Kinetics of Carbon Monoxide Oxidation over CuO Supported on

Ind. Eng. Chem. Res. 2009, 48, 5633–5641

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Kinetics of Carbon Monoxide Oxidation over CuO Supported on Nanosized CeO2 Jose L. Ayastuy,* Anita Gurbani, Marıá P. Gonza´lez-Marcos, and Miguel A. Gutie´rrez-Ortiz Department of Chemical Engineering, Faculty of Science and Technology, UniVersity of the Basque Country, P.O. Box 48080-Bilbao, Spain

A CuO/CeO2 catalyst prepared by incipient wetness impregnation (7 wt % Cu) with well-dispersed CuO on nanosized ceria, has been used to obtain a kinetic expression for CO oxidation, including estimation of kinetic parameters, in the absence of CO2, H2O, or H2 in the feed stream. Experimental data were checked to be collected in the kinetic regime, in the absence of external and intraparticle mass and heat transfer, and in a wide range of experimental conditionsswith CO partial pressure ranging from 0.0015 to 0.0125 atm (that is, including those usually found in PROX reactors) and λ values between 0.35 and 13.3s and were fitted to different rate expressions (power-law and mechanistic) by using a genetic algorithm. Although a power-law rate equation satisfactorily fits the experimental data, it is limited to the experimental range of operation conditions. Further discrimination among mechanistic models indicates that an expression proposed by Liu and Flytzani-Stephanopoulos (Chem. Eng. J. 1996, 64, 283), with a partial reaction order of 0.08 with respect to oxygen, is the most statistically significant, almost indistinguishable from the expression proposed by Mars-Van Krevelen (Spec. Suppl. Chem. Eng. Sci. 1954, 3, 41). Introduction CO oxidation at low temperatures has attracted great attention for its application to selective oxidation of CO in hydrogenrich environments. Copper-based catalysts are found among the most active catalysts, especially if copper is supported onto reducible oxides, such as CeO2, due to their ability to store and release oxygen. Ceria has been widely used in three-way catalysts for automobile exhaust gas emission control1 and, in the past decade, CuO-CeO2 catalysts are being exhaustively studied for their application in low-temperature oxidations and WGS reactions.2,3 The application of such catalysts to the oxidation of CO in hydrogen-rich environments (PROX reaction) for the purification of hydrogen to be fed to PEMFC is attracting much attention.4 Their activity depends strongly on the preparation method, and the intimate contact between copper clusters and ceria is found to be responsible for their good performance. Despite the huge amount of articles concerning preparation and characterization of CuO-CeO2 catalysts,5-8 much less is published related to the kinetic study of CO oxidation9 or PROX reaction10-13 in these systems. For alumina-supported noble metal catalysts, CO oxidation is well-known to occur via Langmuir-Hinshelwood mechanism, where both CO and oxygen compete for NM surface.14 In such systems, due to very strong adsorption of CO, the self-poisoning effect of CO gives negative partial reaction orders in CO. However, in the presence of ceria, which plays an active role in the oxygen supply for oxidations by releasing lattice oxygen, a synergistic effect can be observed when it is on intimate contact with CuO, especially with nanosized ceria,15-17 and a change in the reaction mechanism occurs. Among the mechanisms for CO oxidation over CuO-CeO2 catalysts, the redox-type Mars-Van Krevelen mechanism and that proposed by Liu and FlytzaniStephanopoulos18 are widely accepted: in the former mechanism, redox changes occur in the copper-ceria interface,13 while the latter corresponds to a modification of a Langmuir-Hinshelwood mechanism. * To whom correspondence should be addressed. E-mail: [email protected].

The main aim of this work is to provide a kinetic expression of CO oxidation with CuO/CeO2 catalysts, including estimation of kinetic parameters. According to the literature,11 CO oxidation rate is essentially independent of H2 concentration; thus, the mechanistic rate equation obtained in this work will be valid for the selective oxidation of CO (PROX). The effect of the presence of CO2 and H2O in the feed stream (as in typical PROX reaction) falls out of the aim of this work. Experimental data were checked to be collected in the kinetic regime, in the absence of external and intraparticle mass and heat transfer, and in a wide range of experimental conditions, including temperature and partial pressures of CO and O2 usually found in PROX reactors, and fitted to rate expressions including power law (PLR) and a set of mechanistic rate equations. Genetic algorithm (GA) has been used for optimization of kinetic parameters. Experimental Section Catalyst Preparation and Characterization. A 7 wt % copper CuO/CeO2 catalyst was prepared by conventional incipient wetness impregnation, using Cu(NO3)2 · 3H2O as a precursor. A commercial high surface area CeO2 (Rhodia) was used as support. The as prepared precursor was dried in air at 388 K overnight and calcined in air flow at 773 K for 5 h. The copper content of the catalyst was evaluated by ICPAES (Horiba). Its crystalline structure was analyzed by XRD of sample in finely grounded powder with Cu KR radiation in continuous scan mode from 10 to 80° of 2θ (Philips PW1710). The particle size of CeO2 was calculated by X-ray broadening using Scherrer equation at its most intense peak (28.5°). Specific surface area and pore size distribution of the catalyst was determined by N2 adsorption-desorption isotherms at 78 K (Micromeritics ASAP 2010). Redox properties of support and catalyst were investigated by temperature-programmed reduction with hydrogen. H2-TPR was recorded by heating the sample from 253 to 673 at 10 K/min in a flow of 5% H2-Ar (TPR of ceria was carried out from 253 to 1173 K). The catalyst ability to adsorb CO was analyzed by CO-TPD. The sample was saturated with a flow of 5% CO-He at 273 K for 30 min, and then purged with He for 2 min. After that, the sample was heated at 20 K/min in a flow of

