CO Oxidation at SnO2/Pt3Sn(111) Interfaces

Segregation induced formation of oxide/metal interfaces can significantly influence the catalytic activity of alloy nanoparticles. One example is Pt3Sn nanoparticles, which are known to segregate into SnOX and an Sn deficient alloy phase during typical operating conditions for CO oxidation. Here, we use density functional theory calculations to investigate CO oxidation over Pt3Sn(111) supported SnO2 and (SnO2)3, representing the initial state of segregation. The results are compared to CO oxidation at an interface between bulk-like SnO2 and Pt3Sn(111). The barrier for CO oxidation via a Mars–van Krevelen mechanism is found to be lower on SnO2 and (SnO2)3 as compared to the bulk-like model. However, the regeneration of the finite systems is associated with higher barriers for O2 dissociation which may become the rate limiting step in the low temperature regime where the metal surface can be assumed to be CO covered.


Introduction
Bimetallic alloys represent an important class of catalysts in heterogeneous catalysis where PtSn is one example that has been investigated for low-temperature CO oxidation [1][2][3][4]. The effects of alloying on catalytic properties are generally described in terms of ensemble, ligand and strain effects [5][6][7][8]. Ensemble effects refer to geometric adsorption constraints upon alloying, whereas ligand and strain effects denote changes in the electronic structure. The mixing pattern in metal nanoalloys is generally complex and depends both on the constituent metals and the synthesis method [9][10][11]. Nanoalloys have been investigated extensively 1 3 computationally [9], and recent efforts include examples where the chemical ordering of bimetallic nanoparticles is predicted on the basis of density functional theory (DFT) calculations [12]. Although this approach is suited to explore the homotop distribution for PtSn bimetallic nanoparticles of particular shape and composition [13], it does not describe adsorbate (or reaction) induced segregation phenomena.
The dynamic response of metal nanoparticles to adsorbates makes it challenging to assess the origin of promoting effects upon alloying. Several studies have demonstrated that reactions may drive surface reconstruction and/or segregation [14][15][16]. Also in the case of PtSn nanoparticles, experimental evidence exists for adsorbate induced segregation during typical operating conditions for regular COoxidation and preferential CO-oxidation in the presence of H 2 (PROX) [2][3][4]. In Ref. [3], ambient pressure X-ray photoelectron spectroscopy (XPS) showed the formation of an Sn-oxide phase during CO-oxidation over silica supported PtSn nanoparticles. Based on the experiments, [3] it was suggested that CO oxidation occurs with a low activation barrier at the interface between Pt and SnO x domains via a Mars-Van Krevelen mechanism [17]. Moreover, SnO 2 -phases in contact with a PtSn nanoalloy have been observed during CO electro-oxidation over PtSn with transmission electron microscopy [18]. Furthermore, Pt-Sn segregation during CO exposure over alumina supported PtSn has been inferred from in situ diffuse reflectance IR Fourier transform spectroscopy following the CO stretch vibration [1,2,4]. In the case of Pt 3 Sn, adsorbate induced segregation is not limited to nanoparticles and it has been observed for Pt 3 Sn(111) during CO oxidation with X-ray photoemission spectroscopy (XPS) [19]. In particular, it was proposed that Pt 3 Sn(111) converts to an inverse catalyst with SnO X supported by metallic Pt 3 Sn [19].
Adsorbate induced segregation of bi-metallic nanoparticles leads to the creation of interfaces and during oxidation reactions, metaloxide/metal interfaces or metaloxide/ metaloxide interfaces are formed. It is well known, that such interfaces can have superior catalytic performance in CO oxidation [20][21][22][23][24][25][26]. For example, low temperature activity has been observed for promoted Pt-group metals [23] and in the case of FeO x /Pt(111), the activity was rationalized by the possibility to have CO oxidation at the FeO x /Pt interface [20]. In case of Pt 3 Sn, segregation into an SnO 2 phase supported by a Pt(111)/Pt 3 Sn (Pt-skin) was recently computationally predicted via ab initio thermodynamics [24]. Moreover, the importance of an oxide/metal interface for the reaction kinetics was explored using first principle micro-kinetic modeling for a periodic SnO 2 -rod supported by a Pt-skin [24]. The catalytic role of the SnO X -phase was manifested by a low temperature activity at the interface. The reaction path at the interface is of Mars-van Krevelen type which limits the effect of CO poisoning that controls the low temperature reaction rate on regular Pt, where the reaction proceeds via a Langmuir-Hinshelwood mechanism. Furthermore, the CO-oxidation barriers at the SnO X /Pt interface were found to be lower than the corresponding barrier on the metal-only system. As it is difficult to know the experimental degree of segregation, especially during the initial stages, it is unknown whether the results for the bulk-like SnO 2 interface can be generalized to other SnO x -structures supported on Pt. Therefore, it is desirable to investigate how CO oxidation barriers and their regeneration with O 2 depend on the size of the SnO X -phase.
Herein, we use DFT calculations to investigate CO oxidation and O 2 regeneration over finite size SnO 2 and (SnO 2 ) 3 units supported on Pt 3 Sn(111). The results are compared to oxidation at a bulk-like metal supported SnO 2 -phase, represented by a SnO 2 -rod/Pt 3 Sn(111) model.

