Federica Prinetto

Preparation and characterization of SnO2 and MoOx–SnO2 nanosized powders for thick film gas sensors

Abstract This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

In situ FT-IR and reactivity study of NOx storage over Pt-Ba/Al2O3 catalysts

The mechanism of NOx storage on Pt–Ba/Al2O3 and, for comparison, on Ba/Al2O3 catalysts was investigated. In situ FT-IR spectroscopy was used to follow temporal changes in the types and relative amounts of surface species formed at three different temperatures (423, 523, 623 K) during the reaction with NO, NO/O2, NO2 or NO2/O2 atmospheres. At 623 K the NOx storage capability was also investigated by a mass-spectrometry transient response method (TRM) as a complementary technique. The results show that in absence of oxygen appreciable amounts of NO are adsorbed only on the Pt–Ba/Al2O3 system mainly as ionic nitrites. When oxygen is present, NO is stored according to two different mechanisms, both promoted by Pt. The first occurs through the formation of surface nitrites that evolve to nitrates; the second occurs through the NO oxidation to NO2 over Pt, followed by NO2 migration to the Ba phase and nitrate formation. When NO2 is used with or without O2, an extensive storage in the form of ionic nitrates occurs both on Ba/Al2O3 and Pt–Ba/Al2O3 catalysts at all the temperatures studied, without the formation of nitrites. NO2 storage occurs initially through a very fast mechanism, giving nitrates without NO release. In parallel, it proceeds via a disproportionation reaction of NO2 to nitrate and NO. Both mechanisms are not promoted by Pt.

Comparison of the structural and acid–base properties of Ga- and Al-containing layered double hydroxides obtained by microwave irradiation and conventional ageing of synthesis gels

Structural and acid–base properties of Mg/Al and Mg/Ga layered double hydroxides (LDHs) obtained by microwave irradiation of the co-precipitated gels have been investigated and compared to those of samples conventionally aged by prolonged hydrothermal treatment of the gels. Similar crystallinities and chemical compositions were obtained whatever the synthesis method used. Besides, all samples, and remarkably the Ga-containing LDHs with a molar ratio Mg ∶ Ga = 4.5, exhibited pure lamellar phases. The acid–base properties of the mixed oxides obtained by calcination of the LDHs have been examined by microcalorimetric adsorption of CO2 and by FTIR spectroscopy upon CH3CN interaction. These techniques gave evidence that the number and strength of acid and basic sites were influenced by the nature and amount of the trivalent cation, as well as by the preparation method.

Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples

Abstract The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C.

Relevance of the Nitrite Route in the NOx Adsorption Mechanism over Pt–Ba/Al2O3 NOx Storage Reduction Catalysts Investigated by using Operando FTIR Spectroscopy

Nowadays, severe regulations are in force in the industrialized countries to limit the emission of pollutants from the exhausts of passenger cars and trucks. Today, post-treatment catalytic technologies are necessary to meet the current most strict or the forthcoming emission limits. In traditional gasoline-fueled stoichiometric engines, threeway catalysts are used for the abatement of NOx, unburned hydrocarbons, and CO. However, the need to reduce fuel consumption and the corresponding CO2 emissions has led in the past to an impressive spread in the market of lean-burn engines, such as diesel or direct-injection (DI) gasoline engines. These engines operate in the presence of excess oxygen, in which the current three-way catalysts, optimized for exhausts that fluctuate around oxygen-free conditions, do not ensure acceptable emission levels of NOx. The NH3 selective catalytic reduction (SCR) and the NOx storage–reduction (NSR) or lean NOx trap systems are at present the top contenders for reducing NOx concentration in the exhaust from diesel and lean-burn gasoline direct-injection vehicles. Although the NH3-SCR technology, which accomplishes NOx reduction by injecting urea (a precursor of NH3) in the exhaust gases and requires an onboard urea tank, is preferred for heavy-duty vehicles and minivans, lean NOx traps have been specifically developed for small engines. This technology is based on the use of a suitable catalytic material that consists of an alumina carrier on which alkaliand/or alkali-earth metal compounds (e.g. , K and Ba) and noble metals (e.g. , Pt) are deposited. These catalysts operate under cyclic conditions, which alternate long periods in the presence of excess oxygen during which NOx species are stored on the alkaliand/or alkali-earth metal compounds with short rich phases during which the adsorbed NOx species are reduced to nitrogen on Pt. [2] Although the NSR technology is presently being used on a commercial scale, an agreement and understanding of the mechanistic aspects of the storage of NOx species and of their reduction is still lacking. Regarding the storage phase, many authors suggest that NO is at first oxidized by Pt to NO2, which is then stored onto the alkalior alkali-earth metal compound (e.g. , BaO) in the form of nitrates according to the overall reactions in Equations (1) and (2):

Characterization of materials for gas sensors. Surface chemistry of SnO2 and MoOx–SnO2 nano-sized powders and electrical responses of the related thick films

