A bifunctional catalyst for the single-stage water-gas shift reaction in fuel cell applications. Part 2. Roles of the support and promoter on catalyst activity and stability.

The nature of oxide supports has a crucial effect on the performance of Pt-based catalysts in the water–gas shift reaction. Supports not only determine the activity of the catalyst, but also influence their stability (deactivation mechanism). Among the catalysts studied, Pt/TiO2 was the most active. Pt/CeO2 deactivated with time due to the formation of stable carbonate on the ceria surface. Sintering of Pt was found to be the cause of Pt/TiO2 deactivation. Using mixed oxides as catalyst supports did not improve the activity despite the better red–ox properties of mixed oxides compared with the single-oxide supports. Pt/TiO2 could be stabilized by adding a second metal (Re), which prevented Pt sintering. In addition, Pt–Re/TiO2 was more active than Pt/TiO2. Under WGS conditions, part of the Re was present in oxidizing form (ReOx); we speculate that this helped improve the catalyst activity

Role of Re in Pt-Re/TiO2 catalyst for water gas shift reaction: A mechanistic and kinetic study.

Transient kinetic studies and in situ FTIR spectroscopy were used to follow the reaction sequences that occur during water gas shift (WGS) reaction over Pt–Re/TiO2 catalyst. Results pointed to contributions of an associative formate route with redox regeneration and two classical redox routes involving TiO2 and ReOx, respectively. Under WGS reaction condition rhenium is present at least partly as ReOx providing an additional redox route for WGS reaction in which ReOx is reduced by CO generating CO2 and re-oxidized by H2O forming H2. The overall reaction rate, based on steady state kinetics, was given by rH2=0.075e31 kJmol-1/RTxpH2O x pH2-0.5(1-β), where β is the approach to equilibrium. Results obtained in the study indicated that the reaction between CO adsorbed on Pt and OH groups on titania is the rate-determining step

Bifunctional catalysts for single-stage water-gas shift reaction in fuel cell applications. Part 1. Effect of the support on the reaction sequence.

Oxide support plays a significant role in the mechanistic reaction sequence for the water–gas shift (WGS) reaction over Pt-based catalysts. In situ FTIR spectroscopic and transient kinetic studies have been used to follow the reactions that occur. CeO2-, TiO2-, and ZrO2-supported Pt catalysts have been studied at 300 °C. In all cases, CO is adsorbed on Pt. The role of the support oxide is to activate water, completing the WGS reaction sequence. We have taken into consideration four different pathways that may be involved in the complex WGS reaction scheme: (A) red–ox route, (B) associative formate route, (C) associative formate route with red–ox regeneration of the oxide support, and (D) carbonate route. In the case of Pt/ZrO2, the WGS reaction follows the associative formate route with red–ox regeneration (route C). On Pt/TiO2, both the red–ox route (A) and the associative formate route with red–ox regeneration (C) contribute. The associative formate route (B) is the relevant reaction pathway on Pt/CeO2

Development of active, and stable water-gas-shift reaction catalysts for fuel cell applications

Water-gas-shift (WGS) reaction CO + H2O = CO2 + H2, is a key step in the generation of H2 for fuel cells. Noble metal-based catalysts are promising single stage WGS catalysts because they less sensitive than LTS catalysts (Cu based) and more active than the HTS (Ni) catalysts. High activity in CO conversion at moderate temperatures and stability during start-up - shutdown cycles is essential, especially in transport applications. A series of supported Pt catalysts was studied. Zirconia based catalysts were stable but catalyst activity was low. Pt/TiO2 and Pt/Ti0.5Ce0.5O2 gave the required commercial activity (8.10-5 mole H2 g-1cat/sec, based on a 2 kg catalyst for a 100 kw fuel cell) at 300°C. However, 35 % of initial activity was lost after 24 hr. A new promising catalyst developed has high activity as Pt/TiO2 and was very stable. The reasons for the different activities, deactivation mechanism, and the stability of the newly developed catalysts were discussed. This is an abstract of a paper presented at the 231st ACS National Meeting (Atlanta, GA 3/26-30/2006)