CO Oxidation on Pt: Variable Phasing of

Inputs During Forced Composition Cycling

A combined experimental and theoretical investigation of the effect of
forced feed composition cycling for CO oxidation on platinum has been
performed. A novel approach to forced composition cycling was
examined, in which the phase angle between the two input streams was
varied. Reaction rate enhancement is shown to occur, and by varying
the phasing of the feed streams it is possible to achieve a global
maximum in the time-average reaction rate. This phenomenon can be William R. C. Graham
explained quantitatively by a model based on an adsorbate-induced
David T. Lynch
Department of Chemical Engineering
phase change of the Pt surface combined with CO adsorption self- University of Alberta
exclusion. This mathematical model can also quantitatively describe Edmonton, Alberta, Canada T6G 2G6
the complex steady-state behavior (uniqueness-multiplicity transitions)
observed for this reaction. The predictions of the model have been
validated further through a detailed experimental study of the effects of
feed flow rate, temperature, size of catalyst charge, and cycling
frequency on the instantaneous and time-average conversions during
forced cycling of the feed composition.

Introduction the much less complex phenomenon of steady-state multiplicity.

Despite the apparent simplicity of the reaction, a complete Hegedus et al. (1977) demonstrated that intrapellet diffusional
mechanistic description of the C O oxidation on supported resistances are important, while Chakrabarty et al. (1984)
platinum has yet to be developed. This lack of knowledge is concluded that the interaction of the surface reaction with the
illustrated vividly by the variety of hypotheses proposed to adsorption/desorption processes, in the absence of diffusional
explain the experimentally observed phenomena. For example, limitations, is responsible for the multiplicities. Herskowitz and
for the phenomenon of self-sustained oscillatory behavior it has Kenney (1983) found that each of two quite different Langmuir-
been speculated that the oscillations could be due to: competitive Hinshelwood-Hougen- Watson models could adequately predict
adsorption of different types of surface CO (Hugo and Jakubith, the values of CO concentration at which transitions from low to
1972); coverage-dependent activation energies (Belyaev et al., high conversions occurred. Graham and Lynch (1 987) showed
1974; Pikios and Luss, 1977); adsorption of inert (Eigenberger, that several very different models (standard Langmuir-Hinshel-
1978) or coreacting (Mukesh et al., 1982) species; variation of wood, oxidation-reduction, surface island, CO self-exclusion)
the catalyst surface temperature (Dagonnier et al., 1980; Jensen could quantitatively describe the effect of operating conditions
and Ray, 1982); kinetic nonlinearities in the reaction mecha- on the upper multiplicity boundary (transition from high to low
nism (Morton and Goodman, 198 1); catalyst oxidation/ conversion), but that only a model based on CO adsorption self-
reduction (Sales et al., 1982); variation of the oxygen sticking exclusion could describe the variations in the lower multiplicity
probability as a function of CO coverage (Ertl et al., 1982; boundary (transition from low to high conversions) over a wide
Lynch et al., 1986); interaction of silicon impurities with the range of C O and 0, feed compositions. However, it was found
catalyst surface (Yeates et al., 1985); and diffusion of carbon to that, even with quite extensive data, an essentially infinite
the catalyst surface (Burrows et al., 1987). number of different sets of parameter values could result in
Several alternative hypotheses have been postulated even for excellent agreement between the model predictions and the
steady-state data.
As demonstrated by Kobayashi and Kobayashi (1974) and
Bennett (1976), the use oftransient response methods can often
Correspndence concerning this paper should be addressed to D. T. Lynch.
W. R. C. Graham is currently with AECL Research Ltd.. Chalk River, Ontario KOJ 1JO. help to resolve questions concerning reaction mechanisms.

