Morphology-Dependent Catalytic Surface Chemistry Of Ag Nanocrystals

Heterogeneous catalysis plays a key role in chemical industry,energy conversion and environmental protection.The catalyst efficiency is closely related to the catalyst surface composition and surface structure.Catalysts synthesized by the traditional impregnation method generally exhibit catalyst particles with broad size distributions and irregular morphologies,thus it is difficult to correlate their surface structures with their catalytic properties.Single crystal model catalysts are widely used to study the correlation between the catalytic performance and surface structure under ultra-high vacuum(UHV)conditions due to their well-defined surface structures.However,there are "materials gap"and"pressure gap"between single-crystal model catalysts studied under UHV conditions and working catalysts at atmospheric or higher pressures,which make it difficult to unambiguously correlate between the catalytic performance and the catalyst structure of working catalysts.With the development of nanotechnology,catalyst nanocrystals with uniform morphology and size,which expose specific catalytic activity sites,have been successfully synthesized and used as model catalysts for fundamental studies to correlate the catalytic performance with the catalyst surface structure under the conditions approaching working catalysts as closely as possible.Based on the above ideas,in this thesis,we have prepared ligand-free cubic and octahedral Ag nanocrystals by morphology-preserved reduction of corresponding capping ligand-free cubic and octahedral Ag2O nanocrystals and studied the correlation between their catalytic performances and surface structures in CO2 electroreduction and ethylene partial oxidation reactions.We have also prepared different shaped Ag nanoparticles by methanol vapor reduction of cubic and octahedral Ag2O nanocrystals and studied their catalytic performances in partial oxidation of methanol.The main results are summarized as the following:1.Catalytic performances of capping ligand-free cubic and octahedral Ag nanocrystals in electrochemical reduction of CO2 were studied.The cubic Ag nanocrystals are much more active than the octahedral Ag nanocrystals in CO2 electroreduction reaction and exhibit a stable CO formation rate as high as 142.4 mmolCO·gcat-1·h-1 at-0.95 V(vs.RHE).Ag films transiently formed on Ag2O nanocrystals during the CO2 electroreduction reaction were observed more active than corresponding Ag nanocrystals,suggesting that the existence of Ag2O substrate has a promoting effect on electrocatalytic activity of Ag film.These results unambiguously identify the facet sensitivity of silver nanocrystals in CO2 electroreduction reaction and highlight the importance of surface cleanness of Ag colloids as efficient electrocatalysts.2.Catalytic performances of shaped silver nanoparticles in methanol partial oxidation reaction were studied.Under the harsh reaction conditions of methanol partial oxidation,Ag nanoparticles undergo serious restructuring,but Ag nanoparticles derived from octahedral Ag2O nanocrystals were found to be more intrinsically active than those derived from cubic Ag2O nanocrystals.Ex situ characterization results demonstrated that the Ag nanoparticles derived from octahedral Ag2O nanocrystals are more facile for the formation of active oxygen species than the Ag nanoparticles derived from cubic Ag2O nanocrystals.These results demonstrate that strong correlations between original surface structure and restructured surface structure of catalyst nanoparticles even under very harsh reaction conditions and adds fundamental understandings of methanol partial oxidation over Ag catalysts.3.Catalytic performances of cubic and octahedral Ag nanocrystals in ethylene epoxidation reaction were studied.Cubic Ag nanocrystals enclosed with Ag{100}facets exhibit higher ethylene oxide selectivity than octahedral Ag nanocrystals enclosed with Ag{111}facets.In-situ DRIFTS spectra observe temperature-dependent formations of oxametallacycle intermediate on cubic Ag surface,in which molecularly chemisorbed oxygen-and atomic oxygen-involved oxametallacycle intermediates form respectively at low and high reaction temperatures.Photoillumination can enhance the catalytic activity of both cubic and octahedral Ag nanocrystals but do not affect the selectivity,due to the photon-promoted further reaction of oxametallacycle intermediates.