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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Probing the Reactivity of Pt/Ceria Nanocatalysts Towards Methanol Oxidation: From Ionic Single-Atom Sites to Metallic Nanoparticles Nguyen-Dung Tran, Matteo Farnesi Camellone, and Stefano Fabris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05735 • Publication Date (Web): 08 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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The Journal of Physical Chemistry

Probing the Reactivity of Pt/Ceria Nanocatalysts towards Methanol Oxidation: From Ionic Single-Atom Sites to Metallic Nanoparticles Nguyen-Dung Tran1,a , Matteo Farnesi Camellone1 , and Stefano Fabris1∗ 1

CNR-IOM DEMOCRITOS, Istituto Officina dei Materiali, Consiglio Nazionale delle

Ricerche and SISSA Scuola Internazionale di Studi Superiori Avanzati, Via Bonomea 265, I-34136, Trieste, Italy. E-mail: [email protected]

Abstract Single atom catalysts represent the ultimate extreme in heterogeneous catalysis for maximum dispersion of mononuclear catalytic metal particles on supporting surfaces. Ultra-low Pt loading has been achieved on nanostructured ceria surfaces that allow for stabilizing metallic and ionic Pt sites that are anchored at surface defects. Here we assess the chemical reactivity of these different Pt species, that are experimentally known to co-exist on Pt-ceria nanocatalysts, by taking methanol oxidation as a chemical probe. Our density functional theory calculations demonstrate that Pt2+ and Pt4+ single-ion species do not promote methanol oxidation by themselves. Instead, metallic sites of supported sub-nm Pt particles are always required to promote the oxidation reaction. Our finding generalizes the conclusions of recent photoemission experiments ∗

To whom correspondence should be addressed a Present address: Fracture and Reliability Research Institute, School of Engineering, Tohoku University, Japan.

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ACS Paragon Plus Environment

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in the context of H2 oxidation by ceria/Pt nanocatalysts. Moreover, the simulations predict that surface hydroxide groups may act as co-catalyst for the direct methanol oxidation to formaldehyde, thus proposing a viable strategy for catalyst design.

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1

Introduction

Methanol oxidation is a central chemical reaction in energy-related technologies, as well as in the synthesis of chemical and pharmaceutical products. 1,2 As an example, the oxidative dehydrogenation of methanol leads to formaldehyde, which is the precursor for the synthesis of several materials and chemical compounds, and which has a global production of various million tons per year. Industrial and technological applications involving methanol oxidation require efficient, stable, and selective heterogeneous catalysts, with particular reference to oxide-supported systems. 3 Catalytic oxide supports based on cerium oxide have been shown to lead to highly-active catalysts for methanol oxidation and, more generally, for C-H activation in organic chemistry. 3–9 More recently, ceria-supported Pt nanoparticles were demonstrated to perform as efficient single-atom catalysts for direct methanol fuel cells. 10 These new Pt/ceria nanomaterials are interesting because they allow to greatly reduce the Pt load, which therefore impacts on the cost of the fuel cell device. Experimental analysis and theoretical simulations identified the presence of specific Pt sites on the ceria support, having both metallic and ionic character (Pt4+ , Pt2+ , Pt0 ). 11–16 Here we aim at determining the chemical reactivity of these ceriasupported Pt sites by means of density functional theory calculations (DFT). The adsorption and oxidation of methanol, which is taken as a catalytic surface probe, 17,18 is studied on selected Pt/CeO2 model surfaces that reproduces the electronic and structural properties of the experimentally observed Pt sites. Several theoretical studies adressed the adsorption of methanol with CeO2 surfaces. 19–23 These showed that CH3 OH weakly interacts with the pristine CeO2 (111) surface and that the activation energy for methanol oxidation to formaldehyde is larger than 1 eV on both pristine and defective (111) ceria surfaces. Similar activation energies were predicted for the ceria (110) surface 21 as well as for CeO2 surfaces doped with Zr, Hf and Th. 24 Different experimental studies investigated the interaction between methanol and pure ceria surfaces. Temperature programmed desorption (TPD) experiments 25 showed that, 3



