Optimally Selecting Photo- and Electrocatalysis to Facilitate CH4 Activation on TiO2(110) Surface: Localized Photoexcitation versus Global Electric-Field Polarization.

Photo- and electrocatalytic technologies hold great promise for activating inert chemical bonds under mild conditions, but rationally selecting a more suitable method in between to maximize the performance remains an open issue, which requires a fundamental understanding of their different catalytic mechanisms. Herein, by first-principles calculations, we systematically compare the activation mechanisms for the C-H bond of the CH4 molecule on TiO2(110) under the photo- and electrocatalytic modes without or with water involved. It quantitatively reveals that the activation barrier of the C-H bond decreases dramatically with a surprising 74% scale by photoexcitation relative to that in thermocatalysis (1.12 eV), while the barrier varies with a maximum promotion of only 5% even under -1 V/Å external electric field (EEF). By detailed geometric/electronic analysis, the superior photocatalytic activity is traced to the highly oxidative lattice Obr •- radical excited by a photohole (h +), which motivates the homolytic C-H bond scission. However, under EEF from -1 V/Å to 1 V/Å, it gives a relatively mild charge polarization on the TiO2(110) surface region and thus a limited promotion for breaking the weakly polar C-H bond. By contrast, in the presence of water, we find that EEF can facilitate CH4 activation indirectly assisted by the surface radical-like OH* species from the oxidative water cleavage at high oxidative potential (>1.85 V vs SHE), which explains the high energy cost to drive electrocatalytic CH4 conversion in experiment. Alternatively, we demonstrate that more efficient CH4 activation could be also achieved at much lower oxidative potential when integrating the light irradiation. In such a circumstance, EEF can not only promote the h + accumulation at the catalyst surface but also help H2O deprotonation to form hydroxide, which can serve as an efficient hole-trapper to generate OH• radical (OH- + h + → OH•), unveiling an interesting synergistic photoelectrocatalytic effect. This work could provide a fundamental insight into the different characteristics of photo- and electrocatalysis in modulating chemical bond cleavage.

Introduction

To achieve the efficient activation of inert C–H bond of methane (CH4) at mild temperature is highly desired for facilitating the selective and controllable conversion of CH4 into a value-added commodity, which constitutes one of the most fundamental and challenging tasks in the chemical community.[1−4] In this regard, the transition metal oxide (TMO) catalysts have drawn enormous attention in practice with the unsaturated metallic site and lattice oxygen exposed simultaneously;[5−7] particularly, the photo- and electrocatalytic technologies are recognized as promising approaches, with merits of altering reaction thermodynamics and kinetics under mild temperature driven by the external light and electricity energy supply. Significantly, some experiments have demonstrated the feasibility of photo- and electrocatalytic CH4 conversion.[8−11] For example, Yoshida et al. reported that Pt/TiO2 photocatalyst displayed high activity for CH4 oxidation around room temperature (ca. 323 K).[12] The TiO2 electrode decorated with RuO2 and V2O5 was found capable of electrocatalytically converting CH4 into methanol with a selectivity of 57% at an applied voltage of 2.25 V (vs standard hydrogen electrode, SHE).[13]

Despite these important findings, the insight into the activation of the first C–H bond in the CH4 molecule under light irradiation and the electric-field (EF) polarization imposed by the cell potential has been not fully understood at the atomic level, which largely differs from the common thermocatalytic mode.[10,14] For example, qualitatively, the photoexcited polarons are believed to play an important role in the promotion effect with uncertainty. Wei et al.[15] proposed that the hole-trapped lattice oxygen radical was the main species for CH4 activation over the β-Ga2O3 photocatalyst under 254 nm light irradiation, which was also suggested on other metal oxide catalysts such as Ag-decorated ZnO, SrCO3/SrTiO3, and so on.[16−18] However, the photoelectron could be also controversially responsible for the enhanced CH4 activation at mild conditions.[19−21] Thus, a quantitative description of the activation process incorporating the kinetic barrier for CH4 activation in the photocatalytic mode remains an unresolved essential issue.

