Single Pt atom species on CeO(111) surface

To investigate the nature of the stability of a single Pt atom deposited on the CeO(111) surface,10, 11, 29 all possible models of single Pt atom species were considered, including adsorbed Pt atom (Pt), (PtOH),(PtO), (PtOH), Pt substitutional to Ce site ((Pt)) and Pt substitutional to O site ((Pt)). These models have been proposed based on the experimental results by state-of-the-art equipment.24, 30 To evaluate the chemical state of a single Pt atom species, we define the formal charge of a single Pt atom model based on its chelation with extra O atoms. In particular, 0 for adatom (Pt), I for (PtOH) and (PtO) species, II for (PtOH) and (PtO) species; for Pt substitutional to Ce and O sites, the formal charge of Pt is the same as these cations and anions, i.e., +IV and –II for (Pt) and (Pt), respectively. The characteristics of these single Pt atom species are described as follows.

An adsorbed Pt atom, (Pt) can stabilize on different sites on the CeO(111) surface, consisting of hollow, top O and top Ce sites (Figure 1). We considered these sites and showed that the most stable site is the hollow site. However, Pt does not reside at the center of the hollow site it spontaneously moves and is strongly bound to two O surface atoms with a binding energy of 3.17 eV and with a Pt-O bond length of 3.278 Å (Figure 2a). The Bader charge of (Pt) is 0.04 |e|, indicating the neutral charge of Pt. This is consistent with our designation of a formal charge of 0 for the (Pt) structure.

The single Pt atom can reside on CeO(111) as a (PtOH) structure if Pt interacts with one OH group on the CeO(111) surface. In this configuration, Pt is bound to two O atoms from the CeO(111) surface with distances of 2.058 and 2.283 Å and bound to one extra O (of OH group) with a distance of 2.039 Å (Table 1 and Figure 2b). We can see that the Pt bound is much stronger on the CeO(111) surface as a Pt(OH) structure with a binding energy of 5.23 eV, as evidenced by the reduced Pt-O bond length compared to the (Pt) structure and the higher oxidation state of Pt due to charge transfer from Pt to CeO(111), which is confirmed by the Bader charge of Pt of 0.38 |e| (Table 1). The interaction between Pt and OH groups can give rise to a condensation process, resulting in the formation of (PtO) species and 1/2 H in the gas phase. In (PtO) species, Pt is also strongly bound to the CeO(111) surface and has extra O with an adsorption energy of 6.00 eV. Similar to the (PtOH) species, Pt is bound to two surface lattice O atoms with bond lengths of 2.022 and 2.324 Å and bound to one extra O atom with a bond length of 1.887 Å (Table 1 and Figure 2c). However, a larger charge was observed for Pt in (PtO) with a Bader charge of 0.57 |e| compared to (PtOH) species (0.38 |e|), Table 1. This indicates that the oxidation state of Pt is I in good agreement with previous studies of Pt on TiO. 24, 26

The Pt atom can anchor on the CeO(111) surface by interacting with two OH groups, forming the (PtOH) structure. In this configuration, Pt is bound to 4 atoms, two from surface lattice atoms with a distance of 2.024 Å and two extra O atoms with a distance of 2.063 Å (Table 1 and Figure 2d). The increasing number of coordinates of Pt with O atoms results in an increasing charge of Pt, illustrated by the Bader charge of 0.75 |e|, which was designed as the oxidation state of II (Table 1). In this structure, Pt is very strongly bound to the CeO(111) surface with a binding energy of 7.95 eV (Table 1). Similar to the case of (PtO), Pt interacts with two OH groups that can induce (PtO) and H molecules, resulting from the condensation process. In the (PtO) structure, Pt is also strongly bound to the extra O atoms with a binding energy of 8.82 eV, indicated by reducing the Pt-O distance to 1.843 and 1.856 Å compared to those in the (PtOH) structure (2.063 Å) and increasing the bond length between Pt and lattice O with distances of 2.101 and 2.169 Å, Table 1 and Figure 2e.

Furthermore, a single Pt atom can exist at either the Ce or O vacant site. These defect sites were formed during the synthetic or catalytic process.31 Therefore, we also considered these configurations. At the Ce vacancy or Pt substitutional to the Ce site, (Pt), Pt bound to six lattice O atoms with Pt-O bond lengths from 2.098 to 2.175 Å (Table 1 and Figure 2f). Among single Pt atom species, Pt bound to the largest number of O atoms, resulting in the largest Bader charge of 1.35 |e| and the largest binding energy of 10.60 eV (Table 1).

The last configuration is Pt substitutional to lattice O atoms, (Pt). At this site, the Pt atom moves up to the surface, resulting in a large distance between Pt and O of 3.290 Å (Figure 2g). In contrast to other single Pt atom structures, Pt accepts the charge from O vacancies, as evidenced by the negative Bader charge of -0.86 |e|, Table 1. At this site, Pt is bound to the CeO(111) surface with a binding energy of 4.11 eV.

CO adsorption properties on supported single Pt atom structures

To evaluate the electronic properties of single Pt atom species deposited on the CeO(111) surface, CO was used as a probe molecule because CO is sensitive to changes in the electronic structure of materials.32 Moreover, the CO molecule has been experimentally applied in many oxidation processes based on these catalysts.

First, we considered the CO adsorption properties on the pristine CeO(111) surface, in which CO physisorbed on the Ce(IV) site with an adsorption energy of 0.31 eV. The weak adsorption of CO on pristine material is illustrated by a large distance of 2.82 Å between CO and Ce(IV), Table 1 and Figure 3a. The CO bound to the Ce(IV) site is mainly controlled by the electrostatic interaction generated by the local electronic field of the Ce(IV) cation and CO molecule.33 This gives rise to a reduction in the CO bond length to 1.142 Å compared to CO in the gas phase (1.144 Å) at this theoretical level. As a consequence, the stretching frequency of CO is redshifted by 25 cm with respect to the free CO molecule.