at T≈300K, methanol dissociatively adsorbs on a CeO2 (111)-like thin film supported on Cu(111). The main products of the dissociation detected in these TPD experiments were CO and H2 . Methoxy intermediates of the oxidation reaction, bound on top, bridge and triple surface sites, were identified by infrared and Raman spectroscopies performed on ceria nanoshapes such as rods, cubes and octahedra. 27 Other works 28,29 reported molecular methanol adsorption on thick ceria films for temperatures below 140 K. Photoelectron spectra measured during annealing identify methoxy, CHOO and CO intermediates. 29 Although the mechanisms for methanol oxidation by ceria surfaces are still under debate, it is likely that there exist at least two different channels for methoxy decomposition. Recent studies focused on the adsorption of methanol on Pt nanoparticles supported by a CeO2 (111)/Cu(111) thin film 34 as well as on Pt(111) surfaces. 35 In Ref., 34 Matolin et al. suggest that both systems have similar reactivity toward CH3 OH oxidation. Quite interestingly, it is shown that the rough Pt(111) surfaces can be used as a model catalyst that mimic the properties of small supported Pt particles. It is found that methoxy species are strongly bound on Pt-CeO2 (111) facilitating the subsequent dehydrogenation of methoxy. Thus, the high activity of Pt/CeO2 catalysts in direct fuel cells can be traced back to the efficient decomposition of methanol on this system. Pt-CeO2 catalysts with ultra-low Pt loadings were prepared with radio frequency sputtering and characterised in conditions relevant for fuel cell applications. 10,31 The composition of these novel catalysts was studied with photoelectron spectroscopy measurements. 10,12,13,32 These measurements demonstrated the presence of Pt4+ , Pt2+ , Pt0 species depending on the synthesis conditions, reaction environment, Pt loading and depth from the surface. In this work, we present a detailed and systematic study of methanol adsorption and oxidation on ionic and metallic Pt species dispersed/supported on ceria surfaces. We demonstrate that the ionic species are not catalytically relevant for methanol oxidation and assess the role of surface steps. Moreover we study the reaction mechanism and energetics of methanol oxidation on ceria-supported Pt nanoparticles, identifying strategies to reduce the

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activation energies.

2

Computational Details

2.1

Electronic-structure calculations

The spin polarized Kohn-Sham equations were solved in the plane-wave and pseudo potential framework of density functional theory with the generalized gradient-corrected approximation (GGA) for the exchange and correlation functional proposed by Perdew, Burke and Ernzerhof (PBE). 42 The calculations employed Vanderbilt ultrasoft pseudo potentials 43 and represented the wave-function and augmentation charge density with a basis set limited by energy cut-off of 40 Ry and 320 Ry, respectively. Pure and Pt-ceria surfaces were simulated with the PBE+U approach, which includes an additional Hubbard-U term to the Kohn-Sham functional acting on the occupancies of the Ce 4f states. 44 It is well established that this approach allows for an accurate description of the electronic structure of both reduced and oxidized ceria systems. 45–48 In line with the recent literature 22,49 and with our previous works 45,46,50,51,53 we have set the value of the parameter U to 4.5 eV. All the calculations were carried out with the Quantum Espresso (QE) package. 38 Effects of long-range dispersion-type interactions on the methanol adsorption and dissociation were assessed by adding to the PBE+U functional the semiempirical C6 /R6 term proposed by Grimme 52 (PBE+U+D2). In these calculations we employed the dispersion parameters for the Ce and O atoms reported in Ref.

22

Flat and stepped stoichiometric Pt/CeO2 systems were modeled with supercells in periodic boundary conditions. The flat (111) surface was described by three-CeO2 -layer supercell slab, with more than 11 ˚ A of vacuum separating the periodic images along the direction perpendicular to the slab. The thickness of the slab and of the vacuum region were tested in a previous work. 46 For the model of surface step, it was selected as the lowest-energy one 5



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labeled as step Type I in Ref. 39 This step separates two (111) terraces, and its step edge is oriented along the [110] direction. The step is modeled as vicinal surface described with ˚2 along the [112] and [110] monoclinic supercells having lateral extension of 17.97×11.67 A directions. The supercell slab comprised nine atomic layers and its surfaces were separated by more than 10 ˚ A of vacuum in the direction perpendicular to the (111) terrace. All the atomistic structures were relaxed according to the calculated Hellman-Feynman forces until the maximum force was less than 0.02 eV/˚ A. During the structural relaxation, the lowest three atomic layers in the slab models for flat surfaces were constrained to their bulk equilibrium coordinates, as well as the Ce atoms in the central O-Ce-O trilayers far from the step edge. Integrations over the Brillouin zone were performed on a (2×2×1) Monkhorst-Pack grid for the (2×2) supercells, whereas the Γ-point sampling was used for all other cases. The results obtained with these settings were converged to 0.01 eV with respect to using denser grids to sample the Brillouin zone (further details in SI). A molecule of methanol was adsorbed over the possible symmetry-independent surface sites and with different rotation and tilt angles around the molecular longitudinal and lateral axis. In this study, coverage effects of adsorbates were taken into account in two ways: either using smaller supercell, or adding more adsorbates in a supercell. For the first way, denser K-point grids was used accordingly for BZ sampling to ensure the convergence of the results. Details of these settings will be given in the main text when necessary. Coverage effects on the computed quantities are presented in the SI. The adsorption energies of molecular and dissociated adsorbates were computed using the formula gas + Esurf ) , Ead = Eads/surf − (Eads

(1)

gas where Eads , Esurf , and Eads/surf are the total energies of the adsorbate in the gas phase, of

the clean surface, and of the combined system, respectively. According to this definition, negative values of Ead indicate preferential binding of adsorbate to the surface. 6


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Activation energies for methanol oxidation were calculated with the climbing-image nudged elastic band (CI-NEB) 40,41 method that allows to identify minimum energy paths on the potential energy surface (PES). These calculations have been performed in the supercells described above including up to 15 replica images.

