Kinetics and Catalysis, Vol. 42, No. 4, 2001, pp. 525–526. Translated from Kinetika i Kataliz, Vol. 42, No. 4, 2001, pp. 579–580.
Original Russian Text Copyright © 2001 by Goryashchenko, Slovetskaya, Slinkin.
The Effect of Preliminary Treatment on the Catalytic Activity
of Cobalt–Silica Gel Catalysts
in the Complete Oxidation of Methane
†
S. S. Goryashchenko, K. I. Slovetskaya, and A. A. Slinkin
Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119992 Russia
Received April 16, 1998
Abstract—The effect of temperature in multiple oxidative–reductive treatments on the activity of cobalt–silica
gel catalysts in the complete oxidation of methane is studied. A decrease in the temperature of oxidative–reduc-
tive treatments from 500°C to 300°C results in an irreversible decrease in the activity of samples prepared by
the impregnation of SiO with cobalt nitrate. A sample prepared from cobalt acetate and calcined at 500°C
2
shows a lower activity, which was close to the activity of samples prepared from nitrates and calcined at 300°C.
INTRODUCTION
It is known that both bulk Co O and supported
umn packed with NaX zeolite. The sensitivities to meth-
–7
ane and oxygen-containing products were 2 × 10 and
3
4
–5
3
× 10 vol %, respectively. The space velocity of the
cobalt oxides are one of the most active catalysts for the
complete oxidation of hydrocarbons [1]. Cobalt oxide
supported on SiO and reduced in H can also be used
–
1
gaseous mixture (V , h ) at 500°C at which 99% meth-
sp
ane conversion was achieved was taken as an activity
2
2
measure. In all runs, CO and ç é were the only prod-
2
2
as a material that readily chemisorbs oxygen [2]. How-
ever, it has been shown that cyclic oxidative–reductive
treatments may substantially affect the phase composi-
tion of the cobalt–silica gel system [2]. Depending on
the pretreatment temperature (500 or 300°ë), either
ëo O or CoO phases can be stabilized on the SiO sur-
ucts.
Catalysts were subjected to cyclic oxidative–reduc-
tive treatments in the same reactor at 300 or 500°ë and
–
1
a space velocity of 1000 h . The samples were calcined
beforehand for 30 min in a flow of hydrogen. Then,
after cooling to room temperature in a flow of hydro-
gen, the latter was replaced by an oxygen flow, heated
to the desired temperature, and treated for 30 min. Air
was cleaned by passing through a NaX-packed column.
Hydrogen was cleaned over a Ni–Cr catalyst and a
chromium silicate sorbent. The residual amount of oxy-
3
4
2
face. This paper deals with the effect of the temperature
of oxidative–reductive treatments on the catalytic activ-
ity of the cobalt–silica gel system in the complete oxi-
dation of methane.
–
6
EXPERIMENTAL
gen was at most 2 × 10 vol %.
Supported cobalt oxide catalysts prepared by two
methods were used. According two one procedure,
samples containing 1.1 to 7.7 wt % Co (based on metal)
were prepared by the incipient-wetness impregnation
of support with aqueous solutions of Co(NO ) . In the
RESULTS AND DISCUSSION
The figure shows the dependence of the catalytic
activity on the concentration of cobalt in the samples
calcined at 500°ë that were not subjected to oxidative–
reductive treatments. The activities of these catalysts
are stable and do not change after calcination for 12 h
at a given temperature. The linear dependence of the
activity on the concentration of CO can be indicative of
the same structure and close parameters of the active-
phase crystals over the whole range of Co concentra-
tions.
3
2
second case, a 5.0 wt % Co/SiO sample was prepared
2
from cobalt acetate. KSK silica gel used in this work
2
had a specific surface area of 250 m /g. All samples
were dried at room temperature for 10 h and then in a
desiccator at 100°ë for 6 h. Then, the catalysts were
calcined in a flow of dry air for 1 h at 300 or 500°ë.
The catalytic activity was determined in a flow-
2
type reactor. The reactor cross-section was 1.8 cm .
The activities of the samples after three cycles of
oxidative–reductive treatment at 500°ë coincide with
the initial activity and are stable. Thus, cyclic treat-
ments at this temperature do not change the states of
The catalyst loading weighed 2 to 3 g and had a bulk
3
density of 0.7 g/cm . The reaction mixture contained
–
2
2
.9 × 10 vol % CH + 17.0 vol % é + balance He.
4 2
Products were analyzed using an LKhM-8MD chro-
matograph with a flame-ionization detector and a col-
samples. As it has been shown earlier [2], the ëo O
3
4
phase is present on the SiO surface in the catalysts of
2
†
Deceased.
this sort. The size of crystals in this phase is ~160 Å.
0
023-1584/01/4204-0525$25.00 © 2001 MAIK “Nauka /Interperiodica”
Goryashchenko
