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J. Phys. Chem. 1996, 100, 744-754
Comparative Study of Carbon Dioxide Reforming of Methane to Synthesis Gas over Ni/La2O3 and Conventional Nickel-Based Catalysts Zhaolong Zhang,† Xenophon E. Verykios,*,† Susan M. MacDonald,‡ and Stanley Affrossman‡ Department of Chemical Engineering and Institute of Chemical Engineering & High Temperature Processes, UniVersity of Patras, P.O. Box 1414, GR-26500 Patras, Greece, and Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow GL 1XL, UK ReceiVed: June 29, 1995; In Final Form: September 27, 1995X
Carbon dioxide reforming of methane to synthesis gas was studied by employing a Ni/La2O3 catalyst as well as conventional nickel-based catalysts, i.e., Ni/γ-Al2O3, Ni/CaO/γ-Al2O3, and Ni/CaO. It is observed that, in contrast to conventional nickel-based catalysts, which exhibit continuous deactivation with time on stream, the rate of reaction over the Ni/La2O3 catalyst increases during the initial 2-5 h and then tends to be essentially invariable with time on stream. X-ray photoelectron spectroscopy (XPS) studies show that the surface carbon on spent Ni/Al2O3 catalyst is dominated by -C-C- species that eventually block the entire Ni surface, leading to total loss of activity. The surface carbon on the working Ni/La2O3 catalyst is found to consist of -C-C- species and a large amount of oxidized carbon. Both XPS and secondary ion mass spectrometry results reveal that a large fraction of surface Ni on the working Ni/La2O3 catalyst is not shielded by carbon deposition. FTIR studies reveal that the enhancement of the rate of reaction over the Ni/La2O3 catalyst during the initial 2-5 h of reaction correlates well with increasing concentrations of La2O2CO3 and formate species on the support, suggesting that these species may participate in the surface chemistry to produce synthesis gas. It is proposed that the interaction between nickel and lanthanum species creates a new type of synergetic sites at the Ni-La2O3 interfacial area, which offer active and stable performance of carbon dioxide reforming of methane to synthesis gas over the stated catalyst.
1. Introduction The process of carbon dioxide reforming of methane (CH4/ CO2), which converts two of the cheapest carbon-containing materials into useful chemical products, i.e., synthesis gas (H2/ CO), has received considerable attention in recent years. Compared to steam reforming of methane (CH4/H2O) to synthesis gas, this process offers several advantages: (1) production of synthesis gas with a lower hydrogen-to-carbon monoxide ratio, which is suitable for use in Fischer-Tropsch synthesis to higher hydrocarbons; (b) utilization of CO2, which is considered to be an important greenhouse gas; (c) better use in chemical energy transmission systems, i.e., solar or nuclear energy or renewable energies.1,2 Numerous supported metal catalysts have been tested, including Ni-based catalysts,3-9 as well as supported noble metal (Rh, Ru, Pd, Pt, and Ir) catalysts,10-19 which have been found to exhibit promising catalytic performance in terms of methane conversion and selectivity to synthesis gas. The major problem encountered in this process is rapid catalyst deactivation by deposition of excess carbon on the catalyst surface. Catalysts based on noble metals are reported to be less sensitive to coking than Ni-based catalysts.6,8,10,14 However, considering the aspects of high cost and limited availability of noble metals, it is more practical, from the industrial standpoint, to develop Ni-based catalysts that are resistant to carbon deposition and exhibit high activity for the conversion of CH4/CO2 to synthesis gas. The reforming reaction of methane with carbon dioxide using a non-noble metal catalyst was first studied by Fischer and Tropsch.3 It was observed that the catalyst deactivated rapidly by formation of carbon on its surface. Rapid deactivation due * To whom correspondence should be addressed. † University of Patras. ‡ University of Strathclyde. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0744$12.00/0
