Evidence for a weak metal-support interaction in thoria-supported nickel catalysts

Reduction of Ni/ThO₂ catalysts at increasing temperatures does not result in a significant variation of the activity (per gram of metallic nickel) towards ethane hydrogenolysis and CO hydrogenation. O₂ treatment followed by H₂ reduction at low temperature does not produce an important increase of the activity either. Then, it is concluded that the Ni/ThO₂ studied does not present the strong metal-support interaction phenomenon (SMSI).

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Ni/ThO₂ ( ) CO. O₂ H₂ . , Ni/ThO₂ .

     

Evidence of a strong metal-support interaction in zirconia-supported nickel catalysts

The reduction of Ni/ZrO₂ catalysts above 700 K results in a decrease of hydrogen adsorption and of the catalytic activity toward ethane hydrogenolysis. O₂ treatments followed by reduction at low temperature of samples reduced at high temperatures restore the normal properties. This behavior is similar to that observed for Ni/SiO₂, Ni/TiO₂ and is characteristic of a strong metal-support interaction similar to that observed on Pt/TiO₂ catalysts. Paradoxically, it is shown that Ni does not interact strongly with ZrO₂ since it is easily reduced and its reduction gives very large metallic particles.

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Ni/ZrO₂ 700 . , , . Ni/SiO₂ Ni/TiO₂ , Pt/TiO₂. , Ni ZrO₂, .. .

     

CeO2-supported nickel catalysts. Evidence of a strong metal-support ineraction

An increase of the reduction temperature of a Ni/CeO₂ solid results in a continuous decrease of the catalytic activity toward C₂H₆ hydrogenolysis and of the quantity of adsorbed H₂. O₂ treatments with subsequent reduction at low temperature restore a part of the initial properties. This behavior, typical of a strong metal support interaction, is similar to that of Ni/TiO₂, Ni/SiO₂ and Ni/ZrO₂ solids and shows that the SMSI phenomenon is general.

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Ni/CeO₂ C₂H₆ H₂. . , (), Ni/TiO₂, Ni/SiO₂ Ni/ZrO₂ , .

     

Kinetics of toluene hydrogenation over platinum-titania catalysts in conditions of strong metal-support interaction

Toluene hydrogenation was investigated over different platinum-titania catalysts. It was found that the form of kinetic equation of the reaction does not depend on the modifications of titania (rutile, anatase) but the catalyst activity is different. The catalyst reduction at temperature higher than 300^oC leads to strong metal-support interaction (SMSI) which is manifested in a decrease of the reaction rate.     

Engineering Pt in ceria for a maximum metal-support interaction in catalysis

Conventional supported metal catalysts are metal nanoparticles deposited on high surface area oxide supports with a poorly defined metal-support interface. Typically, the traditionally prepared Pt/ceria catalyzes both methanation (H2/CO to CH4) and water-gas shift (CO/H2O to CO2/H2) reactions. By using simple nanochemistry techniques, we show for the first time that Pt or PtAu metal can be created inside each CeO2 particle with tailored dimensions. The encapsulated metal is shown to interact with the thin CeO2 overlayer in each single particle in an optimum geometry to create a unique interface, giving high activity and excellent selectivity for the water-gas shift reaction, but is totally inert for methanation. Thus, this work clearly demonstrates the significance of nanoengineering of a single catalyst particle by a bottom-up construction approach in modern catalyst design which could enable exploitation of catalyst site differentiation, leading to new catalytic properties.     

Reaction specificity in catalysts reported to exhibit strong metal-support interactions

The specific activities of a 10% Ni/TiO₂ and a 10% Ni/SiO₂ catalyst for the hydrogenolysis of n-hexane and for the methanation reaction have been investigated. Both catalysts have very similar activities and selectivities for hydrogenolysis. However, the titania-supported catalyst exhibits a considerably greater specific activity for methanation. It is concluded that this enhancement of activity is reaction specific, and we propose a mechanism involving the adsorption of CO onto the surface of titania.

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10% Ni/TiO₂ 10% Ni/SiO₂ - . . , TiO₂ . , . , CO .

     

Boosting selectivity and stability on Pt/BN catalysts for propane dehydrogenation via calcination & reduction-mediated strong metal-support interaction

Propane dehydrogenation(PDH) provides an alternative route for producing propylene. Herein, we demonstrates that h-BN is a promising support of Pt-based catalysts for PDH. The Pt catalysts supported on h-BN were prepared by an impregnation method using Pt(NH₃)₄(NO₃)₂ as metal precursors. It has been found that the Pt/BN catalyst undergoing calcination and reduction is highly stable in both PDH reaction and coke-burning regeneration, together with low c...     

