The transient response study of CO, C[O.sub.2], and [O.sub.2] adsorption and CO Oxidation over [La.sub.0.4][Sr.sub.0.6][Co.sub.0.4][Mn.sub.0.6][O.sub.3].

Li, Rong ; Yu, Shui-Xian ; Wang, Fu-Shan 等

INTRODUCTION

Carbon monoxide, emitted from many industrial processes and automobile exhausts, has been considered as a major air pollution. A number of catalysts have been studied in the past decades (Efstathiou, 1991; Sander et al., 1991; Islam et al., 1996; Eichler and Hafner, 1999; Bak et al., 2000; Martra, 2000). The kinetics of CO oxidation on the perovskite-type oxides catalyst [La.sujb.0.4][Sr.sub.0.6][Co.sub.0.4][Mn.sub.0.6][O.sub.3] has been studied previously and the possible mechanism of CO oxidation was proposed (Men and Xi, 1985; Ma et al., 1993). In this communication, the transient behaviour of components in the reaction system of CO oxidation on this catalyst was investigated by the use of transient response method. Results showed that CO was not adsorbed on the catalyst surface and CO oxidation was carried out between the surface oxygen species on the catalyst and gas phase CO. Meanwhile, C[O.sub.2] can be adsorbed on the catalyst surface but its adsorption site was different from the forming site.

EXPERIMENT

Catalyst

A [La.sub.0.4][Sr.sub.0.6][Co.sub.0.4][Mn.sub.0.6][O.sub.3] catalyst was prepared by the citric acid complex compound method (Kakihana and Okubo, 1999; Julien et al., 2000). Its structure determined by X-ray diffraction found that it was the single crystal and its surface area was 28.4 [m.sup.2]/g with the BET method. The gases for the reaction were commercial cylinders such as CO, C[O.sub.2], [O.sub.2], and ultra high purity [N.sub.2].

Experimental Method and Condition

The reactor for experiments was a pylex 'u' type glass tube (d = 5.5 mm) filled with 12.8 g of 60-80 mesh catalyst. It was put in a thermostat in which the plant oil was used as a thermostatic medium. All response experiments were carried out at 70[degrees]C and 1 atm.

The mixed gases for the reaction were consisted of CO, [N.sub.2], and [O.sub.2]. The disturbance of the gas partial pressure was performed via a four-way valve. The concentrations of two different kinds of mixed gases were contented in all courses except the disturbance gas and [N.sub.2]. The flowing rates of all gases were 180 mL/min during the experiments. The mixed gases after reaction was analyzed by using SC-6-type gas chromatograph fitted with the two columns filled with 5 [A.sup.0] molecular sieves and carbon molecular sieves, respectively. A 16-way sampling valve controlled by a TP-801 PC was used to collect samples conveniently and determine the concentration quickly. Samples collected were put in 16 collectors so that the volume of each one was about 0.2 mL and then checked correspondingly. Figure 1 represented the flow diagram for the transient response.

[FIGURE 1 OMITTED]

RESULT AND DISCUSSION

The case that the concentration change of the component Y in the outlet mixed gases caused by that of X in the inlet was designated as X-Y response. When X increased, X(i)Y; when decreased, X(d)-Y, and when X decreased down to nil, X(d=0)-Y.

To study the CO response behaviour and make the active oxygen species on the catalyst react completely, the catalyst was reduced with the [N.sub.2] mixed with 6% CO. After 15 h, CO concentration in outlet mixed gases was up to 6% with no further change. This meant that there was not any active oxygen species on the catalyst surface. Then the ultra high purity [N.sub.2] replacing the above gases, the CO concentration in outlet mixed gases was examined at different time. The CO(d=0)-CO response curve was obtained. By switching, another curve CO(i=0)-CO was obtained. As can be seen from Figure 2, the change of CO partial pressure from one steady-state to another was almost transient and it exhibited that the adsorption of CO on the catalyst did not take place.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

