J. Phys. Chem. 1992,96, 5668-5669
5668
2p/A1 2p intensity ratios, I believe that the interpretation put forward by HPHH for their quantitative results of low-Fe catalysts’ needs to be reconsidered in light of the above criticism. Registry No. Iron, 7439-89-6.
References and Notes (1) Hoffmann, D. P.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1991, 95, 5552. (2) Kerkhof, F. P.; Moulijn, J. A. J. Phys. Chem. 1979, 83, 1612. 13) Proctor. A.: Hercules. D. M. A o d . Soectrosc. 1984, 38, 505. (4) Tougaard, S.; Ignatiev, A. Surf..’Sci.‘1983, 124, 451. ( 5 ) Powell, C. J.; Erickson, N. E.; Madey, T. E. J. Electron Spectrosc. Relat. Phenom. 1979, 17, 361. (6) Seah, M. P.; Jones, M. E.; Anthony, M. T. Surf.Interface AMI. 1984, 6, 242.
(7) Powell, C. J.; Seah, M. P.; J. Vac. Sci. Technol. 1990, A8, 735. (8) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (9) Tanuma, S.; Powell, C. J.; Penn, D. R. J . Vac. Sci. Technol. 1990, A8, 2213. (10) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (1 l ) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8 , 129. (12) Fadlev. C. S. J . Electron Soectrosc. Relat. Phenom. 1975. 5. 895. (13j Papaiazzo, E. Appl. Surf.sci. 1986, 25, 1. (14) Paparazzo, E. J. Electron Spectrosc. Relat. Phenom. 1987, 43, 97. (15) Paparazzo, E. J. Vac. Sci. Technol. 1987, AS, 1226. (16) Paparazzo, E.; Dormann, J. L.; Fiorani, D. Phys. Reo. 1983, 828, 1154. (17) Paparazzo, E.; Dormann, J. L.; Fiorani, D. J. Electron Spectrosc. Relat. Phenom. 1985, 36, 77. (18) Paparazzo, E.; Dormann, J. L.; Fiorani, D. Solid State Commun. 1984, 50, 919.
Istituto di Struttura della Materia del CNR Via E. Fermi 38 I-00044 Frascati, Italy
E. Paparazzo
Received: August 23, 1991
size measurements”. We have indicated in the paper a preference for a given interpretation that seemed straightforward and consistent with the results. With respect to inclusion of the shake-up satellite associated with the Fe 2p3/, peak in the measurement of the Fe 2~312area, while it is obviously the correct procedure we were reluctant to adopt it because of our desire to restrict the energy window to minimize background subtraction problems and because of the lack of objective criteria for curve-fitting the envelope. However, on the basis of the shape of the Fe 2p envelope (Figure 5, ref l), it is not likely that this should lead to severe underestimation of the Fe 2~312area. It is also worth mentioning that close examination of Figure 3 in ref 1 shows that, for low Fe content, even the relative changes in the Fe 2p3/,/A1 2p intensity ratios with Fe loading are more consistent with those measured for Fe 3p/A12p than the corresponding Fe 2p/A12p values. For example, Fe 2psI2/A1 2p and Fe 3p/A12p intensity ratios deviate from linearity for Fe loadings higher than 6 wt % versus 8 w t % for their Fe 2p/N 2p counterparts. Also,the abrupt increase in the Fe 3p/A1 2p intensity ratio with increasing Fe loading from 8 to 11 wt 7%is better reflected in the corresponding variation of the Fe 2p3/2/Al2p ratios, compared to that measured for Fe 2p/A1 2p ratios. Finally, as stated in the Introduction and indicated by the title of the paper, the main thrust of this work is to offer different approaches for monitoring variations in the particle size of a supported phase by ESCA. This is evidently not affected by the subject of this comment. We hope, however, that our original paper, the remarks of Paparazzo, and the present reply will assist other investigators in elaborating an accurate method for measuring the ESCA Fe 2p peak area. Registry No. Iron, 7439-89-6.
References and Notes ( I ) Hoffmann. D. P.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1991, 95, 5552. (2) Kerkhof, F. P. J. M.; Moulijn, J. A. J. Phys. Chem. 1979, 83, 1612.
