Atomic-Scale Explanation of O2 Activation at the AuâTiO

Subscriber access provided by Kaohsiung Medical University

Article 2

2

Atomic scale explanation of O activation at the Au-TiO interface Niklas Siemer, Alexander Lüken, Michal Zalibera, Johannes Frenzel, Daniel MuñozSantiburcio, Anton Savitsky, Wolfgang Lubitz, Martin Muhler, Dominik Marx, and Jennifer Strunk J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10929 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Atomic scale explanation of O2 activation at the Au-TiO2 interface Niklas Siemer1,‡, Alexander Lüken2,‡, Michal Zalibera3,4, Johannes Frenzel1, Daniel MuñozSantiburcio1,† , Anton Savitsky5, Wolfgang Lubitz3, Martin Muhler2, Dominik Marx1,*, Jennifer Strunk6,* 1

Ruhr-Universität Bochum, Lehrstuhl für Theoretische Chemie, 44780 Bochum, Germany Ruhr-Universität Bochum, Lehrstuhl für Technische Chemie, 44780 Bochum, Germany 3 Max-Planck-Institut für Chemische Energiekonversion, 45470 Mülheim/Ruhr, Germany 4 Slovak University of Technology in Bratislava, Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, SK-812 37 Bratislava, Slovakia 5 Technische Universität Dortmund, Fakultät Physik, 44227 Dortmund, Germany 6 Leibniz-Institut für Katalyse e.V. an der Universität Rostock, 18059 Rostock, Germany 2

KEYWORDS Finite-temperature ab initio simulations; Electron paramagnetic resonance (EPR); selective oxidation of alcohols; photocatalysis; heterogeneous catalysis.

ABSTRACT: By a combination of EPR spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO 2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently than on the bare TiO 2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes.

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Despite the longstanding success story of gold catalysis, the impact of size and shape of the gold nanoparticles[1-4] as well as the nature and role of the Au-TiO2 interface[3-7] in the catalytic performance of Au/TiO2 have remained the focus of vivid scientific discussions. Furthermore, applications of such gold catalysts cover a much broader range today, including photocatalysis[8] and photoelectrochemical applications[9] to name but two. In classical (thermal) heterogeneous catalysis, gold nanoparticles on oxides have primarily received attention due to their unique capability to catalyze the oxidation of CO[4,10-13] and the selective aerobic oxidation of alcohols and alkenes[14-18] at remarkably low temperatures. Both reactions require the activation of molecular oxygen, a step that has consequently been widely studied experimentally and computationally. Although a certain consensus has now been reached that activation of dioxygen is favored at perimeter sites, [19-22] the elementary processes that finally lead to activation and possibly dissociation of dioxygen are still debated. [19,20,23,24] Furthermore, adsorbed dioxygen species such as superoxide[25,26] and hydroperoxide,[27] as well as lattice oxygen species according to the Mars-van Krevelen mechanism[21] have been proposed to be involved. Moreover, co-adsorption of CO was assumed in computational studies to assist in oxygen activation.[19,20] More information on gold catalysts and oxygen activation in classical heterogeneous catalysis is provided in the Supporting Information (SI). Overall, the dioxygen activation step itself is an important, yet open issue in selective oxidation reactions of alcohols. This is the case in thermal catalysis, but even more so in photocatalysis. When Au/TiO2 is used in photocatalysis, the roles of TiO2 and gold differ depending on the light source with which the material is irradiated. Under UV light irradiation, as is performed in the present study, the semiconductor TiO 2 functions as photoabsorber, in which electrons are excited across the band gap. The gold nanoparticles on the surface, like other noble metals on TiO2, aid in the extension of electron-hole lifetime. The photoexcited electrons in the semiconductor preferably migrate to the noble metal nanoparticles so that charges are separated across the Schottky barrier.[28-36] Apart from collecting electrons, the gold nanoparticles can function as co-catalyst for reduction reactions.[37-42] Under anaerobic conditions, Au/TiO2 has frequently been used as co-catalyst for hydrogen evolution from alcohols,[38-43] whereby the alcohol was commonly degraded non-selectively. However, Pt and Pd have often been found superior in catalyzing hydrogen evolution.[38-41] In presence of oxygen, selective oxidations of alcohols have been carried out with Au/TiO2.[24,44-49] In liquid-phase methanol oxidation under irradiation with a full-range Xenon lamp, the reaction mechanism on Au nanoparticles in mesoporous TiO2 interparticle networks was explained with a direct oxidation of methanol on the TiO2 surface. The electrons were transported over quite long distances through the interparticle networks to the Au nanoparticles. At the Au nanoparticles, oxygen reduction was suggested to take place, but the products of this reaction were not further specified.[44] This mechanism was not evidenced at this time, but it corresponded to a mechanism suggested earlier for 2-propanol oxidation on Ag/TiO2 under UV+Vis illumination.[33] For Au-TiO2 snowman-like heterodimer structures, the same mechanism was proposed, but photoluminescence measurements revealed a second possible mechanism in which the photogenerated electrons might rapidly decay into trap states in TiO 2. From the trap states, interfacial charge transfer to gold was less efficient

Page 2 of 17

than from the TiO2 conduction band.[45] Ethanol was photocatalytically oxidized to acetaldehyde with a selectivity up 95% using photodeposited gold nanoparticles on a sol-gel prepared TiO2.[47] In situ infrared spectroscopy revealed that hydrogen bonding interaction of the alcohol was more likely than ethoxy formation on metallized (Au, Pt) TiO 2, and that acetate formation occurred as side product. The selectivity to acetaldehyde was higher for Au/TiO2 than for Pt/TiO2.[48] A high conversion of 96% in the photocatalytic oxidation of 2propanol was achieved with Au/TiO2, and while conversion was higher for the gold-loaded sample, the selectivity to acetone was lower than for pure TiO2. It was again suggested that charge separation was enhanced due to the presence of gold, leading to the better photocatalytic performance.[49] In a previous vibrational spectroscopy study by us employing UV light only (365 nm),[24] evidence has been obtained that in selective 2-propanol oxidation the adsorbed alcohol can react directly with the photogenerated holes on the TiO2 interface, but the corresponding photoexcited electrons react too slowly, leading to an accumulation of excess electrons in TiO2. This accumulation of excess electrons in bare TiO2 was monitored by vibrational spectroscopy. On the contrary, in Au/TiO2, the accumulation of excess electrons in TiO2 is prevented, which can be rationalized by their migration to the gold nanoparticles and the subsequent efficient transfer from gold to the co-reactant oxygen, possibly at interface sites.[15,24] On different forms of pure TiO2, photocatalytic oxidation of 2-propanol has been studied in more detail.[50-54] Early studies suggested that the selective oxidation to acetone proceeded through a hydrogenbonded intermediate, whereas the chemisorbed 2-propoxide was oxidized completely to CO2.[50,51] Undissociated 2propanol was suggested to trap holes directly, and electrons were transfer onto adsorbed molecular oxygen.[52] In later studies it was instead assumed that both dissociated and undissociated strongly bound 2-proponal species were converted to acetone, which may further react to CO2.[53,54] Reaction to acetone occurred both in presence and absence of O 2,[53] again suggesting that holes may directly carry out this reaction. Under irradiation with pure visible light or sunlight, TiO2 is not able to absorb significant parts of the incoming radiation. Instead, gold can absorb visible light by plasmon resonance[36,37,55-62] and processes such as electromagnetic field enhancement,[56,63] hot electron transfer[36,55,57,61,64] or local heating effects[55] may enhance photocatalytic performance compared with bare TiO2. Consequently, the direction of electron transfer and the reaction mechanism may change as a function of the wavelength used for excitation. [36,37,44,57,62,63,65] When electrons are transferred from gold to titania under visible light irradiation, an oxidation reaction on the gold surface instead of on titania must be assumed,[59,60] whereas oxygen reduction should then occur on the titania surface.[37,60] Since the separation of charge carriers across the Schottky barrier favors electron transport in the opposite direction, this process is detrimental for photocatalytic activity under visible light.[61,62] Visible light excitation is not employed in the present study, so the potential processes are not discussed further at this point. Very detailed discussions can be found in Refs. [12,66]. The nature of active (di)oxygen species and the exact directional transfer of excess electrons thus require clarification on the atomic scale. In this contribution, activation of oxygen at the Au-TiO2 interface is explained comprehensively at different lengths scales, down to a molecular orbital picture at finite temperature. From a combination of EPR spectroscopy, ab initio simulations yielding activation free energies, and elec-


