Adsorption of Multiple H2 Molecules on Pd3 and Pd4 Clusters. A Density Functional Study

Abstract

The reaction/adsorption of multiple H2 molecules on Pdn clusters with n = 3 and 4 were studied using the density functional theory. It has been shown that the activation of the first H2 molecule by Pd3 and Pd4 takes place without or with a small energetic barrier (based on ΔH (298.15 K)) and leads to the formation of Pd3(H)2 and Pd4(H)2 complexes, respectively, where the H−H bond is broken. For the Pd4(H)2 complex, various isomeric structures were found in the singlet and triplet states, among which the singlet Pd4_1_c_(e,e‘), with the two H ligands bridging the Pd−Pd edges not sharing the Pd atom, is found to be the most favorable. Unlike the first H2 addition reaction, dissociative adsorption of the second H2 molecule, reactions Pd3(H)2 + H2 → Pd3(H)4 and Pd4(H)2 + H2 → Pd4(H)4, appeared to be thermodynamically and kinetically unfavorable. Instead, molecular adsorption of additional mH2 molecules onto the “naked” Pd centers of the Pd3(H)2 and Pd4(H)2 complexes was shown to be feasible, which is in good agreement with the experimental Pdn + D2 saturation studies. The thermodynamic stabilities of the resulting Pd3(H)2(H2)m (m = 1−3) and Pd4(H)2(H2)m (m = 1−4) species were discussed in terms of the ΔH and ΔG values estimated at T = 298.15 and 70 K.