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Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ru catalysts
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International Journal of Hydrogen Energy 29 (2004) 419 – 427
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Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ru catalysts Dimitris K. Liguras, Katerina Goundani, Xenophon E. Verykios∗ Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece Accepted 17 June 2003
Abstract A series of Ru catalysts supported on cordierite monoliths, ceramic foams and -Al2 O3 pellets were prepared and tested for the production of hydrogen by catalytic partial oxidation of ethanol. The catalyst supported on a cordierite monolith exhibited excellent catalytic performance for a wide variety of process conditions and excellent long-term stability with low coke formation. The e6ect of the steam to ethanol molar ratio on conversion and selectivities was relatively small. A more pronounced e6ect was observed for the oxygen to ethanol ratio. Overall, catalysts on all three supports were able to completely convert ethanol with high selectivities towards the desired products. The Ru supported on a ceramic foam catalyst provided comparatively better performance, probably due to the smaller pore sizes and higher tortuosity of this support. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Catalytic partial oxidation; Hydrogen production; Ethanol; Ruthenium; Monolith; Ceramic foam
1. Introduction In the last few years, hydrogen has emerged as the energy carrier that has the potential to solve two major energy challenges: reducing dependence on fossil fuels and reducing pollution and greenhouse gas emissions. Although the transition to the so-called “hydrogen economy” has been postulated to have already begun, signi>cant technical challenges will make it a slow process. The most important challenges involve the sources of hydrogen and the development of e?cient, safe and a6ordable production processes. The most commonly mentioned sources of hydrogen are natural gas, gasoline, diesel and methanol. All these sources, however, have a major drawback in that they do not address the goal of reducing neither the dependence on fossil fuels nor the emissions of greenhouse gases, such as CO2 . Bio-fuels, on the other hand, are viable alternatives since they o6er high energy density and ease of handling so that they can be used for on-demand production of hydrogen for ∗
Corresponding author. Tel./fax: +30-2610-991527. E-mail address: [email protected] (X.E. Verykios).
automotive and distributed power generation applications. Among the various liquid bio-fuels, bio-ethanol is emerging as a frontrunner. In addition to advantages related to natural availability, storage and handling safety, bio-ethanol is produced renewably from many biomass sources, including energy plants, grains, waste materials from agro industries and even the organic fraction of municipal solid wastes. Furthermore, a bioethanol-to-hydrogen system has the signi>cant advantage of being nearly CO2 neutral since the carbon dioxide produced in the process is consumed for biomass growth, o6ering a nearly closed carbon loop. Hydrogen production from ethanol via steam reforming or catalytic partial oxidation has been the subject of several recent studies [1–6]. The process has been shown to be entirely feasible from a thermodynamic point of view [7–9] and several catalysts have been proposed which show su?cient activity and stability to be further considered for practical applications [10–16]. All of these studies, however, have examined catalysts in powder form. Such a form is unsuitable for practical applications in terms of handling and pressure drop in the reactor. Especially for automotive applications, the catalysts must be deposited on structured supports, which have excellent structural stability in harsh
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(03)00210-6
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environments that include vibrations, thermal cycling and thousands of start-ups and shut-downs. Ethanol steam reforming is a highly endothermic reaction C2 H5 OH + 3H2 O → 2CO2 + 6H2 (KHr = 173:1 kJ mol−1 )
(1)
requiring a great amount of heat to maintain the system at a steady reaction temperature. There are two alternative ways of supplying heat to the system: (i) external heat supplied by burning some type of fuel, or (ii) internal heat supplied by co-feeding oxygen or air and burning a portion of the ethanol at the expense of hydrogen production. At least 0:61 mol of oxygen per mole of ethanol are required to achieve thermal neutrality C2 H5 OH + 0:61O2 + 1:78H2 O → 2CO2 + 4:78H2 (KHr = 0 kJ mol−1 ):
(2)
Depending on the way combustion is accomplished, the systems are either termed auto-thermal (gas phase combustion) or catalytic partial oxidation (catalytic combustion) although combinations of these have been examined. The present study examines the catalytic partial oxidation of ethanol, where both reforming and combustion reactions take place on the same Ru catalysts deposited on di6erent supports. 2. Experimental 2.1. Catalyst preparation A series of Ru catalysts were supported on cordierite monoliths, ceramic foams and pellets. The monoliths were of the type used in automobile exhausts with 400 channels per square inch. The ceramic foams were proprietary formulations of alumina and zirconia with medium porosity (50 pores per square inch). The pellets were 1=16 extrudates of -Al2 O3 (Engelhard) with a speci>c area of 90 m2 g−1 . The Ru=-Al2 O3 catalyst supported on a cordierite monolith was prepared by washcoating. A 5% Ru=-Al2 O3 powder catalyst was prepared by impregnating -Al2 O3 (90 m2 g−1 ) with aqueous solution of Ru(NO)(NO3 )3 followed by overnight drying at 120◦ C and calcination at 550◦ C for 5 h. A dense suspension of this powder in de-ionized water was created and the catalysts were coated by successive immersions in the suspension followed by overnight drying at 120◦ C and calcination at 550◦ C. A >nal calcination at 1000◦ C took place before testing. A total of 1:8 g of catalyst was loaded on a monolith 25 mm in diameter, 40 mm in length and weighing 7:2 g. The same technique and the same 5% Ru=-Al2 O3 powder catalyst was used to washcoat a zirconia–alumina ceramic foam. The foam had a diameter of 25 mm, length of 23 mm and weight of 6:1 g. The loading was 1:7 g of catalyst. Another ceramic foam of similar size was loaded with Ru by wet impregnation using an aqueous solution of
Ru(NO)(NO3 )3: This material was also calcined and reduced in situ after an overnight drying at 120◦ C. The same technique was employed for the preparation of the Ru catalyst on alumina pellets. The metal loading on both pellets and ceramic foam was 5% Ru. As a base case, a plain monolith, with no catalyst, was also tested. 2.2. Apparatus and procedures Catalytic performance tests have been carried out using an apparatus consisting of the Pow system, the reactor unit and the analysis system. The Pow system is equipped with a set of mass-Pow controllers (MKS) and an HPLC pump (Marathon) for feeding liquid reagents (ethanol–H2 O mixtures). The liquid is pumped to a vaporizer where it is evaporated and heated to 150◦ C and where it can be mixed (when desired) with the gas stream coming from the mass-Pow controllers. The resulting gas mixture is then fed to the reactor through stainless steel tubing maintained at 150◦ C by means of heating tapes. The reactor used is shown in Fig 1. Feed and air or O2 are mixed in a small bed of quartz chips before entering the catalyst to achieve complete mixing, to obtain uniform distribution and to minimize homogeneous reactions. The main reactor can easily accommodate catalytic specimens up to 27 mm in diameter. Glass wool provides support and a seal between the catalyst and the reactor walls. Additional support is provided by the quartz funnel shown in Fig. 1. The mouth of the funnel is slightly smaller than the reactor internal diameter and su?cient to support the edge of the catalyst. The stem of the funnel is of much smaller diameter (8 mm OD) and acts as a sampling port. Most of the reaction products also exit through this path so as to reduce their residence time in the hot reactor and minimize any gas phase reactions. The rest of the products exit through the second port shown. Dual thermocouples run the length of the reactor, passing through the catalyst, to monitor the temperature pro>le. The reactor is placed inside an electric furnace. The analysis system consists of two gas chromatographs (Shimadzu). The >rst one is equipped with two packed columns (Porapak-Q and Carbosieve) and two detectors (TCD, FID) and operates with He as the carrier gas.
