(704f) Water As Poison for H2 Activation Sites at Au/TiO2 Interface: Implications for Prox of H2 in Water-Gas Shift Streams | AIChE

(704f) Water As Poison for H2 Activation Sites at Au/TiO2 Interface: Implications for Prox of H2 in Water-Gas Shift Streams


Whittaker, T., Trinity University
Chandler, B. D., Trinity University
Grabow, L., University of Houston

Steam reforming is the primary
route for production of hydrogen (H2). Hydrogen is industrially used
for hydrotreating, the production of ammonia, and is considered a potential
clean transportation fuel for the future. A two stage water-gas shift reactor
is used to enrich synthesis gas mixtures with H2 and eliminate CO,
but the effluent hydrogen stream still contains 1% CO. If pure H2 is
needed, for example in fuel cells with platinum electrodes, the CO content must
be further lowered to 0.2 ppm to prevent CO poisoning.(1) The
problem is currently addressed by CO methanation, but this process consumes
5-15% of the generated hydrogen. A more attractive and energy efficient
solution is the preferential oxidation (PROX) of CO with O2 in
hydrogen rich streams. Saavedra et al. showed that water enhances CO oxidation
activity at low temperatures through activation of O2 to OOH*
on Au/TiO2.(2) Under the right conditions, mixtures of H2,
O2, and H2O may form reaction intermediates that mimic
those derived from H2O2, a potent and green chemical
oxidant. This type of chemistry is dominantly reported for Au/TiO2
catalysts at low temperatures, and the interplay between active sites on the
metal and the oxide support plays a crucial role in reported mechanisms for CO
oxidation,3 hydrocarbon oxidation4 and the water-gas
shift reaction.5 While the role of water on CO oxidation is well
studied,(2,3) there has not been much research on the effects of
water on H2 oxidation.

Herein, we elucidate
the role of water in H2 oxidation on Au/TiO2 employing
both experiments and density functional theory(DFT) calculations. The
experimentally calculated reaction orders shown in Table 1 reveal that H2
oxidation is limited by activation of H2 owing to its higher
reaction order compared to O2 and H2O. H2
activation has been studied using DFT on all the possible active sites on gold
and Au/TiO2 interface. The reaction energies and activation barriers
calculated using DFT presented in Table 2 indicate the interface sites of gold
nanoparticles and hydroxyl groups bound to coordinatively unsaturated (cus) Ti
atoms on TiO2 to be most active towards H2 activation.
The role of bridging and cus-hydroxyl groups on TiO2(110) towards H2
activation is also examined in the limit of low water pressures. The higher
activity of the interface sites compared to gold sites is explained by a Bader charge
analysis of the transition state, which discloses the heterolytic nature of H2
activation at the interface compared to homolytic activation on gold sites. The
charge density plots in Figure 1 show the basic nature of cus-hydroxyl groups
on TiO2 while the hydrogen atom binding to gold having a negative
charge indicative of a hydride. We believe this stabilizing effect of the
interface sites results in an exothermic H2 activation on the
cus-hydroxyl groups at Au/TiO2 interface and thus a lower barrier.

Table 1. Summary of H2 and CO oxidation reaction


PH2O (Torr.)

H2 Oxidation

CO Oxidation







0.60 ± 0.06

0.90 ± 0.10




0.70 ± 0.08

0.92 ± 0.07


0.66 ± 0.06

0.74 ± 0.05



0.21 ± 0.01

0.37 ± 0.09




0.12 ± 0.01

0.29 ± 0.08


0.28 ± 0.07

0.40 ± 0.10



-0.64 ± 0.02

-0.70 ± 0.03



-1.41± 0.06

-1.5 ± 0.2



Table 2.  Reaction
energies (DE) and activation energies (Ea)
for H2 activation on various sites on Au/TiO2.

Activation Site

DE (eV)

Ea (eV)













Figure 1. Charge density
difference plots for the transition state of H2 dissociation

We have also
observed an increase of 30 kJ/mol in the barrier for H2 activation
across the Au/TiO2 interface in the presence of a layer of water
above the active interface sites on hydroxylated TiO2 surface. Even though
the nature of H2 dissociation is still heterolytic in the presence
of an extra water molecule at the interface, the unstable H3O+ type
transition state results in a higher barrier. We support this theoretical
finding with the experimentally calculated negative reaction orders of water
for H2 oxidation. It must be noticed that water has a positive
reaction order towards CO oxidation as advertised earlier. Thus, water while
enhancing CO oxidation through O2 activation, also poison the active
Au/TiO2 interface sites for H2 oxidation making it a
perfect co-catalyst for PrOx of H2 in H2/CO mixtures. The
findings of this study explain the underlying reasons for the activity of
(Au/TiO2 + water) system towards PrOx of H2 in water-gas
shift streams from both theoretical and experimental standpoints.

Keywords: Heterolytic
activation, Metal-support interface, Reaction orders


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St-Pierre.; Electrochim. Acta. 55 (2010) 4208-4211

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Tran, H. V.; Doan, H. A.; Chandler, B. D.; Grabow, L. C.; Curr.
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Sakurai, H.; Ueda, A.; Kobayashi, T.; Haruta, M.; Chem.
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(6)  Saavedra,
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