Giuliano Moretti

Auger parameter and Wagner plot in the characterization of chemical states by X-ray photoelectron spectroscopy: a review

Abstract In this review we describe the Auger parameter concept and its relation to the final state relaxation energy. After a historical introduction we illustrate the several models proposed to calculate relaxation energies and Auger parameter shifts. It is stressed how the initial empirical approach of the Auger parameter concept developed by Wagner in the early 1970s is now changed thanks to the elaboration of theoretical models based on classical electrostatic and quantum chemistry. The Auger parameter permits to obtain information on the local chemical environment of core-ionized atoms. Even more information can be obtained arranging photoemission and Auger lines in the format of the so called Wagner plots (chemical state plots). These plots contain information on the initial state effects, i.e. the Madelung potential and the ground state valence atomic charge at the core-ionized atom, and on the final state relaxation energy. By means of the point–charge potential approximation, the Auger parameter and the Wagner plot, we describe a simple semiquantitative model useful to rationalize the dependence of the local electronic structure on the atomic environment. The Auger parameter is useful to study the screening mechanism in the final state of the photoemission process. In the case of the non-local screening mechanism (polarization of the charge from the nearest-neighbour ligands towards extended empty orbitals of the core-ionized atom) a simple electrostatic model permits the estimation of the Auger parameter shifts. It is shown that the Auger parameter shift is a function of the number, distance, electronic polarizability and local geometry of the nearest-neighbour ligands around the core-ionized atom. The electrostatic model has been applied successfully for interpreting Auger parameter shifts of core-ionized atoms in insulators and semiconductors compounds. On the other hand, when the electrostatic model cannot be applied, as in the case of the local screening mechanism (complete tranfer of one electron towards empty and localized orbitals of the core-ionized atom), the extended potential model of the Auger parameter is able to relate the potential in an atomic core to the atomic valence charge. According to this model the Auger parameter shift depends on atomic properties of the core-ionized atom, obtained from atomic structure calculations, on the valence charge in the initial state and on the screening charge in the final state. It has been applied successfully for interpreting metal-free atom and alloy–pure metal Auger parameter shifts. Other models of the Auger parameter based on electrostatics, quantum chemistry and thermochemistry are also reviewed.

Epoxidation on titanium-containing silicates: do structural features really affect the catalytic performance?

Five titanium-containing silicates, with different structural features, were compared: Ti–MCM-41, ordered titanium-grafted mesoporous silica, Ti–SiO2 Davison, nonordered titanium-grafted porous silica, Ti–SiO2 Aerosil, nonporous pyrogenic titanium-grafted silica, MST, nonordered in-framework mesoporous material, and TiO2–SiO2 Grace, commercial amorphous porous mixed oxide. They were tested in the liquid-phase epoxidation reaction on six unsaturated cyclic terpenes. Good performances were obtained on the commercial mixed oxide and also on the three grafted silicates. The in-framework MST showed the worst activity results. Under these conditions, the porosity features do not affect the catalytic performances noticeably and the use of an ordered mesoporous material is not strictly required. Likewise, a very high surface area is not mandatory in order to have an efficient titanium-grafted catalyst in the epoxidation of these substrates.

Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts

Copper and zinc hydroxycarbonates with Cu/Zn atomic ratios of 100/0, 92/8, 85/15, 77/23, 67/33, and 50/50 were prepared by the coprecipitation method. The samples with the ratios 100/0, 92/8 and 85/15 were monophasic with the monoclinic P2/sub 1//a malachite structure, whereas the richer zinc samples contained in addition to the malachite phase the orthorhombic B22/sub 1/2 aurichalcite, whose concentration was very low in the 77/23 specimen but increased continuously with zinc content. The amount of aurichalcite in the biphasic powders has been estimated by means of quantitative X-ray diffraction and differential thermal analysis. With only those X-ray reflections belonging to the malachite structure taken into account, the unit cell constants for all the solid solution hydroxycarbonates have been determined, showing that as a whole the cell volume of malachite decreases continuously with increasing zinc content. This result is related, as shown also by DTA measurements and thermochemical literature data, to an increasing stability of compounds richer in zinc, to increasing covalent effects, and/or to less octahedral site distortion around the metal atoms when zinc substitutes for Cu/sup 2 +/ ions in the malachite lattice. Magnetic susceptibility and thermogravimetric measurements together with diffuse reflectance spectra are also presented and discussed.

The catalytic activity of Cu-ZSM-5 and Cu-Y zeolites in NO decomposition: dependence on copper concentration

NO decomposition was studied on Cu-ZSM-5 (Cu exchange extent from 23 to 210%) and Cu-Y (Cu exchange extent from 5 to 105%) catalysts at 773 K. The results show that the activity (NO molecules decomposed per gram of catalyst per second) increases by roughly 100-fold when the extent of exchange with copper in the ZSM-5 framework increases from 80 to 100%. This behaviour shows that not all Cu sites are equivalent in their decomposition activity. Cu-ZSM-5 samples prepared with either H-ZSM-5 or Na-ZSM-5 show the same activity pattern.

