Mechanism of aerobic visible light formic acid oxidation catalyzed by poly(tri-s-triazine) modified titania

Abstract

Visible light aerial oxidation of formic acid catalyzed by N/C-modified titania (TiO₂-N,C) is investigated by wavelength-dependent photocatalytic and photoelectrochemical experiments in the presence of oxygen, tetranitromethane, and methylviologen as electron acceptors. The title reaction is shown to proceed both through oxidative and reductive primary processes. Contrary to the urea-derived (TiO₂-N,C), so-called “N-doped” titania (TiO₂-N) as prepared from ammonia is inactive. In accord with photocurrent action spectra of corresponding powder electrodes, this different activity of the two photocatalysts is traced back to the different chemical nature of the reactive holes localized at the modifier. Hole stabilization by delocalization within an extended poly(tri-s-triazine) network of TiO₂-N,C is proposed to render recombination with conduction band electrons less probable than in TiO₂-N.

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Introduction

The development of novel functional materials capable of solar-driven chemical transformations or production of electricity has attracted significant interest motivated by the need to develop technologies based on a clean and sustainable energy supply.^1,2 In this context, particularly semiconductor photocatalysis is one of the promising approaches since semiconductors, due to their unique electronic and optical properties, enable efficient photon-induced generation and separation of charges that can be utilized for selective oxidation and reduction reactions at the semiconductor's surface. Due to its low cost, non-toxicity and excellent chemical stability, one of the most commonly employed photocatalysts is titanium dioxide. However, utilization of TiO₂ is hampered by the fact that, due to the large bandgap of 3.2 eV (∼390 nm), it can make use of only a very small UV part (about 3%) of solar radiation. Therefore strong efforts are presently being made to shift its photocatalytic activity to the visible spectral region. The fundamental issue at stake here is that all viable strategies for large-scale applications should avoid expensive manufacturing procedures or using rare noble metals like platinum, iridium, palladium, etc. Accordingly, one of the most promising methods seems to be doping and/or surface modification of TiO₂ with nonmetals, such as carbon, nitrogen and sulfur.^3–13

Recently we reported¹⁴ that calcining a mixture of urea and TiO₂ at 400 °C produces poly(amino-tri-s-triazine) derivatives covalently attached to the semiconductor surface (Scheme 1). The resulting visible light-active photocatalysts TiO₂-N,C consist of a titania core covered by a polytriazine shell symbolized as “C,N”. These condensed heteroaromatics were found to be responsible for the visible light activity, contrary to many other proposals made for so-called urea-derived “N-TiO₂” in the literature.^15–30

Scheme 1 Modification of titania with urea affording the core-shell particle TiO₂-N,C. For simplicity only one tri-s-triazine unit (melem) is depicted.¹⁴

TiO₂-N,C exhibits a weak absorption shoulder in the visible region at 400–500 nm and photocatalyzes the aerial oxidation of formic acid to CO₂ and H₂O (λ > 455 nm). The quasi-Fermi level for electrons, which practically merges with the conduction band edge, was −0.48 V compared to −0.56 V for unmodified TiO₂ (pH 7).

Different from TiO₂-N,C, the catalyst TiO₂-N prepared from ammonia by treating anatase powder in the NH₃(67%)/Ar atmosphere at 600 °C for 3 h⁹ was found to be inactive in decomposition of formic acid under visible light irradiation.^25,31 The nature of the nitrogen species responsible for visible light activity of TiO₂-N is still a matter of discussion. Proposals range from N³⁻,^9,32 NO, NO₂, and NO²⁻ to NH_x-like species^32,33 substituting oxide ions or occupying interstitial lattice positions. TiO₂-N exhibits a similar absorption spectrum as the urea-derived catalyst. The distinct difference of TiO₂-N,C and TiO₂-N in the photocatalytic oxidation of formic acid very likely can be attributed to an efficient electron–hole recombination in the latter material although the location of relevant energy states are very similar (Fig. 1). We proposed that this might originate from the different electronic nature of the nitrogen-centered intra-bandgap states present in the two materials. Whereas in TiO₂-N the nitrogen species do not allow for hole stabilization through charge delocalization, this should be possible in TiO₂-N,C.³⁴ This effect is expected to increase the hole lifetime and favor interfacial electron exchange over recombination. The assumption of an energy band-like structure of the intra-bandgap states of TiO₂-N,C, instead of a manifold of discrete levels as drawn for TiO₂-N in Fig. 1, is supported by the fact that pristine poly(amino-tri-s-triazine) derivatives themselves exhibit semiconductor photocatalytic properties.^34,35

