Supporting Information. Water-Gas Shift Activity of Atomically Dispersed Cationic Platinum versus Metallic Platinum Clusters on Titania Supports

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1 Supporting Information Water-Gas Shift Activity of Atomically Dispersed Cationic Platinum versus Metallic Platinum Clusters on Titania Supports Salai Cheettu Ammal and Andreas Heyden * Department of Chemical Engineering, University of South Carolina, 301 South Main Street, Columbia, South Carolina 29208, United States * heyden@cec.sc.edu Supplementary Computational Details Ab Initio Thermodynamic Analysis In order to identify relevant structures that could be present under WGS reaction conditions, Gibbs free energies for the formation of different structures were computed as a function of temperature (T) and partial pressures (P) of various gas molecules. Gibbs free energies calculated with standard DFT (Figure 1) and DFT+U (Figure S1) methodologies provided similar trends for the interaction of a Pt atom on a TiO2 (110) surface. Inclusion of the Hubbard U parameter only affected oxygen vacancy structures somewhat. For example, the free energy for the formation of a Pt-doped structure with an oxygen vacancy (Ti1-xPtTi5cO2-x) calculated with the DFT+U method is 0.25 ev higher than the free energy obtained using the standard DFT method and thus, when using the DFT+U method, this structure becomes thermodynamically less favorable. However, both levels of theory predicted that the Ti1-xPtTi5cO2-2H structure is the most stable structure below 500 K and the Bader charges on Pt obtained from both levels of theory are very similar. Gibbs free energies of different possible structures calculated with PBE functional both in the presence and absence of CO for the Pt-doped TiO2 at the Ti5c and Ti6c positions are 1

2 summarized in Figures S2 and S3, respectively. Comparison of these two figures suggest that the formation of structures with Pt replacing a Ti5c with adsorbed CO and additional H atoms on the TiO2 surface are thermodynamically more favorable than Pt replacing a Ti6c atom. When Pt is in the Ti5c position, adsorption of CO on Pt leads to an octahedral coordination for Pt (Figures S2b, S2d, and S2e) and the positive charge on Pt increases due to back-donation of Pt d electrons to CO. When we add two adsorbents on Pt (Figures S2g and S2h), the local structure of Pt changes back to square planar with only two Pt-O bonds and the charge on Pt becomes closer to that of the Pt +2 ion (Bader charge of for Pt in bulk PtO). 1 Calculated Gibbs free energies clearly indicate that the structures with Pt in a square planar geometry and lower oxidation state are thermodynamically more favorable than the structures with Pt in octahedral geometry and higher oxidation state. Although, the Pt atom in structures Ti1-xPtO2-2H-2CO and Ti1-xPtO2-4H-CO seems to be coming out of the surface leaving a cation vacancy on the surface, it is still connected to two of the surface oxygen atoms to which the Ti5c atom was originally connected. We find that breaking one of these Pt-O bonds in the most stable structure Ti1-xPtO2-4H-CO and forming another Pt-O bond away from the cation vacancy has an activation barrier of 0.94 ev, although it is thermodynamically only slightly endothermic(figure S4). Thus, we believe that this structure is a stable structure under WGS reaction conditions at temperatures below 600 K. Furthermore, our calculations suggested that removing the CO from Ti1-xPtO2-4H-CO to form a Ti1-xPtO2-4H structure with different arrangements of H atoms is highly unfavorable with reaction energies ranging from ev (Figure S5). This again provides strong justification that at least one CO molecule must always reside in our catalyst model throughout the temperature range of K during WGS catalysis. 2

