Nano Sized Gold Catalyst: Catalytic Combustion And Selective Hydrogenation Studies

Information gained during last couple of decades indicates nano sized Aucatalysts present unique catalytic characteristics in catalytic combustion,selective oxidation/hydrogenation and also some coupling reactions.Investigations on the modulation of the catalytic characteristics of nano sizedAu catalysts are of significant importance in both fundamental and applicationresearches of Au catalysis. In this work, catalytic combustion of in-door airpollutants （CO and HCHO） as well as chemo-selective hydrogenation ofα,β-unsaturated carbonyl compounds are employed as index reactions for oureffort to modulate the catalytic characteristics of nano size Au catalysts.In our prior work, we found that the catalytic activity of Au/ZrO₂catalystsfor CO combustion was affected by the content of cationic Au （Au3+/Au ratio）and size of “support”. In this work, we presented a comparative study on theeffects of calcinations temperature and Au loading on the CO and HCHOcombustion reaction over Au/ZrO₂catalysts. Distinct induction period wasfound in CO combustion over Au/ZrO₂catalysts with cationic Au（calcinations temperature no higher than300oC）, and the mass-specificactivity of Au decreased with the increasing Au loading. No induction periodwas observed during the CO combustion under moist condition （2000ppmH₂O） and HCHO combustion （H₂O is a must product in HCHO combustion）over Au/ZrO₂catalysts, and the mass-specific activity of Au for bothreactions was independent with the Au loading. In-situ FTIR analysis revealedthe induction period was related to the partial reduction of cationic Au tometallic Au during CO combustion reaction. The H₂O-assisted rapid reductionof cationic Au would eliminate the induction period. For both reactions, theAu/ZrO₂catalysts with varied calcinations temperatures exhibit the sameactivity order and the catalyst calcinated at200oC appears to show the highestactivity, and the catalyst without cationic Au （calcinated at450oC） showed the lowest activity, which suggested the proper content of cationic Au isnecessary for the high catalytic activity of Au/ZrO₂catalyst.In our prior work, Au/SiO2was found to be very selective forhydrocinnamaldehyde in cinnamaldehyde hydrogenation. In order to improvethe catalytic activity of Au without alternating its unique selectivitypropensity, in this work, we applied the novel Pt-on-Au nanostructures（Ptm^Au, m refers to the atomic Pt/Au ratio） in the chemo-selectivehydrogenation α,β-unsaturated carbonyl compounds （e.g. cinnamaldehyde）,and found the deposition of Pt entities dramatically enhanced the catalyticactivity of Au （up to70-fold to Au/SiO2）, without alternating the selectivitypropensities of Au for these reactions. Investigation of the effects ofsupporting material, reaction temperature, hydrogen pressure and H-source, aswell as the kinetic assessments revealed a synergy between Pt and Au in thePtm^Au catalyzed hydrogenation, in which the Pt entities functioned as theactivation site for hydrogen dissociation, and the H atoms activated on Pt sitesreacted with substrate molecules adsorbed on nearby Au sites.