Inelastic neutron scattering study of ethylene adsorbed on silver — A zeolite

fP/AfP_I:.tRS Inelastic neutron scattering study of ethylene adsorbed on silver- A zeolite Joseph Howard, Keith Robson, Thomas C. Waddington (the late) and Zahrah A. Kadir

Chemistry Department, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK Inelastic neutron scattering (INS) spectra have been measured of C2H 4 and C2D 4 adsorbed, at two different overpressures, onto fully silver-exchanged Linde type A zeolite. The lowest frequency intramolecular modes of the adsorbed gas, and some of the vibrations of the adsorbed gas relative to the zeolite surface, have been observed. Assignment of the latter vibrations has been made on the basis of observed deuteration shifts and by comparison with inelastic neutron scattering spectra of some ethylene-containing organometallic complexes. In contrast with published X-ray crystallographic data we have established the existence of two different sites for the adsorbed ethylene. Some of the conclusions from the INS data have been corroborated by high frequency infrared studies. Keywords: Spectroscopy; neutron scattering; ethylene adsorption; ethylene vibration; silver-A zeolite; cation sites

INTRODUCTION This study of ethylene adsorbed onto silverexchanged Linde type A zeolite (Ag12-A) follows naturally from our work with ethylene adsorbed onto silver-exchanged type 13-X zeolitea. Such studies are designed to investigate the forces between a surface and an adsorbed gas. Force constants and barriers to rotation of the adsorbed species, which in principle can be calculated from our measurements, provide a sensitive test of models of the adsorbate-adsorbent interactions. Although zeolites are chemically more complex than some other adsorbents, they do possess the advantages of high surface area and adsorption sites which are usually well defined. Furthermore, the strength of the zeolite-adsorbed molecule interaction can be altered in a variety of ways including ion-exchange and modification of the Si to A1 ratio 2. It is possible, therefore, to study the interaction of a given adsorbate with a series of different cations, each held within the same framework. It has generally proved difficult, and in many eases impossible, to obtain the Raman spectra of zeolites, and particularly of zeolites containing adsorbed species. Zeolites themselves give rise to very weak Raman scattering and their fluorescence background is often high 3. Because the presence of even small amounts of transition metal ions usually leads to unacceptably high levels of fluorescence 3,4, successful investigations have been mostly confined to alkali and alkaline earth metal exchanged zeolites. Similarly, whilst many near-infrared studies of zeolites, and zeolites containing adsorbed species, have been published, greater difficulty has been experienced with far-infrared studies s. Apart from the intrinsic experimental 0144--2449/82/010002-11 $03.00 © 1982 Butterworth & Co (Publishers) Ltd

2 ZEOLITES, 1982, Vol 2, January

difficulties, the low frequency normal modes of the adsorbed species appear to yield bands which are of, at most, very low intensity relative to the vibrations of the framework. In contrast with Raman and far-infrared techniques, inelastic neutron scattering spectroscopy is well suited to the study of the low frequency vibrations of hydrogeneous adsorbed species. This is a result of the incoherent neutron scattering cross-section of hydrogen being very large in comparison with that of the anhydrous ion-exchanged zeolite substrate. Thus the aluminosilicate lattice of the zeolite is relatively transparent to neutrons and the vibrations observed are those of the adsorbed species. The structure of Ag12-A and in particular the structural changes which occur on dehydration are the subject of some controversy. Because of its importance to this work the structural information available will be reviewed here. Linde type 4 - A zeolite (Na12-A) 6-1° possesses a pseudo unit cell of composition Na12[(A102)12(SiO2) 12]" 27H20, with cubic symmetry. The description 'pseudo unit cell' applies only to the idealized high symmetry structure (Pm3m) which results if framework silicon and aluminium atoms are n o t differentiated 2. In fact, the 'real structure' contains an ordered arrangement of strictly alternating SiO 4 and A104 tetrahedra, which causes the cubic cell constant to be doubled and the symmetry to be lowered to Fm3c. It follows, therefore, that the 'real' unit cell of type A zeolite contains eight formula units of composition Na12[Al12Sia204s ] • 27H20. In c o m m o n with most authors, however, we will describe type A zeolite structures using the concept of the pseudo unit cell.

INS study o f ethylene sorbed in silver-A zeolite: Joseph Howard et al.

The sodalite unit6, ~ is a truncated octahedron or '14-hedron' (8 hexagonal faces and 6 square faces) formed by 24 (Si, A1) atoms present at its vertices linked by 36 oxygen atoms. In Na12-A zeolite, the sodalite units are joined together by four bridging oxygen atoms across square faces of the truncated octahedra. An almost spherical internal cavity 11.4 A in diameter is thereby formed known as an '0~' cage (or 26-hedron of Type 111).This m a y be entered through six approximately circular windows 4.2 A in diameter, the circumferences of which contain eight oxygen atoms and are hence known as '8-rings'. A second set of voids ({3 cages; 6.6 A in diameter) is contained within the sodalite units themselves. These interconnect with the ~ cages though distorted '6-rings' of oxygen atoms 2.2 A in diameter. There are therefore two interconnecting pore systems, one of diameter 11.4 A with 4.2 A constrictions, and the other of alternate 11.4 A and 6.6 A cavities separated by 2.2 A constrictions. The possible cation positions within type A zeolites (based upon the classification system of Barrer 11) are shown in Table 1. Almost total replacement of sodium ions, in Linde type 4 - A (Naa2-A) zeolite, by silver ions may be accomplished 1°. X-ray single crystal determinations of fully hydrated silver type A zeolite (Ag12-A) 12,13 (unit cell = 12.288 A) have shown the cations to be distributed over three positions within the framework (Table 1). Eight silver ions were located on three-fold axes near 6-rings; of these, five were recessed into the large (~) cavity ($2"), each coordinated to at least one water molecule, and the three remaining were displaced into the sodalite (~J) cage ($2') with three water molecules bridging them. A further three silver ions were associated with 8-rings, but off-centre (making closest approach to only three oxygen atoms) and displaced into the o~ cage ($1"). The final cation was found within the large (0~) cavity opposite a 4-ring ($3). As a result of the low overall symmetry of the silver ions in the o~ cavity many different coordination sites for water exist. These have low

