Atomic-Scale Explanation of O2 Activation at the ... - ACS Publications

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Cite This: J. Am. Chem. Soc. 2018, 140, 18082−18092

Atomic-Scale Explanation of O2 Activation at the Au−TiO2 Interface Niklas Siemer,†,∇ Alexander Lüken,‡,∇ Michal Zalibera,§,∥ Johannes Frenzel,† Daniel Muñoz-Santiburcio,†,⊗ Anton Savitsky,⊥ Wolfgang Lubitz,§ Martin Muhler,‡ Dominik Marx,*,† and Jennifer Strunk*,# †

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

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‡

S Supporting Information *

ABSTRACT: By a combination of electron paramagnetic resonance 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 TiO2, 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 in this way than on the bare TiO2 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.

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INTRODUCTION Despite the long-standing success story of gold catalysis, the impact of the size and shape of the gold nanoparticles,1−4 as well as the nature and role of the Au−TiO2 interface,3−7 on the catalytic performance of Au/TiO2 has remained the focus of vivid scientific discussions. Furthermore, applications of such gold catalysts cover a much broader range today, including photocatalysis8 and photoelectrochemical applications9 to name but two. In classical (thermal) heterogeneous catalysis, gold nanoparticles on oxides have received attention primarily due to their unique capability to catalyze the oxidation of CO4,10−13 and the selective aerobic oxidation of alcohols and alkenes14−18 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 superoxide25,26 and hydroperoxide,27 as well as lattice © 2018 American Chemical Society

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 TiO2 functions as a 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 Received: October 10, 2018 Published: November 20, 2018 18082

DOI: 10.1021/jacs.8b10929 J. Am. Chem. Soc. 2018, 140, 18082−18092

Article

Journal of the American Chemical Society

which may further react to CO2.53,54 Reaction to acetone occurred both in the presence and in the absence of O2,53 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 effects55 may enhance photocatalytic performance compared with that observed on 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 and 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 electron paramagnetic resonance (EPR) spectroscopy, ab initio simulations yielding activation free energies, and electronic 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.

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 the 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 as 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 TiO2. From the trap states, interfacial charge transfer to gold was less efficient than that from the TiO2 conduction band.45 Ethanol was photocatalytically oxidized to acetaldehyde with a selectivity up to 95% using photodeposited gold nanoparticles on a sol−gel-prepared TiO2.47 In situ infrared spectroscopy revealed that the alcohol was more likely to undergo hydrogenbonding interaction than ethoxy formation on metallized (Au, Pt) TiO2 and that acetate was formed as a 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 2-propanol 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 (375 nm),24 evidence was 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. In contrast, in Au/ TiO2, the accumulation of excess electrons in TiO2 is prevented, which can be rationalized on the basis of their migration to the gold nanoparticles and 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 hydrogen-bonded intermediate, whereas the chemisorbed 2-propoxide was oxidized completely to CO2.50,51 Undissociated 2-propanol was suggested to trap holes directly, and electrons were transferred 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,

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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 continuous wave (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/TiO2) 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 a vacuum (1 × 10−5 bar) or synthetic air (1 bar). For the samples sealed in air, the amounts of the reaction partners in the EPR tube are estimated to be ∼32 μmol of O2 and ∼260 μmol of 2-propanol. These measurements under approximately 8-fold 2-propanol excess are referred to 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 and 30 min using a custom-designed light-emitting diode 18083


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Figure 1. X-band CW EPR spectra of P25 (TiO2) and Au/TiO2 powders (A) wetted with 2-propanol and (B) without 2-propanol. Samples were sealed under air or in a 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 assignments are 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. (LED) array consisting of six 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 (Thorlabs actinometer). 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 orthorhombic cell parameters a = 13.15 Å and b = 17.80 Å. 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 charges +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 described in ref 73. The density functional theory (DFT)-based calculations were performed using the CPMD74 and Quantum Espresso/PWscf75 codes for dynamic and static computations, respectively. All calculations were performed using spin-polarized GGA+U based on the Perdew− Burke−Ernzerhof exchange correlation functional (PBE).76−78 The spin-polarized Kohn−Sham equations were solved in the plane-wave/ pseudopotential framework using Vanderbilt’s ultrasoft pseudopotentials79 with a cutoff of 25 Ry, where k-point sampling was restricted to the Γ-point. In line with our previous work,22,71,72,80,81 we again made use of our GGA+U implementation82 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 Γ-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 was done using the Car−Parrinello MD algorithm as implemented in CPMD. Nosé−Hoover chain thermostatting86 was used to keep the temperature at 450 K and the average fictitious kinetic energy of the electrons constant (at 0.1 au). A fictitious electron mass of μ = 350 au and a time step of 5.0 au 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. Integration of the resulting average constraint forces yields the free energy profile as a function of the oxygen−oxygen distance, ΔF(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 Figure 3 in the next section, 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.

