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Ind. Eng. Chem. Res. 2009, 48, 2728–2733
Platinum-Loaded NaY Zeolite for Aqueous-Phase Reforming of Methanol and Ethanol to Hydrogen Zhong Tang,† Justin Monroe,‡ Junhang Dong,*,† Tina Nenoff,§ and Donald Weinkauf‡ Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221, Department of Chemical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, and Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico 87185
Platinum-loaded NaY zeolite (Pt/NaY) catalysts were synthesized and tested for the aqueous-phase reforming (APR) of methanol and ethanol solutions to produce hydrogen. The APR performance of the Pt/NaY catalysts was compared to the conventional γ-alumina-supported platinum (Pt/γ-Al2O3) catalysts. The results have shown that the Pt/NaY catalysts have higher catalytic performance for the APR of methanol and ethanol than the Pt/γ-Al2O3 catalysts. 1. Introduction The production of hydrogen as a clean energy carrier from biorenewable sources and organic wastes is considered to be a promising way to mitigate the environmental problems that are associated with fossil fuel combustion and reduce our dependence on the diminishing oil reserves.1,2 A potential strategy for achieving such biorenewable hydrogen is to convert biomassderived intermediates (e.g. bioethanol and methanol) to hydrogen via catalytic reforming, which is also a possible method of distributing H2 production for the emerging fuel-cell applications.3,4 Haryanto et al.5 summarized recent catalyst developments for H2 production via ethanol steam reforming. The processes commonly involve two reactors: one for the high-temperature (>450 °C) steam reforming of C2H5OH to H2, CO2, and CO, and the another for the lower-temperature (98%, Aldrich). The final γ-Al2O3 nanopowders were obtained by drying and then calcining in air at 723 K for 6 h. The Pt/γ-Al2O3 catalysts were prepared by incipient wetness impregnation with [Pt(NH3)4](NO3)2 solutions followed by thermal treatments and reduction processes. The [Pt(NH3)4](NO3)2 solution-to-solid mass ratio used in the wet impregnation process was 0.8 mL solution/g γ-Al2O3 dry powder. The impregnated powders were dried at 373 K for 12 h and then calcined in air at 533 K for 2 h. The platinum loading in Pt/γ-Al2O3 catalysts was varied by controlling the concentration of the [Pt(NH3)4](NO3)2 precursor solution. All catalysts were activated by in situ reduction in a pure H2 flow at 533 K in the reactor. The γ-Al2O3 and NaY particles were examined by X-ray diffraction (XRD) (Rigaku, model D/MAX-II) before and after loading platinum metal. The specific surface area measurement and H2 and CO chemisorption experiments were performed by a Micropore BET/chemisorption analyzer (Micromeritics, model ASAP 2020). The actual platinum loading in the Pt/NaY and Pt/γ-Al2O3 catalysts was determined usign a microprobe (Cam-
Figure 2. Microscopic pictures of the catalysts: (a) scanning tunnelling electron microscopy (STEM) image of Pt/γ-Al2O3 (3 wt % platinum) catalyst and (b) scanning electron microscopy (SEM) image of the 0.5% Pt/NaY catalyst (a high-resolution transmission electron microscopy (HRTEM) image is shown in the inset).