2

Page 3 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tronic structure analyses based on representative ensembles of configurations sampled along the activation pathway, we provide direct evidence for the direction of the electron transfer processes from the catalyst to the adsorbed oxygen molecule. Experimentally, superoxide species are formed in a photoinduced process and the surface chemistry at the Au-TiO2 interfaces favors the splitting of the O-O bond. EXPERIMENTAL AND COMPUTATIONAL DETAILS Commercial Au/TiO2 (Aurolite, STREM Chemicals Inc.) and TiO2 (P25, Evonik Industries AG) were used. Previous studies showed that the commercial Au/TiO2 catalyst has P25 as support.[15,16] P25 is known to be a mixture of rutile, anatase and amorphous TiO2 phases.[67] Both catalysts have been extensively characterized in various studies. [16,68-70] All CW EPR spectra were recorded at 10 K before and after sample irradiation using a Bruker Elexsys E-500 CW X-band spectrometer equipped with an ER 4116DM dual mode resonator operating in perpendicular mode and a He-flow cryostat (Oxford Instruments ESR900). The microwave power and field modulation were fixed for all experiments at 0.2 mW and 0.2 mT, respectively. The g factor scale was calibrated with a LiF standard (g = 2.0023). For the EPR measurements, the dry catalyst powders (10 mg of Evonik P25, or Au/TiO 2) were filled into EPR quartz tubes (15 mm filling height in 3.8/2.6 mm o.d./i.d. tube) and wetted with 20 µL of 2-propanol. Samples were degassed by three freeze-pump-thaw cycles and sealed under vacuum (1×10-5 bar) or synthetic air (1 bar). For the samples sealed in air, an estimation of the amount of the reaction partners in the EPR tube results in ~32 µmol O 2 and ~260 µmol 2-propanol. These measurements under approximately eightfold 2-propanol excess are termed as “simulated reaction conditions”. Control samples were prepared in the same manner but without 2-propanol. All samples were irradiated at room temperature for 15 min and 30 min using a custom-designed LED array consisting of 6 LEDs in one row emitting at a wavelength of 375 nm with a nominal electric power input of 60 mW each. In the center of the LED array, an output power of 5.82 mW/cm2 was measured (Actinometer, Thorlabs). Details of the pulsed EPR experiments are provided in the SI. Our computational model has been introduced and validated in many respects in our previous investigations of titania surfaces and gold/titania systems, as provided in what follows, to which we refer the reader for full details and fundamental references. In short, the supported nanocatalyst model consists of a Au11 cluster on top of an oxygen surface vacancy of the TiO2 support which effectively pins the metal nanoparticle in space.[22,71,72] The supporting titania is modeled by four O−Ti2O2−O trilayers forming a large (6×2) supercell slab with the cell parameters a = 13.15 Å and b = 17.80 Å of the orthorhombic cell. The two trilayers at the bottom of the slab are constrained at their equilibrium positions. To mimic bulk-like behavior the bottom of the slab is passivated with pseudo hydrogen atoms of nuclear charge +4/3 and +2/3. The individual titania slabs are separated by more than 10 Å. Extensive convergence tests including the slab thickness of our previously used smaller (4×2) titania surface model are provided in Ref. [73]. The density functional theory (DFT) based calculations have been performed using the CPMD[74] and Quantum Espresso/PWscf[75] codes for dynamic and static computations, respectively. All calculations have been performed using spinpolarized GGA+U based on the Perdew-Burke-Ernzerhof ex-

change correlation functional (PBE).[76-78] The spin-polarized Kohn-Sham equations were solved in the plane-wave / pseudopotential framework using Vanderbilt’s ultrasoft pseudopotentials[79] with a cutoff of 25 Ry where k–point sampling was restricted to the –point. In line with our previous work,[22,71,72,80,81] we again made use of our GGA+U implementation[82] based on a self-consistent U calculation[83] yielding without any adjustment a value of U = 4.2 eV for the present case.[80] For the static electronic structure calculations, the energy threshold for the convergence of the Kohn-Sham orbitals was set 510−7 Ry. The occupations of the electronic states were smeared out with Gaussian functions with a width of 0.1 mRy to facilitate optimization. For all sampled snapshots, the density of states (DOS) was calculated at the -point. For presentation purposes, the states in DOS plots were broadened by Gaussians with a width of 40.8 meV (3.0 mRy). A projection of the DOS onto atomic orbitals (PDOS) allows one to assign the contributions of individual atoms and provides the corresponding Löwdin population of that atom.[84] For ab initio molecular dynamics (AIMD) simulations[85] the propagation of the systems is done using the Car-Parrinello MD algorithm as implemented in CPMD. Nosé-Hoover chain thermostatting[86] is used to keep the temperature at 450 K and the average fictitious kinetic energy of the electrons constant (at 0.1 a.u.). A fictitious electron mass of μ = 350 a.u. together with a time step of 5.0 a.u. are used. The free energy profile is calculated using the thermodynamic integration technique as described e.g. in Ref. [85]. Here, we use the O−O bond distance which is constrained at different values that carry O2 from the molecular regime beyond the transition state for dissociation. The integration of the resulting average constraint forces yields the free energy profile as a function of the oxygen-oxygen distance, F(dO−O). Each different replica of the system corresponding to a distinct constraint value was carefully equilibrated after which the average constraint force was collected for at least 7.5 ps depending on the convergence of the actual replica as a function of sampling time. This yields the free energy profiles shown in Fig. 3 where the shaded areas indicate the errors obtained by considering the maximum and minimum constraint forces as estimated by plus/minus one standard deviation at each quadrature point. RESULTS AND DISCUSSION In general, EPR spectra of irradiated TiO2 samples can be divided into two magnetic field regions separated by the g = 2 value. EPR signals appearing in the field range corresponding to g > 2 are generally assigned to the oxygen-related species[36,87]. The signals emerging at the fields below g = 2 are associated with various forms of Ti3+, related to the trapped photo-generated electrons.[36,87] Figure 1 displays the X-band CW EPR spectra of the bare TiO2 P25 and the commercial Au/TiO2 catalyst with (Figure 1A) or without (Figure 1B) wetting the samples with 2propanol. Spectra were recorded either in vacuum or in the presence of air. The control experiments on TiO2 and Au/TiO2 samples in the absence of 2-propanol and in vacuum provide EPR signals of negligible intensity even after prolonged irradiation (Figure 1B, bottom), since in the absence of both hole and electron scavengers rapid charge recombination occurs after photoexcitation of TiO2. All EPR spectra measured in the presence of 2-propanol (Figure 1A) are dominated by signals of Ti3+ species, i.e., excess electrons at reduced Ti sites, because the reaction of the


