L4_electrocatalysis_final

Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Electrocatalysis! Shi-Gang Sun Xiamen University !" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Contents! •! Introduction •! Basic concepts •! Electrocatalytic reactions •! Electrocatalysts #" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The important applications of electrocatalysis! •! Electrochemical energy conversion"Fuel cells, hydrogen energy economy#! •! Electrochemical synthesis (green, atomic efficiency#! •! Environment"sensor, water treatment, ozone generation, …#! •! Bioelectrochemistry ! •! Materials! •! Electroanalysis •! Electrochemical industry! •! High technology"MEMS$Lab-on-chips, …#! $" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Electrocatalysis! •! It is an interdiscipline that involves –! Electrochemistry –! Heterogene catalysis –! Surface science •! It includes! Catalysis processes under interfacial electric field Electrode kinetics Electrode reaction mechanism Electrocatalyst %" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Electrocatalysis vs. Catalysis! Electrocatalysis •! Electrocatalysts + Electron transfer! •! Heterogeneous"s|l$s| s, l|l interfaces#! •! Surface effects! •! Altering Electric filed to change the energy of system! Catalysis •! Catalysts + Chemical reactions! •! Heterogeneous (s|g#and homogeneous (solution#! •! Surface effects! •! Altering T, p to change the energy of system! &" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group A brief look at the electrode kinetics '" Electron transfer Reagent Product e-! Mass transport Adsorption/desorption Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group (" The diffusion equations The boundary conditions The current 1. The migration (Electric field action on charged species) 2. The convection (Nature and forced hydrodynamic transport) 3. The diffusion (Gradient of chemical potential) The effect of mass transport Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The effect of activation Gibbs energy The potential effects! )" Reaction coordinate! reagents products nFE !nFE "nFE Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The effects of catalyst ! *" reagents products Heterogenous catalysis reagents products nFE Electrocatalysis 1. Change the reaction route 2. Alter the activation Gibbs energy Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The fundamental of catalysts !+" The parameters affecting #G$0 and pathways •! Interaction of reagent molecules with catalyst surface –! Adsorption, desorption, dissociation, bonding, etc. •! The effects of surface structure of catalysts –! Chemical structure: Chemical composition of bulk (alloys) and surface (modification) –! Electronic structure: surface stat density, work function, etc. –! Geometric structure：Surface atomic arrangement •! The catalytic active centers（sites) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Most reactions in electrocatalysis involve much more than simple electron transfer and mass transport, particularly !! bond cleavage and/or bond formation !! adsorbed intermediates !! multiple electron transfer The rate of such reactions show a strong dependence on reaction conditions and particularly on electrode material. BUT !! such reactions still involve electron transfer – their rate will depend strongly on potential !! there is still a mass transport limitation !!" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group ADSORPTION In electrochemistry, the interaction of species from the solution phase (solvent, electrolyte ions, reactant, intermediates, products, additives etc) with the electrode surface. COVALENT BOND TYPES of INTERACTION ELECTROSTATIC ATTRACTION ions, dipoles with charged surface VAN DER WAALS FORCES “solvophobicity” FACTORS INFLUENCING ADSORPTION Electrode material, potential, solution composition, structure and concentration of adsorbate ! Electrochemistry Group Ministry of Education of the People's Republic of China! !#" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Thinking about adsorption Two competitions Between all species in solution for the limited number of sites on the electrode surface. Between the electrode surface and the solution for all species in the system. Why interest in adsorption? Electrocatalysis New reactions involving surface chemical reactions Inhibition of reactions Additives Factors influencing adsorption Electrode material, potential, solution composition, structure and concentration of adsorbate, etc !$" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Electrocatalysis of hydrogen evolution reaction (HER) The reaction 2H+ + 2e- H2, although relatively simple, it still clearly a multistep sequence and is very slow without the intervention of a catalytic pathway. Adsorbed H atoms provide such pathways, ie. Step A H+ + e- + M M-H Step B 2M-H 2M + H2 or Step C M-H + H+ + e- 2M + H2 Each step may be the rate determining step. !%" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group NOTE – reaction fastest for intermediate bond strength and the surface coverage by adsorbed H species, q. For too weak interaction (low coverage), insufficient adsorbed H to provide viable catalytic pathway. For too strong interaction, rates of steps B and C become too slow to be useful. Step A H+ + e- + M M-H Step B 2M-H 2M + H2 Rate of step 2 favoured by concerted formation of H – H bond as M – H bond is breaking. ie. geometric spacing of active sites M – M – M - M H H H H M – M – M - M M – M – M - M H2 One mechanism !&" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Hydrogen evolution reaction! !'" Catalytic route Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Two parallel routes (written for 1 M acid) O2 2H2O H2O2 2H2O O2 + H2O heterogeneous or homogeneous disproportionation 2H+ + 2e- 2H+ + 2e- 4H+ + 4e- Oxygen reduction reaction (ORR) !(" •! Formation of H2O2 in fuel cells/batteries always leads to loss in cell voltage, therefore energy efficiency. •! All catalysts poor, best Pt - h ~ 400 mV. Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group M – M – M - M M – M – M - M M – M – M - M O2 e- formation of superoxide (generally unfavourable) O O adsorption O O concerted mechanism protonation of oxygen molecule? M – M – M - M O O e- Initial steps in oxygen reduction !)" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Metal and alloy catalysts Electrocatalysis of HER and ORR !