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Hydrogen production by autothermal reforming of kerosene over MgAlOx-supported Rh catalysts
Applied Catalysis A: General 371 (2009) 173–178
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Hydrogen production by autothermal reforming of kerosene over MgAlOx-supported Rh catalysts Makoto Harada a,d,*, Kazuhiro Takanabe b, Jun Kubota b, Kazunari Domen b, Takashi Goto c, Kazuya Akiyama c, Yasunobu Inoue d a
INPEX Corporation, 5-3-1 Akasaka, Minatoku, Tokyo, 107-6332, Japan Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo, 113-8656, Japan Nikki-Universal Co., Ltd., 7-14-1 Shinomiya, Hiratsuka, Kanagawa, 254-0014, Japan d Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomiokacho, Nagaoka, Niigata, 940-2188, Japan b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2009 Received in revised form 2 October 2009 Accepted 5 October 2009 Available online 12 October 2009
Autothermal reforming (ATR) of kerosene for hydrogen production was performed on the MgAlOxsupported Rh catalysts at LHSV of 15–25, S/C of 2.5, O/C of 0.5, at 101 kPa. Sphere-shaped supports with high compressive ultimate strength (0.90 MPa) were obtained from MG-30 hydrotalcite core (3 mm) by thorough heat treatment before impregnation of rhodium. Rhodium was loaded by pore-filling impregnation selectively to the surface of the sphere-shaped supports, confirmed by electron probe microanalysis. Stability tests on the prepared catalysts were performed, focusing on the small amount of C2–C3 hydrocarbons, concentrations of which reflect the catalytic activity and stability; i.e., low rates of C2–C3 olefin formation correspond to high activity of the catalysts. The reactor was designed to measure temperature profiles and gas distributions within the reactor (inner diameter 21.0 mm) and the catalyst bed (length 100 mm). The ATR reactions occur starting with an exothermic combustion of hydrocarbons, followed by an endothermic reforming. The maximum temperature reached 1200 K at the inlet of the catalyst bed and decreased to 1020 K towards the end of the catalyst bed. Among investigated catalysts, the catalysts treated in air at 1223 K gave the best performance for ATR of kerosene, giving H2 production reaching 60% of the exit gas. The concentration of the main byproduct, C2H4, over the optimized catalyst was lower than 0.03% at the exit of the reactor for 50 h of the study. The catalyst showed high tolerance to coking and high stability even at LHSV of 25 and daily start-up and shut-down (DSS) cycles, meeting practical requirements for the ATR catalysts. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Kerosene Autothermal reforming Hydrogen Rh catalyst Hydrotalcite
1. Introduction There is tremendous interest in highly efficient methods for hydrogen production for power generation and fuel cell application. Steam reforming of hydrocarbons, mostly methane and naphtha (lower than C6 hydrocarbons), are generally conducted for industrial hydrogen production [1]. These gaseous resources favor generation of hydrogen near the natural gas and naphtha fields, requiring high-energy-required compression process to liquefy the resource or H2, or expensive pipeline infrastructure for easier transport. Kerosene is a common commodity in some residential areas as a heating fuel, which contains saturated hydrocarbons with typical carbon number between 6 and 16. Kerosene, which can be easily transported, thus is a great candidate as a source of on-site hydrogen
* Corresponding author. Tel.: +81 355720264; fax: +81 355720269. E-mail address: [email protected] (M. Harada). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.10.009
production for residential fuel cell system. Catalytic steam reforming of kerosene can be an attractive method for hydrogen production [2–6]; however, because of its high carbon number, deposition of carbon species on the catalyst surface causes deactivation of the catalysts and plugging of the reactor, which must be avoided completely. High temperature is required to attain high yield of hydrogen because of unavoidable thermodynamic requirement, typically operated at above 1000 K. Steam reforming of hydrocarbons is highly endothermic, demanding extensive external heat supply. Autothermal reforming (ATR) instead provides the required heat for the hydrogen production by concurrent exothermic reactions of hydrocarbon oxidations requiring no external heat [7]. This method has been proven effective for hydrogen production from methane [8,9], higher hydrocarbons [10,11], and biomass-derived oxygenates [12–14]. Addition of O2 to steam also accelerates rates of hydrocarbon conversions, shortening contact time, and also tends to suppress carbon deposition because of high reactivity of oxygen [7,9]. These merits make ATR attractive for conversion of kerosene.
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Hydrotalcite, anionic clay, is known to be an efficient support material for the reforming of hydrocarbons, especially with nickel metals [15]. It is composed of brucite-like sheet with exchangeable anions within the layers of the crystal structure, namely, Mg6Al2(OH)16CO34H2O. Takehira and Shihido and their co-workers extensively studied Ni/hydrotalcite catalysts for reforming of methane and propane, and showed that the reduction at a high temperature leads to the formation of small nickel particles on Ni– Mg–O solid solution with mechanical spacer of Al2O3 or MgAlO4 [15]. This material provides moderately high surface area (100 m2 g 1) even at high reaction temperatures used for the reforming [15]. Various metal ions can intercalate into its layered structure, leading to highly dispersed active components for the reforming after appropriate treatment (e.g., reduction). Rh has been reported as one of the most reactive and stable metals for steam reforming of hydrocarbons [1,2]. The supported Rh catalyst has high tolerance against coke deposition during these reactions. In addition, deep desulfurization process is required before hydrocarbon is passed over the supported metal catalyst because the metals are susceptible to poisoning by sulfur compounds contained in hydrocarbons [1,16]. It was also suggested that the sulfur-poisoned metals are sensitive to carbon deposition [16]. This study deals with kerosene deeply desulfurized prior to ATR using Rh/hydrotalcite catalysts. In a practical point of view, shaping of the catalyst is critical to attain the most of the intrinsic catalyst performance [1]. To avoid plugging of the gases, micro- or milli-meter order sized pellets and spheres are preferred [1]. This size and shape of the catalysts, however, lead to strict internal mass transfer limitations prevalent especially at high temperatures operated during the reforming. Thus the catalysts should be designed to make active components, such as active Rh metals, localized intentionally at the surface of the shaped supports. This design also minimizes the amount of used expensive noble metals, e.g., Rh. The shaped catalyst should also have high physical strengths to be tolerant against high flow rates of reactants and products at the required high temperatures. If residential fuel cells are indeed operative with on-site production of hydrogen production from reforming of hydrocarbons, daily start-up shut-down (DSS) process is inevitable. During these DSS operations, the catalyst is exposed to oxidative conditions in H2O at high temperatures. After the shut-down, the catalyst should be again reactive immediately when hydrogen is needed. The catalyst must possess high stability during such severe reduction (reforming)–oxidation (shut-down in H2O atmosphere) cycles.