10.1021/ie9001603 CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

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He up to 873 K, the exhaust gases being continuously measured by mass spectroscopy. The catalyst ability to store and release oxygen (OSC, defined as the CO2 formed during a CO step after oxidation in O219) was measured at 393 K and a total flow of 60 mL/min, in three consecutive cycles. Each cycle consisted of the following sequence of gases: 5% CO-He (reduction step), 9 min; He, 6 min; 5% O2-He (oxidation step), 9 min; He, 6 min. Mass spectroscopy (MKS Cirrus) was used for continuous measurement of reactor exhaust. H2-TPR, CO-TPD and OSC measurements were conducted in the same experimental equipment (Micromeritics Autochem 2910) with ca. 0.4 g of sample loaded in a quartz reactor. Reactor System and Analytical Methods. Experimental runs to collect intrinsic kinetic data were carried out: at steady-state, in differential regime, at atmospheric pressure, with a total flow of 200 mL/min, several feed stream compositions, and an average catalyst particle size of 0.2 mm (in the range 0.16-0.25 mm). The reaction tests were carried out in a stainless steel tubular reactor whose details can be found elsewhere.20 The catalyst was weighted (ca. 0.02 g, to ensure low CO conversion) and diluted in inert alumina, in order to dissipate the heat released in the reaction, to a total bed height of 15 mm. All gas flows were measured by calibrated gas flow controllers. Prior to a catalytic run, the catalyst was subjected to a mild reduction in a flow of 50% H2:50% He for 30 min at 573 K, and then conditioned to reaction temperature in helium flow. CO + O2 gas mixtures (balanced with He) were prepared so that PCO varied from 1.5 × 10-3 to 1.25 × 10-2 atm, while PO2 varied from 1.75 × 10-3 to 1.5 × 10-2 atm, with λ values (2PO2/PCO) in the range 0.35-13.3. The experiments were carried out isothermally at four temperatures (between 300 and 353 K). Reactor inlet and outlet streams were continuously analyzed by mass spectroscopy (MKS Cirrus) coupled to a NDIR-photometer (Siemens) in order to follow CO, CO2, and O2 concentrations. A preliminary experiment at 3 K/min was carried out for 2% CO and λ ) 1, in order to design further experimental conditions. CO fractional conversion was calculated from the molar flow of CO at inlet (FCO,0) and outlet (FCO) of the reactor, as XCO )

FCO,0 - FCO FCO,0

(1)

Experimental data were obtained in differential regime (assumed for CO conversion below 0.2). Therefore, the reaction rate was calculated as follows

(

(-rCO)

)

FCO,0XCO molCO ) gcat·min W

(2)

where (srCO) is the reaction rate of CO oxidation and W the catalyst weight (in g). To ensure plug-flow conditions, the following criteria were implemented in our experiments:21 (1) catalyst bed height to catalyst particle size ratio (L/dp) g 50 (75 in this work) and (2) internal reactor diameter to catalyst particle size ratio (D/dp) g 10 (40 in this work). Parameter Estimation. The experimental reaction rate data were fitted to a set of rate equations, both empirical PLR and mechanistic. The optimum parameters for each rate equation were obtained by minimizing the objective function, defined as the sum of squared residuals (SSR) of reaction rate, as follows: N

OF ) minimum

∑ ((-r

CO,expt)i

i)1

- (-rCO,calcd)i)2

(3)

where i ) experiment number (N ) 61 in CO oxidation reaction), and subscripts expt and calcd refer to experimental and calculated reaction rates, respectively. The model adequacy was checked by applying F-test to the sum of residual squares at 95% confidence level (R ) 0.05). As few replicated experiments were performed, the applied F-test was based on the regression sum of squares and the sum of squared residuals:21 N

Fc )

N

∑ i)1

(

∑ i)1

(

[(-rCO,calcd)i]2 p

)

[(-rCO,expt)i - (-rCO,calcd)i]2 N-p

)

(4)

where p is the number of parameters of the checked model. When Fc > Ft (p, Nsp; 1sR), then the regression could be considered statistically meaningful (Fc and Ft being calculated and tabulated F values, respectively). The coefficient of determination (R2) was calculated as N

