Published on November 21, 2007
Computational Calculations on Direct Methanol Fuel Cells: Computational Calculations on Direct Methanol Fuel Cells Nicholas Dimakis University of Texas-Pan American Edinburg 2005 Fuel Cells: Fuel Cells Direct methanol Fuel Cell Why do we care ? Petroleum is a limited resource Alternative sources for fueling vehicles need to be explored Must be abundant, at least same efficiency as petroleum Methanol Oxidation on anode surface: Methanol Oxidation on anode surface In Methanol oxidation CO2 is created as CO appears as intermediate poisons the Pt surface. CO becomes CO2 using oxygen supplied by water OR CO binding energy is reduced I.e more available sites Goal: keep CO as less as possible on the anode catalyst surface. Use alloy doping (Ru/Os) in the Pt cluster to “help” attract oxygen (Pure Pt is a poor catalyst) CO-Metal Adsorption Mechanism: CO-Metal Adsorption Mechanism CO Molecular Orbitals Basic idea belongs to G. Blyholder (5 -d donation, 2π*-d back donation with metal) On a CO-Pt cluster, if Pt atoms are replaced by Ru then νCO downshifted 2π*-s,d and 5-s,d are enhanced How ? G. Blyholder, J. Phys. Chem. 68(10), 2772 (1964) 1π 5σ 2π* Hybrid CO-Pt orbitals: Hybrid CO-Pt orbitals 5-d Pt13CO 5-d Pt26CO 2π*-s,p Pt13CO 2π*-s,p Pt26CO If Blyholder mechanism works...: If Blyholder mechanism works... Partially populating the antibonding 2π* CO MO Increase on C-O distance (decrease on CO) Leads to.. Bonding C-Pt is enhanced Decrease on C-Pt distance Causes... Increase on CO binding energy (M.T.M Koper, et al. J. Phys. Chem. B, 106, 686 (2002)) Increased d-band vacancies upon alloying !. A Paradox ? : Increased d-band vacancies upon alloying !. A Paradox ? Experimental XANES shows that average d-band vacancies increases upon alloying Then how is it possible to have more 2π*-d back donation ? Is Blyholder mechanism wrong ? Analyze CO-Pt/alloy using DFT and FEFF8: Analyze CO-Pt/alloy using DFT and FEFF8 DFT (Jaguar-B3LYP) FEFF8 1.Build DFT CO-Pt, CO-Pt-Ru spin-optimized clusters ((100)- face) 2. Determine DFT optimal cluster 3. Calculate Mulliken electron population & analyze CO MO shifts with cluster size and alloying 1. Confirm experimental XANES 2. Calculate LDOS & electron population A.L Ankudinov et al. Phys. Rev. B58, 7565 (1998) Pt Cluster-Size Effect: Pt Cluster-Size Effect Optimal DFT Cluster CO-Pt26 Optimal Slide11: So what DFT Pt-Cluster-size analysis tell us ? 1. CO vary inversely proportional to C-O & C-Pt changes. 2. Spin Multiplicity (2S+1) is increased with cluster-size. 2. Reduction of CO when Pt is alloyed with Ru confirms FTIR spectra. 3. CO binding energy is reduced upon alloying. Optimal Mulliken Electron Population Analysis: Mulliken Electron Population Analysis Upon Alloying ... d 5-s,d s,d s d s,d s d CO chemisorption Ligand Effect “Wall” effect DFT Calculated CO MO Shifts: DFT Calculated CO MO Shifts 5-s (-6.519 eV) 2π*-d (-10.228 eV) HOMO (-5.927 eV) HOMO (-5.478 eV) HOMO (5.356 eV) 5-s (-10.5 1.5 eV) 5-d (-8.08 eV) 2π*-p (-5.371 eV) 5-d (-8.45 2.5 eV) 2π*-d (-11.1 2.0 eV) 5-s (-10.6 1.5 eV) 5-d (-8.35 3.5 eV) New States 2π*-d (-11.1 2.0 eV) 5-d (-7.5 eV) Summary of CO MO Shifts Calculations by DFT alone: Summary of CO MO Shifts Calculations by DFT alone Hybrids 5 and 2π* lower in energy and broaden* (Pt13)CO (Pt1)(Pt12)(Pt13)CO CO (Pt1)(Pt12)(Pt13)CO (Pt1)(Pt12)(Ru4Pt13)CO CO Due to.. Hybrids 5 and 2π* further broaden but do NOT lower in energy * B. Hammer et. al., Catal. Lett., 46, 31, (1997) FEFF8 XANES Calculations agree with experimental XANES : FEFF8 XANES Calculations agree with experimental XANES L-III (2p3/2 d) L-I edge (2s p) L-II (2p1/2 d) Average d-band vacancies increased upon alloying Pt with Ru Confirm DFT using theoretical XANES: Confirm DFT using theoretical XANES Use clusters from DFT to calculate XANES and Density of States (DOS) d-band center is lowered when Pt alloyed with Ru. BUT, more states appear close to HOMO (also confirmed by DFT) This allows more 2π*-d back donation ! Blyholder mechanism has been reconciled with increased d-band vacancies upon alloying Interrelated effects during methanol oxidationExperimental Observations: Interrelated effects during methanol oxidation Experimental Observations CO-CO Interaction Alloy Surface Relaxation Effect of methanol Effect of Potential Dipole-Dipole Interaction νCO ↑ Pt-Pt bonding relaxed at surface Pt-Ru contracts- effect on νCO ? R. Liu et. al., J. Phys. Chem B, 104, 3518 (2000) Potential Funding: Potential Funding National Institute of Health (NIH) “Structural and electronic information for single and multi-nuclear active sites of metalloproteins and hemes” Design XAFS DWF models for metal-amino acid formations Apply models to single and multi-nuclear active sites Examine the effect of the metal d-occupancy on VDOS Correlate Heme structural modes with vibrational and electronic effects Army Research Office (ARO) “Computational Carbon Monoxide Adsorption on Nanosize Catalysts” Using DFT and FEFF8 examine the effects of: CO-CO interaction alloy surface relaxation effect of dielectric on CO stretching frequency Cross-check results by experimental XANES/EXAFS spectra at APS Conclusions: Conclusions Computational DFT/FEFF8 study on Pt-CO and PtRu-CO clusters reconciled the increase of the d-band vacancies upon alloying, the hybridization of the 5/2* CO MOs, and the shift of the d-LDOS center and its apparent asymmetric broadening to explain the systematic reduction of CO as the alloy Pt mole fraction is reduced. CO adsorption on Pt- alloy surface causes: hybridization of 5/2* CO molecular orbitals and thus electron density transfer to and from these states with the electronic bands of the cluster, Shifts the d-band center to lower energy values, and the band is asymmetrically broadened resulting in more 5/2* CO filled states This aggregated with the DFT calculated CO MOs confirming that the broadening of the hybrid 5/2* CO MOs enhances bonding back-bonding mechanism to the CO and downshifts the CO.