We herein demonstrate the study of Rh/hydrotalcite catalysts, which can be directly applied for a practical use of ATR of kerosene. First attention is given to develop a sphere-shaped catalyst possessing a physically strong core of the support and surfaceconcentrated Rh particles with high dispersion. Next ATR performance of the catalyst was examined especially focusing on the formation of C2–C3 hydrocarbon byproducts as an indication of the catalytic activity and stability. The catalysts were also tested for the DSS operations. 2. Experimental 2.1. Material and preparation of the catalyst M-30 hydrotalcite, which originally has a diameter of 4 mm, was obtained from Sasol-Japan. Rh(NO3)3 (Tanaka Kikinzoku Kogyo, 99.99%) was used for pore-filling impregnation method. The concentration of 4.5 wt% Rh(NO3)3 aq. was mixed with 2 wt% NaOH to reach pH 3.2–3.8. Deionized water was added to this solution to attain adequate amount of the solution. The sphereshaped hydrotalcite support treated at 1223–1523 K in static air was put in a round-bottomed flask. The prepared solution was gradually added into the flask while it was shaken manually. When the surface of the support spheres appeared homogeneously colored, the flask was set to a rotary evaporator. The water was removed by heating at 353 K under vacuum. The catalyst was moved to a ceramic dish and treated at 773 K for 3 h in static air. Then the catalyst was completely washed with deionized water to remove Na. The catalyst was again dried at 393 K in an oven, followed by treatment in flowing H2 at 773 K for 2 h and subsequent treatment in flowing N2 at 1223–1523 K. Fig. 1 illustrates the picture and image of the used catalysts sphere. The Rh/Al2O3 catalyst (2.0 wt%-Rh, 3.2 mm diameter-sphere, aAl2O3, core/shell Al2O3/Rh structure) was obtained from Tanaka Kikinzoku Kogyo K.K. and used as a reference. The catalyst was treated at 1223 K in flowing N2 before ATR. 2.2. Catalytic measurements Fig. 2 shows schematic of the experimental setup for ATR. O2 was introduced by using a mass flow controller (Brooks instrument, 5850E). Flow rates of H2O and deeply desulfurized kerosene were controlled using liquid pumps (Flom Corp., KP12-33). H2O gasified in the first carburetor held at 473 K moved to the second carburetor held at 473K where kerosene was injected and gasified.
Fig. 1. Schematic image of the Rh/MgAlOx catalysts with core–shell structure.
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Fig. 2. Schematic representation of the setup for activity test.
This gasified mixture was then mixed with oxygen thoroughly in a static mixer, and introduced to the reactor containing the active catalysts. The reactor used in this study consisted of SUS316 tube (O.D. 25.4 mm, I.D. 21.1 mm). The reactor was insulated thoroughly with an insulation material (diameter 270 mm). The catalyst bed was held on the quartz wool and a dish was put as a separator on the top of the catalyst bed. Length of the catalyst bed was 100 mm and the catalyst volume was 32 cm3. Inlet temperature of the reactor was rigorously controlled at 473 K. Note that this inlet temperature was found to significantly affect inlet temperature of the catalyst bed and thus the catalytic ATR performance. Temperature measurement was operated with a Ktype thermocouple adjustable through the catalyst bed. Reactant and product distribution along the catalyst bed was measured by using a branched-type reactor. The branches were equipped at the positions, 10, 0, 10, 30, 60 and 100 mm from the inlet of the catalyst bed. The remaining reactant and products which can be liquefied at 273 K was condensed and separated from gaseous products using a heat exchanger and a drain trap. Qualification and quantification of the gaseous products were analyzed by a gas chromatograph (GL-Science, CP-4900) equipped with two thermal conductivity detectors. DSS study was carried out by cooling down the catalyst bed to 473 K after the ATR test in flowing H2O. 2.3. Characterization of the catalyst Actual Rh contents loaded on the catalyst support after porefilling impregnation was determined using inductive coupling of plasma (ICP; SSI, SPS7800). Electron probe microanalysis (Shimadzu, EPMA-1600) was carried out to measure the distribution of Rh in the catalyst spheres. Compressive ultimate strength of the catalyst spheres treated at different temperatures was measured. The specific surface area of the catalysts after H2 reduction was determined by a BET method using BELsorp-mini (BEL, Japan). XRD measurement was performed with a Rigaku, RINT-TTR III instrument with monochromatized CuKa. Metal crystallite sizes were calculated from line broadening using the Scherrer’s equation. 3. Results and discussion 3.1. Catalyst properties First the strength against the heat treatment of the support sphere was examined. Fig. 3A shows the apparent volume changes of the catalyst core when treated in air at high temperatures (1223–1523 K) with treatment time. The core volume decreases more significantly with increasing treatment temperature. At each
treatment temperature, the core volume did not change drastically after 8 h treatment, indicating that the temperature was the main factor to determine the core volume rather than treatment time. Fig. 3B shows the changes in surface area with treatment time at different temperatures (1223–1523 K). With heat treatments at 1423 and 1523 K for 8 h, the surface area of the core dropped significantly from 86 to 10 m2 g 1 and 3 m2 g 1, respectively, and remained almost constant for further treatment time up to 40 h. The heat treatments at 1223 and 1323 K gradually reduced the core surface areas with increasing treatment time. After 40 h of the treatment, the surface areas were 55 and 28 m2 g 1 at 1223 and 1323 K, respectively. The apparent volume does not change significantly at such high temperature treatments while surface area significantly decreased, indicating that the pore of the core is blocked with time. High compressive ultimate strength of the spheres is required for the practical application because of the harsh conditions operated during the high-temperature autothermal reforming. As described later, the temperature during the autothermal reforming under the conditions investigated reached 1200 K, and thus the pretreatment at above this temperature was also required to avoid sintering of the support during the reaction. Table 1 compiles compressive ultimate strengths and sphere diameters of the cores, measured after heat treatments at 1223–1523 K for 40 h. The compressive ultimate strength increased with increasing heat treatment temperature. After the heat treatment at 1523 K, the sphere diameter shrank to 2.9 mm and the compressive ultimate strength became 0.90 MPa, reasonably high for the practical use. Based on these considerations, the heat treatment of the core at 1523 K was used for further study described below. Catalyst sphere treated at 1223 K (2.9 mm in diameter) was used for the impregnation by pore-filling method. EPMA result demonstrates that the Rh exists on the surfaces of the sphere in about 50 mm, as illustrated in Fig. 1. This structure ensures the absence of internal mass transfer that otherwise prevails during highly exothermic reactions, such as combustion and autothermal type of reactions. ICP measurement showed that the obtained sample after the impregnation and heat treatment in flowing N2 possessed 0.175 wt% of Rh. The average crystallite size of Rh can be estimated to be 28 and 84 nm after 1223 and 1523 K treatments, respectively, estimated from XRD patterns using Scherrer’s equation. 3.2. Catalytic performance of Rh/MgAlOx catalysts for ATR We discuss the effects of N2 treatment temperature after Rh was loaded on the surface of the hydrotalcite core. Fig. 4A shows