occupancies, and hence no definite positions or occupancies could be determined. Dehydration under vacuum causes Ag12-A zeolite to change colour from pale grey to deep orange. This is a reversible process: Ag12-A becomes bright yellow if allowed to cool in air, and ultimately returns to its initial pale grey state. Matsumoto et al. a4 have concluded that Ag12-A is structurally less stable with respect to temperature than Na12-A zeolite. A single crystal X-ray study has been reported a3 of an Ag12-A zeolite, partially dehydrated by heating at 200°C in a streafh of oxygen for 12 h, and then by evacuating at 350°C and 10 -s tort for 48 h. Eight silver ions were found to be distributed over three-fold axes associated with 6-rings. Of these, three were located in the 0~ cage ($2"), two were situated within the sodalite unit (¢J cage) 0.44 A from the.ring plane ($2'), and three were within the sodahte unit 1.19 A from the ring plane ($2"). Three additional silver ions were found adjacent to 8-rings, but displaced off-centre (as in the ease of hydrated Ag12-A zeolite); two of these d,ccupied positions where the cation was recessed into the c~ cage (SI*), the third lay within the 8-ring plane (S1). The twelfth silver ion was statistically divided among 12 equivalent positions corresponding to a two-fold axis opposite a 4-ring ($3). It was found that even dehydration under vacuum at 350°C was insufficient to remove water molecules present within the {3cavity, and that the partially hydrated Agl2-A zeolite studied corresponded to Ag12-A trihydrate. Additional studieslS, 16 have been carried out on more severely dghydrafed Agl2-A single crystals from which it appears that the eight 6-ring silver ions ($2" and $2' positions of the fully hydrated zeolite) occupy a relatively stable environment, the effect of heat treatment being only to bring these ions closer to the centre of the 6-rings. In contrast, the less energetically favoured 8-ring (SI*) and

Table 1 Possible cation positions in type A zeolites compared with the site occupancies found for fully hydrated and partially dehydrated zeolite Ag12-A

Position 1~

Designation

In an 8-ring Adjacent t o an 8-ring, but displaced into a 26-hedron* (~ cage) In a 6-ring Adjacent to a 6-ring, but displaced into a 26-hedron (c~ cage) Adjacent to a 6-ring, but displaced into a sodalite (/3) cage

Sl

Against a 4-ringt forming one of the ribs of a 26-hedron (e cage) In the centre o f a Sodalite (/3) cage In the centre of a 26-hedron (e cage)

Number of sites (per pseudo unit cell)

Site occupancy in hydrated Ag12-A zeolite 13

Si~e occupancy in partially dehydrated AgI2-A zeolite ~3 1

3 3

2

S2*

5

3

S2' S2"$ S3

3

2 3 1

$1 *

S2

SU $4

12

1

1 1

* A 26-hedron o f Type I ~1 (or c~ cage), such as is present in type A zeolite, consists of eight 6-rings, six 8-rings and twelve 4-rings t In type A zeolite, a '4-ring' site corresponds to one of the four faces each of the cuboids linking adjacent sodalite units possesses opening into a 26-hedron (e cage) Position defined in the text

ZEOLITES, 1982, Vol 2, January 3

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al.

pancy of ~-of the sodalite units by Ag 6 molecules, with the remaining [3 cavities e m p t y of all reduced silver ions. Even more severe conditions (heating above 450°C) give rise to the appearance of crystallites of silver which result from the reduction process proceeding to include 6-ring silver ions, thereby causing the destabilization of the (Ag+)8(ag6) system in some sodalite units.

Figure 1 The octahedral Ag~ molecule (SU) stabilized by coordination to eight silver ions at S2* positions ~6

4-ring ($3) silver ions are progressively reduced. A maximum of four silver ions may therefore be reduced, and these were f o u n d to migrate into the sodalite 03) cavities to form a proposed octahedral Ag6 molecule ls'16 at position SU. A complex of formula (Ag+)8(Ag6) was suggested with the a g 6 molecule enclosed by a 'cube', the comers of which corresponded to the stable silver ions within the centre of 6-rings ($2") (Figure 1). Interaction is assumed to occur between the atoms of the Ag 6 clusters and four equivalent framework oxide ions, the silver atoms behaving as weak Lewis acids with respect to the framework, accepting electron density and delocalizing it through coordination interaction onto the silver ions at the 6-rings. The following scheme has been suggested for silver ion reduction 16 involving initially the residual water molecules found in the sodalite (~3) cages of Ag12-A partially dehydrated at "~ 350°Cla: (Ag +) 12[Si12AlI2048112-. 3H20 _+ (ag0)2(ag+) 10(H+) 2 [Si12A112048 ] 12+ ½02 + 2H20 After depletion of the supply of water molecules, additional silver ions must be reduced b y framework oxide ions. Thus it has been found that evacuation to 10 -5 torr at 425°C for about 10 days leads to all four silver atoms in unfavourable sites being reduced: (Ag0) 2(Ag+)lo(H+)2 [Si12A11204s ] 12(Ag°)4(Ag+)8(H+)2[Si12A112047]l°- + ½0 2 The formation of Ag 6 molecules stabilized by eight silver ions requires more silver atoms than are present in the unit cell. There is thus an occu-

4 ZEOLITES, 1982, Vol 2, January

The almost total reversibility of this dehydration scheme has been demonstrated by X-ray studies carried out on an Ag12-A single crystaP 7. A combination of dehydration and hydrogenation treatments caused complete loss of the zeolite crystalline diffraction pattern and the appearance of lines indicative of silver metal. Oxygen treatment restored the zeolite lattice diffraction pattern, and the silver atoms were re-oxidized to a limit of eleven silver ions per unit cell. The final structure determined showed eight equivalent silver ions on three-fold axes near the centres of 6-rings ($2"), and three equivalent cations in the 8-ring planes, but n o t at their centres (S1). 0.56 silver atoms were located at the neutral silver atom position (SU) where Ag6 clusters would be expected to have formed (i.e. 9% of the sodalite units held Ag6 clusters), whilst the remaining 0.44 silver atoms were found to have left the crystal, presumably as the oxide. Confirmation of the relative stabilities of the cation positions may therefore be seen with the 6-ring and 8-ring sites refilling sequentially, but with the most unfavourable position, the 4-ring, remaining vacant. Gellens et al. 18, disagree with the findings of Kim a n d S e f f 15-17 regarding the formation of Ag6 clusters in the sodalite (~) cages of Ag12-A zeolite upon heat treatment. Whilst admitting that severe dehydration treatments might lead to the formation of the proposed Ag6 clusters, these were not found in studies carried out, by X-ray diffraction and u.v. reflectance spectroscopy, on variously exchanged and variously pre-treated (Ag, Na)12-A zeolites. Instead, at ~ 105°C Gellens et al. observed the formation of a linear Ag+-Ag°-Ag + molecule, m which the two Ag ions were present at $2 sites, and the Ag o atom located in the sodalite unit opposite a framework four-ring. Increased severity of treatment (vacuum dehydration at 375°C), was found to cause the formation of two such clusters. It was suggested that a probable maximum of four A g ÷-Ag o-Ag - + molecules- were hkely • per sodalite (~3) cage, with all the Ag o atoms coplanar. Interaction between clusters was thought possible because the distance between Ag o atoms of different clusters was similar to that of silver metal. •

