Limiting quantum yield measurements for the disappearance of 1-propanol and propanal: an oxidative reaction study employing a TiO₂ based photoreactor

Abstract

Relative initial quantum yields (@A,,,) for the disappearance of I-propanol and propanal were estimated based on total light intensity entering the reaction vessels. Two types of photocatalytic reactor geometries were compared, a commercial prototype flow reactor with immobilized TiO, and a conventional reactor with TiO, dispersion. A maximum relative quantum yield, @I,,, == 1 , was extrapolated for pure propanol and propanal in both geometries. The high quantum yields were attributed to efficient competition of substrate oxidation with electron-hole recombination, at high surface coverage. Propanal was identified as the primary intermediate generated from I-propanol in both reactor systems, but yields of propanal were systematically 10 times lower than I-propanol loss, indicating that there was an effective pathway for propanal oxidation on the surface prior to desorption. The inhibition of I-propanol reaction caused by accumulation of product arises because both reactant and product compete for active sites on the photocatalyst surface. These propositions were confirmed by introducing propanal into the feed. The relative quantum yields converge to unity for pure substrates, suggesting that they are true yields. That is to say, absorption by TiOz of light entering the reactor is efficient in these geometries. GIUSEPPE P. LEPORE, BHUVAN C. PANT et COOPER H. LANGFORI). Can J . Chem. 71, 2051 (1993). En se basant sur I'intensite totale de la lumikre entrant dans les recipients reactionnels, on a tvalue les rendements quantiques relatifs initiaux (@A ,,is) de disparition du 1-propanol et du propanal. On a comparC deux types de gComCtries pour les reacteurs photocatalytiques, un prototype commercial d'un rkacteur Ccoulement avec du TiO, immobilisC et un rkacteur conventionnel avec une dispersion de TiO,. Pour chacune des geometries, on a pu extrapoler un rendement quantique relatif maximal, @I,, .= I , pour le propanol pur ainsi que pour le propanal. On attribue les valeurs Clevtes des rendements quantiques j. une compCtition efficace, a une couverture elevee de la surface, de I'oxydation du substrat avec la recombinaison electron-trou. Pour chacun des reacteurs, on a identifie le propanal comme intermkdiaire primaire genCrC par le 1-propanol, mais les rendements en propanal sont systCmatiquement 10 fois plus lents que les pertes du I-propanol; ces observations indiquent que, avant la dCsorption, il existe j. la surface une voie efficace pour I'oxydation du propanal. L'inhibition de la reaction du I-propanol avec accumulation de produits a t t C attribuCe B une competition entre le rkactif et le produit pour les sites actifs presents B la surface du photocatalyseur. Ces propositions sont confirmCes par I'introduction de propanal dans la source. La convergence des rendements quantiques relatifs vers I'unitC pour les substances pures suggkre que ces valeurs correspondent au rendement quantique vrai, c'est-a-dire que, pour ces gComCtries, I'absorption par le TiO, de la lumikre entrant dans le rCacteur est efficace.[Tr...

Quantitative Spin-Trapping Studies of Weakly Illuminated Titanium Dioxide Sols. Implications for the Mechanism of Photocatalysis

“…The question of why φ apparently reaches a plateau above [DMPO] ≈ 2 mM in phosphate-buffered media,52b at variance with the results of Figure , is intriguing. Two facts remain: (1) Sun et al did not explore the range [DMPO] > 6 mM,52c and (2) Lepore et al also measured quantum yields larger than 0.4 for the photoxidation of propanal at 313 nm in unbuffered TiO 2 sols prepared by a procedure identical to ours. , We notice that the reported [DMPO]-dependence of initial OH-trapping rates in the homogeneous, broadband photolysis of H 2 O 2 in phosphate buffers cannot be accounted for by simple kinetic arguments. For example in ref b, Figure a, the ratio Γ = k (OH+DMPO)[DMPO]/{ k (OH+H 2 O 2 )[H 2 O 2 ]} = 14 at [H 2 O 2 ] = 50 mM and [DMPO] = 2 mM, will ensure quantitative trapping by DMPO, at variance with observations.…”

“…Considering the conflicting reports on the dependence of photocatalytic rates on I a , − and the importance of such dependence on our understanding of this phenomenon, we investigated this effect at [DMPO] = 0.6 and 2.0 mM, in the range 4.5 < I o (10 -10 Einstein cm -2 s -1 ) < 90. The results R het (0) vs I a are shown in Figure .…”

A New Method To Determine the Generation of Hydroxyl Radicals in Illuminated TiO2Suspensions

“…During recent years, several contradicting results dealing with the dependence of photocatalytical rates on the photon flux I 0 were reported. [33][34][35] Okamato found a square root dependence, 36 but recent results favor, at least at lower light intensities, a linear correlation between the photon flux and the quantum yields for the production of hydroxyl radicals in illuminated TiO 2 suspensions. 13 Applying 3-carboxyproxyl (0.8 mM) as the spin trap and using the derived trapping efficiency η ) 0.79, we have calculated the production of • OH radicals at different light intensities (Figure 4) for an air-saturated suspension with TiO 2 (0.2 g L -1 ) at pH ) 3.8.…”

Some Observations on the Development of Superior Photocatalytic Systems for Application to Water Purification by the “Adsorb and Shuttle” or the Interphase Charge Transfer Mechanisms

“…The key point of adsorb and shuttle, A/S, is that the concentration of the substrate close to the TiO 2 surface should greatly increase the probability of meeting between the substrate and a photoactivated site. An elementary illustration of this is the increase of a quantum yield when a favourable substrate concentration is raised, as is the case of an increase of quantum yield from 0.001 to 0.15 as the propanol concentration is raised from 0.001 M to 0.05 M [2]. Moreover, it is known that interfacial redox can compete well with recombination.…”