2.2

Model Pt sites in Pt/CeO2 systems

In this work we probe the reactivity of several Pt sites exposed by ceria surfaces, differing in the charge state and local atomistic structure. To this end, a set of representative stable structures of Pt/CeO2 systems exposing ionic (Pt4+ and Pt2+ ) and metallic (Pt0 ) platinum species have been selected. The model systems are displayed in Figure 1. We take the pristine CeO2 (111) surface as the reference system (Fig. 1a). Stable Pt4+ sites were simulated with a Pt/ceria solid solution in which Pt substitutes for Ce at the CeO2 (111) surface (Fig. 1b). It turns out that this surface does not allow to stabilize Pt2+ species, which can instead be stabilized as Pt adatoms forming PtO4 units at the (110) facets of the stoichiometric step Type I-S (Fig. 1c,d “S” stands for stoichiometric), similarly to what reported for the Pd case. 26 The same Pt2+ species were shown to be even more stable at oxygen-rich step edges (step Type I-O, where “O” stands for oxygen rich), as displayed in Fig. S7 (e, g). 15 The electronic structure and charge analysis that allow to associate the Pt sites in the Pt/CeO2 systems to Pt4+ and Pt2+ ions are reported in the SI (see, Figs. S1), and previously in Ref., 15 respectively. The metallic species are simulated by means of a Pt6 cluster supported on the pristine (111) ceria surface (Fig. 1e). The cluster morphology was fully optimised while its structural, electronic and mobility properties were previously characterized. 53 The Pt6 cluster is strongly bound to the support with adsorption energy of 6.7 eV (per Pt6 ) and the charge transferred from the cluster to the support leads to a reduced ceria substrate with 3 Ce3+ ions.

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3

Results and discussion

3.1

Methanol at pure ceria surface

We start by considering the methanol adsorption and dissociation on the stoichiometric pure CeO2 (111) surface. The interaction between methanol and ceria surfaces has been already extensively characterized with electronic-structure simulations. 19–23,36,37 Here we reconsider these well-studied adsorption configurations so as to validate our computational set up and to provide a valid reference for assessing the reactivity of different Pt species. The results reported here are for coverage θ = 0.25 ML in which we use a (2x2) supercell and a (2x2x1) Monkhorst-Pack grid for the integration over the BZ. Coverage effects are reported (Table S2) and discussed in SI. In the molecular adsorption (MA) configuration (Fig. 2a), methanol binds above a Ce site, with the OH group pointing towards a neighboring surface O atom. The computed PBE+U adsorption energy is found to be - 0.53 eV, in very good agreement with the literature value of - 0.48 eV. 19 In the dissociative adsorption (DA) case (Fig. 2b), the H atom of the methanol hydroxyl group breaks the intramolecular bond with the methanol Om atom and binds to a surface O site forming a surface hydroxyl. The computed adsorption energy, - 0.62 eV, is in good agreement with the literature values - 0.55 eV 19 and - 0.69 eV, 22 and predicts a weak energetic preference for the dissociative with respect to the molecular adsorption of methanol. We remark however that the energy difference between the MA and DA configurations is comparable with the error bar of the present calculations. Another stable MA2 configuration differing by molecular rotation around the main axis is higher in energy by 0.28 eV (see Fig. S4). In this configuration, methanol binds above a O site, with the Om H group pointing right down to the O surface atom. The adsorption energy in this case is Ead = - 0.25 eV. Effects of long-range dispersion-type interactions on the methanol adsorption and dis-

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sociation were assessed by adding to the PBE+U functional the semiempirical C6 /R6 term proposed by Grimme 52 (PBE+U+D2). In these calculations we employed the dispersion parameters for the Ce and O atoms reported in Ref. 22 The inclusion of long-range dispersiontype interactions lowers the adsorption energies of both MA and DA configurations by ≈ 0.2-0.3 eV (Table 1). The computed PBE+U+D adsorption energies are in very good agreement with the previous works, differing by less than 0.1 eV, and predict a slightly increased energetic preference for DA. The analysis of the electronic structure (Bader charge, DOS, and PDOS) of the molecular and dissociative adsorptions are reported in Table 1 and Fig. S2. Particularly, the PDOS of DA in Fig. S2 shows that there is no appearance of new Ce(f) gap state within the band gap. Therefore, methanol interaction does not reduce the stoichiometric CeO2 (111) surface: All Ce ions preserve their formal 4+ oxidation state. Table 1: Adsorption energies (eV) and relevant bond lengths (˚ A) for structures shown in Figs.2, 3, S5. CeO2 (111) Pt4+ @CeO2 (111) Refa Refb MA DA DA MA DA MA DA DA1 Ead - 0.48 - 0.55 - 0.69 - 0.53 - 0.62 - 0.44 - 0.98 - 0.44 ∗ Ead – – - 0.91 - 0.71 - 0.93 - 0.74 - 1.28 - 0.73 d(Ce-Om ) 2.62 2.22 2.21 2.60 2.22 2.62 2.20 2.21 d(O-H) 1.72 1.01 – 1.63 – 1.84 – – d(Om -H) – 1.62 1.65 – 1.68 – 1.74 1.68 MA, DA: molecular, and dissociative adsorptions, respectively. *: PBE+U+D calculation a Ref.: 19 θ = 0.25 ML, b Ref.: 22 θ = 1/16 ML.

