Atomic-scale structure and catalytic reactivity of the RuO(2)(110) surface

Design of Ru-zeolites for hydrogen-free production of conjugated linoleic acids

While conjugated vegetable oils are currently used as additives in the drying agents of oils and paints, they are also attractive molecules for making bio-plastics. Moreover, conjugated oils will soon be accepted as nutritional additives for "functional food" products. While current manufacture of conjugated vegetable oils or conjugated linoleic acids (CLAs) uses a homogeneous base as isomerisation catalyst, a heterogeneous alternative is not available today. This contribution presents the direct production of CLAs over Ru supported on different zeolites, varying in topology (ZSM-5, BETA, Y), Si/Al ratio and countercation (H(+), Na(+), Cs(+)). Ru/Cs-USY, with a Si/Al ratio of 40, was identified as the most active and selective catalyst for isomerisation of methyl linoleate (cis-9,cis-12 (C18:2)) to CLA at 165 °C. Interestingly, no hydrogen pre-treatment of the catalyst or addition of hydrogen donors is required to achieve industrially relevant isomerisation productivities, namely, 0.7 g of CLA per litre of solvent per minute. Moreover, the biologically most active CLA isomers, namely, cis-9,trans-11, trans-10,cis-12 and trans-9,trans-11, were the main products, especially at low catalyst concentrations. Ex situ physicochemical characterisation with CO chemisorption, extended X-ray absorption fine structure measurements, transmission electron microscopy analysis, and temperature-programmed oxidation reveals the presence of highly dispersed RuO(2) species in Ru/Cs-USY(40).

Oscillatory Behavior in the CO-oxidation over Bulk Ruthenium Dioxide — the Effect of the CO/O2 Ratio

CO oxidation over polycrystalline ruthenium dioxide was monitored in an in-situ XRD setup. The evolution of the bulk state of the catalyst was followed by in-situ XRD during reaction, while the surface morphology and chemical state before and after reaction were investigated by HRSEM and EDX. The commercial RuO2 powder was calcined prior reaction to ensure the formation of completely oxidized RuO2. This pre-calcined RuO2 is initially inactive in CO oxidation regardless of the CO/O2 feed ratio and requires an induction period, the length of which strongly depends whether the catalyst is diluted with boron nitride or not. After this induction period oscillations in the CO2 yield occur under O2-rich conditions only. These oscillations exhibit two time constants for the diluted catalyst, while the low frequency oscillations were not observed in the case of undiluted RuO2. Furthermore, the state of the catalyst after activation in O2-rich feed conditions differs dramatically from the state after activation in CO-rich feed conditions. Firstly, the catalyst activated in an O2-rich atmosphere remains inactive under CO-rich conditions in contrast to the catalyst activated in CO-rich conditions which is afterwards active under all feed ratios examined. Secondly, the surface morphology of the catalyst is quite different. While the apical surfaces of the RuO2 crystals become roughened upon activation in the CO-rich feed, they become facetted under O2 rich activation conditions. Therefore, we conclude that at least two different active surface states on the bulk RuO2 catalyst exist.

Reaction kinetics and polarization modulation infrared reflection absorption spectroscopy investigations of CO oxidation over planar Pt-group model catalysts

Microscopic and spectroscopic techniques at near-atmospheric pressures have been used in recent years to investigate CO oxidation over Pt-group metals. New insights have been obtained that allow broadening of the understanding of this reaction beyond the ultrahigh vacuum regime where it is well-understood. However, new issues also have arisen that need clarification. In this paper, we review recent work in our laboratory, using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and reaction kinetics measurements from ultrahigh vacuum to near-atmospheric pressures. These studies reveal a continuity of this reaction with respect to pressure over Pt, Pd, and Rh; that is, Langmuir-Hinshelwood kinetics is exhibited over a wide pressure range with no apparent "pressure gap". The difference between Ru(0001) and other noble metals is well-understood with respect to higher oxygen binding energies and reduced CO inhibition. It is concluded that for all Pt-group metals the most active phase is one saturated with chemisorbed oxygen and with low CO coverages. The significance of oxide phases under most industrially relevant catalytic conditions suggested recently in the literature is not consistent with the experimental data.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

Revelation of multiple underlying RuO2 redox processes associated with pseudocapacitance and electrocatalysis

Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO(2) materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO(2) materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO(2) materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO(2) materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO(2), relevant to pseudocapacitance and electrocatalysis.