1796 December 1990 Vol. 36, No. 12 AIChE Journal

These methods have often been used to examine CO oxidation effective free volume of the reactor, bellows and associated
on platinum, from which it has been determined that the tubing (excluding the volume of the catalyst and bed diluent)
bimolecular surface reaction between adsorbed species is the was determined to be 190 cm’, and the mixing in the vessel was
main reaction path at typically encountered reaction conditions found to closely approximate that of an ideal CSTR. The details
(Engel and Ertl, 1979). Several investigations of CO oxidation of the catalyst characteristics, experimental equipment and feed
have used a variant of the transient response method in which materials have been previously given by Graham and Lynch
the feed composition is periodically manipulated, usually in the (1987), with the exception that modifications were made to the
form of square wave cycles. As shown in modeling studies by feed system to permit the generation of feed composition square
Douglas and Rippin (1966) and by Bailey (1973), and in the waves as described by Graham and Lynch (1988). The C O and
seminal experimental study by Unni et al. (1973), the operation 0, feed streams (specialty mixtures of CO/N, and O,/N, from
of catalytic reactors in a periodic manner can substantially Linde) were each routed through air-actuated four-way valves,
improve the reaction rate and/or selectivity. In addition, peri- the timing of which was controlled with digital outputs from a
odic operation can be used to obtain insights into underlying HP minicomputer to achieve square wave cycling. Frequencies
reaction mechanisms. Cutlip (1 979) found that forced composi- of up to 17 mHz were used in this study. This manner of
tion cycling of the feed stream to a CSTR (continuous stirred generating square-wave composition cycles was found to be
tank reactor) resulted in reaction rate enhancement for C O vastly superior to the alternative method of directly manipulat-
oxidation on a supported platinum catalyst, and for this reaction ing the flow controller setpoints. Steady-state and dynamic
Cutlip et al. ( 1 983) and Graham and Lynch (1984) have shown measurements of the CO, concentration in the reactor effluent
that reaction rate enhancement can be partially explained by were made using an infrared spectrophotometer set a t 2,355
using models based on Langmuir-Hinshelwood-type mecha- cm-’ which was interfaced to the minicomputer for real-time
nisms. Oh et al. (1978), however, have shown that a diffusion- data accumulation and analysis.
reaction model, which accounts for surface accumulation, can
adequately describe the rate enhancement that was observed
when a tubular reactor was used. In a similar fashion, Cho
Dejnition of oxygen phase lead
(1983) found from a mathematical model of a single catalyst In this paper, the reactant feed composition was not main-
pellet that rate enhancement can be due to the interaction of tained at a constant value. Instead, the feed concentration of
diffusion with adsorption-desorption processes, whereas Bar- each reactant was switched periodically between two values.
shad and Gulari (1985) believe that rate enhancement is due to During cycling, the C O feed mole fraction alternated between
an optimization of the amount of reactive CO surface species 2% for half of each cycle and 0% for the other half, with an
relative to nonreactive surface species. average composition of 1%. The oxygen feed mole fraction
In this investigation of C O oxidation on supported platinum, alternated between 1% for half of each cycle and 0% for the
the effect of reactor operating parameters on rate enhancement other half, so that the average feed mole fraction of 0, was 0.5%.
during forced feed composition cycling is examined comprehen- Ultra-high-purity nitrogen (Linde) comprised the balance of the
sively for seven different sets of values of the mass of catalyst, feed stream. Because the two four-way valves in the feed system
the feed flow rate and the reactor temperature. In an earlier could be independently manipulated, it was possible for the
study (Graham and Lynch, 1987), the steady-state rate and oxygen half-cycle to overlap with the C O half-cycle as much or
multiplicity behavior were completely determined for each of as little as was desired. Different ways of cycling the reactants
these sets of reactor operating conditions. In this study, the are shown in Figure 1. The overlap between the oxygen and C O
effect of the frequency of the input cycle on the transient and half-cycles will be defined using the term “oxygen phase lead.”
time-average reaction rates is determined for each of the seven Defining one complete feed cycle as having 360 degrees, the
sets of operating conditions. In addition, a new mode of cyclical oxygen phase lead will refer to the fraction of a cycle, in degrees,
operation is examined in which the phase angle between inputs between the start of an oxygen half-cycle (1% 0,) and the start
is varied. It is shown that the phase angle has a major effect on of a carbon monoxide half-cycle (2% CO). Hence, as shown in
the time-average reaction rate. These data are used to evaluate a Figure la, if the C O is switched off when the oxygen is switched
model combining the features of the surface phase transforma- on, and vice versa, an oxygen phase lead of 180 degrees results
tion model that has been used to explain self-sustained oscilla- and the inputs are said to be out-of-phase. If the C O and 0, are
tory behavior (Lynch et al., 1986) and the CO self-exclusion switched on and off simultaneously, as shown in Figure lb, then
model that has been used to account for steady-state multiplicity the oxygen phase lead is zero degrees (or 360 degrees in-phase
(Graham and Lynch, 1987). It is shown that the combined cycling). If the C O is turned on three-quarters of a cycle after
model is capable of quantitatively describing most of the the oxygen is turned on, as shown in Figure Ic, then the oxygen
experimental observations. Thus, this results in a single consis- phase lead is 270 degrees, whereas if the C O is turned on after
tent explanation for three commonly observed complex phenom- the oxygen has been flowing to the reactor for one-quarter of a
ena: reaction rate resonance during feed composition cycling; cycle, as shown in Figure Id, then the oxygen phase lead is 90
steady-state multiplicity; and self-sustained oscillations that degrees. In general, any amount of oxygen phase lead is possible
occur during C O oxidation on supported platinum. with the experimental equipment.

Experimental Equipment and Materials Experimental Behavior

A 0.5 wt. % Pt/Al,O, catalyst was used in an external recycle The seven sets of reactor operating conditions used in this
reactor system operated at 0.1 MPa total pressure for the study, Table 1, are identical to those used previously by Graham
reaction studies. From frequency response measurements simi- and Lynch (1987), in which the steady-state rates and bifurca-
lar to those described by Lynch and Walters (1990), the tion behavior were determined for this system. The effects of

AIChE Journal December 1990 Vol. 36, No. 12 1797

 1.2 I
1.0
0.8
0.6
a
Q) 0.4
a,
kl 0.2

1.0
0.8
d
Q) 0.6
tln
h 0.4
x Case IV uam -----
0 0.2
Case V AAA

0
k 1.0
Q)

