2.1 CH3OH oxidation pathways on pure Pt(111) surface
Surface and solvation models and computational details are given in Supporting Information. For comparison, CH3OH electrooxidation mechanisms on pure Pt(111) surface in aqueous phase that calculated in our previous work are sketched out to examine the effect of alloying in this paper.31 As can be seen in Figure 1 (a), our previous DFT calculated results indicate that the optimal indirect pathway of CH3OH electrooxidation into CO2 mainly occurs by initialing with C-H bond scission via intermediates CH2OH, CHOH, CHO and CO on pure Pt(111) surface in aqueous phase, in which COH species may be only a spectator. The oxidation rate may be able to be determined by C-H bond cleavage of CH2OH into CHOH intermediate and CO further oxidation into CO2 due to their relatively higher activation barriers. The direct pathways may occur through CHO hydroxylation into HCOOH intermediate and its further oxidation to form CO2, as shown in Figure 1(b). Combined with CHO formation and its further oxidation into CO2, the optimal direct pathway possibly takes place through intermediates CH2OH, CHOH, CHO, HCOOH, HCOO, and COOH on pure Pt(111) surface, in which the rate-determining step is C-H bond cleavage of CH2OH into CHOH species. By comparing formation barrier of CO intermediate with that of HCOOH species during CHO further oxidation (ca. 0.28 eV vs. 0.10 eV), it is found that both are very low and surmountable at room temperature, suggesting that the indirect and direct pathways may be able to parallelly occur on pure Pt(111) surface. However, a high barrier of ca. 0.88 eV is required for CO further oxidation to CO2 product, which can lead to poisoning of surface Pt active sites due to adsorbed CO, explaining why CH3OH electrooxidation experimentally is facile to be suppressed on pure Pt electrodes. The corresponding energetics for possible elementary reaction steps involved in indirect and direct pathways on pure Pt(111) surface in aqueous phase are summarized in Table S1.
2.2 CH3OH oxidation pathways on Pt-Ru alloy surfaces
The optimized structures for various possible adsorbed species on Pt-Ru alloys with different Ru surface compositions are given in Figures S1-3, which are employed to carry out MEP analyses. Formation pathways of key intermediate CO are firstly examined initial with C-H, O-H and C-O bond scissions of CH3OH in this paper. As can be seen in Figures 2 and 3, the activation barriers for C-H bond cleavage to form the adsorbed CH2OH species are calculated as ca. 0.67, 0.27 and 0.28 eV with energy change of reaction of ca. -0.01, -0.16 and -0.35 eV on Pt2Ru(111), PtRu(111) and PtRu2(111), respectively. For formation of adsorbed CH3O species via O-H bond breakage, the calculated barriers are ca. 1.15, 1.11 and 0.58 eV with reaction energies of ca. 0.72, 0.63 and -0.27 eV, respectively. For C-O bond cleavage into the adsorbed CH3 species, the barriers are calculated as ca. 1.50, 1.58 and 0.38 eV with energy change of reaction of ca. 0.50, -0.20 and -0.42 eV, respectively. The above MEP analyses indicate that initial CH3OH oxidation into CH2OH species via C-H bond scission may be the most favorable on Pt-Ru alloys due to significantly lower barriers and more exothermic or less endothermic than those of O-H and C-O bond breakage, which are well agreeable with previous theoretical homogeneous dehydrogenation studies in aqueous phase on pure Pt(111) surface from us, Mattsson et al. and Okamoto et al.31, 52, 53 Thus, only the oxidation pathway that initial with C-H bond breakage of CH3OH is further considered on Pt-Ru alloys with different Ru surface compositions in aqueous phase.