Low-dimensional surface oxides in the oxidation of Rh particles

The oxidation of rhodium particles leads to the formation of low-dimensional nanostructures, namely ultrathin oxide films and stripes adsorbed on the metallic surface. These structures display unique electronic and structural properties, which have been studied in detail experimentally and theoretically in recent years. In this review, the state of research on low-dimensional surface oxides formed on Rh surfaces will be discussed with a special focus on the contributions derived from computational approaches. Several points elucidating the novel properties of the surface oxides will be addressed: (i) the structural relation between the surface oxides and their bulk counterparts, (ii) the electronic properties of the low-dimensional oxide films and (iii) potential catalytic and electronic applications of the surface oxides.

Size effect of ruthenium nanoparticles in catalytic carbon monoxide oxidation

Carbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior. Here we report a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts. Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)(3) precursor in the presence of poly(vinylpyrrolidone) stabilizer. The measurement of catalytic activity of CO oxidation over two-dimensional Ru NPs arrays under oxidizing reaction conditions (40 Torr CO and 100 Torr O(2)) showed an activity dependence on the Ru NP size. The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts. The results gained from this study will provide the scientific basis for future design of Ru-based oxidation catalysts.

Oxide Surface Science

Most metals are oxidized under ambient conditions, and metal oxides show interesting and technologically promising properties. This has motivated much recent research on oxide surfaces. The combination of scanning tunneling microscopy with first-principles density functional theory–based computational techniques provides an atomic-scale view of the properties of metal-oxide materials. Surface polarity is a key concept for predicting the stability of oxide surfaces and is discussed using ZnO as an example. This review also highlights the role of surface defects for surface reactivity, and their interplay with defects in the bulk, for the case of TiO2. Ultrathin metal-oxide films, grown either through reactive evaporation on metal single crystals or through oxidation of metal alloys (such as Al2O3/NiAl), have gained popularity as supports for planar model catalysts. The surface oxides that form upon oxidation on Pt-group metals (e.g., Ru, Rh, Pd, and Pt) are considered as model systems for CO oxidation.

Ultrahigh vacuum/high-pressure flow reactor for surface x-ray diffraction and grazing incidence small angle x-ray scattering studies close to conditions for industrial catalysis

A versatile instrument for the in situ study of catalyst surfaces by surface x-ray diffraction and grazing incidence small angle x-ray scattering in a 13 ml flow reactor combined with reaction product analysis by mass spectrometry has been developed. The instrument bridges the so-called "pressure gap" and "materials gap" at the same time, within one experimental setup. It allows for the preparation and study of catalytically active single crystal surfaces and is also equipped with an evaporator for the deposition of thin, pure metal films, necessary for the formation of small metal particles on oxide supports. Reactions can be studied in flow mode and batch mode in a pressure range of 100-1200 mbar and temperatures up to 950 K. The setup provides a unique combination of sample preparation, characterization, and in situ experiments where the structure and reactivity of both single crystals and supported nanoparticles can be simultaneously determined.

