fuel cells, utilizing hydrogen as energy source. The use of an upstream reformer of a hydrocarbon, such as methanol or ethanol, is among several processes investigated to produce hydrogen for PEM fuel cells. Such processes can provide a H 2 -rich stream having an approximate composition of 45–75% H 2, 15–25% CO 2 , 0.5–2% CO, 10–20% H 2 O and N 2 [1,2]. The presence of CO in the reformer effluent stream, however, constitutes a prob- lem since it acts as a poison of Pt used on the anode of the fuel cell, which requires reducing the CO concentration in the effluent below 20 ppm. This can be achieved by preferentially oxidizing CO to CO 2 in a H 2 -rich stream upstream the fuel cell. The preferen- tial oxidation of CO (PROX) requires highly selective catalyst for CO oxidation and at the same time unselective to the unwanted oxi- dation of H 2 . Pt is known for being an active catalyst in the PROX reac- tion [3–10] as it was first reported for such reaction to purify hydrogen used in ammonia synthesis [11]. Fuel cells, however, al- ready utilize significant amounts of Pt, and therefore the Pt loading * Corresponding author. Fax: +1 574 631 8366. E-mail address: ewolf@nd.edu (E.E. Wolf). must be minimized in a PROX catalyst. One alternative is to use promoters that allow decreasing Pt loading while maintaining high activity and selectivity. Previous works have shown that combin- ing Pt and Sn supported on a carbon support resulted in a better PROX activity than on an unpromoted Pt/Al 2 O 3 catalyst [6,12]. This effect was attributed to the promotion of oxygen adsorption on Sn sites and to the competing adsorption of CO and H 2 on Pt sites [6]. It has also been suggested that promoters can positively affect the catalyst selectivity by blocking the spillover of adsorbed oxygen onto the support and/or hydrogen species, both involved in hy- drogen oxidation [10,13–15]. Minemura et al. [16] reported that during the PROX reaction, a 2% Pt supported on Al 2 O 3 promoted with potassium could reach 100% CO conversion and 50% CO 2 se- lectivity at around 90 ◦ C. In a previous paper [17], we reported an interesting promotion– inhibition effect of adding Nb to Al 2 O 3 -supported Pt catalysts dur- ing the PROX reaction. At low Nb loadings the catalysts were very active and selective for CO oxidation. As Nb loading increased, this effect reversed and in the limit of Nb loading, i.e. Pt supported on niobia, the CO selectivity of Pt was inhibited during the PROX re- action. Interestingly, this latter catalyst is very active for hydrogen oxidation in the presence of significant amounts of CO, i.e. it be- comes resistant to CO poisoning. In this work, we present in situ and operando (under reaction conditions) FTIR and EXAFS/XANES spectroscopy characterization 0021-9517/$ – see front matter  2008 Published by Elsevier Inc. doi:10.1016/j.jcat.2008.12.008