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Gold-based catalysts over different supports were investigated in the oxidative esterification of furfural by employing an efficient and sustainable process. The catalytic performances follow the trend: Zirconia–Au > Ceria–Au ≫ Titania–Au. Zirconia came out to be the best support option to promote activity, selectivity and also stability. The chemical and morphological properties observed for zirconia-supported sample seem to fulfill a good compromise between high gold dispersion and the presence of suitable acid–base properties, for good selectivity. Moreover, stability and recycling of the catalysts were also investigated.

An Au/ZrO 2 catalyst was prepared and characterized in order to study the role of gold nanosize in the conversion of furfural into methyl furoate by oxidative esterification with O 2 and methanol, without NaCH 3 O. The comparison with the Au/TiO 2 reference catalyst provided by the World Gold Council and with an Au/ZrO 2 catalyst treated at 600 °C in oxygen emphasized very good catalytic performances. Highly dispersed Au was evidenced on Au/ZrO 2 by quantitative CO chemisorption, HRTEM analysis and FTIR spectroscopy of adsorbed CO. On the contrary, the thermal treatment at 600 °C results in the presence of nanoparticles larger than 4 nm that make the catalyst poorly active and selective. The Au clusters and their ability in the activation of the O 2 molecule play a key role in the reaction.

Hydrogen activation is a key step in many reactions like hydrogenations and other processes as the reverse water gas shift reaction. Therefore, the comprehension of the mechanism of hydrogen activation would sustain the understanding of the whole reaction mechanism as well as of the role played either by the gold or by the support. FTIR spectroscopy has been employed as a tool in order to improve the actual knowledge on these topics. Hydrogen interaction with the Au/TiO 2 reference catalyst provided by the World Gold Council, containing mainly gold nanoparticles, and with two samples, Au/CeO 2 and Au/ZrO 2 , in which gold clusters are present, has been performed. Hydrogen is dissociated at room temperature on the edge and corner sites of both nanoparticles and clusters, independently from the nature of the support. FTIR evidences of the formation of Au–H and Au–OH species are presented for the first time. These species are strongly bonded to Au and compete with CO adsorption. In addition, the spillover of the H atoms takes place on titania, H atoms are ionized to H + and the released electrons populate the conduction band, as testified by a monotonous absorption that grows at increasing contact time and pressure. The species observed in this paper can play a role on the activity and selectivity of gold catalysts in reactions involving hydrogen dissociation.

A treatment of the Temkin model was developed for describing the adsorption of CO on gold. In this presentation, it is made clear that the Temkin thermodynamic model is an extension of the Langmuir...

Gold catalysts were prepared by deposition-precipitation method on mixed Ce–Fe composite oxides, synthesized by the urea gelation coprecipitation pathway. The impact of the support composition on the catalytic performance for the water-gas shift reaction was studied by varying the Ce/(Ce + Fe) ratio. The following activity order was observed: Au/CeO2 > Au/Ce50Fe50 > Au/Ce75Fe25 > Au/Ce25Fe75 > Au/Fe2O3. The differences in the gold particle size (1–25 nm) and in the concentration of surface Ce3+ defect sites could explain the WGS activity order. The analysis of the HRTEM data combined with a FTIR study suggested the presence of highly dispersed gold clusters on the surface of Au/CeO2. Additionally, a higher amount of Ce3+ sites was registered by FTIR spectroscopy on the surface of this catalyst. Mossbauer spectra collected at room temperature and 90 K evidenced that different amounts of cubic CeO2-like solid solution (CeO2FeSS) and hematite coexisted in CeO2–Fe2O3 supports. The analysis of the characterization data suggested that Fe3+ were distributed at Ce4+ and interstitial sites by a dopant compensation mechanism. This mechanism of solid solution formation accounted for the lower oxygen vacancy concentration and Ce3+ amount on all gold catalysts supported on mixed CeO2–Fe2O3 materials, which affected the performance in the WGSR.

Ce–Fe mixed oxides were prepared by urea gelation coprecipitation method and used as supports of gold catalysts. The impact of the support composition on the catalytic performance for the preferential CO oxidation (PROX) was studied by varying the Ce/(Ce + Fe) ratio. A deep characterization study by different tools such as XRD, HRTEM, TPR and FTIR spectroscopy was undertaken in order to correlate the structural characteristics of the catalysts and the gold oxidation state and dispersion with the catalytic properties. The results revealed that the variation of the support composition led to significant differences in the gold particles size (in the range 1–25 nm), which affected strongly the CO oxidation activity of Au/CeO2–Fe2O3 catalysts under PROX conditions. The following activity order was observed: Au/CeO2 ≈ Au/Ce50Fe50 > Au/Ce75Fe25 > Au/Ce25Fe75 > Au/Fe2O3. The support with composition 50 wt.% CeO2–50 wt.% Fe2O3 appeared beneficial not only for nucleation and growth of highly dispersed gold particles (1–1.8 nm), but also for activation of oxygen and its mobility. Moreover, the presence of Fe2O3 in the supports composition improved the resistance towards deactivation by CO2. The CeO2–Fe2O3 supports comprised different amount of two phases: cubic CeO2-like solid solution and hematite. The analysis of the characterization data suggested that the solid solution formation probably proceeded via a dopant interstitial compensation mechanism.

