Indian Journal of ChemistryVol. 35A, June 1996, pp, 443-453

Advances in Contemporary Research

Photocatalytic reduction of dinitrogen to ammonia

K T Ranjit &'8 Viswanathan"

Department of Chemistry, Indian Institute of Technology, Madras 600 036, India

Received 22 August 1995; revised 11 January 1996

The fixation of dinitrogen to ammonia via Haber's process is an energy intensive process and henceit is attractive to search for alternate pathways for the fixation of dinitrogen molecule under milder con-ditions. The photocatalytic reduction of dinitrogen to ammonia represents one such proposition, Thediscovery of this reaction by Schrauzer and Guth in 1977 prompted many workers to study this intrigu-ing reaction and several studies have been reported in literature demonstrating the feasibility of thisprocess. However, the low yields of ammonia obtained thus far, makes this process an uneconomicalproposition. This article discusses the problems associated with the photoreduction of dinitrogen andgives an appraisal of the aspects concerning the photofixation of dinitrogen molecule on semiconductorsystems.

ProfB Viswanathan DrKT Ranjit

Prof B Viswanathan has been with the Department of Chem-istry, Indian Institute of Technology, Madras for the past 26years. He has published more than 300 research papers andedited/authored 10 books. His major areas of research interestincludes heterogeneous catalysis, surface science, solid statechemistry, chemical kinetics and hydrogen energy. He hadpostdoctoral experience at The Fritz Haber Institut der MPG,Berlin and Institute of Catalysis Kokkaido University, Japan.

Dr K T Ranjit obtained his Ph.D from the Indian Institute ofTechnology. Madras in 1995. and is currently working, as Re-search Associate at the Institute of Chemistry and Farkas Cen-tre for light induced processes, The Hebrew University, JeTUSlrlem, Israel. His main research interests are in the areas ofphotocatalysis, photoelectrochemical cells, and other photo-processes,

IntroductionThe reduction of dinitrogen is an important

part of the nitrogen cycle in nature'. The products I

of this process such as hydrazine and ammoniahave enormous economic value. However, to datethe common way of industrial production of am-monia has been the classicial Haber's process/namely direct reaction of gaseous nitrogen and hy-drogen on the surface of a catalyst-".

N2 + 3H2 catalyst" 2NH3 ... (1)

This process requires high pressure and tempera-ture to activate the reactants and thus is energy in-tensive.

Dinitrogen is perhaps the most challenging sub-strate to reduce electrocatalytically. Rupture of thefirst :It bond of Ilhe N - N triple bond requires 130kcal mol- l, and the ionization potential of the din-itrogen molecule is similar to that of argon. The fi-xation of dinitrogen with hydrogen to ammonia isone of the important processes, demanding sub-stantial amounts of natural gas, oil or coal for theproduction of the required hydrogen. It is there-fore attractive to search for alternative pathwaysto produce ammonia.

One of the alternatives would be the catalytic fi-xation process as practiced by nature. One tries tosimulate the mechanism of biochemical processcarried out by dinitrogen fixing microbes, whichoperate at atmospheric pressure and room tem-

444 INDIAN J CHEM, SEe. A, JUNE 1996

perature. The reducing enzyme, commonly knownas nitrogenase, has been isolated" from variousmicrooraganisms e.g. those belonging to the genusazotobacter, as well as from other soil bacteria andblue-green algae. During the past decade much in-formation has been accumulated on the propertiesand reactions of the enzyme?". But, in spite ofconsiderable efforts, most of the observed reac-tions remained obscure and no acceptable me-chanism of the reduction of nitrogen could be for-mulated.

Another alternate path utilizes activation of din-itrogen molecule by compexation with transitionmetals such as Ti, Ru, Os, Rh, Re, Mo and Coand subsequent reduction with strong reducingagents. The discovery of [Ru(NH3)sN1F + complex?prompted several researchers to investigate in thisdirection. Reviews on this and on related aspectshave been presented by Kuchynka'", Fisher andVenuto II , Ozaki and Aika 12 and Chatt et af.13.

The electrolytic and chemical reduction of mo-lecular dinitrogen have been reported by VanTammelen and his coworkers':"!". These workershave used titanium isopropoxide in 1,2-dimethox-yethane based on previous studies oil. dinigrogenfixation via nitrogen metallo-organic complexes 14.

Electrochemical fixation of dinitrogen is also re-ported to be possible if dinitrogen is subjected tosilent discharge prior to bubbling into aqueouselectrolytes of sulphuric acid or potassium hydrox-ide!' but this approach should be excluded fromdinitrogen fixation under mild conditions.

An electrochemical reduction catalyzed byTi(OHkMo(III) system has been performed byGrodyskii et al.lx on a mercury cathode in an alka-line methanol solution under mild conditions. Adirect electrochemical reduction of dinitrogen overiron cathode and stainless steel anode at 31 RKand atmospheric pressure in an aqueous medium(6 M NaOH) has been achieved!". However, inthese methods large amounts of reducing agentssuch as sodium dihydronaphthylide!', metallic alu-minium!" potassium?", Ti(OH)2' vanadium(II), mo-lybdenum(II), niobium(III) and tantalum(III) (refs21-23) have to be used.

The idea that molybdenum as the central atomis responsible for high catalytic activity of the en-zyme nitrogenase prompted the research of manymolybdenum based model systems for the activa-tion of dinitrogen molecule". The system whichmatches the catalytic activity of nitrogenase atequal reaction conditions is that described by Shi-lova and coworkers+:". Strelets et al." have sug-gested an improved route for the electrochemical

synthesis of ammonia based on controlled-potentialelectrolysis,

The possibility of providing. part of the requiredenergy input of this reaction from sunlight hasspurred efforts to use semiconductor materials un-der band gap illumination as photoelectrodes.With illuminated p-gallium phosphide in a non-aqueous electrolyte of titanium isopropoxide andAlCl3 dissolved in 1.2-dimethoxyethane and a sac-rificial aluminium anode, the reduction of nitrogenhas occurred+-" with a production rate of 10 ~4

rnol.'(h ern- electrode surface). In a detailed studyof this reaction?", the possibility of substitutingTiCI3, VCl3 or CrCI3 has been tested. Ammonia isproduced at best at a current efficiency of 0.9%.Traces of hydrazine have also been detected.

A completely new route has been suggested bySchrauzer and Guth31-.". Ammonia is producedfrom dinitrogen and water on wet Ti02 powder ina batch reactor under the stimulus of near UV ra-diation according to the reaction (2)

N2+H20...!:!..Y.2NH3+3/202;~G=678kJmol :, " \.')

~ G is the Gibbs energy stored in one mole of am-monia produced, This reaction is endergonic, thusrequiring an input of energy.

After the above reports, several studies34-1!,

have been made on this photoreaction which hasdefinitively proved its feasibility in heterogeneousconditions.

The advance made in the photocatalytic reduc-tion of dinitrogen forms the subject of this presen-tation. Although at present this reaction is notpractically viable, significant progress has beenmade in devising novel catalysts and in under-standing the reaction mechanism. However, onehas to realise that there are still certain unresolvedproblems in the photocatalytic reduction of dini-trogen.

