R. Jürgen Behm

Active Oxygen on a Au/TiO2 Catalyst: Formation, Stability, and CO Oxidation Activity†

Since their introduction by Haruta, oxide supported Au catalysts with Au nanoparticles (NPs) of few nanometers in diameter have attracted enormous interest because of their high activity for various oxidation and reduction reactions most prominently the CO oxidation reaction. Mechanistic details and hence the physical origin of their high activity, however, are still controversial. Focusing on the CO oxidation reaction, a number of different effects and active sites have been proposed as being responsible for the observed high activity, both from experimental and from theoretical work, but so far agreement has not been reached. Most controversial in oxidation reactions are the activation of molecular oxygen, the active site for this reaction step, and the nature of the catalytically active oxygen species present under working conditions. Stiehl et al. had shown that molecularly adsorbed oxygen can be deposited on both Au(111) and Au NPs supported on TiO2(110) at 77 K [16] and moreover, that this oxygen can directly react with CO. The molecular species desorbs, however, upon heating to 170 K. Carretin et al. could identify a -superoxide and peroxide species on pure and Fe-doped Au/TiO2 catalysts upon interaction with O2 at 253 K at atmospheric pressure, which disappeared when changing to CO/O2 reaction mixtures, implying that these species represent the active oxygen species. Stable, molecularly adsorbed oxygen species, mostly located at the perimeter of the interface between a Au cluster and TiO2 support [10;11;15;18;19] or at low-coordination sites of the Au clusters, were identified also in a number of theoretical studies and proposed as active oxygen species, which can react with coadsorbed CO with rather low activation energies via a coadsorption complex. In most cases, the dissociation of adsorbed O2,ad species without interaction with coadsorbed CO was found to be highly activated. Recently, Kotobuki et al. demonstrated in temporal analysis of products (TAP) reactor measurements that active oxygen species, active for facile reaction with CO, can be deposited on Au/TiO2 catalysts at 80°C by exposure to thermal O2 pulses and that these species are stable against desorption at that temperature. They showed that the oxygen storage capacity (OSC) and also the CO oxidation activity of these catalysts during continuous CO oxidation in a micro reactor scale with the length of the perimeter of the interface between TiO2 support and Au NPs. Accordingly, active oxygen species on sites along the perimeter of the interface between Au NPs and TiO2 support were proposed as active species, both for reaction in the TAP reactor and during continuous reaction at atmospheric pressure in a micro reactor. It is important to realize that this oxygen species can hardly be identical with the molecularly adsorbed oxygen species identified in the above experimental and theoretical studies, since the calculated adsorption energies would be too low to stabilize them at 80°C, and in the work by Stiehl et al., desorption of the molecular O2 species was observed at 170 K. [16;17] Stable active oxygen species and a correlation between OSC and CO oxidation activity were reported also for other oxide supported Au catalysts, indicating that this species, which contrasts most proposals for the CO oxidation mechanism, is a general feature for CO oxidation on oxide supported Au catalysts. However, the nature of the active oxygen species, in particular whether it is a molecular or atomic species, could not be clarified in these studies, leaving this central question unresolved. Because of the very low amount of these oxygen species of about 1% of the total amount of surface oxygen, spectroscopic identification of this species is hardly possible. Furthermore, it is also open whether these oxygen species are adsorbed at the perimeter, or whether they represent surface lattice oxygen adjacent to the Au NPs, which is activated by the presence of the Au NPs. In the present communication, we report new results which allow us to clearly identify the nature of the active oxygen species and which provide strong evidence for their location on the catalyst surface. This is based on multi-pulse measurements performed in a TAP reactor at temperatures between 80°C and 400°C. Prior to each experiment, the Au/TiO2 catalyst was pre-treated by in situ calcination in 10% O2/N2 at 400°C (O400) to prepare a well defined, fully oxidized catalyst. Afterwards, the micro reactor was evacuated, and exposed alternately to sequences of CO/Ar pulses and O2/Ar pulses (1:1, 1 10 molecules per pulse each) to determine the amount of stable adsorbed oxygen that can be reversibly deposited and reactively removed under these conditions (OSC). This procedure allows us to determine even very low amounts of active oxygen stored on a catalysts surface very precisely. Raw data of the signals during the initial reduction and subsequent re-oxidation of the Au/TiO2 catalyst after O400 pretreatment (reaction temperature 80°C) and the following cycle, are shown in Fig. 1. In agreement with previous findings, CO2 is produced solely during the CO/Ar pulses over the oxidized catalyst, and not during the subsequent O2/Ar pulses, indicating that CO is reversibly adsorbed under these conditions and desorbs instantaneously after the CO pulse. The consumption of the educt gases, calculated from the missing mass spectrometric intensity compared to that after saturation, is highest in the beginning of each sequence and decreases with ongoing pulse number, until it reaches the zero level and the oxidation state of the catalyst surface is not changed [ ] D. Widmann, Prof. Dr. R.J. Behm Institute of Surface Chemistry and Catalysis Ulm University Albert-Einstein-Allee 47, 89081 Ulm (Germany) Fax: (+49) 731-502 5452 E-mail: Juergen.Behm@uni-ulm.de Homepage: www.uni-ulm.de/en/nawi/iok

