Density Functional Theory Study of Methanol Steam Reforming on Pt₃Sn(111) and the Promotion Effect of a Surface Hydroxy Group

Abstract

Methanol steam reforming (MSR) is studied on a Pt₃Sn surface using the density functional theory (DFT). An MSR network is mapped out, including several reaction pathways. The main pathway proposed is CH₃OH + OH → CH₃O → CH₂O → CH₂O + OH → CH₂OOH → CHOOH → COOH → COOH + OH → CO₂ + H₂O. The adsorption strengths of CH₃OH, CH₂O, CHOOH, H₂O and CO₂ are relatively weak, while other intermediates are strongly adsorbed on Pt₃Sn(111). H₂O decomposition to OH is the rate-determining step on Pt₃Sn(111). The promotion effect of the OH group is remarkable on the conversions of CH₃OH, CH₂O and trans-COOH. In particular, the activation barriers of the O–H bond cleavage (e.g., CH₃OH → CH₃O and trans-COOH → CO₂) decrease substantially by ~1 eV because of the involvement of OH. Compared with the case of MSR on Pt(111), the generation of OH from H₂O decomposition is more competitive on Pt₃Sn(111), and the presence of abundant OH facilitates the combination of CO with OH to generate COOH, which accounts for the improved CO tolerance of the PtSn alloy over pure Pt.

1. Introduction

Methanol steam reforming (MSR) has been widely accepted as a candidate method of generating hydrogen for the on-board application of direct methanol fuel cells (DMFCs) [1,2]. Platinum (Pt) is generally applied as a DMFC catalyst because of its thermal stability and high catalytic activity [2,3,4,5,6]. However, CO molecules are primarily produced from methanol (CH₃OH) decomposition and gradually accumulate on Pt, which ultimately leads to CO poisoning and the loss of activity of Pt catalysts [7,8]. Alloying is an effective way to enhance the resistance of metal catalysts. Recently, a PtSn alloy showed promise as an efficient DMFC catalyst with considerable CH₃OH electrocatalytic rates compared to Pt [8,9,10,11,12,13,14,15], and it is reported to be active for CO oxidation [16,17,18]. Therefore, an in-depth study of MSR reactions on PtSn is an essential prerequisite to rationally design more efficient and stable PtSn-based catalysts for DMFC applications.

Generally, the MSR process can be summarized as the following two main reaction mechanisms based on previous experimental research studies [19,20,21]. The first mechanism (M1) proceeds with the direct dehydrogenation of CH₃OH and the formation of CO; then, CO is oxidized to CO₂ via the water–gas shift (WGS) reaction (H₂O + CO → H₂ + CO₂) [22,23]. The second mechanism (M2) includes the reactions of intermediates with adsorbed OH, which is generated from water decomposition (H₂O → OH + H), to yield CH₂OO, CHOOH, CHOO and, finally, H₂ and CO₂ [24,25,26]. From the perspective of theoretical research, different catalyst models account for different intermediates and MSR mechanisms. Using density functional theory (DFT) calculations, Luo et al. [27] investigated MSR reactions on Co(0001) and Co(111), and their results showed that the direct decomposition of CH₂O to CO is favored rather than CH₂OOH formation, indicating the preference of the M1 mechanism. Fajín and Cordeiro [28] performed a DFT investigation on bimetallic Ni−Cu alloy surfaces and also confirmed the M1 mechanism. They found that the MSR evolves mostly through CH₃OH decomposition followed by the WGS reaction. In these studies, the surface OH group did not take part in the main reaction pathway, but it can become involved in or influence the MSR process on other metal and alloy surfaces. Lin et al. proposed that MSR reactions on Cu(111) [25,29] and PdZn(111) [26,30] follow the M2 mechanism, that is, the stepwise dehydrogenation of CH₃OH occurs first, followed by CH₂O formation; then, CH₂O combines with OH, which produces a CH₂OOH intermediate. Finally, CH₂OOH is further dehydrogenated to yield CO₂. CH₃O dehydrogenation is identified as the rate-determining step on both Cu(111) and PdZn(111) surfaces. Li et al. [31] also confirmed the M2 mechanism of the MSR on an α-MoC(100) surface using DFT calculations. The results suggest that the stepwise O−H and C−H bond scissions of CH₃OH yield CH₂O. Then, CH₂OOH is formed through the combination of CH₂O and OH, which is preferred over the decomposition path of CH₂O to CHO and H. In addition to its direct involvement in the MSR reaction pathway, the surface OH group can also exert an important influence on the MSR process. Huang et al. [32] studied CH₃OH decomposition on PdZn(111) using the DFT and found that the presence of co-adsorbed OH species would hinder C–H bond scission while significantly reducing the energy barrier of the O–H bond scission. Thus, CH₃OH preferentially undergoes O–H bond scission to form CH₃O because of the influence of OH. Although a great number of efforts have been made to determine the MSR mechanisms of various catalyst models, the detailed MSR process, as well as intermediate information, has not been unambiguously elucidated for specific new catalyst models. At present, there are no theoretical reports available to elucidate the complete MSR mechanism on a PtSn alloy surface. Furthermore, the effect of OH species on the MSR process should also be clarified.

