Photocatalytic reactions hydrogenation, alkene

In thermocatalytic process, H2 is dissociatively adsorbed on Pd surface, and thus formed H species (as H-Pd) are incorporated into a C=C double bond. Based on the mechanism of thermocatalytic hydrogenation over Pd catalyst, the mechanism of photocatalytic hydrogenation without the use of H2 can be considered as follows (1) are reduced by photogenerated electrons on Pd surface to give active H species (probably as H-Pd) and (2) this active H species on Pd are inserted into a C=C double bond, resulting in ethylbenzene formation. The active species formed in thermocatalytic and photocatalytic reactions would be essentially same because H2 was evolved after consumption of styrene (Fig. 9.12a). It is apparent that H species photocatalytically formed on Pd are very active for hydrogenation of alkenes. 

Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-l-enes over Wilkinson s catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh ] in benzene is quite selective (i.e. was 45 times faster than the cis-aIk-2-ene [15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4 -diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. 

Applicability of the photocatalytic hydrogenation of alkenes to alkanes was investigated using various aromatic or aliphatic alkenes, and Table 9.1 shows results of the photocatalytic hydrogenation in methanolic suspensions of Pd-Ti02 particles under deaerated conditions. Hydrogenation reaction proceeded in all cases, and high yield was obtained. 

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