Fig. 1. Schematic diagram of the reactor employed for the ethanol partial oxidation studies.
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Porapak-Q is used for the separation of C2 H5 OH, H2 O, CH3 CHO, CH4 , C2 H4 and C2 H6 , while Carbosieve is used for the separation of CO, CO2 and CH4 . Since the presence of large amounts of water hinders analysis in the Carbosieve column, a condenser is placed before its inlet, in order to condense and remove H2 O from the gas stream. The second GC is equipped with a Carbosieve column, connected to the exit of the condenser, and a TCD detector. This chromatograph uses N2 as the carrier gas and is used solely to determine the H2 concentration in the reformate. Determination of the response factors of the TCD and FID detectors has been achieved with the use of gas streams of known composition (Scott specialty gas mixtures, self-prepared EtOH=H2 O=CH3 CHO mixtures, etc.). Since several substances (CO, CO2 , O2 , H2 ) can only be analyzed after the condenser, an internal standard is used to account for volume change. Reaction gases are supplied from high-pressure gas cylinders (Air Liquide) and are of ultrahigh purity. Analytic grade ethanol was obtained from Merck. Catalytic activity is evaluated in terms of ethanol conversion. Selectivities are de>ned as the ratios of the product moles to the consumed moles of ethanol, accounting for stoichiometry. In a typical experiment, the fresh catalyst is placed in the reactor and reduced in situ at 750◦ C for 2 h under hydrogen Pow. After reduction, the reactor is heated to the desired reaction temperature under He Pow. When the system has equilibrated at the desired temperature, the reactant streams are introduced to the reactor and the conversion of reactants and selectivities towards reaction products are determined using the analysis system described above. All experiments were performed at atmospheric pressure. 3. Results and discussion All catalysts described above were tested for the catalytic partial oxidation of ethanol. We will >rst discuss in detail the behavior of the 5% Ru=-Al2 O3 powder catalyst washcoated on the cordierite monolith and to this we will compare the performance of the other structured catalysts, deposited on di6erent supports. 3.1. E3ect of reaction temperature on ethanol conversion and product distribution The e6ect of temperature on the catalytic activity and product distribution of the 5% Ru=-Al2 O3 =monolith catalyst is presented in Fig. 2A, where the conversion of ethanol (XEtOH ) and selectivities to reaction products (Si ) are plotted as functions of reaction temperature. Very high ethanol conversions are evident at all temperatures tested with ethanol completely converted at furnace temperatures above 650◦ C. This is achieved with very high selectivities towards hydrogen (¿ 90%) with the main by-product being methane.
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Fig. 2. E6ect of reaction temperature on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: HSV = 9055 h−1 ; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
Other byproducts, such as CH3 CHO, C2 H4 and C2 H6 are also observed but at much smaller concentrations. The catalyst also exhibits good activity for the water gas shift reaction as shown by the selectivities towards CO and CO2 . Although both CO and CO2 have been reported as primary products of ethanol reforming [13], the observed selectivities are close to the thermodynamic equilibrium, indicating su?cient promotion of the WGS reaction. Furnace temperatures are used as set points since the combustion reactions make attainment of constant temperatures at the catalyst inlet impractical, if not infeasible. The experimental setup, however, allows monitoring of the temperatures along the centerline of the reactor and typical temperature pro>les are shown in Fig. 2B. The monolith is approximately 4 cm long and the air enters the quartz chip bed 2 cm before the catalyst. Although the furnace temperature is set at 600 –750◦ C, the temperature at the mixing point is much lower, 410 –560◦ C, due to the low temperatures of the incoming streams and the heat losses of the system. Heat conduction and radiation and, possibly, initiation of the combustion reactions cause a rapid increase in temperature. The fact that the temperature maxima develop well inside the monolith indicates that the majority of the combustion reactions take place inside the catalyst.
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Fig. 3. E6ect of space velocity on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
Oxygen is consumed and these reactions appear to terminate in a short Pame front as evident by the very high temperatures attained. As expected, raising the furnace set point raises the whole temperature pro>le, although the rise is not directly proportional due to heat losses in the non-adiabatic system. Loss of the heat source, combustion, and the heat requirements of the reforming reactions lead to the subsequent temperature drop. 3.2. E3ect of space velocity on catalytic activity and product distribution The e6ect of space velocity on catalytic activity and product distribution was examined by varying the feed Pow while maintaining constant feed composition and furnace set point (600◦ C). As shown in Fig. 3A, there is a very small but measurable drop in ethanol conversion as the space velocity increases from 3620 to 9055 h−1 . Interestingly, there is also a small increase in selectivity towards hydrogen accompanied by a small decrease towards the main byproduct, methane. The other byproducts appear to go through a weak maximum at 5435 h−1 as does CO2 , while CO goes through a weak minimum. The explanation may lie in the temperature pro>les shown in Fig. 3B. As the Pow increases, feed and air enter the system at progressively lower temperatures since
the heat supplied in the preheating zone is maintained constant. With the exception of the lowest Pow, all combined streams enter the catalyst at approximately the same temperature indicating a balance between the heat required to raise the stream temperature and the heat conducted and/or radiated back from the catalyst. At all points after entering the catalyst, the temperatures increase with increasing Pow. A larger amount of ethanol is combusted within the same catalyst volume, releasing greater amounts of heat and raising the temperatures higher. At the same time, the maxima are moved slightly in the direction of Pow as more heat is conveyed downstream by the larger masses passing through the catalyst. Heat losses also increase with increasing temperature so that the pro>les approach each other for the higher Pows. The pro>le at the lowest Pow rate is of interest since it is “Patter” than the others, indicating that combustion takes place to a signi>cant extend before the catalyst. This, in addition to the much lower temperatures inside the catalyst may explain the small variations in product selectivities. It is apparent from the above that two e6ects are operable in the experiments: the enhancement of Pow rate decreases the contact time in the monolithic reactor, which would decrease ethanol conversion and hydrogen selectivity. On the other hand, enhancement of Pow rate raises the temperature pro>le within the monolith, which would increase conversion and selectivity. Because the reaction is, in all cases, at a rather insensitive state (i.e. almost complete conversion and over 90% H2 selectivity) the two factors seem to balance out and the net e6ect is very small variations in catalytic performance with increasing feed Pow rate. This, of course, would not be the case under extreme conditions of very high or very low Pow rates. 3.3. E3ect of steam to ethanol ratio The stoichiometry of the steam reforming reaction dictates that 3 mole of H2 O per mole of ethanol are required for complete reformation to CO2 and H2 , while less than 2 mole are required for the partial oxidation reaction. The e6ect of this ratio was examined by varying the relative concentrations of steam and ethanol in the feed while maintaining the total Pow constant. The air supply was adjusted to provide the necessary 0:61 mole of oxygen per mole of ethanol and the furnace temperature was kept constant at 650◦ C. The results, shown in Fig 4A, point to no clear bene>t from increasing the molar ratio from 3/1 to 4/1. Lowering the ratio to 2/1, however, results in slightly lower ethanol conversion. Selectivity towards H2 drops more substantially, accompanied by a similar increase in selectivity towards CH4 . More importantly, other byproducts, such as CH3 CHO, C2 H4 and C2 H6 , appear in small but signi>cant amounts in the reformate. Finally, the selectivity towards CO2 drops while that towards CO increases. The last observation may rePect the e6ect of the lower steam concentration on the water gas shift reaction and the shift of the equilibrium distribution towards lower CO2 levels.
