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The effect of CO 2 and H 2 O on the NO/O 2 storage over a Pt-Ba/Al 2 O 3 LNT catalyst was investigated in the temperature range 150–350 °C by combining FT-IR spectroscopy and microreactor flow experiments. It is found that the presence of water shows negligible effect on the NO x storage at all the temperatures studied. Otherwise, the presence of CO 2 inhibits the “nitrite route” (initial formation of surface nitrites and their subsequent evolution to nitrates), whereas the “nitrate route” (involving NO oxidation to NO 2 over Pt and its subsequent adsorption on Ba in the form of nitrates) proceeds as in the absence of CO 2 . The inhibition effect of CO 2 on nitrite formation increases on increasing the temperature. This behavior is likely related to the different thermal stability of nitrites and carbonates and, in particular, to the decreasing ability of nitrites to displace carbonates on increasing temperature. Simultaneously, the presence of CO 2 in the feed significantly shortens or eliminates the dead time in the NO x breakthrough at the reactor outlet. As a consequence, the presence of dead time in the absence of CO 2 has been correlated to the occurrence of the nitrite route.

Supported Ni catalysts (2 wt.% Ni) were investigated by FT-IR and UV–vis-NIR spectroscopy, using CO2, CH3CN and CO as probe molecules. The supports studied range from acidic (SiO2), via amphoteric (Al2O3, Mg(Al)O) to basic oxides (MgO, CaO). CO2 adsorption experiments allowed to obtain the following qualitative scale for the basic strength of O2− sites and Mn+O2− pairs: Ni/CaO > Ni/MgO ≥ Ni/Mg(Al)O ≫ Ni/Al2O3. On Ni/SiO2 these sites are absent. CH3CN adsorption allowed to reveal acidic and strong basic Lewis sites with the following results: Ni/SiO2 does not contain basic or acidic Lewis sites. Ni/Al2O3 contains acidic Lewis sites but no strong basic Lewis sites. The ratio between the strong basic and the acidic Lewis sites decreases in the order: Ni/MgO > Ni/CaO > Ni/Mg(Al)O. The strength of Lewis acid sites decreases in the order Ni/Al2O3 ≫ Ni/Mg(Al)O > Ni/MgO > Ni/CaO. Interaction of CO at room temperature results into formation of Ni(CO)4 and/or Ni(CO)y

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In this paper the reactivity of NOx stored at 350 °C onto Pt–Ba/Al2O3 LNT catalyst in the reduction by H2, CO and CO/H2O mixture is investigated by means of transient experiments and complementary FT-IR analyses. Hydrogen is found to be the best reductant, being characterized by the highest NOx removal efficiency in the range 150–350 °C. It is proposed that the reduction by hydrogen proceeds according to a dual-steps mechanism in which NOx are first reduced to ammonia, which in turn further reacts with other nitrate species leading selectively to N2. Conversely, when CO is used as reducing agent under dry conditions only a part of the initially stored NOx is removed as N2, the other fraction being reduced to adsorbed cyanates/isocyanates species. It is proposed that the cyanate/isocyanate ad-species lead to the formation of nitrogen by oxidation involving other nitrate species; this reaction is self-poisoned by CO. In the presence of water, CO reduced nitrates into cyanates/isocyanates ad-species that were readily hydrolyzed to ammonia; this species is hence precursor in N2 formation upon reaction with residual nitrate species. Alternatively, stored nitrates are reduced by H2 formed via the water gas shift (WGS) reaction.

Mo{sub 1-x}W{sub x}O{sub 3} oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO{sub 3} and WO{sub 3} were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo{sub 1-x}W{sub x}O{sub 3} in oxidizing atmosphere to Mo{sub 1-x}W{sub x}O{sub 3-{delta}} in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperaturemore » are peculiar of each mixed oxide. - Graphical abstract: The combined use of different techniques provides new insight into structural, optical and electronic properties of semiconductor Mo{sub 1-x}W{sub x}O{sub 3-{delta}} mixed oxides.« less

In this work we report the synthesis, microstructure, electric and spectroscopic properties, and sensing performances of Ti x Sn 1 − x O 2 ( x = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) nano-powders and of SnO 2 and TiO 2 reference samples, prepared via sol-gel route starting from metal-organic precursors working in hydro-alcoholic media. Actually, the attention is particularly focused on properties of the sample with x = 0.2, in comparison with ones of the other solid solutions and of the single oxides. Indeed, this solid solution showed a borderline behaviour between that of the solid solution with x = 0.1 and that of the other solid solutions with x ≥ 0.3. An abrupt change in the structural, electrical and spectroscopic properties has been observed, passing from sample with x = 0.1, showing a behaviour very similar to that of SnO 2 , to one x = 0.3 showing a behaviour very similar to that of TiO 2 . The borderline properties of the mixed oxide with x = 0.2 represent the expected continuous transition among the two behaviours.

The reduction by CO of NO x species stored over Pt–Ba/Al2O3 Lean NO x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N2 formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.

