Catalysts working by self-activation Review Article

The idea of self-activation for the working catalyst, formation of active sites or active compounds during catalysis or in pretreatment, is proposed. Isomerization and hydrogenation reactions of olefin on MoS2 are catalyzed by the formation of monohydride and dihydride sites in the presence of H2. Similarly, olefin metathesis reaction on MoOx is catalyzed by the formation of MoCHR sites form adsorbed olefins. When MoCHR sites are not formed on the surface from adsorbed olefins, no metathesis reaction will be catalyzed on it. However, If MoCHR sites are provided on this inactive surface by other methods, the surface will be an active catalyst. It was demonstrated that an inert MoOx film sublimated on quartz changes to a super active catalyst by preparing MoCHR sites on it. On an activated MoOx catalyst, a total mechanism of propene metathesis reaction was established by elucidating the conformation of alkyl substituted metalla-cyclobutane intermediates. Pt/Rh bimetallic catalyst for the reaction of NO + H2 is also activated during the reaction. This reaction is highly structure sensitive on Pt and Rh single crystal surfaces, Pt(1 0 0) ⪢ Pt(1 1 0) and Rh(1 1 0) > Rh(1 0 0). The catalytic activity of these single crystal surfaces is enhanced dramatically by making bimetal surfaces, and the reaction changes to being structure insensitive on these bimetallic surfaces. Activation of these alloy and bimetallic surfaces is caused by the segregation of Rh atoms during the catalysis of NO + H2. Interestingly, the activated surfaces of the Pt–Rh(1 0 0)–O alloy, and Pt/Rh(1 0 0) and Rh/Pt(1 0 0) bimetallic surfaces give a common p(3 × 1) LEED pattern. The first scanning tunneling microscope (STM) image being discernible, Pt and Rh atoms were obtained for an activated p(3 × 1) Pt–Rh(1 0 0)–O alloy surface, which showed a specific array of Pt and Rh atoms. Formation of active sites with similar local structure is responsible for the structure insensitive catalysis on the alloy and bimetallic surfaces, and models for the activated p(3 × 1) Pt/Rh(1 0 0)–O, c(2 × 4) Pt/Rh(1 1 0)–O and c(2 × 2) Rh/Pt(1 1 0)–O surfaces were proposed.