Surface Science 108 (1981) L465-L469

North-Holland Publishing Company

L465

SURFACE SCIENCE LETTERS

OXYGEN MIGRATION AND ISOTOPE EXCHANGE DURING PHOTOASSISTED SPLITTING OF WATER VAPOUR ON TiO*-CARBON MIXTURES

Joseph CUNNINGHAM and John P.J. TOBIN Chemistry Department, University College, Cork, Ireland

and

Paul MERIAUDEAU Institut de Recherche sur la Catalyse. CNRS, Villeurbanne, France

Received 16 March 1981

Since the initial experiments demonstrating activity of TiOz substrates for pho- toassisted coversion of liquid or gaseous water to products of nominally higher

energy [ 1,2], this metal oxide has continued to feature prominently in research on

photoelectrochemistry and other aspects of solar energy conversion [3,4]. Two lines of development which have received considerable experimental attention involve efforts to enhance the lifetime or oxidising power of photogenerated holes (or conversely the lifetime or reducing power of accompanying electrons) through contact of the TiOz surfaces with another solid, such as platinum [5] or RuOz [6]

or (carbon + RuOz) [7]. In cases where a promoter is incorporated into a com- pound catalyst in the form of small particles, such as RuOz and/or carbon dispersed onto TiOz substrates of high surface area, an understanding of the interaction between photoexcited TiOz and the other dispersed materials (i.e., the promoters) will be important. When the present study commenced, little published work had appeared dealing with the nature and efficiency of Promoter/Photoactivated Sub- strate Interactions (PPSI) or with possible differences between these and the more conventional Promoter/Thermally-activated Substrate Interactions (PTSI). Some

evidence of the development of interest in such PPSI effects in complex TiOz pho- tocatalysts under illumination is provided by the very recent publication [8] of an

investigation of possible effects of promoter particle size upon efficiency of photo- assisted water splitting by Pt dispersed on TiOa. The present communication examines another aspect of complex TiOz photocatalysts under illumination, viz. the nature and efficiency of chemical changes deriving from PPSI and involving amorphous carbon-13 added as a promoter/reactant to ruthenium-doped TiOz catalyst. Kawai and Sakata [7] had reported that addition of amorphous carbon-12 enhanced the photocatalytic activity of ruthenium-doped TiOz.

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Our reexamination of these systems differed initially from previous studies in the method used for incorporating ruthenium, viz. by the incipient-wetness tech- nique rather than as a mechanical mixture of RuOz and TiOz. A small volume of an aqueous solution of RuCls, containing r-uthenium equivalent to 10% or 2% of the weight’ of the TiOz, was used to wet the TiOz (Degussa PX). For the first set of experiments 0.174 g of this material. after calculation in air at 600 K for 3 days. was finely ground in an agate mortar with a 0.015 g of amorphous carbon-13

(Pro&em.. 90% 13Cj and taken up into a thick paste with triply distilled water. This paste was then coated as a thin polycrystalline layer onto a quartz tube on

which it was dried and outgassed overnight on the vacuum system at 400 K at residual pressure of 1O-6 Torr protected by liquid nitrogen traps. The resultant sample may be designated by (Ru”+- Ti02/r3C). The role and the valence states of the ruthenium were not established in the present study but are under contin- uing study. Following equilibration with H2 160 at ca. 20 Torr for 3 h at room

temperature in the dark, three successive overnight illuminations at 300 K by the output of a 500 W Hg arc lamp produced increasing yields, as follows, of gases non-

condensible at 77 K (n.c. at 77 K): 2.22 gmol; 3.9 and 6.6 pmol. An initially auto- catalytic nature of the photoassisted production of the products is indicated by these data. Mass spectrometric analysis after the third overnight illumination, indicated: 68% Hz, 14% NZ, 5% “C”O, 4% 13C160 and 2% CH4. The predotni- nance of hydrogen in the product fraction n.c. at 77 K was consistent with the results of Kawai and Sakata and was further supported by our experiments with

