Supported Single Pt 1 /Au 1 Atoms for Methanol Steam Reforming Xiang-Kui Gu,, Botao Qiao,,, Chuan-Qi Huang, Wu-Chen Ding, Keju Sun, Ensheng Zhan,, Tao Zhang, Jingyue Liu*,,, and Wei-Xue Li*, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academic of Sciences, Dalian, 116023, China Department of Physics, Arizona State University, Tempe, Arizona 85287, United states of America Corresponding Author jingyue.liu@asu.edu and wxli@dicp.ac.cn Author Contributions These authors contributed equally. Supporting Information 1
Content: Theoretical and Experimental Setup Table S1: Calculated binding energy of the reactants and intermediates Table S2: Calculated reaction barriers and reaction energetics Table S3: Test calculations of DFT+U/van der Waals interaction Figure S1: HAADF images for Pt 1 /Au 1 /ZnO NWs Figure S2: Calculated charge density difference for ZnO and Pt 1 /Au 1 /ZnO { 1010} Figure S3: Optimized structure for reactants/intermediates on ZnO and Pt 1 /Au 1 /ZnO{ 1010} Figure S4: Calculated projected density of states for ZnO and Pt 1 /Au 1 /ZnO{ 1010} Figure S5: Calculated charge density difference for CH3O* on ZnO and Pt 1 /Au 1 /ZnO{ 1010} Figure S6: Transition states for TS4, TS5 and TS6 on ZnO{ 1010} 2
1. Computational methods Spin-polarized DFT calculations were performed by VASP 1 using PAW potential, and the generalized gradient approximation in form of Perdew-Wang 91 functional. 2 The plane wave cutoff of 400 ev was used. The optimized lattice constants of bulk ZnO were a=3.28 Å and c= 5.30 Å, which agree well with the experiments (a=3.25 Å and c= 5.21 Å). ZnO{ 1010} were modeled by a six-layer slab model with the (3 1) periodicity separated by 12 Å vacuum along z-direction. One third of the surface Zn atoms was substituted by Pt or Au atom, and the adsorbates and intermediates were placed on one side of the slab with dipole correction. The (2 2 1) k-point mesh was used to sample the Brillouin zone. The bottomfour layers of the slab were fixed, while the remained atoms were relaxed until the residual forces less than 0.02 ev/å. The transition states were identified by the climbing-image nudged elastic band (CI- NEB) method 3, and the barriers for the elementary reaction steps were calculated as the energy difference between transition states and the most stable adsorbed/coadsorbed reactants without zero point correction. Influence of DFT+U correction 4 was tested (Table S3). When U value varied from 2.0 to 6.0 ev, the change of the calculated binding energies for H 2 O and CH 3 OH and the corresponding dissociation barriers is very small, and the maximum change of the binding energies for CH 3 O is 0.26 ev. Moreover, the influence of van der Waals interaction on binding energy and reaction barrier was also tested (Table S3), and the largest change is 0.24 ev. Importantly, the relative trend for the binding energy and reaction barriers was little influenced by the method used (DFT, DFT+U, and van der Waals), which agree well with previous work. 5-6 References (1) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169-11186. (2) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244-13249. (3) Henkelman, G.; Jonsson, H. J. Chem. Phys. 2000, 113, 9978-9985. (4) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505-1509. (5) Pala, R. G. S.; Metiu, H. J. Catal. 2008, 254, 325-331. (6) Meyer, B.; Rabaa, H.; Marx, D. Phys. Chem. Chem. Phys. 2006, 8, 1513-20. 2. Catalyst preparation The ZnO nanowires were synthesized by a modified physical vapor deposition method. The Au 1 and Pt 1 on ZnO-nws were prepared by adsorption method. Specifically, a desired amount of ZnO-nws were dispersed in deionized water and then the calculated amount of HAuCl 4 or H 2 PtCl 4 solution were added into the ZnO-nws suspension under stirring at room temperature. After stirring and aging for 2 hours, the suspension were filtered and washed with deionized water for several times. The samples were then dried at 60 o C for 5 h to form the Pt 1 /ZnO and Au 1 /ZnO catalysts without any heat treatment thereafter. The final Pt 1 /ZnO and Au 1 /ZnO catalysts contain about 0.0125 wt% of metal. 3. Electron microscopy characterization 3