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Fig. 4. E6ect of the steam to ethanol molar ratio on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 650◦ C; HSV = 7245 h−1 ; O2 : EtOH = 0:61:1.
Fig. 5. E6ect of the oxygen to ethanol molar ratio on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 650◦ C; H2 O : EtOH = 3:1.
The temperature pro>les, shown in Fig. 4B, may again provide an explanation for the results of Fig. 4A. Decreasing the steam/ethanol molar ratio from 4/1 to 2/1, causes a shift of the pro>les towards the front of the reactor, even though the temperature, for all three ratios, is essentially the same at the point where air is introduced. The maximum temperature for the low ratio has in fact moved out of the monolithic catalyst, into the quartz chip bed where air and feed mix. Repeated e6orts to “push” the maximum inside the catalyst were unsuccessful, as it would always stabilize in the position shown. The feed mixture with the higher ethanol concentration lights o6 very quickly and produces the largest exotherm, possibly due to lower heat absorption by water. In fact, the maximum temperatures reached, monotonically decrease as the ratio is raised.
invariant at 650◦ C. Ethanol conversion is unchanged and complete as the ratio increases from 0.61 to 0.80 (Fig. 5A) while there is a small but noticeable increase in selectivity towards hydrogen formation, which reaches ¿ 99%, and a simultaneous decrease of the selectivity towards methane. Decreasing the oxygen-to-ethanol ratio to 0.40, however, produces a signi>cant change. Ethanol conversion drops to approximately 85% and selectivity towards hydrogen drops to 89%. More methane is present in the reformate and the other byproducts appear in signi>cant amounts. Selectivities towards both CO and CO2 trend lower, as there are other dispositions for the available carbon. The temperature pro>les (Fig. 5B) assist in providing an explanation for the above results. Although there is a small temperature variation in the mixing zone, the combined stream enters the monolith at essentially the same temperature in all three cases. For the low ratio, however, the substantial exotherm at the top of the reactor is followed by a signi>cant drop in temperature with the product stream exiting the monolith at 650◦ C, which corresponds to the furnace temperature. Combustion of a smaller part of the ethanol does not provide su?cient heat to drive the reforming reactions to completion, leading to lower conversion and more byproducts at the exit. The high ratio, on the
3.4. E3ect of air to ethanol ratio Reaction (2) indicates that 0:61 mole of oxygen are required per mole of ethanol to achieve thermal neutrality (KHr ∼ 0). The e6ect of the oxygen-to-ethanol ratio on the performance of the monolithic catalyst was examined by maintaining constant feed Pow while varying the amount of air supplied to the system. The furnace set point was
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Fig. 6. Long-term stability test of the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 600◦ C; HSV = 7245 h−1 ; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
other hand, consumes so much ethanol that the temperatures along the monolith are much higher and allow not only complete conversion of ethanol but also complete reforming of all byproducts, including methane. 3.5. Catalyst stability The 5% Ru=-Al2 O3 =monolith catalyst underwent a longterm stability test remaining on stream for 77 h (Fig. 6). There were multiple start-ups and shut-downs and one regeneration during this period. Although Fig. 6 shows only the data pertaining to the stability test conditions, between these tests, the conditions varied signi>cantly. Feed Pow changed by a factor of 2.5 and the furnace temperatures ranged from 600◦ C to 750◦ C. The catalyst shows remarkable stability through all these. Ethanol conversion is essentially complete while selectivity towards hydrogen remains above 90%. The only sign of catalyst ageing is the small shift in selectivities towards CO and CO2 . Even though there was no apparent deactivation of the catalyst, a regeneration was performed after 70 h on stream to test catalyst regenerability. Carbon was removed under controlled air supply and temperature conditions as to approximate commercial procedures. The elusion of carbon oxides was monitored during the burn and provided an estimate of the carbon accumulation on the catalyst. Based on these results, we calculated that 0:08 g of carbon had been deposited on the catalyst for every liter of ethanol processed. The catalyst was reduced after regeneration and was put back to service. As evident in Fig. 6, regeneration did not a6ect either the activity or the selectivity of the catalyst. 3.6. Catalyst characterization The catalyst was examined for changes in its crystalline structure during the various operations via XRD. Fig. 7 presents the pattern of the -Al2 O3 used to produce the Ru=-Al2 O3 catalyst, the pattern for a freshly reduced sample
Fig. 7. XRD measurements of the -Al2 O3 support, a fresh reduced sample of the Ru=-Al2 O3 catalyst and the Ru=-Al2 O3 catalyst after the long-term stability test.
