- Email: [email protected]
The effect of oxygen vacancy at CO oxidation on anatase (001)-supported single-Au catalyst
Materials Chemistry and Physics 240 (2020) 122291
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The effect of oxygen vacancy at CO oxidation on anatase (001)-supported single-Au catalyst Lei Li a, *, Wenshi Li a, Canyan Zhu a, Ling-Feng Mao b, ** a b
Institute of Intelligent Structure and System & School of Electronics and Information Engineering, Soochow University, Suzhou, 215006, People’s Republic of China School of Computer and Communication Engineering, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China
H I G H L I G H T S
� A stable carbonate occurs at the site of extra O vacancy. � O vacancy dimer reduces the catalytical performance of CO oxidation. � Synergistic effects exist between a negatively charged Au atom and a Ti3þ ion. A R T I C L E I N F O
A B S T R A C T
Keywords: Gold adatom Anatase TiO2(001) surface O vacancy CO oxidation Adsorption energy
The catalytic performance of CO oxidation on the TiO2-supported gold catalysts would be affected by the O vacancies on a TiO2 surface. First-principle studies here are focused on how the O vacancy addresses the per formance of single atom catalysts on CO oxidation. Three possible configurations of gold adatoms and O va cancies on anatase TiO2(001) support are considered as follows: A gold adatom above an O vacancy, a gold adatom or two gold adatoms above an O vacancy dimer. The O vacancy dimer favors the binding of negatively charged gold adatoms on anatase TiO2(001) support. However, the dual gold adatoms slightly increase the barrier energy in CO oxidation through the Au-assisted Mars-van Krevelen mechanism. The formation of a stable carbonate on the initial-existing O vacancy reveals that the extra O vacancy deteriorates the catalytical performance of CO oxidation. In final, the high catalytic performances of TiO2/Au catalyst are attributed to the synergistic effects between a negatively charged Au adatom and a lower-coordinated Ti3þ ion.
1. Introduction Recently, CO oxidation on single-atom catalysts has been extensively investigated in the heterogeneous catalysis, since the first single-atom catalyst Pt1/FeOx was proposed by Qiao et al. [1,2]. For the Pt1/FeOx catalyst, one oxygen vacancy formed in the pre-reduction process opens the possible channels for the catalytic cycle of CO oxidation [1]. The oxygen vacancy produced by a CO adsorbate abstracting the lattice oxygen atom on the surface acts as the medium to complete the reaction cycle in the Pt1/CeO2 single-atom catalyst [3]. Thereby, to understand how an oxygen vacancy impacts on the activity of a single atom catalyst is of fundamental importance to know the chemical reactivity in CO oxidation. In general, these oxygen vacancies are easily produced in a bulk or a surface of the transition-metal oxides, e.g., TiO2, WO3, NiO, Fe2O3, and
CeO2. In this case, it thus introduces some defective electronic states in the gap below the conduction band minimum. The unpaired excess electrons left behind by these oxygen vacancies would modify the electronic properties of the oxide. For example, the transition between the insulator and conductor would occur due to the mobility of the larger amounts of the oxygen vacancies in rutile TiO2 under the higher electric field [4,5]. The occurrence of oxygen vacancies in TiO2 always leads to the reduction of the Ti4þions to the Ti3þ ions (Ti4þ(3 d0) þ e → Ti3þ(3 d1)), where the partial unpaired electrons are localized. Similar situation exists in CeO2 and ZrO2. These electrons prefer to localize at the lower-coordinated Zr3þ and Ce3þ ions, of which the magnetic properties were detected by electron paramagnetic resonance [6–8]. These unpaired excess electrons localized at Ti3þ ions nearest or nearer neighbor to the oxygen vacancies would reconstruct the surface by the atomic relaxation and certainly modify the properties of the surface.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Li), [email protected] (L.-F. Mao). https://doi.org/10.1016/j.matchemphys.2019.122291 Received 29 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 Available online 10 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Recent experimental and theoretical results show that the Au clusters were strongly anchored on a MgO support with F-centers and such that promoted the catalytic activity on CO oxidation [9,10]. The reduced Ti atoms enhanced the strength of the bonding to Au atoms on the highly reduced TiO2-x support and determined the shape and electronic prop erties of Au atoms [11–13]. Zeng et al. revealed that the activation of CO2 was evidently enhanced with the increasing of oxygen vacancy concentration on the oxygen vacancy-rich ZnO nanosheets by the H2 plasma treatment [14]. Yoon et al. emphasized the importance of oxy gen vacancies in the oxide support for the high rate of CO oxidation [6]. Pre-reduced solid catalysts based on ZrO2 nanoparticles displayed a better activity in the transformation of biomass into fuels [15,16]. In addition, reduced ZrO2-x was shown to be photo-catalytically active in H2 production under solar light while the stoichiometric ZrO2 was inactive [17]. For a single-atom catalyst, single Au adatom would be deposited on the surface with various configurations. Accordingly, the possible interaction between the single Au adatom and the oxygen vacancy or the reduced Ti3þ ions would be concerned, which results in the electrons redistribution on the surface and further leads to the modification of the CO catalytical properties. Herein, we perform the first-principle calcu lations on the basis of density functional theory corrected by on-site Coulomb interactions (DFT þ U) to investigate the oxygen vacancy effort at the catalytic properties of CO oxidation on the anatase TiO2(001)-supported single-gold catalyst. As reported by Wan et al., one Au adatom locates above one oxygen vacancy and binds with two nearest-neighbor Ti ions [18]. On an actual surface in experiments, it should have some oxygen vacancies unfilled by the Au adatoms, thus the correlation between the oxygen vacancy and Au adatom would be concerned. Besides, the dual oxygen vacancies are produced on a sur face, and then the Au dimer would seal these dual oxygen vacancies. Hence, we would compare the electronic structures on our three pre-built catalysts and the subsequent energy evolution in catalytic cy cles of CO oxidation. As a prediction of the behavior of Au adatoms on the defective anatase TiO2(001) surface, our aims are essentially to provide a primary evaluation on the effect of oxygen vacancy on the anatase TiO2(001)-supported single-gold catalyst.
with the recent work published by Wan et al., we present results of DFT þ U-based calculation on a (3 � 3) unreconstructed anatase TiO2(001) surface [18]. An anatase TiO2(001) surface in five-atomic layers was thus selected to be constructed by a total of 125 atoms in 3 � 3 supercell. A vacuum layer in 15 Å was used for separating the slabs along the perpendicular Z-direction. During the slab relaxation (also defined as geometry optimization) and the subsequent adsorption of the Au atoms or a molecule CO, the bottom-two layers were always fixed to simulate the bulk properties, while the upper-three layers and the ad sorbates (Au and CO) were completely relaxed. An oxygen vacancy on a TiO2(001) surface marked by a blue ball in Fig. 1(a) was generated from a twofold coordinated oxygen atom and later replaced by the Au atom labeled by a yellow ball in Fig. 1(c). The combination of a single Au atom and two nearest-neighbor Ti ions forms a Ti–Au–Ti structure on the defective TiO2(001) surface [29], as the structure shown in Fig. 1(c), which is defined as TiO2/Au. To construct an oxygen vacancy dimer on a surface, a second oxygen vacancy was created on another twofold coordinated oxygen atom adjacent to the first oxygen vacancy, as shown in Fig. 1(b). The defect formation en ergies for a single oxygen vacancy and dimer are 3.6 eV and 4.03 eV, respectively, of which for the single oxygen vacancy consisting with the value (3.59 eV) as reported by Tang et al. [34] and however slightly smaller than that of 3.75 eV reported by Ammal and Heyden [35]. One Au atom or dual Au atoms are respectively located above one oxygen vacancy or one oxygen vacancy dimer, as shown in Fig. 1(d) or 1(e), thereby termed as TiO2-x/Au or TiO2/2Au, respectively. Compared with the case of TiO2/Au, there is one additional oxygen vacancy on a TiO2-x/Au surface or one additional Au adatom on a TiO2/2Au surface, of which the binding energies of an Au adatom are 4.31 eV and 4.32 eV, much lower than that of 3.95 eV in TiO2/Au. This difference shows us that the additional oxygen vacancies enhance the anchor strength of Au adatoms on a surface, consistence well with our previous work [24]. It should be noted that the additional oxygen vacancy in Ref. [24] were produced based on the structure of TiO2/Au, whereas the oxygen vacancies here were generated before the adsorption of an Au atom. In other words, the TiO2/Au model in Ref. [24] was firstly built, and then, the oxygen vacancy replaced the oxygen atoms adjacent to the gold adatom in TiO2/Au to produce the models of TiO2-x1-Au, TiO2-x2-Au, and TiO2-x3-Au. In this work, one and two oxygen vacancies were produced on the TiO2(001) surface, respectively; the Au atoms were later located on these surfaces to form TiO2/Au, TiO2-x/Au, and TiO2/2Au, respectively, as shown in Fig. 1. Accordingly, the difference of the model in this work to that in ref. 24 was the production order of the oxygen vacancy and the gold atom. The attractive forces between the dual Au adatoms in TiO2/2Au shorten the distance between them (3.048 Å) compared to the initial distance of 3.782 Å. However, the Au–Au bonding rarely occurs there, as shown in Fig. 2(c). Subsequent analysis of Bader charge reveal that the single Au adatom or the dual Au adatoms on TiO2/Au, TiO2-x/Au and TiO2/2Au are slightly negatively charged (Auδ , δ- ¼ 0.54 |e|, 0.57 | e| and 0.56/-0.48 |e|, respectively), which demonstrate the charge transfer from the oxide support to Au adatoms happens upon the adsorption of Au adatoms. The electron density differences in Fig. 2 indicate the Au adatoms obtain some partial electrons marked in yellow, suggesting the formation of directional Au–Ti covalent bonds, identical to that in Au clusters supported reduced TiO2 [36]. Here, following the works in Refs. [37,38], the electron density differences are evaluated within the formula Δρ1 ¼ ρ(TiO2/Au) – ρ(TiO2) – ρ(Au), where ρ(TiO2/Au), ρ(TiO2), and ρ(Au) are the electron density of the whole model, the sup port, and the Au adatom, respectively. Labeled by the black-down arrow in Fig. 3, the overlaps of Au-5d electronic states (marked by red and green-shade) and 3d electronic states of Ti40 in blue (also together with Ti43 in TiO2/2Au) illustrate the hybridization between Au adatom and Ti-ions on these surfaces. The definitions of Ti40 and Ti43 are found in Fig. 1(c). Obviously, some of the electrons are localized at a Ti3þ ion (Ti43) on the TiO2-x/Au surface in
2. Methodology All DFT calculations were performed via Vienna ab initio Simulation Package (VASP) [19–23]. The more basic sets of these calculations could be found in our previous work [24]. In this work, the computations on a molecule CO and O2 were performed in a unit cell at 10 Å � 10 Å � 10 Å at the Gamma-point. The bond lengths in our computational results at 1.14 Å in a molecule CO and 1.23 Å in a molecule O2 consist well with the experimental values of 1.13 Å [25] and 1.21 Å [26], respectively. To determine the location and energy of transition states (TS), the climbing image nudged elastic-band (CI-NEB) method with the convergence tolerance of 0.05 eV/Å was used, in which almost five images were set [27]. 3. Results and discussions 3.1. Au adatom Due to the unsaturated bonds on an anatase TiO2(001) surface, the gold atoms will be strongly adsorbed on an anatase TiO2(001) surface instead of on an anatase TiO2(101) surface [28]. Based on the experi mental conditions, unreconstructed and reconstructed (1 � 4) anatase TiO2(001) could be applied [29–32]. The unsaturated atoms on an surface—five-coordinated Ti ions and anatase TiO2(001) two-coordinated O ions—tend to strongly bind with Au atoms; mean while, the Ti–O bonds on an anatase TiO2(001) surface much weaker than that in bulk phase or on anatase TiO2(101) surface would poten tially break when bonding with Au atoms [33]. Herein, following up 2
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Fig. 1. Structural configuration of TiO2(001) surface with one oxygen vacancy in (a) and an oxygen vacancy dimer in (b); (c) TiO2/Au: One Au adatom above the oxygen vacancy in (a); (d) TiO2-x/Au: One Au adatom above one oxygen vacancy in (b); (e) TiO2/2Au: Two Au adatoms above the oxygen vacancy dimer in (b). Figures at side-view and top-view list on the top and below, respectively. Models in line-style and ball-style indicate the bottom-two fixed and top-three optimized layers, respectively. The atoms of Ti40, Ti41, Ti43, and Ti44 are marked by the numbers in white with background filled in green. Red: O; Grey: Ti; Yellow: Au; Blue: oxygen vacancy. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 2. Calculated electron density differences for Au adatoms in (a) TiO2/Au, (b) TiO2-x/Au, and (c) TiO2/2Au. Areas in yellow and green represent the accumulation and depletion of electrons, respectively. The cutoff values of the iso-surfaces in the electron density differences are set to 0.008 electrons/Å3. One Ti3þ ion (Ti3þ 43 ) with the localized electrons in yellow is defined by a black arrow. The subfigures in two different views are listed on left and right, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2(b), which has a lower positive charge (þ1.73 |e|) than that on a TiO2/Au surface (þ2.08 |e|) and a TiO2/2Au surface (þ1.90 |e|), consistent well with the sole presence of electronic state of Ti43 below the Fermi level as the blue-shade shown in Fig. 3(b). Although the binding energy of single Au adatom is higher in TiO2-x/ Au, the valley resulted from the formation of the second oxygen vacancy provides a possible path for the migration of this Au adatoms to the position above the adjacent oxygen vacancy. Fig. S1 confirm that a barrier energy of 1.08 eV prevents the diffusion of this single Au atom and it would be firmly anchored in the bonding of Ti–Au–Ti. The larger
barrier energy is derived from the bonding between the threefold co ordinated oxygen ions (marked in black-dash circle) and the single Au adatom, as the transition state (TS) shown in Fig. S1. 3.2. First CO oxidation The total oxidation procedures of a molecule CO through the Auassisted Mars-van Krevelen (MvK) mechanism could be summarized [39,40]:
3
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Fig. 3. Partial density of states in (a) TiO2/Au, (b) TiO2-x/Au, and (c) TiO2/2Au. The electronic density of state of the top surfaces, Au adatoms, Ti43 ions, and Ti40 ions are listed from top to bottom, respectively. The down-black arrows indicate the overlap of electronic density of states of Au adatoms and Ti40 ion or Ti43 ion. All energies are referred to the Fermi level defined by the dash-lines.