3


alcohol with the photogenerated hole on the TiO2 surface is facile.[24] The EPR spectra consist of overlapping EPR signals showing contributions from multiple Ti3+ species, and most of them were assigned using data from the literature, see Table 1. The identified Ti3+ centers are located in the anatase lattice (g⊥ = 1.990, g∥ = 1.960),[36,88-92] in a distorted interfacial region between rutile and anatase (g⊥ = 1.978 and g∥ = 1.960),[93] as well as at the surface of anatase (broad line, g ≈ 1.93).[88,90,92] The surface character of the latter Ti3+ species was recently confirmed using an elegant strategy of 17O surface labelling, combined with electron spin echo envelope modulation (ESEEM) hyperfine spectroscopy,[59] and is in line with our own 1H ENDOR (electron nuclear double resonance) results (see SI for details). The broad character of the EPR line at g ≈ 1.93 was previously explained by the g strain effects associated with a substantial heterogeneity of the paramagnetic centres and related to the variety of crystal faces and morphological defects present at the surface.[92,94] Further detailed information on the Ti3+-related signals is provided in the SI. Samples irradiated in the presence of 2-propanol under air atmosphere (Figure 1A, top; simulated reaction conditions) show considerably weaker Ti3+ signal intensities than samples irradiated in vacuum (Figure 1A, bottom). The pure TiO2 sample wetted with 2-propanol and measured in vacuum turned blue upon irradiation, a typical characteristic of a selfdoped TiO2-x material[95,96], where the blue color stems from the Ti3+ defects in the vicinity of oxygen vacancies. The spectrum of this sample (Figure 1A, bottom) showed two additional spectral features due to Ti3+ species, one of which (g⊥ = 1.975, g∥ = 1.940) has previously been assigned to Ti3+ species in the rutile lattice.[90] A second Ti3+ species appears with g⊥ = 1.971 and g∥ = 1.945. This species is stable upon sample annealing in vacuum, but disappears together with the blue color upon exposure of the sample to ambient air (Figure S1). In accordance with previous works, [25] this EPR signal is assigned to Ti3+ with oxygen vacancies in their coordination sphere located in the rutile phase (see SI for details). It is important to note that the broad signal at g ≈ 1.93 assigned to anatase surface Ti3+, i.e., surface-trapped electrons, was still detectable in the air-treated pure TiO2 sample (Figure S1) and under simulated reaction conditions of selective alcohol oxidation (Figure 1A, top; wetted with 2-propanol and measured in air). This underlines our previous observation that a reaction of excited electrons with adsorbed dioxygen is slow in case of bare TiO2.[24] Although the catalytic cycle is closed in Table 1: EPR Parameters of Ti3+ centers in TiO2 System/ assignment Ti3+ lattice anatase Ti3+ lattice rutile in P25 Ti3+ surface anatase Ti3+ surface anatase in P25 Ti3+ 4 coordinated at the rutile-anatase interface in P25 Ti3+ in rutile after photocatalytic H2O splitting

g⊥

g∥

Reference

1.990 1.975 1.930

1.960 1.940 1.885 1928

[88-92,97-99] [90,99] [88,89,91,92] [90,97-100]

1.979

-

[89,97]

1.972

1.949

[101]

presence of both 2-propanol and oxygen, a net excess of electrons occurs, since the consumption of the electrons by the formation of superoxide is likely the slowest step (see below). In stark contrast, EPR spectra of Au/TiO2 under simulated

Page 4 of 17

reaction conditions (Figure 1A, top) contain a smaller contribution from Ti3+ signals and the anatase surface Ti3+ signal (g ≈ 1.93) is almost completely absent. This observation indicates a removal of excess (surface) charge from TiO2 due to the presence of the gold nanoparticles. Since, in quantitative terms, the broad signal of anatase surface Ti3+ at g ≈ 1.93 represents the major contribution in the spectra shown in Figure 1A, we have performed several pulse EPR experiments to characterize it, and the Ti3+ species involved, in more details. The electron spin echo detected EPR spectra (ESE-EPR) of the Au/TiO2 sample irradiated in vacuum in the presence of 2-propanol (Fig S2), showed variation in shape and signal magnetic field position depending on the microwave pulse powers applied. The optimal power (or length) of a microwave pulse, in a pulse EPR experiment is determined by the spin state of the species under study, and the discussed changes thus indicate the presence of Ti 3+ species with different total spin multiplicities. Detailed information on the electron spin state can be obtained from the transient nutation EPR experiment (TN).[102-104] Very recently, this approach was used to identify high-spin states in reduced anatase TiO2[105] and a MgO catalyst.[106] The 2D-TN EPR spectrum of the Au/TiO2 vacuum sample is shown in Figure 2. The obtained signals can be rationalized by using Equation (1),[102]  nut (mS , mS  1) 

g e B1  , h

  S (S  1)  ms (ms  1)

(1)

which provides a simple relationship between the observed nutation frequency and the quantum numbers S and mS of a given spin state transition. Three distinct ridges can be recognized in the spectrum, with maximum signal intensities at different g values and corresponding to νnut of 0.92 MHz, 1.85 MHz and 2.80 MHz, respectively. The ratio of the nutation frequencies 1:2:3 resembles the ratio of the α–factors √1:√4:√9, Eq. (1). Assuming that the signal with the lowest nutation frequency originates from the|-1/2⟩→|1/2⟩ EPR transition of an isolated Ti3+ center with the total electron spin of S = 1/2, the two remaining signals can be identified as |1/2⟩→|1/2⟩ transitions of the S = 3/2 and S = 5/2 spin manifolds, respectively. In line with this interpretation, the ridges assigned to the species with electron spin S = 3/2 and S = 5/2 show additional signals at lower nutation frequencies. These stem from the electron transitions involving higher spin levels (e.g. |–3/2⟩→|–1/2⟩, |–1/2⟩→|3/2⟩, with α = √3 for the S =3/2 and α = √8 for the S = 5/2, respectively). Thus, the TN experiment shows that in addition to the isolated Ti 3+ centers with S = 1/2, clusters containing three to five (S = 3/2, 5/2) ferromagnetically coupled Ti3+ ions are formed at the surface of the studied Au/TiO2 when oxygen is not available to close the catalytic cycle. For these clusters the broadening of the EPR line is not only caused by the g strain effects but additionally involves contributions from the zero field splitting (ZFS) which makes them difficult to characterize by conventional CW EPR. Unfortunately the ZFS patterns are not sufficiently resolved and do not allow to estimate the inter-spin distances in these clusters. The observation of clusters of reduced Ti3+ species in Au/TiO2 photocatalysts is qualitatively in line with the (thermally or light-induced) formation of a reduced rutile phase in the vicinity of the gold nanoparticles, as was observed previously in spatially resolved Raman spectra and ultra-high vacuum FT-IR spectra of the same catalyst.[70] The significant extent of reduction observed in the sample upon


