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1 DOI: /NCHEM.1066 Facile removal of stabiliser-ligands from supported gold nanoparticles Jose A. Lopez-Sanchez 1, Nikolaos Dimitratos 1, Ceri Hammond 1, Gemma L. Brett 1, Lokesh Kesavan 1, Saul White 1, Peter Miedziak 1, Ramchandra Tiruvalam 2, Robert L. Jenkins 1, Albert F. Carley 1, David Knight 1, Christopher J. Kiely 2 and Graham J. Hutchings 1 * 1 School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT UK hutch@cf.ac.uk 2 Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, Pennsylvania18015, USA. * To whom correspondence should be addressed. hutch@cf.ac.uk NATURE CHEMISTRY 1

2 Contents 1. Experimental Details 1.1. Catalyst preparation 1.2. Reflux method 2. Catalytic testing 2.1. CO oxidation 2.2. Alcohol Oxidation Benzyl Alcohol Oxidation Glycerol Oxidation 3. Characterization Methods 3.1. Laser Raman spectroscopy 3.2. Elemental Analysis 3.3. Electron Microscopy 3.4. X-Ray Photoelectron Spectroscopy 3.5. Thermogravimetric analysis 4. Results 4.1. Characterization Elemental Analysis (Tables S1 & S2) Raman Spectroscopy (Tables S3 & S4) STEM analysis of Au-PVA/TiO 2 materials (Figs. S1, S2, S3 and Table S5) STEM analysis of AuPd-PVA/TiO 2 materials (Figs S4, S5 and S6) Thermogravimetric analysis (Fig. S7) 4.2. Catalytic data CO oxidation using Au-PVA/TiO 2 (Figs. S8-S15) Alcohol/polyol oxidation using Au and AuPd-PVA/TiO 2 and Au-PVA/TiO 2 (Tables S6 and S7) NATURE CHEMISTRY 2

3 1. Experimental Details 1.1. Catalyst preparation For the preparation of supported Au-Pd colloidal materials, an aqueous solution of PdCl 2 and HAuCl 4. 3H 2 O of the desired concentration were prepared. Fresh solutions of poly (vinyl alcohol) (PVA) (1 wt% aqueous solution, Aldrich, MW=10 000, 80% hydrolyzed) and an aqueous solution of NaBH 4 (0.1 M) were also prepared. For example, a catalyst comprising Au-Pd nanoparticles with 1 wt% total metal loading on a TiO 2 support was prepared as follows: To an aqueous PdCl 2 and HAuCl 4 solution of the desired concentration, the required amount of a PVA solution (1 wt %) was added (PVA/(Au+Pd) (w/w) = 1.2); a freshly prepared solution of NaBH 4 (0.1 M, NaBH 4 /(Au +Pd)(mol/mol)=5) was then added to form a dark-brown sol. After 30 min of sol generation, the colloid was immobilized by adding TiO 2 (acidified at ph 1 by sulphuric acid) under vigorous stirring conditions. The amount of support material required was calculated to have a total final metal loading of 1 wt%. After 2 h, the slurry was filtered and the catalyst was washed thoroughly with distilled water (neutral mother liquors) and dried at 120 C overnight. Monometallic gold catalysts were also prepared using a similar methodology. The calcined catalysts were pre-treated at o C under static air for 3h using a heating rate 5 C/min Reflux method Typically, 1g of catalyst was placed in a round bottom flask and the desired volume of solvent (typically water or (ethanol/thf) was added into the flask. The round bottom flask was connected to a reflux condenser and placed in an oil bath which was heated at 90 C, under vigorous stirring (500 rpm); the solution was left to reflux for periods of time between 30 min and 2 h. After the desired reflux period (typically, 2 h) the slurry was filtered and washed thoroughly with distilled water (2L) and dried at 120 C overnight. In order to optimize the catalyst performance in later studies the stirring rate during the water treatment procedure was increased to 1200 rpm. NATURE CHEMISTRY 3

