Theoretical Studies of the Adsorption of CO and C on Ni(111) and Ni

Article pubs.acs.org/JPCC

Theoretical Studies of the Adsorption of CO and C on Ni(111) and Ni/ CeO2(111): Evidence of a Strong Metal−Support Interaction Javier Carrasco,† Laura Barrio,†,‡ Ping Liu,‡ José A. Rodriguez,‡ and M. Verónica Ganduglia-Pirovano*,† †

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, E-28049, Spain Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

ABSTRACT: The catalytic CO methanation reaction on Ni/CeO2(111) systems is known to depend on Ni coverage: at medium and large coverages, Ni/CeO2(111) surfaces are able to catalyze methane production, whereas at small coverage they become efficient catalysts for the water−gas shift reaction. Electronic structure, geometries, and the adsorption of C and CO on small Nin (n = 1 and 4) particles deposited on CeO2(111) have been studied using density functional theory (DFT) with the DFT+U approach and compared with Ni(111) and CeO2(111). The most stable Ni4 cluster has a pyramidal structure (pyr-Ni4), and a planar rhombohedral structure (r-Ni4) is less stable by ∼0.2 eV. Metallic Ni particles are partially oxidized (Ni2+/Ni1+) upon deposition on the ceria support, which is partially reduced. C species are strongly bound on Ni(111), whereas on Ni/CeO2(111), and on the bare support, oxidative adsorption (C + CeO2 → CO + CeO2−x) is mostly preferred, opening a Mars− van Krevelen mechanism to prevent coke formation. The exothermicity of nonoxidative adsorption of C on nickel sites follows the trend: Ni1/CeO2(111) < pyr-Ni4/CeO2(111) < Ni(111). On these systems, CO adsorption is nonoxidative. The C−O bond strength follows the inverse trend of the nonoxidative adsorption of C: Ni(111) < pyr-Ni4/CeO2(111) < Ni1/CeO2(111). The stronger C−O bond found for the CO/Ni1/CeO2(111) system compared with CO/Ni(111) provides an explanation of the Ni coverage dependence reported for the CO methanation reaction on Ni/CeO2(111) catalysts. The strong electronic perturbations in the Ni1 adatoms produce a drastic change in their chemical properties.

1. INTRODUCTION

One of the challenges of catalyst’s design is the substitution of scarce noble metals such as Pt or Rh for more common and less expensive alternatives while maintaining catalytic performance. Ni-based catalysts are widely employed in many industrial process such as the reforming of alcohols, hydrogenation reactions, hydro-cracking, and also oxidation processes.10,11 Recent work at Brookhaven National Laboratory has proven the high activity and stability of Ni-CeO2 systems for both the WGS12 and the ethanol steam reforming13 reactions. It is quite remarkable that a low activity for the CO methanation reaction is observed on the Ni-CeO2 systems,13 in contrast with the high activity seen for the bulk metallic Ni.14,15 The reaction of low concentrations of CO in a mixture with H2 to form CH4 was developed as a gas purification process in the 1950s.14,16At the present time, the methanation reaction has a critical role in the production of synthetic natural gas from hydrogen-deficient carbonaceous materials.15,16 Ni-based catalysts are frequently used in the CO methanation reaction.17−19 Goodman and Kelley compared the rates of CO methanation on Ni(100) and Ni(111) surfaces.17 At a given temperature, the rate of production of CH4 over an initially clean catalyst crystal was constant, with no apparent induction period.17 Relative small

Ceria (CeO2)-supported systems are superior catalysts in many chemical processes due to their outstanding oxygen storage capabilities associated with the easy conversion between Ce4+ and Ce3+ oxidation states.1 In addition, ceria often enhances the performance of late transition metals by making them more resistant to sintering due to strong metal−support interactions (SMSI), which is now a well-known concept and can affect the activity and selectivity significantly.2 Local bonding and local strain effects are important when determining the morphology, stability, nucleation, and growth of late transition-metal nanoparticles. Recent works have extended the role of ceria to a chemically active participant in many reactions by providing lattice oxygen atoms when required. For example, the active role of ceria in the catalysis of metal/ceria systems for the water gas shift (WGS, CO + H2O → H2 + CO2) reaction has been widely reported.3,4 Ceria by itself is not a catalytically active system for this reaction but combined with metals has been shown as an essential component for high WGS activity.3,5 In many cases, metal−ceria interactions have been proven to be crucial to achieving high catalytic performance.2,6,7 However, the exact role played by both the oxide and the metal in the WGS reaction is still a matter of debate. In practical terms, one wants to manipulate metal ↔ oxide support interactions to improve catalytic activity and selectivity.2 © 2013 American Chemical Society

