Supporting Information. for Room Temperature CO Oxidation

Transcription

1 Supporting Information Constructing Hierarchical Interfaces: TiO 2 -Supported PtFe-FeO x Nanowires for Room Temperature CO Oxidation Huiyuan Zhu, *, Zili Wu,, Dong Su, Gabriel M. Veith, Hanfeng Lu, # Pengfei Zhang, Song- Hai Chai, ǁ and Sheng Dai *,,ǁ Chemical Sciences Division, Center for Nanophase Materials Sciences, and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ǁ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States # Institute of Catalytic Reaction Engineering, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang , China Experimental Section Chemicals and Materials. The following chemicals were used as received. Oleylamine (OAm, >70%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), Fe(CO) 5, Pt(acac) 2 (acac = aceylacetonate), born tert-butylamine complex (BTB), hexane, ethanol, isopropanol and acetic acid were purchased from Sigma-Aldrich. Sodium oleate (97%) was obtained from Tokyo Chemical Industry CO., LTD. S1

2 Synthesis of nm PtFe NWs: nm PtFe NWs were synthesized as seeds according to the previous method. 1 To synthesize nm PtFe NWs, 0.25 mmol Pt(acac) 2, OA (1.5 ml) and OAm (2 ml) were mixed with ODE (10 ml) under a gentle N 2 flow. The mixture was magnetically stirred and heated to 115 o C for 30 mins. 30 mg of nm PtFe NWs in hexane was injected into the solution, followed by the injection of 0.25 mmol Fe(CO) 5. Under a N 2 blanket, the solution was heated up to 220 o C at a rate of 3-4 o C/min and kept at this temperature for 30 mins. Then the mixture was cooled down to room temperature and the PtFe NWs were precipitated by adding 30 ml isopropanol and centrifugation at 9000 rpm for 8 mins. The product was further washed with ethanol for three times and redispersed in hexane for following study. Synthesis of 4 nm Pt NPs: 0.5 mmol Pt(acac) 2 was mixed with OAm (15 ml) and heated to 60 o C for 20 mins to remove the dissolved oxygen and moisture. 2 mmol BTB was dissolved in 3 ml OAm and added into the solution at 60 o C. The reaction solution was further heated to 180 o C at a ramping rate of 5 o C/min and kept there for 30 mins. The purification process was similar to the one in PtFe NW synthesis, except that instead of isopropanol, only ethanol was used. Synthesis of 5 nm PtFe NPs and 12 nm FeNPs: 5 nm PtFe NPs were synthesized according to a previous method nm Fe NPs were prepared according to a reported process. 3 Preparation of PtFe-FeO x /TiO 2, Pt/TiO 2, PtFe-FeO x NPs/TiO 2, FeO x /TiO 2 and PtFe- FeO x /SiO 2 catalysts: 100 mg TiO 2 was first mixed with 25 ml hexane and sonicated for 30 mins to form a homogeneous suspension. Under a magnetic stirring, the as-synthesized PtFe NWs, Pt NPs, PtFe NPs and Fe NPs were slowly added into the TiO 2 dispersion. The NWs and NPs immediately adsorbed onto TiO 2. The mixture was stirred overnight for a complete assembly on TiO 2 and washed twice with ethanol (25 ml). The final loading was determined by ICP-OES. Based on the Pt amount, 2.5 wt. %, 4 wt. %, 5 wt. % was obtained in PtFe NWs/TiO 2, Pt/TiO 2, PtFe NPs/TiO 2 respectively. The Fe loading of Fe/TiO 2 was calculated to be 7 wt. %. The PtFe NWs/TiO 2, Pt/TiO 2, PtFe NPs/TiO 2 and Fe/TiO 2 were dried under vacuum overnight and calcined in air at 200 o C to remove surfactants and to produce PtFe-FeO x /TiO 2 and S2

3 FeO x /TiO 2. Under the same condition, PtFe NWs were deposited on SiO 2 (Cab-O-Sil, surface area 175 m 2 /g) with a Pt loading of 4 wt. % and annealed in air to form PtFe-FeO x /SiO 2. Removal of surface FeO x in PtFe-FeO x /TiO 2 : 40 mg PtFe-FeO x /TiO 2 were mixed with 30 ml acetic acid and heated to 70 o C overnight under a gentle N 2 flow. After cooled down to room temperature, the PtFe-FeO x /TiO 2 was precipitated by adding 20 ml ethanol and centrifugation at 5000 rpm for 5 mins. The sample was further washed with distilled water to remove excess acid and dried under vacuum. Characterization: TEM and HRTEM images were obtained on a JEOL 1400 (120 kv) and a JEOL 2100F (200 kv), respectively. Quantitative elemental analysis was done on an inductively coupled plasma optical emission spectrometry (ICP-OES) on a Perkin-Elmer Optima 2000 DV ICP spectrometer. X-ray photoelectron spectroscopy (XPS) was obtained using a PHI 3056 spectrometer with an Al anode source operated at 15 kv and an applied power of 350 W. STEM analyses were obtained on a Hitachi HD2700C (200 kv) with a probe aberration-correction in the Center for Functional Nanomaterial at Brookhaven National Laboratory. The EELS mapping and spectra were collected on a high resolution Gatan-Enfina ER with a probe size of 1.3 Å. X- ray diffraction was performed on a Panalytical Empyrean diffractometer with Cu Ka radiation k = A operating at 45 kv and 40 ma. Fourier-transform infrared spectrum (FTIR) was collected on a PerkinElmer Frontier FTIR spectrometer. Thermogravimetric analysis was conducted on a thermalgravimetric analyzer coupled with a Mass Spectrometer (TA Discovery TGA-MS). Catalytic tests: CO oxidation was carried out in a temperature-controlled microreactor (Altamira AMI 200) equipped with an on-line gas chromatograph. All experiments were S3

