Photothermal Catalysis over Non-plasmonic Pt/TiO 2 Studied by Operando HERFD-XANES, Resonant XES and DRIFTS

Transcription

1 Supporting Information Photothermal Catalysis over Non-plasmonic Pt/TiO 2 Studied by Operando HERFD-XANES, Resonant XES and DRIFTS Ying Zhou a,b *, Dmitry E. Doronkin a,c, Ziyan Zhao a,b, Philipp N. Plessow c, Jelena Jelic c, Blanka Detlefs d, Tim Pruessmann, a,c Felix Studt, a,c and Jan-Dierk Grunwaldt a,c* a) Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany b) School of Materials Science and Engineering, Southwest Petroleum University, Chengdu , China c) Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany d) European Synchrotron Radiation Facility (ESRF), Grenoble, France *To whom correspondence should be addressed grunwaldt@kit.edu (J.-D. Grunwaldt); yzhou@swpu.edu.cn (Y. Zhou) S1

2 Experimental Section 1. Catalyst synthesis TiO 2 nanoparticles dominated with {101} facets were prepared according to our previous work. [1] Typically, 2.0 g of the fresh Ti(OH) 4 precursor and 0.2 g of NH 4 Cl were added into a mixture of 15mL of water and 15 ml of isopropyl alcohol in a 50 ml Teflon-lined autoclave. After stirring for 30 min, the suspension was heated to 180 C and kept under these conditions for 24 h. After the reaction, the products were collected by filtration and washed with deionized water several times. Pt/TiO 2 was prepared via wet impregnation. 500 mg prepared TiO 2 with {101} facets was added to a mixture of 30 ml of water and 5 ml of H 2 PtCl 6 (2 mg/ml) aqueous solution and stirred for 6 h. The initial mass ratio of Pt to TiO 2 is 2%. After centrifugation, the product was collected and dried in an oven at 65 C for 24 h. 2. Photothermal catalytic performance evaluation The photothermal catalytic reactions were performed in a Praying Mantis TM high temperature reaction chamber from Harrick with a modified top cover with a CaF 2 window (Fig. 1, Fig. S4). [2] The temperature of the catalyst bed in the cell was calibrated by an IR camera (InfraTec Image IR 3) and Schott KL 2500 LCD light source (with an Osram Xenophot HLX250 W tungsten-halogen lamp and a 1m glass fiber light guide) was used as a light source for photocatalysis in the light-assisted experiments with a typical intensity of 683 mw cm -2 (intensity distribution corresponds to a black body radiation at 3300K). Light intensity was measured by a Thorlabs PM100D power meter equipped with a photodiode arranged in the same geometry as the catalyst sample. The catalyst layer thickness was approx. 1 mm (approx. 20 mg) and silica with the same grain size ( μm) was used to fill the space between the catalyst layer and the bottom of the sample holder (2 mm). The catalysts were pretreated inside the cell in 5% H 2 in N 2 with flow rate of 100 ml/min at 180 C for 1 h. After that, the catalysts were cooled to room temperature with the same gas flow condition for catalysis reaction. CO oxidation experiments were performed on a gas composition of 1000 ppm of CO and 10% O 2 in N 2 at a flow rate of 50 S2

3 ml/min (GHSV = ~ h -1 ). The temperature and light intensity were varied accordingly and the concentration of outlet gases (CO and CO 2 ) was measured by NDIR continuous analyzer (Hartmann & Braun Uras 10E). It continuously measures absorption at specific wavelengths and produces an analog voltage signal which is sampled every 2 seconds with an ADC. A special setup was built to measure temperature of the catalyst bed with/without light irradiation with 10 Hz time resolution and 0.1 C precision. The same Harrick DRIFTS cell, the same Pt/TiO 2 catalyst and the same gas flow was used as for catalytic experiments. Additional 0.4 mm thick N-type thermocouple was fixed with its tip at 0.5 mm below the upper surface of the catalyst layer, at a distance of 1 mm from the wall of the sample cup. The thermocouple voltage was read out by a fast data logger based on a Meilhaus Electronic RedLab-TC data acquisition board. CO conversion was determined from as X CO = C CO2 out / C CO in, where C CO2 out is the concentration of produced CO 2 and C CO in the concentration of CO at the inlet of the reaction cell. 3. In situ DRIFTS measurements In situ DRIFTS investigations were carried out on a VERTEX 70 FTIR spectrometer (Bruker) equipped with an in situ diffuse-reflectance cell (Harrick) with a non-modified dome cover. [3] Two KBr windows are for IR measurements, while a glass window is for light irradiation using a Schott KL 2500 LCD lamp with a typical intensity of 683 mw/cm 2. The catalyst bed and gas flow condition are the same with that for photothermal catalytic activity investigation. The IR scanning range was cm 1 with averaging over 100 scans. KBr was used for obtaining background spectra. 4. Operando HERFD-XANES / XES measurements Operando HERFD-XANES/XES measurements were performed at ID20 beamline (ESRF, Grenoble, France, Fig. S9). X-ray beam generated by 3 u26 undulators was monochromatized using Si (111) double crystal and Si (311) channel-cut pre- and post-monochromators. A secondary emission spectrometer (Rowland geometry) using a S3