Received: January 14, 2013 Revised: March 10, 2013 Published: March 18, 2013 8241

dx.doi.org/10.1021/jp400430r | J. Phys. Chem. C 2013, 117, 8241−8250

The Journal of Physical Chemistry C

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methane formation. In the bottom panel of Figure 1, the estimated turnover number (TON) for the methanation of CO on Ni/CeO2(111) surfaces with an admetal coverage below 1 ML is shown.7 For Ni(100) and Ni(111), under identical reaction conditions, a TON of ∼0.7 CH4 molecules site−1 s−1 has been measured.17 This value is comparable to the range of TONs estimated for Ni coverages of 0.5 to 1 ML on CeO2(111): 0.72 to 0.84 CH4 molecules site−1 s−1. The drastic drop in methanation activity seen in Figure 1 at small coverages of Ni is a very surprising result, considering the high methanation activity observed for extended Ni surfaces and Ni nanoparticles supported on alumina.17,19 The observed lack of activity suggests the existence of strong metal−support interactions in the Ni/CeO2(111) system.18 The nature of these interactions is unknown. Although these interactions have a negative impact on the CO methanation activity of Ni, they are highly beneficial for the WGS reaction12,20 and for the steam reforming of ethanol (C2H5OH + 3H2O → 6H2 + 2CO2).13 In general, a fundamental understanding of the nature of Ni ↔ oxide interactions is necessary for a rational design of better nickel-based catalysts.10,16,20−23 In this article, we use density functional theory (DFT) calculations to investigate the interaction of highly dispersed Ni species with model CeO2 (111) surfaces. We will analyze charge transfers, changes in oxidation state, and perturbations in the Ni density of states associated with the formation of Ni−ceria bonds. We will also compare the adsorption of CO and C on the Ni/CeO2 structures considered as well as on the bare Ni(111) and CeO2 (111) surfaces. We are interested in the binding of C because on the one hand it is a product of the decomposition of CO, that is, the initial step in the methanation reaction and on the other hand the formation of coke layers on the surface of the catalysts affects its catalytic performance. We will focus our analysis on the study of the metal ↔ support interaction (variations in density-of-states and charge transfer between nickel and ceria) as a function of nickel coverage. Finally, we will also consider the relevant role of ceria in the Ni/CeO2 interaction with C and CO.

differences were found among the reaction rates on Ni(111), the more open Ni(100) surface, and Ni particles supported on alumina.17 More recent studies by Andersson et al.19 indicate that steps are the active sites for CO dissociation on clean Ni(111). After observing different reaction rates on Ni surfaces and on Ni particles with different shapes, it was suggested that the CO methanation reaction is structure-sensitive and that highly under-coordinated Ni sites are the active sites.19 The top panel in Figure 1 shows data for the production of CH4 from CO and H2 over a series of Ni/CeO2(111) surfaces as a function of Ni coverage.20 For Ni coverages below 0.2 ML, the production of methane is very small. At Ni coverages above 0.2 ML, there was a substantial enhancement in the rate for

2. THEORETICAL METHODS AND COMPUTATIONAL DETAILS Spin-polarized DFT and supercell periodic models were used within the Vienna ab initio simulation package (VASP, version 5.2.12).24−26 We treated explicitly the C (2s, 2p), O (2s, 2p), Ni (3p, 3d, 4s), and Ce (4f, 5s, 5p, 5d, 6s) electrons as valence states expanded in plane-waves with a cutoff energy of 415 eV, whereas the remaining electrons were kept frozen as core states in the projector-augmented wave (PAW) method.27 Total energies and electron densities were computed within the DFT +U approach of Dudarev et al.,28 in which a Hubbard U-like term (Ueff = U − J, that is, the difference between the Coulomb U and exchange J parameters, hereinafter referred to as simply U) is added to the Perdew−Burke−Ernzerhof (PBE) generalized-gradient approximation (GGA) functional.29 We used a value of U = 4.5 eV for Ce atoms, which was calculated self-consistently by Fabris et al.30 using a linear response approach.31 The O-terminated CeO2(111) and Ni(111) surfaces were modeled by (3 × 3) unit cells, containing six atomic layers separated by at least 18 Å of vacuum. Monkhorst-Pack grids with 2 × 2 × 1 and 4 × 4 × 1 k-point sampling were used for the CeO2(111) and Ni(111) surfaces, respectively. All of the