The photoreduction of dinitrogen is difficultdue to the following reasons:

1. The reduction of one molecule of dinitrogenvia equations 3A and 3B

N2 + 6H+ + 6r~ -+ 2NH3

3H20 + 6H+ -+ 3/202 + 6H+

". (3A)

... (3B)

involves a transfer of six electrons.2. Even though the energetics of dinitrogen re-

duction is similar to that of water oxidation theformer reaction experiences a higher activationbarrier as compared to other reactions.

3. Dinitrogen chemisorption and activation are

RANJIT et af.: PHOTOCATALYTIC REDUCTION OF DlNITROGEN TO AMMONIA 445

essential steps in the production of ammonia.Most of the photocatalysis used in dinitrogen re-duction possess poor dinitrogen chemisorption ca-pacity.

4. Photogenerated ammonia accumulates in thesolution and participates in hack reaction-. This i...typically true when the reactions are carried out illbatch reactors.

Catalytic Nature of the ProcessVan Damme and HaJ]·17cast doubts on the ca-

talytic nature of the reaction discovered hySchrauzer and Guth. The formation of ammoniawhich corresponds to few OH layers, the signili-cant decline of the reaction rate after few hoursand the relatively low turnover number as com-pared to the value obtained for the photoassistcddecomposition of water in a photoelectrochcmicalcell (four orders of magnitude higher) are all till'reasons which suggest the non-catalytic nature ()rthe reaction.

The unfavourahle !1G value (~G2'1K.C = 183 kcalmol' I) of this reaction has prompted Forrncnuar 1 Teichner" to criticize the photoproduction orammonia.

Recently, Edwards et a/.NAII and others41.4c havereported their inefficiency to photo reduce molecu-lar dinitrogen on heterogeneous surfaces. Augugli-aro and Soria+' have argued that the inability ofEdwards et al. to photoproduce ammonia may bedue to the fact that most of their photoreductionexperiments were conducted with "xerogels"whose photocatalytic activity has not been docu-mented in literature. Further they stress the pointthat the specimen preparation is crucial for thesuccess of the process.

In support of the arguments advanced byAugugliaro and Soria", Palmisano et al." statethat the experimental conditions employed by Ed-wards et al. would have been a prime factor fortheir inability to observe ammonia. Furthermore,they point out that the photocatalytic reduction ofdinitrogen to ammonia in combination with photo-electrolysis of water is thermodynamically a feasi-ble reaction. This reaction is possible because thepotential corresponding to the band gap inTi02(Ech) is negative vs the normal hydrogen elec-trode (NHE XEcb< EH+ 1H

2)' From

the Pourbaix diagram'", the reduction poltentialE~2!NH is positive vs NHE (ENZ,NH

J>EH+ 'H;/

Since the formation of NH1 according to Eq. (1) IS

known, it follows that irrespective of the mechan-ism, the photoreduction of dinitrogen according toEq. (2) is feasible given the position of the bandedges of TiOe.

Davies and Edwards"; however, remain skepticabout the results obtained thus far and still main-tain that no more effect should be expended onproduction of tiny amounts of ammonia from nat-ural nitrogen.

The claim that ammonia can be synthesized ca-t.ilytically from nitrogen and water in the presenceof simple oxide based semiconductors suggests aprocess of overwhelming importance, Hence, it isenticing for many researchers to conduct a largenumber of experiments with various semiconduc-tor catalyst« hut they appear to produce very smallvicldx of ammonia.. In view of the controversies existing in literature.and the luct that no significant breakthrough hashccn achieved in enhancing the yield of ammonia,till' 1I11l'stion whether the process is really catalyticor no: seems to be still not answered. However,the f;lct that many laboratories have reported thephotoreduction of dinitrogen under similar sets ofexperimental conditions points out that the trendsobserved in photoreactivity are not due to anycontamination problems. Thus from the analysisof the above factors no firm statement can be giv-en on the catalytic nature of this intriguing reac-tion,

The P hotoactivity of T i02 Based CatalystsAs early as 1941 it has been reported"? that

TiO, in soil could increase the velocity of dini-trogen fixation under sunlight. Schrauzer andGuth" in reinvestigating the photoreduction ofC2Hc on Ti02 have reported that the hydrocarbonproduct distribution changed when the argon gaswas replaced by nitrogen as the protective gas,This suggested that chemisorbed N2 was reducedunder these conditions. The detection of ammoniaand hydrazine confirmed this hypothesis and thusthis discovery led to the development of proto-types of nitrogen reducing solar cells. The authorsalso have found that the activity of TiO 1 de-pended on the partial pressure of ammonia, calci-nation temperature of the catalysts, reaction time,temperature and the nature of metal dopants. Thesalient results of their study are given in Tables 1and 2 respectively.

An important observation of their study is thatrutile phase of Ti02 is more active than the ana-tase form. This is a rather surprising result giventhe fact that the conduction band edge of anataselies at a more negative potential compared to ru-tile. If this is indeed true, the position of the con-duction band edge (reducing ability) would onlyhave a marginal influence on the yield of ammoniafor semiconductors which have almost similar flat-

446 INDIAN J CHEM, SEC. A, JUNE 1996

Table I-Yields of ammonia and hydrazine from nitrogen onexposure of undoped and Fe doped TiOz to intermittentsunlight at temperatures between 25 and 35°C (From ref. 31]

Conditions and catalysts Yield in !.l moles

Ammonia Hydrazine

TiO,. I aim ArTiO~. 0.5 atm NlTiO" 1 atrn NzTiO~/0.05 wt% Fe,O,1 I atrn ArTi0110.05 wt% Fe~OJ I atrn N~

TiOzIO.2 wl% Fe~O,1 1 atrn NlTi01/0.2 wt% Fe,O,/ 1atrn airTiO, 10.2 wt% Fe,O, 11 atm Ar

0.000.741.55

0.003.204.481.89

0.00

0.000.050.17

0.000.090.120.07

0.00

hand potential at a particular pH. This suggeststhat the photoexcitation process is independent ofactivation of dinigrogen molecule and the key stepis probably the activation of the dinitrogen mole-cule. This necessarily means that the reductioncenters designed in the semiconductor should becapable of activation of dinitrogen. However, theunderstanding of bonding of dinitrogen at the re-duction centers of the hitherto designed semicon-ductors systems has not been examined in detail.

Activity oj iron doped Ti02 catalystsThe conventional Haber's process employs an

iron based catalyst and since iron oxide is knownto activate the dinitrogen molecule, iron dopedTiOz catalysts have been examined in detail forthe photocatalytic reduction of dinitrogen. Someof the salient features4R-54 observed are:

(i) the reaction does not occur in the absence ofthe catalyst;

(ii) ammonia production declines significantly inthe absence of radiation (these two points indicatethat both light and catalyst are required for thisreaction i.e. it is truly a photocatalytic reaction);

(iii) specimens containing the Fe2TiOs phaseshow only slight activity; and

(iv) only the specimens containing iron ions insolid solution in the Ti02 lattice (irrespective ofthe Ti02 phase, anatase or rutile) in an amount0.4-0.5 at % are active, with the coprecipitatedspecimens being more active than the impregnatedones. Ammonia production is of the order of 50-100 ILgh - I per gram of catalyst. The role of ironions in solid solution is to improve the charge car-riers by trapping the electrons.

