Time-resolved Infrared Reflection Absorption Spectroscopy Research Articles

Phosphonic Acids on an Atomically Defined Oxide Surface: The Binding Motif Changes with Surface Coverage.

We have studied the anchoring mechanism of a phosphonic acid on an atomically defined oxide surface. Using time-resolved infrared reflection absorption spectroscopy, we investigated the reaction of deuterated phenylphosphonic acid (DPPA, C6H5PO3D2) with an atomically defined Co3O4(111) surface in situ during film growth by physical vapor deposition. We show that the binding motif of the phosphonate anchor group changes as a function of coverage. At low coverage, DPPA binds in the form of a chelating tridentate phosphonate, while a transition to a chelating bidentate occurs close to monolayer saturation coverage. However, the coverage-dependent change in the binding motif is not associated with a major change of the molecular orientation, suggesting that the rigid phosphonate linker always maintains the DPPA in a strongly tilted orientation irrespective of the surface coverage.

Anchoring of a Carboxyl-Functionalized Norbornadiene Derivative to an Atomically Defined Cobalt Oxide Surface

We have investigated the anchoring of the molecular energy carrier norbornadiene (NBD) to an atomically defined oxide surface. To this end, we synthesized a carboxyl-functionalized NBD derivative, namely 1-(2′-norbornadienyl)pentanoic acid (NBDA), and deposited it by physical vapor deposition (PVD) under ultrahigh vacuum (UHV) conditions onto a well-ordered Co3O4(111) film grown on Ir(100). In addition, we performed a comparative growth study with benzoic acid (BA) under identical conditions which was used as a reference. The interaction and orientation of NBDA and BA with the oxide surface were monitored in situ during film growth by isothermal time-resolved infrared reflection–absorption spectroscopy (TR-IRAS), both below and above the multilayer desorption temperature. The thermal behavior and stability of the films were investigated by temperature-programmed IRAS (TP-IRAS), with help of density functional (DF) calculations. BA binds to Co3O4(111) under formation of a symmetric chelating carboxylate wi...

Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111)

Thin films of ionic liquids (ILs) can be used to tune the activity and selectivity of heterogeneous catalysts and electrocatalysts (solid catalysts with IL layer, SCILL). In several cases it has been found that these IL layers have a strong beneficial effect on the selectivity. To explore the molecular origin of this phenomenon, we have performed a model study on ultrahigh-vacuum conditions. We have investigated the coadsorption of CO and the room-temperature IL [C2C1Im][OTf] (1-ethyl-3-methylimidazolium trifluoromethanesulfonate) on Pd(111) by time-resolved infrared reflection–absorption spectroscopy, temperature-programmed reflection absorption spectroscopy, and temperature-programmed desorption. We find that the [OTf]− anion adsorbs specifically to the Pd(111) surface via the SO3– group, thereby adopting a well-defined orientation with the molecular axis oriented perpendicular to the surface. At higher IL coverage, unspecific but oriented adsorption occurs, before the orientation is successively lost i...

Functionalized Porphyrins on an Atomically Defined Oxide Surface: Anchoring and Coverage-Dependent Reorientation of MCTPP on Co3O4(111)

We have studied the adsorption of tetraphenylporphyrin (2HTPP) and its carboxylated counterpart mono-para-carboxyphenyltriphenylporphyrin (MCTPP) on an atomically defined Co3O4(111) film under ultrahigh vacuum (UHV) conditions. Using time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we show that 2HTPP adsorbs molecularly in a flat-lying orientation, whereas MCTPP binds to the surface via formation of a chelating bidentate carboxylate upon deposition at 400 K. Combining TR-IRAS and density-functional theory (DFT), we determine the molecular tilting angle as a function of coverage. We show that the MCTPP adsorption geometry changes from a nearly flat-lying orientation (tilting angle <30°) at low coverage to a nearly perfectly upright-standing orientation (tilting angle of approximately 80°) in the full monolayer.

Quantitative analysis of desorption and decomposition kinetics of formic acid on Cu(111): The importance of hydrogen bonding between adsorbed species.