4 2. Catalytic testing 2.1. CO oxidation Catalyst samples were evaluated for CO oxidation using a fixed-bed laboratory microreactor operated at atmospheric pressure. Typically CO (0.5 % in synthetic air) was fed to the reactor at a controlled rate of ml min 1 using mass flow controllers and passed over the catalyst (35-50 mg). The catalyst temperature was maintained at 25 C by immersing the quartz bed in a thermostatically controlled water bath. The products were analyzed using online gas chromatography with a 1.5 m packed carbosieve column and thermal conductivity detector. The conditions are equivalent to a total gas space velocity (GHSV) of h -1. The catalysts, after being used in the reactor, were stored at room-temperature in sealed containers before spectroscopic analysis. Under such conditions, the state of the catalyst surface should be unaffected and in particular the oxidation state of the gold particles will mainly remain unchanged Alcohol Oxidation Benzyl alcohol oxidation Benzyl alcohol oxidation was carried out in a stirred reactor (100 ml, Parr Instruments). The vessel was charged with benzyl alcohol (40 ml) and catalyst (0.05 g). The autoclave was then purged 5 times with oxygen leaving the vessel at 10 bar gauge. The stirrer was set at 1500 rpm and the reaction mixture was raised to the required temperature and the reaction time was started as soon as the required reaction temperature was reached. Samples from the reactor were taken periodically, via a sampling system. For the analysis of the products a GC-MS and GC (a Varian star 3400 cx with a 30m CP-Wax 52 CB column) were employed, and the products were identified by comparison with known standards. NATURE CHEMISTRY 4

5 For the quantification of the amounts of reactants consumed and products generated, an external calibration method was used Glycerol oxidation Catalytic reactions were carried out using a 50 mlglass/autoclave reactor. The glycerol solution (0.3 M and NaOH/glycerol molar ratio = 2, mol/mol) was added into the reactor and the desired amount of catalyst (glycerol/metal molar ratio = , mol/mol) was suspended in the solution. The glass reactor was purged with oxygen three times and pressurised to the desired level with oxygen (3 bar of pressure) and heated at the desired temperature. The reaction mixture was stirred at 1500 rpm for the required amount of time (0.5-24h). The reactor vessel was cooled to room temperature and the reaction mixture was analysed by HPLC. Analysis was carried out using HPLC with ultraviolet and refractive index detectors. Reactants and products were separated using a Metacarb 67H column. The eluent was an aqueous solution of H 3 PO 4 (0.01 M) and the flow was 0.45 ml min 1. Samples of the reaction mixture (0.5 ml) were diluted (to 5 ml) using the eluent. For the quantification of the amounts of reactants consumed and products generated, an external calibration method was used. NATURE CHEMISTRY 5

6 3. Characterization 3.1. Laser Raman Spectroscopy Raman spectroscopy was used for analyzing the amount of PVA remained before and after the reflux method. Typically, a sample of catalyst (approx 0.1g) was placed on a metal slide inside the spectrometer (Renishaw in-via Ramascope). The powder was analysed under an IR class laser (785nm) with a laser intensity of 0.5% or 5%. The sample was scanned at an attenuation time of 10 seconds and 10 scans were carried out to give a spectrum Elemental Analysis Elemental analysis was carried out on 1% Au/TiO 2 and catalyst, to confirm whether or not the reflux process facilitates the removal of the PVA polymer (stabilizer) and in addition to confirm if the reflux process leads to metal leaching, by measuring the percentage of carbon and of metal that is remained after the reflux process and comparing it to the original fresh sample and the calcined samples Electron Microscopy Samples of sol-immobilized catalysts were prepared for TEM/STEM analysis by dry dispersing the catalyst powder onto a holey carbon TEM grid. In the case of the starting Au-PVA and Au+Pd- PVA sols, a drop of the colloidal sol was deposited, and then allowed to evaporate, onto a 300-mesh copper TEM grid covered with an ultrathin continuous C film. High-angle annular dark field (HAADF) imaging experiments were carried out using a 200kV JEOL 2200FS transmission electron microscope equipped with a CEOS aberration corrector. NATURE CHEMISTRY 6

7 3.4. X-Ray Photoelectron Spectroscopy X-ray photoelectron spectra were recorded on a Kratos Axis Ultra DLD spectrometer employing a monochromatic AlK α X-ray source ( W) and analyser pass energies of 160 ev (for survey scans) or 40 ev (for detailed scans). Samples were mounted using double-sided adhesive tape and binding energies referenced to the C(1s) binding energy of adventitious carbon contamination which was taken to be ev Thermogravimetric Analysis The thermogravimetric (TGA) and differential thermal (DSC) analysis measurements were performed using a TG-DSC 111 apparatus. The TGA and DSC experiments were carried out under oxygen using mg samples and a heating rate of 10 ºC/min. Quartz cells were used as sample holders with α-al 2 O 3 as reference. The temperature range studied was ºC. NATURE CHEMISTRY 7