of Ru=-Al2 O3 and the pattern obtained from the Ru=-Al2 O3 catalyst at the end of the experiment. The later consisted of powder scraped o6 the washcoat of the monolith after the long-term stability study mentioned above. It is worthy noting that, even after a long period of exposure to severe process conditions, the catalyst appears to retain the shape and size of the Ru crystallites as shown by the intensity of the Ruthenium peak at 2 = 44◦ . An equally important observation is that the -Al2 O3 carrier appears to retain its state. Exposure of -Al2 O3 to temperatures above 1000◦ C is expected to transform it into -Al2 O3 to a large degree. Bonding of -Al2 O3 to the monolithic substrate seems to hinder such transformation. 3.7. Comparison of the di3erent structured catalysts In addition to the monolithic catalyst described above, where the powdered 5% Ru=-Al2 O3 catalyst was washcoated on the monolith, three more structured catalysts were prepared and tested: (a) a zirconia–alumina ceramic foam washcoated with the same powdered catalyst, (b) a zirconia– alumina ceramic foam impregnated with Ru and (c) -Al2 O3 pellets impregnated with Ru. A plain monolith, without catalyst, was also tested and will be used as a base case. The feed in all cases was a mixture of water and ethanol at a 3/1 molar ratio, while air was supplied at a Pow providing 0.61 mole of O2 per mole of ethanol. The catalysts were tested under di6erent conditions, but the comparisons presented here focus on their behavior versus space velocity (HSV) with the furnace set point maintained at 600◦ C. Although the diameter of the samples was essentially the same, their length and volume varied, resulting in di6erent space velocities at the same Pow rate. Apparatus Pow limitations did not allow scanning of all space velocities for all samples. The activity of the di6erent structured catalysts is presented in Fig. 8A. All samples were able to achieve ethanol conversions greater than 95%. Only the ceramic foams showed lower conversion and only at the highest end of
D.K. Liguras et al. / International Journal of Hydrogen Energy 29 (2004) 419 – 427
425
Fig. 8. Catalyst comparison: ethanol conversion (A) and selectivity towards hydrogen (B) as functions of space velocity. Experimental conditions: Tfurnace =600◦ C; H2 O : EtOH=3:1; O2 : EtOH=0:61:1.
Fig. 9. Catalyst comparison: selectivity towards methane (A) and towards acetaldehyde (B) as functions of space velocity. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
space velocities tested. Surprisingly, the monolith without catalyst achieved reasonably high conversions, which increased with increasing Pow rate. This is due to the higher temperatures reached inside the monolith at the higher Pows and is the same phenomenon we discussed previously. As we have reported previously [6], ethanol can decompose completely at high temperatures even in the absence of catalyst. Catalysts promote ethanol conversion but more importantly, have a pronounced e6ect on the selectivities towards the desired products (hydrogen). Selectivities towards H2 are shown in Fig. 8B. It is evident that although the plain monolith (without catalyst) gives selectivities greater than 70%, the catalytic materials push these selectivities above 87%. Pellets and the impregnated foam produce the highest values. Hydrogen selectivities over the monolith increase with increasing Pow, as shown previously, due to the higher temperatures reached and their bene>cial e6ect on the reformation of the byproducts. The main byproduct is methane and the monolith without catalyst produces the most (Fig. 9A). The impregnated foam and pellets show the lowest selectivities towards methane, while the washcoated monolith has high selectivity at low space velocity, but improves substantially as the Pow increases. The washcoated foam exhibits the lowest activity for methane reformation resulting in the most methane in the reformate.
Similar patterns are observed with CH3 CHO production (Fig. 9B). Acetaldehyde is produced by ethanol dehydrogenation and is subsequently reformed. Most of the catalysts are able to reform it to a large extend, with the washcoated foam exhibiting again the lowest activity. Very low residence times also result in greater amounts of CH3 CHO remaining unconverted. Ethylene is one of the most troublesome byproducts of ethanol reforming and partial oxidation since it can deactivate the catalyst by carbon deposition. It is formed via ethanol dehydration over the acidic sites of the catalysts, but most catalysts are able to reform it easily [14]. Ethylene selectivities of the catalysts tested were generally low (Fig. 10A). The monolith and the impregnated foam were the most active in its conversion with the pellets and the washcoated foam producing the larger amounts, possibly due to a greater exposed alumina area. A slightly di6erent picture is presented for the selectivities towards ethane (Fig. 10B). Here, the impregnated pellets exhibit the lowest selectivity with the order being plain monolith ¿ washcoated foam ¿ monolith ¿ impregnated foam ¿ pellets. It should be noted that a similar trend was observed for CH3 CHO production. Ethane is, most probably, the product of ethylene hydrogenation, while acetaldehyde is the product of ethanol dehydrogenation. It appears that the metal has been dispersed on the pellets in such a state as not to favor these reactions.
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Fig. 10. Catalyst comparison: selectivity towards ethylene (A) and towards ethane (B) as functions of space velocity. Experimental conditions: Tfurnace =600◦ C; H2 O : EtOH=3:1; O2 : EtOH=0:61:1.
The selectivities towards CO and CO2 (Figs. 11A and B) may rePect the ability of the catalysts to promote the water gas shift reaction. The impregnated foam and pellets again exhibited the highest activity with only the former being a6ected by the space velocity, possibly due to its small size. The very high selectivity towards CO, and, correspondingly, the very low activity towards CO2 , exhibited by the plain monolith tend to support a mechanism where CO is the primary product of ethanol conversion and CO2 is produced via the water gas shift reaction. Overall, the impregnated foam and pellets showed the best activity and selectivity towards the desired products. The chemical nature of the foam, its relatively low pore size and open volume and its high tortuosity appear to be bene>cial for its catalytic performance. The washcoated monolith also produced very good results, especially at the higher space velocities tested. The washcoated foam was the least active and selective. Given the nature of this support, we can easily visualize pore blockage and active phase inaccessibility occurring, as a result of the particular preparation method. This method, therefore, cannot be recommended. Among the most active catalysts, pellets may be unsuitable for mobile applications due to high attrition rates in an environment of constant vibrations. Such attrition would lead to catalyst collapse and very high pressure drops. The impregnated foam emerges as the best candidate, while the
Fig. 11. Catalyst comparison: selectivity towards CO (A) and towards CO2 (B) as functions of space velocity. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
versatility and wide use of monoliths make them a very good alternative. 4. Conclusions All Ru-containing catalysts, supported on cordierite monoliths, ceramic foams and -Al2 O3 pellets, which were tested during this study, are good candidates for the process of hydrogen production for fuel cell applications via catalytic partial oxidation of ethanol. The most extensively tested catalyst, Ru supported on a cordierite monolith, exhibited excellent catalytic performance for a wide variety of process conditions and excellent long-term stability with low amounts of coke deposition. Furthermore, its nature, as deduced from the XRD analyses, does not appear to be affected by exposure to severe operating conditions. Varying the steam-to-ethanol molar ratio of the feed stream resulted in relatively small changes in conversion and selectivities. More signi>cant e6ects were observed for changes in the feed oxygen-to-ethanol ratio. Comparing all catalytic systems, we may conclude that, catalysts deposited on all three supports were able to completely convert ethanol with high selectivities towards the desired products. Ru-containing catalysts supported on alumina pellets showed very good catalytic behavior, but they might be unsuitable for mobile applications. The Ru supported on a ceramic foam
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catalyst provided comparatively better performance probably due to the smaller pore size and higher tortuosity of this support. This catalyst might be the preferred candidate for the process of hydrogen production for fuel cell applications by catalytic partial oxidation of ethanol in automotive applications.
