Theoretical study on the leaching of palladium in a CO atmosphere

We studied computationally the leaching of palladium from the ideal surface Pd(111) and its various structural defects at different coverages of CO, using density functional calculations on slab models. Accordingly, the energy required for leaching of a single Pd atom from a bare surface is quite large, at least ∼270 kJ mol^-1. In a CO atmosphere at low density, PdCO is predicted to be the leaching species; this process was calculated to require at least 225 kJ mol^-1, somewhat less than the leaching of a bare metal adatom, 268 kJ mol^-1. The energies required for either leaching process (at low CO density), yielding single Pd atoms or PdCO subcarbonyl, correlate in a linear fashion with the coordination number of the Pd center to be leached. At high CO coverage, leaching of Pd subcarbonyl species, Pd(CO) _x (x = 2, 3), was calculated to be thermodynamically favorable in several cases, providing direct theoretical evidence for the feasibility of Pd leaching in a dense CO atmosphere. In a qualitative fashion, we also explored possible leaching mechanisms, comprising one or two steps.