∑ [(-r

- (-rCO,calcd)i]2

∑ [(-r

- (-rjCO,expt)]

CO,expt)i

R2 )

i)1 N

(5) CO,expt)i

2

i)1

where (-rjCO,expt) is the average experimental rate. Kinetic and adsorption constants in rate equations were assumed to follow Arrhenius-type and Van′t Hoff-type dependencies with temperature, respectively. GA, developed on Matlab 7.0 commercial software (The MathWorks, Natick, MA), was used for minimization of SSR,22,23 with the following specifications: real-value coding for parameters, initial population of 100 individuals, 200 generations, 10 individuals guaranteed to survive to the next generation, mutation fraction of 0.1 (crossover fraction of 0.9), and proportional fitness scaling. Initial population, number of generations, and surviving individuals were optimized previously. An improvement for the initial population was developed by dividing the total population into 5 subpopulations of 20 individuals. Each individual “chromosomes” are defined by a set of values for each parameter of the model. Kinetic Models. A PLR equation and a set of rate equations derived from mechanistic models (given in Table 1) were used to fit the steady state CO oxidation data: CO + 1/2O2 f CO2

∆Hr0 ) -283 kJ/mol

ModelLH1isderivedfromasingle-siteLangmuir-Hinshelwood mechanism, where adsorption of molecular oxygen (step (ii)) is the rate determining step (rds) and adsorbed carbon monoxide (CO*) the most abundant rate intermediate (mari). Such a mechanism is commonly stated for noble metal supported on nonactive supports (i.e., Pt/Al2O3),24 and leads to a partial order with respect to CO and O2 of -1 and +1, respectively. Model LH2, proposed by Nibbelke et al.,25 corresponds also to a single-site Langmuir-Hinshelwood mechanism, where dissociative chemisorption of oxygen over two adjacent catalytic sites in a single step26 (step ii) is the rds and CO* the mari. This mechanism leads to a partial order with respect to CO and O2 of -2 and +1, respectively. Many authors agree with copper-ceria interface playing an active role in CO oxidation.27-30 According to them, the proposed reaction mechanism should consider such behavior.


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Table 1. Rate Models Based on Mechanistic Models Tested in This Work model

description of the mechanism

rate equation

LH1

(i) CO + * f CO* (ii) O2 + * f O*O (iii) O*O + * f 2O* (iv) CO* + O* f CO2 + 2* single-site, step (ii) being the rds and CO* the mari.

(-rCO) )

LH2

(i) CO + * f CO* (ii) O2 + 2* f 2O* (iii) CO* + O* f CO2 + 2* single-site, step (ii) being the rds and CO* the mari.

(-rCO) )

LH3

(i) CO + * f CO* (ii) O2 + 2X f 2OX (iii) OX + * f O* + X (iv) CO* + O* f CO2 + 2* dual-site (*, Cu site; X, ceria site), step (ii) being the rds and CO* the mari.

(-rCO) )

LH4

both CO and oxygen (dissociated) are adsorbed at lattice vacancies, addition of the first oxygen adatom being the rds.

LH5

Bimolecular surface reaction between CO* (on Cu) and oxygen (adsorbed on lattice vacancies), only a single oxygen atom being added to give CO2.

(-rCO) )

ref

kO2PO2 1 + KCOPCO

24

kO2PO2 (1 + KCOPCO)2

kO2 1 + KCOPCO

26

26

kCOPCOPO0.52 (1 + KCOPCO + √KO2PO2)2

55

(-rCO) )

LFS

Dual-site where ceria provides oxygen sources and copper provides sites for CO adsorption; both species react at the boundary. Step (iv) is the rds. No lattice oxygen is involved. CO* is the mari.

MVK

redox model, where catalyst is reduced by CO and oxidized by ambient oxygen.

In the Langmuir-Hinshelwood-type LH3 mechanism, oxygen is first dissociatively adsorbed on ceria (X) and, then, oxygen adatoms migrate to copper oxide-ceria interface, where they are exchanged (stage iii, taken as rds) and then react with CO adsorbed on Cu (CO*). CO* is considered as mari, and OX is considered to be independent of gas phase oxygen partial pressure. This four-step mechanism has been reported for noble metal supported on reducible supports, such as Pt/CeO2 catalysts,26 and leads to a partial reaction order with respect to CO and O2 of -1 and 0, respectively. LH4 corresponds to a Langmuir-Hinshelwood-type rate equation with adsorption of both CO and oxygen at the lattice vacancies, the addition of the first oxygen adatom (OX) being the rds. This mechanism leads to a partial order with respect to CO and O2 of -1 and -0.5, respectively. On the other hand, LH5 mechanism is a bimolecular surface reaction between CO* (adsorbed on copper sites) and oxygen adsorbed on lattice vacancies, only a single oxygen atom being added to give CO2. From this equation, a partial reaction order of 0 is derived both with respect to CO and oxygen. A modified Langmuir-Hinshelwood-type (LFS) model has been proposed by Liu and Flytzani-Stephanopoulos for CO

kCOPCOPO0.52

55

(1 + KCOPCO)(1 + √KO2PO2)

