J. Phys. Chem. C 2010, 114, 11441–11447
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Two-Site Mechanism for the Oxidation Reaction of Methane on Oxidized Palladium Joaquı´n Corte´s,* Eliana Valencia, and Paulo Araya Facultad de Ciencias Fı´sicas y Matema´ticas, UniVersidad de Chile, P.O Box 2777, Santiago, Chile ReceiVed: October 6, 2009; ReVised Manuscript ReceiVed: May 12, 2010
Kinetics mechanisms with two independent sites are proposed for the adsorption steps of CH4 and O2 in the CH4-O2 reaction on an oxidized metal, assuming noncompetitive adsorption of the reactants in agreement with the experimental observations. This condition cannot be incorporated in the previous models with a single type of active site. Studies of the kinetics equations and Monte Carlo simulations of those mechanisms interpret reasonably well the experimental results of the literature for the reaction on oxidized Pd. Introduction Just as in past decades, the study of the catalytic reactions of oxidation of CO and reduction of NO by CO has had great importance because of their relevance in automotive exhaust emission control;1 over the past years, the study of the catalytic oxidation of hydrocarbons has gained much interest, both for pollution abatement as well as for power generation. Among the latter reactions, the oxidation of methane (CH4-O2 reaction) has become especially prominent, and in recent years a large number of papers have appeared because methane is a wellknown greenhouse gas and the most stable and abundant alkane.2,3 In spite of the large amount of experimental work done on the CH4-O2 reaction, the same has not happened with the theoretical interpretation of the microscopic characteristics of the reaction mechanism. Although there is no agreement in the literature on a kinetic mechanism for this reaction, attempts as simple as the Langmuir-Hinshelwood (LH) model shown in Scheme 1 have been made, which Ma et al.4 find interprets well the experimental results for the oxidation of methane, ethane, and propane over platinum supported on alumina. A list of theoretical equations for the oxidation rate of methane has been published by Hurtado et al.5 assuming simple models of the LH, Eley-Rideal (ER), and Mars-van Krevelen (MVK)6 type, concluding that their experiments with palladium supported on alumina can be modeled more successfully by an MVK mechanism. There seems to be agreement that the oxidation of methane takes place by direct interaction of the reactants with the oxygen of the lattice by a MVK3 type of mechanism. Iglesia et al.7 proposed a more complex mechanism with a sequence of elemental steps that “resembles Mars von Krevelen reduction oxidation pathways” for this reaction on PdOx (x < 1) supported on ZrO2, although the way in which it is presented does not seem distinguishable from an LH mechanism. In general, Pd is the most active catalyst for the oxidation of methane among the transition metals, its action depending markedly on the reaction conditions and on the type of support. Isotopic studies made later by Iglesia’s group showed the combination of kinetic and thermodynamic effects, justifying the irreversibility or reversibility of the different steps proposed for the mechanism.8 * To whom correspondence should be addressed. E-mail: jcortes@ dqb.uchile.cl.
Considering Iglesia’s mechanism, our group recently published for the first time in the literature the topic of Monte Carlo (MC) simulation of the behavior of the CH4-O2 reaction.9 This type of kinetic MC simulation of surface reactions has a long history in the literature and includes the work of authors such as Evans, Ziff, Albano, and others10 as well as our own.11 In this paper, beside proposing a kinetics mechanism for the CH4-O2 reaction that includes more than one kind of active surface site that modifies the traditional criterion of assuming the existence of only one type of site, the simulation of a surface reaction that takes place through a redox or MVK type mechanism is established, in contrast with previous simulations that normally assume a LH type mechanism. Two-Site Model for the Catalytic Oxidation of Methane It has been determined experimentally that the CH4-O2 reaction is more efficient on a catalyst in the metallic state in the case of Pt, while Pd is more active in the oxidized state. With completely reduced palladium, the reaction occurs with the existence of a single type of active Pd°, so that the methane and oxygen molecules must compete for the same sites, which results in a negative order for the oxygen. Also, that surface is not affected by poisoning with water.3,12 The situation is very different in the case of the surface of oxidized palladium, PdOx, which experimentally has a small positive or zero order for oxygen and negative for water.7,13,14 In this case, there is no competition between the adsorptions of methane and oxygen on the surface, which is in agreement with experiments, where negative orders have not been observed with respect to oxygen, which never inhibits the reaction. This noncompeting condition cannot be incorporated in the classical kinetics with a single type of active site. Models of this type can present adequate behaviors with respect to the O2 under particular circumstances, such as, for example, in the mechanism of Iglesia et al.8 when SCHEME 1: LH Mechanism for the Oxidation of Methanea
aX (g) refer to the gas phase, Y(a) refer to the adsorbed phase, Ki are equilibrium constants, k3 is the rate constant, and S is an active site on the surface.