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Fig. 3. Relative volumes and specific surface areas of the hydrotalcite support core treated at different temperatures (1223–1523 K) in flowing air.
the H2 concentration as a function of time on stream for the Rh catalysts treated at different temperatures (1223–1523 K). H2 formation rate decreased with increasing the treatment temperature. For the catalyst treated at 1223 and 1323 K as well as the Rh/ Al2O3 reference catalyst, the H2 formation rate remained constant at 60% among gaseous products for 60 h period of this study. This decrease in H2 formation rate by high temperature heat treatment is attributable to the decrease in the number of Rh active sites on the catalyst surfaces by the treatment at such high temperatures, consistent with the increased crystallite sizes with the treatment temperature, estimated by XRD measurement. At 1523 K, the catalyst slightly deactivated at initial period of time on stream and then gave stable rates for H2 formation. No deposited coke on all the catalyst was detected after the reaction, confirming that the catalyst shows strong tolerance against coking. Fig. 4B shows the concentrations of C2H4 as a function of time on stream. The C2H4 concentrations increased with increasing N2 treatment temperature, consistent with the decrease in H2 concentrations (Fig. 4A). The higher C2H4 concentration implies a lack of steam reforming activity of the catalyst since C2H4 should also be reformed to COx species. The sample treated at 1223 K maintained the concentration of C2H4 of <0.01% for 60 h. The C2H4 concentration on the Rh/Al2O3 reference catalyst (treated at 1223 K) increased with increasing time on stream, and gave higher concentration of C2H4 than the Rh/MgAlOx sample treated at 1223 K. This increase of C2H4 concentration implies the catalyst deactivation, probably due to sintering of the active metal; i.e., the decrease in the number of active site. This result clearly indicates the beneficial effects of hydrotalcite as a starting support material to disperse an active metal component, and avoid sintering by strong interaction with the support material [15]. Steady-state H2 formation rates and C2H4 concentrations measured after 5 h are plotted in Fig. 5 as a function of different treatment temperature in N2 for the impregnated catalyst at LHSVs of 15, 20 and 25. It can be seen from Fig. 5A that H2 formation rates monotonically increased with decreasing N2 treatment temperature, and with increasing LHSV. On contrary, C2H4 concentrations Table 1 Core diameters and compressive ultimate strengths of the hydrotalcite support core treated at different temperatures (1223–1523 K) in flowing air. N2 treatment temperature (K)
Core diameter [5_TD$IF](mm Ø)
Compressive ultimate strength (MPa)
1223 1323 1423 1523
3.4 3.3 3.1 2.9
0.43 0.50 0.63 0.90
increased with increasing N2 treatment temperature and with decreasing LHSV. These results show that the catalyst treated at 1223 K give the highest productivity of hydrogen per mass of the catalyst. Fig. 6 shows the temperature profile and reactant and product distribution along the catalyst bed containing the catalyst treated at 1223 K. The temperature (inlet: 473 K) started increasing before reaching the catalyst bed at the position of 20 mm. Since no reaction occurred at this point (i.e., no thermal decomposition of kerosene occurred in the gas phase), this temperature rise was ascribed to radiant heat from the catalyst bed. At the very inlet of the catalyst bed, the temperature reached at 1020 K, and reached the maximum temperature of 1200 K at 10 mm from the inlet. After this point, temperature decreased along the catalyst bed. At the exit of the catalyst bed (at 100 mm from the inlet), the temperature was 1070 K, where thermodynamic equilibrium concentrations of H2, CO2, CO and CH4 are 71, 15, 14 and 0.02%, respectively. It is notable that the O2 consumption is rapid: at 10 mm of the catalyst bed, 97% of O2 was already consumed (Fig. 6). This rapid exothermic oxidation reaction is consistent with the temperature rise measured at the very inlet of the catalyst bed. At 30 mm from the inlet, O2 was completely consumed, wherein steam reforming and water gas shift were considered to take place. At this point, kerosene conversion is also considered to reach 100% based on the calculation in view of carbon balance. Major products obtained were H2, CO2 and CO. The concentration of H2 increased along the catalyst bed from 10 to 30 mm from the inlet and remained almost the same after 30 mm from the inlet. At 10 mm from the inlet, almost equivalent amounts of CO and CO2 were observed (20%). The CO concentration further decreased at 30 mm from the inlet due to water gas shift, consistent with the increase in H2 concentration at this point. The major byproducts observed were C2H4 (5%) followed by CH4 (3%), C3H6 (2%), C2H6 (0.5%) and C3H8 (0.1%) at 10 mm from the inlet of the catalyst bed. These byproducts are not favored from the thermodynamic point of view; the highest concentration of CH4 expected is only 0.02%. The concentrations of C2 and C3 byproducts decreased at 30 mm position, and diminished at 60 mm position (< 0.01%). These results indicate that the C2–C3 byproducts can be steam-reformed to generate more H2. Among the byproducts, the alkenes, C2H4 and C3H6, which can be formed from dehydrogenation of the cracking of higher hydrocarbons, are known to lead to carbon formation more easily than light alkanes, such as CH4. The reaction pathways to form these alkenes remain unclear: the alkenes can be formed as a result of cracking on the acid sites (mainly on the oxide support) or hydrogenolysis type of
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Fig. 4. H2 concentrations (A) and C2H4 concentrations (B) as a function of time on stream for ATR of kerosene at LHSV of 20 on the Rh/MgAlOx catalysts treated at different temperatures (1223–1523 K) in flowing N2 (S/C = 2.5, O2/C = 0.5, 101 kPa).