Figure 1. Top panel: CO methanation activity of model Ni/ CeO2(111) catalysts as a function of admetal coverage. Each Ni/ CeO2(111) surface was exposed to a mixture of 24 Torr of CO and 96 Torr of H2 at 625 K. The reported values correspond to the number of CH4 molecules produced after a reaction time of 5 min under steadystate conditions. Bottom panel: Estimated turnover number for CO methanation over Ni/CeO2(111) catalysts with a nickel coverage of 0.05 to 1.0 ML. Reprinted with permission from ref 18. Copyright 2011 Springer. 8242



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Table 1. Summary of the Total Magnetic Moment (μB), Number of Ce3+ Ions, and Total Number of Unpaired Electrons within the CeO2(111)-Supported Ni Clusters before and after Adsorption of CO and C Speciesa system

total magnetic moment (μB)

no. of Ce3+ ions

no. of unpaired e− on the Ni cluster

Ni1/CeO2(111) pyr-Ni4/CeO2(111) r-Ni4/CeO2(111) CO on Ni1/CeO2(111) nonoxidative ads. CO on pyr-Ni4/CeO2(111) nonoxidative ads. C on Ni1/CeO2(111) oxidative ads. C on Ni1/CeO2(111) nonoxidative ads. C on pyr-Ni4/CeO2(111), oxidative ads. C on pyr-Ni4/CeO2(111), nonoxidative ads.

4.0 6.0 6.0 2.0 4.0 2.0 2.0 6.0 2.0

2 2 2 1 2 2 2 4 1

2 4 4 1 2 0 0 2 1

a In each case, only the most preferred adsorption configuration is considered. (See the text.) Prefixes “r” and “pyr” stand for rhombohedral and pyramidal Ni4 clusters, respectively.

the magnetic moment is ∼1 μB. Hence, those ions are referred to as Ce3+. Because the calculations are restricted to high-spin solutions, the number of unpaired electrons on the Ni clusters is calculated as the difference between the total magnetization of the unit cell (a well-defined magnitude) and the contribution of each Ce3+ ion in the system. (See Table 1.) Once the number of unpaired electrons per Ni atom is known, its oxidation state is inferred, taking into account the fact that the electronic configurations of Ni0, Ni1+, and Ni2+ species are 3d84s2, 3d94s0, and 3d84s0, respectively. Notice that Ni1+ species have only one unpaired electron, whereas both Ni0 and Ni2+ have two unpaired electrons. Differentiating between Ni0 and Ni2+ species is straightforward because only Ni2+ ions can be linked to the presence of Ce3+ ions in the system, whose extra electrons are precisely transferred from Ni 4s states.