Time
a 0.8
0.6
Figure 1. Feed cycling strategies.
Oxygen phase leads: a. 180 degrees; b. 0 degrees; c. 270 degrees; d. 0.4
90 degrees Case VI o a m -----
0.2
0.0
operating conditions (mass of catalyst, temperature, and flow 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
rate) on the multiplicity boundaries are summarized in Figure 2.
In the cusp-shaped regions of Figure 2, multiple steady states Percent CO in t h e Feed
exist. To the left of the multiplicity region only high-conversion
Figure 2. Steady-state bifurcation behavior.
steady states occur, while to the right of the multiplicity region
a. Effect of mass of catalyst
only low-conversion steady states are present. Open symbols b. Effect of temperature
indicate the highest CO concentrations for which unique, c. Effect of flow rate (seeTable 1 for operating parameters)
high-conversion steady states were observed, whereas the solid
symbols indicate the lowest CO concentrations for which unique,
low-conversion steady states were found. The half-filled symbols out-of-phase. The effects of size of catalyst charge, temperature,
mark the boundaries of the observed region of multiplicity. (A flow rate and frequency on the average conversion were exam-
low-to-high conversion transition occurs at a concentration ined. For out-of-phase feed switching, the (CO, 0,) feed
bracketed by an open and a half-filled symbol, whereas a composition alternates between (O%, 1%) for the first half-cycle
high-to-low conversion transition occurs between each pair of and (2%, 0%) for the second half-cycle. The average feed
half-filled and filled symbols.) See Graham and Lynch (1 987) composition of (1%, 0.5%) is in the unique low-conversion region
for additional information concerning the steady-state behavior in Figure 2 for all seven sets of operating conditions. For cycling
for the seven sets of reactor operating conditions. The curves in in this manner, the feed alternates between the high and low
Figure 2 are model predictions which will be described later. conversion sides of the multiplicity region. A somewhat similar
In the first part of this study, the oxygen phase lead was switching between the two sides of the multiplicity region occurs
maintained constant a t 180 degrees, i.e., the inputs were if the oxygen feed composition is held constant with only the CO
feed composition cycled. This type of single-input cycling has
Table 1. Reactor Operating Conditions been compared to 180-degrees out-of-phase cycling of the two
feed streams by Graham and Lynch (1988), and it was found
Mass of Cat. Flow Rate that both methods of feed cycling resulted in approximately
Case g Temp. "C mol/s equal time-average reaction rates. In the following, all feed
cycling involved variation of both feed streams (CO and 0,
I 14.6 90 205 x
cycling).
I1 4.95 90 68 x
I11 43.6 90 615 x In the second part of this study, the effect on the average
IV 14.6 70 205 x conversion of the phase angle between inputs was examined for
V 14.6 110 205 x the base case (case I in Table 1) operating conditions. For
VI 14.6 90 68 x cycling of this type the (CO, 0,) feed composition switched
VII 14.6 90 615 x
sequentially from (O%, 1%) to (2%, I%), (2%, O%), and (O%,

1798 December 1990 Vol. 36,No. 12 AIChE Journal

0%) for oxygen-phase leads between 0 and 180 degrees (see 3. The CO, concentration falls as the reaction rate goes to
Figure Id) and from (0%. 1%) to (O%, O%), (2%, O%), and (2%, zero. The gas-phase CO, concentration is not a direct indication
1%) for phase leads between 180 and 360 degrees (see Figure of the reaction rate, because the alumina support has a large
Ic). In every case an average feed composition of (1% CO, 0.5% capacity for CO, adsorption-desorption. During this interval,
0,) was maintained. the metal surface becomes saturated with CO.
4. When the CO feed is switched off and 0, is switched on,
the reaction rate does not immediately rise because the surface
Out-of-phasefeed cycling is almost entirely covered with CO, leaving very few pairs of
In the initial experiments, the reaction behavior during empty sites for oxygen adsorption. The gas-phase CO concentra-
180-degrees out-of-phase feed switching was determined. The tion decrease is accompanied by a slow decrease in the surface
dynamic COz exit composition during out-of-phase cycling is coverage of CO due to both CO desorption and surface reaction
shown in Figure 3 for case I operating conditions. At low processes.
frequencies (e.g., w = 1.67 mHz in Figure 3a) there are six 5. When sufficient pairs of active sites become available for
distinct regions of interest during a single cycle. These regions significant oxygen adsorption, there is a very sharp rise in the
are labeled in Figure 3a and are characterized as follows: CO, concentration as most of the surface CO is eliminated by
1. There is a rapid increase in CO, concentration as soon as reaction with adsorbing oxygen.
the CO is switched on. At the start of this region, the catalyst 6 . In the final region, the remainder of the surface CO reacts
surface is almost entirely covered with oxygen and the reactor to form CO,, and the CO, concentration falls as the oxygen
oxygen composition is approximately 1%. During this period, surface coverage increases. The desorption of CO, from the
almost all the CO entering the reactor adsorbs on the surface alumina support again (as in region 3) prevents the reactor CO,
and reacts with adsorbed oxygen, which is being replenished concentration from falling to zero.
from the gas phase. This period lasts until there is no gas-phase As the frequency increases from that in Figure 3a, the peaks
oxygen left in the reactor. in the CO, concentration begin to merge and the different
2. The rate of increase in CO, concentration decreases after regions start to overlap or disappear. The double peak per cycle
the gas-phase oxygen has been depleted. During this interval, becomes a single peak per cycle, and, for high frequencies, the
the CO entering the reactor adsorbs on the catalyst surface and high-rate behavior disappears altogether. The time-average con-
cleans the surface of the remaining oxygen. version for each cycling frequency is determined by integrating
the area under the CO, concentration curve. Average conver-
sions for the five cases in Figure 3, as well as for several
additional frequencies, are shown in Figure 4,where, at very low
frequencies, the time-average conversion increases almost lin-
early from zero. As the frequency increases, the time-average