Similar to the photocatalysis, the electrocatalytic approach is usually believed to be capable of modulating the chemical bond breaking/reforming through the EF effect,[22] but to our knowledge, how largely the EF polarization effect directly affects the C–H activation on the semiconductor is less understood, despite that some important progress on the EF tuning effect has been achieved on the transition metal catalysts (e.g., Ni, Pt, Co, and so on).[23−29] Moreover, as an extended fundamental question, one may also wonder: theoretically, which catalytic mode is more efficient in facilitating the C–H bond and other chemical bonds? Interestingly, the current research implies that the electrochemical oxidation of CH4 appears to be kinetically slow and energy intensive.[30−33] For example, only when the applied potential (Uapp) increases to 2.25 V (vs SHE) can the efficiency of electrocatalytic CH4 conversion increase rapidly with a selectivity toward methanol reaching 57% over a TiO2/RuO2/V2O5 electrode; with a low Uapp (<2.25 V), the improvement of the EF-imposed catalytic efficiency becomes relatively limited.[13,34] Remarkably, when combining the electrocatalytic condition with the light irradiation, the activation of CH4 can occur on pure TiO2 catalyst at a much reduced Uapp; as shown from the experimental fact, the selectivity of CH4 steam reforming toward CO reached up to 81.9% over TiO2 surface under −0.41 V vs SHE.[9,35] Rationalizing these questions is not straightforward, and evidently, the photo- and electrocatalytic modes exhibit distinct catalytic characters. Some key questions are as follows: How does the EF imposed by the external voltage affect the C–H activation below and above 2.25 V (vs SHE) on TiO2 catalysts? What are the unique characteristics of photo- and electrocatalysis, respectively? How can they cooperate with each other?

To quantitatively understand the photo- and electrocatalytic characteristics on CH4 activation and rationalize their essential differences, here we carried out systematic DFT+U calculations with the PBE functional benchmarked against the advanced HSE06 hybrid functional for selected geometries on CH4 activation over the rutile TiO2(110) surface, a cross-sectional oxide support in photoelectrocatalytic reaction. The results unambiguously disclose and explain the unequivalent effects of light and electric field on CH4 activation. Light irradiation can remarkably promote the C–H bond activation directly by the localized photogenerated hole, while the direct EF polarization effect on C–H cleavage in the electrochemical mode is unexpectedly weak. Mechanistically, we found that the C–H activation undergoes different mechanisms in diverse external driving forces. To activate the C–H bond on TiO2(110), the electrochemical mode could have to indirectly make use of the radical-like OH* intermediate from environment H2O dissociation via the proton coupled electron transfer (PCET) mechanism. Moreover, we also demonstrated that these two distinct function mechanisms of photocatalysis and electrocatalysis will give an improved activity when integrated cooperatively.

Results and Discussion

Thermocatalytic Activation of CH4

To begin with, the adsorption and activation of the CH4 molecule on the TiO2(110) surface in the thermocatalytic condition were first calculated as a benchmark. The rutile TiO2(110) surface comprises two types of main active sites: the two-coordinated bridge oxygen and five-coordinated Ti cation (Figure a), denoted as Obr and Ti5c, respectively.[36] Because of the geometrically high symmetry of the CH4 molecule, it can weakly adsorb on the TiO2(110) surface with an adsorption energy (Eads) of −0.32 eV, wherein CH4 interacts mainly with d orbital of the surface Ti5c atom.[37,38] As the transition state (TS1) shows in Figure b, when C–H bond activation of CH4 follows the homolytic cleavage mode at the Obr site (i.e., the so-called hydrogen atom transfer (HAT) mechanism),[6,39] it gives a reaction barrier as high as 1.83 eV, which corresponds to CH4 + Ti5c4+ + Obr2– → CH3• + Ti5c3+ + ObrH– with the CH3• radical formed and the released electron being trapped at Ti5c4+ into Ti5c3+ at TS1. By comparison, in response to the synergistic coupling of Ti5c3+ with the CH3• radical, the C–H bond activation prefers to proceed via the proton coupled electron transfer (PCET) mechanism[40] (CH4 + Ti5c4+ + Obr2– → Ti5c4+–CH3– + ObrH–, see TS2 in Figure b) at the Ti5c and Obr dual sites with a lower barrier of 1.12 eV, which can be apparently taken as a heterolytic cleavage mechanism. Specifically, as the spin charge distribution of the transition state (TS2) illustrates in Figure b, the Obr site captures the dissociated hydrogen, and Ti5c as a Lewis acidic center stabilizes the methyl group, yielding a proton and a methyl anion (corresponding to Bader charges of 1.000 and −0.376 |e|, respectively), respectively. The forming H–Obr and C–Ti5c bonds in the TS2 are measured to be 1.200 and 2.347 Å, respectively. Nevertheless, overcoming the activation barrier of 1.12 eV is kinetically difficult at room temperature, which accords with the fact that CH4 activation generally has to be operated at high temperature.[41,42]

