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NO Oxidation on Platinum Group Metals Oxides: First Principles Calculations Combined with Microkinetic Analysis Hai-Feng Wang,†,‡ Yang-Long Guo,† Guanzhong Lu,*,† and P. Hu*,‡ Laboratories for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China, and School of Chemistry and Chemical Engineering, The Queen’s UniVersity of Belfast, Belfast, BT9 5AG, U.K. ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: July 21, 2009
By combining density functional theory calculation and microkinetic analysis, NO oxidation on the platinum group metal oxides (PtO2, IrO2, OsO2) is investigated, aiming at shedding light on the activities of metal oxides and exploring the activity variations of metal oxides compared to their corresponding metals. A microkinetic model, taking into account the possible low diffusion of surface species on metal oxide surfaces, is proposed for NO oxidation. The resultant turnover frequencies of NO oxidation show that under the typical experimental condition, T ) 600 K, pO2 ) 0.1 atm, pNO ) 3 × 10-4 atm, pNO2 ) 1.7 × 10-4 atm; (i) IrO2(110) exhibits higher activity than PtO2(110) and OsO2(110), and (ii) compared to the corresponding metallic Pt, Ir, and Os, the activity of PtO2 to catalyze NO oxidation is lower, but interestingly IrO2 and OsO2 exhibit higher activities. The reasons for the activity differences between the metals and oxides are addressed. Moreover, other possible reaction pathways of NO oxidation on PtO2(110), involving O2 molecule (NO + O2 f OONO) and lattice bridge-O2c, are also found to give low activities. The origin of the Pt catalyst deactivation is also discussed. 1. Introduction The removal of nitrogen oxides (NOx) under lean-burn condition is one of the central issues in autoemission control. Unfortunately, conventional three-way catalyst is ineffective to remove NOx under such conditions.1,2 To resolve this problem, two alternative approaches, selective catalytic reduction (SCR)3-6 and NOx storage reduction (NSR),7-12 have been proposed. In both approaches, NO oxidation to form NO2 is a fundamental step to facilitate the ultimate conversion of NOx.13-17 Thus, exploring suitable catalyst for NO oxidation and understanding the mechanism are of great importance. In our previous work, the origin of activity variation of different metals (Pt, Pd, Ir, Rh, Os, Ru) for NO oxidation was kinetically analyzed.18 However, an important issue remains to be tackled. Under excess-oxygen conditions, the oxidation states of the metal catalysts themselves would potentially change, leading to oxide formation, and the formation of metal oxide could in turn affect the activities of catalysts for NO oxidation. In this contribution, we focus on the intrinsic properties of metal oxides that determine the varied catalytic activity for NO oxidation compared with their corresponding metallic states. The fact that the formation of metal oxide affects considerably the catalytic activity on CO oxidation has been well elucidated in which one well-known case is that RuO2 exhibits enhanced catalytic activity toward CO oxidation at high pressure relative to the low activity of metallic Ru.19,20 A comparative study of CO oxidations on the late transition-metal surfaces and their corresponding oxide surfaces has also been performed by Gong et al.21 However, in comparison to CO, NO molecule has one unpaired electron in its 2π* orbital and there is a large binding * To whom correspondence should be addressed. E-mail: (P.H.) p.hu@ qub.ac.uk; (G.L.)
[email protected]. † East China University of Science and Technology. ‡ The Queen’s University of Belfast.