Beginning with further CH2OH oxidation, two possibilities are considered. An activation barrier of ca. 0.42, 0.15 and 0.38 eV is required for C-H bond scission to form CHOH species on Pt2Ru(111), PtRu(111) and PtRu2(111) with the reaction energies of ca. 0.30, -0.43 and -0.73 eV, respectively, whereas formation of CH2O species via O-H bond scission need to overcome the barrier of ca. 0.94, 0.73 and 0.34 eV with energy change of reaction of ca. 0.87, 0.46 and -0.40 eV, respectively. We find that CH2OH intermediate can be facile to further oxidize to form the adsorbed CHOH intermediate via C-H bond activation step on Pt2Ru(111) and PtRu(111), as demonstrated by significantly lower barrier and more exothermic values of reaction energies compared to those of O-H bond cleavage to form adsorbed CH2O species. However, CH2OH intermediate further oxidizes through C-H and O-H bond activation steps to form the adsorbed CHOH and CH2O intermediates both have low and approximately equal activation barriers with exothermic values of reaction energies on PtRu2(111), suggesting that CHOH and CH2O intermediates may be able to be simultaneously formed and both pathways may be able to parallelly occur on PtRu2(111).
The adsorbed CHO or COH species may be able to be further formed by either O-H or C-H bond breakage steps of CHOH species formed by CH2OH oxidation. The calculated barrier is ca. 0.50, 0.43 and 0.26 eV for O-H bond scission into CHO species with reaction energies of ca. 0.32, 0.20 and -0.44 eV on Pt2Ru(111), PtRu(111) and PtRu2(111), respectively. C-H bond cleavage to COH requires the barriers of ca. 0.16, 0.03 and 0.03 eV, respectively. The corresponding reaction energies are ca. -0.19, -0.48 and -1.28 eV. It can be found that significantly lower barriers are required for COH formation than that of CHO on Pt2Ru(111), PtRu(111) and PtRu2(111), which are almost non-activated processes with exothermicity and can be overcome at room temperature, suggesting that C-H bond breakage into COH may be more facile to occur. However, we also observe that CHO formation barrier is also very low and can be easily overcome with exothermicity on PtRu2(111), suggesting that O-H bond scission in CHOH species is also possibly to occur. The produced CH2O by CH2OH oxidation can also react via C-H bond cleavage step to form adsorbed CHO on PtRu2(111). The required barrier is extremely low (0.10 eV) with a exothermic reaction energy of -0.47 eV. Thus, the results showed that CHO can be facile to form through CHOH and CH2O further oxidation, which maybe parallel pathways on PtRu2(111).
Table 1 Activation barriers (Eact, eV) and reaction energies (Ereac, eV) for possible elementary reaction steps involved during CH3OH oxidation into CO on Pt2Ru(111), PtRu(111) and PtRu2(111)
Elementary Reaction Steps
|
Pt2Ru(111)
|
PtRu(111)
|
PtRu2(111)
|
Eact, eV
|
Ereac, eV
|
Eact, eV
|
Ereac, eV
|
Eact, eV
|
Ereac, eV
|
CH3OH* → CH2OH* + (H+ + e-)
|
0.67
|
-0.01
|
0.27
|
-0.16
|
0.28
|
-0.35
|
CH3OH* → CH3O* + (H+ + e-)
|
1.15
|
0.72
|
1.11
|
0.63
|
0.58
|
-0.27
|
CH3OH* → (CH3 + OH)*
|
1.50
|
0.50
|
1.58
|
-0.20
|
0.38
|
-0.42
|
