Nature of Atomically Dispersed Ru on Anatase TiO2: Revisiting Old

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Nature of Atomically Dispersed Ru on Anatase TiO: Revisiting Old Data Based on DFT Calculations Ho Viet Thang, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00977 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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The Journal of Physical Chemistry

Nature of Atomically Dispersed Ru on Anatase TiO2: Revisiting Old Data Based on DFT Calculations Ho Viet Thang, Gianfranco Pacchioni* Departimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy Version 27.2.2019 Abstract – In recent years, a lot of emphasis has been given to the concept of single atom catalyst. In this paper we show that some of these species have been observed long time ago. Active catalysts based on highly dispersed Ru species on the surface of TiO2 have been widely studied in the past and various techniques have been used to characterize the active species, but so far an atomistic assignment is missing. We have performed an extensive set of DFT+U calculations, considering various potential candidates for stable Ru single atom catalysts on the TiO2 (101) regular and (145) stepped surfaces. The sites include a Ru adatom, (Ru)ad, (RuO)ad, and (RuO2)ad units which are formally derived from the interaction of Ru with one or two hydroxyl groups, respectively, and Ru atoms substituting lattice Ti and O atoms. The properties of isolated Ru species have been compared with those of a supported Ru4 cluster to mimic formation of small Ru nanoparticles. Studying the dependence of the vibrational and binding properties of CO probe molecules on the various Ru/TiO2 species, and comparing the DFT results with temperature-dependent IR spectra, we come to the conclusion that the active species present on the surface of Ru/TiO2 catalysts, and observed since a long time, consists of single Ru atoms stabilized at the surface via interaction with a surface O probably originating from an OH group.

*

Corresponding author: [email protected] 1 ACS Paragon Plus Environment

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1. Introduction Many active catalysts consists of precious metals deposited on a binary oxide, such as Ru/TiO2,1,2 Ru/CeO2,3,4 Pt/TiO2,5,6 Au/TiO2,7,8 Au/ZrO29,10 etc. The catalyst activity and selectivity are connected to the formation of nanoparticles11 that can be activated via charge transfer from the support.12 However, in some cases highly dispersed forms of metal deposits can be achieved, depending on the preparation conditions. In the end, this may lead to atomically dispersed species on the surface of the support. The term single atom catalyst (SAC) has been recently coined to identify catalysts where the active site consists of an heteroatom stabilized on a support,13-17 although these species are known since a long time and, perhaps, named differently. Clearly, individual metal atoms interact with the surrounding, and their activity is the result of the atom electronic structure and of the modifications induced by the interaction with the support. In this respect, there are no “single atoms catalysts”, but rather single atom active sites. A necessary prerequisite for SAC to be technologically relevant is to exhibit a high thermal stability. In particular, under catalytic conditions the species have to be sufficiently stable to avoid sintering and aggregation, or diffusion into the bulk of the material. The notion of SAC is gaining momentum and has been proposed in connection to various supports, including metal oxides,18,19,20,21,22 metal alloys23,24,25,26 or hybrid materials.27,28,29 However, as we mentioned above, a throughout analysis of the existing literature shows that isolated atomic species are present in many catalysts studied in the past, only their nature has not been recognized or simply no particular emphasis was given to the “single atom catalyst” nature of the species. In this context, one of the metal species that has received more attention is Ru. The nature and the catalytic activity of highly dispersed Ru species has been studied on various supports, including CeO2,30 defective graphene31,32, layered double hydroxides,33 and anatase TiO2.34,35,36 Identifying unambiguously the location, coordination, and charge state of SAC with experimental tools is challenging due to the elusive nature of the species. In this work we concentrate on the Ru/TiO2 catalyst.6,37 The first studies on this system date back to the 1990s, but the system has attracted interest even in recent years. Most of these studies point to a similar nature of the supported species. For instance, adsorption of CO on Ru/TiO2 reduced at 573 K produces features at 2138, 2075, 2049, and 1990 cm-1, plus some features at lower frequencies.38 Similar IR bands have been reported in other studies indicating the formantion of a stable species on the surface of anatase.39-47 The discussion of these bands 2 ACS Paragon Plus Environment