An exclusive kinetic model for the methylcyclohexane dehydrogenation over alumina-supported Pt catalysts

The experimental data of six different types of Pt-containing alumina catalysts are used to study the detailed and rigorous kinetics of the methylcyclohexane dehydrogenation reaction. A large number of kinetic rate equations were formulated using the power law kinetics and the Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics, which were tested against the experimental data. For each catalyst, the elementary reaction step “the loss of first molecular hydrogen” in the LHHW single-site surface reaction mechanism was observed to be the rate-determining step. The form of the kinetic rate model developed in the study is believed to be applicable to any Pt-loaded alumina catalyst.     

Characterization of strong metal-support interaction in Pt/TiO2 catalysts by thermal programmed desorption of benzene

The Pt–TiO₂ interaction after HTR was studied by the TPD of benzene. Shifts in desorption temperatures are detected after HTR. They may be ascribed to electronic and morphologic changes.

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Pt/TiO₂ . . .

     

Grafting and anchoring of molecular complexes as metal precursors for alumina-supported Pd catalysts

Abstract

The surface coordination chemistry of Pd complexes on alumina has been studied in the framework of synthesizing Pd/γ-Al₂O₃ catalytic materials. Two methodologies were explored: the direct grafting of Pd complexes on hydroxyl functions present at the alumina surface and the anchoring of the precursors via amine-bearing silanes previously grafted on the support. Suitable conditions to graft and anchor Pd complexes on alumina surface were found and experimental proofs of grafting and anchoring processes are provided. The results show that covalent grafting indeed took place for samples prepared in acetonitrile with [Pd(CF₃CO₂)₂(bipy)] and [PdCl₂(PhCN)₂] complexes or with [Pd(OAc)₂] and [Pd(CF₃CO₂)₂] in acetone. The anchoring was successful for catalysts prepared in acetone with 1 wt.% of [Pd(CF₃CO₂)₂] loading. Grafting and anchoring were found to stabilize palladium in its Pd(II) oxidation state. This has an adverse effect on the activation step that should lead to reduction of the complex to give the metallic catalytic supported active phase.     

Alumina as a nickel catalysts support for steam reforming of hydrocarbons

The coking resistance of six alumina supported nickel catalysts in n-butane steam reforming mainly depends on the average size of nickel crystallites. Thus, by using suitable preparative methods, it is possible to produce good, coking resistant nickel catalysts even with Al₂O₃ support.

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Al₂O₃ -, , . . ., , - Al₂O₃.

     

Manifestation of the effect of strong metal-support interaction in a Pt/TiO2 rutile catalyst

Pt/TiO₂ catalysts supported on ultradispersed rutile are prepared. The effect of strong metal-support interaction (SMSI) in these catalysts is more pronounced than in Pt/TiO₂ catalysts with greater fractions of an anatase phase in the support. Mild oxidation of the rutile catalyst eliminates the SMSI effect.     

Stabilization of hexagonal close-packed metallic nickel for alumina-supported systems prepared from Ni(II) glycinate

The decomposition in flowing argon of the neutral complex [Ni^II(glycinate)₂(H₂O)₂] leads to a mixture of face-centered cubic (fcc) and hexagonal close-packed (hcp) metallic nickel. The latter is the main phase when the Ni(II) complex is supported on alumina. Unlike most hexagonal Ni phases described earlier, and similar to hexagonal Ni₃C, the unit cell parameters (a=0.2493 and ) lead to Ni-Ni distances equal to those encountered in fcc Ni. TEM shows that the nanoparticles are protected by graphite layers, whose elimination by heating in hydrogen results in transformation to the fcc phase and crystal growth. Magnetic measurements provide evidence of the coexistence of superparamagnetic and ferromagnetic nanoparticles. This result is in line with the broad size distribution observed by TEM and is interpreted on the basis of the metallic character of hcp Ni particles.     

New synthesis method for nickel phosphide hydrotreating catalysts

Nickel phosphide particles on silica and alumina support were prepared from metal or metal oxide particles by treatment with phosphine and hydrogen at moderate temperature, resulting in small particle sizes equivalent to that of the precursor particle size.     

Nature of the metal-support interaction in bifunctional catalytic Pt/H-ZSM-5 zeolite

The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Br?nsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Br?nsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Br?nsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.     

