Calcination and reduction

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

Since the catalytically active phase is frequently quite expensive (e.g. noble metals) it is clear that it is in principle advantageous to prepare catalysts with high, approaching 100%, catalyst dispersion Dc. This can be usually accomplished without much difficulty by impregnating the porous carrier with an aqueous solution of a soluble compound (acid or salt) of the active metal followed by drying, calcination and reduction.1... 

In this work, various conditions have been used for preparing catalysts to screening out a proper one mainly for CTA hydrogenation. The palladium precursor, impregnation time, calcination and reduction will be taken into account. 

Preparation conditions of Pd/CNFs by wet impregnation method, such as palladium precursor, impregnation time, calcinations and reduction, are proved to have profound effect on the catalytic property. The catalyst prqjared by impregnating HzPdCLi precursor in an hour, then calcinated in air and reduced in 20%H2/Ar is believed to perform better in CTA hydropurification than the industrial Pd/C under laboratory conditions. 

PtCU- helps to disperse the platinum evenly overthe support, while the adsorbed chlorine enhances the acidity of the support. Calcination and reduction produce the eventual catalyst. 

Copper ore containing a deposit of aurlchalclte was obtained from Wards Natural Science Establishment. The mineral aurlchalclte crystallites were gently scraped from the ore and rinsed In ethanol prior to use. The synthetic precursor was prepared by copreclpltatlon from a mixture of IM Cu and IM Zn nitrate solutions, such that a Cu/Zn mole ratio of 30/70 was prepared, by dropwlse addition of IM Na2C03 at 90 C until the pH Increased from approximately 3 to 7. Calcination and reduction of the mineral were performed as In standard catalyst preparation procedures, which have been described In detail earlier (jL). ... 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

In heterogeneous catalysis post-impregnation treatments such as calcination and reduction are usually performed to remove ligands or their residues. In some cases the removal of heteroatoms such as carbon, sulfur or chloride is tedious and can... 

Unpromoted Cu/Si02 is found to have a low activity for methanol synthesis from H2/CO mixtures, whereas an increased activity from H2/CO2. Alkali metal promotion increases the activity for methanol synthesis from the H2/CO mixtures, probably due to the increase in surface OH groups engaged in the formation of the formate species which are the precursors to the methanol. Cu/Si02 powder catalysts (with 5 wt% Cu) can be prepared by ion exchange of silica with Cu(NOs)2 in aqueous solution, followed by calcination and reduction. Such preparations contain very fine Cu particles ( 0.5 nm) on a powdered silica support as revealed by HRTEM. 

It was at this point in the Spring of 1773 that Lavoisier embarked on a systematic study of the fixation and liberation of airs, a topic much pursued in other countries but The French Chemists alone seem not to take any part in these important inquiries. The results of this intensely pursued task were published in January of 1774 as Opuscules physiques et chymiques (Physical and Chemical Essays). In the first half of this book, Lavoisier thoroughly reviewed the work previously published on the topic in the second part he described his own experiments, consisting chiefly of repetitions of earlier work, though often with different observations and interpretations. Here for the first time he put into print his view of calcination and reduction of metals. 

Catalysts pre-treatment (calcination and reduction) was performed in the same testing system or in a parallel automatic activation system prior to reaction test Calcination is carried out at 600 °C under airflow for 8 h and reduction at 250 °C for 2 h under hydrogen flow. Catalytic tests were carried out at 30 bar total pressure, temperature range 200-240°C, and 2.26h-1 WHSV, H2/hydrocarbons molar ratio of 2.93. Each fixed bed microreactor contained 500 mg of catalyst (particle size 0.4—0.6 mm, for which there are no internal diffusion limitations). Reaction products distribution are analysed using a gas chromatograph (Varian 3380GC) equipped with a Plot Alumina capillary column. 

The catalyst (Pd/ZnO) was introduced by wash coating prior to the mounting, which was performed by electron beam welding. Calcination and reduction of the catalyst were performed after the welding procedure. The total heating power of the six heating cartridges was 1.5 kW. Bores were introduced for temperature determination at various positions. The feed inlet temperature was set to 140 °C. 

Figure 6 Changes occurring during the calcination and reduction of Ni-Al coprecipitates...

Keegan et al. studied the calcination and reduction of the same Pt(NH3)4 on HZSM-5 system with energy dispersive Extended X-ray Absorption Fine Structure (EXAFS). They showed that during calcination the Pt-Pt coordination rises, indicating agglomeration. The final metal particle size obtained after direct reduction (no calcination prior to the reduction) was smaller than the particle size obtained after calcination prior to the reduction. The authors did not clarify the chemistry of the pretreatment process. 

Another important zeolite catalyst is the so-called bifunctional catalyst. The thermal reduction of zeolites previously exchanged with metals is the method currently used for the preparation of bifunctional catalysts for hydrocarbon conversion. The bifunctional zeolite catalysts are composed of both acidic sites and metal clusters. The preparation methods of these catalysts encompass three steps ion exchange, calcinations, and reduction, (Section 3.2.1.4) [123,127],... 

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

Catalyst sulfation was also carried out by impregnation with ammonium sulfate (2 wt % sulfur). The data reported in Table 3 indicate that a large part of sulfur is eliminated during the calcination and reduction steps at 500 C and that ceria containing samples retain more sulfur. This result is in agreement with the stability of... 

In summary, the way is paved to look at oxide-supported metal nanoparticles, prepared in solution, and to understand the formation of MNPs through calcination and reduction. However, there is still a way to go to identify the elementary steps in the interaction of the species from solution at the solid-liquid interface. Of course, this is what we really want. 

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