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DFT insight into the high stability of single Pt atoms on the CeO2(111) surface






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Abstract

Introduction: Single Pt atom catalysts supported on CeO2(111) surfaces have attracted considerable attention in recent years due to their high reactivity and selectivity in important reactions, such as water–gas shift reactions for hydrogen production and CO oxidation. However, the geometrical and electronic structure of Pt/CeO2(111) at the atomic level is still not clearly understood. In this study, we aim to gain insight into the geometrical and electronic structure of a single Pt atom on the CeO2(111) surface.


Methodology: Various single Pt atom species, including (Pt)ads, (PtOH)ads, (PtO)ads, (PtO2H2)ads, (PtO2)ads, (Pt)subCe, and (Pt)subO, deposited on the CeO2(111) surface were investigated by the DFT+U method with dispersion corrections. Furthermore, the CO molecule was adopted to evaluate the electronic structures of these single Pt atoms. In addition, the supported Pt3 cluster was applied as a metallic model to distinguish it from single Pt atom models.


Results: By comparing the CO adsorption properties of single Pt atom models and Pt3 cluster models to those of experimental observations in the literature, the stretching frequency of CO on (PtO2)ads species agrees well with the experimental results, while the frequencies of CO on other single Pt atom structures and on Pt3 clusters largely differ from the experimental results.


Conclusion: Based on the DFT calculated results, we conclude that the activity and high stability of single Pt atom species that were observed from the experiment are formed via the interaction between Pt and two extra O atoms on the CeO2(111) surface.

INTRODUCTION

Heterocatalysts consisting of precious metals deposited on metal oxide surfaces have attracted considerable attention for environmental remediation, such as CO 2 conversion and CO oxidation. 1 , 2 , 3 , 4 Depending on the synthesis and catalytic processes, different sizes of precious metals were deposited on the metal oxide support. However, in many cases, precious metals cannot be used efficiently (i.e., 100%), and the selectivity is difficult to evaluate. Therefore, reducing the size of precious metals to obtain the maximum efficiency and high selectivity in catalytic processes is essential. This boosts the scientific communities in the world and has spent much effort on this field in recent years aiming at reducing the size of precious metals as little as possible. As a result, the size of supported precious metals was reduced from nanoparticles to cluster species, and the ultimate size was atomic species. 5 The pioneering work has obtained the atomic size of precious metal Pt dispersed on FeO x materials 6 , and it has shown an excellent catalytic activity and high selectivity for CO oxidation compared to Pt clusters or Pt nanoparticles. This paves the way for numerous studies of different single metal catalysts supported on different supporting materials, including metal oxides, metals, MOFs, graphene, and zeolites. 7 However, the high surface energy of these single atom species gives rise to the aggregation forming particles. To avoid this obstacle, many techniques have been proposed and successfully synthesized, such as creating void space, chelating, functional groups, etc. 8

Among metal catalysts, Pt supported on CeO 2 materials has been widely used in the control of vehicle exhaust. 9 Owing to its high thermal stability, catalytic activity and selectivity. Furthermore, the atomic dispersion of Pt on CeO 2 has been experimentally synthesized and demonstrated the efficient utilization of Pt atom catalysts. 10 , 11 , 12 However, the electronic nature of a single Pt atom deposited on CeO 2 , such as the local structure, chemical state and dynamics during catalytic processes, is still not clearly understood even with state-of-the-art experimental methods. To shed light on the high stability and catalytic activity of a single Pt atom on the CeO 2 supporting material, we systematically investigated the atomic dispersion of Pt species on the most stable CeO 2 (111) surface by applying the DFT+U approach with dispersion correction. The geometrical and electronic properties of various single Pt atom configurations were evaluated and probed by the adsorption energy and stretching frequencies of CO.

COMPUTATIONAL METHODS AND MODELS

Spin-polarized density functional theory (DFT) calculations were carried out by using the Vienna ab initio simulation package (VASP 5.4.1). 13 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional 14 was adopted to evaluate the electron exchange-correlation. The core electrons and nuclei interactions were described by the projector augmented wave (PAW) 15 , while the valence electrons of atoms explicitly considered are Ce (5s, 5p, 6s, 4f, 5d), Pt (6s, 5d), H (1s), O (2s, 2p) and C (2s, 2p). To partly compensate for the lack of self-interaction in the DFT method, we applied DFT+U as suggested by Dudarev et al.(16) In this work, Hubbard’s parameter U 16 , 17 of 4.5 eV was applied for the Ce 4f state 18 , and with this U parameter, we reproduced the cell parameters (a=b=c=5.472 Å) in good agreement with that of the experiment (a=b=c=5.411 Å). 19 Furthermore, the dispersion correction was also included by using the D3 scheme of Grimme. 20 All calculations were considered at the G point only with a plane wave basis set within a cutoff energy of 400 eV. 21 The geometrical structure optimization was obtained when the ionic forces were less than 0.01 eV/Å.