To collect more evidences to demonstrate the above point, the CO-C[O.sub.2] response experiments were performed at different conditions and the results were shown in Figure 3. During experiments, the state A represented the mixed gases (Pco=4.65 kPa, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] flowing in the catalyst bed. In Figure 3, two response curves were obtained, respectively, by switching from the state A to the mixed gases [O.sub.2] + [N.sub.2] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and to ultra high purity [N.sub.2] almost overlapped in the experiment error. This also proved that CO cannot be adsorbed on this catalyst and the result was similar to that of CO adsorption behaviour on the catalyst [Pb.sub.3] [O.sub.4] (Boudart et al., 1966; Kobayashi and Kobayashi, 1975).

C[O.sub.2] (i=0)-C[O.sub.2] and C[O.sub.2] (d=0)-C[O.sub.2] response curves in Figure 4 were obtained by switching between two steady states reached by two different mixed gases [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] From Figure 4, it can be known that C[O.sub.2] was adsorbed on this catalyst as it requires much time from one state to another. The adsorption amount and the desorption amount for C[O.sub.2], obtained by integral of the above curves, were 3.3 x [10.sup.-7] mol/g and 2.6 x [10.sup.-7] mol/g, respectively. The fact that the adsorption amount was clearly higher than the desorption amount indicated that the adsorption of parts of CO2 on the catalyst surface was irreversible.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

To investigate the relationship between the adsorption of C[O.sub.2] and C[O.sub.2] partial pressure and the gas nature, C[O.sub.2](d=0)-C[O.sub.2] response experiments were done via switching from one state obtained by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] to another obtained by pure [O.sub.2] and pure [N.sub.2], respectively. From the result shown in Figure 5, it can be seen that two response curves almost overlap. This presented that the adsorption of C[O.sub.2] in two kinds of courses were same and the desorption of C[O.sub.2] did not depend on the gas nature in inlet mixed gases. Also, the concentration of C[O.sub.2] in inlet mixed gases (Pco = 4.94 kPa, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] did not apparently influence the CO conversion rate (as can be seen in Figure 6). So it indicated that C[O.sub.2] did not restrain the CO oxidation on the catalyst and the adsorption site of C[O.sub.2] on the catalyst was different from its forming site.

When switching from one state containing Pco = 5.55 kPa, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] to another Pco = 3.29 kPa, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], we obtained CO(i)-C[O.sub.2] response curve as shown in Figure 7. In outlet gas mixture, C[O.sub.2] partial pressure was up to the maximum in 5 min and then down quickly. This presented that the active oxygen species for CO oxidation was not the gas phase oxygen but the surface lattice one.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

CONCLUSIONS

Through the experiments we reach the following conclusions:

1. The adsorption of CO on the catalyst did not take place.

2. The C[O.sub.2] adsorption amount was clearly higher than the desorption amount indicated that the adsorption of parts of C[O.sub.2] on the catalyst surface was irreversible.

3. The adsorption of C[O.sub.2] in two kinds of courses were same and the desorption of C[O.sub.2] did not depend on the gas nature in inlet mixed gases.

4. C[O.sub.2] did not restrain the CO oxidation on the catalyst and the adsorption site of C[O.sub.2] on the catalyst was different from its forming site.

5. The active oxygen species for CO oxidation was not the gas-phase oxygen but the surface lattice one.

ACKNOWLEDGEMENTS

Financial support by the Jinchuan Company for the present work is gratefully acknowledged.

Manuscript received November 19, 2006; revised manuscript received August 24, 2007; accepted for publication September 12, 2007.

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Rong Li, * Shui-Xian Yu, Fu-Shan Wang, Sheng-Lian Sun and Jian-Tai Ma College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China

* Author to whom correspondence may be addressed.

E-mail address: liyirong@lzu.edu.cn

DOI 10.1022/cjce.20022