SEARCH

SEARCH BY CITATION

Keywords:

  • alcohols;
  • biomass;
  • heterogeneous catalysis;
  • nanoparticles;
  • X-ray absorption spectroscopy

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Bimetallic Pt–Re nanoparticles supported on Norit carbon were effective at converting aqueous glycerol to 1,3 (34 %) and 1,2 (33 %) propanediol at 443 K under 4 MPa of H2. Results from X-ray absorption spectroscopy and analytical transmission electron microscopy confirmed that the nanoparticles were indeed bimetallic, with a particle size less than 2 nm in diameter. The Pt LIII near edge spectra indicated that the Pt was reduced to the metallic state by treatment in H2 at 473 K, but that a partial positive charge remained on the Re. These oxidized Re species could be associated with charged Re atoms dispersed on the carbon support because spillover of H atoms from Pt was required to reduce Re in the bimetallic particles.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

The conversion of molecules derived from renewable biomass to value-added products is an area of growing interest because of the uncertainty of fossil carbon sources. Thus, an abundance of glycerol produced by the biodiesel industry motivated its conversion to synthesis gas1, 2 at temperatures above 540 K and to propanediols37 at lower temperatures. Although the focus of most of the prior work involved the catalytic hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO or propylene glycol), formation of higher value 1,3-propanediol (1,3-PDO) has recently been obtained with improved selectivity.811 The comonomer 1,3-PDO is combined with terephthalic acid to produce polytrimethylene terephthalate, a polymer valued for its chemical resistance, light stability, elastic recovery, and dyeability.3 Although 1,3-PDO has been traditionally produced from ethylene oxide or acrolein, Dupont Tate & Lyle recently commercialized a biorenewable process that utilizes genetically-engineered E. coli to transform D-glucose derived from corn.12, 13 The literature also describes a two-step route to convert glycerol to 1,3-PDO by conversion to acrolein over a solid acid catalyst followed by hydration and hydrogenation.14 Although the multistep conversion of glycerol to 1,3-PDO is reasonably effective, there is a strong motivation to discover a catalyst capable of the single-step hydrogenolysis of glycerol to 1,3-PDO.

Homogeneous metal complexes of Rh, Pd, and Ru in various organic solvents catalyze the formation of 1,3-PDO from glycerol.1517 For example, a Pd-1,2-bis(1,5-cyclooctylenephosphino) ethane complex in sulfolane/water solvent together with of 1-methyl-pyrrolidinone (MPI) at 413 K under 6 MPa of a CO/H2 gas mixture converted glycerol to 1,3-PDO with 30.8 % selectivity.16 However, a selective solid catalyst is highly preferable to facilitate catalyst recovery. Exceptional examples noted in Table 1 include the use of Pt/WO3/ZrO3 in 1,3-dimethyl-2-imidazolidinone (DMI) and Rh/C in sulfolane/water, which achieved 1,3-PDO selectivity of 27–28 %. Additionally, Huang et al. obtained 32 % selectivity to 1,3-PDO using a solvent-free vapor phase glycerol feed in a fixed-bed reactor over Cu-H4SiW12O40/SiO2 at 0.54MPa H2 pressure at 483 K. In this case, the catalyst was designed to facilitate the coupled sequential reaction of acid-catalyzed dehydration of glycerol to 3-hydroxypropanal followed by hydrogenation to 1,3-PDO. Many of the selective catalytic reactions (ca. 30 % selectivity to 1,3-PDO) employ organic solvents such as sulfolane, MPI, and DMI9, 16, 23 whereas those that use water as a solvent often suffer from low selectivity to 1,3-PDO.1822, 24, 25 In addition, water lowers the selectivity to 1,3-PDO during the vapor phase dehydration/hydrogenation of glycerol over Cu-H4SiW12O40/SiO2, which is problematic because water is a product of the reaction sequence. The modification of Rh/SiO2 with Re, Mo, or W has been shown to increase the activity for glycerol hydrogenolysis in water while giving 1,3-PDO selectivity comparable to systems using nonaqueous solvents.10, 11 Herein, we describe the catalytic performance of Pt–Re bimetallic catalyst for glycerol conversion to propanediol and detailed characterization of the catalyst by X-ray absorption spectroscopy at the Pt and Re LIII edges.