The photocatalytic reactor configuration is alsoan important parameter which determines the effi-ciency of any light assisted reaction. When aphotocatalytic reaction is carried out in a gas-solid

Table 2-Jiffect of metals on nitrogen photoreduction. Yieldsof ammonia after 3 h of UV radiationa t 40''1', containing 0.2 g

of doped titania and Nz at 1 atm (from ref. 31]

Metal Rutile/ anatase NH J yield (!.l mol)

Fe -95:5 6.40

Co 30:70 3.80Mo 30:70 4.00Ni 10:90 1.76

None 95:5 1.35Pd - 5:95 0.65Pt - 5:95 0.43Ag - 5:95 0.21.Au - 5:95 0.25V - 5:95 0.25Cr - 5:95 0.22Pb - 5:95 0.19Cu - 5:95 0.17

reactor, in addition to the requirement of goodcontact between reactants and catalysts, it is alsonecessary to achieve efficient exposure of the .ca-talyst to light irradiation. The packed bed type )freactor, so often used for gas-solid. reactions, isnot suitable at all because light energy cannot pen-etrate into the interior of the bed. Fluidising thecatalyst will not only enhance light penetration,but also bring more catalyst particles into contactwith light. In order to improve the fluidisationcharacteristics, Yue et al.55.50 supported TiOz pow-der with y-alumina.

Since the preparation conditions influence theyield of ammonia new routes for the preparationof metal doped Ti02 powders have been devised.A completely new route has been suggested byRadford and Francis". The preparation of ironclusters via the metal vapour synthesis of a ther-mally unstable organometallic complex appears toprovide a facile new route for doping titanium di-oxide to produce photoactive materials for dini-trogen reduction.

An attempt to interpret the photocatalytic re-sults requires a detailed knowledge of the structu-ral features of the semiconductor. Among theseare the phase composition, solubility, location anddispersion of dopant ions. Schiavello et al.41.5H-6Xhave attempted to rationalize the results obtainedfrom the photocatalytic studies with the physico-chemical properties of the catalysts. Some aspectsof the photocatalytic data may be rationalizedlooking at the structural features of the FeZO)-TiOz system. The phase composition of the catal-yst affects the photocatalytic yield ESR spectraand Mossbauer spectroscopy have been used to

RANJIT et al.: PHOTOCATALYTIC REDUCTION OF DINITROGEN TO AMMONIA

analyze the phase composition. It has been foundthat ferric ions in solid solution in the Ti02 latticeseemed to be responsible for the photoactivity.

Since only non-surface iron ions are active,their role must be to influence the photogenera-tion/ and or the evolution of electrons and holes inthe Ti02 grains. This can be explained by assum-ing 'Fe3 + ions acting as traps for the electrons, al-lowing the holes to reach the surface and give riseto highly reactive OH radicals which promote thechemical reactions at the surface. These steps canbe represented as: .

Ti02 h Ti02 ed, + Ti02Hv+b ... (4)

.. , (5)

... (6)h+ +OHsurf-OH.urf

This electron trapping role for the Fe.1+ ions wasconfirmed by ESR results. This process can beirreversible if the holes generate species adsorbedon the surface. The charge separation favoured byFe' + ions, helps the nitrogen adsorption by itsreaction with surface species activated by holes.The role of Fe:+ ions is therefore to help the se-partion of electrons and holes by trapping the for-mer and allowing the generated surface OH tohave an increased liftetime thus making it possiblefor these highly reactive radicals to give at the sur-face some type of activated [N2 •••. 'OHj* complex

N2+OH·-[N2·····OHj* .. , (7)

The hypothesis of the existence of a specificN2 .••• 'OHinteraction was supported by results ofIR studies. The overall turnover number for dini-trogen reduction involving as high as six electronsper atom can then be reached before deactivationshowing the catalytic character of the participationof Fe in this process.

Bickley et al. in a recent study?" have correlatedthe surface properties of iron doped Ti02 catal-ysts with their photocatalytic properties for reduc-tion of dinitrogen to ammonia. The introductionof Fe3+ from 0.5 to 10 atom % into the Ti02 ma-trix has a drastic effect on the structural and mag-netic properties of the materials. In their study7o,71

the complexities of solid state chemistry have beenrelated to (i) the phase transition from anatase torutile, (ii) the difference in the solubility limits forthe dispersed form of Fe' + in these phases, (iii)precipitation of an iron(IIl) oxide phase and (iv)conversion of iron(II1) oxide into pseudo-brookitephase by interaction with TiOz lattice at elevatedtemperatures. Thus it is difficult for one to deline-ate the observed photocatalytic activity with fund-

447

amental characteristics of the doped system. How-ever, from the results obtained by various groups,one can conclude that only single-phase systemsare active. In addition, the inactivity of the multi-phasic solids has been considered in terms of themasking of the active phase by surface layerswhich create electronic heterojunctions that en-courage electron-hole recombination. Energy dis-persive X ray analysis (EDX) measurements revealinhomogenities in the distribution of surface ionicspecies in such cases, which are consistent withthe trends observed in photocatalytic activity.

Activity of other doped 1'i02 catalystsSince it is demonstrated that the presence of

dopants is essential for the photocatalytic reduc-tion of dinitrogen to ammonia, it is of interest tosee if the nature of the dopant had any effect onthe photocatalytic activity. Since the ionic radii ofCr.1+ is similar to that of Ti4 +, Cr doped systemshave been investigated in de.tail72.7.1 and the mainconclusions can be summarized as follows:

(i) Incorporation of chromium decreases thestrength of surface Lewis acid sites. Bronsted sitesare associated with the presence of Cr, since theyare not detected in pure titania.

(ii) Interaction of Cr ions with OH groups mod-ifies the surface properties of the support and con-sequently influences the photocatalytic activity.

(iii) The diffusion length of the minority carriersis strongly influenced by the presence of chromiumspecies?" (0.2 urn for Cr ion-doped Ti02 against 1urn for pure Ti02).

(iv) It improves the capability to adsorb N2molecules. From a chemical point of view, dopingof Ti02 with p-type or lower altervalent cationsuch as Cr3 + is equivalent to the introduction ofdefects, i.e., TP + in the lattice. Since it has beenreported''! that Ti02 samples containing TP + areable to activate N 2 at room temperature, dopinghas a positive effect for the activation of dinitrog-en molecule which is probably one of the keysteps in the reduction process. However, the activ-ity of the chromium doped systems has beenfound to be lower compared to the iron dopedsystems. The observed difference in activity maybe explained by intrinsic features of the system.The diffusion length of the minority carriers has avalue?" of 0.2 urn for Cr doped Ti02 as against 2urn for iron doped Ti02• This indicates that theelectron-hole recombination is higher for chrom-ium doped Ti02 catalysts than for iron doped ca-talysts with the consequence of lower photoactivityof Cr doped Ti02 catalysts.