Tin–titanium solid solutions at increasing Ti molar fraction, x = 0.1, 0.3, 0.5, 0.7 and 0.9, were prepared by symplectic gel co-precipitation (SGC) method from stoichiometric hydroalcoholic solutions of Ti and Sn metalalkoxides. Structural, spectroscopic and electrical analyses were performed and the results have been correlated. The peaks of the XRD patterns change from the positions of SnO 2 cassiterite to those of TiO 2 rutile, when increasing the Ti content in the solid solution. Photoluminescence (PL), FT-IR and UV–vis-NIR characterizations are in agreement with the formation of solid solutions: PL peak falls either in the visible or IR region, depending on the Ti content in the solid solution; FT-IR spectra reveal a progressive upward shift of the high-frequency absorption edge of the skeletal vibration modes from the value of SnO 2 to one of TiO 2 on increasing the Ti content; UV–vis-NIR spectra reveal a progressive downward shift of the VB–CB absorption edge on increasing Ti content. The changes induced by CO interaction at increasing temperature in the FT-IR and UV–vis-NIR spectra revealed a different behaviour of the materials following the Ti content: for the lowest Ti molar fraction ( x = 0.1) as well as for SnO 2 the reduction gives rise to mono-ionized oxygen vacancies, for the other solid solution the loss of reticular oxygen gives rise to doubly ionized oxygen vacancies and to Ti 3+ ions. From the electrical-conductance point of view, two groups of films can be identified: pure SnO 2 and the solid solution with x = 0.1, and all the other solutions including pure titania, in agreement with the results of FT-IR investigations. The best response to carbon monoxide was achieved by the solutions with x = 0.3 and x = 0.5. Opposite to electrical properties, exhibiting good responses to reducing gases, these agents did not appreciably modify the PL yield of the solid solutions. Conversely, PL emissions were sensitive to NO 2 , particularly for Ti 0.5 Sn 0.5 O 2 .

Three types of multifunctional supported Pd catalysts (0.2 wt.% Pd) have been obtained from layered double hydroxides (LDH) precursors by: (i) impregnation with Pd(acac) 2 of a Mg(Al)O support, (ii) coprecipitation of a multicationic Pd/Mg/Al LDH, and (iii) synthesis of a Pd colloid Mg/Al LDH nanocomposite. The precursors have then been calcined at 773 K and reduced at 523 K to obtain the catalysts, which exhibit very different basic properties and Pd particle size distributions. Their potential was evaluated in the one-pot synthesis of 2-methyl-3-phenyl-propanal (MPPAL) from benzaldehyde and propanal. The full one-pot synthesis of MPPAL was performed sequentially operating in two stages, i.e. for 20 h in N 2 , then for 7 h in H 2 atmospheres. The activities of the catalysts in the first stage of the reaction (condensation and dehydration steps) are related to their reconstruction ability in the propanol/water solvent to give Bronsted basic sites. The hydrogenation ability of the catalysts decreases when the average Pd 0 particle size decreases. The specific behavior of the nanosized particles accounts for a decrease of the adsorption strength of C C bond due to the enhancement of the metal–support interaction. The catalyst prepared from a Pd nanocomposite precursor ensures the best balance between the different functions. It leads to a MPPAL selectivity of 77% at 64% benzaldehyde conversion. Benzyl alcohol formed by hydrogenation of benzaldehyde on Pd and hydrogen transfer from propanol is the main secondary product. However, the process is negatively affected by the high consumption of propanal from self-condensation.

This work gives results about the characterisation of MoO3–WO3 binary systems in powder or deposited in thin films on alumina and silicon substrates. Morphological and structural characterisation was carried out by XRD and SEM techniques, the powder texture by volumetric measurements. Absorbance FT-IR and diffuse reflectance UV–vis–NIR spectroscopies were employed to study the influence of different atmospheres on the electronic properties of the materials. Spectra were recorded after treatments in O2, in vacuum, in NO2/O2 and in CO/O2 at increasing temperature. The spectra of all the samples show, both in the vis–NIR and medium IR regions, the increase of a variety of broad absorptions after reducing atmospheres and the decrease of these absorptions after oxidising ones, so that they are all assignable to electronic transitions related to oxygen defects. Some electrical measurements on thin film supported on alumina are also presented. The electrical response toward CO at increasing temperature can be correlated with the intensity changes in the vis–NIR absorptions observed after interaction with CO/O2 at increasing temperature.

The influence of the nature of the Zn, Al and Pt precursors, and of the temperature of precipitation and aging have been studied in connection with the preparation of Zn/Al and Pt/Zn/Al layered double hydroxides (LDH) by the sol–gel method. Whatever the precursors the XRD analysis shows that LDH is formed at the expense of ZnO when the precipitation and aging temperatures decrease from 353 K to 273 K. Moreover, chemical composition and TG analysis suggest the presence of weak amounts of hydrozincite, hydrozincite-like and Al(OH)3 phases. When the precursors are Zn acetate-2-hydrate or Al acetylacetonate the amount of LDH reaches a maximum of 50 mol% at 273 K. At variance, about 90 mol% of LDH is obtained when using Zn acetylacetonate and Al isopropoxide as precursors, which are precipitated at 273 K. This proportion is slightly improved for the Pt-containing sample prepared under the same conditions. The specific surface areas of the different samples obtained after calcination at 723 K increase with their LDH content, reaching values of 110–120 m2 g−1. They make them particularly attractive for catalytic applications.