(-rCO) )

(-rCO) )

kCOKCOPCOPOn 2 1 + KCOPCO

9

kCOkO2PCOPOn 2 0.5kCOPCO + kO2POn 2

13

oxidation over Cu-CeO2 catalysts.9 This model assumes that no lattice oxygen is involved in CO oxidation, while atomic oxygen is provided by the amount adsorbed onto ceria surface. Mars-Van Krevelen model (MVK) is often used to describe the reduction-oxidation cycle of the catalyst surface in partial oxidation of hydrocarbons.31,32 For CO oxidation, a redox mechanism is also supported by Martıńez-Arias et al.28 and Dow et al.33 This redox model consists of two steps: (i) CO reduces the oxidized catalyst while CO2 is formed; (ii) reduced catalyst reoxidizes with oxygen. Step ii is the rds. Results and Discussion Catalyst Preparation and Characterization. Some measured properties for support and catalyst are listed in Table 2. The actual copper loading was 6.8 wt %, which corresponds to an atomic Ce/Cu ratio of 4.9. Compared to the pure support, impregnation with CuO slightly decreases BET specific surface area. XRD pattern (not shown) for the catalyst is very similar to that corresponding to fluorite-type CeO2 cubic structure (PDF files 34-0394): the absence of the CuO peaks suggests the size of copper clusters is very small. The ceria crystallite size

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Table 2. Properties of CeO2 Support and CuO/CeO2 Catalyst sample CeO2 CuO/CeO2

Cu BET areaa dporea dCeO2b CO2 desorbedc H2 uptaked (wt %) (m2/g) (nm) (nm) (µmol CO2/g) (µmol/g) 6.8

164 150

4.0 4.2

6.9 6.9

35.9

530 1487

a From N2 adsorption isotherms at 77 K. b From XRD peak at 28.5° with Scherrer equation. c From CO-TPD spectra. d From H2-TPR spectra.

estimated by Scherrer equation resulted in 6.9 nm, confirmed by TEM observations (not shown), in which nanoparticles of ceria can be observed to form aggregates. Figure 1a shows H2-TPR and cumulative H2 consumption in the 253-673 K temperature range. H2-TPR pattern for pure ceria presents two characteristic broad reduction peaks: the first one around 763 K, associated to surface capping oxygen, and the second one centered around 1107 K, associated to reduction of bulk ceria;34 while for pure CuO, a single TPR peak has been reported at 653-665 K.35 CuO addition to the support shifted these peaks to considerably lower temperature, as shown in Figure 1a. The catalyst reduction pattern presents two peaks: a sharp and intense peak centered at 358 K and a broader peak centered at 456 K, which starts at 411 K and finishes at 491 K. The former peak is attributed to reduction of highly dispersed CuO species30 and amounts to 561.8 µmol H2/g; while the latter peak is attributed to simultaneous reduction of CeO2 and bulk CuO. Better catalytic results are known to be achieved on catalysts with easily reducible and finely dispersed copper species interacting with ceria36,37 and, as a consequence, our catalyst should be highly active for CO oxidation at low temperature. Figure 1b shows the CO-TPD pattern for the catalyst. A single CO2 (m/z ) 44) desorption peak at 398 K (starting at 335 K and finishing at 522 K) was observed during CO-TPD (no m/z ) 28 signal, corresponding to CO, is observed), in accordance to the literature.28 In this experiment, as no molecular oxygen is fed to the sample, oxygen for CO2 formation is necessarily released by the catalyst. OSC results at 393 K are shown in Figure 2. Once the CO step starts, CO signal decreases and, simultaneously, CO2 signals increases (formed with oxygen supplied by the catalyst). When the catalyst has supplied all available oxygen, the corresponding CO2 signal returns back to the baseline. As expected, the area below CO2 peak agrees (within 5%) with the area above CO signal (up to 5 vol %). During the oxidation stage, only oxygen is detected. This pattern is repeated in the three cycles. The amount of CO2 formed during reduction was (in µmol CO2/g): 245 (first cycle), 291 (second cycle), and 283 (third cycle); which gives an average OSC of 2.61 µmol O/m2 ceria (calculated by equation given in ref 38). Compared to the results of Ozawa et al. for CeO2/Al2O3, our results represent about 50% of the maximum OSC per unit ceria surface area (about 5.0 ( 0.6 µmol O/m2 ceria, measured at 873 K40), rather high, considering that our OSC measurements were carried out at much lower temperature (393 K), and must be attributed to the presence of well-dispersed CuO. Heat and Mass Transfer Limitations. The absence of internal mass-transfer limitations was checked with Weisz-Prater criterion, which expresses the ratio of chemical reaction rate to diffusive flux. Assuming spherical catalyst particles:39 CWP )

