Reaction between nitric oxide and ammonia on polycrystalline Pt: A mathematical model of rate oscillations

Chemical Engineering Science. Vol. Printed in Great Britain.

40. No. 9. pp. I75 t -I 757,

1985.

ooo9-2509/85 S3.00+ 0.00 Pergamon Press Ltd.

REACTION BETWEEN NITRIC OXIDE AND AMMONIA ON POLYCRYSTALLINE Pt: A MATHEMATICAL MODEL OF RATE OSCILLATIONS PETER

NOWOBILSKI

and CHRISTOS

G. TAKOUDIS

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, U.S.A. (Received

29 November

1983)

Abstract-The reduction of nitric oxide by ammonia over polycrystaliine platinum has been seen to exhibit oscillatory reaction rates at reactor pressures between 0.2 and 1.0 torr, catalyst temperatures between 360 and 55O”C, and feed gas compositions of NO between 60 and 80 y0 v/v. In this paper we present a model of this reaction system with realistic reaction and reactor parameters, at the conditions mentioned above. It is shown that a precursor state in the adsorption of nitric oxide and slow adsorption and desorptionof nitrogen (the product) yield a mathematicalmodel that can predict several featuresof the oscillatory behaviour of the

reductionof nitricoxide by ammonia reported previously.Our mathematicalmodel of this systemassumes no transport limitations,clean catalyst surface, morphologically unchanged platinum surface and constant

catalyst temperature. The model we propose for this reaction system can also predict the steady state nitrogen and nitrous oxide formation rate expressions reported in the literature.

INTRODUCTION Temporal oscillations in the rate of several heterogeneously catalyzed reactions have been reported in the literature; the oxidations of hydrogen [l-5] and of carbon monoxide C&-8] have been the most extensively studied. Numerous theoretical models of such systems have been developed [9]. Reviews of this literature have been presented by several investigators [6,10,11]. Most experimental studies so far were performed with ill-defined surfaces, at fairly high pressures (2 10 torr), with mass and/or heat transport limitations, with almost no direct additional information supporting the origin(s) of the oscillations. In all studies, no in situ information on surface concentrations of adsorbed reactants and reaction intermediates has been available. Among heterogeneous catalytic systems exhibiting oscillatory states on fairly well-defined cataIysts (e.g. see [2,12-141) is the reduction of nitric oxide by ammonia on platinum [13]_ This reaction system exhibits oscillations at temperatures between 360 and 55O”C, pressures between 0.2 and 1.0 torr and feed gas concentrations PNOJPNHJ,, of 1.54.0 [13]. In this paper, we attempt to model the reaction between nitric oxide and ammonia on platinum with realistic reaction and reactor parameters, at the conditions mentioned previously. BACKGROUND The experimental apparatus has been described in detail elsewhere [15, 161. The platinum wires used had surface areas of 0.6-1.3 cm2. The reactor had a volume of 380 cm3, and at the conditions mentioned previously, it behaved as a well mixed reactor. Further experimental details are given in [15, 161. 7 Author to whom correspondenceshould be addressed. CES 4o:L)-I

1751

A succession of mechanisms has been developed, describing the heterogeneous reaction between nitric oxide and ammonia on platinum. The formation of surface complexes on the catalyst surface was postulated by Otto, Shelef and Kummer [17, 181 as the major intermediate in the reaction toward products. 15N isotope labeled experiments were conducted over supported platinum at relatively high pressures (420450 torr) and moderate temperatures (200-250°C). Their conclusions are based on the product distributions they observed. A simpler reaction mechanism was postulated by Gland and Korchak [19] with data from Auger electron spectroscopy, low energy electron diffraction and mass spectrometry. Their scheme involved simple Langmuir-Hinshelwood kinetics. Their experimental system was operated at very low pressures (10 - ’ torr) and over a range of temperatures (150-500°C) on a stepped Pt single crystal. No N,O production was observed. Their conclusions are based on fitting possible reaction schemes to empirically derived reaction rate parameters. Takoudis and Schmidt [16] performed similar experiments on polycrystalline platinum wire at moderate pressures (0.2-1.0 torr) and over a wide range of temperatures (20&1ooO”C). Under such conditions both N, and N,O were observed as products. Their mechanism (Table 1) was postulated on the basis of correlating experimentally and theoretically derived rate parameters. Takoudis and Schmidt showed further that the data of Otto et al. and Gland and Korchak were consistent with the mechanism outlined in Table 1. The sustained rate oscillations observed during the reaction between ammonia and nitric oxide were strongly dependent on the surface temperature, feed gas composition and total pressure. Oscillatory behaviour was stable over long periods of time.

