Kinetics of the Preferential Oxidation of CO over CuO/CeO2 Catalysts in H2-RichGases

Tiziana Caputo,† Luciana Lisi,‡ Raffaele Pirone,*,‡ and Gennaro Russo†

Dipartimento di Ingegneria Chimica, UniVersita Federico II, Napoli, Italy, and Istituto di Ricerche sullaCombustione, CNR, Napoli, Italy

The influence of reaction parameters such as temperature, contact time, and O2, CO, H2, CO2, and H2Opartial pressures on the catalytic performances of 4 wt % CuO/CeO2 in the CO-PROX process (the preferentialoxidation of CO) has been investigated in a fixed-bed reactor. Catalyst appears very active (light-off at 70°C)and selective; reaction kinetics is weakly depressed by CO2 and H2O and not significantly increased withincreasing O2 concentration, while the reaction order in CO is variable with temperature ranging from values<1 up to 110°C and>1 at higher temperatures. A power-law rate equation fitted the experimental dataunder typical CO-PROX conditions in order to evidence the lower activation energy of the catalytic oxidationof CO with respect to H2. A Langmuir-Hinshelwood type reaction rate is able to describe the catalyticbehavior in the whole field of experimental conditions explored.

Introduction

The development of polymer electrolyte membrane (PEM)fuel-cell technology requires the production of CO-free hydrogensince the functionality of Pt catalysts for the electrodes is greatlyhindered by even traces of CO (tolerance limited to 10-100ppm). Actually, the gas produced in a fuel processor by auto-thermal reforming and, subsequently, water gas shift stages istypically composed by 50-75 vol % H2, 10-20 vol % CO2,5-10 vol % H2O, and 0.5-1 vol % CO. Thus, the COconcentration must be significantly reduced. For nonstationaryapplications, the catalytic abatement of CO is the only feasibleway, because of the high costs and dimensions of the compressorin the pressure-swing adsorption and because of the presenceof high CO2 concentrations that may complicate methanation.Such a catalytic process should operate in a range of temperaturefixed by the fuel processor at 80-200 °C and must be veryactive and selective toward the oxidation of CO more than ofH2. Moreover, the catalyst should not catalyze the reverse watergas shift (WGS) reaction in order to prevent limitations to themaximum CO conversion.

Supported noble metals catalysts such as Pt1-5 or Au6-10 wereproposed for the CO-PROX process (the preferential oxidationof CO). Nowadays, the most investigated alternative systemsto noble metals for CO-PROX are CuO/CeO2 catalysts.11-16

Because of their high activity and selectivity toward COoxidation, these materials seem to attract the major interest ofthe scientific community. However, despite the large researcheffort devoted to such a topic, a specific kinetic study of COoxidation in H2-rich streams over CuO/CeO2 is still missing. Alarge number of investigations has been devoted to the prepara-tion of catalysts samples,12,16-18 to the identification of activespecies or Cu oxidation state,19-21 and even to the reactionmechanism,13 but a quite complete experimental characterizationof the reaction kinetics, although not modeled, has been reportedonly by Ratnasamy et al.22 Sedmak et al.23 also studied thekinetics of CO-PROX over a nanostructured Cu0.1Ce0.9O2-y

catalyst but subsequently modeled only the CO oxidation (in

the absence of H2),24 elaborating a Mars-Van Krevelen rateequation on the basis of low-temperature data at 100% selectiv-ity.23

In this paper, we have experimentally investigated thepreferential oxidation of CO in large H2 excess over a CuO/CeO2 catalyst by studying the effects of the reaction parametersand of the presence of reformate species as CO2 and H2O onthe reaction rate. Particular care has been devoted to thecompetition between the two reactions of oxidation of CO andH2 by exploring a quite large field of possible temperatures ofinterest and a wide range of contact times, large enough toeffectively test the actual potentialities of the catalyst in reducingthe CO concentration to a few ppm. The kinetics of bothreactions have been preliminarily modeled, i.e., with empiricalpower-law rate equations to give more insight on the competitionbetween CO and H2 oxidation, and finally, more mechanicisticrate equations have been also tested to describe the reactionkinetics.