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has been interpreted in terms of formation of well-dispersed TiO2/Ru+(CO)x (with x  2) complexes via reaction involving zero-valent Ru, CO and surface hydroxyl groups. The precise nature of the complexes, however, remains undefined. Theory can provide an useful complement to experiment, as it allows to explore a variety of possible situations and to simulate properties that can then be directly measured. A few succesfull cases of this approach have been recently reported, including some from our group. In particular, we have shed light on the nature of SAC consisting of Ru anchored on ZrO248 and of Pt on anatase TiO2 surfaces.49 The work was based on a combination of DFT calculations with thermal programmed desorption (TPD) and Fourier-Transform Infra-Red (FT-IR) measurements of adsorbed CO probe molecules. The nature of Ru single atom on ZrO248 is different from that of Pt on TiO2.49 While on ZrO2 Ru is stabilized in form of a (RuO)ad unit, formally derived from the interaction of Ru with an OH group on the ZrO2 surface, atomic Pt on TiO2 anatase is present as (PtO2)ad, derived from the interaction of a Pt atom with two OH groups, and an a lattice O2c atom. 49 In order to identify the nature of atomic Ru species on the TiO2 (101) surface, DFT+U calculations have been carried out. We have analyzed a Ru ad-atom, (Ru)ad, the species which are obtained from the interaction of Ru with hydroxyl groups, (RuO)ad and (RuO2)ad, and substitutional Ru to Ti and O lattice sites. For comparison, Ru4 sub-nanometer clusters deposited on TiO2 were also considered. CO probe molecules have been widely used to characterize the electronic properties and active sites on metals, oxides, supported catalysts, etc.48,49 In fact, CO is very sensitive to the electronic structure and the local environment. By using both frequency and adsorption energy of CO and comparing to literature data,38,45,48 we propose that (RuO)ad species are excellent candidates for the stable Ru SAC anchored on TiO2. The paper is organized as follows. In Section 2 we provide the computational details. Section 3, is divided in 3.1, dedicated to Ru atoms stabilized at the (101) TiO2 surface and the properties of adsorbed CO, and Section 3.2 where the same analysis is extended to step sites of anatase. Section 3.3 analyzes the results for a small Ru4/TiO2 cluster, while a general discussion and comparison with existing IR data are reported in the concluding Section. 2. Computational Methods 3 ACS Paragon Plus Environment


Spin-polarized density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).50 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional51 was used to treat exchange-correlation. The Projector Augmented Wave (PAW) 52,53 method was adopted to describe the core electrons while the valence electrons explicitly treated are Ti (3s2, 3p6, 3d2, 4s2), Ru (4p6, 4d7, 5s1), O (2s2, 2p4) and C (2s2, 2p2). The self-interaction problem typical of DFT was partly compensated by following the DFT+U approach as suggested by Dudarev at al.54 Here, an Hubbard’s parameter U54,55 was empirically set to 3 eV for the Ti 3d states.36 A plane wave basis set with cutoff energy of 400 eV was adopted, and all the calculation were done at the gamma point only. The structures were optimized, with all atoms relaxed until the ionic forces are smaller than 0.01 eV/Å. The regular TiO2 (101) and the stepped TiO2 (145) anatase surfaces were modeled by a five-atomic layers slab, with a (3x1) supercell for the surface, (Ti60O120), and a (2x1) supercell for the step, (Ti88O176). A vacuum region of more than 15 Å was used to avoid the interaction between two neighboring slabs. Further details can be found in Ref. 56. On the TiO2 (101) surface, there are two types of Ti cations, a 5-fold coordinated (Ti5c) and a 6-fold coordinated Ti (T6c), and two types of O anion, 2-fold coordinated (O2c) and 3-fold coordinated O sites (O3c), Figure S1. The adsorption energies of CO molecules, in eV, were calculated as the difference between the energy of the isolated species and of the adsorption complexes (positive values indicate stable adsorbates). The CO stretching frequencies were computed within the harmonic approximation, in which CO and its nearest neighboring atoms were included in the calculation. The bond length (1.144 Å) and stretching frequency (2125 cm-1) of gas-phase CO are obtained at the PBE level. To facilitate comparison with experimental IR frequencies, a scaling factor  = 2143/2125 = 1.0085 has been applied to all calculated frequencies, where 2143 cm-1 is the experimental frequency of CO in gas phase.57 The effective charge of Ru atoms was evaluated by using the Bader method.58-61