Quantitative volumetric measurements of low temperature CO chemisorption on Au/ZrO 2 catalysts and Fourier transform infrared measurements of adsorbed CO in well defined and controlled conditions of temperature and pressure are here presented as widely accessible and reproducible methods for determining the concentration of gold sites able to chemisorb CO on Au/ZrO 2 catalysts. HRTEM analyses were also performed in order to support the validity of the measurements.

An FTIR study of CO adsorption from 120 K up to room temperature on a series of Au–ceria samples is presented. Samples with low gold content (0.7 and 0.6 at%) were prepared by urea gelation/co-precipitation and by cyanide leaching of the high-gold content (5.8 at%) material prepared by deposition–precipitation on La-doped CeO2. The samples were subjected to different pretreatments to collect information on the surface composition under working conditions. An absorption band at 2130–2140 cm−1, not reversible on outgassing and more resistant to oxidation than the usual carbonyl band on Au0 sites, was present due to CO adsorbed on cationic gold clusters. This highly stable species is relevant for hydrogen gas upgrade by removing CO from reformate-type gases at low temperatures. In addition, a broad absorption band in the 2000–2100 cm−1 range was observed after reduction in hydrogen, due to structural and electronic changes of gold. Interestingly, the reduced gold species in ceria can be reoxidized at mild conditions. Light-off of the CO oxidation reaction took place below room temperature on the metallic gold-containing ceria but was delayed until 310 K on the ionic gold-containing sample. TPR and XPS analysis of the fresh and used catalysts corroborated the stability of ionic gold in ceria up to 393 K in the reaction gas mixture.

Fourier transform infrared spectra of CO adsorption from 120 K up to room temperature on two gold catalysts supported on different mixed ceria-titania oxides are discussed in comparison with those obtained on Au/TiO(2) and Au/Fe(2)O(3) reference catalysts provided by the World Gold Council. The spectra of adsorbed CO, run on the different samples before preliminary treatment, are shown and compared with those of the untreated catalysts and of the samples reduced either in CO or in hydrogen. Big differences have been found between the ceria-titania supported samples and the reference ones: unusual absorption bands, irreversible to outgassing, have been detected after CO interaction on the untreated and oxidized ceria containing samples. These absorptions are assigned to CO on Au(n)(+) small clusters stabilized at the ceria defects. By reduction in hydrogen, negatively charged Au(n)(-) species are produced on the same sample. Oxidized small particles are present on the reference catalysts, but only on the untreated samples; after treatment, only metallic step sites are evident.

The water-gas shift (WGS) activity of different M/ceria catalysts, where M is a metal from IB group (i.e. Au, Ag, Cu) has been studied. Two different techniques were used for the preparation of the catalysts—deposition–precipitation and a modified version of deposition–precipitation. HRTEM combined with EDS, X-ray diffraction, FT-IR of the molecules involved in the WGS reaction at 90 and 300 K as well as of forward and reverse WGS reaction up to 573 K, XPS and H 2 -TPR were employed to obtain information about the effect of different synthesis procedures on the surface and on the bulk structure of the catalysts and to explain the differences observed in the catalytic activity. The influence of each IB metal on the WGS activity has been investigated, too. The experimental results revealed that deposition–precipitation is the preferred method for preparation of active ceria-based catalysts because it allowed most part of IB metal promoters to be exposed at the catalyst surface. WGS reaction proceeds at the boundary between nanosized metallic particles and ceria, where CO adsorption on IB metals and H 2 O dissociation on oxygen-vacancy defects of ceria take place. The role of formate species as intermediates in the WGS reaction is confirmed by FT-IR spectroscopy. The correlation between the stability of formates and the WGS activity suggests that the decomposition of formates is a rate-limiting step.

The effects of ZrO2 content on the CO oxidation activity in a series of CuOx/CexZr1−xO2 (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H2, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.

