International Journal of Hydrogen Energy 33 (2008) 197 – 205 www.elsevier.com/locate/ijhydene An attempt to rank copper-based catalysts used in the CO-PROX reaction Irene López, Teresa Valdés-Solís, Gregorio Marbán ∗ Instituto Nacional del Carbón (CSIC), c/Francisco Pintado Fe, 26, 33011 Oviedo, Spain Received 24 July 2007; received in revised form 11 September 2007; accepted 11 September 2007 Available online 24 October 2007 Abstract This work provides a comparison of the activities of copper-based catalysts in the preferential oxidation of CO in the presence of H 2 . The Liu–Flytzani-Stephanopoulos equation [Liu W, Flytzani-Stephanopoulos M. Total oxidation of carbon monoxide and methane over transition metal-fluorite oxide composite catalysts II. Catalyst characterization and reaction kinetics. J Catal 1995;153:317–332] was used to evaluate catalytic rate constants on the basis of the data reported by various groups and in some cases these data were found to be insufficient. Although the reaction rate constants obtained could in some cases be influenced by experimental artefacts, it was unambiguously established that the addition of modifiers (e.g. Zr, Sn, Co, etc.) to the copper–ceria catalysts does not produce any improvement in the catalytic activity of these catalysts and that chelating methods are the most appropriate procedures for the preparation of CO-PROX catalysts. Deactivation during the testing of catalysts has received little attention in the literature, and yet it is a potential source of uncertainty in the catalytic activities reported.  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Preferential oxidation; CO; Hydrogen; Copper-based catalysts; Catalytic reaction rate In future vehicles powered by low temperature H 2 -fed fuel- cells, hydrogen must be supplied with less than 100 ppm of CO, due to its deactivating effect on the fuel cell electrocata- lysts [1]. Two major solutions for feeding the H 2 fuel are en- visaged: (i) Onboard storage of ultrapure H 2 and (ii) Onboard H 2 production from hydrocarbons. The second option offers several advantages from the point of view of fuel distribution and supply, safety and cost, especially when bio-methanol is the source of H 2 via an onboard steam reforming process [2]. However, the production of H 2 from hydrocarbons results in the undesired generation of CO as a byproduct and consequently a purification step must be introduced prior to the fuel cell stage. The purification process usually consists of a water–gas shift reaction (required only for a high CO concentration) fol- lowed by preferential CO oxidation (PROX) [3], a process by which the residual CO is catalytically oxidised to CO 2 while the simultaneous oxidation of H 2 to water is minimised. There are basically two approaches to PROX catalysts. Initially, no- ble metal catalysts (Au, Ag, Pt, Ru, etc.) were employed as in ∗ Corresponding author. Tel.: +34 985119090; fax: +34 985297662. E-mail address: greca@incar.csic.es (G. Marbán). 0360-3199/$ - see front matter  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.09.011 the case of the catalysts developed for CO purification in the ammonia synthesis process [4]. Later on it was observed that cheaper copper-based catalysts appeared to be equally active and much more selective [5] given the high temperatures re- quired for the onboard process between the H 2 production stage (∼ 250 ◦ C for bio-methanol steam reforming) and the fuel cell stage (70.100 ◦ C). Different compositions of copper-based cat- alysts have been prepared and tested for producing the PROX reaction in the 100.250 ◦ C temperature range, and have been reported in a considerable number of papers. However, other testing conditions reflect a certain degree of diversity between different research works, especially with respect to gas flow rate and amount of catalyst (spatial velocity), thus making it difficult to compare the reported catalytic activities (usually ex- pressed in standard CO conversion values). Consequently there is only a limited number of articles that provide such a com- parison [6,7]. In the majority of works the catalytic activities of a number of different catalysts prepared by the same authors and tested under identical conditions are compared [8–10]. The main reason for the lack of comparative works might be the difficulty in finding a common basis of comparison regard- ing the activity, selectivity and stability of the copper-based catalysts. In addition, in many cases it is difficult to perform a