CO þ Au/TiO2 → CO2 þ Au/TiO2-x
(1)
O2 þ Au/TiO2-x → Au/TiO2 þ Oad
(2)
CO þ Au/TiO2 þ Oad → Au/TiO2 þ CO2
(3)
on the surface, which subsequently leads to the reduction of the Au/TiO2 support; Pathway (2–3) is a molecular oxygen heals the oxygen vacancy and then react with the molecule CO to generate the second molecule CO2 [41]. These prepared surfaces with Au adatoms are ready to be discussed
Pathway (1) is a reaction of a molecule CO with a lattice oxygen ion
Fig. 4. Reaction pathway (1) for the formation of the first CO2 on TiO2/Au, TiO2-x/Au, and TiO2/2Au catalysts, starting from the support in Fig. 1(c–d). The detailed structures shown on both sides and at the bottom correspond to the structures at each step in the reaction pathway. 4
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
on a molecule CO oxidation in pathway (1), as summarized in Fig. 4. The relatively larger adsorption energy (defined as Ead) of a molecule CO at 0.55 eV occurs on a TiO2/2Au surface. The higher activation barrier energy (defined as Eac: 0.99 eV) for the reaction of COad þ OS → CO2 þ Ovac (step TS and III) indicates a molecule CO is converted to a molecule CO2 over a TiO2/2Au surface, which happens at an elevated temperature by contrast to that on other surfaces. At step III, the CO adsorbate attracts a lattice oxygen specie adjacent to one Au adatom or dual Au adatoms to complete the CO oxidation reaction in pathway (1). In the case of TiO2-x/Au with one oxygen vacancy adjacent to the Au adatom, both the adsorption energy (Ead: 0.38 eV) and activation barrier energy (Eac: 0.80 eV) determine that this oxygen vacancy results in a weakly negative impact on the CO poisoning and the oxidization to CO2, in compared to those in TiO2/2Au surface (Ead: 0.32 eV and Eac: 0.70 eV). Moreover, these evitable differences in the activation barrier energies are almost determined by the adsorption energy of a molecule CO in step II. The higher adsorption energies of a molecule CO lead to the higher activation barrier energy [42]. The structural configurations also clarify that the binding properties of a molecule CO to the unsatu rated Ti ions and Au adatoms in step II result in the known adsorption energy differences above. The configuration of Ti–Au–Ti bonds still lo cates in TiO2/Au, as well as that in TiO2-x/2Au, whereas the new bonds of Ti–Au–C–Ti is produced in TiO2/2Au. However, almost the same bonds length of C–OS (1.201 Å) and C–Ti (2.132 Å) are found on these three structures at step TS. The analysis of Bader charge illustrate that the single Au adatom or the dual Au adatoms on the TiO2/Au/CO, TiO2-x/Au/CO and TiO2/2Au/ CO are still negative but with less charges (δ ¼ 0.37 |e|, 0.40 |e| and 0.21/-0.47 |e|, respectively), which reveal the charge transfer from one Au adatom to the CO adsorbate or oxide support upon the adsorp tion of one molecule CO. Further Bader charges of the CO adsorbate would be clarified to show where the detailed charges are transferred from the Au adatoms in final. Thereby these negative charged COδ on TiO2/Au/CO ( 0.13 |e|), TiO2-x/Au/CO ( 0.12 |e|) and TiO2/2Au/CO
( 0.23 |e|) reveal the Au adatoms donate more electrons to CO 2π* antibonding orbitals than to the oxide supports. This also corresponds to the elongated C–O bond lengths in 1.155 Å, 1.164 Å, and 1.175 Å on the TiO2/Au/CO, TiO2-x/Au/CO and TiO2/2Au/CO support with a CO adsorbate, respectively. The elongated C–O bond length and negative charged COδ eventually reveal the contribution of π-back-donation [43, 44]. In addition, due to the longer C–O bond length and more negative charged COδ , the Au1 adatom in TiO2/2Au/CO therefore have more π-back-donation to a CO adsorbate which could be also deduced from the subsequent analysis of density of electronic states where there are larger overlap of the electronic states (marked as blue-shade) between the Au1 adatom and the CO adsorbate below the Fermi level as shown in Fig. 5(a). The larger electron density in Fig. 6(c) marked by the black-arrow between Au1 adatom and the CO adsorbate in TiO2/2Au/CO consists well with the phenomena of the more π-back-donation to a CO adsor bate. Here are the shorter bond lengths of C–Au (2.121 Å) and C–Ti (2.214 Å) in TiO2/2Au than that of C–Ti (2.476 Å) in TiO2/Au and C–Ti (2.233 Å) in TiO2-x/Au. Comparing Fig. 6(c) with Fig. 2(c), due to the adsorption of a molecule CO, the strong repulsion as these dual-Au adatoms at the shorter distance by 2.794 Å (3.048 Å without the CO adsorbate) mainly induces the electron density derives from the core of the Au1 adatom. As mentioned in Fig. S2(c), the axis of the CO adsorbate is tilted at 26.3� with respect to the Z axis of TiO2/2Au, which mainly leads to the larger barrier energy for the diffusion of the CO adsorbate to react with a surface lattice oxygen specie. The presence of the asymmetrical electron density around the Au adatoms further demonstrates that the CO adsorbates reconstruct the surfaces. In comparison to the electron density in Fig. 2(b), the unpaired electrons left by one oxygen vacancy are localized at another nearestneighbor Ti3þ ion (Ti44, set by blue-dot arrow) in Fig. 6(b). The elec tronic states of these surfaces in the gap rightly overlap with that of the Au adatoms and the CO adsorbates, however, except the defect states
Fig. 5. Partial density of states of (a) the Au adatoms and one CO adsorbate, and (b) the top surfaces in TiO2/Au, TiO2-x/Au, and TiO2/2Au. Concerning the special characteristics of the electronic states in one CO adsorbate, the calculated Fermi levels are described in the dash-lines, which differs from that in Fig. 3. 5
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
over the surface. Although one additional oxygen vacancy occurs, we continue to define these three surfaces as TiO2/Au, TiO2-x/Au, and TiO2/ 2Au for the subsequent discussion in pathway (2)–(3) of Au-assisted MvK mechanism. Under pathway (2), the molecule O2 would locate on the surface to fill the site of an oxygen vacancy. For the molecule O2 adsorption, the favorable structure is often characterized by the two oxygen atoms in molecule O2 parallel to the surface [45]. However, hereby, two types of oxygen adsorbates (peroxide) in Fig. S3 are examined in the bonding of Ti–O/O–Ti or Ti–O–O–Ti, where they are perpendicular or parallel to the surface, respectively. In comparison about those adsorption energies in Fig. S4, we select the type of peroxide in the bonding of Ti–O–O–Ti because of the larger adsorption energy, which determine that they steadily fill the site of oxygen vacancies to facilitate CO oxidation. On a TiO2-x/Au surface, it requires two mole cules O2 to fill the original and newly formed oxygen vacancies in pathway (1) of Au-assisted MvK mechanism. In Fig. S3(b), One O adsorbate of molecule O2 (defined as *Om) slightly moves inward toward the surface but the other appears outward, and the former *Om ions locate nearer the Au adatom than the latter one. Accordingly, this im plies that the Au adatom somewhat determines the location of the filled *Om adatoms on surfaces. When the molecule CO binds with the Au adatom, this *Om adatom adjacent the Au adatom completely heals the site of the oxygen vacancy, as the structures shown at step IV in Fig. 7. The bond lengths of *Om–*Om are therefore elongated from almost 1.38 Å to 1.43 Å, which shows much longer than that of 1.236 Å in the gas phase O2, further revealing *Om would be easily activated later [46]. The *Om adatom away from Au adatom participates in the CO oxidation and composes of the molecule CO2 by oxidizing the molecule CO, as the structures shown at step VI in Fig. 7. The next key issues are how the peroxides in the bonding of Ti–*Om–*Om–Ti dissociated into two separated *Om adatoms, one of which eventually fills the oxygen vacancy and forms a twofold coordi nated oxygen in the bonding of Ti–*Om–Ti, and the other locates above a fivefold coordinated Ti ion in the bonding of Ti–*Om to prepare for oxidizing the molecule CO. All dissociation procedures of the peroxides are exothermic with the assistance of molecule CO, including step IV → step V in TiO2/Au and TiO2-x/Au, or step TS3 → step V’ in TiO2/2Au. Moreover, the CO-assistance peroxide dissociation reduces the conver sion energy barrier in step V → step TS3’ for TiO2/Au (0.12 eV) and
Fig. 6. Calculated electron density differences for a CO adsorbate on (a) TiO2/ Au, (b) TiO2-x/Au, and (c) TiO2/2Au at step II in Fig. 4. See the definitions of electrons density differences in the caption of Fig. 2. Another Ti3þ ion (Ti3þ 44 ) with the localized electrons in yellow is defined by a blue-dot arrow. The compasses with the axis labels about the subfigures on left and right are shown on the top-right of (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(defined by blue-shade) of the TiO2-x/Au surface in Fig. 5(b), which consists well with the status of Ti3þ ion in Fig. 6(b). It reveals that these uncertain extra electrons originating from one oxygen vacancy would deteriorate the stability of the TiO2-x/Au/CO surface. 3.3. Second CO oxidation In pathway (1) of Au assisted MvK mechanism, one molecule CO was oxidized to one molecule CO2 and a new oxygen vacancy was formed