Magnesium doped Ti02 has been found to con-

448 INDIAN J CHEM, SEe. A, JUNE 1996

Table 3-Relation between yield of ammonia and conductionband energy of semiconductor catalysts (Irradiation 100 Whigh pressure mercury lamp, reaction vessel 700ml capacity,

5 h irradiation at 38°C) [from ref. 42]

Yield of NH) Conduction band energy V(11 mo1/0.3 g cat) (against NHE)

1.6 - 0.2

2.1 -0.63.2 -0.74.6 -IA

Catalyst

TiO,ZnOCdSGaP

vert dinitrogen to ammonia upon UV irradiation 77.

The efficiency of this catalyst is found to dependon the level of Mg2+ doping mixing temperatureand duration of heating of the sample.

Activityofmetal loaded Ti02A series of metal doped Tia2 catalysts have

been evaluated for the photocatalytic reduction ofdinitrogen to ammonia in an aqueous suspensionof the semiconductor powder?". The yield of am-monia incsreases hy loading with Pt and Pd metalon TiO:, and IS highest (ahout 9 ~g/5 h of illumi-nation) with mixed TiO/ Rua} catalyst. Tiac ca-talysts coated with metals such as silver, mercuryand platinum have been found to produce pre-dominantly ammonia"). Irradiated suspensions of(Fe, Ru, Os)/TiO:, catalysts have been found toreduce N:, tt) NH,. A mechanism involving inter-action of Hilt!, and N:, is proposed to explain thereactivity and deactivation of these catalysts withthe help of ESR measurements. The activity isfound to he in the order Ru> Fe> as while re-verse order is found for the stability. A series ofmetal loaded Tia:, catalysts have been examinedfor the photocatalytic reduction of N~ to NH, byRanjit et al.", In their investigation, they observedthat the yield of ammonia is influenced hy nature,amount and method of metal loading. The opti-mum metal loading varies with the nature of me-tal. A correlation between the ammonia yield andthe intermediary M - H hond strength is esta-blished suggesting that photocatalysts exhibiting ahigh barrier for hydrogen evolution lead to highammonia yield presumably through insertion of ni-trogen into the M - H bond.

Phosoacuvhy ol other SemiconductorsA variety of semiconductors have been exa-

mined for the photocatalystic reduction of dini-trogen to ammonia. Since in most of the cases thepure semiconductor exhibited low activity, thesemiconductors were either loaded with metals orby oxide in order to improve the activity.

Table 4- Yields of ammonia and hydrogen over various catal-ysts I.reaction temperature 4()"C, H,O; 5 ml. N,: O.Y atm.

catalyst 0.5 g) [trom ref. ~31

Photocatalyst Yields in micromol. 2 h

NH, H,

SrTiO, OAI 0,00

BaTiO, 0.K7 U.UU

NiO-SrTiO, ().~Y 5.ISRuO,-NiO-SrTiO, 2.SI 4.74

Titanium exchanged SA zeolite has been usedas catalysts and the rate of ammonia production isfound to increase to a maximum of O.S mg pCI'gram of catalyst per hour'". However, a milk)precipitate is obtained in the reaction vessel afrc:irradiation indicating a net loss of catalyst ()\ cr ,Ilong period of time.

Miyama et al.42 have reported that the yield otammonia was found to increase as the conductionband energy of the semiconductor becomes nega-tive. The results are given in Table 3.

Photocatalytic synthesis of ammonia from nitrogen and water and its photodecomposition havebeen investigated over SrTiO, and BaTiO, ba-edcatalysts":'. It was demonstrated that the Yield oj

ammonia formation increased by use of RuO} andNif) doped three component catalysts. These canbe attribtued to the increased activities of NrO Iorhydrogen evolution and Rua2 for dinitrogen acti-vation. The results are given in Table 4. The ratesof ammonia formation were initially high but ra-pidly dropped. This is attributed to a decrease inphotocatalytic activity and also due to the photo-decomposition of ammonia. This is confirmed hystudying the decomposition of NH4aH. The mo-lar ratio of H2 and N} obtained is the stoichiomet-ric value of three. Thus as ammonia is accumulat-ed, the photodecomposition of ammonia becomesconsiderable.

Lichtin et al.~4-Hl) have studied the photoassistedsolid-catalyzed reduction of H20 of Nz and mix-tures of N2 and ca2• Active catalysts includedCoO, C01a4, Co-Me-Al-oxide, c-FeO, and manyother oxides such as Zn-Fe-oxide, La-Ni-oxideand La-Ti-oxide. An interesing ohservation oftheir studies is that several oxides with band gapenergies smaller in magnitude than EO = 1.23 V forthe six electron reduction of H2O. to NH4aH werefound to be active catalysts. However, there wasno consistent pattern in the variation of plateauyield with the band gap energy. The results aresummarized in Table 5.

Since neither the weight or volume proportion

RANJIT et al.: PHOTOCATALYTIC REDUCTION OF DINITROGEN TO AMMONIA 449

Table 5-Photoassisted reduction of N 2 at 30°C by aqueoussuspensions of illuminated catalysts (illumination by Xe lamp)

[from refs 84-89]

Illumination Band gapPeriod (h) energy (eV)

15 O.R

12 0.912 1.9

16 2.X

16 2.9

14 2.2

22 3.210 1.5

Catalyst(w/v"lo)

Yield of NH,(1-1 mol) .

3.601.70

0.961.601.251.10

0.810.37

0.65

('nOi(U3('0.\0, (l.IO

('r,O,lO.33WO/O.17MoO,!O.17N,I,O\/0.17rso. Il.IO

TeO,/n.IOa-Fe,()\iO.IO 12 2.1

or- catalyst, nor period of illumination is constantover the set of experiments, comparison has to bemade with caution. Also, the authors have deter-mined the formal activation energy for formationof NH3 in 0.1% wt/vol aqueous suspensions of a-Fe.O, suspensions. The apparent activation energyis found to be 46 kJ mol- I. Also under the reac-tion temperatures maintained (40 to 70°C), negli-gible amount of NHJ remains in the reaction cell.Thus this experiment has practical implications. Itdemonstrates not only that the decay of NH3 canbe avoided by raising the reaction temperaturehigh enough to drive it from the solution, but alsothe conversion of N2 to NHJ can be increased byraising the temperature.

