Langmuir 1997, 13, 2055-2058
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CH4 TPR-MS of NiO/MgO Solid Solution Catalysts Yun Hang Hu† and Eli Ruckenstein* Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260 Received October 15, 1996. In Final Form: January 22, 1997X The activation of CH4 over NiO/MgO was investigated via temperature-programmed reaction mass spectroscopy (TPR-MS). Over pure MgO, CH4 is not activated, while over pure NiO, CH4 is activated to CO2 around 660 °C. There are two kinds of CH4 activations over the NiO/MgO solid solution catalysts prepared by the impregnation of MgO with Ni(NO3)2 solutions and calcination at 800 °C: the activation to CO and CO2 by the oxygen of the surface NiO dissolved in MgO, which is characterized by relatively sharp peaks, followed by the slow activation of CH4 by the bulk lattice oxygen bound to the Ni ions. The temperatures corresponding to the sharp peak and to the beginning of the slow activation decrease with increasing NiO content. The activation temperature is lower by about 230 °C over a catalyst prepared by mechanical mixing of NiO and MgO powders followed by calcination at 800 °C than over the catalyst prepared by impregnation, and it is lower by 120 °C than that over pure NiO. This is likely a result of the formation of a thin layer of NiO/MgO solid solution over MgO particles during the calcination of the mixture and of the defects formed in that layer and in the NiO particles.
1. Introduction The oxide solid solutions have important applications,1-11 especially when the solute is a transition metal oxide and the solvent an insulating, diamagnetic oxide. Their usefulness for catalysis was first recognized when the effect of the electronic interactions between cations on the catalytic activity and selectivity was investigated.4,5 The NiO/MgO system, which forms solid solutions over the entire molar fraction range,6 was employed as a catalyst for N2O decomposition, CO or CO2 hydrogenation, and steam reforming.7-11 Recently, we found that NiO/MgO has both excellent activity and selectivity as a CO2reforming catalyst and rather high stability.12,13 This happens because NiO/MgO is very effective in inhibiting the carbon deposition process, which could not be avoided over the other Ni-based catalysts.14,15 Those studies have also revealed that the catalytic activity and selectivity of NiO/MgO are affected by the preparation conditions of the catalyst, such as the precursor employed in the preparation of MgO,16 the Ni loading,13 and the calcination temperature.17 * To whom correspondence should be addressed † Permanent address: Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Kale, G. M. J. Am. Ceram. Soc. 1991, 74 (9), 2209. (2) Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. (a) J. Chem. Soc., Faraday Trans. 1 1984, 80, 1875; (b) J. Chem. Soc., Faraday Trans. 1 1984, 80, 1891. (3) Wang, G. W.; Itoh, H.; Hattori, H.; Tanabe, K. J. Chem. Soc., Faraday Trans. 1, 1983, 79, 1373. (4) Vrieland, E. G.; Selwood, P. W. J. Catal. 1964, 3, 536. (5) Cimino, A.; Schiavello, M.; Stone, F. S. Discuss. Faraday Soc. 1966, 41, 350. (6) Nussler, H. B.; Kubachewski, O. Z. Phys.Chem. (N. F.) 1980, 121, 187. (7) Cimino, A.; Bosco, R.; Indovina, V.; Schiavello, M. J. Catal. 1966, 5, 271. (8) Cimino, A.; Indovina, V.; Pepe, F.; Schiavello, M. J. Catal. 1969, 14, 49. (9) Cimino, A.; Pepe, F. J. Catal. 1972, 25, 362. (10) Takezawa, N.; Terunuma, H.; Shimokawabe, M.; Kobayashi, H. Appl. Catal. 1986, 23, 291. (11) Takahama, T.; Oshima, H.; Ueno, A.; Kotera, Y. Appl. Catal. 1983, 5, 59. (12) Ruckenstein, E.; Hu, Y. H. Appl. Catal. 1995, 133 (1), 149. (13) Hu, Y. H.; Ruckenstein, E. Catal. Lett. 1996, 36 (3&4), 145. (14) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Nature 1991, 352, 225. (15) Rostrup-Nielsen, J. R. Stud. Surf. Sci. Catal. 1988, 36, 73. (16) Ruckenstein, E.; Hu, Y. H. Appl. Catal., in press.