to carbon deposition on supported Ni catalysts during the CH4/ CO2 reaction was later observed by many investigators.4-9 It is generally claimed that catalyst deactivation is due to coke formation within the pores of the catalyst, which leads to breakup of the catalyst particles. Gadalla et al.6,8 studied carbon dioxide reforming of methane over Ni supported on different carriers. They found that the nature of the carrier affects the catalytic performance of Ni to a significant extent and that no carbon deposition was obtained when reaction temperatures higher than 940 °C and CO2/CH4 ratios larger than 2 were applied. An inspection of the carbon limit diagram, obtained from thermodynamic calculations,20 reveals that the carbon-free performance is due to the fact that carbon deposition is thermodynamically unfavorable under these conditions. Rostrup-Nielsen21,22 observed that adsorption of sulfur atoms results in deactivation of the neighboring nickel atoms and that the rate of carbon formation decreases more rapidly with sulfur coverage than the reforming rate. This suggests that the ensemble for the reforming reaction is smaller than that required for nucleation of carbon whiskers.21,22 Based on this finding, the SPARG (sulfur-passivated reforming) process has been developed for CO2 reforming of methane.20 By partially sulfiding the Ni catalyst, the sites for carbon formation are blocked while sufficient sites for the reforming reaction are maintained. This permits the catalytic reaction to take place without significant coking problems. However, catalytic activity is sacrified to a large extent. Addition of an oxide of strong basicity (e.g., alkali, alkaline oxide) to Ni-based catalysts has been known to be an efficient way for reduction of coking. Recently, Yamazaki et al.9 obtained carbon-free operation of carbon dioxide reforming of methane at 850 °C by addition of CaO to Ni/MgO catalyst. Kinetic studies showed that the CaOpromoted catalyst has higher affinity for CO2 chemisorption. It was reasoned that the enhanced CO2 chemisorption may promote © 1996 American Chemical Society
CO2 Reforming of Methane to Synthesis Gas the reaction with coke precursors from methane, thus preventing accumulation of coke. However, a significant reduction in activity of the Ni/MgO catalyst was observed by addition of the strongly basic CaO component. A recent study conducted by Zhang and Verykios23 shows that, in sharp contrast to regular Ni-based catalysts, which show continuous deactivation with time on stream, appropriately activated Ni/La2O3 catalysts exhibit significant enhancement of activity with time on stream during the initial 2-5 h of reaction, which then tends to be essentially invariable with time on stream for at least 100 h of reaction. It was found that the activity of the Ni/La2O3 catalyst is the same order of magnitude as that of Ni/γ-Al2O3 and is approximately 1 order of magnitude higher than that of alkali or alkaline-promoted Ni/γ-Al2O3 catalysts, which are commercial reforming catalysts. Detailed kinetic studies24 suggest that a new surface structure, which is more active toward the CH4/CO2 reaction and more stable, is formed on the Ni/La2O3 catalyst surface following exposure to the reaction mixture. In the present study, the surface and bulk structure of the Ni/La2O3 catalyst was investigated by H2 chemisorption, FTIR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS) techniques. For comparison, similar studies were also conducted on Ni/γ-Al2O3, Ni/CaO, and Ni/CaO/γAl2O3 catalysts, which exhibit different catalytic behavior. The purpose of the present study is to reveal any correlation between activity enhancement during the initial 2-5 h of reaction and alteration of surface and/or bulk structure of the Ni/La2O3 catalyst and to study differences in surface structures between the Ni/La2O3 and the other Ni-based catalysts that might be responsible for the different catalytic behavior. 2. Experimental Section 2.1. Catalyst Preparation. Ni/La2O3, Ni/CaO, Ni/γ-Al2O3, and Ni/CaO (10 mol%))/γ-Al2O3 catalysts, all containing 17 wt% Ni, were prepared by the wet-impregnation method using nitrate salt as the metal precursor.24,25 A weighed amount of nickel nitrate (Alfa Products) was placed in a 100 mL beaker, and 20 mL of distilled water was added. After 30 min, the appropriate weight of the support (La2O3, CaO, γ-Al2O3, or CaO (10 mol %)/γ-Al2O3) was added under continuous stirring. The slurry was heated to ca. 80 °C and maintained at the temperature until the water was evaporated. The residue was then dried at 110 °C for 24 h and was subsequently heated to 500 °C under N2 flow for 2 h for complete decomposition of the nitrate. After this treatment, the catalyst was reduced at 500 °C in H2 flow for at least 5 h. 2.2. Kinetic Measurements. Kinetic studies under differential conditions and studies under integral reactor conditions were conducted in a conventional flow apparatus consisting of a flow measuring and control system, a mixing chamber, a quartz fixed-bed reactor (i.d. 3 