PETERN~WOBILSKIand CHRISTOS G. TAKOUDIS

1752

Table 1. Mechanism of Takoudis and Schmidt [15, 161 NH13+2S NO+S NH,-S + NO-S

* = -+

H-S+NO-S ZHNO-S HNO-S + H-S 2N-S

--t + + +

NH,-S + H-S NO-S N,+H,O+2S HNO-S + S N,O+H,O+2S N-S+H,O+S N,+2S

(I) (11) @Ii) (IV) ivi WI) WI)

Oscillations arose isothermally (typically within f 0.5 K) under conditions where transport limitations were absent and the catalyst surface was fairly welldefined, reproducible and remained unchanged morphologically (Fig. 1) [15]. Under these experimental conditions, no in situ information about the chemical state of the surface was obtained. Ex situ Auger micrographs taken before reaction and at different times after reaction indicated that such surface changes were not expected [15]. Also, in this system there was no homogeneous reaction. The aim of this paper is to show that slight modifications of the mechanism outlined in Table 1 may lead to a system that can predict oscillatory behaviour in the reduction of nitric oxide by ammonia on platinum. The prediction of kinetic oscillations would also further attest to the validity of the mechanism. SUMMARY OF EXPERIMENTAL OBSERVATIONS In two previous publications [ 13, 161, several results of the steady state behaviour of the reduction of nitric oxide by ammonia have been discussed. Here, we summarize results on which no elaboration or justification has been reported before. These experimental observations are mainly pertinent to the oscillatory

PNO= 04

Torr

P,+=

Torr

T

=

0.1 432°C

behaviour of the reaction system mentioned above [ 13, 15, 163:

(0 At constant feed gas concentrations

(ii)

(iii)

(iv)

(v)

(vi) (vii)

(viii)

(ix) (x)

P,,,8/P,,3,,,

the regime of temperatures in which oscillations are observed is moved towards higher temperatures as the reactor pressure increases. At constant total and partial pressures of the reacting species, as the catalyst temperature increases, the period of the oscillations can go through an extremum. The conversion of the limiting reactant during the course of these oscillations may be less than 5-8 %. The N, and N,O formation rates and NO consumption rate are shown to exhibit “breaks” in the temperature range of 40047O”C, at total pressures 0.5 and 1.0 torr, and N,oJP,,, between 2 and 5. These “breaks” are reproducible at all times. Oscillatory behaviour is observed in or about the temperature range that the rate “breaks” mentioned in (iv) are observed. The selectivity to nitrogen increases as the catalyst temperature increases. At constant catalyst temperature and reactor pressure, isothermal rate oscillations are observed in a certain range of feed gas compositions of NO. At constant catalyst temperature and at constant ratio PNO.IIPNH,,ithe period of the oscillations decreases as the reactor pressure increases. Also, oscillatory rates are observed in a certain range of total reactor pressures. Isothermal rate oscillations observed are always single- or double-peaked. The addition of small amounts of nitrogen in the feed stream (_ 10 y0 v/v) influences the existence, the period and the amplitude of the oscillations observed.

