ZnO

Ind. Eng. Chem. Res. 2005, 44, 41-50

41

Kinetics of the Low-Temperature WGS Reaction over a CuO/ZnO/Al2O3 Catalyst J. L. Ayastuy, M. A. Gutie´ rrez-Ortiz, J. A. Gonza´ lez-Marcos, A. Aranzabal, and J. R. Gonza´ lez-Velasco* Group of Chemical Technologies for Environmental Sustainability, Department of Chemical Engineering, Faculty of Sciences, Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea, P.O. Box 644, 48080 Bilbao, Spain

The kinetics of the water-gas shift reaction (WGSR) over a commercial copper/zinc oxide/alumina catalyst has been studied. Experiments were carried out in an isothermal plug-flow reactor at temperatures ranging from 453 to 490 K and a pressure of 0.3 MPa. Sixteen redox and associative models were proposed to obtain rate equations to which the obtained experimental data were fitted. Both the differential reactor approach and the integral reactor approach were used in obtaining the experimental data. The discrimination between models was based on the F test. Two statistically indistinguishable Langmuir-Hinshelwood models were found to be the best models, with apparent activation energies of 57.5 and 50.1 kJ/mol. Also, experimental data were fitted to a simple power-law rate equation, resulting in partial reaction orders of 0.47, 0.72, -0.65, and -0.38 for CO, H2O, H2, and CO2, respectively, with an apparent activation energy of 79.7 kJ/mol and high statistical significance. Introduction The water-gas shift reaction (WGSR)

CO + H2O / CO2 + H2

(∆H0 ) -41.1 kJ/mol) (1)

has a very broad application in industry,1,2 such as in ammonia synthesis and hydrogen production via the steam reforming of hydrocarbons. The main objective of the WGSR is to increase and adjust the H2/CO molar ratio in the synthesis gas and to remove CO from the gas effluents.3,4 In recent years, there has been renewed interest in the WGSR in fuel cell power generation systems. Proton-exchange membrane fuel cells (PEMFCs) requires CO concentrations below 20 ppm to avoid irreversible damage and hindering of the electrochemical reaction for Pt-based electrodes.5 In a major transportation application, PEMFCs operate with hydrogen produced by an on-board reforming unit followed by a WGSR stage to reduce the CO concentration.6 However, this CO concentration is too high to feed the cell, so that a preferential oxidation is required, with the inconvenient conversion to water of part of the H2 produced.7 Thus, numerous studies are being performed to develop new catalysts8-11 as well as new operational approaches12-14 to improve the WGSR. Although the WGSR is catalyzed by many materials,15-18 industrially, it is carried out in two adiabatic stages with two catalysts.1,19 The first stage uses an iron-based catalyst20-22 that operates at 573-723 K and converts most of the CO, up to the thermodynamically limited equilibrium limit. The second stage employs copper-based catalysts, also known as low-temperature (LTWGS) catalysts, because their activity occurs at 453-503 K and is thermodynamically favored, thus allowing the attainment of effluents with CO concentra* To whom correspondence should be addressed. Tel.: +3494-6012681. Fax: +34-94-6015963. E-mail: [email protected].

tions lower than 0.3 vol %.23-25 The composition of the copper-based catalysts is usually CuO/ZnO/Al2O3, and such catalysts are very sensitive to chloride or sulfur poisoning and to sinterization.4,26 Among the numerous studies concerning the mechanism of the WGSR,27-32 basically two types of mechanisms are distinguished. First, Langmuir-Hinshelwood mechanisms (also known as associative mechanisms) involve the formation of surface formate intermediates.18,33-39 On the other hand, redox or surface regenerative mechanisms assume the presence of oxygen adatoms on the active site.40-46 Grenoble et al.18 reported on WGSR chemistry and the catalytic activity of many alumina-supported metals. An associative mechanism with formic acid as an intermediate was proposed for the studied temperature range (403-523 K). Amadeo et al.35,36 studied the WGSR kinetic on a commercial CuO/ZnO/Al2O3 catalyst and fitted the experimental data obtained at 0.2 MPa and 453-503 K to both an associative and a redox mechanism, finding the associative mechanism to be more probable. Also, Van Herwijnen at al.,37 Yoshihara et al.,38 and Edwards et al.,39 among others, have proposed associative mechanisms for the reverse WGSR over polycrystalline copper, as they observed surface formate species but no oxygen adatoms. Ovesen et al.44 and Nakamura et al.45 studied the WGSR over clean Cu(110) and Cu(111) surfaces according to a microkinetic analysis. Both groups proposed that the rate-controlling step of the WGSR was the dissociation of water to give adsorbed oxygen atoms. Gine´s et al.46 studied the reverse WGSR on a CuO/ZnO/ Al2O3 catalyst and found the dissociation of CO2 to be the rate-controlling step when the H2 concentration was higher than the CO2 concentration; however, when this ratio was reversed, the main reaction rate was controlled by water formation. More studies concerning the mechanism of WGSR over different catalysts, with particular emphasis on single crystals, have recently been reported. However,

