Kinetics of catalytic oxidation of CO over copper-manganese oxide catalyst

Separation and Purification Technology 57 (2007) 147–151

Kinetics of catalytic oxidation of CO over copper-manganese oxide catalyst Ming Li a,b , Dong-Hui Wang a,∗ , Xi-Cheng Shi a , Ze-Ting Zhang b , Tong-Xin Dong a a

b

Research Institute of Chemical Defence, Beijing 100083, China Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Received 28 November 2006; received in revised form 21 March 2007; accepted 23 March 2007

Abstract The hopcalite-like catalyst was prepared by co-precipitation, which showed excellent activity and stability for CO oxidation at room-temperature. The activity of carbon monoxide oxidation was investigated at different temperature. The kinetics of carbon monoxide oxidation over coppermanganese oxides catalysts was obtained by means of recycling batch reactor. As a result, CO oxidation followed different reaction mechanism at a broad range of the experimental temperature. The reaction activation energy of CO oxidation was found to be 64.08 kJ mol−1 for 273–313 K and 12.15 kJ mol−1 for 333–373 K, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper-manganese oxides; Carbon monoxide; Kinetic model; Reaction mechanism

1. Introduction Carbon monoxide is a toxic component of air. Catalytic oxidation of carbon monoxide to carbon dioxide at ambient temperature and pressure is an important process for respiratory protection. In particular, the process is widely adopted by mining industries and has also found applications in deep-sea diving, space exploration, carbon dioxide lasers and CO gas sensors. The development of catalytic carbon monoxide oxidation catalysts has become an important research topic during the last 17 years since Haruta et al. [1] demonstrated that gold, highly dispersed on various oxides, forms catalysts active at sub-ambient temperatures. Precious metal catalysts, such as Au/MgO, Au/TiO2 , Au/Fe2 O3 , Au/Al2 O3 , Au/CeO2 , Au/ZnO, Au/MOx/Al2 O3 , Pt/CeO2 , Pt/CeO2 -Al2 O3 , Pt/SnO2 TiO2 , Pd/CeO2 , Pd/Ce0.8 Tb0.2 O2−x /La2 O3 -Al2 O3 , Ir/TiO2 , Ir/Al2 O3 , Ir/Fe2 O3 , and Ag/MnOx /perovskites, have been studied for CO oxidation and showed high catalytic activities [2–16]. However, the high cost of precious metals and their sensitivity to sulfur poisoning have long motivated the search for substitute catalysts. The so-called base metal catalysts deserve more attention for their significant activities and lower cost. Therefore, the mixed of transition metal and transition metal oxides has been widely studied in recent years [17–19]. ∗

Corresponding author. Tel.: +86 10 66748486 810. E-mail address: [email protected] (D.-H. Wang).

1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.03.016

Hopcalite – an oxide mixture based on copper and manganese oxides – has been successfully applied as a catalytic material for the removal of toxic gases for about 80 years [20,21]. Due to the complexity of the preparation methods of hopcalite, the attempts have been not stopped to find new preparation methods for more active hopcalite. Copper-manganese oxide catalysts prepared by co-precipitation was described by Taylor and coworkers [22,23]. They showed that the effect of a broad range of precipitation parameters, such as ageing time, ageing pH, ageing temperature, copper/manganese molar ratio and calcination temperature are of crucial importance in controlling the catalytic performance of CO oxidation at ambient temperature. Our present work is to prepare copper-manganese oxide used co-precipitation that simplifies the preparation procedures yielding simultaneously a catalyst with higher activity. The purpose of this work is to investigate the intrinsic reaction kinetics of CO oxidation on the copper-manganese oxide catalysts. 2. Experimental 2.1. Catalyst preparation Copper-manganese mixed oxide catalysts were prepared by co-precipitation procedure. Aqueous solutions of Cu(NO3 )2 (0.25 M) and Mn(NO3 )2 (0.25 M) were pre-mixed and the resulting solution was added to aqueous Na2 CO3 (0.25 M) which was continuously stirred. After dropped completely, the precipitate

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M. Li et al. / Separation and Purification Technology 57 (2007) 147–151