8 4. Results 4.1. Characterization Elemental analysis Table S1. Elemental analysis of 1 wt % Au/TiO 2 (non-reflux, 30 min water reflux, and 60 min water reflux). Carbon, hydrogen, nitrogen were analysed using a CE440 elemental analyser, chlorine using an oxygen flask determination methodology and ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) was carried out for Au. Elemental Analysis Non-Reflux 30 min reflux 60 min reflux wt% wt% wt% Carbon Hydrogen Nitrogen Chlorine Gold NATURE CHEMISTRY 8

9 Table S2. Carbon analysis of 1 wt % Au/TiO 2 (non-reflux, 30 min water reflux and 60 min water reflux) and calcined samples (200, 300 and 400 ºC in air). Carbon Analysis Non-Reflux 30 min reflux 60 min reflux 200 ºC 300 ºC 400 ºC wt% wt% wt% wt% wt% wt% Carbon The elemental analysis carried out on the catalyst shows that there is 0.59% carbon present in the nonreflux catalyst, 0.57% present in the 30 min refluxed catalyst and 0.44% present in the 60 min refluxed catalyst. These results demonstrate that the reflux method is partially removing the PVA and after 60 min reflux the amount of carbon residues have decreased by 20%. For the calcined samples, a heat treatment above 300 ºC is necessary for efficient removal of PVA. NATURE CHEMISTRY 9

10 Raman Spectroscopy Table S3. Band assignments for the Raman spectrum of polyvinylalcohol (PVA). PVA Band assignments Frequency (cm 1 ) Assignment 855 C C stretch 917 C C stretch 1073 C O stretch, O H bend 1092 C O stretch, O H bend 1147 C O stretch, C C stretch 1360 C H bend, O H bend 1477 C H bend, O H bend 1602 C-H stretch, O-H bend 1778 C-H stretch 2250 C-O stretch, C-H bend 2600 C-H stretch Structure of PVA The structure of PVA is shown here. The band assignments in Table S1 correspond to peaks in Figure 1 and show C-C, C-H and C-O bonds, all of which can only be found in the PVA molecule. As a decrease in these peaks is seen in Figure 1 with water reflux treatment, it indicates that a significant fraction of the PVA is being removed. NATURE CHEMISTRY 10

11 Table S4. Band assignments for the Raman spectrum of TiO 2 (P25). TiO 2 Band assignments Frequency (cm 1 ) Assignment 144 Anatase crystalline phase short range order of the octahedrally co-ordinated titanium short range order of the octahedrally co-ordinated titanium NATURE CHEMISTRY 11

12 STEM analysis of Au-PVA/TiO 2 materials Starting Au-PVA colloidal sol An HAADF image of the starting Au-PVA colloid deposited onto a continuous carbon film is shown in Fig. S1 (a). The corresponding particle size distribution, presented in Fig. S2 (b), shows that the median particle size of the sol is 2.7nm Au-PVA sol immobilized on TiO 2 and dried at 120 o C A representative HAADF image of the Au-PVA sol immobilized onto TiO 2 and dried at 120 o C is shown in Fig. 2(b). The corresponding particle size distribution, presented in Fig. S2 (b), shows that the median particle size has increased slightly to 3.5nm during the drying process. Atomic resolution HAADF images of the Au particles in this sample (Fig. 2) show that they display a mixture of cuboctahedral (Fig. 2a), singly twinned (Fig. 2b) and multiply twinned (Fig. 2c) morphologies, with the twinned variants dominating. NATURE CHEMISTRY 12

