Light, Catalyst, Activation: Boosting Catalytic Oxygen Activation Using

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Light, Catalyst, Activation: Boosting catalytic oxygen activation using a light pre-treatment approach Wibawa Hendra Saputera, Jason Anthony Scott, Hassan A. Tahini, Gary K-C Low, Xin Tan, Sean C. Smith, Da-Wei Wang, and Rose Amal ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00700 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Light, Catalyst, Activation: Boosting catalytic oxygen activation using a light pre-treatment approach Wibawa H. Saputera†, Jason Scott†*, Hassan Tahini§, Gary-K.C. Low†, Xin Tan§, Sean Smith§, Da-Wei Wang†, Rose Amal†* †

Particle and Catalysis Research Group, School of Chemical Engineering, The University of

New South Wales, Sydney, Australia. §

Integrated Materials Design Centre, School of Chemical Engineering, The University of New

South Wales, Sydney, Australia.

ACS Paragon Plus Environment

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ABSTRACT

Oxygen activation is a key reaction step in assorted thermal-catalytic processes. Here we use light pre-treatment to boost oxygen activation by platinum, palladium or gold loaded on TiO2. Light pre-treatment improved catalyst performance by altering metal oxidation state and/or activated oxygen (Oads) generation. For Pt/TiO2 and Pd/TiO2, light pre-treatment initially promoted Oads accumulation on the metal. In time, the metal deposits underwent oxidation with a concomitant decrease in the light pre-treatment benefit. Au/TiO2 differed in that no Au oxidation was observed. Electrocatalytic assessment indicated light pre-treatment lowered the energy for oxygen activation and that Au was less able to activate oxygen overall. First principle calculations demonstrated that light pre-treatment reduces the oxygen dissociation barrier as each metal becomes negatively charged via electron injection from the TiO2 conduction bands. The calculations also illustrated the ability of Pt and Pd to stabilize the oxygen once dissociated, a trait not imbued by Au.

Keywords: Oxygen activation, light pre-treatment, TiO2, noble metals, dark catalysis

1. INTRODUCTION Oxygen activation is an important reaction for various catalytic processes in the areas of energy conversion, fine chemical synthesis and environmental remediation. For example, it is a key process at the cathode within a fuel cell,1-4 is important for pharmaceutical manufacture5 and is of interest for NO6 and ammonia oxidation.7 Activation involves the reduction and dissociation of molecular oxygen over a metal catalyst to produce O-ads species (Eq. 1). O2(ads) + 2e- → 2O-ads

(1)


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Platinum is the most favoured metal for catalyzing oxygen activation8 although other metals such as gold and palladium have been found capable of promoting the process as well.9 In the context of fuel cells, the cathodic (electro)catalyst most typically comprises Pt loaded on a carbon-based support such as carbon black,10 carbon nanotubes,11 graphene12 and modified analogues of these materials.13 Carbon-based materials are the support of choice as their high specific surface areas facilitate platinum deposit dispersion. Metal oxide supports, while typically not possessing the high surface areas offered by carbon-based materials, can introduce other benefits including strong metal-support interaction (SMSI) and optical activation properties (i.e. semiconductors) which, if effectively captured, can be beneficial for a reaction process. In regards to the latter, optically activated supports can induce reactivity via photocatalysis, often in conjunction with a metallic co-catalyst, which has been well-explored over the past three to four decades.14 Much less considered though is using light to activate a metal-on-metal oxide semiconductor as a means of boosting its (thermal) catalytic activity. Pre-treating Pt/TiO2 with UV light has been reported to improve its catalytic performance for CO oxidation15-16 as well as formic acid oxidation.17-19 Of these, Einaga et al.16 was the only study which probed the species active for the oxidation reaction. They reported Pt deposits on TiO2 stabilized O- and O3- species which formed on the Pt/TiO2 during UV-light pre-treatment when in a humidified oxygen environment. The generated O3- species were then assigned to be responsible for catalytic CO oxidation in the dark in dry air. More recently, it was observed that the light pre-treatment effect for formic acid oxidation by Pt/TiO2 necessitated the presence of oxygen in the system and that the effect decayed with time (on an hour-based time-scale).20 While the earlier observational studies touch on the idea of light pre-treatment for boosting oxidation reactions, there is little understanding regarding the mechanism governing the effect


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nor is it known whether the effect is specific only to platinum or if it presents itself in other metals. In this paper, we establish the potential of light pre-treatment to boost oxygen activation by metal catalysts (Pt, Au, Pd) supported on a semiconductor (TiO2). The light pre-treatment period is demonstrated to be important for catalyst performance as it governs the level of metal catalyst oxidation as well as the oxygen state prior to reaction. Formic acid oxidation (aqueous phase) was utilized as the test reaction for probing oxygen activation and on coupling with characterization results, electrocatalytic studies and first principle calculations, a mechanism explaining the activation process is established. The study initially focuses on Pt/TiO2 as the catalyst with the effect then shown to occur for Au/TiO2 and Pd/TiO2 catalysts. Differences in the oxygen activation behavior by Au relative to Pt and Pd are also accounted for. 2. EXPERIMENTAL SECTION 2.1. Catalyst