A series of Pt and Pt,Cu supported catalysts were prepared by wet impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds. These novel NO x storage-reduction (NO x SR) catalysts show improved performances in NO x storage than Pt,Ba/alumina NO x SR catalysts at reaction temperatures lower than 200 °C. These catalysts show also improved resistance to deactivation by SO 2 . The effect is attributed to the formation of well dispersed Mg(Al)O particles which show good NO x storage properties. The promoted low temperature activity is explained by the lower basicity of the Mg(Al)O mixed oxide in comparison to BaO, which induces on one hand a lower inhibition on Pt activity (NO to NO 2 oxidation and/or hydrocarbon oxidation) due to electronic effect, and on the other hand a lower thermal stability of the stored NO x . The presence of Cu slightly inhibits activity at low temperature, although improves activity and resistance to deactivation at 300 °C. On these catalysts FT-IR characterization evidences the formation of a Pt–Cu alloy after reduction.

Metal-oxide semiconductors are widely used as functional materials for gas sensing because of their chemoresistive effect when interacting with ambient gases. For this work, nanosized Ti x Sn 1− x O 2 have been synthesized at increasing Ti molar ratio ( x = 0, 0.1, 0.2, 0.25, 0.3, 0.5, 0.7, 0.8, 0.9, 0.95, 1) and extensively investigated through a wide variety of characterizations. It turned out that the merging process of the two single oxides formed new compounds with improved gas responses compared to pure TiO 2 and also to pure SnO 2 . Moreover, all investigated physico-chemical characteristics resulted tuneable through the titanium content in the solid solution. Each characterization carried out onto Ti x Sn 1− x O 2 solid solutions made it possible to identify two classes of materials with SnO 2 -like or TiO 2 -like behaviours, the best CO response being on the intersection of the two species of materials. Moreover, a very interesting analysis on the generally accepted electrical transport model through polycrystalline semiconductors has been carried out. Indeed, the comparison with experimental evidences has highlighted a conduction mechanism not previously considered.

Well-crystallized layered double hydroxides (LDH) containing Mg2+ or Ni2+ as divalent cations and Al3+ as trivalent cations were obtained either by coprecipitation (cp) or with the sol−gel (sg) method. After calcination, mixed oxide structures were achieved, as shown by XRD. Their acid−base properties were investigated through CO2 and NH3 TPD experiments and FT-IR spectroscopy, using CO2 and NH3 as probe molecules. Basic and acidic sites of medium-high strength were simultaneously revealed: the basic sites were mainly O2- atoms and Mn+−O2- pairs, while the Lewis acid sites were provided mainly by Al3+ in Mg-containing materials and by Al3+ and Ni2+ in Ni-containing materials. Several parameters, such as the nature of cations and anions involved in the LDH structure, the M2+/M3+ ratio, the synthesis method, and the activation conditions, influenced the acid−base properties of the mixed oxides. In particular, (i) Mg-containing samples showed higher basicity than Ni-containing samples, (ii) sg materials wer...

Mg/Al and Ni/Al layered double hydroxides (LDHs) were prepared by a sol–gel (sg) route performing hydrolysis of alkoxide or acetylacetonate precursors with HCl or HNO 3 . For comparison, other samples were prepared from nitrate salts by the classical co-precipitation (cp) method at constant pH. Pure sg Mg/Al LDHs with a Mg 2+ /Al 3+ ratio in the range 2.47–4.29 were obtained, as shown by XRD. Almost pure sg Ni/Al LDHs resulted when using HCl for the precursor hydrolysis, while unsuccessful trials were done with HNO 3 . The mixed oxides, obtained by calcination of sg materials, exhibited alcogel like texture and specific surface areas, at least 10% higher than those of the cp ones, as evidenced by N 2 volumetric measurements. Furthermore, TEM analysis revealed different morphologic features of the sg and co-precipitated materials, both as-prepared and calcined. TG-DSC analysis and IR spectroscopy evidenced the presence of organic species remaining as compensating anions in the interlayer space, inducing a rather different decomposition process for the sg than for the co-precipitated samples. IR spectroscopy also indicated the higher basicity of the Mg-compared to the Ni-containing LDHs. Furthermore, it evidenced that Al 3+ was more easily segregated from Ni(Al)O than from Mg(Al)O mixed oxides.

Chemical routes such as sol–gel techniques and thermal decomposition of heteronuclear complexes have been used to prepare nanosized powders of n-type (SnO2, TiO2 and In2O3) and p-type (LaFeO3) semiconducting oxides. Thick film gas sensors have been fabricated using these powders, printed on laser pre-cut 96% alumina substrates, each 2×2 mm element being provided with a heater, Au interdigitated contacts and a Pt-100 resistor for controlling the operating temperature. The films have been fired at different temperatures suitable to keep the grain size at the nanometer level. Electrical responses to some major polluting gases (CO, NO, NO2 and O3) obtained in the field have been compared with international standard analytical techniques.

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.

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.

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

VOx/ZrO2 samples were prepared by three methods: (i) adsorption from aqueous solutions of ammonium metavanadate (AV) at pH 1 or 4; (ii) dry impregnation with AV aqueous solutions at pH 4; and (iii...