Fig. 1 Energy level schemes of TiO₂-N,C (A) and TiO₂-N (B) at pH = 7. The reactive, surface-trapped holes generated upon UV and Vis irradiation are denoted as h_r,v⁺ and h_r,s⁺, respectively, and the reactive, surface-trapped electron as e_r⁻.³⁴

Although rarely found in literature, detailed studies of the mechanism of the photocatalytic action are crucially important. Such mechanistic knowledge is indispensable for development of further photoactive materials with optimized properties. Accordingly, herein we report in detail our investigations of the mechanism of formic acid photocatalytic oxidation by TiO₂-N,C. We are particularly interested in elucidation of the role of visible light generated holes in TiO₂-N,C because the nature and fate of h_r,s⁺ may have decisive bearings on the general photocatalytic activity. Similarly, the efficiency of dye-sensitized titania solar cells is also known to be dependent on the electronic structure of the visible-light generated dye radical cation.³⁶ In this paper the mechanism of aerial formic acid oxidation is investigated by conducting a series of photochemical and photoelectrochemical experiments in which the electron scavenger oxygen is replaced by tetranitromethane or methylviologen. By combining these techniques we arrive at a conclusive mechanism of formic acid degradation catalyzed by TiO₂-N,C.

Before communicating our results, the mechanism of the photocatalytic oxidation of formic acid over unmodified titanium dioxide irradiated with UV light is briefly discussed. It is generally agreed to proceed by electron transfer to photogenerated valence band holes (eqn (1)) and/or reaction with OH˙ radicals (eqn (2)) produced through (i) water or hydroxyl group oxidation by valence band holes or (ii) dioxygen reduction (eqn (3)) via intermediate O₂⁻, HO₂ and H₂O₂.^37–40

HCOO⁻ + h_r,v⁺→ COO⁻˙ + H⁺ (1)

HCOO⁻ + OH˙→ COO⁻˙ + H₂O (2)

e_r⁻ + O₂→ O₂^-˙ (3)

Importantly, it is well known that the one-electron oxidation product of formic acid can inject a second electron into the conduction band of a semiconductor, which can be directly observed by enhancement of photocurrent and has become known as the so-called ‘photocurrent multiplication (photocurrent doubling) effect’.^41–42,43

Experimental

The catalysts TiO₂-N/C¹⁴ and TiO₂-N⁹ were prepared by calcining TiO₂ in the presence of urea and ammonia, respectively. In both cases Hombikat UV-100 (100% anatase, Sachtleben Chemie) was used as titania source. Methylviologen (1,1′-dimethyl-4,4′-bipyridinium dichloride; Acros Organics) and tetranitromethane (Aldrich) were used as received.

Photomineralization experiments were carried out in a cylindrical 20 mL cuvette attached to an optical train. The reaction mixture was stirred magnetically. Irradiation was performed with an Osram XBO 150 W xenon arc lamp installed in a light-condensing lamp housing (PTI, A1010S). A water filter and an appropriate cut-off filter were placed in front of the cuvette. 20 mL of a 1 g L⁻¹ powder suspension in 10⁻³ mol L⁻¹ formic acid was sonicated for 15 min prior to illumination. When C(NO₂)₄ (10⁻² mol L⁻¹) was used as an electron scavenger, argon was bubbled for 40 min prior and during irradiation. Withdrawn samples were filtered through a syringe filter and subjected to ion chromatography analysis (Dionex DX120, Ion Pac 14 column, conductivity detector; NaHCO₃ : NaCO₃ = 0.001 : 0.0035 mol L⁻¹ as eluent); no oxalate was detectable. Formation of C(NO₂)₃⁻ anion was monitored by electronic spectrum (Varian Cary 50 UV-Vis spectrometer).

