Preparation And Performance Of Pd-based Catalysts Towards Electro-oxidation Of C2 Alcohols In Alkaline Media

Owing to high theoretical energy density,low toxicity and renewability from biomass-based resources,C2 alcohols?including ethanol and ethylene glycol?have great potential applications in electrochemical energy storage and conversion.Unfortunately,in achieving the energy conversion via the direct C2alcohol fuel cells,the kinetics for the relevant C2 alcohol electrooxidation is slow,and the incomplete oxidation processes always predominate.As a result,the specific energy and specific power densities are in fact not high enough.At the same time,the traditional Pt and Pd-based catalysts still lack sufficient activity and stability for C2 alcohol oxidation,hindering the development of C2alcohol fuel cells.Notably,C2 alcohol electrooxidation in alkaline media deserves a greater attention,given its faster reation kinetics,less corrosive attack on catalysts and supports,and far lower membrane crossover rate as compared to those in acidic media.More importantly,Pd shows an increasing activity towards C2 acohol electrooxidation with increasing pH,rendering it to be a promising catalytic metal to substitute more scarce and expensive Pt in alkaline media.Therefore,it is highly desirable to synthesize rationally new efficient Pd-based catalysts based on mechanistic understanding of C2 alcohol oxidations,in order to promote the utilization of C2 alcohols in clean energy convernsion.In this thesis work,we first compared systematically the C2 alcohol electrooxidation behaviors on Pt/C and Pd/C in alkaline media,gaining the essential knowledge regarding the advantages and disadvantages of Pt and Pd catalytic materials.Based on the known mechanisms and the catalyst design guidelines,several carbon supported Pd-based nanoalloys were prepared by aqueous phase synthetic methods,characterized by physicochemical methods and examined as the catalysts for C2 alcohol oxidations in alkaline media.The main contents are summarized as follows.·Comparison of C2 alcohol electro-oxidation on Pd/C and Pt/C in alkaline mediaBy comparing the onset oxidation potentials and the peak currents for the cyclic voltammograms under controlled conditions,we tried to address the following issues:a)electrochemical performances of ethanol and ethylene glycol oxidations?EOR and EGOR?on a same catalyst in a same solution;b)different behaviors of Pt/C and Pd/C in electrocatalyzing EOR or EGOR;c)the effect of OH^-concentration on electrocatalysis of C2 alcohol oxidation.The experimental result showing a higher susceptibility to poisoning for Pt/C in EOR was explained.·Carbon-supported Pd-B nanoalloy as the catalyst for C2 alcohol oxidation in alkaline mediaBased on C2 alcohol electrooxidation mechanism,Pd-B/C containing 6.5at.%of B was synthesized with DMAB and tested as a catalyst.The comparison of C2 alcohol electrooxidation on PdB/C and Pd/C in alkaline media indicated that Pd-B/C exhibited a lower onset oxidation potential and a higher electrocatalytic activity and stability than Pd/C did.·Carbon-supported PdNiB catalyst toward C2 alcohol electrooxidation in alkaline mediaTernary alloy catalysts incorporating non-precious metal and non-metal are expected to adjust the electronic and geometric effects of the catalyst.A well-dispersed PdNiB/C ternary alloy catalyst was synthesized with a mean size of ca.3.8 nm and containing 6.5 at.%of B toward C2 alcohol oxidation in alkaline media.Cyclic voltammograms and chronoamperometric curves indicated that the PdNiB/C catalyst presented a remarkably higher activity than Pd/C,PdB/C and PdNi/C nanocatalysts.Comprehensive XRD,XPS and other characterizations suggested that the superior electrocatalytic performance of PdNiB/C could be attributed to collaborative bifunctional,structure and electronic effects owing to the coexistentence of Ni and B.·Carbon supported Ag rich Pd-Ag bimetallic electrocatalyst for ethylene glycol oxidation in alkaline mediaIn collaboration with a group member,a series of Pd_xAg_y/C?x:y values from 2:1 to 1:4?catalysts were prepared by adding Pd?II?precursor in a preformed Ag colloidal solution,Pd₁Ag₃/C showed the best overall performance for EGOR in alkaline media.Comprehensive XRD,XPS and HR-TEM characterizations indicated that the Pd_xAg_y/C catalysts existed in the alloy form with Pd-rich surfaces and partial electron transfer between Pd and Ag.It was proposed that Ag colloids and Pd deposits were restructured to form smaller bimetallic nanoparticles due to the lattice mismatch of Pd and Ag as well as the galvanic displacement reaction.