Identification of Adsorbed Species by Infrared Spectrometry

Reasons have been given for choosing a reference electrode with liquid junction. A mixed KCl-KN08 salt solution appears to be more useful than the usual saturated KC1 solution. This is evidenced by the equal representation of N a + and C1- upon changing solvent, a lessening of the apparent IC+ error of the N a + glass, and the representation of Ka+ activity in dextrose solution equal to the ratio of N a + to ITater using two sodium salts. The observed potential change attributed to the liquid junction could be explained by the relative rates of migration of the C1-, K+, and KO3- ions in water and alcohol. The faster (in 1%-ater)chloride ion would orient the dipole a t the liquid interface positive toward the reference electrode. Should junction be made with a liquid such as alcohol, in which the migration rate of C1- is more nearly equal to that of K+ (2), the dipole would be lessened and the reference electrode side would be more negative The slower NO3- ion

would reverse the dipole. Should this explanation be true, the mixture would be critical with temperature (4) and would not remedy other factors such as variation of ions in the unknown solution. The latter is said to be minimized by using concentrated salts in the bridge. The estimation of apparent activity of sodium ions should be a valuable tool in the study of the behavior of sodium ions, their association with anions, their competition for association, and the effect of a change in solvent. Since the electrode system appears to be valid in solutions with lowered water activity, and since the sensitivity is adequate, i t should be useful in blood and other biological systems. ACKNOWLEDGMENT

The author is grateful to Hans Hansen and Samuel Nadler of the Biochemistry Research Department of Touro Infirmary and to Benton B. Owen of Yale University for discussion and criti-

cism helpful in this Tyork. The glass used in this study was kindly supplied by G. Eisenman. LITERATURE CITED

(1) Eisenman, G., Rudin, D. D., Casby, J. U., Science 126, 831 (1957). (2) Harned, H. S., Owen, B. B., "The Physical Chemistry of Electrolytic Solutions," 3rd ed.. u. 232, Reinhold, Sew York, 1958. (3) Zbid., chap. 14. (4)Z b i d , p. 236. (5) Leonard, J. E., "The ;Inalyzer," 1'01. 1, KO.4, Beckman Scientific and Process Instruments Division, October 1960. (6) Neuhausen, B. S., Marshall, E. K., Jr., J. Biol. Chem. 53, 365 (1922). (7) Ringer, W. E., 2. physiol. Chem. Hoppe-Seylers 130, 275 (1923).

(8) Scatchard, G., Scheinberg, I. Armstrong, S. H., J . An. C h e m 72, 535 (1950). (9) Wingfield, B., Acree, S. F., J . search A'atl. Bur. Standards 19, (1937).

H.,

SOC.

Re-

163

RECEIVEDfor review August 4, 1961. Bccepted January 2, 1962. Work supported by a grant from the John A. Hartford Foundation.

Identification of Adsorbed Species by Infrared Spectrometry Thio phene-Co ba It Moly bd a te-Moly

bden u m Sulfide Systems

DAN E. NICHOLSON' Humble Oil & Refining Co., Baytown, Tex.

b Infrared spectrometry has been applied to the identification and semiquantitative analysis of several surface species that form when thiophene is adsorbed on desulfurization catalysts. Infrared absorption bands attributable to both two-point and four- or fivepoint structures of thiophene were found, giving a total of three optically distinguishable surface species. The vibrational modes noted correspond to intermediates created through the reaction of one double and two double bonds with the surface. The identifications and semiquantitative analyses derived from infrared spectra were based principally upon the carbonhydrogen stretching modes observed between 31 25 and 2899 cm.-'

I

SPECTROMETRY is being applied successfully in the identification and semiquantitative analysis of structures formed when thiophene reacts with the surface of experimental desulfurization catalysts. Preliminary NFRARED

370

ANALYTICAL CHEMISTRY

research in this area was reported earlier (3). In the current study major emphasis was focused on chemisorbed structures, since the latter are of primary significance in exploratory refining research. EXPERIMENTAL