Table 1. Summary of prior work on glycerol hydrogenolysis to 1,3-propanediol over heterogeneous catalysts.
CatalystP [MPa]T [K]Conv. [%]Sel. [%]Ref.
  1. Solvent/additive: [a] water; [b] water/Amberlyst; [c] water; [d] sulfolane/H2WO4; [e] water/H2WO4; [f] DMI/none; [g] vapor phase, no solvent.

2.5 % Ni/2.5 % Re/C[a]9503474.618
2.5 % Pd/2.5 % Re/C[a]9503265.218
Ru/C[b]83936.45.619
Rh/Cs2.5H0.5[PW12O40][c]0.54536.37.120
Rh/C[b]84136.47.221
Rh/SiO2[b]8393149.822
Rh-ReOx/SiO2[a]84338610.410
5 % Rh/C[d]8453321223
Ru/Al2O3+Re2(CO)10[a]8433531324
Ru-Re/SiO2[a]8433241425
Rh-ReOx/SiO2[a]83933816.110
1 %Rh/Nafion[e]845381923
Rh/C[e]84531.32121
Rh-ReOx/SiO2[a]8393152810
Pt/WO3/ZrO2[f]844386289
Cu-H4SiW12O40/SiO2[g]0.5448383328

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Four different catalysts were prepared and evaluated in the hydrogenolysis of glycerol to propanediols. First, a bimetallic Pt–Re catalyst was synthesized in a 1:1 atomic ratio of metal components on a carbon support. The miscibility gap in the Pt–Re phase diagram at a 1:1 metal ratio might suggest the formation of Pt-rich and Re-rich bimetallic particles. The catalyst was reduced at low temperature (723 K) and at high temperature (973 K) to produce Pt–Re/C and Pt–Re/C (sintered), respectively. Figure 1 presents electron micrographs of the Pt–Re/C catalyst as synthesized (low temperature reduction) and after sintering. The sintering of the catalyst resulted in a slight increase in the mean metal particle size from 1.2 to 1.7 nm, but both catalysts were very highly dispersed. The results from energy-dispersive X-ray spectroscopy (EDS) analysis for the Pt–Re catalyst revealed that the Pt and Re were both present on the carbon. The atomic ratio of Pt/Re determined by standardless EDS analysis was 1.6 and compared well to the value of 1.2 measured by inductively coupled plasma atomic emission spectroscopy (ICP–AES) analysis. Analysis of individual Pt–Re particles was not possible due to the proximity of these particles and their small size, hence the analysis was performed by scanning a region containing a few hundred particles. After sintering, the interparticle spacing had increased so that it was now possible to perform EDS analysis of individual metal particles. Whereas the large area scans revealed no change in composition after sintering, the individual particles were found to have a higher Pt/Re ratio (3:1) than the large area scans. By analyzing sample areas that were devoid of metallic particles, we could confirm the presence of dispersed Re on the surface. The dispersed Re was also found on the sample reduced at the lower temperature. For comparison, monometallic Pt/C and Re/C catalysts were also prepared. Chemisorption of H2 was used to estimate the number of active sites, assuming every adsorbed H atom corresponded to a surface active site. Whereas H2 adsorbed readily onto the Pt/C and Pt–Re/C catalysts, it did not adsorb onto Re/C. A summary of the physical characteristics of the catalysts is presented in Table 2. The decrease in H/Pt ratio for the sintered Pt–Re/C catalysts compared to the as-synthesized bimetallic catalyst is consistent with the particle size determined from transmission electron microscopy (TEM).

thumbnail image

Figure 1. TEM image and the corresponding particle size distribution of a) Pt–Re/C and b) Pt–Re/C (sintered).

Download figure to PowerPoint

Table 2. Physical characteristics of carbon-supported Pt–Re, Pt, and Re catalysts.
MaterialPt loading [wt %]Re loading [wt %]H uptake [μmol g−1]H/Pt
  1. [a] Metal weight loadings were assumed to remain constant upon sintering.