3. Results As we mentioned above, five structural models were investigated: Ru adatom, (Ru)ad; Ru-O and O-Ru-O units, (RuO)ad and (RuO2)ad; Ru replacing a lattice Ti site, (Ru)subTi, or a lattice O site, 4 ACS Paragon Plus Environment

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(Ru)subO. The same definition of the formal charge of Ru was applied as in our previous study of Ru/ZrO2.48 Since the oxidation state of lattice O atoms is –II, the formal charge of Ru is only defined by the extra O atoms formally belonging to surface hydroxyl groups that have reacted with Ru. In this context, we assigned oxidation state 0 to (Ru)ad, +I to (RuO)ad, and +II to (RuO2)ad, while (Ru)subTi and (Ru)subO have the same oxidation state of the lattice atoms they replace, +IV and -II, respectively. 3.1. Ru/TiO2 (101) regular surface In the most stable configuration, the (Ru)ad species resides in the center of the hollow site of the TiO2 (101) surface where is strongly bound to two O2c and two O3c atoms,34,36,62 Figure S2a. The corresponding distances, Ru - O2c and Ru - O3c are 2.03 and 2.16 Å. The adsorption energy of Ru, 3.84 eV, in line with previous DFT reports.62 This is a quite strong adsorption energy for a single atom on a regular, defect free, oxide surface.63 If a Ru atom interacts with an OH group on the TiO2 (101) surface, this can lead to a condensation process leaving a (RuO)ad species plus ½ H2 in the gas phase. In this process, Ru reduces the proton to neutral hydrogen, and is oxidized to a +I oxidation state. The (RuO)ad unit is strongly anchored to the TiO2 (101) surface with an adsorption energy of 3.76 eV (computed with respect to a RuO gas-phase molecule). In this complex, Ru is coordinated to three O atoms, one is the O from the OH group, and two are lattice O2c atoms. The Ru-O distances are 1.927 and 2.120 Å, respectively, Figure S2b. The interaction of Ru with two OH groups is associated with the release of an H2 molecule. In the resulting (RuO2)ad species Ru is anchored to the surface by four O atoms, two from the OH groups, one O2c and one O3c atom from the surface. The corresponding distances are 1.812, 1.922 and 2.003 Å, respectively, Figure S2c. The RuO2 unit is bound by an adsorption energy of 2.46 eV, more than one eV smaller than the RuO unit (3.76 eV) and a Ru adatom (3.84 eV). Also in this case, the reference is an isolated RuO2 molecule. For the (Ru)subTi species, the Ru atom can replace Ti5c, Figure S2d, or Ti6c atoms. In these structures, Ru takes the place of Ti without important changes in the geometry, the Ru-O bond lengths being almost the same as the corresponding Ti-O ones (the changes are less than 3 %).

