S. Srinivasan

The formation of titanium oxide monolayer coatings on silica surfaces

The formation of a dispersed titanium oxide layer on Cabosil-fumed silica and on nonporous silica spheres was studied by infrared and Raman spectroscopies and by transmission electron microscopy (TEM). The procedure for obtaining the titania coatings involved reacting the silanol groups on the silica surface with titanium alkoxides under a N2 atmosphere. This self-limiting reaction led to a coating of dispersed titania on the silica spheres with a weight loading between 0.5 and 1.4 × 10−3 g/m2. The dispersed titanium oxide on the silica spheres was visible as a surface texturing of the silica in TEM images, and led to over two orders of magnitude increase in the reactivity of the silica spheres for 1-propanol dehydration. Raman spectroscopy and TEM confirmed that the dispersed titania was stable to calcination in dry air at 973 K or to heating under a vacuum of 2 × 10−7 Torr up to 1058 K. However, under alcohol dehydration reaction conditions, the dispersed titania transformed into crystals of anatase, 3 nm in diameter. On Cabosil-fumed silica, on the other hand, a similar preparation resulted in a titania loading (per square meter) that was only 7% of that seen on the silica spheres. Higher loadings caused the appearance of bands due to crystalline TiO2 (anatase) in the Raman spectra. The lower monolayer capacity on Cabosil silica can be correlated with the presence of singly bound hydroxyls as seen by IR. The Stober spheres on the other hand show hydroxyl bands that show significant hydrogen bonding.

The morphology of oxide-supported MoS2

MoS{sub 2} supported on oxides such as alumina and promoted with nickel or cobalt constitutes an active catalyst for reactions such as hydrogenation, hydrodesulfurization, and methanation. Recent work has shown that the support oxide has a marked effect on the specific activity of the sulfide catalysts, and it has been suggested that these activity differences may be related to the morphology of the MoS{sub 2} on the surface. In order to study the role the surface texture and particle size, the authors have examined the morphology of MoS{sub 2} on low-surface-area model alumina and silica powders and compared it with MoS{sub 2} supported on titania. These model supports on titania. These model supports allows them to separate the role of surface area and morphology from that of oxide surface chemistry, since all of these supports have comparable surface areas. The objective of this work is to document differences between the morphology of MoS{sub 2} supported on silica, alumina, and titania when powders of comparable surface area are used. Results suggest that the surface texture of the oxide support may be an important factor affecting the morphology, and indirectly the reactivity, of the dispersed phase in a heterogeneous catalyst.

The effect of alumina structure on surface sites for alcohol dehydration

We have studied the effect of alumina form(-/, 5, and a), and impurities, on 2-propanol dehydration activity. In a steady-state flow reactor experiment, α-alumina was approximately 1000 times less reactive (in terms of moles propene formed/s/m2) than reforming grade γ-alumina. The structure of the transitional alumina (γ vs δ) played a smaller role than Na contamination in determining alcohol dehydration activity. For instance, δ-alumina was ≈3–6 times less reactive than reforming grade γ-alumina in its activity for alcohol conversion, while Na-poisoned γ-alumina was ≈180 times less reactive. The number of surface sites on these aluminas were determined using temperature-programmed reaction (TPR) of 2-propanol. It was found that ≈1–2 × 1018 alcohol molecules/m2 reacted to form propene on all of the aluminas independent of alumina form or impurity concentration. The temperature at which the alcohol reacted to form propene in the TPR experiment correlated inversely with steady-state alcohol dehydration activity. The peak desorption temperature (TP) during TPR ranged from 440 K for reforming grade γ-alumina to 600 K for α-alumina. These results are discussed in terms of a model for the nature of reactive sites on the surface of alumina.