atoms in the three bottom layers were fixed at their optimized bulk-truncated (aCeO2 = 5.485 Å; aNi = 3.518 Å) positions during geometry optimization, whereas the rest of atoms were allowed to fully relax. All of the reaction energies (Ereac) were referenced to C, CO, or CO2 as a gas-phase species for which Γ-point calculations were performed in at least 12 × 12 × 12 Å3 boxes. A similar computational setup was used for the calculation of pure gas-phase Ni1, pyramidal Ni4, and rhombohedral Ni4 clusters. The adsorption of C and CO species was studied on the bare CeO2(111) and Ni(111) surfaces and on supported single Ni atoms and small Ni4 clusters on the CeO2(111) surface. Notice that in Figure 1 the strongest perturbations in the chemical properties of Ni are observed at very low coverages of the admetal, justifying our interest in examining the interaction of Ni atoms and small Ni clusters with CeO2(111). Experimentally, it has been found that at very small coverages of Ni on CeO2(111) the admetal is present on the oxide as Ni2+ species (i.e., Ni−O bonding), not forming Ni aggregates (no Ni−Ni bonding). For all of the systems investigated a range of different initial positions and orientations was considered, and the structures were relaxed using a conjugate-gradient algorithm with a residual force threshold of 0.05 eV/Å. Total energies were converged better than 10−4 eV per unit cell in each selfconsistent field (SCF) cycle. Upon the adsorption of Ni species, Ce3+ ions are generated in most cases. Many possible locations for the Ce3+ ions were considered. In addition, special attention was paid to the convergence to the 4f state of the lowest energy associated with each Ce3+ ion. In some occasions it was observed that broadly used SCF density mixers such as the Pulay method32 may end up in electronic local minima solutions up to 0.2 eV higher than the actual lowest-energy 4f state. In such cases, we found that simple straight linear mixing is less efficient than Pulay’s mixer but robust enough to solve the problem. Moreover, in many of the considered models, a large number of possible spin states exists; therefore, we restricted our discussion to high-spin (ferromagnetic-like) states for the sake of simplicity; indeed, we found that the energy difference between the high-spin state and any other spin state is typically 0.2 eV)50−52 and indeed we found that a CO molecule placed at the top of a surface oxygen atom is readily oxidized, leading to a CO2 molecule “floating” on the surface and the formation of an oxygen vacancy. This exothermic process releases −0.72 eV. A similar behavior is observed for the adsorption of a C atom, where the formation of a CO molecule and one oxygen vacancy is thermodynamically much more favorable (−5.95 eV) than the adsorption on top of a surface oxygen atom (−3.26 eV), cf. Table 2. B. Ni1 and Ni1/CeO2(111) Surface. We consider now the adsorption of CO and C species on Ni1 and on the Ni1/ CeO2(111) system investigated in Section 3.1.A. On a nonsupported free Ni atom the CO bonding energy is −2.86 eV. The low-coordination number of the isolated Ni atom makes it more reactive than the atoms in a Ni(111) surface, which have a CO adsorption energy of −1.90 eV. On Ni1/ CeO2(111), the CO molecule prefers to sit slightly tilted on top of the supported Ni atom (Figure 5a) with an adsorption energy of −1.39 eV (Table 2). The strong effects of the Ni ↔ ceria support interactions in the Ni1/CeO2 system reduce the CO binding from that on both the gas-phase Ni atom (−2.86 eV) and the Ni(111) surface (−1.90 eV). Interestingly, the supported Ni2+ ion becomes partially reduced upon CO adsorption to Ni1+. An oxidative adsorption reaction of the CO + Ni/CeO2(111) → CO2,ads + CeO2−x(111) type, resulting in the formation of an adsorbed CO2 and one oxygen vacancy (Figure 5b), is less favorable (ΔE = −1.00 eV). We note that NiO(100) surfaces adsorb CO in a similar tilted configuration than the Ni1/CeO2 system, but the adsorption energy determined experimentally (−0.3 eV)53 or theoretically (−0.8 eV)54 is ∼0.6 to 1.0 eV weaker. C atoms on Ni1/CeO2 are readily oxidized to CO upon adsorption (−7.70 eV), abducting an oxygen atom from the ceria surface and leaving one oxygen vacancy behind (Figure 5c). This oxidative adsorption is uniquely associated with the presence of the ceria support; that is, such a mechanism is not an alternative for extended Ni(111) surfaces on which C species strongly bind (−6.77 eV). In this process, the initially