The nine-atomic layer slab of the (2x2) supercell corresponding to the stoichiometric formula of Ce 48 O 96 was modeled for the most stable CeO 2 (111) surface. 22 A vacuum thickness of 15 Ă… was included in the slab to minimize the interaction between the neighboring slabs. The structural model is shown in Figure 1 . Compared to O and Ce in the bulk materials, where the coordinate numbers of O and Ce are 4 and 8, respectively, the coordinate numbers of surface O and Ce are 3 and 7, respectively.

The binding energies (BE in eV) of the single Pt atoms on the CeO 2 (111) surface were computed using the following formula:

BE = E(Pt) + E (suf) – E(Pt/surf) 23

where E(Pt), E(surf) and E(Pt/surf) are the total energies of the isolated Pt atoms, CeO 2 (111) surface and combined Pt/CeO2(111) surface, respectively.

The CO adsorption energies, E ads (in eV), were computed as follows:

E ads = E(catal) - E(CO) - E(CO_catal) 12 , 24 , 25

where E(catal), E(CO), and E(CO_catal) are the total energies of single Pt atom species on CeO 2 (111) or CeO 2 (111) surfaces, of CO molecules and of CO adsorption complexes on single Pt atom species on CeO 2 (111) or CeO 2 (111) surfaces, respectively. According to this calculation, the positive values imply stable CO adsorption. The stretching frequencies of adsorbed CO molecules were evaluated within the harmonic approximation, in which CO and its nearest neighboring atoms were taken into account in this calculation. The CO bond length of 1.144 Ă… and stretching frequency of 2125 cm -1 are obtained for the free CO molecule by PBE functional theory. For comparison with the stretching frequencies of those in the experiments, a scaling factor a = 2143/2125 = 1.0085 was used for all theoretical frequencies, 24 , 26 in which the experimental frequency of the free CO molecule was 2143 cm -1 . 27 The effective atomic charge was calculated using the Bader method, which was developed by Henkelman et al. 28

RESULTS

Single Pt atom species on CeO2(111) surface

To investigate the nature of the stability of a single Pt atom deposited on the CeO 2 (111) surface, 10 , 11 , 29 all possible models of single Pt atom species were considered, including adsorbed Pt atom (Pt) ads , (PtOH) ads ,(PtO 2 ) ads , (PtO 2 H 2 ) ads , Pt substitutional to Ce site ((Pt) subCe ) and Pt substitutional to O site ((Pt) subO ). 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) ads , I for (PtOH) ads and (PtO) ads species, II for (PtO 2 H 2 ) ads and (PtO 2 ) ads 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) subCe and (Pt) subO , respectively. The characteristics of these single Pt atom species are described as follows.

An adsorbed Pt atom, (Pt) ads , can stabilize on different sites on the CeO 2 (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 Ă… ( Figure 2 a). The Bader charge of (Pt) ads 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) ads structure.

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

The Pt atom can anchor on the CeO 2 (111) surface by interacting with two OH groups, forming the (PtO 2 H 2 ) ads structure. In this configuration, Pt is bound to 4 atoms, two from surface lattice atoms with a distance of 2.024 Ă… and two extra O atoms with a distance of 2.063 Ă… ( Table 1 and Figure 2 d). 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 2 (111) surface with a binding energy of 7.95 eV ( Table 1 ). Similar to the case of (PtO) ads , Pt interacts with two OH groups that can induce (PtO 2 ) ads and H 2 molecules, resulting from the condensation process. In the (PtO 2 ) ads 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 Ă… compared to those in the (PtO 2 H 2 ) ads structure (2.063 Ă…) and increasing the bond length between Pt and lattice O with distances of 2.101 and 2.169 Ă…, Table 1 and Figure 2 e.

Figure 1 . a) Side view and b) top view of all positions of a single Pt atom residing on the CeO 2 (111) surface. (1) hollow, (2) top O and (3) top Ce sites. O and Ce are red and purple spheres, respectively. To distinguish O atoms in the first and other layers, the O atoms in the second layer are depicted in yellow.

