  Hydrogen production as a clean, sustainable replacement for fossil fuels is
gathering pace. Doubling the capacity of Paris-CDG airport has been halted,
even with the upcoming Olympic Games, until hydrogen powered planes can be
used. It is thus timely to work on catalytic selective hydrogen production and
optimise catalyst structure. Over 90 % of all chemical manufacture uses a solid
catalyst. This work describes adsorption of carbon-monoxide (CO) on platinum
thin films, supported by cheap Al(111). CO reacts with water to produce
hydrogen (water-gas shift). Quantum Monte Carlo methods are the only ones
accurate enough to investigate the early steps of this catalysed reaction at
close-packed Pt/Al(111).
  Many chemical reactions involve bond-dissociation. This process is often the
key to rate-limiting reaction steps at solid surfaces. Since bond-breaking is
poorly described by Hartree-Fock and DFT methods, our embedded active site
approach is used This work demonstrates a novel Quantum Monte Carlo (QMC)
methodology.
  The water-gas shift reaction step studied is water addition to CO
pre-adsorbed on a Pt-monolayer supported by Al(111). The water molecule is only
partially dissociated. Its oxygen atom binds to CO giving adsorbed COOH and
Pt-H. This concerted addition is rate-limiting. In subsequent steps, the
adsorbed formate species (with acidic hydrogen) decomposes to carbon dioxide
and, after proton migration to Pt-H, the clean product H$_2$ is obtained.
  The QMC activation barrier found is 64.8 $\pm$ 1.5 kJ/mol. Thus, QMC is shown
to be encouraging for investigating similar catalytic systems.