Quantitative analysis of desorption and decomposition kinetics of formic acid (HCOOH) on Cu(111) was performed by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy, and time-resolved infrared reflection absorption spectroscopy. The activation energy for desorption is estimated to be 53-75 kJ/mol by the threshold TPD method as a function of coverage. Vibrational spectra of the first layer HCOOH at 155.3 K show that adsorbed molecules form a polymeric structure via the hydrogen bonding network. Adsorbed HCOOH molecules are dissociated gradually into monodentate formate species. The activation energy for the dissociation into monodentate formate species is estimated to be 65.0 kJ/mol at a submonolayer coverage (0.26 molecules/surface Cu atom). The hydrogen bonding between adsorbed HCOOH species plays an important role in the stabilization of HCOOH on Cu(111). The monodentate formate species are stabilized at higher coverages, because of the lack of vacant sites for the bidentate formation.

Benzoic Acid and Phthalic Acid on Atomically Well-Defined MgO(100) Thin Films: Adsorption, Interface Reaction, and Thin Film Growth

To better understand the interaction and the growth of thin films of functionalized organic molecules on oxide surfaces, we have studied the adsorption, reaction, and desorption of benzoic acid (BA) and phthalic acid (PA) on a well-ordered MgO(100) thin film grown on a Ag(100) single crystal surface. We have applied isothermal time-resolved infrared reflection–absorption spectroscopy (TR-IRAS) and temperature-programmed IRAS (TP-IRAS) under ultrahigh-vacuum (UHV) conditions. BA is dosed using a supersonic molecular beam (SSMB) source while PA is deposited by physical vapor deposition (PVD). For both molecules we have explored the film growth as a function of temperature, both in the monolayer and in the multilayer regime. We have also investigated structural transitions and desorption by temperature-programmed experiments in the range from 100 to 400 K. In addition, we carried out density-functional (DF) calculations. We find that both molecules BA and PA bind through the carboxyl groups to the MgO(100) s...

Interactions Between the Room-Temperature Ionic Liquid [C2C1Im][OTf] and Pd(111), Well-Ordered Al2O3, and Supported Pd Model Catalysts from IR Spectroscopy

The interactions between ionic liquids and their supports determine many of their applications. The adsorption of the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] on Pd(111), ordered Al2O3/NiAl(110), and Pd nanoparticles supported on Al2O3/NiAl(110) was investigated under ultrahigh vacuum (UHV) conditions using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). On Pd, the [OTf]− anion stands up with its CF3 group directed toward the vacuum, whereas the anion is less clearly oriented on the oxide. We also find that strong interactions of the IL with the Pd result in migration of the IL from the oxide to the metal nanoparticles.

Ionic liquid based model catalysis: interaction of [BMIM][Tf2N] with Pd nanoparticles supported on an ordered alumina film

Towards a better understanding of novel catalytic materials consisting of supported noble metal catalysts modified by an ionic liquid (IL) film, we have performed a study under ultrahigh-vacuum (UHV) conditions. The model surface consists of Pd nanoparticles grown in UHV on an ordered alumina film on NiAl(110). Thin films of the room temperature IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][Tf(2)N] are deposited onto this surface by means of physical vapor deposition (PVD). The interaction of the IL with clean and CO-covered Pd/Al(2)O(3)/NiAl(110) at 300 K and the thermal behavior of the deposited IL films on Pd/Al(2)O(3)/NiAl(110) are investigated by time-resolved infrared reflection absorption spectroscopy (TR-IRAS) and X-ray photoelectron spectroscopy (XPS). At 300 K, the IL adsorbs molecularly both onto the Pd particles and onto the alumina. The IR spectra suggest that the [Tf(2)N](-) anions interact with Pd sites preferentially via the sulfonyl groups. CO pre-adsorbed on the Pd particles is partially displaced by the IL, even at 300 K, and only the part of CO adsorbed onto hollow sites on (111) facets of the Pd particles remains in place. Upon heating to temperatures higher than the desorption temperature of the IL (>400 K), molecular desorption of the IL competes with decomposition. The decomposition products, atomic species and small fragments, remain preferentially adsorbed onto the Pd nanoparticles and strongly modify their surface properties. Most of the decomposition products originate from the [BMIM](+) cations, whereas the [Tf(2)N](-) anions desorb for the most part.