Figure 1

(a) Surface structure of optimized rutile TiO2 (110), with the corresponding O–Ti bonds defined as d1 and d2. (b) Two possible mechanisms for CH4 activation in thermocatalysis incorporating the spin density distributions of initial state (IS), transition state (TS), and final state (FS) at isovalue = 0.005. Some key bond lengths and Bader charges are marked in black and blue, respectively. Bader charge unit: |e|. Red, blue, gray, and white balls represent O, Ti, C, and H atoms, respectively. These notations are used throughout the paper. (c) Energy profiles of CH4 activation in three different external driving forces (light, electricity, and thermal energy).

Photocatalytic Activation of CH4

Under light irradiation, the photogenerated hole/electron pairs are generated and could be separated and localized at the different regions of the TiO2(110) surface, where the photogenerated hole tends to be trapped by surface Obr to generate Obr•– radical (i.e., Obr2– + h+ → Obr•–), while the photogenerated electron can be localized at Ti5c4+ to form the Ti5c3+ radical (i.e., Ti5c4+ + e– → Ti5c3+), which has been demonstrated in our previous studies and others.[43−45] As displayed in Figure S1, with the assistance of Ti5c3+, the barrier of CH4 activation is decreased a little to 0.99 eV with the strengthened stabilization of the CH3 intermediate by the Ti5c3+ site, and the reaction behavior is the same as that in thermocatalysis with a similar TS structure (see in Figure S1). Interestingly, in the presence of Obr•– radical, the barrier of C–H bond cleavage of the CH4 molecule (i.e., CH4 + Obr•– → HObr– + CH3•) can be greatly reduced to as low as 0.29 eV (see energy profile in Figure c). In comparison with the thermocatalytic case, the promotion scale of the activation barrier is quantitatively estimated to be an order of 74% (0.29 eV versus 1.12 eV). One can thus anticipate that the photocatalytic conversion would be an efficient way for activating CH4 on TiO2(110) if there are sufficient surface Obr•– species by increasing light absorption and decreasing the recombination of hole/electron pairs.

By analyzing the geometric/electronic structure of the resulting TS (Figure ), one can see that the C–H cleavage mode changes evidently owing to the participation of Obr•– species; specifically, the TS changes from the original dihapto configuration (with the CH3 and H atom in CH4 bonded to the Lewis acid/base site, Ti5c/Obr, respectively) in the thermocatalytic case to the monohapto configuration, in which the H atom is captured by the Obr•– site and the methyl radical suspends above the TiO2 surface. The bond lengths of the CH3···H and H···Obr in the TS are 1.227 and 1.339 Å, respectively. In other word, it exhibits a radical-like TS type, and the radical nature of CH3 species is also confirmed by a Bader charge of 0.090 |e| and a magnetic moment of 0.43 μB (Table S3). Therefore, the C–H bond activation modulated by the photoexcited hole essentially follows a homolytic cleavage mechanism or the so-called hydrogen-atom transfer mechanism.[40]

Figure 2

Geometry structures and spin density distributions of IS, TS, and FS for CH4 activation on TiO2(110) with the assistance of photogenerated hole.

