3998
GEORGE BLYHOLDER AND MARVIN C. ALLEN
a’ is the “distanc.e of closest approach” parameter
(here taken to be 1.5 for all electrolytes); and Dh is the Debye-Huckel term for a 1-1 electrolyte (Dh = -0.509 d U ( 1 a’ 4 7 ) ) . By combining the equations for individual ions to give mean ionic activity coefficients, we obtain for a pair of 1-1 electrolytes
+
log Yo1 = (’/2)(log
710
+ 1%
720)
(5)
of primary isopiestic standards2 and of extrapolations to infinite dilution (to which this computation is sensitive) of two-component data; two-thirds of the computed values lie within 1mv. of the linear extrapolation and most values are much better than would be obtained by using’ the value of log Y * N ~ C ~at the same ionic strength ( i e . , assuming aI2 = 0). Agreement is worst with NaC104 and not very good at some ionic strengths with sodium acetate, Na2S04, and BaCI2.
and for a 1-1 in the presence of a 2-1 electrolyte log Yo1 = (l/J(log
710)
+ (3/8)(10g 720) (l/12) (Dh)
(6) Comparison of experimental and computed values are shown in Figures 1-3 (at 1111 = 0). Agreement is very good in most cases, especially in view of uncertainties in literature values of the two-component data, both
Acknowledgments. The author wishes to acknowledge the very sizable contribution of Dr. J. S. Johnson to this paper. His advice and assistance have greatly improved the final presentation. Thanks are also due to Prof. George Scatchard and to Dr. K. A. Kraus for many helpful discussions during the course of this work.
Infrared Spectra and Bonding of Nitric Oxide Adsorbed on Nickel and Iron
by George Blyholder and Marvin C. Allen Department of Chemistry, University of Arkanaas, Fayetteuille, Arkansaa
(Received J u n e $1, 1966)
Infrared spectra of NO adsorbed on Ni and Fe have been obtained. The bands for the main NO on Ni structure are at 1840, 650, and 625 em.-’. These are assigned as the N-0 stretch, Ni-N stretch, and Ni-N-0 bend, respectively. This assignment gives force constants of 13.0, 5.7, and 0.56 mdynes/A. for these bonds, respectively. A weak band at 2205 em.-‘ is tentatively assigned to an adsorbed NO molecule on a site different from that producing the 1840-cm.-l band. Variations in NO stretching frequencies as the NO is adsorbed on different sites are discussed in terms of a molecular orbital model of the Ni-N-0 system. NO adsorbed on Fe produces two main bands at 1810 and 1720 cm.-l. On both Ni and Fe some of the NO dissociates to produce a surface oxide which gives very broad infrared bands around 460 and 600 cm.-l, respectively. On neither metal did the addition of Hz or 0 2 to the adsorbed NO interact to produce any new surface species.