Figure 1 (111) surface. (1) hollow, (2) top O and (3) top Ce sites. O and Ce are red and purple spheres, respectively. To distinguish O atoms in the first and other layers, the O atoms in the second layer are depicted in yellow.

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Table 1 Bond length between Pt and O, r(Pt-O), Bader charge, Q(Pt) and binding energies, BE of single atom Pt species deposited on CeO 2 (111) surface.

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) subCe , Pt bound to six lattice O atoms with Pt-O bond lengths from 2.098 to 2.175 Ă… ( Table 1 and Figure 2 f). 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) subO . At this site, the Pt atom moves up to the surface, resulting in a large distance between Pt and O of 3.290 Ă… ( Figure 2 g). 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 2 (111) surface with a binding energy of 4.11 eV.

Figure 2 . Side view (left) and top view (right) of a single Pt atom species on the CeO 2 (111) surface. (a) (Pt) ads , (b) (PtOH) ads , (c) (PtO) ads , (d) (PtO 2 H 2 ) ads , (e) (PtO 2 ) ads , (f) (Pt) subCe , (g) (Pt) subO . O, Ce and Pt are red, purple and gray spheres, respectively.

Figure 2 . O, Ce and Pt are red, purple and gray spheres, respectively.

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CO adsorption properties on supported single Pt atom structures

To evaluate the electronic properties of single Pt atom species deposited on the CeO 2 (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.

Figure 3 . Side view (left) and top view (right) of CO adsorption on pristine and on single Pt atom species on CeO 2 (111) surface. (a) Pristine CeO 2 (111), (b) (Pt) ads , (c) (PtOH) ads , (d) (PtO) ads , (e) (PtO 2 H 2 ) ads , (f) (PtO 2 ) ads , (g) (Pt) subCe , (h) (Pt) subO . O, Ce,Pt and C are red, purple, gray and green spheres, respectively.

Figure 3 . O, Ce,Pt and C are red, purple, gray and green spheres, respectively.

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First, we considered the CO adsorption properties on the pristine CeO 2 (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 Ă… between CO and Ce(IV), Table 1 and Figure 3 a. 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 Ă… compared to CO in the gas phase (1.144 Ă…) at this theoretical level. As a consequence, the stretching frequency of CO is redshifted by 25 cm -1 with respect to the free CO molecule.

Different from the pristine CeO 2 (111) surface, a strong adsorption of CO was found when CO was placed on (Pt) ads 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 -1 ) in the stretching frequency of CO (2047 cm -1 ) with respect to free CO (2143 cm -1 ) ( Table 2 ). Furthermore, the strong adsorption of CO on (Pt) ads species was demonstrated by a shortened C-Pt distance (1.812 Å) compared to C-Ce(IV) (2.820 Å), Table 2 and Figure 3 b.

Compared to the adsorption of CO on (Pt) ads , a smaller adsorption energy (2.62 eV) was found for CO bound to (PtOH) ads species. This is due to the lower charge density of Pt in the (PtOH) ads 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) ads species, resulting in a stretching frequency of CO (2077 cm -1 ) on (PtOH) ads that is 30 cm -1 larger than that on (Pt) ads (2047 cm -1 ), as shown in Table 2 . In addition, the smaller adsorption energy of CO on (PtOH) ads is evidenced by a larger C-Pt distance (1.837 Å) compared to that of CO on (Pt) ads (1.837 Å), Table 2 and Figure 3 c.

Table 2 CO adsorption site, CO adsorption energy, E ads (eV), C-O bond length, R CO (Ă…), C-M (M=Pt,Ce) distance, R CM (Ă…), Bader charge of Pt (before) after CO adsorption, Q(Pt) (|e|), harmonic CO stretching frequency, (cm -1 ) and frequency shift with respect to the gas-phase, (cm -1 ) for CO adsorption on various single Pt atom species on CeO 2 (111) surfaces.

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

A significant difference in adsorption energy was observed for CO on (PtO 2 H 2 ) ads 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 -1 , while a blueshift of the stretching frequency by +25 cm -1 was found for CO on the pristine material ( Table 2 ). This is demonstrated further by the larger CO bond length (1.150 Ă… vs. 1.142 Ă…) and smaller C-Pt distance (1.875 Ă… vs. 2.820 Ă…), Table 2 and Figure 3 e. Once the condensation process occurs, forming a H 2 molecule and leaving (PtO 2 ) ads species on the CeO 2 (111) surface, CO was strongly bound to these (PtO 2 ) ads species with an adsorption energy of 1.37 eV, although the CO bond length (1.150 Ă…) and C-Pt distance (1.864 Ă…) are similar to those of CO on (PtO 2 H 2 ) ads .

