(307a) Study of Water-Gas Shift (Wgs) and Methanation Reaction on Supported Group Viiib Catalysts

Study of water?gas shift (WGS) and methanation
reaction on supported group VIIIB catalysts

A. A. Phatak, K. M. Lee, P. Figaro, S. Rai, G. Zhu,  K. T.
Thomson, W.N. Delgass and F. H. Ribeiro^*

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA.

^*Corresponding author (fabio@purdue.edu)

Our research is directed to developing fundamental
understanding of the water?gas shift (WGS) process for the production of
hydrogen for fuel cell applications. Hydrogen (H₂) with low CO
concentration is necessary for efficient and long?term performance of proton
exchange membrane fuel cells (PEMFCs). The conventional process for hydrogen
production involves fuel reforming followed by hydrogen enrichment using the
WGS reaction. The industrial Cu?based low temperature WGS (LT?WGS) catalyst has
an adequate rate per unit of volume but is unstable under frequent shutdown and
startup situations. Thus, an alternative robust and active catalyst for WGS is
an important challenge in generating hydrogen for fuel cell applications.

Our approach involves a systematic study to determine the
rates, reaction orders and activation energies for WGS and methanation reaction
on the various supported Pt group catalysts studied under realistic fuel
processing conditions. Previous literature results show that Pt and Pd catalysts
supported on various oxides are stable for WGS. We observed, as have several
other groups, that the turnover rate (TOR) on a Pt catalyst was 40 times higher
when supported on ceria (CeO₂) than on alumina (Al₂O₃).
There are at least two issues on these catalysts: low rate of reaction and
reaction selectivity.  Because of the large concentration of H₂
present during WGS, hydrogen-consuming side reactions such as methanation need
to be suppressed. For alumina supported catalysts, WGS rate per gram decreases
as Pt >> Pd, Ni and methanation rate per gram decreases as Ni > Pd
> Pt. We found that the addition of basic oxides to Pt?Al₂O₃
catalyst suppressed the methanation TOR by a factor of at least 5 and increased
the WGS TOR by a factor of ~2. Similarly, addition of basic oxides to Pd?Al₂O₃
catalyst resulted into suppression of methanation TOR by at least a factor of 2
and increased the WGS TOR by at least 8 times. We also observed that addition
of acidic oxide such as niobium (Nb) to Pd?Al₂O₃ increased
the reaction rate for WGS by 4 times and for methanation by 7 times. Thus,
group VIIIB metals show opposite trends in reaction rate per gram for WGS and
methanation. Additions of basic oxides to Pt and Pd catalysts help suppress
methanation while addition of acidic oxide increases the methanation TOR.