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International Journal of Hydrogen Energy 29 (2004) 419 – 427
www.elsevier.com/locate/ijhydene
Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ru catalysts Dimitris K. Liguras, Katerina Goundani, Xenophon E. Verykios∗ Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece Accepted 17 June 2003
Abstract A series of Ru catalysts supported on cordierite monoliths, ceramic foams and -Al2 O3 pellets were prepared and tested for the production of hydrogen by catalytic partial oxidation of ethanol. The catalyst supported on a cordierite monolith exhibited excellent catalytic performance for a wide variety of process conditions and excellent long-term stability with low coke formation. The e6ect of the steam to ethanol molar ratio on conversion and selectivities was relatively small. A more pronounced e6ect was observed for the oxygen to ethanol ratio. Overall, catalysts on all three supports were able to completely convert ethanol with high selectivities towards the desired products. The Ru supported on a ceramic foam catalyst provided comparatively better performance, probably due to the smaller pore sizes and higher tortuosity of this support. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Catalytic partial oxidation; Hydrogen production; Ethanol; Ruthenium; Monolith; Ceramic foam
1. Introduction In the last few years, hydrogen has emerged as the energy carrier that has the potential to solve two major energy challenges: reducing dependence on fossil fuels and reducing pollution and greenhouse gas emissions. Although the transition to the so-called “hydrogen economy” has been postulated to have already begun, signi>cant technical challenges will make it a slow process. The most important challenges involve the sources of hydrogen and the development of e?cient, safe and a6ordable production processes. The most commonly mentioned sources of hydrogen are natural gas, gasoline, diesel and methanol. All these sources, however, have a major drawback in that they do not address the goal of reducing neither the dependence on fossil fuels nor the emissions of greenhouse gases, such as CO2 . Bio-fuels, on the other hand, are viable alternatives since they o6er high energy density and ease of handling so that they can be used for on-demand production of hydrogen for ∗
Corresponding author. Tel./fax: +30-2610-991527. E-mail address: [email protected] (X.E. Verykios).
automotive and distributed power generation applications. Among the various liquid bio-fuels, bio-ethanol is emerging as a frontrunner. In addition to advantages related to natural availability, storage and handling safety, bio-ethanol is produced renewably from many biomass sources, including energy plants, grains, waste materials from agro industries and even the organic fraction of municipal solid wastes. Furthermore, a bioethanol-to-hydrogen system has the signi>cant advantage of being nearly CO2 neutral since the carbon dioxide produced in the process is consumed for biomass growth, o6ering a nearly closed carbon loop. Hydrogen production from ethanol via steam reforming or catalytic partial oxidation has been the subject of several recent studies [1–6]. The process has been shown to be entirely feasible from a thermodynamic point of view [7–9] and several catalysts have been proposed which show su?cient activity and stability to be further considered for practical applications [10–16]. All of these studies, however, have examined catalysts in powder form. Such a form is unsuitable for practical applications in terms of handling and pressure drop in the reactor. Especially for automotive applications, the catalysts must be deposited on structured supports, which have excellent structural stability in harsh
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(03)00210-6
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environments that include vibrations, thermal cycling and thousands of start-ups and shut-downs. Ethanol steam reforming is a highly endothermic reaction C2 H5 OH + 3H2 O → 2CO2 + 6H2 (KHr = 173:1 kJ mol−1 )
(1)
requiring a great amount of heat to maintain the system at a steady reaction temperature. There are two alternative ways of supplying heat to the system: (i) external heat supplied by burning some type of fuel, or (ii) internal heat supplied by co-feeding oxygen or air and burning a portion of the ethanol at the expense of hydrogen production. At least 0:61 mol of oxygen per mole of ethanol are required to achieve thermal neutrality C2 H5 OH + 0:61O2 + 1:78H2 O → 2CO2 + 4:78H2 (KHr = 0 kJ mol−1 ):
(2)
Depending on the way combustion is accomplished, the systems are either termed auto-thermal (gas phase combustion) or catalytic partial oxidation (catalytic combustion) although combinations of these have been examined. The present study examines the catalytic partial oxidation of ethanol, where both reforming and combustion reactions take place on the same Ru catalysts deposited on di6erent supports. 2. Experimental 2.1. Catalyst preparation A series of Ru catalysts were supported on cordierite monoliths, ceramic foams and pellets. The monoliths were of the type used in automobile exhausts with 400 channels per square inch. The ceramic foams were proprietary formulations of alumina and zirconia with medium porosity (50 pores per square inch). The pellets were 1=16 extrudates of -Al2 O3 (Engelhard) with a speci>c area of 90 m2 g−1 . The Ru=-Al2 O3 catalyst supported on a cordierite monolith was prepared by washcoating. A 5% Ru=-Al2 O3 powder catalyst was prepared by impregnating -Al2 O3 (90 m2 g−1 ) with aqueous solution of Ru(NO)(NO3 )3 followed by overnight drying at 120◦ C and calcination at 550◦ C for 5 h. A dense suspension of this powder in de-ionized water was created and the catalysts were coated by successive immersions in the suspension followed by overnight drying at 120◦ C and calcination at 550◦ C. A >nal calcination at 1000◦ C took place before testing. A total of 1:8 g of catalyst was loaded on a monolith 25 mm in diameter, 40 mm in length and weighing 7:2 g. The same technique and the same 5% Ru=-Al2 O3 powder catalyst was used to washcoat a zirconia–alumina ceramic foam. The foam had a diameter of 25 mm, length of 23 mm and weight of 6:1 g. The loading was 1:7 g of catalyst. Another ceramic foam of similar size was loaded with Ru by wet impregnation using an aqueous solution of
Ru(NO)(NO3 )3: This material was also calcined and reduced in situ after an overnight drying at 120◦ C. The same technique was employed for the preparation of the Ru catalyst on alumina pellets. The metal loading on both pellets and ceramic foam was 5% Ru. As a base case, a plain monolith, with no catalyst, was also tested. 2.2. Apparatus and procedures Catalytic performance tests have been carried out using an apparatus consisting of the Pow system, the reactor unit and the analysis system. The Pow system is equipped with a set of mass-Pow controllers (MKS) and an HPLC pump (Marathon) for feeding liquid reagents (ethanol–H2 O mixtures). The liquid is pumped to a vaporizer where it is evaporated and heated to 150◦ C and where it can be mixed (when desired) with the gas stream coming from the mass-Pow controllers. The resulting gas mixture is then fed to the reactor through stainless steel tubing maintained at 150◦ C by means of heating tapes. The reactor used is shown in Fig 1. Feed and air or O2 are mixed in a small bed of quartz chips before entering the catalyst to achieve complete mixing, to obtain uniform distribution and to minimize homogeneous reactions. The main reactor can easily accommodate catalytic specimens up to 27 mm in diameter. Glass wool provides support and a seal between the catalyst and the reactor walls. Additional support is provided by the quartz funnel shown in Fig. 1. The mouth of the funnel is slightly smaller than the reactor internal diameter and su?cient to support the edge of the catalyst. The stem of the funnel is of much smaller diameter (8 mm OD) and acts as a sampling port. Most of the reaction products also exit through this path so as to reduce their residence time in the hot reactor and minimize any gas phase reactions. The rest of the products exit through the second port shown. Dual thermocouples run the length of the reactor, passing through the catalyst, to monitor the temperature pro>le. The reactor is placed inside an electric furnace. The analysis system consists of two gas chromatographs (Shimadzu). The >rst one is equipped with two packed columns (Porapak-Q and Carbosieve) and two detectors (TCD, FID) and operates with He as the carrier gas.