Materials and Methods

Preparation of Catalyst. A commercial CeO2 (Grace,SBET

) 56 m2/g) was used as support. The CuO/CeO2 catalyst wasprepared by wet impregnation of ceria with a (CH3COO)2Cu‚H2O (Aldrich, 99.8%) solution with a suitable concentration toobtain a 4 wt %CuO/CeO2 sample. The material was driedovernight at 120°C and calcined at 450°C under dry air flow(5 Nl/h) for 3 h.

Catalysts Characterization.The copper content was evalu-ated by spectrophotometric analysis using a UV Hach-Drel/2000spectrophotometer at 560 nm. X-ray diffraction (XRD) analysiswas performed with a Philips PW 1710 diffractometer withrotating anode generators and a monochromatic detector usingCu KR radiation. Specific surface area was determined by themultipoints Brunauer-Emmett-Teller (BET) method with aCarlo Erba Sorptomatic 1900.

Catalytic Activity Tests. Catalytic tests were carried out ina fixed-bed reactor, pretreating the sample in air at 400°C for0.5 h and subsequently purging with N2 at the reactiontemperature. The standard composition of the feed was 0.5 vol% CO, 0.5 vol % O2, and 50 vol % H2 in N2. A high-purity 5vol % CO/N2 mixture and pure CO2 and O2 were used for the

* To whom correspondence should be addressed. E-mail: pirone@irc.cnr.it.

† UniversitaFederico II.‡ Istituto di Ricerche sulla Combustione.

6793Ind. Eng. Chem. Res.2007,46, 6793-6800

10.1021/ie0616951 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 09/13/2007

feed mixture, whereas a hydrogen generator (CLAIND) provideda pure H2 stream. Water was added by saturating the reactionmixture at a given temperature by assuming that H2O concentra-tion corresponds to the water vapor partial pressure at thetemperature of the saturator. The reaction temperature rangedfrom 70 to 200°C, and the contact timeW/F ranged from 0.02to 0.085 g‚s/cm3.

The catalyst (φ ) 150-212 µm) was placed in a tubularquartz reactor. The gases were analyzed with a EmersonNGA2000 analyzer equipped with four channels for thesimultaneous analysis of CO and CO2 (IR detector), H2 (thermalconductivity detector (TCD)), and O2 (paramagnetic detector).

Kinetic Model

The reactor model has been developed on the basis of thefollowing hypotheses:

(1) isothermicity;(2) pressure constancy;(3) no axial dispersion along the catalyst bed (L/dp > 50)

and perfect mixing along the radius (D/dp > 10);(4) no effects of intraparticle diffusion; and(5) no diffusional resistance through the gas film surrounding

the catalyst particle (pseudo-homogeneous model).All these assumptions have been suggested and justified by

experimental results. The reactor isothermicity is assured by(i) the quite low adiabatic temperature rise (∼40 °C at completeCO conversion) and (ii) the use of a microreactor with annularcross section (higher external surface per unit volume) charac-terized by a high heat dissipation toward its surroundings(constituted by a “three -zone-controlled” electrical furnace).Moreover, a uniform temperature profile has been revealed bythe internal thermocouple placed along the reactor axis.

The estimation of the reactor Peclet number (Pe) VgasL/Da)allowed us to neglect the axial back-diffusion (Da),

whereL ) 0.5-1 cm, dp ) 150-212 µm, andPedp ) 2 forcatalytic fixed-bed reactors forRep > 0.01-0.1.25

The assumption of neglecting intra- and extraparticle diffusionterms has been checked by carrying out reaction tests withdifferent catalyst particle sizes and/or flow rates while keepingthe same contact time, as well as by theoretical estimates.

The measured values of pressure drop in the microreactorwere variable with the experimental conditions but always withinmaximum valuese0.1 atm, as expected by the Ergun Law26

for low values of Reynolds number (Rep ≈ 10). The modelequations are mass-balance relationships for CO and H2 andconsist of a system of two differential equations.

In the parameters estimation code, the reactor model is asubroutine that expresses the value of CO and O2 conversionsexpected by the model as functions of the vector of parametervaluesυ ) (ki, Kj) and experimental conditions involved in eachsingle test:x ) (T, P°CO, P°C2

, P°CO2, τ) in order to generate the

following function Φ, which should be minimized by meansof another algorithm to find the “best-fitting” parameter valuesυ,

whereN is the number of experiments done;Xi,CO andXi,O2 arethe conversions of CO and O2, respectively, in theith experi-

ment; and the superscripts exp and calc denote the experimentaland the calculated (by the reactor model) values, respectively.