(-rCO,obs)RP2 < 0.3 CiDeff

(8)

where (-rCO,obs) is the observed reaction rate per catalyst volume, RP is the mean catalyst particle radius (0.1 mm,

geometric average), Ci is the gas concentration of i compound at the catalyst surface (assumed to be equal to the bulk concentration) and Deff is the effective gas-phase diffusivity. In the conditions of the present work, both CO and O2 (reactants) diffusion is governed by Knudsen diffusion (the mean free path for both gas-phase diffusivities is much higher than catalyst average pore diameter). Average CWP values of 4.9 × 10-3 and 3.7 × 10-3 were obtained for CO and O2, respectively, which widely satisfied the Weisz-Prater criterion (also in experiments conducted at high temperatures). The absence of external diffusion limitations was checked by applying the Mears criterion:40 CM,mass )

(-rCO,obs)RPe < 0.15 kg,iCi

(9)

where e is the whole reaction order (for preliminary investigation taken as 0.3) and kg,i is the mass-transfer coefficient through the film for i compound, estimated through the jD Colburn factor:41 jD )

kg,iFie (Sci)2/3 Go

(10)

The Chilton-Colburn analogy for packed beds and ReP < 50 was used to estimate jD:42 jD ) 0.91(Re)-0.51ψ

(11)

where Ψ represents the sphericity (assumed to be unity in this work). The average CM,mass values obtained (0.13 and 0.06 for CO and O2, respectively) assured that the resistance to film mass transfer does not affect reaction rate, although in the case of CO the criterion is barely satisfied in experiments carried out at high temperature. A Mears (1971) criterion for spherical pellets was used to ensure the absence of heat transfer resistance:43 CM,heat )

(-rCO,obs)RPEa(∆Hr) hT2R

< 0.15

(12)

where Ea is the activation energy of the reaction (taken to be 51.5 kJ/mol, from the PLR equation), ∆Hr is the reaction heat, R is the universal constant for gases, and h is the heat transfer coefficient, calculated by the following correlation for spherical solids:44 Nu )

hdP ) 2 + ReP0.5Pr1/3 k

(13)

where dP is the catalyst particle diameter, k is the conductive heat transfer coefficient, ReP is the particle-based Reynolds number and Pr is the Prandlt number. For gas viscosity estimations at a given temperature, Sutherland’s formula was used, with reference viscosity and temperatures from ref 45. Substituting the variables in eq 11, we have estimated CM,heat ) 3.1 × 10-4, which is far enough from 0.15 to consider that heat transport limitation does not set in. As a result of the above calculations, our kinetic data, used to check the rate equations, can be considered to be intrinsic reaction rate data. Catalytic Activity. To have preliminary CO conversion versus temperature data, a catalyst activity test for CO oxidation was carried out at a heating ramp of 3 K/min. Figure 3 shows the light-off curve obtained for a total gas flow rate of 200 mL/ min and 2 vol % of CO, operating at λ ) 1 (continuous line is the curve predicted by LFS model). Total CO conversion in these conditions is achieved at 385 K, which makes our catalyst


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Figure 1. (a) Cumulative H2 consumption and H2-TPR profile of CuO/CeO2, (b) CO2 signal (m/z ) 44) during CO-TPD of CuO/CeO2.

Figure 2. OSC measurement (cycled CO-He-O2 steps) at 393 K of CuO/ CeO2; 28, 32, and 44 m/z signals, corresponding to CO, O2 and CO2, are plotted.

Figure 4. Effect of partial pressures and temperature on CO oxidation rate with CuO/CeO2, in the 300-353 K range. (a) Effect of CO partial pressure at a constant PO2 ) 0.01 atm. (b) Effect of O2 partial pressure at a constant PCO ) 0.01 atm. Figure 3. Light-off curve for 2% CO oxidation, operating at λ ) 1, with CuO/CeO2. Open symbols correspond to an experiment measured at 3 K/min, while the continuous line is the curve simulated with LFS model.

very promising to be used in systems requiring complete CO removal (from a mixture with 2 vol % CO) at low temperature. Kinetic Results. Figure 4 shows the effect of CO and oxygen partial pressures on the reaction rate at different temperatures. In these kinetic experiments, temperature was varied in the range 300-353 K in order to keep CO conversion below 20%, that

is, differential reactor approach could be assumed. In Figure 4a, the reaction rate is shown to increase monotonically with CO partial pressure, this indicating that the reaction is not inhibited by CO (which would occur in the typical LH model, with single site for CO and O2 adsorption, typical mechanism for noble-metals supported on an inert material26,46). On the other hand, Figure 4b shows that reaction rate is almost independent of oxygen partial pressure. At low partial pressures, the CO oxidation rate increases more rapidly with oxygen than with CO, while at higher partial pressures the effect of oxygen on CO reaction rate is less pronounced than the effect of CO.

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Figure 5. Log-log plot of reaction rate vs reactant partial pressure for CO (a) and oxygen (b), with CuO/CeO2.