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The characteristics of the NiO/MgO system were investigated in some detail. They depend upon the calcination temperature (Tc)18 employed after impregnation. When Tc was in the range 400-600 °C, large NiO particles were formed over the surface of MgO. For Tc larger than 600 °C, the Ni2+ ions diffused into the matrix of MgO and the system evolved progressively toward a bulk NixMg(1-x)O solid solution. The surface chemical composition indicated that for Tc ) 800 °C a homogeneous solid solution was formed, which exhibited only a slight surface Ni segregation.18 The reduction of the Ni2+ of the NiO/MgO solid solution with H2 differed from that of the pure NiO.18-20 In NiO/MgO solid solutions, the Ni2+ cations located on the surface were reduced to Ni0 at about 500 °C and those located in the immediate subsurface layer at about 650 °C.19 Zecchina et al.20 concluded, using diffuse reflectance spectroscopy (DRS) measurements, that the Ni2+ ions located at the edges and corners of (100) terraces on the MgO surface were reduced at 400 °C, followed by the reduction of the five-coordinated squarepyramidal Ni ions at 500-700 °C. The reduction of the subsurface Ni2+ started only at 820 °C. The effects of the calcination temperature, treatment time, and Ni loading on the H2 reducibility of NiO have been studied by Parmaliana et al.21 Although NiO/MgO was employed and investigated as a catalyst in the CO2 or steam-reforming of CH4, the interactions between CH4 and NiO/MgO were not yet investigated. However, such a study is relevant, because it can allow the understanding of the role of the interactions between NiO and MgO in the oxidation process. In the present paper, the methane activation over NiO/MgO was studied via temperature-programmed reaction mass spectroscopy (TPR-MS). 2. Experimental Section 2.1. Catalyst Preparation. NiO Catalyst Preparation. NiO was prepared by the decomposition of Ni nitrate in air at 800 °C for 1.5 h. (17) Hu, Y. H.; Ruckenstein, E. Catal. Lett., in press. (18) Arena, F.; Horrell, B. A.; Cocke, D. L.; Parmaliana, A. J. Catal. 1991, 132, 58. (19) Highfield, J. G.; Bossi, A.; Stone, F. S. In Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 181. (20) Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1891. (21) Parmaliana, A.; Area, F.; Frusteri, F.; Giordano, N. J. Chem. Soc., Faraday Trans. 1990, 86 (14), 2663.
© 1997 American Chemical Society
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Figure 1. Temperature-programmed reaction mass spectrum of CH4 over pure NiO: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2. NiO/MgO Solid Solution Preparation by Impregnation. NiO/ MgO catalysts were prepared by impregnating a MgO powder (Aldrich Chemicals, 325 mesh, purity > 99%) with an aqueous solution of nickel nitrate. The obtained paste was dried at room temperature in air and then decomposed and calcined at 800 °C in air for 1.5 h. Longer calcination times have not affected the process. XRD investigations demonstrated that, under the preceding conditions, solid solutions of NiO/MgO were formed. Calcination of a Mechanical Mixture of NiO and MgO. The mechanical mixture of NiO and MgO powders was heated in air at a rate of 27 °C/min from 25 to 800 °C and further calcined at 800 °C for 1.5 h. 2.2. BET Surface Area. The surface areas were determined via nitrogen adsorption (BET method), using a Micromeritics ASAP 2000 instrument. 2.3. Temperature-Programmed Reaction of CH4. The catalyst (0.02 g), held on quartz wool in a vertical quartz tube (2 mm inside diameter), was degassed with He (40 mL/min) at 800 °C for 2 h, and then the temperature was allowed to decrease to room temperature. The temperature-programmed reaction was performed with a 25 mL/min mixture of CH4/He (1% CH4), at a rate of 10 °C/min. The analysis of gases was carried out continuously with an on line mass spectrometer (HP Quadrupole, 5971 Series Mass Selective Detector) equipped with a fast response inlet capillary system. From the obtained curves, the reducibility of the catalysts was calculated. Calculations also showed that the carbon balance was satisfied. The very small CO2 peaks below 500 °C. They are due to the presence of a trace amount of CO2 in CH4, were disregarded.
3. Results 3.1. Temperature-Programmed Reaction of CH4. The CH4 activation over MgO and NiO/MgO was investigated via temperature-programmed reaction mass spectroscopy (TPR-MS). The TPR-MS curve for pure MgO does not exhibit any CO2 or CO peaks. The pure NiO has one wide CO peak overlapped over one wide CO2 peak at about 700 °C (from 650 to 750 °C); the latter peak is much higher than the former (Figure 1). For NiO/MgO, the CH4 TPR-MS curve depends upon the composition and preparation conditions. For the 2.6 mol % NiO catalyst, prepared by impregnation and calcined at 800 °C, the TPR-MS curve indicates extremely low amounts of CO and CO2 only above 750 °C. The 9.7 and 13.9 mol % NiO catalysts have almost the same TPRMS curves: they exhibit a low CO2 peak and a high CO
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Figure 2. Temperature-programmed reaction mass spectrum of CH4 over 9.7 mol % NiO/MgO: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2.