Kinetics of the reverse water-gas shift reaction over Cu(110)

The reverse water-gas shift reaction (CO2 + H2 → H2O + CO) has been studied over a clean Cu(110) single-crystal model catalyst at temperatures between 573 and 723 K. The steady-state kinetic measurements were carried out at medium pressures (10–2000 Torr) in a microreactor after cleaning and characterization of the sample under UHV conditions. The H2/CO2-pressure ratios varied from 1000: 1 to 1 : 10. The product buildup was monitored with a gas chromatograph (GC). The apparent activation energy is about 18 kcal/mol, and the reaction orders in H2 and CO2 depend strongly on the H2/CO2 ratio and temperature. The steady-state kinetics are compared favorably with the rates of elementary steps potentially involved in a “surface redox” reaction mechanism of the reverse and forward water-gas shift reaction involving the formation and removal of oxygen adatoms. Kinetic evidence that is tentatively attributed to a hydrogen-induced surface phase transition that affects the reaction rate, is also presented.

Turnover frequency for NO decomposition over Cu-ZSM-5 catalysts: insight into the reaction mechanism

In this letter we present a simple model useful to understand the relationship between the turnover frequency for NO decomposition over Cu-ZSM-5 catalysts (NCu), the number of Al atoms per unit cell of the ZSM-5 zeolite (p),and the copper loading expressed as percent of exchange (E). Our simple model is able to explain the literature data. We show that: (1) on catalysts with the highest activity (Cu exchange levelsE>90%),NCu increases withp (i.e. decreasing the Si/A1 ratio) indicating that the most active sites may contain two close copper ions; (2) at low Cu exchange levels (E<80%) the catalysts have lower activity and, moreover,NCu decreases withP, according to previous results of Iwamoto et al. (1986). The present results are also in agreement with the evidence that the redox couple Cu2–/Cu– play a key role in the reaction mechanism.

Effects of the Si/Al atomic ratio on the activity of Cu-ZSM-5 catalysts for nitric oxide decomposition

A review of literature data for nitric oxide decomposition over Cu-ZSM-5 catalysts leads us to conclude that the turnover frequency depends on the Si/Al atomic ratio in a way opposite to the trend suggested by Iwamoto and co-workers and considered correct by Shelef in a recent letter published in this Journal. In particular we show that the turnover frequency increases with the number of Al atoms per unit cell (i.e. decreasing the Si/Al atomic ratio). This result suggests that the most active sites for NO decomposition over Cu-ZSM-5 catalysts may contain two close copper ions.

Preparation and characterisation of mesoporous silica–alumina and silica–titania with a narrow pore size distribution

Abstract Amorphous mesoporous materials with a different degree of order in the arrangement of pores are outlined. Particularly, the synthesis of a class of mesoporous silica–alumina (MSA) materials with narrow pore size distribution and a disordered arrangement of pores is reported and discussed. Likewise, the preparation of titanium-containing ordered mesoporous silicates (Ti-MCM-41) and disordered mesoporous silica–titania (MST) are also described in detail. The structural properties of the solids are compared by means of X-ray diffraction and UV-Vis diffuse reflectance spectroscopy. The nitrogen adsorption–desorption measurements were performed and the textural properties are evaluated by the BET, DFT, BJH and t -plot methods. The high specific surface area and pore volume, as well as the acidity, make MSA solids interesting catalysts in several petrochemical transformations, i.e. oligomerisation, alkylation, hydroisomerisation, rearrangement reactions. Besides, thanks to the width of the mesopores of such solids, the catalytic activity of titanium-containing silicates may have a potential application in the epoxidation of bulky unsaturated fine chemical substrates.

Nitric oxide decomposition over Cu-exchanged ZSM-5 with high Si/Al ratio

Abstract The catalytic properties of Cu-exchanged H-ZSM-5 ( Si Al = ca. 80 ) in NO decomposition were investigated. It was found that, both in the presence and in the absence of oxygen in the feed, the rate of NO decomposition increased with the copper content up to about 640% Cu over-exchange, while the turnover frequency reached a maximum value at about 500% over-exchange. The reaction order of NO decomposition in the absence of O2 was 1.6 in the range of NO concentration from 1000 to 5000 ppm. Addition of 1 % O2 in the feed resulted in the decrease of NO conversion to N2 depending on the contact time. In all the range of Cu% exchange the catalysts were active for the catalytic oxidation of NO to NO2, reaching the equilibrium conversion both in the absence (above 350°C) and in the presence (above 300°C) of O2 in the feed.

Preparation and characterisation of cobalt–copper hydroxysalts and their oxide products of decomposition

Three cobalt–copper hydroxysalts with chemical formulae Co(CO3)0.5(OH)1.0· 0.11H2O, Co0.67Cu0.33(CO3)0.4(OH)1.2 and Co0.49Cu0.51(CO3)0.43(OH)1.14 have been obtained and characterised by powder X-ray diffraction (XRD), thermal analysis (TG, DTG and DTA), diffuse reflectance spectroscopy (DRS), magnetic susceptibility and X-ray photoelectron spectroscopy (XPS). The above precursors treated at 723 K for 24 h both in N2 and in air give several cobalt and copper oxides which were characterised by magnetic susceptibility, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance. Depending on the Co : Cu atomic ratios and on the treatments with various gases, the products consist of oxides such as CoO, CuxCo1–xO, CuCoO2 and CuxCo3–xO4 for the samples calcined in N2, and Co3O4, CuxCo3–xO4 and CuO for those calcined in air.