10.1021/jp909575z 2010 American Chemical Society Published on Web 06/10/2010
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SCHEME 2: Two-Site Mechanism (S1, S2) for the Oxidation of Methane
Corte´s et al. however, do not require the reversibility conditions needed to solve the kinetics equations of the previous model. Situations with and without diffusion of superficial O* oxygen will also be considered, as will be seen later. Resolution of Mean Field Kinetics Equations for Two-Site Models in the Catalytic Oxidation of Methane
the surface is largely poisoned with OH groups, in which case the model predicts zero order for O2. However, this does not happen in general with that mechanism, as shown in previous work by our group, where an order of -0.62 for O2 was obtained with the kinetics parameters considered in that paper.9 One way of overcoming the above situation is to consider a theoretical approach that assumes the existence of different sites for the adsorption of methane and oxygen on the surface, as in the simple case of Scheme 2 that will be analyzed in detail below by solving the mean field kinetics equations. This mechanism is a simplified version whose kinetics equations allow an analytic solution that does not consider, for example, the linear adsorption of O2 prior to its dissociation or the sequence of steps that lead to the products, as in the mechanism of Scheme 3 that will be discussed below. The proposed two-site mechanism in which there is no competition between CH4 and O2 for the same sites is applicable, for example, in the case of oxidized Pd, whose PdOx (x < 1) surface can be considered as having two types of sites: one of them metallic Pd (S1) on which CH4 is adsorbed, and the other an oxygen vacancy (S2) on which oxygen is adsorbed dissociatively. The proposed mechanism does not set a difference between the adsorbed oxygen and that belonging to the superficial lattice. It seems reasonable, on the other hand, to consider the latter sites, S2, for step 3, since, in the case of a metallic Pd surface, it is not affected by poisoning with water.12 This is also considered by Iglesia et al.,7,8 whose mechanism requires the existence of oxygen vacancies on the PdOx (x < 1) surface, showing how their density decreases with increasing concentration of H2O. Although in steps 3 and 4 there are reactions between particles that are neighboring in the surface, the sequence of steps of Scheme 2 resembles MVK reduction-oxidation pathways, because step 4 leads to the reduction of the catalyst, and step 2 recovers the oxidation of the surface. An MC simulation of the CH4-O2 reaction is also made in this work, introducing this concept of two sites in the modified model of Iglesia7,8 that we had used previously.9 The resultant mechanism is shown in Scheme 3, where the two types of sites, S1 and S2, already considered in Scheme 2 are indicated in the corresponding steps proposed in this paper. The simulations,
The analytic solution of the kinetics equations corresponding to the mechanism of Scheme 2 assumes a quasi-equilibrium situation for the CH4 physisorption step (1) as considered in the original model of Iglesia et al.7 On the other hand, isotopic studies made by Iglesia’s group establish that step 2 behaves irreversibly. In our analysis, this situation is taken into account considering that step in equilibrium to solve the equations, but with a high value for the equilibrium constant, ensuring negligible desorption of the O* species. The equilibrium of the reaction of water in step 3, on the other hand, is considered reversible according to Iglesia et al.,8 because otherwise the water in the gas phase would not have an effect on the reaction rate nor on the composition of the superficial species, as is the case. Let us consider first Scheme 2 without step 3 for the water, which would be the model corresponding to Scheme 1 in the case of two sites. The equilibrium assumption for the first two steps allows writing the following relations:
k1PCH4θS1 ) k2θCH4
(1)
k3PO2θS22 ) k4θO2
(2)
where the covered fractions θ(i) of the superficial species (i) comply with the following independent normalization equations:
θS1 + θCH4 ) 1
(3)
θS2 + θO ) 1
(4)
From eqs 1 and 3 we directly obtain the fraction of the surface covered with methane
θCH4 )
k1PCH4 k1PCH4 + k2
On the other hand, relations 2 and 4 lead to the equation
SCHEME 3: Two-Site Mechanism (S1, S2) for the Oxidation of Methane on PdOx Used in the MC Simulations
(5)
Two-Site Mechanism for CH4-O2 Reaction on Oxidized Pd
