ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2010, том 84, № 3, с. 471-474
ХИМИЧЕСКАЯ КИНЕТИКА И КАТАЛИЗ
УДК 541.128
CATALYTIC PROPERTIES of La0.8Sr0.2CG0.5M0.5O3 (M = Cc, Ni, Cu)
IN METHANE COMBUSTION
© 2010 Wu Yue-huia, Luo Lai-taob, Liu Weib
aDepartment of Materials Chemistry, Nanchang Hangkong University, Jiangxi Nanchang 330063 China bInstitute of Applied Chemistry,Nanchang University, Jiangxi Nanchang 330031 China E-mail: <wuyuehui_c@sina.com> Received Wareh 23.2009
Abstract — A set of perovskite-type catalysts of general formula La0.8Sr0.2Co0.5M0.5O3 (M = Co, Ni, Cu) were prepared by alanine solution combustion method and used successfully for the methane combustion.The property of these materials were characterized by XRD and TPR measurements.The effects of transition-metal ions on site B on structure and performance of the catalysts were studied. The result indicated that the structure and catalytic activities of the catalysts can remarkably affect by transition-metal ions on site B, the decrease of the electrons filled in d orbitals in the atom on site B is propitious to increase the catalytic activity for methane combustion, and the sequences of catalysts activities are La0 8Sr0 2CoO3 > Ce0 8Sr0 2CoO3 > Ndo.sSro.2CoO3.
Due to more stringent legislation concerning emissions of NOX, CO and HC during the last two decades, catalytic combustion has emerged as a very promising technique for gas turbine applications. Several reviews published in this area clearly show the potential of this technique to achieve ultra-low levels of emissions. The most common fuel for gas turbines is natural gas, which mainly consists of methane. Compared with flame combustion, the temperature needed for catalytic combustion of natural gas is lower, and this leads to a decrease the formation of thermal NOx [1—5]. So far, the catalysts employed for catalytic combustion reactions are based on supported noble metals. However, besides being expensive, the latter easily sinterise and can form harmful volatile compounds. Among the possible substitutes for noble metals, perovskite-type transition metal mixed oxides, if prepared in the appropriate way, showed comparatively active, highly resistant to deactivation and much cheaper. Perovskites are mixed oxides of general formula ABO3, where A is usually a lanthanide ion and B is a transition metal ion. Both A and B can be partially substituted [6—8], leading to a wide variety of compounds of general formula
Aj _xAxBj _ B;O3, characterised by structural and electronic defects, owing to their non-stoichiometry. For the complete oxidation reactions, such as the catalytic combustion, the metal B ion is considered the responsible of catalytic activity, while the cation A, especially when partially substituted with an ion A' of different valence, determines the formation of crystal lattice vacancies and can stabilise unusual oxidation states for B, leading to different catalytic performance.
The activity of perovskites has been ascribed to their ionic conductivity [9, 10], to oxygen mobility within the lattice, to their reducibility and to oxygen sorption property. All selected perovskite-type compositions, a priori expected to be good catalysts, comprise transition metal oxides that are well known to confer a high oxidation activity through their easy change between two oxidation states and a certain degree of oxygen nonstoichiometry.
In this paper, perovskite La0.8Sr0.2Co0.5M0.5O3 (M = Co, Ni, Cu) mixed oxides were prepared by alanine solution combustion method and characterized by XRD and TPR measurements; their catalytic activities were determined in CH4 combustion; in addition, effects of transition-metal ions on site B on structure and performances of the catalysts were studied.
EXPERIMENTAL
Catalyst Preparation. Alanine solution combustion method: Briefly, Lanthanum, strontium and cobalt nitrates in a desired molar ratio were dissolved in a deionized water with constant stirring. To this solution, DL-alanine was added, keeping the organic agent : cobalt nitrate mole ratio as 2.4 : 1. DL-alanine react with the metal nitrates and form complexes with metal cations in aqueous solution. This guarantees a good solution homogeneity, probably avoiding the preferential precipitation of ionic species. The overall combustion reactions can be written as follows:
0.8La(NO3)3 + 0.2Sr(NO3)2 + Co(NO3)3 +
• Lao.8Sro.2CoO3 +
+ nC3H7NO2 + (3.75n - 5.7)O2
+ 3nCO2 + 3.5nH2O + (2.4 + 0.5n)N2.
20
30
40
50
60
70 80 29, deg
The catalysts T50 T100 kJ/mol Д110), nm (£2)1/2 x 103
La0. 8Sr0.2CoO3 470 550 66.6 13.04 4.55
La0. 8Sr0.2CO0.5Ni0.5O3 545 630 74.8 22.32 4.15
La0. 8Sr0.2Co0.5Cu0.5O3 555 660 75.9 23.89 4.06
Temperature-programmed reduction (TPR) was carried out in an in-house apparatus over 0.1 g catalyst. The samples were heated from room temperature to 700°C in N2 (35 ml/min) at a rate of 10 K/min in order to remove any impurities. After cooling to room temperature under N2, a gas mixture consisting of H2 and N2 (5 : 95 v/v) was introduced into the system and heated at a rate of 10 K/min for recording the TPR spectra.
RESULTS AND DISCUSSION
Fig. 1. XRD patterns of the catalysts; (a) La0 ^S^ 2CoO3; (b) La0.8Sr0.2Co0.5Ni0.5O3; (c) La0.8Sr0.2Co0.5Cu0.5O3.
