Minimized Pt usage and improved catalytic stability

Transcription

1 Supporting Information Pt modified ZnO/Al2O3 for Propane dehydrogenation: Minimized Pt usage and improved catalytic stability Gang Liu, Liang Zeng, Zhi-Jian Zhao, Hao Tian, Tengfang Wu, and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin , China * S1

2 Table of Contents 1. Experimental methods and DFT calculations 2. XRD patterns of the fresh catalysts 3. UV-Visible patterns of the fresh catalysts 4. FT-IR of CO adsorption 5. Catalytic performance 6. FT-IR of pyridine adsorption 7. TGA profile of spent catalysts 8. TPR results 9. Fitting results of XPS spectra 10. H2O profile during reaction 11. XPS overview spectrum 12. XPS spectra of the Al 2P (including Pt 4f) 13. TPSR profile 14. Stability test over the 15Zn0.1Pt catalyst 15. Surface chemical composition 16. Details of DFT calculated elementary steps for propane dehydrogenation and H2, H2O formation S2

3 1. Experimental methods and DFT calculations Catalyst preparation The catalysts were prepared by the incipient wetness impregnation method. H2PtCl6 6H2O (Tianjin Kaiyingte chemical trade Co., Ltd, 99.9%), Zn(NO3)2 6H2O (Alfa Aesar 98%) were used as precursors and γ-al2o3 (Sinopharm chemical reagent Co.) was used as the support. After impregnation, the solids were dried at 80 C overnight, and calcined at 500 C for 2 h. The element loading was based on the weight ratio between Pt/Zn and Al2O3. Catalyst characterization XRD measurements were performed with 2θ values between 10 and 90 by using a Rigaku C / max-2500 diffractometer with the graphite filtered Cu Kα radiation (λ= Å). The optical absorbance measurement of the catalysts was performed using a Shimadzu UV-2550 spectrophotometer. Textual properties of the catalysts were measured with a Micromeritics Tristar 3000 analyzer by nitrogen adsorption at -196 C. The samples were outgassed at 300 C for 3 h before measurements. This instrument employed the Brunauer- Emmett-Teller (BET) method by measuring the quantity of nitrogen adsorbed at -196 C. TGA (STA449F3 NETZSCH Corp.) was used to investigate the carbon deposition of spent catalysts. The sample was preheated at 80 C for 0.5 h in N2 (50 ml/min), then the sample was heated to 700 C at a rate of 10 C/min in air (100 ml/min). Elemental composition of the prepared catalysts was determined by the ICP-OES (VISTA-MPX, Varian). Prior to measurements, the samples were digested in H2SO4 and H3PO4 mixed solutions. Fouriertransform infrared (FT-IR) spectra of chemisorbed pyridine and CO was collected on a Thermo Scientific Nicolet 6700 spectrometer. 30 mg catalysts were loaded into in-situ cell equipped with ZnSe windows after compressing, then were preheated at 500 C under H2/Ar to simulate reaction condition. Pyridine and CO were adsorbed at 50 and 30 C respectively, and the spectra was recorded after adsorption. The XPS analysis of the catalysts was performed using Al KR X-ray source (E= ev) on a Perkin-Elmer PHI 1600 ESCA system operated at a pass energy of ev for survey spectra. All binding energy (BE) values were referenced to the C 1s peak at ev. High angel annular dark field-scanning transmission electron microscopy (HAADF-STEM) was performed on JEM-2100F trans mission electron S3