Different cobalt containing materials obtained from the thermal decomposition of two organic precursors, cobalt–acetylacetonate and cobalt–tetrametoxyphenylporphyrin, and Co 3 O 4 prepared by thermal decomposition of cobalt carbonate have been characterised by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and high resolution transmission microscopy (HRTEM). It has been found that the cobalt precursor, the organic macrocycle and the active carbon are simultaneously needed during the heat treatment in inhert atmosphere. Their contemporaneous presence is useful to obtain a material on which the dispersed metal is able to catalyse reactions that produce pores or tunnels inside the carbon, thus resulting in an enhanced contact area. The organic macrocycle is only partially decomposed and the residual fraction could enhance the conductivity.

A catalytic study of the hydrogen production by CO water gas shift reaction (WGSR) on gold, silver and copper particles supported on TiO2 has been carried out. A deep characterisation of the catalysts by TPR and FTIR has been performed. Silver catalyst exhibits no catalytic activity, copper and gold catalysts show intermediate and very high performances, respectively. These strong differences have been interpreted on the basis of FTIR data of CO adsorption at 90 K and on the effect of coadsorbed species. Gold and copper catalysts, either oxidised or reduced, are able to adsorb CO. Reduced silver catalyst does not adsorb CO at all, while oxidised silver catalyst does quite strongly.

The catalytic activity and stability of Au/TiO 2 have been examined under ambient conditions. The best performance was obtained for a catalyst calcined at 473 K and left at room temperature for a few days. The activity for CO oxidation was greatly influenced by moisture in the reactant gas and a maximum was observed at around 200 ppm. We have also succeeded in regenerating the catalysts exposed to various environments, such as rest room, using irradiation with light. The above results show that the Au/TiO 2 catalysts are very useful for indoor air quality control at room temperature without using thermal energy for regeneration.

Infrared spectra of CO adsorbed on nanometric gold particles supported on oxidised and reduced TiO 2 are reported. Depending on which pre-treatment is used different stretching bands related to gold carbonyls have been found. On calcined samples the predominant absorption band is at 2110–2120 cm −1 , assigned to CO adsorbed linearly on nanometric gold metallic particles; on highly dehydrated, oxidised samples, a band at 2154 cm −1 is detected, ascribed to CO adsorbed on gold oxidised sites; on reduced samples some new bands in the frequency range 2050 to 1900 cm −1 are produced, assigned to CO adsorbed linearly and bridge-bonded on gold small clusters, perturbed by the negative charge produced in support reduction.

The Ru/ZrO 2 catalyst's surface properties have been studied by quantitative chemisorption experiments on carbon monoxide and oxygen at different temperatures, by FTIR studies of oxygen, carbon monoxide, and nitric oxide adsorptions, and by FTIR studies of the CO–NO reaction. On the oxidized sample at least three FTIR absorptions are evident at 1008, 980, and 950–940 cm −1 . All the observed bands can be assigned to Ru–O surface species, differing in the oxidation state of ruthenium directly bonded to oxygen, the degree of metal–oxygen π bonding, and the coordination of the Ru atoms with the support. The larger heterogeneity of the bands observed on Ru/ZrO 2 catalyst, with respect to that previously detected on Ru/ZnO, can be taken as an indication that a significant role is played by the coordination properties and the surface features of the support. The strong increase of the band at 1008 cm −1 at temperatures higher than 573 K can be ascribed to the increase of the surface concentration of Ru–O surface species bonded to an Ru atom with an oxidation state higher than 4. The CO adsorption on the oxidized sample evidences the full reactivity of Ru–O species toward CO between room temperature (RT) and 473. CO and NO chemisorptions on reduced samples show that the two molecules are both molecularly and dissociatively adsorbed on the catalysts. The NO admission on preadsorbed CO produces immediately a significant modification of the adsorbed CO spectrum, i.e., a reduction of the intensity of the band assigned to CO adsorbed on Ru° sites and the appearence of additional bands that can be assigned to N 2 O, NO, and carbonate species. These features indicate that NO is immediately dissociated on Ru already in the presence of preadsorbed CO and gives rise to N 2 O and CO 2 already at RT. Upon heating at 373 K the N 2 O absorptions and the bands ascribed to CO adsorbed species are depleted and that of CO 2 increases, indicating that the reaction 2CO+2NO→N 2 +2CO 2 may prevail on the catalyst.