DEVELOPMENT OF THE MATHEMATICAL MODEL A simple Langrnuir-Hinshelwood model quantitatively explains the steady state behaviour [15], but it fails to predict the observed oscillatory phenomena. Evidence provided in [16, 20, 211 suggests that at pressures between lo- 3 and 1 torr, nitric oxide adsorbs on platinum in accordance with a precursor mechanism. Therefore, Step II of the mechanism of Table 1 would be broken up into two steps (IIa and IIb). NO + S* z+ NO-S* NO-S*

t

lmin TIME

Fig. 1. N, and N,O formation rate, NO consumption rate and electric current oscillations in isothermal reaction at 432°C for PNo,,/PNH,,, = 4.

+ S G= NO-S

(IIa) + S*.

(IIb)

It has also been noted [19] that when the reaction rate increases rapidly with catalyst temperature, the nitrogen-containing surface species present during reaction is large. This may be caused by the presence of a nonreactive surface species. In fact, the surface species may be adsorbed nitrogen (the product). Inclusion of this observation requires the elementary step VII of Table 1 to be reversible.

17.53

Reaction between nitric oxide and ammonia on polycrystalline Pt Further experimental evidence [ 15,193 suggests that the NH, and H surface concentrations should be similar because they are determined by the gas-phase ammonia pressure; NO-S and N-S appear to be the dominant adsorbed species; and the mass balance of the precursor state NO-S* is at pseudo-steady state. Hence, under the conditions of the experiments reported in [ 13,151, the mass balances of the adsorbed species NO--S and N-S are:

dt

;(P,,,i-P,,)+; +k, K;“P;&e,,es)

(k,e:--k-,

PN2%

k,, P,o

+ k - za + ‘G’s

+ k_

and

(6)

a,%o

+ k,)K:‘2P~~,e,,e,

-(k,

(1)

and

de,& dt

6

(3He

HNO

+

2k,

P&

- 2k,e:,

(2)

pseudo-steady where eNHI= eH = (K, PNH,)ltZe,, state is assumed for the mass balance of oHNO;and 8, assume very small values so that 8,, + ON and 8 HNO e ,““et, ‘v 1. We start with this two equation model [eqs (1) and (2)] in order to keep our modeling efforts as simple as possible without losing any qualitative features of our reaction system. It is easy to show that the model described by eqs (1) and (2) can exhibit oscillatory states for wide ranges of the parameters ki, Pi and K,. In this model, perhaps the only questionable assumptions are those concerned with the kinetics of the various steps. But there is a great deal of knowledge on the kinetics of this reaction system [15, 17, 191. The existence of the adsorbed species HNO-S has been questioned by some investigators, particularly in studies on the reaction NO + Hz on Pt [22]. However, the adsorbed species HNO-S is not critical to the issue of oscillation in this case. Equations (1) and (2) can be modified in accordance to intermediates proposed by Otto et al. [ 17, 181 or Gland and Korchak [ 191 with no significant effect on the model [23]. Although in many of the experiments reported in [15] the conversion was low, if the reactor mass balances of the gas-phase species are not accounted for perhaps the only comparison between experimental observations and model predictions should be in terms of the periods of the oscillations. However, the incorporation of gas-phase mass balances does significantly affect the periods and amplitudes of oscillations obtained through model equations on mass balances of adsorbed species [23]. Furthermore, there exists an interesting selectivity problem in this system (N, vs N20) and extensive data exist in terms of the gas-phase concentrations. Hence, it was decided to include the reactor mass balances of nitric oxide, nitrogen, nitrous oxide and ammonia:

k2ak2bPNoes-k_-k_2beN0 x

khP,,+k_2,+k2be,+k_2beNo

(4)

dP,zo dt

kkk2bPNoes-k-2,k-zbeNo

dB,,= dc

dP,I =

(3)

