Dynamic Chemical Processes in Catalysis

Abstract

In conventional thinking, catalytic activity of metals is tacitly considered as an intrinsic property of metals such that Pt is active, but Au is inactive. On the other hand, we know experimentally and theoretically that the adsorption bond of molecules on metals takes perturbation of additives. Based on them, the role of promoting materials is so often explained by the term “synergetic effect” or “activation of metals,” but this is awkward explanation as proved in “selective oxidation of CO in H2 improved by H2O” mentioned in Sect. 10.4. I could say that true mechanism of catalysis is difficult to deduce without clarifying the dynamics of chemical processes including the roles of active sites or active materials on the surface. One important strategy to make clear the mechanism of catalysis is how to remove unnecessary migration of intermediates on the surface during catalysis. One prominent example is the “hydrogenation of olefins” and simultaneously catalyzed isomerization of olefins. As proved in Sects. 10.1 and 10.2, the two reactions are evidently distinguished by preventing unnecessary migration of H atoms over the catalyst. Another example is the “selective oxidation of CO in H2 improved by H2O” on a Pt/CNT catalyst discussed in Sect. 10.4, on which the migration of OH– ion from promoting materials to Pt and Au particles is indispensable process. “Metathesis reaction of olefins” is another type of dynamic processes as mentioned in Sect. 10.3. That is, the metathesis reaction of α-olefins (CH2=CHR) is catalyzed by alternative reaction of olefin with two active intermediates, Mo=CH2 and Mo=CHR. However, the hidden metathesis reaction named “degenerate metathesis” proceeds on each Mo=CH2 and Mo=CHR sites. Interestingly, this degenerate metathesis of propene, CH2=CH−CH3 + CD2=CD−CD3 → CD2=CH−CH3 + CH2=CD−CD3, on the Mo=CH–CH3 sites takes place 100–900 times more frequently compared to that on the Mo=CH2 sites. These practical reactions prove that the catalysis cannot be rationalized without clarifying the dynamic chemical processes on the surface.