The responses of semiconductor gas sensors to the reducing gases are affected by the changes in charge carrier density arising from addition of dopants or catalysts. The aim of our work has been to obtain a better understanding of the influence of Pd on the electrical and electronic properties of thick SnO2 films. We investigated the responses of SnO2 and Pd/SnO2 films to CO in dry and wet (50% RH) air at temperatures increasing from 250°C up to 450°C. The samples were obtained from commercial and laboratory powders. FT-IR spectroscopy has been employed here to provide information on the changes of the electronic properties of the differently prepared samples, and the results have been compared to d.c. electrical measurements. Films prepared with pure tin oxide, both from commercial and laboratory powders, show a maximum of electrical response at 350°C, and show lower responses in wet air than in dry air at temperature lower than 400°C, while the responses is higher in wet air at 400–450°C. Pd always decreases the temperature of the maximum response to CO, both in dry and wet air. Pd addition decreases the response to CO of the films prepared from commercial powders at temperature ≥300°C, both in dry and wet air, while at 250°C there is a moderate increase of the response. In the case of films prepared from laboratory powder, Pd addition does not show a clear trend. Some comparisons with the responses to CH4 analyzed in a previous paper are also reported.

Catalysts at different Cr loading ( x , in the range 0.18–3.8 wt% Cr) were prepared by (i) impregnation of SiO 2 Aerosil directly with the waste water (SC x (I) samples); or (ii) mixing Aerosil with chromium oxo-hydroxides, precipitated from the waste water to eliminate the sulphate and sodium ions (SC x (II) and SC x (III) samples). Preparations (II) and (III) only differ for the accuracy in sulphate and sodium ions elimination. Catalyst surface characterization was performed by FT-IR spectroscopy using CO, adsorbed at room temperature, as test molecule, and by UV-Vis-NIR DR spectroscopy. Their activity towards ethylene polymerization were tested by FT-IR spectroscopy, monitoring the time dependence of the CH 2 stretching modes intensity. The obtained results were compared with those obtained for simplified Phillips catalysts with comparable chromium contents, SC x (R). SC x (I) catalysts were not active in the ethylene polymerization, while the SC x (II) and SC x (III) ones showed lower activity (1/10 at maximum) compared to that of simplified Phillips catalysts with comparable chromium contents. SC x (II) and SC x (III) catalysts were active in the isobutane dehydrogenation reaction and showed activity in the range of those of standard catalysts, while SC x (I) ones showed activity at 873 K, but very low at the usual temperatures (773–823 K).

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.

The temporal trends of ten elements, namely Cl, Cu, Cr, Fe, Mg, Ni, P, S, Zn and Zr, in airborne particulate matter sampled in the urban area of Turin (Piedmont, Italy) were determined by wavelength dispersive X-ray fluorescence for the period from 2002 to 2005. The average daily concentrations were as follows: Cl 322 ng/m 3 ; Cr 16.4 ng/m 3 ; Cu 8.19 ng/m 3 ; Fe 274 ng/m 3 ; Mg 1501 ng/m 3 ; Ni 7.33 ng/m 3 ; P 19.8 ng/m 3 ; S 1206 ng/m 3 ; Zn 333 ng/m 3 and Zr 8.15 ng/m 3 . Some elements, namely Al, Ba, Ca, K, Na, Si and Sr, were not quantifiable using XRF because of the too high contribution from the filter blank. A clear seasonal pattern with higher concentrations of most of the elements in the cold periods was observed. Moreover, no marked change in the element distribution in atmospheric particulate matter over 4 years was evidenced during the sampling period, to indicate that similar sources have influenced the elemental composition of it in these last years. The application of multivariate chemometric techniques (principal component analysis and hierarchical cluster analysis) to the experimental results allowed us to identify correlations among the investigated elements and to reveal similarities and differences between sampling sites.

The reduction of NO x stored over a Pt–Ba/Al2O3 Lean NO x Trap is analysed when H2, CO or heptane are used as reductants. In all cases, the reduction proceeds via a Pt-catalyzed process involving the formation of intermediate species like ammonia and isocyanates in the case of H2 and CO, respectively. No specific intermediates have been observed when heptane is used as reductant. It is claimed that the role of the reductant is to keep Pt in a reduced state; this favours nitrate decomposition and reduction over the Pt sites. The effect of water on the reaction is also investigated.

Pure SnO2 and Pd/SnO2 (0.4 wt.%) pastes have been prepared starting from SnO2 powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450°C. By alternating wet and dry air, it has been observed that the conductance of pure SnO2 samples changes in a reversible way only for temperatures over approximately 200°C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnO2 samples treated in wet air at temperatures above 200°C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5−0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200°C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick films.

Supported nickel catalysts were obtained by exchange of Mg/Al Layered Double Hydroxides (LDHs) compensated with NO 3 - ions with negatively charged Ni complexes, followed by thermal reduction. With this aim, suspensions of Ni complexes were prepared by controlled hydroxylation of Ni2+ cations in the presence of citrate (obtaining [ Ni ( C 6 H 5 O 7 ) ( OH ) ] y 2 y - species) or chloride (obtaining [NiCl4]2− species) complexing ions. For comparative purposes, other two supported Ni catalysts were prepared starting from Mg/Al LDHs compensated with Cl− ions and thereafter exchanged with [ Ni ( C 6 H 5 O 7 ) ( OH ) ] y 2 y - species (nominal degree of exchange: 100% and 20%). XRD and HRTEM techniques were employed for studying nickel phases and dispersion. CH3CN adsorption at RT followed by FT-IR spectroscopy gave information on surface acid–base properties. CO adsorption at 77 K followed by FT-IR spectroscopy was employed to deepen the characterization of the metal phase. On the basis of the results obtained, it is possible to evidence a correlation between the metal phase morphologies and the surface acid–base properties of the reduced catalysts. High surface basicity revealed by the presence of strongly basic O2− sites stabilizes small nickel particles ( 11 Ni wt.%), whereas, when the basicity is suppressed by Cl− ions, a dramatic Ni sintering occurs even at low metal loading (in the range 3–10 Ni wt.%).