CH2OH* → CHOH* + (H+ + e-)
|
0.42
|
0.30
|
0.15
|
-0.43
|
0.38
|
-0.73
|
CH2OH* → CH2O* + (H+ + e-)
|
0.94
|
0.87
|
0.73
|
0.46
|
0.34
|
-0.40
|
CHOH* → CHO* + (H+ + e-)
|
0.50
|
0.32
|
0.43
|
0.20
|
0.30
|
-0.44
|
CHOH* → COH* + (H+ + e-)
|
0.16
|
-0.19
|
0.03
|
-0.48
|
0.03
|
-1.28
|
CH2O* → CHO* + (H+ + e-)
|
\
|
\
|
\
|
\
|
0.10
|
-0.47
|
CHO* → CO* + (H+ + e-)
|
\
|
\
|
\
|
\
|
0.02
|
-1.03
|
COH* → CO* + (H+ + e-)
|
0.08
|
-0.64
|
0.39
|
-0.39
|
0.16
|
-0.67
|
(CO + OH)* → CO2* + (H+ + e-)
|
0.68
|
0.11
|
0.68
|
0.72
|
1.21
|
0.84
|
The further oxidation of adsorbed COH species to form key intermediate CO need to overcome barriers of ca. 0.08, 0.39 and 0.16 eV with negative reaction energies of ca. -0.64, -0.39 and -0.67 eV on Pt2Ru(111), PtRu(111) and PtRu2(111), respectively. Conversion of adsorbed CHO produced by CH2O species to form stable adsorbed CO requires a barrier of ca. 0.02 eV with strong exothermicity of ca. -1.03 eV on PtRu2(111). The results show that intermediate CO can produced by COH further oxidation on Pt2Ru(111), PtRu(111) and PtRu2(111) and CHO further oxidation on PtRu2(111) due to the surmountable barriers at room temperature, as shown in Figures 2 and 3. The corresponding energetics for possible elementary reaction steps involved during CH3OH oxidation into CO on Pt-Ru alloys with different Ru surface compositions are summarized in Table 1.
In indirect pathway, the formed key intermediate CO further oxidation can lead to production of CO2 product. The corresponding activation barrier is ca. 0.68, 0.90 and 1.21 eV on Pt2Ru(111), PtRu(111) and PtRu2(111) with the reaction energy of ca. 0.11, 0.72 and 0.84 eV (See Table 1), respectively. Our present proposed optimal CH3OH oxidation pathways to produce CO2 products on Pt-Ru(111) alloys with different Ru surface compositions initial with C-H bond scission are given in Figure 4. As is well-known, CHO species, as a generally accepted reactive intermediate in indirect and direct pathways for CH3OH oxidation, may be able to lead to formation of HCOOH intermediate in the direct pathway via hydroxylation. And then, the final production CO2 can be produced via successive oxidation of HCOOH. Formation of CHO species is unfavorable on Pt2Ru(111) and PtRu(111) due to significantly higher barriers than those of COH (See Table 1), suggesting that the direct pathways via HCOOH intermediate is impossible to occur. Formation pathways of CHO and COH species may be able to parallelly occur on PtRu2(111) due to the low and almost identical barriers. Thus, it can be speculated that the direct pathways via HCOOH intermediate may be able to occur on PtRu2(111). However, an extremely high barrier of ca. 1.50 eV is required for CHO hydroxylation into HCOOH with strong endothermic process of ca. 1.31 eV, suggesting that the direct pathway is also difficult to occur and CHO species only can lead to CO formation on PtRu2(111). The most recent experimental study from Wei et al. also showed that the pathways via CO intermediate is more favorable on the Ru-rich Pt-Ru alloy electrocatalysts,40 further validating accuracy of our present theoretical studies.