synthesis. The Pt (1.5 wt% nominal) was loaded onto the TiO2 support

(Aeroxide® TiO2 P25 (primary particle size ∼ 25 to 30 nm, surface area ∼50 m2/g, anatase to rutile ratio of 4:1)) via impregnation. Typically, 4 g of the TiO2 powder was dispersed in 400 mL ultrapure water. Then 102.5 mL of a 2000 ppm H2PtCl6 (99 %, Sigma–Aldrich®) solution was added drop wise to the suspension for 30 minutes while stirred at 500 rpm. The impregnated suspension was dried in an oven at 110 oC for 12 h. The recovered powder was then calcined and reduced in a horizontal tube furnace (Labec). Calcination involved heating the sample under 50 mL/min N2 at a rate of 5oC/min to 300oC and holding for 30 min. The N2 was replaced with 10% H2 in N2 (50 mL/min) and the temperature further increased to 500oC at a rate of 5oC/min where it was maintained for 3 h. The sample was then cooled to ambient temperature under a N2 flow. Au and Pd were loaded on the TiO2 in a similar fashion to the Pt with HAuCl4 used as the


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precursor during Au impregnation and Pd(NO3)2 used as the precursor during Pd impregnation. The light pre-treatment process was conducted immediately prior to catalyst activity assessment so is described in the appropriate section below. Catalyst characterization. The morphologies of the samples were investigated by high resolution transmission electron microscopy (HR-TEM), using either a Philips CM200 equipped with EDAX energy dispersive x-ray spectroscopy system and SIS CCD camera for direct recording of digital images or a JEOL JEM-ARM200F equipped with an EDS detector and a Cold Field Emission Gun with a resolution of 0.3 eV, which was coupled with a GIF Quantum imaging filter. Metal deposit size distributions were evaluated using 300 deposit counts. High resolution high angle annular dark field imaging scanning transmission electron microscopy (HR-HAADF-STEM) and fast Fourier transform analyses were conducted using a JEOL JEMARM200F. The crystal structure of samples was evaluated by X-ray diffraction (XRD) using a PANalytical xpert multipurpose x-ray diffraction system instrument. Data were collected by varying 2ϴ between 20° and 80° with a step size of 0.026°. The loading of each metal on TiO2 was determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) using a Perkin Elmer OPTIMA 7300. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo ESCALAB250i. The radiation was provided by a monochromatic X-Ray source (Al Kα, 1486.68 eV) operated at 164W emission power. Pt4f and O1s core-level spectra were recorded and the corresponding binding energies were referenced to the C1s peak at 285eV from surface carbon. The ORR electrocatalytic experiments necessitated preparing the catalyst in an electrode form. A catalyst paste was formulated by grinding 500 mg of the as prepared Pt/TiO2 with 500µL of


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absolute ethanol, 500µL of glacial acetic acid and 20 mg of chitosan. Ultrapure water and additional ethanol were then added until the paste possessed a homogeneous consistency. The electrode was prepared by transferring the paste onto a FTO glass slide using a doctor blading method. Neat TiO2, Au/TiO2 and Pd/TiO2 electrodes were prepared in the same manner. The ORR activity was assessed using a three electrode system consisting of the as-prepared Pt catalyst, Ag/AgCl and Pt foil as the working, reference and counter electrodes, respectively. The electrolyte comprised 0.1M KOH and a 300W Xenon lamp (PEC-CELL) was used as the light source. The I-V (current-voltage) profile was obtained using a potentiostat (PGSTAT302N, Autolab) controlled by GPES interface software. Linear sweep voltammetry (LSV) was performed from 0 V to -1.0 V after an initial 20 min N2 purge as the baseline, followed by a second measurement after 20 min of air purging. Light pre-treatment for 0 – 120 min was performed following the air purging step, and the current density was corrected to the corresponding baseline. To confirm there were no electric double layer or mass transfer effects during the analysis, electrochemical ORR and LSV were conducted for the as-prepared neat TiO2 and Pt/TiO2 at 1600 rpm using a glass carbon rotating ring-disk electrode (RRDE) comprising a computer controlled potentiostat (CHI 750E, CH Instrument) and Hg/HgO and Pt wires as the reference and counter electrodes, respectively. The sample (5.0 mg) was dispersed in a 1 ml mixture of water, ethanol and nafion (at a ratio of 16:4:1). A 7μl sub-sample of the homogenous suspension was dropped onto the RDE and then dried in the open air. The electrochemical tests were conducted in an O2-saturated electrolyte (0.1 M KOH) at a scan rate of 5 mV s-1 and a rotation speed of 1600 rpm.


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Photoluminescence (PL) measurements performed using a Fluoromax-4 (Horiba, Japan) spectrofluorometer. A 150W Xenon lamp with a spectral range of 200-1800 nm was used as the irradiation source. 2.2. Theoretical

calculations. Theoretical modelling of the O2 dissociation on the metal

surfaces and the metal oxidation were performed using density functional theory as implemented in Vienna ab initio simulation package (VASP).21-22 Electronic exchange-interaction was described using the Perdew, Burke, and Ernzerhof (PBE) functional.23 Wavefunctions expansion cut-off was set to 400 eV. Metal surfaces were modelled using 4 × 4 and 2 × 2 cells consisting of 6 layers, keeping the bottom three layers fixed to their ideal bulk positions. These slabs were separated by 15 Å to minimise image interactions. Brillouin iseone was sampled using a 4 × 4 × 1 grid. For structure relaxations, forces were relaxed to < 0.01 eV Åିଵ while total energies were converged to within 1 × 10ିହ eV. O2 dissociation energies and metal oxidation barriers were calculated using climbing-image nudege elastic band (CI-NEB) method. The role of TiO2 is to provide photoexcited electrons to the metal nanoparticles. This is effect was captured by adding electrons to the simulation cell. 2.3. Catalyst

