CO Adsorption and Disproportionation on Smooth and Defect-Rich Ir(111)

CO adsorption and dissociation on "perfect" and "defect-rich" Ir(111) surfaces were studied by a combination of surface-analytical techniques, including polarization-dependent (PPP and SSP) sum frequency generation (SFG) vibrational spectroscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. CO was found to be ordered and tilted from the surface normal at high coverage on the "perfect" surface (e.g., θ = 30° at 0.70 ML), whereas it was less ordered and preferentially upright (θ = 4-10°) on the "defect-rich" surface for coverages of 0.55-0.70 ML. SFG, LEED, and XPS revealed that CO adsorption at low pressure/high temperature and high pressure/low temperature was reversible. In contrast, upon heating to ∼600 K in near mbar CO pressure, "perfect" and even more "defect-rich" Ir(111) surfaces were irreversibly modified by carbon deposits, which, according to DFT, result from CO disproportionation.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

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.

Sum frequency generation spectroscopy in heterogeneous model catalysis: a minireview of CO-related processes

Sum frequency generation (SFG) vibrational spectroscopy is applied to ambient pressure surface science studies of adsorption and catalytic reactions at solid/gas interfaces.

Chemical and Laser Ablation Synthesis of Monometallic and Bimetallic Ni-Based Nanoparticles

The catalytic properties of nanoparticles depend on their size, shape and surface/defect structure, with the entire catalyst performance being governed by the corresponding distributions. Herein, we present two routes of mono- and bimetallic nanoparticle synthesis that enable control of the structural parameters, i.e., wet-chemical synthesis and laser ablation in liquid-phase. The latter is particularly suited to create defect-rich nanoparticles. Impregnation routes were applied to prepare Ni and NiCu nanoparticles, whereas nano- and femtosecond laser ablation in liquid-phase were employed to prepare Ni and NiAu nanoparticles. The effects of the Ni:Cu ratio in impregnation and of laser fluence and liquid-medium on laser ablation are discussed. The atomic structure and (surface) composition of the nanoparticles were characterized by electron microscopic (BF-TEM, DF-TEM, HRTEM) and spectroscopic/diffraction techniques (EDX, SAED, XPS, IR), complemented by theory (DFT). The chemically synthesized bimetallic NiCu nanoparticles initially had Cu-rich surfaces, which changed to Ni-rich upon reaction. For laser ablation, depending on conditions (fluence, type of liquid), highly defective, ordered, or core/shell-like nanoparticles were produced. The case studies highlight the specific benefits of each preparation method for catalyst synthesis and discuss the potential of nanoparticles produced by pulsed laser ablation for catalytic applications.

An ultrahigh vacuum-compatible reaction cell for model catalysis under atmospheric pressure flow conditions

Atmospheric pressure reactions on model catalysts are typically performed in so-called high-pressure cells, with product analysis performed by gas chromatography (GC) or mass spectrometry (MS). However, in most cases, these cells have a large volume (liters) so that the reactions on catalysts with only cm2 surface area can be carried out only in the (recirculated) batch mode to accumulate sufficient product amounts. Herein, we describe a novel small-volume (milliliters) catalytic reactor that enables kinetic studies under atmospheric pressure flow conditions. The cell is located inside an ultrahigh vacuum chamber that is deliberately limited to basic functions. Model catalyst samples are mounted inside the reactor cell, which is locked to an oven for external heating and closed by using an extendable/retractable gas dosing tube. Reactant and product analyses are performed by both micro-GC and MS. The functionality of the new design is demonstrated by catalytic ethylene (C2H4) hydrogenation on polycrystalline Pt and Pd foils.

Roughening of Copper (100) at Elevated CO Pressure: Cu Adatom and Cluster Formation Enable CO Dissociation

Carbon monoxide participates in many copper-catalyzed reactions, which makes CO-induced structural changes of Cu catalysts key for important industrial processes. We have studied the interaction of carbon monoxide with the Cu(100) single crystal termination at 120, 200, and 300 K by means of low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), and density functional theory (DFT) calculations. The absorption band of CO (2082-2112 cm^-1) at elevated gas pressure (up to 5 mbar) and at 200/300 K was found at a higher wavenumber than the characteristic band of the c(2 × 2)CO structure and was consistent with CO adsorbed on low-coordinated Cu atoms. The combined PM-IRAS/DFT analysis revealed that exposure to CO induced surface roughening through the formation of Cu adatoms and clusters on the (100) terraces. The roughened surface seemed surprisingly active for CO dissociation, which indicates its unique catalytic properties.

The interaction of ethylene with free gold cluster cations: infrared photodissociation spectroscopy combined with electronic and vibrational structure calculations

The interaction of ethylene with free gold clusters of different sizes and charge states has been previously shown theoretically to involve two different adsorption modes of the C₂H₄ molecule, namely: the di-σ- and π-bonded ethylene adsorption configurations. Here, we present the first experimental investigation of the structure of a series of gas-phase gold-ethylene complexes, [Formula: see text]. By employing infrared multiple-photon dissociation spectroscopy in conjunction with first-principles calculations it is revealed that up to three C₂H₄ molecules preferably bind to gold cations in a π-bonded configuration. The binding of all ethylene molecules is found to be dominated by partial electron donation from the ethylene molecules to the gold clusters leading to an activation of the C-C bond. The cooperative action of multiple coadsorbed C₂H₄ on [Formula: see text] is shown to enable additional charge back-donation and an enhanced C-C bond activation. In contrast, the strong C-H bond is not weakened and the experimental spectra do not give any indication for C-H bond dissociation. The possible correlations of the C-C bond stretch vibration with the C-C bond length and the net charge transfer are discussed.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions

