Influence Of Metal-support Interactions Research Articles

Tailoring the Brønsted acidity of Ti-OH species by regulating Pt-TiO2 interaction.

Bifunctional catalysts comprising metal and acid sites are commonly used for many reactions. Interfacial acid sites impact intermediate reactions more than other sites. However, controlling the type and amounts of interfacial acid sites by regulating metal-support interaction (MSI) via traditional methods is difficult. Thus, the influence of MSI on interfacial acid sites remains unclear. We prepared Pt-mTiO2/α-Al2O3 (m represents the cycle number of TiO2) catalysts via atomic layer deposition (ALD). New Brønsted acid sites were generated via Pt-TiO2 interaction, and the acidity was precisely regulated by regulating Pt-TiO2 interaction by changing the TiO2 nanolayer thickness. We chose levulinic acid (LA) hydrogenation as a model reaction. The catalytic activity varied with the TiO2 nanolayer thickness and was linearly correlated with the Ti-OH species (Brønsted acid) content. Pt-40TiO2/α-Al2O3, with the highest acid site content of 0.486 mmol/g, exhibited the best catalytic activity. Hydrogen spillover and water dissociation at the Pt-TiO2 interface promoted Ti-OH species generation.

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO2 thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions. The hydrogenation is performed under isothermal reaction conditions using a pulsed molecular beam reactive scattering (pMBRS) technique. Besides the activity and selectivity of clean Pd particles also the impact of dehydrogenated species intentionally introduced a priori is elucidated, whereas the active phase of the catalyst is additionally characterized by CO infrared reflection-absorption spectroscopy (IRRAS) and post-mortem temperature-programmed reaction (TPR). Metal-support interactions are found to influence the catalytic properties of Pd particles by charge-transfer, where positive charging leads to increased activity for acetylene hydrogenation. However, the increased activity is accompanied by formation of undesired byproducts. The active sites for acetylene and ethylene hydrogenation are shown to be different as previously proposed by the A and E model. The availability of the two different active sites on the Pd nanoparticles is determined by dehydrogenated species, whose nature and stability can be tuned by metal-support interactions. Based on these findings an electronic model is proposed how selectivity for acetylene hydrogenation can be steered solely by metal-support interactions leading to blocking of unselective sites in situ.

Influence of metal-support interactions on reaction pathways over Ni/CeZrOx–Al2O3 catalysts for ethanol steam reforming

This work investigates selective Ni locations over Ni/CeZrOx–Al2O3 catalysts at different Ni loading contents and their influences on reaction pathways in ethanol steam reforming (ESR). Depending on the Ni loading contents, the added Ni selectively interacts with CeZrOx–Al2O3, resulting in the stepwise locations of Ni over CeZrOx–Al2O3. This behavior induces a remarkable difference in hydrogen production and coke formation in ESR. The selective interaction between Ni and CeZrOx for 10-wt.% Ni generates more oxygen vacancies in the CeZrOx lattice. The Ni sites near the oxygen vacancies enhance reforming via steam activation, resulting in the highest hydrogen production rate of 1863.0 μmol/gcat·min. In contrast, for 15 and 20-wt.% Ni, excessive Ni is additionally deposited on Al2O3 after the saturation of Ni–CeZrOx interactions. These Ni sites on Al2O3 accelerate coking from the ethylene produced on the acidic sites, resulting in a high coke amount of 19.1 mgc/gcat·h (20Ni/CZ-Al).

