Spinel-type phases

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

The TPR profiles of calcined LDHs precursors show two peaks of H2 consumption (Fig.l). The first peak around 570 K corresponds to the release of NO3 anions as NO2, and their subsequent reduction to NO and N2O as identified by mass spectrometry [9]. The second peak with maxima at 705,920 and 1000 K for HA, HC and HG samples respectively, corresponds to the reduction of NiO particles. These experiments show that the reducibility of the nickel oxide particles decreases when the Mg content increases. This could be compared with the decrease of the Ni crystal size measured by XRD in HC and the lack of detection of these particles in HG samples. This behaviour has been attributed to the formation of excess Ni aluminate and Ni spinel type phases decreasing the size of the mixed oxide particles and hindering their reducibility [6]. 

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]. 

Figure 1. PXRD patterns of samples Ni/Mg/Al (left), Rh/Ni/Mg/Al (right). HTlc precursors BOl and B02 (a), SAl and SA2 (b). Calcined samples BOl and B02 (c), SAl and SA2 (d). After reaction samples BOl and B02 (e), SAl and SA2 (f). MgO-type phase o spinel-type phase Ni metal phase...

After calcination at 900°C, both MgO and spinel-type phases are formed (Fig. Ic and Id). In contrast with the trends observed for the as-synthesized samples, the solids derived firom precursors submitted to MWHT show a lower degree of crystallinity if compared to the samples aged by stirring for the same period of time (Table 1). The a parameter of the rock-salt type phase confirms the hosting... 

By calcining precipitates 2-5 at increasing temperature, we observed at first (at about 573K) the formation of amorphous dichromate-type phases [13], which successively decomposed to CuO and cubic spinel-type phases, in analogy to that previously reported in the literature [14,15]. 

At higher calcination temperatures, spinel-type phase. 

However, for samples 2-5, the amounts of CuO detected by quantitative XRD analysis (18) approach a maximum of 50% of the value calculated on the basis of a phase composition CuO + stoichiometric spinels, showing the existence of a consistent fraction of copper ions which escape XRD detection, probably present inside the spinel-type phase or strongly interacting with it (19). 

Figure 2. Composition of the spinel-type phases obtained eifter heating the Cu/Cd/Cr precipitate at 753K in air (A) or in vacuum (B), determined hy linear interpolation on the basis of the a values ( ) [CuCr204 cubic ICDD 26-509 and CdCr204 ICDD 2-1000] or by quantitative XRD ( ) or chemical (O) analyses.

I2 Picmonicse, M., Trillro, F., Vaccari, A., Forcsti, E. and Gazzano. M. (1991). Synthesis of non-stoichiometric spinel-type phases - A key tool for the preparation of tailored catalysts with specific activity. In G. Poncelet, P.A. Jacobs. P. Grange and B. Dcimon (Eds.). Preparation of Catalyts V (pp. 49-58). Amsterdam, Elsevier. 

Rebours, B., D-espinose Dc La Caillcric. J-B. and Clause, O. (1994). Decoration of nickel and magnesium oxide crystallites with spinel-type phases. J. Amer. Chem. Soc.. 116, 1707-1717. 

In previous papers [18,19,39], a model was proposed for Ni/Al mixed oxides obtained from HT precursors, involving the formation of NiO (containing a very small amount of Ap ions) and Ni-doped alumina phases, which strongly interact with a spinel-type phase present at their interface. The spinel-type phase, probably a non-stoichiometric-type, is responsible for the thermal properties of the calcined precursors, hindering the growth and sintering of the NiO crystallites. Likewise, the reduction of the NiO phase is hindered by the presence of the... 

In all cases, the evolution of the spinel-type phases towards the smichiometric forms, which takes place with a consistent segregation of oxide phases, destroys diis system and leads to considerable modifications in the properties of the oxides obtained. In particular, for the Ni-rich samples an increase in the reducibility of the main fraction of die Ni " ions was observed, with a behaviour similar to that of free NiO, while for Mg-rich samples the reduction of the Ni " " ions is further hindered by die formation of NiO/MgO solid solutions, with reduction temperatures very similar to those reprated for nickel-magnesia catalysts. 

