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, [Pd 4 H] + and [Pd 6 H] + , 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 NH 3 (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 [Pd n H x ] x + species are formed.