Infrared absorption spectroscopy has proven to be one of the most powerful spectroscopic techniques available for the characterization of catalytic systems. Although the history of IR absorption spectroscopy in catalysis is long, the technique continues to provide key fundamental information about a variety of catalysts and catalytic reactions, and to also offer novel options for the acquisition of new information on both reaction mechanisms and the nature of the solids used as catalysts. In this review, an overview is provided of the main contributions that have been derived from IR absorption spectroscopy studies of catalytic systems, and a discussion is included on new trends and new potential directions of research involving IR in catalysis. We start by briefly describing the power of Fourier-transform IR (FTIR) instruments and the main experimental IR setups available, namely, transmission (TIR), diffuse reflectance (DRIFTS), attenuated total reflection (ATR-IR), and reflection-absorption (RAIRS), for advancing research in catalysis. We then discuss the different environments under which IR characterization of catalysts is carried out, including in situ and operando studies of typical catalytic processes in gas-phase, research with model catalysts in ultrahigh vacuum (UHV) and so-called high-pressure cell instruments, and work involving liquid/solid interfaces. A presentation of the type of information extracted from IR data follows in terms of the identification of adsorbed intermediates, the characterization of the surfaces of the catalysts themselves, the quantitation of IR intensities to extract surface coverages, and the use of probe molecules to identify and titrate specific catalytic sites. Finally, the different options for carrying out kinetic studies with temporal resolution such as rapid-scan FTIR, step-scan FTIR, and the use of tunable lasers or synchrotron sources, and to obtain spatially resolved spectra, by sample rastering or by 2D imaging, are introduced.

Spectral evidence for hydrogen-induced reversible segregation of CO adsorbed on titania-supported rhodium

Adsorption of hydrogen on titania-supported Rh nanoparticles partially covered by CO causes compression of the CO adlayer.

Kinetically limited CO adsorption: spill-over as a highly effective adsorption pathway on bimetallic surfaces

We demonstrate that the (local) adsorbed carbon monoxide, COad , coverage on the Pt-free areas of bimetallic Pt/Ru(0001) surfaces (a Ru(0001) substrate partly covered by Pt monolayer islands) can be increased to ∼0.80 monolayers (ML), well above the established saturation COad coverage of 0.68 ML, even under ultrahigh vacuum conditions by using spill-over of CO adsorbed on the Pt islands to the Ru areas as an highly effective adsorption channel. The apparent COad saturation coverage of 0.68 ML on pure Ru(0001) is identified as due to kinetic limitations, hindering further uptake from the gas phase, rather than being caused by thermodynamic reasons. This spill-over mechanism is proposed to be a general phenomenon for adsorption on bimetallic surfaces.

A new dual-purpose ultrahigh vacuum infrared spectroscopy apparatus optimized for grazing-incidence reflection as well as for transmission geometries

A newly designed ultrahigh vacuum (UHV) infrared spectroscopy apparatus dedicated to the spectroscopic characterization of oxides, singles crystals as well as powders, is described. It combines a state-of-the-art vacuum Fourier transform infrared (FTIR) spectrometer (Bruker, VERTEX 80v) with a novel UHV system (PREVAC) consisting of load-lock, distribution, measurement, and magazine chambers. The innovative design allows carrying out both reflection-absorption IR spectroscopy experiments at grazing incidence on well-defined oxide single crystal surfaces and FTIR transmission measurements for powder particles. A further unique feature of the apparatus is the entirely evacuated optical path to avoid background signals from gas phase H(2)O, CO(2), and other species, thus creating the possibility to record high-quality IR data with high sensitivity and stability, an essential prerequisite for monitoring molecular species adsorbed on oxide single-crystal surfaces. The unique performance of this new apparatus with regard to the spectroscopic characterization of adsorbates on oxide single crystals as well as on powder particles is demonstrated by case studies for two different materials, TiO(2) and ZnO.

Kinetic measurements of hydrocarbon conversion reactions on model metal surfaces

Examples from recent studies in our laboratory are presented to illustrate the main tools available to surface scientists for the determination of the kinetics of surface reactions. Emphasis is given here to hydrocarbon conversions and studies that rely on the use of model systems, typically single crystals and controlled (ultrahigh vacuum) environments. A detailed discussion is provided on the use of temperature-programmed desorption for the determination of activation energies as well as for product identification and yield estimations. Isothermal kinetic measurements are addressed next by focusing on studies under vacuum using molecular beams and surface-sensitive spectroscopies. That is followed by a review of the usefulness of high-pressure cells and other reactor designs for the emulation of realistic catalytic conditions. Finally, an analysis of the power of isotope labeling and chemical substitutions in mechanistic research on surface reactions is presented.

Generation and Detection of Single Metal Nanoparticles Using Scanning Electrochemical Microscopy Techniques

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.

From Pd nanoparticles to single crystals: 1,3-butadiene hydrogenation on well-defined model catalysts

Although 1,3-butadiene hydrogenation is known to be a structure-sensitive reaction, correlation of the catalytic activity with the exact Pd particle surface structure shows that the reaction is in fact particle size independent.

In Situ Studies of Methanol Decomposition and Oxidation on Pd(111) by PM-IRAS and XPS Spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.