Role of Heteronuclear Interactions in Selective H 2 Formation from HCOOH Decomposition on Bimetallic Pd/M (M=Late Transition FCC Metals) Catalysts

Transcription

1 Supporting Information Role of Heteronuclear Interactions in Selective H 2 Formation from HCOOH Decomposition on Bimetallic Pd/M (M=Late Transition FCC Metals) Catalysts Jinwon Cho 1, #, Sangheon Lee 1,4,#, Sung Pil Yoon 1, Jonghee Han 1,3, Suk Woo Nam 1,3, Kwan-Young Lee 3,*, and Hyung Chul Ham 1,2,* 1 Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu, Seoul , Republic of Korea 2 Clean Energy and Chemical Engineering, Korea University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon, , Republic of Korea 3 Green School (Graduate School of Energy and Environment), Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, , Republic of Korea 4 Department of Chemical Engineering and Materials Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul, 03760, Republic of Korea * Corresponding Authors: Dr. Hyung Chul Ham, Prof. Kwan-Young Lee hchahm@kist.re.kr, kylee@korea.ac.kr [#] These authors equally contributed as the first authors

2 A. Coverage effects on the selective H2 production via HCOOH decomposition Table S-1. Calculated Reaction Energy Changes (ΔE) and Activation Energies (Ea) for Each Dehydrogenation and Dehydration Pathway with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2 2 and 3 3 surface unit cell) on the 2 2 and 3 3 Surface Unit Cells [Pd(111), Pd/Ag(111)]. Unit is given in ev. PW E /(Ea) Pd/Ag(111) Pd(111) Pd/Ag(111) Pd(111) HCOOH HCOO + H HCOO CO2 + H 0.07/(0.69) 0.24/(0.64) 0.04/(0.72) 0.14/(0.68) 0.78/(0.78) 0.43/(0.87) 0.58/(0.81) 0.41/(0.90) HCOOH HCO + OH 0.75/(1.12) 0.66/(1.02) 0.96/(1.19) 0.90/(1.08) HCO + OH CO + H2O Table S-2. Comparison of Binding Energy of HCO and OH on the Pd(111), Pd/Cu(111) and Pd/Ag(111) between 2 2 and 3 3 Surface Unit Cells. Unit is given in ev 2 2 Pd Pd/Cu Pd/Ag OH (at bri) OH (at fcc) OH (at hcp) OH (at top) HCO (at top) HCO (at bri) HCO (at fcc) HCO (at hcp) Pd Pd/Cu Pd/Ag OH (at bri) OH (at fcc) OH (at hcp) OH (at top) HCO (at top) HCO (at bri) HCO (at fcc) HCO (at hcp)

3 B. Stability of the bimetallic core shell catalysts In fact, some experimental studies found surface segregation in homogeneous PdxAg1 x alloy nanoparticle under heat treatment. 1 On the other hand, for the fully covered Ag-Pd core shell nanoparticle no surface phase segregation was found after heat treatment according to the analysis of XRD patterns. 2 The experimental study by Tedsree et al. also demonstrated that the core shell structure was maintained with no exposure of Ag core even after many hours of HCOOH decomposition reaction. 2 These results indicate that once the Ag core is fully covered by the Pd monolayers, the core shell nanoparticles become kinetically stable under the extreme conditions. 2 Thus, we believe that the Ag-Pd core shell nanoparticle is indeed a stable catalyst under the reaction conditions. Another study of surface stabilization is also observed in the Au-Pd core-shell nanoparticle. Gu et al. experimentally conducted the H2 production via HCOOH decomposition on Au-Pd core-shell structure and reported the improved reactivity of Au-Pd core-shell catalyst toward H2 production compared to monometallic Pd catalyst with little change of catalytic activity, suggesting that the structure of Au-Pd core-shell catalyst can be kinetically stable under real reaction conditions. 3 Similar surface stabilization effects were also demonstrated by Larsen et al. for the submonolayer Pd films deposited on V, Mo, W, and Au foils. 4 These Ag-Pd and Au-Pd cases are the clear examples where the kinetic stabilization effect by the thin, uniform Pd skin layer can suppress the thermodynamic driving force toward the surface segregation moves of the core metals (such as Au, Ag) at low temperatures. We think that the kinetic stabilization effect also works in the other Pd/M catalysts, although the long term durability of the real Pd/M catalysts still needs further experimental validation, which is beyond the scope of the present work. To evaluate the kinetic stabilization effect, we calculate the reaction energy changes and activation energy barriers for the surface segregation moves of the core metal in the HCOO covered Pd/M model surfaces with a Pd vacancy introduced into the topmost Pd monolayer. We also predict the binding energy of HCOO for the initial (M at the subsurface layer) and segregated (M at the top surface layer) Pd/M catalysts. As shown in Figure S-1 below, the calculated segregation energies and energy barriers are 0.75/0.97, 1.27/1.75, 1.62/1.89, 0.28/0.87, 0.16/0.29, and 0.00/0.36 ev for Pd/Cu, Pd/Rh, Pd/Ir, Pd/Pt, Pd/Au, and Pd/Ag, respectively and we expect much greater activation energy barriers for the defect free Pd overlayer cases. This is also consistent with the variation of the binding energy of HCOO as the subsurface M atom migrated from the subsurface to the top surface layer. Note the reduction of binding energy of HCOO after the segregation of M atom by 0.23~0.07eV compared to the initial (M at the subsurface layer) Pd/M catalysts. Considering that the calculated activation energy barriers for the surface segregation moves are greater than or comparable to the experimentally demonstrated Pd/Ag and Pd/Au cases and the HCOOH decomposition occurs at relatively lower temperatures (< 100 C) in HCOOH based low temperature fuel cells, these surface segregation moves are hardly likely to occur in real operating conditions