13 Au-PVA immobilized on TiO 2, and water washed at 90 o C for 30, 60 and 120mins Representative low magnification HAADF images of the Au immobilized on TiO 2, and water washed at 90 o C for time periods of 30, 60 and 120mins are presented in Figs. S1c, d and e respectively. The corresponding particle size distributions are shown in Figs. S2c, d and e. It is clear that the median particle size increases only marginally from 3.7 to 4.2 to 5.0 nm across this series of samples (see also Table 1). A montage of atomic resolution HAADF images of the samples subjected to the water washing treatments (Fig. 2) show that they all still retain a mixture of cub-octahedral, singly twinned and multiply twinned particles. The distribution of the various Au particle morphologies still strongly resembles that displayed by the 120 o C dried sample. It is also noticeable that those samples washed for the longer time periods seem to show more distinctly facetted surfaces, presumably as a consequence of restructuring after losing a significant fraction of their protective PVA ligands Au-PVA immobilized on TiO 2 and calcined at 200 o C, 300 o C, 400 o C for 3h Representative low magnification HAADF images of the Au immobilized on TiO 2, and calcined for 3h at temperatures of 200, 300 and 400 o C are presented in Figs. S1f, g and h respectively. The corresponding particle size distributions are shown in Figs. S2 f, g and h. It is evident that the median particle size progressively increases from 4.7 to 5.1 to 10.2 nm across this series of samples (see also Table 1). The dramatic increase in size between 300 and 400 o C suggests that the ligands are efficiently removed somewhere in this temperature interval, and the elevated ambient temperature is favourable for unhindered sintering to rapidly occur. Atomic resolution HAADF images of the samples subjected to calcination treatments at different temperatures (Fig. S3) show that they all still retain a mixture of cub-octahedral, decahedral and icosahedral Au particles. However, the distribution of the various Au particle morphologies now becomes markedly different from that observed in the 120 o C dried sample, with the cub-octahedral becoming progressively more dominant as the calcination temperature is NATURE CHEMISTRY 13

14 increased. It is also noticeable that the calcined samples show an increased tendency to form a flatter and more extended interface structures with the underlying TiO 2 support particles. (a) Starting Au-PVA colloid (b) Immobilized & dried at 120 o C (c) Immobilized & water refluxed (90 o C: 30min) (d) Immobilized & water refluxed (90 o C: 60min) NATURE CHEMISTRY 14

15 (e) Immobilized & water refluxed (90 o C: 120min) (f) Immobilized and calcined (200 o C:3hr) (g) Immobilized and calcined (300 o C:3hr) (h) Immobilized and calcined (400 o C:3hr) Figure S1. Representative low magnification STEM-HAADF images of (a) starting Au-PVA sol deposited on a continuous C film; (b) Au immobilized on TiO 2 & dried at 120 o C, (c) Au immobilized on TiO 2 & water refluxed at 0 o C,30min; (d) Au immobilized on TiO 2 & water refluxed at 90 o C,60min; (e) Au immobilized on TiO 2 & water refluxed at 90 o C,120min; (f) Au immobilized on TiO 2 & calcined at 200 o C,3h; (g) Au immobilized on TiO 2 & calcined at 300 o C,3h; and (h) Au immobilized on TiO 2 & calcined at 400 o C,3h. NATURE CHEMISTRY 15

16 (a) Starting Au colloid (b) Immobilized & dried at 120 o C (c) Immobilized & water refluxed (90 o C: 30min) (d) Immobilized & water refluxed (90 o C: 60min) (e) Immobilized & water refluxed (90 o C: 120min) (f) Immobilized and calcined (200 o C:3hr) NATURE CHEMISTRY 16

17 (g) Immobilized and calcined (300 o C:3hr) (h) Immobilized and calcined (400 o C:3hr) Figure S2. Particle size distributions derived from STEM-HAADF images of (a) starting Au-PVA sol deposited on a continuous C film; (b) Au immobilized on TiO 2 & dried at 120 o C, (c) Au immobilized on TiO 2 & water refluxed at 90 o C,30min; (d) Au immobilized on TiO 2 & water refluxed at 90 o C,60min; (e) Au immobilized on TiO 2 & water refluxed at 90 o C,120min; (f) Au immobilized on TiO 2 & calcined at 200 o C,3h; (g) Au immobilized on TiO 2 & calcined at 300 o C,3h; and (h) Au immobilized on TiO 2 & calcined at 400 o C,3h. NATURE CHEMISTRY 17