After all, it should be emphasized that the only concentrations monitored in situ in this reaction system were the gas-phase ones! Notice that the addition of eqs (3)-(6) to eqs (1) and (2) introduces two new parameters, Q/V and A/V, whose values are known from the experiments reported in the literature [ 13, 15, 16, 241. Also, the mass balance of water is not considered since it does not affect the oscillations predicted by eqs (l)-(6) [23] and the partial pressure of water has not been monitored continuously [15]. Furthermore, P,*., has been used in eq. (4) simply because there exist data on dilution experiments with nitrogen. The values of the reaction rate constants ki were chosen to be consistent with the available kinetic data and other experimental evidence related to this reaction system [is, 16,241. Thus, these rate constants are assumed to be functions of the catalyst temperature as indicated in Table 2. Note that the adsorptions of nitric oxide and nitrogen to the platinum surface are nonactivated processes. It is emphasized that no systematic attempt was made to optimize the values of the unknown parameters in order to improve the agreement between the observed amplitudes and frequencies, and the predicted ones. RESULTS

The model consisting of eqs (l)-(6) may predict single-peaked oscillatory states over fairly wide range of its parameters. Double-peaked rate oscillations are predicted over narrow ranges though. Figure 2(a) illustrates simulated double-peaked oscillations which are similar to those observed experimentally at a higher temperature (Fig. 1). In Fig. 2(a) all oscillating bulk phase and surface concentrations are in phase as time

Table 2. Catalyst temperature dependence of the rate constants ki K”2 1 kza kbz,, t_zt.,

= = = = =

k: k, k, k, k-7

= = = = =

=

3.7 x 10m4 exp (3000/T) 30 6.5 x 10;exp (-8000/r) 3.7 x lOa exp (-7500/T) 1.8 x 10 exp(-7500/T) 1.35 x lo9 exp (-9000/T) 1.7 x lO’exp(-7500/T) 4.5 x lo9 exp (-9000/T) 1.7~ 10Sexp(--10000/T) 5.5 x 10’ exp ( -6000/T) 1.35

1754

PETER

NOWOBILSKI~~~

CHRISTOSG. TAKOUDIS

P NO 0.36

P N2

i_

I3NO

Time

(minutes)

Fig. 2. Simulation gas phase oscillations of NO, N, and N,O and surface coverage oscillations of NO-S and N-S us time for the model eqs (1 j(6). P,.,n,,, = 0.1 torr, PNO,i= 0.4 torr, T = 402°C Aj V = 0.605, and Q/ V = 0.86 (a); Q/V = 0.5 (b).

goes on. However, Fig. 1 indicates that the gas-phase concentration of N,O is out of phase. If the mass balance of the adsorbed species HNO-S is accounted for, then that difference in time-phase can be made up [23], although no further effort is pursued here on a model with one more equation. Note that the time average conversion of nitric oxide is about 5 o/0 [Fig. 2(a)]. Figure 2(b) shows single-peaked rate oscillations of the model eqs (l)-(6). The time average conversion

of nitric oxide is now about 7 o/0 and the average selectivity of nitrogen/nitrous oxide is about two, in agreement with experimental observations at 402°C [15, 161. It should be emphasized that starting from different initial conditions, we may have transients of a duration of as much as 5 min, in agreement with observations reported in the literature [13, IS]. In Fig. 3(a), the effect of the total reactor pressure on the simulated (and observed) oscillations is shown at

Pressure

(torr)

30 (b)

20 _.

.. 10-f 0 50

0; 0

*.

. .

I

0.4

Q/V

0.8

(sex-‘)

':, '.

-._ -... -.... '...

.f.

_.~ _. *~.-:

ii,

,

-. .

-I (C

_._ _/ ,_' __I' . ./I' :*

'. , -m----I~----m-~t--~----\

I

1.2

0

0.04

0.02

A/V

(tori-1

Fig. 3. Period and amplitude of isothermal oscillations (a) us reactor pressure for PNOJPNH,,i = 4, Q/V = 0.5 and A/V = 0.005;(b) us Q/V for A/ V = 0.605, PNO.,= 0.4 torr and PNH,,,= 0.1 torr; and (c) vs A,JV for = 0.1torr. ---- Model predictions at T = 402°C; --model predictions Q/V = 0.5, PNO,i= 0.4 torr and P NH,,i at 427°C; l data at 402°C; and l data at 427°C.

