Reflection-absorption infrared spectroscopy of adsorbates on a Cu(111) single crystal surface

A Fourier Transform infrared spectrometer has been attached to an ultrahigh vacuum (UHV) apparatus in order to perform reflection-absorption infrared Spectroscopy (RAIRS) of adsorbed species on well-defined surfaces.An infrared spectrum of carbon monoxide (CO) adsorbed at 90 K on Cu(111) has been measured using a resolution of 2 cm⁻¹ and a measuring time of 60 s. Coverages below 1 % of a monolayer are easily detectable.Tetracyanoethylene (TCNE) has been adsorbed at various coverages at 100 K on Cu(111). Strongly red-shifted CN stretchings modes due to charged TCNE adspecies are observed at low coverage. The RAIRS spectrum of the condensed phase is characteristic of crystalline TCNE.Finally, isotopically labeled ¹²C and ¹³C acetonitrile (CH₃CN) has been adsorbed on Cu(111) as multilayers. Shifts caused by isotopic labeling as small as 3 cm⁻¹ are easily detected.     

Adsorption of carbon monoxide on tungsten (210)

Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β₂ and β₁ binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β₂ and β₁ states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.     

Adsorption and desorption of oxygen and carbon monoxide on the Si(111) surface studied by ion scattering spectroscopy

The adsorption of O₂ and CO on the Si(111) surface was studied by low-energy helium ion scattering. The adsorption consists of a fast adsorption stage followed by a much slower Sorption process. In the final uptake region CO has a faster rate of increase than O. There is no evidence of He⁺ scattering from C atoms. This fact excludes the CO molecule having its axis parallel to the surface. A comparison of the intensities of the substrate (Si) signals, for the same recorded oxygen content on the surface, shows that carbon monoxide shadows the Si atoms more than oxygen does. An increase in the oxygen signal was observed even after exposures in the range of 10¹⁴–10¹⁵ molecules cm^?2. No substantial diffusion of CO into the bulk can be deduced from these results. Desorption of oxygen by He⁺ ions was observed by following the adsorbate and substrate signals as a function of time. The sputtering cross-section has a maximum for an impact angle of 25° relative to the surface.     

Mechanisms of the catalytic carbon monoxide oxidation on Pt (110)

The kinetics of the carbon monoxide oxidation on a clean Pt (110) crystal were investigated in an ultra-high vacuum system by utilizing Auger electron spectroscopy, low-energy electron diffraction and residual gas analysis. Two different catalytic reaction mechanisms were found to prevail for the experimental conditions chosen. In the temperature range, 100 < T ? 220°C, where essentially CO was preadsorbed on the Pt surface the subsequent adsorption of O₂ was competitive and the reaction exhibited the characteristics of a Langmuir-Hinshelwood mechanism. In this case the onset of the CO₂ formation was delayed by a characteristic time which depended strongly on temperature (“induction period”). A simple model for the Langmuir-Hinshelwood reaction was developed which permitted a more detailed evaluation of the kinetic curves yielding an activation energy for the catalytic reaction of 2.9 kcal/mole. On the other hand, when oxygen was preadsorbed on the Pt surface (T > 90°C) the subsequent reaction with CO occurred immediately and was temperature independent. This behavior was interpreted in terms of an Eley-Rideal mechanism. Both reactions were used for titration of the adsorbed species. From area measurements under the titration curves it was concluded that the saturation coverage for CO and oxygen on Pt(110) is approximately the same.     

Adsorption of carbon monoxide on the molybdenum (100) surface

The interaction of CO with Mo(100) has been studied by means of thermal desorption spectrometry, work function measurements and electron stimulated desorption, in conjunction with LEED and AES. Results have been obtained for adsorption at room temperature and at temperatures down to 200 K. The study confirms previous results, showing that the β-states formed at room temperature are atomic. The thermal desorption data for the β-states are analyzed to give directly the desorption activation energy as a function of coverage. This energy is found to vary smoothly from an initial value of 3.7 to a final value of 2.9 eV molecule, indicating an average repulsive interaction between a pair of adjacent adatoms of 0.2 eV. The data at low temperature indicate that a molecular state, virgin-CO, is produced in competition with β-CO and probably one other state, from a common precursor. The step leading to virgin-CO has both a low activation energy and a low pre-exponential factor, suggesting that a reorientation of the molecule is required.     

