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
