J. Phys. Chem. 1985,89, 3243-3251 blocked. In pure solvents such as the linear alcohols and their deuterated analogues, rrotfor oxazine showed a linear dependence on viscosity, assuming a prolate shape for stick boundary conditions.z2 In this case, no anomalous nonhydrodynamic behavior was observed in over most of the mole fraction range for alcohol/water mixtures. However, deviations from hydrodynamjc behavior were seen in the range 0 < xROH I0.1. While the shortening of the excited-state lifetime observed for 0 < xROH 0.01 could give rise to artifactual shortening of T~~~in this smaller region, the arguments that the deviations were actually due to changes in liquid structure in the region up to xROH = 0.1 were quite strong. These changes would correspond to large disruptions of the alcohol structure in the presence of small amounts of water. The specific cresyl violet/alcohol interactions are believed to increase the effective volume of the molecule, leading to slower rotation than predicted by v, while in oxazine the amino interaction site is blocked leading to effectively a faster rotation than expected for stick boundary conditions.2s2z A microscopic but nonquantitative model has been put forward and called the solvent torque It is proposed that the slip boundary condition should be applied generally to describe rotational motion, and that deviations from slip are due to specific torques exerted on the molecule by the hydrogen-bonded solvent. In the hydrogen-bonding solvents, it is the interaction between the bound molecules of the first shell and the next solvation layer which determines rrot,and hence some correlation with rd might be expected. This model predicts that in aprotic solvents, such as D M F and Me2S0, positively charged ions should have longer rrotvalues compared to anions with equal steric hindrance, while in the alcohols the reorientation times of the anion and cation should be similar, and comparable in value to those of the cation in the aprotic solvents. This arises because of the differences in the polarizability of the solute cation and anion, and accessibility
(22) L. A. Philips, S.P. Webb, S. W. Yeh, and J. H.Clark, J . Phys. Chem., 89, 17 (1985). (23) K. G. Spears and L. E. Cramer, Chem. Phys., 30, 1 (1978). (24) Resorufin is a prolate spheroid for hydrodynamic calculations since it has one 16ng axis.
3243
of polarizable negative centers of the solvent. We are presently testing these. predictions using the probe molecule thionin, which, while structurally very similar to resorufin, possesses a positively charged nitrogen center instead of the negatively charged oxygen. Preliminary data indicate that in fact the rotational reorientation in alcohols is comparatively insensitive to the charge on the probe. In summary, the structural features experienced by a dye molecule in a pure liquid or in a binary system must depend on both the magnitude and duration of the site-specific solutesolvent interactions in comparison to the bulk solventsolvent interactions. The correlation times of such interactions must now be included to make a more quantitative model and we are currently developing this approach. In resorufin,a rigid probe molecule with an anionic 0- site, the water:propanol rmtindicates that the local solvation structure undergoes significant change as water is added to propano!. The peak in excess viscqsity in Figure 7 at x~~~= 0.7 is a reflection of the nonideality of the system and corresponds to the extremum on the curve in Figure 5. A comparison of recent studies of the viscosity dependence of the reorientational times in alcohols observed by varying either the alcohol (i.e., the structure), the temperature? or the pressurezzconfirms the general conclusion that the'same macroscopic viscosity (prepared in three ~ differences ~ . different ways) does not lead to the same T ~ The in the rotation times observed reflect the changing microscopic pafameters, which include boundary conditions for angular momentum transfer, as is particularly apparent for resorufin as an anionic probe of binary polai fluids. Further work is in progress in order to determine the effect of changes in charge and structure of the probe molecule on the microscopic interactions involved in rotational relaxation.
Acknowledgment. We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Office of Naval Research (U.S.A.). E.F.G.T. acknowledges the receipt of an NSERC Canada postdoctoral fellowship. Registry No. MeOH, 67-56-1; EtOH, 64-17-5; 1-PrOH, 71-23-8; 2-PrOH, 67-63-0; 1-BuOH, 71-36-3; 2-Me-l-PrOH, 78-83-1;CH3CN, 75-05-8; DMF, 68-12-2; Me2S0, 67-68-5; HCONH2, 75-12-7; H20, 7732-18-5;resorufin sodium salt, 34994-50-8.