(e) w=6.45 mHz

1
2 5 ;
I
I j 0 2 4 6 a
0 I . . ' . " . I . . "

0 200 400 600 800 1000

Frequency (mHz)
Time, seconds Figure 4. Predicted vs. experimental time-average conver-
Figure 3. Instantaneous CO conversion during 180-de- sions for 180-degrees out-of-phase forced cy-
grees out-of-phase forced cycling: Case I data. cling.
Dashed lines indicate the timing of step changes in the feed CO 0, case I data; ... .., standard Langmuir-Hinshelwood model;
concentrations with oxygen stepchanges 180 degrees out-of-phase - _ _ ,_CO self-exclusion model; ___ ,Eqs. 4 to 10

AIChE Journal December 1990 Vol. 36,No. 12 1799

conversion increases to a maximum and then decreases until a The critical frequency above which significant rate enhance-
critical frequency is reached, following which the time-average ment does not occur is determined primarily from mixing
conversion drops to a low value that approximately equals the considerations and the steady-state bifurcation diagram (Figure
steady-state conversion for a 1.0% CO and 0.5% 0, feed. The 2 ) . In the low-conversion steady-state region, the reaction rate
curves in Figure 4 represent model predictions, which will be does not significantly affect the reactant concentrations (conver-
discussed later. Out-of-phase feed switching experiments were sion <lo%, except for case VI). If it is assumed that the reactor
performed for several frequencies for each of the cases in Table concentration is not affected by the reaction, then for square-
1, and the time-average conversions are summarized in Figure 5 . wave feed cycling to a CSTR the reactant concentrations will
For comparison, the base case (case I) results from Figure 4 are alternately increase and decrease in an exponential manner
included in each of the panels of Figure 5. solely due to mixing effects. For a cycling frequency w with
Shown in Figure 5a is the effect of the mass of catalyst on the switching between 0% and 2% CO in the feed, the extreme mole
time-average conversion (with the ratio of feed flow rate to fractions of CO in the reactor will be
catalyst mass held constant). Although the steady-state bifurca-
tion behavior was identical for cases I, I1 and 111 (see Figure 2a),
the time-average behavior is quite different. Large catalyst
charges lead to higher maximum average conversions during where 7 , = V/Qo.The reactor (CO, 0,)composition will remain
forced cycling, and the cycling frequency, at which the maxi- on a line between (O%, 1%) and (2%, 0%) during cycling. As
mum average conversion occurs, increases with increasing long as the reactor compositions stay to the right of the
catalyst mass. In Figure 5b it is seen that increasing tempera- low-to-high-conversion bifurcation curve, the reactor will re-
ture increases the maximum time-average conversion, while the main in a low-conversion state. If the reactor crosses to the left
data in Figure 5c show that decreasing the flow rate will increase of the low-to-high-conversion bifurcation curve, the possibility
the maximum time-average conversion. of increased reaction rates will exist. To reach the high conver-
sion branch of the rate curve, however, the surface must be
cleaned of CO. The time required to clean the surface of CO will
be approximately equal to U , L , / ~ ~ ~ Q ~(see
[CO the] ~
Notation
0.8
section for the definition of symbols). If FcOcrit
is defined as the
CO composition (in percent) at the point where the line between
0.6
(CO, 0,)compositions of (O%, 1%) and (2%, 0%) intersects the
low-to-high-conversion bifurcation curve, then to a first approx-
0.4 imation,
F:
0 0.2
.A
74.
b 0.0
For five of the seven cases in Table 1 (cases I-V), the observed
critical frequency was within 15% of that predicted by Eq. 1.
Case IV -----
0.6 The largest difference was for case VI, where the predicted
critical frequency was 55% higher than the experimental value
0.4 (the steady-state conversion, fss, for case VI is not negligible,
thus violating the stated assumptions). Thus, the critical fre-
0.2 quency during out-of-phase cycling is strongly related to the
time required for the concentrations to cross the low-to-high-
0.0 conversion bifurcation boundary on the steady-state multiplicity
diagram (Figure 2).
In the low-frequency regions of Figure 5 , the frequency-
conversion curve is linear. The slope is primarily a function of
flow rate, reactor volume and catalyst surface area. For long
cycle periods (low frequencies), the conversion will be limited by
the amount of CO, produced by the alternate cleaning of the
surface of adsorbed carbon monoxide and oxygen (the two peaks
per cycle behavior shown in Figure 3a). The first peak, associ-
ated with switching on the CO feed, will contain an amount of
CO, limited by the sum of one monolayer of oxygen on the
0 2 4 6 8 10 12 14 16 18 catalyst surface and the gas-phase oxygen contained in the
reactor volume. The second peak, which occurs after the oxygen
Frequency (mHz) feed is switched on, will contain an amount of CO, approxi-
Figure 5. Effect of frequency on the time-average conver- mately equal to the CO in one surface monolayer (most of the
sion during 180-degrees out-of-phase cycling. gas-phase CO has left in the reactor effluent). The total CO,
Curves from Eqs. 4 to 10 produced during the two peaks will thus be approximately equal
a. Effect of mass of catalyst to two monolayers plus twice (due to the stoichiometry) the feed
b. Effect of temperature
c. Effect of Row rate (see Table 1 for operating parameters) oxygen concentration multiplied by the reactor volume. The