activity assessment. Aqueous-phase formic acid (> 98%, Riedel–de Haën)

oxidation was employed as the test reaction to assess the catalytic oxidation performance of Pt/TiO2, Au/TiO2 and Pd/TiO2. Light pre-treatment and the ensuing formic acid oxidation reaction were performed in a glass spiral reactor (described elsewhere24) which had been modified to include a cell for measuring dissolved oxygen content of the suspension. Preparation of the catalyst suspension involved dispersing 50 mg of the catalyst powder in 50 mL of ultrapure water followed by 15 min of ultrasonic treatment. The pH of the suspension was then adjusted to 3±0.05 using 0.5 M perchloric acid (70 %, Frederick Chemicals) to ensure


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mineralized carbon from the oxidation reaction was expelled from the water into the gas phase (as CO2). The suspension was loaded into the spiral reactor where it was continually circulated through the system by a peristaltic pump. Light pre-treatment involved the suspension being illuminated with UV-light (NEC, 20 W black light blue, maximum emission = 365nm) for a set period of time (light pre-treatment period) after which the light was turned off. Earlier studies indicated that the light pre-treatment step in water promoted H2O2 formation although this oxidant was found not to contribute to the ensuing oxidation process in the dark.20 The system was allowed to continue circulating for a further 10 min so as to air-equilibrate and then 100 µmol of formic acid was added. Formic acid mineralization was monitored in-situ using a Jenway 3540 conductivity probe and dissolved oxygen content was also measured in-situ using a GOnDO PDO-408 dissolved oxygen probe. Catalyst performance was compared on the basis of the rate at which 50 % of the formic acid was mineralized (R50). The rate of dissolved oxygen consumption was compared at the time when 50 % of the formic acid had been mineralized (DOR50). Experiments performed under anoxic conditions followed the same procedure described above except the solution was purged with N2 gas in the period between turning off the light and adding the formic acid. 3. RESULTS AND DISCUSSION 3.1. Catalyst

Activity. The light pretreatment period strongly influenced Pt catalyst

performance as depicted in Figure 1a. The optimum pre-treatment time was 30 minutes which enhanced the formic acid oxidation rate (R50) by a factor of ~8 relative to the non-illuminated (i.e. t = 0 min) case. Dissolved oxygen consumption (i.e. DOR50) in relation to the light pretreatment period mimicked the formic acid oxidation profile (Figure 1b), clearly illustrating the


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involvement of oxygen in the reaction. The importance of oxygen is further displayed in Figure 1c where anoxic conditions invoked a substantial drop in activity implying DO is needed to replenish oxygen consumed during the reaction. Initially removing the oxygen decreased the R50 by more than 30 times for the 30 minute light pre-treatment condition.

Figure 1 (a) Fifty percent mineralization rate (R50) of formic acid by Pt/TiO2 with increasing light pre-treatment time; (b) dissolved oxygen consumption rate by Pt/TiO2 at the time when 50% of the formic acid had been mineralized (DOR50) for various light pre-treatment periods; (c) impact of light pre-treatment period on R50 under a deoxygenated condition. Initial amount of formic acid injected = 100 µmol; suspension volume = 50 mL; catalyst loading = 1 g/L;


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suspension pH = 3 ± 0.05; air equilibration time= 10 minutes. The suspension was not illuminated during or after formic acid addition. 3.2. Catalyst

Characteristics. The Pt deposits were shown from HR-HAADF-STEM

(Figure 2a) and HR-TEM imaging (Figure 2b) to be well dispersed across the TiO2 surface, possessing diameters of 1.7 ± 0.1 nm with a narrow size distribution ranging from 1 to 5 nm (Figure 2c). The Pt loading on the TiO2 was determined from ICP-OES to be 0.78 at%. The Aeroxide P25 TiO2 maintained its crystallinity and relative crystal phases during the impregnation process (XRD profiles, Supplementary Information Figure S1a). Following the impregnation process, peaks at ~2ϴ = 39.7o and 46.4o appeared in the XRD spectrum which are indexed to Pt (111) and Pt (200), respectively (JCPDF 01-1194), possessing a face-centered cubic (fcc) structure with an Fm3m space group. XPS of the region encompassing the Pt peaks (Figure 2d) can be separated into Pt4f5/2 at a binding energy (BE) of 74.6 eV and Pt4f7/2 at a BE of 71.3 eV. The peaks representing the two orbital configurations can be de-convoluted into three overlapping peaks representing Pt0 (metallic Pt), PtOads (Pt with surface-adsorbed oxygen) and Ptox (oxidized Pt (i.e. Pt2+ and Pt4+)).16 The spectrum for the as-prepared Pt/TiO2 indicates the Pt deposits are initially present as a mixture of Pt0 (65.2%), PtOads (21.7%) and Ptox (13.1%). Direct evidence of oxygen activation can be obtained by electrocatalytic studies measuring the occurrence of the ORR as seen in Figure 2e for neat TiO2 and Pt/TiO2. The profile for Pt/TiO2 exhibits two main features: (i) an initial slow increase in current density (over the applied voltage range -0.25 V – -0.63 V) attributed to the reduction of Oads to produce HOads which is a reflection of the Oads species already present on the surface;25 (ii) an ensuing sharp increase in current density (applied voltage < -0.63 V) attributed to the activation of dissolved