18 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure S3. Representative HAADF STEM images of the immobilized sol on TiO 2 (a, b, c) calcined at 200 o C, 3h; (d, e, f) calcined at 300 o C, 3h; and (g, h, i) calcined at 400 o C, 120, 3h. NATURE CHEMISTRY 18

19 STEM analysis of AuPd-PVA/TiO 2 materials Starting Au+Pd PVA colloidal sol HAADF images of the starting Au+Pd-PVA colloid deposited onto a continuous carbon film are shown in Figs. S4 (a) and (b). The corresponding particle size distribution, presented in Fig. S4 (c), shows that the median particle size of the sol is 3.2 nm Au+Pd PVA sol immobilized on TiO 2 and dried at 120 o C A low magnification HAADF image and particle size distribution for the Au+Pd PVA sample immobilized on TiO 2 and dried at 120 o C are presented in Figs. S5 (a) and (b). The immobilization and drying process only causes a minor increase in the median particle size to 3.5nm Au+Pd PVA sol immobilized on TiO 2 and hot washed at 90 o C for 60 min A low magnification HAADF image and particle size distribution for the Au+Pd PVA sample immobilized on TiO 2 and washed with hot water at 90 o C for 60 min are presented in Figs. S6 (a) and (b). The hot water washing process causes a further minor increase in the median particle size up to 4.2nm. It is clear once again that the water washing treatment does not dramatically increase the metal nanoparticle size distribution while it efficiently and effectively removes the protective PVA ligands. Representative HAADF and HREM images (Figs. S6 (c) and (d) respectively) of this sample clearly illustrate the tendency of the AuPd particles to facet and wet the TiO 2 forming a flat extended interface with the support upon removal of the PVA ligands. NATURE CHEMISTRY 19

20 (a) (b) (c) Figure S4. Low (a) and high (b) magnification STEM-HAADF images of the starting Au+Pd-PVA sol deposited onto a continuous carbon film. (c) The corresponding particle size distribution of the Au+Pd- PVA sol. NATURE CHEMISTRY 20

21 (a) (b) Figure S5. Low magnification STEM-HAADF image (a) and corresponding particle size distribution (b) of the Au+Pd-PVA sol immobilized on TiO 2 and dried at 120 o C. NATURE CHEMISTRY 21

22 (a) (b) (c) (d) Figure S6. (a) STEM HAADF image and particle size distribution (b) of the Au+Pd PVA sol immobilized on TiO 2 and washed with hot water at 90 o C for 60 min. (c) STEM-HAADF image and (d) HREM image of typical AuPd alloy particles in this sample. NATURE CHEMISTRY 22

23 TGA DSC Figure S7. TGA and DSC curves of a PVA sample. Thermal decomposition of PVA occurs in the ºC temperature range. NATURE CHEMISTRY 23

24 4.2. Catalytic data CO oxidation using Au-PVA/TiO 2 Figs. S8-S15 show the catalytic performance of Au-PVA/TiO 2 material by varying the following parameters for optimizing the CO conversion level and demonstrating the efficacy of the methodology: effect of reflux treatment (Fig. S8); effect of flow rate (Fig. S9); comparison of reflux treatment and heat treatment in N 2 (Fig. S10); reproducibility of CO conversion levels (Fig. S11); effect of extended reaction time on CO oxidation performance (Fig. S12); effect of volume of reflux solvent used on CO oxidation (Fig. S13); CO oxidation performance of water refluxed 1 wt% Au-PVA/TiO 2, 60 min, using an increased rate of stirring (1200 rpm) for the reflux method (Fig. S14). Catalytic activity of 1 wt% Au/TiO 2 after a series of pre-treatments using oxygen and hydrogen (Fig. S15). An additional experiment was performed in which Au/TiO 2 was treated with potassium permanganate. For treatment with KMnO 4, Au/TiO 2 (0.1 g) was suspended in H 2 O (20 ml) and 1 ml (0.05M KMnO 4 ) solution was added. The slurry solution was stirred at room temperature for 20 h, filtered and washed with H 2 O. The product was dried at 70 o C for 10 h and when tested for CO oxidation under standard conditions it was found to be inactive. NATURE CHEMISTRY 24

25 Figure S8. Effect of reflux treatment for CO oxidation using PVA stabilized 1wt% Au/TiO 2 catalyst, GHSV of 12,000 h -1, catalyst (50 mg). NATURE CHEMISTRY 25