3 (a) (b) (c) Pt Ti5c O 2 = +1.68) Pt +TiO 2 (b)+2h Pt Ti6c O 2 = +1.79) Pt +TiO 2 (b)+2h 2 TiO 2 -Pt = +0.09) (S)+ Pt -Pt ΔG (ev) (c) TiO 2 -Pt Ti C H O Pt O vacancy (d) (e) Temperature (K) (f) (g) Pt Ti5c O 2-x = +1.12) (S)+ Pt + H 2 PtO TiO 2 (b)+ H 2 TiO 2-x -Pt = -0.69) (S)+ Pt + H 2 O 127 -Pt + H 2 O TiO 2 -Pt-2H = -0.67) (S)+ Pt + H 2 -Pt-2H Pt Ti5c O 2-2H = +1.07) Pt -2H + TiO 2 (b) + H 2 Figure S1. Gibbs free energies ( G) for the interaction of a Pt atom with a TiO2(110) surface calculated with DFT+U method (UTi = 2.5 ev). Side view of the optimized structures together with the reactions used to calculate Gibbs free energies are provided in the inset. Ti64O128(s) and TiO2(b) are the clean TiO2(110) surface and a TiO2 bulk unit, respectively. QPt is the Bader charge on the Pt atom. Reducing conditions of PH2O=0.2 and PH2=0.4 atm were used to calculate G. 3

4 (a) (b) (c) (d) = +1.66) Pt +TiO 2 (b)+2h 2 -CO = +1.77) (S)+Pt+2H 2 O +C Pt -CO+TiO 2 (b)+2h 2 -x = +1.11) (S)+ Pt + H 2 PtO TiO 2 (b)+ H 2 -x -CO = +1.29) (S)+Pt+H 2 O+C PtO 127 -CO+TiO 2 (b)+h 2 ΔG (ev) Temperature (K) (e) -2H-CO = +1.28) Pt -2H-CO+TiO 2 (b) +H 2 Ti C H O Pt O vacancy (f) (g) (h) -2H = +1.08) Pt -2H + TiO 2 (b) + H 2-2H-2CO = +1.16) O + 2C Pt -2H-2CO + TiO 2 (b)+ H 2-4H-CO = +0.75) O +C Pt -4H-CO + TiO 2 (b) Figure S2. Gibbs free energies ( G) calculated with PBE functional for replacing a Ti5c atom by Pt on a TiO2(110) surface under WGS reaction conditions. Ti64O128(s) and TiO2(b) are the clean TiO2(110) surface and a TiO2 bulk unit, respectively. QPt is the Bader charge on the Pt atom. Reaction conditions of PCO=0.1, PH2O=0.2, and PH2=0.4 atm were used to calculate G. Similar structures with and without CO are shown in solid and dashed lines, respectively. 4

5 (a) (b) = +1.80) Pt +TiO 2 (b)+2h Ti C H O Pt O vacancy -2H = +1.49) Pt -2H + TiO 2 (b) + H 2 ΔG (ev) (c) Temperature (K) (d) -x = +1.11) (S)+ Pt + H 2 PtO TiO 2 (b)+ H 2 -x -CO = +0.74) (S)+Pt+H 2 O+C PtO 127 -CO+TiO 2 (b)+h 2 Figure S3. Gibbs free energies ( G) calculated with PBE functional for replacing a Ti6c atom by Pt on a TiO2(110) surface under WGS reaction conditions. Ti64O128(s) and TiO2(b) are the clean TiO2 (110) surface and a TiO2 bulk unit, respectively. QPt is the Bader charge on the Pt atom. Reaction conditions of PCO=0.1, PH2O=0.2, and PH2=0.4 atm were used to calculate G. Similar structures with and without CO are shown in solid and dashed lines, respectively. 5

6 Ti C H O Pt E act = 0.94 ev -4H-CO(1) E = 0.00 ev -4H-CO(2) E = 0.06 ev Figure S4. Migration of a H-Pt-CO unit along the Ti1-xO2 (110) surface. The relative energies ( E) and activation barrier (E act ) are not zero-point energy corrected. Ti C H O Pt -4H (1) = +0.62) E = 2.68 ev -CO -4H-CO = +0.75) -4H (2) = +1.47) -4H (3) = +1.06) Figure S5. Different possibilities to desorb CO from the Ti1-xPtO2-4H-CO structure. Zero-point energy correction is not included in the energies provided here. QPt is the Bader charge on the Pt atom. 6

