Toward a Comprehensive Understanding of Enhanced Photocatalytic

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Towards a comprehensive understanding of enhanced photocatalytic activity of bimetallic PdAu/TiO catalyst for selective oxidation of methanol to methyl formate 2

Kamil Czelej, Karol #wieka, Juan Carlos Colmenares, Krzysztof J. Kurzydlowski, and Yi-Jun Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08158 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Applied Materials & Interfaces

Towards a comprehensive understanding of enhanced photocatalytic activity of bimetallic PdAu/TiO2 catalyst for selective oxidation of methanol to methyl formate Kamil Czelej*,†, Karol Cwieka†, Juan C. Colmenares*,‡, Krzysztof J. Kurzydlowski†, Yi-Jun Xu¶,§ †Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Woloska Str., 02-507 Warsaw, Poland ‡Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka Str., 01-224 Warsaw, Poland ¶State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P. R. China §College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents energy efficient and sustainable route for synthesis of esters. Specifically, the bimetallic PdAu/TiO2 system was found to be highly active and selective towards photocatalytic production of methyl formate (MF) from gas–phase methanol. In the current paper, we applied the electronic structure density functional theory (DFT) method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over PdAu/TiO2 catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanocluster to the interface of PdAu/TiO2 and subsequent formation of the unique structure, resembling core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron–hole separation capability of PdAu nanoparticles deposited onto anatase surface and hence, the remarkably enhanced photocatalytic activity, in comparison to its monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO2 is responsible for total oxidation of methanol to CO2 (no MF formation) in gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles – TiO2 catalysts for the photocatalytic selective oxidation of methanol to MF. Keywords: photocatalysis; selective oxidation; titania; methanol; methyl formate; DFT


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1. Introduction Titania-based photocatalytic technology has focused considerable attention as a consequence of its potential applications in different fields such as green chemistry synthesis, environmental protection or renewable energy. The main driving forces behind the development of TiO2-based photocatalytic technology are low cost, easy reuse of catalytic materials and high reactivity. In the last three decades, TiO2 has been extensively investigated as an eminent material for environmental protection, because of its remarkable chemical stability with minimum photocorrosion, outstanding photocatalytic activity, cost effectiveness and strong oxidizing power of the photogenerated species1–3. Additionally, it is important to highlight that the research on heterogeneous photocatalysis is of particular significance for the organic synthesis and the design of chemical processes that are atom-efficient1,2,4–7. Such studies make valuable contribution to improvement of photocatalytic performance of designed sustainable materials through identification of relationships between the photocatalytic properties and the structure. The preparation route of photocatalysts with desired physico-chemical properties is therefore attributed to the term “photocatalyst design” which became widely accepted in the last decades. Nevertheless, further progress in this field is required, especially considering the scientific outcomes derived from theoretical calculations combined with experimental measurements. Despite aforementioned desirable properties of titania, its widespread use in practical applications is indeed hampered6. In fact, photo-generated electrons and holes tend to recombine rapidly resulting in a poor quantum yield of titania-based catalytic systems. In this regard, doping, semiconductor coupling, and surface sensitization method as efficient ways of enhancing the photocatalytic activity of TiO2 are broadly scrutinized in numerous recent works1,2,4–7. One promising way of improving titania photocatalytic performance is by using alloyed nanoparticle


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support composed of two noble metals. Such a combination of two different noble elements in the nanoparticle may lead to unique electronic structure and as a result, interesting optical and (photo)catalytic properties that are absent in the corresponding monometallic counterparts8–13. As reported in literature, the physico-chemical properties of bimetallic nanoparticles depend strongly on chemical composition and particle size14,15. Selective oxidation of alcohols to aldehydes, ketones and carboxylic acids plays a crucial role in organic synthesis16–21. Specifically, catalytic selective photo-oxidation of biomass (including primary alcohols) can be used for the sustainable synthesis of high value-added chemical compounds, such as MF, aliphatic esters, crotonaldehyde or acetone16–18,21. One of the most difficult challenges in the photocatalytic selective oxidation is the design of photocatalysts and development of robust synthesis methods, ensuring high selectivity to desired final products. It turned out that titania supported metallic nanoparticle systems can be successfully designed as a photocatalysts for selective transformation of methanol to MF22,23. Methanol (in general, alcohols) is regarded as a very attractive starting material for the preparation of different fuels and chemical compounds24–27, as well as represents a typical volatile organic compound (VOC) model

