Bifunctional alloys for the electroreduction of CO 2 and CO

Transcription

1 Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics. This journal is the Owner Societies 2016 Bifunctional alloys for the electroreduction of CO 2 and CO Heine A.Hansen a, Chuan Shi a, Adam C. Lausche b, Andrew A. Peterson a, Jens K. Nørskov a,b a ) SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA. b ) SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Definition of Formation Energies We have defined the formation energy of COOH*, E COOH, as the DFT energy of the reaction 1 CO 2 (g) + * + ½ H 2 (g) COOH*. The formation energy of CO*, E CO, is the reaction energy for CO desorption 1,2 CO(g) + * CO*. We define formation energies including adsorbate stabilization due to the solvent as 1 E COOH(solv) = E COOH 0.25 ev and E CO(solv) = E CO 0.1 ev The formation energy of CHO*, E CHO, is calculated as the reaction energy of 2,3 C + H 2 O(g) + * CHO* + ½ H 2 (g), where C is carbon in graphene. With the RPBE functional C + H 2 O(g) CO(g) + H 2 (g) has the reaction energy E CO(g) =1.75 ev, 3 which allows calculation of E CHO without explicit calculation of graphene. If CO(g) + * + ½ H 2 (g) CHO* has reaction energy ΔE 9, then E CHO = E CO(g) + ΔE 9. The formation energy of OH*, E OH, is defined using H 2 O and H 2 as reference 2,3 H 2 O(g) + * OH* + ½ H 2 (g). The formation energy of OCHO* is defined using CO 2 and H 2 as reference

2 CO 2 (g) + * + ½ H 2 (g) OCHO*. Limiting Potentials 0.2 OH*! H 2 O CO*! CHO* CO(g)! CHO* UL(CHO) (V vs RHE) Ir 3 Cr Ir 3 Mo Pt 3 Cr U L (OH) (V vs RHE) Figure S1: CO reduction on (111) facets approximated by the limiting potentials for OH removal and CHO formation. The potential limiting step is indicated in the legend. Alloys unstable at 0 V vs RHE and ph 7 are shown in gray with smaller symbols. Cd 3 Pt Cd 3 Pd

3 UL CHO COOH (V vs RHE) Au 3 Zn Zn 3 Pt OH* H 2 O CO* CHO* CO(g) CHO* CO 2 (g) COOH* OCHO* HCOO Pt 3 Ni Pt 3 Fe Au 3 Cd Zn 3 Cu Ag 3 Ga Au 3 Ag Pd 3 Fe Cu 3 Ga Cu 3 Zn Pd 3 Au Co 3 Pt Pd 3 Hg Cu 3 Ge Ni 3 Pt Pt 3 Sb Pd 3 Bi Au 3 Pd Co 3 Rh Cd 3 Pd Au Au 3 Sn Ag 3 Zn 3 In Rh 3 Rh Pd 3 Sb Cu 3 Cu Pd 3 Cd Pd 3 Ag U L OH OCHO (V vs RHE) Figure S2: CO 2 reduction on (211) facets approximated by the limiting potentials for OH and OCHO removal and CHO and COOH formation. The potential limiting step is indicated. Alloys unstable at 0 V vs RHE and ph 7 are shown in gray with smaller symbols. 0 2 Au 3 Zn OH* H 2 O CO* CHO* CO(g) CHO* UL CHO (V vs RHE) Au 3 Cd Au 3 Ag Pt 3 Ni Pt 3 Fe Ag 3 Ga Zn 3 Cu Fe 3 Pt Cu 3 Ge Au 3 Sn Pd 3 Fe Cu 3 Ga Au 3 In Cu 3 Zn Co 3 Pt Cu 3 Au Co 3 Rh Ni 3 Pt Pt 3 Sb Zn 3 Au Cd 3 Pd Ag 3 Zn Pd 3 Sb Pd 3 Bi Rh 3 Rh Pd Ir 3 Ru 3 Cd Zn Cu 3 Cu 3 Ni Pd 3 Zn Pd Pd 3 Cu 3 Ag Pd 3 As U L OH (V vs RHE) Zn 3 Pt Figure S3: CO reduction on (211) facets approximated by the limiting potentials for