Reply to the Comment on “Monitoring Particle Size Changes of a Supported Phase by ESCA” Sir; The objective of our paper’ was to provide various means for monitoring particle size changes of a supported phase by ESCA. A series of Fe/A1203 catalysts were used for illustrative purposes. A survey of our ESCA Fe 2p spectra (Figure 4 in ref 1) clearly showed variation of the inelastic background contribution as a function of Fe loading. This problem was severe for the Fe 2p region, thus complicating any procedure for background subtraction. In contrast, the shape of the Fe 2p3/2 region was not significantly affected by the Fe loading. Furthermore, ESCA Fe 2p3,,/A1 2p intensity ratios relative to the monolayer line derived from the Kerkhof-Moulijn (K-M) model2 were more consistent with the corresponding Fe 3p/A1 2p ratios than their Fe 2p counterparts which appear to overestimate surface Fe content. Taking into consideration the fact that the ESCA Fe 3p area can be accurately measured, we have proposed the use of the Fe 2p3/2 peak area (instead of the total Fe 2p area) for monitoring changes in the particle size of the Fe phase. Paparazzo believes that the above observations are not sufficient to conclude that ESCA Fe 2p areas overestimate the surface Fe content. He proposes that the apparent overestimation of the Fe 2p peak areas is probably due to uncertainties in the parameters used for the determination of the monolayer line from the K-M model (e.g., assumed linearity of the overall transmission/detector efficiency with the photoelectron kinetic energy). Paparazzo also attributes the observed agreement between the ESCA Fe 2py2/Al 2p intensity ratios and the corresponding Fe 3p/A1 2p ratios to fortuitous canceling of errors in the determination of the monolayer line and the measurement of the Fe 2 ~ , / ~ / A 1 2intensity p ratios. We are fully aware of the limitations of the K-M model (see section 3.1 in ref 1). In fact, we have stated in that section that because of the number of assumptions involved in the determination of the monolayer line, the K-M model should “be used whenever possible in conjunction with other methods for particle 0022-3654/92/2096-5668$03.00/0
Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania 15260
Douglas P. Hoffmann Andrew Proctor Marwan Houak David M. Hercules*
Received: January 17, 1992; In Final Form: April 6, 1992
Comment on “Novel Kinetic Scheme for the Ammonium Perchlorate Gas Phase” Sir; In the referenced paper, Sahu et al.’ have applied a mechanism consisting of 22 homogeneous reactions to model the gas-phase combustion kinetics of ammonium perchlorate. The authors are aware of the complications of multiphase chemistry but argue that their simple, homogeneous mechanism produces the observed products without involving the species C10, previously regarded as critical to the gas-phase kinetic scheme. They also stress the significance of the fact that their temperature-time profiles do not change “even when the reaction pathways of the earlier schemes are included. This indicates that the present scheme is the fastest among all the reaction pathways and the one that would actually occur.” The starting temperature for the computations appears to be To= 870 K po is not stated, but based on previous work, values in the range 10-100 atm seem appropriate. The starting composition, based on eq 15 of their ref 2 is [NH,], = [HC10410 = 0.2, [H2Ol0 = 1.62, [ O ~ =O 1.015, [HClIo = 0.76, [NJo = 0.265, [NO10 = 0.23, [Cl,]o = [N@Io = 0.12 in relative moles. These figures assume that 80% of the NH4C104has decomposed to give the known indicated products, and the rest has simply vaporized. The fact that adding other reactions does not accelerate the temperature rise may support the argument that “the present 0 1992 American Chemical Society
J . Phys. Chem. 1992, 96, 5669-5670
scheme is the fastest ...” but certainly does not prove that the scheme is correct. In fact, the kinetics of ref 1 are very sensitive to one reaction rate in particular: k= NH3 N O + NH, H N O 8 x 10” exp(-2OOO/Rn cm3/(mol.s) (19) However, this reaction is approximately 59 kcal/mol endothermic, and the chosen Arrhenius parameters would make the reverse rate coefficient at 870 K some 14 orders of magnitude faster than gas kinetic. (The reverse is included specifically as reaction 11; the ratio of k19to k l l differs from the equilibrium constant by as many orders of magnitude.) Muzio et al.3 (the source for reaction 19 and its rate coefficient in ref 1) in fact had acknowledged that their “...attempt at modeling N O reduction with ammonia has not yet proved successful in that the mechanism employed required that the rate constants for [NH3 + N O and NH, NO] are inordinately large.” When the value of this rate coefficient is reduced to a physically possible one, the reaction time is lengthened drastically: the maximum in the slope of dT/dt occurs after hundreds rather than tenths of a microsecond. The contribution of this moleculemolecule reaction to the overall mechanism is greatly reduced, and the contention of Sahu et al. that this pathway would “... proceed immediately ...due to the large availability of N O and 0,” [the latter to react with NH2] is considerably weakened. A second serious flaw in the mechanism is that the only reaction to take C103 to products is k = 1 X l O I 4 exp(-l1930/RT) (6) 2C103 C12 30,
+
+
+
-
+