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oxygen in the solution (i.e. the ORR). A comparison between the neat TiO2 and Pt/TiO2 profiles indicates oxygen activation is enhanced by the presence of Pt deposits and dissolved oxygen. RRDE measurements at 1600 rpm was conducted to determine whether there were any diffusion and/or electric double layer influences on the results are provided in Figure S1b for neat TiO2 and Pt/TiO2. The current density in the RRDE measurements (Figure S1b) was higher when compared to the stationary electrode measurements for both neat TiO2 and Pt/TiO2 (Figure 2e) indicating that the diffusion resistance is minimised and the mass transport of reactants is actively stimulated under these conditions.26 Additionally, the RRDE measurement was in agreement with the stationary electrode measurement wherein ORR process comprises a twostep mechanism, as indicated by dual plateau pathways observed for both profiles. The ring current from the RRDE measurement at the peak maxima at an applied voltage of ~0.52 eV (vs Ag/AgCl) also indicated HOads was being produced.27 Consequently, as the ORR mechanism is similar for both the RRDE and stationary electrode measurements, any influence of the electrical double layer can be considered as negligible.28 PL spectroscopy can define the extent of electron/hole recombination occurring within a material following light excitation. Emission peaks at 374 nm, 396 nm and 413 nm have been reported to represent recombination within TiO2 following UV-light excitation.29 Figure 2f shows in the presence of Pt deposits and following excitation (λ = 350 nm) the emission peak intensity exhibited by the TiO2 is lower which indicates a lesser degree of electron/hole recombination and signifies electron capture by the Pt deposits.


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Figure 2 (a) HR-HAADF-STEM image of as prepared Pt/TiO2 including lattice fringe measurements and FFT patterns (insets); (b) HR-TEM image of Pt deposits on TiO2 including (c) deposit size distribution (300 Pt deposit count); (d) XPS profile of Pt/TiO2 - Pt0 (red) represents


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metallic Pt, Ptox (pink) represents oxidized Pt (Pt2+ and Pt4+), PtOads (blue) represents Pt with surface-adsorbed oxygen. The solid black profile describes the original XPS spectrum; (e) electrocatalytic ORR profiles for neat TiO2 and Pt/TiO2 under ambient and deoxygenated conditions. The darker grey-shaded area represents the potential region where Oads already present on the surface is reduced to HOads. The lighter grey-shaded area represents the potential region where the activation of oxygen dissolved in the solution occurs (i.e. the ORR)21; (f) photoluminescence spectra of neat TiO2 and Pt/TiO2 following illumination at λ = 350 nm. Increasing the light pre-treatment period alters the relative portion of the three types of Pt species, as seen in Figure 3. Most noticeable is the variation in PtOads content mirrors the change in R50 values (Figure 1a) irrespective of whether ambient (Figure 3a) or deoxygenated (Figure 3b) conditions are used. That is, the greatest percentage of PtOads occurs with 30 minutes of light pre-treatment. The same variation in atomic oxygen (O-ads) species on the Pt/TiO2 surface can be seen for the de-convoluted O1s region of the XPS spectra (XPS profile, Supplementary Information Figure S1c and Figure S1d). It is also apparent from Figure 3 that the Ptox content decreases over the first 30 minutes of light pre-treatment. Beyond 30 minutes the amount of Ptox then increases at the expense of the PtOads (and Pt0). The fraction of Pt0 in the deposits is observed to consistently decrease with increasing light pre-treatment time.


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Figure 3 Impact of light pre-treatment period on Pt deposit oxidation state for Pt/TiO2 under (a) ambient and (b) deoxygenated conditions. Pto represents metallic Pt, PtOads represents Pt with surface-adsorbed oxygen, Ptox represents Pt in the +2 and +4 oxidation states The influence of light pre-treatment time on Pt deposit size was also examined with the associated HR-TEM images provided in (Figure S2). The micrographs and corresponding Pt deposit size distributions indicate there is a minor increase in Pt deposit size with light pretreatment time which is not anticipated to be a significant contributor to the variation in activity performance exhibited by the Pt/TiO2.


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Light pre-treatment alters the capacity of Pt/TiO2 to activate molecular oxygen (i.e. ORR) as is illustrated in Figure 4a whereby a 30 min light pre-treatment period facilitates oxygen activation to the greatest extent. In addition, the current density profiles (Supplementary Information, Figure S1e) exhibit two additional features following light pre-treatment - the light pre-treatment time: (i) reduces and then extends the ORR onset potential; (ii) increases and then halts the level of Oads that is reduced prior to the ORR onset (as indicated by the slope of the profile in the dark grey region). A maximum level of Oads reduction is observed for 30 minutes of light pre-treatment suggesting either an increased presence of Oads on the Pt deposits or facilitation of the reduction process. Following light pre-treatment times of 60 minutes and beyond it appears that either the Oads presence has been diminished or its reduction has been quenched. Analysis of the 396 nm emission peak from PL spectroscopy (Figure 4b) shows that electron/hole recombination within the Pt/TiO2 decreases over the initial 30 min of light pretreatment after which it increases. Both the extent of the ORR and variation in electron/hole recombination with the light pre-treatment time echo the R50 reaction profile in Figure 1a. Pre-treating Pt/TiO2 with light improves its capacity for oxygen activation. The findings indicate atomic oxygen on the Pt deposits (PtOads) is the active species for the reaction with the light pre-treatment boosting its presence. A clear relationship exists between the light pretreatment period, the catalytic oxidation rate, the (post-reaction) dissolved oxygen concentration and the presence of PtOads. Increasing the light pre-treatment period concomitantly increases each of these characteristics, although a time is reached (30 min) after which the effect begins to diminish. The decrease in performance correlates with a growing presence of oxidized Pt (i.e. Ptox - Pt2+ and Pt4+) in the Pt deposits.