"

-F

•

t

The X-ray structure of ethylene adsorbed onto partially decomposed Ag12-A zeolite has been reported 19. A single crystal was dehydrated at 400°C and 5 X 10 -6 torr for 4 days and then exposed to 120 torr of C2H4 gas at 23°C. The d e h y d r a t i o n produced approximately 2.76 silver atoms per unit cell, which were believed, b y analogy with previous studies ls-17, to result from the reduction of the 4-ring silver ion and 1.76 of

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al.

the three 8-ring cations. On the assumption that these had formed neutral Ag 6 molecules, it was calculated that 46% of the unit cells in the crystal contained Ag6 molecules in sodalite (~) cavities (at site SU).

I I

H

~

H /

Sorption of ethylene had little effect on the Ag6 clusters, their numbers remaining constant and their coordinates scarcely changing. There was, however, a change in the number of coordinated 6-ring ($2") silver ions, from the anticipated value of eight (for the complex (Ag+)s(A~6)) with the formation of the new complex (Ag)6(Ag6) (Figure 2). The removal of two silver ions from coordination with Ag 6 caused the Ag-Ag bond length (the edge of the Ag6 octahedron) to decrease, consistent with diminished ability to withdraw electron density from the Ag 6 cluster, and the Ag-Ag + distance to shorten indicating stronger interaction. The two remaining silver ions located within this unit cell were associated with 8-rings (S 1); these were found not to complex with ethylene. In the remaining 54% of unit cells whose sodalite (~) cavities did not contain Ag6 molecules, one out of eight 6-ring silver ions was observed, upon ethylene adsorption, to recede ~ 1 A into the sodalite unit, thereby taking up an $2' position. No ethylene molecule could be located in coordination with this ion, although this was possibly the result of inadequate experimental resolution. Conversely, each of the remaining seven 6-ring cations, n o w protruding 1.2 A into the a cage ($2"), was found to form a ~r-complex with one ethylene molecules, thus adopting a tetrahedral configuration. The carbon atoms of the complexed ethylene molecule were equivalent, each 2.54 )1, from a silver ion and, whilst not accurately determined, the C = C bond length did not appear to be significandy different from that of ethylene gas

~"

/

(

/

I I

2

~

I

Zeolite surface

H

/

/

//

/ /

C I

Zeolite surface

Figure 3 The three hindered rotations, Tx, r y (antisymmetric stretch) and ~z, and the three hindered translations, tx, ty and tz (symmetric stretch), of ethylene relative to the surface

(1.334 A2°). No significant approach was made by ethylene to the zeolite framework: at the smallest C--O distance (3.76 A) the hydrogen atoms were considered too far from the nearest oxygen atoms to interact with them. As with the 46% of unit ceils whose sodalite cavities contained Ag6 molecules, two silver ions were located per unit cell positioned within 8-rings ($1). Again, these appeared not to coordinate to ethylene. Although no previous infrared or Raman spectroscopic studies have been made of t y p e A zeoliteabsorbed ethylene systems, some insight into these systems may be obtained from earlier investigations of ethylene adsorbed onto type 13-X zeolites2a-23, and in particular the silver exchanged form. Carter et al. 22 have reported infrared spectra in the region 1300-3300 cm -1 for ethylene adsorbed on Li +, Na +, K +, Ag +, Ca 2+, Ba 2+ and Cd 2+ exchanged type 13-X zeolites. In all spectra, the formally infrared inactive (in the isolated CzH4 molecule) vibrations u2, (C= C stretch) and p a (CH2 symmetric deformation) featured quite strongly, showing that olefin interaction with the surface had taken place 24, and that dissociation had not occurred. Bonding was shown to be weak by the complete removal of adsorbed gas, ~apon evacuation at room temperature, from all but the silver and cadmium exchanged forms. For the latter two zeolites stronger interaction was indicated, with evacuation at temperatures in excess of 200°C necessary to displace ethylene from Cd13-X zeolite, and still more stringent conditions required in the case of A g l 3 - X zeolite. A scheme 22,23 was proposed for cation-ethylene interaction in which a a bond is formed by overlap of the olefin lr orbitals with either a 5s or a 5sp orbital of the cation. Additionally, in the case of silver, back donation was suggested to occur from filled silver 4d orbitals into the vacant 7r* orbitals of ethylene according to the Chatt-Dewar-Duncanson model2S, 26. As the interaction of the silver ion with the franSework is partly covalent, a necessary consequence is that its d orbitals are fixed in space and thus olefin rotation about the z axis (Figure 3) results in bond-breaking.

Figure 2 The octahedral Ag 6 molecule (SU) stabilized by coordination to six silver ions at $2" positions 19

Of immediate relevance to our present work on the Agx~-A + ethylene system are previous INS spectroscopic investigations of the Agl 3-X + ethylene

ZEOLITES, 1982, Vol 2, January 5

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al,

system1, 2~. Zeolite samples, prepared b y evacuation to 5 × 10 -6 torr at 450°C, had ethylene (C2H4 and C2D4) adsorbed on them to a pressure of 500 torr. Spectra were then measured at this overpressure, which was equivalent to the high coverage case of Yates 21,23, where 8 ethylene molecules were located per supercage, occupying sites II and III. The samples were then re-evacuated for 30 min at room temperature, and INS spectra recorded corresponding to the low coverage case (where 4.4 ethylene molecules had been found per supercage adsorbed on type III sites21,23). From measurements of the full width at half maximum (FWHM) of the elastic peak, it was established, in agreement with the work of Yates et al. 22,23, that the ethylene molecule was not freely rotating. Analysis of the vibrational part of the spectrum was carried out using intensity relations and deuteration shifts predicted for two models of the adsorbed ethylene molecule: (i) a planar (gaseous) configuration 2s, and (ii) a nonplanar (Zeise's salt) structure 29 in which the hydrogen atoms were bent out of the plane and away from the cation. As a result (Table 2), three frequencies were assigned corresponding to the torsional modes rx, ry and f= of ethylene relative to the silver cation, a n d a fourth identified as a hindered translation (Figure 3). In the high coverage case only, the band due to f~, the torsional mode of the ethylene molecule about an axis projected from the cation through the centre of the C--C bond, was split into two. These were assigned to the in-phase and out-of-phase torsions of ethylene molecules on neighbouring sites II and III. Using the observed values of f~ at each coverage barriers to rotation were calculated.