Three 8 wt% copper catalysts supported on SiO{sub 2} and on two SiO{sub 2}/TiO{sub 2} powders with different TiO{sub 2} content have been prepared by a chemisorption-hydrolysis method. Transmission electron microscopy indicated that after calcination the three catalysts contain supported particles which are small and quite homogeneous in size (mean diameter, d{sub m} = 3.0 nm). These particles slightly increase in size after reduction in H{sub 2} up to 773 K (d{sub m} = 3.5 nm). On the basis of the diffuse reflectance UV-Vis-NIR spectra CuO and Cu{sub 2}O were found to be present after calcination, while the electronic spectra were dominated by the features due to metallic Cu particles already after a mild reduction in H{sub 2} at 523 K. FTIR spectra of CO adsorbed onto the three catalysts reduced at 523 K appeared very similar. By a spectral fitting procedure, four different carbonylic species were evidenced, three assigned to carbonylic adducts on different types of microfacets exposed at the surface of three-dimensional Cu particles and one related to CO molecules adsorbed on plate-like two-dimensional copper particles. By increasing the reduction temperature an overall decrease in intensity of the bands due to CO adsorbed on copper supported on silica-titaniamore » carriers was observed, probably because of the formation of titanium suboxides that can cover a fraction of the copper sites. Furthermore, the CO-O{sub 2} and CO-NO reactions were studied by FTIR spectroscopy of the adsorbed species and quadrupole mass analysis of the gas phase over the catalysts. This allows the elucidation of the nature of the surface sites involved in the activation of these molecules and the nature of the intermediates present at the surface of the catalysts during the reactions. The role played in these reactions by the uncoordinated copper surface atoms exposed at the surface of the two different types of Cu particles will be discussed.« less

A Fourier transform infrared (FTIR), UV−vis, and high-resolution transmission electron microscopy (HRTEM) study of fresh and used Au/ZrO2 catalyst is reported. FTIR data show that on a fresh sample CO is adsorbed on regular gold metallic sites, while on the used sample the adsorption sites are bidimensional gold clusters made positive by interaction with the support. NIR−UV−vis reflectance spectra show that the gold plasmonic peak appears broader and the ZrO2 absorption edge weaker on the used sample with respect to the fresh one. HRTEM indicates the disappearance of the gold metallic particles in the used catalysts. Moreover, a strong reduction in the CO oxidation activity has been evidenced on the used sample and ascribed to the positivization of gold and/or to the decrease of the number of highly uncoordinated gold sites, consistently with the spreading of gold at the support surface.

The surface activity of a Fe–K/ZrO2sample prepared from iron citrate and potassium carbonate has been investigated by the FTIR spectroscopy of adsorbed CO and NO and compared with the behavior of a Fe/ZrO2sample. The addition of potassium produces a strong decrease of the BET area and of the amounts of the adsorbed molecules. At the same time the spectrum of the sample reduced with H2at 573–773 K shows, under anhydrous conditions, the presence of small patches of reduced Fe0, not easily formed on Fe/ZrO2under the same thermal and reducing conditions. The formation of new phases containing iron and potassium in close combination, such as mixed iron–potassium oxalates and potassium ferrites that fully cover the zirconia surface, was inferred from FTIR data. CO adsorption favors the partial surface reduction of these phases producing small Fe-carbonyl clusters in tight contact with potassium and Fe,K-carboxylate surface species. The adsorption of NO, producing nitrosyl species on the reduced iron particles and nitrite groups on the potassium phase is in good agreement with the results of adsorbed CO. These results are confirmed by quantitative determinations of adsorbed gases and by TPR experiments.

A FTIR study of CO adsorbed on Au ZrO 2 and on some Au  Pt ZrO 2 bimetallic samples, prepared with different methods, subjected to different pretreatments and to oxidation with O2 is reported. CO adsorption at 298 K on Au ZrO 2 produces a band at 2110–2118 cm−1 that is reversible upon outgassing at room temperature, while on Pt ZrO 2 the adsorption of CO produces an absorption band at 2080-2060 cm−1 that is irreversible on outgassing at room temperature. On the mixed Au  Pt ZrO 2 bimetallic samples, bands are detected at 2190, 2110-2060 cm−1, whose behaviour upon outgassing and with respect to oxygen interaction is strongly dependent on the preparation method, on the pretreatment and on the surface coverage. The gold and platinum adsorption sites appear independent and uncoupled, however, 12CO13CO exchange experiments and COO2 interactions indicate an higher lability and reactivity of the CO adsorbed on Pt sites.

The reactivity of different gold‐based catalysts has been examined and re‐discussed on the basis of the most recent literature data, mainly looking at the characterization data obtained by FTIR spectroscopy and HRTEM microscopy. In particular, the CO oxidation reactions at 90 K and at room temperature have been discussed. By comparing samples with different gold particle sizes and different supports it has been deduced that the sites responsible for the adsorption and the reactive activation of CO and O2 are the gold edge sites, in proximity to the support oxygen‐vacancy defects. These defects may produce electron‐rich gold atoms, which may be relevant for oxygen activation and dissociation. As for the water–gas shift reaction, an important role of support sites at the borderline with the gold sites, where water and CO in mutual interaction may be activated, has been proposed. Moreover, a role of oxygen adsorbed on gold particles near oxygen vacancies of the support in the oxidative dehydrogenation of methanol and in the furfural oxidative esterification has been discussed. All the above summarized points confirm the importance of the dual perimeter sites in the catalytic activity of gold catalysts.