Pt–Re/C5.74.62330.80
Pt–Re/C[a] sintered5.74.61180.41
Pt/C6.11800.57
Re/C4.7

The reactivity results from glycerol hydrogenolysis over the four catalysts are provided in Table 3. In general, glycerol was converted in aqueous solution to a variety of products at 473 K and 4.0 MPa H2, such as ethylene glycol (EG), 1,2-PDO, 1,3-PDO, 1-propanol (1-PrOH), 2-propanol (2-PrOH), and ethanol (EtOH). Typically, 1,2-PDO, 1,3-PDO, and 1-PrOH constituted greater than 85 % of the products (Scheme 1). Monometallic Pt/C produced only 1,2-PDO and EG at 473 K, with a turnover frequency (TOF) that was an order of magnitude lower than that of Pt–Re/C. The activity and selectivity for glycerol hydrogenolysis over Pt/C compared well with previously reported values.7 Lowering the temperature of the reaction from 473 to 443 K increased the 1,3-PDO/1,2-PDO ratio from 0.46 to 0.63 at 20 % conversion over Pt–Re/C catalyst. Both monometallic Pt/C and Re/C exhibited no detectable activity at 443 K. The selectivity to 1,3-PDO at 443 K improved from 26 % to 34 % (at 20 % conversion of glycerol) when Pt–Re/C was sintered at high temperature. Because highly-dispersed Re was observed by EDS analysis of regions between metal particles, we suspect that the increase in selectivity to 1,3-propanediol with the sintered sample resulted from a higher level of Re incorporation into the bimetallic Pt–Re particles after high temperature treatment compared to the as-synthesized sample.

Table 3. Results from glycerol hydrogenolysis over carbon-supported Pt, Re, and Pt–Re catalysts.[a]
CatalystT [K]BaseC Balance[b] [%]TOF[c] [s−1]Conv. [%]t [h]Selectivities
  Condition    EG1,2-PDO1,3-PDO1-PrOH2-PrOHEtOHLAFA
  1. [a] 1 wt % aqueous glycerol solution, substrate/metal adsorption site (S/Mads)=350, H2 Pressure=4 MPa [b] Carbon balance=moles of C at the end of the reaction/moles of C initial [c] TOF of glycerol calculated at 20 % conversion. S/Mads=350 [d] 50 wt % glycerol solution [e] Substrate/metal ads. site=1000 [f] Pressurized with N2.

Pt–Re/C473820.039200.540.100.390.180.200.060.07
5050.080.360.140.250.080.09
               
Pt/C473940.0021050.230.74
               
Pt/C443NaOH 0.8 M990.012201.560.080.430.460.03
6880.070.440.440.04
               
Pt–Re/C443NaOH 0.8 M920.033200.590.140.460.270.12
9380.040.290.050.050.130.250.13
               
Pt–Re/C443910.008202.530.040.410.260.220.07
3480.030.390.250.250.08
               
Pt–Re/C Sintered443900.009202.060.330.340.220.11
4580.270.290.300.14
               
Pt/C443100008
               
Re/C44394048
               
Pt–Re/C[d]443930.006[e]20110.020.220.270.400.090.01
33240.010.200.250.410.090.02
               
Pt–Re/C[f]443900.0032180.170.610.040.030.070.02
thumbnail image

Scheme 1. Primary hydrogenolysis products from glycerol over Pt–Re/C at 443 K.

Download figure to PowerPoint

The effects of solution pH, glycerol concentration, and gas composition on the conversion of glycerol over Pt–Re/C were explored. Previous work has demonstrated the promotional role of adding base to aqueous solutions of glycerol for the production of 1,2-PDO.7 However, addition of a base also produces the salts of organic acids such as lactate (LA) and formate (FA). As indicated in Table 3, addition of NaOH to the glycerol reactant solution increased the rate of glycerol conversion over Pt–Re/C to 1,2-PDO and EG, but eliminated the formation of 1,3-PDO. Results in Table 3 also show that a high selectivity to 1,3-PDO was observed when the concentration of the glycerol solution was increased from 1 to 50 wt % glycerol (0.11 to 5.43 M). However, replacing H2 with N2 eliminated the reaction path that formed 1,3-PDO.

