Terminal CO Species Research Articles

Characterization of secondary phosphine oxide ligands on the surface of iridium nanoparticles.

The synthesis of iridium nanoparticles (IrNPs) ligated by various secondary phosphine oxides (SPOs) is described. This highly reproducible and simple method via H2 reduction produces well dispersed, small nanoparticles (NPs), which were characterized by the state-of-the-art techniques, such as TEM, HRTEM, WAXS and ATR FT-IR spectroscopy. In particular, multinuclear solid state MAS-NMR spectroscopy with and without cross polarization (CP) enabled us to investigate the different binding modes adopted by the ligand at the nanoparticle surface, suggesting the presence of three possible types of coordination: as a purely anionic ligand Ir-P(O)R2, as a neutral acid R2P-O-H and as a monoanionic bidentate H-bonded dimer R2P-O-HO[double bond, length as m-dash]PR2. Specifically, the higher basicity of the dicyclohexyl system leads to the formation of IrNPs in which the bidentate binding mode is most abundant. Such cyclohexyl groups are bent towards the edges, as is suggested by the study of 13CO coordination on the NP surface. This study also showed that the number of surface sites on faces available for bridging CO molecules is higher than the number of sites for terminal CO species on edges and apices, which is unexpected taking into account the small size of the nanoparticles. In addition, the IrNPs present a high chemoselectivity in the hydrogenation of cinnamaldehyde to the unsaturated alcohol.

Effect of Li additive on CO 2 hydrogenation reactivity of zeolite supported Rh catalysts

We performed CO 2 hydrogenation over Li-promoted Rh ion-exchanged zeolite catalysts (Li/RhY). As the amount of added Li increased, the main product transformed from methane to CO. Simultaneously, we observed promotion of ethanol production, as well as methanol production. Moreover, when 1.8% of CO was mixed with the reactant gas, the selectivity for ethanol was increased to as high as 13% on Li/RhY (Li/Rh=10), but no influence was observed on Rh ion-exchanged zeolite catalyst (RhY). We studied the mechanism of the promotion effect of Li by means of in situ FT-IR observations. IR spectra of CO/CO 2 adsorption and those for subsequent TPR were observed. On RhY, CO 2 molecules were adsorbed as terminal CO species which were the same as those found in CO adsorption. However, on Li/RhY, a new peak appeared at 1618 cm −1 by CO 2 adsorption and relatively stable species were formed on Li/RhY (Li/Rh=10). The results indicate that the Li additives created a new reaction path on the catalyst surface, where CO 2 adsorption enhancement as well as stabilization of adsorbed CO species occurred.

Experimental evidence on molecular interaction in desorption and adsorption of CO molecules on metal surfaces

Adsorption and desorption of CO on Ni(100) and Pt(111) surfaces are presented. At the thermodynamic equilibrium, the site occupation between the terminal and the bridged sites are described with the free energy of the system, including the vibrational entropy. Adsorption of CO onto a cold surface as 20 K has also been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of bridged CO to terminal CO species on Ni(100) at 20 K ranges from ∼ 2·8 to 0·7 at the total coverage from 0·003 to 0·15 ML. Such strong coverage dependence of the occupation ratio even at small coverages suggests that the interaction between CO molecules operates at relatively long range (> 10 Å). The isotope experiments suggest that there is substantial interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species. Desorption process is also affected by the interaction between the adsorbed CO and the incoming species. The effect of temporal bimolecular CO interaction on the desorption kinetics is also discussed.

Direct and indirect mechanisms in site occupation of CO molecules on Ni(100) and Pt(111)

Adsorption of CO onto a cold surface at 20 K has been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of bridged CO to terminal CO species on Ni(100) at 20 K ranges from ∼3.1 to 0.8 at the total coverage from 0.003 to 0.15 ML. Two adsorption processes of direct adsorption of CO molecules on the bare metal surface and indirect collisional site conversion are discussed. At zero-coverage limit, CO tends to accommodate on the site with the larger adsorption energy. Strong coverage dependence of the occupation ratio even at small coverages suggests that the interaction between CO molecules operates at relatively long range ( > 10 A ̊ ). The isotope experiments suggest that there is substantial interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species.

Initial adsorption sites of CO on Pt(111) and Ni(100) at low temperature

Adsorption of CO on Pt(111) at 25 K and Ni(100) at 18 K has been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of terminal CO to bridged CO species on Pt(111) at 25 K ranges from ∼2.3 to 1.2 at total coverages from 0.03 to 0.13 ML, and that of bridged CO to terminal CO species on Ni(100) at 18 K ranges from ∼3.2 to 0.7 at total coverages from 0.003 to 0.15 ML. Such strong coverage dependence of the occupation ratio, even at small coverages, suggests that the substrate-mediated interaction between CO molecules operates at a relatively long range ( > 10 A ̊ ). The isotope experiments suggest that there is dynamic interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species.

Interactions between chemisorbed CO and oxygen on Ni(111)

Infrared reflection absorption spectroscopy (IRAS), low energy electron diffraction (LEED), work function (ΔΦ), and temperature programmed desorption (TPD) have been used to characterize the interactions between chemisorbed CO and oxygen on Ni(111). It has been found that chemisorbed oxygen exhibits both a long range and a short range effect on chemisorbed CO on Ni(111). The short range interaction between chemisorbed oxygen and chemisorbed CO produces a perturbed terminal-CO species, designated as CO∗, with a singleton frequency 41 cm −1 higher than that of the unperturbed terminal-CO. The CO∗ species is found to adsorb on Ni sites which are the nearest neighbors to the oxygen adatoms adsorbed in the threefold sites. The long range interaction between the two coadsorbates also produces a blue-shift in the singleton frequency of the bridged-CO, and a reduction in the CO saturation coverage. A comparison of the interactional effect of oxygen and sulfur on chemisorbed CO indicates that the size of an electronegative species and its projection above the surface is more important than the electronegativity in determining the strength of the poisoning effect of the electronegative species on chemisorbed CO.