Activity, Selectivity, and Long-Term Stability of Different Metal Oxide Supported Gold Catalysts for the Preferential CO Oxidation in H2-Rich Gas

A comparative study of the catalytic performance and long-term stability of various metal oxide supported gold catalysts during preferential CO oxidation at 80°C in a H2-containing atmosphere (PROX) reveals significant support effects. Compared to Au/γ-Al2O3, where the support is believed to behave neutrally in the reaction process, catalysts supported on reducible transition metal oxides, such as Fe2O3, CeO2, or TiO2, exhibit a CO oxidation activity of up to one magnitude higher at comparable gold particle sizes. The selectivity is also found to strongly depend on the employed metal oxide, amounting, e.g., up to 75% for Au/Co3O4 and down to 35% over Au/SnO2. The deactivation, which is observed for all samples with increasing time on stream, except for Au/γ-Al2O3, is related to the build-up of surface carbonate species. The long-term stability of the investigated catalysts in simulated methanol reformate depends crucially on the ability to form such by-products, with magnesia and Co3O4 supported catalysts being most negatively affected. Overall, Au/CeO2 and, in particular, Au/α-Fe2O3 represent the best compromise under the applied reaction conditions, especially due to the superior activity and the easily reversible deactivation of the latter catalyst.

Imaging an Ionic Liquid Adlayer by Scanning Tunneling Microscopy at the Solid|Vacuum Interface

Ionic liquids (ILs), which are salts usually consisting of stericallyhindered organic ions with melting points below 1008C, are ofhigh interest because of a number of distinct physical proper-ties such as ionic conductivity, electrochemical stability in awide potential window, a very low vapor pressure or low flam-mability.

Toward the Microscopic Identification of Anions and Cations at the Ionic Liquid|Ag(111) Interface: A Combined Experimental and Theoretical Investigation

The interaction between an adsorbed 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSA], ionic liquid (IL) layer and a Ag(111) substrate, under ultrahigh-vacuum conditions, was investigated in a combined experimental and theoretical approach, by high-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory calculations (DFT-D). Most importantly, we succeeded in unambiguously identifying cations and anions in the adlayer by comparing experimental images with submolecular resolution and simulated STM images based on DFT calculations, and these findings are in perfect agreement with the 1:1 ratio of anions and cations adsorbed on the metal derived from XPS measurements. Different adlayer phases include a mobile 2D liquid phase at room temperature and two 2D solid phases at around 100 K, i.e., a 2D glass phase with short-range order and some residual, but very limited mobility and a long-range ordered 2D crystalline phase. The mobility in the different adlayer phases, including melting of the 2D crystalline phase, was evaluated by dynamic STM imaging. The DFT-D calculations show that the interaction with the substrate is composed of mainly van der Waals and weak electrostatic (dipole-induced dipole) interactions and that upon adsorption most of the charge remains at the IL, leading to attractive electrostatic interactions between the adsorbed species.

Performance study of magnesium–sulfur battery using a graphene based sulfur composite cathode electrode and a non-nucleophilic Mg electrolyte

Here we report for the first time the development of a Mg rechargeable battery using a graphene-sulfur nanocomposite as the cathode, a Mg-carbon composite as the anode and a non-nucleophilic Mg based complex in tetraglyme solvent as the electrolyte. The graphene-sulfur nanocomposites are prepared through a new pathway by the combination of thermal and chemical precipitation methods. The Mg/S cell delivers a higher reversible capacity (448 mA h g(-1)), a longer cyclability (236 mA h g(-1) at the end of the 50(th) cycle) and a better rate capability than previously described cells. The dissolution of Mg polysulfides to the anode side was studied by X-ray photoelectron spectroscopy. The use of a graphene-sulfur composite cathode electrode, with the properties of a high surface area, a porous morphology, a very good electronic conductivity and the presence of oxygen functional groups, along with a non-nucleophilic Mg electrolyte gives an improved battery performance.

