3494
NOTES
I and NIBS IA standards sent to us by the IAEA in Vienna. There was agreement between the old and new standards. Actually it is only the ratio between the deuterium concentration of the two standards which enters into the value for a. Should the value for this ratio be changed after repeated measurements, the same relative change should be applied to CY 1.
Table I: Spectra of Surface Species on Cobalt
Adsorbates
1860 w 1700 vw 1600 vw 1060 sh 1038 m 2010 sh 1955 m 1845 m Ethanol
by G. Blyholder’ and Laurence D. Neff
1112 s(liq), 1057 s(gas) 1030 s
1089 s 1050 s 880 s 802 w 657 s 433 m
In order to examine the validity of using an analogy between coordination complexes and heterogeneous reactions on metals, we are examining the structure and reactions of surface species on Co surfaces and comparing them to similar reactions of Co complexes. In order for the comparisons to be meaningful the structure of surface species must be det’ermined. Whereas the structures of a vast number of coordination complexes are well known, relatively little is known about the structural details of species adsorbed on metal surfaces. The interaction of alcohols and aldehydes with Fe2 and Nia which are adjacent to Co in the periodic table produce quite different surface species. Thus the surface species produced on Co are important in further defining the role of the number of “d” electrons in determining the structure of surface species. The surface species on cobalt are also important for the understanding of the oxo or hydroformylation reaction which has received much attention.4 In a previous study the infrared spectra of alcohols adsorbed on silica-supported Co has been reported.6 Because the silica support permits only the C-H and C=O stretching regions to be examined, the assignment of the principal bands to an alkoxide structure cannot be considered to be firmly based except in the one case of a methoxide. A wide spectral range experimental technique, which has been described in detail elsewhere6 was used here. It consists of evaporating Co from an electrically heated tungsten filament in the presence of a small The Journal of Physical Chemistry
2010 m 1950 m 1850 m Ethylene oxide
CO stJr. Added CO Added CO Added CO
Acyl CO str CHa rock Skeletal str Skeletal str CHz O’H bend M-0 str Skeletal bend Added CO Added CO Added CO
Ethanol 1995 vw 1088 w 1030 w 885 w
Acetaldehyde 1150 vw 1090 w 1040 w 885 w a
Chemisorbed CO Acyl CO str Acyl CO str CH3 rock
Chemisorbed CO Acyl CO str
483 vw
Department of Chemistry, University of Arkansas, Fayetteville, Arkansas 73701 (Received March 15, 1069)
Assignments
Ethanol5 1890 vw 1700-1750 vw 1600 vw 1093 m 1047 m 890 m
Structure of Surface Species on Cobalt
cm-l---Model compounds Methanol“
Methanol
-
Acknowledgments. The author wishes to express his appreciation to Professor Th. Sigurgeirsson, head of the Physics Division of the Science Institute, for his advice and encouragement, and to Mr. Th. Bddlason, of the Mathematics Division, for many fruitful discussions. The author is also indebted to Dr. I. Friedman a t the U. S. Geological Survey for reading the manuscript and offering valuable suggestions.
----Freauencies. Surface species
1149 w 1089 s 1050 s 880 s 802 w 657 s 433 m Ethanol 1149 w 1089 s 1050 s 880 s
Chemisorbed CO CHI rock CHa rock Skeletal str Skeletal str CH2 OH bend Skeletal bend CH3 rock CH3 rock Skeletal str Skeletal str
Reference 7.
pressure of helium. The metal particles formed in the gas phase deposit in a purified hydrocarbon oil film on the salt windows of an infrared cell. The gas to be studied is then admitted to the cell and the spectrum of the chemisorbed species obtained. Spectra are recorded before and after admission of the gas to the cell. (1) Address inquiries to this author at the University of Arkansas. (2) 6. Blyholder and L. D. Neff, J . Phys. Chem., 70, 893 (1966). (3) G.Blyholder and L. D. Neff, {bid.,70,1738 (1966). (4) R.F.Heck and D. S. Breslow, “Actes du Deuxieme International de Catalyse,” Editions Technip, Paris, 1961,p 671. (5) G.Blyholder and W. V. Wyatt, J . Phys. Chem., 70, 1745 (1966). (6) G.Blyholder, J . Chem. Phys., 36,2036 (1962). (7) C.Tanaka, Nippon Kagaku Zasshi, 83,792 (1962).