Different from the pristine CeO(111) surface, a strong adsorption of CO was found when CO was placed on (Pt) species with an energy of 3.52 eV. The strong interaction between CO and Pt atoms is due to the effect of “-back donation” from the 4d electrons of Pt to the antibonding 2* orbital of the CO molecule.34 As a result, the CO bond length is prolonged to 1.169 Å compared to free CO (1.144 Å), resulting in a large redshift (-96 cm) in the stretching frequency of CO (2047 cm) with respect to free CO (2143 cm) (Table 2). Furthermore, the strong adsorption of CO on (Pt) species was demonstrated by a shortened C-Pt distance (1.812 Å) compared to C-Ce(IV) (2.820 Å), Table 2 and Figure 3b.

Compared to the adsorption of CO on (Pt), a smaller adsorption energy (2.62 eV) was found for CO bound to (PtOH) species. This is due to the lower charge density of Pt in the (PtOH) configuration (Bader charge of 0.38 |e|), giving rise to a reducing effect of “-back donation” from electrons in the 4d orbital of Pt to the antibonding 2* orbital of the CO molecule. Therefore, the CO bond length (1.160 Å) is shorter than that of CO (1.169 Å) on (Pt) species, resulting in a stretching frequency of CO (2077 cm) on (PtOH) that is 30 cm larger than that on (Pt) (2047 cm), as shown in Table 2. In addition, the smaller adsorption energy of CO on (PtOH) is evidenced by a larger C-Pt distance (1.837 Å) compared to that of CO on (Pt) (1.837 Å), Table 2 and Figure 3c.

A similar observation was found for CO bound to (PtO) with an adsorption energy of 2.63 eV. This is due to the similar charge density of Pt in the (PtOH) (Bader charge of 0.75 |e|) and (PtO) (Bader charge of 0.74 |e|) species. Furthermore, the CO bond length (1.160 Å) and C-Pt distance (1.840 Å) are also similar to those of CO on (PtOH) species (Table 2 and Figure 3d). This observation indicates that the two structures could have the same population on the CeO(111) surface.

A significant difference in adsorption energy was observed for CO on (PtOH) with a physisorption energy of 0.38 eV. This is similar to CO bound to the Ce(IV) site on the pristine material however, the vibrational stretching of CO is redshifted by 13 cm, while a blueshift of the stretching frequency by +25 cm was found for CO on the pristine material (Table 2). This is demonstrated further by the larger CO bond length (1.150 Å vs. 1.142 Å) and smaller C-Pt distance (1.875 Å vs. 2.820 Å), Table 2 and Figure 3e. Once the condensation process occurs, forming a H molecule and leaving (PtO) species on the CeO(111) surface, CO was strongly bound to these (PtO) species with an adsorption energy of 1.37 eV, although the CO bond length (1.150 Å) and C-Pt distance (1.864 Å) are similar to those of CO on (PtOH).

For CO bound to (Pt) structures, a weak adsorption energy of 0.18 eV was observed, indicating that this CO complex could not be observed from the experiment existing in the literature10, 11, 30, 35 at room temperature. The physisorption of CO on this site is demonstrated by a large C-Pt distance of 3.43 Å and the small blueshift of the CO frequency (+9 cm) with respect to the free CO molecule (Table 3 and Figure 3g). In contrast, CO is strongly bound to the (Pt) structure with an adsorption energy of 1.12 eV. This is due to a larger charge density on the Pt center (Bader charge of -0.86 |e|) transferring to CO, resulting in a prolonged CO bond length compared to that of CO in the gas phase (1.162 Å vs. 1.144 Å) (Table 3 and Figure 3h). As a consequence, the CO frequency is largely redshifted by -80 cm compared to free CO.

CO adsorption on supported metallic Ptclusters

For comparison to those of single Pt atom structures, the Pt cluster deposited on the CeO(111) surface was adopted as a metallic model. The optimized structure of the Pt/CeO(111) surface is shown in Figure 4a, in which two bottom Pt atoms bound to surface O atoms with a distance of 1.93 Å and the third Pt atom is on the top forming a triangular Pt cluster on the CeO(111) surface. The metallic properties of the Pt cluster on the CeO(111) surface were confirmed by the Bader charge of 0.06 |e|. Furthermore, the interaction between Pt-Pt atoms in the Pt cluster is also strong, with cohesive energies of 7.5 eV, but less than the interaction between a single Pt atom and the CeO(111) surface in the (PtO) species (8.82 eV), Table 1. This implies that isolated Pt atoms in (PtO) are stable against the agglomeration of Pt.

To probe the electronic properties of the supported Pt cluster, all possible CO adsorption configurations on the Pt cluster were considered, including Pt top, bridge and Pt interface sites (Figure 4). In either structure, CO is strongly bound to the supported Pt cluster with similar adsorption energies in the range from 2.38 to 2.49 eV (Table 2). These results are in line with previous studies of CO adsorption on Pt/TiO(101) surfaces.24 The strong binding of CO on the Pt cluster is related to a large charge transfer from Pt to CO, which prolonged the CO bond length in the range of 1.167- 1.191 Å. As a result, the large redshift of CO frequencies (-109, -115 and -270 cm for CO on Pt top, bridge and Pt interface sites, respectively, Table 2). These observations are in good agreement with the experiment in reference10. This obviously indicates that the difference in electronic structure of single Pt atoms and their metallic counterparts can be clearly distinguished.