θO2(k3PO2 - k4) - 2k3PO2θO + k3PO2 ) 0
(6)
whose solution for the fraction covered with oxygen atoms is
θO )
k3PO2 - (k3k4PO2)1/2 k3PO2 + k4
(7)
where only the negative sign must be considered, as will be seen below. The reaction rate r, for this first model is therefore obtained from step 4 and eqs 5 and 7:
r ) kθCH4θO
(8)
Scheme 2 additionally includes step 3 for the adsorption of water proposed in the literature,2,9 which is also included in the model of Iglesia et al.7 To solve this case, we also have eqs 1 and 2 and the equilibrium assumed for step 3, from which we can write
k5PH2OθOθS2 ) k6θOH2
(9)
The existence of OH groups as a new superficial species that occupies only S2 sites, modifies normalization eq 4, keeping eq 3, that is,
θS2 + θO + θOH ) 1
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θOH )
A (C + A)
(16)
θO )
1 (C + A)
(17)
and therefore
This allows having expressions for the covered fractions of the superficial species as a function of the pressures Pi of the reactants in the gas phase and the rate constants ki, and the reaction rate can be calculated again from eq 8. It is interesting to note that when PH2O f 0, the expressions should coincide in both cases, since when PH2O f 0, B f ∞ and A f 0, then θO ) 1/C, which is identical to relation 7 for θO considering the negative sign of the solution of expression 6, confirming the sign used in eq 7. Monte Carlo Simulation The MC algorithm developed in this paper for two sites according to the mechanism shown in Scheme 3 is similar to that used previously by our group for other systems11 and for the combustion of methane considering a single type of active site S on the surface by our group9 and by Zhdanov15 recently. A simplified scheme of the surface has been assumed consisting of a hexagonal lattice of S1 sites, with the S2 sites at the centers of triangles formed by the centers of three S1 sites, so that the number of S1 sites is equal to the number of S2 sites. The
(10)
From relations 2 and 10 we can write
(
) ( )
k4 1 θOH ) θO θO k3PO2
1/2
+1)C
(11)
On the other hand, from eqs 9 and 10 we get
θO 2 θOH
-
( ) θO θOH
2
-
θO k6 ) )B θOH k5PH2O
(12)
If we define A ) θOH/θO, it is possible to write from eqs 11 and 12 the following system of equations for A and θOH:
A -A)C θOH
(13)
1 1 2 1 - )B AθOH A A
(14)
()
from which we get
A2 )
(C - 1) B
(15)
Figure 1. (a) Production of CO2, and the surface concentration θi of species i versus concentration yO2 (phase diagram) obtained by solving the kinetics equations of the model of Scheme 2 for the catalytic combustion of methane at total pressure P ) 70 Torr and PH2O ) 1 Torr with the parameters defined in the text. The lines have been drawn to guide the eyes. θO ([); θOH(]); θCH4 (2); θS1(∆); θS2 (0); RCO2 (s). (b) The same as panel a without considering step 3 for water.
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Figure 2. Reaction order of (a) CH4, (b) O2, and (c) H2O for the catalytic combustion of methane obtained by solving the kinetics equations of the model of Scheme 2: PCH4 ) 16 Torr, PO2) 160 Torr, and PH2O) 1 Torr. (d) RCO2 production versus the equilibrium constant K3 of step 3 (The inset shows the order of H2O versus the equilibrium constant K3).
simulation process begins by choosing an event of the mechanism (adsorption, desorption, dissociation, reaction, or diffusion) according to the probability pi of the event defined by
pi )
ki
∑ ki
(18)
In the case of steps that involve the adsorption of gas j, the step’s rate constant is calculated from the expression of the kinetic theory of gases:
kads(j) ) σ(2πMjRT)-1/2
(19)
where Mj is the molecular mass of j, coefficient σ is the area occupied by 1 mol of superficial site, and the corresponding sticking coefficient is assumed to be equal to 1 in all cases. The remaining ki constants will be discussed later. We will designate as lattice 1 that part of the surface that includes S1 sites, and lattice 2 that which includes S2 sites. If the adsorption of CH4 is chosen, one site of lattice 1 is selected randomly. If the site is occupied, the attempt is ended, but if it is vacant, a molecular CH4 particle will be adsorbed. S1 sites, which are considered for the adsorption of CH4, can only be occupied by CHx*, CO*, CO2*, CO3* and CHO* particles during the kinetic process. If the selected event corresponds to the adsorption of O2, a site of lattice 2 is chosen randomly on the surface. If the site is occupied, the attempt is ended, but if it is vacant, a molecular oxygen particle, O2*, will be adsorbed.
