Impregnation

Impregnation is a common technique for distributing active metals within the pores of a catalyst support. Calcined supports are especially porous. Like sponges, they use capillary action to suck up aqueous solutions containing 

In the incipient wetness method, precise amounts of solution are added -just enough to leave the support dry to the touch. After a drying step, addition solution may be added to increase loading of the same or different active metal. 

In the case of wet impregnation, a fourth process is operative, viz. transport of solute to the outer particle surface. Depending on the process conditions, different profiles of the active phase over the support body will be obtained. For instance, depending on the pH, the interaction with the support can be strong or weak, and even repulsion can exist. 

Soluble catalyst precursors are fixed to the support either by reaction, exchange with surface OH groups, and/or by adsorption. 

In the former case, the concentration (density) of surface OH groups, which depend on the pre-treatment of the support, is crucial. In the latter case, the surface charge plays an important role. At a pH value of the so-called Point of Zero Charge (PZC) the surface is electrically neutral. At pH values above PZC, the surface is negatively charged, while at pH values below PZC the surface is positively charged (see Figs. 1.2 and 1.3). 

For silica this can be illustrated as follows. At pH = 3 the surface is neutral. In a mildly basic environment is removed, and, as a result, the surface is negatively charged. In an acid environment the surface will become protonated. If it is intended to deposit anions onto the carrier surface, the preparation should proceed at pH values below the PZC, whereas if cations are to be deposited, a pH value above that of the PZC is preferred (Fig. 1.2). Table 1.1 gives PZC values for alumina, silica, and a mixture of alumina and silica. 

It should be mentioned that the exact PZC values not only depend on the chemical nature of the carrier, but also on its history and the method by which it was prepared. Of course, for the solid 

From a chemical point of view, impregnation and precipitation represent two extreme cases  

In this chapter, the most common techniques for preparation of supported metal catalysts will be discussed, including impregnation, coprecipitation, homogeneous deposition precipitation, and precipitation at constant pH. In principle, these techniques can all be used to attach the active phase to supports, some preferably in the form of a powder, others in the form of a pre-shaped body. First, a general description of the techniques will be presented. Then, the techniques are illustrated by specific examples of the preparation of metallic catalysts. In view of the expertise of the authors of this chapter, Pt, Au, and Ag as the active metal phases will be emphasized. The last two examples are focused on the production of propene oxide and, as a consequence, they refer to an unresolved research issue. The results on the Ag catalysis have not been published elsewhere, and are therefore treated extensively. 

For silica, this can be illustrated as follows. At pH = 3, the surface is neutral. In a mildly basic environment, H+ions are removed, and, as a result, the surface is negatively charged. In an acidic environment, the 

Due to the ease of preparation, impregnation is one of the most commonly used techniques to fabricate catalysts. High-surface-area carbon black can be impregnated with catalyst precursors by mixing the two in an aqueous solution 

The most common platinum precursors used for impregnation are chloride salts however, it has been argued that the metal chloride salts might lead to 

7 SOME PRACTICAL APPLICATIONS AND IMPLICATIONS OF WETTING IMPREGNATION, FLOTATION, 

FIGURE 8.10 (a) Capillary suppression and (b) rise in a capillary determining penetration 

In this section, some of the most important methods for catalyst preparation including impregnation, grafting, precipitation, and chemical vapor deposition (CVD) are discussed [6-18], 

This is a process designed to cover a catalyst support, such as silica, alumina, mesoporous molecular sieves, or other supports with a metallic catalyst, or other catalytically active materials. The process is carried out by contacting the solid support, for a precise time, with a solution containing the active elements, to introduce a solution of the precursor into the pores of the support. During the impregnation process, the support can be completely free of the solvent when the precursor is dissolved. In this 

The Physical Chemistry of Materials Energy and Environmental Applications 

Washing, drying, and frequent calcination occurs between and after impregnations [7,8] and in order to prepare metallic-supported catalysts, it is as well necessary to carry out a reduction process [9], The most common reducing agent is H2. 

