Hydrogen spillover phenomena on Pt/ZrO2

A Pt/ZrO₂ catalyst has been investigated by temperature-programmed reduction and temperature-programmed desorption of hydrogen. Hydrogen spills over from Pt onto the ZrO₂ surface at about 550°C. One part of spillover hydrogen is consumed by a partial reduction of zirconia. The other part is adsorbed on the surface and is desorbed at about 650°C. This desorption is a reversible one, i.e. it can be followed by a renewed uptake of spillover hydrogen. No connection between dehydroxylable OH groups and spillover hydrogen adsorption has been observed. The adsorption sites for the reversibly bound spillover hydrogen were possibly formed during the reducing hydrogen treatment.     

Improving then-pentane hydroisomerization of Pt/SO 4 2− SiO2) catalysts through mixing with ZrO2

The performance of Pt catalysts supported on sulfated zirconia-silica with different stoichiometries is investigated in then-pentane hydroisomerization reaction. Comparatively, with respect to the Pt/SO ₄ ^2– -SiO₂ or Pt/SO ₄ ^2– -ZrO₂ catalysts, the sulfated mixed oxides show an enhancement of the catalytic activity that increases with the content of ZrO₂, reaching its maximum at values between 10 and 15 wt% zirconia. The characterization of the samples reveals that at this stoichiometry occurs the highest H2-consumption of the samples as well as the top value of strong Brónsted acid sites according to the TPD-H₂ and FTIR measurements of absorbed pyridine respectively. That is, close to these percents of zirconia content one has a compound that is homogeneously mixed and above those values the segregations of the single oxides occur as verified by X-ray diffraction characterization.     

Effect of lithium and residual nitrate species on platinum dispersion in Pt/Al2O3 catalysts

The effect of lithium (ex LiNO₃) on the metallic dispersion of 0.8 wt% Pt/Al₂O₃ catalysts, prepared by different impregnation techniques, was investigated by temperature programmed reduction (TPR) and the frontal method of H₂ chemisorption. The temperature at which platinum precursor is reduced at a maximum rate (543 K) was not modified by 0.1 wt% lithium addition, whatever the preparation technique used. The dispersion values of platinum (70–90%), after reduction at 773 K, were slightly dependent on the preparation procedure. After the addition of 0.8 wt% lithium the TPR profile presented two well defind peaks and the dispersion values (20–50%), measured after reduction at 773 K, presented a significant decrease. The results are linked with the presence of residual nitrate ions, that had not been eliminated during calcination at 773 K in air, but had been decomposed under the reducing atmosphere of the TPR experiment.     

Metal-support effects in Pt/L-zeolite catalysts

A series of Pt/L-zeolites with different cations (varying acidity) were prepared and characterized by H₂ and CO chemisorption and competitive toluene/benzene hydrogenation. The ratio of adsorption equilibrium constants for toluene/benzene was extracted from rates of hydrogenation. The variation of this ratio across a series of alkaline-earth exchanged Pt/L-zeolites indicates that there is a varying support interaction with Pt clusters in the zeolite and suggests that the clusters interact electronically with the support.     

Effect of support modification on the chlorobenzene hydrodechlorination activity on Pt/Al2O3 catalysts

Al₂O₃ was modified with TiO₂ and ZrO₂ using organometallic precursors and is used in the preparation of supported platinum catalysts. The catalysts have been characterised by nitrogen adsorption, hydrogen chemisorption and X-ray diffraction and were tested for their activity in the hydrodechlorination of chlorobenzene. The investigations show that support modification controls the catalyst deactivation remarkably and the catalysts were found to be highly active and selective. This revised version was published online in July 2006 with corrections to the Cover Date.     

A pathway of NO–H2–O2 reaction over Pt/ZrO2 through ammonium nitrate

The relationship between the product selectivity for the NO–H₂–O₂ reaction and characteristics of the catalyst, Pt/ZrO₂, was investigated. From the results of activity tests and characterizations, such as CO adsorption and TEM, the catalysts with high Pt dispersions showed high NH₃ selectivities, and those with significantly agglomerated Pt particles by high temperature calcination exhibited higher N₂ formation. The effect of the reduction and oxidation pretreatment was also investigated. The reduced Pt sites promoted the formation of NH₃. The pretreatment condition influenced not only on the amount of accumulated nitrogen-containing species during the reaction but also on the decomposition of ammonium nitrate, which was suspected to be an accumulated species. The decomposition of ammonium nitrate would be involved in the NO–H₂–O₂ reaction.     

EXAFS investigations of lanthana-promoted Rh/SiO2 catalysts

EXAFS investigations at the Rh K edge of lanthana-promoted Rh/SiO₂ catalysts showed that the local environment of the Rh ions in the oxidic catalyst precursor state did not depend on the La₂O₃ content and resembled that of Rh₂O₃. No LaRhO₃ formation could be detected. In the reduced state, EXAFS as well as H₂ and CO chemisorption demonstrated that La₂O₃ increased the Rh dispersion. Covering of the Rh metal particles by La₂O₃ was minor, because during catalyst preparation, La was impregnated prior to Rh.     