28–30

. A few studies addressing the photocatalytic

selective oxidation of methanol over TiO2 catalyst put emphasis on poor selectivity of intermediate products and total oxidation to carbon dioxide31,32. Amongst the diverse routes for methanol conversion and utilization, the selective photooxidation of methanol to MF represents cheap and environment-friendly process to get precious methanol-downstream products; therefore, its efficient optimisation is of paramount significance in chemical industry. Following along these lines, Kominami et al.33 conducted the pioneering experimental work on the selective photocatalytic oxidation of gaseous methanol to MF under UV illumination conditions, using


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TiO2 photocatalyst in anatase phase. The reported selectivity to MF at room temperature reached 91% while total conversion of methanol was 8% and raised up to 28% when the temperature was 250°C. Formaldehyde, carbon monoxide and carbon dioxide in turn were detected as residual products of the photocatalytic reaction. In other studies, Guo et al.34 have investigated the formation of methyl formate on TiO2 under light illumination at 400 nm wavelength, while Phillips et al.35, using mass spectrometry and scanning tunneling microscopy techniques and analysing the reaction mechanism, have proposed two distinguishable photo-oxidation steps leading to MF from methanol. Han et al.22 demonstrated Au-Ag nanoparticles immobilized onto titanium dioxide which exhibit very high (>90%) methanol conversion and selectivity >85% for the selective photooxidation of methanol to MF. Besides all these investigations focusing on effective photocatalytical synthesis of MF from methanol in gas phase, there is a lack of a comprehensive understanding of enhanced photocatalytic activity of bimetallic systems, especially an intimate correlation of experimental results with well-established theoretical calculations. In the light of our previous experimental investigation23 related to PdAu/TiO2 (bimetallic photocatalyst prepared by our sonophotodeposition method), our present work addresses the electronic structure density functional theory calculations to understand the photocatalytic activity and selectivity in the selective oxidation of methanol to MF in flow gas phase. In fact, an accurate description of photoexcited systems is beyond the scope of methodology we applied and time-dependent density functional theory (TD-DFT) has to be used to accurately account for photoexcitation36,37. For instance, oxygen dissociation and methanol dissociative adsorption are affected by light that cannot be addressed in our work36. Nevertheless, the state-of-the-art TD-DFT methods are computationally too demanding for the investigation of systems consisting of more than several dozen atoms. To the best of our


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knowledge, this is the first comprehensive theoretical analysis on the plausible activation mechanism of (methanol + oxygen) ↔ (PdAu/TiO2) under photon irradiation and the explanation of its enhanced photocatalytic activity and selectivity. A profound understanding of activation processes in photocatalysis is essential for improving photocatalytic materials, optimizing process conditions, and for circumventing the problems related to premature photocatalyst degradation. 2. Computational details We carried out our plane-wave first-principle calculations in the framework of spin-polarized DFT as implemented in the Vienna ab initio Simulation Package (VASP)38 program. We applied the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional to compute the ground-state charge density of the systems. The core electrons were treated using the projector-augmented wave (PAW) method40. The plane-wave energy cutoff was set to 450 eV. On-site Coulomb interactions effect in the Ti 3d orbitals was taken into account following the DFT+U method proposed by Dudarev et al.41. The Ueff= 3.5 eV was selected to reproduce the experimental band structure of TiO2, cf.42,43. The TiO2 anatase (101) surface was described by supercell model containing 288 atoms in 3 TiO2 layers, separated by a vacuum space of 20 Å. Subsequently, the relaxed TiO2 anatase (101) surface was loaded with Au, Pd and PdAu nanoparticles containing 10 and 19 atoms. The dipole induced by the asymmetric slab was cancelled by incorporating a linear electrostatic potential into the local potential44. Because of the large geometrical model considered, a Γ point sampling of the first Brillouin zone was applied to compute the total energy. For geometry optimization, the bottom TiO2 layer of slab was kept frozen in its lattice position, whereas the other 2 layers were allowed to relax. Optimized structures were obtained by minimizing the forces until they fall below the threshold of


