www.sciencemag.org/cgi/content/full/science.aan6515/DC1

Supplementary Materials for

Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with

O2 under mild conditions

Nishtha Agarwal, Simon J. Freakley, Rebecca U. McVicker, Sultan M. Althahban,

Nikolaos Dimitratos, Qian He, David J. Morgan, Robert L. Jenkins, David J. Willock,

Stuart H. Taylor, Christopher J. Kiely, Graham J. Hutchings*

*Corresponding author. Email: hutch@cardiff.ac.uk

Published 7 September 2017 on Science First Release

DOI: 10.1126/science.aan6515

This PDF file includes:

Materials and Methods

Figs. S1 to S10

Tables S1 to S7

Materials and Methods Catalyst Preparation Monometallic gold, monometallic palladium and bimetallic Au-Pd nanoparticles were prepared by standard colloidal methods. An aqueous solution of HAuCl4 precursor (Sigma Aldrich) and acidic solution of PdCl2 (Sigma Aldrich) precursor (in 0.5 M HCl) were dissolved in 800 mL of de-ionized water to give a total metal concentration of 0.16 mmol / L. Polyvinylpyrrolidone (PVP, average molecular weight 1,300,000, Sigma Aldrich) was added as a stabilizer to give the required metal-to-PVP ratio (typically 1:1.2). After 2-3 min of stirring, freshly prepared 0.1 M sodium borohydride (NaBH4, Sigma Aldrich) solution was added such that the molar ratio of NaBH4-to-metal was 5. The required 6.6 ml of 0.1M NaBH4 was added as two 3.3 ml aliquots. This produced a dark brown colloid which was the left stirring for 30 minutes to ensure all the metal precursors were reduced to metallic form. The colloid was concentrated using a roto-evaporator to give a nominal metal loading of 0.66 mmol / L. The colloid was stored in glass media bottles prior to use.

For supported catalysts, the sol prepared as described above was immobilized onto a TiO2 support material consisting of a mixture rutile and anatase phases (P25, Degussa, 1.98 g). A sufficient amount of support material was added to ensure a 1% wt metal loading and the solution was acidified to pH 1 using sulphuric acid to achieve more homogeneous deposition of nanoparticles. The supernatant solution became clear over a 1 h period after support addition, indicating the deposition process was complete. The sol-immobilized catalyst was then filtered, washed thoroughly with distilled water and then left to dry in an oven at 110 ºC overnight.

Catalyst Testing Methane oxidation was carried out in a 50 mL glass-lined stainless steel Parr autoclave reactor. The reactor was charged with 10 mL of colloidal catalyst (6.6 µmol metal equivalent) and different amounts of H2O2 (Sigma Aldrich, 50% wt in water). The charged autoclave was sealed and purged three times with methane (99.999%, Air Products). It was then pressurized with methane (30 bar) and oxygen (5 bar, BOC) to remain within oxygen lean reaction limits. The mixture was stirred at 1500 rpm and heated to the desired reaction temperature (usually 50ºC) at the ramp rate of 2.25 °C/min and maintained at the reaction temperature for a fixed time (usually 30 min). At the end of reaction, the autoclave was cooled in ice to a temperature below 10ºC in order to minimize the loss of volatile products. The reaction gas was removed for analysis in a gas sampling bag. For isotopic reactions, BOC specialty gases > = 98% 18O enriched O2 gas and > = 98% 13C enriched CH4 were used. In cases where sol-immobilized catalysts were used, the reactor was charged with 100 mg of catalyst in 10 mL solution of de-ionized water and H2O2. At the end of the test, the reaction mixture was filtered before analysis. Product Analysis The 1H-NMR studies were carried out using a Bruker 500 MHz NMR equipped with a solvent suppression system. It was used to quantify the amounts of liquid phase products using TMS in CDCl3 as an internal standard. The H2O2 concentration was determined using the titanium oxalate spectrophotometric method (Agilent, Cary 60). In this, 0.05 to 1.0 mL of reaction sample was acidified using dilute H2SO4 before adding potassium titanium oxalate solution (0.5% wt in water, Sigma Aldrich) to form the yellow pertitanic acid complex with absorption