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Evidence supporting the idea that light pre-treatment is promoting oxygen activation is provided by the electrocatalytic ORR and PL findings. The increase in current density and the decrease in electron/hole recombination imply an enhanced charge trapping effect is occurring for up to 30 min light pre-treatment. A direct correlation exists between the ORR, electron/hole recombination (PL) and R50 values demonstrating oxygen activation (Eq. 1) is in effect.

Figure 4 (a) Extent of electrocatalytic ORR activity (vs Ag/AgCl); (b) change in 396 nm PL peak intensity for Pt/TiO2 following different pre-light treatment times under ambient conditions. 3.3. O2

dissociation on Pt(111). It is known that Pt is able to catalyze the reduction and

dissociation of molecular oxygen with first principle studies demonstrating Pt surfaces30 and Pt


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clusters31 can drive this reaction. We believe the same process is occurring in our system with the phenomenon being amplified by light pre-treatment. That is, light pre-treatment is intensifying the electron presence within the Pt deposits through the deliverance of photoexcited electrons from the TiO2 support. The increased electron density within the Pt deposits then facilitates oxygen activation and promotes the formic acid oxidation reaction. Theoretical calculations were performed to confirm our view whereby the impact of adding electrons to a Pt (111) surface on molecular oxygen reduction and dissociation was studied. Adding electrons to the Pt (111) mimics the injection of photoexcited electrons into the Pt deposit from the TiO2 during the light pre-treatment stage. One of the most stable O2 adsorption sites on a Pt (111) surface is the top-bridge-top configuration in which the O2 molecule dissociates into atomic O that adsorb onto fcc sites. The energy barrier for this process was calculated to be ~0.52 eV for a 2 × 2 supercell (Figure 5a) which is comparable to other theoretical and experimental findings.32 In addition, the ‘hump-like’ nature of the profile indicates that once the oxygen has dissociated there is a resistance to its re-association suggesting a degree of activated oxygen stability exists. Upon adding an extra electron to the simulation cell, the barrier decreased to 0.35 eV. This supports the experimental findings in that treating the sample with light can promote an increase in oxygen dissociation to provide additional adsorbed O species. For a 4x4 supercell (Figure 5b) the same effect is in place with a single electron injection into the cell lowering the dissociation barrier from 0.18 eV to 0.10 eV. A further three-electron addition to the 4x4 cell sees the dissociation barrier become almost non-existent. Also apparent, when comparing the energy barriers for dissociation for the 2x2 and the 4x4 cells, is that an increasing presence of oxygen atoms relative to the number of surface Pt atoms (i.e. 0.5ML and 0.125ML for the 2x2 and 4x4 cells, respectively) raises the barrier for oxygen dissociation.


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Consequently, the ease of oxygen dissociation on the Pt is seen to be governed by both the level of charge within the Pt and the oxygen content on the Pt surface. The postulation of ensuing oxidation of the Pt at longer light pre-treatment times (> 30 min) requires an adsorbed O atom to move into a subsurface region in which the most energetically stable site is the one with a fourfold-coordinated tetrahedral arrangement. This site lies directly below the hcp site33 and thus requires that O atoms adsorbed at fcc sites to first hop to hcp sites before Pt oxidation can take place. The calculated barriers for the movement of an adsorbed O from an fcc to an hcp site and then into the subsurface tetrahedral site (2 × 2 supercell) for the neutral and for the negatively charged case are shown in Figure 5c. It is apparent that the additional electron lowers the barrier to O site hopping by ~0.1 eV while raising the barrier to O subsurface diffusion by the same amount. The greater ease by which the O can hop from an fcc to an hcp site may assist oxygen dissociation by freeing up occupied fcc sites at a greater rate. Given the overall barrier to O subsurface diffusion is ~2.6 eV in the neutral case a 0.1 eV increase upon single charge addition does not represent a significant change to this step. In any case, the O subsurface diffusion hump for the 2x2 supercell indicates that once the barrier has been overcome, there is a resistance to the reverse process (i.e. liberation of the O atom from the fourfold-coordinated tetrahedral Pt site) occurring. To assess whether the extent of O coverage was responsible for stabilizing Pt oxidation the subsurface O site stability for a 2x2 supercell with an O coverage over the range 0.25 to 1.0 ML was evaluated (Figure 5d). It is apparent that when the O monolayer coverage increases from 0.25 ML to 0.5 ML the subsurface O site stabilizes. Figure 5d also illustrates that as the surplus charge density within the Pt cell is increased from neutral to four the stability of the subsurface O site weakens reflecting an increased favoritism for PtOox photoreduction. The reduced stability at


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1.0 ML coverage derives from the increased O-O repulsion at the surface sites.21 The calculations indicate that the extent of PtOox deriving from light pre-treatment is governed by a shift in the balance between excess charge within the Pt deposits and active oxygen species on the Pt surface. That is, over the initial 30 min of light pre-treatment the excess charge is dominant while beyond 30 min activated oxygen coverage is the controlling factor.

