Kinetics and Catalysis, Vol. 45, No. 4, 2004, pp. 618–620. Translated from Kinetika i Kataliz, Vol. 45, No. 4, 2004, pp. 654–656.
Original Russian Text Copyright © 2004 by Rozovskii, Kipnis, Volnina, Lin, Samokhin.
LETTER TO THE EDITOR
Selective Oxidation of CO under Conditions
of Catalyst Surface Ignition
A. Ya. Rozovskii, M. A. Kipnis, E. A. Volnina, G. I. Lin, and P. V. Samokhin
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
Received November 25, 2003
The oxidation of CO in the presence of hydrogen with the use of a BINOS 100 IR analyzer (error of
(selective oxidation of CO) has been intensely studied <1 ppm).
in recent years in the context of hydrogen purification
We found that the oxidation of CO in reactor 1
for fuel cells (for example, see [1, 2] and references
occurred in a kinetic region. In reactor 2, the ignition of
therein). Many researchers observed a rapid increase in
a catalyst surface was observed, and the reaction
abruptly changed to an outer-diffusion region. Figure 1
illustrates the ignition and quenching of the surface. At
a low temperature, the reaction occurs in a kinetic
region; the temperature in the catalyst bed increased in
parallel with the temperature of the furnace until the
critical temperature of surface ignition was reached. On
reaching a critical value, the catalyst bed temperature
spontaneously increased at a constant furnace tempera-
ture with a simultaneous decrease in the residual con-
centration of CO. Once the surface ignition mode was
established, a change in the furnace temperature only
slightly affected the gas temperature in the catalyst bed
and the residual CO content of the gas mixture until the
critical temperature of surface quenching was reached
(with decreasing temperature). On reaching this critical
value, the temperature in the catalyst bed abruptly
decreased, and the CO content increased; both of these
parameters changed to a level that occurred before igni-
tion. In this case, a hysteresis was observed, which is
characteristic of the phenomenon of surface ignition
(Fig. 1).
the conversion of CO with temperature. This phenome-
non was usually defined by the term ignition. Note that,
as a rule, ignition cannot be implemented in heteroge-
neous catalysis. However, the spontaneous change of a
reaction to an outer-diffusion region (Frank-
Kamenetskii [3] named it catalyst surface ignition) can
be observed. According to Frank-Kamenetskii [3], this
change is observed in exothermic reactions in the case
that the positive heat flow q+ due to the reaction
becomes equal to the negative heat flow q– related to
heat removal, q+ = q–. In combination with the con-
straint dq+/dT > dq–/dT, this condition becomes crucial,
and the reaction abruptly changes to an outer-diffusion
region as the temperature is increased; this is accompa-
nied by a spontaneous increase in the reaction temper-
ature [3] (see also [4]).
To understand the essence of the above processes,
we studied the selective oxidation of CO on a platinum-
containing catalyst in an excess of hydrogen in the pres-
ence (and in the absence) of CO2 and water vapor in
flow reactors with dramatically different conditions of
heat removal.
Reactor 1 (with a high rate of heat removal) was
close to an isothermal reactor. The reaction was per-
formed in a cylindrical metal reactor with a coaxially
placed metal tube. A catalyst sample was diluted with
an inert material in a ratio of 1 : 10 and loaded in an
annular gap between the tube and the reactor walls.
The apparent activation energy was determined by
two methods: from the dependence of the critical tem-
perature of surface ignition on the space velocity of a
gas flow (see [4, p. 193]) and from data obtained in an
isothermal reactor using a traditional method. Both of
the values were practically equal (13 and 14 kcal/mol).
Cylindrical quartz reactor 2 (with a low rate of heat
removal) approached an adiabatic reactor, so that the
temperature of a gas mixture increased as the mixture
moved along the axis of the reactor. A catalyst sample
was placed in the reactor without dilution with an inert
material. The temperature of the gas mixture was mea-
sured to within 0.1 K at the inlet and outlet of the cata-
lyst bed (thermocouples in a thin metal jacket in the cat-
alyst bed). The furnace temperature was specified with
the use of a special programmer.
It is of importance that in all cases the residual CO
content under optimum conditions was much lower in a
surface ignition mode than that in an isothermal reactor,
where the reaction occurred in a kinetic region.
Figure 2 demonstrates the temperature dependence
of the residual CO concentration in an isothermal reac-
tor (curve 1) and in a “hot spot” at the outlet of the cat-
alyst bed in reactor 2 (curves 2, 3). It can be seen that
the residual CO concentration at the same temperatures
The residual concentration of CO in a dried gas mix- and space velocities was lower by one order of magni-
ture was measured (and recorded) in an on-line mode tude under conditions of catalyst surface ignition.
0023-1584/04/4504-0618 © 2004 MAIK “Nauka /Interperiodica”
Rozovskii