The atomic structure and chemical state of the Pt–Re/C catalyst were evaluated by X-ray absorption spectroscopy (XAS). The Pt LIII near edge spectra for the bimetallic sample in air and after reduction in situ are compared in Figure 2 to Pt foil and PtO2. The decrease in the white line intensity above the edge for the bimetallic catalyst sample after H2 treatment was consistent with reduction of Pt to the metallic state. However, the position of the edge was clearly shifted to higher energy compared to Pt foil (and Pt/C, not shown) indicating a possible role of the Re on the electronic structure of Pt in the bimetallic catalyst, which has been reported previously.26 The derivatives of the Pt LIII-edge for Pt–Re/C in H2 at 473 K and Pt foil are given in Figure 3 along with the derivatives of the X-ray absorption near edge structure (XANES) for Pt/C in H2 and Pt–Re/C (sintered). The inflection point in Figure 2 (maximum in Figure 3) of the Pt LIII-edge XANES was shifted to higher energy by approximately 1 eV for the Pt–Re samples under H2 whereas the XANES of Pt/C under H2 coincided with Pt foil. The Re LIII-edge spectra presented in Figure 4 indicate that Re in Pt–Re/C in air was fully oxidized (Re 7+) but was partially reduced by H2 at 473 K. The Re/C sample could not be reduced under the same conditions (remained Re 7+), demonstrating the important role of Pt on the reduction of Re in the bimetallic catalyst. Figure 5 correlates the edge position (inflection point) to the oxidation state of standard Re compounds. Plotting the edge position of the reduced Pt–Re/C catalyst and the sintered analogue confirmed that Re was mostly reduced in the bimetallic sample by H2 at 473 K and is consistent with the edge position of Re in Pt–Re bimetallic clusters reported previously.2 The intensity of the white line and the edge position of the Re LIII edge for the Pt–Re/C in H2 both suggest that although the Re was mostly reduced, the average oxidation state was not equivalent to the Re metal. It is possible that the nearly atomically dispersed Re on the carbon (as detected by EDS) remained oxidized, whereas Re associated with the bimetallic particles was reduced after treatment in H2.

thumbnail image

Figure 2. X-ray absorption near edge structure (XANES) at the Pt LIII edge of the Pt samples. μ(E)=absorption coefficient.

Download figure to PowerPoint

thumbnail image

Figure 3. Derivative of μ(E) at the Pt LIII-edge of Pt foil compared to supported Pt samples.

Download figure to PowerPoint

thumbnail image

Figure 4. X-ray absorption near edge structure (XANES) of the Re LIII edge of the Re samples.

Download figure to PowerPoint

thumbnail image

Figure 5. Correlation of the Re LIII-edge energy (inflection point) to the oxidation state of reference compounds (triangles) and supported catalysts (circles).

Download figure to PowerPoint

Fourier transforms (FTs) of the Pt LIII and Re LIII extended X-ray absorption fine structure (EXAFS) are presented in Figure 6 and Figure 7. Although the Fourier transforms are qualitatively similar to those obtained from Pt–Re clusters supported on Vulcan XC-72 carbon black, curve-fitting analysis of the EXAFS from the bimetallic catalyst was not reported.2 The coordination numbers (CN), interatomic distances (r), and Debye–Waller factors (σ2) obtained by curve fitting the EXAFS results in Figure 6 and Figure 7 are summarized in Table 4. Example curve fits associated with the results in Table 4 are compared to the experimental EXAFS for Pt and Re LIII EXAFS of Pt–Re/C (sintered) in Figure 8 and Figure 9. The Re EXAFS of Pt–Re/C and Re/C samples in air were indistinguishable from the Re EXAFS of ammonium perrhenate (Figure 7). However, metal–metal contributions in the Re EXAFS of the bimetallic catalyst were observed after reduction of the sample in H2. It should be noted that the low intensity of the metal–metal peaks in the Fourier transform of the reduced catalyst given in Figure 6 and Figure 7 results from both a low coordination number relative to the bulk standard and the elevated temperature used to acquire the spectra. Because the backscattering functions of Pt and Re are similar, it was not possible to distinguish the different atom types in the EXAFS fitting. The small coordination number for both Pt and Re in Pt–Re/C after reduction (5 and 6, respectively) indicated the particles were very small (about 1 nm in diameter or less),27 which is consistent with the particles imaged in Figure 1 and the high hydrogen chemisorption capacity in Table 2. The nearly equivalent coordination numbers for Pt and Re suggest that both components are present in the surface layer of the particles. However, because particles of this size expose nearly all of their atoms to the surface, severe segregation to the surface is not possible. The coordination numbers for Pt and Re in the sintered bimetallic catalyst increased from 5 and 6 to 8 and 10, respectively, confirming the growth of the metal particles by the thermal treatment. Sintering of metal particles is consistent with the decreased H2 chemisorption (Table 2) and slightly larger particles observed by electron microscopy (Figure 1). All of the supported Pt/C and Pt–Re/C catalysts revealed metal[BOND]metal bond distances contracted from the bulk metals, which is consistent with a substantial fraction of the atoms residing in a contracted surface layer.

