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Sol−gel boria−alumina mixed oxides with a wide range of B/Al atomic compositions have been characterized as dried xerogels in the preceding paper in which a structural model with four domains of composition was proposed. The present paper presents the results obtained on the same series of solids after calcination at 500 °C. It was found that the structure of the dried xerogels has a strong influence on the structure of the final oxides. In particular, in the dried xerogel state from B/Al ≈ 0.15, BO3 chains crossed over the solid matrix, and this ratio was also a crucial limit for the calcined solids structure. Indeed, during calcination some BO3 chains were volatilized, giving a measured B/Al ratio lower than the theoretical one; the oxides were dislocated and their SSA brutally increasing from ~500 m²·g^-1 up to ~650 m²·g^-1. Cleavage resulted in the exposition of new external surfaces that exhibited BO3OH species incrusted in the host alumina matrix, conferring then an epitactic character to the solids. Further, the ¹¹B magic-angle spinning (MAS) NMR spectra of the solids exhibited a feature constituted of BO3 and BO4 (hydrated surface BO3 species) components that were resolved by a software simulation. While for B/Al < 0.15 the quadrupolar interaction on the BO4 species was of about 0.4 MHz, it increased up to about 0.9 MHz for B/Al > 0.15. This increase originated in a strain on the new BO3 species trapped along the cleaved surfaces. Further, X-ray diffraction and X-ray photoelectron spectroscopy results showed that, while for B/Al < 0.15, the system can be considered as an alumina matrix locally modified by insertion of BO3 species, for B/Al > 0.15 a mixed phase was progressively formed. In good agreement, the ²⁷Al MAS NMR spectra of the solids with high B/Al ratio were similar to that expected for model boria-alumina mixed phases. In addition, the BO4/BO3 ratios calculated by simulation of the ¹¹B MAS NMR spectra were consistent with the XPS results. We calculated the proportion of B species (in wt %) present on the surface of the solids (i.e., BO4 species), which can be potentially involved in catalytic reactions. It was remarkable that the tendency observed for the BO4 wt % as a function of the B/Al ratio was consistent with a boria−alumina phase diagram previously proposed by Gielisse et al.

While boria−alumina mixed oxides have been widely used for various catalytic reactions, their structure has never been satisfactory elucidated and is still a matter of debates. The present paper deals with the elucidation of the structure of boria−alumina prepared by a sol−gel method with B/Al atomic ratio varying from 0.013 to 1.643. The powders were prepared by hydrolysis of aluminum tri-sec-butoxide in the presence of (NH4)2B4O7·4H2O. Then, the solid-state magic angle spinning (MAS) NMR spectroscopy was used to characterize the obtained solids in the dried state (xerogels), the calcined state being the object of a next paper. Both ²⁷Al and ¹¹B MAS NMR spectra were recorded with subsequent simulation of these last ones. This allowed to build a consistent structural model of these xerogels, taking into account the evolutions of calculated parameters such as the quadrupolar interaction, the real chemical shift, and the relative quantity of, respectively, BO3, BO4, tetrahedral, pentahedral, and octahedral aluminum species as a function of the B/Al atomic ratio. As a result, it was found that, for B/Al < 0.06, presence of boron induces the creation of bulk and surface tetrahedral aluminum species. Then, for B/Al > 0.06, BO3 chains attached to these tetrahedral aluminum species are formed and start to grow through the matrix. Further, for B/Al > 0.15, when the boron loading increases, the BO3 chains progressively emerge outside of the matrix, crossing it over. Furthermore, for B/Al > 0.26, some pentahedral aluminum species are formed supposedly due to the considerable steric strains afforded by some particular aluminum atoms. In brief, the presented model elucidates the structure of the dried alumina-based xerogels and is the key starting point to explain the structure of the oxides obtained after calcination and presented in the following paper.

A series of bimetallic catalysts Pt−Mo deposited on silica, with the same (Pt + Mo) metal atom content but various Mo/Mo + Pt ratios, has been synthesized and characterized by XPS, ESR, hydrogen chemisorption, and oxygen uptake during their reoxidation after reduction, in order to determine the oxidation state of Pt and Mo and the catalyst surface compositions. They have been further tested for their activities in the dehydrogenation of cyclohexane, the hydrogenation of benzene, the hydrogenolysis of butane, and the reactions of hexane, on the one hand, and of 3-methylpentane, on the other hand, with hydrogen. Finally the kinetics of butane hydrogenolysis has been extensively studied in order to determine the equilibrium constants of butane adsorption and the rate constants of the steps of C−C bond breaking on all the Pt−Mo/SiO2 samples. The main results are that, while Pt is completely reduced in a metal state, only a fraction of Mo, which depends only on the reduction temperature, is reduced into metal. Mo at various oxidation states is present in the same proportion for all the bimetallic catalysts. A Mo surface enrichment has been evidenced after reduction in hydrogen, but it has been shown that this enrichment decreases or even disappears in the presence of oxygen and of aromatic hydrocarbons strongly adsorbed on the metal particle surfaces. Two kinds of reactions are distinguished:  cyclohexane dehydrogenation, alkanes isomerization, and cyclization, with a rate-determining step occurring preferably on Pt, and alkanes hydrogenolysis, which is much faster on Pt−Mo because of preferential adsorption on a mixed Pt−Mo site. The nature of Pt and Mo species participating to this mixed site is discussed.

In this paper we calculate the effect of surface OH/OH− on the simultaneous adsorption of H2 and O2 on ZnO. A quantitative comparison between H2 and CO oxidation rates shows that the two mechanisms are similar for the same water recovery on ZnO.