F.M. Hoffmann

Infrared reflection-absorption spectroscopy of adsorbed molecules

Infrared Reflection-Absorption Spectroscopy (IRAS) is used to perform vibrational spectroscopy of absorbed molecules in submonolayer quantities on well-defined, single crystal metal surfaces. Theoretical and experimental aspects of this method in a single reflection mode are discussed. Apart from its potential to identify adsorbed species through their vibrational spectra, the high sensitivity (< 5/1000 of a CO monolayer) and resolution (1–3 cm-1) make this method particularly suitable to study the interaction between metal substrate and adsorbed molecule as well as intermolecular interactions within an adsorbed layer of molecules. This is illustrated with examples of CO adsorption on metals, where vibrational frequencies of the C-O stretch allow determination of adsorption sites. Intermolecular interactions within the adsorbed layer, which lead to the formation of ordered islands and incommensurate compression structures, can be monitored by characteristic frequency shifts. Further insight into these interactions can be gained by observing the linewidth and intensity as a function of surface coverage and order. Finally examples of other adsorption systems are given to demonstrate the wide range of molecules which have been studied in the past and the potential of IRAS as a method to bridge studies of adsorption of molecules on metal single crystals at UHV pressures to supported catalysts at high, catalytically relevant pressures.

Potassium promoted C–O bond weakening on Ru(001). I. Through‐metal interaction at low potassium precoverage

The interaction of carbon monoxide and submonolayer coverages of potassium on Ru(001) has been investigated with high resolution electron energy loss spectroscopy, LEED, Auger electron spectroscopy, thermal desorption, and work function measurements. The adsorption of potassium on ruthenium at submonolayer coverages (θk≤0.15) is characterized by ionic, mutually repulsive potassium atoms as evidenced by a strong work function decrease (−4.3 eV), various LEED patterns and a large desorption energy (65 kcal/mol) at low coverage. The adsorption of CO on a potassium precovered surface (θk=0.10) is nondissociative and reversible with an initial increase in the activation energy for desorption from 40 kcal/mol for clean Ru(001) to 50 kcal/mol on the potassium precovered surface. The C–O bond is anomalously weak as evidenced from vibrational spectroscopy (EELS), where C–O stretch frequencies in the range of 1400 to 1970 cm−1 are found. Observation of the first vibrational overtone indicates a strong anharmonicity...

The interaction of methanol with Ru(001)

The adsorption, desorption, and decomposition of methanol on a clean Ru(001) surface at 85 K has been examined with electron energy loss spectroscopy, multiple mass thermal desorption spectroscopy, low energy electron diffraction, and a work function probe. Methanol adsorbs readily on Ru(001) and is found to decompose at submonolayer coverages even at low temperature (85 K). Two decomposition pathways are observed: oxygen–hydrogen bond breaking (CH3OH→CH3O–M+H–M) and carbon–oxygen bond breaking (CH3OH→H2O+C–M+2H–M). The methoxy species either recombines with hydrogen and desorbs as methanol between 220 and 250 K via second order reaction kinetics (n=1.85; E*D≂14 kcal/mol; ν(2)=10−2 cm−2 s−1); or further decomposes to form carbon monoxide and hydrogen. The conversion of the methoxy species into carbon monoxide begins at 220 K and is completed at 300 K. The methoxy conversion is accompanied by the gradual formation of a p(2×2) LEED pattern which disappears after CO desorption. The second reaction channel, i...

In-situ FT-IRAS study of the CO oxidation reaction over Ru(001): I. Evidence for an Eley-Rideal mechanism at high pressures?

Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction on a Ru(001) surface in-situ at high pressures. The vibrational spectra allow us to characterize the nature of the surface during reaction and qualitatively determine steady-state coverages of CO and oxygen. Under oxidizing conditions (COO2 ratios 2), the steady-state coverage of oxygen decreases with decreasing oxygen partial pressure concurrent with a large reduction in reaction rates. Vibrational data reveal a steady-state coverage of CO (θCo ≃ 0.11, Eads(CO) ≃ 25 kcalmol) adsorbed on an O−(2 × 1)−Ru(001) surface (θO = 0.5). Severely reducing conditions lead to low steady-state oxygen coverages < 12 ML and island formation of oxygen. Th implications of the various CO species and of the oxygen island formation are discussed in relation to the reaction mechanisms suggested by our previous kinetic study. In particular, we propose for reaction under oxidizing conditions an Eley-Rideal mechanism involving reaction between gas-phase or weakly adsorbed CO and the O−(1 × 1)−Ru(001) surface. Under reducing conditions on the O−(2 × 1) surface the reactions proceed via a Langmuir-Hinshelwood mechanism between chemisorbed CO and oxygen.

