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Surface Models in Heterogeneous Catalysis the Synthesis of Ammonia and the Conversion of Carbon Monoxide

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JOURNAL OF CATALYSIS 8, 29-35 (1967) Surface Models in Heterogeneous Catalysis The Synthesis of Ammonia and the Conversion of Carbon Monoxide G. PARRAVANO From the Deparzment qf Chemical and Metallurgical Engineering, University qf Michigan Ann Arbor, Machigan Received November 16, 1966, revised January 16, 1967 The establishment of chemical equilibria between gas and solid catalysts during the synthesis of NH3 and the conversion of CO is considered. Suitable solid-gas equilibrium reactions are discussed. With the correct choice of the rate-controlling step, the equilib- rium surface models are able to reproduce the experimental expressions for the rates of reaction. These models represent a drastic departure from the concept of a fixed, hetero- geneous surface that has been extensively used in the past for the interpretation of the experimental results on the above reactions. The two approaches, however, complement each other and the validity of each one may well be justified under different experimental conditions. Thus, a model may be considered the extension of the other by modification of some of the parameters of the system. Over the past several years many investi- view and it is certainly invalid for relatively gations have been carried out on the catalytic long periods of catalyst operation. It is, synthesis of NH3 and on the conversion of therefore, of interest to consider other CO. The studies have generally been directed possibilities more physically justifiable. toward the understanding of the nature, In this direction it seems particularly properties, and operation of the catalytic important to consider the possibility that surface through the analysis of experimental equilibration reactions between surface and results of reaction rates. gas phase modify the chemical composition While the nature of the preponderant of the surface and, as a result, its chemical surface intermediates is still uncertain, the reactivity and catalytic activity (1). The. physical model of the surface is similar for model of an equilibrium surface must also all the reaction schemes advanced so far. be based upon several assumptions and, in Essentially, the surface is considered as a some instances, may not turn out to be more fixed array of atomic centers with catalytic compelling than its “frozen” counterpart, properties controlled by the physicochemical but it offers a more plausible surface picture, manipulation of the catalyst prior to use. in accord with the increased knowledge of These reaction schemes assume that at surface transport processes and consistent synthesis conditions no modification of with the realization of the ready occur- surface properties occurs. rence of the latter even at moderately low At the temperatures in which both re- temperatures (2). actions have been studied, there is likelihood The basic ideas on the role of surface of ready surface mobility bringing redistri- equilibration reactions in catalytic kinetics bution of matter between various parts of were formulated several years ago (S), but the surface and between the surface and the no attempt has been made to apply these gas phase. The model of aLLfrozen” surface, concepts to the reactions of NH, synthesis although mathematically successful, is prob- and of CO conversion. For the latter reaction, ably unrealistic from a physical point of a recent, study (4) employing a kinetic 29 30 G. PARRAVANO relaxation method has shown the correctness brings us to consider the following reaction of treating the overall reaction as a sequence sequence : of two steps, one of which represents the equilibrium between surface and gas phase. N&g) + Hdg) -+ &K44 (4 In this paper we wish to show that, by N2H2(a) + 2Hz(g) + 2NH3(g) (b) employing a surface model in which equili- bration reactions with the gas phase take To bring out explicitly the possible influences place concurrently with the catalytic reac- of reaction steps (a) and (b) upon the tion, and thereby modify the properties of activity of the surface, the rates of steps (a) the surface, it is possible to obtain a satis- and (b), V~ and 2)b,are expressed in terms of factory agreement with the experimental first order dependence upon the gas partial rate results. pressures. Surface effects upon the rate of reaction are considered by means of a AMMONIA SYNTHESIS function of the concentration of surface A large number of studies on the reaction intermediates, f([N2H2]), or Nz + 3Hz + 2NH3 0) va/A = kf,([N&l)p~,p~, - ~‘J’aWzHzl) (3) have shown that the experimental observa- tions can be satisfactorily represented by a vt,/A = k&([&&])&~ - ~‘bf’b([Ndbl) rate expression x PRHl (4) where I&, k’,, kb, k’b are the rate of reaction dpNHa/dt = ~~N,@&~HI)‘-~ (2) steps (a) and (b) per unit surface area, A, where /3 = 0.5 for a typical iron catalyst gas partial pressure, and [N2H2]. For step (a) under conditions sufficiently removed from at complete equilibration let us set equilibrium (5). The generally accepted interpretation of Eq. (2) considers the [N2H2lequil(a) = &&'N (5) overall reaction occurring as a sequence of elementary steps taking place on a non- where K, is the equilibrium constant for re- uniform catalytic surface. The surface is action step (a). Substituting the equilibrium pictured as an array of sites with reactivity condition (5) into the rate equation (3) and varying according to an ad hoc distribution using the expression of the rate at equi- function, which controls the chemisorption librium va = 0, Eq. (3) becomes of Nz. The result is a Freundlich isotherm, whose pressure dependence is assumed to be valA = k.fd[NzHzl)~PN~PH, the correct value to fit the experimentally - [NJ3lequil(a)lKaj (6) determined dependence of the rate upon the Similarly for reaction step (b), the equi- partial pressure of Nz. This scheme results librium condition gives in a large degree of consistency among various experimental measurements of reac- [NdLIequil(b) = @/Kb) (Pm/P& (7) tion rates, adsorption heats, and isotope where Kb is the equilibrium constant of effects. However, it is not possible to provide an independent physical justification for reaction step (b). Substituting the equi- librium condition (7) into the rate expression the assumed distribution function of the Freundlich isotherm. (4) and using the expression for the rate at equilibrium vb = 0, it is found In the present discussion, we shall retain two of the features common to the majority h/A = h?b([Ndhl) ( &, of the analytical treatments suggested so - (PaHsIKb[N2H2Iequil(b)) ] (3) far, namely that the kinetically determining step is the chemisorption of Nz and that the In a catalytic flow reactor steady state preponderant surface intermediates are imine conditions prevail, and, as a result, the radicals, NH, or closely analogous species concentration of adsorbed species at steady containing nitrogen and hydrogen (6). This state [NzH&, must lie between [N2H2]equii(a) SYNTHESIS OF NH3 AND CONVERSION OF CO 31 and [NzH2]equii(b).Since the majority of the “TB-” H-N=yN-H experimental results indicate that k, < kb, Fe Fe Fe we shall assume the limiting case W&.lss E PLEJrqoiuw (9) (I) (II) The steady state rate of reaction (1) is us8% va. In the rate equation (6) an explicit Alternatively, direct r-bond complexing form of the function f([N2H2]) should be with surface Fe atoms may be considered introduced. Since the rate of formation of (II). There is increasing evidence for the role NzHz [reaction step (a)] is adversely influ- of this type of surface bond in several enced by [NZHS],it is possible to set heterogeneous catalytic reactions (hydro- fd[N&l) = [NdWm~ (10) genation, isomerization, polymerization) (7) , but it is less likely for bonding Nz, since the where m, is a constant. If reaction step (a) corresponding ?r orbitals are energetically is the slowest of the sequence, its reverse more stable. The atomic structure of the reaction may be neglected, and from Eq. (6), catalytic surface is unknown, but in the (7), (9), and (lo), one gets presence of impurities of various kinds (catalyst, gas phase) at the temperatures employed for the synthesis, it is likely that = k,Kb”aPN,(p~,/paHs)m’ (11) a relatively thick surface layer compound (or alloy) may be formed with structure and For m, = 0.5, the rate expressions (11) and (2) are similar. properties closer to those of a metallic salt To carry further the analysis of the (nitride, hydride, nitrohydride) than to reactive surface, let us focus the discussion those of metallic Fe. Strong support for this upon step (a) and suggest some possibilities view was obtained by measuring the adsorp- for its occurrence. Equation (11) is rewritten tion of Nz during the decomposition of NH,. as follows : On W it was found that the amount adsorbed was several times as much as that required to completely saturate the surface (8). The thickness of the nitride layer is controlled by the relative rates of nitride where koxpti = k~b0.6(pNH~/pN2)-’ is the rate formation and decomposition, and the coefficient obtained directly from the experi- temperatures prevailing during synthesis mental results and k, is the true rate con- conditions would favor relatively thick stant, independent of the partial pressure of surface nitride layers. It is also pertinent to the gaseouscomponents. recall the well-known work on the effect of The molecular structure of the dimer promoters, showing that nonreducible oxides radical is unknown, but likely it involves tend to concentrate on the surface of the Q and ?r bonds, by sp hybridization of the crystallites (9). corresponding atomic orbitals of Nz, giving If this picture of the surface is accepted, a flat, linear species. The -N=N- bond the equilibrium of formation of the surface distance is = 1.20 A” [1.24 A0 in Nz(CH,)z, compound by means of reaction step (a) 1.13 A” in diazomethane].
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