10.1021/ie049886w CCC: $30.25 © 2005 American Chemical Society Published on Web 12/08/2004

42

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005

Figure 1. Experimental setup.

few can be found reporting on copper-based catalysts.30,36,40,41,46,47 The aim of this paper is to determine the best mechanism and kinetics for the WGSR carried out on a commercial CuO/ZnO/Al2O3 catalyst that allows the obtained experimental data to be fit with a high statistical significance. Experimental Section Catalyst Characterization. The catalyst was the commercial product ICI 53-1, industrially obtained by the coprecipitation of copper, zinc, and aluminum nitrates with sodium carbonate, followed by sodium ions elimination, drying, and calcination, resulting in a ternary mixture of the corresponding oxides with the composition CuO/ZnO/Al2O3 ) 24.9/43.7/31.4 wt %. Textural properties of the precursor were evaluated from the nitrogen adsorption-desorption isotherms at 77 K, resulting in the following values: surface area, 91.9 m2/g; pore volume, 0.29 cm3/g; predominant pore diameters, between 10 and 20 nm. The copper surface area was measured by gas chemisorption (oxygen at 77 K, carbon monoxide at 293 K, and N2O pulses at 393 K), resulting in a mean value of 9.4 m2/g if a copper coverage of 1.35 × 1019 molecules/ m2 is assumed.48 The copper crystallite, calculated assuming spherical particles, resulted in a mean value of 14.2 nm, which corresponds to 6.8% of the copper particles being exposed to reactants. Experimental Setup. The experiments were carried out in an isothermal plug-flow reactor, a schematic of

which is shown in Figure 1. It consists of four sections: the feed section, the reaction section, the separation section, and the analysis section. The reaction mixture was composed of nitrogen (>99.9% purity), carbon monoxide (>99.9% purity), carbon dioxide (>99.5% purity), and hydrogen (>99.9% purity), all supplied by Argon, and ultrapure deionized water. The feed section consists of a three independent gas lines and a liquid line. Carbon monoxide, nitrogen, and a preprepared customized gas mixture containing carbon monoxide, nitrogen, hydrogen, and carbon dioxide were supplied using the gas piping. Water was pumped into a preheater, where it was vaporized. The reaction section consists of a stainless steel tubular reactor placed inside a furnace. The reactor was loaded with 0.25-0.40-mm-diameter catalyst particles, diluted with Pyrex glass of the same size to avoid temperature gradients along the bed. The bed temperature was measured with K-type thermocouples. Analysis of the feed and effluent gas was performed after water condensation by gas chromatograph (GC). The reaction products CO, H2, and N2 were separated by a molecular sieve column, whereas a Porapak column was used to separate CO2. The products were detected by a thermal conductivity detector (TCD). Helium was used as the eluent and nitrogen as an internal standard for the determination of CO conversion. The catalyst was activated “in situ” by reduction in a gas mixture of 4 vol % hydrogen in nitrogen that was passed through the catalyst bed for 12 h at 473 K and

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 43

used as constraints for the kinetic study, that is

Table 1. Experimental Conditions variable

operating range

variable

operating range

P (MPa) T (K) W (g) dP (mm)

0.3 453-490 0.10-1.40 0.25-0.40

y0CO 0 yH O 02 yH2 0 yCO2

0.065-0.358 0.043-0.270 0-0.531 0-0.267

1 h at 493 K. To avoid overheating of the catalyst bed and subsequent sintering of the copper particles, strict temperature and the hydrogen flow control was adopted. The experimental conditions, summarized in Table 1, ensure the absence of internal mass and temperature gradients (estimated in 0.12% for CO partial pressure and 0.14% for temperature) and axial gradients (a pressure loss of 2.6% of the total pressure through the bed). No side reactions were observed during experimental runs, and only CO, H2O, H2, CO2, and N2 were detected. The time duration of the experiments was 90 h, with the activity of the catalyst decaying gradually during the first 30-40 h until stabilization, as a result of initial contact of water vapor with the catalyst. Thus, we considered 50 h after the experiment initiation as the reference point for the kinetic study. During the period from 50 to 90 h, the activity remained constant, as chlorine-free water was used in our experiments to avoid chlorine poisoning. Constraints on Kinetic and Adsorption Constants. A total of 16 mechanistic models, listed in Table 2, were initially proposed, denoted as LH for a Langmuir-Hinshelwood mechanism, R for a redox-type mechanism, and ER for an Eley-Rideal mechanism. Assuming a rate-determining step for each mechanism, 70 rate equations were generated, each with the general form described by Hougen and Watson49