Fig. 5. H2 concentrations (A) and C2H4 yields (B) as a function of treatment temperature of the Rh/MgAlOx catalysts in flowing N2 for ATR of kerosene at LHSV of 15-25 (S/ C = 2.5, O2/C = 0.5, 101 kPa).
Fig. 6. Temperature and O2, H2, CO2 and CO concentrations (A) and CH4, C2H4, C2H6, C3H6 concentrations (B) as a function of position from the inlet of the catalyst bed for the ATR of kerosene on Rh/MgAlOx catalysts treated at 1223 K in flowing N2 (LHSV = 20, S/C = 2.5, O2/C = 0.5, 101 kPa).
reactions that occur on the Rh metal surfaces. The possibility of these alkene formations in the solely gas-phase reactions can be excluded because no O2 consumption and no product formation were observed at 0 mm from the inlet of the catalyst bed (i.e., no catalyst present). The abilities of minimizing formation of these
alkenes as well as of maximizing reactivity for steam reforming of alkenes without carbon deposition are required for stable and coke-tolerant catalysts. The DSS experiments were carried out to investigate tolerance of the catalyst for cooling–heating and oxidation–reduction cycles.
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the results of byproducts (C2–C3 hydrocarbons) observed during the DSS study. Consistent with the unchanged formation rates for H2, CO and CO2, the concentrations of C2–C3 product remained very low (<0.015%). The byproducts, especially olefins favor strong adsorption on surfaces, polymerizing to become carbon deposits to cause deactivation for ATR. The concentrations of these olefins are thus closely related to catalyst durability. The formation rates of C2H4 and C2H6, as major byproducts, increased only slightly, indicating that the catalyst has quite high tolerance for DSS cycles. Any notable structural changes were observed after the DSS operation by XRD measurements, also consistent with the stable catalytic performance. The Rh/MgAlOx catalyst developed in this study is a strong candidate of a highly stable catalyst for practical use of ATR of kerosene. 4. Conclusions [42_TD$IF]Pore-filling impregnation method achieved loading of Rh only on the surface of the sphere-shaped MG-30 hydrotalcite (diameter 3 mm), of which high compressive ultimate strength was attained (0.9 MPa) by the pretreatment at 1323 K in flowing air. EPMA measurements confirmed that the rhodium was successfully localized on the surface of the spheres (50 mm thickness). Among investigated catalysts, the impregnated catalysts treated in flowing N2 at 1223 K after the impregnation showed the best performance for ATR of kerosene. H2 production rate reached close to the value expected from thermodynamic equilibrium under the conditions used (60%). The maximum temperature reached 1200 K at the inlet of the catalyst bed and decreased to 1020 K towards the end of catalyst bed. The byproducts (mainly C1–C3 paraffins and olefins) over the optimized catalyst were significantly low (< 0.03%), indicative of high activity of the used sample. Stable and high rates for hydrogen production, close to the value expected from thermodynamic equilibrium, were still attained even at LHSV of 25. The catalyst durability against DSS shows high tolerance to coking and stable ATR performance, meeting practical requirements. Acknowledgement The part of this study contents were carried out under auspices of NEDO sponsored research project, entitled development of the basic technology of hydrogen production by ATR reforming. References
Fig. 7. H2, CH4, CO2 and CO concentrations (A) and C2H4, C2H6, and C3H6 concentrations (B) as a function of time on stream for ATR of kerosene on the Rh/ MgAlOx catalysts treated at 1223 K in flowing N2 during DSS cycles, and temperature vs. position from the inlet of he catalyst bed for the first DSS cycle (open circle) and the 6th DSS cycle (filled square) (C) (LHSV = 20, S/C = 2.5, O2/ C = 0.5, 101 kPa).
