Nickel aluminate

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

With nickel/alumina catalysts (cf. 4 ) preparation by coprecipitation or by the decomposition of a high dispersion of nickel hydroxide on fresh alumina hydrogel, yields nickel aluminate exclusively. On the other hand, when, as in impregnation, larger particles of nickel compound are deposited, the calcination product is a mixture of nickel oxide and nickel aluminate. The proportion of nickel oxide increases when occlusion of the impregnation solution leads to a very nonuniform distribution (49). 

Morikawa et al. (42) suggest that nickel aluminate itself undergoes hydrogen reduction only to a superficial extent, and then produces extremely small nickel particles as the reduction product. In this circumstance, the nickel particle size distribution in a reduced nickel/alumina catalyst will obviously be much dependent on the preparative details that control the proportions nickel oxide and nickel aluminate and the size of the particles in which these substances exist before reduction. 

Moreau, C. Bekakra, L. Olive, J. L., and Geneste, P., Hydrodenitrogenation of Quinoline and Phenanthridine in the Presence of 2, 6-Diethylaniline and Hydrogen-Sulfide Over Molybdenum and Nickel-Molybdenum Sulfides Supported on Zirconia, Titania, Nickel-Aluminates and Magnesium-Aluminates. Bulletin Des Societes Chimiques Beiges, 1991. 100(11-12) pp. 841-847. 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

An analysis of the thermodynamic stability models of various nickel minerals and solution species indicates that nickel ferrite is the solid species that will most likely precipitate in soils (Sadiq and Enfield 1984a). Experiments on 21 mineral soils supported its formation in soil suspensions following nickel adsorption (Sadiq and Enfield 1984b). The formation of nickel aluminate, phosphate, or silicate was not significant. Ni and Ni(OHX are major components of the soil solution in alkaline soils. In acid soils, the predominant solution species will probably be NE, NiS04°, and NiHP04° (Sadiq and Enfield 1984a). 

Kiviat and Petrakis (80) found the infrared specta for adsorbed pyridine on NiMo/Al to be only slightly different than that on CoMo/Al. Both showed evidence of Lewis and Brpnsted acid sites, but the Ni catalyst had a somewhat higher Brpnsted-to-Lewis acidity ratio. Similar results were reported by Mone and Moscu (75), who also found that higher calcination temperatures produced more nickel aluminate spinel. 

Reflection spectra for the MoNi-153 catalysts are shown in Figure 10. The 480°C calcined catalyst shows the characteristic absorption band (25) of octahedrally coordinated nickel ions. The 650°C calcined catalyst shows the characteristic spectrum of nickel aluminate. These reflection spectra indicate that the nickel ions migrate from the catalyst surface into the alumina, as has been observed also for the cobalt-molybdenum-alumina catalysts. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

The reappearance of Brdnsted acid sites has been observed for the high calcined nickel-molybdenum-alumina catalysts. The presence of a nickel aluminate phase has been concluded from the reflectance spectra. The second Lewis band (1612 cm l) has a very low intensity, in comparison with the cobalt containing catalysts of a same composition and after the same calcination conditions. 

Similar interactions have been observed for the nickel promoted catalyst. However, the degree of interaction depends on the calcination temperature. This interaction disappears for a great part at increasing temperatures. This is ascribed to bulk nickel aluminate formation. 

The variance of the lattice parameters in the 111 direction for this Ni/Al203 catalyst, and the paracrystal size in this direction were found to be very similar to those that had been found earlier for an NH3 synthesis catalyst. It was suggested that there could be a common mechanism for the structural perturbation in each catalyst involving solution of a spinel phase in the host metal. Since the ratio of the volume of a Ni atom in bulk Ni to that of an oxygen anion in nickel aluminate is 1.5 1, it was suggested that the substitution could involve one (A102) group for three nickel atoms. 

C02 Reforming of Methane to Syngas Deactivation Behavior of Nickel Aluminate Spinel Catalysts... 

Keywords Selective Hydrogenation of Acetylene, Coking, Pretreatment, Nickel Aluminate... 