mm), and an on-line gas chromatograph.24,25 For the differential operation, the feed flow rate was adjusted to be between 100 and 400 mL/min, while the amount of catalyst loaded was between 5 to 10 mg. The catalyst was in powder form (e0.02 mm) and was diluted with R-Al2O3 (one portion of catalyst was diluted with 2-3 portions of R-Al2O3). Rate limitation by external or internal mass transfer was proven to be negligible by applying suitable criteria. For the integral operation, smaller flow rates and larger amounts of catalyst (30-50 mg) were employed. The temperature of the catalyst bed was measured by a chromel-alumel thermocouple, and it was kept constant to within (2 °C. Analysis of the feed stream and reaction mixture was performed using the thermal conductivity detector of a gas chromatograph. A
J. Phys. Chem., Vol. 100, No. 2, 1996 745 carbosieve S-II 100/120 column was used to separate H2, N2, CO, CH4, CO2, and H2O. Prior to reaction, the catalyst was reduced again, in situ, at 750 °C in H2 flow for 1-2 h. 2.3. Catalyst Characterization. H2 Chemisorption. Nickel dispersion was determined by static equilibrium adsorption of H2 at room temperature. A constant volume high-vacuum apparatus (Micromeritics, Accusorb 2100 E) was used for this purpose. The catalyst that had been freshly reduced at 700750 °C in H2 flow for 2 h in a separate apparatus was placed under dynamic vacuum at 200 °C for 2 h to desorb gases adsorbed on the surface. The sample was then exposed to H2 (200 Torr) for 1 h at 400 °C and subsequently placed under dynamic vacuum for approximately 10 h. It was then cooled to 25 °C, at which temperature hydrogen chemisorption isotherms were obtained. The adsorption isotherm was obtained in the H2 pressure range of 10-300 Torr. The quantity of H2 adsorbed at monolayer coverage was estimated by extrapolation of the linear portion of the isotherms to zero pressure, which is a standard procedure. XRD Measurements. A Philips PW 1840 X-ray diffractometer was used to identify the main phases of the Ni catalysts. Anode Cu KR (40 mV, 30 mA) was used as the X-ray source. 2.4. Surface Analysis. FTIR Study. A Nicolet 740 FTIR spectrometer equipped with a DRIFT (diffuse reflectance infrared Fourier transform) cell was used for the measurement of surface species formed on the Ni/La2O3, Ni/γ-Al2O3, and Ni/CaO/γ-Al2O3 catalysts. The cell, containing ZnSe windows, which were cooled by water circulating through blocks in thermal contact with the windows, allowed collection of spectra over the temperature range 25-700 °C and at atmospheric pressure. For all the spectra recorded, a 32-scan data accumulation was carried out at a resolution of 4.0 cm-1. An IR spectrum obtained under Ar flow (before the reaction) was used as the background to which the spectra, after reaction, were ratioed. For the Ni/γ-Al2O3 and Ni/CaO/γ-Al2O3 catalysts, both in situ and ex situ measurements were done, and the same results concerning surface species were obtained. Because the surface species on the working Ni/La2O3 catalyst require a long time (approximately 5h) to reach a stable level and because the IR cell cannot be exposed to the reaction conditions for a long period of time, measurements were carried out ex situ to follow the change of the surface species with time on stream, i.e., the treated sample was quickly quenched to room temperature and transferred to the FTIR sample holder for measurements. XPS and SIMS Studies. Samples were mounted on doublesided adhesive tape. For static SIMS samples were irradiated with a Vacuum Science Workshop ion gun using a 3 keV argon ion beam with a current of 2 × 10-10 A measured at the gun exit over an area of ca. 5 mm2. Spectra were obtained with a Vacuum Generators 12-12 quadrupole. Sample charging was compensated by an electron flood gun at 30 eV. XPS data were obtained with a Vacuum Science Workshop X-ray anode using magnesium KR radiation and a 100 mm hemispherical analyzer. The binding energies were corrected for charging by reference to adventitious carbon at 284.8 eV, and signal intensities were corrected for cross-section and escape depth using Wagner’s sensitivity factors.26 The La 3d, Al 2p, Ni 2p, O 1s, and C 1s signals were measured at a takeoff angle normal to the sample. C 1s spectra were deconvoluted using the procedure of Evans.27 3. Results The kinetic behavior obtained over the Ni/La2O3 catalyst is compared to that obtained over conventional Ni-based catalysts (i.e., Ni/CaO, Ni/γ-Al2O3, and Ni/CaO/γ-Al2O3). The chemical
746 J. Phys. Chem., Vol. 100, No. 2, 1996
Zhang et al.