Introduction In studying the structure of species adsorbed on metal surfaces, a large amount of attention has been focused on chemisorbed carbon monoxide with the result that considerable understanding of the nature of the carbon-metal bond formed upon chemisorption has been The Journal o.f Physical Chemistry
gained, as well as information on the effect of chemisorption upon the Carbon-OxYgen bond. The NO molecule has an electronic structure which greatly resembles co and Yet differs significantly. Thus, NO is a good molecule to use in adsorption studies to see if ideas specifically developed for chemisorbed CO are valid
IKFRARED SPECTRA AND BONDING OF S O ADSORBED ON Ki AND Fe
for other systems and to examine the effect of changing the electronic structure of the adsorbent upon the structure of the adsorbed species while still keeping a relatively similar structure. The molecular orbital view of KO differs from that for CO only in that KO has one electron in an antibonding H orbital that is empty for CO. Thus, for S O which has an excess of two bonding electrons in the u system and four electrons in two H-bonding orbitals, together with the one electron in a n-antibonding orbital, the bond order is 2.5. For CO with the same electron count except for the antibonding H electron the bond order is 3. The view of chemisorbed CO that has been developed in this laboratory considers the H system for a linear 11-C-0 structure.' Variations in C-0 stretching frequencies on a single sample are explained in terms of different electron densities in the H system as the adsorbent metal atom occupies different surface sites. We wish here to examine the ability of this model to account for data for the adsorption of SO on metals. There has been very little work on the adsorption of X O on metals reported in the literature. Terenin and Roe$ have investigated the infrared spectra in the 4000- to 1350-cm.-' region of NO adsorbed on several alumina-supported metals. They have assigned bands to structures described as ionic, covalent, or coordinative on the basis of comparison of the X-0 stretching frequency observed to frequencies for nitrosyl-metal complexes. I n a discussion of the paper by Terenin and Roev, Sachtler, has reported that YO partially dissociates on S i and postulated the existence of both chemisorbed NO and osygen on the basis of mass spectroscopic analysis of the gas phase. Aiso, Dunkel and Hobert4 have reported infrared bands for KO adsorbed on silica-supported Pt and Pd. Infrared spectral studies of gases adsorbed on silicaor alumina-supported metals have the disadvantage that the support limits the spectral range that can be studied to approximately from 4000 to 1360 cm.-'. Drawing structural conclusions from such a limited range has disadvantages and eliminates any possibility of observing bands for metal-nitrogen bonds. A technique which allows observation of the entire infrared spectral range for species adsorbed on metals has been developed in this laboratory and is used in this study.
Experimental Section The wide spectral range experimental technique, which has been described in detail elsewhere,5 consists of evaporating Ki from an electrically heated tungsten filament in the presence of a small pressure of helium. The metal particles formed in the gas phase deposit in
3999
an 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 is obtained. Spectra are recorded before and after admission of the gas to the cell. Five minutes of pumping has been found sufficient to remove all spectra due to gas phase molecules. Some spectra were also run of species adsorbed on silica-supported metals. The experimental technique, cells, and sample preparation have previously been described in detaiL6 Essentially, a nietal nitrate is dispersed on silica (Cab-0-Sil, donated by Godfrey L. Cabot, Inc., Boston, Mass.), pressed into a disk, and the disk is placed in a vacuum cell. The metal nitrate is reduced to metal in a stream of Hz at teniperatures up to 400". This saniple transmits infrared radiation over the range from 4000 to 1300 em. -I. Hs, 0 2 , and CO were obtained from the Matheson Co., Inc. S O was prepared by the reaction of IiS02, KSO,, and Crz03 as given by Ray and Ogg.7 All gases were dried by passage through a cold trap. Hz was passed over hot copper turnings to remove O s ; CO was passed through an activated-charcoal trap cooled with liquid air. Spectra were obtained on a Perkin-Elmar Model 337 and a Perkin-Elmer Model 21 spectrophotometer. The latter is equipped with NaCl and CsBr prisms.
Results The spectrum of S O chemisorbed on S i evaporated into hydrocarbon oil is shown in Figure 1. Bands are recorded a t 2203 (w), 1810 (s), 630 (w), 623 (in), and about 460 cm.-' (svb). The intensity symbols are s, strong, m, medium, w, weak, v, very, and b, broad. I n all cases the spectra shown are those recorded after the gas phase indicated in the figures has been pumped out. The effect of exposure of chemisorbed S O to 10 mm. of O2 pressure is to leave the band at 2203 cni.-I unchanged, reduce the intensity of the 1840-cm.band to about 10% of its original intensity, eliminate the bands a t 6.50 and 625 cm.-', produce absorption in the 1300-1600-~ni.-~range, and increase the intensity of the broad band around 460 cin. -I.
(1) G. Blyholder, J . Phys. Chem., 68, 2772 (1964). (2) A. Terenin and L. Roev, Actes Congr. Intern. Catnlyse, 2.. Paris, 1960, 2183 (1961). H. 'ISachtler, . ibid., 2197 (1961). (3) W.> (4) H. Dunkel and H. Hobert, 2.Chem., 3, 398 (1963). ( 5 ) G. Blyholder, J . Chem. Phys., 36, 2036 (1962). (8) G. Blyholder and L.D. Neff, J . Phys. Chem., 66, 1464 (1962). (7) J. D. Ray and R. A. Ogg, J . Am. C h m . Sac., 78, 5993 (1956).
