Langmuir 1985, 1 , 768-770
768
emission. We can therefore assign the 5.8 f 0.3 eV binding energy photoemission feature to the Br(4pz). With dissociative Br, adsorption on Fe(110) the binding energy of the B r ( 4 ~ , , ~is) notably coverage-dependent ranging from 5.4 f 0.2 eV at low coverage to 4.3 f 0.1 eV at high coverage when the vector potential A of the incident light is parallel to the (100) direction.' The Br(4pJ orbital is not observed to have a pronounced binding energy coverage dependence and varies in value between 5.7 and 6.1 eV, at most, with dissociative Br, adsorption on Fe(l10).7 No pronounced coverage dependence of the Br(4pZ)and Br(4p,,) orbitals was observed with dissociative CBr, adsorption (Figure 1). The absence of marked coverage-dependant binding energies of the bromine orbitals with dissociative CBr4 adsorption may also be a result of different sequence of overlayer structures, as compared to dissociative Br2 adsorption, as the bromine coverage increases. The carbon may also influence bonding and surface adsorption sites of the bromine overlayer resulting in some differences in the photoemission spectra for dissociative CBr, and Br, overlayers. The binding energies of the Br(4pJ and Br(4pxJ)orbitals at 5.8 f 0.3 and 4.5 f 0.2 eV, respectively, observed following dissociative CBr, adsorption do, however, resemble the binding energies of the Br(4p2)and Br(4pJ orbitals at 6.0 f 0.2 and 4.3 f 0.1 eV, respectively, observed following saturation of the bromine ov'erlayer with dissociative Br2 adsorption on Fe(l10h7 The small difference in binding energy of the Br(4p,,) orbital at saturation of the dissociative CBr, overlayer as compared to the dissociative Br, overlayer is likely to be a result of differences in saturation coverage. Dissociative CBr, adsorption has been
observed to end at a somewhat smaller bromine saturation coverage on Fe(100) than that for dissociative Br, adsorption., For the free bromine atom the bromine 4p orbitals are degenerate. Since the Br(4pJ orbital has a greater binding energy than the Br(4pJ orbitals at i=' (normal emission), this suggests that following dissociative CBr4 adsorption, the bromine is bound to the surface via the Br(4pZ)orbital while the B r ( 4 ~ ~orbitals , ~ ) are nonbonding. A similar bonding behavior has also been observed with bromine adsorption on Pd(lll).17 For bromine on Cu(lOO)'* and bromine on Ni(100),16bonding occurs via the B r ( 4 ~ ~ , ~ ) orbitals while the Br(4p,) is nonbonding. The broad half-width of the bromine emissions for bromine bound to the Fe(ll0) surface is probably attributable to lifetime broadening. Similar lifetime broadening of chemisorbed bromine photoemission features has also been observed for bromine on Ni(100).16 In conclusion, we find that CBr, adsorption on Fe(ll0) at 300 K is completely dissociative. A bromine overlayer is formed with the bromine bonding to the surface via the Br(4p2) orbital.
Acknowledgment. We thank the staff of the Synchroton Radiation Center of the University of Wisconsin, Madison for their excellent support. The Tantalus I storage ring is supported by NSF Grant DMR 77-21888. This research is supported by Syracuse University and the Research Corporation, as well as the University of Cornel1 Material Science Center (NSF-DMR 82-17227 and NSFDMR 83-13742). Registry No. CBr4, 558-13-4;Fe, 7439-89-6.
Transient-Kinetic Study of Catalysts Aging: Methanation over Raney Nickel Y. Soong and P. Biloen* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received March 25, 1985 Abrupt switches in the isotopic composition of the reactants have been utilized to abstract transient-kinetic information at, essentially, steady state. In the context of methanation over Raney nickel this method demonstrates that aging is a process which originates in the catalyst itself rather than in its adlayer. We report on the use of non-steady-state kinetic methods for the study of one of the less tractable problems in heterogeneous catalysis: catalyst deactivation. A Raney nickel catalyst, when exposed to hydrogen-lean syngas, initially declines at a rate of approximately 0.2% /h. After a period of approximately 120 h, the residual decline rate is only approximately 0.03% /h. On the basis of the existing methanation literature'" and without further evidence we tentatively conclude that the catalyst's adlayer slowly deteriorates. Less reactive surface species are being slowly produced and removed, with a steady-state level of these surface-blocking species being attained after some 120 h. Below we show this tentative conclusion to be incorrect: aging originates in the catalyst itself rather than in the adlayer.
* To whom correspondence
should be addressed.
