450
1.E. DENBESTEX;, P. G, FOX, AND P. 8’. ~ S L W O O P
indeed Characteristic for this type of electronic transition. This also may lead to an unequivocal
Vol. 66
criterion for the identification of C.T,T.S. spectra.
Such work in being carried out in our Department.
THE MECHANISRI: OF CHEMISORPTION: CARBON MONOXIDE AND CARBOX DIOXIDE O S NCKEL BY I. E. DENBESTEN,P. G. Fox,
AKD
P. W. SELVOOD
Chemical Laboratorg of Northwestern University, Evanston, Ill. Reeeaved September 11, 1961
Magnetization-volume isotherms on nickel-silica have been obtained for carbon monoxide and carbon dioxide by the low freuuency a x . permeameter method. The mode of bonding of carbon monoxide is dependent on nickel particle size and on surface coverage, in qualitative agreement with the infrared absorption studies by Yates and Garland. The magnetic data suggest linear or bridged bonding at low coverages, depending on nickel particle size, and bridged bonding to nickel atoms already bonded to at least one carbon monoxide molecule at higher coverages. Most of the high-coverage carbon monoxide may be desorbed as such, but the low coverage carbon monoxide may not be desorbed without disproportionation. Carbon dioxide is chemisorbed on nickel, There appear to be rather more than two bonds formed for every molecule adsorbed, but the maximum volume which may be chemisorbed at room temperature is only about one-eighth the volume of hydrogen which may be chemisorbed on the same surface.
Introduction The purpose of this work was to extend to carbon monoxide and carbon dioxide the method of magnetization-volume isotherms previously applied to hydrogen,l ethylene,2 ~ x y g e nbenzene4 ,~ and other adsorbates. The theory of the method already has been de~cribed.~ Experimental A11 magnetic measurements were made on the low frequency a.c. permeameter previously described.’ The adsorbent was either Universal Oil Products (UOP) nickel-kieselguhr hydrogenation catalyst containing about 52% nickel, or a coprecipitated nickel-silica containing about 30% nickel and prepared as described by Van Eyk Van Voorthuisjen and Franzen,e and designated by them CLA 5421. Both of these catalysts had been used extensively in this Laboratory in previous investigations. The samples were reduced in flowing purified hydrogen for 15 to 24 hr. a t 350-370’, and then were evacuated at the same temperature for 3 to 4 hr., and cooled in vacuo. Commercial tank carbon monoxide was purified by passage over hot glass beads and then through a Dry Ice trap. Tank carbon dioxide was found to be sufficiently pure for the purposes of this work and was used without further purification. Gas analyses were made as required on a Consolidated Electrodynamics Model 21-611 mass spectrometer.
Results Carbon Monoxide.-Magnetization-volume isotherms for carbon monoxide and for hydrogen a t -78” are shown in Fig. 1. The corresponding pressure-volume isotherms also are shown. It is well known that a nickel surface will sorb a volume of carbon monoxide considerably greater than the maximum volume of hydrogen which may be taken up by the same surface. This effect and the corresponding magnetic changes a t 25” are shown in Fig. 2.
Figure 3 shows a magnetization-volume isotherm for carbon monoxide as compared with that of hydrogen at 24” on the coprecipitated catalyst. (1) P. W.Selwood, J . Am. Chem. Soe., 78, 3893 (1956). (2) P. W. Selwood, ibzd., 83, 2863 (1961). (3) R. J. Leak and P.W. Selvood, J . Phys. Chem., 64, 1114 (1960). (4) J. A. Silvent and P. W. Betwood, J . Am. Chem. Sot., 83, 1033 (1961). (5) P. mi. Selwood. “Actes du DeuxiAme Congrbs International de CataIyse,” Paris, 1960, p. 1795. (6) J. J. B. Van Eyk Van Voorthuisjsn and P. Branzsn, Rec. trau. chim. 70,793(1951).
Figure 4 gives similar data for a COP catalyst sample which had been sintered a t 600” for 2 hr. to increase the nickel particle size. It will be noted that the magnetizatioiz-volume isotherms in Fig. 2-4 suffer a rather abrupt change of slope as the volume adsorbed exceeds that corresponding to surface saturation by hydrogen. It will be convenient to refer to that part of the isotherm prior to the change of slope as the “lowcoverage” part, and to that beyond the change of slope as the “high-coverage” part. At low coverage the fraction of carbon monoxide which may be removed by evacuation a t room temperature did not exceed 5% of the whole. Removal of this fraction was accompanied by a corresponding slight increase of magnetization. At high coverage the situation was quite different. Evacuation at room temperature removes as such a substantial fraction of the carbon monoxide originally adsorbed beyond the change of slope. Raising the temperature during the evacuation ultimately mill remove most of this carbon monoxide, although some of it comes off as carbon dioxide formed, presumably, by disproportionation. An example of these changes is shown in Table I. TABLE I C.4RBON MGSOXIDEON NICKELAT 25” 29.4 cc. per g. Ni Total volume of CO adsorbed “High-coverage” volume 16 2 5 0 (95-98% CO) Volume desorbed at 25” 9 5 (92-9570 CO)5 Volume desorbed at 150” a More gas may be removed by raising the temperature to 360’ but this is about 90% carbon dioxide.
