E. Guglielminotti

Surface characterization of the Ru3(CO)12Al2O3 system: I. Interaction with the hydroxylated surface

Ru3(CO)12 adsorbed on hydrated Al2O3 is transformed under the action of atomspheric oxygen into an anchored mononuclear dicarbonylic species (RuB). In the absence of O2, OH groups of the surface act as oxidizing agents at T > 373 K leading to a variety of oxidized species, whose structure and relative concentration depend upon the treatment conditions. In vacuo and in the temperature range 673-423 K, dicarbonylic RuA, RuB, and RuC anchored species characterized by well-defined ir bands at 2138-2075, 2072-2005, 2054-1977 cm−1, respectively, are prevalently formed. At higher temperature the decarbonylation is complete but the ruthenium is prevalently in an oxidized form. Decarbonylation in a flow of hydrogen favours the formation of RuC species at intermediate temperatures and of metallic ruthenium at the highest temperatures.

Surface characterization of the Ru3(CO)12Al2O3 system: II. Structure and reactivity of the surface carbonylic complexes

Abstract The structure of the anchored carbonylic complexes (Ru A , Ru B , and Ru C ) formed by interaction of Ru 3 (CO) 12 with Al 2 O 3 is further investigated by 13 CO 12 CO isotopic substitution. All complexes contain two coupled CO oscillators per ruthenium centre and the local structure is tetrahedral (Ru A ) or octahedral (Ru B and Ru C ). The reactivity of these species towards simple molecules is also tested and the interconversion of the Ru B and Ru C species in reducing and oxidizing atmospheres is established. The oxidation states III, II, and 0 are proposed for Ru in the Ru A , Ru B , and Ru C species, respectively.

Surface characterization of the Ru3(CO)12Al2O2 system: III. Surface properties after full decarbonylation and reduction

The structure of dispersed Ru obtained by full decarbonylation of the Ru3(CO)12Al2O3 system is discussed. Samples decarbonylated in vacuo are appreciably oxidized and trivalent anchored species are the main oxidized product. These Ru ions can act as nucleation centres for the formation of very small metallic particles which, due to their extremely reduced dimensions, can be broken up by CO at 473 K with formation of low-nuclearity anchored carbonylic complexes. By successive decarbonylation in vacuo, these complexes give back the initial metallic particles, so showing that highly dispersed Ru is mobile at 473 K. Samples decarbonylated in flowing H2 do not show these features and this is explained by the larger dimensions of the metallic particles, which resist the CO attack.

Infrared study of CO adsorption on magnesium oxide

The adsorption of CO at room temperature on well outgassed specimens of MgO gives rise to a large number of bands in the 2200–1000 cm–1 range, which can be divided into two main groups. The bands of the former group are destroyed by oxygen at room temperature: some react instantaneously and are associated with a marked pink colour of the sample; the others are less reactive as they require prolonged contact time in order to be completely oxidized at room temperature (r.t.). The bands of the latter group, far from being destroyed by oxygen, grow when the oxygen-sensitive species are depleted. The oxygen-sensitive species are thought to be negatively charged polymeric CO structures (CO clusters) of the type (CO)x–n, where x= 2 or 4 and n is > 2. The simplest Co clusters (dimers) can be transformed into larger polymers by further CO addition. Under the correct conditions the reverse process can also be carried out. The oxygen-insensitive species have a carbonate-like structure and are present on the surface in fairly constant ratios with respect to the former group species. A chemisorption mechanism leading both to oxidized (carbonate-like) and to reduced (negative CO polymers) species is proposed. The active centres for CO chemisorption consist of groups of ions (both positive and negative) in strongly uncoordinated situations.

Effect of oxidation–reduction treatments on the infrared spectra of carbon monoxide chemisorbed on a Ru/TiO2 catalyst

The measurement of IR spectra of CO adsorbed on H2-reduced Ru/TiO2(1% Ru, Degussa P-25 TiO2) is made difficult by the very low transparency, caused by partial reduction of the support; reduction at 758 K leads to complete opacity, but spectra can be obtained by multiple scanning on a slightly oxidised sample (transparency 0.07%). Reduction at either 433 or 623 K, however, gives products with which spectra are readily observed. On samples well reduced at 623 K the adsorption of CO gives as prevailing species Ru0—CO with ν(CO) at 2045 cm–1 at full coverage, shifting to lower frequency up to 1990 cm–1 as the coverage is decreased. At high pressures at room temperature, and especially on samples not well reduced and containing Cl–, bands are observed at 2151–2132 and 2090–2072 cm–1 which are attributed to Ru3+(CO)2 or Ru2+(CO)3 species. These species are removed by evacuation at 473 K, whereas Ru2+(CO)2 species (bands at 2101–2080 and 2038–2023 cm–1) are stable up to 573 K. At 473 K CO is an effective reductant for incompletely reduced samples. The Ru2+(CO)2 species are intermediates in the reduction by CO of the Ru/TiO2 system: at 573–623 K all the oxidised Ru species are reduced to Ru0. Spectra obtained by CO adsorption on samples reduced at 433 K are similar, save for a shoulder on the low-frequency side of the 2045 cm–1 linear Ru0—CO band. The presence of a low-frequency component at 1990 cm–1 is the fingerprint of isolated Ru0—CO species formed on very small and less perfect Ru crystals. This suggests that Ru is well dispersed. Evacuation at 758 K of unreduced RuCl3/TiO2 also results in the formation of some Ru0.