eca, model SX-100). The morphology of the particulate catalysts was observed by a field-emission SEM (FESEM) system (Hitachi, model S-800) and a field-emission STEM (FASTEM) system (EOL, model 2010F). 2.2. Aqueous-Phase Reforming (APR). The APR experiments were conducted in a fixed-bed reactor system. One gram of the particulate catalyst was packed in a stainless steel tube reactor (with an inner diameter (ID) of 5 mm and a length of 10 cm), placed vertically in a temperature-programmable oven with feed entering from the bottom end. The catalyst was activated by calcining at 533 K in a continuous flow of O2/He (1:9) gas mixture (total flow rate ) 40 cm3/min) for 2 h and then reducing in pure H2 flow (25 mL/min) at 533 K for 2 h after the catalyst was purged with pure N2 gas at 50 °C (50 cm3/min for 12 h). The heating and cooling rates used in this program were 0.5 K/min. The 1 wt % alcohol solution was fed continuously into the reactor, using a high-pressure syringe pump (Teledyne Isco, model D500) through a preheating coil. The feed flow rate was 0.8 mL/h, which gave a weight hourly space velocity (WHSV) of 0.008 h-1 for comparison with literature data.17 The exiting stream from the reactor was separated at room temperature in a 70-cm3 flash tank. The gas flow from the separator was
2730 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 Table 1. Results of H2 and CO Chemisorption on Different Catalysts H/Pt
CO/Pt
sample
H2 uptake (µmol/g-cat)
Aa
Bb
A-B
CO uptake (µmol/g-cat)
Aa
Bb
A-B
0.25%Pt/NaY 0.4%Pt/NaY 0.5%Pt/NaY 3%Pt/γ-Al2O3
8.4 12.9 16.0 26.8
1.30 1.26 1.25 0.36
0.39 0.37 0.35 0.10
0.91 0.89 0.90 0.26
8.8 13.4 16.4 26.7
0.75 0.70 0.68 0.20
0.06 0.05 0.04 0.02
0.69 0.65 0.64 0.18
a A represents the first (total) adsorbance measured at 300 K. after the sample was evacuated for 15 min.
b
B represents the second adsorption quantity, measured following the first adsorption
analyzed using online gas chromatography (GC) (HewlettPackard, model HP-5890, equipped with a Hayesep DS column and a thermal conductivity detector (TCD)). The total organic carbon (TOC) of the liquid samples in the separator was analyzed using a total carbon analyzer (Shimadzu, model TOCV), to verify the conversion calculated from gas compositions. The organic compounds in the liquid phase were also analyzed using a gas chromatography/mass spectroscopy (GC/MS) system (Agilent, model 6890N GC-5975B MSD). In all APR experiments, the reactor pressure was maintained slightly above the autogenous vapor pressure of the feed solutions. APR experiments were first performed for methanol at 498 and 538 K on the Pt/NaY and Pt/γ-Al2O3 catalysts. The Pt/NaY catalyst exhibiting the best catalytic performance was then further tested for APR of ethanol solutions. The materials used include methanol (99.93%, Aldrich), ethanol (>99.95%, Aldrich), and deionized (DI) water. 3. Results and Discussion 3.1. Properties of Catalysts. The XRD examination confirmed that the catalyst supports were pure NaY zeolite and γ-Al2O3. No characteristic peaks of platinum metal were appreciable in the XRD patterns of the Pt/NaY catalysts, because of the low load of platinum metal and its high dispersion states.27,29 The platinum was distinguishable in the XRD pattern of the γ-alumina-supported catalyst load with 3% platinum. Figure 1 shows the XRD patterns of the catalysts. The scanning tunnelling electron microscopy (STEM) image of the Pt/γ-Al2O3 (3 wt % Pt) catalyst revealed uniformly distributed platinum nanoparticles (1.5-2 nm in size) on the γ-Al2O3 surface (see Figure 2a). Figure 2b shows SEM and highresolution transmission electron microscopy (HRTEM) pictures of the Pt/NaY catalyst. The platinum in NaY seemed to have an individual particle size of ∼1 nm. The sizes of the platinum nanoparticles on the γ-Al2O3 surface and in the NaY zeolite agree well with those reported in the literature obtained by similar preparation methods.30,31 The BET surface areas of the NaY zeolite and γ-Al2O3 powders were 715 and 259 m2/g, respectively. Pt/NaY catalysts with three different platinum loading levels (0.25, 0.40, and 0.5 wt %) were obtained. The specific areas of the platinum-loaded NaY zeolites were essentially unchanged, with values in a range of 715-728 m2/g. The platinum loading in the zeolite was kept low in this study, to avoid complication of possible zeolite structure destruction under high loading levels.27 The Pt/γ-Al2O3 catalysts with three different platinum loadings (0.5, 1.1, and 3 wt %) were also prepared. The results of H2 and CO chemisorption measurements for the catalysts at 300 K are given in Table 1. The number of hydrogen atoms or CO molecules chemisorbed per Pt atom (i.e., H/Pt and CO/Pt) was calculated from the adsorption data. Because the physical adsorption of H2 on Pt is negligible,32 the first absorbance of hydrogen (A) can be taken as the chemical adsorbance, with the second adsorption (B) being considered as weak chemisorption.