In addition, by analyzing the Bader charge of the Obr•– center, it is found that it holds less electron than the Obr2– site in thermocatalysis (−0.690 vs −1.082 |e|). In this sense, we can rationalize that the Obr•– center, as a strong oxidative site with an unoccupied 2p orbital, can more easily capture electrons from the dissociated hydrogen atom than Obr2–, thereby leading to a much decreased barrier of C–H bond activation. By contrast, in the thermocatalytic condition, the oxidizability of the close-shell Obr2– is too weak to break the strong C–H bond in the CH4 molecule.

Electrocatalytic Activation of CH4

As a quantitative comparison, the adsorption and activation of CH4 in a series of electric-field intensities (−1 V/Å ≤ F ≤ 1 V/Å) were calculated and are shown in Table . When the negative EEFs are applied, the CH4 adsorption and activation progresses can be facilitated. For example, in comparison with the zero EEF, the adsorption energy of CH4 at −1 V/Å increases from −0.32 eV to −0.41 eV, and the CH4 activation barrier deceases from 1.12 to 1.06 eV (Figure a). However, to our surprise, the contribution of EEFs on CH4 activation is relatively weak (on an order of only 5% in −1 V/Å relative to that in 0 V/Å), whereas the positive EEFs suppress this progress. Specifically, the activation barrier increases gradually with the increasing of the applied electric fields from −1 V/Å to 1 V/Å (Figure b). Moreover, by fitting the activation barriers (Ea) versus the EEF intensities (F), we can obtain two independent linear relations with different slopes in the negative and positive EEF regions:

Table 1

Adsorption Energies of the CH4 Molecule on the TiO2(110) Surface (Eads), as well as the Activation Barriers (Ea) and Enthalpy Changes (ΔH) of CH4 Activation, and the Optimized Average O–Ti bond lengths (d1 and d2), Bader Charges of Obr and Ti5c Sites, and -ICOHP Values of C–Ti5c and H–Obr Bonds in TSs, Obtained in Different EEFs (F) Using DFT+U

				bond length		Bader charge		-ICOHP (eV)
EEF (V/Å)	Eads (eV)	Ea (eV)	ΔH (eV)	d1 (Å)	d2 (Å)	Obr	Ti5c	C–Ti5c	H–Obr
–1.00	–0.41	1.06	0.32	1.840	1.863	–1.048	2.432	1.516	2.452
–0.50	–0.35	1.10	0.45	1.846	1.854	–1.066	2.434	1.409	2.465
–0.35	–0.35	1.11	0.48	1.848	1.853	–1.066	2.409	1.391	2.468
0.00	–0.32	1.12	0.54	1.854	1.847	–1.082	2.404	1.341	2.468
0.35	–0.27	1.14	0.57	1.859	1.841	–1.094	2.405	1.246	2.463
0.50	–0.24	1.15	0.59	1.860	1.838	–1.114	2.402	1.216	2.471
1.00	–0.18	1.15	0.62	1.868	1.831	–1.135	2.397	1.113	2.482

Figure 3

(a) Structures and spin density distributions of IS, TS, and FS for CH4 activation at −1 V/Å. (b) Correlations between Ea and F, as well as the mechanism schemes for CH4 oxidation modulated by negative (left) and positive (right) EEFs, in which red arrows indicate the promotion effect, whereas blue arrows signify the suppression effect. (c) Schematic diagram for surface charge polarization and atomic fluctuation at different EEFs. (d, e) Bond length changes of d1 and d2, as well as Bader charge changes of Obr and Ti5c versus F.

Obviously, the slope in negative EEFs is more than twice than that in positive EEFs, indicating that the negative EFs possess a more sensitive influence on promoting the CH4 activation progress. Furthermore, it is worth noting that the C–H bond dissociation mechanism under EEF is the same as that in thermocatalytic conditions, maintaining a two-site assisted TS configuration. Taking the reaction in −1 V/Å as an example, the CH3···Ti5c bond is 2.280 Å, which is slightly shorter than that without the EF effect (2.347 Å). Additionally, the changes of CH3···H and Obr···H bonds are also tiny (1.440 vs 1.446 Å and 1.200 vs 1.200 Å, respectively, Figure a).