Infra-red study of nitric oxide adsorption on magnesium oxide

An i.r. study of nitric oxide adsorption on high area MgO (previously degassed at 800°C) at room temperature is reported. The adsorption of NO onto MgO produces i.r. bands at 800–900 cm–1, 1100–1250 cm–1, and 1350–1450 cm–1. These absorption bands were assigned to several surface species: NO–2 ions with various configurations and NO– and N2O2–2 ions, co-ordinated to Mg2+ ions. The adsorption (3.3 × 1013 molecule cm–2, at 10.4 × 103 N m–2 of NO) occurs in limited surface zones.

Infrared study of carbon monoxide adsorption on calcium and strontium oxides

CO adsorption on CaO and SrO takes place, as on MgO, via a disproportionation reaction leading to both surface carbonates and unusual surface species with a complex vibrational spectrum in the low-frequency range. These are thought to be negatively charged CO polymers, the simplest ones being (CO)2–2. A strong electrostatic interaction between negative species and surface cations accounts for the marked dependence of the infrared signal on the lattice parameter of the solids. The increasing basicity along the series MgO, CaO, SrO causes: (i) a marked increase in the total adsorptive capacity; (ii) an increase in the relative population of polymers with respect to dimers; (iii) an increase in the importance of a Boudouart-like reaction upon desorption.

An IR investigation of CO chemisorbed on ruthenium microcrystals supported on silica

Abstract The IR spectra of CO adsorbed on silica-supported Ru in highly dispersed and sintered form have been measured and compared with the spectra obtained on flat and stepped faces of single crystals. Sintered Ru particles show hexagonal shape with predominance of the {001} facelets: this fact explains the similarity between the spectrum of CO adsorbed on them and on {001} single crystal faces. Due to the presence of well shape crystallites, sintered samples are suitable for spectroscopic investigations concerning adsorbate-adsorbate interactions of dynamic and static type. The separation between the dynamic and static contributions has been made by using isotopic 12CO 13CO mixtures.

Effects of structural defects and alloying on the FTIR spectra of CO adsorbed on PtZnO

The curve fitting analysis of the FTIR spectra of two differently pretreated PtZnO dispersed samples is discussed. Carbon monoxide adsorbed on a photodeposited PtZnO sample mildly reduced in a 0.5% H2N2 mixture at 493 K shows a main band at 2100 cm−1, shifting to the red and reducing its intensity by decreasing the surface coverage. All the spectra are well fitted by the addition of five Lorentzian bands. Looking at the literature, the five components, on the basis of their spectroscopic features can be assigned to Ptx+ sites, Pt0(111) terrace sites with coordination number 9, Pt0(100) terrace sites with coordination number 8, and Pt0 step sites with coordination number 7. The spectra of CO adsorbed on the same PtZnO sample reduced in pure H2 at 523 K exhibit a main band at ≈ 2070 cm−1, at lower frequency, significantly less intense and further red-shifting by decreasing the coverage. In this case the band appears as the superposition of two bands, not completely well fitted by Lorentzian bands. The maximum of these bands appears related to Pt atoms with coordination numbers 6 and 5. It must be stressed that by increasing the temperature of reduction in the case of Pt supported on refractory oxides, usually prepared by ionic exchange, a sintering and clustering occurs; these processes convert almost isolated low coordinated metal sites into terrace sites and induce a blue-shift of the absorption band. Our mildly reduced samples, as a consequence of the photoreductive method, are made of well crystallized Pt small particles. By increasing the reduction temperature, the alloying of Pt with Zn atoms coming from the support occurs; the opposite frequency shift observed and the change in the shape of the band can be ascribed to the chemical effect of the alloying of platinum with zinc. This hypothesis is confirmed by the analysis of isotopic mixtures experiments.

Infrared evidence of metal-semiconductor interaction in a Ru/ZnO system

The new Ru/ZnO system, that should have applications in catalysis or as gas sensor, is studied by IR spectroscopy. On oxidized samples a Ru=O species ( v = 972 cm -1 is present, readely reduced by CO at RT to Ru(0). A residual IR transparency of reduced samples, unlike the case of pure ZnO, shows that an electron transfer from ZnO donor centers to Ru(0) occurs. The loss in the IR transparency after H 2 adsorption at RT is due to the filling of the ZnO donor centers, as a consequence of the spillover of H atoms, dissociated by ruthenium and acting as ionized donors on the ZnO. CO adsorption leads either to linearly or bridged carbonyl species, the latter ones are usually not observed on Ru(0).