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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 18084


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Journal of the American Chemical Society are associated with various forms of Ti3+, related to the trapped photogenerated 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 a vacuum or in the presence of air. The control experiments on TiO2 and Au/ TiO2 samples in the absence of 2-propanol and in a 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 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.

exposure of the sample to ambient air (Figure S1). In accordance with previous work,25 this EPR signal is assigned to Ti3+ with oxygen vacancies in the 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 the reaction of excited electrons with adsorbed dioxygen is slow in the case of bare TiO2.24 Although the catalytic cycle is closed in the 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 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 a vacuum in the presence of 2-propanol (Figure S2) showed variations in the 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 changes as discussed thus indicate the presence of Ti3+ species with different total spin multiplicities. Detailed information on the electron spin state can be obtained from the transient nutation (TN) EPR experiment.102−104 Very recently, this approach was used to identify high-spin states in reduced anatase TiO2105 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

Table 1. EPR Parameters of Ti3+ Centers in TiO2 g⊥

g∥

refs

Ti3+ lattice anatase

1.990

1.960

Ti3+ lattice rutile in P25 Ti3+ surface anatase

1.975 1.930

1.940 1.885

Ti3+ surface anatase in P25 Ti3+ 4-coordinated at the rutile−anatase interface in P25 Ti3+ in rutile after photocatalytic H2O splitting

1.928 1.979 −

88−92, 97−99 90, 99 88, 89, 91, 92 90, 97−100 89, 97

1.972

101

system/assignment

1.949

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 labeling, combined with electron spin echo envelope modulation (ESEEM) hyperfine spectroscopy,59 and is in line with our own 1H electron nuclear double resonance (ENDOR) results (see SI for details). The broad character of the EPR line at g ≈ 1.93 was previously explained on the basis of the g strain effects associated with a substantial heterogeneity of the paramagnetic centers 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 a vacuum (Figure 1A, bottom). The pure TiO2 sample wetted with 2-propanol and measured in a vacuum turned blue upon irradiation, typical of a self-doped 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 a vacuum but disappears together with the blue color upon

vnut(ms , ms + 1) = α=

gβeB1 h

α,

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 = 0.92, 1.85, and 2.80 MHz, respectively. The ratio of the nutation frequencies 1:2:3 resembles the ratio of the α-factors √1:√4:√9, Equation 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., 18085


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|−3/2⟩ → |−1/2⟩, |−1/2⟩ → |3/2⟩, with α = √3 for S = 3/2 and α = √8 for S = 5/2). Thus, the TN experiment shows that, in addition to the isolated Ti3+ 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 not only is 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 us to estimate the interspin distances in these clusters. The observation of clusters of reduced Ti3+ species in Au/TiO2 photocatalysts is qualitatively in line with the (thermally induced 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 exposure to only 2-propanol is a clear indication that 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 =

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 10 K 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 a perimeter site (red) and far from the Au cluster (green); the shaded areas provide an estimate of the statistical error (see Computational Details). Representative configuration snapshots show a top view of both adsorption sites (mid left and right) and charge density differences upon addition of the O2 molecule to the surface at different O−O bond lengths as indicated in the graph (Δρ = ρO2/Au/TiO2 − ρAu/TiO2 − ρO2; upper and lower panels). Au, Ti, and O atoms and the O2 molecule are depicted in yellow, cyan, red, and brown, respectively. Purple and blue isosurfaces show charge accumulation and depletion, respectively, at an isovalue of ±0.01 |e| Å−3. 18086


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Figure 4. Bonding charge difference Δρ 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 Δρ for an oxygen molecule bound at a surface position far from the Au cluster. (a) Realspace isosurface representation Δρ(x,y,z) of one representative snapshot at stage A at an isovalue of ±0.1 |e| Å−3; see Figure 3 for color coding and for similar snapshots at the two later stages of the dissociation process. Panels b−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, Δρ(z), plotted as a function of the height z relative to the titania surface. The lower panels (e−h) provide the corresponding plots for an O2 molecule bound at the Au/TiO2 perimeter. 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 (“Au11”) and the oxygen molecule (“O2”), respectively.