-rCO )

(kinetic group)(driving-force group)

)

(adsorption group)n kΦ(P,yj0,XCO,Ke) Φ(P,y0j ,XCO,Kj,a)n

(2)

The Arrhenius equation was used to describe the variation of the rate constant with temperature

A0 ) exp(∆S0r /R)

k ) A0 exp(-Ea/RT)

(3)

To reduce the correlation between the preexponential factor (A0) and the activation energy (Ea), a reparametrization was applied by choosing an appropriate mean temperature, Tm, that transforms the rate constant expression into the form

[ (

)]

Ea 1 1 k ) km exp R T Tm

)

[

exp ln km -

(

)]

Ea 1 1 R T Tm

(4)

On the other hand, the adsorption equilibrium constants were defined by the van’t Hoff equation 0 /RT) Kj,a ) Aj exp(-∆Hj,a

0 Aj ) exp(∆Sj,a /R)

(5)

where the preexponential factor (Aj) has to satisfy certain thermodynamic criteria for the mechanisms to be physically meaningful.50,51 These criteria should be

0 0 (i) ∆Sj,a < 0 or exp(∆Sj,a /R) ) Aj < 1

(6)

and the adsorption entropy must satisfy 0 0 (ii) -∆Sj,a < Sj,gas

(7)

0 (0.1 MPa, 298 K) values of 198.1, 188.9, 130.8, Sj,gas and 213.9 J/mol‚K were found in the literature52 for CO, H2O, H2, and CO2, respectively. Thus, from inequalities 6 and 7, the following numerical expressions can be formulated

0 /R) ) ACO > 4.064 × 10-11 exp(∆SCO,a 0 exp(∆SH /R) ) AH2O > 1.228 × 10-10 2O,a 0 exp(∆SH /R) ) AH2 > 1.385 × 10-7 2,a 0 exp(∆SCO /R) ) ACO2 > 6.014 × 10-12 2,a

In addition, the following rule for the adsorption entropy has to be observed51 0 0 (iii) 41.8 e -∆Sj,a e 51.04 - 0.0014∆Hj,a

(8)

For dissociative adsorptions, the same criterion is applicable, taking into account the following relation 0 0 0 ∆Sj,a ) 2Sj-diss,a - Sj,gas

(9)

Results Mechanistic Model. (i) Preliminary Model Discrimination: Differential Reactor Approach. For a preliminary discrimination of the proposed rate equations, the differential reactor approach was used. Assuming CO conversions of below 10%, the partial pressures become

PCO ≈ P0CO 0 PH2O ≈ PH 2O

P H2 ≈ 0 PCO2 ≈ 0

and the initial reaction rate can be calculated as

(-rCO)0 )

XCO W/F0CO

(10)

This approach allowed the initially proposed 70 mechanistical rate equations to be represented by 28 initial reaction rate equations. Data fitting was performed for three sets of experimental data obtained at constant temperatures of 467, 473, and 490 K. Linearized initial rate equations were checked by simple or multiple linear regression using the SPSS 9.0 software package. At this stage, 14 of the 28 initial rate equations were rejected because of either unacceptable values of the adsorption and kinetic constants or the lack of fit of experimental data by the linearized model; the remaining 14 initial rate equations, listed in Table 3, were accepted. (ii) Further Model Discrimination and Parameter Estimation: Integral Reactor Analysis. Figure

44


Table 2. Proposed LTWGSR Mechanisms mechanism

elementary steps

mechanism

LH1

LH1.1 LH1.2 LH1.3 LH1.4 LH1.5

CO + s / COs H2O + s / H2Os COs + H2Os / CO2s + H2s H2s / H2 + s CO2s / CO2 + s

LH2

LH2.1 LH2.2 LH2.3 LH2.4 LH2.5 LH2.6

CO + s / COs H2O + 2s / Hs + OHs COs + OHs / CO2s + Hs 2Hs / H2s H2s / H2 + s CO2s / CO2 + s

LH3.1 LH3.2 LH3.3 LH3.4 LH3.5

CO + s / COs H2O + r / H2Or COs + H2Or / CO2r + H2r H2r / H2 + r CO2s / CO2 + s

LH4.1 LH4.2 LH4.3 LH4.4 LH4.5

CO + s / COs H2O + 2s / Hs + OHs COs + OHs / COOHs + s COOHs / CO2 + Hs 2Hs / H2 + 2s

LH5.1 LH5.2 LH5.3 LH5.4

CO + s / COs COs+ H2O + s / COOHs + Hs COOHs / CO2 + Hs 2Hs / H2 + 2s

LH6.1 LH6.2 LH6.3 LH6.4 LH6.5

H2O + 2s / Hs + OHs CO + Hs / COHs COHs + OHs / COOHs + Hs COOHs / CO2 + Hs 2Hs / H2 + 2s