The catalytic activity of CrOx/ZrO2 (Cr content, 0.26 to 1.80 atoms nm−2) for the reduction of nitric oxide with hydrogen was investigated in a flow apparatus at 473 to 773 K and in a circulation apparatus at 453 to 593 K. Before catalysis, the average oxidation state of Cr, n, was controlled by redox treatments with oxygen at 773 K (n=5.5) or hydrogen at 623 K (n=2.5). No ammonia was detected at any temperature, N2, N2O and H2O being the only reaction products. The catalytic activity of all samples was stable with time-on-stream. The selectivity to nitrogen increased with a rise in reaction temperature (90% at 773 K in flow experiments). The activity of the ZrO2 support is one to two orders of magnitude lower than that of CrOx/ZrO2 (ZC) samples. In ZC samples, the activity was proportional to the chromium content. At a given temperature, irrespective of the chromium content, turnover frequencies, calculated with respect to the total chromium (NCr/molecules s−1 atom−1) have almost the same value. This suggests that the active sites are mononuclear chromium species. The surface species formed on nitric oxide adsorption or in the presence of NO+H2 reactants were characterised by Fourier transform IR spectroscopy. The chromyl species (CrV, mononuclear on ESR evidence) and chromates were detected in the oxidised ZC samples. In the presence of NO or NO+H2, nitrosyls of CrII and CrIII, nitrites, nitrates and water were identified. On the basis of the thermal stability and reactivity with hydrogen of CrIII dinitrosyls, these species may well play an important role in the NO+H2 reaction.

MoO x /ZrO 2 samples (Mo content, 0.7 to 4.0 atoms nm -2 ) have been investigated by XPS and Fourier transform Infrared (FTIR) spectroscopies. After heating in O 2 (773 K), XPS shows the presence of Mo VI species; while the IR analysis shows bands in the 830-1012 cm -1 region, arising from Mo VI species of various nuclearity. In the early stages of the reduction (H 2 at 500 K) both XPS and IR show the formation of Mo V . In agreement with previous ESR results, the intensity of IR and XPS bands from Mo V does not appreciably change by further increasing the extent of reduction up to 803 K

The NO x storage-reduction over the single Pt–Ba/Al 2 O 3 and Fe-ZSM-5 systems and over the hybrid Pt–Ba/Al 2 O 3 + Fe-ZSM-5 systems, both physically mixed and in dual bed configuration with the SCR catalyst placed downstream of the LNT catalyst, is systematically investigated by Isothermal Step Concentration (ISC) experiments and FTIR spectroscopy. The study is accomplished using H 2 as the reductant, in the presence and absence of CO 2 and H 2 O and in a wide temperature interval (150–350 °C). The results are explained at the quantitative level by applying a scheme of the reduction of stored NO x , which implies at first the reduction of nitrates to give NH 3 , followed by the reaction of ammonia with NO x stored downstream in the reactor to give N 2 . The following main points have been clarified: (1) the storage of NO x occurs primarily onto Pt–Ba/Al 2 O 3 and is lower in the presence of CO 2 and H 2 O because BaCO 3 is formed which retards NO x adsorption; (2) NH 3 formed as intermediate in the reduction of stored NO x is trapped onto the Fe-ZSM-5 catalyst. The trapping of NH 3 is most favoured when the particles of Pt–Ba/Al 2 O 3 and of Fe-ZSM-5 are extensively in contact, as in the LNT/SCR physical mixture, and at low temperature. NH 3 trapped onto Fe-ZSM-5 reacts during the subsequent lean phase with gaseous NO x slipped from the Pt–Ba/Al 2 O 3 particles to give N 2 via SCR; (3) the NO x removal efficiency is always higher for both LNT/SCR dual bed and physical mixture compared to single LNT, due to the additional contribution of SCR over Fe-ZSM-5 during the lean phase; (4) in the case of the LNT/SCR physical mixture, NH 3 trapped onto the Fe-ZSM-5 particles can be oxidised over the LNT particles upon admission of oxygen to give N 2 , N 2 O and NO. The selectivity to nitrogen is almost complete at any temperature in the presence of CO 2 and H 2 O, while in the absence of CO 2 and H 2 O, it increases significantly with temperature and is almost complete at 350 °C; (5) the ammonia slip is low to the best over the LNT/SCR dual bed at any temperature in the absence of CO 2 + H 2 O. In the presence of CO 2 and H 2 O, the ammonia slip is nil at any temperature over the LNT/SCR dual bed.