The primary experimental problem is the preparation of the sample for spectroscopic analysis. The basic essential is introducing a sufficient concentration of the surface species into the light path of the infrared spectrometer to yield absorption bands detectable above the background noise. The noise level is, generally, very high in these studies. Sample densities between approximately 10 and 20 mg. per sq. cm. were satisfactory in most instances. The best specimens, as judged from the criterion of minimum noise level for a given peak height, were prepared employing a combination of sedimentation techniques and spraying. After each spraying the catalyst coating deposited from a slurry of isopropyl alcohol and water on a

calcium fluoride plate was allowed to approach dryness before the next series of coatings was applied. This step in sample preparation was very important to prevent flaking and to secure uniform deposits. The calcium fluoride plates coated with catalyst were dried in a specially constructed infrared cell under vacuum for 16 hours at 110" C. The dried catalysts were then reduced a t 400" to 500" C. in a n atmosphere composed of 98% hydrogen and 2% hydrogen sulfide for 8 to 16 hours or until no further weight changes were observed by means of the quartz-spring balance system used with the infrared cell to denote weight changes in the catalyst during the experiments. At the conclusion of the reduction period the temperature was lowered to 300"to 325" C. and the excess adsorbed hydrogen and hydrogen sulfide removed by evacuation for about 15 minutes. Spectra were recorded using a Beckman IR-7 and a Perkin-Elmer Model 112 infrared spectrometer. The latter 1 Present address, Loa Alamos Scientific Laboratory, Los Alamos, N. M.

Table 1.

Infrared Spectrum of Thiophene

Band Position Cm.-' Vapor Liquid

8

,.:. ...~

, ~

liz?LE..:",

Figure 1.

'---U-

CII-'

s:,

Thiophene(1)

(NBS 99.99%)

instrument mas modified in a manner similar to that used by Eischens and collaborators ( 1 ) and others (2, 4, 6). Pressed disks were prepared in some supplementary comparison experiments for determining whether or not the sulfides hydrolyzed during the sedimentation procedure. Identical infrared spectra were obtained for a given catalyst irrespective of whether the sedimentation or pressed-disk technique was employed. However, when pressures approaching 100,000 p.s.i. were applied to the die in disk construction, some of the solid samples became impermeable to penetration by gaseous thiophene under the conditions of the investigation. The surface areas of the samples of molybdenum disulfide were 50 sq.

n I ri x i

Figure

605 605 710 710 836 836 872 870 909 904 ~~. . . 1035 1035 1077 1080 1252 1255 ... 1290 1405 1405 .. . 1479 .. . 1565 ... 1590 3078 3110 3108

Assignment" 1 22 3. 24 10 11 2

~

~~

4 14 12 6

Combination Combination 15

Resonance 8 and 17 Overtone-third Harmonic of 12 a See discussion in (6) for explanation of the vibrational modes referred to here.

meters per gram. Gravimetric absorption data obtained simultaneously with the infrared studies showed that the surface coverage was approximately one tenth of a monolayer or less in all of the studies reported here. These data indicated that the type of bonding of thiophene to the catalyst surface was essentially independent of the extent of surface coverage when taken with the spectra to be discussed below. ?With sparse coverage of the surface there might be four-point coverage, changing to two-point coverage as the surface became more crowded under isothermal conditions. DISCUSSION AND RESULTS

Figure 3.

Goin:

Certain new types of adsorbed species have now been identified. Infrared bands attributable to both two-point and four-point- structures have been observed, giving a total of three discrete forms of the surface species. The vibrational modes noted correspond to intermediates formed through the reaction of one double bond of thiophene and of two double bonds with the catalyst surface,

Thiacyclopentane(1)

TO%

Period: 32

5855.

I r $ r m e n ? : Eechmon IR-7.

Y I 3300

I

330

I 3200

I

3150

I

I

I

1100 3030 3333 F E E O J E I L I , CM.1

Figure 4.

I 3131

!

3129

233: iFE3tL'C".

I

2853

I 2933

Thiocyclopentane

/

2810

I

2822

CY

1 -..

&--

-'

Figure 5. Thiophene adsorbed on molybdenum SUIfide with cobalt (Mo/Co

= 20/1)

VOL. 34, NO. 3, MARCH 1962

a

371

' c . Thiophene a t 7 M m . , ofter Heating. I I

I 3134

Figure 6. disulfide

I

I 3029 FREOJENCY, C M - '

3134

2934

Thiophene adsorbed

3029

I

I

2934

2841

FREOUENCY, C M - '

2841

Figure 7. Thiophene adsorbed on molybdenum disulfide with cobalt

on molybdenum

(MofCo = 67/11

A summary of the infrared spectrum of gaseous and liquid thiophene from earlier investigations (2) is presented in Table I. I n addition to the infrared active fundamental modes described there, Raman bands are observed a t 375, 435, 565, 686, and 748 ern.-' In conjunction with the present studies, the infrared spectrum of thiophene was recorded as shown in Figures 1 and 2 in solution in carbon disulfide and in the gaseous state. Similar spectra of thiacyclopentane are given in Figures 3 and 4. The molecular motions responsible for the absorption spectra of thiophene near 3100 cm.-l can be represented approximately by the scheme below.