Reaction betweennitric oxide and ammonia on polycrystalline Pt 402 and 427°C. No rate oscillations are predicted at a reactor pressure of 1 torr and 402°C and at 0.2 torr and 427”C, as observed. The period and amplitude decrease as the pressure increases, in agreement with experimental observations 113, 151. Everything else being constant, oscillatory states are predicted for a certain range of reactor pressures at each temperature, as observed. Furthermore, Fig. 3(a) indicates that as the catalyst temperature increases, the regime of reactor pressures over which rate oscillations are predicted moves towards higher pressures, in agreement with experimental results reported in [13, 15, 161. Figure 3(b) illustrates the dependence of the amplitude and period of the observed and simulated oscillations on the total volumetric flow rate Q, in the form of Q/V where V is the reactor volume, at a reactor pressure of 0.5 torr. As the volumetric flow rate increases, the period first decreases and then increases while the amplitude decreases, in agreement with experimental data [13, 15, 16, 241. In Fig. 3(c) the effect of the catalyst surface area, in the form of A/V which is proportional to the catalytic area, on simulated and observed rate oscillations is illustrated at 402°C. Data indicate that oscillatory states are observed for 0.003 < A/V c 0.007 [13, 151, in agreement with predictions of the mode1 eqs (l)-(6). Higher or lower values of A/ Vwere not tested experimentally because of other limitations [ 1 S]. In Fig. 4(a), the effect of the feed gas composition of nitric oxide on the simulated oscillatory states is shown at 402 and 427°C at a reactor pressure 0.5 torr. These results are then compared to experimental data reported in the literature [Fig. 4(b)] [13, 15, 161. Everything else being the same, oscillations are predicted for a certain range of feed gas compositions of nitric oxide, as observed [13, 151. As the feed composition of NO increases, the period increases while the amplitude does not change much in the simulations shown in Fig. 4(a). This agrees with observations in terms of the period [Fig. 4(b)]. But experimental results indicate that the amplitude also increases as the feed composition of NO increases. Note that the amplitude is defined as [ (PN,,,_. - P,,,m,,)/P,,,,] x 100. Also, Fig. 4(a) indicates the influence of catalyst temperature on the effect of nitric oxide feed composition on simulated rate oscillations. It is seen that as the catalyst temperature increases, the regime of nitric oxide feed compositions in which oscillatory states are predicted moves to higher values. Such detailed experimental data were not available for comparison [Fig. 4(b)]. Figure 4(c) illustrates the dependence of simulated rate oscillations on nitric oxide feed compositions at reactor pressures of 0.5 and 0.25 torr. It is seen that the lower the reactor pressure is the smaller the region of NO feed compositions where rate oscillations are predicted becomes, in agreement with experimental data in [ 13, IS]. Furthermore, it is indicated that as the total pressure decreases, the period of predicted rate oscillations increases and so does the amplitude. These agree with data reported in [13, 15-J. Notice that there are no data points in Fig.

I755

15

,___..___ ..~.A..~. . . . . . i :

(a)

Amplitude

(%)

i:

10. 5. /

0 Period

I

; j I j I

!;

500.

,,;: :: ,/ .‘,!

(WC? 0

Amplitude (%)

1 % NO in Feed

Fig. 4. Isothermal period and amplitude vs % v/v of NO in the feed, Q/V = 0.5 and A/V = 0.005. (a) PT = 0.5 torr, ----

model predictionsat 402°C; -- model predictionsat 427°C; (b) PT 2 0.5 torr. 0 data at T = 402”C, i data at T = 427°C; and (c)T = 402°C ---- model predictionsat PT = 0.5 torr, -model predictionsat 0.25 torr.