For CO bound to (Pt subCe ) 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 literature 10 , 11 , 30 , 35 at room temperature. The physisorption of CO on this site is demonstrated by a large C-Pt distance of 3.43 Ă… and the small blueshift of the CO frequency (+9 cm -1 ) with respect to the free CO molecule (Table 3 and Figure 3 g). In contrast, CO is strongly bound to the (Pt subO ) 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 Ă… vs. 1.144 Ă…) (Table 3 and Figure 3 h). As a consequence, the CO frequency is largely redshifted by -80 cm -1 compared to free CO.

CO adsorption on supported metallic Pt3 clusters

For comparison to those of single Pt atom structures, the Pt 3 cluster deposited on the CeO 2 (111) surface was adopted as a metallic model. The optimized structure of the Pt 3 /CeO 2 (111) surface is shown in Figure 4 a, in which two bottom Pt atoms bound to surface O atoms with a distance of 1.93 Ă… and the third Pt atom is on the top forming a triangular Pt 3 cluster on the CeO 2 (111) surface. The metallic properties of the Pt 3 cluster on the CeO 2 (111) surface were confirmed by the Bader charge of 0.06 |e|. Furthermore, the interaction between Pt-Pt atoms in the Pt 3 cluster is also strong, with cohesive energies of 7.5 eV, but less than the interaction between a single Pt atom and the CeO 2 (111) surface in the (PtO 2 ) ads species (8.82 eV), Table 1 . This implies that isolated Pt atoms in (PtO 2 ) ads are stable against the agglomeration of Pt.

To probe the electronic properties of the supported Pt 3 cluster, all possible CO adsorption configurations on the Pt 3 cluster were considered, including Pt top, bridge and Pt interface sites ( Figure 4 ). In either structure, CO is strongly bound to the supported Pt 3 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 3 /TiO 2 (101) surfaces. 24 The strong binding of CO on the Pt 3 cluster is related to a large charge transfer from Pt 3 to CO, which prolonged the CO bond length in the range of 1.167- 1.191 Ă…. As a result, the large redshift of CO frequencies (-109, -115 and -270 cm -1 for CO on Pt top, bridge and Pt interface sites, respectively, Table 2 ). These observations are in good agreement with the experiment in reference 10 . This obviously indicates that the difference in electronic structure of single Pt atoms and their metallic counterparts can be clearly distinguished.

Figure 4 . Side view (left) and top view (right) of the Pt 3 cluster and CO adsorption on Pt 3 adsorbed on the CeO 2 (111) surface. (a) Pt 3 , (b) CO at Pt top, (c) CO at bridge and (d) CO at Pt interface sites. O, Ce, Pt and C are red, purple, gray and green spheres, respectively.

Figure 4 , (b) CO at Pt top, (c) CO at bridge and (d) CO at Pt interface sites. O, Ce, Pt and C are red, purple, gray and green spheres, respectively.

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DISCUSSION

The nature of the high stability of single Pt atoms on the ceria surface has been theoretically studied on the basis of metal-support interactions between Pt atoms and the ceria surface, and they pointed out that the high thermal stability of the single Pt atoms on the ceria substrate is due to Pt atoms substitutional to Ce cations 35 , 36 or located at step edges. 37 In these defect sites, Pt is bound strongly with lattice O atoms forming a planar structure, resulting in the high thermal stability of single Pt atoms, which was experimentally demonstrated above 700 K. 37 Furthermore, such systems were found to have high selectivity for CO oxidation reactions via the Mars-van Krevelen mechanism. This is due to the active center of single Pt atoms, which can act as both an oxidant agent and reductive agent. Different from the previous theoretical studies mentioned above, here, to gain insight into the electronic and geometrical structure of a single Pt atom anchored on the CeO 2 (111) surface, we mainly focus on the vibrational frequencies of CO adsorption on single Pt atom structures and compare them to experimental results existing from the literature. 10 , 11 , 12 , 30 , 38 There is a consistency from various experimental studies in the literature that the stretching frequencies of CO on single Pt atom species dispersed on CeO 2 (111) surface are in the range from 2092-2102 cm -1 , 10 , 11 , 12 , 30 , 38 which is completely different from those of CO on Pt metallic clusters 10 and on bare CeO 2 (111) surface (2159-2168 cm -1 ). 10 , 30 Therefore, we can compare our results with these experimental results existing from the literature and provide in detail the electronic structures of single Pt atoms deposited on the real CeO 2 catalyst.