Figure 3. Effect of platinum loading on the catalytic performance for the aqueous-phase reforming (APR) of methanol at 493 K.
Both the H2 and CO uptakes in the Pt/NaY catalysts increased as the platinum loading increased. However, the H/Pt and CO/ Pt numbers had very small changes relative to variation of the platinum loading in the Pt/NaY samples. The high values of H/Pt and CO/Pt of the Pt/NaY catalysts are consistent with the literature findings.33 The H/Pt number of the zeolite-supported platinum catalysts is known to be a function of the support ionicity, as well as a result of hydrogen spillover effect, which enhances the hydrogenolysis activity of the catalyst.30,34 The 3% Pt/γ-Al2O3 catalyst had higher H2 and CO uptakes but significantly lower H/Pt and CO/Pt values, compared to the Pt/ NaY catalysts. The higher CO/Pt values in the Pt/NaY are associated with the higher electron density on the platinum particles in the zeolite micropores, compared to the platinum that was supported on the mesopore and macropore surfaces.31 Although the H/Pt and CO/Pt values are not direct measures of platinum dispersion when the supports are different,30 the large differences in these numbers between the NaY and γ-Al2O3 supported catalysts are relevant to the variation of their catalytic performances. 3.2. Aqueous-Phase Reforming of Alcohols. 3.2.1. APR of Methanol. The APR experiments were performed on all six Pt/NaY and Pt/γ-Al2O3 catalysts at 493 K and 2.65 MPa for the 1 wt % methanol solution, which was fed at a WHSV value of 0.008 h-1. The results are presented in Figure 3. For each catalyst, the data were taken for a period of 24 h after reaching steady state. The selectivity of H2 is calculated from the number of moles of H2 (nH2) and C atoms (nC) in the gas phase, according to the reaction stoichiometry CH3OH + H2O S CO2 + 3H2 (i.e., H2 selectivity ) 1/3nH2/nC). For both the Pt/γ-Al2O3 and Pt/NaY catalysts, increasing the platinum loading increased the methanol conversion but had almost no influence on the H2 selectivity for same type of catalyst. With the same platinum loading, the 0.5% Pt/NaY achieved significantly higher conversion and slightly higher H2
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2731 Table 2. Comparison of Catalytic Performance between 3% Pt/γ-Al2O3 and 0.5% Pt/NaY Reference Catalysta
3% Pt/Al2O3 Catalyst
0.5% Pt/NaY Catalyst
parameter
T ) 498 K
T ) 538 K
T ) 498 K
T ) 538 K
T ) 498 K
T ) 538 K
pressure, P (MPa) total organic carbon (TOC) in liquid (%) conversion (%) composition of gas products (mol %) H2 CO2 CH4 H2 selectivityb (%) alkane selectivityc (%) H2 productivityd (mmol g-1 h-1) turnover frequency, TOFe (min-1)
2.86 6.5 94
5.53 6.4 94
2.81 19.5 78.9
5.51 4.8 95.5
2.81 18.6 81.0
5.51 0.8 98.8
74.6 25.0 0.4 99 1.7 0.70 0.16
74.8 24.6 0.6 99 2.7 0.70 0.16
74.6 24.9 0.5 97.9 2.0 0.57 0.16
74.5 24.9 0.6 97.4 2.4 0.71 0.20
74.6 24.8 0.6 97.9 2.4 0.58 0.82
74.4 24.8 0.8 96.9 3.0 0.74 1.04
a Described in ref 17. b Based on a hydrogen balance. The selectivity of hydrogen ) [(number of moles of H2)/3]/(number of moles of C atoms in the gas phase). c Based on a carbon balance. The selectivity of alkane ) (number of moles of CH4)/[number of moles of (CH4 + CO2)]. d The weight hourly space velocity (WHSV) value is 0.008 g of methanol per gram of catalyst per hour. e Normalized by the number of surface metal atoms, as determined from an irreversible uptake of CO at 300 K.
