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RESEARCH NOTES Kinetics of Steam Reforming of Ethanol over a Ru/Al2O3 Catalyst Prakash D. Vaidya and Alirio E. Rodrigues* Laboratory of Separation and Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto, Porto 4200-465, Porto, Portugal
Catalytic steam reforming of ethanol over a Ru/γ-Al2O3 catalyst in the temperature range 873-973 K was studied. The influence of inlet ethanol concentration on ethanol conversion and hydrogen yield was investigated. The conversion vs space time data was subjected to the integral method of analysis. The results show that the reaction order with respect to ethanol is 1. An activation energy of 96 kJ mol-1 was obtained. A possible reaction sequence for ethanol steam reforming was suggested. A rate expression was derived assuming that the decomposition of an activated complex formed during reaction into intermediate products was the ratedetermining step. Introduction Fuel cells (FCs) are electrochemical devices that convert the chemical energy of a fuel and an oxidant directly into electricity and heat on a continuous basis. FCs use hydrogen as a fuel, which results in the formation of water vapor only, and thus, they provide clean energy. Ethanol is a promising source of hydrogen for fuel cells as it is a renewable resource. Its production from biomass is simple and cheap, and hence, catalytic steam reforming of ethanol to produce hydrogen for fuel cells will assume importance in the future. Bioethanol, which contains water in large excess, can be directly subjected to steam reforming, thereby eliminating one unit operation of distillation required to produce pure ethanol. The entire process could, therefore, be economically attractive. Hydrogen thus produced can be used to generate electricity for stationary applications and mobile electric vehicle operations. The steam reforming reaction is strongly endothermic and produces only H2 and CO2 if ethanol reacts in the most desirable way:
C2H5OH + 3H2O f 2CO2 + 6H2 (∆H0298 ) 174 kJ mol-1) (1) Steam reforming of ethanol over Ni, Co, Ni/Cu, and noble metals (Pd, Pt, and Rh) has been extensively studied. Vaidya and Rodrigues1 have recently presented a comprehensive review on this topic. Among noble metals, Ru is a versatile catalyst having high activity in steam reforming of hydrocarbons. Increasing attention is, therefore, being focused on developing Ru-based reforming catalysts. For instance, a reforming catalyst developed by Idemitsu is a ruthenium catalyst called ISR-7G. It is possible to enhance the activity and stability of Ru-based catalysts and suppress carbon deposition by the addition of suitable promoters. Berman et al.2 have studied steam reforming of methane over a Ru/Al2O3 catalyst promoted with Mn oxides. Fierro et al.3 showed that the order of performance of alumina * Corresponding author. E-mail:
[email protected]. Tel.: +351 22 5081671. Fax: +351 22 5081674.
supported noble metal catalysts (5% metal loading) for bioethanol oxidative steam reforming at 973 K was Pt < Pd , Ru < Rh. Liguras et al.4 studied steam reforming of ethanol over Ru/Al2O3 in the range of temperatures 873-1123 K and found a marked increase in ethanol conversion and selectivity to H2 with an increase in the Ru content. At a high Ru loading (5 wt %), the performance of Ru was comparable to that of Rh. The catalyst was stable and had activity and selectivity higher than Ru/MgO and Ru/TiO2. Thus, the activity of Rh and Ru catalysts are close to each other, but the cost of Ru is significantly lower. A catalyst based on Ru is, therefore, expected to be far less expensive. In the present work, the intrinsic kinetics of the reaction over a commercial Ru/γ-Al2O3 catalyst was studied. Attempts were made to properly interpret the reaction kinetic data. Experimental Section Materials. Ethanol (absolute GR) used in all experiments was purchased from Merck. Nitrogen from a cylinder with an ultrahigh purity was obtained from M/s. Air Liquide and used as the carrier gas during experimentation. The commercial Ru/ γ-Al2O3 catalyst (Noblyst 3500) was procured from M/s. Degussa AG. The catalyst had the following characteristics: Brunauer-Emmett-Teller (BET) surface area, 250 m2 g-1; specific pore volume, 0.7 mL g-1; Ru content, 1% (wt). The surface morphological characterization of the Ru/Al2O3 catalyst was carried out by scanning electron microscopy. A JEOL JSM6301F scanning electron microscope was used for this purpose. The sample was earlier pretreated to make it conducting. The scanning electron micrograph shown in Figure 1 corresponds to the fresh catalyst sample before reaction. Clusters of alumina could be seen. An in-depth characterization of the catalyst was excluded from the scope of the present study. Experimental Procedure. All experiments were conducted at atmospheric pressure in a fixed-bed down-flow reactor made of a SS-316 tube (i.d. ) 4.75 mm, L ) 45 mm), which was placed inside a temperature-controlled heating furnace (Termolab, Fornos Electricos Lda). The liquid feed consisting of an ethanol-water mixture was introduced by using an HPLC pump
10.1021/ie051342m CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006
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Figure 1. SEM image of commercial 1% Ru/Al2O3 catalyst.
Figure 2. Fractional conversion of ethanol as a function of W/Qo at various temperatures (points, experimental data; curves, kinetic model predictions (eq 7)).
Table 1. Experimental Results yAO
yBO
Qo (cm3 min-1)
W/FAO (g h mol-1)
T (K)
X
0.043 0.058 0.066 0.058 0.043 0.058 0.066 0.058 0.043 0.058 0.066 0.071 0.043 0.058 0.066 0.066
0.43 0.58 0.66 0.58 0.43 0.58 0.66 0.58 0.43 0.58 0.66 0.71 0.43 0.58 0.66 0.66
246.1 362.9 479.7 362.9 246.1 362.9 479.7 362.9 246.1 362.9 479.7 595.7 246.1 362.9 479.7 479.7
2.04 1.02 0.68 0.54 2.04 1.02 0.68 0.54 2.04 1.02 0.68 0.54 2.04 1.02 0.68 0.36
873 873 873 873 923 923 923 923 943 943 943 943 973 973 973 973
0.43 0.30 0.22 0.16 0.59 0.41 0.28 0.24 0.84 0.63 0.49 0.37 0.94 0.78 0.63 0.35
(Merck L-2130). Nitrogen used as a carrier gas during experimentation was introduced from a high-pressure cylinder at a predetermined flow rate. Prior to the reactor inlet, the premixed liquid reactants were vaporized and mixed with nitrogen to ensure a homogeneous vapor mixture. This mixture was then contacted with the catalyst placed inside the reactor. The product vapors exiting the reactor were passed through a condenser. The off-gas exiting the condenser flowed through a back-pressure regulator which controlled the reactor pressure. The liquid products were periodically collected, while the gaseous products were analyzed on-line by gas chromatography. The initial waterto-ethanol molar ratio used in all experiments was 10:1. Before each experiment, catalyst was reduced under flowing hydrogen (30 cm3 min-1) at 873 K for 1 h. The noncondensable gases were analyzed using gas chromatography (GC 1000, Dani Chromatographs) equipped with an on-line multiport valve (Valco Instruments Company, Inc.) for sample injection. A Poraplot-U column (27.5 m length) was used for this purpose. H2 was detected using a thermal conductivity detector (TCD), while the various intermediates, such as CH4, C2H4, and C2H6, were detected by using a flame ionization detector (FID). COX formed during the reaction could, however, not be detected using this technique. Ethanol and acetaldehyde present in the condensed liquids were analyzed separately by GC using the same Poraplot-U column and FID.
A steady-state condition was achieved within 1 h. The reproducibility of results was checked, and the error in the experimental measurements was