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In the last cases, (Ru)subO2c and (Ru)subO3c species, the Ru atom is simply substituting O2c, and O3c atoms, respectively. Due to the larger atomic size of Ru compared to O, Ru is displaced from the original O site. This results in longer Ru-Ti5c and Ru-Ti6c bonds (2.28 Å and 2.34 Å) than the corresponding O-Ti5c and O-Ti6c ones ( 1.85 Å and 1.88 Å), respectively, Figure S2e. The same is true for (Ru)subO3c species where Ru-Ti5c and Ru-Ti6c bond lengths are prolonged from 2.05 Å to 2.46 Å, and from 1.97 to 2.35 Å, respectively, Figure S2f. 3.1.1. Ru(CO)/TiO2 (101) On the bare TiO2 (101) surface CO is weakly bound to a lattice Ti5c atom with an adsorption energy of 0.34 eV, Table 1; the CO stretching frequency is shifted by +44 cm-1 compared to gasphase CO,56 in close agreement with experimental measurements.64 This is due to the electrostatic interaction of the CO multiple moments with the local electric field generated by the Ti4+ cation.65 As a consequence, there is a shortening of the CO bond length from 1.144 Å (gasphase) to 1.138 Å (surface), Table 1. When a CO is adsorbed on the (Ru)ad site, Figure 1a, the adsorption energy (2.36 eV) is much stronger than on the Ti5c sites (0.34 eV), Table 1. The CO bonding to (Ruad) is also stronger than on the analogous surface complex formed on the ZrO2 surface (1.86 eV),48 indicating a non-negligible role of the support. The interaction is dominated by the -back donation mechanism from the Ru 4d levels to the antibonding CO 2* states, resulting in an elongation of the CO bond length to 1.174 Å and a large red-shift of the CO frequency (-180 cm1)

with respect to gas-phase CO, Table 1.


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Figure 1. Side view (left) and top view (right) of CO adsorption structures on a) (Ru)ad, b) (RuO)ad, c) (RuO2)ad, d) (Ru)subTi5c, e) (Ru)subO2c, f) (Ru)subO3c on the TiO2 (101) surface. Ti, O, Ru and C are blue, red, green and gold spheres, respectively. Also for CO adsorbed on (RuO)ads species, Figure 1b, the adsorption energy is large, 2.55 eV, even larger, by 0.5 eV, than on the analogous (RuO)ad species formed on ZrO2 (2.05 eV).48 The Bader charge of Ru in (RuO)ad is +1.25 |e| and reflects the positive oxidation state of the atom. Not surprisingly, this results in a smaller charge transfer to CO than in the (Ru)ad case. The CO bond length (1.169 Å) is elongated compared to free CO and the CO stretching frequency is red-shifted, (-144 cm-1), but less than on (Ru)ad (-180 cm-1), Table 1. Notice that despite the 7 ACS Paragon Plus Environment


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positive oxidation state of Ru in (RuO)ad, the bonding mechanism is dominated by the Ru to CO charge transfer mechanism, sign that a significant amount of electron density is present on the (RuO)ad species.

Table 1. CO adsorption on single Ru atom species deposited on TiO2 (101) surface: formal

oxidation state of Ru adsorption site, adsorption energy, Eads (eV), C-O bond length, RCO (Å), CRu distance, RCRu (Å), Bader charge of Ru, Q (|e|), harmonic CO stretching frequency, e (cm-1) and frequency shift with respect to the gas-phase, e (cm-1) (scaled frequencies). e

e

(cm-1)

(cm-1)

-

2169

+44

1.831

+0.66

1963

-180

1.169

1.831

+1.25

1999

-144

2.06

1.161

1.851

+1.57

2046

(Ru)subTi5c(Ti59O120)

Ru+IV

1.45

1.151

1.918

+1.69

2089

-97 -54

(Ru)subO2c(Ti60O119)

Ru-II

3.41

1.177

1.826

-0.10

1948

-195


Ru-II

3.22

1.176

1.820

-0.10

1949

-194

2.36

1.170

1.825

+0.68

2020

-123

2.21

1.170

1.825

+0.68

1955

-188

2.55

1.161

1.848

+1.27

2074

-69

1.76

1.161

1.848

+1.27

2010

-133

3.41

1.163

1.833

+0.09

2010

-133

1.04

1.173

1.951

+0.09

1952

-191

2.36

1.164

1.844

+0.71

2037

-106

2.21

1.168

1.844

+0.71

1976

-167

0.77

1.168

1.975

+0.71

1968

-175

2.55

1.155

1.874

+1.27

2125

-18

CO

Eads

RCO

RCRu

Site

(eV)

(Å)

(Å)

(TiO2)60a

Ti+IV

0.34

1.138

2.467

(Ru)ad(TiO2)60

Ru0

2.36

1.174

(RuO)ad(TiO2)60

Ru+I

2.55

(RuO2)ads(TiO2)60

Ru+II

System

Q (|e|)