difference between NO and CO on catalyst surfaces. In addition, the reaction of NO + O f NO2 is less exothermic relative to CO + O f CO2 and is prone to equilibrium-limiting over temperatures of practical interest. The conclusion obtained from CO oxidation is not necessarily applicable to NO oxidation.22 For example, RuO2 is inert for NO oxidation despite its high activity for CO oxidation. Thus, a systematic investigation on the activities of metal oxides and their intrinsic properties for NO oxidation is desirable. Recently, experimental and theoretical work on NO oxidation has focused mostly on metallic substrates in particularly on the Pt catalyst.23-31 Theoretical investigations on the activities of metal oxides for NO oxidation are rare, while increasing experimental evidence indicated that the formation of metal oxide noticeably affects the activities of metal catalysts.6,32-34 For example, the deactivation of the most extensively used catalyst for NO oxidation, Pt, is often ascribed as Pt oxide formation. Fridell et al. suggested that Pt oxide formation decreases oxidation activity of NOx storage catalyst (Pt/BaO/ Al2O3) and served as a possible deactivation mechanism.35 Olsson et al. also proposed the Pt oxide formation as cause for the decrease of NO oxidation activity with time for Pt/Al2O3 and Pt/TiO2.8,14 Likewise, it was reported that the oxidation of Pt gives rise to activity decrease in propane combustion on Pt/ SiO2-Al2O3 catalyst.36 Despite these developments, the deactivation mechanism owing to Pt oxide formation remains elusive. To provide insight into the activity of metal oxide for NO oxidation and explore the activity difference between metals and their oxides, in this paper we investigate the detailed reaction pathways of NO oxidation catalyzed by the transition metal oxides, PtO2, IrO2, OsO2, using density functional theory (DFT) calculations. Furthermore, a microkinetic analysis, which is particularly adjusted to apply to oxide surfaces by taking into account the low diffusion of surface species, is made to obtain an estimation of turnover frequencies for the overall reaction
10.1021/jp904371f CCC: $40.75 2009 American Chemical Society Published on Web 09/30/2009
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and to address the key factor in determining catalytic activity. In particular, we examine almost all the possible pathways of NO oxidation to explore the activity of PtO2(110) in order to uncover the deactivation mechanism of Pt. This paper is organized as followings. Structure models and calculation details are given in Section 2. This is followed by a presentation of the results in Section 3 and their discussions are in Section 4. Finally, our conclusions are summarized in Section 5. 2. Model and Calculation method In this work, the SIESTA code37-39 was used with TroullierMartins norm-conserving scalar relativistic pseudopotentials.40 Total energy calculations were performed using the DFT-slab approach with the GGA-PBE functional,41 in which the Kohnsham orbitals were expanded in a localized basis set with the mesh cutoff of 200 Ry. A double-ζ plus polarization (DZP) basis set was utilized. The localization radii of the basis functions were determined from an energy shift of 0.01 eV. The Broyden method was employed for geometry relaxation until the maximal forces on each relaxed atom was less than 0.05 eV/Å. Considering that the rutile structures are stable phases for PtO2, IrO2, and OsO2, these oxides were modeled by the rutile structure, the most exposed (110) surface of which was hence calculated. The lattice constants of bulk PtO2, IrO2, and OsO2 were preoptimized. The surface was modeled as a periodic slab with nine layers of oxide, and the vacuum between slabs is ∼15 Å. A p(2 × 1) surface slab with a corresponding 3 × 3 × 1 k-point mesh was used. During structural optimization, the bottom six layers of the slab were fixed at bulk truncated position and the top three layers and the adsorbates were fully relaxed. The transition states (TS) were searched using a constrained optimization scheme.42 The distance between the two atoms that will form a new bond is constrained at an estimated value and the total energy of the system is minimized with respect to the other degrees of freedom. The TSs can be located via changing the fixed distance and was verified when (i) all forces on atoms are small (the criterion is set as 0.05 eV/ Å), and (ii) the total energy is a maximum along the reaction coordination but a minimum with respect to the rest of the degrees of freedom. Previous work43 has shown that the above DFT setup affords a good accuracy, especially for the calculation of reaction barriers in heterogeneous catalysis. The accuracy of the SIESTA method was carefully benchmarked with a plane-wave methodology.44 3. Results 3.1. Thermodynamics of Metal Oxide Formation. Under realistic conditions of NO oxidation, the presence of the excess oxygen can potentially oxidize metals into oxides. In order to obtain a quantitative estimation of this probability, a simple phase diagram was calculated by considering the thermodynamic equilibrium between the two states, M and MO2 (M ) Pt, Ir, Os). At a given T and O2 partial pressure, the Gibbs free energy change from the metal to metal oxide can be written as bulk bulk ∆G(T, pO2) ) GMO (T, pO2) - GM (T, pO2) - µO2(T, pO2) 2 (1) bulk bulk (T,pO2) and GM where GMO (T,pO2) are the Gibbs free energies 2 for metal oxide MO2 and metal M, respectively. µO2(T,pO2)is the chemical potential of gas-phase O2 at the given temperature T and partial pressure pO2. Considering that the entropy changes
Figure 1. Phase diagram of metals (Pt, Ir, and Os) in the presence of O2, plotted against oxygen partial pressure pO2 and temperature T. The green, brown, and blue lines represent thermodynamic boundaries for Pt/PtO2, Ir/IrO2, and Os/OsO2, respectively, above which the corresponding metal oxide is favored. Dash lines show the typical experimental condition.