Effects of elevated hydrogen pressures on carbon monoxide chemisorbed on Ni(111)

Abstract The influence of hydrogen gas on CO chemisorbed on Ni(111) has been studied by infrared reflection absorption spectroscopy, temperature-programmed desorption, and low-energy electron diffraction. For CO coverages below 0.5 ML, both bridge-bonded and terminal-CO species coexist on Ni(111). Under the influence of elevated pressures of hydrogen, the terminal-CO species disappear, and CO is compressed into a disordered phase containing only bridge-bonded species. Simultaneously, a high-frequency CO species is produced by short-range interactions with electronegative coadsorbed hydrogen. These changes are all reversible, and only take place above a threshold temperature ( ∼ 240 K) and hydrogen pressure ( ∼ 10 −3 mbar). The experimental observations point to an activated motion of the CO molecules from their normal adsorption sites under the influence of a weakly-bound hydrogen species in equilibrium with H 2 (g).

Effects of elevated hydrogen pressures on carbon monoxide chemisorbed on Ni(111)

The influence of hydrogen gas on CO chemisorbed on Ni(111) has been studied by infrared reflection absorption spectroscopy, temperature-programmed desorption, and low-energy electron diffraction. For CO coverages below 0.5 ML, both bridge-bonded and terminal-CO species coexist on Ni(111). Under the influence of elevated pressures of hydrogen, the terminal-CO species disappear, and CO is compressed into a disordered phase containing only bridge-bonded species. Simultaneously, a high-frequency CO species is produced by short-range interactions with electronegative coadsorbed hydrogen. These changes are all reversible, and only take place above a threshold temperature ( ∼ 240 K) and hydrogen pressure ( ∼ 10 −3 mbar). The experimental observations point to an activated motion of the CO molecules from their normal adsorption sites under the influence of a weakly-bound hydrogen species in equilibrium with H 2(g).

IRAS study of ESD behavior of CO on Ni(111) at 90 K

The effect of electron bombardment (270 eV) on chemisorbed CO species on Ni(111) has been studied by reflection infrared and thermal desorption methods. These methods permit the study of the detailed behavior of different hybridization states of chemisorbed CO during electron-stimulated desorption (ESD). Terminal-CO species are found to be removed exclusively from the surface by an electron-induced desorption process with a cross section of Q T =1.26×10 −17 cm 2. During the removal of terminal-CO by ESD, little change occurs in the coverage of bridged-CO. After ∼ 75% of the terminal-CO has desorbed. bridged-CO begins to dissociate and to desorb under the influence of electron bombardment. The total cross section for bridged-CO depletion is found to be Q B =1.27×10 −18 cm 2, which is one-tenth that of terminal-CO species. The dissociation cro section of the bridged-CO is found to increase with decreasing bridged-CO coverage, and the desorption cross section of the bridged-CO is found to decrease with decreasing bridged-CO coverage. Thus the relative efficiency of the desorption and dissociation processes is found to depend on the state of hybridization of the CO species as well as the coverage.

A rairs observation of the local interaction between coadsorbed NO and CO on Ni(111)

The local interaction between coadsorbed NO and CO on a Ni(111) surface has been investigated using FT-reflection absorption infrared (FT-RAIR) spectroscopy. The adsorption of NO on a CO-predosed Ni(111) surface at 85 K gives rise to the conversion of bridge-CO → terminal-CO. The production of the terminal-CO species could be related to the formation of CO islands upon the post-adsorption of NO. On the other hand, the adsorption of CO on a NO-predosed Ni(111) surface is mainly characterized by the RAIR observation of two NO-induced terminal-CO species with ν(CO) vibrational frequencies observed at ~ 2065 and ~ 2115 cm −1, respectively. These values are higher than that of the terminal ν(CO) mode observed on clean Ni(111) at ~ 2054 cm −1. These results can be explained by the hypothesis that the coadsorbed NO and CO on Ni(111) compete for the surface electron density available for the backdonation into the corresponding 2π ∗ orbitals.

Compressed CO overlayers on Pt(111) — Evidence for tilted CO species at high coverages by digital ESDIAD

The existence of tilted CO molecules in compressed CO overlayers on Pt(111) has been detected using the digital ESDIAD method with enhanced resolution achieved by applying a retarding potential procedure to separate CO +, CO ∗, and O + ESD products. Digital symmetrization was used for analysis of the ESDIAD data collected under field-free conditions. The CO tilting is consistent with the building of antiphase domain boundaries of close-packed terminal-CO species. It has been found that CO molecules tilt about 6° off-normal at CO coverages higher than 0.6 CO/Pt. The azimuthal directions (equivalent to [1 1 0]) of CO tilting indicate that the close-packed CO molecules are repelled along the nearest Pt-Pt neighbor directions. Studies of the thermal broadening of the CO + ESDIAD patterns due to the tilted-CO molecules have shown that the maximum amplitude of the vibrational motion of the tilted-CO species occurs along the directions normal to the tilting plane. A comparison of the CO + and CO ∗ ESDIAD patterns has permitted approximate estimation of the distance of the image plane from the center of the charge for the CO + ESD product at the point of its origin.