Since it is assumed that the dissociation of O2* is an instantaneous process, it is immediately revised to see if in any of the nearest six sites of lattice 2 there are one or more empty sites. If it is so, one is chosen randomly and the O2 dissociates, leaving both sites occupied with O*; otherwise the O2* remains on the surface. Since O* participates in the fast reactions, it is necessary to revise all the neighbors of both O* that have come from the dissociation, and then carry out all the possible reactions (4a-4f) and derived steps. If the adsorption of H2O is chosen and the site of lattice 2 chosen randomly on the surface is empty, one of its six nearest neighbors in that lattice is selected randomly, and if it is occupied by O*, both sites are occupied by OH* particles. If no O* particle is found, the attempt is ended. If O2 desorption is chosen, a surface site of lattice 2 is selected randomly. If it is occupied by a particle different from O2* or it is vacant, the attempt is ended. However, if it is occupied by an O2* particle, desorption occurs and the particle is replaced by a vacant site. The procedure is analogous in the case of choosing the desorption of CH4 and CO2. In the case of the desorption of CO3*, a surface site of lattice 1 is selected randomly; if is is occupied by a CO3* particle, a site of lattice 2 is chosen randomly among the neighbors of CO3*, and if it is empty, desorption occurs, leaving an adsorbed atomic oxygen, O*, in the latter, and a molecule of CO2 leaves the surface, leaving the first site empty. In the case of reaction 4, a site is chosen in lattice 1. If it is not occupied by CH4, the event ends. If it corresponds to CH4, a site is chosen in the neighboring lattice 2 and if it has oxygen, O*, adsorbed, a particle of CH3* remains in the first site and one of OH* in the second. Since CH3* participates in one of
Two-Site Mechanism for CH4-O2 Reaction on Oxidized Pd
Figure 3. (a) The same as Figure 1, but obtained by MC simulation in the steady state at total pressure P ) 160 Torr and PH2O ) 1 Torr with the parameters defined in the text. θO ([); θOH (]); θCH4 (2); θS1 (∆); θS2 (0); RCO2 (s). (b) Snapshot illustrating a fragment of the lattice corresponding to yO2 ) 0.794 of Figure 3a. O ([); CH4 (2); S1 (∆); OH (]); CO3 (9); S2 (0).
the abstraction reactions (4a-f), which are instantaneous, it is necessary to revise if, in the three sites of lattice 2 neighboring it, there are one or more adsorbed atomic oxygens. If it is so, all the possible reactions of this series occur. In the case in which the selected event is the direct reaction 5, a site of lattice 2 is first chosen randomly. If it is occupied by an OH* particle, a neighboring site of that lattice is then chosen randomly next to the first site. If the latter is occupied by the other OH* particle, the event is successful, and one molecule of H2O leaves the surface and a particle of O* remains in the first site. If the chosen event is the diffusion of O*, a site of lattice 2 is selected, and if it is occupied by O*, a site is chosen randomly among the six nearest neighbors of that lattice, and if this site is empty, the O* occupies it, leaving the first site empty. Results of the Resolution of the Kinetics Equations An important difficulty for making microscopic interpretations of the catalysis experiments is having reliable and universal values of the kinetics constants ki, especially if it is considered that most of the experimental information is obtained on supported catalysts that are difficult to reproduce. In this work we have chosen to use some approximations for certain