LH7

LH7.1 LH7.2 LH7.3 LH7.4 LH7.5

CO + s / COs H2O + s / H2Os COs + H2Os / COOHs + Hs COOHs / CO2 + Hs 2Hs / H2 + 2s

LH8

LH8.1 LH8.2 LH8.3 LH8.4 LH8.5 LH8.6

CO + s / COs H2O + 2s / OHs + Hs OHs + COs / HCOOs + s HCOOs + s / CO2s + Hs 2Hs / H2 + 2s CO2s / CO2 + s

LH3

LH4

LH5

LH6

2 shows an example of carbon monoxide conversion vs space time measurements made for a set of four different temperatures. For an integral plug-flow reactor, the reaction rate is calculated as

-rCO )

dXCO d(W/F0CO)

(11)

The kinetic parameters, estimated using the Levenberg-Marquardt nonlinear regression algorithm, were used to solve the mass balance for the isothermal plugflow reactor

W/F0CO ) f(XCO,k,Kj,...)

(12)

R1

R1.1 R1.2 R1.3 R1.4 R1.5 R1.6

CO + s / COs H2O + s / H2Os H2Os + s / OHs + Hs COs + Os / CO2 + 2s OHs + s / Os + Hs 2Hs / H2 + 2s

R2

R2.1 R2.2 R2.3 R2.4 R2.5 R2.6 R2.7 R2.8

CO + s / COs H2O + s / H2Os H2Os + s / OHs + Os 2OHs / Os + Hs OHs + s / Os + Hs COs + Os / CO2s + s 2Hs / H2 + 2s CO2s / CO2 + s

R3

R3.1 R3.2

H2O + s / H2 + Os CO + Os / CO2 + s

ER1

ER1.1 ER1.2 ER1.3

H2O + s / H2Os H2Os + CO / H2 + CO2s CO2s / CO2 + s

ER2

ER2.1 ER2.2 ER2.3

H2O + s / H2Os H2Os + CO / CO2 + H2s H2s / H2 + s

ER3

ER3.1 ER3.2 ER3.3

CO + s / COs COs + H2O / CO2s + H2 CO2s / CO2 + s

ER4

ER4.1 ER4.2 ER4.3

CO + s / COs COs + H2O / H2s + CO2 H 2 s / H2 + s

ER5

ER5.1 ER5.2 ER5.3

CO + s / COs COs + H2O / HCOOHs HCOOHs / CO2 + H2 + s

were used for the discrimination. The computer program checked that model parameters satisfied eqs 6-9. The initial rate equations accepted by the differential reactor approach could represent 25 of the 70 initially proposed rate equations for the studied mechanisms, namely, those listed in the last column of Table 3. The discrimination between rival models was done by applying the F test on the sum of residual squares at the 95% confidence level. As few replicated experiments were performed, the applied F test was based on the regression sum of squares and the sum of residual squares53 N

∑ i)1

F) N

The integration of the mass balance was realized by an explicit fourth-order Runge-Kutta method. The optimum parameters were obtained by minimizing the objective function, the sum of residual squares, N (XCO - X ˆ CO)2. A total of 199 experimental data ∑i)1

elementary steps

∑ i)1

X ˆ CO2 p

(XCO - X ˆ CO)2

(13)

N-p

If the calculated F value is higher than the tabulated ones, the regression is statistically meaningful. The

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 45 Table 3. Accepted Initial Rate Equations by Isothermal Data Fitting in Differential Reactor Approach initial rate equation

Ea (kJ/mol)

ln A0

related rate-determining steps

63.6

16.65

LH3.2, LH6.1, E1.1, E2.1, R3.2

78.0

20.40

LH1.2

76.7

20.06

LH2.2, LH4.2

90.9

24.78

LH5.2

62.1

17.25

LH6.3

66.9

18.47

E1.2, E2.2, R3.1

73.3

22.25

LH1.3, LH4.3, LH7.3

38.0

11.47

LH3.5, E1.3

62.7

16.84

R1.3, R2.3

92.9

24.06

LH8.4

74.3

22.50

LH4.5, LH5.4

62.5

15.21

LH2.6

28.5

10.18

LH6.5

75.0

22.25

LH7.5

0 (-rCO)0 ) kPH 2O

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

(-rCO)0 )

