(386a) The Kinetics of H2-D2 Exchange Over H2S-Poisoned Pd-Alloy Hydrogen Separation Membrane Surfaces

Authors: 
Miller, J. B., Carnegie Mellon University
O'Brien, C. P., Carnegie Mellon University
Gellman, A. J., Carnegie Mellon University


Pd's
unique ability to dissociatively adsorb hydrogen and absorb
H-atoms for transport through its bulk has led to its use as a material for
membrane separation in advanced coal gasification processes.  Pd is often alloyed with minor
components, such as Cu or Au, to improve robustness or resistance to
deactivation by exposure to S compounds. 
The kinetics of H2 dissociation over the pure component
metals is well understood. Dissociation over alloy surfaces, especially in the
presence of contaminants such as H2S, is, in contrast, poorly
understood.  In this work, H2-D2
exchange experiments, both with and without an H2S contaminant, were performed
over fixed beds of Pd, Cu and PdCu alloys. A microkinetic model was applied to experimental results to quantify
the effect of alloying and H2S exposure on the dissociative adsorption and recombinative desorption elementary steps.

In the absence of H2S, dissociation
over Cu is limited by a large adsorption activation barrier (0.54 eV). 
Dissociation barriers on Pd and Pd70Cu30, and Pd47Cu53
alloy surfaces are relatively small; in these cases the exchange reaction
is limited by desorption of HD product. The crystal structures of Pd
(β-Pd-hydride and α-Pd-hydride) and of Pd47Cu53
(body-centered-cubic and face-centered-cubic) have a significant impact on the
kinetics of H2-D2 exchange.  The desorption barriers obtained from microkinetic
analysis of H2-D2 exchange over Pd, Pd70Cu30,
and Pd47Cu53 were as follows: 0.63 eV
for β-Pd-hydride, 0.68 eV for α-Pd-hydride,
0.52 eV for Pd70Cu30, 0.67 eV for body-centered-cubic Pd47Cu53,
and 0.46 eV for face-centered-cubic Pd47Cu53.   To the best of our knowledge,
these are the first reported measurements of the activation barrier for H2
desorption from Pd70Cu30 and Pd47Cu53.

H2S
exposure induces formation of a thick sulfide scale, Pd4S, on the surface
of pure Pd, which, like Cu, is characterized by a large H2 dissociation
barrier (0.8 eV). The mode of response of alloys to H2S
exposure depends on both alloy composition and temperature—bulk sulfide
formation is kinetically limited at high Cu content/low temperature. In either
case, H2S exposure increases dissociation barriers on the alloy
surface substantially. For Pd47Cu53, which does not form
a bulk sulfide, surface poisoning by H2S causes the adsorption
barrier to increase to over 1.0 eV. In such cases dissociation
rates approach H2 fluxes measured in permeation experiments
conducted in the presence of H2S, suggesting that H2
dissociation can become the rate-limiting step in the separation sequence in an
S environment.