Titania adsorption properties

Morales F., de Smit E., de Groot F.M.F., Visser T., and Weckhuysen B.M. 2007. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 246 91-99. 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

Figure 7a shows the powder XRD pattern of the as-prepared mesostructured titania from titanium butoxide. The mesostructure was collapsed at 400 °C for 5 h, indicated by powder XRD diffraction and adsorption properties. 

Zirconia and titania both contain Lewis-acid and Lewis-base sites, with the latter having stronger adsorption properties. The titania phase also has strong Brpnsted acidic sites. Basic compounds are less retained on zirconia and titania phases, due to their basic nature. Neutral compounds such as poly aromatic hydrocarbons (PAH), due to their rr-electron system, behave as Lewis bases and the interactions with Lewis acid sites on the zirconia and titania packing materials become dominant for retention. 

In terms of this adsorption, properties of various adsorbents, among them the inorganic sorbents can be determined. It must be emphasized that inorganic sorbents such as silica, alumina, titania, complex carbon - mineral sorbents, apatites, e.t.c., are both structurally and energetically heterogeneous. Their total heterogeneity may be described by the kinds of adsorption potential distribution function which is one of the most significant characteristics of the aforementioned solids. 

Thermal stability of ion exchange and adsorption properties of titania gels prepared from titanous chloride and hydrogen peroxide... 

Hydrous titanium oxides are important ion exchangers with different selectivities to different ions, exhibiting certain desirable catalytic and adsorptive properties [1-6]. One particular advantage to these hydrous oxides is their good thermal and radiation stabilities. Studies undertaken by a number of workers have shown that hydrous titanium oxide has ion exchange characteristics that are highly dependent on preparative conditions[7-8], and that the titania surface displayed amphoteric characteristics [9-10]. 

Titania is often combined with other oxides to form binary systems. Cao et al. [248,249] and Yoda et al. [250,251] have investigated the applicability of titania and titania-silica aerogels as potential oxidation catalysts for removal organics from air (VOCs) or water. The concept is to combine the gas adsorption properties of silica with the photocatalytic activity of titania in a mixed catalysis system. Bismuth-molybdenum-titanium xerogel and aerogels are promising catalysts for the oxidation of butadiene to ftiran, where reported activities and selectivities are comparable to state-of-the-art industrial catalysts [252]. 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Liu etal. [32] reported the characteristics and reactivity of highly ordered mesoporous carbon-titania hybrid materials synthesized via organic-inorganic-amphiphilic coassembly followed by in situ crystallization. In the degradation of Rhodamine B these materials also show enhanced properties due to the dispersion/stabilization of small titania nanocrystals and the adsorptive capacity of the nanocarbon. 

Kraemer, S.M. Hering, J.G. (1997) Influence of solution saturation state on the kinetics of ligand-controlled dissolution of oxide phases. Geochim. Cosmochim. Acta 61 2855-2866 Kraemer, S.M., Xu, J., Raymond, K.N. Spo-sito, G. (2002) Adsorption of Pb(II) and Eu(III) by oxide minerals in the presence of natural and synthetic hydroxamate sidero-phores. Environ. Sd. Technol. 36 1287-1291 Kraemer, S.M. Cheah, S.-F. Zapf, R. Xu, J. Raymond, KN. Sposito, G. (1999) Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goefhite. Geochim. Cosmochim. Acta 63 3003—3008 Kratohvil, S. Matijevic, E. (1987) Preparation and properties of coated uniform colloidal partides. I. Aluminum (hydrous) oxide on hematite, diromia, and titania. Adv Ceram. Mater. 2 798-803... 

The samples taken to the experiments were not purified with the method advised by Parfitt [165]. The Parfit s opinion on acidity or alkalinity of the hydroxyl group of both form of the titania comes from the position of the pHpzc. This value is determined by the adsorption/desorption reactions of the proton and adsorption of the background electrolyte ions. The oxide surface charge depends on the alkali-acid properties of the hydroxyl groups and confine only to the analysis of the acid properties of the oxide surface group, do not allow to predict the shift of pHpzc properly. 

Interest in the composition and structure of submonolayer metal oxide deposits on metals has developed as a consequence of growing evidence that such deposits influence the adsorptive and catalytic properties of the substrate metal [see for example ref. (1)]. In particular, it has been shown that titania deposited on a Ni(l 11) (2) surface and on the surface of Pt and Rh foils (3.41 will enhance the activity of the metal for CO hydrogenation. Similar results have also been reported for niobia deposited on a Pt foil (5). Hie present paper discusses the characterization of titania overlayers deposited on the surface of a polycrystalline Rh foil and a Rh(lll) surface. 

Control of the ionic properties of the polyelectrolyte, e.g. charge density and acid-base strength, is possible by simple proton adsorption because the charge carriers, commonly -NH3+X groups, are directly located along the polymer backbone. For numerous inorganic oxides, for example the silica or titania used, the adsorption of the PVAm can be carried out under condi-... 

The CO-H2 synthesis properties of metal/titania catalysts have been found in several studies to be essentially unaffected by the temperature of reduction, in contrast to the strong effect this factor has on chemisorption properties. This problem has focused attention on the special nature of the metal-titania contact perimeter. Reduction of titania, undoubtedly through hydrogen spillover, begins there. It is important to note that Ti J+ cations are produced by reduction temperatures as low as 473K, as shown by 02 adsorption/H20 decomposition measurements (36) or by temperature programmed reduction. In the latter study, the amount of-TiJ+ produced at temperatures below 503K was equivalent to a TiJ+/Pt atom ratio of 0.6 (37). 

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