Figure 5 Energy barriers for the dissociation of O2 into atomic oxygen: (a) The 2 × 2 surface; and (b) the 4 × 4 surface. The dissociation energies decrease as the electron density is increased. (c) The energy barriers for an fcc adsorbed O atom to hop to an hcp site before migrating to a subsurface tetrahedral site (2 × 2 surface). (d) Change in subsurface site stability with increasing activated oxygen coverage on, and increasing charge density within, the Pt (2 × 2 supercell). A positive binding energy value indicates site stability.


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O2 dissociation mechanism on Pt deposit. Based on the experimental

findings and first principle calculations, a mechanism explaining the effect of light pre-treatment on oxygen activation by the Pt deposits on TiO2 can be envisaged (Figure 6). Initially, as demonstrated by XPS (Figure 3a), the Pt deposits are dominated by Pt0, although a mix of PtOads and Ptox is also present (Figure 6a). The electrocatalytic findings (Supplementary Information, Figure S1e) also indicated an initial presence of Oads on the Pt deposit surface. Upon illumination (Figure 6b), photoexcited electrons transfer from the TiO2 into the Pt deposits. The increase in charge has two competing effects: (i) a portion of the O atoms originally located in the subsurface tetrahedral Pt sites are destabilized leading to a decrease in the Ptox content (from XPS, Figure 3a); (ii) O2 dissociation on the fcc Pt sites is facilitated via a lowering of the associated energy barrier (first principle calculations, Figure 5a and 5b). The continual decrease in Pt metal content with increasing light pre-treatment time implies the latter process is more dominant while the reduced photogenerated electron-hole recombination (from PL, Figure 4b) highlights the increased uptake in charge by the process. The dissociated O atoms then begin to accumulate and the Pt fcc sites become increasingly occupied. The effect of O atom accumulation is twofold: (i) the energy barrier for O2 dissociation rises (first principle calculations, Figure 5a and 5b), offsetting the increased charge effect; (ii) the energy barrier to O atom hopping from the fcc sites to hcp sites is eventually surmounted as overcrowding sees the O atom ‘pushed off’ the fcc sites. The O atom hopping process appears to be mildly aided by the elevated charge in the Pt (first principle calculations, Figure 5c). Ultimately the hcp sites become saturated by O atoms whereby O atom diffusion into a subsurface tetrahedral Pt site becomes viable (Figure 6c) and stable according to the electrocatalytic ORR profiles (Supplementary Information, Figure S1e), leading to the onset of Pt oxidation (from XPS,


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Figure 3a). The experimental findings indicate this point is reached at between 30 and 60 min of light pre-treatment. Once a tetrahedral Pt site has been filled by an O (i.e. Ptox has formed) the hcp site is no longer able to accept further O atoms and the sites effectively become deactivated with an accompanying loss in activity. The loss of active metallic Pt sites also lowers the capacity of the Pt deposit to accept electrons from the TiO2 which is reflected as an increase in photogenerated electron-hole recombination within the particle (from PL, Figure 4b).

Figure 6 Proposed effect of light pre-treatment time on oxygen activation and fate: (a) With no light pre-treatment; (b) Up to 30 min light pre-treatment; (c) Beyond 30 min light pre-treatment.


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3.5. Pd/TiO2

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and Au/TiO2 activity. We investigated whether the light pre-treatment effect

can benefit oxygen activation by other metal catalysts and found it can occur for Au and Pd deposited on TiO2. Without light pre-treatment the Pd/TiO2 catalyst exhibited negligible activity while light pre-treatment induced activity (Figure 7a) with an optimum rate achieved for a 60 min light pre-treatment period. In the case of Au/TiO2, 30 min of light pre-treatment invoked an almost eight-fold increase in catalyst activity (Figure 7b), albeit considerably less active than Pt/TiO2. Characteristics of the Pd and Au deposits pre- and post-light pre-treatment are provided as Supplementary Information (Figure S3 – S4) and highlight similarities and differences for the two metals when compared with Pt. HR-HAADF-STEM and HR-TEM imaging (Supplementary Information, Figure S3a and S3b, respectively) indicates the Pd deposits ranged in size from 1 to 5 nm in diameter with an average size of 2.3 ± 0.2 nm (Figure S3c). ICP-OES indicated the catalyst had a Pd loading of 0.68 at%. The XPS analysis showed the Pd deposits (Supplementary Information, Figure S3d) initially comprised a mixture of Pd0, PdO and PdO2 with light pre-treatment changing the relative portions of each oxidation state. The change in Pd oxidation state with increasing light pre-treatment time exhibited similarities to the Pt case with a decline in the Pd0 content with time and a PdO maxima occurring at the point when the highest activity occurred. It is also apparent the PdO2 presence in the deposits increases with light pre-treatment time. The similarities suggest a process analogous to that observed for the Pt is occurring for the Pd with the PdO imitating the PtOads and the PdO2 growth roughly equivalent to the PtO formation (especially at longer pre-treatment times). While the XPS profiles for Pd do not give direct evidence of a PdOads presence, verification of an initial Oads presence (or absence) can be seen in the O1s region of its spectra as well as the dark grey region of its electrocatalytic ORR profiles


