Thermal Stability of Au Nanoparticles in O2 and Air on Fully Oxidized TiO2(110) Substrates at Elevated Pressures. An AFM/XPS Study of Au/TiO2 Model Systems

Temperature-driven evolution of ceria-zirconia-supported AuPd and AuRu bimetallic catalysts under different atmospheres: insights from IL-STEM studies

The evolution of the structure and composition of the system of particles in two Ce0.62Zr0.38O2-supported bimetallic catalysts based on Au and a 4d metal (Ru or Pd) under high temperature conditions and different reducing and oxidizing environments has been followed by means of Identical Location Scanning Transmission Electron Microscopy (IL-STEM). As an alternative to in situ microscopy, this technique offers valuable insights into the structural modifications occurring in chemical environments with the characteristics of a macro-scale reactor. By tracking exactly the same areas on a large number of metallic entities, it has been possible to reveal the influence of particle size and the nature of the redox environment on the temperature-driven mobilization of the different metals involved. Thus, oxidizing environments evidenced a much higher capacity to mobilize the three metals, preferentially Au. Moreover, the typical storage conditions (under air) of catalysts during the prolonged exposure time has been proved to induce significant modifications in these bimetallic systems, even at room temperature. Regardless of the type of redox environment, bimetallic systems showed better thermal resistance, which demonstrates a beneficial effect of the second metal. In summary, IL-STEM is an invaluable and complementary methodology for characterizing heterogeneous catalysts under realistic reaction conditions and is within the reach of most laboratories.

CO Oxidation on Planar Au/TiO2 Model Catalysts under Realistic Conditions: A Combined Kinetic and IR Study

The oxidation of CO on planar Au/TiO2 model catalysts was investigated under pressure and temperature conditions similar to those for experiments with more realistic Au/TiO2 powder catalysts. The effects of a change of temperature, pressure, and gold coverage on the CO oxidation activity were studied. Additionally, the reasons for the deactivation of the catalysts were examined in long-term experiments. From kinetic measurements, the activation energy and the reaction order for the CO oxidation reaction were derived and a close correspondence with results of powder catalysts was found, although the overall turnover frequency (TOF) measured in our experiments was around one order of magnitude lower compared to results of powder catalysts under similar conditions. Furthermore, long-term experiments at 80 °C showed a decrease of the activity of the model catalysts after some hours. Simultaneous in-situ IR experiments revealed a decrease of the signal intensity of the CO vibration band, while the tendency for the build-up of side products (e. g. carbonates, carboxylates) of the CO oxidation reaction on the surface of the planar model catalysts was rather low.

Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode | electrolyte interface in Li-ion batteries

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]⁺[TFSI]^-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO₂(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li₃N, Li₂S, Li_xSO_y, and Li₂O. In the absence of a [BMP]⁺[TFSI]^- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO₂) above a critical temperature, forming LiO_x and Ti³⁺ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li⁺ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

Temperature-dependent insertion and adsorption of lithium on spinel Li4Ti5O12(111) thin films - an angle-resolved XPS study

Aiming at a detailed understanding of the Li adsorption and insertion behavior on/into lithium titanate (Li4Ti5O12, LTO), which is a promising anode material in Li-ion batteries, we have investigated the interaction of vapor deposited Li with LTO in the temperature range between 80 K and room temperature by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The experiments were performed under ultrahigh vacuum (UHV) conditions, and the presence of additional Li species was detected by the formation of Ti3+ in the Ti 2p core level signal due to charge transfer from adsorbed/inserted Li to Ti. Even at 80 K most of the deposited Li diffuses into the bulk of LTO, reflecting the facile insertion and diffusion of Li into and in LTO. Deposition of up to 3 monolayers equivalent (MLE) of Li at 80 K results in an increase in Li concentration in the surface region (topmost 6 nm), up to a stoichiometry of Li4+xTi5O12 with x > 0.3, and slightly lower values in the near-surface region (topmost 1 nm). For higher Li doses, the amount of Li in the near-surface region, including adsorbed Li, increases more than the concentration in the underlying surface region. This is attributed to the blocking of diffusion channels by inserted Li at a stoichiometry of Li4.3Ti5O12 and above. Upon increasing the temperature, Li+ starts to diffuse into the LTO bulk at temperatures between 120 K and 175 K, depending on the concentration in the surface region. It has completely disappeared at 260 K. The consequences of these results for the understanding of physical Li insertion will be discussed.

Achieving nano-gold stability through rational design

When Au is subdivided to the nanoscale its reactivity changes from an inert nature to one of incredible reactivity which is not replicated by other catalysts. When dispersed onto metal oxides such as TiO2, nano-Au has shown high reactivities for a multitude of reduction and oxidation reactions of industrial importance with potential and current uses such as, CO oxidation, NO x reduction, purification of hydrogen for fuel cells, water gas shift reactions, abatement of volatile organic compounds (VOC's) as well as pollution and emission control systems such as autocatalysts. However, many industrially important reactions and applications operate under harsh conditions where the catalyst is exposed to high temperatures and further needs to operate for extended periods of time. These conditions cause Au nanoparticle sintering whereby small, highly active clusters form large clusters which are catalytically inactive. For this reason, research into stabilizing Au nanoparticles has abounded with a goal of producing durable, thermally stable catalysts for industrial applications. Here we show a durable, thermally stable Au-TiO2 catalyst which has been developed by rational design. The catalyst exhibits a 3-dimensional, radially aligned nanorod structure, already locked into the thermodynamically stable polymorph, via a scalable and facile synthesis, with Au nanoparticles isolated on the support structure. As the Au nanoparticles are highly stable the new catalyst is able to maintain light-off for CO oxidation below 115 °C even after multiple cycles at 800 °C. This ability of the catalyst to resist multiple thermal cycles to high temperature while remaining active at low temperatures shows promise for various industrial applications. The thermal stability of the catalyst is investigated and characterized through morphological and structural studies.