Single CO molecule

Two CO molecules (Ru1)ads(TiO2)60

Ru0

(RuO)ads(TiO2)60

Ru+I


Ru-II

Three CO molecules (Ru)ad(TiO2)60

(RuO)ad(TiO2)60

Ru0

Ru+I


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(Ru)subO2c(Ti60O119) a

Ru-II

1.76

1.155

1.875

+1.27

2051

-92

1.78

1.158

1.909

+1.27

2028

-115

3.41

1.160

1.859

+0.19

2031

-112

1.04

1.173

1.881

+0.19

1949

-194

1.31

1.192

1.932

+0.19

1806

-337

Ref. 56

Next we consider a CO molecule bound to (RuO2)ad, Figure 1c. Also in this case the adsorption energy is substantial, Eads = 2.06 eV, and the stretching frequency exhibits a large redshift,  = -97 cm-1. These data are consistent with the higher oxidation state of Ru in (RuO2)ad species, resulting in lower charge transfer to CO (Bader charge of (RuO2)ad +1.57 |e| compared to (RuO)ad, +1.25 |e| and (Ru)ad, +0.66 |e|). Once more, notice that to a formal +II oxidation state for Ru corresponds a CO red-shift in the vibrational frequency of about 100 cm-1. Next, the incorporation of Ru into the lattice replacing Ti5c and Ti6c sites was considered. For the Ti6c case we observed the spontaneous formation of CO2 upon CO adsorption, due to the interaction with a lattice O. This indicates an unstable structure for Ru atom placed at this site. The system can also be seen as a Ru-doped TiO2 surface, where doping results in the easier formation of oxygen vacancies.66 Therefore, we only discuss for the case of Ru incorporated at Ti5c site, Figure S2d. We assigned to this structure an oxidation state +IV, the same of Ti; of course, the Bader charge is much smaller (+1.66 |e|), Table 1. On this site, (Ru)subTi5c, CO has a moderate binding energy (1.45 eV) and small red-shift (-54 cm-1), Table 1 and Figure 1d. Even the formal +IV oxidation state of Ru substitutional to Ti in TiO2 does not result in a blue shift of the CO frequency, but in a red-shift, due to the presence of residual electrons in the Ru 4d orbitals. The last case is that of Ru replacing an O atom (which is equivalent to a Ru atom adsorbed on a neutral oxygen vacancy). Here, we considered two positions, O2c, Figure S2e, and O3c, Figure S2f. On both sites the CO adsorption properties are identical, Table 1. In particular, we observe a very strong adsorption energy (3.24 eV) and a very large red-shift (-195 cm-1). This is consistent with a negative charge on Ru (formal charge –II, Bader charge -0.1 |e|). The large charge transfer from Ru 4d orbitals to CO leads to a significant elongation of the CO bond



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length, 1.177 Å, and a reduction of the CO-Ru distance, 1.820 Å, Table 1 (see also Figure 1e for (Ru)subO2c and Figure 1f for (Ru)subO3c). 3.1.2. Ru(CO)2/TiO2 (101) The formation of stable geminal CO complexes has been observed only on (Ru)ad, (RuO)ad, and Ru substitutional to a lattice oxygen atom, (Ru)subO2c. In the other cases, the second CO molecule interacts with other O atoms and forms CO2. For (RuO2)ad the second CO spontaneously interacts with one of the two extra O atoms, while for (Ru)subTi5c the reaction involves a lattice oxygen. On the (Ru)ad species, the second CO molecule is strongly bound, by 2.21 eV, a value comparable to that of the first CO molecule (2.36 eV). In a TPD experiment, the first and the second CO molecule would desorb more or less at the same temperature, giving rise to a single peak in TPD. Using a Redhead Equation,67 𝐸 𝑅𝑇𝑝

= ln

𝜈𝑇𝑝

( ) ― 3.64

(1)