7 Microkinetic Model The microkinetic modeling approach used in this study is very similar to the approach used in our earlier work on our Pt8/TiO2 interface model. 2,3 Harmonic transition state theory and collision theory with a sticking coefficient of 1 were used to calculate rate constants for elementary surface reactions and adsorption processes, respectively. The forward (kk ffffff ) and reverse (kk rrrrrr ) rate constants of a surface reaction were calculated as, EE aa ffffff kk ffffff = AA ffffff ee kk BBTT ; kk rrrrrr = AA rrrrrr ee EE aa rrrrrr kk BB TT (1) aa aa where EE ffffff and EE rrrrrr correspond to the zero-point energy (ZPE) corrected forward and reverse activation barriers, respectively. The ZPE was calculated as 1 2 hυυ ii from calculated vibrational frequencies, υυ ii. The adsorbed CO and H species and the surface oxygen atoms surrounding the doped Pt were included in the frequency calculations (Figure S6). Displacements of ±0.02 Å were used along the x, y and z directions for all Hessian constructions from analytic gradients. Since the harmonic approximation is least accurate for small vibrational frequencies, we shifted all (real) frequencies below 50 cm -1 to 50 cm -1 to ensure that for surface reactions low frequency modes have no effect on reaction energies and rate constants (these frequencies in effect cancel out). Frequency factors (A) were calculated from the vibrational partition functions (qvib) using the following expressions: AA ffffff = kk BBTT qq vvvvvv,tttt ; AA h qq rrrrrr = kk BBTT vvvvvv,aa h qq vvvvvv,tttt ; (qq qq vvvvvv ) = ii hυυ vvvvvv,bb ii (2) 1 1 ee kk BB TT 7

8 Ti O Pt Figure S6. Optimized structure of our Ti1-xPtO2(110) model (side view). The highlighted areas correspond to the atoms that are displaced during the vibrational frequency calculations. The forward rate constant for an adsorption process was calculated using, kk ffffff = ππmm AA kk BB TT SS uuuuuuuu(ss 1 aaaaaa 1 ), where ma is the molecular weight of adsorbed species A and kb is the Boltzmann s constant. Sunit is the adsorption area of the active site which we approximate to be m 2 which is the same adsorption area we used previously for our active site model of an interface, corner Pt atom of a Pt8 cluster on TiO2(110). 4 The reverse rate constant was obtained from the equilibrium constant, KK = kk ffffff kk rrrrrr, which was calculated from the ZPE-corrected adsorption energy ( Eads) and the entropy factors of gas molecule (A) using the expression, KK = EE qq aaaaaa vvvvvv,aa kk ee BBTT. qq vvvvvv, (qq vvvvvv qq rrrrrr qq tttttttttt ) AA After calculating the forward and reverse rate constants for each elementary reaction we constructed a Master equation and solved for the steady-state solution of probability densities for the system to occupy each discrete state. 2 These probability densities are referred to in the 8