Model NOx Storage Materials at Realistic NO2 Pressures

AbstractThe interaction of NO2 with single‐crystal‐based model NOx storage materials, consisting of barium aluminate nanoparticles on Al2O3/NiAl(110), are investigated by time‐resolved infrared reflection absorption spectroscopy (TR‐IRAS) at realistic NO2 partial pressures up to 1.75 mbar. The data is compared to spectra obtained under ultrahigh vacuum (UHV) conditions on the same model system. At 300 K, the NO2 uptake at pressures around 1 mbar proceeds through rapid initial formation of surface nitrites and nitrates, similar to that under UHV conditions. The vibrational spectra of the surface species formed at realistic NO2 pressures are comparable to those for species formed under UHV conditions. Beyond the formation of surface species, the formation of bulk nitrates occurs, but is kinetically strongly hindered. At a very low rate, the formation of a disordered barium bulk nitrate is detected. At 500 K, this kinetic hindrance is overcome and the available Ba2+ is quantitatively converted to bulk Ba(NO3)2. The IRAS spectrum of these Ba(NO3)2 particles differs characteristically from those obtained for nitrate multilayers formed upon incomplete conversion under UHV conditions. In addition to the formation of bulk Ba(NO3)2, a more weakly bound disordered nitrate species is formed. This species gives rise to a dynamic NO2 uptake and release well below the decomposition temperature of bulk Ba(NO3)2. The experiments show that model studies under UHV conditions mainly provide information on the initial reaction mechanism, whereas the observation of actual bulk NOx storage phases requires experiments at realistic temperatures and pressures.

Particle-Size-Dependent Interaction of NO2 with Pd Nanoparticles Supported on Model NOx Storage Materials

Combining scanning tunneling microscopy, molecular beam methods, and time-resolved infrared reflection absorption spectroscopy, we investigate the structure and reactivity of Pd nanoparticles suppo...

Adsorption and reaction of NO 2 on ordered alumina films and mixed baria–alumina nanoparticles: Cooperative versus non-cooperative reaction mechanisms

In order to obtain insights into the mechanism and kinetics of adsorption and reaction of NO 2 on aluminum and barium oxide surfaces, we have performed systematic adsorption experiments on single-crystal-based model materials. As a model surface, we use BaO containing nanoparticles grown by physical vapor deposition (PVD) of Ba under ultrahigh vacuum (UHV) conditions on an ordered Al 2O 3 film on NiAl(110) and subsequent oxidation and annealing. The growth behavior, the morphology and the chemical composition of the three-dimensional mixed barium aluminum oxide particles (BaAl 2 x O 1 + 3 x ) formed via this procedure have previously been characterized by scanning tunneling microscopy (STM) and high-resolution photoelectron spectroscopy (HR-PES). In order to monitor adsorption and reaction processes on these systems, we perform time-resolved infrared reflection absorption spectroscopy (TR-IRAS) during exposure to a molecular beam (MB) of NO 2. In a first step, the interaction of NO 2 with the Al 2O 3/NiAl(110) model support is probed. At 100 K, only molecular adsorption occurs in form of the D 2h dimer (N 2O 4) in all coverage regions from the submonolayer up to multilayers. At 300 K, a slow surface reaction occurs, initially leading to the formation of surface nitrites and, subsequently, of surface nitrates in bridging adsorption geometry. On the BaAl 2 x O 1 + 3 x particles on Al 2O 3/NiAl(110), NO 2 shows a very different and strongly temperature-dependent behavior. At 100 K, molecular adsorption of the D 2h dimer (N 2O 4) is accompanied by a highly efficient reaction channel, leading to the formation of surface nitrites and nitrates. With the surface temperature increasing to 300 K, however, the reaction probability decreases by several orders of magnitude. In contrast to reaction at 100 K, surface nitrites in flat-lying adsorption geometry are the only primary product, indicating a temperature-dependent change in the reaction mechanism from a cooperative to a non-cooperative pathway. With increasing exposure, the nitrite coverage increases and, finally, the surface nitrites are converted into bridging and monodentate surface nitrates. The latter reaction shows a complex kinetics, including an initial induction period. For temperatures of 400 K and above, nitrate formation becomes more efficient, eventually resulting in the formation of ionic nitrates. The vibrational properties of these ionic species sensitively depend on the reaction temperature, indicating the formation of well-defined structures in a narrow temperature interval around 500 K. The mechanism of the NO x storage process and the vibrational assignments of the related surface species are critically discussed on the basis of the present results.