Lopez-Agudo et al. (69) reported that the sulfidation of nickel in NiP/Al catalysts, measured by XPS, is not influenced by phosphorus addition. On the other hand, Iwamoto and Grimblot (67) found that phosphorus increases sulfidation of nickel in NiP/Al at 400°C because phosphorus prevents the formation of stable nickel aluminate species. A similar explanation was also proposed for nickel reduction 102). 

Al-Ubaid, A. Wolf, E.E. (1988) Steam Reforming of Methane on Reduced Non-Stoichiometric Nickel Aluminate Catalysts. Appl. Catal., 40, 73-85. 

What is certain Is that the addition of foreign Ions can lead to new compounds. Nickel aluminate has been shown to be fornied from Che calcination of Ni/Al O catalysts, and the aluminate has been suggested to be responsible, at least in part, for the observed extra stability [19p 19(a)]< Lanthanum aluminate is also known to be formed on calcination of La doped aluminas... 

In addition to the above reported synthesis of ferrites our search has revealed that aluminates [119], nickelates [120], and manganates [121], have also been prepared by the sonochemical method. Nanosized nickel aluminate spinel particles have been synthesized [119] with the aid of ultrasound radiation by a precursor approach. Sonicating an aqueous solution of nickel nitrate, aluminum nitrate, and urea yields a precursor which, on heating at 950 °C for 14 h yields nanosized N1A1204 particles with a size of ca. 13 nm and with a surface area of about 108 m g-i. 

Freshly prepared Pd-Ru and Pd-Ni on a monolithic support converted all the NO (lOOOp.p.m.) with less than 5% ammonia formation in 0.4% O2 (1% CO, 250p.p.m. CaHg) at 753 and 873 K, respectively. As a further example of the metal-support interactions discussed in Section 6, the well known formation of nickel aluminate at high temperatures (in part) caused substantial deactivation of Ni-Pd in real exhaust, whereas Ni-Al204 was a more stable non-interacting support allowing a better performance to be maintained. After lOOh under net reducing conditions there was still a 50% loss of Ru from Pd-Ru catalysts. 

Summarizing all the information obtained above, the course of formation of the nickel catalyst supported on AI2O3 is pictured in Fig. 5. The dried nickel hydroxide decomposes into nickel oxide, a part of which combines with carrier alumina and forms nickel aluminate in the interface of the two solid phases. 

Nickel oxide in the upper phase is reduced with hydrogen to metallic nickel, which has the ability to hydrodecompose the C—C bond of hydrocarbons. This nickel oxide is dissolved away in a dilute hydrochloric acid and leaves the exposed surface of nickel aluminate. This nickel aluminate is slowly decomposed so as to form very fine particles of nickel on heating over 500° in a stream of hydrogen. 

The rate of decomposition is much higher in hydrogen than in the air, and the nickel particles formed on the surface are so fine and so closely surrounded by the nickel aluminate carrier that the catalytic activity of the reduced nickel is so modified that it loses the ability to hydro-decompose the C—C bond completely but still retains the high activity to hydrogenate the benzene nucleus. The number of fine nickel particles produced on the surface by the reduction at 500-600° should be very small, because they are completely poisoned by the addition of a minute amount of sulfur compound in hydrogen. 

The active nickel jiarticles produced on the surface by the reduction of nickel aluminate or silicate are only a small fraction of the total nickel contained in this catalyst. It was desirable to devise a new method that gives a catalyst with high availability of the nickel. We searched for a way to form only active metal sites on the surface of a carrier that had a large specific surface area. 

Nickel aluminate is a well-defined spinel that is difficult to reduce. Formation of pseudospinels are not limited, however, to stoichiometric... 

Various solid state reactions are involved in the process. In addition to those governing sintering, catalyst-support interactions may alter the nature and catalytic activity of the solid and may stabilise or destabilise the solid towards sintering. Thus, for example, the formation of nickel aluminate, NiAlgO, is well established in steam reforming catalysts [21,22], and this compound is catalytically inactive. However, its presence may affect the thermal stability of the solid [23], as is the case in cobalt-molybdenum and nickel-molybdenum based catalysts supported on alumina and used for hydro-treating [24]. 

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