Quasi-Fermi potentials of electrons (_nE_F*) were measured according to the literature⁴⁴ using methylviologen dichloride ((MV)Cl₂, E°_MV^2+/+˙ = −0.445 V vs. NHE) as a pH-independent redox system. In a typical experiment 50 mg of catalyst and 10 mg of methylviologen dichloride were suspended and sonicated for 15 min prior to illumination in a 100 mL two-necked flask in 50 mL of 0.1 mol L⁻¹ KNO₃. Only a water filter was placed in front of the flask for UV-Vis tests, whereas a 420 nm cut-off filter was added for the Vis measurements. The tests were performed in the presence or absence of formic acid (10⁻³ mol L⁻¹). A platinum flag and Ag/AgCl served as working and reference electrodes and a pH meter for recording the proton concentration. Initially the pH of the suspension was adjusted using HNO₃ to pH ca. 2.5 before measurement. Stable photovoltages were recorded about 20 min after changing the pH value. The suspension was magnetically stirred and purged with nitrogen gas about 30 min prior and throughout the experiment. Titration was performed using NaOH (0.1, 0.01 and 0.001 mol L⁻¹), also purged with nitrogen gas. The light source was the same as used in photodegradation experiments.

For the photostability test, 20 mL of a 1 g L⁻¹TiO₂-N,C suspension in 4.6 × 10⁻⁴ mol L⁻¹ formic acid was sonicated for 15 min prior to illumination in a centrifuge glass. After centrifugation, 2 mL of liquid was withdrawn, filtered through a syringe filter and subjected to ion chromatography analysis (see above). Subsequently, the suspension was sonicated for 1 min and irradiated with visible light (λ≥ 420 nm). After 4 h of irradiation the suspension was again centrifuged, 2 mL of liquid were withdrawn, filtered through a syringe filter and analyzed by ion chromatography. The powder suspension was filled up with 4 mL of concentrated formic acid to achieve the initial concentration and volume of suspension. This procedure was repeated three times. The light source was the same as used in photomineralization.

For photocurrent measurements, electrodes consisting of a material film deposited on FTO–glass were prepared. The conducting FTO–glass substrate (Solaronix, sheet resistance of ∼10 Ω per square) was first cut into 2.5 × 1.5 cm pieces and then subsequently degreased by sonicating in acetone and boiling NaOH (0.1 mol L⁻¹), rinsed with demineralized water, and blown dry in a nitrogen stream. A suspension of 200 mg of a catalyst in 1 mL of ethanol was sonicated for 20 min and then deposited onto the FTO glass by doctor blading using scotch tape as frame and spacer. The electrodes were then dried in air, covered with a glass plate, and pressed for 3 min at a pressure of 200 kg cm⁻² using an IR pressing tool (Paul Weber, Stuttgart, Germany). Such a procedure yields an opaque layer of the investigated material having an excellent mechanical stability. Photocurrent experiments were performed with a tunable monochromatic light source provided with a 1000 W Xenon lamp and a universal grating monochromator Multimode 4 (AMKO, Germany) with a bandwidth of 10 nm. The electrochemical setup consisted of a BAS Epsilon Electrochemistry potentiostat and a three-electrode cell using a platinum counter electrode and a Ag/AgCl (3 mol L⁻¹ KCl) reference electrode. During photoelectrochemical measurements the electrodes were pressed against an O-ring of an electrochemical cell leaving a working area of 0.636 cm². The photocurrent experiments were carried out in a LiClO₄ (0.1 mol L⁻¹) containing formic acid (10⁻³ mol L⁻¹) or potasium iodide (0.1 mol L⁻¹) as hole scavengers. Nitrogen was passed through the electrolyte prior to the experiment, whereas it was supplied only to the gas phase above the electrolyte during the experiment. The wavelength dependence of photocurrent was measured at a constant potential of 0.5 V vs. Ag/AgCl. The electrodes were irradiated from the backside (through the FTO glass) with light and dark phases of 5 and 10 s, respectively. The value of photocurrent density was taken as the difference between current density under irradiation and in the dark. The incident photon-to-current efficiency (IPCE; the number of electrons generated in the external circuit divided by the number of incident photons) for each wavelength was calculated according to equation IPCE (%) = (i_phhc)/(λPq)×100, where i_ph is the photocurrent density, h is Planck's constant, c velocity of light, P the light power density, λ is the irradiation wavelength, and q is the elementary charge. The spectral dependence of lamp power density was measured by the optical power meter Oriel 70260 (Oriel, Stratford, USA).