\‘OLUMES
DESORBABLE AFTER CHEMISORPTIGh G F
If the evacuation is continued up to 350” it is possible to remove more carbon in the form of carbon dioxide, together with some hydrogen. and the magnetization as measured at room temperature rises to 90% of its initial value. But the various reactions which occur at elevated temperatures, including disproportionation, carbidiiig, thermal decomposition of carbide, and reaction with residual water in the silica, are related more to the bulk metal than to its surface.
March, 1962
CHEMISOKPTION OF CARBON MONOXIDE AND CARBON DIOXIDEON SICKEL 45 1
90.
BO -
.. .?O
-
I' z
.so -
50
5 I5'
40--'C
'
20
2%
30
I
35
vo1. ads cc /g. Ni ISCI.
Fig. 3.-Isotherms
on a coprecipitated nickel-silica a t 24'.
Fig. 1.-Magnetization-volume isotherms for carbon monoxide and for hydrogen at -78" on UOP nickel-lrieselguhr. Pressure-volume isotherms also are shown.
4: 400
.96
300
- 600
-5 94
2
200 a
- 500
92 100
- 4005
.90
E
- 300 E.
V o l a d s , cc l q NI (SCl
Fig. 4.--Isotherms
on a sintered UOP nickel-kieselguhr at 24'.
- 200 100
Vol Cd'ods
Fig, 2.-Isot,herms
,cc 1pNi (SC) 25
30
over a inore extended pressure range a t 25".
Carbon Dioxide.--A magnetization-volume isotherm for carbon dioxide on UOP catalyst a t 25" is shown in Fig. 5 . It may be expected that carbon dioxide would exhibit a fairly large van der Waal's adsorption on a high area (-175 m.2/g., BET Ns) catalyst a t room temperature. The magnetic data show that this is the case. If, after the pressure had reached 237 mm. (corresponding to the maximum volume shown in Fig. 5 ) the sample was evacuated, it was found that about 80% of the carbon dioxide could be desorbed as such without change of magnetization. The final residue was then approximately 2 cc. (sc) of chemisorbed carbon dioxide per g. of Ni and this caused a 6.0% loss of magnetization. This is moderately larger than the fractioiial loss caused by the same volume of hydrogen. I t will be noted that the maximum volume of carbon dioxide which may be chemisorbed on nickel under these conditions is only about oneeighth of the volume of hydrogen which may be chemisorbetl.7 Discussion of Results There have been numerous suggestions concerning the mechanism of carbon monoxide chemisorption on nickel. Some of the earlier work is reviewed by Gundry and Tompkins.8 The work most nearly
related to that described here is the recent paper of Yates and Garland,g who observed the infrared absorption of carbon monoxide adsorbed on alumina-supported nickel and, in part, on silica-supported nickel. Yates and Garland identified several bands as probably being associated with the structures shown belon?, and being formed on nickel crystallites of several sizes, the largest being called "crystalline" and estimated by X-ray line width broadening as being 38 8.in diameter. Yates and Garland show that adsorption on crystalline sites occurs at very low pressures, adsorption on semi-crystalline and on dispersed sites
(7) This result is eonfirmed by heats of adsorption studies made b y Trapnell. (Personal communication from B. M. W.Trapnell.)
(8) P. M. Gundry and E. C . Tompkins, Quart. Revs., 14, 257 (1960). (9) J. T. Yates and C . W. Garland, J . Phys. Chem., 66, 617 (1961).
2
4
6
e
Vol ads., c c . / g sample (SC).
Fig. 5.-Magnetization-volume isotherm for carbon dioxide a t 25' on UOP nickel-kieselguhr.