The ATR reactions were conducted for 5 h followed by cooling down to 473 K in flowing H2O, and the ATR reaction was repeated. This cycle was conducted 6 times. Fig. 7A shows the concentrations of H2, CO2, CO and CH4 as a function of time on stream of the DSS cycles. Concentrations of H2, CO2 and CO were unchanged throughout the catalytic test for 6 cycles, indicating the high stability of the prepared catalyst for DSS operations. Level of CH4 concentration was also unchanged at 0.15–0.20%. Fig. 7B shows
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Hydrogen production by autothermal reforming of kerosene over MgAlOx-supported Rh catalysts Makoto Harada a,d,*, Kazuhiro Takanabe b, Jun Kubota b, Kazunari Domen b, Takashi Goto c, Kazuya Akiyama c, Yasunobu Inoue d a
INPEX Corporation, 5-3-1 Akasaka, Minatoku, Tokyo, 107-6332, Japan Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo, 113-8656, Japan Nikki-Universal Co., Ltd., 7-14-1 Shinomiya, Hiratsuka, Kanagawa, 254-0014, Japan d Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomiokacho, Nagaoka, Niigata, 940-2188, Japan b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2009 Received in revised form 2 October 2009 Accepted 5 October 2009 Available online 12 October 2009
Autothermal reforming (ATR) of kerosene for hydrogen production was performed on the MgAlOxsupported Rh catalysts at LHSV of 15–25, S/C of 2.5, O/C of 0.5, at 101 kPa. Sphere-shaped supports with high compressive ultimate strength (0.90 MPa) were obtained from MG-30 hydrotalcite core (3 mm) by thorough heat treatment before impregnation of rhodium. Rhodium was loaded by pore-filling impregnation selectively to the surface of the sphere-shaped supports, confirmed by electron probe microanalysis. Stability tests on the prepared catalysts were performed, focusing on the small amount of C2–C3 hydrocarbons, concentrations of which reflect the catalytic activity and stability; i.e., low rates of C2–C3 olefin formation correspond to high activity of the catalysts. The reactor was designed to measure temperature profiles and gas distributions within the reactor (inner diameter 21.0 mm) and the catalyst bed (length 100 mm). The ATR reactions occur starting with an exothermic combustion of hydrocarbons, followed by an endothermic reforming. The maximum temperature reached 1200 K at the inlet of the catalyst bed and decreased to 1020 K towards the end of the catalyst bed. Among investigated catalysts, the catalysts treated in air at 1223 K gave the best performance for ATR of kerosene, giving H2 production reaching 60% of the exit gas. The concentration of the main byproduct, C2H4, over the optimized catalyst was lower than 0.03% at the exit of the reactor for 50 h of the study. The catalyst showed high tolerance to coking and high stability even at LHSV of 25 and daily start-up and shut-down (DSS) cycles, meeting practical requirements for the ATR catalysts. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Kerosene Autothermal reforming Hydrogen Rh catalyst Hydrotalcite
1. Introduction There is tremendous interest in highly efficient methods for hydrogen production for power generation and fuel cell application. Steam reforming of hydrocarbons, mostly methane and naphtha (lower than C6 hydrocarbons), are generally conducted for industrial hydrogen production [1]. These gaseous resources favor generation of hydrogen near the natural gas and naphtha fields, requiring high-energy-required compression process to liquefy the resource or H2, or expensive pipeline infrastructure for easier transport. Kerosene is a common commodity in some residential areas as a heating fuel, which contains saturated hydrocarbons with typical carbon number between 6 and 16. Kerosene, which can be easily transported, thus is a great candidate as a source of on-site hydrogen
* Corresponding author. Tel.: +81 355720264; fax: +81 355720269. E-mail address: [email protected] (M. Harada). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.10.009
production for residential fuel cell system. Catalytic steam reforming of kerosene can be an attractive method for hydrogen production [2–6]; however, because of its high carbon number, deposition of carbon species on the catalyst surface causes deactivation of the catalysts and plugging of the reactor, which must be avoided completely. High temperature is required to attain high yield of hydrogen because of unavoidable thermodynamic requirement, typically operated at above 1000 K. Steam reforming of hydrocarbons is highly endothermic, demanding extensive external heat supply. Autothermal reforming (ATR) instead provides the required heat for the hydrogen production by concurrent exothermic reactions of hydrocarbon oxidations requiring no external heat [7]. This method has been proven effective for hydrogen production from methane [8,9], higher hydrocarbons [10,11], and biomass-derived oxygenates [12–14]. Addition of O2 to steam also accelerates rates of hydrocarbon conversions, shortening contact time, and also tends to suppress carbon deposition because of high reactivity of oxygen [7,9]. These merits make ATR attractive for conversion of kerosene.
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Hydrotalcite, anionic clay, is known to be an efficient support material for the reforming of hydrocarbons, especially with nickel metals [15]. It is composed of brucite-like sheet with exchangeable anions within the layers of the crystal structure, namely, Mg6Al2(OH)16CO34H2O. Takehira and Shihido and their co-workers extensively studied Ni/hydrotalcite catalysts for reforming of methane and propane, and showed that the reduction at a high temperature leads to the formation of small nickel particles on Ni– Mg–O solid solution with mechanical spacer of Al2O3 or MgAlO4 [15]. This material provides moderately high surface area (100 m2 g 1) even at high reaction temperatures used for the reforming [15]. Various metal ions can intercalate into its layered structure, leading to highly dispersed active components for the reforming after appropriate treatment (e.g., reduction). Rh has been reported as one of the most reactive and stable metals for steam reforming of hydrocarbons [1,2]. The supported Rh catalyst has high tolerance against coke deposition during these reactions. In addition, deep desulfurization process is required before hydrocarbon is passed over the supported metal catalyst because the metals are susceptible to poisoning by sulfur compounds contained in hydrocarbons [1,16]. It was also suggested that the sulfur-poisoned metals are sensitive to carbon deposition [16]. This study deals with kerosene deeply desulfurized prior to ATR using Rh/hydrotalcite catalysts. In a practical point of view, shaping of the catalyst is critical to attain the most of the intrinsic catalyst performance [1]. To avoid plugging of the gases, micro- or milli-meter order sized pellets and spheres are preferred [1]. This size and shape of the catalysts, however, lead to strict internal mass transfer limitations prevalent especially at high temperatures operated during the reforming. Thus the catalysts should be designed to make active components, such as active Rh metals, localized intentionally at the surface of the shaped supports. This design also minimizes the amount of used expensive noble metals, e.g., Rh. The shaped catalyst should also have high physical strengths to be tolerant against high flow rates of reactants and products at the required high temperatures. If residential fuel cells are indeed operative with on-site production of hydrogen production from reforming of hydrocarbons, daily start-up shut-down (DSS) process is inevitable. During these DSS operations, the catalyst is exposed to oxidative conditions in H2O at high temperatures. After the shut-down, the catalyst should be again reactive immediately when hydrogen is needed. The catalyst must possess high stability during such severe reduction (reforming)–oxidation (shut-down in H2O atmosphere) cycles.