Influence of metal-support interaction on nitrate hydrogenation over Rh and Rh-Cu nanoparticles dispersed on Al2O3 and TiO2 supports

Well-defined Rh and Rh-Cu nanoparticles (NP's) of 1.6nm and 1.3nm, respectively, were synthesized by alkaline polyol method and then dispersed on insulating (Al2O3) and semiconducting (TiO2) supports. Both colloidal NP's and supported NP's were characterized using various experimental methods (TEM, XPS, XRD, etc.) to gather information about their specific morphology, structure and chemical state. The effects of size and support on the catalytic behavior of NP's for nitrate hydrogenation reaction were analyzed. Oxide supports, especially TiO2, were found to have a strong positive effect on the catalytic activity of metallic NP's. The non-supported, colloidal, Rh and Rh-Cu NP's are either inactive or posses very low hydrogenation activity. For supported materials, the intimate contact between two metals (i.e. Rh-Cu) is required to attain good hydrogenation activity. The strong metal-support interaction, induced by hydrogen spillover, is a key point in determining hydrogenation activity. The Rh-Cu NP's dispersed on TiO2 are extremely active for NO3− and NO2− (intermediate) deep hydrogenation, with high selectivity for NH4+. The hydrogenation activity of Rh-Cu NP's supported on Al2O3 is hindered considerably, the main products of NO3− hydrogenation being NO2− intermediate.

ChemInform Abstract: The Influence of MSI (Metal—Support Interactions) on Activity and Selectivity in the Hydrogenation of Aldehydes and Ketones

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The influence of MSI (metal-support interactions) on phenylacetaldehyde hydrogenation over Pt catalysts

TiO 2-supported Pt, after a high-temperature reduction (HTR) at 773 K to induce the SMSI state, had a turnover frequency (TOF) for phenylacetylaldehyde hydrogenation that was 15–20 times higher than TOF values for Pt dispersed on either SiO 2 or η-Al 2O 3 and, since all supported catalysts had Pt dispersions of unity (1 nm crystallites), the TOF for Pt TiO 2 (HTR) based on complete dispersion was still 2–5 times greater than the other supported catalysts. More importantly, selectivity to 2-phenylethanol was markedly enhanced over Pt TiO 2 (HTR), comprising 70% of the product at conversions as high as 60%. Pt TiO 2 after a low temperature reduction (LTR) at 473 K gave neither the high TOFs nor the enhanced selectivity. Among these catalysts the highest activation energy of 12.4 kcal/mol was attained with Pt TiO 2 (HTR) and the reaction order on phenylacetaldehyde was between 0 and 1 2 while that on H 2 was around 1 2 or higher. At higher conversions (over 50%) significant hydrogenolysis activity to form benzene and toluene occurred on all Pt catalysts except Pt TiO 2 (HTR). The higher hydrogenation activity and suppressed hydrogenolysis capability with the Pt TiO 2 (HTR) catalyst is attributed to TiO x species migrating onto the Pt surface to create active sites at the Pt-titania interface while simultaneously destroying large ensembles of Pt atoms required for hydrogenolysis reactions; however, competitive desorption/readsorption processes must also play a significant role in enhancing selectivity to the intermediate phenylethanol product.

The influence of MSI (metal–support interactions) on activity and selectivity in the hydrogenation of aldehydes and ketones

The vapor–phase hydrogenation of a group of aldehydes and ketones which contain unsaturated C=C bonds has been studied over a family of supported Pt catalysts. Turnover frequencies, activation energies and reaction orders for crotonaldehyde, benzaldehyde, phenylacetaldehyde and acetophenone are provided and compared to those for butyraldehyde, benzyl alcohol, 1–phenylethanol, acetylcyclohexane and acetone. Metal–support interactions (MSI) induced in Pt/TiO2 not only enhance specific activity but also markedly shift selectivity because hydrogenation of the carbonyl bond is favored. The retention of high selectivity to the intermediate unsaturated alcohols shows that adsorption properties, as well as kinetic parameters, are altered. A model is discussed which describes the sites responsible for this behavior.