Both the reactor model and the minimization algorithm havebeen processed by means of the ODE45 and FMINS subroutinesof the commercial software Matlab, respectively.

Results and Discussion

The actual copper content of the sample investigated in thiswork is 4.2 wt % CuO, as detected by the spectrophotometricanalysis. No CuO aggregates were detected by XRD analysis,although a slight reduction of the original surface area of thesupport is observed (50 m2/g). The absence of both internal andexternal diffusion limitations was theoretically and experimen-tally verified.

In Figure 1, CO and O2 conversion and O2 selectivity to CO2

are reported as functions of the temperature at constantW/F.The catalyst exhibits high activity and selectivity toward COoxidation with 100% selectivity up to 100°C.

All data reported here are steady-state results; no catalystdeactivation was observed also for long-run tests. Furthermore,under the reaction conditions investigated in this paper, thetransient period to reach a steady state after the introduction ofreactants is very short, with final values of conversion andselectivity being reached after a few minutes, if not seconds.

The light-off of CO oxidation lies at∼70 °C, a temperatureeven lower than that of many noble metals for the CO-PROX

Figure 1. CO (a) and O2 (b) conversions and O2 selectivity to CO2 (c) asfunctions of the reaction temperature (W/F ) 0.023 g‚s/cm3). Continuouslines represent the numerical fitting with the power-law rate equations.Pe) Pedp

‚ Ldp

≈ 80

Φ ) ∑i)1

N

(Xi,COexp - Xi,CO

calc)2 + ∑i)1

N

(Xi,O2

exp - Xi,O2

calc)2

6794 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

reaction (Au excluded). At higher temperatures, the oxidationof hydrogen also starts (light-off at about 120°C), thus reducingselectivity, whereas a maximum CO conversion is observed.The appearance of such a maximum in CO-PROX tests is quitefrequent even over catalytic systems very different in theirchemical nature. The possible occurrence of reverse WGSreaction, disallowing CO conversion to be complete, has beensupposed for other catalytic systems; however, this was excludedover CuO/CeO2 by carrying out suitable tests.15 Moreover, H2

and CO temperature-programmed reduction (TPR) experi-ments23,27 showed that CO is able to reduce the catalyst attemperatures much lower than H2, confirming the differentaffinity of the two molecules toward catalyst sites, which isprobably the basic reason for the high selectivity. Hence, theappearance of the maximum of CO conversion with thetemperature could be attributed to the competition between thetwo oxidation reactions taking place when the oxidation of H2

starts to be activated, as we will demonstrate in the followingsections.

In Figure 2, the effect of contact time, evaluated at twodifferent temperatures, is shown. The results show that, despitethe increase of CO conversion and, hence, the consequentdecrease of the CO/H2 ratio in the gas phase, the selectivityremains quite constant in the larger part of the range of contacttime values explored, confirming that its value is basicallydependent on the reaction temperature, as already put intoevidence in previous work.15

As a consequence, in theory, it could be possible to obtainhigh CO conversion at high selectivity by just operating at lowtemperature and very long contact time, because of the quiteslow reaction rate. On the other hand, even if the selectivity issomewhat lowered at increased temperatures, in such conditionsthe system is able to accomplish the desired CO abatement forpractical applications with reasonable values of the reaction rate

and selectivity that, even if lower than 100%, are still muchlarger than the values claimed for Pt in commercial units (20-25%). Data collected atW/F ) 0.50 g‚s/cm3 are reported inTable 1, where it is evident that CO concentration can bereduced to values as low as 10 ppm at 110°C, with a verynegligible amount of H2 being oxidized. Nevertheless, the COconversion obtained at 92°C (77%), when H2 oxidation isnegligible, can be further enhanced by increasingW/F sinceO2 is still available for CO oxidation.

The effect of the partial pressure of oxygen in the feed hasalso been investigated, using a reacting mixture with variableoxygen concentrations at two different contact times and twotemperatures. Under all experimental conditions explored, COconversion does not significantly change, suggesting that theCO oxidation kinetics is quite independent of the oxygen partialpressure in a region of oxygen concentration higher than thestoichiometric value. On the other hand, the selectivity to CO2

slightly decreases, suggesting there is a very weak positive effectof larger O2 concentrations on the oxidation of H2, even at lowtemperature (92°C).

