Radiotracers in fluorine chemistry. Part VI, Surface areas of ionic metal fluorides. Effect of pretreatment

JournalofFluorineChemistry, 14(1979)331-338 0 Elsevier Sequoia S.A.,Iausanne - Printedinthe Netherlands

331

Received: May 31,1979

RADIOTRACERS AREAS

IN FLUORINE: CHEMISTRY,

OF IONIC METAL

GEORGE

FLUORIDES,

GEOFFREY

A, KOLTA+,

Chemistry.Department, Gt.

EFFECT

WEBB

SURFACE

PART VI,

OF PRETREATMENT

and JOHN M. WINFIELD

University

of Glasgow,

Glasgow

G12 8QQ,

Britain

SUMMARY Determination B.E,T,

method

using

and TlF samples either

of CsF, TlF, and RbF surface 85

by heating

in vacua -in acetonitrile,

acetone effective

markedly

of CsF

areas

by sample

or by reaction

pretreatment,

with hexafluoro-

The latter method

RbF samples

for CsF,

Surface

Kr is described,

are increased

areas by the

is particularly

are less sensitive

to pre-

treatment,

INTRODUCTION

Ionic, conditions, requires effect

metal

fluorides

A complete

a knowledge

on surface

of their

fluoride

although

for some metal

chemistry

between

exchange

with

carbonyl

fluoride

[l],

pretreatment used

CsF in some reactions

t Permanent Egypt.

address:

areas,

there

sulphur

We now report

to be a

to undergo

the effect

and thallium(I)

Research

attention,

tetrafluoride

fluoride,

the

This aspect

appears

areas of caesium

[2], rubidium

National

including

little

area and ability

[3,1],

heterogeneous

of their behaviour

has received

labelled

on the surface catalyst

used under

pretreatment,

fluorides

surface

fluorine

widely

surface

area of sample

of ionic

correlation

are widely

understanding

18-

or

of sample

fluoride,

a

an alternative fluoride

Centre,

Dokki,

which

Cairo,

to

332

has no catalytic SClF5

[I].

measuring

activity,

Experimental small

surface

at least in the reaction

SF4 + ClF --t

aspects areas

of the method used for (cl m 2 g -1) are described,

EXPERIPIEKTAL

&face

area determination

Surface

areas were determined

by the B,%,T,

the radio-isotope

85-kyrpton

using

gas is t!lat small

radioactive

be determined The apparatus, described

relatively evacuable

by Aylmore

the handling schematically

as adsorbate,

rapidly to ~10~~

and Jepson

of hygroscopic

changes

and with

method

in pressure

of

can

a high precision,

torr, was similar

141, modified

solids,

using

The advantage

to that

to facilitate

It is shown

in the Figure,

FIGURE B.E.T.

apparatus,

Volumes

of the

various

sections

were

as follows:

bulb 1, 63.07

cm3

2, 23.53 cm3 3,

8.79 cm3

4,

5.19 cm3

5, 17~18 cm3 6, 42.39

cm3

7, lo*90

cm3

8,

6.32

cm3

S,

8.97 cm3

C, 50.08 cm3

333

The calibrated

system

constructed

of two sets of bulbs, reservoir, vessel

bulb

(C) and the adsorbent

Centre,

Amersham)

glass,

1 - 4 and 5 - 8, each with

a 85Kr storage

was also connected

from Pyrex

a mercury

(B), a thin-walled

sample

to reservoirs

(S), containing 85

bulb

and non-radioactive

consisted

counting The manifold

Kr (Radiochemical

Kr (B.O.C.Ltd~), and to a

Diluted trap containing charcoal activated at 578 K --• in vacua 85 Kr was prepared in the latter, to give a working activity of ~a.