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0.03 eV/Å, and energy convergence criterion of 1×10−5 eV is satisfied. A climbing image nudged elastic band (CI-NEB) method was used to find the minimum energy pathway (MEP) for oxygen dissociation reactions. A Bader analysis was carried out in order to quantify the electron transfer between the TiO2 anatase (101) surface and the metal clusters45. 3. Results and discussion 3.1. Representation of the Au/TiO2, PdAu/TiO2 and Pd/TiO2 interfaces. Our recent high resolution transmission electron microscopy (HRTEM) investigation has shown that the interface between TiO2 and metallic particles is predominantly composed of anatase (101) oriented crystals and (111) oriented fcc phase of Au, PdAu and Pd crystals23. Accordingly, the interface is modeled by (111)-terminated Au, PdAu and Pd nanoparticles deposited onto the TiO2(101) surface, in order to reflect the experimental observation. To capture an essential chemistry of the interfaces and, at the same time, keep the numerical model tractable, we have used relatively big 26.6×15.4×29.8 Å supercells consisting of over 300 atoms. The smaller 10 atomic metal clusters were selected to investigate their interaction with oxygen and the surface induced segregation phenomena in PdAu/TiO2 system. Subsequently, the electronic structure of the interfaces was analyzed using the larger 19 atomic metal clusters. In the case of PdAu alloys, it has been reported that Pd:Au ~ 50:50 % composition provides the highest catalytic activity23,46,47, therefore, Pd5Au5 and Pd10Au9 nanoparticles have been taken into account. All geometrical models employed throughout this study are shown in the Supporting Information, Figure S1, S2 and S3. 3.2. Interaction of the metallic nanoparticles with TiO2(101) surface. Complex nature of the metal–titanium oxide interface drives the exceptional photocatalytic activity of PdAu/TiO2 system. Thus, a thorough insight into the electronic structure of the interface is crucial for the


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design of highly active and selective photocatalysts. There are free types of interactions responsible for the formation of interfacial electronic states in the bandgap of ionic oxide supported with a metal cluster. The first type is the metal-induced gap states (MIGS) characterized by continuum of states in the bandgap of the oxide, caused by an exponential decay of delocalized metal orbitals in insulator48,49. Another origin of the interfacial states arises from the bonds between metal atoms and the lattice oxygen anions at the interface50. Those states are localized in the vicinity of both valence band maximum (VBM) and conduction band minimum (CBM) of the ionic oxide. The third phenomenon originates from the polarization of metal electrons by the electric field of the adjacent insulator51. Due to the polarization effect, a charge rearrangement at the interface occurs and electrons begin to populate or deplete the space between the metal atoms and the oxide ions. As a result of the charge transfer through the interface a dipole moment perpendicular to the interface is created52. For all considered metallic nanoparticles the Fermi level lies higher than the CBM of TiO2(101), therefore electrons move from the nanoparticles to the oxide surface until the Fermi energy levels of both are equalized. The experimental measurements based on photoelectron spectroscopy have also revealed charge transfer from Au to the stoichiometric (110) surface of TiO253,54. The Bader charges of the Au19, Pd10Au9 and Pd19 nanoparticle atoms adjacent to TiO2(101) are shown in Figure 1.


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Fig. 1. Bader charges of the a) Au19, b) Pd10Au9 and c) Pd19 nanoparticle atoms adjacent to TiO2(101).


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As can be noticed, cationic and anionic species of Au depending on their relative position on TiO2(101) substrate can be distinguished. The electron redistribution due to interface formation (see Figure 2) indicates that charge density is accumulated in the space right above Ti5C and depleted right above O2C, which is consistent with sign of the Bader charges calculated for the interfacial metal atoms.

Fig. 2. The charge redistribution upon the interface formation: a) (-101) view, b) (0-10) view. Red lobes present electron accumulation while blue lobes present electron depletion. Relaxation of the Au, PdAu and Pd nanoparticles on TiO2(101) surface is leading to a significant distortion of atoms in the clusters (see Supporting Information). In particular, the nanoparticles containing Pd are strongly affected by the interface formation. Our calculations indicate that the initially mixed Pd5Au5 nanoparticle undergoes the oxygen-induced preferential segregation of Pd atoms to the perimeter of PdAu/TiO2(101) interface. The driving force for the preferential segregation is the oxygen affinity of Pd atoms, which is much higher than the oxygen affinity of Au. Similar results were presented by Zhang et al.55, where the same effect was investigated for bimetallic nanoclusters deposited onto CeO2. Interestingly, the Pd–rich interface of PdAu/TiO2 system was also observed under transmission electron microscopy