Table 3 Experimental conditions under which spectra were measured

Gas C2H4 C2H4 C2H4 C=H 4 C2D 4 6284 C2D4

0204

Residual overpressure o f gas (Torr)

Instrument

Temperature (K)

100 0.01 100 0.003 70 0.01 100 0.003

IN4* IN4* Dido Dido ]N4t IN4f Pluto Pluto

5 5 77 77 5 5 77 77

BFD BFD

BF D BFD

* Incident neutron energies of 12.5 and 22.9 meV employed 1 Incident neutron energies of 12.6 and 22.7 meV employed

equivalence of the sodium ions to be exchanged) and a temperature of 25°C 1°. The sample was washed thoroughly and the degree of exchange was determined by chemical analysis. It was then degassed to a pressure below 10 -s torr at 450°C.

EXPERIMENTAL

The dehydrated zeolite powder was transferred, under vacuum, into an aluminium sample cell and the INS spectrum of the cell plus zeolite measured as a background. Ethylene gas, either cylinder C2H4 (> 99.9% purity) or C2D4 supplied b y Merck, Sharp and Dohme Ltd. (99 atom % D), was then admitted into the sample cell (at room temperature) via a glass beaker seal, and adsorbed at an overpressure of ~ 450 torr. Upon adsorption being completed, the cell and zeolite were evacuated to 100 torr and sealed off from the vacuum line. After the high overpressure (h.o.) INS spectrum had been recorded, the cell was reattached to the vacuum line b y means of a second glass breaker seal, and the sample re-evacuated for ~ 30 min at r o o m temperature. The low overpressure (1.o.) INS spectrum was then measured.

Ag12-A zeolite was prepared from Naa2-A (BDH Chemicals Ltd.) b y ion-exchange employing a 0.2 M solution of AgNO3 (containing the exact

A summary of the gas overpressures, sample temperatures and spectrometers used to study the samples is given in Table 3.

Table 2 Comparison of the neutron scattering data and assignments f o r ethylene adsorbed o n t o Ag~2-A and A g 1 3 - X zeolites A g 1 3 - X + ethylene (low coverage)

AgI=-A + ethylene

Frequencies/cm -x

Frequencies/era -~

BFD

C2H 4 t.o.f.

BFD

C2D 4 t.o.f,

Observed Predicted deuteration deuteration shift shiftt Assignment

6 0 0 + 10 540 + 10

456 -+ 8 417-+8 278 -+ 8 224-+8 74-+8

298-+ 10 240 -+ 10 188-+ 10 92 + 1 73±1 55 -+ 2 30-+2

39-+10

( 8 1 -+ 1 /58-+2 | 47 -+ 2 t, 26-+ 2

0.71 -+0.04 0.86 + 0.06 0.84-+0.08 0.88 -+ 0.02 0.79-+0.04 0.85 + 0.07 0.87-+0.13

0.73 0.86 0.86 0.94 0.84 0.84 0.94

* A t high coverage r H splits into components, at 22 and 56 cm-X t Calculated using the Zeise's salt configuration 29

6 ZEOLITES, 1982, Vol 2, January

BFD

(ul0 internal C=D 4 mode) combination band (TD(I) + r~(I)) 7H(ll) rx(I ) 418-+28 Ty(I) 258 -+ 21 ~y(ll) tz(I) rZ(I) rz(ll) ty(I)

C2H 4 t.o.f.

BFD

C2D 4 t.o.f,

276 -+ 20 199 -+ 20 80 39.5 -+ 1.8

Assignment

rx Ty 75.0 -+ 5 35.0 -+ 1.8

tz ~z*

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al.

Except where stated otherwise, all spectra have had a background, corresponding to the sample cell plus dehydrated Ag12-A zeolite, subtracted. Assuming the backgrounds indicated on the spectra, Gaussians have been fitted to the data using a Du Pont curve resolver and intensity ratios measured. These have then been compared with the intensity ratios predicted on the basis of the suggested assignments. Neutron energy gain time-of-flight (t.o.f.) spectra were measured using the 4H5 and 6H spectrometers on the Dido reactor at AERE Harwell. Further details of these spectrometers may be found elsewhere a°-a2. Neutron energy loss t.o.f. spectra were obtained using the IN4 rotating crystal spectrometer on the High Flux reactor at the Institut Laue-Langevin, Grenoble a3. IN4 is used to best advantage in the down scattering mode, and in this configuration is capable of much better energy resolution than either 4H5 or 6H. This arises because the relation between energy transfer and time-of-flight is not linear. Thus, taking the example of neutron energy loss spectroscopy, as time-of-flight increases, the energy spread within each channel becomes progressively smaller, and resolution consequently better. Increases in the time-of-flight should therefore be maximized so that features of interest in the spectrum of the sample are in the region of the best resolution of the instrument. In order to operate IN4 in this way, the approximate frequencies of the transitions to be investigated must be known in advance, so that a suitable incident wavelength may be selected. The time,of-flight data are usually transformed to produce the function P(oe, ~) at each scattering angle a4. P(o~,~) is defined by: P(c~, ~) = 2~ sin h(~12) exp (2W(g)) - o/

where ~3= hcg/KT, o~= h2Q2/2MKT, hQ is momentum transfer, g is energy transfer in cm -a, M is the mass of the scattering nucleus or ligand, K is Boltzmann's constant, T is the absolute temperature and exp ( - 2W(ff)) is the energy dependent Debye-Waller factor (usually set to unity). The error quoted for a transition in the t.o.f. spectra represents the maximum energy separation between the data point taken as the band centre and an adjacent point. It does not therefore necessarily represent the true error in the transition frequency. Neutron energy loss spectra, at higher energy transfers, were measured on the beryllium filter detector (BFD) spectrometers on the Pluto and Dido reactors, at AERE Harwell. Accounts of the Pluto BFD spectrometer have been given elsewhere as. The resolution of the BFD spectrometer has recently been discussed in the literature 36. The minimum full width at half maximum (FWHM) of a band observed using this type of instrument is 35 cm -1, this value increasing with energy if the same m o n o c h r o m a t o r plane is used.