The weak dependence of CO oxidation on the oxygen partialpressure is in good agreement with the data previously reportedby Liu and Flytzani-Stephanopoulos.28 More recently, Sedmaket al.23 specified that, on CuO/CeO2 catalysts, the oxygenconcentration does not significantly affect the CO oxidation ratein CO-PROX conditions for concentrations higher than thedoubled stoichiometric value.

On the other hand, the weak effect of O2 partial pressure onCO conversion was confirmed by the results of catalytic testscarried out at different CO inlet concentrations but given O2

inlet concentration, providing trends very similar to thoseobserved at constant O2/CO ratio (and, for this reason, not shownhere).

The effect of CO inlet concentration on the reaction kineticswas investigated at both constant O2/CO ratio and oxygen partialpressure (not shown). At constant O2/CO ratio (Figure 3), theCO conversion decreases with increasing CO inlet concentrationin the region of low temperature investigated. On the contrary,at higher temperature, the CO conversion increases with the

Figure 2. CO conversion (full symbols) and selectivity to CO2 (opensymbols) as functions of contact time at 92 (circle), 98 (triangle), and 110°C (square).

Figure 3. CO conversion (full symbols) and selectivity to CO2 (opensymbols) as functions of CO inlet concentration at 92 (circle) and 132°C(square) (W/F ) 0.032 g‚s/cm3, O2/CO ) 1.25)

Table 1. CO Conversion, Selectivity to CO2, and ResidualConcentration of Unconverted CO (C0 ) 5000 ppm) at 92, 98, and110 °C for W/F ) 0.50 g‚s/cm3

T (°C) XCO (%) SCO2 (%)residual COconc. (ppm)

92 77.0 97 115098 89.5 86 525

110 99.8 66 10

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076795

CO inlet concentration. This should imply a singular phenom-enon of apparent and overall reaction order<1 at lowtemperature and>1 at high temperature. For catalytic reactions,it usually happens that the overall reaction order tends to unityby increasing or decreasing either the temperature or thereactants partial pressure, coming from values always strictlyhigher or lower than 1 in the other regions.

On the other hand, in contrast with data reported above atvariable contact time (Figure 2), selectivity is not constant butslowly decreases with decreasing CO partial pressure at bothtemperatures investigated, although remaining at values thatshould be considered relatively high, if compared to thoseattained on noble metal based catalysts. This phenomenon isprobably due to the different temperature profiles establishedunder the two different conditions explored, characterized bydifferent reactants partial pressure and, consequently, differentthermal effects on the lab-scale reactor.

The dependence of the CO conversion on the CO inletconcentration observed at low temperature is in agreement withprevious literature studies on CuO/CeO2 catalysts. Liu andFlytzani-Stephanopoulos28 observed that the CO reaction orderseems to decrease from 1 to 0, increasingPCO at a constantPO2. Sedmak et al.23 showed that the oxidation rate increasesless than linearly with CO partial pressure and levels off at highvalues of CO concentration. Such an occurrence happens athigher concentration with increasing temperature, although onlythe low-temperature region was explored, in which selectivitystrictly remains 100%.

Figure 4 shows the effect of H2 inlet concentration on theCO conversion and selectivity to CO2. The CO conversion iscompletely unaffected by the hydrogen partial pressure exceptat 200°C, because, at such a high temperature, the dramaticincrease of the H2 oxidation reaction rate embezzles the oxygen,thus limiting CO conversion and selectivity as well.

Similar observations were reported by Sedmak et al.,23 whofound that, on CuO/CeO2 catalysts, the CO conversion is

independent of the hydrogen partial pressure in the wholetemperature range investigated (T e 150°C) as long as there isenough oxygen fed to the reactor.

The effect of the presence of H2O and CO2 in the feed onthe conversion of CO and selectivity of the CO-PROX processwas also examined, with those species being not only thereactions products but also present in significant amounts inthe reformed gases in the practical applications of the process.

CO2 and H2O were separately added to the standard feedmixture at different reaction temperatures (92, 116, 132, and200 °C). The CO2 inlet concentration was varied between 0and 20 vol %, while H2O concentration was varied between 0and 10 vol %.

Subsequently, the effect of the contemporary addition of CO2

and H2O was studied by changing CO2 concentration in therange 0-20 vol % for two different H2O partial pressures (0.5and 1.5 vol %).