4 microscope at 200 kv. Hydrogen-oxygen titration method was used to study the dispersion of platinum, and the method was based on prior works 1. For each test, 100 mg sample was pre-treated with 10 vol % H2/Ar at 500 C for 1 h, then cooled to 50 C. Subsequently, 10 vol % O2/He was admitted to the sample by injection pulses until the consumption peaks became stable. Then, the H2 chemisorption was carried out by injection pulses of 10 vol % H2/Ar. It can be assumed that the adsorption stoichiometry factor Pt/H2 = 2/3. The metal dispersion is calculated by the Eq. (1): Dispersion (%) = 100 VH2 2/3 MWPt / (WPt 22414). (1) Where VH2 is the volume of adsorbed H2 (ml), MWPt is the atomic weight of Pt (g/mol), and WPt is the weight of supported Pt on the sample (g). NH3 and H2-TPD experiments were performed on a Micromeritics AutoChem 2920 apparatus. After pretreating at 500 C for 1h under 10 vol % H2/Ar and cooling to 50 C under Ar, NH3 or H2 was injected until adsorption saturation occurred, followed by purging with He for 1 h. The temperature was then raised at 10 C/min, NH3 and H2 desorbed were detected by thermal conductivity detector and HIDEN QIC-20 mass spectrometer (m/e=2) respectively. For TPSR experiment, 50 mg fresh catalyst was first pretreated at 300 C under Ar to remove water. Upon cooling to 100 C, a flow rate of 10 ml/min of reaction gas (N2/H2/C3H8=22/14/14) was used, and the temperature was increased linearly from 100 to 700 C at 10 C/min. Mass spectrometer signals at m/e of 44, 41 and 2 were monitored. The H2O profile was recorded during reaction at 600 C by MS. Specifically, 50 mg fresh catalyst was heated to 600 C under Ar, then the flow was changed to reaction gas (N2/H2/C3H8=22/14/14) at a flow rate of 10 ml/min. Simultaneously, MS was turn on to record the signal with m/e 44, 36 and 18. Catalytic test Catalytic tests were carried out at the atmospheric pressure and 600 C in a quartz fixed-bed reactor with 8 mm inner diameter and 24 cm length. 0.5 g catalyst with mesh size distribution was loaded in the quartz tubular reactor. The catalyst was first heated to 600 C under N2, afterward N2 was replaced by PDH reaction mixture of C3H8 (28 vol %) and H2 (28 vol %) in N2 at a total flow of 50 ml/min. the weight hourly space velocity (WHSV) of propane was 3 h -1. The product gas was analyzed by an online GC equipped with a S4

5 flame ionization detector (Chromosorb 102 column) and a thermal conductivity detector (Al2O3 Plot column). The selectivity to propylene was determined from Eq. (2): Sel (%) = 100 ni [Fi]out / ( ni [Fi]out). (2) Where i represents the hydrocarbon products in the effluent gas, ni is the number of carbon atoms of the component i, and Fi is the corresponding flow rate. Deactivation of the catalysts was calculated by comparing the percentage change in propylene concentration at the outlet between 20 and 240 min time on stream using following Eq. (3): D (%) =100 { [F(C3H6)]20min [F(C3H6)]240min}/[F(C3H6)]20min. (3) Where [F(C3H6)]20min and [F(C3H6)]40min are the C3H6prduction rates at 20 and 240 min. DFT calculations DFT calculations were performed with the plane-wave based Vienna ab initio simulation package VASP with BEEF exchange-correlation functional. 2-4 The (2x2) ZnO(1010) was represented by periodic slab models of three layer thickness. The valence wave functions were expanded in a plane-wave basis with a cutoff energy of 400 ev, while the interaction between the atomic cores and the electrons was described by the projector augmented wave (PAW) method. 5 A Monkhorst-Pack mesh of was used to sample the Brillouin zone. 6 A U-J = 4.7 ev was used for Zn atoms to correct the on-site Coulomb repulsion of 3d electrons via the DFT+U method. [5] S5

6 2. XRD patterns of the fresh catalysts Figure S1. XRD patterns of fresh ZnO/Al 2O 3 and Pt-ZnO/Al 2O 3 catalysts with different Zn loadings. Only diffraction lines of γ-al2o3 (JCPDS ) were detected, suggesting that ZnO were well dispersed over the Al2O3. No diffraction peaks of Pt were detected due to its extremely low loading and small particle size. S6

7 3. UV-Visible patterns of the fresh catalysts Figure S2. UV-visible absorption spectra of fresh ZnO/Al 2O 3 and Pt-ZnO/Al 2O 3 with different Zn loadings. After subtracting the optical absorption of Al2O3 support, no absorption band at about 370 nm corresponding to the band gap width of ZnO macrocrystalline was observed, and the absorption edge between 240 and 270 nm can be associated with small ZnO clusters around 1 nm. 7, 8 S7

8 4. FT-IR spectra of CO adsorption Figure S3. FT-IR spectra of CO adsorbed on the 15Zn, 15Zn0.1Pt and 0.1Pt catalysts. 5. Catalytic performance Figure S4. Catalytic activity and selectivity of the 0.1Pt, 1Zn0.1Pt, 10Zn0.05Pt, 10Zn0.15Pt, 15Zn0.15Pt catalysts (T = 600 C, atmospheric pressure, WHSV propane = 3 h 1, 500 mg of catalyst, C 3H 8/H 2 = 1/1, with balance N 2 for total flow rate of 50 ml/min). S8