Figure 2. Models of stoichiometric PtO2(110) surface in a perspective view (a) and the top view (b). In (b), the letters A-C denote possible adsorption sites for NO and O; A, the top site of Pt5c, B, the bridge site, C, the top site of bridge-O2c. The dark blue and red balls are Pt and O, respectively.
of the solids as a function of temperature are negligible compared to the gas-phase molecule, the free energies of bulk MO2 and M can be substituted by the DFT-calculated total energies. The chemical potential µO2(T,pO2) was calculated based on the thermodynamic equation:
µO2(T, pO2) ) µO2(T, p°) + RT ln
pO2 p°
) µO2(0 K, p°) -
TSO2(T f 0 K, p°) + RT ln
pO2 p°
(2)
where µO2(0 K, p°) is calculated from DFT including the zeropoint energy, and the entropy changes were taken from the experimental data.45 When ∆G(T, pO2) < 0, the oxide formation is favored and correspondingly we can obtain the phase diagram of Pt/PtO2, Ir/IrO2, Os/OsO2, shown in Figure 1. We can see that under the typical experimental condition (T ) ∼600 K, pO2/p° ) ∼0.2), these three transition metals (Pt, Ir, and Os) are inclined to form metal oxides thermodynamically. Therefore, it is crucial to study the catalytic activity of metal oxides in order to understand the metal catalyst performance under realistic conditions. 3.2. Structure of Clean MO2(110). The most stable surface of MO2, (110) surface, is illustrated in Figure 2 in which PtO2(110) is shown. Ir/O2(110) and OsO2(110) are similar to PtO2(110) and thus not shown here. From Figure 2 we can see that the surface is terminated by 2-fold coordinated bridge oxygen (O2c), and there are two types of Pt atoms, the 6-fold
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TABLE 1: Adsorption Energies (eV) of NO and O at Different Sites on PtO2(110), IrO2(110), and OsO2(110) from Our Calculationsa NO
O O2 NO2
adsorption site
PtO2(110)
IrO2(110)
OsO2(110)
A: on-top of M5c B: bridge site, upright C: bridge, tilted D: on-top of bridge O2c A: on-top of M5c B: bridge site bridge site bridge site
-0.19 -0.74 -1.49 -1.51 1.03 1.83 -0.17 -1.85
-1.92 -1.05 -2.09 -1.36 -0.62 0.84 -1.41 -1.77
-3.17 -0.93 -3.14 weak -1.88 -0.04 -1.94 -1.68
a The adsorption data of O2 and NO2 are also included. Various sites are shown in Figure 2b.
coordinated Pt6c and 5-fold coordinated Pt5c linked by the 3-fold coordinated O3c. The coordinatively unsaturated Pt5c is also denoted as cus-Pt5c and generally considered to be vital for catalytic activity, which provides basic adsorption sites for NO and O. The distance between the nearest Pt5c-Pt5c is 3.28 Å, larger than the Pt-Pt distance (2.84 Å) on Pt(111), and the cusPt5c row are separated by the bridge O2c, as shown in Figure 2. 3.3. Chemisorption of Various Species. To examine the adsorption energetics and geometries of NO, NO2, and O, the adsorption energies (Ead) on several sites of the three oxides were calculated and the results are summarized in Table 1. The adsorption structures of NO and NO2 on PtO2(110) are shown in Figure 3. The following standard equation was used to calculate adsorption energies
Ead ) E(adsorbates/oxide) - E(oxide) - E(adsorbates) where the adsorbates denote the surface species such as NO, NO2, and O. Here, a negative value of Ead means that the adsorption is exothermic. It should be noted that the adsorption energy of O on the surface is relative to the gas-phase O2 molecule. The total energies of NO, O2, and NO2 were evaluated in a (15 Å × 15 Å × 15 Å) supercell using spin-polarized calculation where necessary. From Figure 3 and Table 1, we can see the following features. On PtO2(110), NO prefers to adsorb at the bridge site of two cus-Pt5c in a tilted manner (Figure 3c) with the adsorption energy of -1.49 eV. The adsorption configuration of NO at the bridge site and on the top site of cus-Pt5c with an upright configuration is much weaker, which is different from CO adsorption on PtO2(110), since CO prefers to adsorb on the top site of cusPt5c perpendicular to the surface. It is worth noting that NO can also adsorb on the top site of bridge O2c (Figure 3d) with a high adsorption energy of -1.51 eV. However, in such adsorption configuration, it is likely that NO cannot react with bridge-O2c to form NO2 (no transition state was located in our calculations). Thus, we exclude the contribution of this adsorption configuration to the overall reaction activity. O prefers to chemisorb on the top site of cus-Pt5c with an adsorption energy of 1.03 eV, being reasonably consistent with the results (0.92 eV) reported by Gong et al.46 It should be noted that O adsorption is endothermic relatively to the gas-phase O2, indicating the difficulty in O2 dissociation and the formation of surface O species. Molecular adsorption of O2 has a low adsorption energy at the bridge site with the O-O bond parallel with the surface. NO2 adsorbs at the bridge site in a µ-N,Onitro configuration, with one N-O bond essentially parallel to the surface and the other N-O bond nearly normal to the surface. These species take similar configurations on IrO2(110)