thumbnail image

Figure 6. Magnitude of the Fourier transforms (not corrected for phase shifts) of the k3-weighted Pt LIII-edge EXAFS of the standard compounds (Pt foil and PtO2) compared to supported Pt and Pt–Re. Samples in H2 were recorded at 473 K.

Download figure to PowerPoint

thumbnail image

Figure 7. Magnitude of the Fourier transforms (not corrected for phase shifts) of the k3-weighted Re LIII-edge EXAFS of the standard compounds (Re foil and ammonium perrhenate) compared to supported Re and Pt–Re. Samples in H2 were recorded at 473 K.

Download figure to PowerPoint

Table 4. Results from the analysis of Pt and Re EXAFS.
MaterialShellFT range ΔK [Å−1]Fitting range Δr [Å]CNr [Å]σ2 [Å2]ΔE [eV]equation imageR-factor
  1. [a] Absorber–backscatter pair, where the backscatter M could be either Pt or Re, but was fitted with only one type of function associated with either Pt or Re. [b] ΔE was restrained between −15 and +15 eV.

Pt–Re/C in airRe–O2–150.8–3.04.5±0.31.73±0.0050.001±0.00511±10.6770.02
Pt–Re/C in H2Re–MRe[a]2–141.6–3.46±12.55±0.009[b]0.010±0.001−15±2[b]0.8910.04
 Pt–MPt[a]2–91.6–3.45±12.69±0.0100.008±0.0103±20.7200.04
Pt–Re/C in H2Re–MRe[a]2–141.6–3.410±22.67±0.0100.010±0.001−1±20.8910.07
(sintered)Pt–MPt[a]2–91.6–3.48±12.72±0.0100.008±0.0025±10.7200.03
Pt/C in H2Pt–Pt2–141.0–3.48±12.69±0.0090.012±0.0013±10.8290.07
Re/C in H2Re–O2–180.8–3.03.9±0.31.72±0.0050.001±0.0059±10.6770.04
thumbnail image

Figure 8. Comparison of curve fit (dashed line) to experimental data (solid line) from Pt LIII EXAFS of Pt–Re/C (sintered) in H2 at 473 K. (a) Fourier transform of EXAFS (b) Back transform over fitting range in Table 4.

Download figure to PowerPoint

thumbnail image

Figure 9. Comparison of curve fit (dashed line) to experimental data (solid line) from Re LIII EXAFS of Pt–Re/C (sintered) in H2 at 473 K. (a) Fourier transform of EXAFS (b) Back transform over fitting range in Table 4.

Download figure to PowerPoint

In a recent paper, Kunkes et al. speculate on the nature of the active sites on Pt–Re/C for the glycerol conversion to syngas.2 In particular, they suggest that higher catalytic activity of Pt–Re/C compared to Pt may result from less extensive blocking of surface sites by reaction intermediates and/or products such as CO, which can adsorb strongly on Pt.2 Because CO is not a major reaction product in the present work, we suggest that addition of Re to Pt accelerates the glycerol hydrogenolysis by direct promotion of the reaction. The Re component of the catalyst is oxophilic and is likely to be coordinated to OH when the catalyst is in aqueous solution. Shinmi et al. also suggest that Re in Rh–Re bimetallic catalyst for glycerol hydrogenolysis in water exists initially as Re[BOND]OH.11 However, they speculate that the role of Re is to anchor glycerol to the bimetallic surface so that adjacent Rh sites can proceed to deoxygenate the bound glycerol with hydrogen to form an adsorbed diol.11 We suggest that another possible role of surface Re[BOND]OH species present on a bimetallic surface is to facilitate hydrogenolysis of glycerol by direct activation of a C[BOND]OH bond. Future studies with ab initio quantum chemical calculations will be performed to help elucidate the role of oxophilic metal promoters in bimetallic catalysts for carbohydrate hydrogenolysis.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