TABLE 1: Characterization of Supported Ni Catalysts (before Reaction) catalyst with 17 wt % Ni γ-Al2O3 CaO/γ-Al2O3 CaO La2O3
statea
crystalline phasesb
metal dispersionc, %
reduced oxidized reduced oxidized reduced oxidized reduced oxidized
γ-Al2O3(+++), NiAl2O4(+++) γ-Al2O3(+++), NiAl2O4(+++) Ni(++), γ-Al2O3(+++), NiAl2O4(++) NiO(+), γ-Al2O3(+++), NiAl2O4(+++) Ni(+++), CaO(+++), Ca(OH)2(+), CaCO3(+) NiO(+++), CaO(+++), CaCO3(++) Ni(+++), La2O3(+++) NiO(+++), La2O3(+++)
4.0 3.0 0.5 1.0
a The reduced state implies that the catalyst was reduced in H flow at 750 °C for 2 h; the oxidized state means that the catalyst was oxidized 2 in O2 flow at 750 °C for 2 h. b Determined by XRD measurements: (+) weak intensity; (++) medium intensity; (+++) strong intensity. c Determined by H2 chemisorption assuming that each surface metal atom chemisorbs one hydrogen atom, i.e., H/Nisurface ) 1.
Figure 1. Alteration of the rate of reaction of carbon dioxide reforming of methane to synthesis gas as a function of time on stream over Ni/ γ-Al2O3, Ni/CaO (10 mol %)/γ-Al2O3, Ni/CaO, and Ni/La2O3 catalysts: T ) 750 °C, CH4/CO2/He ) 20/20/60 vol %, mcat ) 10 mg, and metal loading ) 17 wt %.
and physicochemical properties of the Ni/La2O3 and the other Ni-based catalysts, i.e., metal dispersion and bulk phase and surface composition of the working catalysts as revealed by H2 chemisorption, XRD, FTIR, XPS, and SIMS techniques, are presented. 3.1. Catalytic Properties. Kinetic studies of the CH4/CO2 reaction were carried out at 750 °C. Figure 1 shows the alteration of reaction rate (in units of mmol/g/s) obtained under differential reaction conditions over various Ni-based catalysts as a function of time on stream. The ratio of H2/CO obtained under the stated conditions was found to vary between 0.7 and 0.9. The deviation from the stoichiometric ratio (i.e., H2/CO ) 1) may be due to the occurrence of the reverse water-gas shift reaction. During the test (after several hours of reaction), the value of the H2/CO ratio was kept approximately constant. As shown in Figure 1, the Ni/γ-Al2O3 catalyst exhibits a higher reaction rate compared to the Ni/CaO catalyst. However, the deactivation rate of the Ni/γ-Al2O3 catalyst is also relatively higher than that of Ni/CaO. As has already been demonstrated in the process of steam reforming of methane, addition of alkali and/or alkaline promoters to Ni/γ-Al2O3 can significantly reduce coke formation on the surface while the reaction rate is sacrified to a certain extent.28,29 In the present study, 10 mol % CaO was added to the Ni/γ-Al2O3 catalyst. It is observed that the deactivation rate is indeed reduced by adding CaO promoter (Figure 1), as expected. The reaction rate obtained over the CaO-promoted Ni/γ-Al2O3 catalyst is found to be slightly higher than that over the Ni/γ-Al2O3 catalyst (Figure 1), a result that is unexpected in view of previous observations that the presence of basic promoters leads to a reduction of reaction rate.28,29 As will be demonstrated in the following section, this might be due to the presence of larger amounts of metallic Ni crystallites on the CaO/γ-Al2O3 support since basic CaO inhibits the reaction between NiO and Al2O3, which is forming a stable and inactive Ni2Al2O4 compound. Although their reaction and deactivation rates differ to a significant extent, the Ni/γ-Al2O3,
Ni/CaO, and Ni/CaO/γ-Al2O3 catalysts all exhibit continuous deactivation with time on stream. The primary reason for deactivation of these three Ni-based catalysts can be generally attributed to carbon deposition on the Ni crystallites.3-9,25 In sharp contrast to the behavior of the Ni-based catalysts described above, when Ni is supported onto a La2O3 carrier, it exhibits unique catalytic performance. The alteration of reaction rate obtained over the Ni/La2O3 catalyst as a function of time on stream is also shown in Figure 1. It is observed that the reaction rate over the Ni/La2O3 catalyst first increases with time on stream during the initial 2-5 h of reaction and then tends to be essentially invariable with time on stream, at least during 100 h of reaction. This leads to the suggestion that new catalytic sites, which are more active