The sulfur atom ( 0 ) is not involved in the motions described above; hence, it would be expected that thiophene adsorbed through one-point attachment would exhibit spectra essentially identical with those of pure gaseous or liquid toluene in the carbon-hydrogen stretching region. The identifications and semiquantitative analyses based on infrared spectrometry in the current study are based principally 1 pon the stretching modes between 3175 to 2900 em.-' From a priori considerations i t could be expected that thiophene might exist in a minimum of three forms on a surface.

H

n

H

I

1

3134

3029 FREQUENCY, CM-'

l

S

I

I

2934

264

Figure 8. Thiophene adsorbed on supported molybdenum disulfide (420s)

372

ANALYTICAL CHEMISTRY

q

H

H

(3) (1) One-Point Form ( 2 ) Two-Point or Side Form (3) Four-Point or Flat Form

I n addition to these three species, the possibility of physically held entities exists and structures similar to the twoand four-point forms in which there is a sulfur-surface bond. The distinction between the two-point form and a three-point structure could be made by noting the appearance of a sudacesulfur mode. The latter would be active in the far-infrared between about 400 to 500 em.-' Infrared spectra of thiophene adsorbed on a series of molybdenum

3C29 'FLCUE'.CY.

2934

2841

CM-'

Figure 9. Infrared spectrum of thiophene adsorbed on cobalt molybdate

Table 11. Summary of Adsorbed Species in Thiophene-Cobalt Molybdate-Molybdenum Sulfide Systems ( a ) Band Po-

sition, Cm.-1

Structural Type

-

a,%

3086

l

Mid

1 3 ' 34

I 7-23

__

.-..-:LdE.,tv,

I

I

2334

2eLl

or gas or physically absorbedwhich would exhibit essentially the same infrared spectrum aa liquid thiophene

:y

Figure 10. Thiophene adsorbed on cobalt molybdate saturated with hydrogen

3005

disulfide catalysts are reproduced in Figures 5 through 8. The influence of the support, gamma alumina, in changing the structure of adsorbed thiophene becomes apparent from Figure 8 and the comparisons are summarized in Table 11. The identifications by infrared spectroscopy were based on spectra recorded under a standard set of experimental conditions. These parameters involved uniform pretreating steps, degassing, and reaction of thiophene with the surface a t 375' to 400" C. for 1 minute. The infrared cell waa cooled rapidly to room temperature for recording spectra. One sample of molybdenum disulfide (Figure 5 ) contained absorption bands near 3086, 3005, and 2960 em.-' ascribable to thiophene attached to the solid through the sulfur atom (3086 crn.-I) or physically adsorbed, the two-point structure (3005 crn.-I) and the four-point or flat form (2960 cm.-l). A higher concentration of the two-point species than of the flat species was found. Samples of pure molybdenum disulfide, BS shown in Figure 6, convert thiophene primarily to the four-point species, al-

1

j

0. cell a t

i

1

3134

3000 C. w i t h 9 M m . Thiophene

though perceptible amounts of the sidetype configuration are indicated. The catalyst having a molybdenum to cobalt ratio of 67 to 1 forms appreciable amounts of both the flat and edgewise forms of surface species of thiophene. In the case of the supported samples, there is an increase in the concentration of the flat form relative to that of the two-point form attributable to the influence of gamma alumina. Analysis of the desorbed gas confirmed that only thiophene was evolved. The desorption was carried out by pumping for approximately 5 minutes a t room temperature and collecting the gas evolved from the infrared cell in a cold trap. Gravimetric adsorption measurements showed that a t least 99.9% of the adsorbate Eas removed. The evolved gas was analyzed by infrared and mass spectrometry. The infrared spectrum of the catalyst recorded after the desorption step was identical with the background spectrum of the same catalyst recorded prior to the initial adsorption of thiophene. This background spectrum of the catalyst after desorption coupled with the gravimetric balance readings prored that polymers or de-

?