The transmission function of beryllium is not a simple rectangle. In addition, the efficiency of the neutron detectors is also energy dependent. These factors result in the observed band maximum in a BFD spectrum being displaced from the true transition frequency. Correction factors have, however, been calculated 35 and are applied in this paper. The data were collected by counting scattered neutrons for a constant number of neutron monitor counts at each incident energy, the neutron monitor being placed in the neutron beam between the sample and the source. Under these conditions, the BFD spectrum is directly proportional to g(~), the amplitude weighted density of states at non-zero m o m e n t u m transfer a4,37 multiplied b y the Debye-Waller factor. It has been shown that BFD and t.o.f. (P(0~, (3)) data may be compared directly 3s. The infrared measurements were made on a Perkin Elmer 580B spectrophotometer using a selfsupporting zeolite disc ('," 10 mg cm -2) degassed in an all-metal infrared cell. While the sample pretreatment and overpressures were similar to those used in the INS experiments, the infrared spectra were obtained at room temperature. RESULTS A N D DISCUSSION

INS spectra of Zeise's salt and dimer t show the two lowest frequency internal ethylene vibrations (the CH 2 rocking modes v17 and v22 Ref. 39) to occur at ~ 718 and ~ 830 cm -a respectively. On weaker complexation with silver, these will shift to higher frequency, since, for gaseous ethylene, the equivalent vibrations (vl0and v6) occur at 810 and 1236 cm -1 (Ref. 40). Thus, the lowest frequency intramolecular mode of C2H4 adsorbed within Ag12-A zeolite will probably be found at ~ 800 cm -1, and the analogous mode of C2D4 at ~ 586 cm '1 (Ref. 40). INS bands observed at frequencies below those arising from internal C2H4 and C2D4 normal modes must result from either hindered rotations and translations (Figure 3) of the adsorbed molecule, or amplified surface modes 41. In order to assign these lower frequency bands, we will use predicted intensity ratios and deuteration shifts calculated on the basis of a 'Zeise's salt configuration '29, in which the four hydrogen atoms bend out of the 'gas phase plane' and away from the cation with which interaction is occurring. (The C--D bond length has been assumed to be 0.01 A shorter than that of C--H.) It has been shown 34 that the intensity, in the density of states obtained from the INS spectrum, of a translation is proportional to aim and that of a rotation to ar2/IR. In the case of ethylene adsorbed onto Ag12-A zeolite, M is the mass of ethylene, r is the perpendicular distance b e t w e e n the scattering atom and the axis of rotation, IR is the m o m e n t of inertia of ethylene about the rotation axis and a is the degeneracy of the mode. Theoretical intensity ratios (Table 4) show that the three ethylene-metal vibrations predicted to have

ZEOLITES, 1982, Vol 2, January 7

INS study o f ethylene sorbed in silver-A zeolite: Joseph Howard et al.

Table 4 The intensity ratios and deuteration shifts predicted f o r zeolite--ethylene vibrations assuming a 'Zeise's. Salt' geometry for the adsorbed ethylene molecule Zeise's salt geometry =9

Intensity ratio Mode* for C=H4adsorption

Intensity ratio for C=D4 adsorption

Deuteration shift

rx ~y rz

3.8 2.1 2.4 1.0 1.0 1.0

0.73 0.86 0.84 0.94 0.94 0.94

tX

ty tz

6.7 2.6 3.0 1.0 1.0 1 .o

* The three hindered rotations and three hindered translations of ethylene relative to the zeolite surface are defined in Figure 3

point of the four displaced. Secondly, bands in the region 200-400 cm -1 (incident neutron energy) of the spectrum of Ag12-A + C2D4 (I.o.) are better resolved than those in the comparable h.o. spectrum. The inferior resolution in the h.o. case probably results from the ethylene molecules constituting the overpressure being non-specifically adsorbed within the 0~ cage of the zeolite• In the region above 200 cm -1, the BFD spectrum of C2H4 adsorbed (h.o.) onto Ag12-A zeolite (Figure 4a) contains three bands, at 224,278 and 417 cm -1. By matching bands of equivalent intensity, and noting that within the simple harmonic oscillator model the largest deuteration shift is 0.71, we correlate these with transitions in the

Table 5 Frequencies of the hindered torsional modes of the C=H4 I[gand in some organometallic complexes (cm -~) Complex

~z

~y

rx

Zeises satt a, ~ 9 Zeises dimer ~,3~ 1 ] AgNO~ -3C=H4

185 170 140

490 487 --

1180 1176 --

u

greatest intensity are rx, ry and r~ (Figure 3). The two latter torsions have been observed in INS spectra 1, and the former in infrared spectra 39, of Zeise's salt and dimer (Table 5). In the compound AgNO3 • gC2H4 1 1, 7"z occurs at lower frequency (140 cm -1) than for the platinum complexes, reflecting the weaker g-bonding present in this salt between ethylene and the silver cation. Similarly, in INS studies of ethylene adsorbed onto Agl 3-X zeolite t,27, the same order of r e l a t i v e frequencies of the torsional modes, ~'x (418 cm-X), ry (258 cm -~) and rz (39.5 cm-1), has been estabhshed as for Zeise's salt and dimer, but in c o m m o n with AgNO3 • gC2H4 ~ 1, the frequencies were shifted to lower energy.

~-xxx

I

100

8 ZEOLITES, 1982, Vol 2, January

I

200

[

x I

I

300 400 500 Incident neutron energy (cm -I )

600

Figure 4 Dido BFD spectra (77 K) of C2H4adsorbed onto Ag~2-A zeolite at overpressures of (a) 100 Torr and (b) 3 X 10 -3 Torr. o and X denote data collected using the A1 (111 ) and A1 (311 ) monochromator planes respectively

BFD spectra of ethylene adsorbed onto Ag12-A zeolite The BFD spectra of ethylene adsorbed at both high and low overpressures (h.o. and 1.o. respectively) onto Ag12-A zeolite are shown in Figures 4 and 5, and the frequencies derived from these are displayed in Table 2. It may be seen that the spectra are essentially independent of the overpressure of gas present and, wherever possible, we will discuss both sets of data together. Some of the differences which do exist, however, between the spectra obtained at different overpressures may be immediately explained. Firstly, the 518 cm -~ feature in the spectrum of Agx2-A + C2H4 (1.o.) is an artifact. Between the first point of the feature and the point immediately preceding, the reactor had 'tripped'. Consultation with the reactor log showed that upon the 'trip' occurring, the reactor loading had been changed, with the removal of neutron absorbing cobalt isotrope rigs and that these had not been replaced until the scan had reached approximately the final

...

