Nickel aluminate, formation

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 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. 

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). 

There is only a limited amount of information on the deactivation mechanisms and rates of Vanadium and Nickel. The formation of metal silicates and/or aluminates have been proposed [26, 33, 34, 36], which seem to form more easily by reduction and oxidation cycles [37]. 

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). 

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 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]. 

Catalyst deactivation and resistance to coking are two important issues of the methane reforming reaction with CO2 over Ni based catalysts because of their potential industrial application. Chen and Wren (9) and Bhattacharya and Chang (10) have recently proposed that the nickel aluminate spinel produced by interaction between nickel and alumina has a positive effect on the suppression of carbon deposition in CO2 reforming of methane. On the other hand, the formation of various types of nickel silicate species between the nickel and the support, attributed to the strong metal-support interaction, has been reported in Ni-silica catalysts (11,12). From these conclusions, it seems interesting to study the influence of Ni-silica interaction on carbon deposition. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

The reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

In conclusion, the addition of phosphorus may increase the sulfidation of nickel or cobalt in Ni/Al or Co/Al samples at temperatures up to about 700°C as a consequence of the decreasing amounts of stable aluminates that are formed at higher temperatures the decrease is caused by the formation of phosphides. However, the Ni2P and C02P phases might be... 

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

There is only a limited amount of information on the deactivation mechanisms and rates of vanadium and nickel migration. The formation of metal silicates and/or aluminates has been proposed, as they seem to form more easily by reduction and oxidation cycles. Rajagopalan et al. [8] confirm that methods involving cyclic redox aging of metals in the presence of sulfiir are needed for screening metals-tolerant catalyst. They propose a cyclic test (the cyclic propylene steam method), which addresses the redox aging of the metal, but not the nonuniform laydown and age distribution of metals on the catalyst. 

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]. 

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