3.2

Methanol at Pt4+ sites

Surface Pt4+ species (Fig. 1 b) are formed in solid solutions when Pt atoms substitute for Ce ions at the CeO2 (111) surface. These ionic species are experimentally observed in Pt/CeO2 9



thin films prepared with magnetron sputtering deposition. 16,33 Our model of Pt-CeO2 (111) solid solution was a (3x3) supercell in which one surface Ce ion is substituted by a Pt atom (Fig.1 b). The analysis of the DOS and PDOS of the Pt-CeO2 (111) solid solution (Fig. S1) shows that the Ce(f) states are above the Fermi level, confirming that there is no surface reduction induced by Pt doping and that the Pt species are in 4+ oxidation state. In this model, the distance between stable Pt4+ species and the nearest-neighboring surface O atoms (denoted as OP t ) is 2.19 ˚ A, thus shorter than Ce-O bond which is 2.38 ˚ A. The methanol adsorption and dissociation on the Pt-CeO2 (111) solid solution surface were considered at coverage θ = 1/9 ML. Denser coverage effect is reported (see, Table S2) and discussed in SI. We find that methanol molecules do not bind directly to these Pt4+ sites regardless of the molecular orientation. A spontaneous formation of DA is instead predicted when a methanol molecule is placed above Ce sites next nearest-neighbouring (NNN) to the Pt4+ species, denoted as CePt . The final DA configuration (Fig. 3b) involves the formation of a surface OPt -H group and of a methoxyl group bound through its Om atom to the surface CePt ion. The calculated adsorption energy of this DA configuration, - 0.98 eV (see Table 1), is considerably lower than the corresponding value for DA on the pristine surface, - 0.62 eV. In the DA configuration, the Om -H and Ce-Om distances are 1.74 ˚ A and 2.20 ˚ A (see Table 1), respectively. This preferential DA on the CeP t ions NNN to the Pt4+ sites originates from a modification of the local electronic structure induced by Pt doping. A substitutional Pt4+ ion yields a charge depletion on the OPt atoms: The Bader charge of the OPt atoms (-1.03 |e|) is substantially smaller than that one of the unperturbed O surface atoms (-1.21 |e|) (full Bader charge analysis in Table S1). The Pt4+ defect induces specific electronic states right below the Fermi level, arising from Pt-d and OPt -p states (see DOS and PDOS in Fig. S1, left panel). Due to this decreased charge, the OPt sites act as stronger Brönsted bases than the O sites of the pristine surface, hence they promote the methanol dehydrogenation. This interpretation can be proved by changing the initial orientation of the methanol

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molecule above the CePt site: when its H atom points towards OPt the dehydrogenation is spontaneous, while two higher-energy MA (Fig. 3a) and DA1 (Fig. S5) metastable configurations could be stabilized by rotating the methanol OH group towards a surface O atom that is not bound to the Pt4+ ion. This is a further proof that the dissociation is controlled by the local electronic properties of the OPt site. In the MA configuration, the Om -H and Ce-Om distances are 1.84 ˚ A and 2.62 ˚ A while in the DA1 configuration, they are 1.68 ˚ A and 2.21 ˚ A, respectively. The adsorption energies Ead of the MA and DA1 are equal to - 0.44 eV, which are comparable to the corresponding ones (MA and DA) on the pristine ceria surface 19 (Table 1), thus showing that the effect of Pt4+ species on methanol dissociation is limited to the first neighboring OPt shell. Similar to the pristine CeO2 (111) case above, the methanol dissociations (DA and DA1) do not induce surface reduction: all Ce and Pt ions remain in 4+ oxidation state, as demonstrated by the DOS and PDOS in Fig. S3. Finally, we conclude that Pt4+ surface sites on ceria surfaces promote the heterolytic CH3 OH dissociation yielding stable methoxyl and hydroxyl surface species.