Fig. 1. Schematic diagram of the reactor employed for the ethanol partial oxidation studies.
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Porapak-Q is used for the separation of C2 H5 OH, H2 O, CH3 CHO, CH4 , C2 H4 and C2 H6 , while Carbosieve is used for the separation of CO, CO2 and CH4 . Since the presence of large amounts of water hinders analysis in the Carbosieve column, a condenser is placed before its inlet, in order to condense and remove H2 O from the gas stream. The second GC is equipped with a Carbosieve column, connected to the exit of the condenser, and a TCD detector. This chromatograph uses N2 as the carrier gas and is used solely to determine the H2 concentration in the reformate. Determination of the response factors of the TCD and FID detectors has been achieved with the use of gas streams of known composition (Scott specialty gas mixtures, self-prepared EtOH=H2 O=CH3 CHO mixtures, etc.). Since several substances (CO, CO2 , O2 , H2 ) can only be analyzed after the condenser, an internal standard is used to account for volume change. Reaction gases are supplied from high-pressure gas cylinders (Air Liquide) and are of ultrahigh purity. Analytic grade ethanol was obtained from Merck. Catalytic activity is evaluated in terms of ethanol conversion. Selectivities are de>ned as the ratios of the product moles to the consumed moles of ethanol, accounting for stoichiometry. In a typical experiment, the fresh catalyst is placed in the reactor and reduced in situ at 750◦ C for 2 h under hydrogen Pow. After reduction, the reactor is heated to the desired reaction temperature under He Pow. When the system has equilibrated at the desired temperature, the reactant streams are introduced to the reactor and the conversion of reactants and selectivities towards reaction products are determined using the analysis system described above. All experiments were performed at atmospheric pressure. 3. Results and discussion All catalysts described above were tested for the catalytic partial oxidation of ethanol. We will >rst discuss in detail the behavior of the 5% Ru=-Al2 O3 powder catalyst washcoated on the cordierite monolith and to this we will compare the performance of the other structured catalysts, deposited on di6erent supports. 3.1. E3ect of reaction temperature on ethanol conversion and product distribution The e6ect of temperature on the catalytic activity and product distribution of the 5% Ru=-Al2 O3 =monolith catalyst is presented in Fig. 2A, where the conversion of ethanol (XEtOH ) and selectivities to reaction products (Si ) are plotted as functions of reaction temperature. Very high ethanol conversions are evident at all temperatures tested with ethanol completely converted at furnace temperatures above 650◦ C. This is achieved with very high selectivities towards hydrogen (¿ 90%) with the main by-product being methane.
421
Fig. 2. E6ect of reaction temperature on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: HSV = 9055 h−1 ; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
Other byproducts, such as CH3 CHO, C2 H4 and C2 H6 are also observed but at much smaller concentrations. The catalyst also exhibits good activity for the water gas shift reaction as shown by the selectivities towards CO and CO2 . Although both CO and CO2 have been reported as primary products of ethanol reforming [13], the observed selectivities are close to the thermodynamic equilibrium, indicating su?cient promotion of the WGS reaction. Furnace temperatures are used as set points since the combustion reactions make attainment of constant temperatures at the catalyst inlet impractical, if not infeasible. The experimental setup, however, allows monitoring of the temperatures along the centerline of the reactor and typical temperature pro>les are shown in Fig. 2B. The monolith is approximately 4 cm long and the air enters the quartz chip bed 2 cm before the catalyst. Although the furnace temperature is set at 600 –750◦ C, the temperature at the mixing point is much lower, 410 –560◦ C, due to the low temperatures of the incoming streams and the heat losses of the system. Heat conduction and radiation and, possibly, initiation of the combustion reactions cause a rapid increase in temperature. The fact that the temperature maxima develop well inside the monolith indicates that the majority of the combustion reactions take place inside the catalyst.
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Fig. 3. E6ect of space velocity on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
Oxygen is consumed and these reactions appear to terminate in a short Pame front as evident by the very high temperatures attained. As expected, raising the furnace set point raises the whole temperature pro>le, although the rise is not directly proportional due to heat losses in the non-adiabatic system. Loss of the heat source, combustion, and the heat requirements of the reforming reactions lead to the subsequent temperature drop. 3.2. E3ect of space velocity on catalytic activity and product distribution The e6ect of space velocity on catalytic activity and product distribution was examined by varying the feed Pow while maintaining constant feed composition and furnace set point (600◦ C). As shown in Fig. 3A, there is a very small but measurable drop in ethanol conversion as the space velocity increases from 3620 to 9055 h−1 . Interestingly, there is also a small increase in selectivity towards hydrogen accompanied by a small decrease towards the main byproduct, methane. The other byproducts appear to go through a weak maximum at 5435 h−1 as does CO2 , while CO goes through a weak minimum. The explanation may lie in the temperature pro>les shown in Fig. 3B. As the Pow increases, feed and air enter the system at progressively lower temperatures since
the heat supplied in the preheating zone is maintained constant. With the exception of the lowest Pow, all combined streams enter the catalyst at approximately the same temperature indicating a balance between the heat required to raise the stream temperature and the heat conducted and/or radiated back from the catalyst. At all points after entering the catalyst, the temperatures increase with increasing Pow. A larger amount of ethanol is combusted within the same catalyst volume, releasing greater amounts of heat and raising the temperatures higher. At the same time, the maxima are moved slightly in the direction of Pow as more heat is conveyed downstream by the larger masses passing through the catalyst. Heat losses also increase with increasing temperature so that the pro>les approach each other for the higher Pows. The pro>le at the lowest Pow rate is of interest since it is “Patter” than the others, indicating that combustion takes place to a signi>cant extend before the catalyst. This, in addition to the much lower temperatures inside the catalyst may explain the small variations in product selectivities. It is apparent from the above that two e6ects are operable in the experiments: the enhancement of Pow rate decreases the contact time in the monolithic reactor, which would decrease ethanol conversion and hydrogen selectivity. On the other hand, enhancement of Pow rate raises the temperature pro>le within the monolith, which would increase conversion and selectivity. Because the reaction is, in all cases, at a rather insensitive state (i.e. almost complete conversion and over 90% H2 selectivity) the two factors seem to balance out and the net e6ect is very small variations in catalytic performance with increasing feed Pow rate. This, of course, would not be the case under extreme conditions of very high or very low Pow rates. 3.3. E3ect of steam to ethanol ratio The stoichiometry of the steam reforming reaction dictates that 3 mole of H2 O per mole of ethanol are required for complete reformation to CO2 and H2 , while less than 2 mole are required for the partial oxidation reaction. The e6ect of this ratio was examined by varying the relative concentrations of steam and ethanol in the feed while maintaining the total Pow constant. The air supply was adjusted to provide the necessary 0:61 mole of oxygen per mole of ethanol and the furnace temperature was kept constant at 650◦ C. The results, shown in Fig 4A, point to no clear bene>t from increasing the molar ratio from 3/1 to 4/1. Lowering the ratio to 2/1, however, results in slightly lower ethanol conversion. Selectivity towards H2 drops more substantially, accompanied by a similar increase in selectivity towards CH4 . More importantly, other byproducts, such as CH3 CHO, C2 H4 and C2 H6 , appear in small but signi>cant amounts in the reformate. Finally, the selectivity towards CO2 drops while that towards CO increases. The last observation may rePect the e6ect of the lower steam concentration on the water gas shift reaction and the shift of the equilibrium distribution towards lower CO2 levels.