The vibrational spectra of chemisorbed molecular clusters: H2O on Ru(001)

Electron energy loss spectroscopy and thermal desorption mass spectroscopy have been used to correlate vibrational spectra of H2O on Ru(001) with ESDIAD patterns reported recently. We find that the ‘‘extended bilayer’’ is characterized by a sharp fundamental O–H vibration at 3500–3565 cm−1 which is assigned to nonhydrogen‐bonded OH bonds of molecularly adsorbed water. Hydrogen bonds within the well‐ordered bilayer give rise to features at ∼3290–3450 cm−1 which are of relatively weak intensity in electron scattering due to the orientations of these bonds. In addition, very small clusters exhibit a weak vibrational mode at ∼2935 cm−1 which possibly represents H2O molecules with one O–H bond pointing into the surface. Isolated H2O molecules at low temperature and low coverage exhibit a fundamental O–H vibration at 3600 cm−1. A reinterpretation of the thermal desorption spectra is suggested.

The observation of direct attractive interactions between potassium and carbon monoxide coadsorbed on Ru(001)

Abstract We report a study of the interaction between carbon monoxide and metallic (monolayer) potassium on Ru(001). This system is characterized by simultaneous “autocatalytic” desorption of CO and K at 680 K and vibrational CO stretch frequencies of 1350 and 1490 cm −1 . The results suggest direct attractive interaction between the potassium and carbon monoxide.

In-situ FT-IRAS study of the CO oxidation reaction over Ru(001) : II. Coadsorption of carbon monoxide and oxygen

Abstract Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), Thermal Desorption Mass Spectrometry (TDMS), and Low-Energy Electron Diffraction (LEED), we have investigated the coadsorption of CO and O on a Ru(001) surface under UHV conditions. Preadsorption of oxygen is found to weaken the adsorption energy of CO from 38 kcal mol (clean Ru) to kcal mol (θ o → 1) . CO adsorption on the O−(1 × 1) monolayer is suppressed completely even at 85 K. Vibrational data show in agreement with earlier work that this reduction in the adsorption energy results from a reduction of back-donation from the metal substrate into the 2π∗-orbital of CO. Vibrational and thermal desorption data show the existence of distinctly different CO adsorption states on the O−(2 × 2) and O−(2 × 1) surfaces, where CO is found to order in linear on-top sites with identical (2 × 2) structures, but with different numbers of oxygen neighbors surrounding each CO molecule. Infrared lineshapes reveal a high degree of CO ordering on the O−(2 × 2) surface in contrast to the O−(2 × 1) surface which appears to be less well ordered. At higher CO coverage in both cases a compressed CO layer is observed with drastically reduced adsorption energy and the additional occupation of bridging adsorption sites. At intermediate oxygen coverages mixed phases of clean Ru, O−(2 × 2) and O−(2 × 1) are observed. Vibrational coupling observed at higher CO coverage, however, suggests rather small domain sizes.

The formation and decomposition of KOH on Ru(001)

Abstract Potassium hydroxide is formed when water is adsorbed at 80 K on a Ru(001) surface precovered with potassium. The vibrational spectrum of KOH obtained with EELS is characterized by an OH stretch at 3570–3620 cm −1 ; losses at 1340–1500 cm −1 and 200–800 cm −1 are tentatively assigned to KOH bending and KO stretching vibrations respectively. Heating the surface to 500–600 K results in decomposition of KOH and desorption of the dissociation products. The decomposition pathway is found to depend strongly on the potassium precoverage.

A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001) : I. The structural and electronic properties of Cu

We have used high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe to investigate the structural and electronic properties of well annealed copper layers adsorbed on a Ru(001) substrate. Adsorption of CO on annealed Cu–Ru (001) surfaces (θCu<1) at 85 K results in occupation of both Ru and Cu sites which exhibit characteristic C–O stretching frequencies as a function of CO coverage. The latter also indicates formation of Cu islands for 0.25<θCu<1 and chemical modification of the copper film by the underlying Ru, which leads to increased Cu–CO backdonation. From IR line shapes of the C–O stretch from CO‐√3×√3‐R30° on Ru sites, domain sizes are estimated for the bare Ru patches. Weak chemical modification is still observed for a 3 ML thick Cu film, whereas a 8 ML thick film exhibits all properties of bulk Cu(111), i.e., a coverage dependent red shift (2077–2075 cm−1), a (1×1) LEED pattern and characteristic CO‐TDMS features. In contrast to the modification of Cu by Ru, ther...

The coadsorption of oxygen and potassium on Ru(001): Evidence for the formation of K–O compounds

In order to study an adsorption system where bond formation between coadsorbates competes with the adsorbate–metal substrate bond, we have investigated the interaction between oxygen and a potassium monolayer on Ru(001). At low exposures of oxygen (0.4 L), vibrational, photoemission, Auger, and workfunction data indicate the formation of a KO2 species. EELS spectra are characterized by an intense K–O stretch at 240 cm−1 and the absence of Ru–O stretching modes. Auger spectra indicate a K:O stoichiometry of 1:2 and photoemission spectra contain features indicative of O–O bond formation. Subsequent exposure of oxygen results in the adsorption of atomic oxygen with a characteristic Ru–O stretching vibration at 615 cm−1. Annealing of the K–O layer results in the decomposition of KO2 to Kad+Oad, both being bonded more strongly to the metal substrate than the individually adsorbed species and thus indicating through‐metal interactions between atomic oxygen and potassium.