Tackling CO Poisoning with Single Atom Alloy Catalysts

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1 S1 Supporting Information Tackling CO Poisoning with Single Atom Alloy Catalysts Jilei Liu 1, Felicia R. Lucci 2, Ming Yang 1, Sungsik Lee 3, Matthew D. Marcinkowski 2, Andrew J. Therrien 2, Christopher T. Williams 4, E. Charles H. Sykes 2 *, Maria Flytzani-Stephanopoulos 1 * 1 Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, MA USA 2 Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, MA USA 3 X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL USA 4 Department of Chemical Engineering, University of South Carolina, 301 Main Street, Columbia, SC USA Corresponding Author *maria.flytzani-stephanopoulos@tufts.edu, *charles.sykes@tufts.edu Author Contributions These authors contributed equally. Supplementary methods The method to prepare Pt-Cu SAA NPs is adapted from Lucci et al. 1 Briefly, an aqueous solution of Cu(NO 3) 2 and PVP (200:1 molar ratio of Cu to PVP, 1:1 wt/wt) was reduced by adding 0.1 M NaBH 4 solution dropwise to form Cu NPs. γ-al 2O 3 (2 g, ultra-pure grade 99.99%, surface area m 2 /g, Inframat Advanced Materials; heat treated in air at 400 C) was suspended in 100 ml of DI water under magnetic stirring and added to the Cu NPs colloidal solution. The resulting materials were filtered and washed 3 times with DI water and dried in vacuum for 24 h followed by calcination in air at 350 C for 4 h. Galvanic replacement (GR) took place in aqueous suspension of Cu NPs (pre-reduced in 100% H2 at 300 C) containing HCl (2mM) under nitrogen protection with constant stirring and refluxing at 100 C. The desired amount of H2PtCl6 was added. After 20 min, the resulting material was filtered, washed and dried in vacuum. The sample composition is summarized in Table S1. X-ray absorption spectroscopy (XAS) was performed at beamline 12-BM at Argonne National Laboratory. Pt 0.008Cu-SAA was reduced in H2 at 350 C in situ prior to the tests. Then 0.5 % CO was introduced to the gas mixture at ambient temperature and the

2 S2 catalysts sample was heated to 150 C at 10 C/min. Extended X-ray absorption fine structure (EXAFS) was collected in situ at 150 C after the temperature is stable for 30 min. The spectra were recorded in the fluorescence mode. XAS data were processed and analyzed using Athena and Artemis. 2,3 The EXAFS data were fitted in r-space with the models based on metallic Pt and PtCu 3. FTIR spectra were collected using a Nicolet Nexus 470 spectrometer equipped with a MCT-B detector in the single beam absorbance mode at a resolution of 2 cm -1. Prior to the IR experiments, the samples were reduced in pure H 2 at 300 (Cu-NP), 350 (Pt 0.008Cu- SAA and Pt 0.39Cu-bimetallic) or 400 (Pt-NP) C in a quartz microreactor and sealed in the glass vials. The powder sample of 0.02 g was made into a thin pellet (~ 1cm in diameter). The sample was then loaded into a flow cell sealed with NaCl windows and allows the CO flow through the catalyst pellet. In a typical CO adsorption IR experiment, the sample was first reduced in 50 ml/min 5 % H 2-He for 1 hr at 200 o C followed with the He purge to cool down to 25 o C. Baseline was collected at 25 o C under He purge after the catalyst has been swept for at least 30 min. The sample was then exposed to 1 % CO-He flow at the same temperature for 45 min, and was purged again with He for at least another 30 min until the gas phase CO signal was fully diminished (rather quickly within 5 min of purge) and the catalyst surface adsorption spectra has minimal difference between the IR scans. The IR spectra shown in the main text were collected afterwards at 25, 40, 60, 80, and 100 C. A ramping rate of 5 o C/min was used. The corresponding spectra at each temperature were collected after the sample was stabilized at the target temperature for 10 min. H 2-D 2 exchange was conducted in a packed-bed flow microreactor (L=22 inch, O.D.=1/2 inch) with mg of catalyst diluted by 0.5 g of quartz particles. The samples were pre-reduced in 100% H2 (at 350 C for Pt 0.008Cu-SAA and 400 C for Pt-NP) and then cooled to desired temperature before the reaction gas mixture (33% H2, 33% D 2, 3.3% CO and balance Ar. Flow rate=50ml/min) was introduced. For the isothermal H 2-D 2 exchange, CO was introduced to the gas phase and turned off at the certain time points while H 2 and D 2 are flowing. To do temperature programmed H 2-D 2 exchange, a gas mixture of 33% H2, 33% D 2, 3.3% (or 667ppm) CO and balance Ar (flow rate=50ml/min) was introduced at 120 C and temperature was increased to 200 C at 5 C/min. The gas effluent from the reactor was analyzed by a mass-spectrometer. Acetylene hydrogenation was performed in a packed-bed flow microreactor (L=22 inch, O.D.=1/2 inch) at 70 and