Theoretical Methodology
DFT is applied with the gradient corrected exchange-correlation functional according to Perdew, Burke and Ernzerhof [27]. In particular, the Vienna Ab Initio Simulation Package [28,29] is used. The one-electron Kohn-Sham orbitals are expanded in a plane-wave basis-set with a kinetic energy cutoff of 450 eV. PAW potentials are employed to describe the interaction between the valence electrons and the core [30]. Reciprocal space integration over the Brillouin zone is approximated with finite sampling using Monkhorst-Pack grids [31,32]. Bulk calculations of Pt, Pt 3 Sn, SnO 2 are performed with a k-point grid of at least 12 × 12 × 12. Surface calculations are performed using 5 layered orthorhombic slabs. ( 4 × 2 √ 3) rect or p(4 × 4) surface cells are used for the metal-only surfaces and the metal-supported SnO 2 and (SnO 2 ) 3 units. A ( 6 × 2 √ 3) rect surface cell is used to model a metal-supported periodic (SnO 2 ) 12 -rod. k-point grids are employed depending on the size of the surface cell being either 3 × 3 × 1 or 2 × 3 × 1. A vacuum layer of at least 20 Å is used in the calculations.
The systems are structurally optimized until the largest force is smaller than 0.03 eV/Å. Transition states are obtained initially with the climbing image nudged elastic band method [33,34] and further refined with the dimer method [35]. The convergence criteria for the transition state searches are set to at least 0.03 eV/Å. For the O 2 dissociation reactions, spin polarized calculations are employed for both reactants and transition states. To verify transition states and local minima, a partial Hessian vibrational analysis (PHVA) is employed. The PHVA is performed only for the surface species while keeping the rest of the system fixed. The numerical partial Hessian is calculated by displacements in x, y and z-directions of ± 0.02 Å.

3
The stability of the segregated systems of the form (SnO 2 ) X is assessed by referencing to SnO 2 -bulk. The stability (ΔE) is calculated according to: Here, E (SnO 2 ) x /Pt(111)∕Pt 3 Sn is the energy of the system under consideration, E SnO 2 , bulk is the energy of the SnO 2 -bulk and E Pt(111)∕Pt 3 Sn is the energy of a Pt 3 Sn slab with the top layer being only Pt (a Pt-skin).

Results and Discussion
The reaction induced segregation process is complex on Pt 3 Sn-nanoparticles. From experiments, it is clear that SnO X is formed during CO oxidation, [3,19] however, the morphology is unknown. Here, we present first different SnO 2 /Pt interface models and their relative stability with respect to bulk SnO 2 (Sect. 3.1). Thereafter, CO adsorption, the stability of the interface O atoms and CO oxidation are investigated (Sect. 3.2). The closing of the catalytic cycle by regeneration of SnO X via O 2 dissociation is investigated in Sect. 3.3.