The Influence of Metal-Support Interactions on the Accurate Determination of Ru Dispersion for Ru/TiO2

Titania-supported Ru catalysts have been characterized by TEM, 1H NMR, and H2 chemisorption to determine the metal particle size, the fraction of the metal surface available for H2 chemisorption, and the H2 adsorption capacity of the catalyst, as functions of the reduction temperature. TEM micrographs show that as the reduction temperature rises from 573 to 773 K, the average particle size of Ru remains the same but the surface of the particles is covered to an increasing extent by an amorphous layer of titania. Quantitative estimates of the fraction of the Ru particle surface available for H2 chemisorption were obtained by 1H NMR. The NMR spectra also show that a fraction of the adsorbed H2 spills over onto the support and that as a consequence measurements of total H2 chemisorption overestimate the number of Ru sites available for H2 adsorption. The implications of these results for the correct calculation of Ru dispersion and the determination of turnover frequencies for reactions carried out over Ru/TiO2 are discussed.

The Influence of Metal-Support Interactions on the Whiteline Intensity

The whiteline intensity of Pt/K-LTL catalysts reduced at 300, 450, and 600°C decreases with increasing reduction temperature. This change in whiteline intensity was ascribed to the removal of hydrogen from the metal-support interface by reduction at higher temperature.

The influence of metal-support interactions on the reaction of n-hexane over supported Pt catalysts

The hydrogenolysis and skeletal rearrangement reactions of n-hexane have been investigated on Pt catalysts supported on silica and titania. The results show that the product distribution is dependent on the choice of support but that Pt/titania catalysts in the “normal” state are similar to Pt/titania catalysts in the “strong metal-support interaction” (SMSI) state. It is observed that in the SMSI state, when more than 99% of the Pt is inaccessible to hydrogen, the activity is only slightly reduced. Surprisingly, the selectivity for hydrogenolysis versus skeletal isomerisation is almost the same in normal and in SMSI catalysts. The results provide evidence that the same active centre is involved in both the hydrogenolysis and the skeletal rearrangement reactions, and do not support the contention that hydrogenolysis requires a larger ensemble of Pt atoms than skeletal rearrangement. The results are interpreted in terms of an active centre consisting of a single Pt atom located in the planes of the small Pt crystallites. The precise selectivity observed may depend on the local environment of the Pt atom during the sojourn of the n-hexane molecule on the active Centre.

The influence of metal-support interactions on the reaction of methylcyclopentane over supported Pt catalysts

Silica- and titania-supported Pt catalysts have been prepared and their reactivity in the hydrogenolysis of methylcyclopentane has been studied as a function of pretreatment. The Pt/silica catalysts are little affected by an increase in reduction temperature. When the Pt/titania catalysts are reduced at low temperature they have normal chemisorption properties. However, although the activity is similar to that of the Pt/silica catalysts, the product distributions are substantially different. In particular the Pt/titania catalysts are more selective for the formation of n-hexane. After reduction at high temperature the Pt/titania catalysts no longer adsorb hydrogen and the catalytic activity is totally eliminated. When a reoxidation is performed at 298 K there is a partial recovery of the hydrogen chemisorption capacity and the turnover number of the Pt returns to its original value. However, in this “intermediate” state the catalyst produces even more n-hexane and less 2-methylpentane. From an analysis of the activation energy data it is concluded that the nature of the active centres is not affected by reduction and reoxidation treatments. Instead, the change in product distribution appears to parallel a variation in the relative proportions of the various types of active centre present. The results are discussed in terms of various mechanisms for the methylcyclopentane reaction. It is concluded that the increased selectivity for the formation of n-hexane is consistent with a reaction at the metal-titania interface, possibly involving adsorption at a PtTi 3+ site. This site is believed to exist even in Pt/titania catalysts reduced at low temperatures.