4 performed under atmospheric pressure with a flow rate of 10 ml/min of 1% CO balanced in dry air. For the kinetic measurements, the amount of PtFe-FeO x /TiO 2 and Pt/TiO 2 was reduced to 1 mg and 2 mg, respectively, to ensure the CO conversion is below 15%. The CO conversion was averaged at 5, 10, 20, 30 and 40 mins to calculate the reaction rate. The reaction rate (r) was calculated as follows: r = CO conversion rate [CO]in n(pt) Here, CO conversion rate is the percentage of CO oxidized to CO 2 after the reaction. [CO] in is the total molar flow of CO per second. n(pt) stands for total moles of Pt atoms. In situ diffuse reflectance infrared spectroscopy (DRIFTS) measurements: The sample (ca. 15 mg) was calcined in situ in the DRIFTS cell at 200 o C in either 5% 16 O 2 /He or 2% 18 O 2 /He (flow rate 25 ml/min) for 1h, then cooled down to room temperature under a gentle He flow for CO adsorption or to reaction temperature for CO oxidation. IR background spectrum was collected in He flow prior to the adsorption and reaction. CO adsorption was conducted at room temperature in flow mode. After flowing 2% CO/2%Ar/He for 10 min, either He or O 2 /He was switched back to allow adsorbed CO to desorb or react. The sample was then heated to 120 o C in He or O 2 /He for CO desorption study, which was monitored by both IR and QMS. For CO oxidation reaction at 60 o C and 90 o C, the sample was exposed to CO- 16 O 2 or CO- 18 O 2 (CO: O 2 =1: 4) mixture with a flow rate of 25 ml/min. IR spectra were collected continuously to track the surface species, while QMS profiles were recorded for the different CO 2 isotopes. CO-O 2 copulse and sequential pulse experiments were also conducted in the IR-QMS system at 60 o C and 90 o C using a pulse volume of 0.5 ml. In the copulse mode, a mixture of 2% CO/2%Ar/He (5 S4

5 ml/min) + 5% O 2 /He (20 ml/min) was pulsed repeatedly to the sample, with delays between pulses sufficient for the QMS signals of CO and CO 2 to return to baseline. In the sequential pulse mode, a pulse of 2% CO/Ar/He (5 ml/min) + He (20 ml/min) mixture was first pulsed over the sample, then 5% O 2 /He (20 ml/min) + He (5 ml/min) mixture was pulsed to the sample after the QMS signals of CO and CO 2 traces level off. Each pulse experiment was repeated for 5 times. All reported IR spectra are difference spectra referenced to the background spectrum collected in He at room temperature or reaction temperature after pretreatment. [1] Zhu, H.; Zhang, S.; Guo, J.; Su, D.; Sun, S. J. Am. Chem. Soc. 2013, 135, [2] Nakaya, M.; Kanehara, M.; Teranishi, T. Langmuir 2006, 22, [3] Zhang, S.; Jiang, G.; Filsinger, G. T.; Wu, L.; Zhu, H.; Lee, J.; Wu, Z.; Sun, S. Nanoscale 2014, 6, S5

6 Figure S1. TEM image of Pt NPs. Figure S2. (A) XRD patterns of PtFe (width: 2.5 nm), PtFe (width: 4.5 nm) NWs and Pt NPs. (B) TGA-MS spectra of as-synthesized PtFe NWs during oxidation. S6

7 Figure S3. FTIR spectra of OAm, OAc, as-synthesized PtFe NWs and PtFe/TiO 2 after 200 o C annealing. Figure S4. (A) 2D EELS elemental mapping of Fe (red) and Pt (green). (B) EELS line-scan across one PtFe-FeO x NW. (Inset shows the NW scanned.) S7

8 Figure S5. XRD pattern of PtFe (width: 4.5 nm) NWs after annealing in air for 3h. (The PtFe NWs were directly heated without TiO 2 support.) Figure S6. CO oxidation light-off curve for PtFe-FeO x /SiO 2. S8

9 Figure S7. TEM image of as-synthesized 12 ± 0.3 nm Fe NPs (A) and 5 ± 0.2 nm PtFe NPs (B). (C) CO oxidation light-off curves for PtFe-FeO x NPs/TiO 2 and FeO x /TiO 2. Figure S8. IR spectra collected during CO oxidation with 18 O 2 over PtFe-Fe 16 O x /TiO 2 at 90 o C. S9

10 Figure S9. The CO 2 production during copulse and sequential pulse of CO and O 2 at 60 o C (A) and 90 o C (B). (black squares: CO 2 from copulse; red squares: CO 2 from sequential pulse; blue triangles:percentage of MvK reaction pathway.) Figure S10. The CO 2 production during CO pulse at 60 o C on PtFe/TiO 2 (A) and Pt/TiO 2 (B) pretreated with 18 O 2 at 200 o C. S10

11 Figure S11. IR spectra of CO adsorbed on PtFe-FeO x /TiO 2 (A) Pt/TiO 2 (B) at room temperature and subsequent temperature-programmed desorption in He. Figure S12. CO-TPD in He on PtFe-FeO x /TiO 2 and Pt/TiO 2. S11

12 Figure S13. (A) CO oxidation activity on PtFe-FeO x /TiO 2 declined after 40 mins in the reaction stream at room temperature. The room temperature activity fully recovered after purging with O 2 at 40 o C for 1h. (B) Schematic illustration of CO poisoning and recovering of the NW catalyst. Figure S14. CO conversion of PtFe-FeO x /TiO 2 at 40 o C as a function of time-on-stream. Figure S15. TEM image of PtFe-FeO x /TiO 2 after the stability test (30 h). S12