We herein demonstrate the study of Rh/hydrotalcite catalysts, which can be directly applied for a practical use of ATR of kerosene. First attention is given to develop a sphere-shaped catalyst possessing a physically strong core of the support and surfaceconcentrated Rh particles with high dispersion. Next ATR performance of the catalyst was examined especially focusing on the formation of C2–C3 hydrocarbon byproducts as an indication of the catalytic activity and stability. The catalysts were also tested for the DSS operations. 2. Experimental 2.1. Material and preparation of the catalyst M-30 hydrotalcite, which originally has a diameter of 4 mm, was obtained from Sasol-Japan. Rh(NO3)3 (Tanaka Kikinzoku Kogyo, 99.99%) was used for pore-filling impregnation method. The concentration of 4.5 wt% Rh(NO3)3 aq. was mixed with 2 wt% NaOH to reach pH 3.2–3.8. Deionized water was added to this solution to attain adequate amount of the solution. The sphereshaped hydrotalcite support treated at 1223–1523 K in static air was put in a round-bottomed flask. The prepared solution was gradually added into the flask while it was shaken manually. When the surface of the support spheres appeared homogeneously colored, the flask was set to a rotary evaporator. The water was removed by heating at 353 K under vacuum. The catalyst was moved to a ceramic dish and treated at 773 K for 3 h in static air. Then the catalyst was completely washed with deionized water to remove Na. The catalyst was again dried at 393 K in an oven, followed by treatment in flowing H2 at 773 K for 2 h and subsequent treatment in flowing N2 at 1223–1523 K. Fig. 1 illustrates the picture and image of the used catalysts sphere. The Rh/Al2O3 catalyst (2.0 wt%-Rh, 3.2 mm diameter-sphere, aAl2O3, core/shell Al2O3/Rh structure) was obtained from Tanaka Kikinzoku Kogyo K.K. and used as a reference. The catalyst was treated at 1223 K in flowing N2 before ATR. 2.2. Catalytic measurements Fig. 2 shows schematic of the experimental setup for ATR. O2 was introduced by using a mass flow controller (Brooks instrument, 5850E). Flow rates of H2O and deeply desulfurized kerosene were controlled using liquid pumps (Flom Corp., KP12-33). H2O gasified in the first carburetor held at 473 K moved to the second carburetor held at 473K where kerosene was injected and gasified.
Fig. 1. Schematic image of the Rh/MgAlOx catalysts with core–shell structure.
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Fig. 2. Schematic representation of the setup for activity test.
This gasified mixture was then mixed with oxygen thoroughly in a static mixer, and introduced to the reactor containing the active catalysts. The reactor used in this study consisted of SUS316 tube (O.D. 25.4 mm, I.D. 21.1 mm). The reactor was insulated thoroughly with an insulation material (diameter 270 mm). The catalyst bed was held on the quartz wool and a dish was put as a separator on the top of the catalyst bed. Length of the catalyst bed was 100 mm and the catalyst volume was 32 cm3. Inlet temperature of the reactor was rigorously controlled at 473 K. Note that this inlet temperature was found to significantly affect inlet temperature of the catalyst bed and thus the catalytic ATR performance. Temperature measurement was operated with a Ktype thermocouple adjustable through the catalyst bed. Reactant and product distribution along the catalyst bed was measured by using a branched-type reactor. The branches were equipped at the positions, 10, 0, 10, 30, 60 and 100 mm from the inlet of the catalyst bed. The remaining reactant and products which can be liquefied at 273 K was condensed and separated from gaseous products using a heat exchanger and a drain trap. Qualification and quantification of the gaseous products were analyzed by a gas chromatograph (GL-Science, CP-4900) equipped with two thermal conductivity detectors. DSS study was carried out by cooling down the catalyst bed to 473 K after the ATR test in flowing H2O. 2.3. Characterization of the catalyst Actual Rh contents loaded on the catalyst support after porefilling impregnation was determined using inductive coupling of plasma (ICP; SSI, SPS7800). Electron probe microanalysis (Shimadzu, EPMA-1600) was carried out to measure the distribution of Rh in the catalyst spheres. Compressive ultimate strength of the catalyst spheres treated at different temperatures was measured. The specific surface area of the catalysts after H2 reduction was determined by a BET method using BELsorp-mini (BEL, Japan). XRD measurement was performed with a Rigaku, RINT-TTR III instrument with monochromatized CuKa. Metal crystallite sizes were calculated from line broadening using the Scherrer’s equation. 3. Results and discussion 3.1. Catalyst properties First the strength against the heat treatment of the support sphere was examined. Fig. 3A shows the apparent volume changes of the catalyst core when treated in air at high temperatures (1223–1523 K) with treatment time. The core volume decreases more significantly with increasing treatment temperature. At each
treatment temperature, the core volume did not change drastically after 8 h treatment, indicating that the temperature was the main factor to determine the core volume rather than treatment time. Fig. 3B shows the changes in surface area with treatment time at different temperatures (1223–1523 K). With heat treatments at 1423 and 1523 K for 8 h, the surface area of the core dropped significantly from 86 to 10 m2 g 1 and 3 m2 g 1, respectively, and remained almost constant for further treatment time up to 40 h. The heat treatments at 1223 and 1323 K gradually reduced the core surface areas with increasing treatment time. After 40 h of the treatment, the surface areas were 55 and 28 m2 g 1 at 1223 and 1323 K, respectively. The apparent volume does not change significantly at such high temperature treatments while surface area significantly decreased, indicating that the pore of the core is blocked with time. High compressive ultimate strength of the spheres is required for the practical application because of the harsh conditions operated during the high-temperature autothermal reforming. As described later, the temperature during the autothermal reforming under the conditions investigated reached 1200 K, and thus the pretreatment at above this temperature was also required to avoid sintering of the support during the reaction. Table 1 compiles compressive ultimate strengths and sphere diameters of the cores, measured after heat treatments at 1223–1523 K for 40 h. The compressive ultimate strength increased with increasing heat treatment temperature. After the heat treatment at 1523 K, the sphere diameter shrank to 2.9 mm and the compressive ultimate strength became 0.90 MPa, reasonably high for the practical use. Based on these considerations, the heat treatment of the core at 1523 K was used for further study described below. Catalyst sphere treated at 1223 K (2.9 mm in diameter) was used for the impregnation by pore-filling method. EPMA result demonstrates that the Rh exists on the surfaces of the sphere in about 50 mm, as illustrated in Fig. 1. This structure ensures the absence of internal mass transfer that otherwise prevails during highly exothermic reactions, such as combustion and autothermal type of reactions. ICP measurement showed that the obtained sample after the impregnation and heat treatment in flowing N2 possessed 0.175 wt% of Rh. The average crystallite size of Rh can be estimated to be 28 and 84 nm after 1223 and 1523 K treatments, respectively, estimated from XRD patterns using Scherrer’s equation. 3.2. Catalytic performance of Rh/MgAlOx catalysts for ATR We discuss the effects of N2 treatment temperature after Rh was loaded on the surface of the hydrotalcite core. Fig. 4A shows