The effect of CO2 addition to the reaction mixture onconversion and selectivity is shown in Figure 5. The presenceof CO2 has a negative effect on the performance of the samples,lowering CO conversion. As also observed by Avgouropoulosand Ioannides,29 this effect is more pronounced at low temper-ature, while at 200°C, CO conversion is substantially unaffectedby the presence of CO2.

Nevertheless, the presence of CO2 does not significantlychange the selectivity, if comparing it at equal CO conversion,suggesting that both CO and H2 oxidation are depressed in thesame extent. This negative effect is probably due to thecompetitive adsorption of CO2 on catalyst sites inhibiting theoxidation reactions. At high temperatures, when adsorption ishindered, the effect of CO2 is negligible.

Water content is one of the most critical factors in the CO-PROX reaction and is very important for a proper operation ofthe fuel-cell stack. Contradictory viewpoints still exist on itsinfluence on the CO-PROX process. Actually, the presence ofwater has different effects depending on the nature of thecatalysts.

Many authors reported that water enhances the rate of COoxidation on Pt catalysts.3,30-32 Three different explanations were

Figure 4. CO conversion and selectivity to CO2 as a function of H2 inletconcentration at 92, 132, and 200°C (W/F ) 0.032 g‚s/cm3).

Figure 5. CO conversion and selectivity to CO2 as a function of the CO2inlet concentration in the absence of H2O at 92, 116, 132, and 200°C (W/F) 0.032 g‚s/cm3).

6796 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

suggested for that: (i) promotion of reverse WGS reaction, (ii)role of the adsorbed group, or (iii) change of Pt metal state. Onthe other hand, Korotkikh and Farrauto2 reported that theaddition of a small amount of water (3 vol %) decreases theCO conversion on gold catalysts. Grisel and Nieuwenhuys,10

Zou et al.,14 and Avgouropoulos and co-workers11,12 alsoreported that H2O has a detrimental effect on the CO oxidationactivity of Au based catalysts and on CuO/CeO2 catalysts.

The effect of H2O in the feed on our catalyst is shown inFigure 6. The presence of water vapor gives rise to an evidentdecrease of the catalyst activity at low temperature. Uponincreasing the temperature, this effect becomes less relevant,so that, at 200°C, the CO conversion is unaffected by thepresence of H2O. The selectivity is not influenced by waterpartial pressure. Therefore, as observed for CO2, water alsocontributes to the active sites coverage, giving rise to an activityloss but not making the oxidation of CO less preferential thanthat of H2.

Moreover, the effect of the contemporary presence of carbondioxide and water in the feed has been also studied. CO2 inletconcentration has been changed at two different levels of H2Opartial pressure (0.5 and 1.5 vol %), showing that the negativeeffect of CO2 and H2O on CO conversion is about cumulative,as also reported by Avgouropoulos and Ioannides,29 whileselectivity is unaffected.

A preliminary attempt to explain the observed effects througha kinetic model has been made by using two power-law rateequations under the hypothesis of an isothermal fixed-bed plug-flow reactor operating at constant pressure. Although this modelis simply based on two empirical rate equations with no otherambition than a mere data correlation, it can fairly elucidatethe reaction scheme and the competition between the twooxidation reactions, also providing a good estimation of theactivation energy values and identification of the nature of themaximum of CO conversion reported with increasing temper-ature. Moreover, the mathematical effort in doing this is nottrivial, with the reaction scheme being constituted by two parallel

reactions and the parameters of the model being estimated byfitting both CO and O2 conversions.

Hence, the only reactions considered have been the following:

The following empirical expression has been used for the rateof CO oxidation,

where PCO and PO2 are the CO and O2 partial pressures,respectively;R andâ are the apparent reaction orders for COand oxygen, respectively; andkCO is the kinetic constant forCO oxidation.

The H2 oxidation (eq 2) in a hydrogen rich environment canbe modeled using the empirical rate expression

where the kinetic constantkH2 embeds the dependence onhydrogen partial pressure andγ is the apparent reaction orderof O2. Such a choice is motivated by the scarce effect reportedfor H2 partial pressure on the overall PROX kinetics (Figure4), the lack of (or too few) data with H2 concentration otherthan 50%, and the substantial constancy of H2 partial pressurein all experiments carried out regardless of the conversion andselectivity attained.