The procedures leading to the preservation of catalytic performances of Au/ZrO2 samples have been investigated. The three potential causes of deactivation, namely the particle growth by sintering of gold nanoparticles, the metal leaching and the formation of un-reactive species which inhibit the reaction, have been evaluated. In particular, this paper deals with the stability of gold nanoparticles: (1) under storage conditions; (2) with time on stream for a gas phase reaction (LT-WGSR); (3) with time on stream for a liquid phase reaction (furfural oxidative esterification).

Highly dispersed gold based catalysts supported on zirconia were employed in the oxidative esterification of furfural by an efficient and sustainable process. Au/ZrO 2 catalysts were calcined at different temperatures in order to modulate gold nanosize. A detailed characterization was carried out for the sake of ascertaining if micro structural changes occurred, and the size issue was discussed. Catalysts stability and recycling were investigated too, and the opportunity of reusability by thermal oxidation at a proper temperature was successfully proved.

The curve fitting analysis of the FTIR spectra of two differently pretreated Pt ZnO dispersed samples is discussed. Carbon monoxide adsorbed on a photodeposited Pt ZnO sample mildly reduced in a 0.5% H 2 N 2 mixture at 493 K shows a main band at 2100 cm−1, shifting to the red and reducing its intensity by decreasing the surface coverage. All the spectra are well fitted by the addition of five Lorentzian bands. Looking at the literature, the five components, on the basis of their spectroscopic features can be assigned to Ptx+ sites, Pt0(111) terrace sites with coordination number 9, Pt0(100) terrace sites with coordination number 8, and Pt0 step sites with coordination number 7. The spectra of CO adsorbed on the same Pt ZnO sample reduced in pure H2 at 523 K exhibit a main band at ≈ 2070 cm−1, at lower frequency, significantly less intense and further red-shifting by decreasing the coverage. In this case the band appears as the superposition of two bands, not completely well fitted by Lorentzian bands. The maximum of these bands appears related to Pt atoms with coordination numbers 6 and 5. It must be stressed that by increasing the temperature of reduction in the case of Pt supported on refractory oxides, usually prepared by ionic exchange, a sintering and clustering occurs; these processes convert almost isolated low coordinated metal sites into terrace sites and induce a blue-shift of the absorption band. Our mildly reduced samples, as a consequence of the photoreductive method, are made of well crystallized Pt small particles. By increasing the reduction temperature, the alloying of Pt with Zn atoms coming from the support occurs; the opposite frequency shift observed and the change in the shape of the band can be ascribed to the chemical effect of the alloying of platinum with zinc. This hypothesis is confirmed by the analysis of isotopic mixtures experiments.

An FTIR and quadrupole mass spectroscopic study of CO adsorption and oxidation with 16O2 and 18O2 on copper supported on ZnO and TiO2 is presented. The experimental results indicate that CO is adsorbed on the metallic particles dispersed on both oxides on two kinds of sites, on the normal terrace sites and on sites at the borderline of the particles. Moreover, on titania, a band at 2126 cm-1, assigned to CO adsorbed on isolated Cu atoms and/or on two-dimensional small clusters, is detected. A frequency shift of the bands of CO adsorbed on the metallic particles observed in the CO−O2 coadsorption experiments and the occurrence of a scrambling reaction between CO and 18O2 reveal that on all these samples, there are metallic sites which are able to adsorb at the same time oxygen atoms and carbon monoxide. Carbon dioxide and carbonate-like species are formed: the asymmetric stretching frequencies of CO2 and the quadrupole mass spectroscopic analysis reveal that with 18O2, different isotopic molecular CO2's a...

Mixtures of Ag2SO4 and Ag2S with a silver sulphate content varying from 16.67–50.0 mol.% were heated in air. The diffraction pattern of a sample obtained by the heating of a physical mixture comprising 33.33 mol.% of Ag2SO4 consists of diffraction lines that could not be attributed to any of the other phases existing in the AgOS system. The lines have been ascribed to a new phase of monoclinic system having the formula, Ag6S3O4, which has been ascertained by chemical analysis.