9 following as surface coverages, θ. The steady-state equations were solved using the BzzMath library developed by Buzzi-Ferraris 4 in order to get the surface coverages. We note here that although some of the reactant or product states in the elementary reactions are represented for clarity as having multiple species, in the Master equation each reactant or product state constitutes one discrete state. For example, the steady state equations for the net rates of intermediates (IM4, and IM11) are written as (see Table S1 for the elementary reactions), ddθθ (HH CCCC)PPPP VV bb1 (IIII4) dddd = kk 3 θθ (HH CCCC CCCC2)PPPP OOOO1 (IIII3) kk 3 θθ (HH CCCC)PPPP VV bb1 (IIII4)PP CCCC2 kk 4 θθ (HH CCCC)PPPP VV bb1 (IIII4)PP HH2OO + kk 4 θθ (HH CCCC)PPPP HH2OO bb1 (IIII5) = 0 (3) ddθθ (CCCC CCCC)PPPP OO bb1 HH(IIII11) dddd = kk 11 θθ CCCCPPPP OO bb1 HH(IIII10)PP CCCC kk 11 θθ (CCCC CCCC)PPPP OO bb1 HH(IIII11) kk 12 θθ (CCCC CCCC)PPPP OO bb1 HH(IIII11) + kk 12 θθ (CCCC CCCCCCCC)PPPP OOOO1 (IIII12) kk 17 θθ (CCCC CCCC)PPPP OO bb1 HH(IIII11) + kk 17 θθ (CCCC CCCCCC)PPPP (IIII16) = 0 (4) Similar steady state equations were constructed for the intermediates IM2-IM25 and the fraction of empty surface sites (θθ (HH CCCC)PPPP (IIII1)) is calculated from the site balance. θθ IIII1 = 1 θθ IIII2 θθ IIII3 θθ IIII4 θθ IIII5 θθ IIII6 θθ IIII7 θθ IIII8 θθ IIII9 θθ IIII10 θθ IIII11 θθ IIII12 θθ IIII13 θθ IIII14 θθ IIII15 θθ IIII16 θθ IIII17 θθ IIII18 θθ IIII19 θθ IIII20 θθ IIII21 θθ IIII22 θθ IIII23 θθ IIII24 θθ IIII25 (5) The 25 steady state equations together with the site balance are numerically solved (without any assumptions) using the BzzMath library to get surface coverages of all intermediates. The reaction rate of each elementary reaction is then calculated from these surface coverages (e.g. rr 1 = kk 1 θθ IIII1 kk 2 θθ IIII2 ). The overall rate (turnover frequency) is the sum of the rates of the three pathways (redox, carboxyl, and formate). Supplementary Discussion Multiple possibilities for each elementary step have been examined and only the route with lowest energy pathway is included in the microkinetic model. For example, the activation barrier 9

10 for H2O dissociation in the redox pathway by transferring one H atom to Pt was found to be 0.66 ev higher than the process of transferring that H atom to a surface oxygen (R5). Furthermore, our NEB calculations suggested that the second H-transfer to Pt in the intermediate (H-CO)Pt-Ob1H- Ob2H (IM7) was favored by about 1.7 ev from Ob1 rather than Ob2. The associative carboxyl and formate pathways with redox regeneration start with a transfer of an H atom from Pt to the neighboring oxygen (Ob1) leading to the formation of a nearly neutral Pt (QPt = +0.30) which is 0.22 ev endothermic with an activation barrier of 0.41 ev (R10). In the carboxyl pathway, a second CO molecule adsorbs on the Pt and reacts with the neighboring -Ob1H to form a COOH intermediate (IM12). In the formate pathway, CO abstracts H from - Ob1H forming a CHO intermediate which further spills over to Ob1 producing a HCOO intermediate (IM17). Since direct dissociation of this HCOO species by transferring the H to Pt and forming CO2 requires an activation barrier of 1.51 ev, we considered the formation of a HCOOH intermediate and further dissociation of this intermediate leading to the formation of CO2 and H2 as shown in Figure S9. Metal particles O O Reducible Oxide ± H 2 H H Metal particles O O Reducible Oxide Metal particles +CO C O O O Reducible Oxide (a) CO 2 +H 2 O -H 2 Metal particles +H 2 O V O Reducible Oxide (b) CO 2 H 2 +CO C O H H Metal particles O O Reducible Oxide Figure S7. Proposed reaction pathways for the WGS at the three-phase boundary that involve surface oxygen atoms. (a) Classical redox pathway. (b) Associative pathway with redox regeneration. 10

11 Relative (free) energy (ev) CO Pt -O b1 H (IM10) (CO-CO) Pt -O b1 H (IM11) (CO-COOH) Pt-Ob1 (IM12) IM1 Energy Free Energy TS10 +CO IM10 IM11 TS12 IM12 TS13 IM13 TS14 -CO 2 Ti C H O Pt O vacancy +H 2 O IM14 IM15 TS16 IM7 (CO-CO 2 ) Pt-Ob1 O b2 H (IM13) CO Pt -V b1 O b2 H (IM14) CO Pt -H 2 O b1 O b2 H (IM15) Figure S8. (Free) energy profiles for the carboxyl pathway with redox regeneration (T = 500 K; Pj(gas) = 1 atm). All energies are with reference to the sum of the energies of the initial state ((H- CO)Pt, IM1) and the reactant gas molecules. The insets provide a side view of the optimized structures of the intermediates. 11