and OsO2(110) to PtO2(110), except that on OsO2(110) NO prefers to adsorb on the top site of cus-Os5c. 3.4. Determination of Reaction Barriers. The transition states for O2 dissociation and NO + O f NO2 were searched, and the corresponding reaction barriers and geometric parameters are listed in Table 2. We can see from the table that the O2 dissociation on PtO2(110) has to overcome a barrier of 1.78 eV, being much larger than the one (0.49 eV) on Pt(111).18 However, the surface reaction of NO + O possesses a small barrier (0.10 eV), much smaller than 1.42 eV for the same reaction on Pt(111).18 To evaluate quantitatively the activity difference between Pt and PtO2, kinetic analysis is necessary (see next section). On the oxide surfaces, the chemisorption energies of NO and O at different sites of the cus-Pt5c row (Figure 2) span a wide range, as shown in Table 1. This means that the diffusions of adsorbed NO and O along the cus-Pt5c row are relatively restricted. The high diffusion barriers would give rise to some difficulty in directly applying mean-field-based microkinetics that requires the fast diffusion of the surface species. To overcome this problem, the concept of reactant pair used in kinetic Monto Carlo (KMC)47 is used, which means that only the nearest-neighbored NO and O can react with each other. Therefore, the surface reaction can be described as follows; adsorbed NO and O approaches each other with a diffusion barrier to form a coadsorbed state or complex, denoted as I, and then I transforms to NO2. The diffusion barrier was roughly estimated by comparing the adsorption energies of NO or O on different sites along the cus-Pt5c row. For example, on PtO2(110) the NO diffusion barrier was estimated to be 1.30 eV (the NO adsorption energy difference between the top site (Figure 3a) and the bridge site (Figure 3c)) and the O diffusion barrier was estimated to be 0.80 eV in a similar way. Consequently, the reaction barrier of NO* + O* f I is determined to be 0.80 eV, which is estimated following Ediff ) min[Ediff(NO), Ediff(O)]. The structure of I and the transition state of NO + O reaction on IrO2(110) are shown in Figure 4, which are similar to those on OsO2(110). For the complex I on PtO2(110), which is different from that on OsO2(110) and IrO2(110), the favored NO configuration is tilted to bond with O, similar to the transition state of NO+O. 3.5. Kinetic Estimation. In order to obtain a quantitative estimation of the rate of NO oxidation on the surfaces of PtO2, IrO2, and OsO2, and compare with the activities on the corresponding metal surfaces, microkinetics was used. Elementary reaction steps following the Langmuir-Hinshelwood mechanism are shown in Scheme 1. NO and O2 adsorb at cusM5c site (denoted as NO* and O2*), which is considered to be in equilibrium, and adsorbed O2 molecule dissociates into atomic oxygen O*. Then NO* and O* diffuse along the cus-M5c row to approach each other and form complex I, which finally transforms into NO2 desorbing from the surfaces. * denotes the coordinatively unsaturated cus-M5c site. Using the De Donder relations,48 the net rate for elementary step i in terms of the forward rate constant ki+, the coverage of the reactant θj and the reversibility Zi can be expressed as
ri ) ki+
∏ θjV (1 - Zi) ij
(3)
j
where Zi ) ∏jθVj ij/Kieq, which approaches zero as step i becomes irreversible and approaches unity as step i becomes quasiequilibrated. Kieq is the equilibrium constant of step i, determined by the standard state Gibbs free energy change of the reaction,
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Figure 3. Optimized structures of NO and O adsorption at different sites on PtO2(110), (a-d) for NO adsorption and (e,f) for O adsorption. The most stable configuration of O2 and NO2 molecules are shown in (g,h), respectively. The dark blue, blue, and red balls denote Pt, N, and O, respectively.
TABLE 2: Reaction Barriers (eV) and Transition State Structures (Å) of O2 Dissociation and NO2 Formation on PtO2(110), IrO2(110) and OsO2(110) from Our Calculationsa TS(O2* f 2O*) PtO2(110) IrO2(110) OsO2(110)
Ediff
TS(I f NO2)
Eb1
d(O-O)
(NO* + O* f I)
Eb2
d(O-NO)
1.78 0.44 0.02
3.106 1.841 1.546
0.80 1.04 1.84
0.10 0.19 1.70
2.475 2.029 1.627
a Diffusion barriers along the M5c-row (M ) Pt, Ir, and Os) relating to the formation of co-adsorbed I (NO/O) are also given, estimated by the adsorption energy difference of NO or O at different sites of M5c-row (see the text).