0 kPH 2O

1 + k1P0CO 0 kPH 2O

(1 + k1P0CO)2 kP0CO (1 + k1P0CO)2 0 kP0CO PH 2O 0 (1 + k1PH )2 2O 0 kP0CO PH 2O 0 1 + k1PH 2O 0 kP0CO PH 2O 0 (1 + k1P0CO + k2PH )2 2O 0 kP0CO PH 2O 0 0 1 + k1PH + k2P0CO PH 2O 2O 0 kPH 2O 0 (1 + k1P0CO + k2PH )2 2O 0 kP0CO PH 2O 0 (1 + k1P0CO + k2P0CO PH )2 2O 0 kP0CO PH 2O 0 [1 + k1P0CO + k2(P0CO PH )0.5]2 2O 0 kP0CO PH 2O 0 0 1 + k1P0CO + k2(PH )0.5 + k3P0CO PH 2O 2O 0 kP0CO PH 2O 0 0 [1 + k1(P0CO PH )0.5 + k2P0CO(P0CO PH )0.5]2 2O 2O 0 kP0CO PH 2O 0 0 [1 + k1P0CO + k2PH + k3(P0CO PH )0.5]2 2O 2O

highest calculated F value corresponds to the model that best fits the experimental data. Also, the t test was applied at the 95% confidence level to check whether each parameter value differed significantly from zero. Parameters for which the t value approached zero or the confidence interval was large and included zero could be omitted from the model. Results and Discussion Comparison of F values of the checked kinetic models identifies LH1.3 as the best rate model, with a calculated 10-parameter F value of 1.52 × 104 and a variance of 6.821 × 10-5. The second-best rate model is the 10parameter LH7.3, with a calculated F value of 1.21 × 104 and a variance of 8.568 × 10-5. The improvement of the calculated F value of LH1.3 with respect to LH7.3 is 1.25. As the tabulated F value for the 95% confidence level was 1.27, which is higher than the calculated values, the two models can be considered as statistically

indistinguishable. Both rate equations correspond to associative or Langmuir-Hinshelwood mechanisms. On the other hand, the best rate equation derived from a redox-type mechanism is the four-parameter R3.2, with a calculated F value of 2.25 × 103 and a variance of 1.177 × 10-3. The selected rate model LH1.3 is derived from the mechanism LH1 with a surface reaction of molecularly adsorbed reactants as the rate-determining step (CO-s + H2O-s f CO2-s + H2-s)

-rCO )

(

k PCOPH2O -

)

PCO2PH2 Ke

(1 + KCOPCO + KH2OPH2O + KH2PH2 + KCO2PCO2)2 (14) This rate equation model was reported earlier by

46

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 Table 4. Calculated Model Parameters for the LH1.3, LH7.3, R3.2, and Power-Law Rate Equations parameter

Figure 2. Conversion of carbon monoxide vs space time at 0 different reaction temperatures: P ) 0.3 MPa, y0CO ) 0.075, yH 2O 0 0 ) 0.101, yH2 ) 0.273, and yCO2 ) 0.102; GHSV ) 9700 1/h.

Amadeo et al.36 for the same type of catalyst and similar ranges of operating conditions, as well as by Campbell et al.2 and Podolski et al.54 for iron-based catalysts. Figure 2 shows that the experimental data are very well fitted with this rate equation (solid line). Similarly, the LH7.3 kinetic model was derived from the LH7 mechanism assuming that the rate-determining step was the surface reaction between molecularly adsorbed water and carbon monoxide to give a formate intermediate and atomically adsorbed hydrogen (CO-s + H2O-s f COOH-s + H-s) -rCO )

(

k PCOPH2O -

)

PCO2PH2 Ke

(1 + KCOPCO + KH2OPH2O + KH20.5PH20.5 + KCO2PCO2PH20.5)2

(15) The best redox mechanism, R3.2, was derived assuming rate control by the reduction of oxidized copper (CO + O-s f CO2 + s). The rate equation derived for this model is

-rCO )

(

k PH2O -

)

PCO2PH2

PCOK 1 + KCOPCO2/PCO

(16)

The model parameters obtained for rate eqs 14-16 are listed in Table 4. The relative variances for the three models are 0.78%, 3.72%, and 2.44%, respectively. As none of the parameters have a significatively low t value and none of the confidence intervals include zero, all parameters should be considered as statistically significant. Parity plots of the LH1.3, LH7.3, and R3.2 rate equations are shown in Figure 3. Gine´s et al.46 studied the reverse WGSR over CuO/ ZnO/Al2O3 catalyst and found the reaction pathway to proceed via redox mechanism R1, with CO2 dissociative adsorption as the rate-determining step under hydrogenrich conditions. By the principle of microscopic reversibility, the forward reaction mechanism and ratedetermining step could be the same, R1.4, but this

mean

standard deviation

95% confidence level interval

|t value|

ln Ao Ea/R (K) ln ACO ∆HCO/R (K) ln AH2O ∆HH2O/R (K) ln AH2 ∆HH2/R (K) ln ACO2 ∆HCO2/R (K)

18.38 6940.6 -1.735 1370.5 -3.051 1800.0 -3.100 1743.7 -3.322 1978.0

LH1.3 0.26 103.96 0.18 70.8 0.27 109.52 0.30 120.42 0.53 210.29

ln Ao Ea/R (K) ln ACO ∆HCO/R (K) ln AH2O ∆HH2O/R (K) ln AH2 ∆HH2/R (K) ln ACO2 ∆HCO2/R (K)