Results and discussion

To address the problem of photostability and catalytic function of TiO₂-N,C in the mineralization of formic acid, one unique sample was subjected to a twelve hour irradiation experiment (λ≥ 420 nm). After every fourth hour the irradiation was interrupted for formic acid measurement followed by addition of a new portion of formic acid solution (Fig. 2, see Experimental). Complete photomineralization occurred even after the third irradiation cycle evidencing the photocatalytic function of TiO₂-N,C. This finding corroborates also the recently reported photostability of polymeric melon modified with Pt for photocatalytic (λ≥ 420 nm) H₂ production in the presence of a reducing agent.³⁵

Fig. 2 Decrease of concentration upon addition of formic acid after every fourth hour (dashed lines) during irradiation of TiO₂-N,C (λ≥ 420 nm). See text.

As already mentioned, contrary to TiO₂-N, the suspension of TiO₂-N,C is highly active in the mineralization of formic acid under visible (λ≥ 420 nm) light irradiation in the presence of dissolved oxygen (Fig. 3, curves a and c). It is reasonable to assume that in the presence of oxygen this will be reduced by e_r⁻according to eqn (3). Subsequent protonation of superoxide affords HO₂ that disproportionates to O₂ and H₂O₂. Photoreduction of the latter finally produces OH⁻ and OH˙ radicals.⁴⁵ These are able (E°_OH˙/H2O≈ 2.8 V)^46,47 to oxidize formic acid (E°_{CO2⁻˙/HCO2⁻}≈ 1.90 V).³¹ The initial pH in the photoactivity tests was ca. 3–4 and increased in the course of the photocatalytic reaction to values of ca. 7. The resulting COO⁻˙ can either reduce oxygen to superoxide^48–50 (eqn (4)) or increase the number of reactive electrons through electron injection into the conduction band of TiO₂-N,C (E°_CO2/CO2⁻˙≈−1.90 V,^51,52eqn (5)). Alternatively and less likely, it can react with H₂O₂ and give rise to OH˙ radicals (eqn (6)).⁴⁸ No experimental evidence for oxalic acid formation could be found, which is in line with the fact that the COO⁻˙ radical is known to dimerize only in neutral and alkaline solution.⁵³

COO⁻˙ + O₂→ CO₂ + O₂⁻˙ (4)

COO⁻˙→ CO₂ + e_r⁻ (5)

COO⁻˙ + H₂O₂→ CO₂ + OH⁻ + OH˙ (6)

Fig. 3 Photomineralization of formic acid (c = 1 × 10⁻³ mol L⁻¹); c₀, c_t are concentrations at times 0 and t; (a) TiO₂-N,C in the presence of oxygen, (b) TiO₂-N,C with C(NO₂)₄ in deoxygenated system, (c) TiO₂-N, in the presence of oxygen; λ≥ 420 nm.

In order to differentiate the role of visible light generated holes and electrons the reaction was carried out in oxygen-free catalyst suspension containing tetranitromethane as electron acceptor. This hinders the formation of OH˙ radicals through oxygen reduction and subsequent reactions, hence the reductive photodegradation pathway is effectively “switched-off”. Under such conditions the mineralization of formic acid is still observed (Fig. 3, curve b). The flattening of the degradation curve observed after two hours is most likely due to depletion of tetranitromethane and increasing competitive light absorption and photodegradation of the C(NO₂)₃⁻ product. Clearly, under the Vis irradiation the reactive holes h_r,s⁺ are able to oxidize formic acid either directly (eqn (7)) or indirectly through OH˙ radical formed by hole oxidation of surface-bound OH groups. The latter possibility can be excluded since TiO₂-N,C in pristine water does not enable visible light reduction of methylviologen.⁴² This becomes possible only in the presence of formic acid. In other words, the surface-bound OH groups do not scavenge holes induced by visible light. Degradation is slower by a factor of ∼2–3, suggesting that the reductive pathway via intermediate superoxide plays an essential role in formic acid degradation. In the presence of tetranitromethane the photogenerated electrons e_r⁻ reduce the nitroalkane to C(NO₂)₃⁻ and NO₂ (eqn (8)). The COO⁻˙ radical, in addition to the reaction described by eqn (5), may also reduce tetranitromethane according to eqn (9).