26 Figure S9. Effect of varying flow rate for CO oxidation on a standard Au-PVA/TiO 2 catalyst refluxed at 90 o Cin water for 60 min. GHSV between 12,000 h -1 and 6,000 h -1 catalyst (50 mg). NATURE CHEMISTRY 26

27 Figure S10. Effect of volume of reflux solvent used on CO oxidation performance using PVA stabilized 1wt% Au/TiO 2 catalyst. Reaction conditions: GHSV of 12,000 h -1 catalyst (50 mg). Figure S11. Comparison of reflux treatment and heat treatment in N 2 on Au-PVA/TiO 2 material: refluxed in water 90 o C, 60 min, 200 o C N o C N 2, (heated in flowing N 2 at 200, and 300 o C for 3 h), GHSV of 12,000 h -1, catalyst (50 mg). NATURE CHEMISTRY 27

28 Figure S12. Reproducibility of CO conversion levels for a 1wt% Au-PVA/TiO 2 water refluxed catalyst, 60 min, using 3 different batches. Batch A, Batch B, Batch C, GHSV of 12,000 h -1, catalyst (50 mg). NATURE CHEMISTRY 28

29 Figure S13. Effect of extended reaction time on CO oxidation performance of water refluxed 1 wt% Au-PVA/TiO 2, 60 min, GHSV of 6,000 h -1, catalyst (50 mg). NATURE CHEMISTRY 29

30 Figure S14. CO oxidation performance of water refluxed 1 wt% Au-PVA/TiO 2, 60 min, using an increased stirring rate (1200 rpm) for the reflux method. GHSV of h -1, catalyst (50 mg). NATURE CHEMISTRY 30

31 Figure S15. Catalytic activity of 1 wt% Au/TiO 2 after a series of pre-treatments. Entry A: Calcination (200 o C, 3 h, static air), B: Reflux (90 o C, 1 h), C: Reflux and calcination (200 o C, 3 h, static air); D: Reflux and heat treatment with H 2 (200 o C, 3 h, 5% H 2 /Ar), GHSV = h -1, catalyst (50 mg). NATURE CHEMISTRY 31

32 Oxidation of alcohols Table S6. Effect of water reflux on various alcohol oxidation reactions using 1 wt% (Au-Pd)- PVA/TiO 2 prepared by the sol-immobilisation method a,b Reactant Catalyst Benzyl alcohol 1%(Au-Pd)/TiO 2 No water reflux Benzyl alcohol 1%(Au-Pd)/TiO 2 30 min reflux Benzyl alcohol 1%(Au-Pd)/TiO 2 60 min reflux Selectivity for benzyl alcohol oxidation (%) Time Conv. Benzoic Benzyl Toluene Benzaldehyde (h) (%) acid benzoate Selectivity for glycerol oxidation (%) glycolate/ glycerate oxalate tartronate formate Glycerol 1%(Au-Pd)/TiO 2 No water reflux Glycerol 1%(Au-Pd)/TiO 2 60 min reflux a Reaction conditions: benzyl alcohol, 0.05g of catalyst, T = 120 ºC, po 2 = 150 psi, stirring rate 1500 rpm. b Reaction conditions: glycerol (0.6M, 20 ml), NaOH: glycerol molar ratio = 2, glycerol/metal mol ratio = 1000, 30 ºC, po 2 = 10 bar, stirring rate = 1000 rpm, catalyst (55mg). NATURE CHEMISTRY 32

33 Table S7. Effect of water reflux and calcination treatments on the glycerol oxidation reaction using 1 wt% Au-PVA/TiO 2 prepared by the sol-immobilisation method a Reactant Catalyst Time (h) Conv. (%) Selectivity for glycerol oxidation (%) glycerate oxalate tartronate glycolate/ formate Glycerol 1%Au/TiO 2 dry 110 C %Au/TiO 2 60 min reflux %AuTiO 2 Calcined /200 C %AuTiO 2 Calcined /400 C a Reaction conditions: glycerol (0.6M, 20 ml), NaOH: glycerol molar ratio = 2, glycerol/metal mol ratio = 1000, 30 ºC, po 2 = 10 bar, stirring rate = 1000 rpm, catalyst (46mg). NATURE CHEMISTRY 33