(134f) High Purity Hydrogen by Water – Gas Shift Reaction Over Substituted Nanocrystalline Oxides

Water ? gas shift reaction is used for enriching the syn ? gas in process industries. It is aimed to minimize the CO concentration and the outlet gas is rich in hydrogen. This process becomes especially important when the gas is used for fuel cell applications where CO concentration is required to be in ppm level. This is possible in case of low temperature water ? gas shift reaction where the inlet CO concentration is less (2-3%). Use of noble metals has been reported for catalyzing the water ? gas shift reaction¹. TiO₂ is one of the potential oxide support materials for its use as low temperature WGS catalyst.

In this study we have synthesized Pd and Pt ion substituted TiO₂, and CeO₂ oxides modified by Ti using solution combustion technique which resulted in the formation of nanocrystalline solids. The catalysts were characterized by powder XRD and XPS spectroscopy. CeO₂ due to its high oxygen storage capacity (OSC) acts as a good support material for oxidation reactions. Substitution of a metal ion in the lattice results in the formation of oxide ion vacancies both in CeO₂ and TiO₂ and the oxidation catalytic properties get enhanced due to substitution. Further enhancement in the OSC of CeO₂ can be brought about by introduction of a foreign element like Ti in the crystal lattice at Ce positions. It should be noted that TiO₂ itself does not show high OSC but its solid solution with CeO₂ results in an increased OSC. Experimental as well as theoretical density functional calculations proofs are there that the introduction of Ti in CeO₂ as solid solution enhances the OSC and reducibility². Both CeO₂ and TiO₂ are reducible but the solid solution Ce_1-xTi_xO₂ has reducibility higher than both of the individual pure oxides. Moreover, substitution of a metal ion further creates oxide ion vacancy defects. We have made use of the above to design the low temperature water ? gas shift catalysts for high purity hydrogen production.

Water - gas shift reaction involves oxidation of CO and therefore corresponding reduction of the support material takes place. The effectiveness of the catalyst depends greatly upon the ease of reducibility and access to the redox cycles. Therefore, it is desirable to synthesize compounds for WGS with high OSC and high reducibility. The solution combustion technique results in the synthesis of substituted CeO₂ and TiO₂ oxides with the crystallite size in nanometer range. This imparts good properties to the solids for their use in catalytic applications. The nanocrystalline nature of the catalyst provides high surface area. The metal ion has an influence on the adsorption of the reactant gases. Therefore, the oxidation state of the metal in the solid is the key to the proper working of the catalyst. This makes the currently synthesized catalysts different from those used conventionally over the supports in which the metal is impregnated in zero oxidation state. Enhanced redox properties and substitution of noble metals in ionic form make the catalyst unique. To the best of our knowledge, ours is the first attempt to show the water ? gas shift reaction over these catalysts. We could achieve high CO conversions at low temperatures. This becomes especially important when it is desired to utilize hydrogen for fuel cell purposes. The equilibrium conversion goes on decreasing with temperature. Therefore with higher temperature, pure products can not be obtained. The reduction in temperatures resulted in achievement of higher conversions as the maximum conversions were limited by the equilibrium conversion. The stability of the catalyst was tested for long term operation using daily start up ? shut down technique. The catalysts were found to retain their activity for the complete time interval of study. Therefore, the catalysts synthesized in this study can act as catalysts for production of fuel cell grade hydrogen.

1. Fu Q, Saltsburg H., Flytzani-Stephanopoulos M. Active nonmetallic Au and Pt species on ceria-based water-gas shift catalysts. Science. 2003;301:935-938.

2. Baidya T, Dutta G, Hegde MS, Waghmare UV. Noble metal ionic catalysts: Correlation of increase in CO oxidation activity with increasing effective charge on Pd ion in Pd ion substituted Ce_1-xM_xO₂ (M = Ti, Zr and Hf). Dalton Transactions 2009;3:455-464.