Hydrotalcite-type phases

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

Modified from Ref. T35. "Other components mainly P2O5 (0.2%) other phases mainly alkalis (adsorbed or in solution), insoluble residue and PjO (form of combination unknown). Fe HP = hydrogarnet-type product from ferrite phase. Mg HP = hydrotalcite-type phase. AFm and Mg HP include poorly crystalline materials intimately mixed with C-S-H. Discrepancies in totals arise from rounding. 

C-S-H having Si/Ca 0.62, Al/Ca 0.09, with a hydrotalcite-type phase having Al/Mg 0.38. As with Portland cement pastes, the material described in Fig. 9.1 as C-S-H may really be an intimate mixture of the latter with AFm and. to a minor extent, hydrotalcite-type structures. 

Tanaka et at. (T45) partially masked the surfaces of disks of slag glass with gold coatings the disks were then embedded in cement pastes for various periods, after which they were removed and examined by SEM. This showed that two layers of in situ product were formed. The innermost one had Si/Ca 0.64, Al/Ca 0.32 and Mg/Ca 0.34 the outer one had Si/Ca 0.61, Al/Ca 0.24, Mg/Ca 0.20. The product formed immediately outside the original boundary of the slag had Si/Ca 0.48, Al/Ca 0.09, Mg/Ca 0.17. The two layers of in situ product could have been mixtures in differing proportions of C-S-H. a hydrotalcite-type phase and possibly AFm phase. 

HT= hydrotalcite-type phase. M.O. = MgO-type mixed oxide phase. SP = spinel-type phase... 

All samples show the typical XRD patterns of hydrotalcite type phases. They contain both NO3 and CO3 compensating anions coming from the starting salts and the surrounding atmosphere respectively. The specific surface areas in the range of 15-35 m g-i in the non calcined samples increase up to a maximum value of 190-210 m g- after calcination at 623 K. This has been attributed to a craterisation phenomenon due to the decomposition of the vaporisable anions. No micropore was identified in the samples. 

The XRD spectra of the precipitates dried at 363K show the presence, for all samples, of only a well crystallized hydrotalcite-type phase (Table 1), in agreement with the M(II)/M(III) ratio and nature of the ions [7,9,10]. The crystallographic parameters a and c, calculated for an hexagonal cell on the basis of rhombohedral symmetry, show that the structure of Mg/Al... 

A suspension containing the recently precipitated LDH, or a suspension of the oxides in a solution of the anion to be intercalated, is submitted to hydrothermal treatment for varying periods of time. The hydrotalcite-type phase is formed, and the small crystallites existing in the suspension are converted into larger crystals, while amorphous precipitates crystallise. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

The key parameters in the preparation of active and stable catalysts for N2O decomposition under conditions of industrial interest were examined using mixed oxides obtained form hydrotalcite-type (HT) precursors. The relationship between activity and composition of the catalysts was studied, with attention focused on the nature and amount of metal ions present in the HT precursor (Rh/Mg/Al or Rh/Co/Al). An activation procedure was developed which makes it possible to prepare catalysts, active and stable even under the severe reaction conditions of industrial interest. The relationship between active phase and composition of the bulk matrix was also investigated. Moreover, the flexible structure of the HT precursor made it possible to investigate the use of other active elements (Pd or La) to develop synergetic effects, without any interactions due to side phase segregation, or lack of homogeneity. 

In line with the XRD results, the DSC/FTIR study of CR also suggests the formation of hydrotalcite-like phase [13]. High Mg-containing CS is more thermally stable than CR. With increase in calcination temperatures, the FTIR spectrum evolution reveals the formation of mixed metal oxides, i.e. spinel-type phase [40]. The decomposition of catalyst precursors is also reflected in the XRD study which shows that the resultant oxides are largely amorphous although weak spinel-type feature can be observed [41]. 

Thevenot, F. 1989. Synthbse et caract sation cristallochimique de prdcurseurs d oxydes mixtes divisds hydroxycarbonates de type hydrotalcite et phases d6riv6es. University Blaise Pascal Thbse dTJniversit6. 

The most active Pd based catalysts in the table have been reported by Shishido et al.51 who have employed hydrotalcite precursors, using a solid phase crystallisation (SPC) method. Comparisons made with catalysts prepared via impregnation showed SPC to afford higher activity catalysts, which was ascribed to better dispersion and also easy desorption of CO. The inclusion of a Cr component in these types of catalyst has also been reported to be beneficial.52 In terms of the desorption of CO, isotopic studies performed by McKee53 over ruthenium and platinum catalysts have indicated that the rate determining step of methanol decomposition is the fission of a... 

The electrochemistry of LDH-attached species has received relatively little attention compared with that of zeolite-associated species. The electrochemistry of LDH-supported bis(2-mercapto-2,2-diphenyl-ethanoate) dioxomolybdate(VI) complex, Mo O2( TI A)2l (TEA = O2CC(S)Ph2) is representative, however, of the capabilities of electrochemical methods for obtaining information on this type of system. The mineralogical composition of the used Zn-Al hydrotalcite was Zn2 2oA1o 79(OH)g(N03)o 79 H2O. X-ray absorption spectroscopy and other techniques (Cervilla et al., 1994a), suggested that the complex, which, in solid phase exhibits... 

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