Volume 69, Number 11 .voaembeT 1966
GEORGE BLYHOLDER AND MARVIN C. ALLEN
4000
I'
1000 c L L
-w
_ _ _,'-. .___- -'., Q ..
$80. 0
e
s
,.
\ .
\
2 2 0 0 2100 2000 1900
-...-.
--
\
-
1800
crn" NO a n d C O on N i
_ c
b __._.Figure c----
2. NO and GO chemisorbed on Ni.
m80 -
When chemisorbed S O on Ni evaporated into a fluorocarbon oil is exposed to 10 mm. of 0 2 , the results are the same as above except that the absorption in the 1300- 1600-cm.-' range is absent. The infrared region below 1300 cm.-' is not observable because of fluorocarbon absorption so the bands in the far-infrared region cannot be checked. Adsorption of NO on silica-supported Ni produced bands at 2220 (w) and 1840 cm." (vs). The band at 2220 was removed by 3 hr. of pumping on the cell a t mm. The addition of 10 mm. of Hz to a sample of NO adsorbed on evaporated S i results in the disappearance of the 2205-cm.-l band. The intensity of the band originally at 1840 cm.-l is reduced to about half of its original value, and the band maximum is shifted to 1800 cm.-l. The intensities of the bands at 625 and 650 cm.-l are cut about in half, and the broad 460-cm.-' band is unchanged. While the band maximum of the 1800-cm.-* band is shifted from 1840 cm.-', the envelope for the 1800-cm.-' band is entirely within the envelope for the original 1840-cm.-' band. The spectrum of NO chemisorbed on nickel in hydrocarbon oil with CO chemisorbed immediately afterward on the same evaporated nickel sample can be seen in Figure 2. Peaks are recorded at 2180 (w), 2021 (s), 190.5 (w), and 1840 cm.-l (vs). Only the spectrum from 4000 to 1200 cm.-' was recorded since this region was the only region of any real interest. The Jozirnal of Physical Chemistry
IC 4 1
P
-201
&Q
2500
2000-,
1150
1500
cm
Figure 3. NO chemisorbed on Fe: ( a ) background, (b) after exposure to 10 mm. of NO for 1 hr., (c) after exposure to 15 mm. of HZfor 1 hr.
The spectrum of NO chemisorbed at 10 mm. pressure on evaporated Fe is shown in Figure 3. Bands are recorded at 1810 (s), 1720 (m), and 600 cm.-' (vb). Addition of 10 mm. of HB resulted in a small decrease in intensity and a shift of about 10 cm.-l to lower frequencies by the two high-frequency bands. Addition of 10 mm. of 0 2 to NO adsorbed on a freshly evaporated Fe sample resulted in the intensities of the 1810- and 1720-cm. -l bands dropping to about one-third
INFRARED SPECTRA AND BONDING OF NO ADSORBED ON Ni AND Fe
2000 1750 crn-' 1500 NO CHEMISORBED ON SILICA SUPPORTED Fe
Figure 4. NO chemisorbed on silica-supported Fe: (a) background, (b) after exposure to 8 111111. of NO for 1 hr.
of their original value while the very broad 600-cm.-' band increased in intensity. No additional bands were detected after addition of CO to a sample with NO chemisorbed on Fe. The spectrum of NO chemisorbed on silica-supported Fe is shown in Figure 4 to have bands a t 1820 and 1720 cm.-', approximately the same as for evaporated Fe. However, the 1820em.-' band has broadened so that the 1720-crn.-' band appears only as a shoulder.