Fig. 7. Reaction pathway (3) for the formation of the second CO2 on TiO2/Au, TiO2-x/Au, and TiO2/2Au catalysts, starting from the support at the end of the pathway (1) as shown in Fig. 4. The detailed structures shown on the right side correspond to the structures at each step in the reaction pathway. 6
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
variable Ti3þ ions and easily weaken its stability, as those mentioned above. However, one identical feather appears that the steady Ti3þ 41 ion precisely locates adjacent the Au adatoms at the net charge of þ1.92 |e| in Fig. 2(b), þ1.77 |e| in Fig. 6(b), and þ1.71 |e| in Fig. 8, respectively. This reveals that the Au adatoms constrain the electron steadily local ized on the nearest-neighbor Ti3þ 41 ion although some partial electrons would be localized at other Ti3þ ions either on the surface or in the subsurface. These synergistic effects between the Au adatoms and lowercoordinated Ti3þ 41 ion determine the catalytical properties in the TiO2/Au support. Comparing the structure configurations in Figs. S5(a) and S5(c), the two peroxides in green balls on TiO2-x/Au/2(O2) exhibit the distinctive properties when the CO adsorbates occur above them in TiO2-x/Au/2 (O2)2(CO). One CO adsorbate binding to the peroxide form a carbonate specie, and the other CO adsorbate pushes one oxygen atom in the peroxide to fill the oxygen vacancy and another oxygen atom to move to the site above a fivefold coordinated Ti ion, as we discussed above. The reasons ascribed for these differences would be discussed by the net charges of these Ti-ions, O adatoms and Au adatoms in Table 1. The lower net charge of Ti41 ion and the negative charge of the Au adatom appears in the TiO2-x/Au/2(O2) surface (Fig. S5(a)) and TiO2-x/Au/2 (O2)2(CO) surface (Fig. S5(c)), which illustrates that the partial elec trons still are localized at a Ti41 ion and an Au adatom. Accordingly, as the electron buffer, the Au–Ti41 group provides some electrons to the O1 ion, and helps it fill the oxygen vacancy and elongate the bonding of O1–O2 for the dissociation of peroxide in TiO2-x/Au/2(O2)2(CO). The slightly positive charge of the CO adsorbate at þ0.02 |e| determines that the C atom only transfers a few electrons to the Au adatom. By contrast, the more negative charges of the O3 ion and O4 ion in TiO2-x/Au/2(O2)2 (CO) indicate that they capture more electrons directly from the CO adsorbate (þ1.08 |e|) to stabilize the carbonate specie. These feathers clarify that more electrons the O ion of the peroxides captures, more stable it appears. Thereby, the behavior of the peroxide adjacent the Au–Ti41 group eventually attributes to the partial localized electrons in the Au–Ti41 group, which supplies the fewer electrons than that of the CO adsorbate in carbonate specie. The single Au adsorbate coordinated with the unsaturated Ti3þ ion is proposed as the active sites for the peroxide dissociation and defined as a restrictive electron buffer, which means the transfer electrons should be neither too much nor too little, just right. Direct experimental evidence via in situ/operando EXAFS, and in situ DRIFTS on Au@TiO2-x/ZnO catalysts substantiated Auδ –Ov–Ti3þ served as a dual-active site and contributed to the superior catalytic performance in water-gas shift reaction [52], as well as the Niδ –Ov–Ti3þ site in Ni@TiO2-x catalyst [53]. Idrissi et al. reported some theoretical results on the quaternary Heusler compound [54,55]. It re minders us that the alloy compound may be used as a support since the bonding of Au–Ti or Ni–Ti in this work sometimes could be considered as the alloy formation on the surface.
TiO2-x/Au (0.08 eV), whereas the existence of the peroxide increases the barrier energy in step IV → step TS3 for TiO2/2Au (0.22 eV). In contrast, the peroxide dissociation on a TiO2/Au support without the CO adsor bate has a barrier energy of 0.16 eV, as shown in Fig. S6. All conversion energy barriers are less than 0.5 eV [47], suggesting the promoting role of CO adsorbate in the O2 activation over TiO2/Au, TiO2-x/Au, and TiO2/2Au. In the case of TiO2-x/Au, one additional peroxide results in that the CO adsorbate would bind to the extra peroxide and thus a carbonate (COx3 ) is produced, as shown in Fig. S5(b) (defined as TiO2-x/Au/(O2) (CO3)), of which the adsorption energy is 4.26 eV, twenty times larger than that of a CO adsorbate binding to an Au adatom at step IV in Fig. 7. This means that the carbonate species are highly stabilized above the site of oxygen vacancies on the surface. Accordingly, the subsequent calculations are considered to determine the effect of this carbonate on the binding strength of CO adsorbate to an Au adsorbate on the support of TiO2-x/Au/(O2)(CO3). Thereby, an additional CO adsorbate is artifi cially put above the Au adatom on the TiO2-x/Au/(O2)(CO3) support in the same way as that at step IV in Fig. 7, as the final relaxed surface shown in Fig. S5(c). Its adsorption energy is 0.25 eV, almost identical to that of the CO adsorbate at step IV in Fig. 7, providing a further sight that the binding strength of a CO adsorbate to an Au adatom are inde pendent of the appearance of a carbonate on the support. Therefore, we can ignore the effect of the carbonate on the free energy diagram in a TiO2-x/Au support. Gong et al. suggested that the carbonate species were generated by the surface oxygen reacting with the bent COδ2 species produced in the pathway of first CO oxidation [48]. Many studies demonstrated that the deactivation of Au/TiO2 catalyst attributed to the production of car bonate species, which competed with the CO2 desorption and lead to the support poisoning [49–51]. Those phenomena in our calculation reveal that a molecule O2 would heal the oxygen vacancy originating from the different procedures, then the peroxides adatoms would react with the CO adsorbates to generate the highly stable carbonate or the molecule CO2, and the carbonate would reduce the utilization of reacted mole cules CO and the productivity of the molecules CO2 in final. Accordingly, since the pre-existence of one oxygen vacancy, a TiO2-x/Au support seems not the satisfied candidate for the catalyst of CO oxidation due to the formation of the carbonate in the CO re-oxidation procedures. After the pathway (1), there are finally two oxygen vacancies on the TiO2-x/Au surface (hereby defined as TiO2-2x/Au). Its electron density difference is shown in Fig. 8. Interestingly, a new Ti3þ ion pointed by the black-arrow locates in the subsurface of TiO2-2x/Au rather than on its surface, which greatly differs from that in Figs. 2(b) and Figure 6(b). It illustrates that the oxygen vacancies reconstruct the surface with the
4. Conclusions First-principle calculations were performed to compare the proper ties of CO oxidation on three kinds of anatase TiO2(001)-supported gold catalysts. The three configurations were defined in one gold adatom located above the site of an oxygen vacancy on TiO2 support, one gold adatom on one site of dual oxygen vacancies, and two gold adatoms above two sites of dual oxygen vacancies. In comparison, the Au ada toms were strongly anchored on the support with an additional oxygen vacancy in second and third case. The strong bonding of Ti–Au–C–Ti with more π-back-donation from an Au adatom to a CO adsorbate occurs when a CO molecular adsorbed on the two Au adatoms, which leaded to a larger activation barrier energy. The CO adsorbate promoted the molecule O2 dissolution in all three configurations at the second stage of Au assistance Mvk mechanism. However, the carbonate formed on the site of pre-existence of one oxygen vacancy in second case results in the support poisoning. As shown, the dual oxygen vacancies on the original
Fig. 8. Calculated electron density difference of one Au adatom on the TiO2-x/ Au support after the first catalytic cycle in Fig. 4. Two oxygen vacancies are marked by two blue-balls. See the definitions of electrons density differences in the caption of Fig. 2. Underneath the site of Ti40 ion, another Ti3þ ion with the localized electrons in yellow is defined by a blue-dot arrow. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 7
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Table 1 Net charges of Ti ions, O-ions, and Au adatoms on TiO2-x/Au/2(O2) support with or without two CO adsorbates. TiO2-x/Au/2(O2) TiO2-x/Au/2(O2)2(CO)
Ti40
Ti41
Ti43
Ti44
þ1.95 þ1.91
þ1.77 þ1.78
þ2.02 þ2.05
þ2.02 þ2.05
supports in second and third case deteriorated the performance of the TiO2/Au catalyst. Finally, we proposed that the single Auδ adatom coordinated with the unsaturated Ti3þ ion was served as the active sites in CO oxidation on TiO2/Au, that is the first case we mentioned. Hopefully, these calculations can provide the meaningful guidance to improve the activity and selectivity of single atom catalysts.
