The successive increase in BE values of hexavalent chromium species with
increasing calcination temperature from PCP120 to PC800 is plotted in Fig. 3.
The chromate species [Cr(VI)O x,surf ] on the catalyst became more and more
electron deficient with increased calcination temperatures from 200
C to 800
C.
This correlates well with the typical polymerization behavior of Phillips
catalysts, i.e., that the polymerization activity increases with an increase in
calcination temperature. The dominant supporting of bulky CrO 3 on a silica
gel surface and simultaneous dehydroxylation of the silica gel surface at
120–300
C account for the drastic increase in BE values of hexavalent chromate
species in this temperature range for isothermal calcination. The slowly increasing of BE values of chromate species from 300
C to 600
C was solely derived
from dehydroxylating residual surface hydroxyl groups. Another enhancement of
the increase in BE values of chromate species from 600
C to 800
C might come
from further dehydroxylation of residual hydroxyl groups, as well as enhancement of surface tension from easier relaxation of surface siloxane groups induced
by high temperature.
The dependence of BE values on the XPS acquisition time during the XPS
measurement indicated that further increasing the XPS acquisition from 10 to 30 or
120 min may lead to the catalyst being reduced by the soft X-ray irradiation during
the XPS measurement, which provides a good method for evaluation of the
photostability of Phillips catalysts prepared under different conditions. The
photostability of surface chromate species on Phillips catalysts was found to be
significantly dependent on the calcination temperature used for catalyst preparation
(see Fig. 2); the sample calcined at moderate temperatures (400–600
C) showed the
highest photostability [70].
Fig. 3 Dependence of
binding energy [Cr 2p (3/2)]
of surface Cr
6+ species of
various Phillips catalysts on
calcination temperature for
preparation of the catalysts
from PCP120 precursor
146
R. Cheng et al.
increasing calcination temperature from PCP120 to PC800 is plotted in Fig. 3.
The chromate species [Cr(VI)O x,surf ] on the catalyst became more and more
electron deficient with increased calcination temperatures from 200
C to 800
C.
This correlates well with the typical polymerization behavior of Phillips
catalysts, i.e., that the polymerization activity increases with an increase in
calcination temperature. The dominant supporting of bulky CrO 3 on a silica
gel surface and simultaneous dehydroxylation of the silica gel surface at
120–300
C account for the drastic increase in BE values of hexavalent chromate
species in this temperature range for isothermal calcination. The slowly increasing of BE values of chromate species from 300
C to 600
C was solely derived
from dehydroxylating residual surface hydroxyl groups. Another enhancement of
the increase in BE values of chromate species from 600
C to 800
C might come
from further dehydroxylation of residual hydroxyl groups, as well as enhancement of surface tension from easier relaxation of surface siloxane groups induced
by high temperature.
The dependence of BE values on the XPS acquisition time during the XPS
measurement indicated that further increasing the XPS acquisition from 10 to 30 or
120 min may lead to the catalyst being reduced by the soft X-ray irradiation during
the XPS measurement, which provides a good method for evaluation of the
photostability of Phillips catalysts prepared under different conditions. The
photostability of surface chromate species on Phillips catalysts was found to be
significantly dependent on the calcination temperature used for catalyst preparation
(see Fig. 2); the sample calcined at moderate temperatures (400–600
C) showed the
highest photostability [70].
Fig. 3 Dependence of
binding energy [Cr 2p (3/2)]
of surface Cr
6+ species of
various Phillips catalysts on
calcination temperature for
preparation of the catalysts
from PCP120 precursor
146
R. Cheng et al.