Discussion The results for NO adsorbed on Ni show five bands. The very broad band around 460 em.-' has been found to be characteristic of chemisorbed oxygen.8 This assignment is supported by the fact that the addition of H2 to chemisorbed NO changes the intensity of the other bands while leaving the 460-cm.-' band unchanged, whereas addition of 0 2 enhances the intensity of the 460-cm.-' band. Thus, one cannot obtain chemisorbed NO as the only adsorbate on a metal surface because NO partially oxidizes the surface so that adsorbed NO must coexist with adsorbed oxygen. This substantiates Sachtler's3 assumption that there is oxygen on the surface since he detected ?izin the gas phase after exposing a Xi surface to NO. The results indicate that the weak 2205-crn.-' band, found at 2220 cm.-' on silica-supported Ni, behaves independently of the other bands and so should be assigned to an independent structure. This independence is shown by the facts that oxygen treatment greatly reduces the intensity of the 1840-cm.-' band while leaving the 2205-cm.-' band unchanged and that prolonged pumpj ng on the silica-supported Ni sample removes the 2220-crn.-' band.
4001
The three bands at 1840, 650, and 625 em.-' behave as a unit and so are presumed to belong to a single structure. The hydrogen and oxygen treatments which vary the relative intensities of some bands greatly always make the same relative intendty changes in these three bands. Owing to the similarity in frequency of the 1840-cm.-' band to that of gas phase NO, this band is presumed to be due to an N-0 stretching mode of a chemisorbed species of NO. We turn now to the problem of the structure of this species. In view of the general unsuitability of molecules adsorbed on metal surfaces for X-ray and electron diffraction studies of structure, we shall have to rely on analogies to coordination complexes as a starting point in considering the structure of chemisorbed NO. It is generally assumed that nitrosyls have a linear 34-N-0 group, but few direct experimental investigations have been made. Microwave data indicate that A the Ni-N-0 linkage is linear in T-C~H&S~NO.~ preliminary report of preliminary X-ray work on [(CH&NCSZ]&ONOhas suggested a nonlinear Co-N-0 linkage, but this has not been substantiated.'O I n an electron diffraction study, a linear Co-N-0 linkage was found to fit the experimental data quite well for A . normal coordinate vibrational analCO(C~)~NO ~~ ysis has also been made of this molecule.'2 The K-0 stretch, Co-N stretch, and Co-N-0 bend have been assigned to bands at 1822 (s), 594 (w), and 565 (m), respectively. These assignments lead to a K-0 stretching force constant of 14.0 mdynes/A. !nd a Co-N stretching force constant of 3.87 mdynes/A. Comparison of the three observed bands for NO on Ni at 1840 (s), 650 (w), and 625 cm.-l (m) with the three NO-related bands in CO(CO)~NO shows a strong correlation. On the basis of this comparison and the general expectation from nitrosyl complexes of a linear system, the chemisorbed species is presumed to have a linear M-N-0 linkage. Comparison of both the relative frequencies and intensities of the bending and stretching modes for the adsorbed species to CO(CO)~NOsuggests the assignment of the bands at 1840, 650, and 625 cm.-l to X-0 stretching, Xi-N stretching, and Ni-K-0 bending modes, respectively. The possibility of a bridging NO structure has been (8) G. Blyholder, Proc. Intern. Congr. Catalysis, Srd, Amsterdam, 1964, paper 1-38. (9) A. P. Cox, L. F. Thomas, and J. Sheridan. Nature, 181, 1157 (1958). (10) P. R. H.Olderman and P. G . Ourston, ibid., 178, 1071 (1956). (11) L. D. Brockway and J. S. Anderson, TTan8. Faraday Soc., 3 3 , 1233 (1937). (12) R. S. McDowell, W. D. Horrocks, Jr., and J. T. Yates, J . Chem. Phys., 34, 530 (1961).