Moreover, the following expressions for kinetic constantshave been considered:

The above rate expressions do not take into account the effectof CO2 and H2O on the reaction kinetics; hence, only thecatalytic tests performed in the absence of reformate specieshave been considered in the data fitting, which gives theempirical rate expressions for CO and H2 oxidation reported inTable 2.

In Figure 1, the values of CO and O2 experimental conver-sions are compared with the numerical curve fitting (R2 )0.995). The results of the simulation suggested that the oxidationof CO exhibited a first-order apparent dependence on CO partialpressure (R ) 1.02) and, in the region of overstoichiometricvalues explored, a practical independence on O2 partial pressure(â ) 10-4). On the contrary, a less weak dependence (γ ) 0.2)on the O2 partial pressure was estimated for H2 oxidation, aswas expected by the weak loss of selectivity measured withincreasing the O2 concentration that we have experimentallyobserved. Remarkably, the estimated activation energy for COoxidation (20 kcal/mol), lower than the activation energy forH2 oxidation (36 kcal/mol), is the reason for the genesis of themaximum in the conversion of CO with increasing temperature,which is due to the competition of the two reactions of oxidationwhose rates grow in a very different way by raising thetemperature. CO oxidation is sensibly faster at low temperature,being “selectively” catalyzed by CuO/CeO2 that, in fact, stronglylowers its activation energy, but the rate of H2 oxidation is

Figure 6. CO conversion and selectivity to CO2 as a function of the H2Oinlet concentration in the absence of CO2 at 116, 132, and 200°C (W/F )0.032 g‚s/cm3).

CO + 12O2 f CO2 (1)

H2 + 12O2 f H2O (2)

rCO ) kCOPCOR PO2

â (3)

rH2) kH2

PO2

γ (4)

kCO ) k°CO e-(ECO/RT) (5)

kH2) k°H2

e-(EH2/RT) (6)

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076797

increased much more by increasing the temperature because ofthe higher activation energy with respect to CO oxidation.

The nature of such a high selectivity exhibited by CuO/CeO2

in the CO-PROX process can be better understood by makinga comparison with noble metals based catalysts. Differently fromceria supported CuO systems, for Pt based catalysts, theselectivity is strongly dependent on CO/H2 ratio, since thetendency to oxidize CO rather than hydrogen is due to the abilityto strongly adsorb carbon monoxide on the catalyst surface,33

thus preventing H2 from reaching the active centers. For thesereasons, hence, the selectivity is significantly reduced at lowCO partial pressures, when the surface coverage is weak or atleast not enough pronounced to disallow hydrogen reacting withPt active sites.34-36

A completely different CO oxidation mechanism occurs ongold based systems studied as catalysts for the CO preferentialoxidation6-10 because of their high activity in CO oxidation evenat room temperature.37 Indeed, CO adsorption on gold is weakerthan that on Pt,7,38,39 but the former catalysts activate COoxidation at lower temperature and with a faster rate thanhydrogen oxidation. So, Au should be regarded as an “intrinsic”catalyst for the process, because it activates the desired oxygen.On the other hand, oxygen has a very low sticking coefficienton gold. For this reason, the use of “active” supports like Fe2O3,TiO2, and CeO2 was demonstrated by Schubert and co-workers3,4

to increase the CO oxidation rate by providing oxygen adsorp-tion sites and, thus, supplying O2 during the reaction.

CuO/CeO2 catalysts seem to resemble much more Au thanPt as an “intrinsic” catalyst for CO-PROX. The selectivity,which is very large and quite independent of the CO/H2 ratio,is an intrinsic feature of the kinetics of the two reactions, asconfirmed by the possibility of reaching the target result (10ppm residual CO) at the reactor exit with relatively largeselectivity (∼70%, Table 1).

Of course, the power-law rate equations adopted in thepreliminary modeling are unsatisfactory by other points of view.In fact, it should represent a sort of contradiction to estimatean overall order for CO oxidation) 1, while having experi-mentally demonstrated that it is instead a variable function oftemperature (going from lower to higher values with increasingtemperature, Figure 3). Nevertheless, although a somewhataverage or compromise reaction order value was actuallyestimated (R ) 1), the conclusion reached about the nature ofCO maximum at increasing temperature resulted to be demon-

strated also using such an oversimplified model and basicallyattributed to the different activation energies of the two reactions.Moreover, such a simplified kinetic law is unable to reproducethe effects of CO2 and H2O, which can also be important in theexperiments carried out when they are absent in the feed, withboth being the reaction products of desired and undesiredreactions.