Figure 4. Structures of initial state (a) and transition state (b) of NO + O reaction on IrO2(110). Red, light blue, and deep blue ball represent O, N, and Ir, respectively.
SCHEME 1: Elementary Steps of NO Oxidation Based on the Langmuir-Hinshelwood Mechanism on the cus-M5c Row on MO2 (M ) Pt, Ir, Os)
Kieq ) exp(-∆Gi/RT). Vij are the stoichiometric coefficients for the j reactants or products of step i. The forward rate constant k for the elementary step can be determined by the Arrhenius equation and the transition state theory. The standard equilibrium constant Kieq and rate constant k for each step can be calculated using the DFT data including the entropy effect. Zi can be solved by following the steady state condition. Applying the condition of θO + θNO+ θI + θNO2 ) 1, we can obtain the coverages of
all the species. Then, the total reaction rate can be calculated. Other detailed derivations can be found in our previous paper.18 Taking the typical experimental condition,23 T ) 600 K, pO2 ) 0.1 atm, pNO ) 3 × 10-4 atm, pNO2 ) 1.7 × 10-4 atm, and with the entropy effect of gas-phase O2, NO, and NO2 included in which the entropies are taken from ref 45, the turnover frequencies (TOF) of NO oxidation on the PtO2(11), IrO2(110), and OsO2(110) can be estimated, which are shown in Table 3. 4. Discussion 4.1. Activity Trend of Oxides for NO Oxidation. As shown in Table 3, among the three oxides IrO2(110) possesses the highest activity for NO oxidation, which can be understood as follows. IrO2(110) has a moderate binding strength for NO and O, and the barriers for both O2 dissociation and conversion from complex I (NO + O) to NO2 are low (0.44 and 0.19 eV, respectively). In other words, the formation of surface O is relatively fast, while the removal of surface O by reacting with NO is also fast, resulting in enough free sites18 for O2 dissociation and adsorption of NO. Thus, the overall reaction rate is high. PtO2(110) has a rather weak ability to bind NO and O, leading to that O2 dissociation is pretty difficult with a high barrier (1.78 eV) and a low activity. In comparison to IrO2(110), the adsorption of NO and O is too strong on OsO2(110), which gives rise to two adverse effects for the low activity of OsO2(110), (i) the diffusion of NO and O toward each other to form complex I is difficult, reducing the paring probability of NO and O, and (ii) the reaction of NO with O requires to overcome a rather high barrier (1.70 eV). 4.2. Activity Difference between Metal Oxides and the Corresponding Metals. The activity changes of metal oxides for NO oxidation relative to their corresponding metals are shown in Figure 5. From Figure 5, we can see the following features: (i) The activities trend of metal oxides do not follow the curve of metal systems, mainly owing to the different dependence of the reaction barriers on the reaction enthalpies; and (ii) the binding ability of oxide toward O becomes weak compared to the corresponding metal. PtO2(110) possesses a rather low activity relative to the Pt(111), which is in agreement with experimental XPS measurement that suggested that overoxidation of Pt is concomitant with a TOF decrease.23 Interestingly, IrO2(110) and OsO2(110) exhibit higher activity than Ir(111) and Os(0001), respectively, which can be understood as follows: For the metal surfaces, the optimum adsorption energy of O atom is about 1.0 eV at the peak of the volcano
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TABLE 3: The Kinetic Data for NO Oxidation Catalyzed by PtO2(110), IrO2(110), and OsO2(110), Obtained from Microkinetics Using DFT Data PtO2(110) k3+ k4+ k5+ Z3 Z4 Z5 θ
-2
1.36 × 10 2.39 × 106 2.93 × 1012 3.00 × 10-2 2.92 × 10-5 ∼1 9.99 × 10-1
IrO2(110)
OsO2(110)
2.38 × 10 2.14 × 104 3.05 × 1011 ∼1 5.05 × 10-6 ∼1 1.16 × 10-2
8.16 × 10 4.44 × 10-2 6.91 × 10-2 1.67 × 10-2 4.11 × 10-2 8.98 × 10-4 1.10 × 10-11
9
12
