PEM type fuel cells, which are accepted to be the best option for small scale applications, are prune to CO poisoning due to their low operation temperatures; the Pt anode of PEM has severe stability problems when H2 feed has CO levels above 10 ppm. Fuel processors are devices which convert various hydrocarbon fuels into CO-free hydrogen-to be used in fuel cells. A typical fuel processor has three catalytic units, namely reformer, water gas shift and preferential oxidation units, in series. The last two units are used to enrich the hydrogen produced by reforming; WGS increases the hydrogen concentration while suppressing CO level and PROX eliminates the CO left in the hydrogen produced. WGS is a well known industrial reaction; on the other hand, due to its exothermic and reversible nature, equilibrium-limitations impose low reaction temperatures for decreasing CO level down below 1%. Thus, WGS is normally performed in two steps in a fuel processor. In the first step a high-temperature shift (HTS) catalyst is used for WGS performed at 340–530oC, and the second step is performed over a low temperature shift (LTS) catalyst at 180–230oC. WGS catalysts for fuel cell applications should be active, thermally stable, resistant to poisoning and highly selective for a wide range of H2O/CO ratios. The fact that conventional high temperature (Fe–Cr oxide) and low temperature (Cu–Zn–Al2O3) WGS catalysts cannot be used in combined fuel processor-fuel cell systems has led to an interest in the development of catalyst formulations, which are based on active noble metals supported on metal oxide carriers. Among the noble metals studied, Pt has found to be highly active. Al2O3, which is a very high surface area support, has been reported to result in significant enhancement of activity, selectivity and stability of dispersed noble metals for a number of catalytic reactions.

In the present study, water-gas shift reaction was studied on a series of Pt-Ni/Al2O3 catalysts for medium temperature range WGS reaction (200–450oC). Bimetallic catalysts with different Ni/Pt ratios were prepared by sequential impregnation method and they were characterized by x-ray spectroscopy, scanning electron microscopy and back scattering electron imaging. The effects of the Ni content, GHSV and H2O/CO ratio on the WGS activity of the catalysts were investigated. The inhibition effect of CO2 and H2 in the feed was also studied aiming to understand the performance characteristics of the catalysts for the feed compositions similar to reformer effluent of a typical fuel processor. CO conversions levels were found to increase with increasing Ni/Pt ratio of the catalyst and increasing H2O/CO ratio in the feed. The studies revealed that the bimetallic Pt-Ni/Al2O3 catalysts are very active, selective and stable for WGS reaction. Since the Pt-Ni bimetallic system had been proven to be high performance autothermal reforming catalysts previously [1-3], the current results show that those catalysts can be a good candidate to be used for both reforming and WGS stages of a fuel processor. Additionally, considering near equilibrium conversions reached by the Pt-Ni system in WGS at 200-450 C range, the Pt-Ni catalysts are thought to offer an opportunity for performing HTS and LTS in a single unit upon the optimization of their properties and the reaction conditions in a combined fashion.

Keywords: WGS, High temperature shift (HTS), Pt-Ni

References:

1. Çağlayan, B. S., Avcı, A. K., Önsan, Z. İ., Aksoylu, A. E., “Production of Hydrogen over Bimetallic Pt-Ni/dAl2O3: I. Indirect Partial Oxidation of Propane”, Applied Catalysis A: General, 280 (2) (2005) 181-188

2. Çağlayan, B. S., Önsan, Z. İ., Aksoylu, A. E., “Production of Hydrogen over Bimetallic Pt-Ni/dAl2O3: II. Indirect Partial Oxidation of LPG”, Catalysis Letters, 102 (2005) 63-67

3. Gökaliler, F., Selen Çağlayan, B., Önsan, Z. İ., Aksoylu, A. E., “Hydrogen production by autothermal reforming of LPG for PEM fuel cell applications”, International Journal of Hydrogen Energy, 33(4) (2008) 1383-1391

Acknowledgement: The financial support provided by State Planning Organization of Turkey through project DPT-07K120630 and Turkish Scientific research Council through project TUBITAK-105M282.