First of all, in order to quantify the apparent variable valuesof the global reaction order, we have still used the power-lawrate equations to fit the data at low and high temperatures,separately. We chose the value of 115°C as the limit betweenwhat we have called “low” and “high” temperature zones. Theparameters identification gave the results reported in Tables 3and 4.

The findings obtained are only apparently trivial. Actually,the estimated reaction order is in line with that expected (theorder for CO is 0.7 and 1.3 at low and high temperatures,respectively), but the substantial constancy of the activationenergy of both reactions and the reaction order for O2 aresomewhat unpredictable results. Of course, such a power-lawequation with variable reaction order fits the data in a verysatisfactorily way; also, the effects of O2 and CO partialpressures in the feed are well-reproduced by the model.

For all these reasons, we have tried to model the oxidationof CO with a set of more complex and not empirical rateequations. In particular, we have tested several kinetic modelsof Langmuir-Hinshelwood (LH) and Mars-Van Krevelen type,

whereR, γ, andδ have alternatively assumed the values of 1and 2 whileâ has been set equal to 0.5 (dissociative adsorptionof oxygen) or 1, while the addendum “1” can be present or notin the denominator.

Among such kinetic models, the best fit was given by theequation

whose values of kinetic parameters have been estimated andreported in Table 5.

Table 2. Estimated Parameters for the Power-Law Kinetic Model

rate equation R â γ ko (mol‚h-1‚g-1‚atm-(R+â)) E (cal‚mol-1) R2

rCO ) kCOPCO1.02PO2

10-4 1.02 10-4 1.4× 1012 19 900 0.995kCO ) k°CO e-(ECO/RT) 1.02 10-4 1.4× 1012 19 900 0.995rH2 ) kH2PO2

0.2 0.2 1.6× 1017 35 000 0.995kH2 ) k°H2

e-(EH2/RT) 0.2 1.6× 1017 35 000 0.995

Table 3. Estimated Parameters for the Power-Law Kinetic Model, Data atT < 115 °C Only

rate equation R â γ ko (mol‚h-1‚g-1‚atm-(R+â)) E (cal‚mol-1) R2

rCO ) kCOPCO0.7PO2

0.07 0.7 0.07 1.3× 1012 20 900 0.992kCO ) k°CO e-(ECO/RT) 0.7 0.07 1.3× 1012 20 900 0.992rH2 ) kH2PO2

0.1 0.1 1.1× 1017 36 250 0.992kH2 ) k°H2

e-(EH2/RT) 0.1 1.1× 1017 36 250 0.992

Table 4. Estimated Parameters for the Power-Law Kinetic Model, Data atT > 115 °C Only

rate equation R â γ ko (mol‚h-1‚g-1‚atm-(R+â)) E (cal‚mol-1) R2

rCO ) kCOPCO1.3PO2

0.04 1.3 0.04 1.6× 1012 18 300 0.998kCO ) k°CO e-(ECO/RT) 1.3 0.04 1.6× 1012 18 300 0.998rH2 ) kH2PO2

0.2 0.2 1.5× 1017 35 100 0.998kH2 ) k°H2

e-(EH2/RT) 0.2 1.5× 1017 35 100 0.998

rCO )kcKCOKO2

PCOR PO2

â

((1) + KCOPCOR + KCOPO2

â + KCO2PCO2

γ )δ

rCO )kcKCOKO2

PCOPO2

1/2

(1 + KCOPCO + KO2PO2

1/2 + KCO2PCO2

)2(7)

6798 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

However, the differences in the fitting between the LH modeland the Mars-Van Krevelen one are very insignificant, and atthis stage, we have no reason to choose one reaction mechanismto discuss with respect to the other, having postponed a deeperinvestigation on this matter to future works. The aim of thisapproach is merely to prove that, by also modeling the kineticsof CO oxidation with a plausible reaction mechanism-basedrate equation, the competition with the oxidation of H2 issatisfactorily reproduced (and the presence of the maximum forthe conversion of CO with increasing temperature is well-explained).