Studies have also been performed using semi-conductors which absorb in the visible region. Theuse of WO, materials for this purpose has beensuggested by Braman et a/.~IIwhere these are usedas agents for the adsorption of NHJ from the gasphase before its determination. This has promptedEndoh et al'" to investigate the reduction of N,using sub-stoichiometric tungsten oxides and tung:stcn trioxide. The yields of NHJ with W03_x andWO~<)(,are found to be proportional to time and isabout 0.1 !!g/h. However, the authors concludethat ammonia production for dispersions ofWO, _x in an aqueous phase occurs by a thermalrather than a photochemical reaction. In order toelucidate the relation between the structural pro-perties and NH J photoproduction, Ladoucer etal." have studied the NH3 photoproduction onsemiconductor electrodes obtained by plasmaspray of W01 powder on Ti substrates. Smallamounts of NH3 are detected but X-ray absorp-tion specuoscopic (XANES and EXAFS) studiesindicate that the. reaction is not photocatalytic. Ithas been proposed that N~ is reduced by hydrog-

en released from the reaction of adsorbed waterwith oxygen vacancies.

Tennakone et al." describe an unusual photoca-talytic property of hydrous cuprous oxide. Anaqueous suspension of hydrous cuprous oxide isfound to photogenerate hydrogen from water andphotoreduce dinitrogen to ammonia with self des-truction. The loss of ammonia on prolonged irra-diation is a result of its oxidation (to water and ni-trogen) with concomitant reduction of CuO toCu20 (unhydrated). In order to see if a further en-hancement is possible, CU~O'xH20 was impreg-nated with CuCI, and an enhanced yield of ammo-nia was obtained". The higher yield of ammoniaobtained can be accounted for on the basis of theseparation of oxidation and water reduction sitesand chemisorption of N2 on Cu20·xH20 which isreduction site.

Iron oxide is another widely investigated semi-conductor. Khader el al'" have reported thephotoassisted catalytic dissociation of water andreduction of N~ to NH, on partially reduced ferricoxide. The initial rate of formation of NH, was 10IJ. moll h g of catalyst. An aqueous suspension oflFe20,(H20)nl and Fe(O)OH was found'":"? tophotoreduce N2 to NH,. The catalytic activityseems to originate from its strong negative flat-band potential and chemisorption of N,. Ironcompounds are known to chemisorb N-, withweakening of the N - N bond. The preferential N,photoreduction (i.e. no Hz was detected in this ex=periment) could be a consequence of this pheno-menon. Another noteworthy observation is theability of [FezO,(HzO)nl to photoreduce water inthe absence of promoters (e.g. Pt or other oxides).

The drawback of this catalyst is that the yield ofammonia decreases with increase in time of irradi-ation.'However, loading of the ferric oxide pan.ul-ly with Ti'v results in enhanced yields of ammoniawith no nitrate observed even after 12 h of irradi-ation'". In this composite [Ti02(H20)n] ....l(Fe20,(H~O )n] catalyst electrons concentrated inferric oxide reduce chemisorbed nitrogen whileholes accumulated in Ti02 oxidize water. Vanadi-um substituted hydrous oxide has also been exa-mined by the same authors and an enhanced activ-ity was attributed to the same reason as in thecase of the composite [TiO~(H:"O)nl...[Fe20,(H20)nlcatalyst.

The photoreduction of dinitrogen to ammoniahas also been reported on iron oxide semiconduc-tors encapsulated in a polymer (nafion film 117,Dupont) and montmorillonite clayl}~-IO().The draw-back of such catalysts is the reduced light absorp-

450 INDIAN J CHEM, SEe. A, JUNE 1996

t~on capability and hence such systems may notfmd extensive practical applications.

A number of rare earth oxides have also beenexamined. Hydrous oxides of samarium(III) andeuropium(IlI) have been employed by Tennakoneet al}OI.102These hydrous oxides are presumed tohave layered structure. The layered structure ofthe hydrous oxide lattice is believed to allow en-trance of N2. In these materials, it is suggested thatN2 is initially reduced to an unstable diazine(N2Hz) which later decomposes to give ammonia;

... (8)

Many other semiconductor photocatalysts such asLuRhOj (ref. 103). a-Fe203: Nd203 (ref. 104,105), SrTi03-Fe203-Eu203 (ref. 106), illemenite(ref. 107), Ti02 coated Si02 (ref. 108), Agl (ref.99), WO)-Ti02, W03-CdO, W03-a-Fe20j andWOj-V20S (ref. 100) have also been examined inliterature. However, no explanation has been givenfor the enhanced activity of the composite catal-ysts for photoreduction to ammonia.

One of the major drawbacks of the systems dis-cussed so far has been the non-availability of elec-trons of desired potential for the reduction of din-itrogen molecule. The solution lies in the develop-ment of a photocatalytic system in which the elec-trons generated at the expense of light energywould reduce the coordinated dinitrogen mole-cule. Taqui Khan et al.I09·113have reported the ca-talytic fixation of dinitrogen in aqueous solutionby [Ru(HEDTA)N2l complex under ambient con-ditions in the presence of CdS/Pt/Ru02 particul-ate system. A tentative mechanism has been pro-posed for the formation of ammonia which in-volves in situ formation of a dinitrogen complex ofruthenium(II). The N - N triple bond in this com-plex presumably undergoes stepwise reduction toammonia via diazinido and hydrazido intermedi-ates which have been characterized by NMRspectra of the irradiated solution.

k[LRull(H:PW + N2 k~ [LRull(N2W (9)

[LRull(N2W + Hz ~lLRull(H)(Nz)J2- + H+ (10)

slow[LRull(H)(N2)f--"k lLRu- N=NHj2- ... (11)

(1) (2)

fast II J-[LRu-N=NHj2- + ~ [LRu -NH-NHl

H + 2eeb

(2) (3)

- [LRu - N =NH2P-(4)

[LRu- N=NH2P- +.H+ + 2eeb ....•[LRu- Nr-

... (12).

(4) (5)+NH3 .,. (13)

[LRu - Nl4- + 3H+ ~ [LRu(H20)t + NH3... (14)

Scheme 1

The rate determining step is the transfer of hy-drogen atom from (1) to yield a diazinido complex(2). The formation of (2), (3) and (4) is speculativeand mainly inferred from IH NMR spectrum ofthe irradiated mixture.

The basic disadvantage of a CdS based semi-conductor catalyst is the photocorrosion of CdSwhich decreases the yield of ammonia after a cer-tain period. In order to circumvent the problemassociated with stability, Taqui Khan et al. havestudied the photosensitized reduction of N byR lI(b' )2+ 2u ipy 3 absorbed on the surface of Pt/TiO/Ru02 semiconductor particulate system containingRUIlI_EDTA complex and i-ascorbic acid'!', Animprovement in the yield of ammonia has beenobserved.

Two general modes of binding of dinitrogen arewell known, end-on binding to a single metal siteand bridge bonded species between two metalcentres. The yield of ammonia is found to dependon the type of dinitrogen binding; more with mon-onuclear end-on complex as compared to dinuc-l~ar bridged .dinitrogen complex. UV-vis absorp-non spectra IS used us to distinguish between thetwo types of binding of dinitrogen and it is foundthat the dinitrogen in mononuclear end-on com-plex is more preferred in the photocatalytic route.

Reaction MechanismVan Damme and Hall:" have proposed a me-

chanism applicable in the gas-solid regime wherethe hydroxylated surface of Ti02 Ti4 + - 01-1-.. ' ~ 'isurf),would be Involved In the photosplitting of waterreleasing H2 for N2 reduction. The regeneration ofthe surface by hydrolysis of Ti4 + - °-Ti4 + beingslow and incomplete would be responsible for theloss of activity of the oxide after a few hours ofreaction. A similar explanation is afforded by For-menti and Teichner ".