4(c) for comparison, because the reactor pressure 0.20 torr at which data are available is just outside the region of simulated oscillations [Fig. 3(a)]. Figure 5 shows the effect of the catalyst temperature on the amplitude, period and average conversion

(\ e.g.

2(pN2,i3vg. + pN~O,avg. -

pN,,i)

x100 \ at 0.5 torr. .-. r N0.i + rNH3,i / The predictions of our model indicate that there is a “break” in the average conversion versus temperature plot, and the region of oscillations is in or about that break. Furthermore, the period of predicted oscillatory states may go through an extremum, in agreement with experimental data presented in Fig. 5 and in [ 13, 15, 161. Predictions of our mode1 indicate that the periods of the oscillations increase as the total pressure decreases, and the region of temperatures over which oscillatory states are observed moves to higher values as the reactor pressure increases [23] in agreement with, experimental observations [13, 15). Also, addition of 10 y0 v/v nitrogen in the feed stream affects the period and amplitude of the oscillations observed (Fig. 5) as well as the average conversion of reactants at reactor pressures of 0.5 torr (Fig. 5), in agreement with the predictions of the model consisting of eqs (l)-(6). Summarizing our results, we see that the model

PETER

NOWOBILSKI and CHRISTOS

G. TAKOUDIS

reaction with NO-S. Thus, the reaction rate is high and it produces high concentrations of nitrogen. This leads to nitrogen adsorption. At the same time, low coverages of NO-S lead to increasing chemisorption of nitric oxide. Thus, the availability of empty sites to NH, adsorption goes down and the reaction rate is low. Low reaction rates produce small amounts of N2, so that N-S starts desorbing. Desorption of N-S leads to slowly higher chemisorption rates of NO through the precursor state and rapidly higher reaction rates because of higher availability of empty sites to NH, and higher coverages of NO-S. High reaction rates lead eventually to low eNO, while desorption of N-S leads to low &.,. Now, the cycle starts all over again. Summarizing these steps we have: desorption of adsorbed nitrogen, increasing reaction rates 500

600 Temperature’

700

800

(K)

Fig. 5. Period, average conversion and amplitude of isothermal oscilfatory states us catalysttemperature. Q/V = 0.5 and A/ Y = 0.005. --model predictions with &,o., = 0.4 torr and PNH,,i = 0.1 torr; --- model predictions with PNo,! = 0.36 torr, PNH,,, = 0.09 torr and PN,,, = 0.05 torr; l data with PNO,~= 0.4 torr and PNH,.,= 0.1 torr; o data with PNO.# = 0.36 torr, PNH,,i = 0.09 torr and PNI,,= 0.05 torr.

consisting of eqs (l)-(6) predicts rate oscillations over a wide range of system parameters. This model does predict several features of the oscillations observed during the reduction of nitric oxide by ammonia over polycrystalline Pt wires. It was verified that for given feed compositions, space velocity and catalyst temperature, the predicted limit cycle frequency and amplitude do not depend on the initial conditions of the numerical integration. DISCUSSION

Numerous publications have dealt with oscillatory states in heterogeneous catalytic systems. But the main feature of such studies has been a disagreement between experimental data and theoretical predictions, whenever a quantitative model has been attempted. In fact, very few quantitative mathematical models have been reported so far [25-271, and some of these appear to suffer from oversimplifications which may significantly affect reported results 125271. The mathematical model proposed here for the rate oscillations during the reduction of nitric oxide by ammonia on platinum deals with one of few fairly well characterized experiments on kinetic instabilities in heterogeneous catalytic systems [ 13,151. It also shows satisfactory agreement between experimental observations and theoretical predictions. When the feed concentration of nitric oxide is above a critical value, the surface coverages of NO-S and N-S may become low so that many empty sites are available for fast ammonia adsorption and consequent

T low reaction rates, high coverages of NO-S and N-S, low availability of empty sites

high reaction rates, low coverages of NO-S and N-S, high availability of empty sites /

.