Figure 3. Temperature-programmed reaction mass spectrum of CH4 over 13.9 mol % NiO/MgO: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2.
peak, both at 760 °C, followed by a CO tail up to 800 °C(Figures 2 and 3). The CH4 curve indicates that the activation of CH4 occurs around 760 °C, followed by a tail of CH4 consumption up to 800 °C. When the NiO content increases to 35 mol %, both a low CO2 peak and a high CO peak are present at 670 °C, followed by a CO tail up to 800 °C. The CH4 curve indicates that the CH4 activation occurs around 670 °C, followed by a tail of CH4 consumption (Figure 4). After the 9.7 mol % NiO catalyst, prepared by impregnation and calcined at 800 °C, was additionally calcined at 1100 °C for 1.5 h, the temperature of the peak remained unchanged, but the amount of CH4 activated decreased (Figure 5). The 9.7 mol % NiO catalyst, prepared by calcining the physical mixture of NiO and MgO powder at 800 °C,
CH4 TPR-MS of NiO/MgO Solid Solution Catalysts
Figure 4. Temperature-programmed reaction mass spectrum of CH4 over 35 mol % NiO: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2.
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Figure 6. Temperature-programmed reaction mass spectrum of CH4 over 9.7 mol % NiO prepared by the calcination of a physical mixture of NiO and MgO: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2. Table 1. BET Surface Area of NiO/MgO Catalysts and the Reducibilty of Their NiO BET surface reduced NiO reducibility area (mmol/g of cat.) (%) (m2/g)
catalyst (mol % NiO) 0 (MgOa) 2.6 9.7 9.7 (additionally calcined at 1100 °C) 13.9 35 9.7 (mechanical mixture)b 100 (NiO)a
0 ≈0 0.006 83 0.003 29
0 ≈0 15.3 7.4
21 37 50 23
0.005 07 0.012 28 0.030 58 0.044 11
8.2 9.2 68.7 99.1
31 20 28 3
a Calcined at 800 °C for 1.5 h. b The mixture of NiO and MgO powders was calcined at 800 °C for 1.5 h.
4. Discussion
Figure 5. Temperature-programmed reaction mass spectrum of CH4 over 9.7 mol % NiO calcined at 1100 °C for 1.5 h: (s) CH4; (- ‚ -) CO; (‚‚‚) CO2.
exhibits one CO2 peak and one CO peak at 530 °C, followed by a CO tail up to 800 °C. The CH4 activation occurs around 530 °C, followed by a tail of CH4 consumption (Figure 6). The activation temperature is in this case lower by 230 °C than that of the corresponding catalyst prepared by impregnation and lower by 120 °C than that of pure NiO. It is important to notice that CO2 was detected first and CO later, for all NiO/MgO catalysts. 3.2. Surface Area. The pure NiO and MgO have surface areas of 3 and 21 m2/g, respectively, whereas the surface area of NiO/MgO prepared by impregnation depends on composition (Table 1). For the 9.7 mol % NiO catalyst prepared by calcining the physical mixture of NiO and MgO at 800 °C, the surface area was 28 m2/g, which is larger than the sum (18 m2/g) of the pure NiO and MgO surface areas.
Over the pure NiO, a large amount of CH4 is activated and transformed mostly to CO2 and to a small amount of CO, between 650 and 750 °C (Figure 1). It is clear that the activation is due in this case mostly to the reaction between CH4 and the bulk oxygen. The initial temperature of CH4 activation over NiO/MgO (9.7-35 mol % NiO) is higher than that over pure NiO. Over the 9.7 and 13.9 mol % NiO catalysts, the lattice oxygen of NiO begins to react with CH4 at 750 °C, whereas, over the 35 mol % NiO, the lattice oxygen begins to react with CH4 at 670 °C (Figures 2-4). Over the 2.6 mol % NiO/MgO catalyst, there is hardly any CH4 activation. Over all the 2.6-35 mol % NiO/MgO catalysts, the reducibilities of NiO are lower than 20% (Table 1); i.e., up to 800 °C most of the NiO remains unreduced by CH4. The above observations indicate that the oxidative activation of CH4 by the lattice oxygen occurs with more difficulty over the NiO/MgO solid solution than over the pure NiO. This can be explained as follows: The electronegativity of Mg (1.293) is lower than that of Ni (1.8);22 hence, the binding of oxygen to Mg is stronger than that to Ni. Therefore, when NiO and (22) Albright, T. A.; Burdett, J. K. Problems in Molecular Orbital Theory; Oxford University Press: New York, 1992; p 22.