We tested the "adlayer deterioration" hypothesis by following the line out with transient-kinetic experiments. Thereto we switched repeatedly the feed from 12CO/H2 to 13CO/Hzand vice versa. This leads to a transient-kinetic phenomenon: a decay of the 12CH4production, compensated by the concurrent increase of the W H , production. In what follows we call F13(t) and Fl2(t) isotopic transients, in which
(1)McCarty, J. G.; Wise, H. (2) (3) (4) (5)
J. Cotal. 1979, 57, 406. Wentrcek, P. R.; Wood, B. J.; Wise, H. J . Catal. 1976, 43, 363. Araki, 3. M.; Ponec, V. J. Catal. 1976, 44, 439. Biloen, P.; Helle, J. N.; Sachtler, W. M. H. J. Catal. 1979,58,95. Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1980, 91, 499.
0 1985 American Chemical Society 0743-7463/S5/2401-076~~0~.50/0
Langmuir, Vol. 1, No. 6, 1985 769
Letters
,
I I
r !
I
Preexposure 2000 sec
I
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10-8i
b j0
F
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I
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$3
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Preexposure 120 sec
-
Time on-stream
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I
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1800
200
400
800
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05hr 16hr 40 hr 64 hr 313 hr
1200
1400
1600 .la00
Time (sec)
Time (sec)
Figure 1. In ‘%O/Hz the production of ‘VH, continues for aome
-
Figure 2. Whereas catalyst aging affects the lifetime of inter-
time, fed by l%-containing surface intermediates. F is the fraction of W H , in CHI. Exposure sequence: 13CO/Hz(2000s) 12CO/Hz.
mediates accumulated over a 2000-8 period (Figure 1) this is much less the case for intermediates accumulated in only 120 s. Exposure sequence: 13CO/Hz(120s) 12CO/H,.
For a single reservoir, containing intermediates of the 13C and 12Cvariety at a coverage 813 and e, respectively, we have
obtained after a 2OOO-s preexposure develop a tail (Figure 1). We would like to reemphasize that the only perturbation is in the isotopic composition of the CO. The ratio H2/C0 is kept constant over the full 313 h, and steady state therefore is maintained throughout. The transients in Figure 2, just as those in Figure 1, have been obtained for different times on stream. However, the preexposures now are only 120 s instead of 2000 s. It emerges that the shape of these (normalized) transients does not change with aging and that they do not have a tail. One way of summarizing our observations is (1) the shape of the transienh depends on the extent of aging and (2) for a given extent of aging their shape depends on the length of the preceding exposure. A theoretical analysis which focuses on (2) furnishes proof for the foll~wing:~ (a) This behavior cannot be shown by a single pool of intermediates. A variation of shape with length of preexposure can only be produced by a combination of two or more pools. (b) In view of the (differences in the) global shapes of the transients in Figures 1 and 2, the pools have to be positioned in parallel rather than in series. (c) Irrespective of the detailed interaction between pools in parallel the combined transient consists of sums of exponentials.
e13w + e l m = 8
(1)
TOF~,= e13/7
(2)
F13
= e13/e
(3)
Specifying the reaction atmosphere to be a l2CO/HZatmosphere, we further have (4)
(5) One of the unique and most powerful aspects of isotopic transients is that in eq 2, 4, and 5, the lifetime 7 is essentially independent of 813 and F13F 7 of a surface intermediate is independent of whether that surface intermediate is being surrounded by 13C- or 12C-containing species. Accordingly, eq 5 integrates into F13
= (Fl3It=,&/‘ = (F13)t,oe-kt
(6)
Equation 6 demonstrates that for an irreversible reaction encompassing only a single reservoir of intermediates k (and 0) can be obtained straightforwardly from isotopic Figure 1depicts the isotopic “downcoming” transients recorded for different times on stream, ti = 0.5,16,40,64, and 313 h. All these transients have been recorded in l2CO/H2 atmosphere. Accordingly, the downcoming transients ( U / d t < 0) are the F13ones, and the upcoming transients the F12ones. The 13CH4produced in 12CO/H2 atmosphere derives from carbon 13C-containingintermediates accumulated at the surface in the l3CO/H2exposure period, preceding recording of the transients. Hereafter, we call these the “precedingexposures”. For the transients shown in Figure 1, all the preceding exposures had a length of 2000 s. With increasing time on stream the transients (6) Biloen, P.; Helle, J. N.; van den Berg, F. G. A.; Sachtler, W. M. H. J. Catal. 1983, 81, 450. (7) Yang, C-H.; Soong, Y.; Biloen, P. h o c . Int. Congr. Catal., 8th 1984, 2, 3.
(8)Happel, J.; Suzuki,J.; Kokayeff, P.; Fthenakis, V. J . Catal. 1980,
65, 59.
-
(d) The relative contribution, Ci, of the pools with the smaller 7i value can be enhanced by shortening the preceding exposure. Based on the foregoing we deconvoluted our normalized transients into a sum of two exponentials and a background F(t) = C,e-(t/d
+ Cze-(t/Tz)+ C3
(8)
with the smaller of the two 7’s being obtained from Figure n. A.