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423
Fig. 4. E6ect of the steam to ethanol molar ratio on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 650◦ C; HSV = 7245 h−1 ; O2 : EtOH = 0:61:1.
Fig. 5. E6ect of the oxygen to ethanol molar ratio on the conversion of ethanol (XEtOH ) and on the selectivities (Si ) towards reaction products (A) and the associated temperature pro>les (B) over the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 650◦ C; H2 O : EtOH = 3:1.
The temperature pro>les, shown in Fig. 4B, may again provide an explanation for the results of Fig. 4A. Decreasing the steam/ethanol molar ratio from 4/1 to 2/1, causes a shift of the pro>les towards the front of the reactor, even though the temperature, for all three ratios, is essentially the same at the point where air is introduced. The maximum temperature for the low ratio has in fact moved out of the monolithic catalyst, into the quartz chip bed where air and feed mix. Repeated e6orts to “push” the maximum inside the catalyst were unsuccessful, as it would always stabilize in the position shown. The feed mixture with the higher ethanol concentration lights o6 very quickly and produces the largest exotherm, possibly due to lower heat absorption by water. In fact, the maximum temperatures reached, monotonically decrease as the ratio is raised.
invariant at 650◦ C. Ethanol conversion is unchanged and complete as the ratio increases from 0.61 to 0.80 (Fig. 5A) while there is a small but noticeable increase in selectivity towards hydrogen formation, which reaches ¿ 99%, and a simultaneous decrease of the selectivity towards methane. Decreasing the oxygen-to-ethanol ratio to 0.40, however, produces a signi>cant change. Ethanol conversion drops to approximately 85% and selectivity towards hydrogen drops to 89%. More methane is present in the reformate and the other byproducts appear in signi>cant amounts. Selectivities towards both CO and CO2 trend lower, as there are other dispositions for the available carbon. The temperature pro>les (Fig. 5B) assist in providing an explanation for the above results. Although there is a small temperature variation in the mixing zone, the combined stream enters the monolith at essentially the same temperature in all three cases. For the low ratio, however, the substantial exotherm at the top of the reactor is followed by a signi>cant drop in temperature with the product stream exiting the monolith at 650◦ C, which corresponds to the furnace temperature. Combustion of a smaller part of the ethanol does not provide su?cient heat to drive the reforming reactions to completion, leading to lower conversion and more byproducts at the exit. The high ratio, on the
3.4. E3ect of air to ethanol ratio Reaction (2) indicates that 0:61 mole of oxygen are required per mole of ethanol to achieve thermal neutrality (KHr ∼ 0). The e6ect of the oxygen-to-ethanol ratio on the performance of the monolithic catalyst was examined by maintaining constant feed Pow while varying the amount of air supplied to the system. The furnace set point was
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Fig. 6. Long-term stability test of the 5% Ru=-Al2 O3 catalyst supported on a cordierite monolith. Experimental conditions: Tfurnace = 600◦ C; HSV = 7245 h−1 ; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
other hand, consumes so much ethanol that the temperatures along the monolith are much higher and allow not only complete conversion of ethanol but also complete reforming of all byproducts, including methane. 3.5. Catalyst stability The 5% Ru=-Al2 O3 =monolith catalyst underwent a longterm stability test remaining on stream for 77 h (Fig. 6). There were multiple start-ups and shut-downs and one regeneration during this period. Although Fig. 6 shows only the data pertaining to the stability test conditions, between these tests, the conditions varied signi>cantly. Feed Pow changed by a factor of 2.5 and the furnace temperatures ranged from 600◦ C to 750◦ C. The catalyst shows remarkable stability through all these. Ethanol conversion is essentially complete while selectivity towards hydrogen remains above 90%. The only sign of catalyst ageing is the small shift in selectivities towards CO and CO2 . Even though there was no apparent deactivation of the catalyst, a regeneration was performed after 70 h on stream to test catalyst regenerability. Carbon was removed under controlled air supply and temperature conditions as to approximate commercial procedures. The elusion of carbon oxides was monitored during the burn and provided an estimate of the carbon accumulation on the catalyst. Based on these results, we calculated that 0:08 g of carbon had been deposited on the catalyst for every liter of ethanol processed. The catalyst was reduced after regeneration and was put back to service. As evident in Fig. 6, regeneration did not a6ect either the activity or the selectivity of the catalyst. 3.6. Catalyst characterization The catalyst was examined for changes in its crystalline structure during the various operations via XRD. Fig. 7 presents the pattern of the -Al2 O3 used to produce the Ru=-Al2 O3 catalyst, the pattern for a freshly reduced sample
Fig. 7. XRD measurements of the -Al2 O3 support, a fresh reduced sample of the Ru=-Al2 O3 catalyst and the Ru=-Al2 O3 catalyst after the long-term stability test.