Supported copper-chromium oxide catalysts. The non-modified support after its thermal treatment at 773 K, if coated with a 7% (Cu+Cr) mixture, seems to contain a series of spinel-type phases on the base of the support and copper chromite structures as well. Then diffractogramms of the sample are characterized by distorted lines of the support only. After thermal treatment at 1273 K, there coexist a-Al203 with the increased cell parameter and aluminium-copper-chromium spinel with a = 8.098 A, which is typical for Cu(Ali 8Cro.2)04 composition. No lines of copper (+1), i.e. Cu2Cr204 and CU2AI2O4, are observed. 

Ni- and Rh-containing HT phases are useful preeursors of catalysts for SR and CPO of methane, and may be eleetrosynthesized in situ at r.t. on the surfaee of FeCrAlY foam pellets from nitrate solutions. Thermal treatment results in the formation of well adherent oxide coatings (rock-salt-type and spinel-type phases) eontaining active sites. The rate of base generation and electrosynthesis ean be eontrolled by changing the potential applied, while the layer thickness is controlled by tuning the synthesis time. 

SYNTHESIS OF NON-STOICHIOMETRIC SPINEL-TYPE PHASES A KEY TOOL FOR THE PREPARATION OF TAILORED CATALYSTS WITH SPECIFIC ACTIVITY... 

The preparation, stability and catalytic activity of non-stoichiometric spinel-type phases used in the synthesis of methanol were investigated as a function of the composition, heating temperature and atmosphere. It was shown that these phases formed mainly via amorphous chromates, especially for copper-rich catalysts. High activities in the synthesis of methanol were observed for zinc-rich samples (with a maximum for a catalyst in which 20% of the zinc ions were substituted by copper ions) and associated with the presence of a non-stoichiometric spinel-type phase, stable also in the reaction conditions. On the other hand, the low activity of copper-rich catalysts was attributed to the instability of the spinel-type phase where much of the copper segregates into well crystallized metallic copper, with a further poisoning effect by zinc and cobalt. 

Figures 2a and b report the XRD powder patterns of the precipitates heated at 653K in air and in a reducing atmosphere (H2 N2= 10 90 v/v), respectively. Calcined samples (Fig. 2a) show the presence only of spinel-type phases, whose XRD patterns become more and more broad as the copper content increases. IR spectra confirm the presence, for all calcined samples, of spinel phases, and also show he presence of dichromate-type phases (25), the amounts of which increase with increasing copper content. In previous papers it was shown that non-stoichiometric Zn/Cr spinel-type phases formed by decomposition of amorphous chromates and that some amounts of residual Cr ions are present in these phases (8,15). Taking into account that copper and zinc may form mixed spinel-type phases (with cubic symmetry for high zinc contents) (20,24), we may hypothesize the formation up to a ratio Cu/Cu-i-Zn= 0.5 of cubic non-stoichiometric spinel-type phases, containing both elements and characterized by an excess of bivalent ions. On the other hand, on the basis of the XRD spectra of Figure 2a, we cannot speculate about the number and/or nature of the phases present in the copper-rich catalysts.

After the samples had been heated in an H2/N2 atmosphere, the XRD powder patterns (Fig. 2b) again showed the presence only of spinel-type phases for Cu/Cu-t-Zn < 0.5, while for the copper-rich samples the main phases present were Cu (Cat D and E) or CuO (Cat F). The lack of reoxidation for the metallic copper in Cat D and E, cannot be justified on the basis of differences of crystal size, but most probably can be attributed to the formation of copper-rich alloys at the surface of the particles. The presence of small amounts of zinc or cobalt does not modify the XRD powder pattern of the copper particles, but may strongly influence their physicochemical or catalytic properties (25-27). For all catalysts, the IR spectra show the presence, together with small amounts of residual carbonates, of the typical bands of spinels (even if not well resolved), except the Cu/Cr= 1.0 sample (Cat F) (Fig. 3) for which only... 

Table 2 confirmes the presence in the calcined samples of increasing amounts of chromates, with a maximum for Cat F in which ca 43% of the total chromium is present as Cr ions. Up to a Cu/Cu+Zn ratio< 0.5, the amount of CuO extracted is lower than both the theoretical value and the chromate content, and does not depend directly on the latter. This is particulary true for Cat B, taking into account also the values of the samples heated at 653K in N2. Therefore, two Cu containing fractions are present in these spinel-type phases, which show different solubilities in the NH4OH/NH4NO3 solution, but... 

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