4

Page 5 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exposure to 2-proponol only is a clear indication that the direct oxidation of the alcohol with the photogenerated hole is a feasible process. Furthermore, electron transfer to gold nanoparticles ceases at some point, presumably when the Fermi levels are equilibrated, so further accumulation of electrons is expected to occur equally in both Au and TiO2. When dry TiO2 or Au/TiO2 is exposed to air (Figure 1B, top; 2-propanol not present) the signal of a superoxide species located at the TiO2 surface (gz=2.025, gy=2.009, gx=2.003)[65,89,107,108] is observed for both samples. The gy value allows clear assignment to a superoxide species, and rules out O- and O3- species observed in a previous EPR study on Au/TiO2 with a different type of titania support (TiO2 P90).[36] The superoxide signal is significantly more intense for Au/TiO2 compared with bare TiO2 indicating a more efficient electron transfer to dioxygen in the presence of gold nanoparticles. Together with the observed removal of excess charge from TiO2 by gold, a facilitated transfer of the photo-generated electrons to molecular oxygen due to Au nanoparticles on the TiO2 support is indicated, which supports our suggested mechanism of selective 2-propanol oxidation.[24] For a detailed discussion of the EPR signal of superoxide we refer to the SI. These experimental results clearly show an enhanced catalytic activity towards O2 activation at Au/TiO2. A number of studies have been devoted to provide an understanding at the atomistic scale of such enhanced O2 activation at Au/TiO2 by means of theoretical investigations employing different models for the catalyst and co–reactants.[23,109,110] These studies show the O2 activation barrier to depend on its adsorption site and also on the Au/TiO2 perimeter structure. However, these activation barriers do not account for thermal fluctuation effects known to significantly modulate the shape of the supported gold nanoparticles[110,111] in particular at the elevated temperatures relevant to (photo)catalysis. These fluctuations, of course, induce significant dynamical changes of the Au/TiO2 perimeter structure as well (see Figure S4 for representative snapshots). To capture this key phenomenon, we carried out finite-temperature ab initio simulations using a validated model of the nanocatalyst consisting of a gold cluster consisting of 11 atoms, Au11, pinned to an oxygen vacancy of the rutile TiO2(110) surface (see computational details).[22,72] An oxygen molecule was adsorbed either at a perimeter site of the Au/TiO2 nanocatalyst or at a free surface position (Figure 3). This reduced surface hosts thermalized excess electrons located at Ti3+ sites, which is the same situation as encountered in the EPR experiments after UV irradiation and subsequent ultrafast relaxation of the photogenerated electrons to the electronic ground state. Therefore, our computational modeling does not include the initial electronic relaxation processes involving hot electrons, but rather is exclusively focused on the slowest chemical reaction step observed in the experiment, which is oxygen activation by thermalized excess electrons. The catalytic O2 activation due to the supported Au nanoparticle is directly elucidated by comparing the splitting reaction one-to-one when using the two different aforementioned adsorption sites close to and far from the gold cluster. For both O2 binding sites, we analyzed the bonding charge difference () upon adding the O2 molecule to the Au/TiO2 system which directly probes the charge rearrangement due to the adsorbed O2. As usual, this quantity is calculated as ∆ρ=ρ(O2/Au/TiO2)-ρ(O2)-ρ(Au/TiO2), where (O2/Au/TiO2),

(O2), and (Au/TiO2) are the densities of the whole system, the separated oxygen molecule with the conserved bond length, and the Au/TiO2 surface without the oxygen as obtained from single-point electronic structure calculations while keeping all atomic positions and the supercell size fixed. Using representative snapshots in the first step, this analysis reveals a charge transfer from the Ti atoms towards the oxygen molecule (Figure 3 A and A’). For the oxygen atom in direct contact with the gold cluster, additional charge is transferred from the closest Au atom. The amount of charge transferred to the O2 molecule increases as the O−O bond elongates (see Figure 3 A–C’). In addition to such visualization of (x, y, z) in terms of real-space (isosurface) plots, it is possible to integrate  in the (x,y)–plane at constant z, thus providing the change of the charge difference normal to the surface at the level of an ensemble average at finite temperature. The resulting quantity (z), averaged over all statistically independent configuration that have been sampled at each dissociation stage (i.e. A, B, C and A’, B’, C’ according to Fig. 3), provides us with a chemically intuitive picture of the charge rearrangements in the Au/TiO2 nanocatalyst due to the presence of O2 taking thermal fluctuations explicitly into account. Being far from the perimeter site, the average (z) density difference corresponding to the reactant state A’ shows charge accumulation at the height position of the O2 molecule (Figure 4b). Concurrently, the main charge depletion is observed at this stage close to and above those Ti atoms to which O 2 is adsorbed. When adsorbed at the Au/TiO2 interfacial site, the bottom of the Au cluster also provides additional charge to the O2 molecule at this initial stage of the activation process (Fig. 4f). The amount of charge transferred increases with elongation of the O–O bond for both O2 adsorption sites, i.e. upon proceeding from stage A/A’ to B/B’. For O2 dissociation close to the Au cluster, (zO2) increases from 0.2 to 0.3 to finally 0.4 |e−| Å−1 at stages A, B and C, respectively (Fig. 4f to h). Along this dissociation process, the charge depletion found in the nanoparticle increases systematically as seen by the increasing negative (z) at z-values where the cluster is located. Being far from the Au cluster, the charge on the O2 molecule increases from 0.3 to 0.4, and 0.6 |e −|Å−1 at stages A’, B’ and C’, respectively (Fig. 4b to d). In that situation, the charge of the gold cluster barely changes, (z) is essentially zero, which implies that O2 at this adsorption site is indeed decoupled from the nanoparticle. This supports the use of that adsorption site to provide an internally consistent reference for studying uncatalyzed O2 cleavage using the same Au/TiO2 setup. In conclusion, significant Au–O2 charge transfer occurs upon O2 dissociation if that takes place at Au/TiO2 interfacial sites and increases with O–O bond elongation. This additional Au−O2 charge transfer is crucial for the enhanced oxygen activation at the Au/TiO2 catalyst. To quantify this effect, the activation free energy is extracted from the free energy profiles of O2 dissociation at both binding sites obtained from ab initio free energy sampling (see computational details). This demanding procedure yields an activation free energy of about 80 kJ mol-1 for O2 dissociation far from the nanoparticle, whereas it is significantly decreased to roughly 25 kJ mol-1 at the fluctuating perimeter. In addition to disclosing the catalytic effect in terms of greatly reducing the activation barrier, these simulations show that close contact with the Au cluster causes the O2 dissociation to happen at shorter O−O bond lengths as a result of strong electronic couplings of


5


O2 with the gold cluster (Figure 3) in accord with our experimental findings In order to fully elucidate the mechanistic details of the O 2 activation at the Au/TiO2 catalyst, we analyzed the electronic structure for many statistically independent configurations sampled from the AIMD as reported in the SI. In the following, we restrict our discussion to representative snapshots in order to present an intuitive picture. However, it is important to note that all data relies on sampling at finite temperature. The corresponding averaged quantities in terms of charge transfer, structural features in the close neighborhood of the O2 molecules, and charge rearrangement of the whole catalytic surface are compiled in the SI to complement the proceeding electronic structure analysis. Deep analysis of the electronic structure reveals the mechanistic pathway of the oxygen activation at Au/TiO 2 nanocatalysts. When adsorbed far from the Au cluster, the lowest unoccupied molecular orbital of O2 contains contributions from the oxygens’ p orbitals along the O−O bond and from the d atomic orbitals of both Ti atoms to which O2 is bound (see Figure 5B’). All these atomic orbitals form two -d-p bonding interactions between the Ti and the O atoms and one -p-p anti-bonding interaction within the O2 molecule. Therefore, the occupation of this molecular orbital would weaken the O−O bond, facilitating dissociation. However, when O 2 is decoupled from the gold cluster, this orbital cannot be easily populated. Dissociation only takes place upon significant elongation of the O−O bond, which is when the orbital becomes occupied (see Figure 5C’). On the other hand, when O2 is adsorbed at the Au-TiO2 perimeter it also interacts with the Au atoms, which transfer some charge to O2 as explained above. Here, the electronic structure analysis reveals that this additional charge populates the -p-p anti-bonding orbital of O2. As visualized in Figure 5I, this orbital is already occupied at small O−O bond lengths. Occupation increases as the O−O distance gets elongated, thus clearly facilitating O2 dissociation (see SI for additional analyses). Therefore, the origin of the enhanced O2 activation at the Au/TiO2 interface results from charge transfer from the Au cluster to the -p-p antibonding orbital of adsorbed O2. Our experimental and theoretical results thus underline the previously suggested mode of action of gold as a co-catalyst for superoxide formation in selective 2-propanol oxidation, and they clarify electron transport and oxygen activation on the atomic scale. While our present studies on commercial samples addresses predominantly gold deposited on the rutile phase, it may be interesting to elucidate the influence of the support structure in future studies.