The prepared aqueous solutions were transferred into a ceramic dish and placed into an oven, preheated at 450°C for 1 h. Then the powdered catalysts were calcined at 700°C for 2 h. The synthesized powders were pressed and sieved to a size of 60—80 meshes for the activity evaluation.
Measurement of Catalytic Activities. The catalytic combustion of methane were performed in a continuous microreactor by feeding a gaseous mixture of CH4 (2 vol. %), O2 (12.5 vol. %) and N2 (rest) over 100 mg catalyst with 20000 h-1 GHSV and 350-650°C reaction temperature. The gas composition was analyzed before and after the reaction by an online gas chromatography with thermal conductor detector (TCD), connected with a computer integrator system and Po-rapak Q column. The activity of the catalysts was assigned as CH4 conversion.
Characterisation. Powder X-ray diffraction (XRD) data were obtained using an X-ray diffractometer (type D8/ADVNCE made in German) over the range 20° < 20 < 80°, at room temperature, operating at 40 kV and 30 mA, using CuKa radiation combined with the nick-le filter.
Catalytic activity, apparent activation energy (Ea), average
crystal size (L(110)) and lattice distortion ((s2)1/2) of the catalysts
Structure of the Catalysts. The X-ray diffraction patterns of La08Sr02CoO3, La0.8Sr02Co0.5Ni05O3 and La0.8Sr02Co0.5Cu05O3 mixed oxides are shown in Fig. 1. The diffraction peaks are observed with 20 values of 23.4, 33.3, 40.9, 47.8 and 59.4°, which correspond to (012), (110), (202), (024) and (214) planes of La0.8Sr0.2CoO3 mixed oxides, respectively. The results of XRD clearly indicate that all the samples with different transition-metal ions on site B have the rhom-bohedral distorted perovskite structure [11]. For the evaluation of average crystallite size, Scherrer equation L = 0.9X/(p cos 0) was used. Here, L is the average crystallite size, X, the X-ray wavelength (0.154 nm), p, the half-peak width and 0, the diffraction angle. Table shows the average crystallite size of the catalysts. Comparing the catalysts with different transition-metal ions on site B, we find that the order of the X-ray particle size of catalysts are Lao.8Sra2CoO3 < La0.8Sr02Co0.5Ni05O3 < La0.8Sr02Co0.5Cu05O3, indicating the X-ray particle size of La08Sr02CoO3 catalyst is the smallest.
H2-TPR Studies. H2-TPR can provide information concerning the reducibility of different chemical species presented in the catalyst as well as the degree of interaction between metal-metal and metal-support. The TPR profiles ofLao.8Sr02CoO3, La0.8Sr02Co0.5Ni05O3 and La0.8Sr0.2Co0.5Cu0.5O3 mixed oxides are shown in Fig. 2 and there are two H2-reduction peaks (a and p), respectively, the low temperature peak (a peak) and the high temperature peak (p peak). Because La3+ and Sr2+ of A-site were not reduced by H2 in the scope of experiment, TPR profiles ofLao.8Sr02CoO3, La0.8Sr02Co0.5Ni05O3 and La0.8Sr02Co0.5Cu05O3 provide useful information about the reducibility of transition-metal ions on site B in the catalysts. The peak a may be attributed to chemi-sorption oxygen and the partial reduction of Co3+, Ni3+, Cu3+, i.e., Co3+ — Co2+, Ni3+ — Ni2+, Ni2+ —- Ni0, Cu3+ — Cu2+, Cu2+ — Cu0, with the catalysts still preserved in the perovskite-type phase structure as a whole. The temperature of a-peak reflects binding capacity between transition-metal ions on site B and oxygen. Most likely, peak p corresponds to the reduction of Co2+ to Co0, which leads to the breakdown of the perovskite-type phase. Because Ni3+, Cu3+ is more reducible than Co3+. Clearly, however, the further detailed study of the correlation between the transition-
CATALYTIC PROPERTIES OF La0.8Sr02Co0.5M05O3
473
в
a
a
b
c 1 1 1 1 1 1 1
a, % 100
80
60
40
20
0
400
500
600
700 T, °C
200
400
600 T, °C
Fig. 3. CH4 conversion a as a function of temperature over different catalysts; (a)—(c) — see Fig. 1.
Fig. 2. H2-TPR profiles of the catalysts; (a)—(c) Fig. 1.
metal ions on site B and the reducibility of the catalysts are necessary.
As illustrated in Fig. 2, the temperature of the a peaks of the catalyst increase gradually at the order of
La0.8Sr0.2CoO3,
La0.8Sr0.2Co0.5Ni0.5O3
and
La0.8Sr02Co0.5Cu0.5O3, demonstrating that the activities of oxygen vacancies and lattice oxygen over La08Sr02CoO3 catalysts are higher and the binding capacity between Co3+ and oxygen is lower. Thus, chemisorbed and lattice oxygen over La0.8Sr0.2CoO3 move easily, which is favorable to methane oxidation.
Activity in Methane Combustion. The conversion curves of CH4 over these prepared samples are presented in Fig. 3 and the results of oxidation of CH4 over La08Sr02CoO3, La0.8Sr0.2Co0.5Ni0.5O3 and La0.8Sr02Co0.5Cu05O3, catal
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