Quantum Chemstry Study on the Interactions between Palladium and Alkaline Earth Cations to Explain Cationic Effects on CO Hydrogenation over M⁺⁺-Pd/SiO₂

In this paper, both of the SCC-DV-X_α （Self-Consistent-Charge-Decrete-Variation-X_α） quantum chemcal calculations and the transition metal bonding functions were used to study the interactions between palladium and alkaline earth cations (M⁺⁺), in which a model of metal-promoter-support interaction has been proposed our results are satisfactiory to characterize. Our experimental facts measured by ESR、XPS、 TPD-MS and chemical trapping methods , and to explain the cations affect CO hydrogenation over various M⁺⁺-Pd/SiO₂ catalysts.
According to the concepts of transition metal bonding functions, the interactions of Pd-M can be divided into the direct Pd 4d-M ns interaction between Pd and alkaline earth cation and indirect Pd 4d-M np interaction via the bridge atoms of O 2p and Cl 2p. Some of our experimental facts which are very difficult to be understood by the classical concepts of Pd-M interactions, were explained satisfactorily by the above concepts of bonding functions and were checked by the results of SCC-DV-X_α culculations. For example, the abundance of Pd（Ⅰ） was observed on both of Mg and Ba promoted Pd/ SiO₂ catalysts, behavhy the turnover frequency for CO hydrogenation to MeOH is one order greater on Mg-Pd/SiO₂ than that on Ba-Pd/SiO₂? Our explaination is that, Mg3s（-7,65eV） is better matching with Pd 5s （-7.52eV） than that with Ba 6s （-5.12eV） thus the direct interaction between Mg 3s and Pd 5s causes the electron of Pd 4s （the occupied electron of d band ） to be transfered to Mg 3s via the Pd 5s （vacant orbital）, forming the positive d hole of metal Pd, presenting the Pd（Ⅰ） ESR singnal producting an adsorption center of H^δ+ ;whereas the direct interaction between Ba 6s and Pd 5s, the potentioal energy is not favourable for the electon transfer from Pd 5s to Ba 6s but is favourable for the electron transfer Ba 6s to Pd 5s if the vacant Ba 6s can obtain electron from the indirect interaction between Pd 4d and Ba 5p via O 2p and Cl 2p by election -electron-perturbations. Because O 2p （13.6eV） and Cl 2p（13,0eV） are better matching with Ba 5p （-16eV） than that with Mg 2p（-54eV）, thus the occupied electron of Pd d-band is more easily to be transfered from Pd 4d to Pd 5s vis O 2^p-Cl 2^p-Ba 5^p-Ba 6s path forming the inner d hole, presenting the Pd（Ⅰ） ESR signal, but it is unfavourable to form H^δ+ due to the present of pd 5s electron the controlling step of CO hydrogenation to MeOH is the hydrogenolysis and hydrogenation of HCOO by H^δ+, thus the turnover frequency of CO hydrogenation to MeOH is one order greater over Mg⁺⁺-Pd/SiO₂ than on Ba⁺⁺-Pd/SiO₂.