On the CO-Oxidation over Oxygenated Ruthenium

The oxidation of carbon monoxide over polycrystalline ruthenium dioxide (RuO2) powder was studied in a packed-bed reactor and by bulk and surface analytical methods. Activity data were correlated with bulk phases in an in-situ X-ray diffraction (XRD) setup at atmospheric pressure. Ruthenium dioxide was pre-calcined in pure oxygen at 1073 K. At this stage RuO2 is completely inactive in the oxidation of CO. After a long induction period in the feed at 503 K RuO2 becomes active with 100% conversion, while in-situ XRD reveals no changes in the RuO2 diffraction pattern. At this stage selective roughening of apical RuO2 facets was observed by scanning electron microscopy (SEM). Seldom also single lateral facets are roughened. EDX indicated higher oxygen content in the following order: flat lateral facets > rough lateral facets > rough apical facets. Further, experiments in the packed bed reactor indicated oscillations in the CO2 formation rate. At even higher temperatures in reducing feed (533–543 K) the sample reduces to ruthenium metal according to XRD. The reduced particles exhibiting lower ignition temperature are very rough with cracks and deep star-shaped holes. An Arrhenius plot of the CO2 formation rate below the ignition temperature reveals the reduced samples to be significantly more active based on mass unit and shows lower apparent activation energy than the activated oxidized sample. Micro-spot X-ray photoelectron spectroscopy (XPS) and XPS microscopy experiments were carried out on a Ru(0001) single crystal exposed to oxygen at different temperature. Although low energy electron diffraction (LEED) images show a strong 1×1 pattern, the XPS data indicated a wide lateral inhomogeneity with different degree of oxygen dissolved in the subsurface layers. All these and the literature data are discussed in the context of different active states and transport issues, and the metastable nature of a phase mixture under conditions of high catalytic activity.

Adsorption of Methane and Ethane on RuO2(110) Surfaces

The adsorption of methane and ethane on the stoichiometric and O-rich RuO2(110) surfaces, respectively, was studied by thermal desorption and high-resolution electron energy loss spectroscopy. The results support weak adsorption (physisorption) of these molecules on the surface, regardless of the presence of undercoordinated Ru and oxygen surface sites. The vibrational spectra recorded at about 90 K are compared with spectra calculated for gaseous CH4 and C2H6. The nearly complete agreement of frequencies and relative intensities is consistent with physisorption. No evidence of adsorption induced molecule activation is found.

Structure-Activity Correlations for the Oxidation of CO over Polycrystalline RuO2 Powder Derived from Steady-State and Transient Kinetic Experiments

The oxidation of carbon monoxide was studied at atmospheric pressure in a plug-flow reactor over polycrystalline ruthenium dioxide powder in the temperature range from 363 to 453 K as a function of the pretreatment. Calcining RuO2 in flowing oxygen resulted in purified bulk RuO2, whereas reduction in hydrogen led to bulk Ru metal, which was partially oxidized again in flowing oxygen at increasing temperatures (T ox) up to 573 K to obtain RuO2/Ru shell-core particles with increasing RuO2 shell thickness. Using the TPR technique subsequent to steady-state CO oxidation to monitor the degree of oxidation, the most active and stable state of the unsupported ruthenium catalysts was identified as an ultra-thin RuO2 layer covering a metallic Ru core in agreement with the shell-core model established for supported Ru catalysts. Steady-state turnover frequencies (TOFs) obtained with the ultra-thin RuO2 films are in good agreement with TOFs reported for studies on Ru single crystal surfaces and with supported Ru catalysts. Only for RuO2 films thicker than 1 nm (T ox ≥ 473 K) and for fully oxidized RuO2 deactivation was observed, presumably due to the formation of inactive RuO2 surfaces such as the RuO2(100)-c(2×2) facet. Moreover, it was demonstrated that the presence of moisture in the reactant feed inhibits the oxidation of CO completely.