Sub-angstrom resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images were obtained on the JEM-ARM200F TEM/STEM with a guaranteed resolution of 0.08 nm. Before microscopy examination, the samples were ultrasonically dispersed in ethanol and then a drop of the solution was put onto a copper grid coated with a thin lacey carbon film. 4. Catalytic performance test The methanol steam reforming (MSR) reaction test was carried out in a fixed-bed reactor with 50 mg of catalyst in about 60-80 mesh size. The CH 3 OH and deionized water were premixed and pumped to a helium (He) carrier gas and vaporized at 180 o C. The final feed gas composition was 8 vol% CH 3 OH + 12 vol% H 2 O and He balance. The total gas flow rate was 46 ml/min., providing a space velocity of 55,200 ml h -1 g -1 -cat. The reaction products were on-line analyzed by a gas chromatograph (Agilent 7890A, HS-D 100/120 column and thermal conductivity detector). The products detected are H 2, CO and CO 2 and no other products wered etected. The conversion rate of the MSR reaction was calculated based on the carbon balance. The specific rates were tested for the methanol conversions below 20%. The TOF of the ZnO was calculated by assuming all the Zn sites in the topmost layer of the ZnO surface are active center, and the TOF of the Au and Pt metal was calculated from specific rate by using 100% dispersion by substring the contribution of the support. 4
Table S1. The calculated binding energies of the reactants and key intermediates involved in MSR on pristine and Pt 1 /Au 1 embedded ZnO{ 1010}. binding energy (ev) Intermediate Pt 1 /ZnO Au 1 /ZnO ZnO H 2 O -1.17-1.12-1.06 CH 3 OH -1.19-1.11-1.04 OH -3.55-2.93-1.93 CH 3 O -2.78-2.18-1.15 HCHO -1.89-1.36-0.90 HCO -3.05-2.47-0.63 CO -1.82-1.31-0.33 CO 2-0.90-0.87-0.82 5
Table S2. The calculated barriers (E a in ev) and reaction energies (ΔE in ev) of the elementary steps for MSR on the pristine and Pt 1 /Au 1 embedded ZnO{ 1010} surfaces. Bold font represents favorable reaction pathway for TS4. ZnO Pt 1 /ZnO Au 1 /ZnO Elementary Reaction E a ΔE E a ΔE E a ΔE TS1 HO 2 * + * OH* + H* 0.28 0.21 0.44-0.27 0.65 0.19 TS2 CH3OH * * CH3O* H * + + 0.31 0.23 0.49-0.25 0.66 0.18 TS3 CH3 O* + * HCHO * + H * 0.19-1.14 1.05-1.11 0.63-1.58 TS4 HCHO * + * HCO * + H * 1.51 0.29 1.31-0.72 1.40-0.88 TS4 HCHO* OH* H2COOH * * + + 0.51-0.62 1.00 0.65 1.75 0.75 TS5 HCOOH 2 * + 2* HCOO* + 2 H* 0.57-2.43 - - - - TS6 HCOO* * CO2 ( gas) H * * + + + 1.11-0.46 - - - - TS5 HCO * + * CO * + H * - - 0.52-1.32 0.32-2.13 TS6 CO* OH* CO2 ( g) H * * + + + - - 1.24 0.42 0.64-2.39 6
Table S3. Comparison of different calculation methods, including DFT, DFT+U and van der Waals interaction, for binding energy and reaction barrier (in ev) on ZnO{ 1010}. Binding energies DFT DFT+U van der Waals 2.0 ev 4.0 ev 6.0 ev CO* -0.33-0.33-0.32-0.32-0.36 H 2 O* -1.06-1.12-1.12-1.11-1.11 CH 3 OH* -1.04-1.04-1.04-1.04-1.16 CH 3 O* -1.15-1.05-0.96-0.89-1.39 Reaction barriers HO 2 * + * OH* + H* 0.28 0.29 0.30 0.31 0.20 CH OH * + * CH O* + H * 0.31 0.32 0.33 0.34 0.18 3 3 7
Figure S1a. Low (left) and high (right) magnification HAADF images show the ZnO nanowires and the relatively flat and clean ZnO { 1010} surfaces. 8
Figure S1b: High magnification HAADF-STEM images of M 1 /ZnO nanowires to reveal the embedded Au 1 atom (indicated by the yellow arrow) with the electron beam close to the! " 1020# $zone axis of ZnO (a) and corresponding projected ball-stick model (b), and the embedded Pt 1 atom with the electron beam close to the! " 1010# $direction (c) and the corresponding ball-stick model (d). In the ball-stick models, only cations are plotted: larger and smaller green balls for the embedded Au 1 /Pt 1 atom and Zn in the topmost layer, blue ball for Zn in the second layer, gray ball for Zn in the third and fourth layer, respectively. 9
Figure S1c. HAADF images show the individual Pt atoms dispersed onto the ZnO { 1010} surfaces. 10
Figure S1d. HAADF images show the individual Au atoms dispersed onto the ZnO { 1010} surfaces. 11
Figure S2. The calculated charge density difference of pristine and Pt 1 /Au 1 substituted ZnO. The red and blue regions represent electron accumulation and depletion, respectively. 12
Figure S3. The most stable geometric structures of the key intermediates on pristine and Pt 1 /Au 1 embedded ZnO{ 10 10} surfaces. The blue, green, red, gray, and white spheres represent Pt 1 /Au 1, Zn, O, C, and H atoms respectively. 13
Figure S4. The projected density of states (PDOS) for Zn-3d, Au-5d, and Pt-5d in pristine and Au 1 /Pt 1 /ZnO{ 1010}, repsectively. 14
Figure S5. The charge density difference for CH 3 O* on pristine and Au 1 /Pt 1 embedded ZnO{ 1010}, repsectively. The red and blue regions represent the charge accumulation and depletion, respectively. 15
Figure S6. The schematic structure for the transition state structures TS4, TS5, and TS6 on the pristine ZnO{ 1010}.The transition state for dehydrogenation of CH 2 O to HCO is given at the end. The green, red, gray, and white spheres represent Zn, O, C, and H atoms, respectively. 16