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

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
orders

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

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

References

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

(2)  Saavedra,
J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.; Chandler, B. D.; Science.
345 (2014) 1599-1602

(3) 
Tran, H. V.; Doan, H. A.; Chandler, B. D.; Grabow, L. C.; Curr.
Opin. Chem. Eng. 13 (2016)
100-108

(4) 
Walther, G.; Jones, G.; Jensen, S.; Quaade, U. J.; Horch, S.; Catal.
Today. 142 (2009) 24-29

(5) 
Sakurai, H.; Ueda, A.; Kobayashi, T.; Haruta, M.; Chem.
Commun. 3 (1997) 271-272

(6)  Saavedra,
J.; Whittaker, T.; Chen, Z.; Pursell, C. J.; Rioux, R. M.; Chandler, B. D.; Nat
Chem. 8(2016) 584-589