An FTIR and quadrupole mass study of the adsorption and oxidation of CO on an Au/ZnO sample with 16 O 2 and 18 O 2 is presented. The results indicate that CO adsorbed on gold sites at the perimeter interface with ZnO reacts with surface lattice oxygens at the oxide surface to form carbonate-like intermediates. This reaction and the decomposition of the intermediates can be assumed to be mainly responsible for the conductivity changes of the semiconductive metal oxide caused by CO in air. Oxygen, present in the mixture and adsorbed on gold vicinal sites, makes the nucleophilic attack of surface oxygens on the CO easier as a consequence of an electron-withdrawing effect. This effect produces a positivization of the carbon of adsorbed CO by an inductive effect and activates the reactivity of support lattice oxygens on the CO.

Pd–Fe/SiO2 bimetallic catalysts, prepared by sequential wet impregnation, with a constant 2 wt.% Pd loading and with atomic ratios Fe/Pd = 0.5–2.1, and monometallic samples have been studied by quantitative chemisorption and by FTIR spectroscopy of adsorbed CO and NO. The FTIR results indicate that on the examined bimetallic samples CO is selectively adsorbed on Pdo surface sites and NO is mainly adsorbed on Fe2+ surface sites; furthermore, readsorption and coadsorption experiments reveal that Pd and Fe are interdispersed on an atomic level. The quantitative chemisorption data show that the CO/Pd ratio on all the bimetallic samples is lower than on the pure Pd sample (

Two different synthesis routes, in particular deposition-precipitation (DP) and modified deposition-precipitation (MDP) have been used for preparation of gold/ceria catalysts. The catalysts have exhibited significant differences in CO, CH3OH and (CH3)2O oxidation. The most pronounced effect of different preparation methods was registered in the CO oxidation reaction. DP-prepared catalyst showed high ability to oxidize CO at room temperature. Microscopic measurements have evidenced formation of very small gold particles (d below 1 nm) on the surface of DPprepared catalyst. In contrast, only large gold particles with average size about 15 nm were observed in MDP-prepared sample. A detailed characterization by FTIR spectroscopy after CO adsorption at 90 K and TPR measurements has been carried out with aim to unravel the impact of preparation method on the structural and catalytic properties of the catalysts. The catalytic behavior was explained with the large differences in the gold dispersion and in the number of active gold sites in intimate contact with ceria surface defects.

Pd catalysts supported on silica were tested for H2O2 direct synthesis under very mild (1 bar, 20 °C, no halide addition) and nonexplosive conditions. Other catalytic tests were performed at higher pressure using a solvent expanded with CO2. The optimum Pd content is 1.5 wt.%. Calcination is mandatory to preserve catalytic performance and recycle the catalyst. A comparison among Pd samples with similar metal content over different supports was performed under identical experimental conditions, showing that Pd/SiO2 gives the highest selectivity (constant at 60%). At the same time, the productivity follows the trend: Pd / SiO 2 > Pd / ZrO 2 / SO 4 = > Pd / ZrO 2 > Pd / CeO 2 / SO 4 = > Pd / CeO 2 . HRTEM and chemisorption results clearly indicate that a high Pd dispersion is not advisable to observe good catalytic performance. On the contrary, the presence of less energetic Pd sites exposed at the surface of big particles of Pd/SiO2, where oxygen can chemisorb without dissociation, is a necessary condition to produce H2O2.

The adsorption of CO on the supported gold nanoparticle catalysts Au/TiO2, Au/Fe2O3, and Au/ZrO2 was examined using infrared transmission spectroscopy to quantify the isobaric CO coverage as a function of temperature. The Temkin adsorbate interaction model was then applied to account for the adsorption behavior. To test the general applicability of the Temkin model, this treatment was also applied to three data sets from the literature. This included another real-world catalyst and two model catalysts. All data sets were accurately represented by the Temkin adsorbate interaction model. The resulting thermodynamic metrics are consistent with previous determinations and reflect a particle size-dependence. In particular, the intrinsic adsorption enthalpy at zero CO coverage varies almost linearly with Au particle size, and this trend appears to be correlated with the abundance of low-coordinate Au sites (cf., CN = 6 and 7 for corners and edges, respectively). For very small particles with mostly CN = 6 corne...

New gold catalysts supported on ceria modified by addition of ZrO 2 have been synthesized and tested in the water-gas shift reaction (WGSR) at low temperature, where they displayed better catalytic activity than gold supported on pure ceria, following the trend: AuCe50Zr50 > AuCe80Zr20 > AuCe. Morphologic, textural, structural and spectroscopic characterisation showed that the addition of zirconia to ceria leads to the formation of mixture of tetragonal ceria–zirconia phases. Moreover, depending on the zirconia amount, a different Au dispersion is observed. Au agglomerates (10–20 nm) and nanoparticles (about 2.5 nm) have been detected by HRTEM on AuCe80Zr20, while only nanoparticles have been found on AuCe50Zr50. FTIR spectroscopy of CO adsorbed at 120 K showed also the presence of Au clusters, more abundant on AuCe50Zr50 than on AuCe80Zr20. However, at low temperature the best catalytic activity of AuCe50Zr50 is correlated not only to the abundance of gold clusters and nanoparticles, but also to the effect of ZrO 2 addition that influences the acid/base surface properties of ceria, as successfully demonstrated by adsorption and surface reaction of acetone vapor at room temperature. FTIR measurements performed before and after reaction showed that the carbonate-like species have the lowest stability on the AuCe50Zr50 surface. This feature, along with the presence of a high gold dispersion, makes AuCe50Zr50 the best catalyst in terms of activity and stability.