To conclude, this method for metal catalyst preparation consists of the impregnation of a preformed support with metal precursors and subsequent calcination and reduction. 

Preformed, absorbent supports are uniformly saturated with a solution of the catalytic metals. Several impregnations may be needed to obtain the required metal loading. Supports must be strong enough to be immersed directly into the solution and any dust forming should not contaminate the catalyst surface. To avoid these difficulties, the supports can be sprayed with just enough solution to completely fill the pores. At this point the support suddenly appears to be wet and the procedure is known as incipient wetness. In some cases, when the support has been impregnated, the metals may be precipitated by immersion in a second solution. 

After impregnation catalysts are carefully dried and most are calcined before use to decompose the metal salts to oxides. Care is necessary to avoid high concentrations of metals forming at the support surface as the solution evaporates. Most types of supports can be used to produce impregnated catalysts, although alumina or silica, in various forms, is usually chosen. The support should not, of course, react with the metal solution. For example, if the support is soluble in acid, there is a possibility of re-precipitation in an undesirable form. 

Rovings and mats are the most widely used reinforcements in the production of flat sections with thin walls. Very often, manufacturers insert a roving layer between two mat layers, to give a sandwich structure. The mat does not actually have enough resistance to resin impregnation without the additional support of roving. 

This issue, particularly relevant in the past, is now perceived less as a problem. The end of a mat roll can be sewed onto the initial part of the successive roll, and this is done by hand. Every roll lasts from 1 to 3 h at an average speed this means that, if many rolls are used simultaneously, the sewing operation can be required at such a frequency that it becomes very expensive. Besides, the mat should be handled and directed with greater care than roving. 

Pultruded resins are, in the majority of cases, made of unsaturated polyester, but there can also be cases of epoxy or silicon resins for specific and limited applications. Polyester-based matrices are made of esters between unsaturated acids and glycols, diluted in a polymerizable monomer (styrene), which allows the formation of a three-dimensional grating for polymerization. 

Epoxy resins are mainly used for carbon strengthening. Alternatively, the use of a polyester matrix with this kind of strengthening would lead to a composite with dramatically lower mechanical properties. Specifically, it would result in a reduction 

Alongside the method just described as an example, different impregnation systems are available. In 1971, a company patented a system of impregnation based on resin injection with a drawing machine under 6 atm pressure. This system has been widely applied for the production of pipes wifli average mechanical properties, pigmented with a gel-coat layer. 

The basic principle behind supported metal oxide catalyst preparation involves deposition followed by activation. According to The International Union of Pure and Applied Chemistry (lUPAC) recommendations, deposition is defined as the application of the catalytic component on to [sic] a separately produced support, and activation as the transformation of the precursor to the active phase, usually entailing the calcination of the composite material [17]. There are several well-studied deposition steps, with impregnation being the most common method for preparing industrial supported metal oxides. 

Contacting the support material with a solution that contains the supported metal 

As there are no qualitative differences between the various noble metals as far as the chemistry involved in placing them on the various supports is concerned, we will limit our discussion to just one of them, viz. platinum. 

Alumina-supported Pt catalyst are conventionally made via impregnation of Y-AI2O3 extrudates with an aqueous solution of chloroplatinic acid, H2PtCl6. The acid adsorbs strongly on the alumina surface and, in view of its being relatively irreversible, can be described as involving an inner sphere complex according to  

The Al-OH groups involved in this reaction are likely to be the basic ones, since the adsorption reaction is probably to be thought of as analogous to the hydrolysis of the PtClk2 ) species that occurs in aqueous solution at higher pH values. The fact that the platinum adsorption capacity of a typical y-alumina is of the order of 1.5 pmol/m, while the amount of basic OH groups is typically 3 to 3.5 pmol/m2, as determined by titration with Mo02(acetylacetonate)2, fits in nicely with this idea. 