Page 6 of 17

extent even in Au/TiO2, leading to the observation of clusters of Ti3+ species in TN EPR spectra. This indicates that once the gold nanoparticles are saturated with electrons after equilibration of the Fermi levels, electron accumulation takes place in both Au and TiO2. When Au/TiO2 is contacted with air, the more intense signal of a superoxide species evidences the more efficient oxygen activation in comparison to bare TiO 2. Deeper insight into the role of gold is provided by finitetemperature ab initio simulations, and electronic structure analyses. While in principle also possible on bare TiO 2, oxygen activation is significantly favored on the TiO 2 surface at perimeter sites of gold nanoparticles. At such sites, the intramolecular O-O bond of the oxygen molecule is cleaved at significantly shorter bond lengths with a significantly reduced free energy barrier. Our results have important implications for the understanding of the mode of action of Au/TiO 2 catalysts in thermal catalysis and photocatalysis, since we provide direct evidence for the charge transfer pathways of excess electrons. Furthermore, it becomes clear that the catalytic properties of the Au-TiO2 interface, well known to be beneficial for oxygen activation in classical thermal catalysis, also account for the improved photocatalytic performance of such systems in selective oxidation reactions. ASSOCIATED CONTENT Supporting Information. Detailed discussion of previous works on oxygen activation on Au/TiO2; EPR spectroscopy – Detailed discussion of Ti3+-related signals and superoxide signal; Supporting analyses – Electronic structure analyses of O2 along dissociation at finite temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Prof. Dr. Dominik Marx, Ruhr-Universität Bochum, Lehrstuhl für Theoretische Chemie, 44780 Bochum, Germany, [email protected]; *Prof. Dr. Jennifer Strunk, Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany, [email protected].

Present Addresses † CIC nanoGUNE, 20018 San Sebastián, Spain.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally.

Funding Sources

CONCLUSIONS In conclusion, this study clearly shows that excess negative charge, accumulating in TiO2 upon consumption of photoexcited holes by a selective oxidation reaction, is efficiently transferred to gold nanoparticles located on the TiO2 surface. From there, charge can be transferred to oxygen molecules adsorbed at the Au-TiO2 interface, leading to the formation of superoxide species. The experimental confirmation of these processes has been performed by EPR spectroscopy. In presence of the gold nanoparticles, the intensity of the paramagnetic signals associated with (surface) Ti3+ species was significantly decreased. In absence of oxygen as electron scavenger, however, reduction of TiO2 can take place to a significant

Cluster of Excellence RESOLV (EXC 1069); Max-PlanckSociety; Slovak Research and Development Agency (APVV-150053), Slovak Scientific Grant Agency VEGA (1/0416/17, 1/0466/18).

ACKNOWLEDGMENT This work has been supported by the Cluster of Excellence RESOLV funded by the Deutsche Forschungsgemeinschaft, and by the Max Planck Society. M. Z. acknowledges support from the Slovak Research and Development Agency (APVV-15-0053) and Slovak Scientific Grant Agency VEGA (1/0416/17, 1/0466/18). Computational resources were provided by LRZ München on


6

Page 7 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SuperMUC as well as by HPC@ZEMOS, HPC-RESOLV, BOVILAB@RUB, and RV-NRW. We thank Priv. Doz. Dr. Gerald Mathias (High Performance Computing Group at LRZ München) for his help with the optimization of the Hubbard U routine of the CPMD program package.

ABBREVIATIONS CW, Continuous wave; DFT, Density Functional Theory; DOS, density of states, EPR, Electron Paramagnetic Resonance; NEB, nudged elastic band; PBE, Perdew-Burke-Ernzerhof exchange correlation functional.

REFERENCES [1] L. Li, Y. Gao, Y. Zhao, Y. Pei, Z. Chen, X.C. Zeng, CO Oxidation on TiO2 (110) Supported Subnanometer Gold Clusters: Size and Shape Effects, J. Am. Chem. Soc. 2013, 135, 19336. [2] M. Shekhar, J. Wang, W.-S. Lee, W.D. Williams, S.M. Kim, E.A. Stach, J.T. Miller, W.N. Delgass, F.H. Ribeiro, Size and Support Effects for the Water–Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al2O3 and TiO2, J. Am. Chem. Soc. 2012, 134, 4700. [3] Y. Kuwauchi, H. Yoshida, T. Akita, M. Haruta, S. Takeda, Intrinsic Catalytic Structure of Gold Nanoparticles Supported on TiO2, Angew. Chem. Int. Ed. 51 (2012) 7729. [4] M. Haruta, Size- and support-dependency in the catalysis of gold, Catalysis Today 1997, 36, 153. [5] I.X. Green, W. Tang, M. Neurock, J.T. Yates Jr., Low‐ Temperature Catalytic H2 Oxidation over Au Nanoparticle/TiO2 Dual Perimeter Sites, Angew. Chem. Int. Ed. 50 (2011) 10186. [6] Y. Maeda, Y. Iizuka, M. Kohyama, Generation of Oxygen Vacancies at a Au/TiO2 Perimeter Interface during CO Oxidation Detected by in Situ Electrical Conductance Measurement, J. Am. Chem. Soc. 2013, 135, 906. [7] T. Fujitani, I. Nakamura, Mechanism and Active Sites of the Oxidation of CO over Au/TiO2, Angew. Chem. Int. Ed. 2011, 50, 10144. [8] M. Murdoch, G.I.N. Waterhouse, M.A. Nadeem, J.B. Metson, M.A. Keane, R.F. Howe, J. Llorca, H. Idriss, The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles, Nat. Chem. 2011, 3, 489. [9] R. Negishi, S. Naya, H. Kobayashi, H. Tada, Gold(Core)–Lead(Shell) Nanoparticle‐ Loaded Titanium(IV) Oxide Prepared by Underpotential Photodeposition: Plasmonic Water Oxidation, Angew. Chem. Int. Ed. 2017, 56, 10347. [10] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O4, J. Catal. 1993, 144, 175. [11] G.C. Bond, D.T. Thompson, Gold-catalysed oxidation of carbon monoxide, Gold Bull. 2000, 33, 41. [12] D.A. Panayotov, J.R. Morris, Surface chemistry of Au/TiO2: Thermally and photolytically activated reactions, Surf. Sci. Rep. 2016, 71, 77. [13] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide, J. Catal. 1989, 115, 301.

[14] T. Hayashi, K. Tanaka, M. Haruta, Selective VaporPhase Epoxidation of Propylene over Au/TiO2 Catalysts in the Presence of Oxygen and Hydrogen, J. Catal. 1998, 178, 566. [15] M. C. Holz, K. Kähler, K. Tölle, A. C. van Veen, M. Muhler, Gas‐ phase oxidation of 2‐ propanol over Au/TiO2 catalysts to probe metal–support interactions, Phys. Status Solidi B 2013, 250, 1094. [16] D. Heeskens, P. Aghaei, S. Kaluza, J. Strunk, M. Muhler, Selective oxidation of ethanol in the liquid phase over Au/TiO2, Phys. Status Solidi B 2013, 250, 1107. [17] V.I. Sobolev, K.Yu. Koltunov, O.A. Simakova, A.-R. Leino, D.Yu. Murzin, Appl. Catal. A: General 2012, 433-434, 88. [18] B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Reactivity of the Gold/Water Interface During Selective Oxidation Catalysis, Science 2010, 330, 74. [19] I.X. Green, W. Tang, M. Neurock, J.T. Yates Jr., Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst, Science 2011, 333, 736. [20] Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C. Sanchez, P. Hu, Catalytic Role of Metal Oxides in Gold-Based Catalysts: A First Principles Study of CO Oxidation on TiO2 Supported Au, Phys. Rev. Lett. 2003, 91, 266102. [21] D. Widmann, R.J. Behm, Active Oxygen on a Au/TiO2 Catalyst: Formation, Stability, and CO Oxidation Activity, Angew. Chem. Int. Ed. 2011, 50, 10241. [22] M. Farnesi Camellone, J. Zhao, L. Jin, Y. Wang, M. Muhler, D. Marx, Molecular Understanding of Reactivity and Selectivity for Methanol Oxidation at the Au/TiO2 Interface Angew. Chem. Int. Ed. 2013, 52, 5780. [23] M.A. Saqlain, F.P. Novais Antunes, A. Hussain, M. Siddiq, A.A. Leitão, Adsorption of oxygen and CO oxidation on Au/anatase(001) catalysts. A DFT+U study, New J. Chem. 2017, 41, 2073. [24] A. Lüken, M. Muhler, J. Strunk, On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO2, Phys. Chem. Chem. Phys. 2015, 17, 10391. [25] M. Okumura, J.M. Coronado, J. Soria, M. Haruta, J.C. Conesa, EPR Study of CO and O2 Interaction with Supported Au Catalysts, J. Catal. 2001, 203, 168. [26] Z. Hao, L. Fen, G.Q. Lu, J. Liu, L. An, H. Wang, In situ electron paramagnetic resonance (EPR) study of surface oxygen species on Au/ZnO catalyst for low-temperature carbon monoxide oxidation, Appl. Catal. A: General 2001, 213, 173. [27] J. Saavedra, H.A. Doan, C.J. Pursell, L.C. Grabow, B.D. Chandler, The critical role of water at the gold-titania interface in catalytic CO oxidation, Science 2014, 345, 1599. [28] Z. Zhang, J.T.Yates, Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces, Chem. Rev. 2012, 112, 5520. [29] I. Izumi, W.W. Dunn, K.O. Wilburn, F.-R.F. Fan, A.J. Bard, Heterogeneous Photocatalytic Oxidation of Hydrocarbons on Platinized TiO2 Powders, J. Phys. Chem. 1980, 84, 3207. [30] E. Smotkin, A.J. Bard, A. Campion, M.A. Fox, T. Mallouk, S.E. Webber, J.M. White, Bipolar TiO2/Pt Semiconductor Photoelectrodes and Multielectrode Arrays forUnassisted Photolytic Water Splitting, J. Phys. Chem. 1986, 90, 4604. [31] A.L.Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 1995, 95, 735.