Pd-Fe/SiO2 bimetallic catalysts with a constant amount of Pd and a different Fe/Pd ratio were prepared by sequential wet impregnation and the hydrogenation of 2,4-dinitrotoluene was carried out at 25°C in a batch reactor. The presence of Fe enhances the Pd catalytic activity. The formation of a dispersed Pd-Fe alloy was evidenced through X-ray powder diffraction and is consistent with the data observed by FTIR analysis, TPR, and chemisorption features. The increase of the Fe total content of the catalysts results in the increase of the dispersion of the metallic phase and in the presence of partially oxidized Fe species. FTIR analysis shows that nitrocompounds chemisorb mainly on the support rather than on the metal surface. It is suggested that the Pd-Fe alloy formation is responsible for the increase of the catalytic activity because of a cooperative effect of Fe in the hydrogen transfer step.

FTIR study of CO adsorption on Au/TiO2 samples prepared by a deposition -precipitation technique is presented. The absorption spectra of 12 CO and 12 CO - 13 CO mixture are examined at different equilibrium pressures at room temperature. From these data two kinds of chemically active sites, strongly coupled and interacting were evidenced. Chemical effects, depending on the CO coverage, were also observed.

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.

Characterization by high-resolution transmission electron microscopy of differently pretreated Cu/TiO[sub 2] samples and IR and mass spectroscopic studies of NO-CO interaction on them from room temperature to 473 K are presented. All samples are characterized by Cu particles well dispersed on TiO[sub 2] surface. The adsorption of CO produces an absorption in the 2112-2130 cm[sup [minus]1] frequency range; the intensity and position of the band strongly depends on the sample pretreatment, and it is strongly affected by NO interaction. NO dosing on a sample reduced at low temperature (LTR) precovered with CO leads to the formation of some CO[sub 2] but the reaction is not complete after heating at 473 K; on a sample reduced at high temperature (HTR) large amounts of N[sub 2]O and Co[sub 2] are detected already after short contact times at room temperature. By heating at 473 K an almost complete conversion to CO[sub 2] and N[sub 2] is observed. On both LTR and HTR samples neither NO nor isocyanate-adsorbed species are detected. Reaction products are observed if the sample HTR is contacted, after outgassing of a previous interaction mixture without desorption of the oxygen atoms produced in the NO dissociation, with a 1:1 NO-CO mixturemore » or with CO and then with NO. If NO is preadsorbed prior to CO dosing in a second run of the reaction, almost no N[sub 2]O and CO[sub 2] are observed. The experimental data indicate that HTR Cu/TiO[sub 2] is significantly more active than LTR Cu/TiO[sub 2], the activity being related to an easy dissociation of the NO molecules, determined by an electron transfer from reduced titania to antibonding orbitals of the NO. Free reduced copper sites are needed for CO activation, the reaction being to some extent inhibited by NO oxidation of the copper. 20 refs., 9 figs., 1 tab.« less

Pd-Fe/SiO2 bimetallic catalysts with a constant amount of Pd and a different Fe/Pd ratio were prepared by sequential wet impregnation and the hydrogenation of 2,4-dinitrotoluene was carried out at 25°C in a batch reactor. The presence of Fe enhances the Pd catalytic activity. The formation of a dispersed Pd-Fe alloy was evidenced through X-ray powder diffraction and is consistent with the data observed by FTIR analysis, TPR, and chemisorption features. The increase of the Fe total content of the catalysts results in the increase of the dispersion of the metallic phase and in the presence of partially oxidized Fe species. FTIR analysis shows that nitrocompounds chemisorb mainly on the support rather than on the metal surface. It is suggested that the Pd-Fe alloy formation is responsible for the increase of the catalytic activity because of a cooperative effect of Fe in the hydrogen transfer step.

