Indian Journal of ChemistryVol. 22A, February 1983, pp. 138-139

Not e sFormation of Surface Nitrate Complexesby Adsorption of Nitric Oxide & NitrogenDioxide on Asbestos Mineral Chrysotile

R A ROSS

Alcan International Limited, Kingston Laboratories,Box 8400, Kingston, Canada

and

V VISHWANATHAN*

Department of Chemistry, University of West Indies,Mona, Kingston 7, Jamaica

Received 3May 1982 ; revised 26 June 1982 ;accepted 17 July 1982

Infrared spectra of NO and NO. adsorbed on asbestosmineral chrysotile exhibit strong peaks at 1380 and 1630 cm='assignable to nitrate as well as nitro and nitrito species sugges-ting that such species are formed by the oxidation of NO andNO. by surface oxygen. Thermal desorption spectra of adsor-bed species support the suggestion that surface nitratecomplexes may be formed in the initial stages of adsorption.

Infrared spectra of nitrogen oxides adsorbed onmetals and metal oxides have shown that both NOand N02, like CO and CO2 (ref. 1) react with surfaceoxide ions to form surface nitrate. Earlier Ross andcoworkers from our laboratories studied the adsorp-tion of sulphur dioxide on porous solids such asy-alumina2, activated charcoal" and on asbestosmineral chrysotile" -a hydrous silicate with empiricalformula, Mg3(Si205)(OH)4' The identification ofsurface nitrate complexes arising from the adsorptionof nitric oxide and nitrogen dioxide on chrysotile,employing IR and mass spectrometric techniques,forms the subject matter of the present note. Thesorption study may also be of interest to investigatorsconcerned with the evaluation of the carcinogenicpotential of asbestos" since their surface propertiesmight have some influence on their toxicity.

Chrysotile (Lake Asbestos, Black Lake, Quebec)with surface area of 40 m2g-1 (BET; nitrogen) wasused. Nitric oxide and nitrogen dioxide, anhydrousgrade (>99.99% pure), were supplied by Mathesonof Canada.

Adsorption studies were carried out at theadsorbate pressures 5-30 torr for NO and 10-1-4 torrfor N02• The desorbed species were analysed by aVacuum Generator Micromass Q7 mass spectrometerattached to a circulation loop via a capillary leakand operated at 10-5 torr, The IR spectra wererecorded (1 wt % chrysotile) on a Beckmann IR 12spectrophotometer. For a comparison study of theabsorption bands of nitrate formed during theexposure of NO and N02 on the chrysotile surface,

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the original sample was treated with O.lM aqueoussolution of magnesium nitrate. The suspension wasstirred for 12 hr at 298 K, filtered and then dried inan oven at 423 K overnight. Magnesium was chosenas a suitable cation in Mg(N03)2 solution since thecrystallographic structure of chrysotile has a brucitelayer, Mg(OHh, on the top.

IR spectra were obtained for (i) original sample ofchrysolite; (ii) sample exposed to NO at 298 K for6 hr followed by evacuation for 3 hr; (iii) sampleexposed to N02 at 298 K for 6 hr followed byevacuation for 3 hr; (iv) sample exposed to 30 torrNO at 298 K for 16 hr prior to evacuation for 3 hrand (v) sample treated with magnesium nitratesolution. All the spectra exhibited two distinct peaksat 1380 and 1630 crrr ', their intensity dependingupon the nature of the samples. For example thetwo peaks obtained for sample (ii) and (iii) wereof equal intensity where as for sample (iv) the peakat 1380 cm-1 was of greater intensity. These peakswere assigned" to the vibrational frequencies ofvarious nitrate as well as nitro and nitritocomplexes+s '. These observations suggest that NOmay be oxidised by surface oxygen to form N02(ads)which ultimately yields a surface nitrate, NO;,species perhaps via a nitrite, NO;, intermediate.On the oxidised surface of chrysotile, the active sitesavailable for the catalysed reaction are the same asthe surface active oxygen and hence the amount ofN02(ads) formed from adsorbed NO might beproportional to the amount of surface active oxygenas shown in scheme 1.

o o-Il I

·-'--M·-- M----: !. .

0-I

/N,o 0

NO _ I I- [ J - ----~----~----

Scheme 1

The formation of similar nitrate species from N02adsorption may be envisaged in an analogous waythrough interaction of the adsorbed molecule withcentres such as 0-, the bridged oxygen atoms. Thevery strong peak at 1380 cnr? exhibited by thesample treated with magnesium nitrate solution tendsto support these proposals since vibrational frequencyof surface nitrate species in some inorganic complexeshas been reported to lie at 1380 crrr+.

The thermal desorption spectra obtained from massspectrometric analysis of (a) as received sample ofchrysotile; (b) sampJe treated with 30 torr NO at298 K for 16 hr and (c) sample (b) evacuated for 3 hrshowed that as received sample consisted of H20(mlz 18), CO (m/z 28) and 02(m/z 32) while sample(b) gave an additional peak at mjz 30. From themass spectral tracking pattern, this peak could be

assigned to NO. The evolution of NO and O2 withsimilar maxima at the same temperature suggeststhat two species (NO and O2) could have dissociatedfrom a single parent molecule such as N02• Tracesof N2 (m/z 28) and NaO (m/z 44) have been reported'!in similar studies. The disappearance of the peak(m/z 44) for N20 in the present study suggests thatfurther dissociation of this molecule might haveoccurred giving rise to N2 and NO. This couldaccount for the increase in intensity of the peak atmlz 28 and a contribution to the peak mjz 30.

The presence of surface nitrate species was alsoconfirmed by classical spot test with brucine-" reagentwhich gave a red colouration when its sulphuric acidsolution is mixed even with minute amounts« 0.06 fl-g) of nitrates. The as received chrysotilesample gave a negative test, samples with pre-adsorbed NO at low coverage gave red colourationwhich changed to yellow after 15 min, and thesamples with N02 pre-adsorbed behaved like thosetreated with Mg(N03)2 solution gave an instantane-ous colour change from red to yellow on contactwith the reagent.

NOTES

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