Origins behind the Differences between Photo- and Electro-catalysis

Intuitively, the electrocatalytic approach is similar to photocatalysis, except for the energy source which comes from EEFs.[46] However, why are the EEFs far less efficient than light irradiation on CH4 activation, and how do the EEFs affect the CH4 activation progress? To solve these puzzles and unveil the inherent mechanisms of EEF and light on CH4 activation, the geometric and electronic structure analyses on TiO2(110) catalyst under different external driving forces were carried out.

First, the structural responses and charge states of Obr and Ti5c sites, as the main reactive centers on TiO2(110), were investigated. In photocatalysis, the hole activated Obr and electron activated Ti5c are featured with an obvious extension of Obr–Ti and Ti5c–O bonds (denoted as d1 and d2 in Figure a, respectively) by 0.196 and 0.205 Å, respectively (Figures b and 2).[36,47] Interestingly, under different EEFs from 1 V/Å to −1 V/Å, we found that the d2 increased, whereas the d1 decreased gradually (see Table and Figure d), with the increment (decrement) magnitude of d2 (d1) being only 0.03 Å. Noteworthily, this finding seems to be contrary to our general cognition that, with the negative EEF imposed (corresponding to the oxidative potential), the surface is polarized to be electron-deficient (Figure c), which is favorable to the activation of Obr, and d1 should be elongated compared to the inactivated ones.

To shed light on these different behaviors, we conducted the detailed electronic structure analysis, including plane-averaged electrostatic potential (PAEP) distribution and Bader charge analysis. As illustrated in Figure S2 for the PAEPs of the TiO2(110) surface under different EEFs, the main difference of electrostatic potential appears at the surface region and above, indicating the relatively stronger polarization of EEF on the catalyst surface than in the bulk. Nevertheless, by Bader charge analysis on surface atoms (Figure e and Table ), we found that the most negative EEF (−1 V/Å) only led to a small fraction of charge depletion for both Obr and Ti5c atoms, which existed as Obr–2+α and Ti5c+4+β (0 < α, β ≪ 1), respectively. Thus, we can deduce that the electronic field line interacts weakly with groups of positively charged Ti5c and negatively charged Obr atoms, respectively, and relaxes themselves along the electronic field line to minimize the potential energy, which contributes to the weak fluctuation of surface atoms, i.e., the decrement/increment of d1/d2 (see the illustration in Figure c). This is different from the photocatalytic mechanism: accompanying the localization of a photoexcited hole on Obr, the interaction between this specific hole-trapped Obr and substrate is strongly weakened, and the charge changes a lot from −1.082 |e| to −0.690 |e|, which is much larger than the effect of EEF.

Second, with the above basic understandings, we are now at the position to discuss why the barrier of CH4 activation in positive EEF increases much slower than that in negative EEF, as Figure b shows. The periodic natural bond orbital (NBO) analysis[48,49] was performed to show the bonding nature of two-site assisted TS complex in thermocatalysis. As can be seen from the H–Obr NBOs in Figure a (with details labeled), the H–Obr σ bond is largely composed of the hydrogen s orbital and oxygen p orbitals, which corresponds to the characteristic peak at the energy level of −5.70 eV from the PDOS of TS (Figure b). Noticeably, a tiny characteristic peak of the Ti5c–CH3 σ bond appears below the Fermi level at the energy level of −5.20 eV with an extremely small occupied area of only 0.008 |e|, which implies a weak C–Ti5c bond. One can thus speculate that the activation of CH4 depends mainly on the reactivity of the Obr site, whereas the Ti5c site contributes secondarily. The higher value of integrated crystal orbital Hamilton population (-ICOHP)[50] on the H–Obr (2.468 eV/bond) than that on the C–Ti5c bond (1.341 eV/bond) confirms this speculation (see Table ). Thus, the observation that a smaller slope exists in positive EEFs can be explained. When the positive EEF is exposed, the surface is in an electron-rich state, which reduces the reactivity of the Obr site for accepting H; simultaneously, the Ti5c site, as a secondary reaction site, is activated and facilitates binding with the CH3 group, thus leading to an overall slow increment of reaction barrier with the increment of positive EEFs (see inserted mechanism scheme in Figure b). By contrast, the Obr is activated while Ti5c is inactivated in the negative EFF, and a relatively larger variation of the reaction barrier can be expected.