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(TEM)23,56. As a result of Pd segregation to the interface, Au becomes separated from Pd and we are locally dealing with a nanoparticle resembling Au@Pd core@shell structure. We will demonstrate in following paragraph that this particular segregation has a strong impact on the position of Fermi level and optimal photocatalytic performance. 3.3. Photocatalytic performance of Au/TiO2, PdAu/TiO2 and Pd/TiO2 systems. We have already demonstrated that interface formation between metallic nanoparticles and TiO2(101) surface induces the charge transfer, making the interface negatively charged. When the metalsupported TiO2 catalyst is exposed to continuous illumination, the photoexcitation takes place and the photoexcited electrons in the conduction band of titanium dioxide can be instantly transferred to the surface electronic states of the metal, leaving behind hole localized at the valence band of TiO222,23,57–59. The photoinduced electron transfer from TiO2 to metallic clusters is a very fast phenomenon in comparison to the interfacial charge transfer from metallic clusters to TiO246. According to the experimental measurements conducted by Su et al.46 the kred/krev ≈ 2.5 for AuPd core@shell immobilized on TiO2, where kred is the rate constant related to the electron transfer from TiO2 to metallic clusters and krev is the reversed transportation rate constant. As a result, the accumulated electrons get trapped at the unoccupied acceptor levels in the bandgap of TiO2 during a reverse transportation process, and the holes exhibit prolong lifetime, improving the efficiency of photocatalytic oxidation reaction46,60. On this basis, the optimal photocatalyst requires a fast hole-mediated oxidation reaction and a slow reverse electron transportation process46. The band diagram with the calculated energy levels in the gap for Au/TiO2(101), PdAu/TiO2(101), Pd/TiO2(101) and bare TiO2(101) is depicted in Figure 3. As can be seen, the PdAu/TiO2(101) is characterized by the deepest position of Fermi level amongst the catalysts


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considered, as well as possesses the highest number of unoccupied acceptor states, which are capable of trapping photoexcited electrons. The Fermi level of metal-supported TiO2 depends on how many electrons from metal-surface states can populate the acceptor-like states induced in the band gap of TiO2, until the chemical potential of the entire system will reach equilibrium. In the case of bimetallic Au@Pd core@shell structures deposited onto TiO2, the interface is composed of 3 distinguishable layers: Au-rich core | Pd shell | TiO2. As can be seen, the Pd shell is in direct contact with TiO2 surface; therefore, the electron density of Pd shell layer limits the electron flow to the acceptor-like states induced in the band gap of TiO2. Due to lower work function value of Pd (W= 5.00 eV) in comparison to Au (W= 5.32 eV), the Au core of AuPd nanopartice polarizes the electrons in Pd layer resulting in electron density depletion in the shell. Interestingly, the electron density depletion on the surface of AuPd nanoparticles has been reported in ref.46. As a result of the polarization effect from Au-rich core, causing the electron density depletion in the Pd shell, fewer electrons are capable of flowing to the TiO2 band gap-induced states. Hence, the Fermi level of PdAu/TiO2 is lower than Pd/TiO2 and Au/TiO2. We can assume that photocatalytic performance increases with the increasing number of the unoccupied states above Fermi level. The analysis of the calculated band gap diagram provides an explanation of the superior electron–hole separation capability of PdAu nanoparticles deposited onto anatase surface, in comparison to its monometallic counterparts.


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Fig. 3. Energy band diagrams calculated for bare TiO2(101) slab as well as all the 19 atomic metal clusters supported on TiO2(101) considered in this study. The light orange shaded area represents the experimental band structure of anatase, whereas, the light blue shaded area indicates the occupied energy levels. The energy is referenced to the vacuum level.


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3.4. Adsorption of oxygen and its impact on the selective photo-oxidation of CH3OH to HCOOCH3. To understand the observed differences in conversion of methanol and its selectivity to MF for monometallic Pd/TiO2 and bimetallic PdAu/TiO2 we have investigated the adsorption, activation and dissociation of O2 at the metal-oxide interface. It has been well established that oxygen partial pressure has a remarkable impact on the selective photo– oxidation of CH3OH to HCOOCH3 for noble metal–supported TiO2. According to Han et al.22, oxygen atoms from dissociative adsorption of O2 on AgAu/TiO2 catalyst repeal the H2O formed from the surface hydroxyls and replenish the oxygen vacancies on the surface of titanium dioxide; therefore; they are beneficial for the formation of the coordinated formaldehyde and coordinated methoxy. On this basis it is interesting to gain some insight into the interaction of molecular oxygen with the catalysts investigated. Due to complex geometry of the relaxed metallic cluster on TiO2(101) a large number of adsorption sites can be distinguished. Therefore, only selected ground–state adsorption conformations of O2 are displayed in Figure 4 and their properties are juxtaposed in Table 1. In addition, average adsorption energy on top and interface for all considered systems as a function of Au concentration in the cluster is presented in Figure 5.


