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The IR spectroscopic study showed that nitrous oxide, which is used as a probe for supported transition metal catalysts, is weakly chemisorbed without dissociation on two different sites of alumina.

Differently treated Pt/ZnO samples were studied with FTIR, microgravimetric experiments and TEM analysis. On the vacuum treated samples the spectral range and the behaviour to the outgassing of adsorbed CO species are the same as in the case of CO adsorbed on Pt(111) planes. On H2 reduced samples the surface composition of the metal particles, as indicated by 12CO/13CO mixture spectra, changes from pure Pt to PtZn alloy, with surface segregation of Zn in the most strongly reduced samples. This phenomenon can be at the origin of the SMSI behaviour of this system.

CO reactions with hydroxyls and oxygens on reduced Cu/Zno systems are studied by IR transmission spectroscopy: formates, carbonates and bicarbonates species adsorbed on both the oxidic and the metallic phases have been identified. On the species adsorbed on the metal the “metal surface selection rule” is acting. Many of these surface products are produced at the contact line between the two phases, where also a special four-electron donor carbonyl has been detected.

Infrared transmission spectra and ultraviolet–visible–near-infrared diffuse reflectance spectra are reported for microcrystalline ZnO samples in contact with CO at different temperatures. The absorptions observed after interaction at 300 K are ascribed to the formation of oxidized chemisorbed CO and surface point defects on ZnO. Interaction at 473 K produces extensive reduction of the ZnO and the population of the conduction band, and phonon–plasmon coupling is observed. After cooling at 300 K, electrons from the conduction band populate bulk defects.

The modern low-pressure methanol synthesis catalysts are based on copper-containing systems such as Cu/ZnO/Al2O3 and Cu/ZnO/Cr2O3 with various compositions. These catalysts are also highly active for the low-temperature CO shift reaction. For both reactions the nature of the active sites is still an open question.

The Ru/ZnO (0.5% Ru) system, prepared by decomposition of Ru 3 (CO) 12 on ZnO, was studied. The IR spectrum of CO adsorbed on H 2 reduced samples gives evidence of Ru x+ (CO) n (x,n=2,3) surface complexes. After heating at 423–523 K the CO further reduces the Ru x+ (CO) n complexes. The spectrum of CO adsorbed after this treatment can be interpreted in term of linear Ru°-CO (γ=2000 cm −1 ) on very small clusters and Ru-CO bridged anionic surface species (v = 1900–1820 cm −1 ). The formation of the last species, unusual on Ru, is favored in this case by an electron transfer from the reduced n-ZnO semiconductor to the small Ru particles.

The infrared spectrum (700–4000 cm–1) of quasi-stoichiometric microcrystalline ZnO has been recorded at various temperatures (97–523 K) and following various redox treatments.A broad band centred at ca. 1500 cm–1, produced on samples cooled down to 97 K in oxygen, is ascribed to the increasing concentration of un-ionized donor centres.The disappearance of all phonon and surface impurity (OH, carbonate-like species) bands, caused by heating the samples to 433 K both in vacuo and in a hydrogen atmosphere, is ascribed to the phonon–plasmon coupling in microcrystals possessing a rather high concentration of electrons in the conduction band.

The state of copper in Cu/ZnO catalysts at different reduction stages were studied, mainly by chemisorption of CO at 77 K. The spectroscopic data of fully oxidized samples indicate that a large part of copper ions can be readily reduced to Cu(o) or Cu(I) by CO at 77 K. Two different Cu(I) CO complexes are present, absorbing at 2130 and 2110 cm −1 respectively. Cupric ions strongly resistant to the reduction give rise to Cu(II) CO complex absorbing ble in vacuum. Carbonate- like species appear as oxidation products, mainly adsorbed on Zn(II) and Cu(II) ions. On fully reduced samples two chemisorbed species are present. One absorbing at 2098 cm −1 , is assigned to CO chemisorbed on high index microfacets of metal particles. The other, absorbing at 2070 cm −1 , is assigned to CO chemisorbed on edges and corners of metal particles.

Several α-Al2O3 specimens were prepared by thermal treatment of pure transition aluminas, already in the form of i.r. pellets. When the starting material is a high external area phase, the resulting sample is still suitable for i.r. work. In particular two preparations are considered, which only differ in the number of hours of treatment at 1150°C. Although the X-ray pattern is the same in both cases, great differences are revealed by the i.r. spectra, especially in the stretching region of the surface hydroxyl groups. It is suggested that the sample treated for the shorter time either maintains or simulates some surface characteristics of the starting transition phase.

An i.r. study is made of the room-temperature adsorption of pure CO, H 2 and successive O 2 interaction on SnO 2 powder at different degrees of hydroxylation. Both sharp bands, due to different adsorbed carbonate-like species produced by surface reactions, and a very broad absorption, due to electron transition from the second level of oxygen vacancies to the conduction band, are measured in the case of the CO interaction. A wider variety (bicarbonates, unidentate and bidentate carbonates) and larger amounts of such intermediates are measured on highly-hydroxylated surfaces (h.h.s.) than on less-hydroxylated (l.h.) ones. At the same time, the intensity of the electronic transition is higher for h.h.s. than l.h. ones. A very broad absorption due to the same electronic transition can be detected for H 2 admission; however, no increase in the band intensities due to the vibrations of surface hydroxyls can be measured.

