Electron-deficient palladium clusters in zeolites and their complexes with probe CO molecules. A density functional model cluster study

Tetrahedral and octahedral palladium clusters with an entrapped proton, [Pd4H](+) and [Pd6H](+), were considered as models of electron-deficient palladium species encaged in a zeolite matrix. Density functional studies employing a gradient-corrected exchange-correlation potential have been carried out on the bare and protonated Pd-4 and Pd-6 clusters as well as on their complexes with a CO molecule adsorbed at the three-fold hollow position. In line with the experimental data it is found that the protonation of palladium clusters leads to a reduced CO adsorption energy and an increased vibrational frequency of the adsorbed CO. The protonation energies of the clusters Pd-4 and Pd-6 were calculated to 9.4 eV and 9.9 eV, respectively. These large values are comparable to the proton affinity of such a strong base as NH3 (calc. 9.2 eV) and support the hypothesis that, as a result of the interaction of the guest metal particles with zeolitic protons, electron-deficient [PdnHx](x+) species are formed.