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Indian Journal of ChemistryVol. 17A. April 1979. pp. 344-347

Monolayer Parameters of Chemisorbed Films of Hydrogen,Formaldehyde, Formic Acid, Methanol, Ethanol & Ethylene Glycol

from Anodic Galvanostatic Charging CurvesP. SIDHESWARANt

Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400076

Received 20 August 1978; revised and accepted 16 October 1978

The monolayer parameters of the chemisorbed fUnIS of hydrogen. formaldehyde. methanol.formic acid. ethanol, ethylene glycol and reduced carbon dioxide on platinized-platinum elec-trodes have been evaluated from the anodic gatvanostatlc charging curves. In this study ageneral equation for electro chemisorption has also been identified. The proposed method forevaluating monolayer parameters has been shown to be superior to the catalytic methods.

IT has been well demonstrated=" that themethod of charging curves could be con-veniently utilized for the determination of theadsorption coeffscient , (K), coverage with the chemi-sorbed organic intermediate=+ (6org), the structureof the chemisorbed state, the kinetics of the oxida-tion of the chemisorbed residues>", and the energyof oxidation of the chemisorbed intermediatesv".However, an insight into the molecular cross-section,the monolayer capacities of the chemisorbed inter-mediates and t he like parameters, was not dearlybrought out thus far. In the present investigationan attempt had been made to show how thesemonolayer parameters could be evaluated and tocompare them with a few representative data avail-able in literature.

Materials and MethodsHydrogen was electrolytically generated and

purified as described elsewhere-". All the reagentsused were of AR grade. Doubly distilled methan?land ethanol were used. Formaldehyde and formicacid were used after twice distilling under reducedpressure. Ethylene glycol was used as such. Carbondioxide was generated from a Kipps and purifiedby the procedure described elsewhere+'P.

The method of preparation of the electrodes, thecell used-", the method of formation of the chemi-sorbed organic films, the washing procedure, themethod of evaluation of the organic coverages weredescribed earlier2.4.1l.l2.

Results and DiscussionDetermination of the specific surface area - The

charging curves method furnishes a route for theevaluation of the true surface area from the hydrogenarrest of the galvanostatic charging curves. Forthis purpose the identity 210 x l 0-6 coulomb = 1true em- is usedl4•l5. The platinized-platinum(2 X 1X 0·1 em") electrodes gave a true surface areaof about 2000 true cm2 per em- of geometric area

tPresentlv at Hindustan Lever Research Centre, Chakala,Bombay 400093.

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initially and after considerable ageing it decreasedto about 1000 true em", In the anodic galvano-static charging curves technique the true surfacearea is usually reported. However, in the catalyticresearch often 5, the specific surface area, is usedand expressed as m2 g-l. Hence the differencebetween the true surface and the specific surfaceremains in. dimensions. Whereas the true surface, •e?'Pressed 1ll the charging curves route, is irrespec-.tive of the mass taken, the specific surface isobtained per unit mass. Incidentally the platingprocedure provided a route for the evaluation ofthe plated mass. Hence for 2 mA per geometricern- current density, and 4 geometric em" electrodearea, and 3 hr plating period 86·4 coulombs ofelectricity was passed and it should plate out4·365 xl0-2 g of platinum as per Faraday's Law.Hence the catalytic parameter, specific surface area(coulombic) for platinum (Spt) , was 18·33 m2 g-l forfreshly plated electrodes with 2000 true em- areaand was 9·164 m2g-l for normally aged electrodes(of roughness factor 1000) and 3·67 m2g-l for ex-tremely thermally deactivated (sintered in boilingwater) electrodes-with 400 true em" surface area.

An estimate of the average particle size and porevolume - An estimate of the average particle size(ae) of the plated platinum crystallites could beevaluated from the following formula'":

5 1ae=-.-

SPt PPtw~er~ PPt is the density of platin ~m (. 21·45 at20 ) in g em-3 and the shape-coefficient 1S assumedto be 5. Hence, for an electrode of roughness factor1000, an ae of 254 A was obtained. The ae of~reshly plated electrode was about 127 A and itincreased to 635 A upon thermal deactivation.This agreed with our concept that sintering occurredduring the ageing of the electrodes-t.

The pore volume per gram (Vg) and the surfacearea per gram of the electrode are related-" throughthe a, by Eq. (2).

2Vga--e r»: SPt

•.. (1)

•.. (2)

SIDHESWARAN: MONOLAYER PARAMETERS OF CHEMISORBED FILMS

For the equilibrium electrode of 1000 roughnessfactor, the pore volume was estimated to be 0·116em" g-l.

