Incipient wetness support effect

Highly Lewis-acidic chloroaluminate ionic liquids (ILs) are well known to be both versatile solvents and effective catalysts for Friedel-Crafts reactions [1,2]. Tailoring the physical and chemical properties of the ILs to the needs of a specific reaction allows for a high diversity of applications [3,4]. We could show that immobilising these ILs on inorganic supports yields very active catalysts for alkylation reactions. The immobilisation of ionic liquids leads to novel Lewis-acidic catalysts (NLACs). The methods presented include the method of incipient wetness (method 1, further on called NLAC I), which has been introduced in detail by Hoelderich et al. f5], but focus of this presentation lies on the methods 2 (NLAC II) and 3 (NLAC III). 

A series of 7-aiumina- and silica-supported oxides as MnzOs, C03O4 and Sm-Mn, Sm-Co bimetallic combinations was prepared by incipient wetness impregnation of the supports with aqueous solutions containing the nitrates of the metals and citric acid. The results obtained with these catalysts in the acetone complete oxidation reaction have shown a predominant effect of the support and the transition metal nature. The silica-supported oxides exhibited a bulk-like behaviour with relatively little difference between the Mn and Co catalysts. In contrast, when supported on alumina the performance of the Mn catalysts was remarkably better than that of the Co-based ones. In this case, whereas the Sm-Mn catalysts maintained a rather good performance relative to that of Mn/Al203, the temperatures required for acetone combustion over the Sm-Co catalysts were significantly lower than the ones over C0/AI2O3. 

On the other hand, cerium has been shown to be an effective oxygen reservoir, enhancing the activity of many oxidation catalysts. Due to this property, cerium oxide is also considered to potentially enhance the thioresistance of the catatysts. This aspect is of great practical importance, since the use of palladium catalysts is hindered by the poisonous effect of sulphur compoimds, often present in off gases. Most works dealing with ceria-zirconia catalysts have been carried out with catalysts prepared by coprecipitation methods, whereas in this work an ahemative procedure, based on the incipient wetness technique is used to incorporate ceria to the zirconia support. The aim is to maintain the advantages of zirconia supports, especially the thermal stability. 

Supported nickel catalysts are widely used for hydrogenation reactions. The most common supports are silica and activated carbon, however several studies have shown that clays are rather more effective in maintaining high metal surface area and can impart useful selectivity to the catalyst.18 Deposition of nickel(II) by the incipient wetness technique [in which the volume of nickel(II) solution used is equal to the pore volume of the clay and then dried] followed by H2 reduction was found to be the most effective method of generating a high dispersion of metal particles (< 10 nm in size) within the clay. 

To make effective use of the pore structure of the support, incipient-wetness impregnation can be performed in which the metal precursor is dissolved in just... 

A similar Rh complex, [HRh(CO)(PPh3)3], immobilized onto AC and CNTs (ends-opened) by an incipient wetness technique was tested as a catalyst for propene hydroformylation [70]. Activity assay of the catalysts showed that the CNT-supported Rh complex displayed not only high activity for propene conversion but also excellent regioselectivity to the butyraldehyde product. Under the experimental conditions used, the molar ratio of normal to branched aldehydes reached 12 to 13 at a TOP of 0.12 s corresponding to a propene conversion of 32%. To understand the excellent catalytic behavior of the CNT-supported catalyst, an ends-unopened CNT was also used as support and its catalytic properties tested In this particular case, propene conversion and the nii ratio reached only 17.2% (corresponding to a TOP of 0.06 s ) and 6.0, respectively. On the basis of this result, the authors concluded that the high propene conversion and excellent regioselectivity demonstrated by ends-opened CNT-supported catalyst was due mainly to the confinement effect induced by the presence of a Rh-phosphine complex in the inner surface of the tubular nanochannels of CNT [70]. 

A study of the metal-support interaction effect has been carried out for a Pt-Ti02 catalyst prepared by microemulsion. Isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane were chosen as model reactions for studying the influence of the catalyst nature on the strong metal-support interaction (SMSI) effect. A comparison between the cata-lyticbehaviour of similar catalysts prepared by microemulsion and incipient wetness method,was reported. The catalysts were reduced at 200 and 390 °C since it is well known that the reduction temperature plays a predominant role in the SMSI effect l The behaviour of thePt-TiO microemulsion-prepared catalyst was found to be similar to that of a Pt/AbOs catalyst. Also, it was found that the microemulsion-prepared catalysts require a higher temperature of reduction to induce metal-support interactions. 

The effect of 25 ppm SO2 with respect to the complete oxidation of methane over supported palladium catalysts has been examined. Pd nitrate was impregnated onto alumina, Ba-modified alumina and La-modified alumina by incipient wetness impregnation. The calculated metal loading was 2.5 wt%. Two different sets of catalysts have been prepared one calcined at 500 C and the other at 1000 C. The main purpose of these two calcinations temperatures was to obtain different metal particle sizes. The results show that the combustion activity is strongly affected by the nature of the support when large particles are present. On the other hand, the small Pd particles exhibit similar behavior regardless of the nature of the support. When SO2 is added to the gas stream, a strong deactivation is observed and the presence of additive to stabilize the support increases the loss of activity of the supported Pd particles. 

In the simplest cases, a proper choice of pH should allow one to select the most abundant species in solution to be deposited onto the support. HNO3, carboxylic acids or NH3 are usually chosen as pH adjusters, because these molecules decompose or desorb during thermal treatments. However, in the case of incipient wetness impregnation involving solutions that are neither very acidic nor very basic, the pH in the pore space is regulated by the ad do-basic surface hydroxyls of the support ( buffer effect ) [59] it is thus brought close to the point of zero charge (PZC) of the oxide (see also Chapter 3). 

Pdll deposition was effected by incipient wetness impregnation of either the M0/AI2O3 precursors or the bare support with aqueous solutions of either [Pd(H20)4]2+ nitrate (prepared by dilution of a commercial solution immediately before use to avoid Pd hydrolysis - procedure denoted PdN) or of [Pd(NH3)4]2+ chloride (PdA, for Palladium tetraAmmine). Samples mentioned in this study... 

The purpose of the presem investigation was to elucidate the effect of oxidizing modification of the support (activated carbon) surface on the active phase during the preparation of the Cu/activated carbon catalysts, by depositing copper nitrate from aqueous and methanolic media using the incipient wetness technique. 

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