9 6. FT-IR of pyridine adsorption Figure S5. Pyridine IR spectra of the 15Zn and 15Zn0.1Pt catalysts. Three infrared bands at 1450, 1490, and 1610 cm -1 which are assigned to pyridine adsorbed on Lewis acid sites, while no Bronsted acid was detected. The integral peak areas were used to obtain the semiquantitative results of the total acidity, i.e., 470 and 484 for the 15Zn and 15Zn0.1Pt catalysts. Clearly, the presence of Pt had a negligible influence on the total amount of acidity (i.e., 1:1 for the 15Zn and 15Zn0.1Pt catalysts). S9

10 7. TG profile of spent catalysts Figure S6. TGA profiles of spent catalysts. Thermogravimetry (TG) tests were performed to evaluate the extent of coke deposition over spent catalysts. Apparently, only a small amount of coke was detected for all the catalysts. Additionally, ZnO/Al2O3 and Pt-ZnO/Al2O3 catalysts exhibited similar amount of coke, i.e., g/gcat for 10Zn, g/gcat for 10Zn0.1Pt, g/gcat for 15Zn and g/gcat for 15Zn0.1Pt. S10

11 8. TPR results Figure S7. TPR profiles of fresh 0.1Pt, 15Zn and 15Zn0.1Pt catalysts. During the TPR experiment, the consumption of hydrogen was very small for all the samples, indicating the catalysts were hard to be reduced. For the 0.1Pt catalyst, no obvious peak was observed because the concentration of Pt was low or metallic Pt was on the catalyst. The peaks around 550 over the 15Zn and 15Zn0.1Pt catalysts were assigned to zinc oxide reduction, indicating very small amount of ZnO can be reduced by hydrogen. S11

12 9. Fitting results of XPS spectra Figure S8. Deconvolution of the fit of the Zn L 3M 4.5M 4.5 Auger peak for the spent 15Zn catalyst. 10. H2O profile during reaction Figure S9. H 2O profile of the 15Zn and 15Zn0.1Pt catalysts during propane dehydrogenation reaction S12

13 11. XPS overview spectrum Figure S10. XPS overview spectrum of fresh and spent ZnAl/PtZnAl catalysts. S13

14 12. XPS spectra of the Al 2P (including Pt 4f) Figure S11. XPS spectra of the Al 2P (including Pt 4f) on the fresh a) 15Zn and c) 15Zn0.1Pt catalyst, b) spent 15Zn and d) 15Zn0.1Pt catalyst. No Pt signal was detected because the concentration was too low or Pt was covered. S14

15 13. TPSR profile Figure S12. TPSR profile of a) the 15Zn catalyst and b) the 15Zn0.1Pt catalyst. S15

16 14. Stability test over the 15Zn0.1Pt catalyst Figure S13. Catalytic performance over the 15Zn0.1Pt catalyst (T = 550 C, atmospheric pressure, WHSV propane = 3 h 1, 500 mg of catalyst, C 3H 8/H 2 = 1/1, with balance N 2 for total flow rate of 50 ml/min). 15. Surface chemical composition Table S1. Surface chemical composition of fresh 15Zn and 15Zn 0.1Pt catalysts measured by XPS. Sample Al (wt%) O (wt%) Zn (wt%) Pt (wt%) 15Zn Zn0.1Pt n.d XPS measurements showed that the concentration of Zn on the surface of fresh 15Zn and 15Zn0.1Pt catalysts were 14.1% and 13.2% respectively, which was slightly lower as compared to the catalyst as a whole (15.5% and 14.4%, obtained by ICP-OES respectively). This result implied that Zn was mainly on the surface of Al2O3. S16

17 16. Details of DFT calculated elementary steps for propane dehydrogenation and H2, H2O formation Clean ZnO(101 0): Figure S14. Top view (left) and side view (right) of ZnO(1010) surface. Color: O-red, Frist dehydrogenation reaction Zn-violet The first dehydrogenation step is slightly endothermic by 0.17 ev, with a barrier of 1.19 ev. In the transition state, the C-chain is nearly perpendicular to the ZnO surface (Figure. S14). The dissociating H atom is located in between of the O and C atom, forming C-H and O-H interaction at 148 and 123 pm, respectively. In the final state, the propyl group binds to Zn atom, forming 199 pm C-Zn bond (Figure. S15). The dissociated H attached to the adjacent O atom, forming surface OH group. Figure S15. Top view (left) and side view (right) of first propane dehydrogenation transition state on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white S17

