Atomic absorption catalysts

Composition. The results of elemental analyses are almost always included among the specifications for a commercial catalyst. Depending on the accuracy desired and whether or not the catalyst can be rendered soluble without great difficulty, elemental analysis may be performed by x-ray methods, by one of the procedures based on atomic absorption, or by traditional wet-chemical methods. Erequentiy it is important to determine and report trace element components that may have an effect on catalyst performance. 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

Iron was present as Fe " in the calcined precursors. For all the catalysts the reduction procedure described in Sec. 2.1 resulted in incomplete reduction of the Fe to metallic iron. This is in agreement with the findings of previous authors [6,11]. The individual percentage reductions of Fe to Fe°, as determined by the separate gravimetric and volumetric measurements (Sec. 2.2), are shown in Table 1. The values are calculated on the assumption that all the Fe is reduced to Fe prior to the onset of reduction to Fe°. There is good agreement between the two methods. Table 1 also records the actual Fe/(Fe + Mg) ratio in the catalysts as determined by atomic absorption spectroscopy (AAS) on the calcined precursors. 

V0x/Zr02 catalysts were designated as ZVx(y)pHz, where x gives the analytical vanadium content (weight percent), y specifies the preparation method (a, adsorption, i, impregnation or acac, acetylacetonate) and z the AV solution pH. The V-content was determined by atomic absorption (Varian Spectra AA-30) after the sample had been dissolved in a concentrated (40%) HF solution. 

The rhodium loss to the hydrocarbon phase was analyzed by atomic absorption. We found that for a thermomorphic catalyst solution that was cycled three times that the rhodium loss was below the 0.1 ppm detection limit of their instrument. 

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

Dendrimer encapsulated Pt nanoparticles (DENs) were prepared via literature methods (1, 11). PtCl42 and dendrimer solutions (20 1 Pt2+ dendrimer molar ratio) were mixed and stirred under N2 at room temperature for 3 days. After reduction with 30 equivalents of BH4 overnight, dialysis of the resulted light brown solution (2 days) yielded Pt2o nanoparticle stock solution. The stock solution was filtered through a fine frit and Pt concentration was determined with Atomic Absorption Spectroscopy (11). Details on catalyst characterization and activity measurements have been published previously (11). 

Since mild activation conditions appear to be important, a number of solution activation conditions were tested. PAMAM dendrimers are comprised of amide bonds, so the favorable conditions for refro-Michael addition reactions, (low pH, high temperature and the presence of water) may be able to cleave these bonds. Table 1 shows a series of reaction tests using various acid/solvent combinations to activate the dendrimer amide bonds. Characterization of the solution-activated catalysts with Atomic Absorption spectroscopy, FTIR spectroscopy and FTIR spectroscopy of adsorbed CO indicated that the solution activation generally resulted in Pt loss. Appropriate choice of solvent and acid, particularly EtOH/HOAc, minimized the leaching. FTIR spectra of these samples indicate that a substantial portion of the dendrimer amide bonds was removed by solution activation (note the small y-axis value in Figure 4 relative... 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

Even with the simple laboratory equipment used in these experiments, the CESS procedure allowed quantitative recovery of the product free of solvent, and with rhodium contents ranging from 0.36-1.94 ppm (determined by atomic absorption measurements). Furthermore, using this approach removal of unreacted starting material or side products from the product is possible during extraction from the catalyst, since even small structural differences can result in significant differences in... 

Asymmetric diarylmethanes, hydrogenolytic behaviors, 29 229-270, 247-252 catalytic hydrogenolysis, 29 243-258 kinetics and scheme, 29 252-258 M0O3-AI2O3 catalyst, 29 259-269 relative reactivity, 29 255-257 schematic model, 29 254 Asymmetric hydrogenations, 42 490-491 Asymmetric synthesis, 25 82, 83 examples of, 25 82 Asymmetry factor, 42 123-124 Atom-by-species matrix, 32 302-303, 318-319 Atomic absorption, 27 317 Atomic catalytic activities of sites, 34 183 Atomic displacements, induced by adsorption, 21 212, 213 Atomic rate or reaction definition, 36 72-73 structure sensitivity and, 36 86-87 Atomic species, see also specific elements adsorbed... 

Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10 - lO " M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. 

