  Over 90 % of all chemical manufacture uses a solid catalyst. Related work
thus responds to major societal demand. This study is of water-gas shift on
platinum for hydrogen production. The close-packed Pt(111) surface catalyses
this process.
  Many chemical reactions involve bond-dissociation. This is also true for
reactions at solid surfaces, in which the dissociation step is often limiting
but facilitated in comparison to gas phase reaction channels. Since
bond-breaking is poorly described by Hartree-Fock and DFT methods, this work
adopts Quantum Monte Carlo (QMC) methodology. QMC is a stochastic approach to
solving the Schr\"odinger equation recently came of age for heterogeneous
systems involving solids.
  The present work considers co-adsorption of water and carbon monoxide on
Pt(111). The water is partially dissociated while its oxygen atom binds to CO
losing a hydrogen atom. This concerted step is rate-limiting. The resulting
adsorbed formate species then decomposes to readily eliminated carbon dioxide
and the clean-fuel product is H$_2$.
  The Transition-State geometry can be optimized using molecular Quantum Monte
Carlo force constants, on the basis of our earlier work using the CASINO
software.
  Our embedded active site approach is used. This allows a high-level
configuration interaction (CI) wave-function to be used, expanded in
plane-waves and embedded in the metal lattice exposing its close-packed face.
The resulting periodic function is used to guide the Quantum Monte Carlo
calculation.
  Results are given here on mechanism and QMC activation barrier for water
addition to CO pre-adsorbed on Pt(111) of 17 +/- 0.2 c.f. apparent measured
value of 17.05 kcal/mol. They are encouraging for investigating similar or
complex catalytic systems.