Manipulating the charge state of Au clusters on rutile TiO2(110) single crystal surfaces through molecular reactions probed by infrared spectroscopy

The charge state of Au clusters on TiO₂(110) is manipulated through (NO)₂ → N₂O + O_a reactions: (1) interfacial Au atoms by intensely direct charge transfer; (2) top Au atoms by weakly indirect charge transfer.

Interaction of the ionic liquid [BMP][TFSA] with rutile TiO2(110) and coadsorbed lithium

The interaction of lithium, [BMP][TFSA] and their mixture with rutile TiO₂(110), the thermal stability of the adlayers and the resulting reaction products are investigated under UHV conditions by STM and XPS.

Nanostructured, mesoporous Au/TiO(2) model catalysts - structure, stability and catalytic properties

Aiming at model systems with close-to-realistic transport properties, we have prepared and studied planar Au/TiO(2) thin-film model catalysts consisting of a thin mesoporous TiO(2) film of 200-400 nm thickness with Au nanoparticles, with a mean particle size of ~2 nm diameter, homogeneously distributed therein. The systems were prepared by spin-coating of a mesoporous TiO(2) film from solutions of ethanolic titanium tetraisopropoxide and Pluronic P123 on planar Si(100) substrates, calcination at 350 °C and subsequent Au loading by a deposition-precipitation procedure, followed by a final calcination step for catalyst activation. The structural and chemical properties of these model systems were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N(2) adsorption, inductively coupled plasma ionization spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS). The catalytic properties were evaluated through the oxidation of CO as a test reaction, and reactivities were measured directly above the film with a scanning mass spectrometer. We can demonstrate that the thin-film model catalysts closely resemble dispersed Au/TiO(2) supported catalysts in their characteristic structural and catalytic properties, and hence can be considered as suitable for catalytic model studies. The linear increase of the catalytic activity with film thickness indicates that transport limitations inside the Au/TiO(2) film catalyst are negligible, i.e., below the detection limit.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

On the morphology and stability of Au nanoparticles on TiO2(110) prepared from micelle-stabilized precursors

The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.

Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water

Au catalysis has been one of the hottest topics in chemistry in the last 10 years or so. How O2 is supplied and what role water plays in CO oxidation are the two challenging issues in the field at the moment. In this study, using density functional theory we show that these two issues are in fact related to each other. The following observations are revealed: (i) water that can dissociate readily into OH groups can facilitate O2 adsorption on TiO2; (ii) the effect of OH group on the O2 adsorption is surprisingly long-ranged; and (iii) O2 can also diffuse along the channel of Ti (5c) atoms on TiO2(110), and this may well be the rate-limiting step for the CO oxidation. We provide direct evidence that O2 is supplied by O2 adsorption on TiO2 in the presence of OH and can diffuse to the interface of Au/TiO2 to participate in CO oxidation. Furthermore, the physical origin of the water effects on Au catalysis has been identified by electronic structure analyses: There is a charge transfer from TiO2 in the presence of OH to O2, and the O2 adsorption energy depends linearly on the O2 charge. These results are of importance to understand water effects in general in heterogeneous catalysis.

Aerosol Assisted Chemical Vapor Deposition Using Nanoparticle Precursors: A Route to Nanocomposite Thin Films

Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from co-deposition with [W(OPh)(6)], while gold particles in a host titania matrix resulted from co-deposition with [Ti(O(i)Pr)(4)]. The density of Au nanoparticles within the film could be varied by changing the Au colloid concentration in the original precursor solution. Titania/gold composite films were intensely colored and showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be readily extended to produce other nanocomposite films by varying the colloids and precursors used, and it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.

The Detection of Nitrate Using in-situ Copper Nanoparticle Deposition at a Boron Doped Diamond Electrode

Electrochemical deposition from a 0.1 M sodium sulphate solution, containing Cu2+ (adjusted to pH 3 with hydrochloric acid) produced a well defined copper nanoparticle deposit on the surface of a boron doped diamond electrode. Changing conditions such as potential (-0.8, -1.0 and -1.2 V), time (5, 2 and 0.5 s) and concentration of Cu2+ (500, 250 and 100 microM) was found to give copper nanoparticles of varying size and particle density. The electrocatalytic properties of the copper surface towards nitrate reduction were explored. An in-situ copper nanoparticle production method was developed for the detection of nitrate; this involves electrodeposition, followed by linear sweep voltammetry for the reduction of nitrate and then application of a stripping potential to renew the electrode surface. The linear sweep was discovered to have homogenised the size of the nanoparticles but their number density was still dependant on the initial conditions of deposition. Some particles were still present at the surface after the stripping potential had been applied but repetitions of the procedure showed these did not have an effect on subsequent deposits. Optimisation of the method lead to applying a deposition potential of -0.8 V, at a BDD electrode for 5 s in a 0.1 M sodium sulphate solution (pH 3) containing 100 microM Cu2+ followed by a linear sweep at 1 V/s; this yielded a limit of detection of 1.5 microM nitrate. The analytical applicability of the technique was evaluated for nitrate detection in a natural mineral water sample and was found to agree well with that stated by the manufacturer.