Guest Commentary pubs.acs.org/JPCL
Undesired Role of Sacrificial Reagents in Photocatalysis
O
barrier for the undesired reverse consumption of the generated H2 gas.5 Two possible mechanisms have been proposed for the photocatalytic oxidation of methanol, (1) the direct oxidation by photogenerated holes and (2) the indirect oxidation via interfacially formed •OH radicals that are products of the trapping of VB holes by surface −OH groups or adsorbed water molecules.6−9 It is still a challenge to distinguish between the two mechanisms in practice due to the lack of suitable probe techniques. Wang et al.10 have reported that the methanol photooxidation pathway, direct or indirect, depends on the molecular species adsorbed at the TiO2 surface. These authors studied the competitive adsorption process between water and methanol on TiO2 through the in situ use of sum frequency generation, a nonlinear spectroscopic technique. Accordingly, they concluded that the indirect oxidation by •OH radicals is the mechanism when water is the dominant surface species, with the critical molar ratio between water and methanol for the •OH radical mechanism to become the dominant process being approximately 300. Such a high ratio apparently applies to the photooxidation of methanol by TiO2 in aqueous systems. If the water content is lower than this critical value, the direct oxidation of methanol by photogenerated holes will be the predominant process at the TiO2 surface. Hydroxyl radicals, •OH, are known to react with methanol mainly through the abstraction of a hydrogen atom from the C− H bond. Sun and Bolton11 have therefore used the photocatalytic oxidation of methanol to determine the quantum yield for the photochemical generation of •OH radicals in TiO2 suspensions. The •OH radical generation rate is determined through the R−H atom abstraction from methanol by these •OH radicals (eq 5), followed by monitoring the formation rate of the first principal stable product, that is, formaldehyde (eq 6).
ne of the most important limitations of the application of photocatalysis for water decomposition is that the process employing pure water is usually rather inefficient or is not even operative at all. This is related to the fact that the simultaneous reduction and oxidation of water is a complex multistep reaction involving four electrons. Using sacrificial molecules as electron donors can, for example, remarkably improve the H 2 production,1 as holes are scavenged by these molecules and charge carrier recombination can be greatly reduced. Furthermore, as O2 is not produced, the back reaction to produce water is suppressed, increasing the H2 yield and avoiding a subsequent gas separation stage. However, it should be noted here also that the yield of the H2 formation can subsequently be reduced by competing reduction reactions with the products formed upon the oxidation of the sacrificial reagents. These sacrificial reagents can be divided into organic and inorganic electron donors. Sacrif icial Organic Electron Donors. Various organic compounds such as alcohols, organic acids, and hydrocarbons have been and are being employed as hole scavengers (i.e., as electron donors) for photocatalytic H2 generation.2 In particular, methanol is frequently used as a sacrificial reagent. For practical applications, the utilization of methanol will only be environmentally sensible provided that it is derived from biomass or from toxic residues that must be disposed of. Adding methanol as an electron donor to react irreversibly with the photogenerated valence band (VB) holes can enhance the photocatalytic electron/hole separation efficiency, resulting in higher quantum efficiencies. Because electron donors are consumed in the photocatalytic reaction, their continuous addition is required to sustain H2 production. Kawai et al.3 proposed the following overall methanol decomposition reaction mechanism, resulting in a lower energy that can be stored as compared with the cyclic splitting of water4
CH3OH + •OH → •CH 2OH + H 2O
hν ,TiO2
CH3OH(l) ⎯⎯⎯⎯⎯⎯⎯→ HCHO(g) + H 2(g) ΔG° = 64.1 kJ mol−1
•
CH 2OH + O2 → HCHO + HO•2
(1)
hν ,TiO2
ΔG° = 47.8 kJ mol
(2)
hν ,TiO2
HCOOH(l) ⎯⎯⎯⎯⎯⎯⎯→ CO2 (g) + H 2(g) ΔG° = −95.8 kJ mol−1
(3)
with the overall reaction being hν ,TiO2
CH3OH(l) + H 2O(l) ⎯⎯⎯⎯⎯⎯⎯→ CO2 (g) + 3H 2(g) ΔG° = 16.1 kJ mol−1
(4)