O1 0.55 0.61
O2 0.45 0.45
O3 0.43 1.09
O4 0.52 1.09
Au 0.50 0.41
[16] G. Pacchioni, Ketonization of carboxylic acids in biomass conversion over TiO2 and ZrO2 surfaces: a DFT perspective, ACS Catal. 4 (2014) 2874–2888. [17] A. Sinhamahapatra, J.P. Leon, J. Kang, B. Han, J.S. Yu, Oxygen-deficient zirconia (ZrO2 x): a new material for solar light absorption, Sci. Rep. 6 (2016) 27218–27225. [18] J. Wan, W. Chen, C. Jia, L. Zheng, J. Dong, X. Zheng, Y. Wang, W. Yan, C. Chen, Q. Peng, D. Wang, Y. Li, Defect effects on TiO2 nanosheets: stabilizing single atomic site Au and promoting catalytic properties, Adv. Mater. 30 (2018) 1705369. [19] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [20] J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond, J. Comput. Chem. 29 (2008) 2044–2078. [21] L. Vo�cadlo, D. Alf�e, M.J. Gillan, I.G. Wood, J.P. Brodholt, G.D. Price, Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth’s core, Nature 424 (2003) 536. [22] D. Alfe, M.J. Gillan, G.D. Price, Constraints on the composition of the Earth’s core from ab initio calculations, Nature 405 (2000) 172–175. [23] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758. [24] L. Li, W. Li, C. Zhu, L.-F. Mao, DFT calculation about oxygen vacancy to promote adsorption of a CO molecule on single Au-supported titanium dioxide, Phys. Status Solidi B 256 (2019) 1800386. [25] J.A. Dean, Lange’s Handbook of Chemistry, fifteenth ed., MacGrawHill, New York, 1999. [26] E. Wiberg, A.F. Holleman, N. Wiberg, Lehrbuch Der Anorganischen Chemie, De Gruyter, Berlin, 2007. [27] G. Henkelman, B.P. Uberuaga, H. J� onsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901–9904. [28] C. Sun, S.C. Smith, Strong interaction between gold and anatase TiO2 (001) predicted by first principle studies, J. Phys. Chem. C 116 (2012) 3524–3531. [29] R. Hengerer, B. Bolliger, M. Erbudak, M. Gr€ atzel, Structure and stability of the anatase TiO2 (101) and (001) surfaces, Surf. Sci. 460 (2000) 162–169. [30] G. Shukri, H. Kasai, Density functional theory study of ethylene adsorption on clean anatase TiO2 (001) surface, Surf. Sci. 619 (2014) 59. [31] M. Lazzeri, A. Selloni, Stress-driven reconstruction of an oxide surface: the anatase TiO2 (001) (1� 4) surface, Phys. Rev. Lett. 87 (2001) 266105. [32] N.H. Linha, T.Q. Nguyen, W.A. Di~ no, H. Kasai, Effect of oxygen vacancy on the adsorption of O2 on anatase TiO2 (001): a DFT-based study, Surf. Sci. 633 (2015) 38–45. [33] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638–641. [34] Y. Tang, S. Zhao, B. Long, J.-C. Liu, J. Li, On the nature of support effects of metal dioxides MO2 (M¼ Ti, Zr, Hf, Ce, Th) in single-atom gold catalysts: importance of quantum primogenic effect, J. Phys. Chem. C 120 (2016) 17514–17526. [35] S.C. Ammal, A. Heyden, Modeling the noble metal/TiO2 (110) interface with hybrid DFT functionals: a periodic electrostatic embedded cluster model study, J. Chem. Phys. 133 (2010) 164703–164717. [36] D. Matthey, J.G. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Laegsgaard, B. Hammer, F. Besenbacher, Enhanced bonding of gold nanoparticles on oxidized TiO2 (110), Science 315 (2007) 1692–1696. [37] Z. Lu, P. Lv, Z. Yang, S. Li, D. Ma, R. Wu, A promising single atom catalyst for CO oxidation: Ag on boron vacancies of h-BN sheets, Phys. Chem. Chem. Phys. 19 (2017) 16795–16805. [38] Z. Lu, P. Lv, Y. Liang, D. Ma, Y. Zhang, W. Zhang, X. Yang, Z. Yang, CO oxidation catalyzed by the single Co atom embedded hexagonal boron nitride nanosheet: a DFT-D study, Phys. Chem. Chem. Phys. 18 (2016) 21865–21870. [39] D. Widmann, R.J. Behm, Active oxygen on an Au/TiO2 catalyst - formation, stability and CO oxidation activity, Angew. Chem. Int. Ed. 50 (2011) 10241–10245. [40] E.J. Peterson, A.T. DeLaRiva, S. Lin, R.S. Johnson, H. Guo, J.T. Miller, J.H. Kwak, C.H.F. Peden, B. Kiefer, L.F. Allard, F.H. Ribeiro, A.K. Datye, Low-temperature carbon monoxide oxidation catalyzed by regenerable atomically dispersed palladium on alumina, Nat. Commun. 5 (2014) 4885. [41] D. Widmann, A. Krautsieder, P. Walter, A. Brückner, R.J. Behm, How temperature affects the mechanism of CO oxidation on Au/TiO2: a combined EPR and TAP reactor study of the reactive removal of TiO2 surface lattice oxygen in Au/TiO2 by CO, ACS Catal. 6 (2016) 5005–5011. [42] J. Liang, Q. Yu, X. Yang, T. Zhang, J. Li, A systematic theoretical study on FeOxsupported single atom catalysts: M1/FeOx for CO oxidation, Nano Res. 11 (2018) 1599–1611. [43] B. Yoon, H. H€ akkinen, U.L. man, A.S. Worz, J.-M. Antonietti, S. Abbet, K. Judai, U. Heiz, Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO, Science 307 (2005) 403–407. [44] L. Lian, P.A. Hackett, D.M. Rayner, Relativistic effects in reactions of the coinage metal dimers in the gas phase, J. Chem. Phys. 99 (1993) 2583–2590.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China under grant Nos. 61774014, and 61272105, Natural Science Foundation of Jiangsu Province of China under grant No. BK20141196. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122291. References [1] B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Single-atom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634–641. [2] X.-F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Single atom catalysts: a new frontier in heterogeneous catalysis, Acc. Chem. Res. 46 (2013) 1740–1748. [3] Y. Tang, Y.-G. Wang, J. Li, Theoretical investigations of Pt1@ CeO2 single-atom catalyst for CO oxidation, J. Phys. Chem. C 121 (2017) 11281–11289. [4] J.J. Yang, M.D. Pickett, X.M. Li, D.A.A. Ohlberg, D.R. Stewart, R.S. Williams, Memristive switching mechanism for metal/oxide/metal nanodevices, Nat. Nanotechnol. 3 (2008) 429–433. [5] D.-H. Kwon, K.M. Kim, J.H. Jang, J.M. Jeon, M.H. Lee, G.H. Kim, X.-S. Li, G.S. Park, B. Lee, S. Han, M. Kim, C.S. Hwang, Atomic structure of conducting nanofilaments in TiO2 resistive switching memory, Nat. Nanotechnol. 5 (2010) 148–153. [6] A. Ruiz Puigdollers, F. Illas, G. Pacchioni, Effect of nanostructuring on the reactivity of zirconia: a DFTþU study of Au atom adsorption, J. Phys. Chem. C 120 (2016) 4392–4402. [7] C. Loschen, A. Migani, S.T. Bromley, F. Illas, K.M. Neyman, Density functional studies of model cerium oxide nanoparticles, Phys. Chem. Chem. Phys. 10 (2008) 5730–5738. [8] C. Loschen, S.T. Bromley, K.M. Neyman, F. Illas, Understanding ceria nanoparticles from first-principles calculations, J. Phys. Chem. C 111 (2007) 10142–10145. [9] Z. Yan, S. Chinta, A.A. Mohamed, J.P. Fackler, D.W. Goodman, The role of Fcenters in catalysis by Au supported on MgO, J. Am. Chem. Soc. 127 (2005) 1604–1605. [10] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hakkinen, R.N. Barnett, U. Landman, When gold is not noble: nanoscale gold catalysts, J. Phys. Chem. A 103 (1999) 9573–9578. [11] M.S. Chen, D.W. Goodman, The structure of catalytically active gold on titania, Science 306 (2004) 252–255. [12] J.A. Rodriguez, G. Liu, T. Jirsak, J. Hrbek, Z. Chang, J. Dvorak, A. Maiti, Activation of gold on Titania: adsorption and reaction of SO2 on Au/TiO2(110), J. Am. Chem. Soc. 124 (2002) 5242–5250. [13] N. Lopez, T.V.W. Janssens, B.S. Clausen, Y. Xu, M. Mavrikakis, T. Bligaard, J. K. Norskov, On the origin of the catalytic activity of gold nanoparticles for lowtemperature CO oxidation, J. Catal. 223 (2004) 232–235. [14] Z. Geng, X. Kong, W. Chen, H. Su, Y. Liu, F. Cai, G. Wang, J. Zeng, Oxygen vacancies in ZnO nanosheets enhance CO2 electrochemical reduction to CO, Angew. Chem. Int. Ed. 57 (2018) 6054–6059. [15] T.N. Pham, T. Sooknoi, S.P. Crossley, D.E. Resaco, Increasing oxide reducibility: the role of metal/oxide interfaces in the formation of oxygen vacancies, ACS Catal. 3 (2013) 2456–2473.
8
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
[45] F. Li, Y. Li, X.C. Zeng, Z. Chen, Exploration of high-performance single-atom catalysts on support M1/FeOx for CO oxidation via computational study, ACS Catal. 5 (2014) 544–552. [46] T. Yang, R. Fukuda, S. Hosokawa, T. Tanaka, S. Sakaki, M. Ehara, A theoretical investigation on CO oxidation by single-atom catalysts M1/γ-Al2O3 (M¼ Pd, Fe, Co, and Ni), ChemCatChem 9 (2017) 1222–1229. [47] I.X. Green, W. Tang, M. Neurock, J.T. Yates, Spectroscopic observation of dual catalytic sites during oxidation of CO on an Au/TiO2 catalyst, Science 333 (2011) 736–739. [48] F. Chen, D. Liu, J. Zhang, P. Hu, X.-Q. Gong, G. Lu, A DFT þ U study of the lattice oxygen reactivity toward direct CO oxidation on the CeO2 (111) and (110) surfaces, Phys. Chem. Chem. Phys. 14 (2012) 16573–16580. [49] J. Saavedra, C. Powell, B. Panthi, C.J. Pursell, B.D. Chandler, CO oxidation over Au/TiO2 catalyst: pretreatment effects, catalyst deactivation, and carbonates production, J. Catal. 307 (2013) 37–47. [50] Y. Denkwitz, B. Schumacher, G. Kucerova, R.J. Behm, Activity, stability, and deactivation behavior of supported Au/TiO2 catalysts in the CO oxidation and preferential CO oxidation reaction at elevated temperatures, J. Catal. 267 (2009) 78–88.
[51] Y. Wang, D. Widmann, R.J. Behm, Influence of TiO2 bulk defects on CO adsorption and CO oxidation on Au/TiO2: electronic metal–support interactions (EMSIs) in supported Au catalysts, ACS Catal. 7 (2017) 2339–2345. [52] N. Liu, M. Xu, Y. Yang, S. Zhang, J. Zhang, W. Wang, L. Zheng, S. Hong, M. Wei, Auδ-–Ov–Ti3þ interfacial site: catalytic active center toward low-temperature water gas shift reaction, ACS Catal. 9 (2019) 2707–2717. [53] M. Xu, S. Yao, D. Rao, Y. Niu, N. Liu, Mi Peng, P. Zhai, Yi Man, L. Zheng, B. Wang, B. Zhang, D. Ma, M. Wei, Insights into interfacial synergistic catalysis over Ni@ TiO2-x catalyst toward water-gas shift reaction, J. Am. Chem. Soc. 140 (2018) 11241–11251. [54] S. Idrissi, R. Khalladi, S. Ziti, N. El Mekkaou, S. Mtougui, H. Labrim, I. El Housni, L. Bahma, The electronic and magnetic proprieties of the rare earth-based quaternary Heusler compound LuCoVGe, Phys. B Condens. Matter 562 (2019) 116–123. [55] S. Idrissi, S. Ziti, H. Labrim, R. Khalladi, S. Mtougui, N. El Mekkaoui, I. ElHousni, L. Bahmad, Magnetic properties of the Heusler compound CoFeMnSi:Monte Carlo simulations, Physica A 527 (2019) 121406.