Sintering of Pt/Al2O3 reforming catalysts: EXAFS study of the behavior of metal particles under oxidizing atmosphere

The effect of an oxidative atmosphere (300 °C) is studied on fresh and sintered unchlorinated naphtha reforming catalysts containing 0.6–1% Pt. The TPR profiles show that only one species is formed using our experimental conditions, regardless of the mean crystallite size of the metal particles. The structural information supplied by EXAFS compared with cuboctahedral particle modeling, implies that such species is a surface platinum oxide, the structure of which is close to that of PtO₂, but largely distorted. This is true whether the catalyst is sintered or not.     

Infrared and carbon dioxide chemisorption study of Mo/ZrO2 catalysts

The interaction of Mo with zirconia has been investigated by infrared spectroscopy (IR) and carbon dioxide chemisorption. Quantitative analysis of the IR results indicated that Mo interacts preferentially with the most basic hydroxyl group (high frequency band at 3775 cm^–1). An approximately 79% decrease in the 3775 cm^–1 band is observed vs. 21% for the low frequency band at 3673 cm^–1, with increasing the Mo loading up to 1 wt%. The relative decrease of the IR band at 3775 cm^–1 was identical to that measured for the CO₂ uptake. The Mo cross-sections estimated from CO₂ chemisorption results were much higher than those typically reported for the Mo system. It was concluded that, as previously reported for the Mo/Al₂o₃ system, CO₂ chemisorption overestimates the surface coverage of Mo/ZrO₂ catalysts.     

Effect of the properties of hydrous zirconia prepared under various hydrothermal conditions on the isomerization activity of Pt/WO3-ZrO2

A series of hydrous zirconia samples were prepared by hydrothermal method and the effects of the properties of hydrous zirconia on the catalytic activity of Pt/WO₃-ZrO₂ in the hexane isomerization were investigated. The catalysts were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and differential thermal analysis (TDA), H₂-temperature programmed reduction (H₂-TPR) and NH₃-temperature programmed desorption (NH₃-TPD). The results showed that the hydrothermal treatment under different times and pH values led to remarkable changes in the properties (such as hydroxyl group, ordering degree and thermal stability) of hydrous zirconia. Moreover, the isomerization activity of Pt/WO₃-ZrO₂ varied distinctly with the hydrothermal treatment condition of hydrous zirconia. The correlation between the properties of hydrous zirconia and the isomerization activity of the catalyst was primarily established. It was proposed that the isomerization activity was strongly dependent on the stability and ordering degree of hydrous zirconia, while it was irrelevant to the amount of hydroxyl groups in hydrous zirconia.     

Ni–Pt/H-Y Zeolite Catalysts for Hydroisomerization of n-Hexane and n-Heptane

Ni–Pt/H-Y zeolite catalysts with different Ni contents were prepared and applied to the hydroisomerization of n-hexane and n-heptane in the temperature range 225-375 °C. ESCA studies show the complete reduction of Ni species up to 0.3 wt% Ni addition over 0.1 wt% Pt/H-Y and further addition leads to the occurrence of unreduced nickel species as NiAl₂O₄. A TEM study shows the formation of bimetallic (Ni–Pt) particles of nanoscale size and the average particle size is found to increase with increasing Ni loading. Acidity measurements by NH₃-TPD and pyridine-adsorbed FTIR spectroscopy show the increasing occupation of acid sites by the added nickel when increasing the nickel loading. The catalytic activity of Ni–Pt/H-Y zeolite and Pt/H-Y catalysts was compared and it was found that addition of Ni up to 0.3 wt% increases the n-hexane and n-heptane conversion, multibranched isomer selectivity and sustainability of the catalysts due to better metal-acid synergism, complete reduction of Ni species and the formation of catalytically active Ni–Pt bimetallic particles. Further Ni addition leads to a decrease in conversion and multibranched isomer selectivity and an increase in the cracked products, which may be due to the presence of unreduced Ni species and pore blockage by larger-sized bimetallic particles formed.     

Effect of Pt Incorporation into Ni/ZrO2–SO 4 = /Al2O3 Catalyst for n-Butane Isomerization

Three Ni/ZrO₂–SO₄⁼/Al₂O₃ catalysts with different concentrations of platinum (0.2, 0.3 and 0.4 wt%) were prepare and tested for n-butane isomerization reaction at 338 K, in absence and in presence of hydrogen. The results shown that, at low temperature, platinum contributes to the olefin or butyl ion formation and the reaction follows a bimolecular pathway. However, when the reaction occurs in the presence of hydrogen, the formation of butyl ions is inhibited. The main feature of platinum addition is the stabilization of the catalytic activity, which is indicated by the slow deactivation constants compared to that of the unpromoted catalyst.     