𝛽

assuming a rate constant of ν = 1015 s-1,68,69 and using a heating rate β = 1 K/s, we can estimate a temperature of the desorption peak, Tp, of > 700 K (large error bars are associated to these values due to the assumption on the rate constant and the errors in DFT binding energies). The two CO molecules on (Ru)ad assume a symmetric configuration, Figure 2a, with a C-Ru distance of 1.825 Å and a CO bond length of 1.170 Å, Table 1. The symmetric and antisymmetric stretching frequencies are well separated and expected at 2020 and 1955 cm-1. The corresponding redshifts of CO frequency are thus -123 cm-1 and -188 cm-1. In case of geminal CO molecules on (RuO)ads, the second CO is bound to (RuO)ad with a lower adsorption energy than the first one, 1.76 eV versus 2.55 eV. This suggests that in a TPD experiment the two CO molecules will desorb at distinct temperatures, giving rise to a double peak in the TPD profile at about 520 and 670 K. Also on (RuO)ads we observe a symmetric geminal CO configuration, Figure 2b, with C-Ru distance of 1.848 Å and CO bond length of 1.161 Å, Table 1. The geminal complex results in two well separated CO frequencies, at 2074 cm-1 and 2010 cm-1. Also in this case, despite the presence of two CO molecules and a positive oxidation state of the Ru atom, we observe red-shifts of both the symmetric and antisymmetric stretching modes, -69 cm-1 and -133 cm-1, Table 1. 10 ACS Paragon Plus Environment

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The situation is different on (Ru)subO2c, where the two adsorbed CO molecules are no longer symmetric, Figure 2c. This results in different C-O (1.163 Å and 1.173 Å) and C-Ru (1.951 Å and 1.833 Å) distances, Table 1. Another difference is that while the first CO molecule is strongly bound, by 3.41 eV, the second has binding of 1.04 eV only. Very large red-shifts of the CO frequency were observed, -133 cm-1 and -191 cm-1, respectively, Table 1.

Figure 2. Side view (left) and top view (right) of geminal CO structures on a) (Ru)ad, b) (RuO)ad, c) (Ru)subO2c on the TiO2 (101) surface. Ti, O, Ru and C are blue, red, green and gold spheres, respectively.

3.1.3. Ru(CO)3/TiO2 (101) The analysis of three CO molecules on a Ru single atom species is restricted to the cases where geminal CO complexes are stable, i.e. (Ru)ad, (RuO)ad, and (Ru)subO2c species. On (Ru)ad, the third CO molecule is bound with an adsorption energy of 0.77 eV. This is much smaller than the binding energy of the first (2.36 eV) and the second CO molecules (2.21 eV). Thus, only at low temperatures ( 2)

Terrace Step

2125 2125

RuO(CO)3

Ru+(CO)x (x  2) RuO(CO)3 RuO(CO)2 2074, 2051 2063, 2052

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Ru+(CO)x (x  2) RuO(CO)3 RuO(CO)2 2028, 2010 2042, 2006

Ru+CO RuO(CO) 1999 2018

While a general consensus exist about the isolated nature of the Ru species on the surface of TiO2, a precise atomistic characterization is lacking. In the above sections we have reported the results of CO adsorption on various potential models, including (Ru)ad, (RuO)ad, and (RuO2)ad units, and substitutional Ru to lattice Ti and O sites. The following conclusions can be drawn: 1) Similar to Ru/ZrO2,48 a large red-shift of CO stretching frequencies is always observed; this red-shift remains also when the Ru atom assumes a positive oxidation state. This indicates the presence of a sufficient electron density on Ru which is transferred to the anti-bonding 2* CO orbital. 2) Although (Ru)ad is strongly bound to TiO2, 3.84 eV on the terrace and 3.59 eV at step sites, this is unlikely as the atomic Ru species present on TiO2. In fact, Ru adatoms on TiO2 have high mobility due to the low diffusion barriers of precious metal atoms on the TiO2 (101) surface;74 low diffusion barriers have also been computed for Ru adatoms on ZrO2.75 The computed frequency shifts of adsorbed CO are large and negative, and inconsistent with the FTIR spectra. Of course, even larger redshifts are found when CO binds to small Ru clusters or nanoparticles due to the preference to bind in bridge sites, giving rise to more pronounced C-O bond elongation and change in vibrational frequency. 3) Ru atoms substituting lattice Ti and O atoms (or, stated differently, adsorbed on a Ti or an O vacancy) fulfill the condition for high thermal stability of the Ru single atom catalyst. However, there are various elements that show that this is not a particularly stable species: (Ru)subTi can bind only one CO molecule and does not form geminal complexes, but rather reacts to form spontaneously CO2. When Ru replaces an O atom, (Ru)subO, the vibrational red-shift of adsorbed CO molecules are much larger than experimentally observed.38,45 4) (RuO)ad species are promising candidates for atomic Ru anchored on TiO2. This is based on the adsorption of Ru to a surface O, formally deriving from the interaction between Ru and an 22 ACS Paragon Plus Environment