18 Figure S16. Top view (left) and side view (right) of the first propane dehydrogenation final state, i.e. co-adsorbed H and propyl on ZnO(1010) surface. Color: O-red, Znviolet, C-gray, H-white Second dehydrogenation reaction on the pathway to π-propylene In order to form π-propylene, the propyl need to diffuse to the adjacent Zn site, so that it has an adjacent free O atom. The diffusion barrier is only 0.80 ev, and this step is endothermic by 0.43 ev. In the transition state, the propyl binds to the bridge site of the Zn row, forming two C-Zn interactions (220, 244 pm, Figure S16). Figure S17. Top view (left) and side view (right) of propyl diffusion transition state on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white The second dehydrogenation transition state in the pathway to π-propylene is very similar to the transition state of first dehydrogenation step (Figure. S17). The C-H bond elongated to 150 pm, while the forming O-H bond is 115 pm. This reaction step is strongly endothermic by 1.21 ev, and the barrier is 1.60 ev. The formed π-propylene is only weakly bound to ZnO (Figure. S18), with two C-Zn bonds at 268 and 309 pm. S18

19 Figure S18. Top view (left) and side view (right) of second propane dehydrogenation transition state in the pathway to π-propylene on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Figure S19. Top view (left) and side view (right) of π-propylene on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Second dehydrogenation reaction on the pathway to di-σ-propylene Unlike the pathway to π-propylene, the production of di-σ-propylene do not need additional diffusion step. In the transition state, the propyl directly bent towards the adjacent Zn atom (Figure. S19). The C-H bond elongated to 150 pm, while the forming O-H bond is 125 pm. This reaction step is strongly endothermic by 1.14 ev, and the barrier is as high as 1.88 ev. The formed diσ-propylene binds much stronger than π-propylene by 0.50 ev. The formed two C-Zn bonds are 201 and 203 pm (Figure. S20). S19

20 Figure S20. Top view (left) and side view (right) of second propane dehydrogenation transition state in the pathway to di-σ-propylene on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Figure S21. Top view (left) and side view (right) of di-σ-propylene on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Formation of H2 After desorption of propylene, two H atoms left on ZnO surface in form of OH group. Attempts to locate the transition state of direct formation of H2 from these two OH group always converge to a pathway with additional diffusion steps. The most plausible pathway was shown in Figure S21, where two surface H atoms end in formation of OH and ZnH pair. In the following H2 formation transition state (Figure. S22), the barrier of this reaction is 0.37 ev, and the reaction is slightly exothermic by 0.06 ev. S20

21 Figure S22. Top view binding sites of two H atoms after propylene desorption (left), H diffusion intermediate (middle) and OH, ZnH pair (right) on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Figure S23. Top view (left) and side view (right) of H 2 formation transition state on Formation of H2O ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white Similar as H2 formation, a diffusion step is prior to the formation of H2O in order to shorten the distance between H and OH. However, instead of diffusion of H atom, the whole OH group, including the original surface O atom, moves to an adjacent Zn-Zn bridge site. The transition state of this OH diffusion step was shown in Figure. S23. This step is exothermic by 0.46 ev, with a barrier of 0.46 ev. Subsequently, H2O was formed (Figure. S24) with a similar barrier, 0.39 ev. The last step is endothermic by 0.20 ev. Figure S24. Top view (left) and side view (right) of OH diffusion transition state on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white S21

22 Figure S25. Top view (left) and side view (right) of H 2O formation transition state on ZnO(1010) surface. Color: O-red, Zn-violet, C-gray, H-white References 1. F. Jiang, L. Zeng, S. Li, G. Liu, S. Wang and J. Gong, ACS Catal. 2015, 5, G. Kresse and D. Joubert, Phys. Rev. B 1999, 59, G. Kresse and J. Furthmüller, Phys. Rev. B 1996, 54, J. Wellendorff, K. T. Lundgaard, A. Møgelhøj, V. Petzold, D. D. Landis, J. K. Nørskov, T. Bligaard and K. W. Jacobsen, Phys. Rev. B 2012, 85, P. E. Blöchl, Phys. Rev. B 1994, 50, H. J. Monkhorst and J. D. Pack, Phys. Rev. B 1976, 13, Y. G. Kolyagin, V. V. Ordomsky, Y. Z. Khimyak, A. I. Rebrov, F. Fajula and I. I. Ivanova, J. Catal. 2006, 238, J. Chen, Z. Feng, P. Ying and C. Li, J. Phys. Chem. B, 2004, 108, S22