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11445 parameters in the case of the simple two-site model that appears in Scheme 2, later fitting constant k of step (4) to the experimental results of the literature. The difficulty in interpreting kinetics information in the moderate and high-pressure zones with data obtained in the low-pressure zone has been called the pressure-gap problem. This matter has been discussed in detail by Zhdanov16 in the case of the CO-NO reaction on Rh, with rather discouraging results. In the case of the CH4-O2 reaction, the difficulty is even greater considering the little information existing in the literature. The strategy chosen in this paper, however, has as its objective only to allow having a conceptual understanding of the behavior of the model keeping its relation with the experiment. To that end we have considered the approximation of using eq 19 in the calculation of the adsorption constant, ki(ads), to determine the value of parameters k1, k3, and k5, and we have assumed the reasonable values of K1 ) 0.02 Torr-1, K2 ) 100 Torr-1 for the equilibrium constants of the first two reactions, 1 and 2. This assumes an equilibrium of step 1 displaced toward the gas, and the high value of K2 assures negligible desorption of the O* atoms of the lattice. On the other hand, as indicated by Ciuparu et al.,17 at relatively low temperatures, “most of the water molecules formed in the combustion of methane reaction remain adsorbed on the surface,” so we will first analyze the model using Ciuparu’s high value of the equilibrium constant of water17 for the reaction of step 3 of our mechanism. This means assuming that K3 ) 78677 Torr-1 at a temperature of 550 K. Monteiro et al.18 obtained experimentally a value of 0.59 s-1for the turnover rate, r, of the complete oxidation of methane on Pd at that temperature, 16 Torr of CH4, with an excess of oxygen equal to 160 Torr and 1 Torr of water, and expressed the order of the reactants at 550 K by the equation r ) kapp[CH4]0.7 [O2]0.2 [H2O]-0.9. Using these constants and the resolution of the model of Scheme 2, parameter k of step 4 was fitted to the production data of Monteiro et al., getting a value of 23.4 s-1. Since the equilibrium constants Ki are equal to the quotient of the rate constants of the direct and inverse reactions, we therefore have a complete set of parameters needed to discuss in what follows some aspects of interest of the two-site model of Scheme 2. Figure 1a shows the phase (θi, yO2) and production (r, yO2) diagrams if yO2 is the mole fraction of oxygen in the gas phase assuming a total pressure of 70 Torr. The phase diagram shows, due to the high value of K3, that the part of the surface that generates the S2 sites remains quasi-poisoned with OH groups, keeping a small fraction of oxygen that decreases until the poisoning in yO2 ) 0, explaining the drop of the production curve at low yO2 fractions. At the other end, for high yO2 fractions there is a decrease of the fraction covered with CH4 in that part of the surface corresponding to the S1 sites, due to the lower pressure, PCH4, which gives rise to a continuous decrease of the production whose curve then presents a maximum in the central section of the diagram. It is convenient to recall that the sections corresponding to sites S1 and S2 are normalized independently, as established by eqs 3 and 10. Figure 1b illustrates the phase and production diagram corresponding to the mechanism of Scheme 2, without step 3, so it does not take into account the water of the gas phase or its readsorption from the products. For that reason, the section of the S2 sites is quasi-poisoned with oxygen because no OH groups exist, and production, which shows a maximum at low O2 pressures, takes on a high value at low yO2. Figure 2 shows the diagram of the logarithm of production versus the logarithm of the various pressures of each of the reactants, keeping constant those of the rest. This makes it
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Figure 4. Reaction order of (a,d) CH4, (b,e) O2, (c,f) H2O for the catalytic combustion of methane obtained by MC. (a,b,c) Model of Scheme 3 with dissociative adsorption of O2 and later dissociation. (d,e,f) the same with linear adsorption of O2.
