(611e) Experimental Studies On Water – Gas Shift Kinetics Over Noble Metal – Substituted Nanocrystalline Oxides: Spectroscopic and Mechanistic Insights

Water ? gas shift reaction is a reaction of interest for study both because of its use in industrial processes as well as its presence in more complex reaction networks observed in processes like Fischer ? Trophs synthesis. Much attention has been focused on this reaction with the increasing requirement of high purity hydrogen for applications in fuel cells and the development of technology utilizing hydrogen as a fuel. Since it is a reversible exothermic reaction, it is desirable to have a quantitative description of the reaction for optimal operation of the reactor, the reaction rate being a direct function of temperature with the equilibrium conversion decreasing with temperature.

We have synthesized noble metal substituted ionic catalysts by solution combustion technique and have showed their activity towards the water ? gas shift reaction. There have been reports of non - metallic species, supported by ceria, catalyzing the water ? gas shift reaction¹. The ionic catalysts over titania and zirconia and the mixed oxides of ceria, titania and zirconia have not been much studied. We have synthesized solid solutions of the oxides with noble metals substituted in the crystal lattice. The catalysts were characterized by XRD, XPS and FT - Raman spectroscopy for their complete structure. The metals were found to be in ionic state. For zirconia based catalysts, the surface hydroxyl groups can play an important role in the surface processes and have been reported on from IR spectroscopic studies².

The catalytic activity is dependent upon the nature of the support also. There have been reports showing the differences in catalytic activity for the different supports³. Therefore, we have explored the different supports and have found the activity to be significantly different for the different supports. The use mixed oxide solid solutions result in increased oxygen storage capacity⁴. Therefore, it is desirable to explore the solid solutions as the possible or modifying support material. To have an insight into the reason for high reactivity of the catalysts, we propose a mechanism based on the interaction of the reactive species present on the metal and the support. The catalytic activity was found to be dependent upon the oxidation state of the metal. Especially in case of Pt substituted compounds, Pt in +2 state gave the best results. The change in the oxidation state of the support with the corresponding change in Pt from +2 to +4 state making the metal ? support interaction cycle was proposed as a possible reason for the high activity of the catalyst. The formation of surface species, adsorption over the metal and dissociation of water over the oxide ion vacancies were proposed. The experiments were carried out to deduce the various surface phenomena and were rationalized using spectroscopic studies to have a better insight into the reactivity of the catalyst.

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 Barao M, Vicario M, Llorca J, de Leitenberg C, Dolcetti G, Trovarelli A. A comparative study of water ? gas shift reaction over gold and platinum supported on ZrO₂ and CeO₂ ? ZrO₂ Appl. Catal., B 2009, doi:10.1016/j.apcatb.2008.11.010.

3 Azzam KG, Babich IV, Seshan K, Lefferts L. 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. J. Catal. 2007;251:163?171.

4 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.