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Thermally Activated Formation of Reactive Lattice Oxygen in Titania on Nanoporous Gold Bradley W. Ewers, Andrew S Crampton, Monika M Biener, and Cynthia M. Friend J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06316 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Thermally Activated Formation of Reactive Lattice Oxygen in Titania on Nanoporous Gold Bradley W. Ewers†, Andrew S. Crampton†, Monika M. Biener#, Cynthia M. Friend†

*

†

Harvard University, Department of Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, Massachusetts, 02138 United States. #

Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, Livermore, California 94550, United States ABSTRACT: Nanoporous gold modified by Atomic Layer Deposition (ALD) of titania was employed as a model, inverse supported precious metal catalyst to explore the oxidative reactivity of Au/TiO2 catalysts. The catalyst material is active for oxidation of CO using O from the titania lattice, even at low temperatures. The reactive lattice oxygen available for oxidative reactions was formed by thermal activation, with the total oxidative reactivity of the material showing a strong dependence on the annealing temperature prior to CO exposure. Finally, reduction of the titania via CO oxidation can be reversed under relatively mild conditions by exposure to O2, presenting a viable mechanism for CO oxidation in steady state catalytic conditions.

Introduction Nanoscale gold on metal oxide supports is active for a wide range of selective catalytic reactions.1-6 There is still considerable debate about the origin of the activity of Au nanoparticles and the possible roles of the oxide support. Reducible oxides, such as titania, ceria, and oxides of nickel, cobalt, and iron, dramatically influence the reactivity of Au when employed as supports.7 The TiO2/Au couple is of particular interest owing to its own reactivity and photo-reactivity.8 Titania is also one of the most widely used supports in gold catalysis. One of the key factors in gold catalysis is supplying atomic oxygen to the gold surface. Atomic oxygen on gold drives highly selective partial oxidation and coupling reactions;9-12 however, it is not possible to activate O2 to produce Oads on metallic gold with sufficient efficiency for catalysis.13 Molecular oxygen only weakly binds to metallic gold;14 therefore, there is not an accessible pathway for O2 dissociation despite the robustness of the Au-O bond.15-16 Alternatively, oxygen sources with a higher chemical potential, e.g. ozone or atomic oxygen, can be used to produce adsorbed O; however, these are not viable for catalytic processes. Even so, Au-based materials are used as oxidation catalysts, raising the question of how oxygen is supplied to the material. Other mechanisms have been proposed for the high activity of Au on reducible oxide supports. Early on, quantum size effects related to and/or the presence of a high fraction of undercoordinated Au atoms were proposed to explain the activity of Au nanoparticles supported on reducible oxides.17-22 More recently, reaction at

the interface between the oxide and the Au particle has been invoked to explain the catalytic activity of Au.23-24 A variation on this concept is a modification of the Marsvan Krevelen (MvK) mechanism, in which lattice oxygen of the metal oxide is directly involved in the reaction. This mechanism has, for example, been unambiguously demonstrated for CO oxidation on iron oxide over Pt.25 Theoretical studies of CO oxidation on gold clusters supported on CeO2,26-27 indicate that CO oxidation can occur via reaction with lattice oxygen at step-edge sites and this behavior can be promoted by dopants, as indicated by variations in the oxygen vacancy formation energy of the metal oxide. Temporal Analysis of Products (TAP) was used to demonstrate that oxygen derived from the catalyst support led to transient oxidation of CO, but that the activity quickly deteriorated as reactive lattice oxygen on the catalyst depleted.28 They further demonstrated that activity could be recovered in oxygen, demonstrating viability for steady-state catalytic function. Further evidence for the importance of the Au-metal oxide interface was obtained in studies of “inverse” catalysts, which are model systems comprised of a metal oxide grown on crystalline, metallic Au. 23, 29-32 For example, CeO2 and also mixed CeO2/TiO2 deposited on Au(111) lead to the activation of H2O in the water gas shift reaction. This activation is attributed to interaction of the water at the oxide-Au interface, which yields OH. Similar behavior for the water gas shift reaction has also been recently observed with Ce-TiOx/npAu catalysts, as well as the observation that exposure of reduced titania to oxygen forms a crystalline structure. Subsequent CO exposure then converts titania back to the amorphous structure,