possible to obtain from the slopes of the corresponding straight lines the orders, νi, shown in the figures, defined by the empirical expression used frequently by experimentalists: νCH4 νO2 νH2O r ) kPCH4 PO2 PH2O
(20)
The diagrams correspond to the surroundings of the values of the pressures considered by Monteiro et al.18 at a temperature of 550 K. The straight lines obtained present in all cases an excellent correlation, with orders for methane and oxygen that agree with those of Monteiro and a slightly lower order for water. Figure 2d shows the values of order and production for different values of the equilibrium constant K3 of step 3, keeping all the other constants unchanged. This figure, which establishes the influence of K3 on the activity of the process, also shows in the inset that the order νH2O for water remains negative over the whole range of K3, tending to -0.5 for large values of K3. The order of water in Figure 2d retained its values when constant k was adjusted in each case to keep production constant, for example, at the value of 0.59 s-1 obtained by Monteiro for each value of K3. In the case of the model without the water step, whose graphs are not shown, the same value is obtained for
the order of CH4 as in the case of Figure 2 and an order of zero for oxygen, because it keeps the surface quasi-poisoned over the whole range of the diagram. Monte Carlo Simulations with the Two-Site Model In a manner similar to that of previous work from our group in which MC simulations were used to study the CH4-O2 reaction by our group,9 simulations have been made according to Scheme 3, with the same steps of the previous model but assuming a model of two-sites, S1 and S2, as shown in the scheme. The details of the MC simulation process were explained in a previous section. Just as in the case of the kinetics equations of the simple model of Scheme 2, some approximations have been made for the ki constants. For example, parameter ki(ads) given by eq 19 has also been used to calculate the adsorption of methane, the adsorption of oxygen, and in the reaction of water of step 5. An important difference between the MC simulations and the simple model, however, is that, in this case, it is not necessary to assume the equilibrium approximations used for solving the kinetics equations. In the dissociation of oxygen, where the simple model considered an equilibrium displaced to the right with a high value of the constant, in the simulations it has simply
Two-Site Mechanism for CH4-O2 Reaction on Oxidized Pd been assumed that the superficial oxygen O* is not desorbed. In the case of CH4, a desorption constant equal to 10 times that of adsorption was used. In the case of the reaction of water, eq 19 has been used for the inverse reaction constant kads(H2O), where the water is adsorbed only if there is a neigboring O* particle in the zone of the S2 sites. It has also been assumed that the relation between the reaction rate constant of OH groups, kOH, and the adsorption constant of water, is kOH ) 0.01 kads(H2O). It should be recalled, to avoid confusion that this reaction is written in an inverse way compared to that of the model of Scheme 3 and that of Scheme 2. The constants of steps 6 and 7 have been assumed, as in our previous work,9 to be equal to 50 times kads(O2). In the results shown below, the reaction constant between adsorbed CH4* and superficial oxygen O*, k4, has been fitted, assuming the previous parameters, to the production value obtained by Monteiro et al.18 at the same temperature and pressures of the reactants used in the previous section, getting in this case a value equal to 110000 s-1 for the linear adsorption of O2 and 100000 s-1 for he dissociatively adsorption of O2. These values are naturally different from the previous one considering that the model and the assumed constants are different, and also that in the simulations it is not necessary to impose a priori the equilibrium approximations used in solving the equations. Figure 3a shows the production curve and the phase diagram obtained assuming in Scheme 3 that the oxygen is not adsorbed linearly as in step (1) but it does so dissociatively in a direct way as in step 2 of Scheme 2. The simulations deliver a poisoning of the S2 sites with OH groups lower than in the case of the kinetics equations, because of the different value chosen in this example for the constant of step 5 for the water. This is reflected in the different values of the fractions of covered surface, θOH and θO, of the zone of S2 sites between both cases, as seen in the corresponding phase diagrams. An illustrative snapshot is shown in Figure 3b corresponding to yO2 ) 0.794. Figure 4a-c shows the results of the order of the reactants in simulations where it is assumed that oxygen is adsorbed dissociatively and Figure 4d-f in simulations exactly according to Scheme 3, first linearly, with a later dissociation on the surface with infinite rate if there is a vacant neighboring site. The adsorption constant of oxygen was calculated in this case from eq 19, and the desorption of the adsorbed O2(a) molecules was assumed to have a desorption constant equal to 10 times that of adsorption. The rest of the parameters are the same as those of the previous case. It is seen that no qualitatively substantial differences are obtained in the order of the reactants in both cases. This confirms the observations of Iglesia et al.,8 who comment that neither kinetics results nor isotopic studies “can discern whether O2 dissociation is preceded by quasi-equilibrated molecular adsorption”. These orders are also similar to those that were obtained by solving the kinetics equations, whose graph is not shown, in the case of an equilibrium constant K3 of step 3 of Scheme 2, equal to 100 Torr-1. It is also interesting to comment, as was shown, that the value of the order of the reactants is not substantially different between simulations made with or without diffusion of superficial oxygen O*.