9
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The effect of oxygen vacancy at CO oxidation on anatase (001)-supported single-Au catalyst Lei Li a, *, Wenshi Li a, Canyan Zhu a, Ling-Feng Mao b, ** a b
Institute of Intelligent Structure and System & School of Electronics and Information Engineering, Soochow University, Suzhou, 215006, People’s Republic of China School of Computer and Communication Engineering, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China
H I G H L I G H T S
� A stable carbonate occurs at the site of extra O vacancy. � O vacancy dimer reduces the catalytical performance of CO oxidation. � Synergistic effects exist between a negatively charged Au atom and a Ti3þ ion. A R T I C L E I N F O
A B S T R A C T
Keywords: Gold adatom Anatase TiO2(001) surface O vacancy CO oxidation Adsorption energy
The catalytic performance of CO oxidation on the TiO2-supported gold catalysts would be affected by the O vacancies on a TiO2 surface. First-principle studies here are focused on how the O vacancy addresses the per formance of single atom catalysts on CO oxidation. Three possible configurations of gold adatoms and O va cancies on anatase TiO2(001) support are considered as follows: A gold adatom above an O vacancy, a gold adatom or two gold adatoms above an O vacancy dimer. The O vacancy dimer favors the binding of negatively charged gold adatoms on anatase TiO2(001) support. However, the dual gold adatoms slightly increase the barrier energy in CO oxidation through the Au-assisted Mars-van Krevelen mechanism. The formation of a stable carbonate on the initial-existing O vacancy reveals that the extra O vacancy deteriorates the catalytical performance of CO oxidation. In final, the high catalytic performances of TiO2/Au catalyst are attributed to the synergistic effects between a negatively charged Au adatom and a lower-coordinated Ti3þ ion.
1. Introduction Recently, CO oxidation on single-atom catalysts has been extensively investigated in the heterogeneous catalysis, since the first single-atom catalyst Pt1/FeOx was proposed by Qiao et al. [1,2]. For the Pt1/FeOx catalyst, one oxygen vacancy formed in the pre-reduction process opens the possible channels for the catalytic cycle of CO oxidation [1]. The oxygen vacancy produced by a CO adsorbate abstracting the lattice oxygen atom on the surface acts as the medium to complete the reaction cycle in the Pt1/CeO2 single-atom catalyst [3]. Thereby, to understand how an oxygen vacancy impacts on the activity of a single atom catalyst is of fundamental importance to know the chemical reactivity in CO oxidation. In general, these oxygen vacancies are easily produced in a bulk or a surface of the transition-metal oxides, e.g., TiO2, WO3, NiO, Fe2O3, and
CeO2. In this case, it thus introduces some defective electronic states in the gap below the conduction band minimum. The unpaired excess electrons left behind by these oxygen vacancies would modify the electronic properties of the oxide. For example, the transition between the insulator and conductor would occur due to the mobility of the larger amounts of the oxygen vacancies in rutile TiO2 under the higher electric field [4,5]. The occurrence of oxygen vacancies in TiO2 always leads to the reduction of the Ti4þions to the Ti3þ ions (Ti4þ(3 d0) þ e → Ti3þ(3 d1)), where the partial unpaired electrons are localized. Similar situation exists in CeO2 and ZrO2. These electrons prefer to localize at the lower-coordinated Zr3þ and Ce3þ ions, of which the magnetic properties were detected by electron paramagnetic resonance [6–8]. These unpaired excess electrons localized at Ti3þ ions nearest or nearer neighbor to the oxygen vacancies would reconstruct the surface by the atomic relaxation and certainly modify the properties of the surface.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Li), [email protected] (L.-F. Mao). https://doi.org/10.1016/j.matchemphys.2019.122291 Received 29 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 Available online 10 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Recent experimental and theoretical results show that the Au clusters were strongly anchored on a MgO support with F-centers and such that promoted the catalytic activity on CO oxidation [9,10]. The reduced Ti atoms enhanced the strength of the bonding to Au atoms on the highly reduced TiO2-x support and determined the shape and electronic prop erties of Au atoms [11–13]. Zeng et al. revealed that the activation of CO2 was evidently enhanced with the increasing of oxygen vacancy concentration on the oxygen vacancy-rich ZnO nanosheets by the H2 plasma treatment [14]. Yoon et al. emphasized the importance of oxy gen vacancies in the oxide support for the high rate of CO oxidation [6]. Pre-reduced solid catalysts based on ZrO2 nanoparticles displayed a better activity in the transformation of biomass into fuels [15,16]. In addition, reduced ZrO2-x was shown to be photo-catalytically active in H2 production under solar light while the stoichiometric ZrO2 was inactive [17]. For a single-atom catalyst, single Au adatom would be deposited on the surface with various configurations. Accordingly, the possible interaction between the single Au adatom and the oxygen vacancy or the reduced Ti3þ ions would be concerned, which results in the electrons redistribution on the surface and further leads to the modification of the CO catalytical properties. Herein, we perform the first-principle calcu lations on the basis of density functional theory corrected by on-site Coulomb interactions (DFT þ U) to investigate the oxygen vacancy effort at the catalytic properties of CO oxidation on the anatase TiO2(001)-supported single-gold catalyst. As reported by Wan et al., one Au adatom locates above one oxygen vacancy and binds with two nearest-neighbor Ti ions [18]. On an actual surface in experiments, it should have some oxygen vacancies unfilled by the Au adatoms, thus the correlation between the oxygen vacancy and Au adatom would be concerned. Besides, the dual oxygen vacancies are produced on a sur face, and then the Au dimer would seal these dual oxygen vacancies. Hence, we would compare the electronic structures on our three pre-built catalysts and the subsequent energy evolution in catalytic cy cles of CO oxidation. As a prediction of the behavior of Au adatoms on the defective anatase TiO2(001) surface, our aims are essentially to provide a primary evaluation on the effect of oxygen vacancy on the anatase TiO2(001)-supported single-gold catalyst.
with the recent work published by Wan et al., we present results of DFT þ U-based calculation on a (3 � 3) unreconstructed anatase TiO2(001) surface [18]. An anatase TiO2(001) surface in five-atomic layers was thus selected to be constructed by a total of 125 atoms in 3 � 3 supercell. A vacuum layer in 15 Å was used for separating the slabs along the perpendicular Z-direction. During the slab relaxation (also defined as geometry optimization) and the subsequent adsorption of the Au atoms or a molecule CO, the bottom-two layers were always fixed to simulate the bulk properties, while the upper-three layers and the ad sorbates (Au and CO) were completely relaxed. An oxygen vacancy on a TiO2(001) surface marked by a blue ball in Fig. 1(a) was generated from a twofold coordinated oxygen atom and later replaced by the Au atom labeled by a yellow ball in Fig. 1(c). The combination of a single Au atom and two nearest-neighbor Ti ions forms a Ti–Au–Ti structure on the defective TiO2(001) surface [29], as the structure shown in Fig. 1(c), which is defined as TiO2/Au. To construct an oxygen vacancy dimer on a surface, a second oxygen vacancy was created on another twofold coordinated oxygen atom adjacent to the first oxygen vacancy, as shown in Fig. 1(b). The defect formation en ergies for a single oxygen vacancy and dimer are 3.6 eV and 4.03 eV, respectively, of which for the single oxygen vacancy consisting with the value (3.59 eV) as reported by Tang et al. [34] and however slightly smaller than that of 3.75 eV reported by Ammal and Heyden [35]. One Au atom or dual Au atoms are respectively located above one oxygen vacancy or one oxygen vacancy dimer, as shown in Fig. 1(d) or 1(e), thereby termed as TiO2-x/Au or TiO2/2Au, respectively. Compared with the case of TiO2/Au, there is one additional oxygen vacancy on a TiO2-x/Au surface or one additional Au adatom on a TiO2/2Au surface, of which the binding energies of an Au adatom are 4.31 eV and 4.32 eV, much lower than that of 3.95 eV in TiO2/Au. This difference shows us that the additional oxygen vacancies enhance the anchor strength of Au adatoms on a surface, consistence well with our previous work [24]. It should be noted that the additional oxygen vacancy in Ref. [24] were produced based on the structure of TiO2/Au, whereas the oxygen vacancies here were generated before the adsorption of an Au atom. In other words, the TiO2/Au model in Ref. [24] was firstly built, and then, the oxygen vacancy replaced the oxygen atoms adjacent to the gold adatom in TiO2/Au to produce the models of TiO2-x1-Au, TiO2-x2-Au, and TiO2-x3-Au. In this work, one and two oxygen vacancies were produced on the TiO2(001) surface, respectively; the Au atoms were later located on these surfaces to form TiO2/Au, TiO2-x/Au, and TiO2/2Au, respectively, as shown in Fig. 1. Accordingly, the difference of the model in this work to that in ref. 24 was the production order of the oxygen vacancy and the gold atom. The attractive forces between the dual Au adatoms in TiO2/2Au shorten the distance between them (3.048 Å) compared to the initial distance of 3.782 Å. However, the Au–Au bonding rarely occurs there, as shown in Fig. 2(c). Subsequent analysis of Bader charge reveal that the single Au adatom or the dual Au adatoms on TiO2/Au, TiO2-x/Au and TiO2/2Au are slightly negatively charged (Auδ , δ- ¼ 0.54 |e|, 0.57 | e| and 0.56/-0.48 |e|, respectively), which demonstrate the charge transfer from the oxide support to Au adatoms happens upon the adsorption of Au adatoms. The electron density differences in Fig. 2 indicate the Au adatoms obtain some partial electrons marked in yellow, suggesting the formation of directional Au–Ti covalent bonds, identical to that in Au clusters supported reduced TiO2 [36]. Here, following the works in Refs. [37,38], the electron density differences are evaluated within the formula Δρ1 ¼ ρ(TiO2/Au) – ρ(TiO2) – ρ(Au), where ρ(TiO2/Au), ρ(TiO2), and ρ(Au) are the electron density of the whole model, the sup port, and the Au adatom, respectively. Labeled by the black-down arrow in Fig. 3, the overlaps of Au-5d electronic states (marked by red and green-shade) and 3d electronic states of Ti40 in blue (also together with Ti43 in TiO2/2Au) illustrate the hybridization between Au adatom and Ti-ions on these surfaces. The definitions of Ti40 and Ti43 are found in Fig. 1(c). Obviously, some of the electrons are localized at a Ti3þ ion (Ti43) on the TiO2-x/Au surface in
2. Methodology All DFT calculations were performed via Vienna ab initio Simulation Package (VASP) [19–23]. The more basic sets of these calculations could be found in our previous work [24]. In this work, the computations on a molecule CO and O2 were performed in a unit cell at 10 Å � 10 Å � 10 Å at the Gamma-point. The bond lengths in our computational results at 1.14 Å in a molecule CO and 1.23 Å in a molecule O2 consist well with the experimental values of 1.13 Å [25] and 1.21 Å [26], respectively. To determine the location and energy of transition states (TS), the climbing image nudged elastic-band (CI-NEB) method with the convergence tolerance of 0.05 eV/Å was used, in which almost five images were set [27]. 3. Results and discussions 3.1. Au adatom Due to the unsaturated bonds on an anatase TiO2(001) surface, the gold atoms will be strongly adsorbed on an anatase TiO2(001) surface instead of on an anatase TiO2(101) surface [28]. Based on the experi mental conditions, unreconstructed and reconstructed (1 � 4) anatase TiO2(001) could be applied [29–32]. The unsaturated atoms on an surface—five-coordinated Ti ions and anatase TiO2(001) two-coordinated O ions—tend to strongly bind with Au atoms; mean while, the Ti–O bonds on an anatase TiO2(001) surface much weaker than that in bulk phase or on anatase TiO2(101) surface would poten tially break when bonding with Au atoms [33]. Herein, following up 2
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Fig. 1. Structural configuration of TiO2(001) surface with one oxygen vacancy in (a) and an oxygen vacancy dimer in (b); (c) TiO2/Au: One Au adatom above the oxygen vacancy in (a); (d) TiO2-x/Au: One Au adatom above one oxygen vacancy in (b); (e) TiO2/2Au: Two Au adatoms above the oxygen vacancy dimer in (b). Figures at side-view and top-view list on the top and below, respectively. Models in line-style and ball-style indicate the bottom-two fixed and top-three optimized layers, respectively. The atoms of Ti40, Ti41, Ti43, and Ti44 are marked by the numbers in white with background filled in green. Red: O; Grey: Ti; Yellow: Au; Blue: oxygen vacancy. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 2. Calculated electron density differences for Au adatoms in (a) TiO2/Au, (b) TiO2-x/Au, and (c) TiO2/2Au. Areas in yellow and green represent the accumulation and depletion of electrons, respectively. The cutoff values of the iso-surfaces in the electron density differences are set to 0.008 electrons/Å3. One Ti3þ ion (Ti3þ 43 ) with the localized electrons in yellow is defined by a black arrow. The subfigures in two different views are listed on left and right, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2(b), which has a lower positive charge (þ1.73 |e|) than that on a TiO2/Au surface (þ2.08 |e|) and a TiO2/2Au surface (þ1.90 |e|), consistent well with the sole presence of electronic state of Ti43 below the Fermi level as the blue-shade shown in Fig. 3(b). Although the binding energy of single Au adatom is higher in TiO2-x/ Au, the valley resulted from the formation of the second oxygen vacancy provides a possible path for the migration of this Au adatoms to the position above the adjacent oxygen vacancy. Fig. S1 confirm that a barrier energy of 1.08 eV prevents the diffusion of this single Au atom and it would be firmly anchored in the bonding of Ti–Au–Ti. The larger
barrier energy is derived from the bonding between the threefold co ordinated oxygen ions (marked in black-dash circle) and the single Au adatom, as the transition state (TS) shown in Fig. S1. 3.2. First CO oxidation The total oxidation procedures of a molecule CO through the Auassisted Mars-van Krevelen (MvK) mechanism could be summarized [39,40]:
3
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Fig. 3. Partial density of states in (a) TiO2/Au, (b) TiO2-x/Au, and (c) TiO2/2Au. The electronic density of state of the top surfaces, Au adatoms, Ti43 ions, and Ti40 ions are listed from top to bottom, respectively. The down-black arrows indicate the overlap of electronic density of states of Au adatoms and Ti40 ion or Ti43 ion. All energies are referred to the Fermi level defined by the dash-lines.