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indicating that lattice oxygen in these systems is a reactive species.33 The advantage of an “inverse” catalyst is the potential for increased structural stability compared to supported catalysts. Synthesis of catalytic materials that build on the inverse catalyst concept has been demonstrated for nanoporous Au (npAu), a high surface area material that is a functioning catalyst for oxidation reactions including CO to CO2 and selective oxidation of alcohols.34-36 Further, this material is amenable to investigation with UHV surface science techniques, enabling more detailed investigation of reaction mechanism.34, 37 The nanoporous “inverse” Au/TiO2 catalyst architecture used in this work is synthesized by coating npAu with titania using Atomic Layer Deposition (ALD).38-39 The conformal titania coating breaks apart into nanoparticles upon annealing of the sample in air (at 600˚ C), due to a transformation from amorphous to anatase phase as the majority phase (with small contributions from amorphous and rutile phase). This results in the formation of an inverse supported Au/TiO2 catalyst.40 The presence of the titania also inhibits thermal coarsening38 of npAu and imparts greater mechanical strength, potentially due to step-edge29 and grain boundary41 pinning. This nanoporous Au/TiO2 inverse catalyst oxidizes CO under flow conditions with a rate that is ~2 times greater than over as-prepared npAu38, 42 suggesting that titania augments the ability of npAu to catalyze oxidation reactions.

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viously reported procedures.38, 42 SEM images of the sample before and after the experiments (Fig. 1) indicate that no overall morphological changes in the npAu were observed (e.g. coarsening of npAu) over the course of the experiments. A detailed analysis of the distribution of titania particles was not performed.

Figure 1. SEM images of the npAu@TiO2 before (A) and after (B) the conducted experiments. The titania particles are the light areas in the images shown. No changes in the overall morphology of the npAu were observed.

Herein, we investigate the activity of a npAu/TiO2 inverse catalyst (Fig. 1) for CO oxidation under ultrahigh vacuum conditions in order to better understand how this material functions. Our studies indicate that CO is oxidized by reaction with lattice oxygen of the titania leading to partial reduction of the titania. Thermal activation of the lattice oxygen is required for CO oxidation. We also demonstrate that the overall reduction of the material by CO oxidation can be reversed under relatively mild oxidizing conditions, demonstrating a potential pathway for catalytic function. These results provide insight into the nature of reactivity of the Au/titania couple, suggesting that the interface between these two materials drives the formation of reactive oxygen on the titania surface.

After mounting, the sample was exposed to an ozone (50 g/sccm in O2) stream at room temperature and ~1 atm for one hour at a constant temperature of 425 K to remove carbon contaminants. This treatment was performed in a sample preparation cell and thereafter transferred directly to the main UHV chamber; thus, minimizing recontamination. The ozone is created by treating flowing oxygen with an electrical discharge using a commercial ozone generator (Ozone Engineering LG-7). The sample preparation cell was then evacuated and the temperature of the sample ramped to 725 K and held there briefly to remove adsorbed oxygen, moisture and other contaminants. This procedure was repeated five times and thereafter the sample was transferred to the main analysis chamber.

Experimental All experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of < 5 x 10-10 torr. Samples were in the form of ingots of 6.5 mm diameter and 300 µm thickness. The samples were strapped to an oxidized silicon support piece using gold wire and a gold plated electroformed nickel mesh (Precision E-forming, New York, USA), and a tungsten heating filament and Type K thermocouple were mounted to the back of the silicon support using ceramic paste (Ceramabond 569, Aremco, New York, USA). Demonstration that the sample holder does not contribute to the desorption spectra of the sample is provided in the supporting information (Fig. S1 and S2). Nanoporous Au/TiO2 samples were prepared by applying 30 ALD cycles of titania to npAu ingots prepared by pre-

After transfer and at the beginning of each day, the sample was exposed to ozone (~20,000 L) at 425 K to remove contaminants. Cleanliness was confirmed by Auger spectroscopy in addition to checking for the evolution of H2O, CO, and CO2 from the sample after small ozone exposures. When the sample was clean, only O2 desorption was detected. This treatment was adopted to fully oxidize the titania particles on the Au since vacancies and other defects arising from reduction of the material can influence chemical behavior. The ozone treatment also oxidizes the Au itself, based on studies of pure npAu.43 A film of gold oxide forms that is ~1-2 nm thick, based on environmental TEM and ambient pressure XPS.43 Gas exposures to the sample were made using directed variable leak valves with measured enhancement factors


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of ~50 relative to the change in background pressure. Desorbing gases were analyzed with a Hiden 3F-PIC Series Quadrupole mass spectrometer. The sample was held at 80 V bias during all experiments to inhibit any potential electron-induced reactions from electron sources within the chamber.