3.3

Methanol at ceria step edges and Pt2+ sites

Pt-ceria catalysts with ultra-low Pt content has been related to the presence of Pt2+ surface species, whose stability was shown to require nanostructured ceria surfaces, i.e. exposing steps and corner sites. 15,31 The same Pt2+ species were shown to segregate at the I-S and I-O steps of ceria surfaces. 15 Here, we take these Pt2+ species as model systems to probe the reactivity of Pt2+ sites towards methanol oxidation. In these models, Pt atoms were incorporated at an interstitial surface site of the (110) step edge (Figs. 1 d, S7 e). In order to disentangle the effects of the Pt2+ ions from those of the step defect we analyse in this Section methanol adsorption and dissociation on both Pt2+ -free and Pt2+ -rich CeO2 steps. We remark that pure CeO2 steps have been already characterized by theoretical and experimental studies, 36,56,59–61 but their chemical reactivity has not yet been examined in 11



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depth. Compared to the stoichiometric flat ceria surfaces, where all surface Ce and O sites are symmetrically equivalent, stepped surfaces expose a richer variety of symmetry-inequivalent sites available for molecular adsorption. Also in this case we have performed an initial screening of several binding sites and geometries available in the supercell. We first focus on the stoichiometric step I-S and the Pt2+ species at step I-S. The effects of the step and of the ionic Pt2+ sites are local and limited to a distance of 3 ˚ A around the step edge. We plot in Fig. 4(a) the adsorption energies of the most relevant DA (squares) and MA (circles) configurations as a function of the distance between the adsorption site and the step edge. The indexes in Fig. 4 refer to three adsorption sites, at the step edge (Ces ), at the terrace below and next to the step edge (Cetb ), and at the terrace (Cet ) far away from the step edge, as well as at the Pt2+ site. The MA and DA energies at the step edges are also included in Table 2. Table 2: Methanol adsorption energies (eV) and relevant bond lengths (˚ A) for structures shown in Figs. S6, S7.

Step I-S

Pt2+ @Step I-S

Step I-O

Refa DA MA MA MA DA DA DA Ead - 1.10 - 0.78 - 1.18 - 0.71 - 0.81 - 0.67 - 0.43 ∗ - 1.24 - 1.02 - 1.36 – – Ead – – 2.14 2.23 2.11 d(Ce-Om ) – 2.54 2.67 2.55 – 1.49 1.64 1.62 d(O-H) 0.98 0.99 1.02 d(Om -H) 3.34 1.85 4.39 – – – – MA, DA: molecular, and dissociative adsorptions, respectively. *: PBE+U+D calculation a Ref. 37

Pt2+ @Step-O MA DA - 0.53 - 0.27 – – 2.49 2.87 1.88 1.00 1.76 –

On the pure stepped surface I-S, the calculated MA and DA energies on the Cet and Cetb terrace sites are very similar to the corresponding adsorption energies calculated on a flat 12


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terrace (marked by the dashed horizontal lines): Differences being less than 0.1 eV. Methanol interaction with the step-edge Ces site is instead strongly enhanced, particularly in the DA configuration. There is a clear preference for the methanol DA at the step edge, where the calculated adsorption energy, - 1.18 eV (- 1.36 eV PBE+U+D), is almost twice larger than on the flat ceria (111) terrace (red dashed line). This value is in very good agreement with the recent work by Yang and coworkers, 37 reporting - 1.10 eV (- 1.24 eV PBE+U+D). The equilibrium adsorption geometries at this edge site are displayed in Fig. S6 (a-d) and the corresponding bond lengths could be found in Table 2. The same analysis performed on the stepped surface I-S containing the Pt2+ sites demonstrates the inactivity of these ionic species towards methanol oxidation (Fig. 4b). Methanol molecule does not bind to the Pt2+ step-sites. The strongest adsorption energies are found at the Ces sites neighboring to the Pt2+ ion, Ead = - 0.71 eV for MA and Ead = - 0.81 eV for DA, thus higher than the Pt2+ -free step I-S case by 0.07 eV and 0.37 eV, respectively. The optimized geometries of the MA and DA are shown in Fig. S6 (e-h). It is therefore evident that the ionic Pt2+ sites hinder methanol adsorption and dissociation, and suppress the selective methanol binding at ceria step edges. Methanol can bind to the Cet and Cetb sites neighboring to the Pt2+ ion, but the corresponding adsorption energies are not different than those on the pure stepped I-S surface so much. Next, we focus on methanol interaction at the overstoichiometric step I-O and at the Pt2+ species at step I-O. We do not redo here the screening of adsorption energy as a function of the distance from adsorption site to step edge like before, but consider only the interaction at step edge sites (Pt2+ and Ces sites). Similar to the Pt2+ at step I-S case, methanol does not bind to the Pt2+ ions incorporated in PtO4 moieties at the step I-O. The molecular adsorptions at Ces sites of both the steps are found to be less bound than those at the two previous stoichiometric step edges, but comparable to those at the (111) terrace (- 0.67 eV for step I-O and - 0.53 eV for Pt2+ @ step I-O, see Table 2). We found that both the overstoichiometric step edges suppress the binding preference for the methanol DA