10' counts/sec/torr,

before

determined

using

determined

using

externally

mounted

Approximately attained

a Pirani

after

a Geiger-Muller immediately

each pressure

calibration

was

checked

The sample

bulb

fluoride

vessel

(C),

rates were

adjusted

to give a

errors,

intervals

with

Pressures

to be

The count

being

of diluted

(S) was loaded

(-0.5 g) of metal

the counting

counting

at regular

was

(Plullard ZP 1481)

for equilibrium

change,

sample

pressure

0.09 - 0.29 torr, and 85 Kr activities

below

to minimise

for each fresh

against

counter

the time of counting

count of 0104,

determined

range gauge

0*5 h, was required

reproducible,

sample

of 85 Kr activity

curve

over the pressure

measured

total

at 77 K overnight

use,

A calibration

were

and was equilibrated

The

and was rekrypton,

an accurately

weighed

in a glove box, and was

then outgassed with mercury

for several hours, Bulbs 1 - 8 were filled and 85 Kr admitted to the manifold from (B) to

a pressure less than the S,V,P, of Kr at 77 K+ The relation 85 Kr activity and volume, and hence between pressure

between

(P torr)

and volume

determined,

intercept sample

the final measurement

A plot

empty,

on the V axis corresponding (57.16 + 8.97 cm3),

were obtained

with

an intercept

confirmed Bulb constant,

to 1 - 8

to the dead Since

space f

adsorption the corrected

to the experimental

of - (57.16 + 8*97.T/77).

in a separate

(T IC) was

a negative

(S) held at 77 K,

v vs, T/P plot was a line parallel with

corresponding

of V vs, T/P was linear with

bulb volume

isotherms

(V cm3) at room temperature

line

This was

experiment,

(S) was immersed

in liquid N2, the level being kept

and a second V vs. T/P relationship

determined,

334

For samples 5 - 8 being The slope

of surface

filled with

mercury

of this isotherm

'room temperature'

III*g -' bulbs

areac0.1 before

the run was commenced,

was less than

isotherm,

that of the corrected

the differences

in V(AV

cm3) at The

given

T/P values

number

of molecules adsorbed (x) was (PAV/77)(N/R)(1/760 x 10m3), -1 - Avogadro number and R 1 at mol-' K-' = the gas N mol

where

constant,

being the volumes

1 - 4 were used,

of Kr adsorbed,

The slope of the linear

B,E,T,

vs, P/PO was l/x,, where PO = S,V,P. and xM = number The surface

of Kr molecules

area = xM,19*5

plot P/x(Po-P)

of Kr at 77 K (0.62 torr)

required

x lo-*'/(wt.

to form a monolayer, of sample)m*

g-l, where

19.5 x 10-20 al* = the Kr molecular area, The overall error on -1 , the major source of error the surface areas was + 0.02 III*g being

Sample

graphical,

preparation

CsF and RbF 99%) were

(B.D,H,

stored

Ar or N2 recirculation They were

Co,). mortar

before

Optran

grade)

and manipulated

and purification

thoroughly

use,

and TlF

(Alfa Inorganics

in a glove box equipped

ground

(Lintott

using

Three pretreatment

with

Engineering

an agate pestle

methods

and

were used as

follows: (1)

A sample contained

in a stainless

steel vessel

was

After cooling it was at 423 K -in vacua for 24 h, reground. This procedure was repeated once or twice when

heated

longer

pretreatment

(2)

A sample

times were required, contained

in a Ni boat was heated After

or 773 K in a dry N2 flow for 2 h,

cooling

at 573 K in N2 it

was reground, (3)

A sample

in a stainless ball bearings multiple vacua

("2 g) was shaken

steel vessel with

refluxes

a mixture

for 24 h, at room temperature

containing

of acetonitrile

over CaH2 then P205

over activated

several

46 molecular

(Fluorochem) , purified

by low temperature,

distillation

sufficient

in vacua,

steel

(5 ml), dried by

followed

sieves,

stainless

by storage

&

and hexafluoroacetone trap-to-trap

to produce

an

335

autogeneous removed,

pressure

the vessel

of ~a_ 7 at, was heated

for 10 h, to decompose overnight

any adduct

at room temperature,

Volatile

material

at 373 K under formed

dynamic

was vacuum

[5], and pumped

after which

the sample was

reground,

RESULTS

AND DISCUSSION

CsF and RbP have the rock salt structure, exists

as a distorted

temperature, implied

the presence

[6],

in various

Surface

ways,

while

form of this structure of aspherical

areas

are given

of these

TlF

at room

Tl+ cations

fluorides,

being

pretreated

in the Table,

TABLE Surface

areas

Pretreatment

of metal

fluorides

method

Surface

area

(m2 g -1 ) (a)