In this paper the results obtained in our previous studies on the reduction of nitrates by CO over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts are compared and discussed in order to define the chemistry that leads to nitrogen evolution during the regeneration of the different lean NOx traps. The reaction mechanisms and pathways leading to N2 formation under dry conditions are investigated by complementary transient dynamic experiments and FT-IR analyses. In both cases, the reduction of stored NOx by CO occurs according to a Pt catalyzed surface pathway that does not involve the release of NOx in the gas phase as a first step, is effective already at low temperature and leads to nitrogen. The proposed reaction scheme implies the formation of surface isocyanate species followed by their reaction with residual stored NOx to give nitrogen. This is the exclusive route for nitrogen formation.

The reduction by CO under dry conditions of NOx species stored at 350 °C onto a Pt−K/Al2O3 lean NOx trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FT-IR spectroscopy. The results show that the pathway for the reduction of stored NOx by CO under dry and near isothermal conditions (and in the absence of CO2) is the same as that proposed by some of us in a previous work for a Pt−Ba/Al2O3 catalyst. In particular, (i) the reduction of stored NOx by CO occurs according to a Pt-catalyzed surface pathway that does not involve, as a first step, the release of NOx in the gas phase, is effective already at low temperature, and leads to nitrogen; (ii) the reaction scheme implies the formation of surface isocyanate species, followed by the reaction of these species with residual NOx to give nitrogen; and (iii) the reaction of NCO species with nitrates to give nitrogen is slightly slower than the reduction of nitrates to give NCO species; but on t...

A FT-IR spectroscopic study has been made on two ZnO powders differently prepared: one obtained by thermal decomposition of ZnCO3 (ex-C), the other by zinc metal combustion (Kadox). The morphological differences between the samples have been proposed to be at the origin of the different reactivity and electronic phenomena. In particular, on the more perfect, lower surface area Kadox microcrystals, H2 reacts at RT to give hydroxyls and hydrides mainly reversibly adsorbed (type I): the hydrogen irreversibly adsorbed (type II) is mainly due to an IR inactive species formed by a reductive mechanism releasing electrons to the conduction band. On smaller, less defined, higher surface area ex-carbonate microcrystals type I adsorption occurs in smaller amounts ( 1 3 of the amount on Kadox), type II adsorption is mainly due to the formation of hydroxyls and hydrides, paralleled by release of electrons trapped into defective shallow levels localized in the gap.

The importance of the metal-support interaction on the catalytic properties of an oxide-supported metal has been shown by a great number of experimental data. Structural, chemical and electronic interactions are all very important to realize the catalyst configuration responsible for its high reactivity and/or selectivity. Furthermore the interactions can become more important during the catalytic process itself, and new surface characteristics can be developed. In this paper we examine, with selected examples, the different aspects of the interaction, how they are correlated and how they can be evidenced by different techniques and more specifically by IR or FT-IR spectroscopy.

The reduction by CO under dry condition of NO x species stored at 350 °C onto a Pt Ba/Al 2 O 3 Lean NO x Trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FTIR spectroscopy. It is shown that the reduction by CO under nearly isothermal conditions of nitrates stored onto Pt Ba/Al 2 O 3 occurs through a Pt-catalyzed pathway which leads mainly to nitrogen and does not proceeds via the thermal decomposition of stored nitrates with release of NO x in the gas phase. The CO-TPSR and CO-ISC experiments and the combined FTIR study show that (i) the reduction of nitrates by CO occurs via a consecutive reaction scheme with formation of nitrites and then of surface isocyanate/cyanate species followed by the reaction of these species with residual nitrites to give nitrogen; (ii) the reaction of NCO species with nitrites to give nitrogen is slower than the reduction of nitrates to give nitrites and then NCO species; (iii) upon contact with oxygen at increasing temperature, NCO species can be re-oxidized to surface nitrites at first and then to surface nitrates; (iv) the formation of nitrogen is observed during the reduction of stored NO x by CO, and during the oxidation of surface NCO species upon oxygen addition and upon admission of NO in the presence of excess O 2 . In all cases, the formation of nitrogen can be explained by the reaction between surface NCO species and nitrites. The mechanism for the reduction of stored NO x by CO under dry and near isothermal conditions is discussed.

The introduction of lean-burn engine technology has prompted the development of NOx storage-reduction (NSR or Lean NOx Traps, LNT) catalysts, which are currently based on a PtBa/Al2O3 system. Potassium is another element that has shown potentials as a storage component. This work explores the role of K during the NOx storage phase and the subsequent reduction using transient activity data and FT-IR analyses over a PtK/Al2O3 powdered catalyst. The catalytic behaviour of a PtK/Al2O3 system has been compared with that of a PtBa/Al2O3 catalyst. NOx storage experiments indicate similar storage efficiency in the two cases. The storage pathways are similar on both the storage components: nitrites and nitrates are formed, the first ones only at the beginning of the storage, quickly transformed in nitrates. The reduction of NOx stored over both systems is also analyzed when H2 is used as reductant. An in series 2-steps process is herein reported, involving at first the formation of NH3 upon reaction of nitrates with H2 (step 1), followed by the reaction of NH3 with residual nitrates to give N2 (step 2). The influence of the storage component on the Pt sites modifies the reactivity of H2 towards stored nitrates.