CO þ Au/TiO2 → CO2 þ Au/TiO2-x
(1)
O2 þ Au/TiO2-x → Au/TiO2 þ Oad
(2)
CO þ Au/TiO2 þ Oad → Au/TiO2 þ CO2
(3)
on the surface, which subsequently leads to the reduction of the Au/TiO2 support; Pathway (2–3) is a molecular oxygen heals the oxygen vacancy and then react with the molecule CO to generate the second molecule CO2 [41]. These prepared surfaces with Au adatoms are ready to be discussed
Pathway (1) is a reaction of a molecule CO with a lattice oxygen ion
Fig. 4. Reaction pathway (1) for the formation of the first CO2 on TiO2/Au, TiO2-x/Au, and TiO2/2Au catalysts, starting from the support in Fig. 1(c–d). The detailed structures shown on both sides and at the bottom correspond to the structures at each step in the reaction pathway. 4
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
on a molecule CO oxidation in pathway (1), as summarized in Fig. 4. The relatively larger adsorption energy (defined as Ead) of a molecule CO at 0.55 eV occurs on a TiO2/2Au surface. The higher activation barrier energy (defined as Eac: 0.99 eV) for the reaction of COad þ OS → CO2 þ Ovac (step TS and III) indicates a molecule CO is converted to a molecule CO2 over a TiO2/2Au surface, which happens at an elevated temperature by contrast to that on other surfaces. At step III, the CO adsorbate attracts a lattice oxygen specie adjacent to one Au adatom or dual Au adatoms to complete the CO oxidation reaction in pathway (1). In the case of TiO2-x/Au with one oxygen vacancy adjacent to the Au adatom, both the adsorption energy (Ead: 0.38 eV) and activation barrier energy (Eac: 0.80 eV) determine that this oxygen vacancy results in a weakly negative impact on the CO poisoning and the oxidization to CO2, in compared to those in TiO2/2Au surface (Ead: 0.32 eV and Eac: 0.70 eV). Moreover, these evitable differences in the activation barrier energies are almost determined by the adsorption energy of a molecule CO in step II. The higher adsorption energies of a molecule CO lead to the higher activation barrier energy [42]. The structural configurations also clarify that the binding properties of a molecule CO to the unsatu rated Ti ions and Au adatoms in step II result in the known adsorption energy differences above. The configuration of Ti–Au–Ti bonds still lo cates in TiO2/Au, as well as that in TiO2-x/2Au, whereas the new bonds of Ti–Au–C–Ti is produced in TiO2/2Au. However, almost the same bonds length of C–OS (1.201 Å) and C–Ti (2.132 Å) are found on these three structures at step TS. The analysis of Bader charge illustrate that the single Au adatom or the dual Au adatoms on the TiO2/Au/CO, TiO2-x/Au/CO and TiO2/2Au/ CO are still negative but with less charges (δ ¼ 0.37 |e|, 0.40 |e| and 0.21/-0.47 |e|, respectively), which reveal the charge transfer from one Au adatom to the CO adsorbate or oxide support upon the adsorp tion of one molecule CO. Further Bader charges of the CO adsorbate would be clarified to show where the detailed charges are transferred from the Au adatoms in final. Thereby these negative charged COδ on TiO2/Au/CO ( 0.13 |e|), TiO2-x/Au/CO ( 0.12 |e|) and TiO2/2Au/CO
( 0.23 |e|) reveal the Au adatoms donate more electrons to CO 2π* antibonding orbitals than to the oxide supports. This also corresponds to the elongated C–O bond lengths in 1.155 Å, 1.164 Å, and 1.175 Å on the TiO2/Au/CO, TiO2-x/Au/CO and TiO2/2Au/CO support with a CO adsorbate, respectively. The elongated C–O bond length and negative charged COδ eventually reveal the contribution of π-back-donation [43, 44]. In addition, due to the longer C–O bond length and more negative charged COδ , the Au1 adatom in TiO2/2Au/CO therefore have more π-back-donation to a CO adsorbate which could be also deduced from the subsequent analysis of density of electronic states where there are larger overlap of the electronic states (marked as blue-shade) between the Au1 adatom and the CO adsorbate below the Fermi level as shown in Fig. 5(a). The larger electron density in Fig. 6(c) marked by the black-arrow between Au1 adatom and the CO adsorbate in TiO2/2Au/CO consists well with the phenomena of the more π-back-donation to a CO adsor bate. Here are the shorter bond lengths of C–Au (2.121 Å) and C–Ti (2.214 Å) in TiO2/2Au than that of C–Ti (2.476 Å) in TiO2/Au and C–Ti (2.233 Å) in TiO2-x/Au. Comparing Fig. 6(c) with Fig. 2(c), due to the adsorption of a molecule CO, the strong repulsion as these dual-Au adatoms at the shorter distance by 2.794 Å (3.048 Å without the CO adsorbate) mainly induces the electron density derives from the core of the Au1 adatom. As mentioned in Fig. S2(c), the axis of the CO adsorbate is tilted at 26.3� with respect to the Z axis of TiO2/2Au, which mainly leads to the larger barrier energy for the diffusion of the CO adsorbate to react with a surface lattice oxygen specie. The presence of the asymmetrical electron density around the Au adatoms further demonstrates that the CO adsorbates reconstruct the surfaces. In comparison to the electron density in Fig. 2(b), the unpaired electrons left by one oxygen vacancy are localized at another nearestneighbor Ti3þ ion (Ti44, set by blue-dot arrow) in Fig. 6(b). The elec tronic states of these surfaces in the gap rightly overlap with that of the Au adatoms and the CO adsorbates, however, except the defect states
Fig. 5. Partial density of states of (a) the Au adatoms and one CO adsorbate, and (b) the top surfaces in TiO2/Au, TiO2-x/Au, and TiO2/2Au. Concerning the special characteristics of the electronic states in one CO adsorbate, the calculated Fermi levels are described in the dash-lines, which differs from that in Fig. 3. 5
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
over the surface. Although one additional oxygen vacancy occurs, we continue to define these three surfaces as TiO2/Au, TiO2-x/Au, and TiO2/ 2Au for the subsequent discussion in pathway (2)–(3) of Au-assisted MvK mechanism. Under pathway (2), the molecule O2 would locate on the surface to fill the site of an oxygen vacancy. For the molecule O2 adsorption, the favorable structure is often characterized by the two oxygen atoms in molecule O2 parallel to the surface [45]. However, hereby, two types of oxygen adsorbates (peroxide) in Fig. S3 are examined in the bonding of Ti–O/O–Ti or Ti–O–O–Ti, where they are perpendicular or parallel to the surface, respectively. In comparison about those adsorption energies in Fig. S4, we select the type of peroxide in the bonding of Ti–O–O–Ti because of the larger adsorption energy, which determine that they steadily fill the site of oxygen vacancies to facilitate CO oxidation. On a TiO2-x/Au surface, it requires two mole cules O2 to fill the original and newly formed oxygen vacancies in pathway (1) of Au-assisted MvK mechanism. In Fig. S3(b), One O adsorbate of molecule O2 (defined as *Om) slightly moves inward toward the surface but the other appears outward, and the former *Om ions locate nearer the Au adatom than the latter one. Accordingly, this im plies that the Au adatom somewhat determines the location of the filled *Om adatoms on surfaces. When the molecule CO binds with the Au adatom, this *Om adatom adjacent the Au adatom completely heals the site of the oxygen vacancy, as the structures shown at step IV in Fig. 7. The bond lengths of *Om–*Om are therefore elongated from almost 1.38 Å to 1.43 Å, which shows much longer than that of 1.236 Å in the gas phase O2, further revealing *Om would be easily activated later [46]. The *Om adatom away from Au adatom participates in the CO oxidation and composes of the molecule CO2 by oxidizing the molecule CO, as the structures shown at step VI in Fig. 7. The next key issues are how the peroxides in the bonding of Ti–*Om–*Om–Ti dissociated into two separated *Om adatoms, one of which eventually fills the oxygen vacancy and forms a twofold coordi nated oxygen in the bonding of Ti–*Om–Ti, and the other locates above a fivefold coordinated Ti ion in the bonding of Ti–*Om to prepare for oxidizing the molecule CO. All dissociation procedures of the peroxides are exothermic with the assistance of molecule CO, including step IV → step V in TiO2/Au and TiO2-x/Au, or step TS3 → step V’ in TiO2/2Au. Moreover, the CO-assistance peroxide dissociation reduces the conver sion energy barrier in step V → step TS3’ for TiO2/Au (0.12 eV) and
Fig. 6. Calculated electron density differences for a CO adsorbate on (a) TiO2/ Au, (b) TiO2-x/Au, and (c) TiO2/2Au at step II in Fig. 4. See the definitions of electrons density differences in the caption of Fig. 2. Another Ti3þ ion (Ti3þ 44 ) with the localized electrons in yellow is defined by a blue-dot arrow. The compasses with the axis labels about the subfigures on left and right are shown on the top-right of (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(defined by blue-shade) of the TiO2-x/Au surface in Fig. 5(b), which consists well with the status of Ti3þ ion in Fig. 6(b). It reveals that these uncertain extra electrons originating from one oxygen vacancy would deteriorate the stability of the TiO2-x/Au/CO surface. 3.3. Second CO oxidation In pathway (1) of Au assisted MvK mechanism, one molecule CO was oxidized to one molecule CO2 and a new oxygen vacancy was formed