4 Si (844) reflection of a spherically bent (r = 1m) analyzer crystal and a Maxipix detector was used to record HERFD-XANES and XES at Pt L 3 absorption (scanning incident energy with the emission spectrometer set to the maximum of the Pt Lβ 3 emission line) and Pt Lβ 5 emission lines (keeping incident energy at the maximum of the white line of the absorption spectrum and scanning the energy of the emitted X-rays). Photon flux at the sample position was approx photons/sec and the total instrumental energy bandwidth approx. 1.5 ev. The beam size was approx. 100x100 μm 2. HERFD-XANES spectra were normalized using ATHENA software. [4] Pressed and sieved catalysts ( μm) were loaded in the modified Harrick cell with a 60 μm thick flat mica window (Fig. S9). The light source and catalyst bed configuration are the same as during the photothermal catalytic activity investigation. The catalysts were pretreated (reduced) for 15 min at 180 C in 5% H 2 in He, after which the gas feed was changed to 10% O 2 in He and, subsequently, 1000 ppm CO, 10% O 2 in He (total flow 135 ml/min, GHSV ~ h -1 ), and stepwise heating and switching light on / off was performed. HERFD-XANES and XES spectra were recorded after changing the gas atmosphere and/or temperature. CO conversion was monitored using an online MKS 2030 FTIR gas analyzer. CO conversions slightly changed with time and the value obtained 5 min after establishing desired conditions is reported (HERFD-XANES and XES acquisition took up to 4 h under each set of conditions). Beam effects (so-called beam damage ) were evaluated in the following way. A series of the short XANES spectra (2-3 min each) was recorded and differences between the spectra were evaluated. In this way we observed no beam damage, probably due to relatively thick mica window used which acted as an attenuator (estimated to absorb 60 70% of photons at Pt L 3 edge). At the same time no changes in the CO conversion upon exposure to the X-ray beam was noticed. However beam damage in form of desorption of CO and oxidation of Pt sites has been noticed after long XES scans (30 min per scan). The effect was found to be reversible, i.e. CO was adsorbed and reduced Pt sites within several minutes after turning off the X-ray beam. Two measures were taken to avoid the effect: first, the beamline attenuators were used to be able to record a single XES scan S4

5 without significantly altering the state of Pt. Second, for each consecutive XES scan the X-rays probed a different part of the catalyst (the non-exposed parts could recover during the scan), at least 4 XES scans were recorded in this way and then averaged. 5. Theoretical calculations 5.1 X-Ray spectroscopy Model Pt 3 clusters supported on TiO 2 were built for spectral calculations representing small Pt particles evidenced by TEM and EXAFS analysis of Pt/TiO 2 under CO oxidation conditions obtained in our previous work. [1] The geometry of the model anatase TiO 2 {101} slab with Pt 3 O 3 and Pt 3 (CO) 3 representing Pt sites with adsorbed oxygen and CO was optimized by using the PBE [5,6] density functional with a cutoff of 400 ev, and a -centered k-point-sampling of 3x2 for the 1x4 supercells of the supporting TiO 2 {101} slab. Calculations were carried out with Gaussian smearing, a width of 0.1 ev and the VASP [7,8] code in version 5.4 as well as the standard VASP PAWs. The bulk structure of anatase was optimized with a cutoff of 800 ev, 8x8x8 k-point-sampling resulting in optimized lattice vectors with the lengths (382, 382, 971) pm. HERFD-XANES spectra were modeled using multiple scattering FEFF ab initio code. [9] The radius of the cluster for self-consistent potential calculations (SCF) was set to 5 Å, full multiple scattering calculations (FMS) were computed for the same radius. To simulate high energy resolution the line broadening was reduced by setting vi0 in the EXCHANGE card to 1.6 ev. [10] Resonant XES spectra were obtained using FEFF-RIXS code and the same input parameters as for HERFD-XANES. Spectra were calculated for each inequivalent Pt atom in the model clusters and then averaged with the corresponding weight. 5.2 Vibrational spectroscopy CO vibrational frequencies were calculated using using VASP [7,11] in connection with the Atomic Simulation Environment (ASE). [12] A plane-wave basis set with a cutoff energy of 450 ev, the projector augmented wave method (PAW) [7, 13] and the Bayesian Error Estimation Functional with van der Waals correlations (BEEF-vdW) [14] exchange S5