curve at T ) 600 K.18 The O adsorption energies on Ir and Os surfaces are on the left-hand side of the optimum value in the volcano curve. If the activities of Ir and Os were to be improved, it needed to lower their binding abilities toward surface species in order to achieve the maximum TOF. IrO2 and OsO2 satisfy this requirement and thus exhibit higher activities. But for Pt, which locates on the right-hand side of the volcano curve, it requires a higher binding ability in order to approach the higher activity. However, compared to Pt, PtO2 has a weaker binding strength toward O atom and thus worse activity (see more discussions in the next section). The activity variation from metal oxides to their corresponding metals can be further understood as follows. As suggested in the previous work of our group,18,49,50 many catalytic reactions can be described as a two-step reaction model, that is, the adsorption of the reactant and the desorption of the product. A good catalyst should possess reasonable ability to adsorb the reactant, meanwhile the products can readily desorb from the surface to provide enough free sites leading to a reduction of surface blocking. Accordingly, the activity differences between the metal oxides and metals can be described in the following: (i) From Table 4, we can see that although NO2 formation (I f NO2* + *) on PtO2(110) has a large rate constant (low barrier ∼0.1 eV), PtO2(110) has a too weak ability to adsorb O2/O with very low O2 coverage. Moreover, O2 can hardly dissociate into O with a rate constant as low as 1.36 × 10-2 s-1. Thus, the low activity of PtO2 for NO oxidation could be expected. (ii) IrO2(110) exhibiting higher activity than Ir(111) is mainly ascribed to the low barrier (∼0.2 vs 1.32 eV (Ir)) of adsorbed
Figure 5. Activity trends of NO oxidation on the platinum group metals and their respective oxides (PtO2, IrO2, OsO2). The TOF data of metal systems are taken from our previous paper.18 The dash line indicates the optimum metal catalyst for NO oxidation. The oxygen chemisorption energies were calculated with respect to half an O2 molecule at T ) 600 K.
PtO2(110) θNO θO θO 2 θNO2 θI Log(TOF)
IrO2(110) -4
7.30 × 10 3.82 × 10-16 2.53 × 10-11 5.23 × 10-6 1.47 × 10-12 -12.18
OsO2(110)
-1
9.79 × 10 1.76 × 10-3 7.28 × 10-3 1.12 × 10-5 3.92 × 10-7 1.57
9.91 × 10-1 8.55 × 10-3 2.03 × 10-7 1.84 × 10-15 5.22 × 10-4 -4.44
TABLE 4: Elementary Reaction Steps of NO Oxidation Based on the Mars-van Krevelen Mechanism (Bridge O2c Involved), and the Corresponding Standard Free Energy Changes (∆G°) and Reaction Barriers (Eb) at T ) 600 K Are Listeda Mars-van Krevelen mechanism
∆G°/eV
NO + * f NO* NO* diffuses along the Pt5c row NO* + O2c f NO2* + vac O2 + Vac f O2(vac) NO + Vac f NO(vac) NO* + O2(vac) f NO2* + O2c
-0.05 0 -0.11 0.37 -0.87 -1.60
Eb/eV 1.30 0.27 0.14
a
The adsorption processes are considered to be in equilibrium and the ∆G° of NO and O2 adsorption were calculated including the corresponding gas-phase entropy contributions at T ) 600 K.
NO reacting with O to form NO2, while the O2 dissociation rate constant on IrO2(110) is still high (k3+ ) 2.38 × 109 s-1) and the adsorption energies of O2 and NO are reasonably strong. (iii) On Os(0001), as suggested in our previous work,18 the low activity for NO oxidation are attributed to that O atom adsorption on Os is so strong that the surface is almost covered by O and gives low coverage of free sites, which hinders the adsorption of NO and results in a low coverage product (θNOθO ) 4.68 × 10-6). Compared to Os(0001), the adsorption energy of O on OsO2(110) decreases, which reduces the surface blocking, while the adsorption energy of NO increases, giving rise to an enhanced product θNOθO ) 8.46 × 10-3. In addition, the reaction barrier of NO* + O* f NO2* + * is relatively lowered on OsO2(110) compared to Os(0001) (1.84 (OsO2) vs 1.91 eV (Os)). Thus, the overall reaction rate is enhanced on OsO2(110). 