fully oxidized Ni2+ ion is fully reduced to Ni0 while the atomic C becomes oxidized to C2+. Two Ce3+ ions are created to compensate for the oxygen vacancy formation. We notice that desorption of the so-formed CO molecules is endothermic by 2.37 eV; therefore, CO is expected to remain adsorbed on the surface. The nonoxidative adsorption of C, where Ni does not change its oxidation state (Ni2+), is much less favorable, −3.92 eV (Figure 5d). C. Ni4 and Ni4/CeO2(111) Surface. We consider now the adsorption of CO and C species on the pyr-Ni4 cluster supported on the CeO2(111) surface (Section 3.1 B). A CO molecule prefers to adsorb nonoxidatively on the pyr-Ni4 cluster by −1.90 eV (Table 2), sitting on a three-hollow-like site as it does on Ni(111) (Figure 5e). The Ni cluster is partially reduced upon CO adsorption ([Ni4]4+ → [Ni4]2+) with the formation of two Ce3+ ions (Table 1). Oxidative adsorption (Figure 5f) is much less favorable (−0.21 eV). The CO binding on gas-phase Ni4 clusters that result from the removal of the CeO2(111) support from the CO/Ni4/ CeO2(111) systems, with further geometry optimization, is −2.01 eV, similar to the adsorption on Ni(111) (−1.90 eV). We notice that considering the gas-phase pyr-Ni4 cluster frozen in the geometry it possesses in the supported structure, it has a fairly small effect on the CO adsorption energies (−2.16 eV), showing that a Ni4 cluster already has a chemical reactivity toward CO comparable to that of extended Ni(111) surfaces. The adsorption of a C atom on a relaxed gas-phase pyr-Ni4 cluster is −7.35 eV (−7.50 eV when all Ni atoms are fixed in the geometry they adopt in the adsorption structure). On the ceria-supported pyr-Ni4 cluster, a C atom is readily oxidized (Figure 5g) with an adsorption energy of −7.57 eV. Upon C adsorption, an O vacancy is created on the CeO2(111) surface, a total of four Ce3+ ions are formed, and two unpaired electrons remain within the pyr-Ni4 cluster (Table 1). Desorption of the so-formed CO molecule is endothermic by 1.62 eV. Nonoxidative adsorption (Figure 5h) is less favorable by 1.20 eV. D. Trends in Chemical Reactivity. Above, we have reported the adsorption of a C atom and a CO molecule on the clean Ni(111) and CeO2(111) surfaces as well as on supported Ni1 atoms and pyr-Ni4 clusters on CeO2(111). The main results, which are listed in Table 2 and schematically summarized in Figure 6, are now put in a broader perspective. Comparing the adsorption properties of supported Ni clusters on CeO2(111) with those of the extended Ni(111) surface, two effects have to be considered, namely, the nanostructuring of the metallic phase and the presence of the oxide support. We first consider the adsorption of C atoms. Inspection of Table 2 reveals that C species strongly adsorb on Ni(111) and that ceria in the supported Ni aggregates (for the most part) strongly favors the oxidation of C atoms, resulting in the formation of adsorbed CO species and the generation of an O vacancy on the ceria substrate. Therefore, highly dispersed ceria-supported Ni clusters could prevent coke formation, which is a well-known difficulty with practical use of Ni as a catalyst for methane reforming.15,17,18 We further note that cluster size affects the reactivity toward C adsorption. The preference for the oxidative adsorption of C atoms as compared with the nonoxidative adsorption is 3.78 eV in the case of supported Ni1 adatom (Table 2). When increasing the metal cluster size up to Ni4 (assuming pyr-Ni4 species), the preference for oxidative adsorption still prevails but it is substantially reduced to 1.20 eV (Table 2). In addition, it is important to notice that the clean CeO2(111) surface is 8246



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CO(g) → C(g) +

1 O2 (g) 2

Ereac = 8.48 eV

CO/Ni(111) → C/Ni(111) +

1 O2 (g) 2

(1)

Ereac = 3.61 eV (2)

The comparison of reactions 1 and 2 reflects a considerable weakening in the strength of the C−O bond upon adsorption. The C−O bond strength on the CO/Ni/CeO2(111) systems can be described by the following reactions: CO/Ni1/CeO2 (111) → C/Ni1/CeO2 (111) 1 + O2 (g) Ereac = 5.95 eV 2

(3)

CO/pyr‐Ni4 /CeO2 (111) → C/pyr‐Ni4 /CeO2 (111)

Figure 6. Schematic overview of C and CO adsorption on the clean Ni(111) (right), Ni4 atoms adsorbed on CeO2(111) (center), and a single Ni atom adsorbed on CeO2(111) (left). A large difference between the binding strengths of CO and C favors the methanation reaction. Species that remain adsorbed on the surface are indicated with a star. The generation of an oxygen vacancy upon adsorption is indicated by orange squares.

