Highly Dispersed Metal Catalysts Supported On Nitrogen-doped Carbon Nanosheets And Their Application In Electrocatalysis

Electrocatalytic technology has received fairly extensive attention in the field of energy conversion and environmental protection due to its broad application prospects,because it can not only complete the production,conversion and storage of clean energy,but also realize the control and treatment of water pollution.However,the electrocatalytic efficiency depends largely on the performance of the catalyst.Hence,how to select appropriate electrode material and design specific morphology and structure to optimize its performance and stability in corresponding application is the current focus of electrocatalytic researches.To enhance the mass transfer and energy conversion of electrochemical reactions at the micro-interface,compared with the regulation of macro-scale effect,the finely design and construction of the electrode materials at the micro-nano scale is an important research field of electrode catalysts.Therefore,the current work focus on the design and functional structural optimization of electrode materials from the microscopic geometric scale,the electronic structure and chemical environment of the active metal were optimized through reducing the particle size of the metal and constructing an active interface between the metal and support.As the consequence,three highly dispersed supported catalysts were obtained:CoO_x@Co-NC with dual active centers,Pd-CeO_2-x-NC with high oxidation state Pdⁿ⁺,and Bi-CeO_2-x-NC that have a possibility to promote the production and utilization of active atomic hydrogen（H*）,where NC represents nitrogen-doped carbon nanosheets.The obtained catalysts were applied to electrocatalytic oxygen evolution reaction,hydrogen evolution reaction and electrocatalytic dechlorination of 4-Chlorophenol（4-CP）and Trichloroacetic acid（TCAA）,respectively.Through the analysis of the catalytic reaction process,the correlation between the catalytic activity and the physicochemical properties of the catalyst was explored.Hereby,the main contents and achievements are listed as follows:1.Using readily available urea,oxalic acid and cobalt nitrate as precursors,a bottom-up ligand-mediated strategy was employed to prepare ultrafine CoO_xnanoclusters anchored on a Co-N₄-containing carbon matrix,which was donated as CoO_x@Co-NC.The co-existence of N and O atoms prevented Co atoms from agglomerating into large particles and allowing the formation of ultrafine dispersed Co species with large Co loading（up to 20 wt.%）.Since the relationship between ultrasmall size and large metal loading was well balanced,the CoO_xnanoclusters had no inhibitory effect,but it facilitated the catalytic performance of Co-N₄sites during OER process.Consequently,due to the synergistic effect of ultrafine CoO_xnanoclusters and Co-N₄macrocycles,the synthesized CoO_x@Co-NC exhibited a promising OER activity(η₁₀=370 m V,Tafel plot=40 m V/dec),much better than that of benchmark RuO₂(η₁₀=411 m V,Tafel plot=72 m V/dec).This ligand-mediated strategy to synthesize metal nanocluster materials containing dual active centers with large metal loading is a promising manner for electrocatalytic applications towards developing active and stable catalysts.2.The Pd-CeO_2-x-NC hybrid catalyst with highly dispersed metal species was prepared by one-step thermal polymerization and photodeposition using urea,terephthalaldehyde and cerium nitrate hexahydrate as raw precursors.The CeO_2-xnanoclusters were first dispersed on the nitrogen-doped carbon nanosheets.Further,the active Pd sub-nanoclusters were accurately scattered on the surface of CeO_2-x,ascribing to the strong metal-support interaction（SMSI）between Pd and CeO_2-x.The electronic structure of Pd was optimized to be beneficial to promote the catalytic activity due to the SMSI effect.The high oxidation state of Pdⁿ⁺species were formed due to the electron transfer from Pd to CeO_2-xcaused by the SMSI effect.The acquired overpotential and Tafel slope of Pd-CeO_2-x-NC in the alkaline HER was only115 m V and 58 m V/dec,respectively,showing comparable HER performance to 20wt.%Pt/C.In particular,the HER performance of Pd-CeO_2-x-NC was positively correlated with the ratio of Pdⁿ⁺to some extent,suggesting that Pdⁿ⁺acted as the dominant active species.Besides,the SMSI effect stabilized the valence state of active Pdⁿ⁺species and prevented the sub-nanometer Pd clusters from aggregation.Depending on these properties,it can play a vital role for enhancing the stability of the hybrid catalyst.Noteworthily,Pd-CeO_2-x-NC also exhibited enhanced dechlorination performance in the electrocatalytic dechlorination of 4-CP,which was attributed to the optimized electronic structure and excellent H*production ability of Pd-CeO_2-x-NC.Under the optimized reaction conditions,4-CP could be completely dechlorinated by Pd-CeO_2-x-NC within 4 hours.After 5 times of continuous operation,the dechlorination efficiency of 4-CP could still reach 82.3%.Pd-CeO_2-x-NC showed good catalytic stability.The results of quenching experiments showed that H*was not the main active species during the dechlorination process of 4-CP.However,H*had an important effect on the dechlorination rate of 4-CP,because the participation of H*greatly improved the dechlorination rate of 4-CP.3.Bi-CeO_2-x-NC was prepared by loading Bi on the surface of CeO_2-x-NC nanosheets following the above strategy,and subsequently applied to the dechlorination of TCAA.The results showed that the metal-support interface between Bi and CeO_2-xwas successfully constructed,which promoted the formation of Bi⁵⁺,as well as increased the ratio of Ce³⁺in CeO_2-x.The induced electron-rich Ce³⁺served as the active centers to promote the production of H*and to make H*become effective utilizable before annihilation,thereby greatly improving the dechlorination of TCAA.The kinetic constant of TCAA electroreduction by Bi-CeO_2-x-NC nanosheets was 1.15h^-1and the increment was about 10 times higher than that of CeO_2-x-NC nanosheets.More importantly,compared with CeO_2-x-NC nanosheets,the final dechlorination product of Bi-CeO_2-x-NC was mainly non-toxic acetic acid.Loading active metals on the surface of CeO_2-x-NC nanosheets by thermal polymerization and photodeposition can not only improve the dispersion of active metals on the support,but also enhance the interaction between them.The SMSI between them promotes the transfer of electrons to CeO_2-x,which increases the proportion of high oxidation state M^δ+and Ce³⁺in the catalyst,thus improving the electrocatalytic activity of the catalyst.