NiO-Al₂O₃ catalysts prepared at high pH variation of structure and texture upon thermal treatment

Abstract

A series of five catalysts of NiO-Al₂O₃ of varying content (IA-VA) were prepared at pH ∼ 10 and dried at 80°C using neutral mesoporous alumina composed of μ-Al₂O₃ and poorly crystalline boehmite. Structural changes produced upon heat treatment up to 700° C were followed using XRD, TG and DTA techniques. The surface texture of the samples was investigated by nitrogen adsorption at 77 K. The effect of soaking period on the resulting preparation and on its thermally treated products was also investigated (III Aa).

The presence of nickel in the form of the amine complex had a marked effect on the crystalunity of the support changing the boehmite to the well crystalline form-an effect which was not observed in the absence of the nickel amine complex. The presence of NH₃ alone in the medium resulted in the appearance of two broad bands covering d distance ranges of 0.545 to 0.521 nm and 0.432 to 0.386 nm that also appeared in the XRD patterns of the catalyst samples. The ammonia penetrates between the layer structure of the support thus changing its interlayer distance.

Soaking for 7 days was found to represent non-equilibrium conditions compared to soaking for 15 days.

NiAl₂O₃was characterized in the catalyst samples and NiO appeared in samples heated at 225° C and increased at 325° C resulting from the decomposition of the physically adsorbed surface species [Ni (NH₃)₆](NO₃)₂ and [Ni (NH₃)₆](OH)₂, respectively. The latter became pronounced for higher nickel concentrations as observed from DTA and DTG. The former decomposes exothermally at around 260° C and the latter endothermally at 325° C after losing the NH₃ ligand in the temperature range 245 to 265° C.

At nickel contents greater than 4.6% and at temperatures greater than 225° C a surface compound appeared with characteristic d distances in the ranges 0.202 to 0.2034 nm and 0.2363 to 0.2348 nm depending on treatment temperature. It results from the attack of the nickel amine complex on two adjacent hydroxyl groups.

An increase in the surface parameters of Al₂O₃ was observed upon soaking in NH₄OH solution alone and from pore analysis is found to contain two groups of mesopore sizes. The presence of the least amount of nickel content (1.5%) produced a marked increase in both specific area and total pore volume accompanied by a decrease in average pore radius. These changes reflect the structural changes of the support observed upon impregnation in the nickel-amine complex. Variations of the surface parameters of the catalyst samples with further increase in nickel content depend on the nickel species formed. However, at a nickel content of 9.6% more pores are being exposed that result from the penetration of more nickel ions between the support particulates. At still higher nickel contents blocking of the narrower pores and narrowing of the wider pores occurs as their V _I−t plots indicate. Despite this narrowing the catalyst samples are still predominantly mesoporous retaining, in most cases the existence of two groups of pore sizes.

Variations in soaking period seems to affect the texture of the low temperature samples but for samples treated at temperatures above 500° C the surface parameters are comparable irrespective of the period of soaking.