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Fig. 3. Relative volumes and specific surface areas of the hydrotalcite support core treated at different temperatures (1223–1523 K) in flowing air.
the H2 concentration as a function of time on stream for the Rh catalysts treated at different temperatures (1223–1523 K). H2 formation rate decreased with increasing the treatment temperature. For the catalyst treated at 1223 and 1323 K as well as the Rh/ Al2O3 reference catalyst, the H2 formation rate remained constant at 60% among gaseous products for 60 h period of this study. This decrease in H2 formation rate by high temperature heat treatment is attributable to the decrease in the number of Rh active sites on the catalyst surfaces by the treatment at such high temperatures, consistent with the increased crystallite sizes with the treatment temperature, estimated by XRD measurement. At 1523 K, the catalyst slightly deactivated at initial period of time on stream and then gave stable rates for H2 formation. No deposited coke on all the catalyst was detected after the reaction, confirming that the catalyst shows strong tolerance against coking. Fig. 4B shows the concentrations of C2H4 as a function of time on stream. The C2H4 concentrations increased with increasing N2 treatment temperature, consistent with the decrease in H2 concentrations (Fig. 4A). The higher C2H4 concentration implies a lack of steam reforming activity of the catalyst since C2H4 should also be reformed to COx species. The sample treated at 1223 K maintained the concentration of C2H4 of <0.01% for 60 h. The C2H4 concentration on the Rh/Al2O3 reference catalyst (treated at 1223 K) increased with increasing time on stream, and gave higher concentration of C2H4 than the Rh/MgAlOx sample treated at 1223 K. This increase of C2H4 concentration implies the catalyst deactivation, probably due to sintering of the active metal; i.e., the decrease in the number of active site. This result clearly indicates the beneficial effects of hydrotalcite as a starting support material to disperse an active metal component, and avoid sintering by strong interaction with the support material [15]. Steady-state H2 formation rates and C2H4 concentrations measured after 5 h are plotted in Fig. 5 as a function of different treatment temperature in N2 for the impregnated catalyst at LHSVs of 15, 20 and 25. It can be seen from Fig. 5A that H2 formation rates monotonically increased with decreasing N2 treatment temperature, and with increasing LHSV. On contrary, C2H4 concentrations Table 1 Core diameters and compressive ultimate strengths of the hydrotalcite support core treated at different temperatures (1223–1523 K) in flowing air. N2 treatment temperature (K)
Core diameter [5_TD$IF](mm Ø)
Compressive ultimate strength (MPa)
1223 1323 1423 1523
3.4 3.3 3.1 2.9
0.43 0.50 0.63 0.90
increased with increasing N2 treatment temperature and with decreasing LHSV. These results show that the catalyst treated at 1223 K give the highest productivity of hydrogen per mass of the catalyst. Fig. 6 shows the temperature profile and reactant and product distribution along the catalyst bed containing the catalyst treated at 1223 K. The temperature (inlet: 473 K) started increasing before reaching the catalyst bed at the position of 20 mm. Since no reaction occurred at this point (i.e., no thermal decomposition of kerosene occurred in the gas phase), this temperature rise was ascribed to radiant heat from the catalyst bed. At the very inlet of the catalyst bed, the temperature reached at 1020 K, and reached the maximum temperature of 1200 K at 10 mm from the inlet. After this point, temperature decreased along the catalyst bed. At the exit of the catalyst bed (at 100 mm from the inlet), the temperature was 1070 K, where thermodynamic equilibrium concentrations of H2, CO2, CO and CH4 are 71, 15, 14 and 0.02%, respectively. It is notable that the O2 consumption is rapid: at 10 mm of the catalyst bed, 97% of O2 was already consumed (Fig. 6). This rapid exothermic oxidation reaction is consistent with the temperature rise measured at the very inlet of the catalyst bed. At 30 mm from the inlet, O2 was completely consumed, wherein steam reforming and water gas shift were considered to take place. At this point, kerosene conversion is also considered to reach 100% based on the calculation in view of carbon balance. Major products obtained were H2, CO2 and CO. The concentration of H2 increased along the catalyst bed from 10 to 30 mm from the inlet and remained almost the same after 30 mm from the inlet. At 10 mm from the inlet, almost equivalent amounts of CO and CO2 were observed (20%). The CO concentration further decreased at 30 mm from the inlet due to water gas shift, consistent with the increase in H2 concentration at this point. The major byproducts observed were C2H4 (5%) followed by CH4 (3%), C3H6 (2%), C2H6 (0.5%) and C3H8 (0.1%) at 10 mm from the inlet of the catalyst bed. These byproducts are not favored from the thermodynamic point of view; the highest concentration of CH4 expected is only 0.02%. The concentrations of C2 and C3 byproducts decreased at 30 mm position, and diminished at 60 mm position (< 0.01%). These results indicate that the C2–C3 byproducts can be steam-reformed to generate more H2. Among the byproducts, the alkenes, C2H4 and C3H6, which can be formed from dehydrogenation of the cracking of higher hydrocarbons, are known to lead to carbon formation more easily than light alkanes, such as CH4. The reaction pathways to form these alkenes remain unclear: the alkenes can be formed as a result of cracking on the acid sites (mainly on the oxide support) or hydrogenolysis type of
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Fig. 4. H2 concentrations (A) and C2H4 concentrations (B) as a function of time on stream for ATR of kerosene at LHSV of 20 on the Rh/MgAlOx catalysts treated at different temperatures (1223–1523 K) in flowing N2 (S/C = 2.5, O2/C = 0.5, 101 kPa).