The influence of metal-support interactions on the reaction of n-hexane over supported Rh catalysts

Rh catalysts have been prepared by impregnation of silica or titania and their activities and selectivities determined for the reaction of n-hexane. Titania-supported Rh catalysts are more active than silica-supported and have a different selectivity for isomerization being much less selective for the formation of 2- methylpentane. The results provide evidence of a real support effect even in catalysts reduced at low temperatures. When the Rh/titania catalysts are reduced at high temperatures to induce “Strong Metal-Support Interactions” the turnover number is increased but the selectivity and product distribution are unaffected. When the SMSI Rh/titania catalysts are re-oxidized at 298 K to partially reverse the SMSI state the turnover number decreases by more than two orders of magnitude, but the selectivity does not change. It is concluded that although SMSI catalysts behave differently from “normal” Rh/titania catalysts the creation of the SMSI state is not a prerequisite for obtaining high activity or unusual selectivity. A model is presented in which the active centres for the n-hexane reaction are considered to be Rh atoms in flat surfaces. The reforming of hydrocarbons is a process of major industrial importance. The results obtained demonstrate that metal-support interactions can affect the selectivity of a metal in reforming reactions and provide information on the nature of the active centre required for the skeletal rearrangement of hydrocarbons.

The influence of metal-support interactions on the catalytic properties of Pd/La2O3

Evidence is presented for the existence of metal-support interactions between Pd and La 2 O 3 . XPS spectra indicate that the Pd 3d 5 2 binding energy is shifted below the value of bulk Pd by as much as 0.7 eV. The negative binding energy shift increases with decreasing Pd dispersion. The chemisorption of CO at 298K is suppressed on the Pd microcrystallites, and the amount of suppression also increases with decreasing Pd dispersion. This latter effect exerts a strong influence on the morphology of the Pd surface during methanol synthesis. The turnover frequency for CH 3 OH synthesis from CO and H 2 correlates with the Pd morphology: Pd(100) planes are 2.8 times more active than Pd(111) planes. The division of the Pd surface into a mixture of these two planes is inferred from the infrared spectrum of adsorbed CO. For a fixed Pd morphology, the CH 3 OH turnover frequently is independent of Pd dispersion. By contrast, the CH 4 turnover frequency decreases with increasing Pd dispersion, and it is insensitive to the distribution of Pd(100) and Pd(111) planes.

The influence of metal-support interactions on the catalytic properties of Pd/La 2O 3

Evidence is presented for the existence of metal-support interactions between Pd and La 2O 3. XPS spectra indicate that the Pd 3d 5 2 binding energy is shifted below the value of bulk Pd by as much as 0.7 eV. The negative binding energy shift increases with decreasing Pd dispersion. The chemisorption of CO at 298K is suppressed on the Pd microcrystallites, and the amount of suppression also increases with decreasing Pd dispersion. This latter effect exerts a strong influence on the morphology of the Pd surface during methanol synthesis. The turnover frequency for CH 3OH synthesis from CO and H 2 correlates with the Pd morphology: Pd(100) planes are 2.8 times more active than Pd(111) planes. The division of the Pd surface into a mixture of these two planes is inferred from the infrared spectrum of adsorbed CO. For a fixed Pd morphology, the CH 3OH turnover frequently is independent of Pd dispersion. By contrast, the CH 4 turnover frequency decreases with increasing Pd dispersion, and it is insensitive to the distribution of Pd(100) and Pd(111) planes.

Influence of metal-support interactions on the dispersion, distribution, reducibility and catalytic activity of Ni/SiO 2 catalysts

The influence of metal-support interactions on the dispersion, distribution, reducibility and catalytic activity for benzene hydrogenation of Ni/Si0 2 catalysts, prepared by impregnation or deposition-precipitation, was studied. Weak interact-ions were observed for the former; the latter technique gave rise to strong inter-actions. Nickel contents were from 6–9 wt%. The impregnated precursor featured nickel evenly distributed between the internal (pores) and external surface of the support. Direct reduction did not noticeably change the distribution and a Ni/SiO 2 catalyst formed in which the active phase was finely divided and evenly distributed. Calcination of this precursor caused migration of the nickel to the external surface with the formation there of large crystallites of NiO. The pre-cursor prepared by deposition-precipitation, consisted of nickel hydro-silicate deposited uniquely on the external surface and in the pore mouths. Calcination or reduction of this material did not noticeably change the distribution, but high dispersion was observed in both cases. The stability of catalysts prepared by this technique is discussed in terms of interaction between nickel and silica.