(658c) In Situ XAFS and DRIFTS of Palladium Zinc Water-Gas Shift Catalysts

Authors: 
Williams, W. D., Purdue University
Ratts, J. L., Purdue University
Bollmann, L., Purdue University
Miller, J. T., Argonne National Laboratory
Delgass, W. N., Purdue University
Ribeiro, F. H., Purdue University


To design a water-gas shift catalyst with the robust nature of palladium and the high TOR of copper it is necessary to gain insight into the important characteristics of WGS catalysts. In situ XAFS and DRIFTS experiments have been performed on several WGS catalyst formulations to examine the effect of reactant and product gases on the catalysts. In a previous paper about the effects of zinc on alumina supported palladium catalysts, it was shown that zinc addition could increase the TOR by as much as 20 times under the conditions studied (280°C and 6.8% CO, 11% H2O, 8.5% CO2, 37.3% H2, balance Ar). As reported previously, reduction at 300°C led to an alloy phase for the zinc containing catalysts as evidenced from XAFS experiments. The presence of these alloys was observed to correlate with changes in the linear to bridge ratio of adsorbed CO on the palladium as well as changes in the WGS TOR for the catalysts. In situ XAFS experiments of several PdZn catalysts were performed after reduction at 300oC. The analysis indicates that there is no significant change for the bulk composition of the PdZn particles when 6.4% CO, 6.4% CO with 11% H2O, or 6.4% CO with 11% H2O and 37% H2 are added to an inert feed gas. However, the DRIFTS results show that there is a significant change in the linear to bridge ratio of adsorbed CO with changes in the gas feed. For the 2 wt% Pd, 10 wt% Zn / Al2O3 catalyst, the addition of 37% H2 to the feed gas mixture of 6.8% CO and 11% H2O resulted in an increase of the steady state linear to bridge ratio from 40% linearly bonded CO to 70%. For the same catalyst, increases in the water concentration from 0 to 11% resulted in a decrease of the linearly bonded CO from approximately 80 to 70%. The kinetic analysis on these catalysts show that water had a positive reaction order and hydrogen had a negative reaction order. Consequently, these findings show that although the linear to bridge ratio is an indicator of PdZn coordination it does not correlate well with changes in the WGS rate. In addition, the changes in the bonding of CO to the palladium are most likely the result of surface changes since the bulk PdZn coordination does not change significantly with gas concentration.

Reference:

1. L. Bollmann, J.L. Ratts, A.M. Joshi, W.D. Williams, J. Pazmino, Y.V. Joshi, J.T. Miller, A.J. Kropf, W.N. Delgass, F.H. Ribeiro, J. Catal. 257 (2008) 43-54.