high chemisorption rate of NO and adsorption of N,

Figure 2 confirms this interpretation. Eigenberger [28, 291 has shown that reversible adsorption of an inhibitor can produce oscillations over certain critical range of conditions. Such an inhibitor can be a reactive or inert component of the reaction mixture or a chemisorbed species thereof as long as this species is not, or only slightly, involved in the surface reaction [28]. Thus, the mechanism of the oscillations during the reduction of NO by NH3 proposed previously may be considered as a “real” case of Eigenberger’s theoretical studies [28,29], where the “inhibitor” in this case is N,, one of the products. Observed and simulated oscillations obtained during the dilution experiments with nitrogen in the feed stream corroborate such a proposition [23]. Another aspect in the mathematical modelling of this reaction system is the existence of several different types of surface sites (a minimum of two) with different heats of adsorption for the reactive surface species [19]. Although the evidence available does not favour this case [19], this possibility certainly cannot be ruled out conclusively. At ultra-high vacuum conditions, Pt(ll0) and Pt(lOO)may reconstruct while Pt(ll1) may not [30]. In the reaction system under consideration, a polycrystalline Pt surface was always used. Thus, certain (reversible) phase transition(s) might probably take place under reacting conditions [14]. They were neglected in the models described in this paper. This study relies exclusively on gas-phase concentration measurements, since no in situ information of the adsorbed reactants and reaction intermediates has been available. However, despite the approximations just discussed, the dynamic model consisting of eqs

Reaction

between

nitric oxide and ammonia on polycrystalline Pt

(l)-(6) predicts satisfactorily many features of the rate oscillations observed during the reaction between NO and NH, on Pt at pressures between 0.2 and 1.0 torr.

SUMMARY

A simple mathematical model is discussed as a first attempt to explain the experimentally observed oscillations in the rate of reaction between NO and NH, on polycrystalline platinum in a CSTR at low pressures. The model assumes that the periodic phenomena originate from the adsorption and desorption of nitrogen (the product). The numerical simulations are generally in agreement with several experimental results.

REFERENCES

Zuniga J. E. and Luss D., J. Catal. 1978 53 312-320. Kurtanjek Z.. SheintuchM. and Luss D.. J. Catal. 1980 66 11-27. Sault A. G. and Masel R. I., J. Catal. 1982 73 294-308. Vohs J. M. and Masel R. I., Effectsof feed temperature and compositionon oscillationsduring H, oxidation on Ni powder covered Ni foils. Chem. Engng Comm. preprint. c51 Seebauer E. G., Vohs J. M. and Masel R. I., IEC Fundam. 1984 23 19. l-61Wicke E., Kummann P., Keil W. and SchieflerJ., Ber. Bunsenges.Phys. Chem. 1980 84 315323. Sheintuch M., A.I.Ch.E. J. 1981 27 20-25.

[ij

c91 Cl01

Acknowledgement-This study was partially supported by DuPont, the Atlantic Richfield Foundation, and Purdue University Computing Center.

Cl11 Cl23 t133

NOTATION A

I-S NO-S* ki

k_i pi

t T

V Greek @i

constant, proportional to the catalyst surface area, torr cm’ adsorbed species I on the catalyst surface precursor state in the adsorption of nitric oxide on platinum forward rate coefficient for the step i, s- ’ (except k,, kza: torr-’ s-l) backward rate coefficient for the step i, s-l (except k-,: torr-’ s-l) partial gas-phase pressure of the species i, torr inlet partial pressure of the species 1, torr time average outlet partial pressure of the species 1, torr maximum value of P,, during one period of an oscillatory state, torr minimum value of P,, during one period of an oscillatory state, torr reactor pressure, torr volumetric flow rate, cm3/s empty active site on the catalyst surface empty site available to the precursor state in the adsorption of nitric oxide on platinum time, s temperature, K or “C reactor volume, cm’ symbol surface coverage of the species i

1757

El43 Cl51

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Vayenas C. G., Georgakis C., Michaels J. and Tormo J.,

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