12 (CO-CO) Pt (IM16) CO Pt -HCOO b1 (IM17) CO Pt -HCOO b1 H (IM18) (H-CO) Pt -HCOO b1 (IM19) Energy Free Energy TS17 TS18 TS20 TS21 Ti C H O Pt O vacancy Relative (free) energy (ev) IM11 IM16 IM17 TS19 IM18 IM19 -CO 2 IM20 IM21 - H 2 +H 2 O TS25 IM22 IM23 TS26 IM25 TS IM24 IM1 (H-H-CO) Pt -OCO b1 (IM20) CO Pt -V b1 (IM22) (H-CO) Pt -O b1 H (IM24) (H-CO) Pt -O s H (IM25) Figure S9. (Free) energy profiles for the formate pathway with redox regeneration (T = 500 K; Pj(gas) = 1 atm). All energies are with reference to the sum of the energies of the initial state ((H- CO)Pt, IM1) and the reactant gas molecules. The insets provide a side view of the optimized structures of the intermediates. 12

13 Ti O C Surface Pt Edge interface Pt Corner interface Pt Figure S10. Active site model of TiO2-Pt8-2CO used in our earlier work 3,4 and the kinetic data of which are reported in Table 1 of the paper. 13

14 TS2 Ti C H O Pt TS3 TS5 TS6 TS7 TS9 Figure S11. Optimized structures of the transition states involved in the redox pathway of the WGS reaction on the Ti1-xPtO2-4H-CO model. The relative energies of these TSs are provided in Figure 3 of the paper. 14

15 Table S1: Zero point energy corrected reaction energies and forward activation barriers for the elementary steps considered in the WGS reaction mechanism on the Ti1-xPtO2-4H-CO model. aaaaaa ΔΔΔΔ Reaction ZZZZZZ EE ZZZZZZ (ev) (ev) (R1) (H-CO)Pt (IM1) + CO(g) (H-CO-CO)Pt (IM2) (R2) (H-CO-CO)Pt (IM2) + Ob1 (H-CO-CO2)Pt-Ob1 (IM3) (R3) (H-CO-CO2)Pt-Ob1 (IM3) (H-CO)Pt Vb1 (IM4) + CO2(g) (R4) (H-CO)Pt Vb1 (IM4) + H2O(g) (H-CO)Pt-H2Ob1 (IM5) (R5) (H-CO)Pt-H2Ob1 (IM5) + Os (H-CO)Pt-Ob1H-OsH (IM6) (R6) (H-CO)Pt-Ob1H-OsH (IM6) + Ob2 (H-CO)Pt-Ob1H-Ob2H (IM7) + Os (R7) (H-CO)Pt-Ob1H-Ob2H (IM7) (H-H-CO)Pt-Ob2H (IM8) + Ob (R8) (H-H-CO)Pt-Ob2H (IM8) COPt-Ob2H (IM9) + H2(g) (R9) COPt-Ob2H (IM9) (H-CO)Pt (IM1) + Ob (R10) (H-CO)Pt (IM1) + Ob1 COPt-Ob1H (IM10) (R11) COPt-Ob1H (IM10) + CO(g) (CO-CO)Pt-Ob1H (IM11) (R12) (CO-CO)Pt-Ob1H (IM11) (CO-COOH)Pt-Ob1 (IM12) (R13) (CO-COOH)Pt-Ob1 (IM12) + Ob2 (CO-CO2)Pt-Ob1-Ob2H (IM13) (R14) (CO-CO2)Pt-Ob1-Ob2H (IM13) COPt Vb1-Ob2H (IM14) + CO2(g) (R15) COPt Vb1-Ob2H (IM14) + H2O(g) COPt H2Ob1-Ob2H (IM15) (R16) COPt H2Ob1-Ob2H (IM15) (H-CO)Pt-Ob1H-Ob2H (IM7) (R17) (CO-CO)Pt-Ob1H (IM11) (CO-CHO)Pt (IM16) + Ob (R18) (CO-CHO)Pt (IM16) + Ob1 COPt-HCOOb1 (IM17) (R19) COPt-HCOOb1 (IM17) + Ob3H COPt-HCOOb1H (IM18) + Ob (R20) COPt-HCOOb1H (IM18) (H-CO)Pt-HCOOb1 (IM19) (R21) (H-CO)Pt-HCOOb1 (IM19) (H-H-CO)Pt-OCOb1 (IM20) (R22) (H-H-CO)Pt-OCOb1 (IM20) (H-H-CO)Pt-Vb1 (IM21) + CO2(g) (R23) (H-H-CO)Pt-Vb1 (IM21) COPt Vb1 (IM22) + H2(g) (R24) COPt Vb1 (IM22) + H2O(g) COPt H2Ob1 (IM23) (R25) COPt H2Ob1 (IM23) (H-CO)Pt-Ob1H (IM24) (R26) (H-CO)Pt-Ob1H (IM24) + Os (H-CO)Pt-OsH (IM25) + Ob (R27) (H-CO)Pt-OsH (IM25) + Ob3 (H-CO)Pt (IM1) + Ob3H + Os