High Resolution Electron Energy Loss Spectroscopy on Perfect and Defective Oxide Surfaces

High resolution electron energy loss spectroscopy (HREELS) is a powerful method for the study of vibrational and electronic excitations at solid surfaces and has been extensively applied to metal single crystal surfaces. As a result of experimental difficulties, unfortunately, much less information is available on adsorbate vibrations at oxide surfaces. This review focuses on recent results showing the successful application of HREELS to study adsorption and reaction of molecules on metal oxide single crystal surfaces. The chemical reactivity of perfect surfaces is first investigated systematically using HREELS combined with thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). Furthermore, it is demonstrated that the interaction of adsorbates with surface defects (in particular oxygen vacancies) can also be monitored by vibrational spectroscopy.

Nanoscale analysis of Ru(0001) oxidation using low-energy and photoemission electron microscopy

CO oxidation over oxygen-rich Ru(0001) surfaces is one of the most studied catalytic oxidation reactions in surface science and of widespread interest as a model system for the redox chemistry of transition metal model catalysts. Here, we present an extensive low-energy electron microscopy (LEEM) and photoemission electron microscopy study of the oxidation of Ru(0001), which constitutes a crucial step in understanding the overall surface reaction. After characterizing the different surface nanoscale morphologies observed depending on the oxidation temperature, three distinct oxygen-rich phases are identified by dark-field microscopy and local valence-band spectroscopy. Furthermore, in situ LEEM allows us to follow the growth of single rutile oxide nuclei in real time and determine the relevant activation barriers that induce quasi-one-dimensional growth of oxide nanorods, whose growth rate is limited by O incorporation.

Oxygen-coverage effects on molecular dissociations at a Pt metal surface

The effects of adsorbate coverage on catalytic surface reactions are not well understood. Here, we contrast the rates of O2 and NO2 dissociations, two competing reactions in NO oxidation catalysis, versus oxygen coverage at a Pt(111) surface. In situ x-ray photoelectron spectroscopy experiments show that the NO2 dissociation rate is less sensitive to O coverage than is O2. Density-functional theory simulations reveal an NO2 reaction pathway that is more adaptable to an increasingly crowded surface than is O2 dissociation. While the rates are comparable at low coverage, NO2 dissociation is many orders of magnitude faster at O coverages typical of NO oxidation catalysis.

Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions

Recent advances in colloidal synthesis enabled the precise control of the size, shape and composition of catalytic metal nanoparticles, enabling their use as model catalysts for systematic investigations of the atomic-scale properties affecting catalytic activity and selectivity. The organic capping agents stabilizing colloidal nanoparticles, however, often limit their application in high-temperature catalytic reactions. Here, we report the design of a high-temperature-stable model catalytic system that consists of a Pt metal core coated with a mesoporous silica shell (Pt@mSiO(2)). Inorganic silica shells encaged the Pt cores up to 750 degrees C in air and the mesopores providing direct access to the Pt core made the Pt@mSiO(2) nanoparticles as catalytically active as bare Pt metal for ethylene hydrogenation and CO oxidation. The high thermal stability of Pt@mSiO(2) nanoparticles enabled high-temperature CO oxidation studies, including ignition behaviour, which was not possible for bare Pt nanoparticles because of their deformation or aggregation. The results suggest that the Pt@mSiO(2) nanoparticles are excellent nanocatalytic systems for high-temperature catalytic reactions or surface chemical processes, and the design concept used in the Pt@mSiO(2) core-shell catalyst can be extended to other metal/metal oxide compositions.

Step-orientation-dependent oxidation: from 1D to 2D oxides

Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.

Tuning the stoichiometry of surface oxide phases by step morphology: Ag(511) versus Ag(210)

We show by vibrational and photoemission spectroscopy and by ab initio calculations that in ultrahigh vacuum oxygen adsorption at stepped Ag surfaces causes the formation of surface oxide phases with different stoichiometry, depending on the morphology of the steps involved in the nucleation process. On Ag(511), characterized by close-packed steps and (100) terraces, subsurface O occupies tetrahedral interstitials and an Ag2O-like phase forms; on Ag(210), showing open step edges, subsurface oxygen ends up off center in octahedral sites and an AgO-like structure nucleates.