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

The reaction/adsorption of multiple H 2 molecules on Pd n clusters with n = 3 and 4 were studied using the density functional theory. It has been shown that the activation of the first H 2 molecule by Pd 3 and Pd 4 takes place without or with a small energetic barrier (based on Δ H (298.15 K)) and leads to the formation of Pd 3 (H) 2 and Pd 4 (H) 2 complexes, respectively, where the H−H bond is broken. For the Pd 4 (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 H 2 addition reaction, dissociative adsorption of the second H 2 molecule, reactions Pd 3 (H) 2 + H 2 → Pd 3 (H) 4 and Pd 4 (H) 2 + H 2 → Pd 4 (H) 4, appeared to be thermodynamically and kinetically unfavorable. Instead, molecular adsorption of additional mH 2 molecules onto the “naked” Pd centers of the Pd 3 (H) 2 and Pd 4 (H) 2 complexes was shown to be feasible, which is in good agreement with the experimental Pd n + D 2 saturation studies. The thermodynamic stabilities of the resulting Pd 3 (H) 2 (H 2 ) m ( m = 1−3) and Pd 4 (H) 2 (H 2 ) m ( m = 1−4) species were discussed in terms of the Δ H and Δ G values estimated at T = 298.15 and 70 K.