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Fig. 4. The ground state geometry of the adsorbed O2 on top and interface Table 1 The calculated parameters of adsorbed O2 on Au/TiO2(101), PdAu/TiO2(101), Pd/TiO2(101) and bare TiO2(101)

Au/TiO2(101) AuPd/TiO2(101) Pd/TiO2(101) TiO2(101)

Site

Bond length [Å]

Mag. [µB]

Ead [eV]

Interface Top Interface Top Interface Top

1.45 1.33 1.47 1.31 1.47 1.35

0.00 0.80 0.00 1.08 0.00 0.46

-1.66 -0.20 -1.44 -0.32 -1.80 -1.69

1.23

1.60

-0.07


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Fig. 5. Average adsorption energy of O2 as a function of Au concentration in nanoparticle. Electron rich interface of metal–titanium dioxide catalyst promotes a strong chemisorption of oxygen molecule. On the other hand, the adsorption of O2 on stoichiometric TiO2(101) is extremely week and unlikely to occur under relevant thermodynamic conditions. The most active sites for O2 adsorption at the perimeter of metal–titanium dioxide interface are situated between Ti5C and Pd or Au, where a significant charge enrichment has been observed. A bond length of ~1.47 Å and zero spin state are an indication of peroxide species (O22-) and suggest a charge transfer to the lowest unoccupied molecular orbital (LUMO) of O261. This strong bond activation of O2 indicate a possibility of facile dissociation. In fact, the minimum energy paths (MEP) presented in Figure 6 show that dissociation of O2 adsorbed at the interface is thermodynamically favorable due to negative driving force and have a very small activation barrier: 0.13 eV in the case of Pd/TiO2 and 0.22 eV in the case of PdAu/TiO2. Such a small energy barrier enables a quick dissociation of oxygen at room temperature and will have a significant impact on selectivity to HCOOCH3.


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Fig. 6. Minimum energy path of O2 dissociation at the perimeter of a) Pd/TiO2(101) and b Pd5Au5/TiO2(101) interface. The structure of the initial (IS), transition (TS) and final (FS) states are displayed within the circles. 3.5. Selectivity dependence on the chemical composition of metallic nanoparticle support. According to our experimental investigation, the MF selectivity in the products of selective photo–oxidation of methanol over PdAu/TiO2 catalyst depends strongly on the chemical composition of the metallic support itself. The best photocatalytic performance has been observed for 50÷50 PdAu/TiO2 system with selectivity to MF > 70%, whereas, the monometallic Pd/TiO2 system was exclusively selective for CO2 formation. We now demonstrate that the most likely reason for the tremendous difference in selectivity to MF is associated with in situ oxidation of palladium during the photo–oxidation reaction. As we have shown in previous paragraph, the adsorbed oxygen molecule at the electron–rich interface can easily dissociate into atomic oxygens because of very low activation barrier. This, in turn, may lead to the formation of PdO clusters on the TiO2 surface under oxygen–rich environment. We were even able to find a fingerprint of the in situ oxidation reaction in the experimental profiles of photocatalytic methanol conversion as a function of light irradiation time (see Figure 7). For all investigated specimens, except Pd/TiO2, the conversion curves reach plateau within ~10-15 min. For the


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monometallic Pd/TiO2, however, one can clearly see the linear dependence of MF conversion from ~10 to ~40 min of light irradiation, that indicates a dynamic transformation of the photocatalyst. Combining our theoretical and experimental findings the dynamic transformation can indeed be attributed to in situ oxidation of palladium.

