On the Issue of the Deactivation of Au–Ceria and Pt–Ceria Water–Gas Shift Catalysts in Practical Fuel‐Cell Applications

Abstract

In recent years, nanostructured Au catalysts have attracted a lot of attention as potential candidates for the removal of poisonous carbon monoxide from synthesis gas streams, thus upgrading hydrogen gas to be used as fuel at the anode of lowtemperature fuel cells. The interest in Au nanoparticles has been motivated by the pioneering work of Haruta et al., who demonstrated the extremely high activity of finely dispersed Au on reducible oxide supports for CO oxidation. 2] Gold oxide systems have also been evaluated as potential catalysts for the water–gas shift (WGS) reaction and the preferential oxidation of CO in H2-rich fuel-gas streams. [9–16] Among these catalysts, the combination of Au with nanoscale cerium oxide (ceria) has been shown to be the most active lowtemperature WGS catalyst and one of the most selective preferential oxidation (PROX) catalysts for application at 80–120 8C. Nanocrystalline ceria has unique properties as an oxide support, as it increases the activity of Au toward CO oxidation and the WGS reaction by more than two orders of magnitude relative to microcrystalline ceria. Similar findings have been reported by Gorte and co-workers for Ptgroup metals on nanoscale ceria. Evidence in the literature has been mounting on the participation of the surface oxygen atoms of ceria in the catalysis of CO oxidation by dioxygen or water. Deactivation with time-on-stream and/or in shut-down/ restart operation currently plagues all known WGS catalysts based on ceria or copper oxide. Kim and Thompson reported fast deactivation of their Au–ceria catalyst, which they attributed to the blockage of the active sites by carbonates and/or formates formed during the WGS reaction. However, this fast extensive initial decline of activity of Au–ceria in the WGS reaction was not observed in a less H2-rich gas composition. [5] In their report on the deactivation of Pt–ceria with time-on-stream, Zalc et al. claimed that overreduction of ceria cannot be avoided, thus rendering ceriasupported catalysts impractical for the WGS reaction in fuelcell systems. Furthermore, in cyclic operation, including shut-down to room temperature and start-up of reactors, severe deactivation has been reported both for Pt–ceria and Au–ceria WGS catalysts. The activity of Pd–ceria and Pt– ceria for the WGS reaction can be partially recovered by reoxidation of the catalyst in oxygen at high temperatures (600 8C), but this process is not practical in the continuous operation of a fuel-cell system. To the best of our knowledge, no in situ remedy of the deactivation problem has been reported. Herein, we present a new way to stabilize the activity of Au–ceria catalysts for the WGS and PROX reactions under realistic operating conditions. We started from the observation that Au–ceria catalysts have excellent stability with time-on-stream under PROX conditions at 120 8C. We further studied this type of catalyst in shut-down/start-up cycles of the PROX reactor under conditions that allowed water condensation on the catalyst. Interestingly, this did not affect the subsequent catalyst activity at 120 8C (Figure 1). After reaching steady state at 120 8C in the PROX reaction, the 0.28 AuCe(Gd)Ox catalyst sample was cooled to room temperature and held under a continuous flow of the same gas mixture for 2 h before being reheated to 120 8C. Stable CO conversion was recovered. The sample was then cooled down to room temperature and held in the flowing full gas stream for 6 h; again, no drop of activity was found after reheating at 120 8C. Excellent performance in the shut-down/start-up cycling of the PROX reactor was also observed with another Au–ceria (La-doped) sample (0.57 AuCe(La)Ox ; Figure 1). Remarkable stability was further found for the 0.28 AuCe(Gd)Ox sample at higher conversions of CO when working with a higher O2/CO ratio [*] W. Deng, Dr. M. Flytzani-Stephanopoulos Department of Chemical and Biological Engineering Tufts University Medford, MA 02155 (USA) Fax: (+1)617-627-3991 E-mail: mflytzan@tufts.edu