Therefore, according to these considerations,the likely scheme for the formation of ammoniacan be:

RANJIT et at: PHOTOCATALYTIC REDUCTION OF DINITROGEN TO AMMONIA 451

6Ti4+ -OR~ 6TP+ -OR · .. (15)

6Ti'+ -OR -6Ti'+ :;: 0 + 3H 0 + 3/202 (ads) 2

· .. (16)6Ti3 t == 0 + NZ(ads)+ 6HzO - 6Ti-l + - OH - + 2NH3

· .. (17)

Evidence for the formation of TiH sites is firstprovided by Vishwanathan!". From ESR and ad-sorption of N2 molecule on a Rh/Ti02 catalyst, itis inferred that nitrogen is bound to Ti3+ sites ad-jacent to oxygen vacancies. Support for the factthat the orientiation of Nz on the catalyst is mainlyend-on came from the IR study of the pre-reducedsample.

Although many studies of N 2 photoreductionon semiconductors have been reported in litera-ture, in addition to reduction, oxidation is pre-dominant on illuminated Ti02 surfaces. Thus, theability of Ti02 surfaces to fix dinitrogen oxidat-ively has been established by Bickley et al.II7.IIIJ,

Because Ti02 promotes the photo-oxidation ofammonia by oxygen'?", the oxidation products ofnitrogen observed by several researchers.B(hl.lI7.IIX

could be generated by a route in which initiallyphotogenerated ammonia subsequently undergoesphoto-oxidation. However" since the oxidativephotofixation of nitrogen seems to be more feasi-ble from the thermodynamic viewpoint':", the re-ductive photo-fixation of nitrogen would occurthrough some oxidative pathway implying interme-diate oxidation to nitrogen species (N02-, NO.)etc.) which can be subsequently phtoreduced toammonia. The identity of the photo-oxidativelyfixed nitrogen species has been established indi-rectly. TPD-MS and XPS results suggest that theform of the fixed nitgrogen is one in which theN - N bond is extant, probably as N 20~ -, but thisassignment to hyponitrite species remains tenta-tive. However, the possibility that the adsorbed ni-trogen containing species generated during the ox-idative fixation reaction may undergo oxidation toa more stable species+'" ).120such as NOi or NO;ions!".

However Schrauzer et aI,33(h) and lleperuma et

aUIJ have observed the photo-oxidation of the in-itially formed NH3 to nitrite.

Thus the route through which ammonia isformed is not yet established. It can be either by apath involving photo-oxidative fixation followedby a reduction or by a path involving photoreduc-live fixation followed by an oxidation. It is essen-tial at this stage to refer to the caution advocatedby Edwards et al/" briefly referred to earlier.They have used coprecipitated or surface loaded

iron-doped titania and certain single or binary ox-ide xerogels for performing two types of irradia-tion experiments in the presence of Nz1g)enrichedto 99% with 15N and water vapour or liquid water.Based on their unsuccessful attempts to photopro-duce ammonia:" they have questioned the reprod-ucibility of this intriguing reaction, which has beensuccessfully reproduced in different laborato-ries-lI.42 under similar experimental conditions andwith different Ti02 based photocatalysts.

ConclusionThe photocatalytic fixation of dinitrogen is still

a demanding proposition. A practical and' idealdevice for this purpose should have the ability touse air and water to generate nitrogenous fertiliz-er. Though it has heen demonstrated tnat the re-duction of dinitrogen is feasible at ordinary tem-peratures and pressures in the presence of lightand a catalyst, the low yields of ammonia obtainedthus far, seem to point out that such a processcannot compete with the well established Haberprocess.

The work pursued by many diligent researchershas made enormous ocntributions to our knowl-edge of the chemistry of dinitrogen fixation al-though a practical device is still far fetched.

As of now, photocatalytic route has been suc-cessful only to the extent of producing a fewmolecules of ammonia. Ingenious formulations ofcoupled semiconductor systems with optimised re-dox levels for the stepwise reduction of dinitrog-en in a. way mimicking the natural processes maylead us to a viable low temperature low pressureprocess for the production of ammonia.

References1 Leigh G J, in The chemistry and bio-chemistry of dini-

trogen fixation, edited by J R Postgate (Plenum Press).1971, p. 19.

2 Haber F, Chern Z, 34, (1910) 345.3 Nielsen A, Catal Rev Sci Eng, 23 (1981) 17.4 Merriam J S & Atwood K, in Applied industrial catalysis

(Academic Press, New York), 1984, Vol 3, p 113.5 Kleiner D, Angew Chern Inti Edn, 14 (1975) 80.6 Hardy R W F, Burns R C & Parshall G W, in Inorganic

biochemistry, edited by G Eichhorn (Elsevier, Amster-dam), 1973, Vol 2 p 745.

7 Turner G L & Bergersen F J, Biochem J,. 115 (1969)529.

8 Jackson E K, Parshall G W & Hardy R W F, J bioiChern, 243 (1968) 4952.

9 Allen A D & Senoff C V, .1 chem Soc Chern Commun,621 (1965).

10 Kuchynka K, Catal Rev, 3 (1964) 111.I) Fisher R H & Venuto P B, Bull New Jersey Acad Sci, 15

(1970) I.

452 INDIAN 11 CHEM, SEe. A, JUNE 1996

12 Ozaki' A & Aika K, Catalysis science and techno'fogy, ed­ited y J R Anderson & M Boudart (Springer-Verlag,Berli ),1(1981)87.

13 Chatt J, Dilworth J R & Richards R L, Chern IReI', 78(1978 589.

14 Van T elen E E, Acc chern Res, 3 (1970) 361.15 Van amelen E E & Seeley D A, J Am chern I Soc, 91

(1968 5194.

16 Van elen E E & Akermark B J, JAm chemlsoc, 90(1968 4492.

17 Sasaki K, Kihara T & Shiba H, J chern Soc .l pan, 89

(1968~1201.18 Goro skii A V, Danilin V V, Efimov 0 N, Necfaeva N

p & arev V A, React Kinet Catal Left, 11 (1979) 37.

I9 Sclaf~i A, Augugliaro V & Schiavello M, J ele trochem

Soc, 1 0(1983)734.

20 Kenic i A, Angew Chern Int Ed Engl, 25 (1983) 5518.21 Denis v N T, Shuvalova N I & Shilov A E, Kin~t Katal,26 (1 115)1493.