Pd–Fe/SiO2 bimetallic catalysts, prepared by sequential wet impregnation, with a constant 2 wt.% Pd loading and with atomic ratios Fe/Pd = 0.5–2.1, and monometallic samples have been studied by quantitative chemisorption and by FTIR spectroscopy of adsorbed CO and NO. The FTIR results indicate that on the examined bimetallic samples CO is selectively adsorbed on Pdo surface sites and NO is mainly adsorbed on Fe2+ surface sites; furthermore, readsorption and coadsorption experiments reveal that Pd and Fe are interdispersed on an atomic level. The quantitative chemisorption data show that the CO/Pd ratio on all the bimetallic samples is lower than on the pure Pd sample (

The effect of the addition of various amounts of sulfates to a zirconia support and its possible role during the Au deposition–precipitation step was examined. The high activity showed by the Au/ZrO2 catalysts in the WGS reaction was enhanced by the action of sulfates on the support. SO42− addition to zirconia brings a higher gold dispersion due to (i) the larger surface area and (ii) the positive role of SO42− groups that determine the deposition of Au in the form of highly dispersed non-metallic gold clusters in close contact with the support.

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.

The interaction with NO of differently pretreated ZnO and TiO 2 pressed powders, pure or modified by noble metal addition (Pt and Ru, respectively) is studied, mainly by FT-IR spectroscopy; the analysis of the IR spectra concerns both the vibrational modes of the absorbed species and the variations of the IR transparency related to changes in the electrical properties. The data presented indicate that the NO molecules are dissociated and that electron transfer from the solid to the produced oxygen atoms occurs on prereduced samples, both pure and metal modified; the metal modifiers also adsorb NO in molecular form.

NO adsorption on differently pretreated pure ZnO and Pt/ZnO samples has been investigated by FTIR spectroscopy. Both the vibrational modes of the adsorbed species and the changes in the IR transmission of samples are discussed. On mildly reduced Pt/ZnO samples NO is adsorbed molecularly on Pt sites, while on pure ZnO nitrite and nitrate species are produced; no changes of IR transmission are observed on both samples. On strongly reduced Pt/ZnO and ZnO samples NO adsorption produces a large increase of IR transmission, showing that NO dissociation occurs. Moreover, different species of adsorbed NO are observed on Pt sites modified by alloying with Zn or by O atoms produced in NO dissociation.

An FTIR study of CO adsorption and oxidation on Au/TiO2 is reported. At 90 K different CO−18O2 interactions have been performed, with preadsorbed carbon monoxide or preadsorbed isotopically labeled oxygen, with dry or wet carbon monoxide. From the experimental results it can be inferred that (i) at 90 K carbon monoxide and oxygen are molecularly adsorbed on gold particles, (ii) the reaction occurs if carbon monoxide is preadsorbed (it is almost completely inhibited if oxygen is preadsorbed), and (iii) the reaction occurs more easily in the presence of water. It is proposed that dissociation of oxygen is needed in order to observe the oxidation reaction.

Bimetallic Pd–Au samples supported on zirconia were prepared by different methods and tested for the direct synthesis of hydrogen peroxide under very mild conditions (room temperature and atmospheric pressure), outside the explosion range and without halides addition. Further catalytic tests were performed at higher pressure using solvents expanded with CO2. Samples were characterized by N2 physisorption, metal content analysis, XRD, HRTEM combined with X-ray EDS, TPR, and FTIR. The effect of the addition of gold to Pd in enhancing the yield of H2O2 is sensitive to the preparation method: the best catalytic results were obtained by depositing gold by deposition–precipitation (DP) and by introducing in a second step Pd by incipient wetness impregnation. The origin of the differences between samples is discussed. The role of Au in the catalytic reaction seems to be a complex one, changing the chemical composition of the metallic particles, their morphology, and charge of the exposed Pd sites.

FTIR absorption spectra of CO, of 12CO–13CO isotopic mixtures and of CO–18O2 interaction on gold catalysts supported on group IV oxides are reported together with CO quantitative chemisorption data. On Au/TiO2 two kinds of metallic gold surface sites, mutually interacting, adsorb CO in spite of the low CO/Au ratio (0.03). On Au/ZrO2, where a CO/Au ratio of 0.30 has been determined, mutually interacting corner sites on non-metallic gold nanoclusters are present. Finally, isolated and negatively charged gold nanoclusters have been evidenced on Au/CeO2. Different absorption coefficients have been found. On all samples, by contacting CO–18O2 at 90 K, only C16O18O is produced: gold sites are involved in the activation of both molecules. The largest amount of C16O18O is produced on the CeO2 supported sample, as a consequence of an easier activation of the oxygen on the anionic gold clusters and on the support.

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.

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm 3+ , La 3+ and Zn 2+ ) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H 2 via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO 2 > Au/Sm–CeO 2 > Au/CeO 2 > Au/La–CeO 2 . The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm −1 in the FTIR spectra collected during CO–O 2 interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO 2 and H 2 O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H 2 O already at 90 K.