The influence of strongly reducing conditions on strong metal-support interactions in Cu/ZnO catalysts used for methanol synthesis

A systematic series of binary and ternary copper catalysts was investigated using the methanol synthesis reaction at atmospheric pressure. Strong metal-support interactions between copper and zinc oxide induced by strongly reducing conditions were probed by the adsorption of carbon monoxide, which was monitored qualitatively and quantitatively by a combination of microcalorimetry, temperature-programmed desorption experiments and Fourier transform infrared spectroscopy. For the zinc oxide-containing catalysts, the pretreatment in flowing carbon monoxide at 493 K resulted in a severe decoration of the copper metal particles with ZnOx adspecies, whereas after methanol synthesis at 493 K the state of the copper was essentially identical to that seen after hydrogen reduction. Copper was always found to be present in its zero-valent state.     

CO_2 hydrogenation to methanol over Cu/CeO_2 and Cu/ZrO_2 catalysts:Tuning methanol selectivity via metal-support interaction

Copper-based catalysts for CO_2 hydrogenation to methanol are supported on ZrO_2 and CeO_2, respectively.Reaction results at 3.0 MPa and temperatures between 200 and 300 °C reveal that Cu catalysts supported on ZrO_2 and CeO_2 exhibit better activity and selectivity than pure Cu catalyst due to Cu-support(ZrO_2 and CeO_2) interaction. Combining the structural characterizations with in-situ diffuse reflectance infrared Fourier transform spectroscopy(in-situ DRIFTS), Cu/CeO_2 shows the higher methanol selectivity due to the formation of main carbonates intermediates, which are closely related with the oxygen vacancies over Cu/CeO_2. In contrast, bicarbonate and carboxyl species are observed on Cu/ZrO_2, which originates from the hydroxyl groups presented on catalyst surfaces. Difference in CO_2 adsorption intermediates results in the distinct methanol selectivity over the two catalysts.     

Modification of the surface adsorption properties of alumina-supported Pd catalysts for the electrocatalytic hydrogenation of phenol

The electrocatalytic hydrogenation (ECH) of phenol has been studied using palladium supported on gamma-alumina (10% Pd-Al2O3) catalysts. The catalyst powders were suspended in aqueous supporting electrolyte solutions containing methanol and short-chain aliphatic acids (acetic acid, propionic acid, or butyric acid) and were dynamically circulated through a reticulated vitreous carbon cathode. The efficiency of the hydrogenation process was measured as a function of the total electrolytic charge and was compared for different types of supporting electrolyte and for various solvent compositions. Our results show that these experimental parameters strongly affect the overall ECH efficiency of phenol. The ECH efficiency and yields vary inversely with the quantity of methanol present in the electrolytic solutions, whereas the presence of aliphatic carboxylic acids increased the ECH efficiency in proportion to the chain length of the specific acids employed. In all cases, ECH efficiency was directly correlated with the adsorption properties of phenol onto the Pd-alumina catalyst in the studied electrolyte solution, as measured independently using dynamic adsorption isotherms. It is shown that the alumina surface binds the aliphatic acids via the carboxylate terminations and transforms the catalyst into an organically functionalized material. Temperature-programmed mass spectrometry analysis and diffuse-reflectance infrared spectroscopy measurements confirm that the organic acids are stably bound to the alumina surface below 200 degrees C, with coverages that are independent of the acid chain length. These reproducibly functionalized alumina surfaces control the adsorption/desorption equilibrium of the target phenol molecules and allow us to prepare new electrocatalytic materials to enhance the efficiency of the ECH process. The in situ grafting of specific aliphatic acids on general purpose Pd-alumina catalysts offers a new and flexible mechanism to control the ECH process to enhance the selectivity, efficiency, and yields according to the properties of the specific target molecule.     

Morphological and chemical influences on alumina-supported palladium catalysts active for the gas phase hydrogenation of crotonaldehyde

A series of five alumina-supported palladium catalysts have previously been prepared and characterised by a combination of CO chemisorption and infrared spectroscopy. The reactive attributes of these catalysts are examined using the hydrogenation of crotonaldehyde as a test reaction, using a modified infrared gas cell as a batch reactor. Periodic scanning of the infrared spectrum of the gaseous phase present over the Pd/Al(2)O(3) catalysts was used to construct reaction profiles. Four of the catalysts were able to facilitate a 2-stage hydrogenation process (crotonaldehyde → butanal → butanol), whilst one catalyst was totally selective for the first stage hydrogenation process (crotonaldehyde → butanal). Rate coefficients for the first and second stage hydrogenation processes are normalised to the number of surface palladium atoms for the particular catalyst. Correlation of these kinetic parameters as a function of mean particle size indicates the first stage process to be structure insensitive, whilst the second stage hydrogenation is structure sensitive. Chlorine residues associated with the preparative process of one of the catalysts is seen to selectively poison the second stage hydrogenation process for that catalyst. Structure/activity relationships are considered to explain the observed trends.