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(Supplementary Information, Figure S3e and Figure S3f, respectively). The impact of increasing light pre-treatment period on the O1s region of the Pd/TiO2 XPS profiles reveals a relationship exists between the O-ads and the R50 values (Supplementary Information, Figure S3e) which is similar to that seen for the Pt (Supplementary Information, Figure S1c). PL analysis of the Pd/TiO2 following light pre-treatment (Supplementary Information, Figure S3g) indicates electron-hole recombination is lowest for the light pre-treatment time promoting the greatest activity. Electrocatalytically probing the Pd/TiO2 with increasing light pre-treatment time (Figure 7c) highlights a direct relationship between oxygen activation and catalyst performance. The current density profiles for Pd/TiO2 (Supplementary Information, Figure S3f) show the Oads on the Pd/TiO2 undergoes an initial reduction step during electrocatalytic treatment in a manner similar to Pt/TiO2. That is, the degree of initial Pd reduction is governed by the light pre-treatment period whereby extended light treatment eventually promotes stabilization of the Pd oxide which, from XPS (Supplementary Information, Figure S3d), appears to be in the form of PdO2. First principle calculations (Figure 7e) demonstrated that an increase in electron density in the Pd also lowers the energy barrier for oxygen activation. On comparing the Pt and Pd systems, it is apparent light pre-treatment has a similar effect on both in terms of increasing oxygen activation until a time is reached where the deposits begin to oxidize. Overall though, light pre-treatment has a greater promotional effect on the Pt deposit activity which is reflected in the greater ORR current density it generates as well as the lower O2 dissociation energy barrier it possesses relative to Pd. The Au/TiO2 exhibited similar benefits to Pt/TiO2 and Pd/TiO2 when pre-treated with light although the original activity was considerably lower and some aspects of the Au deposit characteristics were noticeably different. The Au deposits have a size distribution ranging from 4


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to 16 nm with an average size of 8.4 ± 0.3 nm (Figure S4c) which was larger than the other metals (from HR-HAADF-STEM and HR-TEM, Supplementary Information, Figure S4a and S4b, respectively) while ICP-OES indicated the catalyst had an Au loading of 0.91 at%. The larger deposit size of the Au is anticipated to influence the ability of the Au/TiO2 to activate oxygen although, as the purpose of the study is to identify whether the light pre-treatment effect occurs for metals other than platinum and the mechanism by which it occurs, the impact of deposit size is secondary in this instance. The lower activity despite the higher metal loading of the Au relative to the Pt and Pd indicates metal loading, while expected to be impactful for each individual metal, is also a secondary influence when comparing the different metals. The XPS analysis (Supplementary Information, Figure S4d) indicated the Au deposits were metallic prior to and, more interestingly, remained so following light pre-treatment. In contrast, the O1s region of the Au/TiO2 XPS spectra (Supplementary Information, Figure S4e) indicated Oads were initially present on the Au deposits and that light pre-treatment initially promoted and then depleted their presence with time. The current density profiles for Au/TiO2 (Supplementary Information, Figure S4f) display evidence of a small initial presence of Oads on the surface (i.e. the slight decrease in current density within the dark grey region) although the level is substantially lower than for Pt/TiO2 and Pd/TiO2. PL analysis showed a change in electron/hole recombination with light pre-treatment time (Supplementary Information, Figure S4g) giving evidence of some mode charge trapping by the Au deposits. The charge trapping correlates with the variation in O-ads concentration and, in light of the lack of Au deposit oxidation, suggests the electrons are being trapped by the activated oxygen. First principle calculations for Au/TiO2 (Figure 7f) show that light pretreatment lowers the oxygen dissociation barrier as per the Pt/TiO2 and Pd/TiO2 although there are two distinct


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differences in the Au profile. Firstly, the energy needed to overcome the dissociation barrier for Au is substantially greater than both Pt and Pd, explaining the considerably lower R50 values exhibited by the Au. This is further supported by the electrocatalytic assessment of the Au/TiO2 (Figure 7d and Supplementary Information, Figure S4f, light gray shaded region) where the maximum current density is roughly half and one third that of the Pd and Pt, respectively. Secondly the shape of the energy barrier profile once the activation barrier has been overcome is dissimilar. In the case of Au/TiO2 there is no apparent ‘hump’ at the dissociation point with the energy barrier instead plateauing. The plateau infers that once the oxygen has dissociated there is little resistance to its re-association which may explain the lack of Au deposit oxidation with light pre-treatment time. The overall higher energy barrier and lack of resistance to oxygen reassociation may account for the lower activity displayed by the Au/TiO2 as both factors imply oxygen dissociation is not especially facile.