and stable toward the CH4/CO2 reaction, are formed on the Ni/La2O3 catalyst surface after exposure to the reaction mixture. Under integral reactor conditions (methane conversion of approximately 70%), an even faster catalyst deactivation was observed over the Ni/γ-Al2O3, Ni/CaO, and Ni/CaO/γ-Al2O3 catalysts. In certain cases, the reactor was fully blocked by excess carbon deposition. Carbon deposition was also observed on the Ni/La2O3 catalyst when operating under integral reactor conditions. However, the amount of carbon deposited was found to be much less than that on the other Ni-based catalysts. Furthermore, essentially no significant catalyst deactivation was observed over the Ni/La2O3 catalyst during more than 100 h of reaction under integral operating conditions. 3.2. Catalyst Characterization. Bulk Phase Composition. The major crystalline phases of the Ni-based catalysts, in the reduced and oxidized states, were examined by XRD, and the results are presented in Table 1. The oxidized (treated in O2 flow at 750 °C for 2 h) and reduced (treated in H2 flow at 750 °C for 2 h) Ni/γ-Al2O3 catalysts are found to consist of NiAl2O4 and γ-Al2O3 phases. The oxidized Ni/CaO/γ-Al2O3 catalyst consists of NiAl2O4 and γ-Al2O3, as well as a small quantity of NiO crystalline phases. Reduction of the oxidized catalyst leads to conversion of NiO and a fraction of NiAl2O4 to Ni crystallites. NiO, CaO, and CaCO3 phases are found in the oxidized Ni/ CaO catalyst, while Ni, CaO, Ca(OH)2, and CaCO3 phases are contained in the reduced Ni/CaO catalyst. The oxidized Ni/ La2O3 catalyst is found to consist of NiO and La2O3. After reduction, the NiO phase is reduced to Ni crystallites. As can be derived from the results shown in Table 1, there exists no detectable solid reactions between Ni (or NiO) and basic La2O3 or CaO. However, significant amounts of nickel are found in the form of NiAl2O4, a compound resulting from the solid reaction between NiO and the acidic Al2O3 support in the Ni/Al2O3 catalyst. The NiAl2O4, which has a stable spinel structure, is difficult to be reduced, as witnessed by the fact that it essentially survives H2 reduction at 750 °C (Table 1). However, addition of CaO (10 mol %) to γ-Al2O3 significantly affects the basicity/acidity of the support, leading to a profound
CO2 Reforming of Methane to Synthesis Gas
Figure 2. FTIR spectra between 1830 and 2430 cm-1 obtained over the Ni/γ-Al2O3 and Ni/CaO/γ-Al2O3 catalysts after exposure to the CH4/ CO2 mixture for ca. 30 min at various temperatures: (a) room temperature; (b) 300 °C; (c) 400 °C; (d) 450 °C; (e) 500 °C.
change in the bulk phase composition. It is observed that while the intensity of the NiAl2O4 phase is reduced, that of Ni (or NiO) is enhanced. This is probably attributed to the fact that the basic CaO is preferably reacting with the acidic Al2O3 to form stable calcium aluminate, thereby suppressing the reaction between NiO and Al2O3. The presence of detectable Ni crystallites on the CaO/γ-Al2O3 support, compared to that on the γ-Al2O3, may be the reason why the reaction rate over the Ni/CaO/γ-Al2O3 is slightly higher than that over the Ni/γ-Al2O3 catalyst (Figure 1). Since both Ni and/or NiO phases are observed in the Ni/CaO and Ni/La2O3 catalysts, the presence of Ni or NiO is not responsible for the observed difference in kinetic performance between Ni/CaO and Ni/La2O3 (Figure 1). H2 Chemisorption. The dispersion of nickel on the support was roughly estimated by H2 chemisorption at room temperature by assuming that each surface metal atom chemisorbs one hydrogen atom, i.e., H/Nisurface ) 1, and the results are shown in Table 1. It is found that the dispersion of Ni on all supports is very low (e4.0%). Since no large metallic Ni particles are observed by XRD in the Ni/γ-Al2O3 and Ni/CaO/γ-Al2O3 catalysts, the apparent low nickel dispersion on the high surface area of γ-Al2O3 and CaO/γ-Al2O3 carriers is probably due to the formation of NiAl2O4, which is not capable of chemisorbing hydrogen at room temperature. The unusually low nickel dispersion on La2O3 and CaO supports appears, at least partially, to be due to formation of large nickel particles on the relatively low surface area (