Figure 4

(a) NBOs and (b) projected density of state (PDOS) of the TS for CH4 activation on TiO2(110) in thermal conditions. (c, d) PDOSs of Obr-2p and Ti5c-3d bands of the bare surface in different EEFs and light, respectively. (e) Correlation between CH4 activation barrier (Ea) and p-band center (εp) of the Obr site in EEFs (or energy level of characteristic peak of the hole localization, εh, in light).

Third, we did the orbital-projected densities of state (PDOS) of surface Obr-2p and Ti5c-3d bands, which determined the reactivity of the catalyst, to uncover the reactivity trends of photo/electro- versus thermocatalysis. As illustrated in Figure c, with the decrement of EEFs from 1 V/Å to −1 V/Å, the energy level of the Obr-2p band (constituting the valence band maximum, VBM) up-shifts gradually, which indicates a better oxidizability of the Obr site to accept the H atom by forming the polar covalent H–Obr bond. Quantitatively, we calculated the occupied p-band center (εp) of Obr site, an important indicator to describe the reactivity of the reaction center. The εp of the catalyst in different EEFs (+1, +0.5, 0, −0.5, and −1 V/Å) were calculated to be −1.04, −1.01, −0.93, −0.58, and −0.54 eV, respectively, confirming that negative EEFs result in the upshifting of the p-band center of the Obr site. Remarkably, a linear correlation between the activation barriers (Ea) and εp exists (Figure S3), implying that the upshifting of the EEF-induced p-band mainly accounts for the decrement of CH4 activation barrier on TiO2(110) surface.

In the presence of the photoinduced hole, by analyzing the PDOS in Figure d, we found that the formation of Obr•– radical introduced a localized paramagnetic state (1.75 eV above the Fermi level) in the band gap, manifesting itself in the large upward shift relative to the VBM. The localized paramagnetic hole state serves as a springboard to accommodate the released electron from CH4 dissociation (i.e., CH4 + Obr•– → HObr– + CH3•), which is characterized with an extremely strong oxidative ability to facilitate CH4 activation. Interestingly, the decreased Ea with the high-energy-level Obr•– characteristic peak (1.75 eV) can be also well fitted in the linear correlation between Ea and εp in EEFs (Figure e).

Based on these computational results, a picture for CH4 activation on the TiO2(110) surface in different external fields can be proposed. Upon introducing negative/positive EEFs, the 2p-bands of all the surface Obr sites are polarized to a higher/lower energy level, which enhances/depresses the activation of CH4. Similarly, for CH4 activation in light illumination, the unoccupied 2p hole state is the key for surging the catalytic activity. More specifically, the origins behind the superior performance of light than electricity could be understood. In photocatalysis, photogenerated carriers are quantized. These holes activate surface individual lattice oxygen to form Obr•– radicals, which have a strong oxidizability, whereas for electrocatalysis, groups of surface lattice oxygens are influenced gently by the electronic field line owing to the strong Ti–O chemical bond, which contributes to upshift of 2p-band and thereby prompts oxidizability of the Obr site on CH4 activation.

Since the EEFs have a weak contribution to the direct CH4 activation by the TiO2(110) catalyst, one may naturally question the two experimental results mentioned above that (i) the kinetics of electrocatalytic CH4 steam reforming can be accelerated over the TiO2-based electrode, when the potential is above 2.25 V (vs SHE),[13] and (ii) when combining the electrocatalytic condition with the light irradiation, the activation of CH4 can occur even on pure TiO2 catalyst with a much reduced voltage (−0.41 V vs SHE).[35] Considering that the H2O molecule, as the main medium in electrocatalysis and photoelectrocatalysis systems, may have some indispensable roles on CH4 conversion via the dissociated reactive oxygen species, we examined the electrocatalytic H2O dissociation and CH4 activation to answer these two questions.