Fig. 7. Reaction pathway (3) for the formation of the second CO2 on TiO2/Au, TiO2-x/Au, and TiO2/2Au catalysts, starting from the support at the end of the pathway (1) as shown in Fig. 4. The detailed structures shown on the right side correspond to the structures at each step in the reaction pathway. 6
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
variable Ti3þ ions and easily weaken its stability, as those mentioned above. However, one identical feather appears that the steady Ti3þ 41 ion precisely locates adjacent the Au adatoms at the net charge of þ1.92 |e| in Fig. 2(b), þ1.77 |e| in Fig. 6(b), and þ1.71 |e| in Fig. 8, respectively. This reveals that the Au adatoms constrain the electron steadily local ized on the nearest-neighbor Ti3þ 41 ion although some partial electrons would be localized at other Ti3þ ions either on the surface or in the subsurface. These synergistic effects between the Au adatoms and lowercoordinated Ti3þ 41 ion determine the catalytical properties in the TiO2/Au support. Comparing the structure configurations in Figs. S5(a) and S5(c), the two peroxides in green balls on TiO2-x/Au/2(O2) exhibit the distinctive properties when the CO adsorbates occur above them in TiO2-x/Au/2 (O2)2(CO). One CO adsorbate binding to the peroxide form a carbonate specie, and the other CO adsorbate pushes one oxygen atom in the peroxide to fill the oxygen vacancy and another oxygen atom to move to the site above a fivefold coordinated Ti ion, as we discussed above. The reasons ascribed for these differences would be discussed by the net charges of these Ti-ions, O adatoms and Au adatoms in Table 1. The lower net charge of Ti41 ion and the negative charge of the Au adatom appears in the TiO2-x/Au/2(O2) surface (Fig. S5(a)) and TiO2-x/Au/2 (O2)2(CO) surface (Fig. S5(c)), which illustrates that the partial elec trons still are localized at a Ti41 ion and an Au adatom. Accordingly, as the electron buffer, the Au–Ti41 group provides some electrons to the O1 ion, and helps it fill the oxygen vacancy and elongate the bonding of O1–O2 for the dissociation of peroxide in TiO2-x/Au/2(O2)2(CO). The slightly positive charge of the CO adsorbate at þ0.02 |e| determines that the C atom only transfers a few electrons to the Au adatom. By contrast, the more negative charges of the O3 ion and O4 ion in TiO2-x/Au/2(O2)2 (CO) indicate that they capture more electrons directly from the CO adsorbate (þ1.08 |e|) to stabilize the carbonate specie. These feathers clarify that more electrons the O ion of the peroxides captures, more stable it appears. Thereby, the behavior of the peroxide adjacent the Au–Ti41 group eventually attributes to the partial localized electrons in the Au–Ti41 group, which supplies the fewer electrons than that of the CO adsorbate in carbonate specie. The single Au adsorbate coordinated with the unsaturated Ti3þ ion is proposed as the active sites for the peroxide dissociation and defined as a restrictive electron buffer, which means the transfer electrons should be neither too much nor too little, just right. Direct experimental evidence via in situ/operando EXAFS, and in situ DRIFTS on Au@TiO2-x/ZnO catalysts substantiated Auδ –Ov–Ti3þ served as a dual-active site and contributed to the superior catalytic performance in water-gas shift reaction [52], as well as the Niδ –Ov–Ti3þ site in Ni@TiO2-x catalyst [53]. Idrissi et al. reported some theoretical results on the quaternary Heusler compound [54,55]. It re minders us that the alloy compound may be used as a support since the bonding of Au–Ti or Ni–Ti in this work sometimes could be considered as the alloy formation on the surface.
TiO2-x/Au (0.08 eV), whereas the existence of the peroxide increases the barrier energy in step IV → step TS3 for TiO2/2Au (0.22 eV). In contrast, the peroxide dissociation on a TiO2/Au support without the CO adsor bate has a barrier energy of 0.16 eV, as shown in Fig. S6. All conversion energy barriers are less than 0.5 eV [47], suggesting the promoting role of CO adsorbate in the O2 activation over TiO2/Au, TiO2-x/Au, and TiO2/2Au. In the case of TiO2-x/Au, one additional peroxide results in that the CO adsorbate would bind to the extra peroxide and thus a carbonate (COx3 ) is produced, as shown in Fig. S5(b) (defined as TiO2-x/Au/(O2) (CO3)), of which the adsorption energy is 4.26 eV, twenty times larger than that of a CO adsorbate binding to an Au adatom at step IV in Fig. 7. This means that the carbonate species are highly stabilized above the site of oxygen vacancies on the surface. Accordingly, the subsequent calculations are considered to determine the effect of this carbonate on the binding strength of CO adsorbate to an Au adsorbate on the support of TiO2-x/Au/(O2)(CO3). Thereby, an additional CO adsorbate is artifi cially put above the Au adatom on the TiO2-x/Au/(O2)(CO3) support in the same way as that at step IV in Fig. 7, as the final relaxed surface shown in Fig. S5(c). Its adsorption energy is 0.25 eV, almost identical to that of the CO adsorbate at step IV in Fig. 7, providing a further sight that the binding strength of a CO adsorbate to an Au adatom are inde pendent of the appearance of a carbonate on the support. Therefore, we can ignore the effect of the carbonate on the free energy diagram in a TiO2-x/Au support. Gong et al. suggested that the carbonate species were generated by the surface oxygen reacting with the bent COδ2 species produced in the pathway of first CO oxidation [48]. Many studies demonstrated that the deactivation of Au/TiO2 catalyst attributed to the production of car bonate species, which competed with the CO2 desorption and lead to the support poisoning [49–51]. Those phenomena in our calculation reveal that a molecule O2 would heal the oxygen vacancy originating from the different procedures, then the peroxides adatoms would react with the CO adsorbates to generate the highly stable carbonate or the molecule CO2, and the carbonate would reduce the utilization of reacted mole cules CO and the productivity of the molecules CO2 in final. Accordingly, since the pre-existence of one oxygen vacancy, a TiO2-x/Au support seems not the satisfied candidate for the catalyst of CO oxidation due to the formation of the carbonate in the CO re-oxidation procedures. After the pathway (1), there are finally two oxygen vacancies on the TiO2-x/Au surface (hereby defined as TiO2-2x/Au). Its electron density difference is shown in Fig. 8. Interestingly, a new Ti3þ ion pointed by the black-arrow locates in the subsurface of TiO2-2x/Au rather than on its surface, which greatly differs from that in Figs. 2(b) and Figure 6(b). It illustrates that the oxygen vacancies reconstruct the surface with the
4. Conclusions First-principle calculations were performed to compare the proper ties of CO oxidation on three kinds of anatase TiO2(001)-supported gold catalysts. The three configurations were defined in one gold adatom located above the site of an oxygen vacancy on TiO2 support, one gold adatom on one site of dual oxygen vacancies, and two gold adatoms above two sites of dual oxygen vacancies. In comparison, the Au ada toms were strongly anchored on the support with an additional oxygen vacancy in second and third case. The strong bonding of Ti–Au–C–Ti with more π-back-donation from an Au adatom to a CO adsorbate occurs when a CO molecular adsorbed on the two Au adatoms, which leaded to a larger activation barrier energy. The CO adsorbate promoted the molecule O2 dissolution in all three configurations at the second stage of Au assistance Mvk mechanism. However, the carbonate formed on the site of pre-existence of one oxygen vacancy in second case results in the support poisoning. As shown, the dual oxygen vacancies on the original
Fig. 8. Calculated electron density difference of one Au adatom on the TiO2-x/ Au support after the first catalytic cycle in Fig. 4. Two oxygen vacancies are marked by two blue-balls. See the definitions of electrons density differences in the caption of Fig. 2. Underneath the site of Ti40 ion, another Ti3þ ion with the localized electrons in yellow is defined by a blue-dot arrow. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 7
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
Table 1 Net charges of Ti ions, O-ions, and Au adatoms on TiO2-x/Au/2(O2) support with or without two CO adsorbates. TiO2-x/Au/2(O2) TiO2-x/Au/2(O2)2(CO)
Ti40
Ti41
Ti43
Ti44
þ1.95 þ1.91
þ1.77 þ1.78
þ2.02 þ2.05
þ2.02 þ2.05
supports in second and third case deteriorated the performance of the TiO2/Au catalyst. Finally, we proposed that the single Auδ adatom coordinated with the unsaturated Ti3þ ion was served as the active sites in CO oxidation on TiO2/Au, that is the first case we mentioned. Hopefully, these calculations can provide the meaningful guidance to improve the activity and selectivity of single atom catalysts.