4 Binding energy of HCOO [ev] Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd/Au Pd/Ag Initial (M at the subsurface layer) Pd/M(111) slab Segregated PdM alloy/m(111) (M at the top surface layer) slab Figure S-1. (Top) Calculated segregation and activation energies for the vacancy-mediated migration of M atom from the subsurface to the top surface in HCOO-covered Pd/M(M=Rh, Pt, Ir, Cu, Au, and Ag) structures. (Middle) Calculated binding energy of HCOO for the initial (M at the subsurface layer) and segregated (M at the top surface layer) Pd/M catalysts (Bottom) Predicted molecular configuration at the initial, transition, and final states in Pd/Au(111) catalysts. Green, yellow, brown, red and white balls represent Pd, Au, C, O, and H atoms respectively

5 Figure S-2. Reaction pathway of HCCOH decomposition on the Pd/M(111) surface via the carboxyl (COOH) or formate (HCOO) pathway

6 Figure S-3. Potential energy diagram for the HCOOH decomposition to CO2+H via the carboxyl (COOH) (red line) or formate (HCOO) (grey line) pathway on the Pd(111) surface. Ts1 and Ts2 indicates the transition state at each reaction step

7 Table S-3. Calculated Reaction Energy Changes (ΔE) and Activation Energy Barriers (Ea) of HCOOH Decomposition via Carboxyl (COOH) or Formate (HCOO) Pathway with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2 2 surface unit cell). Unit is given in ev. Metals used for M substrates ΔE/(Ea) (ev) Rh Pt Ir Cu Pd Au Ag (DH I) HCOOH HCOO + H (DH II) HCOO CO2 + H (DCO I) HCOOH HCO + OH (DCO II) HCO + OH CO + H2O HCOOH COOH + H COOH CO2 + H COOH CO + OH (RWGS-I) CO2 + H COOH (WGS-I) CO + OH COOH COOH + OH CO2 + H2O CO2 CO2(g) OH + H H2O H2O H2O (g) H+H H /(0.73) 0.30 /(0.87) 1.15 /(1.12) /(0.99) 0.24 /(0.97) 0.08 /(1.66) 0.24 (1.21) 0.08 /(1.58) 0.87 /(0.01) 0.01 /(0.01) 0.63 /(0.37) 0.22 /(0.22) 0.58 /(0.52) 0.33 /(0.51) 0.25 /(0.94) 0.77 /(0.99) /(0.72) 0.18 /(0.98) 0.59 /(1.11) 0.18 /(1.16) 0.59 /(1.70) 0.73 /(0.01) 0.04 /(0.04) 0.05 /(0.56) 0.32 /(0.32) 0.80 /(0.59) 0.07 /(0.71) 0.18 /(0.92) 1.07 /(1.09) /(0.94) 0.19 /(0.98) 0.05 /(1.59) 0.19 /(1.17) 0.05 /(1.54) 0.70 /(0.01) 0.03 /(0.03) 0.51 /(0.41) 0.26 /(0.26) 0.57 /(0.51) 0.21 /(0.78) 0.94 /(0.76) 1.92 /(2.02) /(0.90) 0.63 /(0.78) 0.74 /(1.86) 0.63 /(1.41) 0.74 /(1.12) /(0.05) 0.76 /(0.55) 0.17 /(0.17) 0.86 /(0.98) 0.14 /(0.68) 0.41 /(0.90) 0.90 /(1.08) /(1.03) 0.24 /(0.96) 0.15 /(1.56) 0.24 /(1.20) 0.15 /(1.71) 0.66 /(0.03) 0.05 /(0.05) 0.42 /(0.51) 0.27 /(0.27) 0.99 /(0.60) 0.18 /(0.69) 0.44 /(0.85) 0.91 /(1.06) /(0.87) 0.45 /(0.89) 0.61 /(1.07) 0.45 /(1.34) 0.61 /(1.68) 0.69 /(0.03) 0.05 /(0.05) 0.24 /(0.63) 0.27 /(0.27) 0.88 /(1.09) 0.04 /(0.72) 0.58 /(0.81) 0.97 /(1.19) /(0.93) 0.55 /(0.81) 0.53 /(1.11) 0.55 /(1.36) 0.40 /(1.64) /(0.03) 0.26 /(0.59) 0.22 /(0.22) 0.88 /(1.17) - 7 -