*" Trends in oxygen reduction activity plotted as a function of the oxygen binding energy. J. Phys. Chem. B, Vol. 108, No. 46, 2004 Volcano plot for the HER for various pure metals and metal overlayers Nature Materials 5, 909 - 913 (2006) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Catalytic reactions known to have quite different rates at different single crystal faces. Role for Also grain boundaries etc Role for adsorption of reactant/intermediates. Surface modification, eg. by deposition of partial/full monolayers of underpotential metals, thiols etc With dispersed catalysts, electrocatalysis also influenced by !! catalyst particle size !! substrate for catalyst adatom edge site kink site Edge vacancy Surface vacancy Role of active sites #+" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Single crystal planes #!" Well defined electrocatalysts consisting of known single crystal planes are used to gain knowledge of surface structure-function relationship, and the catalytic active centers Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group An example ##" Pt(111) Pt(331) = 3(111)-(111) = 2(111)-(110) Pt(332) = 6(111)-(111) = 5(111)-(110) Pt(110) Pt(111) Pt(331) Pt(332) Pt(110) Electrocatalytic oxidation of Ethylene Glycol (EG) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Pt(111) Pt(332) Pt(331) Pt(110) Voltammograms of the first cycle of EG oxidation on Pt(111), Pt(332), Pt(331) and Pt(110) electrodes after flame treatment. 0.5 M H2SO4 +0.2 M EG solution, v = 50 mV s -l. The order of electrocatalytic activity Pt(110) > Pt(331) > Pt(332) > Pt(111) #$" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Voltammograms of the 10th cycle of EG oxidation on Pt(111), Pt(332), Pt(331) and Pt(110) electrodes after flame treatment. Pt(111) Pt(332) Pt(331) Pt(110) The order of electrocatalytic activity Pt(331) > Pt(110) > Pt(332) > Pt(111) #%" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The results indicate 1.! The well-defined Pt(110)exhibits the highest activity, but it is not stable 2.! The Pt(331) possesses not only a high activity, but also the highest stability The (111)-(110) chair-sites #&" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group iP was measured at the 10th cycle, i.e. the stable voltammograms of EG oxidation 2(111)- (110) 3(100)- (111) 5(111)- (110) #'" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The characters of catalytic active centers •! 5-6 atoms in stereo structure •! Short range in symmetry order •! Low coordination number of surface atoms •! A large density of step atoms and dangling bonds #(" ! Electrochemistry Group Ministry of Education of the People's Republic of China! An open surface structure Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Geometry of active sites #)" (110)-Chair site (110)+(111) (100)-Chair site (100)+(111) (110)-Chair site (110)+(100) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Geometry of active surface #*" (520)=(310)+(210) (730)=(310)+2(210) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The study of model catalysts provides knowledge of catalytic property dependence with surface atomic arrangements, but single crystal planes can not be used directly in practical applications, due to： 1.! The high price of catalysts of single crystal planes； 2.! The facile reconstruction of single crystal planes under practical catalytic conditions (i.e. they are not stable)； In reality, the catalysts used in fuel cells, electrosynthesis, etc. are often made of nanoparticles that are well-dispersed on conductive substrates of low price, such as carbon materials. Model catalysts vs. Practical catalysts $+" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Tuning the properties of Pt nanoparticle catalysts •! Electronic structure – tuning the chemical composition of nanoparticles •! Surface atomic arrangement and coordination – tuning the shape of nanoparticles $!" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group The crystal growth law $#" •! The growth rate along the direction of high-index planes is much larger than that of the direction low- index planes [210] [100] Surface energy of single crystal planes (hkl) stands for high-index planes with at least one of hkl > 1 Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Cube cuboctahedron octahedron {100} {111} tetrahedron The shapes of nanocrystals (fcc) with low surface energy and low-index facets synthesized by conventional shape-control methods $$" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group $%" •! The study of well-defined Pt electrocatalysts of single crystal planes indicated that the surface structures exhibiting high activity and stability should be those of high-index planes, such as {310}, {210}, {320}, {520}, {730}, etc. •! So the Pt nanoparticles with high catalytic performances should not be those of cubes, cuboctahedra and octahedra synthesized so far by chemical methods, and should be of other complex shapes. Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Possible shapes of nanocrystals bound by high- index facets (fcc lattice) $&" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Surface structure characterization of THH Pt NCs $'" (a) (b) 1.0 nm 50 nm d=0.20 nm (c) (d) {730} {520} {310} {210} The THH Pt crystal is bound by {hk0} facets ({730} and vicinal planes） (310)+(210) (310)+2(210) Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group 50 nm The activity of THH Pt NCs is as 2-3 times higher than that of Pt/C, and the oxidation potential is negatively shifted 60 mV. The catalytic activity of the THH Pt NC’s Electrocatalytic oxidation of HCOOH $(" Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Model and practical catalysts single crystal planes vs nanoparticles’ surface structure! $)" Unit stereographic triangle of fcc single-crystal and models of surface atomic arrangement Unit stereographic triangle of polyhedral nanocrystals bounded by different crystal planes Zhou, Tian, Sun, Faraday Discuss., 2008,140:81–92! Ministry of Education of the People's Republic ! of China" ! Electrochemistry Group Further readings! •! J. Lipkowski, P.N. Ross (eds), Electrocatalysis, Wiley-VCH, New York, 1998 •! . A.Wieckowski$E. Savinova and C. G. Vayenas (eds), Catalysis and electrocatalysis at nanoparticle surfaces$!Marcel Dekker Inc., New York, 2003 •! N. Tian, Z.Y. Zhou and S.G. Sun, Platinum Metal Catalysts of High-Index Surfaces: From Single- Crystal Planes to Electrochemically Shape- Controlled Nanoparticles, J. Phys. Chem. C, Feature Article, 2008, 112 (50): 19801-19817 ! $*" ! Electrochemistry Group Ministry of Education of the People's Republic of China!