I. E.DENBESTEN,P, C. Fox, AND P. W, BELWOQD
452 Band
Wave number, om.-&
A
1915
Struoture
Vo1. 66
Kind of Ni
Adsorption type
cryst.
very strong
cryst.
very strong
cryst.
very strong
semi-cryst. dispersed
mod. strong weak
N1/ \Ni
c
2035
B
1963
D E
2057
pU’i-C-0 0
0
\ /\ / Ni
2082
Ni
Ni-C=O Ni-C=O
occurs a t higher pressures and this carbon monoxide is weakly bonded. The tentative structure assigned to band B is the most interesting for our present purposes. Band B forms on samples containing higher concentrations of nickel, a t carbon monoxide pressures of several millimeters, and after bands A and C have been formed. The conclusions then are that the mechanism of chemisorption is dependent on particle size, that the mechanism is pressure-dependent (and hence dependent on surface coverage) and that there is evidence for a bridged mode of adsorption forming between nickel atoms which already are involved in the linear mode (bands C, D or a). The magnetic data presented in this paper show that the mechanism of chemisorption indeed is dependent on nickel particle size and on surface coverage. On the UOP sample a t -78” and on the coprecipitated sample a t room temperature (wi$h nickel particle diameters of -42 and -25 A,, respectively)1° the initial slope of the magnetization-volume isotherm is almost exactly half that shown by molecular hydrogen. This shows that the carbon monoxide molecule is, under these conditions, attached to one, and only one, nickel atom. It appears from this that the mode of adsorption is linear, corresponding to the Yates and Garland band D, or perhaps to C. Over this region the adsorption bond must be fairly strong because little carbon monoxide may be evacuated at room temperature. If surf ace coverage is increased the magnetization-volume isotherm changes slope rather abruptly. The slope becomes much more nearly parallel to the volume axis and shows that each carbon monoxide molecule is attached on the average to about one-sixth of a nickel atom. Rut it will be noted that the formation of a bridged system (band B) on two nickel atoms already involved in a linear mode (bands C, D or E) would cause no further loss of magnetization. A trifling continued linear adsorption on less accessible sites would account for the slope of the isotherm in this region being slightly greater than zero. A further correlation with the Yates and Garland view is found in Fig. 4. The sintered UOP sample contains nickel particles of about 64 A. diameter,’O and while these are, a t the upper limit of applicability for the low frequency a.c. permeameter method the results apparently show that each carbon monoxide molecule a t low coverage is involved with two nickel atoms. This result is (10) R.
E. Diete and P. W. Gelwood, J . Chem. Phys., 86. 270 (1961).
consistent with the Yates and Garland interpretation of bridged molecules (band A) on “crystalline” nickel a t low coverage and linear molecules (band D) a t higher coverage. If in connection with the above considerations we recall that the ease with which carbon monoxide may be desorbed is markedly greater a t the higher surface coverages, then it appears that carbon monoxide is most readily desorbed from a nickel atom which is attached to more than one such molecule. On the other hand, desorption from nickel atoms attached to only one carbon monoxide molecule is not only difficult but likely to be attended by disproportionation. It might be thought that the carbon dioxide formed by disproportionation would be chemisorbed and then would contribute to the change of magnetization. While this doubtless is true it may be seen from Fig. 5 that the volume of carbon dioxide chemisorbed is quite small in comparison with the volumes of carbon monoxide with which we are concerned. The conclusions which may be reached from the magnetic data alone are: (1) The initial stages of carbon monoxide chemisorption may be linear or bridged and this depends to a degree on the nickel particle size. (2) At higher surface coverages there is an abrupt change in the mode of adsorption. This change is consistent with the view that carbon monoxide may be added to nickel atoms which already are involved with a t least one carbon monoxide molecule. (3) Desorption of carbon monoxide as such is readily observed at high coverages where, presumably, nickel atoms are attached to two or more molecules. (4) Desorption a t low coverages, which involves disproportionatlion, is presumably from nickel atoms attached to only one carbon monoxide molecule. It may be thought that nickel tetracarbonyl formation at the higher pressures of carbon monoxide would be a significant complicating factor, but the rate of formation at room temperature as reported by Yates and Garland seems too low to interfere seriously with the interpretations presented. The conclusions reached with respect to carbon monoxide are in gratifying qualitative agreement with those of Yates and Garland. It is doubtful if quantitative agreement could be expected for such a complicated system unless magnetic and infrared measurements were made simultaneously on the same sample. The chemisorption of carbon dioxide has, in contrast to that of carbon monoxide, received
March, 19862
FLASH-EXCITATION OF ACRIDINE ORANGE IN ACIDICAND BASICSOLVENTS
comparatively little attention. Kokes and Emmett'l have compared the ability of nickel to adsorb carbon dioxide a t -78" before and after evacuation a t room temperature. The results are interpreted to show that carbon dioxide is chemisorbed a t -78" to the extent of 80% of a nitrogen monolayer, Eischens and PliskinlZ have reported a strong infrared band a t 6.4 p and a weaker one at 7.1 for carbon dioxide adsorbed on nickel-silica a t room temperature and a pressure of 1.2 mm. These bands are interpreted as being characteristic of the carboxylate ion, thus
\do
0
I
Ni
The magnetic data show that the volume of carbon dioxide chemisorbed on nickel-silica at room temperature is quite small, although comparable with the volume of ethane chemisorbed under identical conditions.2 The loss of magnetization (11) R. J. Kokes and P. H. Emmett, J . A m . Chem. Soc., 82. 1037 (1960). (12) R. P. Eischens and W. A. Pliskin, Advances in Catalysis,9, 662 (1957).