8 Table S-4. Calculated Reaction Energetics and Activation Barriers of Dehydrogenation and Dehydration on the Pd(111) and Pd/Ag(111) Surfaces using the Two Different PW91 and RPBE Functional with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2 2 surface unit cell) PW91//Energetic/(Energy Barrier) ev Pd(111) Pd/Ag(111) HCOOH to HCOO + H (DH-I) 0.14/(0.78) 0.02/(0.64) HCOO to CO2 + H (DH-II) 0.41/(0.90) 0.58/(0.81) HCOOH to HCO + OH (DCO-I) 0.90/(1.08) 0.98/(1.18) HCO + OH to CO and H2O (DCO-II) RPBE//Energetic/(Energy Barrier) ev Pd(111) Pd/Ag(111) HCOOH to HCOO + H (DH-I) 0.01/(0.69) 0.10/(0.71) HCOO to CO2 + H (DH-II) 0.68/(0.85) 0.84/(0.76) HCOOH to HCO + OH (DCO-I) 0.97/(1.16) 1.19/(1.34) HCO + OH to CO and H2O (DCO-II) 1.79/(0.07) 2.02/(0.01) - 8 -

9 Figure S-4. Calculated intermediate images in the minimum energy pathway of HCOO CO2 + H reaction on the Pd surfaces

10 C. Zero Point Energy Correction and Microkinetic Modeling The Gibbs free energy change (ΔG) is calculated as follows. ΔG = ΔH+ ΔZPE TΔS where ΔH, ΔZPE, and ΔS indicates the change of enthalpy, zero point energy, and vibrational entropy in each reaction step, respectively. Here, ΔH is obtained from the total energy change of a given reaction at T=0K. The vibrational entropy and zero point energy are calculated using the following equations: # of modes Svib = k B { x i i e x i 1 ln(1 e x i)} x i = hv i k B T ZPE = 1 hv 2 i i where k B, h and x i is the Boltzmann constant, Planck's constant and vibrational mode in terms of vibrational frequency, v i, respectively. 5 For elementary reaction A* + B* k for C* + D*, where * symbolizes an adsorbed species, the equilibrium constant, and the rate constant k for is denoted as: (ΔH + ΔZPE) k for = A 0 exp ( ) k b T where A 0 is the frequency factor which is denoted as: A 0 = k bt h exp ( Δs vib.for k b ) The rate for this elementary step, now, can be expressed as: r = r for r rev = k for θ A θ B k rev θ C θ D where r for and r rev is the rate of forward and reserve reaction and θ A, θ B, θ C, and θ D is the surface coverages of the species A, B, C, and D respectively. 5 For the microkinetic modeling of HCOOH decomposition, we have calculated surface coverages of the reaction species of HCOOH, HCOO, HCO, CO2, H2, H2O, CO and OH as follows