Determination of the molecular cross-section (A",) ofthe chemisorbed intermediates - The number of molesof chemisorbed intermediates oxidized could bedetermined from the knowledge of the anodiccharging curves for the chemisorbed intermediatesand the base electrolytes. From the base electro-lyte charging curves,. the true surface area (:At)could be determined. Kmax was available eitherfrom the Brummer's current reversal technique'? orby following K as a function of hydrogen coverage(6H), while varying the concentration of the organicadsorbate- and extrapolating K values to 6H = O.The maximum amount of organic chemisorption(rmax) was obtained+ from the relation (3)org

210 X 10-6 At Kmax (3)rmax_ ...org - nF

where n is the number of electrons involved inoxidizing the chemisorbed organic residue. r;;:.~xcould be related to the At and Am through theAvagadro number by the expression (4)

r::::~x= ~t • 10~16. ~ ... (4)1»

where Am is expressed in A2. From Eqs. (3) and (4)Am is obtained as

nF 1 1 A2' (5)A",~ 210 X 10-6' Kmax . N ...

Upon introducing the constants .we g~t the novelgeneral equation (6) of electrochemisorption.

Am=7'63-n-A2 ...(6)

KmaxIn the case of chemisorbed atomic hydrogen n

=1 and Kmax=l and hence A",=7·63 A2. One couldevaluate the Am from the knowledge of nand Kmax ofthe residues using the general equation for the otherorganic residues. . .

Determination of· the Am for chemisorbed formal-dehyde-Formaldehy.d~ che'mi?Orption. on a 400geometric ern- platinized-platinum gnd electrodehas already been studied and the relevant dataare expressed elsewhere! .. Bot~ at 0 mV and 500mV the chemisorption was investigated. The adsorp-tion coefficient and the maximum number of moles'of organic chemisorption at complete coveragewere ..determined. An avarage value of 12·21 A2was arrived at for chemisorbed formaldehyde.These-films had shown to be constituted by a residuewith' potential-independent chemis.orption cross-sectional area.

Since chemisorbed formaldehyde. films have acomposition of CO, it is of interest to compare' t~e

A value with*that for chemisorbed carbon mono-xIde . for which Hightower et al.20 had regi~t~redA .-...:16·2 A2 BrunaueJ;·et aZ.2I got 16·8 A2 Living-

m , A d E R 1 23ston'e22 reported .16·3 2, an v~. oC{Qv~recorded a value of 15·1 A.2fromf.heIiquid density(0.793 g rnl'"). Eva obtained a value of 8·5 ~2as the, surface area of platinum s~te fro~ th~ ratioof CO/Pt(surfj = 0·55. On a similar lOgIC,.if 15·1

,.

A2 corresponded to the chemisorbed state of, form-aldehyde, then the linear CO fraction should beabout 80% and the bridged CO should be about20% to get J 2·21 A2 as the average surface areaof the chemisorbed formaldehyde. At monolayercoverages the guest (adsorbate) adjusted with thesurface area of the host (adsorbent) ..

Determination of Am of chemisorbed methanol-Methanol dissociatively chemisorbed on platinized-platinum electrodes and the resulting residue CHO

* * *gave' n = 3. Futher the Kmax was found" to be1·12. From these values Am was determined andwas found to be 20·43 A2 This agreed very wellwith the value of 22·22 A2 evaluated from b" van

( b)2/3der Waals constant using Am = 0·96 - , where b. Nfor methanol = 67·1 ml mol-'.

Usefulness of the general equation - The generalEq. (6) could be used to evaluate n in the caseslike glycol, ethanol, formic acid and reduced carbondioxide. For this purpose the Kmax values obtainedfrom our experiments were used (Table 1).

Kmax for reduced CO2 is 1·12 and the chernisorbedresidue could be mostly CO and for it n = 2 and

* *hence Am = 13·62 A2. However, with COOll as

*the chemisorbed residue, we have n = 1 and Am= 6·81 A2. In as much as carbon monoxide itselfoccupied about 16·2 A2, and atomic hydrogen itselfcovered 7·63 A2, the value 6·81 A2 becomes unten-able. This suggests that CO is the chemisorbed..species resulting from carbon dioxide and n = 2and A", = 13·62 A2 appeared to describe appro-priately the parameters of the chemisorbed film ofthe reduced carbon dioxide. It is also of interestto note that the Am for CO2 from adsorption measure-rnentsw is 17 A2. If, however, we assume n = 3(as in the case of CHO) we would get A", value...greater than 17 A2, which cannot be true in thechernisorbed state. Furthermore, CHO cannot be

. * * *mechanistically probable from CO2 during reductivechemisorption despite the fact that such an inter-mediate has been proposed earlier-.

TABLE 1 - CHEMISORPTIONPARAMETERSOF VARIOUSADSORBATES

Adsorbate Kmax n* Am(A2) M 103. x",(g of

residueper g ofPt/Pt)

CH20 1'25 2 12'21 28 3'49CH30H 1'12 3 20'43 29 2·16Reduced CO2 1-12 (2) 13'62 28 3·13Ethanol (O-type) 1'0 (3) 22·89 29 1'93Ethanol (total film) 1'6 (5) .23-84Ethylene glycol 1'46 (4) 20·90 58 4'22HCOOH 1-06 (2) 14·39 44 4·65

*Values in parentheses a~e calculated val~es.