Volume 69, Number 11
November 1966
4002
considered. Bridging NO groups are relatively rare, but recently several compounds have been synthesized which are presumed to have bridging NO groups although no X-ray substantiation of the structures has been published. l 3 In these compounds infrared bands for terminal NO groups have been assigned at 1672 cm.-l and for bridging S O groups at 1505 cm.-l. On the basis of these assignments, we have terminal and not bridging NO groups. I n their work Terenin and Roev2 reported a strong band for YO adsorbed on Ki at 1850 cm.-', which corresponds well to our strong band at 1840 crn.-l, and, in addition, four very weak bands in the 1750to 1620-cm.-' region. We observed no bands in this region and can only suggest that their alumina gel support for their Xi had a different influence on the Ni from our silica support and oil bath support. The separation of the observed frequencies from the upper limit of metal lattice frequencies calculated from the Debye characteristic temperature indicates the adsorbed species may be treated approximately as a free three-atom system. This procedure has received formal mathematical justification.'* For a linear three-atom system only three fundamental frequencies are predicted, and we have found three. If the linear structure is correct, we have then found all of the fundamentals. 'Csing the free three-atom model, the force constants for the N-0 bond stretching, Ni-N bond stretching, and Si-K-0 bending are found to be 13.0, 5.7, and 0.56 mdynes/a., respectively. A Huckel molecular orbital calculation, similar to that done' for CO adsorbed on metals, for the Ni-N-0 7-electron system indicates three molecular orbitals in each of the two planes perpendicular to the bonding axis. An electron count shows that the lowest energy orbital, which is bonding for both the N-0 and Ni-N bonds, would be filled. The highest energy orbital, $3. would be empty. The second lowest orbital, $2, which is antibonding for the N-0 bond but bonding for the Xi-X bond, is a little lower in energy than an isolated metal d orbital. The extent of filling of $2 depends on the interaction of metal d orbitals to produce molecular orbitals that can compete with $2 for the available valence-shell electrons and the electron which may be counted as originally in the NO antibonding orbital. This picture has been presented in somewhat more detail' for chemisorbed GO, to which the comparison of KO is obvious. The frequency of the C-0 or N-0 stretch for the adsorbed species decreases as the amount of charge in $z increases. The amount of charge in $2 is expected to be least on high atom density plane surface sites where the number of metal atoms around the metal atom The JournaE o.f Physical Chemistry
GEORGEBLYHOLDER AND MARVIN C. ALLEN
to which the adsorbed molecule is bound is 8 or 9, thus providing much d orbital interaction to give metal molecular orbitals to compete for the electrons which could go into $2. For adsorption on metal atoms a t edges, corners, and dislocations where the number of surrounding metal atoms is 4 to 6, there is expected t o be less competition and hence more electronic charge in $2. This picture successfully explains a number of features of CO adsorption on metals. For NO adsorption the band at 2205 cm.-l is interpreted as adsorption on high atom density plane sites where $2 is relatively empty and an electron which may be counted as originally in the S O antibonding orbital is contributed to essentially metal orbitals. Thus, the 37-0 stretching frequency is quite similar to that of the NO+ ion near 2200 cni.-'. Although other workers have emphasized the ionic nature of NO species giving a band near 2200 ern.-', we favor an essentially molecular orbital view of the bonding. The band at 2205 cm.-l could be due to an adsorbed NO+ ion held to the surface purely by classical ionic attraction. However, the NO+ ion is isoelectronic with CO, which apparently does form a covalent bond with the surface. It seems reasonable to us, therefore, to treat the bonding of the species giving rise to the 2205-cm.-' band, which has presumably from the position of the infrared band lost about one electron to become formally NO+, within the same molecular orbital framework as is used for CO and "neutral" KO. If one makes the unsupported assumption that the extinction coefficients for the species producing the bands at 2205 and 1840 ern.-' are not unequal, the amount of adsorption on the plane faces of the metal is relatively small. The band at 1840 cm.-' is interpreted as a linear Ni-N-0 system in which the frequency at 1840 em.-', being lower than the gas phase NO band a t 1876 cm.-', indicates back donation of metal electrons into $2 in addition to the NO antibonding electron being retained in $2. This interpretation is consistent with the adsorption site being edge, corner, and dislocation site atoms that have a low coordination number and hence compete less effectively for electrons. I n previous work' the infrared spectrum of CO chemisorbed on Ki has been interpreted in terms of a band around 2060 cm.-l representing CO adsorbed on plane sites and a band around 1940 representing GO adsorbed on edges, corners, and dislocations. To examine the consistency of the NO and CO assignments, CO was adsorbed on a sample which had previously been ex(13) R. B. King and M. B. Bisnette, Inorg. Chem., 3 , 791 (1964). (14) T. B. Grimley, Proc. Phys. Soc. (London), 7 9 , 1203 (1962).