2.3 Effect of Ru surface composition on CH3OH oxidation pathways
As above elaborated, the indirect pathways through CHO and CO intermediates and direct pathways via HCOOH intermediate may be able to parallelly occur on pure Pt(111) surface, as shown in Figure 4(a). After alloying of Pt with Ru, only indirect pathways occur due to difficult formation of HCOOH species, as can be seen in Figure 4 (b) and (c). Thereinto, formation of COH species is more favorable than CHO on Pt2Ru(111) and PtRu(111), whereas COH and CHO species may be able to simultaneously form on PtRu2(111) with higher Ru surface compositions. Thus, the changes of Ru surface compositions may be able to lead to different CH3OH oxidation pathways. In the mean time, it is noted that barriers of some elementary steps during CH3OH oxidation into CO are significantly decreased after alloying of Pt with Ru compared with those of pure Pt surface. C-H bond scission into CH2OH species is the optimal initial CH3OH oxidation pathway on pure Pt(111) and Pt-Ru alloys. The corresponding barrier is ca. 0.80 eV on pure Pt(111). We observe that CH2OH formations barrier is reduced into ca. 0.67 eV when surface composition of Ru is ca. 33%, and it has approximately equal and the lowest value of barrier (ca. 0.28 eV) when surface composition of Ru is ca. 50% and 67%. The most recent experimental results from Wei et al. observed a higher forward peak current for C-H bond scission of CH3OH when surface composition of Ru is ca. 50%, indicating a faster kinetic rate and confirming our present theoretical results.40 CH2OH further oxidation inclines to form CHOH intermediate on pure Pt(111) surface and when surface composition of Ru is ca. 33% and 50%, in which CHOH formation barrier is significantly decreased into ca. 0.42 and 0.15 eV as increasing Ru surface compositions compared with that of pure Pt(111) surface (ca. 0.95 eV). However, intermediates CHOH and CH2O may be able to be simultaneously formed during CH2OH further oxidation due to very low and almost equal activation barriers (ca. 0.38 eV vs. 0.34 eV) when Ru surface composition is increased into ca. 67%, which are also notably lower than that of pure Pt(111) surface. It can be observed that CH2OH further oxidation into CHOH species has lowest barrier when surface composition of Ru is ca. 50%, suggesting that surface compositions of more than 50% will be not favor of CH2OH oxidation.
CHOH further oxidation into CHO and COH species both are facile to occur on pure Pt(111) surface, which have extremely low and almost equal barriers (ca. 0.10 eV) and are almost non-activated processes. However, the barrier is notably increased for CHOH further oxidation into CHO species after alloying of Pt with Ru compared with that of pure Pt(111) surface. CHOH further oxidation into COH species through C-H bond scission is relatively more easily to occur than O-H bond scission into CHO. COH formations are all almost non-activated processes due to extremely low barriers (ca. 0.10 eV) on Pt-Ru alloys with different Ru surface compositions. Moreover, increasing Ru surface compositions make COH formation barriers reduce into below 0.10 eV, namely, COH formations have the lowest values, ca. 0.03 eV when surface composition of Ru is ca. 50% and 67%, being slightly lower than those of pure Pt(111) surface. Simultaneously, we also noted that CHO is also possibly to be formed by CH2O further oxidation due to very low barrier (ca. 0.10 eV) when surface composition of Ru is ca. 67%. After alloying of Pt with Ru, the required barriers for COH and CHO further oxidation into key intermediate CO are also significantly reduced compared with that of pure Pt(111) surface. Thereinto, CO is formed mainly by COH further oxidation with the surmountable barriers at room temperature when surface composition of Ru is ca. 33%, 50% and 67% and CHO further oxidation into CO is also possibly to occur with extremely low barrier (ca. 0.02 eV) when surface composition of Ru is ca. 67%. CO further oxidation can lead production of final CO2 product via hydroxylation, in which OH may be from dissociation of H2O molecule in aqueous phase. The required barrier is reduced to ca. 0.68 eV when surface composition of Ru is ca. 33% compared with that of pure Pt(111) surface (ca. 0.88 eV), as shown in Figure 5(a). The almost identical barrier is observed (ca. 0.90 eV) with that of pure Pt(111) surface when surface composition of Ru is ca. 50%. Ru surface composition of ca. 67% make the barrier notably increase into ca. 1.21 eV. The increasing barrier as increasing Ru surface compositions may be able to be attributed to the increasing barrier for H2O dissociation into adsorbed OH species, as shown in Figure 5(b), which is an essential intermediate for CO hydroxylation in indirect pathway. Furthermore, higher oxygen affinity of Ru make OH adsorption is stronger than that on pure Pt(111), which can lead to more difficult CO hydroxylation on Pt-Ru alloys with high Ru surface compositions. Thus, gradually increased barrier for CO hydroxylation are observed as increasing Ru surface compositions.