_

'~

~

oo

T _

r~

~

uO

~

to)

1 100

E

leo 200

T E

a

0

x T

~

u

cd

1 I I 300 400 500 Incident neutron energy (cm-I)

I 600

] 700

Figure 5 Pluto BFD spectra (77 K) of C=D4 adsorbed onto Ag12-A zeolite at overpressures of (a) 100 Tort and (b) 3 X 10 -3 Torr. o and X denote data collected using the A1 (11 I) and A1 (311) monochromator planes respectively

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al.

equivalent C2D 4 spectrum (Figure 5a) at 1 8 8 , 2 4 0 and 298 cm -1 respectively. From the preceding discussion, the highest energy surface-ethylene vibration of appreciable intensity should be rx. The observed deuteration shift (417 -+ 298 cm -1) of 0.71 is in good agreement with that predicted for rx (0.73), and the frequency of 417 cm -1 is almost identical with that reported for Tx in studies of C2H4 adsorbed onto A g l 3 - X zeolite (418 cm-1) 1. The band at 278 cm -1 in the BFD spectrum of Ag12-A + C2H 4 (h.o.) is in the region of the spectrum where the torsion ry was located in the A g 1 3 - X zeolite + C2H4 system (258 cm-1) 1. Assignment of this band to ry is confirmed b y the observed deuteration shift (278 -+ 240 cm -1) of 0.86, which is identical with that calculated for this mode. Using these assignments we find that the measured relative intensity of r H to rH is identical to that predicted (2.5:1.0) using the equations given earlier. (Surface-ethylene modes involving hydrogeneous and deuterated species will be denoted b y the respective superscripts 'H' or 'D' H e.g. rx will represent the vibration rx of C2H4.) For the comparable BFD spectrum of Ag12-A + C2D4, the observed intensity ratio TD :r D measured by curve resolution, 1.9 : 1.0, also compares very favourably with the calculated value of 1.8 : 1.0. The weak band (Agl2-A + C2H4, h.o.) at 224 cm -1 correlates with a shoulder at 188 cm -1 in the equivalent C2D 4 spectrum. The deuteration shift is 0.84, which is close to those expected for the torsions ry and rz (Table 4). On intensity grounds the 224 cm -1 band cannot be assigned to r H. It is equally improbable that the antisymmetric stretch (ryH) would be split b y 54 cm -1 as a result of interaction b e t w e e n olefin molecules b o n d e d at neighbouring sites, since such large splittings are not observed even for model compounds which have more than one C2H4 ligand b o n d e d to the same metal atom 42. A likely alternative explanation is that the 224 cm -1 transition represents a torsional mode of an ethylene molecule complexed at a second adsorption site. Strong corroborative evidence for the existence of two different adsorption sites is provided from optical studies of this system. Our infrared spectra of Ag12-A + C2H4 at a range of pressures, b e l o w the m a x i m u m of 10 torr studied, c o n t a i n t w o bands in the CH2 deformation region (1460 and 1415 cm-1). This can only be explained b y the existence of t w o different adsorption sites within the zeolite framework. It is still necessary, however, to decide which normal mode the 224 cm -1 transition represents. It is unlikely that an ethylene C2 torsion (rH) would occur at higher frequency than those found for Zeise's salt and dimer (185 and 170 cm -1 respectively) 1 in view of the relatively weak interaction expected between C2H4 and Ag122A zeolite. The most reasonable assignment of the 224 cm -1 band is therefore to the torsion rHv of a C2H4molecule adsorbed on a second site. (We

will designate the sites I and II, hence, for example, the torsion r H at site I will be labelled TH (I).) Returning once again to the BFD spectrum of Ag12-A + C2H 4 at h.o. (Figure 4a) we can see that the band at 417 cm -1 has a distinct tail to the higher frequency side. Using the lower frequency edge as a guide to the true band-shape of the lower frequency c o m p o n e n t we have fitted the observed band using two Gaussians. The higher frequency band was found to be centred at "~456 cm -1 and is of much lower intensity than the 417 cm -1 band. This band (456 cm-1) we assign to TxS(II), that is to r H for the second adsorption site. If the deuteration shift were identical to that measured for rx (I), we would predict rx° (II) to occur at 328 cm -1, which would place it beneath the broad band at 298 cm -1 (Figure 5a). In fact this latter band is non-Gaussian indicating the presence of an additional unresolved component. The remaining band o f high intensity in the BFD spectra of ethylene adsorbed at h.o. onto Ag12-A zeolite (74 cm -1 C2H4/39 cm -1 C2D4) should, b y analogy with the Ag13-X-ethylene system,, Zeise's salt and dimer and the complex AgNO3.½ C2H41,27 represent the torsion, rz. Consideration of this region of the INS spectrum will, however, be deferred until the t.o.f, data are discussed. Apart from a weak feature at 540 cm -1, which represents the combination band rD(1) + ryD(1), there remain three unassigned bands above 150 cm -1 in the BFD spectra of Ag12-A + C2D4 (Figure 5): Of these, that at 600 cm -1 may be assigned to the lowest frequency intramolecular mode of C2D4(Vl0), which has been predicted, in the case of gaseous C2D4, to occur at 586 cm -1 (Ref. 40). The two other bands are more clearly visible in the 1.o. spectrum of Ag12-A + C2D4 (Figure 5b), at 400 and 447 cm -1. Since they occur above the highest frequency zeolite-C2D 4 mode (rxD), and also below the lowest frequency intramolecular mode of C2D4, assignment of them is difficult. They cannot represent any of the hindered rotations and translations of absorbed C2D 4 and, in view of the similarity of the two frequencies to the bands assigned to TH (I) and r H (II) for Ag12-A + C2H4, we considered that they might be due to a small percentage of C2H4 impurity in the C2D 4. The difference in scattering cross-section of hydrogen and deuterium (OH/OD ~ 40) indicates that an ~ 1% impurity level would lead to bands, due to C2H4, of observable intensity. The manufacturers of the C2D4, however, inform us that their product does n o t contain any C2H4, and we do not have a sample of the original gas upon which to make our own measurements of purity. Failing this explanation, we are unable to account for the occurrence of these two bands. Certainly, it is unlikely that they represent amplified zeolite modes, combination bands or overtones. T.o.f. spectra of ethylene adsorbed onto A g l z - A zeolite We have already assigned the normal modes rx and