h_r,s⁺ + HCOO⁻→ COO⁻˙ + H⁺ (7)

e_r⁻ + C(NO₂)₄→ C(NO₂)₃⁻ + NO₂ (8)

COO⁻˙ + C(NO₂)₄→ CO₂ + C(NO₂)₃⁻ + NO₂ (9)

Formation of relatively stable C(NO₂)₃⁻ is confirmed by enhancement of its typical absorbance at 350 nm (Fig. 4).⁵⁴ The peak at time t = 0 min (at λ = 350 nm) is caused by contamination of commercial tetranitromethane with its reduced form. On the contrary, in the case of unmodified TiO₂ no degradation of formic acid and no increasing of absorbance at 350 nm were observed (Fig. 5). The decomposition of C(NO₂)₃⁻ observed after prolonged irradiation in both cases is caused by its photolysis taking place with and without TiO₂.^54,55 Both the formation of C(NO₂)₃⁻ and consumption of formic acid provide clear evidence for interfacial electron transfer reactions occurring at the TiO₂-N,C surface irradiated with visible light. The overall reaction describing processes occurring during oxidation of formic acid in an oxygen free TiO₂-N,C suspension in the presence of tetranitromethane can be summarized according to the eqn (10) (the sum of eqn (7)–(9)).

e_r⁻ + h_r,s⁺ + HCOO⁻ + 2C(NO₂)₄→ CO₂ + 2C(NO₂)₃⁻ + 2NO₂ + H⁺ (10)

Fig. 4 Concentration of C(NO₂)₃⁻ during irradiation (λ≥ 420 nm) of deoxygenated system containing C(NO₂)₄ (10⁻² mol L⁻¹), formic acid (1 × 10⁻³ mol L⁻¹), and TiO₂-N,C (1 g L⁻¹).

Fig. 5 Concentration of C(NO₂)₃⁻ during irradiation (λ≥ 420 nm) of deoxygenated system containing C(NO₂)₄ (10⁻² mol L⁻¹), formic acid (1 × 10⁻³ mol L⁻¹), and TiO₂ (1 g L⁻¹).

Upon UV-Vis (λ≥ 320 nm) light irradiation, as expected, the holes h_r,v⁺ generated in a TiO₂-N,C suspension containing tetranitromethane instead of oxygen can easily oxidize formic acid. As shown in Fig. 6, already within 15 min 75% mineralization is achieved. At the same time increase of absorbance of C(NO₂)₃⁻ is observed (Fig. 7) indicating tetranitromethane reduction by e_r⁻. The faster mineralization is due to a stronger light absorption of TiO₂-N,C resulting in a higher concentration of h_r,v⁺. Interestingly, when the same oxygen-free experiment was carried out in the absence of tetranitromethane, 20% mineralization of formic acid was still detected (Fig. 6). In the course of this reaction the color of suspension changed from yellow to dark blue which is caused by reduction of titania lattice ions by photogenerated e_r^-, since any other electron acceptor is absent.^56,57

Fig. 6 Photomineralization of formic acid (c = 1 × 10⁻³ mol L⁻¹) TiO₂-N,C in a deoxygenated system in the presence (a) and absence (b) of C(NO₂)₄. λ≥ 320 nm.

Fig. 7 Concentration of C(NO₂)₃⁻ anion during irradiation (λ≥ 320 nm) of deoxygenated system containing C(NO₂)₄ (10⁻² mol L⁻¹), formic acid (1 × 10⁻³ mol L⁻¹), and TiO₂-N,C.