7


[32] V. Subramanian, E. Wolf, P.V. Kamat, SemiconductorMetal Composite Nanostructures. To What Extent Do Metal Nanoparticles Improve the Photocatalytic Activity of TiO2 Films? J. Phys. Chem. B 2001, 105, 11439. [33] A. Sclafani, M.N. Mozzanega, P. Pichat, Effect of silver deposits on the photocatalytic activity of titanium-dioxide samples for the dehydrogenation or oxidation of 2-propanol, J. Photochem. Photobiol. A Chem. 1991, 59, 181. [34] J.M. Herrmann, From catalysis by metals to bifunctional photocatalysis, Top. Catal. 2006, 39, 3. [35] H. Choi, W.T. Chen, P.V. Kamat, Know Thy Nano Neighbor. Plasmonic versus Electron Charging Effects of Metal Nanoparticles in Dye-Sensitized Solar Cells, ACS Nano 2012, 6, 4418. [36] I. Caretti, M. Keulemans, S.W. Verbruggen, S. Lenaerts, S. Van Doorslaer, Light-Induced Processes in Plasmonic Gold/TiO2 Photocatalysts Studied by Electron Paramagnetic Resonance, Top. Catal. 2015, 58, 776. [37] E. Kowalska, O.O.P. Mahaney, R. Abe, B. Ohtani, Visible-light-induced photocatalysis through surface plasmon excitation of gold on titania surfaces, Phys. Chem. Chem. Phys. 2010, 12, 2344. [38] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of Au-TiO2 and Pt-TiO2, J. Photochem. Photobiol. A: Chem. 1995, 89, 177. [39] G.L. Chiarello, M.H. Aguirre, E. Selli, Hydrogen production by photocatalytic steam reforming of methanol on noble metal-modified TiO2, J. Catal. 2010, 273, 182. [40] A. Naldoni, M. D’Arienzo, M. Altomare, M. Marelli, R. Scotti, F. Morazzoni, E. Selli, V. Dal Santo, Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency, Appl. Catal. B Environmental 2013, 130, 239. [41] Z.H.N. Al-Azri, W.-T. Chen, A. Chan, V. Jovic, T. Ina, H. Idriss, G.I.N. Waterhouse, The roles of metal co-catalysts and reaction media in photocatalytic hydrogen production: Performance evaluation of M/TiO2 photocatalysts (M = Pd, Pt, Au) in different alcohol-water mixtures, J. Catal. 2015, 329, 355. [42] J.B. Priebe, J. Radnik, A.J.J. Lennox, M.-M. Pohl, M. Karnahl, D. Hollmann, K. Grabow, U. Bentrup, H. Junge, M. Beller, A. Brückner, Solar Hydrogen Production by Plasmonic Au−TiO2 Catalysts: Impact of Synthesis Protocol and TiO2 Phase on Charge Transfer Efficiency and H2 Evolution Rates, ACS Catal. 2015, 5, 2137. [43] M. Bowker, L. Millard, J. Greaves, D. James, J. Soares, Photocatalysis by Au Nanoparticles: Reforming of Methanol, Gold Bulletin 2004, 37, 170. [44] A.A. Ismail, D.W. Bahnemann, I. Bannat, M. Wark, Gold Nanoparticles on Mesoporous Interparticle Networks of Titanium Dioxide Nanocrystals for Enhanced Photonic Efficiencies, J. Phys. Chem. C 2009, 113, 7429. [45] S. Pradhan, D. Ghosh, S. Chen, Janus Nanostructures Based on Au−TiO2 Heterodimers and Their Photocatalytic Activity in the Oxidation of Methanol, ACS Appl. Mater. Interfaces 2009, 1, 2060. [46] A.A. Ismail, S.A. Al-Sayari, D.W. Bahnemann, Photodeposition of precious metals onto mesoporous TiO2 nanocrystals with enhanced their photocatalytic activity for methanol oxidation, Catal. Today, 2013, 209, 2. [47] D. Sannino, V. Vaiano, P. Ciambelli, M.C. Hidalgo, J.J. Murcia, J.A. Navio, Oxidative Dehydrogenation of Etha-

Page 8 of 17

nol over Au/TiO2 Photocatalysts, J. Adv. Oxid. Technol. 2012, 15, 284. [48] J.J. Murcia, M.C. Hidalgo, J.A. Navio, J. Araña, J.M. Doña-Rodriguez, In situ FT-IR study of the adsorption and photocatalytic oxidation of ethanol over sulfated and metallized TiO2, Appl. Catal. B Environmental 2013, 142-143, 205. [49] R. Fiorenza, M. Bellardita, L. Palmisano, S. Scirè, A comparison between photocatalytic and catalytic oxidation of 2-Propanol over Au/TiO2–CeO2 catalysts, J. Mol. Catal. A Chem. 2016, 415, 56. [50] W. Xu, D. Raftery, Photocatalytic Oxidation of 2Propanol on TiO2 Powder and TiO2 Monolayer Catalysts Studied by Solid-State NMR, J. Phys. Chem. B 2001, 105, 4343. [51] W. Xu, D. Raftery, J. S. Francisco, Effect of Irradiation Sources and Oxygen Concentration on the Photocatalytic Oxidation of 2-Propanol and Acetone Studied by in Situ FTIR, J. Phys. Chem. B 2003, 107, 4537 [52] D. Brinkley, T. Engel, Photocatalytic Dehydrogenation of 2-Propanol on TiO2(110), J. Phys. Chem. B 1998, 102, 7596 [53] F. Arsac, D. Bianchi, J.M. Chovelon, C. Ferronato, J.M. Herrmann, Experimental Microkinetic Approach of the Photocatalytic Oxidation of Isopropyl Alcohol on TiO 2. Part 1. Surface Elementary Steps Involving Gaseous and Adsorbed C3HxO Species, J. Phys. Chem. A, 2006, 110, 4202 [54] F. Arsac, D. Bianchi, J. M. Chovelon, C. Ferronato and J. M. Herrmann, Experimental Microkinetic Approach of the Photocatalytic Oxidation of Isopropyl Alcohol on TiO 2. Part 2. From the Surface Elementary Steps to the Rates of Oxidation of the C3HxO Species, J. Phys. Chem. A, 2006, 110, 4213. [55] M.A. El-Sayed, Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes, Acc. Chem. Res. 2001, 34, 257. [56] P. Christopher, H. Xin, A. Marimuthu, S. Linic, Singular characteristics and unique chemical bond activation mechanisms of photocatalytic reactions on plasmonic nanostructures, Nat. Mater. 2012, 11, 1044. [57] J.B. Priebe, M. Karnahl, H. Junge, M. Beller, D. Hollmann, A. Brückner, Water Reduction with Visible Light: Synergy between Optical Transitions and Electron Transfer in Au-TiO2 Catalysts Visualized by In situ EPR Spectroscopy, Angew. Chem. Int. Ed. 2013, 52, 11420. [58] A. Tanaka, K. Hashimoto, H. Kominami, A very simple method for the preparation of Au/TiO2 plasmonic photocatalysts working under irradiation of visible light in the range of 600–700 nm, Chem. Commun. 2017, 53, 4759. [59] Y. Tian, T. Tatsuma, Mechanism and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles, J. Am. Chem. Soc. 2005, 172, 7632. [60] D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation, J. Am. Chem. Soc. 2012, 134, 6309. [61] Y. Shiraishi, N. Yasumoto, J. Imai, H. Sakamoto, S. Tanaka, S. Ichikawa, B. Ohtani, T. Hirai, Quantum tunneling injection of hot electrons in Au/TiO2 plasmonic photocatalysts, Nanoscale 2017, 9, 8349. [62] M.G. Méndez-Medrano, E. Kowalska, A. Lehoux, A. Herissan, B. Ohtani, S. Rau, C. Colbeau-Justin, J.L. Rodríguez-López, H. Remita, Surface Modification of TiO2 with Au Nanoclusters for Efficient Water Treatment and Hy-