Results and Discussion Exposure of npAu/TiO2 to ozone yields atomic O, which recombines and desorbs as O2 at ~580 K (Figure 2). This peak is similar to that measured for O2 formation from O adsorbed on Au single crystals.14, 16, 44 Ozone treatment of npAu produces a thin film oxide that can be either desorbed or removed via reaction with CO.43, 45 Oxygen desorbs from defective sites in rutile TiO2(110) surface at 410 K,46 however the expected phase of titania in this case is anatase,38 which does not support surface oxygen defects, though oxygen can adsorb at near surface defects, producing a bridging O2 species.47-49 These experiments demonstrate that the npAu/TiO2 has capacity for oxygen uptake on exposed Au if the oxygen chemical potential is sufficiently high. Molecular oxygen exposure does not lead to any detectable oxygen adsorption (Fig. 2). The small amount of O2 desorption at low temperatures was not measurably different from that observed for the sample holder alone (Fig. S2). This result indicates that O2 dissociation on the npAu/TiO2 is negligible under these conditions, similar to the low dissociation probability on clean npAu, ~10-7 at 423 K, and the site density for O2 dissociation, ~10-3.45, 50 Stoichiometric titania similarly does not dissociate O2; vacancies are required.51

ature to ensure full oxidation of the titania, with subsequent heating to 750 K to remove atomic oxygen from Au. Control experiments using co-adsorbed 13CO2 demonstrate that the rate of 13CO2 production from 13CO oxidation is limited by reaction, since molecularly adsorbed 13 CO2 desorbs at ~225 K—well below the peak maximum in the temperature programmed data (300 K). Furthermore, the presence of co-adsorbed 13CO2 does not significantly alter the reaction or desorption kinetics of 13CO (Fig. 3A).

Figure 3: Temperature programmed desorption of CO and accompanying production of CO2 demonstrate the oxidation activity of lattice O in the titania. (A) Evolu13 13 tion of CO and CO2 dosed individually and together, demonstrating that CO2 observed from CO oxidation is evolved at higher temperature than the desorption 18 18 temperature of CO2. (B) Evolution of C O and CO O 18 and the lack of production of C O2, indicating that oxygen is derived from the substrate.

Further insight into the mechanism for the CO oxidation was obtained using C18O (Fig. 3B). The exclusive formation of C18OO with the fully oxidized TiO2 (Figure 3B) rules out a mechanism involving CO dissociation as the source of atomic oxygen.

Figure 2. Temperature programmed desorption of O2 (m/z 32) after exposure of npAu/TiO2 to: molecular oxygen (black), and to ozone (red). The sample temperature was 150 K for the O2 exposure compared to 300 K for ozone. The small amount of molecular oxygen desorbed below 300 K is attributed to background. Exposure to ozone yielded a substantial amount of oxygen adsorption, based on the recombination at ~580 K.

Atomic O produced from ozone on the Au leads to C18OO formation at ~175 K peaking at ~225 K (Fig. 3B), precluding this reaction pathway for the production of CO2 at 300 K. Oxygen is not observed to desorb from Au/TiO2 couples below 850 K which also rules out detachment of atomic O from titania onto the gold surface. The similarity in the peak temperatures for desorption of CO2 (Fig. 3A) and for C18OO produced from reaction of C18O with O adsorbed on Au (Fig. 3B) indicates that the lower temperature C18OO production rate is limited by desorption.

CO Oxidation and Annealing Temperature Dependence Temperature programmed reaction following exposure of 13 CO to the npAu/TiO2 yields 13CO2 evolution at 300 K and 13 CO desorption at 250 K (Fig. 3). Prior to exposure to 13 CO, the sample was treated with ozone at room temper-

The activity for CO oxidation depends on the sample annealing temperature prior to CO exposure (with no O2 or O3 treatment in-between cycles), demonstrating that the thermal activation of the material is required to produce reactive O (Fig. 4). In the first 4 trials, the sample was annealed to 775 K prior to exposure, and the observed CO



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conversion was consistently 10-11%. In the subsequent 4 trials, the sample was annealed to only 500 K, and a lower initial conversion (8.5 %) was followed by a continuous decline over the next three cycles. If the sample was subsequently annealed again to 750 K, the activity of the sample recovered to roughly its original state, and the deactivation by lower temperature annealing could be reproduced.