Mechanistic Details of the Heterogeneous Decomposition of Ammonia on Platinum J. J. Vajo, W. Tsai, and W. H.Weinberg* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91 125 (Received: December 26, 1984)
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Absolute reaction rates have been measured for the catalytic decompositionof NH3 and ND, and for the NH, D2exchange and 0.5 torr and temperatures between 400 and reaction over a polycrystalline platinum wire at pressures between 5 X 1200 K in a continuous flow microreactor. At relatively low pressures and/or high temperatures, a primary isotope effect was observed for the decomposition of ND3, indicating that a surface. reaction involving N-H bond cleavage is the rate-limiting step. Under these conditions, the order of the decomposition reaction is unity with respect to apmonia pressure with an apparent activation energy of 4.2 kcal/mol. As coverages increase, corresponding to relatively high pressures and/or low temperatures, the order of the decompositionreaction is zero with respect to ammonia, and the reaction rate becomes controlled by nitrogen desorption. In this case the apparent activation energy of the decomposition reaction is 22 kcal/mol. The kinetics of the NH, + D2exchange reaction have been used, together with data concerning the adsorption-desorption parameters of NH,, H2, and N2as well as the reaction intermediates NH and NH2, to develop a mechanistic model which describes the reaction rate over a wide range of experimental conditions and which includes the energetics of each intermediate step in the decomposition reaction. This model is discussed in terms of a potential energy diagram for ammonia decomposition on platinum.
1. Introduction
Interest in the catalytic decomposition of ammonia on transition-metal surfaces has been motivated by its relative simplicity as a heterogeneous reaction and by its relationship, via microscopic reversibility, to the synthesis of ammonia from nitrogen and hy0022-3654/85/2089-3243$01.50/0
drogen. Although the decomposition reaction on platinum surfaces has been examined e~tensively,'-~ a detailed mechanism which D. G.; Schmidt, L. D. J . Cazal. 1976, 41, 440. (2) Loffler, D. G.; Schmidt, L. D. Surf. Sci. 1976, 59, 195. (1) Loffler,
0 1985 American Chemical Society
3244 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 describes the reaction rate over a wide range of experimental conditions and which includes the energetics of individual reaction steps remains unformulated. LBffler and Schmidt have studied the decomposition kinetics on a platinum wire in a steady-state flow reactor with the ammonia pressure varying from 0.015 to 10.5 t0rr.I In the temperature range between 600 and 1700 K, they were able to fit their data accurately with a single Langmuir-Hinshelwocd rate expression. However, the LangmuirHinshelwcd rate expression provided no microscopic information concerning the individual, elementary reaction steps, and the lack of kinetic parameters for the adsorption and desorption of NH,, H,, and N, precluded a critical evaluation of particular kinetic models. Recently, additional information has become available concerning the adsorption and desorption kinetics of NH,, H,, N,, and probable reaction intermediates (NH and NH,) on platinum surfaces. Thermal desorption mass spectrometry (TDS)'.8 and electron energy loss spectroscopy9of ammonia chemisorbed on Pt(l1 I ) indicate the existence of two molecular states with desorption energies of I(tl2 and 18-20 kcal/mol, respectively. The interaction of hydrogen with Pt(s)-9(11 l)X(lI I), i.e., the Pt(997) surface, has been elegantly studied hy using helium beam scattering.l"." By monitoring the reduction in coherent helium scattering in the presence of disordered hydrogen adatoms, isosteric heats of adsorption of 22 and 19 kcal/mol were obtained for the step edges and the terraces, respectively. Since the activation energy for adsorption of hydrogen on platinum is known to be small,"," at most 1 kcal/mol, the measured isosteric heats of adsorption are essentially equal to the activation energies of desorption. Moreover, "normal* values of the precxponential factor for a second-order desorption reaction of lW3-10-2 cm2/s were obtained." The desorption of atomically adsorbed nitrogen as N2 from a polycrystalline platinum ribbon has been investigated by using TDS." Nitrogen desorption occurred with second-order kinetics, an activation energy of desorption of 19 kcal/mol, and a precxponential factor of the desorption rate coefficient of 4 X 10-8cm2/s. Both of these rate parameters were found not to vary with the fractional surface coverage of nit10gen.l~ The activation energy for dissociative adsorption of nitrogen on iron surfaces has been estimated to be 520 kcal/mol depending on surface orientation and nitrogen merage." Although the adsorption kinetics of N, on platinum surfaces have not been investigated directly, threshold ionization measurements have established the desorption of N2* from a platinum ribbon with 20 kcal/mol of vibrational excitation during ammonia decomposition at a pressure of 0.1-1.4 torr of ammonia and a temperature of 773-1373 K.IS Laserinduced fluorescence has h e n used to determine an apparent activation energy of 63-69 kcal/mol for the desorption of N H radicals from a platinum wire in 0.1 torr of ammonia at 12W14M) K.I6J7 The desorption of NH, radicals was not observed under similar conditions. In the present work, we have measured absolute reaction rates for the catalytic decomposition of NH, and ND,, and for the NH, (3) Gland, J. L.; Kollin, E. B. Sur/. Sei. 1981, 104,478. (4) Guthrie. W.; Sokol, I.; Samorjai, 0. A. Sur/. Sei. 1981. 109, 390. ( 5 ) Robertson. A. I. B.; Willhoft, E. M. A. Trans. Foradoy Soe.1%7,63, "7