453
is such as to suggest the involvement of a t least two (and possibly more) nickel atoms per molecule of carbon dioxide. A possible mode of adsorption is then 0 \
c-0
I
Ni
ki
although this is not in agreement with the available infrared results, and it does not account for the very low maximum coverage possible. The possibility that some of the magnetization loss could be due to polarization as observed for krypton is quite unlikely. The effect on the magnetization of a molecule of carbon dioxide is a t least six times greater than that of a molecule of krypton. The precise mode of adsorption and the reason for the low coverage remain obscure. Acknowledgment.-It is a pleasure to acknowledge support from the Office of Naval Research, the Petroleum Research Fund of the American Chemical Society, E. I. du Poiit de Nemours and Company, and the Abbott Foundation in connection with this work.
FLASH-EXCITATION OF ACRIDINE ORANGE I N ACIDIC AND BASIC SOLVENTS' BY G. BLAUER AND H. LINSCHITZ Department of Chemistry, Brandeis University, Waltham, Mass. Received September 1 1 , 1061
The flash-excitation of acridine orange in degassed basic (triethylamine) or acidic (acetic acid) solvents a t room temperature leads in each case to two types of transient intermediates. The shorter-lived of these is attributed to triplet states on the basis of similarities between the initial flash spectrum and that observed in rigid solvents on cross-illumination. By analogy to previous flash work on fluorescein, the longer-lived products are assumed to be radicals.
The method of flash-excitation is being widely main sharp band was collected, concentrated, and cooled to -20". The resulting precipitate was filtered and dried in used, at present, to study photochemical processes air followed by 2 hr. invacuo a t 70" (m.p. 181-182'). in aromatic molecules.2 I n solutions of sensitizing Triethylamine (TEA) (Eastman-Kodalr, white label) dyes, metnstable triplet states and free radicals was fractionated over potassium hydroxide (boiling range have been observed, particularly by Lindqvist in 0.20). reagents were analytical grade. his carefull and thorough studies on flu~rescein.~ Other Method.-The flash-excitation apparatus, general proI n this paper we present absorption spectra of the cedure and evaluation of data were similar to those deinitial excitation products of acridine orange (AO) scribed previously.' It was found that the freezin thawin acidic and basic media, as well as evidence for the ing, and evaporation operations used in degassing f0 solureversible formation of active intermediates with tions resulted in a loss of absorption even when carried out dim light. The reason for this effect (amounting to sevlonger lifetimes. While the small absorbance in eral per cent.) could not be ascertained. Accordingly, changes associated with these latter substances limit solutions were prepared by dissolving the dry dye in a sidetheir detailed study, the initial photo-products are arm of the sealed-off absorption cell assembly, after all well characterized and the general behavior of the degassin operations had been completed on the pure soldncentrations were determined spectrophotometrisystem is shown to be consistent with that of other vent. cally. Beer's law was verified for A 0 in TEA over the sensitizing dyes. concentration range 1 X 1 0 - 6 to 8 X M (decadic molar extinction coefficient E a t 420 mp = 2.4 X 104) and in acetic Experimental acid over the range 4 X 10-7 to 2 X 10-6M ( E at 493 mp =
3,6-Bisdimethyl-aminoacridine (acridine orange, Allied Chemical and Dye Corp .) was purified by precipitating the free base with dilute sodium hydroxide, dissolving in chloroform (A.R.), and chromatographing on a l ~ m i n a . ~The (1) This w01.k wa8 supported by a grant from the U. 9. Atomic Energy Commission to Brandeis University (AT-(30-1)-2003). (2) J. P.Simons, Quart. Revs. (London), 13, 3 (1959). (3) L. Lindqvist, Arkir Kemi, 16, 79 (1960). (4) V. Zanker, Z. physik. Cham.. 199, 225 (1962).
6.7 x 104). Except for minor solvent shifts, the ground state spectrum of A 0 in acetic acid and in pyridine containing aqueous HC1 was of the same t pe as that given for aqueous solutions of the dye at low p€f(main peak in the visible range near 490 mp). Similarly, the spectrum of A 0 in TEA, in pyridine and toluene containing some TEA, and in ethanol contain(6) H. Linsohita snd K. Sarkanen, J. Am. Chem. SOC.,80, 4826 (1968).