Many of the reactions described above are seen to give less than quantitative recovery of the rhodium catalyst component. The amount of rhodium remaining in a catalyst solution was determined by atomic absorption spectroscopy, and is reported as the percent of the rhodium charged which remains soluble or suspended in the reaction mixture at the end of the reaction (95). After some experiments a wash procedure was employed to dissolve rhodium complexes possibly left in the reactor heating a charge of pure solvent in the reactor under H2/CO pressure sometimes dissolved substantial amounts of rhodium species (94-96, 104, 108, 109). High recoveries of rhodium are essential in a practical process because of the scarcity and high price of this metal (120, 121). 

The nickel and aluminium contents of the catalysts were determined by atomic absorption spectroscopy [AAS). Tin and chlorine contents of the modified catalysts given in Table 2 were determined by AAS and chemical analysis, respectively. 

Main group element analysis was carried out with a Perkin-Elmer CHN elemental analyzer, while molybdenum analysis was performed using atomic absorption spectroscopy. The content of Mo and O on the surface of the catalysts was obtained by X-ray photoelectron spectroscopy (XPS) using a Shimazu ESCA-850 spectrometer with monochromatic MgKa. Since Mo 3p3/2 spectra overlapped with the N Is spectra for the nitrided catalysts, the degree of nitriding (N IsfMo 3d ratio) had to be obtained from a combination of elemental analysis and XPS. 

The characterization of fresh and used automotive catalysts, which includes the examination of poisons accumulated in the catalysts, uses a variety of modem analytical techniques. The two principal tools, besides conventional chemical methods, are atomic absorption and, most important, X-ray fluorescence (XRF). The latter technique has been refined and adapted for the analysis of automotive catalysts to permit rapid and accurate determination of all constituents, including the inadvertent contaminants. An example of a simultaneous XRF analysis of... 

The analytical chemist will choose the appropriate analytical technique (e.g., chromatography, spectroscopy, or titration) to satisfy the technical objective based upon his or her expertise and past experiences with similar analytical problems. Often, however, the analyte itself dictates the kind of analysis method to be used. For example, a residual volatile solvent would most probably be analyzed by gas chromatography (GC), while a residual catalyst, such as palladium, would best be analyzed by atomic absorption or emission spectroscopy. 

It is noteworthy that benzaldehyde can be obtained as a main product with up to 85 % yield with concomitant of 10 % acetophenone and a small amount of benzoic acid if the co-catalyst CuCl is absent by using a biphasic scC02/PEG system, as shown in Scheme 3.5. The presence of C02 could suppress the generation of acetophenone. In addition, the oxidized products could be extracted with scC02, or with diethyl ether, and the PEG phase which immobilized the catalyst is readily reused without further purification or activation. PdCl2 can be recycled for at least five times and the yield of benzaldehyde still reaches over 80 %. Pd leaching is found to be at the level of 0.5 ppm measured by Atomic Absorption Spectroscopy. 

Another batch of the SiC>2 support was ion exchanged with Pt(NH3)4(N03)2. 5 g of the support was suspended in a NH3 solution with pH=9.0. This deprotonates a large part of the silanol surface groups. A Pt(NH3)4(N03)2 solution was slowly added to the suspension under vigorous stirring and allowed to ion exchange with the SiC>2 surface for 12 hrs. Next, the catalyst precursor was washed twice and dried at 120°C overnight. Atomic Absorption... 

The catalyst powder was characterized by atomic absorption (chemical composition), X-ray powder diffraction (structure identification and degree of crystallinity) and nitrogen adsorption/desorption. For the latter method, an automatic Micromeretics ASAP 2000 apparatus was used, which also allowed the determination of the pore size distribution in the mesopore and macropore region (2 nm to 300 nm). 

The importance of catalysts in our energy and pollution conscious age is growing. Many catalysts depend for their activity on low levels of rather exotic metals, while even trace surface levels of elements such as lead may impair their activity. Thus there is plenty of scope for atomic absorption spectrometry. Many homogeneous catalysts are deposited on an alumina base, thus obtaining dissolution is not always easy and some interference in the air/acetylene flame may be encountered. A leaching procedure (e.g. with nitric acid) to dissolve adsorbed trace metals may be used to circumvent these problems. 

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