Figure 6. Preliminary Arrhenius plot for PLR equation.

This suggests that the reaction order with respect to oxygen should fall near zero, while a positive order with respect to CO is expected. For the evaluation of the apparent reaction orders with respect to oxygen and carbon monoxide, the logarithmic dependence of the reaction rate versus partial pressure of each gas has been plotted at different temperatures (Figure 5). Preliminary estimated partial reaction orders with respect to CO and oxygen were 0.36 ( 0.07 and 0.04 ( 0.01, respectively, calculated at the 95% confidence level. They were calculated as the arithmetic average at four temperatures, and used as initial guesses for simultaneous fitting of all data to the PLR model. Similarly, a preliminary evaluation of apparent activation energy and frequency factor was carried out, as shown in Figure 6, where values of Ea/R ) 4127 ( 54 K (i.e., Ea ) 31.2 ( 0.4 kJ/mol) and ln A0 ) 5.75 ( 0.16 were obtained. Power-Law Rate Equation. The whole set of experimental data has been simultaneously fitted to PLR equation of the form m POn 2, where PCO and PO2 are CO and O2 partial (-rCO) ) kPCO

pressures, respectively; m and n are partial reaction orders with respect to CO and O2, respectively; and k is the Arrhenius-type kinetic constant. The above rate expression does not take into account the effect of CO2 on reaction kinetics, which is assumable taking into account that our experiments were carried out free of CO2. The preliminary estimated values of reaction orders with respect to CO and O2 (0.36 and 0.04, respectively) as well as Arrhenius constants (Ea/R ) 4127 K and ln A0 ) 5.75) were taken as central values to randomly generate an initial population around them in the GA programming. Once the number of generations and the crossover fraction were optimized at 200 and 0.9, respectively, the GA was run. In Figure 7a, optimization of the crossover fraction is shown, while Figure 7b plots the fitness value (SSR) vs generation. Those values optimized for PLR equation have been used in all other rate equations. In Figure 7c,d, SSR as a function of the pair m, n and the parity plot of predicted vs experimental reaction rates are shown, respectively. The optimum parameters found for the model are given in Table 3, the most characteristic values being Ea ) 51.5 ( 0.3 kJ/mol; m ) 0.31 ( 0.03; n ) 0.08 ( 0.02. The statistics of the fitting were SSR ) 6.5553 × 10-8, Fc (Ft) ) 799 (2.533) and R2 ) 0.956. Liu et al.9 reported, for Cu-CeO2 prepared by coprecipitation, a LFS mechanism for CO oxidation reaction, which they simplify to a PLR equation with zero order with respect to oxygen and +1 with respect to CO for low CO partial pressures (PCO ) 0.003 atm, in excess oxygen), the apparent activation energy being a function of the Cu/(Cu + Ce) atomic ratio and the catalyst pretreatment. For a catalyst calcined at 923 K with Cu/(Cu + Ce) ) 0.25 (our catalyst is Cu/(Cu + Ce) ) 0.17), the authors report an activation energy of 56 kJ/mol, similar to our values. However, they report a preexponential factor 1 order of magnitude lower than ours, which is related to the high difference in BET area between both catalysts (our catalyst BET area is 5.3 times higher) and the lower Cu surface concentration in their catalyst (related to the preparation procedure). Also, partial reaction orders of +1 and zero with respect to CO and O2, respectively, were reported for CuO/MnOx catalysts prepared by coprecipitation,47 working in high oxygen excess and CO partial pressures below 0.0015 atm. The authors report apparent activation energy decreasing from 64 to 12 kJ/mol as the temperature increased from 273 to 373 K. Compared to our results, which include also data in defect of oxygen and a wider range of partial pressures of CO and oxygen (1.5 × 10-3 atm e PCO e 1.25 × 10-2 atm, 1.75 × 10-3 atm e PO2 e 1.5 × 10-2 atm; 0.35 e λ e 13.3), the reaction order found with respect to oxygen is similar, nearly zero, but the one with respect to CO is much smaller. However, activation energies are similar, considering the range of temperatures studied. When CO oxidation takes place in the presence of hydrogen (PROX reaction), PLR kinetic data for CO with CuO-CeO2 catalysts can be also found in the literature, for comparison. For example, apparent activation energy around 83 kJ/mol and average partial reaction orders of 1.02 with respect to CO (0.7 below 388 K, and increasing with temperature) and zero with respect to oxygen are reported,10 using CuO/CeO2 (4 wt % Cu) prepared by wet impregnation, in the 343-473 K range, PCO e 0.007 atm, in excess oxygen (λ > 1), and H2 concentration e 50 vol %; and Ea ) 94.4 kJ/mol and m ) 0.91 and n ) 0 for CuO/CeO2 prepared by coprecipitation, with Cu/(Cu + Ce) ) 0.2, in the presence of CO2 and H2O,11 in the 373-513 K temperature range, with 50 vol % H2, PCO e 0.04 atm and λ g 1. For similar catalysts, with Cu/(Ce + Cu) between 0.065 and