In summary, Pt–Re bimetallic clusters on Norit carbon were substantially more active for glycerol hydrogenolysis than either Pt or Re and produced 1,3-PDO in water at a selectivity as high as 34 % at 443 K. Characterization by in situ X-ray absorption spectroscopy, transmission electron microscopy, and hydrogen chemisorption revealed that the metal particles were bimetallic and highly dispersed on the carbon support, with an average particle size of less than 2 nanometers. The Re component of the catalysts could only be reduced under H2 at 473 K when Pt was present, presumably by spillover of dissociated H atoms from the Pt component to Re. The catalytic performance of Pt–Re/C was improved by sintering at elevated temperatures, which resulted in better atomic mixing of the Pt and Re components without severely decreasing the dispersion of the metals.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Catalyst Preparation

The monometallic Pt and Re catalysts and the bimetallic Pt–Re catalyst were prepared by incipient wetness impregnation of activated carbon (Norit SX-1G). Aqueous solutions of H2PtCl6⋅6H2O and HReO4 (Strem Chemicals) were used as precursors for Pt and Re impregnation of the activated carbon. A 2:1 ratio of solution mass to carbon support mass was used for impregnation in which the precursor was diluted such that it contained the appropriate amount of metal for a 5 wt % Pt loading for the Pt/C catalyst, a 5 wt % Pt loading with a Pt/Re molar ratio of 1 (ca. 4.9 wt % Re) for the Pt–Re/C catalyst, and a Re loading equal to that of the bimetallic catalyst for the Re/C catalyst. Following impregnation, the catalysts were dried in air overnight at 413 K. The catalysts were reduced under flowing H2 (GT&S, UHP 99.9999 %) at 70 cm3 s−1 while heating at 0.5 K min−1 to 723 K and maintaining that temperature for 3 h. After reduction, the catalysts were cooled to room temperature under flowing H2 prior to being exposed to air. The Pt and Re weight loadings were determined by ICP–AES analysis (Galbraith Laboratories, Knoxville, TN). Prior to ICP–AES analysis the catalysts were dried in air for 3 h at 473 K to remove any adsorbed water.

Chemisorption of H2

The chemisorption analysis was performed on a Micromeritics ASAP 2020 at 308 K over a pressure range of 1–60 kPa. The catalysts were prepared for chemisorption by heating at 4 K min−1 under flowing H2 (GT&S, UHP 99.9999 %) to a reduction temperature of 773 K for Re containing samples or 673 K for a monometallic Pt sample. The samples were reduced at the high temperature in flowing H2 for 90 min followed by evacuation for 120 min, cooling to 308 K, and evacuation for another 120 min. The amount of H2 uptake was determined by extrapolating the linear higher pressure portion of the adsorption isotherm to zero pressure. The number of active sites per gram of catalyst was calculated by assuming each atom of hydrogen adsorbed on an active site.

Transmission Electron Microscopy

Elemental analysis was performed from three types of regions. At low magnification, in TEM mode, large count rates were used to quantify the overall composition of the sample. Individual particles were analyzed using a 1 nm analytical probe. In each case, the beam was rastered in a box centered on the particles and the dwell time was 10 ms per pixel. Analysis of regions that were devoid of metallic particles were performed using the same 1 nm analytical probe and once again scanning the beam in a box that defined the area to be analyzed. In some cases, the analysis was repeated over multiple time intervals to see if the composition changed with time, which would provide evidence that the beam was causing sputtering or preferential loss of individual metal components. Metal particle size distributions were determined using ImageJ software.28

X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) was carried out on a beamline X-18B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The NSLS storage ring was operated at 2.8 GeV with a ring current of about 300 mA. The XAS data were obtained in the transmission mode at the Pt LIII-edge (11.564 keV) and the Re LIII-edge (10.535 keV) with a spot size of 0.5 mm×8 mm.