INFRARED SPECTRA AND BONDING OF KO ADSORBED ON Ni AND Fe
posed to NO. The result, Figure 2, is bands at 2180 (w), 2020 (s), 1905 (w), and 1840 cm.-' (vs). The bands a t 2180 (slightly shifted from 2205) and 1840 cm.-l for adsorbed KO were of about the same intensity as before, indicating that CO did not displace the adsorbed NO. For CO alone, adsorbed on evaporated Ni, the band in the 1900-cm.-' region is more intense than the band above 2000 cm.-l. The intensity of the CO bands, therefore, indicates that most of the edge, corner, and dislocation sites are covered with NO while most of the plane sites are not covered with NO. This is in accord with our previous interpretation of the adsorbed KO bands. The analogy of adsorbed species to coordinatioil complexes has most usually been made on the basis of stretching frequencies for bonds strictly within the adsorbate. Here, we see that comparison of the physical properties of the metal adsorbate, in this case Ni-N stretching and Si-N-0 bending frequencies and force constants, to complexes, in this case Co(CO)3NO, supports the similarity of the bonding. Using this comparison to look a t the Xi-S bond order, it is noted that metal-nitrogen force constants for a number of ammine complexes are from 1 to 3 mdynes/L.l5 Since this bond must be a single bond, the Si-N bond for adsorbed SO is seen to have considerable doublebond character. Admission of Hz at 10 mm. pressure to a cell a t 20" with XO chemisorbed on S i did not produce any infrared evidence of a new surface species formed by KO and Hz interaction. In fact, the H2 apparently caused the desorption of about half of the adsorbed KO responsible for the 1840-cm.-l band and all of that responsible for the 2205-cm.-l band. Hydrogen is generally believed to chemisorb with donation of electrons to the metal. This is expected to decrease the ability of the S i atoms to accept charge from the NO when it chemisorbs. Apparently, this effect is strong enough that all of the S O on the relatively more saturated plane face sites that give rise to the 2205-cm.-l band desorb and only the more strongly held S O on edge sites remain. The molecular orbital picture of the nature of #2 as an antibonding orbital for the NO bond and a bonding orbital for the nietal-nitrogen bond indicates that the K O which is most tightly held to the surface will have the lowest frequency S-0 stretch. The shift in the band maximum from 1840 to 1800 cm.-l as S O desorbs as a consequence of the HZ treatment is then seen to be a result of the more weakly held KO being desorbed first. Addition of 10 mm. of O2 a t 20" to a cell with chemisorbed NO on Xi does not appear to produce any new species. The only effect of O2 observable in our spectra
4003
is to cause desorption of much of the adsorbed NO and the production of more surface oxide. This is in marked contrast to the behavior of CO adsorbed on Ni, which upon exposure to 0 2 is oxidized to a bidentate carbonate complex.* The absorption in the 13001600-cm.-l region produced by adding O2 to chemisorbed NO is assigned to 0 2 and hydrocarbon oil interaction on the Ni surface since absorption in this region is observed when only 0 2 is added to a freshly evaporated S i sample.* This interpretation is further substantiated by the fact that, when NO adsorbed on Ni evaporated into a fluorocarbon oil is exposed to 02, no absorption in the 1300-1600-~m.-~range is observed. I n order to see if the oil was having an effect on the structure of adsorbed XO species, the spectrum of NO adsorbed on silica-supported Xi was obtained. The only change noted was a small shift of the weak band a t 2205 cm.-l to a slightly higher frequency. Since the intense 1840-cm.-1 band is unchanged, the oil is presumed not t o have a substantial effect on the structure of the adsorbed XO on Ni. The spectrum of XO chemisorbed on Fe shows two main bands a t 1810 and 1720 cm.