Further experiments were conducted using methylviologen MV²⁺ as an electron acceptor. In this case the pH dependence of the Fermi potential induces also a pH-dependent formation of MV⁺˙. It can be followed easily by recording the open-circuit photopotential of a platinum electrode immersed into the solution. This experimental set-up is typically used for determination of the quasi-Fermi level of photogenerated electrons.^44,58 In solutions without any added hole scavengers the reduction of methylviologen is observed when TiO₂-N,C is excited with UV-Vis (λ≥ 320 nm) light but is absent within the full pH range under visible light irradiation (λ > 420 nm) (Fig. 8). It means that in the latter case recombination processes overcome the interfacial electron transfer reactions even at pH values enabling the methylviologen reduction. The reason for the inefficiency of MV²⁺ reduction is the absence of a suitable, i.e. more easily oxidizable, hole scavenger than water and surface OH groups. However, when formic acid was added the suspension formed a blue color at pH values higher than ca. 6.0 (Fig. 8), whereby the initially transient green color resulted from superimposition of the colors of yellow photocatalyst with blue MV⁺˙. The titration curve does not have the typical sigmoidal shape, probably due to slow kinetics of formic acid oxidation. Notice that without addition of MV²⁺ to the TiO₂-N,C suspension in formic acid no blue color and no photovoltage change were observed. Therefore, upon visible light irradiation Ti⁴⁺ reduction is negligible and the blue color of the suspension is solely due to MV⁺˙. All these results clearly reveal that under the Vis excitation experiment MV²⁺ is reduced by e⁻_r (eqn (11)) whereas formic acid is oxidized by h_r,s⁺ according to eqn (7). The resulting COO⁻˙ can convert to CO₂ in oxygen free conditions by reducing MV²⁺ directly (eqn (12)) or through intermediate involvement of conduction band electrons (eqn (5)). Thus, the overall reaction occurring during potentiometric titration of oxygen free TiO₂-N,C suspension under visible light irradiation in the presence of methylviologen and formic acid can be summarized according to the eqn (13) (the sum of eqn (7), (11), (12)). Importantly, when TiO₂-N,C was replaced by TiO₂-N in this experiment, no inflection point and no methylviologen reduction were observed.³⁴

e⁻_r+ MV²⁺→ MV⁺˙ (11)

COO⁻˙ + MV²⁺→ CO₂ + MV⁺˙ (12)

e⁻_r + h⁺_r,s + HCOO^- + 2MV²⁺→ CO₂ + 2MV⁺˙ + H⁺ (13)

Fig. 8 Variation of photovoltage with pH value for a suspension of the catalyst in the presence of (MV)Cl₂; (a) TiO₂-N,C, no cut-off filter, no formic acid, (b) TiO₂-N,C, 420 nm cut-off filter, formic acid, (c) TiO₂, 420 nm cut-off filter, formic acid.

The reactivity of photogenerated holes was also examined using wavelength-resolved photocurrent measurements on photocatalyst powders deposited on FTO–glass electrodes. Here it should be noted that a photocurrent is observed only if two processes occur simultaneously—the photogenerated electron is transferred to the FTO electrode and the hole oxidizes a donor species. Since the first process can be assumed to proceed readily at anodically-biased FTO electrodes, it is the latter process which exerts control over the photocurrent response. In other words, under such experimental conditions photocurrents are observed only when the holes can oxidize a suitable reducing agent. Therefore the photocurrent action spectra were recorded with and without addition of potassium iodide and formic acid as different hole scavengers.

As expected, an unmodified TiO₂ photoelectrode oxidizes neither iodide nor formic acid in the visible, as shown in Fig. 9. This is because titania does not absorb visible light and is in agreement with its inactivity in visible light photodegradation of formic acid. However, when TiO₂-N,C is used, both formic acid and iodide can be oxidized in the visible spectral range (Fig. 10). As expected, the incident photon-to-current efficiency is higher for iodide than for formic acid since the standard reduction potential (vs. NHE) is more negative for iodide (ca. 1.3 V) than for formic acid (1.90 V).^31,51 It is noted that the quite small IPCE values at 400–450 nm observed in pristine water are significantly increased upon addition of formic acid (Fig. 10, curves b and c). This is not observed when TiO₂-N is employed as electrode. In this case even a very small decrease is found (Fig. 11, curves b and c). However, a significant visible light response is observed in the presence of iodide (Fig. 11, curve d). This finding is in line with the fact that this material does not exhibit photocatalytic activity towards formic acid in visible light.¹⁴ The rather low IPCE values observed in the range of 400–450 nm in the experiments in the absence of formic acid (curves b in Fig. 10 and 11) may arise from oxidation of water or organic impurities present in TiO₂-N,C.

Fig. 9 Photocurrent action spectra of TiO₂ (Hombikat) measured at 0.5 V vs. Ag/AgCl (3 mol L⁻¹) in LiClO₄ (0.1 mol L⁻¹) electrolyte containing various hole-scavengers: (a) no added hole scavenger, (b) HCOOH (10⁻³ mol L⁻¹), (c) KI (0.1 mol L⁻¹).