5639

Figure 7. (a) Fitness value (SSR) as a function of crossover fraction. (b) SSR evolution with the generation number, during data fitting with GA. (c) SSR contour lines and evolution of SSR through GA run (SSR has been multiplied by 108). (d) Parity plot of calculated (PLR) vs experimental reaction rates. All plots are for PLR. Table 3. Parameters Estimated for the Models and Statistics for the Fitting parameter

PLR

LH1

LH2

LH3

LH4

LH5

LFS

MVK

ln Ao (Ea/R) ln Ao,CO (Ea/R)CO ln Ao,O2 (Ea/R)O2 ln Bo,CO (∆H/R)CO ln Bo,O2 (∆H/R)O2 m n Statistics SSR ( × 108) R2 Fc (Ft)

11.89 ((0.12) 6222 ((36) 0.31 ((0.03) 0.08 ((0.02)

15.81 ((0.23) 6610 ((70) -12.33 ((0.95) -1919 ((344) -

15.81 ((0.23) 6610 ((70) -11.35 ((0.93) -1940 ((333) -

10.80 ((0.14) 6491 ((43) -11.08 ((0.68) -1614 ((237) -

12.96 ((0.12) 3190 ((38) 0.19 ((0.18) -2059 ((55) 0.31 ((0.35) -2598 ((121) -

13.39 ((0.12) 3708 ((35) 1.49 ((0.16) -1500 ((51) -0.06 ((0.27) -3047 ((84) -

11.14 ((0.11) 6372 ((35) 3.71 ((0.25) -746 ((83) 0.08 ((0.02)

14.63 ((0.18) 5679 ((60) 10.43 ((0.18) 6334 ((56) 0.10 ((0.03)

6.5553 0.956 799 (2.533)

63.914 0.528 50.7 (2.553)

63.915 0.528 50.7 (2.553)

9.2755 0.883 272 (2.553)

6.9997 0.952 538 (2.269)

6.7021 0.954 557 (2.269)

6.5476 0.957 603 (2.379)

6.5478 0.957 608 (2.379)

0.25, and prepared via coprecipitation with citrate, Marbań et al.48 report activation energy ranging between 40 and 50 kJ/ mol in the 333-473 K temperature range, working with very diluted reactants (1 vol % H2, PCO ) 0.0003 atm) in excess oxygen (λ ) 2); while Moreno et al.,49 for CuO/CeO2 prepared by urea combustion, report an activation energy of 55 kJ/mol, in the 383-433 K range, and reaction orders of 0.72 with respect to CO and ∼0 with respect to oxygen, with about 50 vol % H2, PCO between 0.01 and 0.025 atm, and λ g 1. Mechanistic-Derived Equations. Although the PLR equation satisfactorily fits the experimental data, it is limited to the experimental range of operation conditions. In Table 3, the parameters obtained with each mechanistic-derived model as well as the statistics for the fittings are listed. LH1 and LH2 models, although both statistically meaningful (their calculated F values are higher than those tabulated), are those worst fitting the data, as expected. Both models are derived assuming that both CO and oxygen compete for copper sites. It is stated that the reaction pathway for CO oxidation involves

surface reaction at the CuO-CeO2 interface where CO adsorbs onto the metallic copper, and reacts with the adsorbed active oxygen species nearby, at the interface around copper particles, or with lattice oxygen. Either lattice oxygen derived from ceria or activated superoxide ions generated on copper-CeO2 interface can react with adsorbing CO to form CO2.50,51 Superoxide species (O2-) formed on ceria from the adsorption of gas oxygen is assumed to be the active species in CO oxidation, due to its fast exchange rate with gas oxygen and easy surface diffusion.9 The requirement of copper-ceria interface for CO oxidation reaction is also demonstrated in refs 30 and 52. LH3 model, dual-site, improves fitting compared to singlesite LH1 and LH2 models (Fc increased from 50.2 to 272.31). However, this model still shows low R2 (0.883), and its fitting is still far from that obtained with PLR. Table 3 shows that statistics for LH4, LH5, LFS, and MVK models are very similar, all of them satisfactorily fitting the experimental data, with R2 above 0.95, and none of them can

5640


Figure 8. (a) Parity plot of calculated (LFS model) vs experimental reaction rates. (b) Discrete points: experimental reaction rate as a function of temperature, for a set of PCO,0 (at constant PO2,0 ) 0.01 atm). Continuous lines represent average reaction rate curves simulated with LFS model: (O) PCO,0 ) 0.0015 atm, (4) PCO,0 ) 0.005 atm, (b) PCO,0 ) 0.0085 atm, (9) PCO,0 ) 0.01 atm).