16.99 6049.2 -2.362 1782.1 -3.403 2088.8 -3.459 2057.7 -5.765 3003.5

LH7.3 1.18 68.96 0.67 38.65 0.90 53.1 1.21 73.55 1.69 108.8

(2.37 (137.72 (1.33 (77.30 (1.81 (106.3 (2.43 (147.1 (3.38 (217.6

14.35 87.85 3.55 46.11 3.79 39.31 2.85 27.98 3.41 27.60

ln Ao Ea/R (K) ln ACO ∆HCO/R (K)

19.12 8681.3 -5.650 3203.9

R3.2 1.45 697.2 0.85 71.9

(2.90 (1394.4 (1.69 (143.9

13.18 12.48 6.68 44.52

a b c d ln A0 Ea (J/mol)

0.47 0.72 -0.65 -0.38 19.25 79.7 × 103

(0.07 (0.05 (0.09 (0.05 (1.05 (4.0 × 103

14.24 27.69 13.83 15.83 36.67 39.50

Power Law 0.04 0.02 0.05 0.02 0.53 2.0 × 103

(0.52 (207.9 (0.36 (141.6 (0.54 (219.1 (0.60 (240.8 (1.07 (420.57

70.69 66.91 9.64 19.36 11.30 16.44 10.33 14.48 6.27 9.41

model was rejected here in the previous model discrimination step. The same conclusion was made by Fujita et al.28 for the reverse WGSR over Cu/ZnO catalyst. Ovesen et al.44 reported an R2 mechanism over Cu(111) and also reported an R2.6 rate-determining step when the water-to-CO ratio is high and an R2.3 ratedetermining step when the water-to-CO ratio is low. We calculated the probability of the R2.3 model (the rate equation with highest F value among the redox models) as being practically null. Campbell et al.29 fitted their experimental data to both mechanisms: if formate is considered in the mechanism, the dissociation of water is the controlling step (LH4.2), and if a redox mechanism is considered, dissociative water adsorption is the controlling step (R1.3). As calculated with our data, the LH4.2 rate equation has a much lower F value (6.15 × 102) than LH1.3 (1.52 × 104), but the R1.3 rate equation has a closer F value (1.89 × 103). Nakamura et al.45 showed strong evidence that redox-type mechanisms with the dissociative adsorption of water are the controlling step, which is the equivalent to the R2.3 rate equation whose low F value (1.51 × 102) permits us to reject this possibility. Empirical Power-Law Model. The design and optimization of an industrial reactor requires high computational efforts that can be facilitated by the use of empirical power-law rate models. To this end, the experimental data were also fitted to the equation

-rCO ) kPCOaPH2ObPH2cPCO2d(1 - β)

(17)

where a, b, c, and d are the apparent reaction orders


Figure 3. Parity plots for the (a) LH1.3, (b) LH7.3, (c) R3.2, and (d) power-law rate equations, with the interval of (10% of error.

for CO, H2O, H2, and CO2, respectively, and

β)1-

PH2PCO2 1 PCOPH2O Ke

represents the approach to equilibrium. Other authors47,55 have introduced a “fudge factor” correcting for the total pressure changes, but this is only necessary in experiments carried out at elevated pressures. Because the pressure in this work was 0.3 MPa, such a correction is not necessary: all calculated fugacity coefficients have a value of 1.0.56 The power-law rate equation was used to fit experiments with nonzero inlet H2 and CO2 partial pressures. The temperature was varied in the interval between 458 0 /P0CO ratio was varied from and 490 K. The initial PH 2O 0.68 to 2.72. The resulting statistical parameters of this model are included at the end of Table 4. As in the case of mechanistic rate model discrimination, the optimum parameter values were determined by searching for the minimum squared sum of residuals of CO conversion. The available number of experimental data points was 89. The calculated F value was 4.12 × 103, and the squared sum of residuals was 1.605 × 10-2, with a variance value of 1.934 × 10-4. The apparent activation energy was 79.7 kJ/mol. The t test was also carried out for each parameter, resulting in all parameters having significance in the checked model. Moreover, because the zero value was not contained in any confidence interval, the fit can also be considered as good.