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dramatically: Adsorption energies being only - 0.43 eV (step I-O) and - 0.27 eV (Pt2+ @ step I-O), even weaker than the binding of DA on pure CeO2 (111) surface. The fully optimized geometries and relevant bond lengths of the MA and DA configurations are shown in Fig. S7, and Table 2. Overall, the above results show that the Pt2+ ions incorporated at PtO4 moieties in ceria step edges are inactive towards methanol oxidation. The inactivity of the atomically dispersed Pt2+ ions was observed also for molecular hydrogen dissociation in a recent photoemission measurement. 30 In this work, a thin film Pt-CeO2 (111) containing a very small concentration of Pt was prepared, where all Pt deposit is oxidized to Pt2+ . The sample was then exposed to gas-phase hydrogen molecule at different temperatures. The dissociation of molecular hydrogen on the sample would lead to the reduction of Ce4+ or Pt2+ , however, no such phenomenon was observed. The finding implies that Pt2+ ions do not promote the dissociation of molecular hydrogen. The authors also have demonstrated that the presence of a relative small amount of metallic Pt0 species in addition to Pt2+ in Pt-CeO2 (111) films, on the other hand, readily activates hydrogen dissociation. 30 The remarkable activity of these ceria-based anodes with ultra-low Pt loading is related to the interplay between two states of the catalysts: atomically dispersed Pt2+ ion and sub-nm Pt NPs. It is suggested that the catalyst can dynamically change between these two states during reaction, through redox processes involving Ce3+ ions. It is however evident experimentally that Pt2+ ions alone do not promote H2 dissociation, but always require the presence of metallic species in small amounts. In supporting the experiment observation, the DFT+U calculation of the activation energy of molecular hydrogen dissociation on small supported metallic cluster Pt6 /CeO2 (111) shows that the metallic cluster lowers the energy barrier substantially, thus facilitating the dissociation, while at the Pt2+ site at the step I-S, the required activation energy is much higher. 30 The above experimental and theoretical investigation suggests that methanol oxidation on Pt/CeO2 (111) surface may also require metallic Pt species.

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3.4

Methanol oxidation by ionic Pt sites

The results presented above identify that the Pt4+ in solid solutions might be the more relevant ionic catalytic sites for the oxidation of methanol. In this case, the DA state is energetically preferred by more than 0.5 eV, which even in the absence of an activation energy barrier represents a substantial energy difference preventing recombination. These sites therefore drive the first dehydrogenation of methanol to methoxy. In the context of direct methanol fuel cells and in other applications, the relevant dehydrogenation is to formaldehyde, CH2 O. Methanol oxidation to formaldehyde at ceria surfaces has been theoretically studied by several works. 21,22,57,58 Recently, the conversion of methanol to formaldehyde and carbon monoxide at doped CeO2 (111) surface with other metals (Zn, Hf, Th), is also carried out. 24 We redo here the CI-NEB calculation for methoxy to formaldehyde reaction at pure ceria and at Pt4+ sites in solid solution surfaces. It turns out that Pt4+ ions do not assist further methanol dehydrogenation to formaldehyde as discussed as the following. All relevant structures are depicted in Fig. 5. On the clean, stoichiometric CeO2(111) surface, Cortada and coauthors 57 have shown that the methoxy to formaldehyde dehydrogenation is energetically favorable, it involves a reaction energy of ∆E = −1.23 eV, but incurs in a high activation energy of E ‡ = 1.03 eV. Other DFT+U+D calculations by Kropp and Paiers, 22 even reported a higher activation energy, e.g E ‡ = 1.25 eV. Our CI-NEB calculation shows that the activation and reaction energies are E ‡ = 0.89 eV and ∆E = −1.31eV (see Fig. 5, red line), respectively. These are in good agreement with Cortada’s observations. 57 On the Pt-CeO2 (111) solid solution surface, the lowest-energy DA structure (see Fig.3 b) is chosen as a reasonable initial state. The final state, where two hydroxyl groups formed on the surface and formaldehyde (FA) bound to a Ce surface atom, is strongly stabilized by ∆E = −1.55 eV with respect to the initial one. Hence the first methanol dehydrogenation on OPt has already taken place in the initial state, the only possibility for the second dehy15



drogenation is on the O surface atom NN to the methoxy. Along this reaction path (see Fig. 5, black line), the activation energy is E ‡ = 0.80 eV, lower than that on pristine surface by only ≈ 0.1 eV. This again reflects the local effect of Pt4+ to the nearest OPt atoms. Similar results are found at the doped Zn-CeO2 and Hf-CeO2 surfaces. 24 During the CH3 OH dehydrogenation reactions, the degree of reduction of both pristine and Pt4+ defective surfaces are the same: from no reduced Ce3+ ions in the initial states to one reduced Ce3+ ion in the transition states, and eventually ending up with two reduced Ce3+ ions in the final states.

