Niculae I. Ionescu

Acid–base properties of supported gallium oxide catalysts

The acid-base properties of supported gallium oxide catalysts have been investigated by microcalorimetry and FTIR spectroscopy of pyridine (acidity) and CO 2 (basicity) adsorption. Pyridine adsorption microcalorimetry has shown that loading supports such as γ-Al 2 O 3 , TiO 2 , and ZrO 2 with gallium oxide in a surface concentration close to the theoretical monolayer, resulted in a decrease of the surface acidity of the catalysts compared to that of the supports while, in the case of SiO 2 , new Lewis acid sites were created. The number of acid sites for the supported and bulk gallium oxide catalysts was in the following order: Ga 2 O 3 /SiO 2 < Ga 2 O 3 /ZrO 2 < Ga 2 O 3 /TiO 2 Ga 2 O 3 < Ga 2 O 3 /Al 2 O 3 . This order is related to the degree of dispersion of gallium oxide on the surface of each support and to the interaction between guest and host oxides. Two types of Lewis acid sites were identified by pyridine adsorption, corresponding to the support and the supported gallium oxide, respectively. The catalysts displayed very weak basicity, except for the samples supported on zirconia and alumina.

Electrical conductivity of γ-Al2O3 at atmospheric pressure under dehydrating/hydrating conditions

Abstract Changes of electrical conductance and capacity of γ-Al 2 O 3 were studied in situ during dehydration/rehydration in gas flow. The measurements were performed at atmospheric pressure by following a standard protocol of experiments, and were coupled with simultaneous monitoring of the composition of the inlet/effluent gas. The electrical properties are dominated by the protonic conduction. It was shown that at low temperature this occurs by vehicle mechanism, being facilitated by the presence of water molecule adsorbed on γ-Al 2 O 3 surface; the decrease of electrical conductance on heating is obviously connected with dehydration. At higher temperature ( t >200°C) conduction is dominated by proton hopping between OH groups (Grotthuss mechanism). It is suggested that in current operating conditions in catalysis, γ-Al 2 O 3 is only partially dehydrated/dehydroxylated, the bulk acting as a source of water/OH groups for the surface; this process controls the acidity of alumina and alumina-supported catalysts.

Calorimetric Study of the Acidity and Interface Effects of Tin Dioxide Layers Deposited on Another Metal Oxide

Samples with various amounts of tin oxide were prepared by impregnating γ-Al2O3, TiO2(anatase), SiO2, ZrO2 and MgO with tin tetrachloride or tributyltin acetate solutions. After drying and calcination, the samples were characterized by chemical analysis, XRD and BET measurements. Ammonia adsorption microcalorimetry was used in order to determine the number, strength and strength distribution of surface acid sites. The influence of the adsorption temperature, evacuation temperature, amount of SnO2 deposited and of the nature of the support on the adsorption properties were studied.

Protonic conductivity of Pt/Al2O3 in hydrogen- and water-containing atmospheres

Abstract Changes of electrical properties of a 0.5% Pt/Al2O3 catalyst and of γ-Al2O3 measured in situ during temperature programmed hydration/dehydration have been correlated with TG–DSC data, obtained in similar conditions. The results are explained in terms of hydrogen spillover to the support facilitated by the presence of traces of water (protonic conduction by “vehicle” mechanism).

Calorimetric Characterization of Surface Reactivity of Supported Ga2O3 Catalysts

The surface properties of supported gallium oxide catalysts prepared by impregnation of various supports (γ-Al2O3, SiO2, TiO2, ZrO2) were investigated by adsorption microcalorimetry, using ammonia and water as probe molecules. In the case of acidic supports (γ-Al2O3, ZrO2, TiO2), the acidic character of supported gallium catalysts always decreased in comparison with gallium-free supports; on very weakly acidic SiO2, new acidic centers were created when depositing Ga2O3. The addition of gallium oxide decreased the hydrophilic properties of alumina, titania and zirconia, but increased the amount of water adsorbed on silica. The catalytic performances in the selective catalytic reduction of NO by C2H4 in excess oxygenwere in the order Ga/Al2O3>Ga/TiO2>Ga/ZrO2>>Ga/SiO2. This order is more related to the quality of the dispersion of Ga2O3 on the support than to the global acidity of the solids.

Surface dynamics in tin dioxide-containing catalysts. I: Surface dynamics of tin dioxide interaction with propene-containing feed in presence of residual water

Abstract Changes in AC electrical conductance were used to obtain information on the peculiarities of propene interaction with the surface of polycrystalline tin dioxide. As results from in situ electrical conductivity data, adsorption of propene seems to prevail over oxygen adsorption, when both reactants are present. The influence of residual humidity suggests the involvement of surface hydroxyl groups in the mechanism of adsorption/oxidation as a possible source of oxygen. The strong adsorption of propene is facilitated by the low mobility of the bulk lattice oxygen of tin dioxide up to 400°C which in turn determines the surface proton mobility coupled with the ability of this material to catalyze dehydrogenation.

EFFECT OF SOME PARASITE SURFACE REACTIONS ON THE VARIATION OF ELECTRICAL CONDUCTANCE IN THE H2-PT/AL2O3 SYSTEM

The charge transfer at the interface H2-0.5% Pt/Al2O3 was studied by using the transient response of the AC electrical conductance. The transient response for water and oxygen contaminated surface was of the overshoot-type.

Some experimental evidences for hydrogen spillover on Pt/Al2O3 catalysts by electrical conductivity transient response

Abstract Transient method and temperature-programmed desorption (TPD) measurements based on the response of the AC electrical conductance (G) have been used to characterise a H 2 –Pt/Al 2 O 3 system. It was shown that if the surface was insufficiently cleaned and contained traces of oxygen, the transient response H 2 (inc., 0) – G was of the overshoot type. Its characteristics are related with the involvement of some competitive processes controlled by the highly exothermic parasite reaction between hydrogen and oxygen. The “ignition” reaction O 2 –H 2 produces the sharp increase of the amount of the spillover hydrogen, which decreases by back spillover and desorption as much as the“extinction” is in progress. The G-TPD profiles, registered after H 2 (inc., 0) – G transient experiments and in conditions simulating the reaction sequence of the proposed mechanism, provide additional support for this statement.

Surface dynamics in tin dioxide-containing catalysts III. Catalysis and the surface conductivity of antimony-doped tin dioxide

Abstract The variation of surface conductivity of SnO 2 , Sb 2 O 3 and of antimony-doped tin dioxide in air, argon, propylene-air, and propylene-argon mixtures has been followed in situ between room temperature and 400°C. The apparent p-type behavior of some of the antimony-doped samples was explained by fluctuations in the concentration of the surface defects caused by variation of the ratio of coexisting Sb 3+ and Sb 5+ ions. Thermogravimetric data indicate increased lattice mobility on antimony doping. It is suggested that the high electronic conductivity resulting from the presence of antimony in the SnO 2 matrix is related with the increased structural disorder; consequently, the increased rate of the reoxidation step prevents surface contamination with strongly adsorbed hydrocarbon species.

Fractal dimensions of lanthanum ferrite samples by adsorption isotherm method

Abstract Fractal dimensions of three different samples of lanthanum ferrite were computed using single adsorption method. The fractal Dubinin–Radushkevitch isotherm was used to fit directly the experimental nitrogen adsorption data. Avnir–Jaroniec method for fractal dimension determination was also used. Low and intermediate fractal dimensions were obtained according to BET specific surface areas.