Monica Caldararu

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.

The structure properties correlation in the Ce-doped SnO2 materials obtained by different synthesis routes

Abstract Ce-doped SnO 2 materials with rutile type structure ( SnO 2 SS ) were obtained as porous ceramic, powders and films from different precursor mixtures and by using different preparation routes. The samples were characterized in terms of their structural, morphological and electrical properties. Depending on the specific precursor mixture, different microstructural parameters were obtained. The lattice parameters of the host oxide (SnO 2 ) decrease by CeO 2 doping and the mean crystallite size D varies in the range of 670–1278 A; different values of lattice strain S were also noticed. The morphological characteristics as shrinkage (Δ l / l ), porosity ( Pa ), density ( d ) and BET surface area were determined. The electrical properties were measured on the porous ceramics and on the powders. All investigated samples showed n-type semiconducting behavior.

Redox processes in Sb-containing mixed oxides used in oxidation catalysis: I. Tin dioxide assisted antimony oxidation in solid state

Abstract The stability of the oxidation state of antimony ion in SnSbO catalysts under the influence of operating conditions was investigated by Mossbauer spectroscopy. It was shown that the presence of the tin dioxide matrix facilitates the oxidation of the antimony ion; at the same time, the Sb5+ ions in the SnO2 rutile matrix show much higher stability under slightly reducing conditions provided by the catalytic test of propylene oxidation.

Effect of frequency on the electrical characteristics of tin-antimony-oxide mixtures

The effect of frequency (100 Hz to 1 MHz) both on complex permittivity and on complex impedance data was used to study the conduction processes in Sn-Sb-O systems between 300 and 873 K. The conductance of tin oxide was increased by the addition of antimony oxide at all frequencies and a peak in conductivity was observed at 5 wt% Sb2O3. The effect of frequency on conductance was in agreement with the universal behaviour of solid dielectrics, whilst the atmosphere-dependent capacitance indicated surface dipole phenomena. The catalytic activity and selectivity patterns for the oxidation of propylene above 650 K may be correlated with the observed electrical behaviour, which is related to the mobility of lattice oxygen in the outer layers of the surface.

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).

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.

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.

Influence of moisture on transient oxidation and reduction of alumina-supported platinum

The transient surface behavior on oxidation and reduction of a partially oxidized alumina-supported platinum catalyst containing residual water have been observed by AC electrical conductance (G) measurements.