The catalysts were pressed into a copper sample holder that was mounted in a cell designed for in situ XAS analysis under flowing gas at elevated temperatures. The amount of catalyst in the cell was adjusted so that the absorption edge had a step height of about one. All samples at any given condition were scanned a minimum of three times. For the Pt LIII-edge scans, a 4 μm thick Pt foil (Goodfellow, 99.99 %) was placed between the transmission and reference ion chambers and Re metal powder (Aldrich, 99.995 %) was used as a reference for the Re LIII-edge scans. The Re powder spectrum was compared to that of 12.5 μm Re foil (Goodfellow, 99.99 %) to ensure that the Re metal powder reference had the same properties as the bulk metal. The catalyst samples were heated at 3 K min−1 to 473 K while flowing 5 % H2 in He. A set of scans was obtained after holding the temperature at 473 K for 2 h. Prior to heating under H2, Pt–Re/C was scanned in air.

The XAS data were processed using the Athena29 software for background removal, post-edge normalization, and XANES analysis. The oxidation states of the samples were determined by comparing the inflection point of the edge from the sample to the inflection point of standards with known oxidation state. Rhenium metal powder, ReO2 (Aldrich, 99.9 %), and ammonium perrhenate (Alfa Aesar, 99.999 %) were used as references for Re 0, +4, and +7 oxidation states. The Pt foil and PtO2 (Aldrich) were used as references for 0 and +4 oxidation states of Pt. The EXAFS was analyzed using Artemis software, which implemented FEFF. The normalized EXAFS data were appropriately k-weighted and then Fourier transformed from k-space to R-space. The Fourier transform ranges are provided in Table 4. The Pt range was rather small because of overlap with the Re LII-edge. The 1st shell metal–metal and metal–oxygen amplitude reduction factors (equation image) were extracted from the metal and metal oxide references, respectively. The equation image values from the references were used as constants when fitting the metal–metal or metal–oxide shells in the synthesized catalyst. Two values of equation image for the Pt[BOND]Pt shell were calculated from the Pt foil, depending on the range of wavevector used for analysis. The overlap of the Re LII edge with the Pt LIII EXAFS required a smaller range of analysis for the bimetallic sample compared to monometallic Pt/C. The EXAFS data were fitted in R-space using single scattering Pt–Pt, Re–Re, and Re–O generated theoretically using FEFF 6.0 code.30

Glycerol Hydrogenolysis

The glycerol solution was prepared by diluting glycerol (Acros, ACS reagent Grade) with distilled, deionized water. The catalyst was loaded into a 300 mL stainless steel batch reactor (Parr Instruments) equipped with a temperature controller, a magnetically driven stirrer and a dip tube for periodic sampling. The amount of catalyst loaded into the reactor was adjusted for the various catalysts to maintain a substrate to surface metal mole ratio of 350:1. The reactor was purged by flowing H2 at 0.2 MPa for 20 min at which time the outlet was closed and the H2 pressure was increased to 1.5 MPa. To re-reduce the catalyst, the reactor was heated to 473 K (6 K min−1) and held for 2 h under about 2.0 MPa prior to adjusting the temperature to the reaction conditions. The glycerol solution was charged into the reactor from an external holding vessel maintained at the reaction temperature and purged with H2. A liquid sample was withdrawn during the transfer for evaluation at time zero and subsequent samples were taken during the course of the reaction. The samples were analyzed using an HPLC (Waters e2695) equipped with an ion exclusion column (Bio Rad Aminex HPX-87 H), a UV/Vis detector (Waters 2489), a refractive index detector (Waters 2414), and Empower 2 software. The HPLC column operated at 318 K using a 5 mM H2SO4 mobile phase flowing at 0.5 mL min−1. The conversion and selectivities were calculated as follows:(1), (2)

  • equation image(1)
  • equation image(2)

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

This material is based upon work supported by the National Science Foundation under grants OISE-0730277 and EEC-0813570. The research at UNM made use of electron microscopy facilities supported by NSF and NASA. Research was carried out in part on beam line X-18B (operated by the Synchrotron Catalysis Consortium, which is funded by the U.S. Department of Energy grant No. DE-FG02-05ER15688) at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886. The authors acknowledge with gratitude the invaluable assistance received from the X-18B beam line personnel, Mr. Syed Khalid and Mr. Nebojsa Marinkovic.