The first two reactions (eqs 1 and 2) have a positive Gibbs free energy; thus, both reactions are thermodynamically unfavorable at room temperature. The photon energy will consequently be used to raise the chemical potential of the reactants, thus driving the reactions to the product side. The third reaction (eq 3) has a large negative Gibbs energy; hence, it intrinsically provides a © 2013 American Chemical Society
(6)
In the presence of molecular oxygen, formaldehyde is formed as the dominant stable product in a quantitative reaction (eq 6), whereas in the absence of O2, formaldehyde is formed through the electron injection into the conduction band of TiO2, a process called “current doubling” (Figure 1a).12,13 In the presence of a cocatalyst such as Pt, these electrons will be utilized to form H2 (Figure 1b), while otherwise, they will be trapped at TiIV, yielding TiIII. The TiIII formation results in a blue coloration of the respective TiO2 suspensions and eventually in the termination of the formaldehyde formation. HCHO can be further oxidized in an analogous manner producing HCOOH and finally CO2.14,15 Asmus et al.16 showed that the efficiency of the reaction of •OH radicals with methanol by R−H abstraction is 93%. The remaining 7% are accounted for by methoxy radicals
HCHO(g) + H 2O(l) ⎯⎯⎯⎯⎯⎯⎯→ HCOOH(l) + H 2(g) −1
(5)
Received: August 26, 2013 Accepted: September 26, 2013 Published: October 17, 2013 3479
dx.doi.org/10.1021/jz4018199 | J. Phys. Chem. Lett. 2013, 4, 3479−3483
The Journal of Physical Chemistry Letters
Guest Commentary
Figure 1. Processes involved in the photocatalytic H2 evolution from aqueous methanol solution on (a) bare TiO2 and (b) Pt-loaded TiO2. (1) Photogeneration of charge carriers, e− and h+; (2) trapping of e− by Ti4+ (a) or by Pt islands (b); (3) first oxidation step of CH3OH; (4) formation of HCHO through e− injection into the conduction band of TiO2 (current doubling); (5) formation of Ti3+ (a) or reduction of H+ (b); (6) recombination channel. Note: For simplicity, the formation of •CH2OH radicals by trapped holes (TiIVOH•+) or by •OH radicals is represented by the hole oxidation step.5
(i.e., through the intermediate formation of S•− and •SO−3 ). Both of these possible free radical intermediates are rather powerful reducing agents, with their one-electron reduction potentials being at −1.7 V (versus NHE) and −2.4 V (versus NHE), respectively.21 Consequently, the likelihood of current doubling processes induced by these radicals is once again very high (and therefore, all arguments given above for the methanol case are 2− valid here as well!). SO2− 3 can be oxidized to both SO4 and 2− S2O2− . Although the formation of S O is thermodynamically 6 2 6 2− less favorable than the conversion of SO2− 3 to SO4 , Bühler et al. 2− observed a significant concentration of S2O6 in the reaction media,26 which they explained by a higher concentration of SO2− 3 ions near the CdS surface as compared with the concentration of OH− ions required for the formation of SO2− 4 . It has been argued that using S2− and/or SO2− 3 as sacrificial reagents instead of alcohols is advantageous because they are more easily oxidizable than the alcohols. Therefore, the undesired photoanodic corrosion reaction of the semiconductor (which is the competing process for the holes) can be reduced, but not prevented entirely.26,27 Another advantage of the use of S2− (as compared with an alcohol) for the photocatalytic H2 generation employing CdS is that due to the presence of sulfide in the surrounding solution, dissolved Cd2+ can react with S2− to rebuild CdS. In the aqueous methanol solution, however, this repair mechanism is not feasible. The photocatalytic efficiency of the H2 production was found to decrease in solutions just containing sulfide ions. This was attributed to the formation of disulfide ions, S2− 2 , which exhibit a less negative reduction potential than the protons (thus competing with their reduction) and are, moreover, able to act as an optical filter, reducing the light absorption by CdS.26 The addition of reducing agents such as SO2− 3 prevents the formation of disulfide ions; therefore, using S2−/SO2− 3 mixtures leads to an enhanced yield of the H2 production from water. For example, Yan et al. obtained the highest quantum yield of 93% for H2 production for a Pt-PdS/CdS photocatalyst in the presence of a S 2− /SO 32− mixture under visible-light irradiation.28 The suggested reaction mechanism for the photocatalytic H 2 evolution in the presence of S2−/SO2− 3 mixtures is given in eqs 8−13.