at 390 nm. Gaseous products were quantified using a Varian 450-GC fitted with a CP-Sil 5CB capillary column (50 m length, 0.32 mm diameter, carrier gas = He), a methaniser unit and both FID and TCD detectors. Analysis of isotope labelling reactions was performed using GC-MS. Methyl hydroperoxide was converted to methanol using NaBH4 before injection into the GC system. The “as-is” reaction mixture was also injected into the GC-MS for comparison.

Primary Oxygenate Selectivity = mol of (CH3OOH + CH3OH)×100

total mol of products

Catalyst Characterization Electron Microscopy: Materials for examination by scanning transmission electron microscopy (STEM) were dispersed onto on a lacey carbon film supported over a 300-mesh copper TEM grid. These materials were examined using BF- and HAADF-STEM imaging mode in an aberration corrected JEOL ARM-200CF scanning transmission electron microscope operating at 200 kV. This microscope was also equipped with a Centurio silicon drift detector (SDD) system for X-ray energy dispersive spectroscopy (XEDS) analysis.

XPS: The composition of the bimetallic samples were analyzed using a Kratos Axis Ultra DLS photoelectron spectrometer utilizing monochromatic Al Kα radiation operating at 120 W power. For analysis, a drop of the colloid was deposited onto a clean Si wafer. The solvent was allowed to evaporate under vacuum overnight in the fast entry lock of the spectrometer. XPS analysis was performed at pass energies of 40 and 160 eV for high resolution and survey scans, with step sizes of 0.1 and 1 eV respectively. All samples were analyzed using a slot aperture and in hybrid spectroscopy mode, which utilizes both magnetic and electrostatic lenses; in this mode, the analysis area is a 700 um x 300 um rectangle. For all samples, the Kratos immersion lens system was used for charge neutralization and the spectra subsequently referenced to the C(1s) line taken to be 285 eV. The sample also exhibited a peak at 99.4 eV which corresponded to the elemental Si(2p) peak arising from the Si substrate.

Figure S1. Systematic study of hydrogen peroxide degradation as a function of time.

Hydrogen peroxide degradation was carried out at ambient pressure and temperature. 1000 µmol H2O2 was added to 10 mL of colloidal catalyst (blue line), or water containing 100 mg of sol immobilized AuPd/TiO2 catalyst (red line), pure water (black line) or bare TiO2 (purple line) in a glass vial. The H2O2 concentration was measured every 10 min in this test.

Figure S2. NMR spectra from the methane oxidation reaction carried out with 10% 13CH4 in 12CH4.

13CH3OOH (δ = 3.90 and δ = 3.66), 13CH3OH (δ = 3.39 and δ = 3.16) and H13COOH (δ = 8.34

and δ = 7.98) satellite peaks are observed along with main 12C peaks (δ = 3.79, δ = 3.28 and δ

= 8.17 respectively). The integrated area of the satellite peaks is about one-tenth that of main 12C peak corresponding to 10% replacement in reaction gas phase mixture.

Table S1. Blank reactions carried out with the AuPd-PVP colloid and the PVP ligand alone. Test conditions: 1000 µmol H2O2, 50ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min. Entry 2: Colloidal AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), [a] Assayed by the titanium oxalate spectrophotometric method [b] Gain factor = moles of oxygenate produced /moles of hydrogen peroxide consumed

Entry Catalyst Product amount (μmol) H2O2

Consumed / % [a]

Gain Factor[b] CH3OOH CH3OH HCOOH CO2

1 PVP 0 0 0 n.d. 2 0

2 AuPd-PVP 0 0 0 n.d. 3 0

Figure S3. Representative XEDS compositional analysis from individual AuPd-PVP

nanoparticles.