3.5

Methanol oxidation by Pt0 sites: clean ceria surface

In this Section we analyze the reactivity of a metallic Pt6 cluster supported on CeO2 (111) surface towards methanol oxidation. The Pt6 @CeO2 (111) model is shown in Fig. 1 (e). The lowest-energy adsorption site for methanol molecule is the apical site of the supported cluster. Our study shows that MA on this site displays rotational selectivity. The strongest MA energy, Ead = - 0.80 eV, is found only when the OH group of CH3 OH points towards the lowest-coordinated Pt atom (see, Fig. S8, left). Other MA configurations on the apical Pt site, which differ in OH rotation, are less bound, with adsorption energies varying from -0.46 eV to -0.58 eV (see, Fig. S8 (middle, right)), thus comparable to MA on the pristine CeO2 (111) surface. In all cases, the bond lengths between the methanol Om atom and the cluster apical Pt atom are around 2.31 ˚ A. Methanol adsorption at the periphery of the metal-oxide interface (Fig. 6a) is particularly relevant for catalytic oxidation. Here the MA energy is - 0.44 eV. A negligible activation energy (E ‡ 6), however, its apical site could be far away from the oxide surface, and facets could be formed. 54,55 Hence, the methanol oxidation on the apical site and at other cluster sites not at interface might be similar to that occurred on Pt surfaces. We therefore will restrict our attention to the methanol dehydrogenation at the metal-oxide interface. There are many reaction channels for methanol dehydrogenation to formaldehyde at metal-oxide interface. Fig. 8 shows the two most interesting ones which are described in 18


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detail as the following. A methanol molecule can bind and directly dissociate to methoxy without an energy barrier at the Pt6 cluster supported by a pristine CeO2 (111) surface. The adsorption and dehydrogenation lower the energy by ∆E = -1.26 eV (see Fig. 8, black line). Further oxidation to FA requires an activation energy E ‡ = 0.58 eV, which is 0.3 eV (33.70%) lower than on a pristine CeO2 (111) surface (Fig.8, red line). The resulting FA and H products bind at different Pt atoms of the cluster periphery (Fig. S9 a). Then there is a strong driving force, ∆E = -1.36 eV, for the spill-over of that H atom to the ceria surface, to form an additional hydroxyl group (FA1, Fig. S9 b). Their detailed geometries are listed in Table 3. Note that the last process is accompanied by the further reduction of the surface, with the appearance of another Ce3+ ion. Note however that the activation energy for the H spillover is likely high because it requires breaking the Pt-H bond, for example, Clelia et. al. illustrated that the barrier is 1.02 eV for the spillover of a H atom from the supported Pt13 cluster to the reducible TiO2 (101) surface. 67 The FA product binds to the Pt6 /CeO2 interface with Ead = -1.18 eV. The calculated activation energy for methanol oxidation to FA is compatible to that on Pt(211) surface (0.54 eV), 64 but higher than that on Pt(111) surface (0.25 eV). 63 A similar barrier (0.60 eV) was reported for Au11 cluster supported on reduced TiO2 (110) surface. 65 TPD experiments shown that the main reaction products desorbing from the surface are formaldehyde and H2 molecules. 25 We therefore calculated the desorption energies for the FA and FA1 products, that are 0.40 eV and 0.81 eV, respectively. These results therefore suggest that hydroxylated ceria surfaces should promote the methanol oxidation to FA. We have shown that DA is not stable in the present of the surface O1H’ group, hence we decided to calculate the MEP connecting the MA and FA configurations (see, Fig. 7). The CINEB calculation reveals that the methyl C-H and hydroxyl O-H bonds of methanol are activated simultaneously so that the methanol oxidation to FA can proceed directly. Quite importantly, this reaction path shows a negligible activation energy (E ‡ < 0.1 eV, see Fig.

19



Page 20 of 38

8, blue line). The calculated desorption energy of the FA, 0.15 eV, is much lower than those of FA and FA1 in the non-hydroxylated case. We conclude that surface hydroxy species near the Pt/ceria interface metal promotes methanol dehydrogenation to formaldehyde and assists FA desorption.

4

Conclusions

Here we investigated, through density functional theory and nudged elastic band calculations, the reactivity of Pt4+ , Pt2+ , and Pt0 species supported on flat and stepped ceria (111) surfaces. Methanol was chosen as a probe molecule, and both its adsorption and dissociation to formaldehyde have been considered. The most important findings of our works can be summarized as as follow: • Pt4+ and Pt2+ ionic sites are catalytically inactive towards methanol oxidation. Methanol does not bind to these cationic sites. Pt4+ promotes the first methanol dehydrogenation at CeP t sites, but the effect is local and limited to the first OP t shell. Pt4+ does not assist further methanol dehydrogenation. • The stoichiometric step edges I-S enhances molecular and dissociative methanol adsorptions at step-edge sites. However, the selective methanol binding is suppressed by the presence of the Pt2+ ionic species that are incorporated in PtO4 units or by precursor peroxide radicals at the O-rich step-edge. The effects of the steps and of the ionic Pt species are also local and limited to a distance within 3 ˚ A about the step-edge. • Supported metallic Pt0 cluster on ceria surface catalyzes methanol oxidation better than the ionic ones. The activation energy required for formaldehyde formation at the metal-oxide interface is lowered by 0.4 eV compared to that on pristine (111) surface. • Surface hydroxyl groups near the supported metal nanoparticle facilitate formaldehyde formation and product desorption. 20