O1 0.55 0.61
O2 0.45 0.45
O3 0.43 1.09
O4 0.52 1.09
Au 0.50 0.41
[16] G. Pacchioni, Ketonization of carboxylic acids in biomass conversion over TiO2 and ZrO2 surfaces: a DFT perspective, ACS Catal. 4 (2014) 2874–2888. [17] A. Sinhamahapatra, J.P. Leon, J. Kang, B. Han, J.S. Yu, Oxygen-deficient zirconia (ZrO2 x): a new material for solar light absorption, Sci. Rep. 6 (2016) 27218–27225. [18] J. Wan, W. Chen, C. Jia, L. Zheng, J. Dong, X. Zheng, Y. Wang, W. Yan, C. Chen, Q. Peng, D. Wang, Y. Li, Defect effects on TiO2 nanosheets: stabilizing single atomic site Au and promoting catalytic properties, Adv. Mater. 30 (2018) 1705369. [19] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [20] J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond, J. Comput. Chem. 29 (2008) 2044–2078. [21] L. Vo�cadlo, D. Alf�e, M.J. Gillan, I.G. Wood, J.P. Brodholt, G.D. Price, Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth’s core, Nature 424 (2003) 536. [22] D. Alfe, M.J. Gillan, G.D. Price, Constraints on the composition of the Earth’s core from ab initio calculations, Nature 405 (2000) 172–175. [23] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758. [24] L. Li, W. Li, C. Zhu, L.-F. Mao, DFT calculation about oxygen vacancy to promote adsorption of a CO molecule on single Au-supported titanium dioxide, Phys. Status Solidi B 256 (2019) 1800386. [25] J.A. Dean, Lange’s Handbook of Chemistry, fifteenth ed., MacGrawHill, New York, 1999. [26] E. Wiberg, A.F. Holleman, N. Wiberg, Lehrbuch Der Anorganischen Chemie, De Gruyter, Berlin, 2007. [27] G. Henkelman, B.P. Uberuaga, H. J� onsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901–9904. [28] C. Sun, S.C. Smith, Strong interaction between gold and anatase TiO2 (001) predicted by first principle studies, J. Phys. Chem. C 116 (2012) 3524–3531. [29] R. Hengerer, B. Bolliger, M. Erbudak, M. Gr€ atzel, Structure and stability of the anatase TiO2 (101) and (001) surfaces, Surf. Sci. 460 (2000) 162–169. [30] G. Shukri, H. Kasai, Density functional theory study of ethylene adsorption on clean anatase TiO2 (001) surface, Surf. Sci. 619 (2014) 59. [31] M. Lazzeri, A. Selloni, Stress-driven reconstruction of an oxide surface: the anatase TiO2 (001) (1� 4) surface, Phys. Rev. Lett. 87 (2001) 266105. [32] N.H. Linha, T.Q. Nguyen, W.A. Di~ no, H. Kasai, Effect of oxygen vacancy on the adsorption of O2 on anatase TiO2 (001): a DFT-based study, Surf. Sci. 633 (2015) 38–45. [33] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638–641. [34] Y. Tang, S. Zhao, B. Long, J.-C. Liu, J. Li, On the nature of support effects of metal dioxides MO2 (M¼ Ti, Zr, Hf, Ce, Th) in single-atom gold catalysts: importance of quantum primogenic effect, J. Phys. Chem. C 120 (2016) 17514–17526. [35] S.C. Ammal, A. Heyden, Modeling the noble metal/TiO2 (110) interface with hybrid DFT functionals: a periodic electrostatic embedded cluster model study, J. Chem. Phys. 133 (2010) 164703–164717. [36] D. Matthey, J.G. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Laegsgaard, B. Hammer, F. Besenbacher, Enhanced bonding of gold nanoparticles on oxidized TiO2 (110), Science 315 (2007) 1692–1696. [37] Z. Lu, P. Lv, Z. Yang, S. Li, D. Ma, R. Wu, A promising single atom catalyst for CO oxidation: Ag on boron vacancies of h-BN sheets, Phys. Chem. Chem. Phys. 19 (2017) 16795–16805. [38] Z. Lu, P. Lv, Y. Liang, D. Ma, Y. Zhang, W. Zhang, X. Yang, Z. Yang, CO oxidation catalyzed by the single Co atom embedded hexagonal boron nitride nanosheet: a DFT-D study, Phys. Chem. Chem. Phys. 18 (2016) 21865–21870. [39] D. Widmann, R.J. Behm, Active oxygen on an Au/TiO2 catalyst - formation, stability and CO oxidation activity, Angew. Chem. Int. Ed. 50 (2011) 10241–10245. [40] E.J. Peterson, A.T. DeLaRiva, S. Lin, R.S. Johnson, H. Guo, J.T. Miller, J.H. Kwak, C.H.F. Peden, B. Kiefer, L.F. Allard, F.H. Ribeiro, A.K. Datye, Low-temperature carbon monoxide oxidation catalyzed by regenerable atomically dispersed palladium on alumina, Nat. Commun. 5 (2014) 4885. [41] D. Widmann, A. Krautsieder, P. Walter, A. Brückner, R.J. Behm, How temperature affects the mechanism of CO oxidation on Au/TiO2: a combined EPR and TAP reactor study of the reactive removal of TiO2 surface lattice oxygen in Au/TiO2 by CO, ACS Catal. 6 (2016) 5005–5011. [42] J. Liang, Q. Yu, X. Yang, T. Zhang, J. Li, A systematic theoretical study on FeOxsupported single atom catalysts: M1/FeOx for CO oxidation, Nano Res. 11 (2018) 1599–1611. [43] B. Yoon, H. H€ akkinen, U.L. man, A.S. Worz, J.-M. Antonietti, S. Abbet, K. Judai, U. Heiz, Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO, Science 307 (2005) 403–407. [44] L. Lian, P.A. Hackett, D.M. Rayner, Relativistic effects in reactions of the coinage metal dimers in the gas phase, J. Chem. Phys. 99 (1993) 2583–2590.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China under grant Nos. 61774014, and 61272105, Natural Science Foundation of Jiangsu Province of China under grant No. BK20141196. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122291. References [1] B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Single-atom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634–641. [2] X.-F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Single atom catalysts: a new frontier in heterogeneous catalysis, Acc. Chem. Res. 46 (2013) 1740–1748. [3] Y. Tang, Y.-G. Wang, J. Li, Theoretical investigations of Pt1@ CeO2 single-atom catalyst for CO oxidation, J. Phys. Chem. C 121 (2017) 11281–11289. [4] J.J. Yang, M.D. Pickett, X.M. Li, D.A.A. Ohlberg, D.R. Stewart, R.S. Williams, Memristive switching mechanism for metal/oxide/metal nanodevices, Nat. Nanotechnol. 3 (2008) 429–433. [5] D.-H. Kwon, K.M. Kim, J.H. Jang, J.M. Jeon, M.H. Lee, G.H. Kim, X.-S. Li, G.S. Park, B. Lee, S. Han, M. Kim, C.S. Hwang, Atomic structure of conducting nanofilaments in TiO2 resistive switching memory, Nat. Nanotechnol. 5 (2010) 148–153. [6] A. Ruiz Puigdollers, F. Illas, G. Pacchioni, Effect of nanostructuring on the reactivity of zirconia: a DFTþU study of Au atom adsorption, J. Phys. Chem. C 120 (2016) 4392–4402. [7] C. Loschen, A. Migani, S.T. Bromley, F. Illas, K.M. Neyman, Density functional studies of model cerium oxide nanoparticles, Phys. Chem. Chem. Phys. 10 (2008) 5730–5738. [8] C. Loschen, S.T. Bromley, K.M. Neyman, F. Illas, Understanding ceria nanoparticles from first-principles calculations, J. Phys. Chem. C 111 (2007) 10142–10145. [9] Z. Yan, S. Chinta, A.A. Mohamed, J.P. Fackler, D.W. Goodman, The role of Fcenters in catalysis by Au supported on MgO, J. Am. Chem. Soc. 127 (2005) 1604–1605. [10] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hakkinen, R.N. Barnett, U. Landman, When gold is not noble: nanoscale gold catalysts, J. Phys. Chem. A 103 (1999) 9573–9578. [11] M.S. Chen, D.W. Goodman, The structure of catalytically active gold on titania, Science 306 (2004) 252–255. [12] J.A. Rodriguez, G. Liu, T. Jirsak, J. Hrbek, Z. Chang, J. Dvorak, A. Maiti, Activation of gold on Titania: adsorption and reaction of SO2 on Au/TiO2(110), J. Am. Chem. Soc. 124 (2002) 5242–5250. [13] N. Lopez, T.V.W. Janssens, B.S. Clausen, Y. Xu, M. Mavrikakis, T. Bligaard, J. K. Norskov, On the origin of the catalytic activity of gold nanoparticles for lowtemperature CO oxidation, J. Catal. 223 (2004) 232–235. [14] Z. Geng, X. Kong, W. Chen, H. Su, Y. Liu, F. Cai, G. Wang, J. Zeng, Oxygen vacancies in ZnO nanosheets enhance CO2 electrochemical reduction to CO, Angew. Chem. Int. Ed. 57 (2018) 6054–6059. [15] T.N. Pham, T. Sooknoi, S.P. Crossley, D.E. Resaco, Increasing oxide reducibility: the role of metal/oxide interfaces in the formation of oxygen vacancies, ACS Catal. 3 (2013) 2456–2473.
8
L. Li et al.
Materials Chemistry and Physics 240 (2020) 122291
[45] F. Li, Y. Li, X.C. Zeng, Z. Chen, Exploration of high-performance single-atom catalysts on support M1/FeOx for CO oxidation via computational study, ACS Catal. 5 (2014) 544–552. [46] T. Yang, R. Fukuda, S. Hosokawa, T. Tanaka, S. Sakaki, M. Ehara, A theoretical investigation on CO oxidation by single-atom catalysts M1/γ-Al2O3 (M¼ Pd, Fe, Co, and Ni), ChemCatChem 9 (2017) 1222–1229. [47] I.X. Green, W. Tang, M. Neurock, J.T. Yates, Spectroscopic observation of dual catalytic sites during oxidation of CO on an Au/TiO2 catalyst, Science 333 (2011) 736–739. [48] F. Chen, D. Liu, J. Zhang, P. Hu, X.-Q. Gong, G. Lu, A DFT þ U study of the lattice oxygen reactivity toward direct CO oxidation on the CeO2 (111) and (110) surfaces, Phys. Chem. Chem. Phys. 14 (2012) 16573–16580. [49] J. Saavedra, C. Powell, B. Panthi, C.J. Pursell, B.D. Chandler, CO oxidation over Au/TiO2 catalyst: pretreatment effects, catalyst deactivation, and carbonates production, J. Catal. 307 (2013) 37–47. [50] Y. Denkwitz, B. Schumacher, G. Kucerova, R.J. Behm, Activity, stability, and deactivation behavior of supported Au/TiO2 catalysts in the CO oxidation and preferential CO oxidation reaction at elevated temperatures, J. Catal. 267 (2009) 78–88.
[51] Y. Wang, D. Widmann, R.J. Behm, Influence of TiO2 bulk defects on CO adsorption and CO oxidation on Au/TiO2: electronic metal–support interactions (EMSIs) in supported Au catalysts, ACS Catal. 7 (2017) 2339–2345. [52] N. Liu, M. Xu, Y. Yang, S. Zhang, J. Zhang, W. Wang, L. Zheng, S. Hong, M. Wei, Auδ-–Ov–Ti3þ interfacial site: catalytic active center toward low-temperature water gas shift reaction, ACS Catal. 9 (2019) 2707–2717. [53] M. Xu, S. Yao, D. Rao, Y. Niu, N. Liu, Mi Peng, P. Zhai, Yi Man, L. Zheng, B. Wang, B. Zhang, D. Ma, M. Wei, Insights into interfacial synergistic catalysis over Ni@ TiO2-x catalyst toward water-gas shift reaction, J. Am. Chem. Soc. 140 (2018) 11241–11251. [54] S. Idrissi, R. Khalladi, S. Ziti, N. El Mekkaou, S. Mtougui, H. Labrim, I. El Housni, L. Bahma, The electronic and magnetic proprieties of the rare earth-based quaternary Heusler compound LuCoVGe, Phys. B Condens. Matter 562 (2019) 116–123. [55] S. Idrissi, S. Ziti, H. Labrim, R. Khalladi, S. Mtougui, N. El Mekkaoui, I. ElHousni, L. Bahmad, Magnetic properties of the Heusler compound CoFeMnSi:Monte Carlo simulations, Physica A 527 (2019) 121406.
9