22 Denis~'v N T, Shuvalova N 1 & Shilov A E, Kin~t Katal,23(190)874.

23 Burlo E M, Lebedevo M I & Denisov N T, Kin~t Katal,27 (1116) 1504.

24 Shilo A E & Chatt J (editors), New trends in th1 chemis­try of itrogen fIXation (Academic Press, London), 980.

25 Dide 0 L P, Gavrilina 0 K, Yablonskaya E E ShilovaA K Shilov A E, NouvJ Chim, 7 (1983) 605.

26 Shilo A K, Mekhaev V D & Shilov A E, D4'kl A kad

Nauk 'SSR, 277 (1984) 1414.27 Strele s V V, Gavrilov A B & Pospisil L, J el ctroanal

Chern 210 (1986) 173. I28 Dicks n C R & Nozik A J, J Am chern Soc; lOp (1978)8007.1 I

29 NOZikt'A J, Boudreaux D S & Chance R R, A~v chern

Ser,14(1980)155.

30 Kozu i M, Yoneyama H & Tamura H, Bull c1em Soc,Japan 54(1981)1682. I

31 Schra zer G N & Guth T D, J Am chern Soc, 9~ (1977)I

7189.1 I

32 SChra!zer G N, in Coord chern, Vol 20, editqd by D

Bane ea (Pergamon), (1980) p 149. I

33 (a) S rauzer G N, Guth T D, Palmer M R & alehi J,

Solar nergy c;emical conversion and storage, edi~,.edby RL Ha tala, R B King & C Kutal (Humana. pub1~ations,Clifto , New York), 1979, p 261. ;

(b) strauzer G N, Guth T D, Salehi J, Str~pach N,Hui N & Palmer M R, NATO ASI Ser, Se~ C. 174(Horn geneous and Heterogeneous PhotocatalysIs) 1986,p506 .

34 Schra zer G N, Strampach N, Hui L N, Palmeri M R &Saleh' J, Proc Natl A cad Sci, 80 (1980) 3874. I

35 Schia ello M & Sclafani A, Photoelectrochemist1', photo­cataly is and photoreactors, fundamentals and IdeveloJr

ments (D Reidel Publishing Co, Dordrecht), 1985, ~ 503.

36 Augu liaro V & Palmisano L, Photocatalysis drd en vi­ronm ntal trends and applications (Kluwer Acad~mic Pu-blishe s, Dordrecht), 1988, p 425. I

37 Van amme H & Hall W K, J Am chern Soc, lQl (1979)4373., :

38 Formt,ti M & Teichner S J, Catalysis, edited i,tbYD ADow n & C Kemball (The Chemical Society, tonQpn),Vol 2, 1978, P 87. ;

39 EdW~' ds J G, Davies K A, Boucher D L & Mdnnad A,

Ange Chern, 31 (1992)489. '

40 Bouc er D L, PhD Thesis; University of TOle10, 1992Diss sstr Int, 53-B (1992) 2853 .•

41 Soria J, Conesa J C, Augugliaro V, Palmisano L, Schia­vello M, Sclafani A, J phys Chern, 95 (1991) 274.

42 Miyama H, Fujii H & Nago Y, Chern phys Left, 74(1980) 523.

43 Augugtiaro V & Soria J, Angew Chern Int Ed Engl, 32(1993) 550.

44 Palmisano L, Schiavello M & Sclafani A, Angew ChernInt Ed Engl, 32 (1993) 551.

45 Pourbaix M, Atlas of electrochemical equilibria in aque­ous solution (Pergamon, London), 1966, p 496.

46 Davies J A & Edwards J G, Angew Chern Int Ed Engl,32 (1993) 552.

47 Dhar N R, Sechacharyubu E V & Mukherji S K, AnnA'8', 1 (1941) 83.

48 Monhert D, Grayer S, Godinger N, Katzir V & HalmannM, Abstracts of the J/th inti conf photochem conversionand storage of solar energy (Osaka, Japan), 1984, p 75.

49 Augugtiaro V, Lauricella M, Rizzutti L, Schiavello M &Sclafani A, Hydrogen energy progress, edited by T NVeziroglu, K Fueki & T Ohta, Proc 3rd world hydrogenenergy conf(Tokyo, Japan), 1980, Vol 2, 589.

50 Augugliaro V, Lauricella M, ·Rizzutti L, Schiavello M &Sclafani A, Int J Hydrogen Energy, 7 (1982) 845.

51 Augugtiaro V, Alba F D, Rizzutti L, Schiavello M &Sclafani A, Int J Hydrogen Energy, 7 (1982) 851.

52 Schiavello M, Rizzutti L, Sclafani A, Majo I, AugugliaroV & Yue P L, Proc 4th World Hydrogen Energy Confe,-..ence(Pasadena, USA), 1982, P 821.

53 Brucato A, Majo I, Sclafani A & Schiavello M, Proc 4thInti Con! on Photochemical Conversion and Storage ofSolar Energy (Jerusalem, Israel) 1982, p 116.

54 Schiavello M, Rizzutti L, Bickley R I, Navio J A & YueP L, Proc 8th Int Cong Catal (Verlag Chemie, Basel) VolIII, p 383.

55 Yue P L, Khan F & Rizzutti L, Chl~m Eng .'lei, 38 (1983)1893.

56 Rizzutti L & Yue P L, Chern EngSci, 38 (1983) 1241.57 Radford P P & Francis C G, J chern Soc Chern commun,

1520 (1983).58 Cordischi D, Burriesci N, Alba F D, Petrera M, Polizzot­

ti G & Schiavello M, J solid state Chem, 56 (1985) 182.59 Cone'sa J C, Soria J, Augugliaro V & Palmisano L, Struc­

ture and reactivity ef surfaces (Elsevier, Amsterdam),1989,p 307.

60 Sclafani A, Palmisano L & Schiavello M, Res Chern In­

termed, 18 (1992) 211.6 I Bickley R I, Palmisano L, Schiavello M & Scalafani A,

Proc 10th Inti Congr Catal, Budapest, Hungary, 1992, p2151.

62 Palmisano L, Schiavello M, Sclafani A, Martin C. MartinI & Rives V, Catal Left, 24 (1964) 303.

63 Bickley R I, Gonzalez-Xarreno T & Palmisano L, Prepof Cat IV Stud Surf Sci Catal (Elsevier Publications, Am­sterdam), 1983, p 297. Augugliaro V & Palmisano L,Phlltocatalysis and environmental trends and applications,edited by M SchiaveUo (KJuwer Academic Publishers,Dordrecht), 1988, p 425.

64 Augugliaro V & Palmisano L, NATO ASI Ser Ser C. 237(Photocatal Environ) 1988.

65 Augugliaro V, Palmisano L, SchiaveUo M, Sclafani A,Conesa J C & Soria J, Proc XI Iberoamerican Symp onCatalysis, Guanjuato, Mexico, 1988, p 217.

66 Palmisano L, Augugliaro V, Sclafani A & Schiavello M, Jphys Chem, 92 (1988) 6710.

67 Sclafani A, Palmisano L & Schiavello M, J phys Chern,94(1990)829.

)..

I "I '.

RANJIT et al.: PHOTOCATALYTIC REDUCTION OF DINITROGEN TO AMMONIA 453

68 Bickley R I, Palmisano L, Schiavello M & Sclafani A,Proc Xth Inil Congr Catal, July 1992, Budapest, Hun­gary, p 2151.

69 'Bickley.R I, Carreno T G, Gonzalez-Elipe A R, Mumue­ra G Palmisano L, J chem Soc Faraday Trans, 90 (1964)2259.