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These results therefore allow us to conclude that the presence of ionic Pt species on ceria surfaces is not alone sufficient to catalyze methanol oxidation. Metallic species, even in very small amounts, are always necessary, in agreement with experimental data in the context of H2 oxidation. 30,66 Our work support the conclusion of these recent studies, explaining the catalytic efficiency of Pt-ceria nanocatalysts in terms of a dynamic coexistence between ionic Pt2+ species and sub nm metallic Pt nanoparticles.

Supporting Information Electronic structure analysis, metastable configurations on flat surfaces, adsorption configurations at step-edges, adsorption configurations at supported cluster, coverage effects and k-point sampling convergence.

Acknowledgement This work was supported by the European Union via the FP7-NMP-2012 project chipCAT under Contract No. 310191. S.F. acknowledges the support provided by the Humboldt Foundation through a Friedrich Wilhelm Bessel Research Award.

21



a) CeO2 (111)

Ce4+ O

b) Pt4+ @CeO2 (111)

c) CeO2 step

Page 22 of 38

d) Pt2+ @CeO2 step

OPt Pt4+ O

Ce3+ Pt2+

e) Pt6 @CeO2 (111)

Pt0

Figure 1: Set of model structures used to study methanol adsorption. Yellow, blue and cyan atoms represent Ce4+ , Ce3+ , and Pt ions, respectively. The O atoms are colored from red to white as a function of their distance from the uppermost surface.

22


CeO2 (111) a) Molecular adsorption b) Dissociative adsorption MA DA

Om

1.68 3

2.22

1.6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.60

Page 23 of 38

Figure 2: a) Molecular (MA) and b) dissociative (DA) adsorption of methanol on the pristine CeO2 (111) surface. The Ce, O, C, H atoms are depicted in yellow, red, light yellow, and green, respectively. O atom of methanol is denoted as Om .

23



1.74 2.20

2.62

Pt4+ @CeO2 (111) b) Dissociative adsorption a) Molecular adsorption DA MA

4 1.8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3: Methanol on Pt-CeO2 solid solution surface: (a) molecular adsorption, and (b) dissociative adsorption. Upper panel: side view, lower panel: top view.

24


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Page 25 of 38

b) Pt2+ @CeO2 step I-S

a) CeO2 step I-S 1.25

1.25

DA DA MA MA

0.75 0.5 0.25

DA DA MA MA

1 E ad (eV )

1 E ad (eV )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cetb

Ces

Cet

0.75 0.5 0.25

0

Ces Pt2+ Cetb

Cet

0 0

2

4

6

8

10

12

0

D istance from step edge (Å)

Ces

Cetb

2

4

6

8

10

12

D istance from step edge (Å)

Ces Pt2+ Cetb

Cet

Cet

Figure 4: Adsorption energies of molecular (circles) and dissociative (squares) adsorptions of methanol on steps as a function of the distance between the step edge and the adsorption site. The dashed red and blue horizontal lines denotes the DA and MA on the (2x2) pristine CeO2 (111) surface.

25



Page 26 of 38

E = 0.89

0 E = 0.80 −0.62 −0.98 Ha

Hb

Ha

Hb

−1.93

−2.53 Figure 5: Energy (in eV) profile for methanol dehydrogenation to formaldehyde on the pristine CeO2 (111) (red line) and the Pt4+ @CeO2 (111) (black line). Ha and Hb are denoted as for hydrogen atoms involved in the first and second dehydrogenations, respectively.

26


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Pt6 @CeO2 (111) a) Molecular adsorption b) Dissociative adsorption MA DA

Figure 6: Methanol molecular (a) and dissociative (b) adsorption around interfacial Pt site of supported cluster surface.

27



Pt6 @CeO2 (111)* a) Molecular adsorption b) Dissociative adsorption MA FA

O2 O1 H' Figure 7: Molecular (a) and formaldehyde (b) adsorption on hydrogenated Pt6 /CeO2 (111) surface.

28


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E = 0.89

0 −0.44 −0.62

E = 0.58

−0.55 −0.99 −1.26

−1.18

−1.93

Figure 8: Energy (in eV) profile for methanol dehydrogenation to formaldehyde on nonhydrogenated (black line), hydrogenated (blue line) Pt6 /CeO2 (111), and clean (111) surface (red line).

29



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Ceria Nanocatalysts ... - ACS Publications

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