The role of sodium and structure on the catalytic behavior of alumina: I. Isopropanol dehydration activity

Abstract Three different transitional aluminas were doped with controlled and measured amounts of Na and were tested for 2-propanol dehydration activity. These alumina samples were activated by heating at 773 K in situ before use. In order to examine the sites responsible for the reaction, simultaneous temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) measurements of 2-propanol were performed on the samples. A typical γ-alumina sample adsorbed ca. 2·10 18 molecules of 2-propanol per m 2 . When 400 ppm (0.04 wt.-%) of Na was added to this sample, the reactivity fell by 75%. Since the Na level corresponds to 0.07·10 18 atoms/m 2 , this suggests that the active sites are much fewer in number than the number of adsorption sites. Besides affecting the active sites, Na also modifies the adsorption sites causing a fraction of the adsorbed 2-propanol to desorb before reaction in the TPD experiment. The effect of added Na was contrasted with the effect of alumina form (γ, δ and α). These alumina samples were also characterized by IR spectroscopy in a companion study (part II of this paper, Appl. Catal. A, 000 (1995) 000) where we found that the added Na impurities affected the hydroxyl groups and adsorbed pyridine. This study provides a comprehensive picture of how Na impurities modify the catalytic behavior of transitional alumina.

A method for measuring the titania surface area on mixed oxides of titania and silica

We have used temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanol to characterize several silica, titania, and silica-supported titania samples. Upon evacuation, most of the 2-propanol desorbed intact from the silica samples. This is in contrast to the results on titania and on silica-supported titania, where a significant amount remained on the surface following evacuation, with a fraction of this reacting to propene and water. The coverages of 2-propanol are approximately proportional to the titania surface area, corresponding to between 2.4 and 6.1×1018 molecules/m2 of titania, depending on the form of the titania. The results suggest that selective adsorption of 2-propanol may be useful for determining the surface area of titania in titania-silicates.

The role of sodium and structure on the catalytic behavior of alumina: II. IR spectroscopy

Abstract A series of alumina samples differing in structure (γ, δ and α) and in Na content, were studied by infrared spectroscopy. The same set of samples was also studied using 2-propanol dehydration as a probe reaction and using temperature-programmed reaction of 2-propanol [S. Srinivasan, C.R. Narayanan, A. Biaglow, R. Gorte and A.K. Datye, Appl. Catal. A, 000 (1995) 000]. Transitional aluminas possess well defined hydroxyl bands whose nature and intensity is affected by alumina structure and impurity content. The most reactive aluminas exhibit a prominent hydroxyl band at 3770 cm−1. Another characteristic of the most reactive aluminas is the formation of a band at 1622 cm−1 after pyridine adsorption and evacuation at room temperature. Addition of Na as well as thermal treatments that transform γ- to δ-alumina have a similar effect on aluminas, namely, an attenuation of the prominent 3770 cm−1 hydroxyl band and the 1622 cm−1 pyridine band. In both cases, the effect appears to be steric in nature, indicative of hindered access of the probe molecule to the reactive sites. These spectroscopy results correlate well with the trends observed in catalytic reactivity.

Transmission electron microscopy of supported molybdenum and vanadium oxides

Transmission electron microscopy has been used to characterize dispersions of molybdena and vanadia on titania and silica supports. When silica spheres of controlled morphology were used as support, the dispersed “monolayer” phase of both oxides could be imaged due to characteristic changes in contrast. In addition to the dispersed phase, we could detect three-dimensional crystallites of V2O5 but in the case of MoO3 only two-dimensional islands were seen. On Degussa P-25 titania, there was no observable contrast change due to the presence of a monolayer of these dispersed oxides. However, exposure to the electron beam caused dramatic changes in the surface texture of the support. Such changes were not seen when blank TiO2 was similarly irradiated. These e-beam induced changes were more pronounced in the vanadia/titania catalysts leading to formation of 1–3 nm clusters of reduced VOx. However, on the MoO3/TiO2 sample, e-beam exposure caused only a pronounced change in texture but no well defined clusters could be detected.