It is evident that, even with a volcano-shaped conversion ofCO with increasing temperature, each of the four kineticparameters has been estimated, with one surface reactionkinetic constant and three adsorption equilibrium constantsthat are monotonically dependent on temperature. In particular,the kinetic constantkc is increasing with temperature, whileKCO, KCO2, and KO2 are decreasing. Moreover, values ofconsequent adsorption heat are reasonable; only the activationenergy assumes a too-high value and deserves further investi-

gation. It is worth noting also that the adsorption constantof CO2, KCO2, is much larger than the other two at 92°C,thus meaning that the only species that significantly contrib-utes to surface sites coverage at low temperature is carbondioxide.

In Figure 7, it is also evident that all the effects of the differentoperating parameters explored are well-reproduced by the LH-type model. Actually, both at high and low temperatures, thekinetic model adequately describes the experimental data. Inparticular, the effect of inlet values of O2 and CO partialpressures, namely, the reaction order, has been described by anoverall exponent of 1.5 in the numerator (as the sum of 1 forCO and 0.5 for oxygen), which is affected by the denominatorat low temperature (when adsorption constants are larger) togive an apparent reaction order< 1. Instead, at high temperature,the apparent reaction order estimated with the power-law rateequation tends to the ideal value of 1.5, since most of the testshave been carried out at constant O2/CO ratio (λ ) 2) and,hence, the exponent evaluated was a global one.

Figure 7. CO conversion as a function of the contact time and CO,λ, and CO2 inlet concentration at 92 and 132°C (W/F ) 0.032 g‚s/cm3, CO inletconcentration) 5000 ppm,λ ) 2, and CO2 absent where not variable).

Table 5. Estimated Parameters for the LH-type Kinetic Model

rate equation T (°C) parameter of the model R2

rCO )kcKCOKO2

PCOPO2

1/2

(1 + KCOPCO + KO2PO2

1/2 + KCO2PCO2

)2

92 kc ) 29.1 h-1 0.99692 KCO) 0.4 atm-1 0.99692 KO2 ) 33.7 atm-0.5 0.99692 KCO2 ) 975.5 atm-1 0.996

132 kc ) 23 040 h-1 0.998132 KCO) 5 × 10-3 atm-1 0.998132 KO2 ) 7.7 atm-0.5 0.998132 KCO2) 7.6 atm-1 0.998

ko (h-1) E (cal/mol) ∆H°CO (cal/mol) ∆S°CO (cal/mol‚K) ∆H°O2(cal/mol) ∆S°O2

(cal/mol) ∆H°CO2(cal/mol) ∆S°CO2

(cal/mol‚K)

6.5× 1030 49 000 -30 000 -90 -5 000 -11 -35 000 -80

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076799

Conclusions

The study of the kinetics of the preferential oxidation of COin H2-rich streams over CuO/CeO2 catalysts has enlightened thenature of the competition between the two different reactionsthat may occur: the desired CO oxidation rather than theundesired oxidation of H2.

Competition favors the CO abatement even down to very fewppm of unconverted carbon monoxide at relatively low tem-perature (70-110°C) and with very high selecitivity (negligibleH2 sacrifice) because of the lower value of the activation energyof CO oxidation catalyzed by CuO/CeO2. With increasingtemperature, hydrogen is oxidized too with a rate increasingfaster than that for CO oxidation, up to the consumption of alloxygen fed, and thus limiting the conversion of CO (andconsequent appearance of a maximum for CO conversion).

The reaction orders between CO and H2 oxidations aredifferent. At low temperature, the conversion of CO decreaseswith increasing CO concentration (while the partial pressure ofoxygen very poorly affects it) with an apparent reaction order< 1 because of the strong adsorption of the CO2 produced bythe reaction. On the contrary, the reverse happens in the high-temperature region (110-200 °C). However, the selectivity ofthe process is basically a function of the sole temperature,because of the weak dependence on CO/H2 ratio observed whensearching for very deep CO abatement.

Such conclusions have been reached with both an experi-mental campaign and a preliminary modeling of the reactionrates of both CO and H2 oxidations. Actually, the numericalapproach involved has shown that nonmonotonous dependenceof CO conversion on temperature can be explained with aconventional and monotonous expression of the reaction rate.

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ReceiVed for reView December 30, 2006ReVised manuscript receiVed August 10, 2007

AcceptedAugust 13, 2007

IE0616951

6800 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007