Characterization of Niobia-Supported Palladium-Cobalt Catalysts F. B. Noronha ² and Martin Schmal* NUCAT-PEQ-COPPE, UniVersidade Federal do Rio de Janeiro, Ilha do Funda ˜ o, COPPE, C.P.68502, CEP 21941, Rio de Janeiro (Brasil) B. Moraweck, P. Deliche ` re, and M. Brun IRC/CNRS, 2 AV. Albert Einstein, 69626 Villeurbanne Cedex, France F. Villain ‡ LURE, Ba ˆ timent 209d, Centre UniVersitaire Paris - Sud, BP 34, 91898 Orsay Cedex, France R. Fre ´ ty Laboratoire d’Application de la Chimie a ` l’EnVironnement, LACE/CNRS, 43 Bd du 11 noVembre 1918, 69622 Villeurbanne Cedex, France ReceiVed: August 5, 1999; In Final Form: February 15, 2000 Niobium oxide supported Pd-Co catalysts were characterized through XPS (X-ray photoelectron spectroscopy), (XPS), temperature-programmed reduction (TPR), magnetic measurements, X-ray diffraction, and extended X-ray absorption fine structure (EXAFS). XPS analyses suggested the presence of Co 3 O 4 particles and Co 2+ surface phase. TPR and magnetic measurements showed that the palladium addition promoted not only the reduction of Co 3 O 4 particles but also the cobalt surface phase. Magnetic measurements and EXAFS analyses revealed the Pd-Co alloy formation during reduction. A model represented by bimetallic particles enriched with palladium and particles containing only cobalt was proposed. Introduction Monometallic cobalt catalysts are effective for the synthesis of long-chain hydrocarbons from hydrogen and carbon monoxide. 1-3 The catalytic hydrogenation (activity, selectivity, and stability) of carbon monoxide on cobalt catalysts has been reported to be affected by two factors: the dispersion and the extent of reduction. 4-7 The reducibility of cobalt is closely related to the nature of the species present on the support. In particular, on supported cobalt catalysts, several cobalt species have been detected as a function of the support. 8-13 At least three different types of cobalt species have been reported on the alumina-supported cobalt catalysts: Co 3 O 4 particles, which are more readily reduced; Co 2+ species and CoAl 2 O 4 , which are hardly reduced or unreduced, respectively. 8,14-16 On niobia- supported cobalt catalysts, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and temperature- programmed reduction (TPR) analysis revealed that catalysts with low cobalt content showed a high amount of Co 2+ species in interaction with the support. For higher cobalt loading, Co 3 O 4 particles were also present, in addition to Co 2+ species. 12 Besides the support, the addition of noble metals to a supported cobalt catalyst can also change the cobalt state and modify its activity and selectivity. 17-21 An enhancement of the activity of cobalt with the addition of small amounts of Pd 20 or Pt 21 has been reported. In general, it has been proposed that the addition of a noble metal promotes the cobalt oxide reduction. 20-27 A larger fraction of metallic cobalt could explain the catalytic behavior of these bimetallic catalysts. Moreover, adding a second metallic element can lead to alloy formation, which could change the catalytic and adsorptive properties. 28,29 For example, the addition of a noble metal to iron or cobalt catalyst increases the selectivity of synthesis gas reaction toward methanol. 17,30-33 The addition of Fe to Pd resulted in significant enhancement of methanol formation which was ascribed to Pd x Fe alloy formation. According to Guczi, 31 the nonreducible oxide-metal interface or the alloy formation induces two effects: modifica- tion of both the CO adsorption mode (CO dissociation or adsorption in a molecular form) and the adsorption of hydrogen. In both cases, the CO + H 2 selectivities should be altered by the bimetallic catalysts. Furthermore, the second metal com- ponent can also modify the deactivation behavior. 34 The addition of noble metal to Co/Nb 2 O 5 catalysts showed interesting selectivity results on the Fischer-Tropsch synthesis. 35 The presence of a noble metal increased the C 5 + selectivity and decreased the methane formation independent of the reduction temperature. However, the characterization of the reduced state was not performed, making difficult a complete comprehension of the catalytic behavior. In fact, relatively few studies have been done to investigate the behavior of bimetallic systems on a support able to promote the strong metal support interaction (SMSI) effect, such as Nb 2 O 5 . 36-38 This may be explained by the difficulties of the niobium oxide supported catalysts characterization. Techniques such as X-ray diffraction (XRD) 39 and energy-dispersive X-ray ² Present address: Instituto Nacional de Tecnologia - INT, Av. Venezuela 82, CEP 20081-310, Rio de Janeiro, Brasil. ‡ Present address: Laboratoire de Chimie Inorganique et Mate ´riaux Mole ´culaires, Case 42, Ba ˆtiment F74, Universite ´ Pierre et Marie Curie, 4 place Jussieu, 75252, Paris Cedex 05, France. * For correspondence. E-mail: schmal@peq.coppe.ufrj.br. 5478 J. Phys. Chem. B 2000, 104, 5478-5485 10.1021/jp992777o CCC: $19.00 © 2000 American Chemical Society Published on Web 05/19/2000