This competitive adsorption drives the platinum deeper into the extrudates, and when sufficient HC1 is added, the PtCl 2 ) adsorption reaction is moderated to such an extent that a reasonably homogeneous distribution is obtained. One can also add acids that adsorb even more strongly than chloroplatinic acid, e.g. oxalic or citric acid, with which Pt profiles such as shown in Fig. 9.6 can be prepared. Profile (b) can be useful when a strongly adsorbing poison is present in the stream 

After impregnation a Pt/A C catalyst is dried and calcined to produce supported Pt-oxide species (some Cl is retained on the alumina), which can then be reduced, usually at relatively mild temperatures (about 300-350°C) in hydrogen to produce small Pt particles (about 10 A). 

Methods of Preparation using Gold Chloride Precursors 

The very first supported gold catalysts were prepared by impregnation (IMP)9,41 since this is the simplest method, and can be used with any support. The precursors most often used were chloroauric acid 

The major utility of PTFE dispersions is that they allow processing of PTFE resin, which cannot be processed as ordinary polymeric melt, because of its extraordinarily high melt viscosity, or as solution, because it is insoluble. Thus, PTFE dispersions can be used to coat fabrics and yams to impregnate libers, nonwoven fabrics, and other porous structures to produce antistick and low-friction coatings on metals and other substrates and to produce cast films. 

To convert a dispersion into a sintered PTFE film, four distinct steps are required (1) casting (dipping or flowing out) onto a supporting surface (2) drying to remove water (3) baking to remove the surfactant and (4) sintering to obtain a clear coherent film. 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be 

This is the classical method used to make supported noble metal catalysts and consists of impregnating a support with a solution of a metal salt. This usually involves suspending the support in a larger volume of solution, from which the solvent is then removed. An alternative variation is the so-called incipient wetness (IW) technique, in which the pores of the support are filled with the solution [61-63]. 

Chloroauric acid (HAuCD) or auric chloride (AuCD or AU2CI6) is usually used but complex salts, such as potassium aurocyanide (KAu(CN)2) and the ethylenediamine complex [Au(en)2]Cl3, are alternatives. Traditional supports are silica, alumina and magnesia, but titania, alumina, boehmite (AIO(OH)), ferric oxide (a-Fe203) and magnesium hydroxide can also be used [18]. 

After drying, the precursor has to be calcined at temperatures as high as 1,073 K and reduced. This reduction can be by hydrogen at 523 K, by aqueous oxalic acid at 313 K or by aqueous magnesium citrate [18]. 

Conventional impregnation techniques result in much larger gold particles than deposition or precipitation methods [5,18,38,39,64]. It is difficult to deposit Au as nanoparticles on metal oxide supports by this method and to 

The resin must be carefully and completely removed before the impregnated specimen is subjected to thermal etching, because the infiltrate will expand as it decomposes under these temperatures, possibly causing the specimen to break apart. 

In order to make the polished section considerably easier to prepare, and to ensure a credible representation of its porosity, the sectioned sample should be impregnated with a synthetic resin of low viscosity. This should ordinarily occur after sectioning, but in the case of very brittle and highly porous samples, it should be performed even before sectioning. Epoxy resins with a low viscosity of approximately 250 cP are espedaUy well suited for use as impregnating media. Aided by capiUary forces, these 

Non-impregnated specimen showing pull-puts and pores 

For purposes of impregnation with epoxy resin, porous ceramic materials can 

The equipment shown in Fig. 9 is an example of a suitable vacuum impregnation apparatus. A motor-driven turntable inside the vacuum vessel allows multiple samples to be impregnated simultaneously. A simple pouring mechanism makes it possible to pour the premixed epoxy resin components (cresin and hardener) over the samples or into the embedding molds. 

There are several other ways of precipitation, for example, formation of anionic species that may be deposited on the surface of the suspensitMi, varying certain parameters, particularly the pH of the solution. 