11 Table S-5 Elementary reaction steps for a microkinetic model No Elementary reaction steps 1 HCOOH (g)+ * HCOOH* 2 HCOOH* + * HCOO*+H* 3 HCOO* + * CO2* + H+ 4 H* + H* H2*+ * 5 CO2* CO2(g) + * 6 H2* H2 (g) + * 7 HCOOH*+ * HCO*+OH* 8 HCO*+OH* H2O* + CO* 9 H2O* H2O (g) + * 10 CO* CO (g) + * 1. HCOOH(g) + θ HCOOH* 2. HCOO* dθ HCOO dt θ HCOOH = P HCOOH θ k 1.eq = r 2 r 3 = k 2 θ HCOOH θ k 3 θ HCOO θ = 0 3. H* 4. HCO*, OH* 5. CO2* dθ HCO dt θ HCOO = k 2 k 3 P HCOOH θ k 1.eq dθ H dt = r 2 + r 3 2r 4 = 0 θ H = ( k 2P HCOOH θ k 1.eq k 4 ) 1 2 θ = r 7 r 8 = k 7 θ HCOOH θ k 8 θ HCO θ OH = 0 θ HCO = θ OH = k 1 7 (P k HCOOH k 1.eq ) 8 2 θ dθ CO 2 dt = r 3 r 5 = k 3 θ HCOO θ k 5 θ CO2 = 0 θ CO2 = k 2 k 5 P HCOOH θ 2 k 1.eq

12 6. H2* dθ H 2 dt = r 4 r 6 = k 4 θ H 2 k 6 θ H2 = 0 θ H2 = k 4 k 6 P HCOOH θ 2 k 1.eq 7. H2O* dθ H 2O dt = r 8 r 9 = k 8 θ HCO θ OH k 9 θ H2 O = 0 θ H2 O = k 7 k 9 P HCOOH θ 2 k 1.eq 8. CO* dθ CO dt = r 8 r 10 = k 8 θ HCO θ OH k 10 θ CO = 0 θ CO = k 7 k 10 P HCOOH θ 2 k 1.eq 9. Site balance equation 1 = θ HCOOH + θ HCOO + θ H + θ CO2 + θ H2 + θ HCO + θ OH + θ H2 O + θ CO For gas phase molecules, the Gibbs free energy, ΔGgas, is obtained as follows ΔGgas (T,P) = ΔH(T,P)+ ΔZPE TΔS at T=323k, P=1bar where ΔH and ΔS can be described as: ΔH = CpΔT ΔS = C p ln ( T T 0 ) k ln ( T T 0 ) where Cp is a heat capacity of gas molecule, and we used experimental data for Cp. For equilibrium constant, K ad, for adsorption and desorption, the following equation is used: K ad = exp ( ΔG ads k b T ) where ΔG ads is Gibbs free energy of an adsorbed molecule on the Pd/M surface

13 Table S-6. Calculated Reaction Energy Changes (ΔG) and Activation Energies (Ga) for Each Dehydrogenation and Dehydration Pathway on Pd/M with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2 2 surface unit cell). ΔG = ΔH+ ΔZPE TΔS at T=323K. Calculated H2 TOF (Turnover frequency) using a Microkinetic Model is given in (hr 1 ). ΔG/(G a ) (ev) HCOOH HCOO +H HCOO CO 2 +H HCOOH HCO+OH HCO+OH CO+H 2 O H 2 TOF (hr 1 ) Metals used for M substrates Rh Pt Ir Cu Pd Au Ag 0.08 /(0.30) 0.43 /(0.74) 1.15 /(1.15) /(0.08) 0.38 /(0.79) 0.77 /(0.89) /(0.25) 0.27 /(0.77) 1.03 /(1.03) /(0.32) 1.08 /(0.66) 1.94 /(1.96) /(0.25) 0.51 /(0.75) 0.90 /(0.97) /(0.20) 0.54 /(0.71) 0.88 /(0.96) /(0.14) 0.69 /(0.69) 0.95 /(1.04)

