Mechanism And Synthesis Of Electrocatalyst For PEMFC

Fuel cells (FCs) are regarded as ideal candidates for stationary and mobile powergeneration due to their high energy conversion efficiency and environmental benefits.However, the high cost and low Pt utilization of electrocatalyst have been recentlyrecognized as the most important issues to be addressed before commercialization. Thusresearches on the effective low-Pt and non-Pt catalysts have attracted more and moreattention. In the present study, we did the following work:1) DFT study on themechanism of methanol reaction catalyzed by Pd(111);2) the behavior of H adsorption,migration and penetration on Pd(111) and the effect of absorbed H on the property ofcatalyst;3) the mechanism of oxygen reduction reaction (ORR) on monolayer platinumover substrate metal (MPt^);4) synthesis of shape-controlled bimetallic PtRunanoparticles with organometallic method.Firstly, a model reflecting the effect of different solution on methanol reactivity onPd(111) has been proposed. The DFT calculation based on the established model hasreached the following conclusions. Pd(111) does not show any activity toward methanoloxidation either in neutral or acid solution. In neutral solution, with the substitution ofone methanol molecule for one water molecule, puckered hexagonal network ofhydrogen bonds has few changes compared with the pure water system. The adsorptionenergy of methanol is28.20kJ mol-1more than that in vacuum. Such a decrease in theadsorption energy is attributed to the contribution of one hydrogen bond betweenmethanol and neighboring water molecules. In acid solution, methanol is stillmolecularly adsorbed on Pd(111) with a slight relaxation of O-C and O-H bonds. Theslight change in methanol structure suggests that methanol in acid solution is notactivated. Different from those in neutral and in acid solution, the reactivity of methanolcatalyzed by Pd(111) in alkaline solution can be categorized into three cases. Theactivation of methanol in every case is indisputable, which is symbolized by elongatedhydroxyl or methoxy formation.Secondly, the interaction of hydrogen atoms with Pd(111) surface is studied withdensity functional theory (DFT) method. Various coverages ranging from0.25to2.00monolayer (ML) are considered. Particular attention is paid to the thermodynamics andkinetics of the adsorption or absorption processes and to the structural and electronicproperties. The results show that the threefold hollow sites, face centered cubic (FCC) and hexaganol close packed (HCP) are most energetically favorable. The diffusionprocessed in the same layer is more preferential than the penetration processed in twodifferent layers. For the coverages more than1.00ML, it is predicted that the additionalhydrogen easily penetrates into subsurface. PDOS research shows that the interaction ofhydrogen atoms with Pd leads to a splitting of d electron density of the outmost Pd(111)and to a gradual decrease of d band center of surface Pd atoms with coveragesincreasing. The Pd(111) occupied by hydrogen atoms shows a little bit less activity thanthe clean Pd(111).Thirdly, DFT is used to calculate the energetics of oxygen reduction reaction (ORR)on MPt^catalysts over substrates including Ir, Pd and Au. The binding energy of bothatomic oxygen and hydroxyl radical is found to correlate well with the d band center ofsurface Pt^. The binding energy of both atomic oxygen and hydroxyl radical increasesas the d band center increases. The relationship between energy barrier of molecularoxygen dissociation and binding energy of oxygen atom on various MPt^bimetalliccatalysts meets the BEP rules. The effect of surface strain, charge transfer and PDOS onthe d band center are well studied, and it can be found that both the surface strain effectand electronic effect will affect the d band center of surface Pt^in bimetallic MPt^catalyst，while the strain effect plays the most important role for the d band center ofPt(111) over various bimetallic MPt^catalysts. Surface compressive strain whichcorresponds to expansive Pt-Pt bond and electrons delation of Pt^drives the d bandcenter of Pt^over Ir substrate downward away from the Fermi level. Surface tensionand surplus electrons of Pt^over Au substrate makes the d band center of Pt^movetowards to the Fermi level. However, Pd substrate makes the d band center of surfacemonolayer Pt of MPt^comparable with the monometallic Pt, bringing similar catalyticactivity to ORR. Their catalytic activities show the volcano relationship as theirpositions in the periodic table, and it can be deemed that the MPt^over Pd substrate willbe the most potential candidate for ORR catalysts.Finally, Ru@Pt core-shell nanoparticles and surfacre tuned T-PtRu bimetalliccatalysts are prepared through the two-step orgnometallic approach. The wide-angleX-ray scattering (WAXS), transmission electron microscope (TEM) and high resolutiontransmission electron microscope (HRTEM) are used to characterize the nanoparticles.It is found that in mild conditions, high dispersed Ru@Pt nanoparticles with a narrowsize distribution are synthesized successfully with two-step orgnometallic approach. Theaverage size diameters are around1.80nm which is about0.20nm smaller than the PtRu nanoparticles synthesized with one-step orgnometallic method. The analysisthrough inductively coupled plasma mass spectrometry (ICP-MS) and energy dispersivespectrum (EDS) further prove our results. HRTEM and the theoretical model analysisshow that the Pt atoms are insufficient to form one monolayer coverage over Ru(0001).Truncated octahedron can be observed obviously through HRTEM when the Ru:Pt ratiois1:2, and the nanoparticles own the character of HCP Ru. Meanwhile, the boundaryatoms among which the distances become smaller show the character of Pt(111). SomePt(111) islands can be determined when the Ru:Pt ratio is1:4. However, it can be foundthat Pt@Ru/PPP nanoparticles can not be synthesized through the two-steporgnometallic method with the precursor of Pt2(dba)3and Ru(COD)(COT), but the CALselective hydrogenation catalyzed by the nanoparticle shows that the surface tunedT-PtRu/PPP nanoparticles have been prepared.