Electrodeposition of a Pt monolayer film: using kinetic limitations for atomic layer epitaxy.

A new and facile one-step method to prepare a smooth Pt monolayer film on a metallic substrate in the absence of underpotential deposition-type stabilizations is presented as a general approach and applied to the growth of Pt monolayer films on Au. The strongly modified electronic properties of these films were demonstrated by in situ IR spectroscopy at the electrified solid-liquid interface with adsorbed carbon monoxide serving as a probe molecule. The Pt monolayer on Au is kinetically stabilized by adsorbed CO, inhibiting further Pt deposition in higher layers.

Pectin, Hemicellulose, or Lignin? Impact of the Biowaste Source on the Performance of Hard Carbons for Sodium-Ion Batteries

Hard carbons are currently the most widely used negative electrode materials in Na-ion batteries. This is due to their promising electrochemical performance with capacities of 200-300 mAh g-1 and stable long-term cycling. However, an abundant and cheap carbon source is necessary in order to comply with the low-cost philosophy of Na-ion technology. Many biological or waste materials have been used to synthesize hard carbons but the impact of the precursors on the final properties of the anode material is not fully understood. In this study the impact of the biomass source on the structural and electrochemical properties of hard carbons is unraveled by using different, representative types of biomass as examples. The systematic structural and electrochemical investigation of hard carbons derived from different sources-namely corncobs, peanut shells, and waste apples, which are representative of hemicellulose-, lignin- and pectin-rich biomass, respectively-enables understanding and interlinking of the structural and electrochemical properties.

Small Addition of Boron in Palladium Catalyst, Big Improvement in Fuel Cell’s Performance: What May Interfacial Spectroelectrochemistry Tell?

Direct formic acid fuel cell (DFAFC) with Pd-based catalyst anode is a promising energy converter to power portable devices. However, its commercialization is entangled with insufficient activity and poor stability of existing anode catalysts. Here we initially report that a DFAFC using facilely synthesized Pd-B/C with ca. 6 at. % B doping as the anode catalyst yields a maximum output power density of 316 mW cm(-2) at 30 °C, twice that with a same DFAFC using otherwise the state-of-the-art Pd/C. More strikingly, at a constant voltage of 0.3 V, the output power of the former cell is ca. 9 times as high as that of the latter after 4.5 h of continuous operation. In situ attenuated total reflection infrared spectroscopy is applied to probe comparatively the interfacial behaviors at Pd-B/C and Pd/C in conditions mimicking those for the DFAFC anode operation, revealing that the significantly improved cell performance correlates well with a substantially lowered CO accumulation at B-doped Pd surfaces.

From Adlayer Islands to Surface Alloy: Structural and Chemical Changes on Bimetallic PtRu/Ru(0001) Surfaces

The correlation between structural and chemical properties of bimetallic PtRu/Ru(0001) model catalysts and their modification upon stepwise annealing of a submonolayer Pt-covered Ru(0001) surface up to the formation of an equilibrated Pt(x)Ru(1-x)/Ru(0001) monolayer surface alloy was investigated by scanning tunneling microscopy and by the adsorption of CO and D(2) probe molecules. Both temperature-programmed desorption and IR measurements demonstrate the influence of the surface structure on the adsorption properties of the bimetallic surface, which can be explained by changes of the composition of the adsorption ensembles (ensemble effects) for D adsorption and by changes in the electronic interaction (ligand effects, strain effects) of the metallic constituents for CO and D adsorption upon alloy formation.

High Selectivity of Supported Ru Catalysts in the Selective CO Methanation—Water Makes the Difference.

The selectivity for CO methanation is a decisive aspect for the practical application of the methanation reaction for the removal of CO from CO2-rich H2 fuel gases produced via hydrocarbon reforming. We show that increasing the water content in the feed gas, up to technically relevant levels of 30%, significantly increases the selectivity of supported Ru catalysts compared with operation in (almost) dry gas, while in operando EXAFS measurements reveal a gradual decrease in the Ru particle size with increasing amounts of water in the gas feed. Consequences of these findings and related IR spectroscopic data for the mechanistic understanding and practical applications are outlined.