6 correlation functional were used. Vibrational frequencies were calculated for two types of systems (Figure S8): one is (2 x 2 x 3) anatase TiO 2 {101} unit cell with small 3 Pt atoms cluster and three CO molecules. For this system the Brillouin zone was sampled with a uniform 3 x 2 x 1 k-point grid. [15] Smaller (1 x 2 x 3) anatase TiO 2 {101} unit cell was used to model CO adsorption on single Pt atom and PtO 2 molecule and for this system we used 5 x 3 x 1 k-point grid. Approximately 20 Å of vacuum was used to separate the slabs. The convergence criterion for geometry optimizations was a maximum force of 0.01 ev/å. Spin polarization was not considered in these calculations. The obtained vibrational frequencies were scaled with respect to the difference between the calculated value for the gas phase CO ( cm -1 ) and the respective experimental value (2143 cm -1 ). References: [1] Zhou, Y.; Doronkin, D. E.; Chen, M. L.; Wei, S. Q.; Grunwaldt, J.-D. Interplay of Pt and Crystal Facets of TiO 2 : CO Oxidation Activity and Operando XAS/DRIFTS Studies. ACS Catal. 2016, 6, [2] Fontaine, C. L.; Barthe, L.; Rochet, A.; Briois, V. X-ray Absorption Spectroscopy and Heterogeneous Catalysis: Performances at the SOLEIL's SAMBA Beamline. Catal. Today. 2013, 205, [3] Zhou, Y.; Zhao, Z. Y.; Wang, F.; Cao, K.; Doronkin, D. E.; Dong, F.; Grunwaldt, J.-D. Facile Synthesis of Surface N-doped Bi 2 O 2 CO 3 : Origin of Visible Light Photocatalytic Activity and In Situ DRIFTS Studies. J. Hazard. Mater. 2016, 307, [4] Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT, J. Synchrotron Radiation 2005, 12, [5] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, [6] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 78, [7] Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, S6

7 [8] Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, [9] Rehr, J. J.; Kas, J. J.; Vila, F. D.; Prange, M. P.; Jorissen, K. Parameter-free Calculations of X-ray Spectra with FEFF9. Phys. Chem. Chem. Phys. 2010, 12, [10] Safonova, O. V.; Tromp, M.; van Bokhoven, J. A.; de Groot, F. M. F.; Evans, J.; Glatzel, P. Identification of CO Adsorption Sites in Supported Pt Catalysts Using High-Energy-Resolution Fluorescence Detection X-ray Spectroscopy. J. Phys. Chem. B 2006, 110, [11] Kresse, G.; Furthmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, [12] Bahn, S. R.; Jacobsen, K. W. An Object-Oriented Scripting Interface to a Legacy Electronic Structure Code. Comput. Sci. Eng. 2002, 4, [13] Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, [14] Wellendorff, J.; Lundgaard, K. T.; Møgelhøj, A.; Petzold, V.; Landis, D. D.; Nørskov, J. K.; Bligaard, T.; Jacobsen, K. W. Density Functionals for Surface Science: Exchange-correlation Model Development with Bayesian Error Estimation. Phys. Rev. B 2012, 85, [15] Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-zone Integrations. Phys. Rev. B 1976, 13, S7

8 Figure S1. XRD patterns of the prepared TiO 2 and as-prepared Pt/TiO 2. Figure S2. Representative (a) TEM and (b) HRTEM images of the obtained Pt/TiO 2 catalyst. S8

9 Figure S3. UV-vis absorption spectra of the prepared TiO 2 and Pt/TiO 2. Figure S4. Photograph of the cell for the photothermal catalytic reaction. S9