Abstract This work gives further results about the properties of SnO2 nano-sized materials, prepared via a sol–gel route, pure or added with Mo at two different Mo loading (1.8 and 4.7 Mo atoms%). FT-IR spectroscopic and electrical measurements are employed on powders and films, respectively, to obtain information on the electronic effects due to the molybdenum addition. FT-IR spectra in air of the powders show that Mo lowers the concentration of the oxygen vacancies. Accordingly, electrical data show that molybdenum lowers the conductance of the films in air. Electrical measurements show that Mo in amount of 1.8 atoms% lowers the ability to sense NO2 of films fired at 650°C and leaves almost unaltered those of films fired at 850°C. At variance Mo in amount of 4.7 atoms% leaves almost unaltered the ability to sense NO2 of films fired at 650°C and enhances the ability to sense NO2 of films fired at 850°C. The sensing temperature of maximum response for all materials is in any case 150°C. FT-IR spectroscopy is not able to distinguish the different ability of the pure and Mo-added materials to sense NO2, although for some samples an electronic response is evident. At variance the spectroscopic technique has been employed to carefully investigate the nature of surface species formed by NO2/O2 interaction with the three materials and their stability at the working temperature.

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O2 mixtures and NO2 on pure ZrO2 and on two series of catalysts supported on ZrO2, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm−2 and ≤5 atoms nm−2, respectively. All samples had been previously submitted to a standard oxidation treatment.On all samples, only extremely minor amounts of NOx surface species are formed by NO interaction at room temperature (RT). NOx surface species are formed in greater amounts on pure ZrO2 when NO and O2 are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O2NO−H)δ− species; when ZrO2 is warmed to 623 K in the NO/O2 mixture, nitrites decrease, nitrates and (O2NO−H)δ− species increase. The same NOx species as on the ZrO2 surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm−2; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm−2 (or ZMo samples with a surface Mo density of 1.5–5 atoms nm−2) when NO and O2 are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO2 surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO2δ+ species, associated with V5+ (or Mo6+) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O2 mixture, nitrites disappear, nitrates increase, NO2δ+ species remain constant or slightly decrease. When NO2 is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O2 mixture is reached. The NOx surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH3, are nitrates, both on ZrO2 and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO2δ+ species are unstable at 623 K.

Effect of water and ammonia on surface species formed during NOx storage–reduction cycles over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts

The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.

Chemical and electronic characterization of pure SnO2 and Cr-doped SnO2 pellets through their different response to NO

The interaction of NO at room temperature (RT) with pure and Cr-doped SnO 2 self-supporting pellets, pretreated in dry oxygen at 673-873 K, has been studied by FT-IR spectroscopy. With both pure and doped pellets treated at 673 K, NO at pressures ≤10 N m -2 causes a reaction (NO⇇NO + +e - ) that is responsible for the increase of a broad electronic absorption, which is irreversible to evacuation at RT, but no peaks due to surface species appear. For pressures >10 N m -2 , peaks due to nitrites, nitrosyls and N 2 O appear, without further increase of the electronic absorption, which actually becomes completely reversible at RT, while surface nitrites and nitrosyls are irreversible under evacuation. The formation of these species is responsible for the shift of the equilibrium involving electrons. The electronic absorption is due to the overlap of two bands : one at 0.1-0.18 eV, assigned to the electron transition from monoionized oxygen vacancies to the conduction band ; the other, at 0.45-0.6 eV, assigned to a similar electronic transition involving oxygen divacancies. Thermal treatment at 873 K affects the electronic response of both the pellets. On Cr-doped pellets the electronic response is completely killed. SnO 2 pellets show the behaviour described for pretreatment at 673 K, except for the shape of the electronic absorption : the relative intensities of the two overlapping bands at 0.1-0.18 and 0.45-0.6 eV bands change in favour of the latter.

INFRARED CHARACTERIZATION OF MOOX/ZRO2 CATALYSTS : THE REDUCIBILITY OF MO(VI) SPECIES BY THERMAL TREATMENT IN H2

Abstract MoOx/ZrO2 samples prepared by different methods, were heated in O2 and subsequently reduced in H2 at increasing temperature. On oxidized samples with Mo-content up to 5 atoms nm−2, prepared by adsorption or impregnation, low and high nuclearity MoVI surface species were present, in relative amounts depending only on the Mo-content. On oxidized samples with Mo-content >5 atoms nm−2 prepared by adsorption or impregnation and on all samples prepared by mechanical mixture, MoO3 and ZrMo2O8 were also revealed. FT-IR results showed that the sample behaviour under the reduction was controlled mainly by the nuclearity of the molybdenum species. From the analysis of the IR ν(MoVI,V,IVO) modes we could evaluate the nature and the relative amounts of the different Mo species present at each reduction stage. A fraction of low nuclearity MoVI species was reduced to MoV, which we were unable to reduce further. In contrast, high nuclearity MoVI species gave MoIV after reduction at 623 K, and further reduced to Mo0 at 773 K. At 773 K, MoO3 and ZrMo2O8 were almost completely reduced to Mo0. To further define the nature of the Mo species formed by reduction, CO and NO were used as probe molecules. MoV was not capable of coordinating CO and it was extensively re-oxidized by NO, whereas MoIV gave dinitrosyl and polycarbonyl species upon NO and CO adsorption, respectively.