+

1 O2 (g) 2

Ereac = 4.01 eV

(4)

where C/Ni/CeO2(111) stands for the nonoxidative adsorption mode in which C species have a very similar spatial structure to that in the corresponding CO/Ni/CeO2(111) systems. For atomically dispersed Ni particles at very low coverage [Ni1/CeO2(111)], the C−O bond strength is considerably higher than that on the Ni (111) surface (cf. reactions 2 and 3). As Ni particles become slightly larger at still low coverage [Ni4/CeO2(111)], for the most stable pyr-Ni4 particles, the C−O bond strength weakens compared with the isolated atoms (cf. reactions 3 and 4) but remains by 0.4 eV larger than on the extended metal surface (cf. reactions 2 and 4). It also must be mentioned that the difference between the binding energies of C and CO is 2.53 eV on Ni1/CeO2(111) and 4.87 eV on Ni(111). The stronger C−O bond for CO species adsorbed on Ni1/CeO2(111) can be correlated with the dramatic difference in activity toward CO methanation observed as a function of Ni loading. Because the energy of CO adsorption does not vary strongly, the variations in the magnitude of the reaction energies describing the C−O bond strength (cf. reactions 1−4) are mostly due to variations in the C nonoxidative binding, which follows the Ni1/CeO2(111) < pyr-Ni4/CeO2(111) < Ni(111) (Table 2); that is, the weaker the C binding, the smaller the driving force for the dissociation of the C−O bond. Figure 3c shows that the top of the occupied Ni d-band is ∼0.7 eV below EF for the Ni1/CeO2(111) system, whereas for the bare Ni(111) surface (Figure 3a) the density of occupied d-states at EF is high. The pyr-Ni4/CeO2(111) system also shows occupied d-states up to EF, resembling already to some extent the scenario of the bare Ni(111) surface. In particular, the trend in reaction energy, Ni1/CeO2(111) < pyrNi4/CeO2(111) < Ni(111), seen for the nonoxidative adsorption of C or CO can be qualitatively understood using the d-band model, which describes the interaction between adsorbate valence states and the s and d states of a transitionmetal surface.56,57 The d-band center of Ni1 supported on CeO2(111) being further away from EF as compared with Ni(111), cf. Table 3, could explain the lower adsorption energies on the supported Ni atoms. In addition, the large Ni positive charge in Ni1/CeO2(111), +2 oxidation state, also hinders the transfer of electrons from the admetal to the 2π* orbitals of CO (the so-called π-backdonation), weakening the strength of the Ni-CO bond.

also able to oxidize directly adsorbed C atoms, but the resulting CO molecule detaches easily from the surface, leaving to the gas phase (Section 3.2A). However, the C oxidation on supported Ni1 and Ni4 species ensures that the resulting CO molecule remains on the surface (Sections 3.2B and 3.2C), and thus it is ready to participate in subsequent reaction steps within, for example, the WGS or ethanol reforming catalytic cycles. It is important to stress that the chemical reactivity toward C of the pyr-Ni4 supported cluster is much closer to that of an extended Ni (111) than to the behavior of atomically dispersed Ni1/CeO2(111) entities. We now consider the adsorption of CO species. As already mentioned in the Introduction, the competition between molecular adsorption of CO and its dissociation to C is a critical step affecting the selectivity for the WGS and methanation reactions. A large difference between the binding strengths of CO and C favors the methanation reaction, whereas a small difference (i.e., no driving force for CO dissociation) improves the selectivity toward the WGS reaction. The activity of the Ni/CeO2(111) systems toward CO methanation depends strongly on the Ni coverage; small Ni coverages result in a drastic drop of the activity toward CH4 formation with respect to that observed for larger coverages (see Figure 1)20 as well as for extended Ni(111) surfaces;17 the WGS reaction benefits instead from the low Ni coverages.12,20 As for the CO adsorption, the investigated Ni/CeO2(111) systems do not favor the oxidation of CO to CO2 as the bare CeO2(111) surface does (Table 2). The CO adsorption energy for the extended Ni(111) surface (−1.90 eV) lies within the energy range from −1.4 to −1.9 eV for the low coverage Ni/ CeO2(111) systems. Hence, the strength of the CO adsorption on its own does not help us to explain the observed reactivity dependence on Ni coverage. We first address the effect on the C−O bond strength upon CO adsorption on the extended Ni(111) surface by considering the following reactions (Ereac experimental value in eq 1 is 8.56 eV55): 8247