Cover Picture: Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism (ChemPhysChem 15/2008)

The cover picture shows a time-resolved infrared reflection absorption spectroscopy experiment during NOx uptake on a single-crystal-based NSR (NOx storage and reduction) model catalyst. By correlation of structural properties (scanning tunneling microscopy, figure background) and in-situ surface spectroscopy, J. Libuda et al. in their paper on page 2191–2197, obtain detailed insights into the mechanism of nitrite and nitrate formation.

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

Using scanning tunneling microscopy (STM), molecular-beam (MB) methods and time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we investigate the mechanism of initial NO(x) uptake on a model nitrogen storage and reduction (NSR) catalyst. The model system is prepared by co-deposition of Pd metal particles and Ba-containing oxide particles onto an ordered alumina film on NiAl(110). We show that the metal-oxide interaction between the active noble metal particles and the NO(x) storage compound in NSR model catalysts plays an important role in the reaction mechanism. We suggest that strong interaction facilitates reverse spillover of activated oxygen species from the NO(x) storage compound to the metal. This process leads to partial oxidation of the metal nanoparticles and simultaneous stabilization of the surface nitrite intermediate.

Microscopic diffusion processes of NO on the Pt(997) surface

The microscopic diffusion processes of NO molecules on Pt(997) at low coverage were investigated using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). When NO molecules adsorb on Pt(997) at low temperature, each molecule transiently migrates on the surface from the first impact point to a possible adsorption site. At 11 K, the molecules are trapped at four adsorption sites on Pt(997): the on-top sites on the (111) terrace (OT), the hollow sites on the (111) terrace (HT), the bridge sites at the step (BS) and the hollow sites at the step downstream (HS). Based on the initial population ratio for these sites, the mean lateral displacement by transient migration is estimated to be 4.1 A. By heating the surface to 45 K, the HS species migrate up to the BS sites; the migration barrier is roughly estimated to be 120 meV. In the temperature range from 70 to 77 K, TR-IRAS measurements were carried out to observe the site change of OT species to the adjacent HT sites at isothermal conditions; the activation barrier and the preexponential factor are estimated to be 200 meV and 2.0 x 10(11) s(-1), respectively. In the temperature range from 100 to 110 K, the HT species migrate across the terrace and finally reach the BS sites. The activation barrier between the HT sites and the preexponential factor are estimated to be 290 meV and 6.5 x 10(11) s(-1), respectively, from the TR-IRAS data together with kinetic Monte Carlo simulations. On the whole, the quantitative microscopic picture of NO migration on Pt(997) has been established.

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe3O4 supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10−6 mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.

CO oxidation on partially oxidized Pd nanoparticles

Combining multi-molecular beam (MB) experiments and in situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we studied the relationship between the formation of Pd surface oxides and the reaction kinetics of CO oxidation on a well-defined Fe 3O 4-supported Pd model catalyst. The model catalyst was prepared in-situ under ultra-high-vacuum (UHV) conditions by Pd deposition onto a well-ordered Fe 3O 4 film on Pt(111). In previous studies, structure, morphology, and adsorption properties of the model system, as well as the formation of Pd oxide species, were characterized in detail. At low reaction temperatures ( T < 450 K ), the CO oxidation activity of partially oxidized Pd particles was found to be significantly lower than that of metallic Pd particles. We address this deactivation of the catalyst to a weak CO adsorption on Pd surface oxides, leading to a very low reaction probability. Even after extended exposure to CO-rich reactants at 400 K, no significant gain in catalytic activity due to reduction of surface oxides was observed. As a result, we conclude that the formation of Pd oxide species at higher temperatures causes long-term deactivation of the catalyst at lower temperatures. At higher reaction temperatures ( T ⩾ 500 K ), however, Pd oxides can be dynamically formed and decomposed, depending on the composition of the reactant environment. Although Pd oxides are formed on the surface under O-rich conditions, such oxide species are decomposed under CO-rich conditions at these temperatures. Using a simple model, we qualitatively analyzed the formation and decomposition process of Pd oxides under reaction conditions. We found that at higher reaction temperatures, partial oxidation of the Pd particles generally led to reduced CO oxidation activity and slow hysteresis effects that were strongly dependent on the pretreatment of the sample.