4.3. Deactivation Mechanism of Pt. We have shown that PtO2(110) exhibits low activity for NO oxidation following the reaction mechanism described in Scheme 1, which is consistent with the experimental results that the formation of platinum oxide lowers the overall activity of Pt catalyst. However, one may ask the following question: Are there other reaction pathways that may give rise to high activity for NO2 formation on PtO2(110)? In order to answer this question, some possible routes were investigated to clarify this issue and the results are the following. (i) Molecular oxygen involved mechanism (NO* + O2* f [OONO] f NO2). Because of the low adsorption energy of O2 (-0.17 eV), especially when entropy of O2 molecule under real temperature (T∆S ) 1.31 eV at T ) 600 K) is included, the coverage of surface O2 species would be very low. Thus this pathway is not expected to give a high turnover frequency for NO2 formation. (ii) Mars-van Krevelen mechanism involving bridge-O2c. The key reaction steps and the corresponding energetics are shown in Table 4, from which we can see that the adsorbed NO* can readily react with lattice bridge-O (O2c) with a low barrier of 0.27 eV and creates a bridge oxygen vacancy. NO* can also
NO Oxidation on Platinum Group Metals Oxides react with O2 molecule adsorbed on the oxygen vacancy with a barrier of 0.14 eV. However, there are two adverse factors that reduce the overall activity. (i) The diffusion of NO* along the cus-Pt5c row is accomplished with a high barrier (1.30 eV), which greatly reduces the possibility of NO pairing with the O2 on the bridge oxygen vacancy (O2(vac)) molecule. (ii) The competitive adsorption of NO and O2 at the oxygen vacancy site with the adsorption energies of 0.37 and -0.87 eV (calculated at T ) 600 K, see Table 4) for O2 and NO, respectively, can lead to the vacancies being almost fully covered by NO. Because of low O adsorption energy on cusPt5c and thus low O coverage as mentioned above, the removal of the NO at the vacancy site through NO + O reaction must be very slow. Rough TOF estimation of the Mars-van Krevelen mechanism shown in Table 4 was carried out, resulting in a low ln(TOF) of -8.96, although the Mars-van Krevelen mechanism gives rise to interestingly higher activity for NO oxidation compared to the Langmuir-Hinshelwood mechanism shown in Scheme 1. Clearly, all the pathways considered above on PtO2(110) give low turnover frequencies for NO oxidation. These results may provide a detailed microscopic explanation for the Pt deactivation under realistic excess-O2 conditions. 5. Conclusions Considering that the metal oxide formation generally is thermodynamically favored under realistic excess-oxygen conditions for NO oxidation, NO oxidation on the platinum group metal oxides (PtO2, IrO2, OsO2) is investigated by combining density functional theory calculations and microkinetic analysis, aiming to provide insight into the activities of metal oxides for NO oxidation and exploring the activity changes compared to their corresponding metals. Our main results can be summarized as follows. (i) Combining DFT data and thermodynamic equations, phase diagrams of M/MO2 (M ) Pt, Ir, Os) are plotted, which confirms that the formation of metal oxide is thermodynamically favored at typical condition for NO oxidation (T ) 600 K, pO2 ) ∼0.2 atm). (ii) The chemisorption energies of species (NO, O, O2, NO2, et al.) at various sites of cus-Pt5c row on the metal oxide surfaces are studied, and the relevant reaction barriers are also calculated. In order to make a quantitative estimation of the reaction rate for NO oxidation on these metal oxide surfaces, an adjusted microkinetic approach, taking into account the possible low diffusion of surface species on the metal oxide surfaces, is used. (iii) Under the typical experimental condition, T ) 600 K, pO2 ) 0.1 atm, pNO ) 3 × 10-4 atm, pNO2 ) 1.7 × 10-4 atm, the kinetic results show the following. First, IrO2(110) exhibits higher activity for NO oxidation than PtO2(110) and OsO2(110). Second, compared to the corresponding metal Pt, Ir, and Os, PtO2 decreases the ability to catalyze NO oxidation, but IrO2 and OsO2 exhibit higher activities. Third, both metallic Ir and IrO2 exhibit high activities for NO oxidation, and this may be one of the reasons for Ir being a good catalyst under excess oxygen conditions. (iv) The reasons for the activity difference between the metal and oxide are also addressed. The decreased activity of PtO2 is owing to the weak binding ability of PtO2(110) toward the species O, O2 and NO. For IrO2(110), the enhanced activity relative to Ir(111) is mainly ascribed to the low barrier of adsorbed NO reacting with O to form NO2, while O2 dissociation on IrO2(110) maintains a high rate constant. The reduced activity