be directly discarded. In fact, Caputo et al.10 report that CO oxidation in a hydrogen environment is well described by the LH4 model. However, comparing LH4 and LH5 (with six adjustable parameters) with LFS and MVK (with five adjustable parameters), shows that introduction of a new parameter does not improve the fitting, but rather the opposite. Moreover, the LH4 model assumes that CO and oxygen are both adsorbed on lattice vacancies, which is against the findings reported in the literature for CuO/CeO2 catalysts,53 where CO adsorbs on Cu+ species, stabilized by interactions between copper oxide clusters and cerium oxide. LFS and MVK are statistically undistinguishable, as shown in Table 3. The parity plot of calculated reaction rates with LFS model and experimental rates is shown in Figure 8a. Lo´pez et al.54 have successfully used LFS model to rank copper-based catalyst activity in CO oxidation, in hydrogenrich environment. Also, Sedmak et al.13 have found that the LFS model adequately fits CO oxidation rates in the hydrogen environment, with nanostructured Cu0.1Ce0.9O2sy catalyst, in the 318-473 K range, with PCO about 0.01 atm and λ g 1. They reported an activation energy of 59 kJ/mol and a reaction order of 0.15 with respect to oxygen, the former very similar to our results, while the latter is somewhat higher. The pioneer work on Cu-CeO2 catalyst of Liu et al.9 reported an apparent activation energy of 56 kJ/mol, similar to our values, and n ≈ 0. For lanthanum-doped Cu-CeO2 catalyst,18 in the absence of hydrogen, values of n ) 0.08 and activation energy of 78 kJ/mol are reported, in the 313-473 K range and PCO e 0.06 atm; however, the heat of CO adsorption they reported (27.9 kJ/mol) exceeds by far the value found in this work (6.2 kJ/ mol) and that reported by Sedmak et al. (8.7 kJ/mol).13 Sedmak et al.13 found also that the MVK model adequately fit the CO oxidation rates, reporting activation energies of 57.2 and 60.2 kJ/mol for CO and oxygen, respectively (the latter slightly exceeding the former), very similar to those obtained in this work (47.2 and 52.6 kJ/mol for CO and oxygen, respectively). They report a somewhat higher reaction order with respect to oxygen (0.2, compared to 0.1), probably related to the different reaction environments: neatly reducing in the work of Sedmark et al., due to the presence of hydrogen, while in this work, without hydrogen, the oxidizing or reducing character is governed by λ (0.35 e λ e 13.3); as the reaction order with respect to oxygen in MVK is related to reoxidation of the catalyst surface.13 In a recent study, the original rate expression of MVK mechanism was found to be inconsistent and incorrect and, in

the words of the author, it should be viewed only as a mathematical function.55 For this reason, we have chosen the LFS model as the best model with physical significance to fit our experimental data. Conclusions In this work, the kinetics of CO oxidation over copper oxide supported on nanosized ceria has been studied. The CuO/CeO2 catalyst prepared by incipient wetness impregnation has been found to be very active for CO oxidation at low temperature, around the operation temperature of PEMFC. The active role of the support supplying oxygen to the reaction has been demonstrated by means of CO-TPD and OSC techniques. In our experimental conditions, the absence of mass and heat transport limitations was checked, so that experimental data in the kinetic regime were used to fit PLR and several mechanisticderived models, with GA. Data fitting to PLR equation resulted in partial reaction orders of 0.31 and 0.08 with respect to CO and O2, and an activation energy of 51.5 kJ/mol. The positive reaction order with respect to CO suggested a reaction mechanism other than typical Langmuir-Hinshelwood with competition between CO and O2 for a single active site. The two most meaningful models, among those analyzed, were those proposed by Liu and FlytzaniStephanopoulos (LFS) and Mars-Van Krevelen (MVK), which produced statistically indistinguishable results. However, the reported incoherence of the latter mechanism convinced us to choose LFS, which corresponds to a dual-site mechanism where ceria provides oxygen and copper provides sites for CO adsorption, and both species react at the boundary. Acknowledgment The authors wish to thank Spanish MEC (Project ENE200767975) for financial support. A.G. thanks MEC for FPU fellowship. Literature Cited (1) Liotta, A.; Macaluso, A.; Longo, A.; Pantaleo, G.; Martorana, A.; Deganello, G. Appl. Catal., A 2003, 240, 295. (2) Hilaire, S.; Wang, X.; Luo, T.; Gorte, R. J.; Wagner, J. Appl. Catal., A 2004, 258, 271. (3) Pintar, A.; Batista, J.; Hocevar, S. J. Colloid Interface Sci. 2007, 307, 145. (4) Jung, C. R.; Han, J.; Nam, S. W.; Lim, T. H.; Hong, S. A.; Lee, H. I. Catal. Today. 2004, 93-95, 183. (5) Avgouropoulos, G.; Ioannides, T. Appl. Catal., B 2006, 67, 1.

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ReceiVed for reView January 29, 2009 ReVised manuscript receiVed April 15, 2009 Accepted April 16, 2009 IE9001603

Kinetics of Carbon Monoxide Oxidation over CuO Supported on

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