Interfacial Dynamics of D2O Coadsorbed on Saturated CO/Pt(100) in Ultrahigh Vacuum(UHV) as Studied by Time Resolved Infrared Reflection Absorption Spectroscopy (TR-IRAS)

not Available.

Surface Dynamics of Coadsorbed CO and D2O on Pt(100) in Ultrahigh Vacuum as Studied by Time-Resolved Infrared Reflection Absorption Spectroscopy

Changes in the properties of CO adsorbed at saturation coverages on Pt(100) induced by subsequent coadsorption of fixed amounts of D2O at 105 K in ultrahigh vacuum (UHV) were monitored by time-resolved infrared reflection absorption spectroscopy (tr-IRAS). The linear- and bridge-bonded CO stretching features were found to change in intensity and shift toward lower energies as a function of time at fixed CO and D2O coverages. Also observed was the development of multiple features in both CO spectral regions depending on the amount of D2O on the surface. These findings indicate that, under the conditions of these experiments, the interfacial dynamics are relatively slow, on the order of minutes, involving a gradual rearrangement of adsorbed CO and D2O on the surface to yield surface solvated CO, as has been suggested in the literature (Kizhakevariam et al. J. Chem. Phys. 1994, 100, 6750). This factor should be considered when comparing, quantitatively, shifts induced by water coadsorption with CO on Pt single crystals in UHV with CO adsorption on the same surfaces in electrochemical environments.

Carbon Monoxide Adsorption on Ru-Modified Pt Surfaces: Time-Resolved Infrared Reflection Absorption Studies in Ultrahigh Vacuum

The vibrational spectra of CO adsorbed on Ru-modified Pt(100) surfaces prepared by chemical vapor deposition (condensation of Ru(3)(CO)(12) at 105 K followed by X-ray irradiation and thermal decomposition at 650 K in ultrahigh vacuum, UHV) was investigated by time-resolved infrared reflection absorption spectroscopy (IRAS) in UHV. Spectra were recorded while Ru/Pt(100) bimetallic surfaces (theta(Ru) = 0.24 and 0.52 by X-ray photoelectron spectroscopy, XPS) were dosed with gas-phase CO. Analysis of the data revealed that for a wide range of calibrated CO exposures, the linear CO-stretching region displays two features: a higher energy peak (2085-2100 cm(-1)), attributed to CO adsorbed on pristine Pt(100) sites, and a lower energy peak (2066-2092 cm(-1)), ascribed to adsorption of CO on sites on the surface induced by the presence of Ru. Similar experiments were performed on bimetallic specimens annealed repeatedly in UHV to 650 K to promote partial Ru dissolution into the lattice and thus render surfaces gradually enriched in Pt. For all surfaces and CO exposures examined, the total integrated area under the two CO spectral features remained fairly constant and equal in value to the corresponding areas found for bare Pt(100). If it is assumed that a fixed exposure leads to a fixed coverage on both bare and Ru-modified Pt(100)surfaces, and the thermal treatment leads to an exchange of Ru by Pt sites without altering significantly the total number of metal sites on the surface, the absorption cross sections for both of these peaks are virtually the same.

Reaction kinetics on supported model catalysts: Molecular beam/in situtime-resolved infrared reflection absorption spectroscopy study of the CO oxidation on alumina supported Pd particles

By combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density, and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ infrared spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurements in combination with TR-IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been proposed previously. Instead, we suggest that the transient behavior is a consequence of the inhibition of the dissociative adsorption of O2 at high CO coverage. Additionally, the inherent heterogeneity of the supported particle system can enhance the observed effect.