Interface Models and Their Relative Stability
Different SnO X model structures are constructed on a Ptskin model, thus a Pt 3 Sn(111) slab with the top-most metal layer being Pt-only. A Pt-skin is used instead of regular Pt 3 Sn(111) to model the Sn-deficiency in the alloy. This model system was previously used to explore CO oxidation routes on PtSn nanoparticles [24]. The studied interface models studied herein are displayed in Fig. 1. ROD represents a (SnO 2 ) 12 -rod, with exposed SnO 2 (110)-surfaces as the (110) facet is known to be the stable SnO 2 surface [36,37]. The TRIMER system is a trimeric (Sn 3 O 3 )O 3 structure and the MONOMER unit is a single Sn ad-atom with two oxygen atoms. For each of the models, a favorable orientation was obtained by optimizing a set of different generated structures. Here, only the lowest energy orientation is considered for further studies.
In earlier work, the exothermicity of segregation was validated for the formation of the rod model onto a five layer Pt 3 Sn slab in absence of CO [24]. It was found that the required oxygen chemical potential for segregation of this slab into an Sn-deficient Pt-skin system and bulk SnO 2 is − 1.53 eV. If instead of bulk SnO 2 a rod-model is formed (Fig. 1a), an oxygen chemical potential of − 1.17 eV is required. With respect to bulk SnO 2 , the stabilities (ΔE) are 0.72 eV (rod), 1.24 eV (trimer) and 1.44 eV (monomer) (see Fig. 1). From the stability difference, the minimum oxygen chemical potential to form the (SnO 2 ) 3 and (SnO 2 ) is calculated to be − 0.91 eV and − 0.81 eV, respectively. In the ΔE = E (SnO 2 ) x /Pt(111)∕Pt 3 Sn − xE SnO 2 , bulk − E Pt(111)∕Pt 3 Sn presence of CO and O 2 , the formation of monomeric and trimeric SnO 2 units should precede the formation of more bulk like SnO 2 -phases, such as the rod model. However, due to the lower stability of the monomer and the trimer, their formation requires higher oxygen chemical potentials and, thus, higher oxygen pressures than the formation of the rod. The minimal oxygen pressure needed to form monomers at a temperature of 400 K is still very low, being ~ 10 −11 bar [24]. The corresponding pressures for the trimer and the rod are about 10 −13 and 10 −19 bar, respectively.

Relative Stability of Interface Oxygens, CO Adsorption and CO Oxidation at SnO X Interfaces
The model systems have, completely oxygen saturated, three or two oxygen atoms available for reaction, see Fig. 1. A measure of their reactivity is given by the stability which could be assessed by calculating the energy required to form oxygen vacancies. The vacancy formation energies, defined as the energy needed to form 1/2 O 2 in the gas phase, are given in Table 1. For rod and trimer models, the oxygen with the lowest vacancy formation energy is denoted position in B. Owing to symmetry, the two atoms (A and C) are equivalent for the monomer. The vacancy formation energies to go from state ABC to A*C are 1.24 and 0.61 eV, for the rod and the trimer, respectively. For the monomer, the vacancy formation energy going from A*C to **C is 0.59 eV. The vacancy formation energies on the trimer and monomer models are, thus, considerably lower than on the rod model. In case of the trimer, vacancy formation from A or C atoms requires more energy than from B, whereas the differences are moderate for the rod.
CO oxidation is one possibility to deplete oxygen atoms by reactions from the SnO X systems. Here, we assume the consumption of the oxygen atom in position B via CO oxidation forming A*C for rod and trimer models and consumption of position A for the monomer model forming **C. CO can adsorb on a multitude of positions on the Pt-skin. In particular, there are different types of sites with respect to the underlying Pt 3 Sn lattice. The different types of hollow sites (fcc-Pt, fcc-Sn, hcp-Pt and hcp-Sn) are visualized in Fig. S1 of the Electronic Supplementary Material (ESM). For example, fcc-Pt refers to a site where a Pt atom is situated in the third layer below the site, for fcc-Sn an Sn atom appears in the third layer. The CO adsorption energies are − 1.99 eV (fcc-Pt, rod), − 1.70 eV (bridge, trimer) and − 1.88 eV (bridge, monomer). The CO adsorption energies near the trimer and the monomer deviate markedly from the adsorption energy of CO on a ( 4 × 2 √ 3) rect Pt-skin system, being − 2.09 eV in the fcc-Pt position which is related to repulsive CO-SnO x interactions. Barriers for all model systems are evaluated with respect to CO adsorbed in an fcc-Pt or bridge position near the consumed oxygen (see * in Fig. 1). The CO oxidation barriers from the (SnO 2 ) X -state [ABC→A*C for rod and trimer, and A*C → **C for monomer] are 0.91 eV, 0.29 eV and 0.63 eV for the rod, trimer and monomer, respectively. If instead the same reference state for CO is used, being CO on the Ptskin far from the metal oxide phase in an fcc-Pt position, the barriers are 1.01 eV (rod), 0.67 eV (trimer) and 0.83 eV (monomer), see Table 1. The CO oxidation barriers on the smaller SnO X models are, thus, considerably lower than for the rod-model. This is evident also in the large variation of the O-CO transition state structures, see Fig. 2. From the A*C configuration, the subsequently consumed oxygen is chosen to be A for the rod and trimer models. While the barrier for (A*C → **C), is about 0.20 eV higher than ABC → B*C, for the rod, the increase is within 0.10 eV for the trimer (see Fig. 4 and Table S1). We conclude that the barrier for CO oxidation is considerably lower on the finite systems than on the rod-model.