Representative XEDS compositional analysis from individual AuPd-PVP nanoparticles from the upper end (A, B), middle (C, D) and smaller end (E, F) of the particle size distribution. It is clear from the comparing the relative heights of the Au Mα and Pd Lα peaks in each XEDS spectrum that the particle composition is relatively invariant with particle size.

0

50

100

150

200

250

300

350

400

0 1 2 3 4

Cou

nts

KeV

0

50

100

150

200

250

300

350

400

0 1 2 3 4C

ount

s

KeV

0

20

40

60

80

100

120

140

0 1 2 3 4

Cou

nts

KeV

B A

C

E

D

F

Au

Au

Au

Pd

Pd

Pd

Figure S4. Representative BF-TEM images and particle size distributions for the AuPd-

PVP/TiO2 sol-immobilized catalyst in the fresh (A, C, E) and used (B, D, F) states.

Representative BF-TEM images (A, B, C, D) and particle size distributions (E, F) for the AuPd-PVP/TiO2 sol-immobilized catalyst in the fresh (A, C, E) and used (B, D, F) states. It should be noted that the AuPd article size on TiO2 is slightly larger than that of the corresponding unsupported sol probably due to the 120 °C drying treatment for 16 h which causes particle to partially wet and spread on the support to form particles with distinct metal/support interface (c.f. initially ‘spherical’ colloid particles). A minor increase in the mean AuPd particle size was noted after use (which was smaller than that noted for the unsupported sol), and no significant morphological changes were detected.

B A

C

E

D

F

Figure S5. XPS analysis of the ‘fresh’ and ‘used’ AuPd-PVP colloid.

XPS spectra of colloidal AuPd-PVP catalyst. (a-b) Au (4f) and (c-d) Pd (3d) spectra of (a, c) ‘fresh’ and (b, d) ‘used’ materials. Plots (c-d) also show the presence of Au (4d) signals. The ‘fresh’ catalyst contains 94% Au metal and 6% cationic gold, whereas the Pd is in metallic form. The ‘used’ catalyst contains 92% Au metal and 8% cationic gold, whereas the Pd is in metallic form. Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

Figure S6. XPS analysis of the ‘fresh’ and ‘used’ AuPd/TiO2 sol-immobilized catalyst.

XPS spectra of supported 1 wt % AuPd/TiO2 catalyst. (a-b) Au (4f) and (c-d) Pd (3d) spectra of (a, c) ‘fresh’ and (b, d) ‘used’ material. Plots (a-b) also show an underlying Ti signal whilst (c-d) also show the presence of the Au (4d) signals. The ‘fresh’ catalyst contains 87% Au metal and 13% cationic gold, whereas all the Pd is in metallic form. The ‘used’ catalyst contains 92% Au metal and 8% cationic gold, whereas all the Pd is in metallic form. Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 100 mg 1 wt % AuPd/TiO2, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

Figure S7. NMR and GC-MS quantification analysis for test reaction solutions.

Liquid phase products were quantified by both mass spectrometry and NMR quantitative analysis of the reaction solutions. Within experimental error, the amount of methylhydroperoxide (blue bar) + methanol (red bar) in the NMR analysis corresponded to the amount of methanol (black ) measured in the GC-MS analysis. Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

Figure S8. GC mass spectrum of methanol formed (m = 32 or 34) during methane oxidation

using a AuPd-PVP colloidal catalyst with 1000 µmol H216O2 + 16O2 (above) or 1000 µmol

H216O2 + 18O2 (below).

Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 1000 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

Table S2. Full breakdown of products generated by methane oxidation using the AuPd-PVP

colloid and varying different amounts of H2O2.