Table 5. Calculated F Value and Squared Sum of Residuals for the WGSR Rate Models Checked by Integral Analysis Assuming a ) 1 parameter

mean

standard deviation

95% confidence level interval

|t value|

b c d ln A0 Ea (kJ/mol)

0.53 -0.51 -0.26 20.32 81.3

0.02 0.03 0.01 0.35 13.6

(0.03 (0.06 (0.03 (0.71 (2.7

16.18 8.69 8.22 28.70 29.88

A comparison of the calculated CO conversion (for the empirical power-law rate) and the measured CO conversion is shown in Figure 3d. The reaction orders for all of the reactant and products were quite different from those reported by Amadeo et al.36 and Ovesen et al.47 over ternary Cubased catalyst. Both reported a unity order with respect to CO. With respect to water vapor, the corresponding published reaction order values were 0.8 and 1.1. In the case of the products, our values were the lowest, as the previously reported results for hydrogen and carbon dioxide were -0.43 and -0.27 and -0.9 and -0.7, respectively. Salmi et al.27 reported variable reaction orders as a function of temperature, ranging from 1 to 0.45 for CO and from 0.55 to 0.07 for water, as the temperature falls from 523 to 473 K. Fishtik and Datta57 found reaction orders on Cu(111) as a function of temperature and reported CO, water, hydrogen, and CO2 orders of close to 0.6, 1, -0.3, and 0, respectively, at 473 K. Our results revealed that the sensitivity of the reaction rate to water pressure must be higher than that to CO, as was also reported by Fishtik and Datta.57 Other authors have reported the contrary. In the case

48


Table 6. Comparison of Our TOF Value with Those Reported in the Literature catalyst

TOF (1/s)

conditions

TOFa (1/s)

Cu(111)

4.0 ×

T ) 588 K PCO ) 3.4 kPa PH2O ) 1.3 kPa

1.2 ×

Cu(111)

1.2 × 10-1

T ) 470 K PCO/PH2O ) 9.9 P ) 0.1 MPa

5.9 × 10-2 c

Ovesen et al.44

Cu(110)

1.2 × 100

T ) 470 K PCO/PH2O ) 9.9 P ) 0.1 MPa

5.6 × 10-1 c

Ovesen et al.44

Cu(111)

9.7 × 10-1

T ) 540 K PCO/PH2O ) 2.6 P ) 0.1 MPa

1.2 × 100 d

Nakamura et al.45

Cu(110)

4.6 × 100

T ) 540 K PCO/PH2O ) 2.6 P ) 0.1 MPa

1.2 × 100 d

Nakamura et al.45

Cu/Al2O3 (10% Cu)

3.4 × 10-3

T ) 573 K PCO ) 24.3 kPa PH2O ) 31.4 kPa

8.0 × 10-4 e

Grenoble et al.18

-

3.0 × 10-1 f

Amadeo et al.35

CuO/ZnO/Al2O3

-

10-2 b

ref

10-2

Campbell et al.29

CuO/ZnO/Al2O3

3.4 × 10-3

T ) 473 K PCO/PH2O ) 0.044 P ) 0.1 MPa

7.0 × 10-3 g

Mellor et al.17

CuO/ZnO/Al2O3

2.5 × 10-1 h

T ) 503 K PCO/PH2O ) 0.33 P ) 0.1 MPa

1.4 × 10-1

Gine´s et al.58

T ) 473 K PCO ) 10.2 kPa PH2O ) 30.6 kPa P ) 0.1 MPa

4.3 × 10-1 i

this work

CuO/ZnO/Al2O3

-

a Values extrapolated to our conditions: T ) 473 K, P b CO ) 10.2 kPa, PH2O ) 30.6 kPa, and P ) 0.1 MPa. Activation energy of 71.1 kJ/mol and reaction orders for CO and H2O of 0 and 0.75, respectively. c Activation energies of 44.3 and 73.7 kJ/mol for Cu(111) and Cu(110), respectively, and reaction orders for CO and H2O of 0.075 and 0.85, respectively. d Activation energy of 41.8 kJ/mol and reaction orders for CO and H2O of 0.2 and 0.9, respectively. SCu ) 1.09 × 1015 Cu atoms/cm2. e Activation energy of 55.6 kJ/mol and reaction orders for CO and H2O of 0.3 and 0.38, respectively. f Calculated from the rate equation assuming SCu ) 13 m2/g. g Assuming reaction orders for both CO and H2O of 1. h Mean value. i Assuming LH1.3 rate equation.

of reaction products, our data revealed that the reaction rate is more sensitive to the hydrogen partial pressure than to the CO2 partial pressure, in agreement with the literature.27,36,47,57 As the CO reaction order is close to 1 over CuO/ZnO/ Al2O3 catalysts,27,36,47 a fitting with this reaction order was also carried out. The parameters calculated with this assumption are listed in Table 5. On the other hand, the reaction order was found to be close to zero when Cu(111) catalyst was employed.29,44,57 Discussion To compare the activity of the CuO/ZnO/Al2O3 catalyst considered in this work with those of other catalysts reported in the literature, turnover frequencies (TOFs) were calculated for some experimental data. The TOF value calculated for our catalyst at 473 K and 0.1 MPa, with PCO and PH2O values of 10.2 and 30.6 kPa, respectively, is 0.43 1/s. TOF values reported in the literature and also values extrapolated to our conditions are listed in Table 6. Ovesen et al.44 and Nakamura et al.45 investigated the reaction over the clean Cu(110) and Cu(111), and both agreed with the idea that the activity of Cu(111) is 1 order of magnitude lower than that of Cu(110). Because the values reported for Cu(110) are closer to our TOF value than the Cu(111) values, this could indicate that our coprecipitated catalyst included many more Cu(110) than Cu(111) planes. The high divergence between the TOF value in this work and that calculated for a similar industrial CuO/

