(209a) Water-Gas Shift Reaction on Metal Oxide Supported Palladium Catalysts

Bollmann, L., Purdue University
Ratts, J. L., Purdue University
Williams, W. D., Purdue University
Ribeiro, F. H., Purdue University
Delgass, W. N., Purdue University

We completed a kinetic
study of Pd supported on different metal oxides.  The kinetic
measurements were carried out at 280°C, 1 bar, 7% CO, 22% H2O, 8% CO2,
37% H2, and balance Ar. The gas composition represents a typical
outlet concentration of a methane steam reformer.  At these conditions the TOR
normalized by the exposed Pd surface area increased as a function of the
support (Al2O3 (2.6 x 10-3 s-1)
< SiO2 (6.6 x 10-3 s-1) < Nb2O5
(7.4 x 10-3 s-1) < Cr2O3 (7.9 x
10-3 s-1) < La2O3 (2.6 x 10-2
s-1) < TiO2 (2.7 x 10-2 s-1)
< ZrO2 (4.9 x 10-2 s-1) < CeO2
(6.7 x 10-2 s-1)).  In the case of La2O3, TiO2, ZrO2
and CeO2
, the TOR changed by less than a factor of 3 suggesting
that support reducibility did not have a strong effect on the TOR under the
conditions studied.  However, catalysts
supported on silica and alumina showed a TOR about 10 times lower than on these
four supports suggesting that alumina and silica have a deleterious
effect on the rates.  In general, the WGS
rate could be described by the power rate law expression rate ~ [CO]0.5
[H2O]0.5 [H2O] -0.5
.  With
the exception of La2O3, a decrease in electronegativity
of the metal M in the metal oxide support MxOy
corresponded to an increase in rate.  There was also a compensation effect
between the measured apparent activation energies and pre-exponential factors. 
For Pd on Al2O3
there was no dependence of particle size (4-14 nm) on the TOR.  The selectivity
towards the formation of methane (CO + 3H2 → CH4 +
H2O) was affected by two factors, namely support and particle size. 
First, at similar particle size and temperatures, supports that had high WGS
TOR showed no methanation activity under the conditions studied compared to
supports with low WGS TOR.  Second, larger particle sizes meant higher
methanation TOR.  For Pd/Al2O3 the methanation TOR
increased by a factor of 8 when the particle size grew from 4 to 14 nm.  It was
found that by tuning the particle size, it is possible to decrease (increase)
the CH4 selectivity on supports with low (high) WGS TORs.  Thus, 12
nm Pd particles on TiO2 showed methanation TOR (8.3 x 10-4
s-1) as high as Pd (14nm) on Al2O3, while 3 nm
Pd particles on SiO2 showed no methanation activity.