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Ind. Eng. Chem. Res. 2008, 47, 1404-1408
Effect of Magnesium Substitution into LaMnAl11O19 Hexaaluminate on the Activity of Methane Catalytic Combustion Tong Li and Yongdan Li* Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China
The effect of the substitution of manganese by magnesium on the activity and physicochemical properties of a LaMn1-xMgxAl11O19 hexaaluminate catalyst was investigated. The samples were prepared using a coprecipitation method and characterized by X-ray diffractometry (XRD), hydrogen temperature-programmed reduction (TPR), oxygen temperature-programmed desorption (TPD), and nitrogen adsorption. The results show that the Mg-substitution enhances the catalytic activity. A sample with a composition of LaMn0.7Mg0.3Al11O19 has a very good activity and T10% ) 704 K. XRD analysis suggests that the partial substitution of manganese by magnesium effectively suppresses the crystal growth along the [001] direction, which leads to an improved surface area. Although the increase in activity is partially attributable to the high surface area, TPD and TPR results confirm that the substitution also affects the oxygen adsorption property and the oxidation state of Mn ions in the hexaaluminate lattice. The reducibility, the amount of Mn3+ species, and the lattice oxygen mobility also are improved. 1. Introduction The combustion of fuels remains the main technology for heat production. Natural gas is the best fossil fuel for environmental requirements, because it gives the lowest CO2 emission for the same amount of energy production. However, its flame produces a substantial amount of NOx. Catalytic flameless combustion improves the efficiency of the energy production and reduces NOx emissions.1,2 Supported noble-metal catalysts, such as Pd/Al2O3 and Pt/Al2O3, show high activity for combustion, even below 623 K;3 however, their commercial exploitation is hampered by the relatively high volatility of their oxides, the ease of sintering at temperatures above 1073 K, and high cost.4 Furthermore, at high conversion levels, it is believed that the complete combustion of residual hydrocarbons is diffusioncontrolled rather than reaction-controlled, so that the reaction rate is independent of the number of active sites but is dependent on the available surface area.5 Therefore, thermally stable catalysts with a high specific surface area (SSA) are desirable. Hexaaluminate (HA) structured compounds are interesting as high-temperature methane combustion (MC) catalysts, because of their excellent thermal stability and high catalytic activity. The high resistance to sintering is attributed to their peculiar layered structure, which consists of γ-Al2O3 spinel structural blocks intercalated by planes (called mirror planes) in which large cations (Ba, Ca, La, and Sr) are located. The structure leads to a thin planar morphology of its microcrystals.6 In many published works, HA retained a large SSA (i.e., >20 m2/g), even at a temperature of ∼1473 K. Preparation techniques have been proven to have a profound effect on the performance of HA. Alkoxide hydrolyzation and carbonate coprecipitation have been the often-employed preparation methods.5,7-9 Among the investigated compositions, La-HAs (hexaaluminates with La as the mirror plane cation) have received much interest, because of their higher catalytic activity and resistance to poisoning by sulfur compounds and carbonates than that of BaHAs and Sr-HAs.10-12 The substitution of transition-metal ions * To whom correspondence should be addressed. Tel.: +86-222740-5613. Fax: +86-22-2740-5243. E-mail address:
[email protected].