It is clear from the results in Table 2 that the stretching frequency of CO adsorbed on the pristine CeO 2 (111) surface (2168 cm -1 ) is blueshifted by 25 cm -1 and in excellent agreement with that observed from experimental studies in references 10 , 30 . Different from CO on the bare CeO 2 (111) surface, a large redshift of CO frequencies with respect to free CO molecules was found for CO adsorbed on (Pt) ads , (PtOH) ads , (PtO) ads , and (Pt) subO by -96 cm -1 , -66 cm -1 , -70 cm -1 and -80 cm -1 , respectively. This is much far from the experimental observation, with a redshift of CO frequencies in the range from -51 cm -1 to -41 cm -1 . 10 , 11 , 12 , 30 , 38 This allows us to rule out these structures as single Pt atom structures, which were experimentally found on the CeO 2 (111) surface in the references 10 , 11 , 12 , 30 , 38 .

For CO adsorption on (PtO 2 H 2 ) ads and (Pt) subCe , the very weak adsorption energy, which cannot form at experimental temperatures above room temperature, as mentioned in the literature 10 , 11 , 30 , and the blueshift of adsorbed CO also allow us to exclude these structures existing as single atom species on the CeO 2 (111) surface.

Only in the case of CO on (PtO 2 ) ads was the stretching frequency of CO (2109 cm -1 ) in good agreement with experimental observations (2190-2102 cm -1 ) in the literature 10 , 11 , 12 , 30 . Furthermore, the Pt center coordinated with four O neighboring atoms that were also in line with experiments in the literature. 10 , 12 Therefore, this (PtO 2 ) ads configuration is a good presentation for the activity and high stability of a single Pt atom on the CeO 2 (111) surface.

It is also obvious that the frequencies (from 1873 cm -1 to 2034 cm -1 ) of CO adsorbed on different sites of the Pt 3 cluster representative of metallic Pt on CeO 2 (111) were completely different from those on single Pt atoms and agree well with experimental data in references 10 , 30 . Therefore, these results can be used to differentiate with those of single Pt atom species.

CONCLUSIONS

The geometrical and electronic structure of a single Pt atom was investigated by means of the DFT+U method with dispersion corrections. Several structures of a single Pt atom have been considered, including (Pt) ads , (PtOH) ads , (PtO) ads , (PtO 2 H 2 ) ads , (PtO 2 ) ads , and (Pt) subCe vĂ  (Pt) subO . Furthermore, the supported Pt 3 cluster for the metallic model was considered for further comparison. To probe the differences in the electronic structure of single Pt atom species and Pt 3 clusters, the adsorption energies and stretching frequencies of CO adsorption on these configurations were evaluated. Based on these results and compared to experiments, we found that the high activity and high stability of a single Pt atom anchored on the CeO 2 (111) surface is formed by the interaction between the Pt atom and two extra O atoms (PtO 2 ) ads , resulting from the condensation process of Pt with two OH groups. Our observation is different from previous theoretical studies, in which they pointed out that the high thermal stability of single Pt atoms anchored on the CeO 2 (111) surface is due to the substitution of Pt into Ce sites 35 , 36 or located at step edges. 37 Therefore, our theoretical results provide another insight into single Pt atom structures on the CeO 2 (111) surface, and this study provides useful information for experiments in synthesis and important applications, especially CO oxidation 38 , 6 and water-gas shift reactions. 39

ABREVIATIONS

DFT: density functional theory

VASP: Vienna Ab initio Simulation Package

COMPETING INTERESTS

The authors declare that there are no conflicts of interest regarding the publication of this paper.

ACKNOWLEDGMENTS

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.06-2020.50.

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Article Details

Issue: Vol 25 No 2 (2022)
Page No.: 2364-2372
Published: May 11, 2022
Section: Section: NATURAL SCIENCES
DOI: https://doi.org/10.32508/stdj.v25i2.3890

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Copyright: The Authors. This is an open access article distributed under the terms of the Creative Commons Attribution License CC-BY 4.0., which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Ho, T., Van, H., & Pham, T. (2022). DFT insight into the high stability of single Pt atoms on the CeO2(111) surface. Science and Technology Development Journal, 25(2), 2364-2372. https://doi.org/https://doi.org/10.32508/stdj.v25i2.3890


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