First, the oxidative water cleavage via the proton coupled electron transfer (PCET) process was tested, including (i) H2O + * → OH* + H+ + e– and (ii) OH* → O* + H+ + e–, where * denotes Ti5c.[51] Following this path, the formed surface OH* and O* intermediate would be radical-like, evidenced with Bader charges of −0.011 |e| and −0.014 |e| on TiO2(110) surface, respectively. Remarkably, our calculations uncover that the OH* and O* species can facilitate CH4 activation greatly with much lower barriers of 0.36 and 0.45 eV, respectively (Figure a). However, this PCET process is thermodynamically difficult to occur, which has to be driven by high oxidative potential. Specifically, the thermodynamic stability of the surface OH*/O* as a function of the applied potential U at pH = 7 is presented in Figure b (see details in Table S4). We can see that only when the potential increases to 1.85 V, the OH* intermediate becomes thermodynamically favored to be formed; when the potential continues to increase to 2.18 eV, the O* begins to be formed. This accords with the electrocatalytic CH4 steam reforming experiment over TiO2/RuO2/V2O5 electrode that CH4 can be activated when the applied potential is above 2.25 V (vs SHE)[13,34] and is also arguably in line with the observation that hydroxide and oxygen adatoms are produced on Ti5c sites under a voltage pulse of 2.4 or 2.8 V on the TiO2(110) surface.[52]

Figure 5

(a) Optimized TSs and barriers for CH4 activation with the assistance of OH•/O• species from H2O dissociation via PCET and OH– intermediate via PT progress, respectively. (b) Thermodynamic phase diagram of H2O conversion on TiO2(110) as a function of the potential at the pH = 7, including (i) H2O + * → OH* + H+ + e– (red), (ii) OH* → O* + H+ + e– (blue), and (iii) H2O + * → O* + 2H+ + 2e– (black). (c) Geometries of PT progress for H2O dissociation with the proton released into the liquid phase.

Thus, we can comprehend that the EEF effect at high oxidative potential can hardly activate CH4 directly but could alternatively activate CH4 (when H2O exists) by resorting to the surface active species (i.e., OH*) from the oxidative water cleavage indirectly. In addition, it may be worth noting that the applied potential is better when it is not higher than 2.18 V, as the reactive O* species can not only facilitate CH4 activation (Ea = 0.45 eV, Figure a) but also simultaneously promote oxygen evolution via O–O coupling.[47] This is in line with the experimental observation over the TiO2/RuO2/V2O5 electrode, that the CH4 oxidation process is prompted below 2.25 V (vs SHE) but is inhibited at a more positive potential with simultaneous oxygen evolution.[13]

Second, the dissociation of water via the deprotonation progress (i.e., proton-transfer mechanism, PT) into the solution was comparatively calculated at the H2O/TiO2(110) interface by the ab initio molecular dynamics (AIMD) simulation[53] (see method details in Supporting Information), which corresponds to Ti5c–H2O → Ti5c–OH– + H+ (Figure c). In comparison with the PCET process, this PT process occurs relatively easily with a low barrier of 0.39 eV and reaction enthalpy of 0.21 eV at the thermal condition. Furthermore, our calculations verified that the potential-induced EF effect would efficiently affect these proton transfer kinetics to form OH– species. Specifically, the negative EFs (corresponding to the oxidative potential) lead to an evident promotional effect on H2O dissociation (Table S5), resulting from the dipole moment and large polarizability of the O–H bond;[54] for example, there is a much decreased barrier of 0.08 eV at −1 V/Å. It implies that a low potential could be strong enough to facilitate the hydroxide OH– formation. However, different from the radical-like OH* species in PCET progress, the formed OH– can hardly facilitate CH4 activation with a high barrier of 0.93 eV, which is comparable with the pristine inert Obr (1.12 eV, Figure a). Thus, the charge state of the OH intermediate on TiO2(110) is critical for modulating its reactivity.

