| The utilization of CO2 and its conversion to value-added chemicals are highly desirable to alleviate the environmental concerns caused by the massive anthropogenic CO2 emission.It also can develop renewable energy and make carbon-neutral come true.Although In2O3/Pd have been employed as efficient catalysts for hydrogenation of CO2 to methanol,the electronic effects by strong metal-support interaction(SMSI)between Pd and In2O3 are poorly understood,which is greatly affected by the morphology of In2O3.Herein,we fabricated a series of Pd-In2O3 catalysts with different structures derived from MIL-68(In)composite materials and combine theoretical calculations and experiments to explore the structure-performance relationship,as well as valuable exploration on the understanding reaction mechanism.Firstly,we use MIL-68(In)nanorod as a morphological template for the synthesis of h-In2O3/Pd.Interestingly,loading Pd on h-In2O3 showed a much higher performance than Iri2O3 with other morphologies,which exhibited almost unchanged CO2 conversion of 10.5%,methanol selectively of 72.4%,and methanol space-time yield of 0.53 gMeOH·h-1·gcat-1 over 100 h on stream at 3 MPa and 295℃.After in-depth characterizations,we found that the different electronic properties of Pd species on In2O3 can be finely tuned by diverse synthetic conditions,which were responsible for high activity and stability.The molar fraction of Pd2+species in the h-In2O3/Pd catalyst reached 67.6%,3.2 times that of the In2O3@Pd catalyst(21.3%),due to the different surface chemistry of In2O3.Density function theory results indicated that the Pd donated more electrons to the curved In2O3(222)surface than the pristine surface,and Pd2+was critical to facilitate H2 adsorption and formation of the surface oxygen vacancy.Combining with in-situ DRIFTS find out the reaction mechanism follow the formate pathway.This work demonstrates that controlling the morphology of In2O3 can modify both the Pd electronic property and SMSI between Pd and In2O3,which are the origins of the high catalytic performance.Besides,Pd/In2O3 catalysts are highly susceptible to the formation of In-Pd bimetallic leads to the deactivation,which is mainly relied on the reduction degree of In2O3.In this work,we elucidate the reduction behavior by using Pd-TCPP@MIL-68(In)as precursors for the synthesis of In2O3-supported Pd catalyst.Pd was immobilizing within MIL-68(In)by competitive coordination between metalloporphyrin(Pd)and organic linker.Confinement of MIL-68(In)enhanced the dispersion of Pd and prevent from excessive reduction to InPd alloys,able to reach a maximum methanol production rate of 0.43 gMeOH·h-1·gcat-1 with selectivity as high as 81%over 50 h on stream.In contrast,the IWI-Pd-In2O3 catalyst showed poor activity due to the formation of InPd/In2O3-x surface structure and deactivation fast.The surface structural investigation reveals that a relationship between the structure of In2O3 and reduction degree,the partially reduced formed Pd0/ln2O3-x surface structure is responsible for highly selective and stability.This work provides a deactivation mechanism of the Pd/In2O3 catalyst and gives a new way to optimized catalytic performance. |
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