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MgO form a solid solution, the lattice oxygen has a higher stability in the NiO/MgO solid solution than in the pure NiO due to the coordination of each O atom to both Mg and Ni. With increasing Ni loading, the coordination number of each O with Mg decreases, whereas the coordination number with Ni increases, resulting in a decrease of the O stability. Consequently, the reaction temperature of the lattice O with CH4 should decrease with increasing Ni loading. Comparing the CH4 consumptions for the 9.7 and 13.9 mol % NiO catalysts (Figures 2 and 3), one can notice that the latter catalyst provides a smaller CH4 consumption and hence a lower NiO reducibility (Table 1), even though the oxygen of the 13.9 mol % NiO catalyst is somewhat more reactive. This happens because the lattice oxygen which reacts with CH4 originates mostly from the surface layer of the NiO/MgO solid solution and the 13.9 mol % NiO catalyst has a smaller surface area than the 9.7 mol % NiO catalyst (Table 1). The 9.7-35 mol % NiO/MgO TPR-MS curves have a long CO tail, after the main CO peak, up to 800 °C. The long CO tail implies a slow oxidation rate of CH4. It is reasonable to consider that the long CO tail is due to the reaction between CH4 and the bulk lattice oxygen, whose rate is affected by the slow diffusion of the oxygen toward the surface. For the 9.7 mol % NiO/MgO prepared by impregnation and calcined at 800 °C, the reducibility of NiO decreased after an additional calcination at 1100 °C for 1.5 h (Table 1). This is due to the decrease of the surface area caused by sintering (Table 1). It is interesting to note that the 9.7 mol % NiO catalyst, prepared by the physical mixing of powders of NiO and MgO followed by calcination at 800 °C in air for 1.5 h, has a relatively low activation temperature (530 °C) (Figure 6). This temperature is lower by 230 °C than that of the corresponding catalyst prepared by impregnation and lower by 120 °C than that of pure NiO. This happens because, in the mechanical mixture, only a small fraction of NiO is dissolved in MgO during calcination, generating a thin NiO/MgO solid solution layer19 over the surface of MgO particles. During this process, a large number of defects is likely to be formed, both in the NiO particles and in the solid solution layer, resulting in numerous sites with unsaturated coordination. Since CH4 can be more easily adsorbed on the unsaturated sites of the NiO with
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numerous defects than on a crystal with fewer defects, its activation is facilitated by the former catalyst. Therefore, the activation temperature can be lower than that for pure NiO. The 68% reducibility of the NiO of the mechanical mixture during the CH4 temperature-programmed reaction (Table 1) indicates that only a fraction of the NiO forms a solid solution with MgO. The TPR-MS curves reveal that the first product is CO2 over all catalysts. This indicates that the initial reaction between CH4 and oxygen takes place via the combustion mechanism
CH4 + 4NiO f CO2 + 2H2O + 4Ni followed by the CH4 activation to CO via the pyrolysis mechanism23
CH4 + Ni f ... f C-Ni + 2H2 C-Ni + NiO f CO-Ni + Ni Ni-CO f CO(g) + Ni Ni-CO + NiO f CO2 + 2Ni 5. Conclusion CH4 is activated to CO and CO2 first by the lattice oxygen of the surface layer of NiO dissolved in MgO and latter by the bulk lattice oxygen of NiO. The activation temperature decreases with increasing NiO content. Since the first product is CO2, the initial reaction between CH4 and oxygen takes place via the combustion mechanism, followed by the CH4 activation to CO via the pyrolysis mechanism. The mechanical mixture of NiO and MgO, calcined at 800 °C, provides a lower activation temperature than pure NiO. This probably happens because defects are generated during calcination both in the NiO particles and in the layer of NiO/MgO solution formed over the MgO particles. LA960986J (23) Hu, Y. H.; Ruckenstein, E. J. Catal. 1996, 158, 260.