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v₁, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v₁ wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

Structure and reactivity of bimetallic electrochemical interfaces: infrared spectroscopic studies of carbon monoxide adsorption and formic acid electrooxidation on antimony-modified Pt(100) and Pt(111)

The influence of predosed antimony on the adlayer structures of carbon monoxide and on the electro-oxidation kinetics of formic acid on Pt(100) and Pt(111) in 0.1M HClO₄ is examined by means of in-situ infrared spectroscopy in conjunction with cyclic voltammetry. Preadsorbed antimony inhibits the adsorption of CO on these surfaces, the attenuation in CO coverage being accompanied by a selective removal of the two-fold bridging geometry as deduced from the relative ν_CO band intensities. At saturation antimony coverages, the CO binding is exclusively terminal on Pt(100) and Pt(111). These findings are consistent with the adsorption of antimony at multi-fold sites, yielding microscopically intermixed adlayers with CO. The electro-oxidation rates of formic acid are enhanced substantially by preadsorbed antimony on Pt(100) and Pt(111). The real-time infrared spectra in the C-O stretching region and the CO coverages thereby deduced in the presence of predosed antimony under reactive voltammetric conditions suggest that the metal adatoms are actively involved in the dissociation of formic acid. The origins of the enhanced electrocatalytic activity of the bimetallic Sb/Pt surfaces are discussed in terms of geometric and chemical effects.     

Vibrational characterization of carbon monoxide oxidation on the Pt(111) surface

The surface reaction between coadsorbed carbon monoxide and atomic oxygen has been characterized using high resolution electron energy loss spectroscopy, coupled with temperature programmed reaction spectroscopy on a Pt(111) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Preferential oxidation of bridge bonded CO is not observed despite the fact that bridge bonded CO is adsorbed less vigorously than linearly bound CO. Saturation of the Pt(111) surface with one quarter of a monolayer of atomic oxygen completely suppresses the adsorption of bridge bonded CO. However, substantial coverages of bridge bonded CO can be coadsorbed if the Pt(111) surface is only partially saturated with atomic oxygen. The vibrational data for reaction of coadsorbed CO and atomic oxygen is consistent with a reaction mechanism involving reaction of mobile CO along oxygen island perimeters.     

Study of oxygen and carbon monoxide adsorption on molybdenum (110) by metastable helium de-excitation spectroscopy

The adsorption of oxygen and carbon monoxide on the (110) face of molybdenum is studied at room temperature by metastable helium atom de-excitation spectroscopy (MDS). On the basis of the interaction mechanism described by H.D. Hagstrum, and which was shown to be valid for the He¹?Mo couple in a previous article and again here, the secondary electron energy spectra are interpretated as relating to the self-convolution product of the electron state density of occupied valence band levels of the solid. The results obtained after deconvolution show the variations in this density of states during oxygen and carbon monoxide adsorption, and they are compared with photoemission spectra (UPS) obtained at the same time as the MDS spectra. They show that MDS is a technique well suited to the study of adsorption phenomena.     

Planar laser-induced fluorescence imaging of carbon monoxide using vibrational (infrared) transitions

We report a new imaging diagnostic suitable for measurements of infrared-active molecules, namely infrared planar laser-induced fluorescence (IR PLIF), in which a tunable infrared source is used to excite vibrational transitions in molecules and vibrational fluorescence is collected by an infrared camera. A nanosecond-pulse Nd:YAG-pumped KTP/KTA OPO/OPA system is used to generate 12 mJ of tunable output near 2.35 μm which excites the 2ν band of carbon monoxide (CO); fluorescence resulting from excited CO is collected at 4.7 μm by using an InSb focal plane array. Quantitative, high-SNR PLIF imaging of gas-phase CO is demonstrated at a 10-Hz acquisition rate with a minimum detection limit of 1350 ppm at 300 K. Received: 30 July 1999 / Published online: 16 September 1999     

A reflection-absorption infrared study of carbon monoxide and nitric oxide adsorption on platinum (100)

The adsorption of NO, CO, and NO/CO mixtures, onto Pt(100), is studied by RAIRS. CO and NO are found to adsorb into islands at 300 K, but the islands breakup upon heating to 400 K. Dosing with a mixture of NO and CO at temperatures below 325 K is found to produce a mixed NO/CO island. There is a shift in the CO peak and NO peak during mixed island formation which is attributed to a strong chemical interaction between the adsorbed NO and CO. This interaction is found to produce an increase in the desorption temperature of CO. Autocatalytic behavior is found to arise because of an enhanced reactivity when CO enters a mobile state. The autocatalytic behavior could be responsible for the “surface explosion” reported by Lesly and Schmidt.     

CO adsorbed on a Xe covered Cu(100) surface studied by infrared spectroscopy

When a Cu(100) surface covered by a full Xe monolayer is exposed to CO at 60 K no infrared absorption associated with the CO vibration is observed. If the temperature is increased to 90 K, when the Xe atoms are desorbed, the ordinary CO spectrum develops. This observation can be explained, if the CO molecules on the Xe covered surface are oriented on the average with their molecular axes parallel to the surface.