71
= 120 f 20 s
We observe 71 to be independent of aging. Deconvolution of the transients of Figure 1 with T~ = 120 s yields 7 2 = 850 f 200 s The value of r2 does not exhibit a trend with aging; however, the values of (C1,C2)do. With increasing aging C1 decreases and C2increases, and they level out concurrent
Book Reviews
770 Langmuir, Vol. I , No. 6, 1985 1
o*
I
e
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- 15
z
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Oqf&
regeneration
i. 0
100
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0 I-
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Time on-stream (hr)
Figure 3. Transient behavior (C,,C,) the steady-state TOF.
lines out concurrent with
with the total activity (Figure 3). In the introduction we speculated that less reactive species slowly build up and that the equilibrium between their deposition and removal reaction is established in
approximately 120 h. This implies that the lifetime itself of these species is of the order of 120 h. What the transients show, however, is that what builds up in a 120-h process are species with a lifetime of only approximately 850 s. W h i c h leads us t o t h e conclusion t h a t t h e 120-h process reflects a change in t h e catalyst itself. On the modified catalyst surface the intermediates have a lifetime of the order of 850 s, on the unmodified a lifetime of 120 s. The 120-h period is one in which we reach an equilibrium distribution of modified and unmodified surface. Further evidence as to the nature of the modification and mathematical proofs of some of the foregoing statements will be given in a forthcoming paper.g
Acknowledgment. It is a pleasure to acknowledge support of this work by the U S . Department of Energy, Office of Basic Energy Sciences (Contract DE-ACO283ER13105), and by the Exxon Education Foundation. Registry No. Ni, 7440-02-0. (9) Soong, Y.; Krishna, K.; Biloen, P. J. Catal., submitted for publication.
Book Reviews Auger Electron Spectroscopy. Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Applications. Volume 74. By Michael Thompson, Mark D. Baker (University of Toronto), Alec Christie (Vacuum Generators Scientific Ltd.),and Julian F. Tyson (University of Technology, England). Edited by P. J. Elving, J. D. Winefordner, and I. M. Kolthoff. John Wiley & Sons, Inc., New York. 1985. 394 pp. ISBN 0-471-04377-X. This is a much more broadly based treatment than the usual review of Auger Spectroscopy,with major sections on the detailed electronic information content of the spectra as well as the usual sections on analytical uses of AES. The heart of the book is in two main chapters: (i) gas phase AES and (ii) applications to metallurgy and material science. The extensive chapter on gas phase spectra is relatively unique among recent reviews and is an excellent summary of the literature through about 1980. The chapter on applications to metallurgy and materials science is essentially just a listing of applications areas with only a cursory treatment of each. Other reviews treat this type of material much better. I have two main problems with this book. First, for a book on Auger Spectroscopy with such a heavy emphasis on electronic information with Auger lineshape analysis (ALA) as a major component of this subject, there is a complete lack of treatment of the problem of backgrounds and electronic energy loss processes that distort solid-phase spectra. Since any detailed comparisons of such spectra to theory or to other experimental results require removal of these artifacts, it is essential that the reader at least know that the problems exist and have sources for more detailed discussion of the subject. Second, the whole area of electronic
structure information content of AES, both in the solid and gas phases, has been moving very rapidly in the past few years and a review with an effective cutoff of 1980 for references is hopelessly out of date. There have been major advances in the understanding of Auger spectra in the past 4 or 5 years, but this is not reflected in the present book. The treatment of the theory of Auger lineshapes, either for gas-phase molecules or for solids, is out of date with no insights given to the reader and in some cases confusion introduced. A particularly poor example is the statement on page 275 that the (CVV) Auger lineshape is complicated by a convolution of the valence band lineshape with the core level lineshape. It is hard to see how the convolution of the valence band with a sharp core line would introduce complications, but it is hard to tell what this means, since this short section on the “Theoretical Aspects” is so terse as to be unusable except as a source of a few outdated references. On a more humorous note, the authors take the first paragraph of the section of the gas-phase spectrum of ammonia to discuss the question of priority of publication among four papers published within 1year. After much discussion of statements of first priority in three of the papers, all of whom missed the “fmt”paper (one of the book’s authors), they end the paragraph by stating that this should cause authors and referees alike a certain amount of embarrassment. The embarrassment continues. The authors own ref 181 at the end of the short section on ammonia, a paper from K. Tamaru’s group, contains, to my knowledge, the first (by 2 years) gas-phase ammonia spectrum. Everyone (including this reviewer) seems to have missed this paper, perhaps because of the obscure nature of the journal in which he published, Physical
Review Letters. Robert R. Rye, Sandia National Laboratories
