Design And Performance Evaluation Of Au Catalysts For Chemical Hydrogen Storage Based On Formic Acid

Owing to the atom-economy,the formic acid-based hydrogen storage system has attracted much interest.While the high thermodynamic stability of CO2 makes its transformation under mild conditions still a challenge.In addition,the C-H dissociation during formic acid dehydrogenation is also a key factor to the efficiency of the system.The present dissertation aims to explore the new methods and procedures for reversible chemical energy storage using heterogeneous Au catalysts under mild conditions,with a focus on the fundamental understanding the C-H dissociation and CO2 activation,as well as the preparation of gold sub-nanoclusters.The detailed results were as follows:(1)A Schiff base functionalized Au nanocatalyst derived from aldimine condensation of(3-aminopropyl)triethoxysilane with HCHO was shown to have excellent activity for pure formic acid(FA)dehydrogenation without any additives.The Schiff base for efficiently deprotonating FA to metal-formate intermediates and for promoting the cleavage of C-H with electronegative gold NPs was suggested due to its unusual catalytic performance in FA dehydrogenation.The record turnover frequency(TOF)was as high as 2882 h-1 in 99%formic acid at a mild temperature of 50 0C.(2)We have delineated a non-bicarbonate route for the direct catalytic hydrogenation of CO2 to formate using a Schiff-base-modified gold catalyst,which is comparable to the fastest known nanocatalysts,with a turnover number of up to 14,470 over 12 h at 90 ℃.Theoretical calculations and in situ DRIFTS results demonstrated that the CO2 molecule is activated by the formation of a weak carbamate zwitterionic intermediate at the gold/Schiff base interface,which plays a crucial role on the subsequent transformation to formate.In contrast,primary amines will protonate the carbamate zwitterion to yield an ionic carbamate,which is less favorable for further hydrogenation.Notably,the electron-rich gold surface,caused by the electron donation from nitrogen groups,might also be beneficial for the hydrogenation,since it could offer a more negative hydride for the nucleophilic attack to the carbon center of CO2.(3)We developed an effective solid reduction method for the preparation of ultrafine Au catalyst.By solid grinding of pre-adsorbed gold precursors with NaBH4,subnanometer-sized Au clusters and isolated Au atoms can be facilely obtained on the SiO2-Schiff by tuning the gold loading.In addition,a strong size effect of Au NPs on their catalytic activity in the hydrogenation of CO2 to formate was revealed,in which subnanometer-sized clusters exhibited the best activity.The size effect was largely attributed to the number of interface Au atoms,as well as the size-dependent electronic structure of the Au species.