Ma. Ermakova et al., XRD studies of evolution of catalytic nickel nanoparticles during synthesis of filamentous carbon from methane, CATAL LETT, 62(2-4), 1999, pp. 93-97
The XDR technique was used for studying a series of high-loaded (90%) nicke
l catalysts with silica as a textural promoter. These were catalysts for di
rect cracking of methane at 550(degrees)C. A relation between the initial a
verage size of active catalyst particles, carbon yield and average methane
conversion was demonstrated. Genesis of these catalysts was studied includi
ng oxide precursors, reduced catalysts prior to the reaction, as well as ca
talysts upon their contacting the reaction medium for various periods of ti
me from 15 min to 2 h. The active catalyst particles were shown to merge or
disperse at the outset of the reaction depending on their initial size. An
yway, close average sizes ranging from 30 to 40 nm were observed by the end
of the first reaction hour for all the catalytic systems providing the car
bon yield of 300-385 g/g Ni. The catalytic system was shown to self-organiz
e in the course of direct methane cracking, i.e., the catalyst particles tr
ansform in response to the reaction conditions. If the size of nickel parti
cles cannot vary, these catalysts are inefficient for the given process.