-', which by analogy to NO adsorbed on Xi are presumed to be due t o linear 11-N-0 systems. The presence of two bands is interpreted, on the basis of the molecular orbital picture of adsorbed NO, to be the result of adsorption on edge, dislocation, and other low coordination number sites t o give the 1720-cm.-l band while the 1810-cm.-' band is due to S O adsorbed on high coordination number plane face sites. The very broad band around 600 cm.-l is taken as indicating dissociation of some of the KO to produce a surface oxide. Oxygen adsorbed on a freshly evaporated Fe sample produces a broad band around 600 cm.-'.16 Since Fe2O3 solid has a broad band near 600 cm.-',17 t,his band for adsorbed oxygen is presumed due to the formation of a surface oxide. NO bands were identified as being due t o the Fe-N stretching or Fe-S-0 bending modes. Presumably, these bands are obscured by the broad oxide band. Exposure of a chemisorbed S O on Fe sample to CO does not produce any bands attributable to adsorbed CO. This is consistent with the interpretation that both the high and low coordination number sites are covered by NO. Neither the addition of H, nor O2to a cell with chemisorbed KO on Fe produced any new surface species (15) K. Nakamoto, "Infrared Spectra of Inorganic and Coordination Compounds," John Wiley and Sons, Inc., New York, N. Y., 1963. (16) G. Blyholder and M. Allen, J . Phys. Chem., in press. (17) E. A. Richardson, Ph.D. Dissertation, University of Arkansas, 1962.
Volume 69,Number 11
November 1966
4004
detectable in our spectra. HZresults in a small shift in the NO stretching frequencies to lower wave numbers which is consistent with the previous interpretation of H2 donating electrons to the metal upon adsorption. The addition of O2resulted in the desorption of much of the N O as indicated by the decrease in the 1810- and 1720-cm.-l bands. 0 2 also as expected increased the intensity of the 6OO-cm.-l band which has been interpreted as a surface oxide. The similarity of the NO stretching bands a t 1820 and 1720 tin.-' for NO adsorbed on silica-supported Fe to the bands for evaporated-into-oil Fe indicates that the oil does not substantially affect the nature of the adsorbed NO. I n view of the previously pointed out problems in obtaining structural information other than infrared, it would be difficult to obtain direct, firm experimental evidence for or against the bands at 2205 and 1840 cm.-l on Ni and a t 1810 and 1720 cm.-l on Fe being, as suggested, due to NO adsorption on different types of sites. This suggestion is largely based on the lack of infrared bands in these regions for other known NO-containing molecules with different structures and the fact that a consistent picture of the electronic structure based on the suggested molecular structure could be built.
The Journal of Phyeical Chemistry
GEORGEBLYHOLDER AND MARVIN C. ALLEN
Over-all, the molecular orbital view seems to furnish a reasonable framework in which to consider the adsorption of NO on Ni and Fe. The finding of infrared frequencies for modes involving primarily the Ni-N bond allowed the nature of the metal-adsorbate bond to be discussed in some detail. Unfortunately, the data for Fe did not turn out to be nearly so complete. If the assumption is made that the u bonds for the Ni-N and Ni-C bonds are equivalent, then the molecular orbital view of the bonding system should furnish a consistent interpretation of the relative values of the frequencies and force constants for the various bonds. Since the X-0 frequency at 1840 cm.-' is less than the C-0 frequency at 1940 cm.-1,8 if the filling of $2 is the dominant factor in this difference, then the Ni-C stretching frequency should be lower than the Ni-N frequency. The Ni-C frequency* at 435 cm.-l and the Ni-N frequency at 650 cm.-' confirm this expectation. As expected, K O produces chemisorbed species quite similar to those of CO and, as hoped for both sets of species, can be set into a single framework.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