An IR spectroscopic study of ethylene hydrogenation on Cu/ZnO based catalysts showed that, under reaction conditions, Cu(0) is at the origin of the H 2 activation process and that Cu(0) and ZnO are both able to activate ethylene as weakly π-bonded species. No evidence of ethylene π-bonded to Cu(I) ions occurred.

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).

Relaxation effects on the infrared spectra of microcrystalline MgO prepared under controlled conditions have been studied both by varying the average particle size and by chemisorption of pyndine. The results are compared with the phonon density of states distribution calculated by other authors.

Publisher Summary This chapter describes a method to study the interaction of Ni(Co) 4 , Fe(Co) 5 , and Co 2 (CO) 8 with MgO and the formation of very small metallic clusters. In this method, the impregnation of a high area support is carried out in conditions of total surface dehydration, absence of solvents, and atmospheric gases. High area MgO samples (∼200 m 2 /g) is obtained directly in the cells by decomposing under high vacuum at 523 K high purity Mg(OH) 2 . Total surface dehydration is achieved by subsequent outgassing at 1073 K under high vacuum conditions. The pellet is then cooled down to room temperature under vacuum and the volatile Ni(CO) 4 , Fe(CO) 5 , and Co 2 (CO) 8 carbonyls is allowed to contact the surface. After each dose, the spectrum of adsorbed species (both I.R. and UV-VIS-Near Infrared diffuse reflectance) is carried out with a Beckman IR 12 and DK-2 spectrometers respectively. The method demonstrate that adsorption of simple carbonyls at R.T. in controlled condition of vacuum and support treatment can produce new polymetallic carbonyls with very few Me atoms. But at higher temperatures (353–523 K) upon CO loss, the new carbonyl clusters can agglomerate (mainly in the Ni case) and/or the metal is oxidized by CO disproportionation.

Cu/ZnO samples with copper fractons ranging from 3 to 50 at % have been studied by diffuse reflectance spectroscopy. Besides the CuO phase absorption, present in all the oxidized samples, only in the 3 at% sample the fundamental direct ZnO absorption is evident. The others show a modified absorption, interpreted as an indirect transition from CuO valence band to ZnO conduction band, as a consequence of a good contact between the phases. Reduction experiments have pointed out the growth of a Cu metal layer on ZnO particles thick enough to be responsible for the disappearance of the oxidic phases without formation of any new phase.

The infrared spectrum (700–4000 cm–1) of quasi-stoichiometric microcrystalline ZnO has been recorded at various temperatures (97–523 K) and following various redox treatments.A broad band centred at ca. 1500 cm–1, produced on samples cooled down to 97 K in oxygen, is ascribed to the increasing concentration of un-ionized donor centres.The disappearance of all phonon and surface impurity (OH, carbonate-like species) bands, caused by heating the samples to 433 K both in vacuo and in a hydrogen atmosphere, is ascribed to the phonon–plasmon coupling in microcrystals possessing a rather high concentration of electrons in the conduction band.

Several α-Al2O3 specimens were prepared by thermal treatment of pure transition aluminas, already in the form of i.r. pellets. When the starting material is a high external area phase, the resulting sample is still suitable for i.r. work. In particular two preparations are considered, which only differ in the number of hours of treatment at 1150°C. Although the X-ray pattern is the same in both cases, great differences are revealed by the i.r. spectra, especially in the stretching region of the surface hydroxyl groups. It is suggested that the sample treated for the shorter time either maintains or simulates some surface characteristics of the starting transition phase.

Hydrogen adsorption on high-surface-area alkaline-earth oxides has been studied by i.r. spectroscopy at 300 and 78 K. In the former case heterolytic dissociation of H2 molecules produces hydride and hydroxy groups which are partially reversible upon evacuation. Low-temperature hydrogen adsorption appears to be undissociative. The active sites are cation–anion couples in low coordination on the surface.On the basis of i.r. evidence a mechanism is proposed for the interaction of oxygen with preadsorbed hydrogen envisaging an electron-transfer process from H– ions to oxygen molecules to give O–2 paramagnetic species.

Solid solutions of ZnO in MgO with larger surface areas than those previously reported in the literature have been prepared and their formation tested by ultraviolet–visible reflectance spectroscopy. The surface chemistry of CO and H2(or D2) interactions has been studied by infrared spectroscopy. Large differences with respect to the surface chemistry of the pure oxides have been found. CO is mainly adsorbed on MgO couples as oxidized carboxylate-type species, which are stable up to 373 K and then transformed into carbonate-type species. H2 is mainly chemisorbed by heterolytic dissociation on three different Zn—O couples.