formed through the H-abstraction reaction from the hydroxyl group (eq 7). CH3OH + •OH → CH3O• + H 2O
(7)
The concentration of HCHO formed photocatalytically divided by a factor of 0.93 is thus used to calculate the corresponding • OH radical concentration. Sun and Bolton11 have used the same factor as that found in the homogeneous system (0.93) to calculate the •OH radical concentration in the case of the heterogeneous TiO2 system. The reaction mechanism depicted in Figure 1b illustrates quite nicely that due to the current doubling effect, at least half of the detected H2 gas that is generated in a system containing methanol as the sacrificial reagent is formed through the action of holes and not that of electrons! Therefore, it is fair to say that the yields reported for the molecular hydrogen formation in such systems cannot (and should not!) be denoted as “water splitting efficiencies”! The same mechanisms are operative for any alcohol carrying a hydrogen atom at the carbon atom in the α-position to its OH group, that is, ethanol, 2-propanol, butanol, polyvinylalcohol, and so forth.17 Moreover, current doubling also occurs when, for example, formate is used as the sacrificial electron donor with the CO2•− radical anion acting as the reducing intermediate in this case. Actually, it is not very easy to find one-electron donors that do not generate strongly reducing radicals following the loss of this first electron! Finally, it should be noted that measuring H2 gas formation in such a sacrificial system no longer generates any mechanistic information, that is, it is not possible to determine whether the electron transfer from the conduction band or from the reducing organic radicals is rate-limiting or whether the overall efficiency might even be limited by the initial hole transfer to the sacrificial reagent. Sacrif icial Inorganic Electron Donors. Sulfide, S2−, and sulfite, SO2− 3 , can act as sacrificial inorganic reagents for the photocatalytic H2 generation because they are very efficient hole acceptors, enabling the effective separation of the charge carriers.18−24 In particular, CdS has been mostly used as a photocatalyst for H2 production in the presence of S2− and/or SO2− 3 . The energy level of the VB of CdS is positive enough (+1.7 V versus NHE) to promote the oxidation of these sulfur compounds.25 The oxidation of S2− and SO2− 3 can either occur by a two-electron transfer process (cf. eqs 9−11) or even through the thermodynamically less favorable one-electron oxidation
2H 2O + 2e− → H 2 + 2OH−
(8)
SO32 − + 2OH− + 2h+ → SO24 − + H 2O
(9)
2SO32 − + 2h+ → S2 O62 − 3480
(10)
dx.doi.org/10.1021/jz4018199 | J. Phys. Chem. Lett. 2013, 4, 3479−3483
The Journal of Physical Chemistry Letters 2S2 − + 2h+ → S22 − SO32 −
2−
(12)
S22 − + SO32 − → S2 O32 − + S2 −
(13)
+ 2h →
hν
e− + C(NO2 )4 → C(NO2 )−3 + NO2
(11)
S2 O32 −
+S
+
Guest Commentary
Also, O2 is not formed when more complex nitroaromatic compounds are employed as electron acceptors.36 Even though all of these studies have been conducted in aqueous solutions, that is, in the presence of 55 M H2O, the photogenerated holes are exclusively reacting with the organic moiety being present in micro- and millimolar amounts at the most. It should be noted here that both the CCl4 and the C(NO2)4 systems initially do not contain any oxidizable organic species. The latter are only formed upon the reductive process. The above discussion suggests that the role of suitable sacrificial electron acceptors such as Ag+ and Fe3+ is highly underestimated. In particular, their possible involvement in the actual water oxidation mechanism has so far not been discussed at all. However, due to the rather high one-electron oxidation potential of the holes photogenerated in most semiconductors that have been found to exhibit high activities for the photocatalytic water oxidation, for example, metal oxides, both Ag+ and Fe3+ can be readily oxidized.
