J. Phys. Chem. 1992, 96, 5667-5668 have different glass transition temperatures, an observation which is confirmed calorimetrically (Table I). Registry No. D-(+)-Gluwse, 50-99-7; D-(+)-galactose, 59-23-4; D(+)-xylose, 58-86-6; D-(-)-arabinose, 10323-20-3; L-(+)-arabinose, 5328-37-0; glucitol, 50-70-4; xylitol, 87-99-0.
References and Notes (1) Kaatze, U. Phys. Med. Biol. 1990, 35, 1663. (2) Aneell. C. A. Annu. Rev. Phvs. Chem. 1983. 34. 593. (3j Abadie, P.; Charbonni5re, R.,Gidel, A.; Girard, P.; Guilbot, A. C. R. SLances Acad. Sci. 1956,242, 1016. (4) Tait, M. J.; Sugget, A.; Franks, F.; Ablett, S.;Quickenden, Y. A. J . Solution Chem. 1972,-i,131. (5) Pottel, R.; Adolph, D.; Kaatze, U. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 278. (6) Angell, C. A,; Smith, D. L. J . Phys. Chem. 1982,86, 3845. (7) Chan, R. K.; Pathmanathan, K.; Johari, G. P. J . Phys. Chem. 1986, 90, 6358.
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(8) Naoki, M.; Katahira, S. J . Phys. Chem. 1991,95, 431. (9) Pissis, P.; Diamanti, D.; Boudouris, G. J . Phys. D Appl. Phys. 1983, 16, 1331. (IO) Roozen, M. J. G. W.; Hemmings, M. A. J . Phys. Chem. 1990, 94, 1326. (1 1) Franks, F. Water, a Comprehensiue Treatise, Plenum: New York, 1982; Vol. 7. ( 1 2) Franks, F. Biophysics and Biochemistry at Low Temperatures; University Press: Cambridge, 1985; Chapter 3. (13) Johari, G. P. J . Chim. Phys. (Paris) 1985,82,283. (14) Ngai, K. L.; Rendell, R. W.; Yee, A. F. Macromolecules 1988,21, 3396. (15) Orford, P. D.; Parker, R.; Ring, S.G. Carbohydr. Res. 1990,196,1 1 . (16) Davidson, D. W.; Cole, R. H. J. J . Chem. Phys. 1951, 19, 1484. (17) Johari, G. P.; Goldstein, M. J . Chem. Phys. 1971,55, 4245. (18) Johari, G. P. In Relaxations in Complex Systems; Ngai, K. L., Wright, G. B., Eds.; Naval Research Laboratory: Washington, DC, 1985; p 17ff. (19) Ahmed, M. S.;Crossley, J.; Hossain, M. S.;Kashem, M. A.; Saleh, M. A.; Walker, S. J . Chem. Phys. 1984,81, 448.
COMMENTS Comment on “Monltorlng Partlcle Slze Changes of a Supported Phase by ESCA”
Sir: In a recent paper,’ Hoffmann, Proctor, Houalla, and Hercules (HPHH) reported on electron spectroscopy for chemical analysis (ESCA) spectra of Fe/A1203 catalysts. Although the paper certainly advances our understanding of the surface composition and structure of supported heterogeneous catalysts, some of the authors’ conclusions appear unwarranted. A central issue in HPHHs paper is monitoring of the Fe-to-Al photoemission intensity ratios as a function of both the particle size and Fe loading. The authors relate the Fe 2p3/2 area-rather than the entire Fe 2p3/2;./2 area-to the growth mode of the active phase up to one monolayer. The rationale is that the Fe 2p/A1 2p intensity ratios measured for low-Fe catalysts systematically overestimate the Fe content relative to the theoretical intensity ratio curve, as derived from the Kerkhof-Moulijn (K-M) model2 (the “monolayer line” in Figure 3 of ref 1). They conjecture that the main source of error derives from the integral background approximation used.3 They also find that this error becomes even greater if a Tougaard background4 is used. I feel that the above experimental results alone are not sufficient to justify the conclusions that the use of the Fe 2p area overestimates the iron content and that the Tougaard background is not accurate enough for these catalyst systems. Such an overestimation of iron is only apparent and may be explained with the following arguments: (i) The first argument concerns some of the parameters (listed in Table I1 of ref 1) that were used to derive the K-M line of Figure 3B,C. A major error can be associated with the intensity-energy response of the photoelectron spectrometer. HPHH assume that the overall response transmission/detector efficiency, D,of their AEI ES 200 instrument-operated in the fixed retardation ratio mode-is proportional to photoelectron kinetic energy. This (theoretical) assumption may be in striking variance with the actual intensity-nergy response of a specific instrument, particularly for such wide energy separations as that existing between the A1 2p peak and the Fe 2p band (around 630 eV).S-7 A minor error may be associated with the electron inelastic mean free paths, A. In Table I1 of their paper HPHH use the Penn formalisms for calculating X values. In fact, it is the electron attenuation length, AL, that is relevant to ESCA.799J0 Although exchange in terminology (and symbols) has often occurred in the literature, X and AL each have a separate physical meaning9
Apart from semantics, and following the assumption of HPHH, ALA’2p = 18 A, use of Penn’s formula gives ALF, 2p = 1 1.2 A, versus the value of 13.3 A derived from the Seah and Dench formalism,1° which provides a more accurate estimate of A L k 7 It should parenthetically be noted that uncertainties in both D and AL(X) affect the accuracy of the calculated mean particle sizes of the active phase which are derived via eqs 2-4.’ (ii) A second argument pertains to the different ways the authors use for measuring the iron photosignal areas, whether referring to the whole Fe 2p or to Fe 2p3 alone (Figure 5 of ref 1). In the first case, they include the high binding energy “shake-up” structure for both the 312 and 112 spin-orbit terms, whereas in the second one they exclude the intensity of the satellite. The latter procedure is wrong. HPHH relate the measured intensities to Scofield’s photoionization cross sections,” which were calculated within the one-electron, frozen-orbital model (Koopmans’ theorem). In this context, sudden approximation arguments12show that all the intensities of a particular photoemission transition must be considered for quantitative purposes, Le., that of the “adiabatic” peak plus that of the satellites originating from electron configuration interaction effects. As a consequence, the Fe 2p/A1 2p intensity ratios reported in Figure 3 are a more faithful witness to the quantitative composition of the materials than the Fe 2p3/,/A1 2p intensity ratios, and their apparent overestimation of the iron content for low-Fe catalysts is probably due to errors associated with the K-M monolayer line, as discussed in paragraph i. Also,the apparent better agreement given by the Fe 2p3/,/Al 2p ratios could result merely from fortuitous canceling out of errors contained in the calculation of the K-M line and in the measurement of the Fe 2~312intensity. Some years ago, I reported on quantitative ESCA results of iron oxides as well as of Fe203+ A1203 mixture^'^-'^ using a “first principles model”’ which included Scofield’s cross sections. I found that both the nO/nFe and nFe/nAl atomic ratios were accurate to &lo% relative error when the “shake-up” structure was considered as a part of the Fe 2~312intensity, whereas severe underestimations of the iron content were obtained when only the intensity of the leading peak was accounted for.I3 These findings were useful for studying the surface composition of Fe/A120316*17 and Fes2B,8/A1203’ssmall particle systems. To conclude, while it is not my intention to establish what is the background to use for measuring the Fe 2p area, nor to settle the question of whether use of the integral background o r use of a Tougaard background introduces less error into the ESCA Fe
0022-3654/92/2096-5661$03.00/0 0 1992 American Chemical Society
J. Phys. Chem. 1992,96, 5668-5669
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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