14 Table S-7. Calculated Enthalpy(ΔHrxn), Gibbs Free Energy (ΔGrxn), Zero Point Energy (ΔZPErxn), Entropy (ΔSrxn) Changes and Activation Energy Barriers (ΔGa, ΔHa) for Each Reaction Step on Pd/M with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2 2 surface unit cell). ΔG = ΔH+ ΔZPE TΔS at T=323K. HCOOH* HCOO*+H* Surface ΔHrxn ΔZPErxn ΔSrxn ΔHa ΔZPEa ΔSa ΔGrxn ΔGa Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag HCOO* CO2*+H* Surface ΔHrxn ΔZPErxn ΔSrxn ΔHa ΔZPEa ΔSa ΔGrxn ΔGa Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag HCOOH* HCO*+OH* Surface ΔHrxn ΔZPErxn ΔSrxn ΔHa ΔZPEa ΔSa ΔGrxn ΔGa Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag H*+H* H2* Surface ΔHrxn ΔZPErxn ΔSrxn ΔHa ΔZPEa ΔSa ΔGrxn ΔGa Pd/Rh Pd/Pt Pd/Ir

15 Pd/Cu Pd Pd/Au Pd/Ag HCO*+OH* CO*+H2O* Surface ΔHrxn ΔZPErxn ΔSrxn ΔGrxn Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag

16 Table S-8. Calculated Enthalpy (ΔH), Gibbs Free Energy (ΔG), Zero Point Energy (ΔZPE), Entropy (ΔS) Change in the Adsorption of Key Intermediates on the Pd/M Surfaces. ΔG = ΔH+ ΔZPE TΔS at T=323K. HCOOH* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag HCOO* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag CO2* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag H* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd

17 Pd/Au Pd/Ag HCO* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag OH* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag CO* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag H2O* Surface ΔH ΔZPE ΔS ΔG Pd/Rh Pd/Pt

18 Pd/Ir Pd/Cu Pd Pd/Au Pd/Ag

19 Table S-9. Real Vibrational Frequencies (cm 1 ) of the Transition State for the DH I, DH II, and DCO I Steps, denoted in Ts1, Ts2, and Ts3 respectively. One Imaginary Number is obtained at Transition State. DH I Pd/M Pd Rh Pt Ir Ag Cu Au Ts DH II Pd/M Pd Rh Pt Ir Ag Cu Au Ts

20 DCO I Pd/M Pd Rh Pt Ir Ag Cu Au Ts

21 Table S-10. Calculated Degree of Rate Control (XRC) based on the TOF of the Overall H2 Production dehydrogenation dehydration Reaction steps Rh Pt Ir Cu Pd Au Ag HCOOH* * HCOO* + H* HCOO* + * CO2* + H* H* + H* H2* + * CO2* CO2(g) + * H2* H2(g) + * HCOOH* + * HCO* + OH* HCO* + OH* H2O* + CO* H2O* H2O(g) + * CO* CO(g) + * Sum of XRC Table S-11. Calculated Degree of Rate Control (XRC) based on the TOF of the Overall CO Production dehydrogenation dehydration Reaction steps Rh Pt Ir Cu Pd Au Ag HCOOH* +* HCOO* + H* HCOO* + * CO2* + H* H* + H* H2* + * CO2* CO2(g) + * H2* H2(g) + * HCOOH* + * HCO* + OH* HCO* + OH* H2O* + CO* H2O* H2O(g) + * CO* CO(g) + * Sum of XRC

22 Based on a developed microkinetic model, we attempted to choose a RDS in dehydrogenation and dehydration steps by calculating the degree of rate control (XRC), which is defined below. Here, the larger the value of XRC is for a given step, the bigger is the impact of its rate constant on the overall reaction rate. 6,7 k i X RC = TOF(H 2 or CO) ( TOF(H 2 or CO) ) k i K i,k j i where, TOF, ki, and Kj are the turnover frequency for the overall H2 or CO production, rate constant of step i, and equilibrium constant of step j, respectively

23 Table S-12. Bond Length of Pd-Pd on Pd/M Surfaces and the Distance between Surface Pd and Subsurface M. Pd/M (Å) Pd Rh Pt Ir Ag Cu Au Pd-Pd Pd M

24 Figure S-5. Projected density of d states for the PdLig M case. The dotted line at 0 ev denotes fermi level position

25 Figure S-6. Effect of surface charge polarization on the d orbitals near the Fermi level ( 0.25 < E Ef < 0). Black squares, red circles and blue triangles represent dz2, dxy+ dx2 y2 and dyz+dxz respectively

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