3 S3 80 C isothermally (20% H 2, 2.2% C 2H 2, 200 ppm CO, 10% Argon, balance He, flow rate=24-48 ml/min) mg catalysts were used. The exit gas stream was analyzed by a mass-spectrometer. Three separate ultra-high vacuum (UHV) chambers were used for temperature programmed desorption (TPD) and scanning tunneling microscopy (STM) studies on model catalysts. Pt-Cu(111) samples were prepared using a similar method in all chambers. Cu(111) was cleaned with repeated cycles of Ar + sputtering (1.5 kev, 15 μa) and annealing to 700 K. Pt was deposited with a flux-monitored EFM 3 electron beam evaporator. To obtain Pt surface coverages between 0.01 and 5 monolayer (ML), Pt was deposited onto Cu(111) at a flux of 0.02 ML per min at a sample temperature of 380 K. For TPD experiments, samples were cooled to 85 K with liquid nitrogen prior to exposure to H 2 (99.9% Airgas), D 2 (99.999% Airgas) and/or CO (99.99% Airgas). CO desorption traces were obtained after a saturation exposure of CO of 10 Langmuir (L) (1 L = 1 x 10-6 torr * s). TPD traces were collected using a quadrupole mass spectrometer to monitor desorption of relevant masses as a function of temperature with a linear heating ramp of 1 K s -1. STM experiments were performed on both variable and low temperature STMs. Images of bare Pt-Cu(111) were obtained using a variable temperature STM at 30 K with tunneling conditions of na and biases between 0.01 and 0.05 V. Atom manipulation studies in Figure 2 were performed on a low temperature STM. The samples were exposed to 20 L H 2 followed by 0.05 L CO at 85 K. The samples were then cooled to 5 K for imaging. STM images were obtained at 30 pa currents and 30 mv. Voltages were increased to 200 mv to remove H adatoms and 5 V with localized pulses to remove CO molecules.

4 S4 Supplementary Figures Figure S1. Temperature effects on the IR intensity of the linearly adsorbed CO on Pt sites of (A) Pt 0.008Cu-SAA, (B) Pt 0.39Cu-bimetallic and (C) Pt-NP. The vertical axis is the ratio of the peak absorbance at the given temperature to the peak absorbance at 25 C. The peak vibrational frequency of each carbonyl is (A) 2088 cm -1, (B) cm -1 and (C) cm -1.

5 Figure S2. Isothermal H 2-D 2 exchange over Pt-NP (A, B, C) and Pt 0.008Cu-SAA (D, E, F) at 120, 150 and 200 C. CO was turned on and off during the experiments. Gas composition: 33% H 2, 33% D 2, 3.3% CO, balance Argon. Total flow rate=50 ml/min. 90 mg catalyst load. S5

6 Figure S3. H 2-D 2 exchange as a function of temperature. Gas composition: 33% H 2, 33% D 2, 667ppm CO (A), or 3.3% CO (B) balance Argon. Total flow rate=50 ml/min. 80 mg catalyst load. S6

7 Figure S4. Reaction rate of acetylene hydrogenation over (A) Pt 0.008Cu-SAA and (B) Pt- NP at different temperatures with and without CO. Gas composition: 20% H 2, 2.2% C 2H 2, 200 ppm CO, 10% Argon, balance He. S7

8 Figure S5. Fourier transform of k 3 -weighted Pt L III EXAFS of Pt foil and Pt 0.008Cu-SAA in CO and H 2 gases mixture plotted in R-space. S8

9 S9 Supplementary Tables Table S1. Sample preparation and properties. Sample Composition [a] Preparation Method Pt Species Cu-NP 14.9 at.% Cu on Al2O3 Colloidal --- Pt-NP 0.12 at.% Pt on Al2O3 IWI [b] Single atom, clusters and NPs Pt0.008Cu-SAA 0.11at.% Pt, 13.7 at.% Cu on Al2O3 GR Single atoms in Cu surface [c] Pt0.39Cu-biletallic 2.2 at.% Pt, 5.6 at.% Cu on Al2O3 GR Single atom, clusters and NPs [d] [a] Determined by ICP-AES. [b] Incipient wetness impregnation (IWI). [c] Determined by STEM, EXAFS in ref. 1 and CO-IR in this work. [d] Determined by EXAFS in ref. 1 and CO-IR in this work

10 S10 Table S2. EXAFS model fitting. Sample Shell CN [a] R [b] (Å) σ 2 (Å 2 ) R-factor Pt foil Pt-Pt ± Pt0.008Cu-SAA H2+CO Pt-Pt Pt-Cu ± ± [a] CN, coordination number. [b] R, distance between absorber and backscattered atoms. R-factor, closeness of the fit, if < 0.05, consistent with broadly correct models.

11 S11 Table S3. CO-IR peak assignments Peak position (cm -1 ) Assignments References Atop CO on Cu species (Cu-NP, Pt0.39Cu-bimetallic, Pt0.008Cu-SAA) 4 7 ca Atop CO on isolated Pt atoms in the surface of Cu NPs (Pt0.008Cu- SAA) Atop CO on extended Pt species (Pt-NP, Pt0.39Cu-bimetallic) Bridged CO on two adjacent Pt sites (Pt-NP, Pt0.39Cu-bimetallic) 9

12 S12 Supplementary Text The IR absorption intensity of atop CO at Pt sites for different samples was compared as temperature is increased from 20 to 100 C (Figure S1). These results corroborate the CO-Pt bond is the weakest on the Pt 0.008Cu-SAA compared to Pt-NP and Pt 0.39Cubimetallic. The EXAFS of Pt 0.008Cu-SAA is plotted in figure S5 and the EXAFS Model fitting results are summarized in table S2. Indeed, no Pt-Pt bonds were formed with CO in the gas phase, which demonstrated the stability of the single Pt atoms. The Pt-Cu coordination number (CN) is for Pt 0.008Cu-SAA. The Pt-Cu interaction distance is Å, which is smaller than that of Pt-Pt (2.77 Å) but greater than that of Cu-Cu (2.56 Å). 1,11 The CO tolerance Pt 0.008Cu-SAA was studied in temperature-programed H 2-D 2 experiments, over a wide temperature range (120 to 200 C) with CO concentrations from a few hundred ppm to 3.3%, which demonstrates the broad operation window of Pt-Cu SAA as a CO-tolerant catalyst (Figure S3).

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