D20 as the gaseous reactant, which yielded D2 as the dominant photoproduct. Our observation of a minor but unmistakeable peak at m/e = 29 confirmed that the

added carbon-l 3 contributed 13C160 as a minor product or intermediate. The simultaneous occurrence of methane as a trace product, without significant forma- tion of other hydrocarbon products such as ethane, appeared strongly reminiscent

of high selectivity in the hydrogenation of surface carbon monoxide to CH4, as reported for TPSI on carbon-supported Ru02 catalyst [9]. However, the very large differences in experimental conditions between the present photoassisted process at ambient temperature and earlier work at 530 K should be borne in mind. The

definite identification of 13C160 as a photoproduct from this H2160(pj/R~“+- Ti02/13C system, and later the appearance of 13C’80 from an H2180/Ru”t-Ti02/ 13C system under illumination (see below), pointed to one PPSI type of process.

It should be noted that the carbon monoxide resulting from this process was an order-of-magnitude less than the hydrogen or carbon dioxide product. The fatter

was released for analysis, after pumping away product n.c. at 77 K, by warming the trap to 150 K. Mass analysis confirmed that this fraction n.c. at 150 K was com-

posed exclusively of C02. However, isotopic distribution between ’ 3C02 and “CO2 varied with duration of illumination, with the ratio 13C02/C’z02 increasing

to 0.52 : 0.48 after the third overnight illumination. This suggested that carbon-12 impurities unavoidably present on the surfaces of the powdered TiOz substrate from the preparation process at 10 -6 Torr were more favourably positioned for

J. Cunningham et al. / Oxygen migration and isotope exchange J.&l

PPSI-type interactions than the carbon-13 introduced as an amorphous solid.

The carbon dioxide photoproduct was similar in magnitude to the Hz product (e.g. 7.1 I.trnol of r2C02 + ‘%Z02 after the third overnight illumination versus 6.6 pmol of H,), but was an order of magnitude greater than carbon monoxide pro- duct. Experiments were made to test the possibility that the thermodynamically favoured water-gas reaction, viz. Hz0 + CO + HZ + CO* for which K,, z 104.98 in standard conditions, might be an important intermediate reaction in the photo- initiated sequence of events at the H20(s)/Ru”+-Ti02/13C interfaces. These experiments involved additions of pure carbon monoxide and the careful compari-

sons of the rates of photoassisted production of CO* and of Hz prior to and after these additions. Since no significant changes were found, it could be concluded that the water-gas reaction made no appreciable contribution to processes in these

systems. The total flux of photons of wavelengths 300-500 nm incident onto the (H,O/

Ru”+-Ti02/13C) system from the 500 W Hg arc lamp via a Pyrex, water-cooling

jacket was measured as 18 X 1Ol7 s-r using potassium ferrioxalate actinometer.

Order-of-magnitude of the photon flux was double-checked with a uranyl-oxalate actinorneter, which adsorbed mainly photons of X = 300-365 nm and yielded a value of 6 X IO” s-r. Based on the latter flux, the apparent quantum efficiencies

of the photoproduct formation during the third overnight illumination were: #(HZ) = 1 X 10-4, @(CO*) = 1 .l X lo4 and @(CO) = 1.5 X lo-‘. These efficien-

cies were disappointing and contrasted with a yield of 2 X IO-* reported by Kawai

and Sakata from photoassisted splitting of liquid water over a mechanically mixed

Ru02-TiO,/Pt catalyst. Experimental tests were therefore made to determine if illumination of water vapour over mechanically mixed Ru-Ti02 or RuOz-TiO&

systems gave greatly enhanced yields of Hz and CO* relative to those just detailed

for Ru”’ -Ti02/C systems. Significant enhancement of yields was not, however, achieved in those other systems. A further disappointing feature of the water

vapour/Ru”+ -Ti02/C systems, in relation to suggestions that such systems may be valuable in solar energy conversion, was that no significant photoassisted splitting

of water vapour resulted whenever near-UV photons were filtered out and the wave- lengths admitted to the H,O/Ru”+ -Ti02/C system were limited to visible photons at h F= 460-640 nm. This indication that photons within or close to the band-edge of TiO, (at ca. 390 nm) were essential for the photoassisted formation of r3C02

and H2 product would be consistent with initiation of the reaction processes by hole-electron pairs photogenerated within TiOz. Involvement of the added 13C in photoassisted production of “CO2 and H2 (without the appearance of signifi- cant r3CO(s) as an intermediate) could then be envisaged according to Scheme 1,

in which photogenerated holes first produce an active monatomic surface oxygen species similar to 0,. This then reacts with added 13C to produce a surface 13CO- anion, whilst water vapour reacts