1800 December 1990 Vol. 36, No. 12 AIChE Journal

time-average conversion of a single cycle is defined as the total CO, 0.5% 0,) feed stream. This combination of two sets of
CO, production divided by the total amount of CO in the feed steady-state operating conditions results in a higher average
stream during a complete cycle. The CO in a single cycle is the conversion than that obtained using 180-degrees out-of-phase
product of the flow rate, the average feed concentration, and the cycling.
period of the cycle. Thus, at low frequencies, the time-average
conversion is approximately given by: Effect of oxygen phase lead on time-average conversion
Following the completion of the first set of experiments, the
effects of the phase angle between the feed square waves were
determined. The phase-angle experiments were performed using
the base case (case I) operating conditions. The time-average
Using experimentally determined values of (fTAQ,[CO],/w) conversion is plotted against the oxygen phase lead for the five
from the approximately linear portions of cases I to VII in frequencies in Figure 6. (The solid lines represent model
Figure 5 and values of (umLm)from hydrogen adsorption predictions which will be discussed later.) At frequencies higher
measurements, a linear regression was performed to obtain than those shown in Figure 6 (e.g., for w > 10 mHz), the
time-average conversion was always similar to that shown in
0.92arnLrn+ 2.571/[0,] Figure 6e for all values of the oxygen phase lead. As the cycle
/>A =
( Qo[COlo
(3) frequency was decreased, a small region of enhanced conver-
sions appeared for oxygen phase leads of less than 180 degrees.
where the 95% confidence intervals for the constant terms are For a period of 120 seconds, as shown in Figure 6d (w = 8.33
0.92 0.20 and 2.57 2 0.83. The first constants in Eqs. 2 and 3 mHz), this region spans from 45 to 150 degrees. The maximum
differ by more than a factor of two, but, considering the time-average conversion attained was greater than 90%, and this
assumptions used to derive Eq. 2, the agreement between these occurred for phase leads between 45 and 90 degrees. For phase
equations is quite good. The (2a,Lrn) term in Eq. 2 is an upper leads less than 45 degrees or greater than 170 degrees, the
limit assuming that two complete monolayers react and that the
number of catalytic sites for reaction is the same as for the
hydrogen adsorption. If fewer than two monolayers react or if
the number of active sites for reaction is less than that for o=l.ll mHz
hydrogen adsorption, then this term will be replaced by a 0.6
smaller one. Herz and Marin (1980) summarized the data that 0.4
oxygen uptakes are typically only about one-half the correspond- 0.2
ing hydrogen uptakes, while Graham and Lynch (1987) ex- 0.0
plained that the ratio of adsorbed C O molecules to surface
platinum atoms can be less than unity. Reduced uptakes of CO
and oxygen relative to hydrogen would decrease the value of the
contrast (2) in the first numerator term in Eq. 2. The second
numerator term in Eq. 2 is based on the assumption that no
gas-phase CO replenishes the surface after the oxygen feed is
switched on. A small fraction of the gas-phase CO is more likely
to adsorb on the surface after the introduction of oxygen, so this
term should be larger than that given in Eq. 2.
From Eqs. 2 and 3, it is seen that large catalyst charges lead to
large slopes, while large flow rates lead to small slopes. Thus, for
operating conditions with the same flow rate and catalyst
charge, the slope in the low-frequency region should be the
same, even if the steady-state behavior is different. This was
observed for reactor operation at different temperatures (cases I,
.
IV and V) as shown in Figure 5b (same initial slope) and Figure 0.2 ;
2b (different steady-state behavior). 0.0 .
The largest average conversion obtained during 180-degrees
out-of-phase cycling was 7 1% for case VI with a frequency of 2.2
mHz. Although this is 4.5 times greater than the steady-state 0.6
conversion for the same average feed composition, it is not 0.4
greater than that which could be obtained from a combination of 0.2
steady states with the same average feed composition. That is, 0.0
this cycling strategy does not represent a global maximum. For 0 90 180 270 360
example, by combining the exit streams from two steady-state
reactors, one of which operates at a 100% conversion level with a Oxygen phase lead (degrees)
(1.7% CO, 1% 0,) feed with the other operating a t 0% Figure 6. Effect of phase angle between CO and 0, feed
conversion with a (0.3%CO, 0%0,)feed with equal space times streams on the time-average conversion during
for both reactors, it is possible to obtain an average CO forced cycling.
conversion of 85% while maintaining an overall combined (1% Curves from Eqs. 4 to 10

