Surface Tuning And Catalytic Performance Study For Small-Sized Platinum-Based Nanomaterials

Platinum(Pt)-based catalyst is one of the state-of-the-art catalysts currently.However,the limited reserve in earth and costly price for Pt have led the design for highly efficient Pt-based catalyst to be severely essential.Reducing the size of catalyst is considered as the most reasonable route for enhancing utilization rate of Pt in catalyst and saving Pt resource.Noting that catalysts like single atom and cluster are easy to agglomerate into larger particles in harsh catalytic environment due to their too small sizes,then leading the stability loss.Thus,developing nano-structures with moderate small-sizes can combine the catalytic stability and enhanced Pt utilization rate,which provides opportunity for developing catalysts with outstanding performance.Besides,catalysis is occurred on catalyst surface,so the surface tuning of catalyst is very important,which can affect the surface electronic structure of catalyst and further achieve the optimization of catalytic performance.Conducting precise surface tuning over small-sized catalyst can effectively enhance catalytic performance under the premise of keeping high Pt utilization rate.The research situations on surface tuning for small-sized Pt-based nanomaterials were reviewed detailly in this thesis.And some challenges were excavated and summarized.Firstly,the precise controls over surface composition,defect and so on for small-sized catalyst need to be further studied due to the size limitation.Secondly,the unique surface structure of intermetallic compounds(IMCs)shows good prospects for catalytic application.While,the universal preparation for small-sized Pt-based IMCs is full of challenges due to the high temperature requirement in the preparation process.Third,there is a lack of effective synthesis strategies for small-sized Pt-based 2D morphologies.Based on these,this thesis aims to study the surface tuning and catalytic performance for small-sized Pt-based nanomaterials.We realized the synthesis of a series of small-sized Pt-based nanomaterials with different dimensions(0D,1D,2D),and researched their applications in electrocatalysis and thermocatalysis.Among them,0D small-sized nanoparticles were used as hydrogenation catalysts in thermocatalysis,and 1D ultrafine nanowires and 2D small-sized nanoplates were used as catalysts for hydrogen evolution reaction(HER),methanol oxidation reaction(MOR)and other reactions in electrocatalysis.(1)The combined method of wet chemical synthesis and annealing was used to prepare 0D small-sized Pt-Cadmium(Cd)nanoparticles(～4 nm)with controllable surface composition which were used as catalysts for selective hydrogenations in thermocatalysis.Firstly,small-sized Pt-Cd alloy nanoparticles(～4 nm)were synthesized by only altering Cd precursor content in wet chemical synthesis,and then loaded on carbon substrate for subsequent annealing.Precise surface composition tuning was achieved successfully by altering annealing temperature over carbon supported nanoparticles due to the difference of Cd evaporation at different temperatures.The composition tuning can alter the desorption energy of styrene on catalyst surface.Thus,one optimal catalyst of small-sized Pt-Cd nanoparticles can be obtained in phenylacetylene hydrogenation for producing styrene through the composition control.It is with high selectivity of 95.2%for styrene as with conversion of 100%.(2)In order to further reduce Pt content in small-sized Pt-based nanoparticles and improve catalytic performance,we prepared 0D small-sized Pt-based IMCs with L10 phase on the basis of synthetic method in(1),and utilized the unique surface characteristics of such IMCs to improve the catalytic performance of selective hydrogenation reaction.In the wet chemical synthesis stage,additional M precursor was also introduced except for Pt and Cd precursors only involved in(1)to prepare small-sized Pt-Cd-M nanoparticles(M=Zn,Fe,Co,etc.).After loaded on carbon substrate,Pt-Cd-M nanoparticles were annealed at 700℃.During annealing,Cd was removed at high temperature,meantime Pt and M atoms diffuse to form small-sized L10 structured PtM IMCs.The method has achieved the preparation of binary,ternary,quaternary,quinary,and senary small-sized L10 Pt based IMCs(15 kinds in total),indicating huge universality in preparation.L10 PtZn IMCs NPs with different sizes were employed to catalyze the hydrogenation of 4-nitrophenylacetylene.Due to the moderate reactant adsorption and more surficial Pt active sites,L10 PtZn IMCs with 4.2 nm size displayed the best catalytic activity and good selectivity of 4-acetylene aniline(selectivity was 95.2%as with 99%conversion).(3)Additionally,considering that 1D materials are less prone to agglomerate in electrocatalysis and with better catalytic stability compared to 0D materials,we prepared 1D small-sized Pt-Nickel(Ni)nanowires modified by Selenium(Se)vacancies,and studied their catalytic performance in alkaline HER and MOR.PtNi nanowires with rich Se vacancies(PtNi-Sev)were obtained by conducting electrochemical etching over PtNi-Se nanowires directly prepared by wet chemical synthesis.Compared with pure PtNi nanowires,Se vacancies modification can weaken H*binding energy and enhance OH*adsorption,leading the improvement of alkaline HER activity.Se vacancies modification can also reduce CO binding,which acts together with enhanced OH*adsorption to promote MOR activity.We coupled HER and MOR reactions to assemble an electrolyzer,which can drive 10 mA cm-2 current density at an extremely low operating voltage of 0.637 V,and the Faraday efficiency of hydrogen production can reach 100%.(4)Finally,considering that 2D materials not only possess the structural advantages of 1D materials,but also more easily integrate with other surface tuning strategies.Thus,2D small-sized PtCdZn nanoplates with an average thickness of only 4.9 nm were synthesized by a simple one-step wet chemical method and used in study of MOR catalyst here.Due to the synergistic effect of more surface tuning strategies like ordered surface atomic arrangement of IMCs,strain engineering,high-index facets and so on,the adsorption of CO intermediates on the surface of PtCdZn nanoplates is effectively weakened,and the toxicity resistance of PtCdZn nanoplates over CO is promoted,enabling PtCdZn to achieve high MOR activity of 2.53A mgPt-1.Meantime,due to the anisotropy of 2D structure,the catalyst also displayed good anti-catalytic agglomeration ability.Besides,the catalyst also showed significantly enhanced HER and oxygen reduction reaction activity compared with commercial Pt/C catalyst.In this thesis,we successfully prepared small-sized Pt-based nanomaterials with different dimensions(0D,1D,2D),and took surface tunings to enhance their catalytic performance in thermocatalysis(selective hydrogenation)and electrocatalysis(HER,MOR,etc.).It lays a good foundation for the design of high-efficient nanocatalysts with low Pt content to solve energy and environment issues in the future.