selectivity than the 0.5% Pt/γ-Al2O3. The methanol conversion and H2 selectivity on the 0.5%Pt/NaY were comparable to that on the 3% Pt/γ-Al2O3. The high catalytic performance of the 0.5% Pt/NaY may be attributed to the high dispersion of platinum in the zeolite cages and, perhaps more importantly, to the synergistic effect between the platinum metal and the microporous zeolite surfaces. The synergistic effects may include the interactions between the platinum nanoparticles and the highionicity microporous zeolite surface, which result in higher electron density on the platinum surface and enhanced hydrogen retention capacity in the Pt/NaY catalysts.30,31,34 The 0.5% Pt/NaY and 3% Pt/γ-Al2O3 were further studied for the APR of methanol at 498 and 538 K, respectively. The reaction was operated continuously for 240 h with no appreciable changes in the performance. Agglomeration or coarsening of the platinum nanoparticles was not observed via STEM and SEM in both the NaY-supported and γ-alumina-supported catalysts after the APR operation. Also, the XRD characteristic peaks of NaY zeolite remained strong with no appearance of platinum peaks after the APR reaction at 538 K, indicating good stability of the Pt/NaY under the reaction conditions. However, the stability of the platinum nanoparticles over a longer time remains to be evaluated. The methanol APR results are given in Table 2, with reference data on a 3 wt % Pt/γ-Al2O3 catalyst17 included for comparison. Large increases in methanol conversion were observed on both 3% Pt/γ-Al2O3 and 0.5% Pt/NaY catalysts when the temperature increased from 498 K to 538 K. This can be explained by the fact that increasing the temperature reduces the Arrhenius term ∆G°/(RT) from the methanol reforming equation and enhances the reaction kinetics. However, this observation is different from the reference work,17 where the methanol conversion remained unchanged at 498 and 538 K. At 538 K, the 0.5% Pt/NaY catalyst is much more active than the γ-Al2O3-supported platinum catalysts for the APR of methanol, as indicated by its significantly higher TOF numbers. 3.2.2. APR of Ethanol. Because of the involvement of C-C bond cleavage in platinum catalysts, dozens of reactions may occur in the APR process of C2H5OH.18 The APR of a 1 wt % ethanol solution was performed on the 0.5% Pt/NaY catalyst at 498 and 538 K, respectively. APR of ethanol was also performed on the 3% Pt/γ-Al2O3 catalyst at 538 K for comparison. At each temperature, the reaction was operated continuously for 120 h after stabilization. No appreciable changes in the catalytic performance were observed during the 120 h of reaction. The results are presented in Table 3.
Table 3. Results of the APR of a 1 wt % Ethanol Solution on the 0.5% Pt/NaY and 3% Pt/γ-Al2O3 Catalysts
parameter pressure, P (MPa) total organic carbon (TOC) in liquid (%) conversiona (%) composition of gas products (mol %) H2 CO2 CH4 C2H6 H2 selectivityb (%) alkane selectivityc (%)
0.5% Pt/NaY Catalyst
3% Pt/γ-Al2O3 Catalyst
T ) 498 K T ) 538 K
T ) 538 K
2.81 12.0
5.53 2.2
5.53 2.4
89.8
97.1
97.2
67.7 22.6 9.4 0.3 69.2 30.7
68.3 22.7 8.8 0.2 71.4 28.8
66.1 22.0 8.6 3.3 59.2 40.9
a The weight hourly space velocity (WHSV) value is 0.008 g of ethanol per gram of catalyst per hour. b Based on a hydrogen balance. The selectivity of hydrogen ) [(number of moles of H2)/3]/[(number of moles of C atoms in the gas phase)/2]. c Based on a carbon balance. The selectivity of alkane ) [number of moles of (CH4 + 2C2H6)]/[number of moles of (CH4 + 2C2H6 + CO2 + CO)].