HREELS/TDS study of NO reaction with hydrogen on Pt(100) surface

NO adsorption on a Pt(100)-(hex) surface and NO_ads reaction with hydrogen at 300 K have been studied by HREELS, LEED, TDS and isothermal desorption. NO adsorbs in molecular form, its molecules gathering in islands with a high local coverage. Surface reconstruction into a (1 × 1) phase proceeds within the boundaries of islands. Reaction NO + H₂ is performed via NO_ads previous heating in vacuum atT _h = 375–425 K. Kinetics of NO_ads titration appears to be autocatalytic. Nitrogen is the major reaction product.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

Oscillations of CO and CO + H2 oxidation in air over supported and unsupported Pt catalysts

Oscillation behaviour of the oxidation of CO (0.2–2.2%) in air over Pt wire coils and over Pt/Al₂O₃ catalysts deposited onto the coils (pellistors) has been investigated. The waveforms differ considerably between the two catalytic systems. Over unsupported Pt at 240–260° C, regular oscillations were accompanied by slowly declining activity and by deposition of carbon. Over supported Pt at 110–180°C, relatively complex but sustained oscillation occurred by a different mechanism. This oscillation was greatly enhanced by H₂ (0.25–1.0%), and may involve fluctuations in the concentrations of CO and H₂ around the supported catalyst.     

On the deactivation of Pt/L-zeolite catalysts

Pt/L-zeolite catalysts have a unique activity forn-hexane aromatization to benzene. There have been proposals which attribute this to electronic and to geometric origins of the L-zeolite. Recently, the uniqueness of the L-zeolite support has been understood to derive from the ability to stabilize very small particles in a non-acidic environment and it has been proposed that a further stabilization against deactivation (by geometric constraint of bimolecular coke precursor reactions) is what distinguishes these catalysts relative to SiO₂ supported small Pt particles. We have investigated the initial deactivation rate of four Pt/L-zeolite catalysts and a Pt/SiO₂ reference during reaction ofn-hexane, neopentane and 2-methyl-2-pentene. In all cases, the relative rates of deactivation correlate with the apparent acidity (as determined by competitive benzene/toluene hydrogenation) suggesting that the deactivation stabilization may have an electronic component.     

Study of alumina-promoted SO2- 4/NiO/ZrO2 catalyst performance

Improvement of the sulfated zirconia catalysts (SZN) was studied by adding different amounts of alumina as follows: 17.5, 29.8, 45.9, and 56wt %. Even though, alumina, is expect to promote higher specific area and increase the amount of acid sites, stability of the zirconia tetragonal phase, the active zirconia phase of the SZN catalyst isisomerization reaction was obtained for catalysts containing between 17.5, and 29.8wt % of alumina, which reflects the promotional effect of alumina. On the contrary, a strong reduction of this reaction rate is observed when higher amounts of alumina are added to the catalyst, which indicates the dilution effect of alumina.This work is dedicated to the memory of Gabriel Aguilar-Rios, researcher at Instituto Mexicano del Petróleo and Professor at Instituto Politécnico Nacional. Who for more than 20 years contributed to catalysis science in Mexico.     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

The NO _x adsorption mechanism on Pt/BaO/Al₂O₃ catalysts was investigated by performing NO _x storage/reduction cycles, NO₂ adsorption and NO + O₂ adsorption on 2%Pt/(x)BaO/Al₂O₃ (x = 2, 8, and 20 wt%) catalysts. NO _x uptake profiles on 2%\Pt/20%BaO/Al₂O₃ at 523 K show complete uptake behavior for almost 5 min, and then the NO _x level starts gradually increasing with time and it reaches 75% of the inlet NO _x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO _x conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO₃)₂. Adsorption studies by using NO₂ and NO + O₂ suggest two different NO _x adsorption mechanisms. The NO₂ uptake profile on 2%Pt/20%BaO/Al₂O₃ shows the absence of a complete NO _x uptake period at the beginning of adsorption and the overall NO _x uptake is controlled by the gas–solid equilibrium between NO₂ and BaO/Ba(NO₃)₂ phase. When we use NO + O₂, complete initial NO _x uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO₃)₂ is independent of the NO concentration. These NO _x uptake characteristics suggest that the NO + O₂ reaction on the surface of Pt particles produces NO₂ that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO _x uptake, this spill-over process is very fast and so it is able to provide complete NO _x storage. However, the NO _x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO₃)₂. The formation of Ba(NO₃)₂ around the Pt particles results in the development of a diffusion barrier for NO₂, and increases the probability of NO₂ desorption and consequently, the beginning of NO _x slip. As NO _x uptake by NO₂ spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO₂ uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO₂ concentration.     

The effect of Mo on the catalytic and surface properties of Rh-Mo/ZrO2 catalysts

A series of Rh-Mo/ZrO₂ catalysts with fixed Rh and different Mo loadings were prepared and characterized by H₂ chemisorption, XRD, TPR, TEM and XPS. The catalysts were studied in two reactions: the hydrogenation of carbon monoxide and the hydrogenation of toluene. The results suggest that the increase in the Mo content produces a partial coverage of the support and the Rh particles. Moreover, at high Mo coverage, an increase of the MoO₃ layer thickness is produced. After being treated in hydrogen, the molybdenum oxide remains as slightly reduced particles, while Rh is essentially as Rh⁰, with only a small contribution of Rh+ species. The Mo promotes the formation of oxygenates in the CO hydrogenation and it does not affect the activity in the hydrogenation of toluene.