AIChE Journal December 1990 Vol. 36,No. 12 1801

time-average conversion was approximately 5%. It is interesting
to note that 180-degrees out-of-phase cycling does not lead to
rate enhancement for this frequency (see case I in Figure 4 with
w = 8.33 mHz).
As shown in Figure 6c for w = 4.17 mHz, the maximum
time-average conversion is approximately 90%. This is greater
than the largest time-average conversion (55% for case I
conditions) obtained with 180-degrees out-of-phase cycling.
Thus, by varying the phase angle between inputs, it is possible to
improve the conversion obtained during feed composition cy-
cling. The highest conversion attainable by combining the
Fractional Surface Coverage of CO, 8,,
steady-state operating conditions of case I is 62.5% [one
steady-state reactor with 100% conversion of a (1.25% CO, 1% Figure 7. Effect of CO coverage on surface reaction and
0,)feed and a second vessel with 0%conversion of a (0.75% CO, 0, adsorption rate constants.
0% 0,) feed stream]. Hence, with variable-phase cyclical
operation it was possible to increase the maximum average
nal (hex) phase. This phase has a low sticking probability for
reaction rate by more than 40%. This is an example of global
oxygen adsorption. When the fractional surface coverage of C O
rate enhancement.
exceeds a critical value, (/3CO)H, the entire surface spontaneously
As shown in Figure 6, as the frequency was decreased, the
transforms to a (1 x 1) phase, i.e., the reconstruction is re-
range of phase angles for which rate enhancement occurred
moved. The (1 x 1) phase has an oxygen sticking probability
increased, the maximum time-average conversion decreased,
orders of magnitude higher than the hex phase. The metal
and a plateau region appeared for large oxygen phase leads. The
surface remains in the (1 x 1) phase until the fractional surface
plateau region is shown clearly in Figure 6a ( w = 1.11 mHz)
coverage of CO drops below a second critical value, (flCJL. The
where it extends from about 180 to 320 degrees of oxygen phase
surface then reconstructs to form the hex phase. These effects
lead (from 180 to 270 degrees for w = 2.78 mHz in Figure 6b).
have been demonstrated using single crystals with well-defined
The plateau conversion is related to the catalyst charge, reactor
faces. The applicability of these effects for supported-metal
volume, feed flow rate, and frequency in the same way as is the
catalysts, such as that used in this study, has received support
conversion in the low-frequency regions of Figure 5, and thus the
from the study by Wang et al. (1985) where it was shown that
plateau conversion can approximately be described by Eq. 3.
supported platinum crystallites form predominantly (100) crys-
Despite a number of experimental studies of 180-degrees
tal planes when grown in hydrogen.
out-of-phase feed composition cycling, this is apparently the
In the mathematical representation of the surface phase-
first-other than the preliminary study by Graham and Lynch
change phenomenon, the surface is considered to be a homoge-
(1988)-experimental report of the effect of phase angle varia-
neous surface, that is, the entire surface is either in phase 1 (hex)
tion during feed composition cycling. However, two somewhat
or phase 2 (1 x 1). In phase 1, the oxygen sticking probability
limited mathematical studies of the effect of phase angle
and the surface reaction rate constant have low values, as shown
variation have been reported. Cho (1983) in his mathematical
in Figure 7. When the catalyst surface transforms to phase 2, the
study of feed cycling to a single catalyst pellet, investigated only
oxygen sticking probability and the surface reaction rate con-
0-, 90- and 180-degrees phase angles and the model predicted
stant both increase by a factor of $, which for this work has been
that the best performance should be achieved with 180-degrees
assigned a value of 250. When the surface transforms back to
out-of-phase cycling which is at odds with our findings. In an
phase 1, the oxygen sticking probability and the surface reaction
examination of an abstract catalytic model for parallel reac-
rate constant both return to their initial values. In Lynch et al.
tions, Zolotarskii et al. (1988) concluded that optimal perfor-
(1986), $ was equal to 150 for the oxygen sticking probability
mance was achieved when two input cycles overlapped to some
and the surface reaction rate constant was assumed to be
extent, which is consistent with this study.
unaffected by the surface-phase transformation. In the initial
attempts to fit the feed cycling data, the surface reaction rate
Model Details constant was kept at the same value for both phases, but it was
The reaction was assumed to proceed via a Langmuir- found that much better agreement with the data could be
Hinshelwood-type bimolecular surface reaction between revers- obtained by allowing the surface reaction rate constant to
ibly adsorbed carbon monoxide and irreversibly adsorbed oxy- change in the same manner as the oxygen sticking probability.
gen (dissociative adsorption). Two key assumptions made When the preceding assumptions are incorporated into a
distinguish the mechanism from a standard L H mechanism. mathematical model for an isothermal CSTR, the following
First, as described by Graham and Lynch (1987), it was equations result:
assumed that CO self-exclusion occurs: on average, each ad-
sorbed CO molecule prevents adsorption of further C O from dX
slightly more than the one surface site occupied by the adsorbed - = X , - Q J - KJ(I - B,-o - 00)
dr
CO molecule. Secondly, in a similar fashion to that described by
Lynch et al. (1986), the catalyst surface was assumed to switch
between two phases of different activity, as shown in Figure 7,
when the critical values of CO surface coverage are reached.
As shown by Behm et al. (1983) and Thiel et al. (1983) for dY
clean surfaces, the Pt( 100) face reconstructs to a quasihexago- -= Y , -
dr
Y K , Y ( ~- eCo - oo)2
Q~- (5)

