(763c) The Role of the Support for Pt Catalysts during the Water-Gas Shift Reaction

The
Role of the Support for Pt Catalysts during the Water-Gas Shift Reaction

Viktor
J. Cybulskis,
Yanran Cui, Mayank Shekhar,
Juan J. Lov—n Quintana, W. Nicholas Delgass, Fabio H.
Ribeiro ^*

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

Because of its relative simplicity, the water-gas
shift (WGS) reaction is
an ideal system for studying catalytic chemistry on supported noble metal
surfaces at the molecular level. There is a general consensus that the most
active WGS catalysts are bi-functional in nature with contributions from both
the dispersed metal and as well as the support in activating CO and H₂O,
respectively [1, 2]. On supported Pt catalysts, the WGS turnover
frequency (TOF) per surface metal has been shown to be independent of the Pt
particle size [3, 4], but can vary by a factor of 100 depending upon the reducibility
of the support as shown in Figure 1. For this study, we combined operando infrared spectroscopy (IR) and
transient kinetics with CO/¹³CO and H₂O/D₂O
isotope switches to examine the carbon and hydrogen reactive pools on Pt/Al₂O₃,
Pt/ZrO₂, Pt/CeO₂, and Na-Pt/Al₂O₃
while correlating catalyst performance with surface features related to CO and
H₂O activation.

The presence of a
hydrogen/deuterium (H/D) kinetic isotope effect (KIE) along with time-resolved
IR spectra of transient surface species, reveals that O-H bond breaking is
involved in the rate-determining step on alkali-free Pt catalysts. The addition
of Na to these Pt-containing materials leads to the formation of new H₂O
activation sites that modify the rate-determining O-H dissociation step such
that nearly all of the CO adsorbed on the Pt surface is able to participate in
the reaction. The implications of these findings on the elementary kinetic
steps for WGS, the number of true catalytic sites, and the variation in WGS TOF
on supported Pt catalysts will be discussed.

Figure 1.
Comparison of WGS TOF per surface Pt at 300 ¡C (22% H₂O, 7% CO, 37%
H₂, 9% CO₂) for various supports.

REFERENCES

[1]   C. M. Kalamaras, P. Panagiotopoulou, D. I. Kondarides,
and A. M. Efstathiou, J. Catal.
264 (2009) 117.

[2]   G. Jacobs, P. M. Patterson, U. M.
Graham, D. E. Sparks, and B. H. Davis, Appl. Catal.
A: Gen. 269 (2004) 63.

[3]   P. Panagiotopoulou,
and D. I. Kondarides, Catal.
Today 112 (2006) 49.

[4]   M. Shekhar,
J. Wang, W.-S. Lee, W. D. Williams, S.M. Kim, E. A. Stach,
J. T. Miller, W. N. Delgass, and F. H. Ribeiro, J. Am. Chem. Soc. 134 (2012)
4700.