70 Bickley R I, Less J S, Tilley R J D, Palmisano L, Chia­vello M & Sc1afani A, J phys Chern, 85 (1991) 274.

71 Bickley R I, Carreno T G & Palmisano L, J Mat chemPhys, 29 (1991) 475.

72 Martin C, Martin I, Rives V, Palmisano L & SchiavelloM,J Catal, 134 (1992) 434.

73 Venezia A M, Palmisano L, Schiavello M, Martin C,Martin 1& Rives V, J Calal, 147 (1994) 115.

74 Maruska H P & Ghosh A K, Solar Energy Mater, 1(1979) 237.

75 Burch R & Flambard A R, J chem Soc Chem Commun,965 (1981).

76 Palmisano L, Schiavello M, Sc1aflini A, Martin C, MartinI & Rives V, Catal Leu, 24 (1994) 303.

77 I1eperuma 0 A, Tennakone K & Dissanayake W D D P,Appl calal, 62 (1990) L1.

78 Endoh E & Bard AJ, NewJ Chern, 11 (1987) 217.79 Ileperuma' 0 A, Weerasinghe F N S & Lewke Bandara T

S, Solar Energy Mater, 19 (1989) 109.80 Rao N N, Dube S; Bala M & Natarajan P, Appl Cillal B,

5 (1994) 33.81 Ranjit K T & Viswanathan B, Unpublished results.82 Khan F, Vue P L, Rizzuti L, Auguliaro V & Schiavello

M, J chem Soc Chem Commun, 1049 (1981).83 Li Q S, Domen K, Naito S, Onishi T & Tarnaru K,

Chem Leu, 321 (1983).84 Lichtin N N & Vijayakumar K M, Abstracts of the 163rd

Meeding of the Electrochemical Society, May 8-13, 1983,P 782.

85 Lichtin N N, US Pat, 4,427,509 (1984).86 Lichtin N N & Vijayakumar K M, US Pat, 4, 427 510

(1984).87 Lichtin N N & Breman E, US Pat, 4, 443, 331 (1984).88 Lichtin N N, Vijayakumar K M & Khader M M, Ex­

tended Abstracts of the 1985 Meeting of the InternationalSolar Energy Society, Montreal, Canada, 1985, p 433.

89 Lichtin N N Vijayakumar K M, J Indian chem Soc, VolLXIfl (1966) 29.

90 Braman R S, Shelley T J & McClenny W A, Anal Chem,54(1982)358.

91 EndoR E, Leland J K & Bard A J, J phys Chern, 90(1986) 6223.

92 Ladoucer M, Dodelet J P, Tourillon G, Parent L & Dal­laire S, J phys Chem, 94 (1990) 4579.

93 Tennakone K, Jayatissa A H, Kekulawala K R S S &Punchihewa S, Chem Phys Lett, 150 (1988) 511.

94 Tennakone K, Punchihewa S & Tantrigoda R, Solar En­ergy Mater, 18 (1988) 217.

95 Khader M M, Lichtin N N, Vurens G H; Salmeron M &Somerji G A, Langmuir, 3 (1987) 303.

96 Tennakone K, Wickrarnanayake S, Fernando CAN,lleperuma 0 A & Punchihewa S, J chem Soc ChemCommun, 1078 (1987).

97 Tennakone K, Bandara J M S, Thaminimulla C T K,Jayatilake W D W, Ketipearachchi U S, I1eperuma 0 A& Priyadarshana M A, Langmuir, 7 (1991) 2166.

98 (a) Tannakone K, Fernando CAN, Wickrarnanayake S,Darnayanthi M W P, Silva L H K, Wijeratne W, I1eperu­ma 0 A & Punchihewa S, Solar Energy Mater, 17 (1988)47.

(b) Tennakone K, Thatninimulla C T K & Bandara,J MS, J Photochem Photobiol A: Chern, 68 (1992) 131.(c) Tannakone K, I1eperuma 0 A, Bandara J M S,Thaminimulla C T K & Ketipearachchi U S, J chem SocChem Commun, 579 (1991).(d) lleperuma 0 A, Kiridena WeB & Dissanayake WD D P,J Photochem PhotobiolA: Chern, 59 (1991) 191.

99 Petriooni G L & Papee H M, Water, A ir and Soil Pol/ut,20 (1983) 276.

100 Li S, Wen X, Cui C & Hua Z, Huaxue Tongbao, 3 (1983)13.

101 Tennakone K, Ileperuma 0 A, Bandara J M S & Tharn­inimulla C T K, Solar Energy Mater, 22 (1991) 319.

102 Tennakone K, Thaminimulla C T K & Kiridena W C B,Langmuir, 9 (1993) 723.

103 Grayer S & Halmann M, J electroanal Chem, 170 (1984)363.

104 Xiao L, Wang D, Yu S, Quan H & Li T, Ziran Zazhu, 8(1985) 920.

105 Xiao L, Wang D, Yu S, Quan H & Li T, TaiyangnengXuebao, 7 (1986) 39.

106 Wang D, Xiao L, Li T & Horu G, TaiyangneICgXuebao,7 (1986) 400.

107 Bahadur K, Ranganayaki S, Verma P, Sinha P M & Ku­~ar S, Phillipps J Sci, 115 (1986) 55.

108 Mazurkevich Y S & Viodarchik R P, Dtkryhya lzobret,34(1990) 101.

109 Taqui Khan M M, Bhardwaj R C & Bhardwaj C, IndianJChern,25A(1986) 1.

110 Taqui Khan M M, Bhardwaj R C & Bhardwaj C, AngewChem Int Ed Engl, 27 (1988) 923.

III TaquiKhanMM& RaoN N,JmolCatal, 52(1989) 15.

112 Taqui Khan M M & Rao N N, J Photochem Photobiol A:Chern, 56 (1991) 101.

113 Taqui Khan M M, Chatterjee D, Krishnaratnarn M & Ba­la M, J mol Catal, 72 (1992) 13.

114 Taqui Khan M M, Bhardwaj R C, Bhardwaj C & Rao NN, J Photochem Photobiol A: Chern, 68 (1992) 137.

115 Rao N N,J mol Catal, 93 (1994) 23.116 Vishwanathan V, J chem Soc Chem COmmun, 848

(1989).117 Bickley RI & Vishwanathan V, Nature, 280 (1979) 306.118 Bickley R I, Jayanty R K M, Vishwanathan V & Navio J

A, Homogeneous and heteropeneous photocatalysis, edit­ed by E Pelizzetti & N Serpone (Reidel, Dordrecht),1986,p 555.

119 Bickley R I, Day R & Vishwanathan V, Proc 9th IntICong Catal, edited by M J Phillips & M Ternan, Vol 4(1988) 1505.

120 Ritchie M & McLean W R,J appl Chern, 15 (1965) 452.

121 Hori Y, Nakatsu A & Suzuki S, Chem Leu, 1429 (1985).