The nature of stored NOx and mechanistic aspects of the reduction of NOx stored over a model Pt K/Al2O3 catalyst sample are investigated in this paper, and a comparison with a model Pt Ba/Al2O3 catalyst is also made. It is found that at 350 °C on both the catalysts the storage proceeds with the initial formation of nitrites, followed by the oxidation of nitrites to nitrates. A parallel pathway involving the direct formation of nitrates species is also apparent; at saturation, only nitrates are present on the catalyst surface over both Pt K/Al2O3 and Pt Ba/Al2O3. However, whereas bidentate nitrates are present in remarkable amounts on Pt K/Al2O3, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on Pt Ba/Al2O3. Under nearly isothermal conditions, the reduction of the stored NOx with H2 occurs via an in series two-steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the Pt Ba/Al2O3 catalyst sample. However, higher N2 selectivity is observed in the case of the Pt K/Al2O3 catalyst due to the similar reactivity of the H2 + nitrate and NH3 + nitrate reactions. Accordingly ammonia, once formed, readily reacts with surface nitrates to give N2, and this drives the selectivity of the reduction process to N2. Notably, a strong inhibition of H2 on the reactivity of NH3 towards nitrates is also pointed out, due to a competition of H2 and NH3 for the activation at the Pt sites. Finally, the effect of water and CO2 on the reduction process is also addressed. Water shows a slight promotion effect on the reduction of the nitrates by H2, and no significant effect on the reduction by ammonia, whereas CO2 has a strong inhibition effect due to poisoning of Pt by CO formed upon CO2 hydrogenation.

The catalytic activity of CrOx/ZrO2 for propene hydrogenation has been investigated at 195 K in a flow apparatus. Samples containing 0.05 to 0.8% chromium by weight (0.1 to 1.9 Cr atoms nm−2) were prepared by the equilibrium adsorption method. After heating in O2 at 773 K (average oxidation number of Cr, n=5.5), samples were reduced to a controlled extent with CO at 393 to 623 K. The activity at 195 K of partially reduced samples (n=4.4) is low, but it increases by nearly two orders of magnitude when n is further decreased by 0.2 units. Thereafter the activity changes only slightly with increasing reduction (down to n=2.5). The concentration of the Crv species observed by ESR after heating the samples with O2 was monitored after each reduction treatment, and it was found to decrease by 17% in the same reduction region in which the catalytic activity increases sharply. The average turnover frequencies per total chromium (NCr/molecules s−1) atom−1 of CrOx/ZrO2 catalysts reduced to the same extent (n=2.5) at 623 K differ by a factor 2.6, in spite of the large variation (factor of 20) of Cr content. The CrIII(NO)2 complex formed on the surface of reduced CrOx/ZrO2 by adsorption of NO at room temperature was reacted with CO at room temperature. By IR spectroscopy, it was found that a fraction of the CrIII(NO)2 species leads to mixed ligand complexes, CrIII(NO)2CO. Relying on IR, ESR and catalytic results, it is inferred that active sites for the low temperature hydrogenation of propene on reduced CrOx/ZrO2 catalysts are surface mononuclear CrIII, species with three coordinative vacancies.

An FTIR study of the NO-CO interaction, from RT up to 533 K, was done on two differently pretreated Pt/ZnO samples: the first, mildly reduced, formed of small pure Pt particles dispersed on ZnO, and the second, strongly reduced, presenting PtZn alloyed metal particles. On the mildly reduced one by heating in NO-CO atmosphere at 373 K a quite strong band assigned to an isocyanate species is formed, produced by reaction of CO molecules with nitrogen atoms, coming from the NO decomposition. At the same time the intensity of the CO band grows up to that of the virgin sample, showing that the isocyanate species spill over from the metal to the support. On strongly reduced samples the isocyanate species band is quite weak and a CO 2 band is produced. The different behaviour will be discussed taking into account the different surface composition and electronic properties of the two samples.

Pure SnO2 and CrOx/SnO2 powders with different crystalline sizes have been prepared and characterized by surface-area measurements, TEM measurements, UV-Vis-NIR diffuse reflectance and FT-IR absorption-transmission spectroscopies. The chemical and electronic responses of the samples on interaction with CO, NO, H2, CH4 at room temperature (RT) have also been studied. The responses are highly affected by the presence of chromium oxide, as revealed by FT-IR measurements. The electronic response to CO and H2 is lowered and no electronic response is detected for CH4 when chromium oxide is present. The electronic response to NO is enhanced or lowered by the presence of chromium oxide, depending on the thermal pretreatment.

The importance of the metal-support interaction on the catalytic properties of an oxide-supported metal has been shown by a great number of experimental data. Structural, chemical and electronic interactions are all very important to realize the catalyst configuration responsible for its high reactivity and/or selectivity. Furthermore the interactions can become more important during the catalytic process itself, and new surface characteristics can be developed. In this paper we examine, with selected examples, the different aspects of the interaction, how they are correlated and how they can be evidenced by different techniques and more specifically by IR or FT-IR spectroscopy.