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Figure 7 Fifty percent mineralization rate (R50) of formic acid by (a) Pd/TiO2 and (b) Au/TiO2 with increasing light pre-treatment time. Initial amount of formic acid injected = 100 µmol; suspension volume = 50 mL; catalyst loading = 1 g/L; suspension pH = 3 ± 0.05; air equilibration time= 10 minutes. The suspension was not illuminated during or after formic acid


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addition. Electrocatalytic ORR results at -1.0V potential (vs Ag/AgCl) for (c) Pd/TiO2 and (d) Au/TiO2 following different pre-light treatment times under ambient conditions. Energy barriers for the dissociation of O2 into atomic oxygen on (e) a 2 × 2 Pd (111) surface and (f) a 2 × 2 Au (111) surface. The dissociation energies decrease as the electron density is increased. 4. CONCLUSION In this work, we have coupled experimental and first principle density functional theory studies to expose the mechanism by which light pre-treatment of Pt/TiO2 promotes catalytic oxygen activation. Pre-treating Pt/TiO2 with UV light for up to 30 minutes promoted its capacity to catalytically activate oxygen by up to 8 times via photogenerated electron injection into the Pt deposits. Light pre-treatment beyond 30 min invoked a decrease in its catalytic oxygen activation capacity with a concomitant partial oxidation of the Pt deposits. It is believed that beyond 30 min the Pt deposit surface becomes over-crowded with activated oxygen atoms with any additional photo-generated oxygen atoms then being forced into subsurface tetrahedral Pt sites. The optimum light pre-treatment time is proposed to be governed by a shift in the balance between excess charge within the Pt deposits and active oxygen species on the Pt surface. We also discovered the light pre-treatment effect can occur in other noble metal deposits (Au and Pd), proceeding via a process analogous to the Pt although, in the instance of Au/TiO2, the effect is not as prevalent. The lesser performance by Au/TiO2 was found to derive from an initially higher energy barrier to oxygen dissociation coupled with a low resistance to activated oxygen reassociation. The findings have positive implications for systems reliant on oxygen activation (e.g. fuel cells) where the light pre-treatment approach can provide a facile, non-thermal means to promote catalyst performance.


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ASSOCIATED CONTENT Supporting Information The supporting information is available for free of charge on the ACS publication website at DOI: Pt/TiO2 characteristic including XRD and XPS results; Pd/TiO2 and Au/TiO2 characteristics including HAADF-STEM, HR-TEM, XPS and PL; Electrocatalytic ORR profiles of Pt/TiO2, Pd/TiO2 and Au/TiO2 following different light pre-treatment times under ambient conditions. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Author Contributions Catalyst synthesis, characterization and activity testing was conducted by W.H.S. First principle calculations were performed by H.T. All other authors provided technical and scientific support as well as played a supervisory role in the research. The manuscript was prepared by J.S. and revised by all authors. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The work was supported by the Australian Research Council (ARC) under the Laureate Fellowship Scheme - FL140100081. W.H. Saputera acknowledges the Indonesia Endowment Fund for Education Scholarship (LPDP), Republic of Indonesia for financing his PhD scholarship. The authors would like to acknowledge the use of facilities (XRD, HR-TEM, XPS) within the UNSW Mark Wainwright Analytical Centre and Dr Bill Bin Gong for his assistance with the XPS analysis. The authors would also like to acknowledge Dr David Mitchell from the Electron Microscopy Centre within the Australian Institute for Innovative Materials, based at the University of Wollongong, for his assistance with the HR-HAADF-STEM and HR-TEM imaging. We also thank Cui Yanglansen for assistance with the RRDE measurement. REFERENCES 1. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B. 2005, 56, 935. 2. Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Chem. Mater. 2016, 28, 16911697. 3. Yu, J.; Chen, G.; Sunarso, J.; Zhu, Y.; Ran, R.; Zhu, Z.; Zhou, W.; Shao, Z. Adv. Sci. 2016, 3, 1600060. 4. Chen, G.; Sunarso, J.; Zhu, Y.; Yu, J.; Zhong, Y.; Zhou, W.; Shao, Z. ChemElectroChem. 2016, 3, 1760-1767. 5. Murphy, Michael P.; Holmgren, A.; Larsson, N.-G.; Halliwell, B.; Chang, Christopher J.; Kalyanaraman, B.; Rhee, Sue G.; Thornalley, Paul J.; Partridge, L.; Gems, D.; Nyström, T.; Belousov, V.; Schumacker, Paul T.; Winterbourn, Christine C. Cell Metab. 2011, 13, 361-366. 6. Xue, E.; Seshan, K.; Ross, J. R. H. Appl. Catal., B. 1996, 11, 65-79. 7. Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Surf. Sci. 2006, 600, 1714-1734. 8. Marković, N. M.; Ross Jr, P. N. Surf. Sci. Rep. 2002, 45, 117-229. 9. Hinde, C. S.; Ansovini, D.; Wells, P. P.; Collins, G.; Aswegen, S. V.; Holmes, J. D.; Hor, T. S. A.; Raja, R. ACS Catal. 2015, 5, 3807-3816. 10. Soboleva, T.; Malek, K.; Xie, Z.; Navessin, T.; Holdcroft, S. ACS Appl. Mater. Interfaces. 2011, 3, 1827-1837. 11. Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou; Sun, G.; Xin, Q. J. Phys. Chem. B. 2003, 107, 6292-6299. 12. Divya, P.; Ramaprabhu, S. J. Mater. Chem. A. 2014, 2, 4912-4918.


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Light, Catalyst, Activation: Boosting Catalytic Oxygen Activation Using

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