Nomenclature CCO Ea k m, n r R2 Se Sr St t T V W

CO concentration (mol L−1 ) reaction activation energy (J mol−1 ) reaction rate constant reaction order CO reaction rate (mol g−1 min−1 ) correlation index residual square error regression square error total square error reaction time (min) temperature of catalyst bed (K) reactor bulk (L) catalyst mass (g)

was aged in mother liquid for 2 h, and then was filtered and washed several times. The precipitate was dried at the temperature of 353 K, and calcined under oxygen flow at 673 K for 1 h. The sample was pressed into a disc and then breaken up to give the final catalyst, with a bulk density of 0.75 g cm−3 .

in 0.32–0.45 mm, was maintained in a quartz tube (inner diameter of 8 mm) at a preset temperature in the water bath. Reaction gas was passed over the catalyst at a certain space velocity. CO concentration was determined with an Agilent GC-6890N gas chromatograph equipped with a thermal conductivity detector. 3. Results and discussion 3.1. Stability of the catalyst Kinetic experimental study in the stable area of catalyst is necessary to obtain any kinetic model. The experiment of stability test was carried out in the same continuous-flow, fixed-bed reactor. 0.25 g of the catalyst in a quartz tube was maintained at 300 K. The space velocity was 45,000 h−1 . Experimental results are shown in Fig. 2. There was a short period to reach steady state activity for copper-manganese oxide. The fluctuation of activity appeared during the initial 50 min. The conversion of CO to CO2 began to reduce after 10 min. However, no remarkable deactivation of the catalyst was observed after the reaction has been carried out for 50 min. That is, the steady state was attained within 1 h. And CO oxidation activity measurements could be performed with good reproducibility at the stable state of the activity.

2.2. Experimental apparatus and analytical method 3.2. Effects of interparticle and extraparticle diffusions A schematic diagram of the experimental facility is shown in Fig. 1. A recycling batch reactor, which is 25 L in volume, was used for the measurement of reaction rate. CO was injected into the reactor with the gas tight syringe. The stabilization of CO concentration was reached by the way of the fan motor, and the initial concentration of CO was about 3 × 10−5 to 5 × 10−5 mol L−1 . Reaction gas was passed the catalyst bed by a gas pump, which was connected with the reactor by a part of soft pipe. Before the catalyst bed, the desiccator was used to remove water in the feed gases. 0.25–0.35 g catalyst with particle size

Fig. 1. Schematic diagram of kinetic apparatus.

In order to ensure the intrinsic reaction kinetics is measured without the mass transfer resistance, the internal and external diffusion effects must be eliminated. For the recycling batch reactor, it was one kind of gradientless reactor. As a result, the external mass resistance could be negligible. The same amount of catalysts with different particle size was used to test the internal mass transfer resistance. The catalyst particle diameter examined ranged from 0.15 to 0.9 mm. The results are shown in Fig. 3. It is observed that the CO conversion is invariable when the catalyst particle diameter is smaller than 0.9 mm. This confirmed that internal diffusion was not a controlling step when the catalyst particle diameter is smaller than 0.9 mm.

Fig. 2. Time on stream experiment, SV = 45,000 h−1 , CCO = 4.5 × 10−5 mol L−1 , T = 300 K.

M. Li et al. / Separation and Purification Technology 57 (2007) 147–151

Fig. 3. Effect of internal diffusion on conversion, SV = 30,000 h−1 , CCO = 4.5 × 10−5 mol L−1 , T = 300 K.

149

Fig. 5. Dependence of reaction rate on the CO conversion at different temperatures.

following kinetic equation will be used: −r = −

Fig. 4. Effect of temperature on the activity of the copper-manganese oxide.

3.3. Parameter estimation The variations of the CO concentration were measured using Fig. 1 type reactor at different temperatures. The results are shown in Fig. 4. Among the reaction rate laws, power-law models often have been found to be adequate enough for preliminary studies. In our experiments, the influence of oxygen can be neglected, as it is present in great excess and its concentration remains practically constant in all the experiments. Taking into account of this consideration and neglecting the influence of the products, the