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rINDIAN J. CHEM., VOL. 17A, APRIL 1979

Determination of Am of chemisorbed ethanol-The a-type (more oxidized type) and the CH-type(less oxidized type) residues'" are the two typesof residues reported for chemisorbed ethanol'".The a-type residue could be isolated and the follow-ing values were obtained: n = 3, Km•x = 1·0;and hence Am = 22·89 A2. It is interesting to notethat similar results have been obtained for ethane-"(22·7 A2) and ethylene28•29 (22·5 M).

The value of n for the complete film of ethanolcould not be obtained. Fortunately this difficultycould be overcome by using the general Eq. (6).For Km•x = 1·66, the Am values of a-type speciesof ethanol, ethane and ethylene are 22·88, 22·7 and22·5 A2 respectively. If we consider the geometriesof the adsorbent. some interesting informationscould be arrived at. The lattice spacing'" is 2·77A2 and hence the area of one atom of the adsorbentis 6·03 A2. For a multi-centred chemisorptioneither twice this area is needed (as with chemisorbedformaldehyde and reduced carbon dioxide) or threetimes the area (as in the case of chemisorbed metha-nol) or four times the area. Four atoms of theadsorbent should expose 24·12 A2. Hence a reason-able value of 23 A 2 was assumed to be coveredby one chemisorbed ethanol residue in the completelayer. With Am = 22·89 A2. n was found to be4·8 and with Am = 20·43 A2. 1£ was found to be 4·28.Hence n = 5 was taken as' acceptable and the Amfor the complete layer was 23·84 A2. .

Evaluation of the Am ethylene glycol and formicacid films - As in the case of chemisorbed ethanollayer here too n was computed by the trial methodfrom Km•x = 1·48; n = 4 and Am = 20·9 A2 wereobtained for glycol films. . ..

If we assume n = 2 in the case of formic acid weget A", = 14·39 A2 with the experimental Km•x =1·06; n = 1 would be impermissible as with reducedcarbon dioxide, Thi.s result suggested that COO. .would be the true chemisorbed state of formicacid.

Determination of the monolayer. capacities - ~hefollowing equation had been used24 for computationof the value of Xm the adsorbate per gram of adsor-bent (Pt/Pt].

2 -1) xmA".NIO-16SPt (m g = MI04 ... (7)

The monolaver capacities of these adsorbatesfor 9·164 m2 per gram electrode area are calculatedfrom the knowledge of A,,. and M values. The~eare presented in Table 1. The Xm value for atomichydrogen was found to be 1·99 X 10-4 ~ of ~ pergram of Pt/Pt. In view of the uncertainty in thenature of the chemisorbed residue of the com-plete ethanol layer, the Xm was n?t evaluated. Forchemisorbed glycol 1£ = 4 would Imply the presenceof HOC- COR as the chemisorbed residue and

hence· ;he· ~orresponding M valu~ was 58 whichgave an x,,. of 4·22 X 10-3 g of residue per gram ofthe plated mass.

Dependence of the open circuit behaviour with thevalue of 1£ - The potential decays under opefol-circuit were larger for formaldehyde and formic

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acid as compared to those of methanol, ethanoland glycol at identical concentration and tempera-tures. Even hydrogen chemisorption showed muchfaster potential decay. These appear to demons-trate that the potential decay is faster for n = 1and n = 2 than for the cases with n = 3 or 4 or 5.This could easily be understood from the fact thatthe probability of finding more number of sites.would be smaller and hence the slower potential.decay was noticed with reactions with larger n.

Furthermore, each of the water dipoles occupied=10·8-14·8 A2 and one water dipole replacement wouldrelease 2 adsorption sites and hence reactions withn = 1 and n = 2 occurred much faster. As 2 or3 water dipoles would have to be released for n = 3,4 or 5 the decay was not faster as anticipated.

Special advantages of the present method - Inthe case of conventional catalytic methods theaccuracy of evaluation depends upon the existenceof the knee, whereas the proposed method is simpleand model-less. In the catalytic methods theremoval of impurities are very difficult, whereasin the electrochemical route the 'double-layer regionof potential (400 mV-700 mV wrt reversible hydro-gen electrode) guarantees extreme cleanliness of theelectrode. Furthermore, there is a remarkable repro-ducibility in the charging curves even after months.Such reproducibility does not exist in the usualcatalytic methods due to prolonged poisoning.

AcknowledgementThe author is grateful to Prof. Hira Lal for his

keen interest in the initial stages of this study. Heis. also indebted to Prof. S. Sathyanarayana forhIS valuable help through discussions at differentstages of the work. He thanks the Director IndianInstitu~e. of Technology, Bombay, for p~ovidingthe facility to carry out this investigation,

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