-0,2-

1 k!-/-‘h+‘x -(~-13c>(H,i03),n.COZ&H20~s)

- Tiz - Ti4+(e-) y - Ti4+(OH)- + iHz Scheme 1

L468 J. Cunningham et al. / Oxygen migration and isotope exchange

with the accompanying electron-excess site in the immediate vicinity to yield hydrogen product and a surface hydroxyl. Production of a carbonic-acid-like SW- face complex then results from interactions with water vapour (together with an additional i H, plus two conduction band electrons not shown in Scheme I or alternatively with formation of additional surface OH-). The CO2 product is envisaged as being in equilibrium with the carbonic-acid-like surface complex, since

evidence for the probable operation of such a fast equilibration process came from the experiments illustrated in fig. 1. These results illustrate the growth of various isotopic COz species having m/z from 44 to 49 (i.e. ‘*C1602 to 13C1802) whenever the water vapour present over the UV illuminated Ru”+-Ti02/13C sample was

enriched to 38% in oxygen-18. It will be noted that each of the probable isotopic species was detected, including those carrying a double 180 label - an observation which would, if equilibration did not occur, exclude any route to CO2 formation which did not allow for incorporation of oxygen-18 at each step of any of the two-

stage addition of oxygen to carbon-13 species. The ratios of the slopes shown in fig. 1 for formation of the 13C-containing CO2 species were 0.47 : 0.39 : 0.14 for m/z 47 : 45 : 49. A similar ratio, viz. 0.48 : 0.37 : 0.15, exists in fig. 1 between the rates of formation of the carbon-12 containing species with m/z 46,44 and 48

10 -

z i ; : h N

8 5-

1 1 5 10

Total flux/photons x 102*

Fig. 1. Rates of growth of various isotopic carbon dioxide products upon prolonged W illumi- nation of water vapour enriched to 38% in oxygen-18 over Ru”+-TiOz/’ 3C sample: (a)

12c1*02; (A) 13c’*o*; (0) 12~160~; (0) l*c’601*0; (z;) 13~1%~; (,)13c’%~*o.

J. Cunningham et al. / Oxygen migration and isotope exchange L469

respectively. In order to demonstrate that this ratio could arise from scrambling of oxygen between a surface complex of (Hz la0 plus CO*) on the surface of the Ru”t-Ti02/13C catalyst under illumination (as envisaged in the last process of Scheme I), unlabelled 12C1602 was admixed with water vapour enriched to 38%

in oxygen-18 and admitted into contact with the catalyst. No significant rate of scrambling of oxygen-18 into CO2 occurred in dark but, upon UV illumination, the CO2 composition rapidly attained isotopic equilibrium characterized by a value of 0.47 : 0.39 : 0.14 for the ratio of ‘2C’60180 : 12C’602 : 13C’802. Since this photoassisted scrambling of oxygen-18 between Hz*‘0 (38%) and C1602 occurred

on a much shorter time scale (typically c,,~ N 1 h for 10 Torr of the mixture), it

became clear that, even if the carbon dioxide photoproduct originated via processes other than Scheme I and were to be formed without full incorporation of oxygen- 18, then such product would rapidly become fully equilibrated by the last process in Scheme I.

The reaction envisaged in Scheme I between the added carbon-13 promoter and a surface oxide ion activated through the capture thereon of a photogenerated hole is similar to photoinitiated steps proposed on TiOz to account for (i) photoassisted oxidation of carbon monoxide [lo], (ii) photoassisted RI-type oxygen isotope exchange [l I], and (iii) photoassisted conversion of oxygen-18 labelled tertiary butanol to unlabelled acetone over preoxidised surfaces of chloride-free rutile [ 121. Good grounds thus exist for recognising the importance of this PPSI-type process in the mechanism of oxidation processes of low efficiency on TiOz surfaces under UV illumination.

References

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