10 Figure S5. The CO concentration and produced CO 2 at room temperature in the dark and under light irradiation over 2 wt % Pt/γ-Al 2 O 3. Figure S6. CO concentration over pristine TiO 2 in the dark and under light irradiation with different intensities ( mw/cm 2 ). The area between vertical lines indicates light irradiation and the corresponding intensity. S10

11 Figure S7. In situ DRIFTS of CO vibrations recorded from TiO 2 support under 1000 ppm of CO, 10 vol % O 2 /N 2 at different temperatures. TiO 2 spectra were recorded after the same reduction pretreatment and under the same conditions as Pt/TiO 2. Figure S8. IR spectra of CO on Pt/γ-Al 2 O 3 recorded at room temperature under light irradiation. S11

12 Figure S9. In situ DRIFTS of CO adsorbed on Pt/TiO 2 under He flow at different temperatures: (a) 45 C; (b) 100 C. Figure S10. In situ DRIFTS of CO adsorbed on Pt/TiO 2 recorded under 1000 ppm of CO, 10 vol % O 2 /N 2. S12

13 CO conversion Rate of CO oxidation / mol/(g cat s) Figure S11. In situ DRIFTS of CO adsorbed on Pt/TiO 2 during photocatalytic reaction process recorded at 45 o C under 1000 ppm of CO, 10 vol % O 2 /N 2 with different light intensities: (a) 972 mw/cm 2 ; (b) 461 mw/cm 2 ; (c) 114 mw/cm 2. (d) photocatalytic rate (subtracting the rate of the thermal process from the rate of the photothermal process) as a function of light intensity over Pt/TiO 2 at 45 o C. a. 3.0 b Temperature / C Temperature / C Figure S12 (a) Conversion of CO and (b) reaction rates achieved over Pt/TiO 2 with and without light during operando measurements. Conditions: 1000 ppm CO, 10% O 2 in He, ~20 mg of catalyst, flow rate 135 ml/min. S13

14 8 a. HERFD-XANES b. XES 7 pretreated in H 2 pretreated in H exposed to O 2 exposed to O 2 and CO exposed to O 2 exposed to O 2 and CO Figure S13. (a) HERFD-XANES and (b) XES spectra of Pt/Al 2 O 3 measured at RT (20 C) after reductive pretreatment under 5%H 2 /He, after exposure to 10% O 2 /He, and 1000 ppm CO/10% O 2 /He. S14

15 CO conversion Rate of CO oxidation / mol/(g cat s) a. HERFD-XANES b. XES RT 80 C 133 C RT 80 C 133 C c. 3.0 d Temperature / C Temperature / C Figure S14 (a) HERFD-XANES and (b) XES spectra of Pt/Al 2 O 3 measured under CO oxidation feed at different temperatures without light. (c) Conversion of CO and (d) reaction rates with and without light during operando measurements. S15

16 a. Calculated XES spectra Pt 3 O 3 Pt 3 (CO) ev ev b. Pt 3 O 3 /TiO 2 c. Pt 3 (CO) 3 /TiO 2 calculated XES spectrum Pt d-dos Ti d-dos O s,p-dos E F calculated XES spectrum Pt d-dos Ti d-dos O s,p-dos C s,p-dos E F Figure S15. (a) Calculated XES spectra of Pt 3 O 3 /TiO 2 and Pt 3 (CO) 3 /TiO 2 model clusters. (b) and (c) d-dos on Pt as well as s- and p-dos on O and C in Pt 3 O 3 /TiO 2 (b) and Pt 3 (CO) 3 /TiO 2 (c). a. HERFD-XANES b. XES 2 RT RT ev ev Figure S16. (a) HERFD-XANES and (b) XES spectra of Pt/TiO 2 measured during CO oxidation at room temperature with and without light. Conditions: 1000 ppm CO, 10% O 2 in He, flow rate 135 ml/min. S16

17 a. HERFD-XANES b. XES RT RT c. HERFD-XANES d. XES C 80 C e. HERFD-XANES f. XES C 133 C Figure S17. HERFD-XANES and XES spectra of Pt/Al 2 O 3 measured during CO oxidation at room temperature (a,b), 80 C (c,d), 133 C (e,f) with and without light. Conditions: 1000 ppm CO, 10% O 2 in He, flow rate 135 ml/min. S17