16 Table S2: Forward rate constants (kfor) and equilibrium constants (K) calculated at different temperatures for the elementary steps considered in the WGS reaction mechanism on the Ti1- xpto2-4h-co model. T = 473 K T = 573 K T = 673 K Reaction kfor (s -1 ) K kfor (s -1 ) K kfor (s -1 ) K (R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13) (R14) (R15) (R16) (R17) (R18) (R19) (R20) (R21) (R22) (R23) (R24) (R25) (R26) (R27)

17 Table S3: Surface coverage (θ) of various intermediates calculated at different temperatures for the WGS reaction on the Ti1-xPtO2-4H-CO model (PCO=0.1, PH2O=0.2, and PH2=0.4 atm). Intermediate Surface Coverage (θ) T=473 K T=573 K T=673 K (H-CO)Pt (IM1) (H-CO-CO)Pt (IM2) (H-CO-CO2)Pt-Ob1 (IM3) (H-CO)Pt Vb1 (IM4) (H-CO)Pt-H2Ob1 (IM5) (H-CO)Pt-Ob1H-OsH (IM6) (H-CO)Pt-Ob1H-Ob2H (IM7) (H-H-CO)Pt-Ob2H (IM8) COPt-Ob2H (IM9) COPt-Ob1H (IM10) (CO-CO)Pt-Ob1H (IM11) (CO-COOH)Pt-Ob1 (IM12) (CO-CO2)Pt-Ob1-Ob2H (IM13) COPt Vb1-Ob2H (IM14) COPt H2Ob1-Ob2H (IM15) (CO-CHO)Pt (IM16) COPt-HCOOb1 (IM17) COPt-HCOOb1H (IM18) (H-CO)Pt-HCOOb1 (IM19) (H-H-CO)Pt-OCOb1 (IM20) (H-H-CO)Pt-Vb1 (IM21) COPt Vb1 (IM22) COPt H2Ob1 (IM23) (H-CO)Pt-Ob1H (IM24) (H-CO)Pt-OsH (IM25)

18 Supplementary References: (1) Zhai, Y. P.; Pierre, D.; Si, R.; Deng, W. L.; Ferrin, P.; Nilekar, A. U.; Peng, G. W.; Herron, J. A.; Bell, D. C.; Saltsburg, H.; Mavrikakis, M.; Flytzani-Stephanopoulos, M. Science 2010, 329, (2) Ammal, S. C.; Heyden, A. J. Catal. 2013, 306, (3) Ammal, S. C.; Heyden, A. ACS Catal. 2014, 4, (4) Buzzi-Ferraris, G. Politecnico di Milano, BzzMath: Numerical libraries in C++, 2012, 18