(e.g., Mn, Fe, Cu) for Al in the spinel blocks enhances activity without decreasing the sintering resistance.13-16 These cations have also a stabilizing effect through a charge compensation mechanism. Particularly, the Mn-substituted HA is a better catalyst than the other metal-substituted ones, because a redox cycle of this element in the crystal lattice between the divalent and trivalent states enhances activity.13 Magnesia has been used as a support for MC catalysts, because of its better thermal stability than alumina.17-20 Groppi et al.21 reported that a HA with a composition of LaMg0.5Mn0.5Al11O19 has a higher specific activity than LaMnAl11O19 in a MC reaction. They proposed that the behavior is associated with an effect of Mg2+ stabilizing Mn ions at a higher oxidation state. Nevertheless, no further work has been published on this topic. The purpose of this work is to elucidate the effect of magnesium substitution on the structure, stability, and catalytic behavior. 2. Experimental Section 2.1. Catalyst Preparation. All the HA samples were prepared by a coprecipitation method. Solutions of the nitrates of lanthanum, aluminum, manganese, and magnesium, with a concentration of 1 M, were mixed in a beaker according to the prescribed atomic ratio. An aqueous solution of ammonium carbonate and the mixed nitrate solution were added dropwise at a same time into a flask at 333 K under vigorous stirring and with the pH maintained at 7.5-8. The formed slurry was aged at 333 K for 4 h and then washed with hot distilled water and filtrated to remove the nitrate and ammonium ions. The material was dried in an oven at 383 K for 24 h and calcined at 1373 K for 5 h. 2.2. Characterization. X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max 2500 v/pc diffractometer, using Cu KR radiation and a scanning rate of 8° 2θ/min. The patterns were compared with the A* Joint Committee on Powder Diffraction Standards (JCPDS) cards. The SSA was measured according to the Brunauer-Emmett-Teller (BET) method with N2 adsorption at 77 K on a CHEMBET-3000 apparatus. The hydrogen temperature-programmed reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) profiles
10.1021/ie070606x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/09/2008
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1405 Table 1. Catalytic Activities and Apparent Activation Energies of the HA Samples sample
composition
T10% (K)
T50% (K)
T90% (K)
Ea (kJ/mol)
1 2 3 4 5 6 7 8 9
LaMnAl11O19 LaMn0.9Mg0.1Al11O19 LaMn0.7Mg0.3Al11O19 LaMn0.6Mg0.4Al11O19 LaMn0.5Mg0.5Al11O19 LaMn0.3Mg0.7Al11O19 LaMn0.1Mg0.9Al11O19 LaMgAl11O19 silica
753 714 704 721 737 757 756 920 1013
868 833 818 829 857 874 889 974 1019
983 929 909 916 958 960 955 984 1023
95.4 74.9 76.2 83.1 80.1 74.5 83.0
were measured using a TPDRO 1100 apparatus with a thermal conductivity detector. During TPR, a 100 mg sample was used, with 6 vol % H2 in argon as the feed, starting from room temperature up to 1173 K at a heating rate of 10 K/min and a flow rate of 20 mL/min (at standard temperature and pressure, STP). The response was calibrated by the reduction of CuO powder. For the TPD measurement, the sample was pretreated in an O2/He mixed gas at 1073 K for 2 h. Subsequently, the samples were heated at a heating rate of 10 K/min and a flow rate of 20 mL/min (STP) in a helium stream from room temperature to 1173 K. Electron paramagnetic resonance (EPR) spectra were collected using a X-band (ν ) 9.78 GHz) EPR spectrometer (Bruker model A320) at room temperature. The magnetic field was swept, starting from 1000 G. The g values were determined from precise frequency and magnetic-field values. 2.3. Catalytic Activity. The MC reaction was conducted in a conventional flow system under atmospheric pressure. The catalytic material was palletized, crushed, and sieved to 4060 mesh. 300 mg of catalyst was loaded into a quartz microreactor with 600 mg of quartz beads of the same particle size. The feed was a mixture of 1.5 vol % CH4 and 98.5 vol % air. The gas hourly space velocity (GHSV) was 60 000 h-1 (STP). The outlet gas was analyzed on-line using a HewlettPackard model HP 4890 gas chromatograph. The activity of the catalyst is evaluated in terms of the temperatures at which the conversions attain 10%, 50% and 90% (i.e., T10%, T50%, and T90%, respectively). 3. Results 3.1. Activity. The composition of the catalysts used in this work and their combustion activities are summarized in Table 1. The conversion, as a function of reaction temperature, is presented in Figure 1. An experiment on the noncatalytic MC reaction was performed by feeding the reactant into the reactor, which was filled with quartz beads (sample 9). In this case, MC seems to be initiated by the radical formation and progresses through chain reactions in the gas phase, as characterized by the high initiation temperature and the steep rise in CH4 conversion. The light-off temperature (viz. T10%) was ∼1013 K. Therefore, it is assumed that the quartz beads and the reactor wall do not contribute to the conversion in a temperature range of interest (i.e., 623-973 K). The MC behavior over sample 8 is similar to that of the quartz beads. The activity of this sample seems to be very low (T10% ) 920 K). Samples 2-7, with 0 < x < 1 in LaMn1-xMgxAl11O19, initiate the reaction at much lower temperatures, with T10% in the range of 704-757 K, and allow complete conversion at T e 1000 K. The light-off temperature of MC decreases with the partial substitution of Mg to Mn, up to x ) 0.3, but increases for further substitutions (i.e., in the range of 0.4 < x < 1).
Figure 1. Methane conversion versus temperature over the hexaaluminate (HA) samples.
Figure 2. Arrhenius plot of the activity data of the samples.
The rate constants were recalculated with a first-order kinetic equation, r ) kPCH4, taking into account the excess O2, and Arrhenius plots were drawn for the catalysts in Figure 2. Good linear correlations are obtained for temperature, up to T ) 873 K, with conversions of