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Table 3. Comparison of the C and CO Adsorption Energies (in electronvolts) with the Position of the Ni d-Band Center of Ni1/CeO2(111), pyr-Ni4/CeO2(111), and Ni(111) Systems system

Ni d-band center

C adsorption energy

CO adsorption energy

Ni1/CeO2(111) pyr-Ni4/CeO2(111) Ni(111)

−2.53 −1.53 −1.70

−3.92 −6.37 −6.77

−1.39 −1.90 −1.90

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 915854631. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EULANEST (MINECO-PIM2010EEUU-00138) for financial support. Computer time provided by the SGAI-CSIC and the RES at CESGA and BSC is acknowledged. This work was granted access to the HPC resources of the RZG made available within the Distributed European Computing Initiative by the PRACE-2IP, receiving funding from the EU’s FP7 Programme under grant agreement no. RI-283493. J.C. is supported by the MINECO through a Ramón y Cajal Fellowship and acknowledges support by the Marie Curie Career Integration Grant FP7-PEOPLE-2011-CIG: Project NanoWGS. L. B. acknowledges support by the JAE-DOC2010 program. P.L. and J.A.R thank the U.S. Department of Energy, Division of Chemical Sciences for support under contract DE-AC02-98CH10886. The COST action CM1104 is gratefully acknowledged.

4. SUMMARY AND CONCLUSIONS Electronic structure, geometries, and the adsorption of C and CO on small Nin (n = 1 and 4) particles deposited on CeO2(111) have been investigated using the DFT+U approach and compared with Ni(111) and CeO2(111). Computational models have been established, the properties of which are consistent with experimental knowledge for model Ni/ceria catalysts.20 Specifically, deposition of small coverages of metallic Ni results in the formation of Ni2+/Ni1+ and Ce3+ species. That is, Ni is electronically strongly perturbed by the presence of the ceria support, which in turn is partially reduced. The most stable Ni4 cluster has a pyramidal structure (pyr-Ni4), and a planar rhombohedral structure (r-Ni4) is ∼0.2 eV less stable. The chemisorption properties of the supported Ni particles are also affected by the ceria support. C species are strongly bound on Ni(111). A major challenge with Ni-based catalysts for reactions involving C species is their tendency to form coke. On CeO2(111), the C adsorption is oxidative with the soformed CO species leaving the surface into the gas phase. The combined effect of nanostructuring the metallic phase and the support actuates the formation of chemisorbed CO species upon the thermodynamically favored oxidative C adsorption, rendering the Ni/CeO2(111) systems less prone to coke formation. The exothermicity of nonoxidative adsorption of C follows Ni1/CeO2(111) < pyr-Ni4/CeO2(111) < Ni(111) in an increasing sequence. In contrast with C species, the oxidative CO adsorption, which produces weakly bound CO2 species on the bare CeO2(111) support and stronger bound species on the Ni/ CeO2(111) systems, is energetically less favored than the nonoxidative adsorption. Ni/CeO2(111) surfaces exhibit a very low activity for CO methanation at small Ni coverages, which increases with increasing Ni loading. This reflects that both the Ni nanostructuring and the support affect the strength of the C−O bond. The CO binding energy remains moderately affected; it is actually the same for the pyr-Ni4/CeO2(111) and Ni(111) systems. The C−O bond strength follows the inverse trend of the nonoxidative adsorption of C; that is, it is the largest for the isolated Ni atoms and the smallest for the extended Ni(111) surface. The Ni−ceria interaction at low Ni concentration significantly increases the C−O bond strength, which is consistent with low activity for CO methanation. The competition between molecular adsorption of CO and its dissociation to C is a critical step affecting the selectivity for the WGS and methanation reactions. With a large difference between the binding strengths of CO and C, Ni4/CeO2(111) and Ni(111) should favor the methanation reaction, whereas corresponding to a small difference, Ni1/CeO2(111) may improve the selectivity toward the WGS reaction.

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Theoretical Studies of the Adsorption of CO and C on Ni(111) and Ni

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