In all cases, the gas products contained mainly H2, CO2, CH4, and C2H6. CO was undetectable by the GC system, whereas CH4 was the main byproduct found in the output gas stream. For both the 0.5% Pt/NaY and 3% Pt/γ-Al2O3 catalysts, the organic carbon in the liquid effluent was mostly ethanol, with trace amounts of acetaldehyde (as found by GC-MS analysis). It was observed that, on the 0.5% Pt/NaY catalyst, increasing the temperature from 498 K to 538 K not only increased the ethanol conversion but also slightly improved the H2 selectivity. The results support the calculations through density functional theory (DFT), which predicts that C-C bond cleavage is faster than C-O bond cleavage in ethanol-derived species with low oxygenation.18 At 538 K, the 0.5% Pt/NaY catalyst achieved similarly high ethanol conversion, as did the 3% Pt/γ-Al2O3 catalyst. However, the 0.5% Pt/NaY catalyst exhibited much higher H2 selectivity and lower alkane selectivity than the 3% Pt/γ-Al2O3 catalyst. The 0.5% Pt/NaY catalyst was further tested for the APR of ethanol solutions with concentrations that varied from 1 wt % to 10 wt %, corresponding to an increase in WHSV from 0.008 g-ethanol/g-cat/h to 0.08 g-ethanol/g-cat/h. The experiments were performed at 538 K and 5.53 MPa, and the results are presented in Figure 4. The ethanol conversion was observed to decrease as the ethanol feed concentration increased. It was also found that the H2 selectivity decreased from ∼71% to 59% and
2732 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009
Figure 4. Graph showing the APR of an ethanol solution, as a function of feed concentration at 538 K and 5.53 MPa.
the alkane selectivity increased from ∼29% to 40% as the feed concentration increased from 1 wt % to 10 wt %. 4. Conclusions This preliminary research showed that the Pt/NaY catalysts are highly active for the aqueous-phase reforming (APR) of alcohol solutions. Note that the APR experiments on the different catalysts were repeated at least once for each catalyst, and the results were determined to have excellent reproducibility. The feasibility of APR of ethanol, which is an important biorenewable compound with a C:O ratio of 2, was demonstrated on the Pt/NaY catalysts. The results were compared with the data on Pt/γ-Al2O3 catalysts. At 538 K, the 0.5 wt % Pt/NaY achieved higher H2 selectivity and lower alkane selectivity than the 3 wt % Pt/γ-Al2O3 catalyst with similarly high ethanol conversion of ∼97% on both catalysts. On the 0.5% Pt/NaY catalyst, the ethanol conversion decreased, whereas the H2 selectivity changed moderately when the feed concentration increased from 1% to 10%. The zeolite-based catalysts, with the advantages of high activity, reduced use of precious metal, and low cost of zeolite materials, deserve further investigations. Research is ongoing into the effect of substrate properties on the catalytic performance of platinum catalysts in APR reactions. Acknowledgment This research was supported by the U.S. DOE/HNIE and Sandia LDRD. Sandia is a multiprogram laboratory operated by Lockheed Martin Co., for the U.S. DOE’s NNSA (Contract No. DE-AC04-94-Al85000). Literature Cited (1) The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs; National Research Council and National Academy of Engineering, The National Academic Press: Washington, DC, 2004; p 256. (2) Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972–974. (3) Ioannides, T. Thermodynamic analysis of ethanol processors for fuel cell applications. J. Power Sources 2001, 92, 17–25. (4) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Steam reforming of ethanol for hydrogen production: thermodynamic analysis. Int. J. Hydrogen Energy 2006, 21, 13–18. (5) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Current status of hydrogen production techniques by steam reforming of ethanol: A review. Energy Fuels 2005, 19, 2098–2106.
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ReceiVed for reView August 8, 2008 ReVised manuscript receiVed January 21, 2009 Accepted January 27, 2009 IE801222F