8

Page 9 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


drogen Generation under Visible Light, J. Phys. Chem. C 2016, 120, 25010. [63] R. Amrollahi, M.S. Hamdy, G. Mul, Understanding promotion of photocatalytic activity of TiO2 by Au nanoparticles J. Catal. 2014, 319, 194. [64] S. Linic, P. Christopher, D. B. Ingram, Plasmonicmetal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 2011, 10, 911. [65] D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation, J. Am. Chem. Soc. 2012, 134, 6309. [66] A. Primo, A. Corma, H. Garcia, Titania supported gold nanoparticles as photocatalyst, Phys. Chem. Chem. Phys. 2011, 13, 886. [67] A. Ulgen, W. F. Hoelderich, Conversion of glycerol to acrolein in the presence of WO3/TiO2 catalysts, Appl. Catal. A Gen. 2011, 400, 34. [68] S. Gaur, H. Wu, G. G. Stanley, K. More, C. S. S. R. Kumar, J. J. Spivey, CO oxidation studies over cluster-derived Au/TiO2 and AUROlite™ Au/TiO2 catalysts using DRIFTS, Catal. Today 2013, 208, 72. [69] N. Kruse, S. Chenakin, XPS characterization of Au/TiO2 catalysts: Binding energy assessment and irradiation effects, Appl. Catal. A Gen. 2011, 391, 367. [70] A. Pougin, A. Lüken, C. Klinkhammer, D. Hiltrop, M. Kauer, K. Tölle, M. Havenith-Newen, K. Morgenstern, W. Grünert, M. Muhler, J. Strunk, Probing Oxide Reduction and Phase Transformations at the AuTiO2 Interface by Vibrational Spectroscopy, Top. Catal. 2017, 60, 1744. [71] M. Farnesi Camellone, D. Marx, On the Impact of Solvation on a Au/TiO2 Nanocatalyst in Contact with Water, J. Phys. Chem. Lett. 2013, 4, 514–518. [72] M. Farnesi Camellone, D. Marx, Nature and Role of Activated Molecular Oxygen Species at the Gold/Titania Interface in the Selective Oxidation of Alcohols, J. Phys. Chem. C 2014, 118, 20989–21000. [73] P. M. Kowalski, B. Meyer, D. Marx, Composition, structure, and stability of the rutile TiO2(110) surface: Oxygen depletion, hydroxylation, hydrogen migration, and water adsorption, Phys. Rev. B 2009, 79, 115410. [74] CPMD: Car-Parrinello Molecular Dynamics; An ab initio Electronic Structure and Molecular Dynamics Program, http://www.cpmd.org. [75] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, R. M. Wentzcovitch, QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, J. Phys.: Condens. Matter 2009, 21, 395502. [76] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 1996, 77, 3865–3868. [77] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)], Phys. Rev. Lett. 1997, 78, 1396–1396.

[78] V. I. Anisimov, F. Aryasetiawan, A. I. Lichtenstein, First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method, J Phys.: Condens. Matter 1997, 9, 767. [79] D. Vanderbilt, Soft self-consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 1990, 41, 7892–7895. [80] P. M. Kowalski, M. F. Camellone, N. N. Nair, B. Meyer, D. Marx, Charge Localization Dynamics Induced by Oxygen Vacancies on the TiO2 (110) Surface, Phys. Rev. Lett. 2010, 105, 146405. [81] M. Farnesi Camellone, P. M. Kowalski, D. Marx, Ideal, defective, and gold-promoted rutile TiO2 (110) surfaces interacting with CO, H2, and H2O: Structures, energies, thermodynamics, and dynamics from PBE+U, Phys. Rev. B 2011, 84, 035413. [82] N. N. Nair, J. Ribas-Arino, V. Staemmler, D. Marx, Magnetostructural Dynamics from Hubbard-U Corrected Spin-Projection: [2Fe−2S] Complex in Ferredoxin, J. Chem. Theory Comput. 2010, 6, 569–575. [83] M. Cococcioni, S. de Gironcoli, Linear response approach to the calculation of the effective interaction parameters in the LDA+U method, Phys. Rev. B 2005, 71, 035105. [84] P.-O. Löwdin, On the Non‐ Orthogonality Problem Connected with the Use of Atomic Wave Functions in the Theory of Molecules and Crystals, J. Chem. Phys 1950, 18, 365–375. [85] D. Marx, J. Hutter, Ab Initio Molecular Dynamics: Basic Theory and Advanced Methods, Cambridge University Press, Cambridge 2009. [86] G. J. Martyna, M. L. Klein, M. Tuckerman, Nosé– Hoover chains: The canonical ensemble via continuous dynamics, J. Chem. Phys. 1992, 97, 2635–2643. [87] T. Rajh, O.G. Poluektov , M.C. Thurnauer, In: Chemical Physics of Nanostructured Semiconductors. A.I: Kokorin, D.W. Bahnemann (Eds.). VSP-Brill Academic Publishers, Utrecht, Boston; 2003. Charge separation in titanium oxide nanocrystalline semiconductors revealed by magnetic resonance; pp. 1–34. [88] R. F. Howe, M. Grätzel, EPR observation of trapped electrons in colloidal titanium dioxide, J. Phys. Chem. 1985, 89, 4495. [89] R. F. Howe, M. Grätzel, EPR study of hydrated anatase under UV irradiation, J. Phys. Chem. 1987, 91, 3906. [90] D. C. Hurum, A. G. Agrios, K. A. Gray, T. Rajh, M. C. Thurnauer, Explaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPR, J. Phys. Chem. B 2003, 107, 4545. [91] C. P. Kumar, N. O. Gopal, T. C. Wang, M.-S. Wong, S. C. Ke, EPR Investigation of TiO2 Nanoparticles with Temperature-Dependent Properties, J. Phys. Chem. B 2006, 110, 5223. [92] S. Livraghi, M. Chiesa, M. C. Paganini, E. Giamello, On the Nature of Reduced States in Titanium Dioxide As Monitored by Electron Paramagnetic Resonance. I: The Anatase Case, J. Phys. Chem. C 2011, 115, 25413. [93] V. Jovic, K. E. Smith, H. Idriss, G. I. N. Waterhouse, Heterojunction Synergies in Titania‐ Supported Gold Photocatalysts: Implications for Solar Hydrogen Production, ChemSusChem 2015, 8, 2551. [94] S. Livraghi, M. Rolando, S. Maurelli, M. Chiesa, M.C. Paganini, E. Giamello, J. Phys. Chem. C 2014, 118, 22141.