Thic step converts C103to final products in a single global step, thus obviating the need for directly invoking C10, ClO,, and other chlorinecontaining intermediates other than C1 atoms, whose only role in this mechanism is the equilibrium between C1 atoms and C12 molecules. (Two other included reactions produce C1 atoms, but the mechanism does not indicate that reverse reactions have been taken into account.) In fact, the mechanism has no chainbranching steps a t all involving chlorine species. Reaction 6 and its rate coefficient are taken from Bodenstein et a1.: who postulated the step to explain observations in their study of the Cl2-03 reaction at 15-50 OC; the activation energy was deduced from an assumed preexponential factor and an observed rate at a single temperature. The conclusions have been criticized elsewhere.5 Most importantly, the reaction is not an elementary one and cannot be inserted with confidence in a completely different system. The self-reaction of HNO (10) is assumed to yield directly H20 and N,O-which seems less likely than some alternative processes. The direct generation of observed products thus circumvents some additional radical steps that would contribute to more chain processes. Of course, it is easier to fault the proposed mechanism than to rectify it with confidence. Inclusion of the reverse reactions -7 and -5 will allow the H/Cl/HCl chain reaction to contribute; inclusion of H + 02 OH 0 (23)
-
+
will make the H/O/OH chain reaction a contributor as well. Omission of both of these important chain processes militates seriously against the correctness of the proposed mechanisma6 As it stands, the mechanism includes only three chain-branching steps: attack on N H 3 by N O (19) and by 0 (20) and attack on NH, by 0 2 . The C10, chemistry is not well understood, but a sequence of steps such as (6’) rather than (6) and (24)-(27) is at least reasonable, if not verified: 2C103 C l o d + C1O2 (6’)
-- + c1 + c10, + - c1 + + - +
2c10,
c103
c10
2c10 0 0
c10
c10,
c10
0, 0,
(24) (25) (26) (27)
Rate coefficients for reactions 25,26, and 27 are at least approximately known;’ (6’) and (24) can be estimated to be the same as (25). Finally, one needs some additional OH reactions, which in ref 1 reacts only with H2 (8), H N O (12), and itself (22), the latter process being chain terminating.
References and Notes (1) Sahu, H.; Sheshadri, T. S.; Jain, V. K. J . Phys. Chem. 1990,94,294. (2) Guirao, C.; Williams, F. A. AIAA J . 1971, 9, 1345. (3) Muzio, L. J.; Arand, J. K.; Teixeira, D. P. Symp. (Int.) Combusr., [Proc.],16th 1976, 199. (4) Bodenstein, M.;Padelt, E.; Schumacher, H. J. Z . Phys. Chem. 1929. BS, 209. ( 5 ) Cohen, N.; Heicklen, J. In Comprehensive Chemical Kinetics;Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1972; Vol. 6, Chapter 1. (6) In an earlier paper [Sheshadri, T. S.; Jain, V. K. Propellants, Explos. Pyrotech. 1989.14, 1931 the authors considered a more detailed mechanism, but the argument that the kinetics are not sensitive to other reactions is
weakened by the unreasonably fast steps noted above. (7) Atkinson, R.; Baulch, D. L.;Cox, R. A.; Hampson, Jr., R. F.; Kerr, J. A.; Troe, J. J . Phys. Chem. Ref.Data 1989, 18, 881.
Environmental Monitoring and Technology Department Space and Environment Technology Center The Aerospace Corporation P.O. Box 92957 Los Angeles, California 90009
N. Cohen
Received: February 3, 1992; In Final Form: March 26, 1992
Halide Ion Quadrupole Relaxation In Aqueous Solutions Containing Organic Cations Sir: Several years ago we observed that, contrary to simple inorganic ions, cations containing hydrophobic groups dramatically increase the rate of quadrupole relaxation of halide ions in aqueous solution.’-3 Other research groupsH made analogous observations and the effects of a large number of cosolutes on C1, Br, and I relaxation were investigated under different conditions. As summarized in ref 7, a consensus was reached attributing these peculiar effects to a type of hydrophobic interaction, where the anion interacts with the apolar chains of the organic ions. Recently Suezawa et al.s in reexamining halide ion quadrupole relaxation in aqueous solutions containing organic cations, have questioned the previous view and argued that the relaxation effect is due to direct interaction between nitrogen cations and the halide ions and make an analysis in terms of contact ion pairing. In our opinion, Suezawa et al. have failed to consider a large number of previous findings, which appear to be inconsistent with their view. While the arguments for attributing the dramatic effect of organic cations on halide ion quadrupole relaxation to an ion-hydrophobic solute interaction rather than direct ion-ion interactions are reviewed in some detail in ref 7, we list the most significant observations here: 1. Originally, these effects were discovered with symmetrical tetraalkylammonium ions, which cause a halide ion relaxation effect orders of magnitude larger than for simple inorganic ions. The effect increases dramatically with the number and length of the alkyl groups (and when the cationic center is closed for approach), as well as with the halogen atomic number (as also found by Suezawa et al.), contrary to what is generally found for ion pairing and direct ion-ion interactions. In nonaqueous solvents, the dependence on cation size is very different and qualitatively consistent with ion-ion effects. 2. Substitution of the cationic atom, for example changing from nitrogen to phosphorus, does not produce any significant change. 3. If the organic groups in the cation are made more polar, the effect is reduced. 4. Direct evidence that the dominant line broadening is due to the nonpolar groups was obtained from the observations that