1802 December 1990 Vol. 36, No. 12 AIChE Journal

 determined by mass balance considerations. Thus, most kinetic
models with correct values for volume, flow rate, and surface
area will describe the slope, so the ability to predict the
conversion in the low-frequency region is not an adequate test of
a model. A better test is to examine the predicted behavior in the
intermediate and high-frequency regions. As shown in Figure 4,
neither the standard L H model nor the C O self-exclusion model
could describe the time-average conversion for other than low
frequencies. As frequency increases from low values, these
models predicted a smooth transition to low time-average rates,
unlike the data which showed an abrupt decrease in conversion
at a critical frequency related to the location of the steady-state
bifurcation boundary. Larger values of K2 and K , are required if
these models are to adequately describe the time-average
behavior. However, increasing K , and K , causes the error in the
predicted steady-state behavior to increase (agreement with
steady-state behavior shown in Figure 2 must be maintained).
The error in the predicted low-to-high-conversion bifurcation
where points can be reduced by increasing the ratio of K, to K-,, but
the error in the predicted high-to-low bifurcation points will
remain unacceptable. If, however, K , and K , are increased only
in the low-conversion, high-CO-coverage region, then it is
possible to describe both the steady-state and the time-average
behavior of the system. This is precisely what occurs when the
surface-phase transformation model is used.
+
In the final modeling attempt, a value of greater than unity
These equations are identical to those used by Graham and
+
was used, in this case = 250. For the parameter values listed
in the Notation section, the predicted steady-state behavior is
Lynch (1987) except that in this model the rate constants for shown in Figure 2. The model describes steady-state data very
oxygen adsorption and surface reaction are given by: well. This is an important point of agreement between the model
and the experimental data, as a model which does not correctly
describe the observed steady-state behavior should not be used.
The predicted case I time-average conversions are shown in
Figure 4, where they are compared to the standard L H model
and the CO self-exclusion model. The new model is superior to
+
where is always unity for surface phase 1 (low C O coverage)
both of the existing models and quantitatively describes all of
the major features of the conversion-frequency curve: the initial
and greater than unity for surface phase 2 (high CO coverage).
This model reduces: to the CO self-exclusion model if +is
slope at low frequency, the magnitude and frequency of the
maximum conversion, and the critical frequency after which
always unity; to the surface-phase transformation model of
significant rate enhancement does not occur. The model was also
Lynch et al. (1986) if Nco = 1; and to the standard L H model if
+
Nco and both are unity. For cycling of the type used in this used to predict the effects of catalyst charge, temperature, and
flow rate on time-average conversion. As shown in Figure 5, the
study, Xo and Yo both alternate between 0 and 2 (but not
together unless the cycles are in phase), and Z , is always zero resulting predictions are in excellent agreement with the data,
because no C 0 2was present in the feed. Kinetic parameters for describing all of the major features of the time-average conver-
sion behavior.
the model were chosen based on two criteria: the model should
describe the steady-state bifurcation behavior (Figure 2); and The model predictions of the dynamic behavior during 180-
the model predictions should match the experimental transient degrees out-of-phase cycling are shown in Figure 8 . Except for
and time-average behavior (Figures 3 and 6 ) as closely as the CO, desorption preexponential factor, the parameters used
possible. for CO, adsorption and desorption on the support are identical
to those used by Lynch et al. (1986). These predictions are for
the conditions of case I. Each panel in Figure 8 can be compared
Model Predictions directly with the corresponding panel in Figure 3. All of the
First attempts at describing the time-average behavior used important features of Figure 3 are also seen in Figure 8 .
the standard LH model and the CO self-exclusion model with The final test of the model was to determine if it could
the rate parameters given by Graham and Lynch (1987), which describe the effect of oxygen phase lead on the time-average
minimized the error in the predicted steady-state bifurcation conversions. The predicted time-average conversion behavior is
behavior. The predicted base case (case I) time-average conver- shown in Figure 6 , where it is compared with the experimental
sions for these models are plotted in Figure 4. At low frequen- results. At low and intermediate frequencies, as shown in
cies, the initial slope of the predicted conversion-frequency curve Figures 6a to 6c, there is good agreement between the model
is approximately in agreement with the data. As previously predictions and the data with disagreement occurring only at
mentioned, however, the slope of the initial linear region is low values of the oxygen phase lead where the model predicts