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11447 Conclusions This paper proposes kinetics mechanisms for the methane oxidation reaction on an oxidized metal catalyst such as PdOx (x < 1), that consider the existence of two types of superficial sites, on one of which methane is adsorbed from the gas phase and the other is occupied by oxygen. This scheme establishes that the adsorptions of oxygen and methane on the surface are not competitive, leading to a positive and small order for oxygen, in agreement with the experiment. On the other hand, a negative and high order for oxygen is obtained with one-site models. The paper analyzes the results of the two-site models by solving the kinetics equations and MC simulations, reasonably interpreting the experimental results of the literature on the CH4-O2 reaction on oxidized Pd. Acknowledgment. The authors acknowledge the financial support of this work by FONDECYT under Project No. 1070351. References and Notes (1) (a) Taylor, K. C. Catal. ReV. Sci. Eng. 1993, 35, 457. (b) Shelef, M.; Graham, G. Catal. ReV. Sci. Eng. 1994, 36, 433. (c) Razon, L. F.; Schmitz, R. A. Catal. ReV. Sci. Eng. 1986, 89, 28. (2) (a) Ge´lin, P.; Primet, M. Appl. Catal., B 2002, 39, 1. (b) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Appl. Catal., B 2002, 234, 1. (c) Parsson, K.; Jansson, K.; Ja¨rås, S. G. J. Catal. 2007, 245, 401. (d) Demoulin, O.; Navez, M.; Ruiz, P. Catal. Today 2006, 112, 153. (e) Okumura, K.; Shinohara, E.; Niwa, M. Catal. Today 2006, 117, 577. (f) Parsson, K.; Pfefferle, L. D.; Schwartz, W.; Ersson, A.; Ja¨rås, S. G. Appl. Catal., B 2007, 74, 242. (3) Ciuparu, D.; Lyubovsky, R. L.; Alman, E.; Pfefferle, L. D.; Datye, A. Catal. ReV. 2002, 44, 593. (4) Ma, L.; Trimm, D. L.; Jiang, C. Appl. Catal., A 1996, 138, 275. (5) Hurtado, P.; Ordo´n˜ez, S.; Sastre, H.; Dı´ez, F. V. Appl. Catal., B 2004, 51, 229. (6) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41. (7) Fujimoto, K.; Ribeiro, F. H.; Avalos-Borja, M.; Iglesia, E. J. Catal. 1998, 179, 431. (8) Au-Yeung, J.; Chen, K.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 188, 132. (9) Corte´s, J.; Valencia, E.; Araya, P. Catal. Lett. 2006, 112, 121. (10) (a) Ziff, R. M.; Gulari, E.; Barshad, Y. Phys. ReV. Lett. 1986, 56, 2553. (b) Evans, J. W. Langmuir 1991, 7, 2514. (c) Zhadanov, V. P.; Kasemo, B. Surf. Sci. Rep. 1994, 20, 111. (d) Albano, E. V. Heterog. Chem. ReV. 1996, 3, 389. (e) Albano, E. V. In Computational Methods in Surface and Colloid Science; Boro´wko, M., Ed.; Marcel Dekker: New York, 2000; Chapter 8, pp 387-437. (11) (a) Corte´s, J.; Puschmann, H.; Valencia, E. J. Chem. Phys. 1998, 109, 6086. (b) Corte´s, J.; Valencia, E.; Puschmann, H. Phys. Chem. Chem. Phys. 1999, 1, 1577. (c) Valencia, E.; Corte´s, J. Surf. Sci. 2000, 470, L109. (d) Corte´s, J.; Valencia, E. Physica A 2002, 309, 26. (e) Corte´s, J.; Valencia, E. Phys. ReV. E 2003, 68, 016111. (f) Corte´s, J.; Valencia, E. J. Phys. Chem. B 2004, 22. (g) Corte´s, J.; Valencia, E. Phys. ReV. E 2005, 71, 1. (12) Lyubovsky, M.; Pfefferle, L. Catal. Today 1999, 47, 29. (13) van Giezen, J. C.; van den Berg, F. R.; Kleinen, J. L.; van Pillen, A. J.; Geus, J. W. Catal. Today 1999, 47, 287. (14) Araya, P.; Guerrero, S.; Robertson, J.; Gracia, F. J. Appl. Catal., A 2005, 283, 225. (15) Zhdanov, V. P.; Carlsson, P. A.; Kasemo, B. J. Chem. Phys. 2007, 126, 234705. (16) Zhdanov, V. P.; Kasemo, B. Surf. Sci. Rep. 1997, 29, 31. (17) Ciuparu, D.; Pfefferle, L. Appl. Catal., A 2001, 209, 415. (18) Monteiro, R. S.; Zemlyanov, D.; Storey, J. M.; Ribeiro, F. H. J. Catal. 2001, 199, 291.
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