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Another method which should be cited apart from the others is to pyrolyze the sample in a hydrogen atmosphere. The sulfur is converted to H2S which darkens lead-acetate-impregnated paper. The speed of darkening, measured by an optical device, provides the concentration measurement. This method attains sensitivity thresholds of 0.02 ppm. 

Using of a source Cs nozzles with the contents of tungsten up to 75 % were investigated. The distributions received for a product without impregnation and after impregnation by tungsten are shown on fig. 1. 

Results of determining a density of a composite material nozzles of the rocket engine before and after impregnation by metal. 

The first phenomenon is sedimentation of developer s particles in a zone impregnated with a penetrant. As a result the thickness of developer s layer h, appearing in formulas, is smaller than the thickness of dry zone. Our experimental results show that in some cases h is 80% smaller than h. The pictures illustrating the sedimentation influence upon the values of thickness for various developers are obtained. The estimation of this influence upon calculated sensitivity is carried out. 

On standing, gelatinous aluminium hydroxide, which may initially have even more water occluded than indicated above, is converted into a form insoluble in both acids and alkalis, which is probably a hydrated form of the oxide AI2O3. Both forms, however, have strong adsorptive power and will adsorb dyes, a property long used by the textile trade to dye rayon. The cloth is first impregnated with an aluminium salt (for example sulphate or acetate) when addition of a little alkali, such as sodium carbonate, causes aluminium hydroxide to deposit in the pores of the material. The presence of this aluminium hydroxide in the cloth helps the dye to bite by ad sorbing it—hence the name mordant (Latin mordere = to bite) dye process. 

Carbon disulphide is an excellent solvent for fats, oils, rubber, sulphur, bromine and iodine, and is used industrially as a solvent for extraction. It is also used in the production of viscose silk, when added to wood cellulose impregnated with sodium hydroxide solution, a viscous solution of cellulose xanthate is formed, and this can be extruded through a fine nozzle into acid, which decomposes the xanthate to give a glossy thread of cellulose. 

The combustion tube. The oxygen is led through the side-arm G (Fig. 85) into the combustion tube L via the horizontal side-arm H which is fused into the combustion tube about 4 cm. from the end (the joint between G and H is also made with impregnated rubber tubing with the two glass tubes touching inside the rubber). The position of H thus allows the combustion... 

All of the pieces pictured here are not going to fit perfectiy into each other and that s going to cause all sorts of leaks. The answer is teflon tape. Strike loves teflon tape This inexpensive product is found in the plumbing department of any hardware store and is the duct tape of the next century. Teflon tape is chemically and thermally indestructible. This stuff is wrapped around any piece of pipe or joint, said part then jammed into its appropriate receptacle and the tape will mold to form a perfect fit. Hell, it can mummify a whoie joint compiex to make it absolutely impregnable. 

Turmeric paper (gives a rose-brown coloration with boric acid) wash tbe ground root of turmeric with water and discard tbe washings. Digest with alcohol and filter, using the clear filtrate to impregnate white, unsized paper, which is then dried. 

Creager and colleagues designed a salicylate ion-selective electrode using a PVC membrane impregnated with tetraalkylammonium salicylate. To determine the ion-selective electrode s selectivity coefficient for benzoate,... 

Impregnating Formulations Based on Furan Chemicals, Technical Bulletin No. 190, Chemicals Division, The Quaker Oats Company, Chicago, lU., 1979. 

Today the most efficient catalysts are complex mixed metal oxides that consist of Bi, Mo, Fe, Ni, and/or Co, K, and either P, B, W, or Sb. Many additional combinations of metals have been patented, along with specific catalyst preparation methods. Most catalysts used commercially today are extmded neat metal oxides as opposed to supported impregnated metal oxides. Propylene conversions are generally better than 93%. Acrolein selectivities of 80 to 90% are typical. 

T. L. Rutt andJ. A. Syne, "Fabrication of Multilayer Ceramic Capacitor by Metal Impregnation," IEEE Trans. Parts Hybrids Packag., PHP-9, 144-147 (1973). 

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