Fig. 10 Photocurrent action spectra of TiO₂ (Hombikat) and TiO₂-N,C measured at 0.5 V vs. Ag/AgCl (3 mol L⁻¹) in LiClO₄ (0.1 mol L⁻¹) electrolyte containing various hole-scavengers: (a) TiO₂, no added hole scavenger; TiO₂-N,C, (b) no added hole scavenger, (c) HCOOH (10⁻³ mol L⁻¹), (d) KI (0.1 mol L⁻¹).

Fig. 11 Photocurrent action spectra of TiO₂ (Hombikat) and TiO₂-N measured at 0.5 V vs. Ag/AgCl (3 mol L⁻¹) in LiClO₄ (0.1 mol L⁻¹) electrolyte containing various hole-scavengers: (a) TiO₂, no added hole scavenger; TiO₂-N, (b) no added hole scavenger, (c) HCOOH (10⁻³ mol L⁻¹), (d) KI (0.1 mol L⁻¹).

Notably, the visible light induced mineralization of formic acid shows maximum reaction rates at pH 3–4. This corresponds with the results reported for the reaction conducted in the presence of UV light and unmodified titania, started at about pH 3.4 and was completely inhibited above pH 6.⁵⁹ It can be rationalized by electrostatic repulsion between formate and the titania surface as indicated by the formic acid pK_a value of 3.75 and by the point of zero charge of ∼5.8 for anatase.⁶⁰ Moreover, it should be noted that the oxidation potential of the OH˙ radical decreases from 2.6 V to 1.9 V upon going from pH 0 to pH 6, and therefore the driving force of formate oxidation is also decreased.^46,61,62 At pH 3.5 the visible light generated holes have enough oxidative force (∼2.2 V) to oxidize formic acid (E°_{HCO2⁻/CO2⁻˙}≈ 1.90 V).³¹ Additionally, protonation of superoxide radical (pK_a = 4.88),⁶³ a reaction necessary for OH˙ radical production on the reductive pathway, is also more effective in acidic solution.

The results reported above can be summarized as follows. Clearly, the separation of photogenerated charges is more efficient in TiO₂-N,C than in TiO₂-N, which makes it manifest both in photocatalytic and photocurrent experiments. This can be rationalized in terms of stabilization of photogenerated holes by delocalization within an extended poly(amino-tri-s-triazine) heterocycle at the surface of TiO₂-N,C, which, in turn, renders electron–hole recombination less favorable. The tri-s-triazine core is based on a cyclic system of twelve C–N bonds which surround the central sp² hybridized N atom. The 14 π-electrons form a doubly cross-conjugated aromatic planar system.⁶⁴ Importantly, melamine rings and related materials are known to be of low flammability. Reasons for this property are the relatively high bond dissociation energy of C–N single and multiple bonds, as well as the relatively high electronegativity of nitrogen atoms, since it causes a partial oxidation of the carbon atoms. Oxidation reactions are therefore less likely for nitrogen rich C/N/(H) compounds as compared to other organic compounds such as hydrocarbons.⁶⁵ It follows that the visible light-generated hole which is delocalized in the tri-s-triazine rings is not expected to induce self-oxidation processes of the TiO₂-N,C shell (i.e. visible light photocorrosion) but more likely causes one-electron oxidation of compounds present in the catalyst suspension. Such hole stabilization is not possible in the case of TiO₂-N since this contains non-aromatic small amidic or oxidic nitrogen species. As a consequence thereof, electron–hole recombination becomes more efficient than interfacial electron exchange and formic acid oxidation is not possible.

Conclusion

The oxidation of formic acid by TiO₂-N,C irradiated with visible light in the presence of oxygen involves both reductive and oxidative primary process. Photocatalytic and photoelectrochemical measurements in the presence of different electron acceptors confirm that the visible light photogenerated holes in TiO₂-N,C are able to oxidize formic acid. This is in contrast to the behavior of conventional TiO₂-N. The reason for this difference most likely is an enhanced stabilization of the photogenerated hole in TiO₂-N,C against recombination with conduction band electrons.

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Footnote

† Electronic supplementary information (ESI) available: Photocurrent measured under intermittent irradiation as a function of irradiated wavelength (Fig. S1). See DOI: 10.1039/b9pp00052f

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