Fig. 7. Photocatalytic methanol conversion as a function of light irradiation time. To understand how the presence of PdO influences the selectivity of the photocatalysts we have investigated the electronic structures of lattice oxygen at the triple point boundary (TPB) region. Based on temperature programmed reaction experiment combined with scanning tunneling microscopy (STM) observation Phillips et al.35 have demonstrated that mechanism of the oxidation of methanol to MF over TiO2(110) photocatalyst includes two photocatalytic steps: formaldehyde formation and subsequent MF formation. They suggested that a hole created on bridging oxygen from UV excitation drives the photo–oxidation reaction. As commonly known,


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CO2 is an ultimate product of methanol oxidation reaction; therefore, the redox potential necessary to oxidize methanol to CO2 is higher than the redox potential needed to selectively obtain MF. Assuming that the oxidizing agent is the hole localized in VBM states of TiO2, the higher the oxidation potential of the hole the deeper its position in the band diagram. The local density of states calculated for O2C at the TPB region are presented in Figure 8. For all metallic nanoparticles supported on TiO2(101) the VBM states associated with O2C are shifted upwards due to strong metal–support interaction effect, and the VBM edges lay above the redox potential of methanol oxidation to CO2. When the PdO is created on the TiO2(101) surface the Fermi energy drops to a lower level and the p states of O2C localized at VBM edge eventually cross over the redox potential of methanol oxidation to CO2, enabling total mineralization. The gold atoms present in PdAu nanoparticles not only improve the photocatalytic performance but also inhibit the in situ oxidation of Pd, keeping the oxidizing power of photogenerated holes low enough to prevent total mineralization. The differences in methanol conversion and selectivity toward MF between the catalysts investigated indicate that the decomposition of oxygen (the dissociated surface oxygen atoms are the acceptor of the hydrogen eliminated from methanol or methoxy, hence they are beneficial for the formation of the coordinated formaldehyde and coordinated methoxy)15,35 is the rate determining step, and the spillover, oxidation by holes, reaction between coordinated methoxy and formaldehyde surface species and diffusion process are fast steps. It is worth mentioning that the photocatalyst synthesis method has a substantial impact on the structural characteristics of the photocatalyst (e.g., the degree of contact between metals and support, particle size control). According to our analysis and previous results9, an extended flat interface between the intimate Au-Pd interaction with the TiO2 surface, achieved by the sonophotodeposition photocatalysts


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synthesis method, results in a sufficient number of boundaries sites essential for photocatalytic selective oxidation of methanol in gas phase.

Fig. 8. Local density of states calculated for O2C at the perimeter of the interface. Only p-states were shown because of the origin of photoexcited holes. Dashed vertical line indicates the redox potential necessary to oxidize methanol to CO2, which is 1.21 eV vs SHE. The energy is referenced to the vacuum level.


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4. Conclusions The electronic structure DFT calculations presented in the current work indicate that the initially mixed PdAu=1:1 nanoparticles undergo the oxygen-induced preferential segregation of Pd atoms to the perimeter of PdAu/TiO2(101) interface. The driving force for the preferential segregation is the oxygen affinity of Pd atoms, which is much higher than the oxygen affinity of Au. As a result of Pd segregation to the interface, Au becomes locally separated from Pd and the unique architecture resembling Au@Pd core@shell structure is formed in the vicinity of PdAu/TiO2(101) interface. We have demonstrated that this particular segregation has strong impact on the position of Fermi level and optimal photocatalytic performance in the selective oxidation of methanol to methyl formate in gas phase. The gold atoms present in PdAu nanoparticles not only improve the photocatalytic performance but also inhibit the in situ oxidation of Pd, keeping the oxidizing power of photogenerated holes low enough to prevent total mineralization of methanol and promote the high level of selectivity to methyl formate. The opposite occurs in the case of the monometallic (Pd/TiO2) system, where Pd is easily oxidized to PdO during the photocatalytic test, giving rise exclusively a photocatalytic system for total oxidation of methanol to CO2 (no methyl formate formation). The presented thorough insight into the electronic structure of the PdAu/TiO2 interface, supported by well-established theoretical calculations and well-designed experimental studies, is crucial for the design of highly active and selective photocatalysts. These results provide useful information on the design and preparation of photocatalysts for the selective oxidation of methanol to methyl formate in a more green and sustainable way.


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ASSOCIATED CONTENT Supporting Information. Detailed geometries of all systems considered in this study: isolated 10- and 19-atom Pd, Au, PdAu clusters; TiO2(101) anatase surface; relaxed geometries of 10and 19-atomic Pd, Au, PdAu clusters deposited on TiO2(101); PdO cluster deposited on TiO2(101). Corresponding Author Kamil Czelej, e-mail: [email protected] Juan Carlos Colmenares, e-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT Computing resources were provided by Poznań Supercomputing and Networking Center (PSNC) under Grant No. 275. We would like to thank Dr Karl Schliep for helping us proofread the manuscript.

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Toward a Comprehensive Understanding of Enhanced Photocatalytic

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