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18751 of OsO2 compared to Os(0001) is mainly due to the decreased surface block owing to the weaker O adsorption energy on OsO2(110). (v) Several possible reaction pathways on PtO2(110) involving O2 molecule (NO + O2 f OONO) or lattice bridge-O2c are also presented, which result in low activities. The origin of the Pt catalyst deactivation, which has been extensively ascribed to the formation of the Pt-oxide, is discussed. Acknowledgment. This work is financially supported by National Basic Research Program (2004CB719500), International Science and Technology Cooperation Program (2006DFA42740) and the 111 Project (B08021). H.F.W. also gratefully thanks China Scholarship Council for Joint-PhD studentship with QUB. References and Notes (1) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283. (2) Fritz, A.; Pitchon, V. Appl. Catal., B 1997, 13, 1. (3) Koebel, M.; Elsener, M.; Kleemann, M. Catal. Today 2000, 59, 335. (4) Heck, R. M. Catal. Today 1999, 53, 519. (5) Amiridis, M. D.; Zhang, T. J.; Farrauto, R. J. Appl. Catal., B 1996, 10, 203. (6) Kung, M. C.; Kung, H. H. Top. Catal. 2004, 28, 105. (7) Miyoshi, N.; Matsumoto, S.; Katoh, K.; Tanaka, T.; Harada, J.; Takahashi, N.; Yokota, K.; Sugiura, M.; Kasahara, K. SAE Technical Paper Series 1995, 950809. (8) Ollsson, L.; Fridell, E. J. Catal. 2002, 210, 340. (9) Shinjoh, H.; Takahashi, N.; Yokota, K.; Sugiura, M. Appl. Catal., B 1998, 15, 189. (10) Sakamoto, Y.; Okumura, K.; Kizaki, Y.; Matsunaga, S.; Takahashi, N.; Shinjoh, H. J. Catal. 2006, 238, 361. (11) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Catal. ReV. Sci. Eng. 2004, 46, 163. (12) Fridell, E.; Skoglundh, M.; Westerberg, B.; Johansson, S.; Smedler, G. J. Catal. 1999, 183, 196. (13) Fridell, E.; Persson, H.; Westerberg, B.; Olsson, L.; Skoglundh, M. Catal. Lett. 2000, 66, 71. (14) Olsson, L.; Abul-Milh, M.; Karlsson, H.; Jobson, E.; Thorma¨hlen, P.; Hinz, A. Top. Catal. 2004, 30-31, 85. (15) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. J. Phys. Chem. B 2001, 105, 12732. (16) Kikuyama, S.; Matsukuma, I.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Appl. Catal., A 2002, 226, 23. (17) Schmitz, P. J.; Baird, R. J. J. Phys. Chem. B 2002, 106, 4172. (18) Wang, H.-F.; Guo, Y.-L.; Lu, G. Z.; Hu, P., J. Chem. Phys., in press. (19) Wang, J.; Fan, C. Y.; Jacobi, K.; Ertl, G. Surf. Sci. 2001, 481, 113. (20) Fan, C. Y.; Wang, J.; Jacobi, K.; Ertl, G. J. Chem. Phys. 2001, 114, 10058. (21) Gong, X.-Q.; Liu, Z.-P.; Raval, R.; Hu, P. J. Am. Chem. Soc. 2004, 126, 8. (22) Hong, S.; Rahman, T. S.; Jacobi, K.; Ertl, G. J. Phys. Chem. C 2007, 111, 12361. (23) Mulla, S. S.; Chen, N.; Delgass, W. N.; Epling, W. S.; Ribeiro, F. H. Catal. Lett. 2005, 100, 267. (24) Ovesson, S.; Lundqvist, B. I.; Schneider, W. F.; Bogicevic, A. Phys. ReV. B 2005, 71, 115406. (25) Smeltz, A. D.; Getman, R. B.; Schneider, W. F.; Ribeiro, F. H. Catal. Today 2008, 136, 84. (26) Marques, R.; Darcy, P.; Da Costa, P.; Mellotte´e, H.; Trichard, J.M.; Dje´ga-Mariadassou, G. J. Mol. Catal. A: Chem 2004, 221, 127. (27) Xu, Y.; Getman, R. B.; Shelton, W. A.; Schneider, W. F. Phys. Chem. Chem. Phys. 2008, 10, 6009. (28) Torres, D.; Gonzalez, S.; Neyman, K. M.; Illas, F. Chem. Phys. Lett. 2006, 422, 412. (29) Kromer, B. R.; Cao, L.; Cumaranatunge, L.; Mulla, S. S.; Ratts, J. L.; Yezerets, A.; Currier, N. W.; Ribeiro, F. H.; Delgass, W. N.; Caruthers, J. M. Catal. Today. 2008, 136, 93. (30) Olsson, L.; Westerberg, B.; Persson, H.; Fridell, E.; Skoglundh, M.; Andersson, B. J. Phys. Chem. B 1999, 103, 10433. (31) Crocoll, M.; Kureti, S.; Weisweiler, W.J. Catal. 2005, 229, 480. (32) Irfan, M. F.; Goo, J. H.; Kim, S. D.Appl. Catal., B 2008, 78, 267.
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