Yonghe Zhang ionocovalent theory applications (31)
American Huilin Institute http://www.amhuilin.com/
On the basis of the ionic-covalent theory (Zhang, 1982) [1,2], the general suitability of the Ag/SiO2 catalyst for the preferential oxidation of CO in H2-rich gases (PROX) reaction, which is used for the purification of H2 feed-gas streams from a methanol reforming for polymer electrolyte membrane (PEM) fuel cell, has been extensively investigated by Qu’s group 
“Two types of Lewis acid sites may be distinguished by their preferential bonding nature to ligands, namely, electrostatic and covalent (Zhang, 1982) [1,2]. Silver cations are considered to be covalent acids, that is to say, they form a covalent bond with ligands. In general, CO can be bonded to covalent Lewis acid sites by both σ-donation and π-back-donation. Many data indicate that the π-back-donation in the Ag+-CO complexes is negligible and the σ-donation bond is predominantly formed. Under the CO oxidation condition, the presence of oxygen species near the silver decreases the electron density on the silver, and therefore the back donation from the silver to the anti bonding π orbital of CO is decreased, resulting in the consequent insignificant increase of the CO frequency. It is suggested that the reaction kinetics and the selectivity in the PROX reaction on an Ag/SiO2 catalyst are largely determined by the steady-state CO coverage on the catalyst surface, and the CO coverage on the silver surface changes significantly with the temperature and CO pressure.”
 Y. Zhang,, Inorg Chem., 1982, 21, 3886.
 Y. Zhang,, Inorg Chem., 1982, 21, 3889.
 Z. Qu, S. Zhou, W. Wu, C. Li, and X. Bao, Catalysis Letters, 2005, 101,1-2, 21-26