Fig. 5. H2 concentrations (A) and C2H4 yields (B) as a function of treatment temperature of the Rh/MgAlOx catalysts in flowing N2 for ATR of kerosene at LHSV of 15-25 (S/ C = 2.5, O2/C = 0.5, 101 kPa).
Fig. 6. Temperature and O2, H2, CO2 and CO concentrations (A) and CH4, C2H4, C2H6, C3H6 concentrations (B) as a function of position from the inlet of the catalyst bed for the ATR of kerosene on Rh/MgAlOx catalysts treated at 1223 K in flowing N2 (LHSV = 20, S/C = 2.5, O2/C = 0.5, 101 kPa).
reactions that occur on the Rh metal surfaces. The possibility of these alkene formations in the solely gas-phase reactions can be excluded because no O2 consumption and no product formation were observed at 0 mm from the inlet of the catalyst bed (i.e., no catalyst present). The abilities of minimizing formation of these
alkenes as well as of maximizing reactivity for steam reforming of alkenes without carbon deposition are required for stable and coke-tolerant catalysts. The DSS experiments were carried out to investigate tolerance of the catalyst for cooling–heating and oxidation–reduction cycles.
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the results of byproducts (C2–C3 hydrocarbons) observed during the DSS study. Consistent with the unchanged formation rates for H2, CO and CO2, the concentrations of C2–C3 product remained very low (<0.015%). The byproducts, especially olefins favor strong adsorption on surfaces, polymerizing to become carbon deposits to cause deactivation for ATR. The concentrations of these olefins are thus closely related to catalyst durability. The formation rates of C2H4 and C2H6, as major byproducts, increased only slightly, indicating that the catalyst has quite high tolerance for DSS cycles. Any notable structural changes were observed after the DSS operation by XRD measurements, also consistent with the stable catalytic performance. The Rh/MgAlOx catalyst developed in this study is a strong candidate of a highly stable catalyst for practical use of ATR of kerosene. 4. Conclusions [42_TD$IF]Pore-filling impregnation method achieved loading of Rh only on the surface of the sphere-shaped MG-30 hydrotalcite (diameter 3 mm), of which high compressive ultimate strength was attained (0.9 MPa) by the pretreatment at 1323 K in flowing air. EPMA measurements confirmed that the rhodium was successfully localized on the surface of the spheres (50 mm thickness). Among investigated catalysts, the impregnated catalysts treated in flowing N2 at 1223 K after the impregnation showed the best performance for ATR of kerosene. H2 production rate reached close to the value expected from thermodynamic equilibrium under the conditions used (60%). The maximum temperature reached 1200 K at the inlet of the catalyst bed and decreased to 1020 K towards the end of catalyst bed. The byproducts (mainly C1–C3 paraffins and olefins) over the optimized catalyst were significantly low (< 0.03%), indicative of high activity of the used sample. Stable and high rates for hydrogen production, close to the value expected from thermodynamic equilibrium, were still attained even at LHSV of 25. The catalyst durability against DSS shows high tolerance to coking and stable ATR performance, meeting practical requirements. Acknowledgement The part of this study contents were carried out under auspices of NEDO sponsored research project, entitled development of the basic technology of hydrogen production by ATR reforming. References
Fig. 7. H2, CH4, CO2 and CO concentrations (A) and C2H4, C2H6, and C3H6 concentrations (B) as a function of time on stream for ATR of kerosene on the Rh/ MgAlOx catalysts treated at 1223 K in flowing N2 during DSS cycles, and temperature vs. position from the inlet of he catalyst bed for the first DSS cycle (open circle) and the 6th DSS cycle (filled square) (C) (LHSV = 20, S/C = 2.5, O2/ C = 0.5, 101 kPa).
The ATR reactions were conducted for 5 h followed by cooling down to 473 K in flowing H2O, and the ATR reaction was repeated. This cycle was conducted 6 times. Fig. 7A shows the concentrations of H2, CO2, CO and CH4 as a function of time on stream of the DSS cycles. Concentrations of H2, CO2 and CO were unchanged throughout the catalytic test for 6 cycles, indicating the high stability of the prepared catalyst for DSS operations. Level of CH4 concentration was also unchanged at 0.15–0.20%. Fig. 7B shows
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