Most certainly, eqs 8−13 are by no means complete; it can rather be expected that there will be various other reactions induced upon the photocatalytic oxidation of sulfide and/or sulfite in aqueous semiconductor suspensions! Usually, most experimentalists are not even interested in these processes because all that they measure is the formation of hydrogen gas. As elaborated above, H2 will be formed through both the direct reductive as well as the indirect oxidative paths! Like in the case of the organic electron donors, any qualitative or even quantitative interpretation of the respective results will therefore be highly speculative. Sacrif icial Electron Acceptors. While the choice of sacrificial electron donors for studies concerning the photocatalytic formation of molecular H2 appears to be rather large, the sacrificial photocatalytic oxidation of water is only reported for a rather limited variety of electron acceptors. By far, the vast majority of research groups working on this topic employ silver cations, Ag+, as electron acceptors, with the involved reactions being proposed as follows29−31
h+ + Ag + → Ag 2 +
+ hν
4e− + 4Ag → 4Ag 0
It is well-known that Ag tends to form peroxides in aqueous solution.
(14)
2Ag 2 + + 2H 2O → Ag 2O2 + 4H+aq
2h+ + Ag 2O2 → 2Ag + + O2
Similar reactions can be envisaged for Fe , which is known to be able to oxidize water, resulting in the formation of hydroxyl radicals. Fe 4 + + H 2O → Fe3 + + H+aq + •OH
Alternatively, further oxidation of Fe via h+ + Fe 4 + → Fe5 +
(26)
appears to be possible followed by reactions such as 2Fe5 + + 2H 2O → [Fe2O2 ]6 + + 4H+aq
(27)
[Fe2O2 ]6 + → 2Fe3 + + O2
(28)
Upon the basis of these possible reactions, it is most certainly highly indicated to study the role of metal cations such as Ag+ and Fe3+ in the photocatalytic water oxidation in detail. The catalytic role proposed here for Ag+ and Fe3+ could be part of a much more general mechanism and open up new design features for photocatalytic and photoelectrochemical energy-to-fuel conversion systems. Finally, another sacrificial electron acceptor is being used in some studies, that is, peroxodisulfate, S2O2− 8 . The anion is indeed an excellent one-electron acceptor. However, the sulfate radical anion formed in the first reduction step (cf. eq 29) is an oxidant that in an aqueous environment is even more powerful than the hydroxyl radical. Indeed, its lifetime in water is very short, with the hydroxyl radical being formed as the next intermediate (cf. eq 30).
(17) (18)
However, no O2 is formed once electron acceptors such as carbon tetrachloride, CCl4, or tetranitromethane, C(NO2)4, are used, even though their irreversible one-electron reduction is readily observed.33−35 hν
e− + CCl4 → •CCl3 + Cl−aq
(25)
4+
hν
hν
(24) 4+
Hence, the photocatalytic formation of molecular oxygen is accompanied by the deposition of metallic silver nanocontacts (Ag0n) on the semiconductor’s surface. Obviously, this will lead to irreversible optical changes of the systems studied here due to the plasmonic absorption band of the silver nanoparticles in the visible spectral region. Moreover, noble metal nanoparticles are known for their catalytic activity, resulting most likely in changes in the chemical and/or photochemical properties of these systems. It is interesting to note that these rather drastic changes are hardly ever discussed in the respective literature nor is any experimental work conducted to ensure that the photocatalytic properties of the systems studied remain unchanged upon the formation of the silver particles. Even though no exact count exists here, it seems fair to say that Ag+ is employed in at least 95% of the published papers dealing with the sacrificial photocatalytic water oxidation. In some cases, it could be shown that molecular oxygen is also formed when ferric ions are employed as sacrificial electron acceptors.32
4e− + 4Fe3 + → 4Fe 2 +
(23)
The subsequent photocatalytic oxidation of these peroxides is then proposed to result in the observed O2 formation.
(15) (16)
4h+ + 2H 2O → O2 + 4H+aq
(22) 2+
hν
nAg 0 → Ag n0
(21)
h+ + Fe3 + → Fe 4 +
hν
4h+ + 2H 2O → O2 + 4H+aq
(20)
2− S2 O82 − + e− → SO•− 4 + SO4
(19) 3481
(29)
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The Journal of Physical Chemistry Letters 2− • + SO•− 4 + H 2O → SO4 + OH + H aq
Guest Commentary
formation of molecular oxygen from water. Interesting examples from the recent literature are presented in this Perspective to illustrate the activity of these novel electron-transfer catalysts. Both Perspective are extremely well written, presenting up-todate overviews on these very important aspects of research in photocatalysis. Hence, their reading is highly recommended.