Entry H2O2 Added (μmol)

Product amount (μmol) Oxygenate Selectivity /

% [a]

Oxygenate Productivity

[b]

H2O2 Used / μmol

[c]

Gain Factor

[d] CH3OOH CH3OH HCOOH CO2

1 2000 11.0 6.0 0.34 2.1 89.2 17.34 1622 0.01

2 1000 27.4 12.0 3.8 2.0 95.6 43.2 638 0.07

3 500 31.4 14.5 4.6 1.5 97.1 50.5 246 0.2

4 100 31.1 10.3 6.4 2.2 95.6 47.8 60 0.80

5 50 18.5 5.4 2.2 1.3 95.3 26.1 22 1.2

6 30 10.2 3.7 1.4 1.3 92.2 15.3 11 1.4

Liquid phase oxidation of methane using AuPd-PVP colloidal catalyst using different amounts of H2O2 with O2. Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 60 min. [a] Oxygenate selectivity = (moles of oxygenate/ total moles of products) * 100 [b] Oxygenate productivity = mol oxygenates kgcat-1 h-1 [c] Assayed by the titanium oxalate spectrophotometric method [d] Gain factor = moles of oxygenate produced / moles of hydrogen peroxide consumed.

Figure S9. GC mass spectrum of methanol formed (m = 32 or 34) during methane oxidation

with AuPd-PVP colloidal catalyst using 50 µmol H216O2 + 16O2 (above) or 50 µmol H216O2 + 18O2 (below).

Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

Table S3. Time-on-line analysis of liquid phase oxidation of methane using AuPd-PVP

colloidal catalyst using H2O2 and O2 for varying times up to 120 mins.

Entry Time Product amount (μmol) Oxygenate

Selectivity / % [a]

H2O2 Used / μmol [b]

CH3OOH CH3OH HCOOH CO2

1 0 6.2 0.3 0.0 n.d. 100 9

2 15 11.9 1.7 0.9 0.1 99.3 13

3 30 15.7 2.8 1.2 0.3 98 16

4 60 18.5 5.4 2.2 1.3 95.3 22

5 90 13.8 11.5 2.6 1.3 95 30

6 120 14.0 10.1 3.9 1.5 94.9 42

Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm. [a] Oxygenate selectivity = (mol of oxygenate/ total mol of products) * 100, [b] Assayed by titanium oxalate spectrophotometric method.

Table S4. Time-on-line and catalyst re-use study of liquid phase oxidation of methane using

AuPd-PVP colloidal catalyst using H2O2 and O2 upon on adding another 50 µmol of H2O2 at

50 °C after 120 min of initial reaction.

Entry Time Product amount (μmol) Oxygenate

Selectivity / % [b] CH3OOH CH3OH HCOOH CO2[a]

1 135 20 10.5 5.1 1.8 95.2

2 150 22.9 11.1 6.4 2.1 95.0

3 180 16.2 19.9 9.5 3 94.0

4 240 13.8 27.6 12.2 3.4 93.8

Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2 (additional 50 µmol was added at 50 °C after initial 120 minutes of reaction), 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm. [a] CO2 = CO2 measured at 120 minutes + CO2 after the end of reaction [b] Oxygenate selectivity = (mol of oxygenate/ total mol of products) * 100

Table S5. Liquid phase oxidation of methane using AuPd-PVP colloidal catalyst using H2O2 with and without O2 at different temperatures.

Entry Conditions

Product amount (μmol) Oxygenate Selectivity /

% [a]

Oxygenate Productivity

[b]

H2O2 Used / μmol [c]

Gain Factor[d]

CH3OOH CH3OH HCOOH CO2

1 No Oxygen T : 50ºC

13.9 3.6 0.34 1.0 94 35.7 41 0.44

2 No Oxygen T : 23 ºC 4.3 0.0 0.0 0.3 93 8.6 9 0.47

3 With Oxygen T : 50ºC

15.7 2.8 1.2 0.3 98 39.4 16 1.23

4 With Oxygen T : 23 ºC 4.7 0.5 0.0 0.2 96 10.4 4 1.35

Test conditions: Pressure (CH4) = 30 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 1500 rpm, 30 min. Entry 1: 50 ºC (ramp rate of 2.25 °C/min), Entry 2: 23 ºC (no heating). Entry 3: 50 ºC (ramp rate of 2.25 °C/min), Pressure (O2) = 5 bar. Entry 4: 23 ºC (no heating), Pressure (O2) = 5 bar. [a] Oxygenate selectivity = (mol of oxygenate/ total mol of products) * 100, [b] Oxygenates productivity = mol oxygenates kgcat-1 h-1, [c] Assayed by the titanium oxalate spectrophotometric method [d] Gain factor = moles of oxygenates produced /moles of hydrogen peroxide consumed.