AIChE Journal December 1990 Vol. 36, No. 12 1803

 100
75 I 0=1.8? mHz
1
experimentally; however, the model predictions indicate that it
would be difficult to observe, because the predicted differences
between the time-average conversions for successive cycles are
not large.
The major disagreement between the model and the experi-
mental observations is seen in Figure 6d where the model
predicts (solid curve) that a low-conversion state will exist for all
values of the oxygen phase lead, whereas experimentally a
0 . region of high time-average conversions was found. The model
100 0=4.17 mHz does predict that a small region of high time-average conver-
75
sions is possible (dotted line) when initial conditions of zero are
50
used for the integration of Eqs. 4 to 10; however, even in this
25
0
case there is disagreement between the values of the phase lead
100 0=5.26 mHz over which high time-average conversions occur. This discrep-
75 ancy could be due either to an incorrect choice of the parameter
50 values or to the use of inappropriate assumptions in the model.
25
For example, as shown by Ladas et al. (1989), the effect of
w=8.45 mHz subsurface oxygen on CO oxidation phenomena may be more
important than surface reconstruction effects (particularly for
50 Pd catalysts). Neither many alternative hypotheses [except as
00
25
1~~ 200 400 600 800 lo00 described by Graham and Lynch (1987) for the steady-state
data] nor alternative sets of parameter values were exhaustively
examined to eliminate this discrepancy, because, given the
Time, seconds relative simplicity of this model, and the large amount of
Figure 8. Predicted transient CO conversion for forced steady-state and dynamic data described here, the overall
cycling. agreement between the model and the experimental data is quite
Conditions as in Figure 3 good.

Conclusions
higher CO conversions than were observed experimentally. The Four main conclusions can be drawn from this study of feed
critical value of the phase lead at which the conversion drops concentration cycling.
from 100% to approximately 5% is affected by the initial 1. Rate enhancement occurs during forced feed composition
conditions used when Eqs. 4 to 10 are integrated. The dotted cycling of the platinum-catalyzed CO oxidation reaction.
curves in Figures 6c and 6d were obtained using initial condi- 2. Variation of the phase angle between the oxygen and CO
tions of zero for all of the integration variables, whereas the solid feed streams can cause increases in the time-average conversions
curves in Figure 6 were obtained with initial conditions of X = 1 over and above what occurs for out-of-phase cycling. The phase
and Oco = 0.995 with all other variables set equal to zero. angle between inputs is an important parameter if maximum
Because both cycling states are stable in the regions of overlap conversion is desired.
between the dotted and solid curves in Figure 6, the model 3. The standard Langmuir-Hinshelwood model, when forced
predicts that a small region of multiplicity can exist for the to match the steady-state bifurcation behavior, is incapable of
time-average conversion depending on the chosen initial condi- describing the observed rate enhancement during forced cycling.
tions. Outside of the overlap regions, the two sets of initial 4. A surface-phase transformation model with the CO self-
conditions produce identical time-average results and both are exclusion can describe both the steady-state data and the rate
represented by the solid curves in Figure 6. The possible enhancement for this system. A simplified form of this model
dependence of the time-average conversion on the initial reactor (Lynch et al., 1986) has already been used to describe oscilla-
and surface conditions was not examined experimentally. All of tory behavior during CO oxidation; thus, the surface-phase
the results presented in Figures 3 to 6 were obtained using a transformation model with CO adsorption self-exclusion pro-
catalyst which was initially covered with CO, thus the experimen- vides a single, consistent explanation for three of the commonly
tal conditions were similar to that used to produce the solid observed forms of complex behavior (steady-state multiplicity,
curves in Figure 6. self-sustained oscillations, and rate enhancement during forced
There is another unusual prediction by the model: over certain cycling) exhibited by the CO oxidation on supported platinum
small regions of phase lead, the model predicts that the catalysts.
time-average conversion can oscillate between several values,
i.e., a “steady-state” cycling state is not reached. This was found Acknowledgment
to occur for several preliminary sets of parameter values that The support of this research by the Natural Sciences and Engineering
were examined when attempting to obtain agreement between Research Council of Canada is gratefully acknowledged. W. R. C.
the model predictions and the experimental observations. This is Graham is grateful for the Province of Alberta Graduate Fellowship.
not surprising as it is well known that the forced cycling of a
system, which can display multiplicity and oscillatory behavior, Notation
can result in diverse chaotic, multipeak and entrainment phenom- a, = total surface area of the supported catalyst, 2.34 m 2for 4.95 g
ena (Cordonier et al., 1990). This phenomenon was not observed charge, 6.92 m2for 14.6 g charge, 20.6 m2for 43.6 g charge

1804 December 1990 Vol. 36. No. 12 AIChE Journal

 a, = total surface area of the support, 1,600 m2for 14.6 g charge Barshad, Y., and E. Gulari, “A Dynamic Study of CO Oxidation on
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1806 December 1990 Vol. 36,No. 12 AIChE Journal