Significantly, when integrating with the light irradiation (see schematic in Figure ), the hydroxide OH– generated under low potential (or low EEF) could bring about an evident synergy effect for CH4 activation. Under a pure photocatalytic condition, it was reported to be only a H2O dissociation probability of about 1.7% and 3.7% on TiO2(110) at the 400 and 355 nm light irradiation, respectively,[52] largely restricted by the high recombination rate of the photoexcited electron–hole (namely, the low concentration of the surface hole)[20,55] and relatively low kinetics of H2O photocatalytic oxidation progress (i.e., H2O + h+ → OH• + H+).[36,47] However, with the cooperation of photo- and electrocatalysis, the photohole can readily trap the OH– species (i.e., OH– + h+ → OH•) generated from electrocatalysis-accelerated H2O deprotonation progress with a hole-trapping capacity of −0.77 eV,[36] to form OH• radical for better CH4 activation. In this sense, one can rationalize that, experimentally, just a low oxidative potential is required to accomplish the efficient CH4 activation when combining with the photocatalysis.[35] In addition, our calculations identify that the imposed negative EEF under the oxidative potential can also enhance the direct distribution of hole/electron along/against the electric-field line, with the hole polaron being preferentially accumulated at the catalyst surface (see details in the Supporting Information, Figures S4 and S5).[56] Accordingly, the synergistic effect of photoelectrocatalalysis can be further enhanced for motivating the formation of reactive OH• radical. It may be worth noting that this finding could also explain the experimental phenomenon that the amount of O2 increases with the increasing of EEFs in photoelectrocatalalysis.[35]

Figure 6

Schematic diagram for OH• formation via PCET and PT+ET progresses. PCET: H2O* → OH• + H+ + e–; PT: H2O* → OH– + H+; and ET: OH– → OH• + e–. Note: 1.64 eV for ET progress; see details in Figure S6.

In brief, the above discussions indicate that the electrocatalytic CH4 conversion eventually needs to indirectly resort to the reactive oxygen species (e.g., OH• radical). As Figure illustrates, the formation of OH• radical can be accomplished from the oxidative cleavage of H2O via the PCET mechanism; however, the required applied oxidative potential is rather high. Alternatively, OH• could be formed via the sequential PT+ET processes, during which the PT process occurs relatively easily kinetically on TiO2(110) and can be efficiently modulated by the low EEF, whereas the ET process is energy-intensive with an energy cost over 1.64 eV (see details in Figure S6). In this circumstance, the light irradiation could be a competent approach to surmount this high energy demand and drive this ET process (OH– + h+ → OH•). In this regard, the integrated photoelectrochemical strategy should be promising for achieving efficient CH4 activation.

Conclusions

In summary, the present study provides a comprehensive comparison between the photo- and electrocatalysis and uncovers their critical roles and unique mechanisms for modulating CH4 oxidation by TiO2(110) in the absence or presence of water. It turns out that a positively polarized surface is helpful for CH4 activation, whereas the contribution is largely inferior to a photogenerated hole in photocatalysis. The origins behind it were revealed: the negative EEFs contribute to the Obr-2p band upshift slightly toward the Fermi level, indicating the more reactivity of lattice oxygen to make a bond with H atom. For comparison, the photocatalysis reaction proceeds with the assistance of a localized gap state as a springboard, whose oxidizing ability is largely greater than that of the electro-activated lattice oxygen. Differently, the essential role of EEFs on CH4 conversion on TiO2(110) can be strengthened in the presence of water. EEF can trigger the oxidative water cleavage via the PCET process under high oxidative potential, resulting in the formation of OH* radical-like species, which are reactive for CH4 activation. Alternatively, EEF can facilitate H2O deprotonation via the PT mechanism to form the surface hydroxide OH–, which can easily trap the photoexcited hole to generate OH•. In addition, it can also will promote the carrier spatial separation by stabilizing the hole–polaron configuration along the electric-field line in photoelectrocatalalysis, rationalizing the synergistic photoelectrocatalytic effect for boosting CH4 activation on TiO2(110) at low applied potential.