4 CHO formation and OH removal. The potential limiting step is indicated. Alloys unstable at 0 V vs RHE and ph 7 are shown in gray with smaller symbols. Figure S4: Free energy diagram for the initial reduction of CO2 to CO and CHO* on Au3Zn, Zn promoted Au and Au(211) at room temperature. Atomic structures are shown to the right. The free energy of CO(g) is shown at 1 atmosphere CO partial pressure. L12 (111) L12 (211) Au3 Zn 02 UL (V vs RHE) 03 Zn3 Pt Ir3 Cr 04 Au3 Cd 05 Zn3 Pd Cd3 Pt Ir3 Mo 06 Pd3 Bi Pd3 Sb Pd3 Cd Zn3 Pd Zn3 Cu Fe3 Pt Pt3 Al 30 Pt3 Sn Pd3 Zn 15 EAlloy (ev) Pt3 Co Cu3 Ga Co3 Pt Cu3 Zn Ag3 Ga Au3 Ag Pd3 Au Cu3 Ge Pt3 Co Pd3 Hg Pt3 Fe Cd3 Pd Pt3 Cr Au3 Pd Ni3 Pt Au3 Sn Co3 Rh Pt3 Sb Pd3 Fe Au3 In Pd3 Ag Pt3 Ni Ag3 Zn Cd3 Pd Cu Zn3 Ni Zn3 Au Pt3 Ni Pd3 Cu Ag3 Pt Cu3 Au Cu3 Pt Pt3 Bi Pt3 Cu Pt3 Ag Pt3 Pb Cu3 Au Ag3 Au Co3 Ir Au3 Pd Cu Pd 3 Pt3 Cd Pt3 Au Fe3 As Pt3 Cu Au3 Pt Ag3 Pd Cu3 Zn

5 Figure S5: The limiting potential for CO reduction on (211) and (111) facets versus the alloy formation energy per formula unit of the L12 alloy. Segregation Figure S6: Atomic structures of (211) facets. (a) Stoichiometric A3B(211) with AB termination. (b) + (c) Surfaces with A atoms along the step edge created from the stoichiometric surface by short-range exchange with atoms at the nearby terrace. (d) Surface with B atoms along the step edge created by short-range exchange with atoms at the terrace. (e)+(f) Surfaces with A and B at the step edge created by long range exchange of step atoms leaving the terrace unchanged. (g)+(f) Surfaces completely covered by A and B respectively. The AB terminated A3B(211) facet contains 4 A atoms and 2 B atoms as shown in Figure S6(a). Segregation energies are calculated from slab calculations based on the reactions below. Swapping B atom at step edge with A atom at nearby terrace A(e)B(e)A3(t)B(t) A2(e)A2(t)B2(t) where (e) and (t) subscripts denotes atoms at step and terrace respectively. In the final state the two possible locations of the swapped B atom are shown in figure S6(b)-(c). Swapping A atom at the step edge with the B atom at the nearby terrace results in the final state shown in figure S6(d) A(e)B(e)A3(t)B(t) B2(e)A4(t) The above processes conserve the stoichiometry of the simulation cell. We also consider phase separation along the step resulting in separate domains with either A or B atoms along the step edges as shown in figure S6(e)-(f).

6 A (e) B (e) A 3(t) B (t) ½ A 2(e) A 2(t) B 2(t) + ½ B 2(e) A 4(t) Finally, we consider complete phase separation on steps and terraces resulting in final states shown in Figures S6(g)-(h) A (e) B (e) A 3(t) B (t) 2/3 A 2(e) A 4(t) + 1/3 B 2(e) B 4(t) Pd 3 Sb 0 5 Pd 3 Bi 0 4 Eseg (ev/atom) Zn 3 Pt Pd 3 Cd Cd 3 Pd Pt 3 Fe Co Fe 3 Pt 3 Pt Pd 3 Hg Ni 3 Pt Au 3 Cd Pt 3 Sb Au 3 In Cu 3 Ga Co 3 Rh 0 0 Cu 3 Zn Pd 3 Fe Au 3 Zn Au 3 Pd Zn 3 Cu Au 3 Ag Ag 3 Zn Cu 3 Ge Ag 3 Ga Pd 3 Au 0 1 swap A enri. Au 3 Sn swap B enri. step phase sep. surf. phase seg. Pd 3 Ag E Alloy (ev) Figure S7: Segregation energies on (211) surfaces versus bulk alloy formation energies per formula unit. CO reduction to CHO

7 30 CO atop X, CHO br XY CO atop X, CHO br YX CO atop X, CHO atop X CO br, CHO br XY other 25 Number U L (V) Figure S8: Histogram of CO and CHO configurations on (211) steps. CO br refers to CO adsorbed at a step bridge site through the C atom. CO atop X refers to CO adsorbed on an atom of type X at the step. CHO br XY refers to CHO adsorbed in a bridging position at the step with the C atom coordinated to an atom of type X and the O atom coordinated to an atom or type Y. The structures in figure 5(e)-(f) in the main text correspond to the class CO atop X, CHO br XY. Only alloys with E CO < 0.05 ev are included in this analysis. References 1. H. A. Hansen, J. B. Varley, A. A. Peterson and J. K. Nørskov, J. Phys. Chem. Lett., 2013, 4, A. A. Peterson and J. K. Nørskov, J. Phys. Chem. Lett., 2012, 3, A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl and J. K. Nørskov, Energy Environ. Sci., 2010, 3,