9


Page 10 of 17

[95] F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt, P. Feng, Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light, J. Am. Chem. Soc. 2010, 132, 11856. [96] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C.L. Bianchi, R. Psaro, V. Dal Santo, Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles, J. Am. Chem. Soc. 2012, 134, 7600. [97] D. C. Hurum, K. A. Gray, T. Rajh, M. C. Thurnauer, J. Phys. Chem. B 2005, 109, 977. [98] D. C. Hurum, K. A. Gray, T. Rajh, M. C. Thurnauer, J. Phys. Chem. B 2004, 108, 16483. [99] D. C. Hurum, A. G. Agrios, S. E. Crist, K. A. Gray, T. Rajh, M. C. Thurnauer, J. Electron Spectros. Relat. Phenomena 2006, 150, 155. [100] O. I. Micic, Y. Zhang, K. R. Cromack, A. D. Trifunac, M. C. Thurnauer, J. Phys. Chem. 1993, 97, 7277. [101] L. Li, J. Yan, T. Wang, Z.J. Zhao, J. Zhang, J. Gong, N. Guan, Nature communications 2015, 6, 5881. [102] A. Schweiger, G. Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance Oxford Univ. Press, Oxford, UK, , 2001. [103] T. Takui, K. Sato, D. Shiomi, K. Itoh, T. Kaneko, E. Tsuchida, H. Nishide, Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals 1996, 279, 155. [104] N. Cox, M. Retegan, F. Neese, D.A. Pantazis, A. Boussac, W. Lubitz, Science 2014, 345, 804. [105] M. Chiesa, S. Livraghi, E. Giamello, E. Albanese, G. Pacchioni, Angew. Chem. Int. Ed. 2017, 56, 2604. [106] P. Schwach, M. Eichelbaum, R. Schlögl, T. Risse, K.P. Dinse, J. Phys. Chem. C 2016, 120, 3781. [107] K. L. Antcliff, D. M. Murphy, E. Griffiths, E. Giamello, Phys. Chem. Chem. Phys. 2003, 5, 4306. [108] E. Carter, A. Carley, D. Murphy, Evidence for O2Radical Stabilization at Surface Oxygen Vacancies on Polycrystalline TiO2, J. Phys. Chem. C 2007, 111, 10630. [109] H. Koga, K. Tada, M. Okumura, Density Functional Theory Study of Active Oxygen at the Perimeter of Au/TiO 2 Catalysts, J. Phys. Chem. C 2015, 119, 25907. [110] Y.-G. Wang, Y. Yoon, V.-A. Glezakou, J. Li, R. Rousseau, The Role of Reducible Oxide–Metal Cluster Charge Transfer in Catalytic Processes: New Insights on the Catalytic Mechanism of CO Oxidation on Au/TiO2 from ab Initio Molecular Dynamics, J. Am. Chem. Soc. 2013, 135, 10673. [111] H. Häkkinen, S. Abbet, A. Sanchez, U. Heiz, U. Landman, Structural, Electronic, and Impurity‐ Doping Effects in Nanoscale Chemistry: Supported Gold Nanoclusters, Angew. Chem. Int. Edit. 2003, 42, 1297.


10

Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions only

Figure 1. X-band CW EPR spectra of P25 (TiO2) and Au/TiO2 powders (A) wetted with 2-propanol and (B) without 2propanol. Samples were sealed under air or vacuum. Measurements in air and in the presence of 2-propanol correspond to simulated reaction conditions of selective alcohol oxidation (Figure 1a, top). All spectra were recorded at 10 K. Black lines – samples prepared under ambient light, Red lines – samples irradiated with UV light for 15 min, Blue lines samples irradiated for 30 min. Signal assignment is given in the text and in Table S1. All EPR spectra were acquired under identical experimental conditions (10 K, microwave power of 0.2 mW, field modulation of 0.2 mT) and are depicted on the same intensity scale.

Figure 2. (Bottom) Contour plot of the field-frequency dependence of TN intensities of the Au/TiO2 powder wetted with 2-propanol after 30 min irradiation of the sample with UV LED at room temperature. The spectrum was recorded at 10K for an evacuated sealed sample using the tprep−Τ−tp−τ−2tp−τ−echo sequence (tprep = 4-480 ns incremented in 2 ns steps, tp = 10 ns, T = 2 ms, τ = 960 ns and microwave power optimized for detection of high spin species). (Top) Cross sections of the 2D nutation spectrum at frequencies marked by the dotted lines in the bottom panel.

Figure 3. Free energy profiles of O2 dissociation at perimeter site (red) and far from the Au cluster (green); the shaded areas provide an estimate of the statistical error (see SI). Representative configuration snapshots show a top view of both adsorption sites (mid left and right) and charge density differences upon adding the O2 molecule to the surface at different O–O bond lengths as indicated in the graph (∆= O2/Au/TiO2 − Au/TiO2 − O2; upper and lower panel). Au, Ti, O atoms and the O2 molecule are depicted in yellow, cyan, red, and brown, respectively. Purple and blue isosurfaces show charge accumulation and depletion at an isovalue of ±0.01 |e|Å−3, respectively.

Figure 4: Bonding charge difference  for an O2 molecule on the Au/TiO2 surface at different O–O bond lengths corresponding to the three stages of the dissociation reaction. The upper panels show  for an oxygen molecule bound at a surface position far from the Au cluster. a) Real-space isosurface representation (x, y, z) of one representative snapshot at stage A at an isovalue of ±0.1 |e−|Å−3; see Fig. 3 for color coding and for similar snapshots at the two later stages of the dissociation process. Panels b) to d) represent the average bonding charge differences (obtained from the snapshots sampled from AIMD simulations as described in the text) integrated in planes parallel to the surface, (z), plotted as a function of the height z relative to the titania surface. The lower panels provide the corresponding plots for an O2 molecule bound at the Au/TiO2 perimeter (e-h). In all cases, z is set to zero at the average height of the bridging oxygen atoms of TiO2. The three horizontal lines denote the average z coordinates of the O and Ti atoms of the titania support (thus the uppermost line at z = 0 Å corresponds to the bridging oxygen rows), whereas the arrows point toward charge depletion and accumulation regions at the gold nanoparticle (“Au 11”) and the oxygen molecule (“O2”), respectively.

Figure 5. Real space representation of Kohn-Sham orbitals (||2) at the O2 molecule energetically close to the Fermi energy. Panel I depicts occupied (bottom row) and unoccupied (upper row) orbitals for O2 dissociation at the Au/TiO2 interface before (B) and after (C) the transition state. Panel II shows the unoccupied orbital before the transition state (upper row, B’) which gets occupied after the transition state (bottom row, C’). Gray and white isosurfaces represent the sign of  at an isovalue of ±0.004 |e|Å−3 for (||2) (see SI for the corresponding density of states of the shown snapshots). The color coding of the elements is identical to that in Figure 3.




Page 12 of 17

Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures only

Figure 1.



Figure 2.

Figure 3.


Page 14 of 17

Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a) Dissociation @TiO2 – Stage A’

b) Stage A’

c) Stage B’

d) Stage C’

e) Dissociation @Au – Stage A

f) Stage A

g) Stage B

h) Stage C

Figure 4.



Figure 5.


Page 16 of 17

Page 17 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


OH O

hν

Au h+ + e-

eTiO2


𝑂2−

𝑂2

Atomic-Scale Explanation of O2 Activation at the AuâTiO

Recommend Documents

Atomic-Scale Explanation of O2 Activation at the AuâTiO