ZEOLITES, 1982, Vol 2, January 9

INS study o f ethylene sorbed in silver-A zeolite: Joseph Howard et al.

x 1.5

Examples of our t.o.f, spectra are shown in Figures 6 and 7 and the data are summarized in Table 2. Once again we can see that the high and low overpressure spectra are essentially identical, the slightly higher intensity above ~ 90 cm -1 in the h.o. cases being almost certainly due to increased backgrounds from the non-specifically adsorbed ethylene. The transition frequencies are, within the statistical accuracy, the same in b o t h the h.o. and 1.o. spectra.

x 1.5 ~z vd

a

b xl.4

xl

cj

°/ 0

I

20

I

I

I

I

I

I

40

60

80

I00

120

140

Energy transfer (cm -I) Figure 6 I N 4 t . o . f , spectra (5 K) o f C2H 4 adsorbed o n t o Ag12-A

Below 100 cm -1 there are four bands in the spectra of b o t h adsorbed C2H4 and C2D 4. Because the m a x i m u m isotopic shift, assuming the harmonic oscillator model, is 0.71, we can correlate the bands in the spectrum of adsorbed C2H 4 with those of the same rank order in the spectrum of adsorbed C2D4 . The measured isotopic shifts are given in Table 2. We can see immediately that two of the bands (73 and 5 5 cm -1 in the C2H4 case) have isotopic shifts very close to that predicted for Tz. F r o m the previous discussion and Table 4 it is obvious that the most intense of these bands (73 cm -1 A g ~ - A + C2H4/58 cm -1 Ag12-A + C2D4) , and indeed the most intense in the t.o.f, region, must represent rz (I). In view of this assignment the bands at 55 (Ag12-A + C2tt4) and 47 cm -1 (Ag12-A + C2D4) must be assigned to f= (II). The remaining bands (92 and 30 cm -1 Ag12-A + C2H4, 81 and 26 cm -1 Ag12-A + C2D4) have isotopic shifts (Table 4) which are very close to that pre-

zeolite (a) 103 °, 100 Torr, (b) 103 °, 10 -2 Torr, (c) 149 °, 100 Torr and (d) 149 °, 10 -= T0rr

ry of ethylene molecules adsorbed onto two different sites within Ag122A. We therefore expect a total of eight bands to be present in the lower frequency (t.o.f.) region, corresponding to the vibrations Tz, tx, t.y and tz on each of the two different adsorption sites. We are, however, unlikely to resolve all of these (even assuming they are non-degenerate) because related studies indicate that these lower frequency modes give rise to bands with appreciable half-widths. In addition we have seen that, for example, the intensity of the band due to ry (I) is several times greater than that assigned to ry (II). Provided that the D e b y e Waller factors for the molecules adsorbed on the two sites are identical, this intensity difference can only be explained b y there being fewer ethylene molecules adsorbed onto type (II) than onto type (I) sites. Consequently, we may predict that, in general, the lower frequency modes of ethylene molecules adsorbed onto type (II) sites will be much less intense than those adsorbed onto type (I) sites. However, we must qualify this statement b y noting that, as hindered rotations give rise to much more intense INS bands than do hindered translations (Table 4), rz (II) may possess an intensity comparable with that of a translational mode on site (I). Hence it is reasonable to assume that we may observe rz (I), ~z (II), tx (I), ty (I) and tz (I), but probably not tx (II), ty (II) or tz (II).

10

ZEOLITES, 1982, Vol 2, January

xl.l

~Q. Xx Xx X

Xxx xl

c X

•

\ ~'= ~

"--.:.. X~,X x

X

•

xl.3

oo

x

x

x 0

x

I

I

I

I

I

I

I

20

40

60

80

I00

120

140

Energy transfer (cm -I) F i g u r e 7 I N 4 t . o . f , s p e c t r a (5 K) of C2D 4 a d s o r b e d o n t o Ag12-A o 0 -2 o

zeolite

(a) 134.3 , 70 Torr, (b) 134.3 , 10 Torr, (c) 152.3,

70 Torr and (d) 152.3 °, 10 -3 Torr

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al.

dicted for a hindered transition. From our earlier discussion of intensities, these bands must represent two of the translational modes expected of ethylene adsorbed o n t o site I. Experience with other systems 1,39,43 indicates that the likely frequency order is ty <~ tx ~ tz and that t= is usually more intense than the other translational modes. It has also usually been found that ty ~ rz < t= and so it seems reasonable to assign the band at 92 cm -1 (Ag12-A + C2H4) {81 cm -1, Agx~-A + C2D4} to tz(I) and that at 30 cm -1 (Ag12-A + C2H4) {26 cm -1, Ag12-a + C2D4} to ty (I). On this basis t~(I) must occur below 92 c m - l b u t is unresolved from the intense features already assigned to rz (I) and rz (II). Barriers to rotation about the axis joining the silver atom to the centre of the C = C b o n d Work carried out on Zeise's salt 44 has shown the potential functions describing the torsion rz (Figure 3) to be of the form:

v(o) =

Vo (1 - cos 20) + ?v4 (1 - c o s 4 0 )

(1)

- cos 2 0 ) + ( 1 - c o s

40)]

We would like to thank the S.R.C. and AERE Harwell for the award of a CASE studentship to one of us (K.R.)., REFERENCES 1 2 3 4

6 7 8

v4 = v0: V0

ACKNOWLEDGEMENT

5

This may be approximated b y a one parameter function for V(O) if the assumption is made that

V(0)= 2 [ ( 1

these sites (if the Debye-Waller factors are assumed to be equal for the two adsorbed species). These intensity ratios have been measured and show significant variation with the mode chosen. It is probable that these discrepancies are associated with the pretreatment conditions of the sample and this strongly implies, in agreement with the findings of Kim and Self is,a6 for Ag12-A , that the relative number of sites changes with heating.