2.003)65,89,107,108 is observed for both samples. The gy value allows clear assignment to a superoxide species and rules out the 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 than for 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 photogenerated 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 A detailed discussion of the EPR signal of superoxide is provided in the SI. These experimental results clearly show an enhanced catalytic activity toward O2 activation at Au/TiO2. A number of studies have been devoted to providing 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 18087


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TiO 2 interfacial sites and increases with O−O bond elongation. This additional Au−O2 charge transfer is crucial for 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 O2 with the gold cluster (Figure 3), in accord with our experimental findings. In order to fully elucidate the mechanistic details of the O2 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 rely 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 following electronic structure analysis. Deep analysis of the electronic structure reveals the mechanistic pathway of the oxygen activation at Au/TiO2 nanocatalysts. When oxygen is 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 σ-d-p bonding interactions between the Ti and the O atoms and one σ-p-p antibonding interaction within the O2

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 (Δρ) upon addition of 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 Δρ = ρ(O2/Au/TiO2) − ρ(O2) − ρ(Au/TiO2), where ρ(O2/ Au/TiO2), ρ(O2), and ρ(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 toward the oxygen molecule (Figure 3A,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). In addition to such visualization of Δρ(x,y,z) in terms of real-space (isosurface) plots, it is possible to integrate Δρ 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 Δρ(z), averaged over all statistically independent configurations that have been sampled at each dissociation stage (i.e., A,B,C and A′,B′,C′ according to Figure 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. The average Δρ(z) density difference corresponding to the reactant state A′, being far from the perimeter site, 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 O2 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 (Figure 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, Δρ(zO2) increases from 0.2 to 0.3 to finally 0.4 |e| Å−1 at stages A, B, and C, respectively (Figure 4f−h). Along this dissociation process, the charge depletion found in the nanoparticle increases systematically, as seen in the increasing negative Δρ(z) at z-values where the cluster is located. The charge on the O2 molecule, being far from the Au cluster, increases from 0.3 to 0.4 and 0.6 |e|Å−1 at stages A′, B′, and C′, respectively (Figure 4b−d). In that situation, the charge of the gold cluster barely changes, and Δρ(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/

Figure 5. Real-space representation of Kohn−Sham orbitals (|Ψ|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 Ψ at an isovalue of ±0.004 |e| Å−3 for (|Ψ|2) (see SI for the corresponding density of states of the snapshots shown). The color coding of the elements is identical to that in Figure 3. 18088



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molecule. Therefore, occupation of this molecular orbital would weaken the O−O bond, facilitating dissociation. However, when O2 is decoupled from the gold cluster, this orbital cannot be easily populated. Dissociation takes place only 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 σ-p-p antibonding orbital of O2. As visualized in Figure 5-I, 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 enhancement of the O2 activation at the Au/TiO2 interface results from charge transfer from the Au cluster to the σ-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 address predominantly gold deposited on the rutile phase, it may be interesting to elucidate the influence of the support structure in future studies.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b10929.

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Detailed discussions of previous works on oxygen activation on Au/TiO2 and of EPR spectroscopy, including Ti3+-related signals and superoxide signal, and electronic structure analyses of O2 along dissociation at finite temperatures, including Figures S1−S11 and Table S1 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Michal Zalibera: 0000-0002-6527-1982 Johannes Frenzel: 0000-0003-0458-5424 Daniel Muñoz-Santiburcio: 0000-0001-9490-5975 Anton Savitsky: 0000-0002-6505-9412 Wolfgang Lubitz: 0000-0001-7059-5327 Martin Muhler: 0000-0001-5343-6922 Jennifer Strunk: 0000-0002-6018-3633

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Present Address ⊗

CONCLUSIONS 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. These processes have been confirmed experimentally by EPR spectroscopy. In the presence of the gold nanoparticles, the intensity of the paramagnetic signals associated with (surface) Ti3+ species was significantly decreased. In the absence of oxygen as electron scavenger, however, reduction of TiO2 can take place to a significant 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 that on bare TiO2. Deeper insight into the role of gold is provided by finite-temperature ab initio simulations and electronic structure analyses. While in principle also possible on bare TiO2, oxygen activation is significantly favored on the TiO2 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 understanding the mode of action of Au/TiO2 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.

CIC nanoGUNE, 20018 San Sebastián, Spain

Author Contributions ∇

N.S. and A.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Cluster of Excellence RESOLV (EXC 2033) 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 SuperMUC as well as by HPC@ZEMOS, HPC-RESOLV, BOVILAB@RUB, and RVNRW. We thank Priv. Doz. Dr. Gerald Mathias (High Performance Computing Group at LRZ München) for help with the optimization of the Hubbard U routine of the CPMD program package.

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