The rate-determining steps may be CH2OH oxidation into CHOH intermediate via C-H bond cleavage and CO further oxidation into CO2 through hydroxylation due to their almost identical and relatively higher activation barriers than other elementary steps on pure Pt(111) surface. CH3OH oxidation rate only is determined by CO further oxidation into CO2 after alloying of Pt with Ru since the barrier for CH2OH oxidation into CHOH intermediate is significantly reduced (See Table 1). Notably, the lowest barrier of ca. 0.15 eV can be obtained when surface composition of Ru is ca. 50%. Simultaneously, we also observed that intermediates CHOH and COH formations have the lowest barriers when surface composition of Ru is ca. 50%. Thus, it can be speculated that the optimal Ru surface composition may be ca. 50% for CH3OH oxidation on Pt-Ru alloys. Although CO further oxidation into CO2 requires almost identical barrier when surface composition of Ru is ca. 50% with that on pure Pt(111) surface, the significantly stronger exothermic process for CO formation make CO2 production be facile to occur (See Figure 2). When surface composition of Ru is ca. 67%, the barriers for some elementary reaction steps are also reduced into the lowest values like formation pathways of CH2OH and COH species and CH3OH oxidation to produce CO intermediate has the strongest exothermicity (See Figure 3). However, it is noted that carbonaceous species, such as CH3 that can poison the surface may be formed easily via C-O bond cleavage in CH3OH with the notably decreased and surmountable barrier of ca. 0.38 eV at room temperature when surface composition of Ru is ca. 67%, suggesting that more than 50% Ru surface composition will be not favor of CH3OH oxidation. Our present calculated Ru surface compositions of ca. 50% exhibits excellent consistency with the previous experimental studies. For example, the studies from Gasteiger et al., Weidner et al. and Wei et al showed that a ca. 50% of Ru surface composition can achieve the best electrocatalytic activity for CH3OH oxidation and anti-CO poisoning ability,37-40 confirming the reasonability of our present theoretical calculations to some degree.
Surface work function W is calculated in the presence of aqueous phase in order to determine origin of the enhanced electrocatalytic activity of Pt-Ru alloys towards CH3OH oxidation in this paper. By definition, W is minimum energy required to extract one electron from surface to an infinite distance, which can be well calculated on the basis of DFT methods.54, 55 The work function can be exactly calculated by equation:56 W = Vvacuum – EF, in which Vvacuum is vacuum level of pure Pt surface and Pt-Ru alloy systems and is defined to be mean electrostatic potential, and EF is the Fermi level. The present calculated surface work function is 5.33, 5.28 and 5.12 eV on Pt2Ru(111), PtRu(111) and PtRu2(111), respectively, which are significantly lower than that of pure Pt(111) (5.50 eV), suggesting more electrons are transferred into various adsorbates on Pt-Ru alloys and facilitating CH3OH oxidation. On this basis, we can approximately obtain the electrode potential (U) by referring the work function (Φ) of Pt-Ru alloy systems to the experimental work function of standard hydrogen electrode (SHE) on the basis of equation,57, 58 U =Φ/e – 4.43. The calculated electrode potential is 0.90, 0.85, and 0.69 V (vs. SHE), which is lower than that of pure Pt(111) surface (1.10 eV), implying that lower overpotentials are required for CH3OH oxidation after alloying of Pt with Ru. Simultaneously, we also noted that the increasing Ru surface compositions can lead to the decreasing potentials. Thus, alloying of Pt with Ru will be favor of CH3OH oxidation, explaining the above analysis of energetics. In fact, the previous experimental study on CH3OH oxidation from Vielstich et al. also showed that alloying of Pt with Ru makes onset potentials decrease,42 further confirming the accuracy of our present calculated results. However, more than 50% Ru surface compositions may go against CH3OH oxidation. For example, C-O bond scission pathway of CH3OH is facile to occur when surface composition of Ru is ca. 67%, which can lead to production of carbonaceous species that can poison surface active sites. Although our present calculated electrode potentials not always quantitatively accurate, the conclusions for effect of potentials on CH3OH oxidation activity in trends is expected to be reasonably qualitatively correct.