ZnO/Al2O3 catalyst used by Mellor et al.17 (60 times lower) might be due to the experimental conditions, as they calculated the rate with a much higher H2O/CO ratio. The very low value calculated for the Grenoble et al.18 data for alumina-supported 10% Cu is due to the low copper content and the very low fraction exposed (0.03). The TOF value calculated in this work is the same order of magnitude as that calculated for catalysts of similar composition (ternary CuO/ZnO/Al2O3), except for that reported by Mellor et al.,17 and 1 order of magnitude higher than those for Cu(111)-face catalysts. The activation energy calculated in this work is 57.5 kJ/mol assuming the LH1.3 model. In the literature, the activation energy range for WGSR on ternary CuO/ZnO/ Al2O3 catalysts is between 45 and 180 kJ/mol, but most of the reported values are close to 80 kJ/mol. Conclusions The mechanism of the WGSR over commercial CuO/ ZnO/Al2O3 catalyst was investigated. At the given operating conditions, the best fit of the experimental data was obtained for two statistically indistinguishable Langmuir-Hinshelwood kinetic models. When molecular adsorption of all reactants and bimolecular surface reaction as the rate-controlling step were assumed, an apparent activation energy of 57.5 kJ/mol and a frequency factor for the kinetic constant of 9.6 × 107 mol/g‚h‚atm2 were obtained, whereas when molecular


adsorption of all reactants except for hydrogen and bimolecular surface reaction as the rate-controlling step were assumed, an apparent activation energy of 50.1 kJ/mol and a frequency factor for the kinetic constant of 2.4 × 107 mol/g‚h‚atm2 were obtained. For further discrimination between the two models, additional information about the absorption of reactive species on the active sites, obtained using spectroscopy techniques, is necessary. Rate equations deduced from the redoxtype mechanism were found to be far less probable, as shown by the statistical F test. An empirical power-law rate equation applied to fit the experimental data resulted in an apparent activation energy value of 79.7 kJ/mol and the reactant reaction orders of 0.47, 0.72, -0.65, and -0.38 for CO, H2O, H2, and CO2, respectively. The turnover frequencies calculated for a wide range of operating conditions varied between 0.03 and 1.7 1/s, values of the same order as those found in the literature, particularly for ternary Cu-based catalysts. Acknowledgment The authors are thankful for Project 9/UPV 13517/ 2001 for financial support. Nomenclature Aj ) preexponential factor of the adsorption constant for component j A0 ) preexponential factor of the kinetic constant a, b, c, d ) reaction orders for CO, H2O, H2, and CO2, respectively dP ) catalyst particle diameter, mm Ea ) activation energy, J/mol ER ) Eley-Rideal mechanism F ) F test value FCO ) molar flow rate of carbon monoxide, mol/h Ke ) equilibrium constant for the WGSR Kj,a ) adsorption constant for component j K1, K2, K3 ) adsorption group constants for initial rate equations k ) kinetic constant for the WGSR LH ) Langmuir-Hinshelwood mechanism N ) number of experimental data points n ) adsorption group exponential P ) pressure, Pa Pj ) partial pressure of component j p ) number of parameters PEMFC ) proton-exchange membrane fuel cell R ) gas constant ) 8.314 J/mol‚K R ) redox mechanism r ) catalyst site -rCO ) reaction rate for the WGSR, mol/g‚h 0 Sj-diss,a ) standard entropy of component j in the dissociatively adsorbed form, J/mol‚K 0 Sj,gas ) standard entropy of component j in the gas phase, J/mol‚K s ) adsorbed, catalyst site T ) temperature, K t ) t test value TOF ) turnover frequency, 1/s W ) catalyst mass, g WGSR ) water-gas shift reaction XCO ) experimental carbon monoxide conversion X ˆ CO) calculated carbon monoxide conversion yj ) mole fraction of component j Greek Letters β ) approach to equilibrium

0 ) standard heat of adsorption of component j ∆Hj,a ∆H0 ) standard heat of reaction, J/mol 0 ∆Sj,a ) standard entropy of adsorption of component j

Subscript m ) mean Superscript 0 ) inlet, initial

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Received for review February 11, 2004 Revised manuscript received September 1, 2004 Accepted October 6, 2004 IE049886W

ZnO

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