V dCCO n = kCCO W dt

(1)

where (−r) is the reaction rate of CO oxidation (mol g−1 min−1 ), V the bulk of recycling batch reactor (L) and W catalyst mass (g), CCO the concentration (mol L−1 ) and k the kinetic constant (mol1−n Ln g−1 min−1 ) which is supposed to have an Arrhenius dependence on temperature, n is the reaction order of CO oxidation. The reaction rate is calculated from the tangent line of the reaction kinetics curves in Fig. 4 using NLCF (non-liner curve fit) function of Origin software. Reaction rate taken at various points of the time versus concentration of CO are shown in Fig. 5. Table 1 shows that the regression equations of curves in Figs. 4 and 5 in the Origin software. Here, exponential function and power function were selected to fit the curves in Figs. 4 and 5, respectively. Table 1 shows that the correlation index R2 was more than 0.9, which means that the regression equation is reliable. The relationship between the reaction rate and the concentration of CO is described by Eq. (1). The constant k of CO oxidation at different temperature can be obtained by fitting Eq. (1) to the data of Fig. 5. The value of n was near 1 which means that the CO oxidation followed the first-order reaction though the value of k increased with the rise of temperature. 3.4. Temperature effect Fig. 6 shows the Arrhenius plot of ln(k) in a wide range of temperature from 273 to 373 K. It is seen that different

Table 1 Data processing results using Origin software T (K)

CCO = a exp(bt)

R2

n r = kCCO

R2

273 300 313 333 353 373

a = 0.00079; b = −0.00037 a = 0.00082; b = −0.00472 a = 0.00092; b = −0.01239 a = 0.00105; b = −0.01932 a = 0.00103; b = −0.02523 a = 0.00114; b = −0.03099

0.97571 0.996 0.99611 0.99814 0.99879 0.99839

k = 0.05469; n = 0.9883 k = 0.76354; n = 0.99714 k = 1.95761; n = 0.995 k = 3.2763; n = 1.00174 k = 4.25206; n = 1.00133 k = 5.24302; n = 1.00156

0.97572 0.99605 0.99619 0.9981 0.99877 0.99834

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tribution and statistic test of Eq. (2) could be attained and the results are shown in Table 2. For square error distribution of linearity equation changed from non-linear equation, F/F␣ should possess a multiply safety factor of 3–10. The value of F is far more than 10F␣ (P, M − P − 1) in Table 2 with the correlation index R2 > 0.9. As a result, the regression equation is reliable and statistically valid. 4. Conclusions

Fig. 6. Relationship between (ln k) and 1/T.

kinetics are operating with markedly different rates and apparent activation energies. From the data in Fig. 6, the ln(k) can be obtained, ln(k) = 25.34 − 7707.24/T from 273 to 313 K, and ln(k) = 5.57 − 1461.46/T from 333 to 373 K, respectively. From the slopes of the coefficient of two straight lines, the activation energies were found to be 64.08 kJ mol−1 for 273–313 K and 12.15 kJ mol−1 for 333–373 K, respectively. 3.5. Kinetic model and model tests

V dCCO = kCCO W dt

(2)

Here, k = 1.02 × 1011 exp(−64.08 × 103 /(R × T)) for 273–313 K k = 264.90 exp(−12.15 × 103 /(R × T)) for 333–373 K For proving the kinetic models, Eq. (2) might be linearized as Eq. (3): ln(CCO ) = −

W kt + δ V

(3)

where δ is the integral constant which is related to the initial concentration of CO. Suppose ln(CCO ) = y, the square error disTable 2 Square error distribution and statistic test of Eq. (2) Item

Expressions

Result

Number of experiments Number of variables

M

P

90 3

St

St =

Sr

Sr =



(y − y¯ )2

j 

(ˆy − y¯ )2



69.922 61.270

j

Se

Se =

j

R2

R2 = 1 −

(y − yˆ )2 Se St

Sr /P Se /(M−P−1)

F

F=

F␣ (P, M − P − 1) F␣ (P, M − P − 1)

α = 0.05 α = 0.1

Acknowledgement This work is financially supported by the National Natural Science Foundation of China (20643005).

The kinetic model might be expressed as Eq. (2): −r = −

The copper-manganese oxides prepared by co-precipitation showed very good performance in CO oxidation at ambient temperature. The kinetic model of carbon monoxide oxidation over copper-manganese oxides was investigated by means of the recycling batch reactor. The square error distribution and statistic test of the kinetic model show that the kinetic model is reliable and statistically valid. It was found that CO oxidation followed the first-order reaction. The reaction mechanism of CO oxidation varied at different temperature range with the activation energy of 64.08 kJ mol−1 for 273–313 K and 12.15 kJ mol−1 for 333–373 K, respectively.

1.001 0.986 1754.674 2.8 2.15

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