NOTES Five minutes of pumping has been found sufficient to remove all spectra due to gas-phase and oil-dissolved light molecules. For 3- and 4-carbon-atom molecules longer pumping times are required. The spectra were obtained using Perkin-Elmer Models 21 and 337 spectrophotometers. The Model 21 is equipped with CsBr optics which permit scanning from 715 to 250 cm-l. The 337, which is a grating instrument, is used to scan the region from 4000 to 400 cm-’. The adsorbates were obtained as reagent grade chemicals from commercial sources. They were degassed by repeated freeze-thaw cycles with pumping and distilled into storage vessels on the vacuum system. The CO was passed through an activated charcoal trap cooled with liquid air. The spectral results for methanol, ethanol, acetaldehyde, and ethylene oxide are shown in Table I. The spectra of the surface species were observed after the adsorbates were pumped out of the cell. In the third column the spectra of model compounds to which comparison is desired are listed. The spectrum of methanol adsorbed on Co indicates a variety of structures. The most intense bands at 1060 and 1038 cm-1 are assigned to a methoxide structure because of the closeness of the bands to CH3 rocking and CO stretching bands of methanol and because of the lack of OH stretching and bending bands. Decomposition of a small amount of the methanol to chemisorbed CO is indicated by the weak band at 1860 cm-l- Exposure of a fresh Co sample to CO results in medium-intensity bands at 1870, 1960, and 2020 cm-’. Similarly to the work on the appearance of the CO band from decomposition at 1860 cm-l is taken as indicating the decomposition occurs on edge, corner, and dislocation sites. For the same reasons as those cited previously3 the very weak 1 7 0 0 - ~ m -band ~ is assigned to an acyl surface structure. The weakness of this band suggests that relatively little acyl structure is formed. The very weak band at 1600 cm-I is tentatively assigned to an acyl structure adsorbed on a different site from that producing the 1700-cm-’ band. In support of this assignment it may be noted that bands for the CO stretch of acyl transition metal compounds have been reported over the range from 1600 to 1725 cm-1.8-11 Further the CO stretching bands for the acyl structures adsorbed on Xi, while having their maxima around 1700 cm-l, do have considerable absorption in the 1600-cm-l region. The addition of CO to the cell at IO-mm pressure results in the desorption of a large part of the alkoxide surface species and the formation of chemisorbed CO as shown by the bands at 2010, 1955, and 1845 cm-l. The results for ethanol closely parallel those of methanol. Again the assignments are the same as those used for adsorbed species on Fez and NLa In the cases of 1-propanol, 1-butanol, 2-propanol, and 2-butanol no bands indicating decomposition to give
3495 chemisorbed CO or an acyl structure were detected. The presence of some alkoxide structure was indicated, but the quantity was very much a function of the conditions. 2-Propanol formed one of the more clear cut examples. Five minutes of pumping after exposing an evaporated-into-oil Ni film to 2-propanol for 1 hr or more reduces the bands for the alcohol dissolved in the oil to around a size that deviates from the background by 3 or 4% transmission. When a Co sample is used in the same way, the bands change shape and relative intensities somewhat and deviate from the background by 30%. This seems to indicate that most of the remaining bands on Co are due to a surface alkoxide structure as is formed in the case of Fe. However, continued pumping reduces the size of the bands until after several hours they show only 4 or 5y0 deviations from background. Thus while alkoxide formation is indicated, for these alcohols, as for methanol and ethanol, the surface bond is weak. Acetone, methyl ethyl ketone, methyl vinyl ether, and t-butyl alcohol likewise gave no infrared evidence of decomposition to chemisorbed CO or an acyl structure. They gave indication of relatively little alkoxide formation. Diethyl ether and tetrahydrofuran gave no infrared evidence of any adsorption. Acetaldehyde and ethylene oxide produced weak bands as shown in Table I indicating formation of a small amount of alkoxide structure. Upon exposure to 10 mm of CO all samples produced bands for chemisorbed CO approximating those of a fresh surface. This indicates the surfaces were in good condition during the previous adsorption experiments. The structures of the surface species found for adsorption on Co are consistent with those found on Fe and Ni. The alkoxide structure was quite stable on Fe but was not found on Ni. Co, occupying a position in the periodic table between Fe and Ni, might be expected to show a stability for the alkoxide structure less than that of Fe, which it does. I n terms of decomposition to CO the behavior of Co is intermediate since Fe adsorption produced no decomposition to CO while Co produced a little and Ni a moderate amount. This seems to be a classic example of regular variation of properties with position in the periodic table. However, such regularity is sometimes absent in surface phenomena.12 Comparisons of the adsorption reactions may be made to the following coordination complex reactions which have been proposedla-16 (8) R. B. King, J . Amer. Chem. SOC.,8 5 , 1918 (1963). (9) F.G.A.Stone, Spectrochim. Acta, 18,686 (1962). (10) W. Hieber, G.Braun. and W. Beck, Chem. Ber., 93,901(1960). (11) W.R. McClellan, J . Amer. Chem. SOC.,83, 1698 (1961). (12) G. C. Bond, “Catalysis by Metals,” Academic Press, New York, N. Y.,1962. (13) R. F. Heck and D. 5. Breslow, J . Amer. Chem. Soc., 82, 4438 (1960).