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OH group. The strong CO adsorption to (RuO)ad, and the CO frequency shifts on terrace (-18, 92, -115 cm-1) and on step sites (-18, -91, -101 cm-1), are in close agreement with experimental data. For instance, a recent study of Ru/ZrO248 where the surface species consists of (RuO)ad units, shows very similar IR bands (-22, -82, -120 cm-1) as for the present Ru/TiO2 system. Staying to the titania surface, the measured red-shifts of CO on 0.5% Ru/TiO2 (-5, -68, -94 cm-1), assigned to multiple CO molecules adsorbed on well dispersed Ru atoms on TiO2 anatase at T < 423 K,38 are consistent with the calculations. Similar values, -1, -59, -107 cm-1, have been attributed to (Ru)n+(CO)3 complexes on TiO2 anatase45 as well as in other studies, see Table 4. When the temperature is raised above 473 K, the band at 2138 cm-1 disappears, and two peaks remain (red-shift -94 and -153 cm-1). Again, this is in agreement with our calculated results for geminal CO on the terrace (-69, -133 cm-1) and step (-80, -137 cm-1). Furthermore, the CO desorption energy deduced from the temperature dependence of the IR spectra (see Fig. 7 in ref. 38), about 1.5-1.6 eV, is consistent with the computed binding energy of the third CO molecule in (RuO)ad(CO)3, 1.6-1.8 eV, Tables 1 and 2. Thus, we suggest that the observed features are due to (RuO)ad(CO)3 species formed at various locations of the surface (but with similar spectral features). At T > 573 K, most of (RuO)ad(CO)2 converts into (RuO)ad(CO) where CO is linearly bound to (RuO)ad, with predicted CO frequency around 1999-2018 cm-1 (DFT) and measured bands at 2000-1990 cm-1 (IR). 6) Finally, also (RuO2)ad species have been considered. However, on this complex the second CO molecule preferentially reacts with an extra O atom, forming CO2 which desorbs in gas-phase. Thus, our models do not support the formation of geminal CO on (RuO2)ad species at variance with the experimental observations which show the formation of this complex on Ru/TiO2 (see above) and on Ru/ZrO2.48 In conclusion, the present analysis of the possible structure assumed by a single Ru atom stabilized on the anatase TiO2 surface, combined with experimental information available on this system and on similar systems, identifies a (RuO)ad species as the most likely species that forms on this oxide under the experimental conditions adopted. We also notice that very similar properties are expected for Ru atoms formed on the regular terraces or on the steps of anatase TiO2, so that more than distinct spectroscopic features, a broadening of the corresponding signals is expected due to the heterogeneity of the surface complexes. 23 ACS Paragon Plus Environment


Conflicts of interest There are no conflicts of interest to declare Supporting Information Figure S1. Side view and top view of anatase TiO2 (101) surface Figure S2. Side view and top view of RuOx species on terrace TiO2 (101) surface Figure S3. Side view and top view of RuOx species on step TiO2 (145) surface

Acknowledgments This work has been supported by the Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS "Solar driven chemistry: new materials for photo– and electro–catalysis". We acknowledge the CINECA facility for the availability of high-performance computing resources.


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Nature of Atomically Dispersed Ru on Anatase TiO2: Revisiting Old

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