of Ru=-Al2 O3 and the pattern obtained from the Ru=-Al2 O3 catalyst at the end of the experiment. The later consisted of powder scraped o6 the washcoat of the monolith after the long-term stability study mentioned above. It is worthy noting that, even after a long period of exposure to severe process conditions, the catalyst appears to retain the shape and size of the Ru crystallites as shown by the intensity of the Ruthenium peak at 2 = 44◦ . An equally important observation is that the -Al2 O3 carrier appears to retain its state. Exposure of -Al2 O3 to temperatures above 1000◦ C is expected to transform it into -Al2 O3 to a large degree. Bonding of -Al2 O3 to the monolithic substrate seems to hinder such transformation. 3.7. Comparison of the di3erent structured catalysts In addition to the monolithic catalyst described above, where the powdered 5% Ru=-Al2 O3 catalyst was washcoated on the monolith, three more structured catalysts were prepared and tested: (a) a zirconia–alumina ceramic foam washcoated with the same powdered catalyst, (b) a zirconia– alumina ceramic foam impregnated with Ru and (c) -Al2 O3 pellets impregnated with Ru. A plain monolith, without catalyst, was also tested and will be used as a base case. The feed in all cases was a mixture of water and ethanol at a 3/1 molar ratio, while air was supplied at a Pow providing 0.61 mole of O2 per mole of ethanol. The catalysts were tested under di6erent conditions, but the comparisons presented here focus on their behavior versus space velocity (HSV) with the furnace set point maintained at 600◦ C. Although the diameter of the samples was essentially the same, their length and volume varied, resulting in di6erent space velocities at the same Pow rate. Apparatus Pow limitations did not allow scanning of all space velocities for all samples. The activity of the di6erent structured catalysts is presented in Fig. 8A. All samples were able to achieve ethanol conversions greater than 95%. Only the ceramic foams showed lower conversion and only at the highest end of
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425
Fig. 8. Catalyst comparison: ethanol conversion (A) and selectivity towards hydrogen (B) as functions of space velocity. Experimental conditions: Tfurnace =600◦ C; H2 O : EtOH=3:1; O2 : EtOH=0:61:1.
Fig. 9. Catalyst comparison: selectivity towards methane (A) and towards acetaldehyde (B) as functions of space velocity. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
space velocities tested. Surprisingly, the monolith without catalyst achieved reasonably high conversions, which increased with increasing Pow rate. This is due to the higher temperatures reached inside the monolith at the higher Pows and is the same phenomenon we discussed previously. As we have reported previously [6], ethanol can decompose completely at high temperatures even in the absence of catalyst. Catalysts promote ethanol conversion but more importantly, have a pronounced e6ect on the selectivities towards the desired products (hydrogen). Selectivities towards H2 are shown in Fig. 8B. It is evident that although the plain monolith (without catalyst) gives selectivities greater than 70%, the catalytic materials push these selectivities above 87%. Pellets and the impregnated foam produce the highest values. Hydrogen selectivities over the monolith increase with increasing Pow, as shown previously, due to the higher temperatures reached and their bene>cial e6ect on the reformation of the byproducts. The main byproduct is methane and the monolith without catalyst produces the most (Fig. 9A). The impregnated foam and pellets show the lowest selectivities towards methane, while the washcoated monolith has high selectivity at low space velocity, but improves substantially as the Pow increases. The washcoated foam exhibits the lowest activity for methane reformation resulting in the most methane in the reformate.
Similar patterns are observed with CH3 CHO production (Fig. 9B). Acetaldehyde is produced by ethanol dehydrogenation and is subsequently reformed. Most of the catalysts are able to reform it to a large extend, with the washcoated foam exhibiting again the lowest activity. Very low residence times also result in greater amounts of CH3 CHO remaining unconverted. Ethylene is one of the most troublesome byproducts of ethanol reforming and partial oxidation since it can deactivate the catalyst by carbon deposition. It is formed via ethanol dehydration over the acidic sites of the catalysts, but most catalysts are able to reform it easily [14]. Ethylene selectivities of the catalysts tested were generally low (Fig. 10A). The monolith and the impregnated foam were the most active in its conversion with the pellets and the washcoated foam producing the larger amounts, possibly due to a greater exposed alumina area. A slightly di6erent picture is presented for the selectivities towards ethane (Fig. 10B). Here, the impregnated pellets exhibit the lowest selectivity with the order being plain monolith ¿ washcoated foam ¿ monolith ¿ impregnated foam ¿ pellets. It should be noted that a similar trend was observed for CH3 CHO production. Ethane is, most probably, the product of ethylene hydrogenation, while acetaldehyde is the product of ethanol dehydrogenation. It appears that the metal has been dispersed on the pellets in such a state as not to favor these reactions.
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Fig. 10. Catalyst comparison: selectivity towards ethylene (A) and towards ethane (B) as functions of space velocity. Experimental conditions: Tfurnace =600◦ C; H2 O : EtOH=3:1; O2 : EtOH=0:61:1.
The selectivities towards CO and CO2 (Figs. 11A and B) may rePect the ability of the catalysts to promote the water gas shift reaction. The impregnated foam and pellets again exhibited the highest activity with only the former being a6ected by the space velocity, possibly due to its small size. The very high selectivity towards CO, and, correspondingly, the very low activity towards CO2 , exhibited by the plain monolith tend to support a mechanism where CO is the primary product of ethanol conversion and CO2 is produced via the water gas shift reaction. Overall, the impregnated foam and pellets showed the best activity and selectivity towards the desired products. The chemical nature of the foam, its relatively low pore size and open volume and its high tortuosity appear to be bene>cial for its catalytic performance. The washcoated monolith also produced very good results, especially at the higher space velocities tested. The washcoated foam was the least active and selective. Given the nature of this support, we can easily visualize pore blockage and active phase inaccessibility occurring, as a result of the particular preparation method. This method, therefore, cannot be recommended. Among the most active catalysts, pellets may be unsuitable for mobile applications due to high attrition rates in an environment of constant vibrations. Such attrition would lead to catalyst collapse and very high pressure drops. The impregnated foam emerges as the best candidate, while the
Fig. 11. Catalyst comparison: selectivity towards CO (A) and towards CO2 (B) as functions of space velocity. Experimental conditions: Tfurnace = 600◦ C; H2 O : EtOH = 3:1; O2 : EtOH = 0:61:1.
versatility and wide use of monoliths make them a very good alternative. 4. Conclusions All Ru-containing catalysts, supported on cordierite monoliths, ceramic foams and -Al2 O3 pellets, which were tested during this study, are good candidates for the process of hydrogen production for fuel cell applications via catalytic partial oxidation of ethanol. The most extensively tested catalyst, Ru supported on a cordierite monolith, exhibited excellent catalytic performance for a wide variety of process conditions and excellent long-term stability with low amounts of coke deposition. Furthermore, its nature, as deduced from the XRD analyses, does not appear to be affected by exposure to severe operating conditions. Varying the steam-to-ethanol molar ratio of the feed stream resulted in relatively small changes in conversion and selectivities. More signi>cant e6ects were observed for changes in the feed oxygen-to-ethanol ratio. Comparing all catalytic systems, we may conclude that, catalysts deposited on all three supports were able to completely convert ethanol with high selectivities towards the desired products. Ru-containing catalysts supported on alumina pellets showed very good catalytic behavior, but they might be unsuitable for mobile applications. The Ru supported on a ceramic foam
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catalyst provided comparatively better performance probably due to the smaller pore size and higher tortuosity of this support. This catalyst might be the preferred candidate for the process of hydrogen production for fuel cell applications by catalytic partial oxidation of ethanol in automotive applications.
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