(30)
In the presence of suitable catalysts, these •OH will react to form H2O2 and subsequently O2, that is, this current doubling process is transforming electrons into the respective oxidation products, making it once again impossible to decide whether the oxygen gas is formed via direct water oxidation or indirectly starting from the reduction of S2O2− 8 . Perspective Articles Published Herein. In their Perspectives published in this issue of the journal entitled “Bioinspired Photocatalytic Water Reduction and Oxidation with EarthAbundant Metal Catalysts” and “Photocatalytically Prepared Metal Nanocluster−Oxide Semiconductor−Carbon Nanocomposite Electrodes for Driving Multielectron Processes”, the authors (S. Fukuzumi et al. and K. Rajeshwar et al., respectively) describe rather complex photocatalytic systems in all of which, however, sacrificial electron donors are also being employed. As shown in Scheme 1 of their work, Fukuzumi et al. (Fukuzumi, S.; Hong, S.; Yamada, Y. Bioinspired Photocatalytic Water Reduction and Oxidation with Earth-Abundant Metal Catalysts. J. Phys. Chem. Lett. 2013, 4, 3458−3467) describe the use of NADH as an electron donor for the formation of molecular hydrogen, while Na2S2O8 is used as a sacrificial electron acceptor whenever the photocatalytic formation of molecular oxygen is reported (cf. Scheme 3 of their Perspective). Rajeshwar and coworkers, on the other hand, describe the reduction of noble metal cations upon the band gap illumination of TiO2 with formate being oxidized concomitantly. This illustrates rather convincingly that these sacrificial systems are very abundant in photocatalysis and that a more detailed and critical discussion of the underlying chemistry seems to be timely. The Perspective of Krishnan Rajeshwar et al. (Rajeshwar, K.; Janaky, C.; Lin, W.-Y.; Roberts, D. A.; Wampler, W. Photocatalytically Prepared Metal Nanocluster−Oxide Semiconductor−Carbon Nanocomposite Electrodes for Driving Multielectron Processes. J. Phys. Chem. Lett. 2013, 4, 3468−3478) focuses on the role of carbon nanocomposite matrixes for both the illuminated photocatalyst as well as the electrocatalysts, enabling multielectron reduction processes. Once both the semiconductor as well as the electron-transfer catalysts are anchored on these carbon supports, vectorial electron-transfer processes are possible, resulting in a charging of the electrocatalyst followed by the respective multielectron transfer processes. Processes that have been observed are the reduction of various noble metal cations forming the respective noble metal nanoparticles and the (subsequent) conversion of carbon dioxide to alcohols. Upon the basis of their own work and that of others, the authors highlight the importance of the nanoarchitecture of these systems for the overall efficiency of the photocatalytic processes. Moreover, the synthetic pathways to “construct” these nanocomposite architectures are also described in detail. The Perspective written by Shunichi Fukuzumi et al. focuses on another important aspect of photocatalytic water splitting systems, that is, the development of new multielectron transfer catalysts replacing rare and expensive noble metals such as platinum. In particular, various systems are presented that employ earth-abundant metals for the photocatalytic water reduction or oxidation. Ruthenium as well as nickel nanoparticles deposited on different supports such as MgO and SiO2 exhibit really promising activities for the sacrificial water reduction, as evinced by good and stable H2 yields. On the other hand, various cobalt and iron complexes and nanoparticles can apparently be used to catalyze the four-electron transfer required for the
Jenny Schneider Detlef W. Bahnemann*
■
Photocatalysis and Nanotechnology Research Unit, Institute of Technical Chemistry, Leibniz University of Hannover, Hannover, Germany
ACKNOWLEDGMENTS The authors gratefully acknowledge the Bildungsministerium für Bildung und Forschung (BMBF) for financial support (Project Number 033RC1012C, HyCats).
■
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Guest Commentary
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