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Density functional calculations suggest that the O vacancy formation energy is reduced along with the energy of Ti interstitials for Au/TiO2(110).56 For the inverted catalyst system, these formation energies could be lowered even further by the increase in the concentration of undercoordinated surface atoms on the TiO2 nanoparticles. It should also be noted that oxidative conditions were required for these calculations to yield activation barriers which were reconcilable with experimental data. This is also supported by our observation of an activity decrease over time, with replenishment of oxygen being necessary to regain activity (See SI). The high temperature activation step is likely required to replenish both the surface and interface with oxygen atoms from the bulk of the nanoparticles, with 500 K being insufficient to induce segregation of O to the surface.

Figure 4. The dependence on the activity for CO oxidation on annealing temperature trajectory is evident from the variation of CO conversion for different heating regimens, with no O2 or O3 treatment in-between cycles. For annealing temperatures of 775 K (red), ~11% of the CO is converted to CO2, whereas for annealing temperatures of 550 K (black), the CO conversion decreases to well below 10%. Conversion is defined as the integrated yield of CO2 divided by the integrated sum of CO+CO2. The activity is recovered by heating to the higher temperature, demonstrating that this is not a continuous degradation in the activity, rather specifically related to the annealing temperature.

Together, these results demonstrate that CO oxidation is occurring via reaction with lattice O from titania and that the oxidation is not occurring on the gold surface but rather at the interface. Similar behavior was also observed on by Shi et al where oxygen exposure of a TiOx/npAu led to a crystallization of titania, and subsequent exposure to CO led back to an amorphous structure.33 This further evidences that lattice oxygen in titania can not only be replenished, but is also an active species for CO oxidation. This leads to a Mars van Krevlen mechanism as the most plausible candidate based on the data presented and previous work. A Mars van Krevlen mechanism has been experimentally observed on titania supported Au nanoparticles, where at T > 350 K the mechanism is purported to be active at the interface.28, 52-53 Theoretical studies have also demonstrated reaction pathways following a Mars van Krevlen mechanistic framework at the Au/TiO2.54-56 The lack of previous evidence for this reaction pathway from UHV studies57 may be due to the ability of an inverted catalyst structure to tune reaction mechanism or to the lower surface area of the single crystal model. In the work described herein, the thermally activated oxygen species is probably associated with the Au/TiO2 interface.

These experiments suggest a different mechanism for catalytic CO oxidation on TiO2-covered nanoporous Au compared to oxide-free npAu. On the pristine material, a AgAu surface alloy leads to O2 dissociation and delivers the reactive atomic O to the surface.34 The presence of the titania nanoparticles opens a second pathway, demonstrated in this work, due to thermal activation of lattice O in the titania particles. The catalytic cycle is sustained because the vacancies created by reaction of the lattice O with CO are replenished by gaseous O2(see Fig. S5 in the SI). The studies suggest that otherwise unreactive metallic Au can be activated by delivery of reactive oxygen via the addition of reducible oxide particles to a catalyst. This mechanism suggests that catalytic activity at lower temperature can be enhanced by tuning the reactivity of the lattice oxygen through modification of the oxide.

Conclusion CO oxidation on npAu@TiO2, an inverse supported metal oxide catalyst, demonstrated a MvK mechanistic pathway in the absence of an external oxygen source, indicating that lattice oxygen from titania is available for oxidation reactions. Additionally, the overall reduction of the catalyst that results from oxidation reactions could be readily reversed by molecular oxygen at moderate temperatures. Together, these two observations suggest that the MvK reaction pathway is viable for this Au/TiO2 composite, consistent with observations of the supported metal catalyst. While it was observed that CO oxidation was spontaneous at low temperatures, the extent of oxidation was greater for higher annealing temperatures. This result indicated that the formation of reactive species, some derivative of the metal oxide lattice oxygen, is a thermally activated process. We propose that this process is driven by the Au/TiO2 interface, and nanoparticle size, which decreases the vacancy formation energy of lattice oxygen, allowing for direct reaction with CO and possibly other target molecules.

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The Supporting Information is available free of charge on the website. CO and O2 TPD from the sample holder (Figure S1 and S2 respectively) and Auger spectra after cleaning treatments (Figure S3); TPRS with different initial CO coverages(Figure S4); CO conversion after different cleaning cycles (Figure S5);

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT We gratefully acknowledge support of this work through the National Science Foundation, Chemical Catalysis program in the Chemistry Division under grant number 1362616. A.S.C. also acknowledges support by a Feodor-Lynen Fellowship from the Alexander von Humboldt Foundation. Work at LLNL was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC5207NA27344.

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