The nature and stability of chromium species on the surface of CrOx/ZrO2 catalyst (Cr content 1.6 wt%) have been investigated using IR spectroscopy. On a sample heated in O2 at 923 K, Cr is present on the surface as Cr(V) (both as mononuclear and polynuclear chromyl species), and as Cr(VI) (both as chromates and polychromates). Upon reduction with CO at increasing temperatures up to 423 K, Cr(V) and polychromates are reduced to Cr(III), even at room temperature. On further reduction at higher temperatures (up to 623 K) chromates and a small fraction of polynuclear Cr(III) are reduced to Cr(II). Thermal treatments in water (up to 623 K) oxidize Cr(II) to Cr(III) without segregation of α-chromia.

Ethylene and CO interact at low pressure on a reduced Phillips catalyst to yield CO/C2H4 mixed complexes involving the most exposed type of Cr(II) (A type). Cr centres responsible for ethylene polymerization form the most stable mixed complexes. At high pressure (⩾ 40 torr), CrA(CO)C2H4 complexes react with a second ethylene molecule to yield a metallacyclohexanone species. After reaction, polymerization is prevented and oligomerization of ethylene is only observed on less reactive sites. A hierarchy of ions CrA may be established in this way. The implications for the polymerization mechanism are discussed.

NO x storage-reduction catalysts based on Pt and Pt/Cu supported on Mg(Al)O mixed oxides obtained from hydrotalcite-type compounds were studied. In particular, NO x storage capability upon admission of NO and NO 2 in the absence or in the presence of excess oxygen at temperatures up to 623 K was investigated by in situ FT-IR spectroscopy. For comparative purposes, the bare mixed oxide support was also investigated. Pure NO 2 is adsorbed according to a dismutation reaction with the simultaneous formation of nitrates and nitrites that progressively evolve to nitrates (“dismutation route”), the nitrite evolution to nitrates is promoted by the metal phase. When metal phase is present, the nitrites oxidation is further accelerated by the oxygen presence. Pure NO is not stored. When O 2 is present, NO is stored according to two different pathways, both promoted by the metal phases: (i) the NO oxidation to nitrites followed by their oxidation to nitrates (“nitrite route”); (ii) the NO oxidation to NO 2 , followed by the “dismutation route”.

Platinum catalysts supported on ordered mesoporous carbons (OMC) are described. The mesoporous carbon support, CMK3 type, was synthesised as an inverse replica of a SBA-15 silica template. The platinum catalysts (i.e. Pt 20 wt% and Pt 10 wt%, respectively), obtained through a conventional wet impregnation method, have been investigated to determine their structural characteristics and electrochemical behaviour. The electro-catalytic performance towards the oxygen reduction reaction (ORR) was compared to those of commercial Pt/C-Vulcan XTC72R (E-Tek) catalysts with the same Pt wt%, under the same experimental conditions. The two catalyst samples have allowed the effect of the variation of both the Pt to Nafion and Pt to the supporting carbon ratios to be studied. Electrochemical tests have been carried out in three different systems: a catalyst ink deposited on a glassy carbon rotating disk electrode (RDE), a gas diffusion electrode (GDE) in a three-electrode cell with H2SO4 as the electrolyte and a complete PEM single fuel cell. The first results indicate that the OMC performs slightly less well than commercial carbon supports, mainly in the complete fuel cell system. The data from the cell tests indicate a less effective distribution of Nafion on the OMC surface which, probably, decreases the platinum utilisation and the proton conductivity.

Morphological, textural, and surface properties of several Pt−Ba/Al2O3 NOx storage reduction (NSR) catalysts at different Ba loading in the range 0−30 wt % were characterized by means of X-ray diffraction, high-resolution transmission electron microscopy, and Fourier-tranform infrared (FT-IR) spectroscopy using CO, CO2, and CH3CN as probe molecules. Upon increasing the Ba loading, the Pt exposure progressively decreased, accounting for sintering and masking of the Pt particles by Ba, whereas the interaction between Pt and the barium oxide phase increased. After a few cycles of heating in NO2 and subsequent evacuation (conditioning treatment), BaCO3 initially present evolved to Ba(NO3)2 and then decomposed into a well-dispersed nanosized BaO phase. Upon conditioning, a slight sintering of Pt is observed. However, the presence of Ba avoided a more stressed sintering, as it occurred for the Pt/Al2O3 sample. Investigation by CO2 and CH3CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity...

Two differently pretreated Pt/ZnO samples were studied with FTIR and HRTEM analysis. HRTEM data show that in mildly reduced samples almost spherical Pt particles are present, while in strongly reduced ones faceted particles of δ-PtZn alloy are formed. The analysis of the IR spectra concerns both the vibrational modes of the adsorbed species and the variations in the IR transparency of the samples, variations related to the electronic properties of the material. IR data indicate that CO adsorption on very mildly reduced samples occurs on stepped Pt particles; on pure H2 reduced ones the surface composition of the metal particles changes from pure Pt to PtZn alloy. Heating in CO atmosphere causes, on mildly prereduced samples, reduction of the ZnO and formation of PtZn particles; on strongly prereduced samples it causes surfaces segregation of Pt on the PtZn alloy. A sensible chemical effect of the alloying on the CO stretching frequency has been put in evidence when Pt surface segregation occurs. The red-shift of the band compared with that of pure Pt indicates that in the alloy the Pt atoms make a bond with CO with a higher backdonation character. The IR transparency changes observed after these treatments have been interpreted as due to changes in the work function of the metallic phase as a consequence of the alloying and of the alloy composition.