Volume 76,Number 10
October 1969
NOTES
3496
+ RCo(C0)d J_ RCOCo(C0)e RCHO + HCo(C0)e RCHzOCo(C0)r (CH2CHz)O + HCo(C0)4 CO
(1) (2)
4
HOCHzCHzCo(C0)4 (3) We have proposed that formation of CO on a surface from alcohol adsorption proceeds via alcohol adsorption to form an alkoxide which rearranges to an acyl structure from which alkyl group migration leaves chemisorbed CO. This mechanism is analogous to the mechanism which has been proposed for the reverse of reaction 1. The adsorption of acetaldehyde to produce an alkoxide structure is analogous to reaction 2. However, in the case of ethylene oxide adsorption we obtain an alkoxide structure rather than the alcohol indicated by reaction 3 for the coordination-complex reaction. As yet there is insufficient information to put the comparison of specific heterogeneous and homogeneous reactions on a quantitative basis but we believe our results demonstrate that this is a feasible and fruitful area of study. (14) R.F,Heck in "Mechanisms of Inorganic Reactions," Advances in Chemistry Series, No. 49, American Chemical Society, Washington, D. C., 1965. (15) R. F.Heck, Advan. Organometal. Chem., 4,246 (1966).
On a Criterion for Rejection of Formation Constants of Weak Complexes
by E. L. Heric Department of Chemistry, Unizersity of Georgia, Athens, Georgia $0601 (Received April 1 , 1969)
The applicability of the Benesi-Hildebrand' equation to the spectrophotometric study of weak 1: 1 complexes formed from negligibly absorbing donor and acceptor, Z[D]O[A]"/A = [ D ] " / e 1/Ke, has been considered by Persona2 (For brevity this equation will be reb.) [D]" and [A]" are the initial ferred to as y = az molar concentration of donor and acceptor and K is the formation constant for the complex DA. A and e are, respectively, the absorbence and the molar absorptivity of DA at fixed wavelength and Z is the path length, all in the Beer-Lambert law, A = eZ[DA]. [DA] is the equilibrium molar concentration of the complex. Person proposed the criterion that application of the Benesi-Hildebrand equation in evaluating e and K when [A]" yn-l > yn-2 >. . . > 91. Assume that a least-squares treatment of the data with the Benesi-Hildebrand equation yields nonzero values of the slope and intercept. This slope and intercept are, of course, the most probable values of these parameters statistically, so that clearly yl, the smallest value of y, is related to b as y1 > b. Let us consider the probability that the set of points from y n through b represents a single population and, thus, that they do not indicate a systematic trend of behavior. I n agreement with Person,z let the basis of establishing probability be the comparison of b U S . y. If we assume that b and the n values of y are members of a single population, the probability that any one yc is greater than b is 100(1/2)o/o. For all n points the corresponding probability becomes 100('/2") yo. The probability that all yc are greater than b is listed in column 2 of Table I for some values of n given in column 1. (1) H. A. Benesi and J. H. Hildebrand, J. Amer. Chem. Soc., 71, 2703 (1949). (2) W.B.Person, ibid., 87,167 (1965). (3) J. Topping, "Errors of Observation and Their Treatment," Reinhold Publishing Corp., New York, N.Y., 1957,pp 48-61.