(2)

This function has a ground state minimum at 0 = 0, a metastable minimum at 0 = 7r/2, and a m a x i m u m at 0m~ = 0.5 cos -x ( - 0.25). The second derivative with respect to 0 of equation (2) is equal to the force constant, k, therefore using the harmonic oscillator approximation yields: 1 (5Vo~ '/2 co = -71" ~-R! where In is the reduced m o m e n t of inertia of the ethylene ligand. Employing the assigned frequencies for r H (I) and r~ (II) of 73 and 55 cm -1, values for V0(I) of 3.8 -+ 0.2 kJ mo1-1 and V0(II ) of 2.2 +- 0.2 kJ mo1-1 may be calculated. These values compare with barrier heights of 23.6 and 14.3 kJ mo1-1 calculated respectively for Zeise's salt 44 and the complex AgNO3. ~C2H41 and 1.1 kJ mo1-1 evaluated for ethylene adsorbed at low coverage on Agl 3 - X zeolite 1. CONCLUSION Several of the normal modes of vibration of ethylene relative to the zeolite surface have been observed and assigned. In contrast with the X-ray crystallographic data of Kim and Seff 19, these data show that there are two different adsorption sites for ethylene in Ag12-A zeolite. The relative intensities of INS bands arising from equivalent normal modes of molecules adsorbed onto the different sites is equal to the relative number of

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Howard, J., Waddington, T. C. and Wright, C. J. J. Chem. Soc. Faraday Trans. I I 1977, 73, 1768 Breck, D. W. 'Zeolite Molecular Sieves'Wiley-lnterscience, London, 1974 AngelI, C. L.J. Phys. Chem. 1 9 7 3 , 7 7 , 2 2 2 Tam, N. T., Cooney, R. P. and Curthoys, G. J. Chem. Soc. Faraday Trans. I 1976, 72, 2577, 2592, 2598 Brodskii, I. A., Zhdanov, S. P. and Stanevich, A. E. Opt. Speckrosk. 1971,30, 58 Venuto, P.B. and Landis, P.S. Adv. Catal. 1 9 6 8 , 1 8 , 2 5 9 Schwochow, F, and Puppe, L.Angew. Chem. 1975, 14,620 Meier, W. M. 'Molecular Sieves' Society Of Chemical Industry, London, 1968, p 10 Pfeifer, H. Krist. Tech. 1976, 11,577 Breck, D. W., Eversole, W. G., Milton, R. M., Reed, T. B. and Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963 Barrer, R. M. 'Zeolites and Clay Minerals as Sorbents and Molecular Sieves' Academic Press, London, 1978 Th6ni, W., Z. Kristallogr., Kristallgeom. Kristallphys., Kristallchem. 1975, 142, 142 Kim, Y. andSeff, K.J. Phys. Chem. 1978,82,1071 Matsumoto, S., Nitta, M., Ogawa, K. and Aomura, K., Bull. Chem. Soc. Jpn. 1975,48, 1169 Kim, Y. and Seff,%.J. Am, Chem. Soc. 1977,99, 7055 Kim, Y. and Seff, K.J. Am. Chem. Soc. 1978,100,6989 K i m , Y . and Seff, K.J. Phys. Chem. 1978,82,921 Gellens, L. et al., in press Kim, Y. andSeff, K,J. Am. Chem. Soc. 1 9 7 8 , 1 0 0 , 1 7 5 Sutton, L. E. 'lnteratOmic Distances and Configurations in Molecules and Ions' The Chemical Society, London, 1958, M/129 Yates, D. J. C. J. Phys. Chem. 1966, 70, 3693 Carter, J. L., Yates, D. J. C., Lucchesi, P. J., Elliott, J. J. and Kevorkian, V. J. Phys. Chem. 1966, 70, 1126 Yates, D. J. C. Chem. Eng. Progr. Symp. Ser. 63, 1967, 73, 56 Sheppard, N. and Yates, D. J. C. Prec. Roy. Soc. A 1956, 238, 69 Dewar, M. J. S. Bull. Soc. Chim. France 1951,18, C71 Chatt, J. and Duncanson, L. A. J. Chem. Soc. 1953, 2939 Howard, J., Waddington, T. C. and Wright, C. J. J. Chem. Soc. Chem. Comm. 1975, 775 Bartell, L. S., Roth, E. A., Hollowell, C. D., Kuchitsu, K. and Young, Y. E. J. Chem. Phys. 1965, 42, 2683 Love, R. A., Koetzle, T. F., Williams, G. J. B., Andrews, L. C. and Bau, R. Inorg, Chem. 1975, 14, 2653 Harris, D. H. C., Cocking, S. J., Egelstaff, P. A. and Webb, F. J. "IAEA Conference on Inelastic Scattering of Neutrons in Solids and Liquids', Chalk River, 1962, IAEA Vienna, published 1963, Vol. 1, p. 107 Bunce, L. J., Harris, D. H. C. and Stirling, G. C. "The Dido (6H) Long Wavelength Inelastic Neutron Spectrometer', AERE Harwell Report R6246, 1970 Reynolds, P. A. and White, J. N. Disc. Faraday Soc. 1970,48, 131 (Ed. B. Maier) 'Neutron Beam Facilities at the HFR Available for Users' Institut Laue-Langevin Report, Genoble, 1973 (Eds. R. J. H. Clark and R. E. Hester) 'Advances in Infrared and Raman Spectroscopy' Heyden, 1980 Vol 7 k

25 26 27 28 29 30

31 32 33 34

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11

INS study of ethylene sorbed in silver-A zeolite: Joseph Howard et al. 35 36 37 38 39 40

Gamlen, P. H., Hall, N. F. and Taylor, A. D. 'Molecular Spectroscopy by the Beryllium Filter Machine' AERE Report, RR L 74/693, 1974 Leuter, H. in press Howard, J.andWaddington, T. C.J. Phys. Chem. 1981,85,2467 Howard, J., Robson, K. and Waddington, T. C. Chem. Phys. in press Hiraishi, J. Spectrochim. Acta 1969, 25A, 749 Arnett, R. L. and Crawford, B. L. J. Chem. Phys. 1950, 18,

12 Z E O L I T E S , 1982, V o l 2, January

41 42 43 44

118 Howard, J., Waddington, T. C. and Wright, C. J. Jo Chem. Phys. 1976, 64, 3897 Howard, J., Waddington, T. C. and Wright, C. J. J. Chem. Soc. Faraday Trans. I I 1976, 72,513 Grogan, M. J. and Nakamoto, K. J. Am. Chem. Soc. 1966, 88, 5454 and J. Am. Chem. Soc. 1968,90, 918 Ghosh, R. E., Waddington, T. C. and Wright, C. J. J. Chem. Soc. Faraday Trans. II 1973, 69,275