G. Centi et al., EFFECT OF AMMONIA CHEMISORPTION ON THE SURFACE REACTIVITY OF V-SB-OXIDE CATALYSTS FOR PROPANE AMMOXIDATION, Applied catalysis. A, General, 149(1), 1997, pp. 225-244
The influence of ammonia on the reactivity of a V-Sb-oxide catalyst in
propane ammoxidation to acrylonitrile (ACN) was studied by infrared s
pectroscopy and flow reactor tests. Ammonia coordinates as such or as
ammonium ions on the catalyst surface in relative amounts which depend
on both the temperature at the time of contact and the hydroxylation
state of the catalyst. The two chemisorbed species influence both the
surface reactivity and the pathways of the reaction in propane ammoxid
ation. NH, chemisorbed on Lewis acid sites inhibits the activation of
propane differently from that chemisorbed on Bronsted sites, The forma
tion of the latter species, however, inhibits the reactivity of the Br
onsted sites in catalysing various side reactions which decrease the s
electivity to ACN. The results of the infrared study are in good agree
ment with the change in surface reactivity observed in flow reactor te
sts as a function of the concentration of ammonia in the feed. Both th
e nature and the amount of the chemisorbed ammonia species thus determ
ine the intrinsic surface reactivity and selectivity of V-Sb-oxide cat
alysts in the synthesis of acrylonitrile.