Figure S10. Kinetic studies showing dependence of product (CH3OOH) formation of A)

amount of catalyst added, B) amount of H2O2 added, C) methane pressure, and D) Arrhenius

plot for the reaction.

Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 10 mL AuPd-PVP colloid, 6.6 µmol of metal (1:1 metal molar ratio), 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min.

B

C D

A

Table S6. Liquid phase oxidation of methane using 1 wt. % AuPd/TiO2 sol immobilised

catalyst using H2O2 and O2.

Entry H2O2 Added (μmol)

Product amount (μmol) Oxygenate Selectivity /

% [a]

Oxygenate Productivity

[b]

H2O2 Used /

%[c]

Gain Factor

[d] CH3OOH CH3OH HCOOH CO2

1 1000 0 0.3 0.7 0.3 76.9 0.026 65 0.001

2 50 0 0.1 0.0 0.1 50.0 0.004 100 0.002

Liquid phase oxidation of methane was carried out using TiO2 supported AuPd colloidal catalyst with different amounts of H2O2 with O2. Test conditions: Pressure (CH4) = 30 bar, Pressure (O2) = 5 bar, catalyst: 100 mg 1 wt % AuPd/TiO2, 6.6 µmol of metal (1:1 metal molar ratio), 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min. [a] Oxygenate selectivity = (mol of oxygenate/ total mol of products) * 100 [b] Oxygenates productivity = mol oxygenates kgcat-1 h-1 [c] Assayed by the titanium oxalate spectrophotometric method [d] Gain factor = moles of oxygenate produced /moles of hydrogen peroxide consumed

Table S7. Liquid phase oxidation of methane using an Fe-based Fenton’s type catalyst using

H2O2.

Entry O2 Added (bar) Product amount (μmol) Oxygenate

Selectivity / % [a]

Oxygenate Productivity

[b]

H2O2 Used /

%[c]

Gain Factor

[d] CH3OOH CH3OH HCOOH CO2

1 0 0.7 1.0 0.0 0.2 89.2 2.1 22 0.1

2 5 1.0 1.3 0 0.3 88.4 2.9 18 0.2

Liquid phase oxidation of methane was carried out using FeCl3.6H2O as a Fenton-type catalyst in the presence of 50 µmol H2O2 and two different amounts of O2. Test conditions: Pressure (CH4) = 30 bar, catalyst: 6.6 µmol of Fe+3 metal in 10 ml of water from solid FeCl3.6H2O,, 50 µmol H2O2, 50 ºC (ramp rate of 2.25 °C/min), 1500 rpm, 30 min. [a] Oxygenate selectivity = (mol of oxygenate/ total mol of products) * 100 [b] Oxygenates productivity = moloxygenates kgcat-1 h-1 [c] Assayed by the titanium oxalate spectrophotometric method [d] Gain Factor = moles of oxygenate produced /moles of hydrogen peroxide consumed.

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Materials and MethodsFigure S1. Systematic study of hydrogen peroxide degradation as a function of time.Figure S2. NMR spectra from the methane oxidation reaction carried out with 10% 13CH4 in 12CH4.Figure S5. XPS analysis of the ‘fresh’ and ‘used’ AuPd-PVP colloid.Figure S6. XPS analysis of the ‘fresh’ and ‘used’ AuPd/TiO2 sol-immobilized catalyst.Table S2. Full breakdown of products generated by methane oxidation using the AuPd-PVP colloid and varying different amounts of H2O2.Figure S9. GC mass spectrum of methanol formed (m = 32 or 34) during methane oxidation with AuPd-PVP colloidal catalyst using 50 µmol H216O2 + 16O2 (above) or 50 µmol H216O2 + 18O2 (below).