propanediol selectivity with a very high TOF by aqueous phase
reforming without the addition of any external hydrogen over
3 wt% Pt supported on hydrotalcite.
were calculated theoretically by FEFF 8.4 code.24 ATOMS25 was
used to obtain the FEFF input code for crystalline materials.
Temperature programmed reduction (TPR) was carried out on a
Micromeritics, Auto Chem II 2920 instrument in the temperature
range of 50–1000 °C with a heating rate of 10 °C min−1 using
10% H2–Ar gas as the carrier gas. The H2 consumed in the TPR
study was measured quantitatively by a thermal conductivity
detector. Before the TPR study, the catalyst was pre-treated at
350 °C for 2 h under a flow of He. Platinum dispersions were
obtained by pulse H2-chemisorption. The experiments were
carried out in the same setup as H2-TPR. Samples were pre-
viously reduced under flowing pure H2 (10 ml min−1) at 500 °C
for 2 h, and was then purged in He at 540 °C for 2 h and cooled
at 25 °C. The pulse size was 0.5 ml H2 and the time between
pulses was 3 min. A Thermo Scientific K-Alpha X-Ray photo-
electron spectrometer was used to study the near surface compo-
sition of the dry reforming catalysts before and after the reaction.
2
Experimental
2.1 Materials and preparation of the catalysts
The support hydrotalcite (HT) material was prepared by the
reported procedure.Pt(NH3)4(NO3)2 was used as the Pt source
and was purchased from Sigma-Aldrich. Glycerol (>99.5%,
spectrophotometric grade) was purchased from Sigma-Aldrich
and used without further purification. Nitrogen (>99%) was pro-
cured from Inox-Air products and used as received.
The Pt supported catalysts were prepared by an incipient
wetness impregnation method using Pt (NH3)4(NO3)2 as the
Pt precursor. In
a typical synthesis procedure 0.185 g
Pt (NH3)4(NO3)2 was dissolved in H2O and 3 g HT was added
to it with constant stirring. Then the mixture was dried in a water
bath (80 °C) followed by drying in an oven at 110 °C overnight,
and further calcined at the required temperature.
2.3 Hydrogenolysis reaction
Hydrogenolysis reactions were conducted in batch mode, in a
160 ml stainless steel autoclave (parr reactor) at various tempera-
tures. The reactor was equipped with a magnetically driven
stirrer, a dip tube and an internal water cooling system. The
temperature was controlled by a programmable PID controller.
Glycerol, catalyst (reduced in H2 at 480 °C) and water were
placed inside the reactor, after which the reactor was closed.
Then the system was purged with nitrogen 4 times to remove the
air. The whole system was pressurized to 45 bar and the heating
was started with a 500 rpm stirring speed. Aliquots (liquid and
gas) were withdrawn through the special sample port attached
within the reactor. At the end of the reaction, the reactor was
cooled down to the room temperature and the pressure was
released very slowly. The catalyst particles were separated by fil-
tration and the product was identified using the GC-MS and ana-
lyzed by an Agilent 7890 gas chromatograph equipped with a
FID (using restek MXT WAX capillary Column 30 m x0.25ID),
the gaseous product was analyzed by an Agilent 7890 gas chro-
matograph equipped with a TCD [using packed column molecu-
lar sieves 13× for H2 and porapack Q for CO, CO2 and alkane
(C1 to C4)].
2.2 Characterization of the catalyst
The physicochemical characterization of the catalyst and support
were characterized by X-ray diffraction (XRD), scanning elec-
tron microscopy (SEM), transmission electron microscopy
(TEM), X-ray photoelectron spectroscopy (XPS), etc. The X-ray
diffraction patterns were obtained using CuKα radiation (40 kV
and 40 mA, Advanced Bruker D8 diffractometer). The SEM
images were realized by a field emission scanning electron
microscope, FEI Quanta 200 F, using a tungsten filament doped
with lanthanum hexaboride (LaB6) as an X-ray source, fitted
with an ETD detector with a high vacuum mode using secondary
electrons and an acceleration tension of 10 or 30 kV. Catalysts
were analyzed by spreading them on carbon tape. Energy-disper-
sive X-ray spectroscopy (EDX) was used in connection with
SEM for elemental analysis. The elemental mapping was con-
ducted with the same spectrometer. The surface area of the HT
and metal loaded HT were examined by N2 adsorption–
desorption isotherms at 77 K (Belsorbmax, BEL, Japan). TEM
images were collected using a JEOL JEM 2100 microscope, and
samples were prepared by mounting an ethanol-dispersed sample
on a lacey carbon Formvar coated Cu grid. Measurements of the
Pt LIII-edge extended X-ray absorption fine structure (EXAFS)
were carried out at the Photon Factory in the Institute of
Materials Structure Science, High Energy Accelerator Research
Organization (KEK-IMSS-PF). The measurements were made in
transmission mode. Spectra were measured at BL-7C and
BL-9C. The electron storage ring was operated at 2.5 GeV and
450 mA. Synchrotron radiation from the storage ring was mono-
chromatized by a Si(111) channel cut crystal. Ionization
chambers, which were used as detectors for incident X-ray (I0)
and transmitted X-ray (I), were filled with an Ar (15%)–N2
mixture gas and Ar (100%) gas, respectively. The EXAFS raw
data were analyzed with an UWXAFS analysis package21 includ-
ing background subtraction program AUTOBK22 and a curve
3
Results and discussion
3.1 Characterization of the catalyst
Typical powder XRD patterns of the hydrotalcite and Pt-loaded
hydrotalcite catalyst are shown in Fig. 1. It was observed that the
hydrotalcite structure remained intact at 280 °C, and the structure
collapsed at 500 °C after calcination in air. We believe that due
to the memory effect of hydrotalcite, small instant peaks appear
at 12, 25, 35.2 (Fig. 1). The BET surface area estimated by nitro-
gen adsorption for hydrotalcite (Al2O3–MgO = 80 : 20) was
220 m2 g−1 and the surface area reduces to 126 m2 g−1 when
3 wt% Pt was loaded.
The morphologies of the Pt-loaded HT catalysts were exam-
ined by a scanning electron microscope (SEM). A representative
SEM image of the catalyst is shown in Fig. 2A. Pt dispersion of
the catalyst was confirmed by EDX (electron dispersive analysis
2
fitting program FEFFIT.23 The amplitude reducing factor, S0
was fixed at 0.95. The backscattering amplitude and phase shift
3108 | Green Chem., 2012, 14, 3107–3113
This journal is © The Royal Society of Chemistry 2012