CsF

TlF

RbF

None

0.04 0.04

o-11 0*13

0.04 0.03

423 K, 24 h, vacua

0*09 0.09

O-34 o-37

0*04 o-04

423 K, 48 h, vacua (b)

0.18 0*19

0.35 0.37

0.04 0.05

423 K, 72 h, vacuotb)

0*18 0*20

0.36 o-37

(c)

573 K, 2 h, N2

0.08 0.08

0.18 O-18

(c)

773 K, 2 h, N2

0.06 0.07

(c)

(cl

0.37 o-41

0.34 0.35

0.08 0.09

(CF3)2C0

+ MeCN

then

373 K, 10 h, vacua

(a) Determined

on different

(b) Two or three (c)Not studied.

periods

samples,

Error

of 24 h; reground

+ 0.02 m2 g after

-1

.

each period,

336

Heating followed

reaction

in vacua,

are commonly

by grinding,

ing CsF prior

with

to its use as a catalyst

The surface

areas

essentially

The fluorides

studied

but otherwise

are unchanged

at 423 K produces

are off-white

of CsF and TlF, the limiting

responsible

periods

increasing

at higher

surface

will

production, important

areas,

in surface

although

be removal

and sintering,

obtained,

The other method (CF ) CO and MeCN con:i?ions

low m-p,,

are

700 Ii

of the adduct, In contrast (CF3)2C0 subjected

between

stabilities

these

Thermal 151. produces CsF which has

in vacua

structure

area.

Apparently

is retained

and the CsF particle

the surface

to thermal

after size is

area of TlF after different

treatment

step is heating

from

in vacua This --. at 373 K rather

TlF and (CF3)2C0,

TlF does not form an adduct

= F 7 CF3 [5,7],

of the

with

and under

and MeC!V is little

that the important

than reaction

pretreatment

and a high surface

decomposition

samples

Because

to give CS+OCF(CF~)~-

open bulk solid

with

For

important

by fusion was not studied,

involves

a relatively

decreased,

out in N2,

of this adduct

appearance

in

H20, dislocation

to be more

at room temperature,

CsF reacts

decomposition a porous

used

in a

the processes

adsorbed

appears

pretreatment

RF

Heating

with 976 K for CsF and 1048 K for RbF L9].

results

thermal

under-

The latter will be particularly

is carried

suggests

area,

small increases

of surface

when pretreatment

those

being

compound

is not so effective

pretreatments

both CsF and TlF, sintering

treatment

in vacua -in the surface areas

for pretreatment

for TlF due to its relatively

compared

pretreatment

Heating

The latter

temperatures

In the thermal

involved

thermal

CsF

factors,

at 356 K and this may be partly

for the increase

observed,

after

increases

48 h for CsF and 24 h, for TlF,

N2 atmosphere

of untreated

by geometric

in appearance,

significant

change

12, 3, 7, 81,

are small and those

and RbF must be determined

goes a phase

(CF ) CO, or fusion 32 used methods for activat-

with F2C0

of M+OCF(RF)2isolation

L7] and as the

adducts

of T10CE(CF3)2

are in the order is not likely,

337

There

is disagreement

concerning

TlF L6, 101 but it is greater

the lattice

to those

of RbF and KF, for which

(CF3)2CO

has been demonstrated

that TlF does not react with thermodynamic

of

adduct

L5J,

formation

It appears

(CF3)2CO

with

therefore

for kinetic

rather

than

reasons,

The thermal effect

energy

than that of CsF and comparable

pretreatment

on the surface

areas

methods

used have

of RbF samples

little

or no

and pretreatment

(CF ) CO and MeCN results in only a small increase, 32 formation therefore does not automatically lead to

with

Adduct

larger

surface

areas, and presumably

and decomposition obtained. factor

determining

RbF compared indicates

poorer

catalytic

between

for technical

G,A,K.

Egypt

the surface

area

ability

thanks

of

CsF and TlF

area is not the sole factor

J. Stoddart

Cairo,

of formation

area of RbF will be one

CsF, but comparison

for support,

Centre,

determine

surface

the generally

with

that surface

We thank S,R,C,

of an adduct

The smaller

the rates

assistance,

the National

involved,

and the Research

for leave of absence,

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