SnO X Regeneration via Molecular O 2 Dissociation
To maintain a catalytic cycle, the SnO x -phases should be regenerated by O 2 adsorption and dissociation. O 2 adsorbs on Pt(111)/Pt 3 Sn with an adsorption energy of − 0.90 eV in a fcc-Pt position (Fig. 3a). The O-O distance is in this state elongated from 1.23 Å in gas phase to 1.41 Å. The transition state for dissociation occurs with an O-O distance of 1.83 Å and a barrier of 0.27 eV (Fig. 3a). The O 2 dissociation is, thus, only weakly activated on the Pt-skin which is in agreement with previous reports for Pt(111) [38]. The barrier calculated with respect to O 2 in the gas phase is − 0.64 eV. At low temperature, the catalyst is expected to be CO poisoned, with only a limited number of empty sites available for O 2 dissociation on the metal phase. Therefore, O 2 dissociation is here investigated at the (SnO 2 )/Pt interface of the different SnO X models in their **C state requiring only one empty metal site, rather than two metal sites. The reaction barrier ΔE ‡ is calculated with respect to CO adsorbed in an fcc-Pt position in a ( 4 × 2 √ 3) rect unit cell, thus in the absence of repulsive interactions with the SnO x -phase. The positions for the oxygen atoms in the rod are labeled according to Fig. 1 We close this section by noting that there might be a possibility to adsorb O 2 at the monomer and trimer, if an O 2 is adsorbed in an on top configuration, requiring only one metal site. The adsorption energy is in this case − 0.29 eV for the trimer and − 0.02 eV for the monomer. The corresponding dissociation energies (with respect to the adsorbed states) are 0.34 eV and 0.47 eV for the trimer and monomer, respectively.
Possible catalytic cycles for CO oxidation over the investigated systems are summarized in Fig. 4. For the rod-system, the CO oxidation reactions proceed with barriers of 1.01 and 1.20 eV, respectively. The corresponding barriers are lower for the monomer and the trimer. Even if the monomer model is regenerated to an SnO 3 state, the CO oxidation proceeds easily for ABC to A*C. The barrier for oxygen regeneration is, instead, higher for the finite systems as compared to the rod. In a previous microkinetic model of CO oxidation at metal supported rodmodel [24], it was found that the O 2 dissociation became slightly rate controlling, about 15%, at 400 K. Because the finite SnO 2 -models have larger O 2 dissociation barriers, they will likely have a higher rate control at similar temperatures.

Conclusions
This work emphasizes interface effects during CO oxidation on Pt-alloys nanoparticles taking place at the SnO 2 / Pt 3 Sn interface. When the Pt phase is CO poisoned, the SnO 2 /Pt 3 Sn interface enables a pathway for low temperature CO oxidation. Dependent on the degree of segregation, different interface effects can be anticipated and here we have investigated monomeric, trimeric and periodic (SnO 2 ) X -rod models. Regardless of the size of the studied (SnO 2 ) X -phase, CO oxidation can proceed with low barriers, which explain the experimentally observed low temperature activity. The reoxidation with O 2 is, however, more activated for the monomer and the trimer than on the rod-model, potentially forming a bottleneck for low temperature CO oxidation on the finite SnO X -units. Summarizing, the co-catalytic role of an SnO X /Pt 3 Sn interface will manifest itself in a low temperature activity provided that the SnO X -phase can be regenerated with oxygen.