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

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.