In this work, a periodic DFT investigation is carried out to elucidate the MSR mechanism on a PtSn alloy surface. Among Pt_xSn catalysts with different Sn contents, Pt₃Sn has been proven to have the best performance for the oxidation of methanol and CO in DMFCs [17,18]. Thus, Pt₃Sn(111) is chosen as a representative PtSn alloy for DFT calculations. The adsorption structures, elementary reactions and potential energy surfaces (PESs) are illustrated for methanol decomposition and steam reformation processes, and the effect of the OH group on the catalytic mechanism is discussed in detailed.

2. Computational Methods

DFT calculations were conducted using the DMol³ program package [33,34,35]. Exchange and correlation effects were treated using the GGA-PW91 functional [36,37,38]. The DSPP method [39] was applied for Pt and Sn atoms, while C, H and O atoms were treated with an all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbitals on a double-numerical basis with polarization functions. A Fermi smearing of 0.005 Hartree and a real-space cutoff of 4.5 Å were used. Spin-polarization was applied in all calculations.

The lattice constant of the Pt₃Sn was calculated to be 4.01 Å, in good agreement with the experimental value of 4.00 Å [40]. The Pt₃Sn(111) surface was built using a p(2 × 2) unit cell with a four-layer slab, and each layer consisted of three Pt atoms and one Sn atom. The height of the vacuum region was set at 12 Å. The reciprocal space was sampled with a (5 × 5 × 1) k-points grid generated automatically using the Monkhorst–Pack method [41]. The uppermost two layers of the slab were relaxed with adsorbates, while two substrate layers were fixed at bulk positions.

High-symmetry sites on Pt₃Sn(111) are presented in Figure 1. The adsorption energies (E_ads) were calculated as follows: [42,43]

E_ads = E_adsorbate + E_slab − E_{adsorbate/slab}

(1)

where E_{adsorbate/slab} is the energy of the adsorbate/slab adsorption system, and E_adsorbate and E_slab are the energies of the free adsorbate and the clean slab, respectively. By this definition, stable adsorption will have a positive adsorption energy.

Transition state (TS) searches were performed at the same theoretical level using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method [22,23,24,44]. In this method, an LST maximization was performed, followed by an energy minimization in directions conjugating to the reaction pathway to obtain an approximated TS. The approximated TS was used to perform a QST maximization, and then another conjugated gradient minimization was performed. This cycle was repeated until a stationary point was located. The convergence criterion for the TS searches was set to 0.01 hartree/Å for the root mean square of the atomic forces. The energy barrier (E_a) was determined as the energy difference between the corresponding TS and the initial state (IS), and the reaction energy (E_r) was defined as the energy difference between the final state (FS) and the IS.

3. Results and Discussion

3.1. Adsorption Structures and Energies

Figure 2 shows the most stable adsorption geometries of intermediates in MSR, and

Table 1. The most stable adsorption sites, geometric parameters (in Å) and energies (in eV) for MSR intermediates on Pt₃Sn(111).

Species	Site	Mode	dC/O-Pt/Sn	Eads
CH3OH	TSn	η1(O)	2.64	0.47
CH3O	TSn	η1(O)	2.05	1.71
CH2OOH	BPtSn	η1(O)-η1(O)	2.15, 2.31	1.89
CH2OO	F2PtSn	η2(O)-η1(O)	2.09, 2.26, 2.27	3.24
CH2OH	TPt	η1(C)	2.14	1.94
CH2O	F2PtSn	η1(C)-η2(O)	2.13, 2.28, 2.43	0.38
HCOOH	TSn	η1(O)	2.58	0.49
CHOO	BPtSn	η1(O)-η1(O)	2.17, 2.29	2.52
CHOH	B2Pt	η2(C)	2.09, 2.12	3.14
cis-COOH	TPt	η1(C)	2.03	2.48
trans-COOH	BPtSn	η1(C)-η1(O)	2.04, 2.55	2.41
CHO	TPt	η1(C)	2.01	2.28
COH	H3Pt	η3(C)	2.04, 2.06, 2.11	4.05
CO2	BPtSn	-	-	0.11
H2O	TSn	-	-	0.01
OH	B2Pt	η2(O)	2.23, 2.24	2.51
O	F2PtSn	η3(O)	2.13, 2.14, 2.14	4.12

shows the corresponding adsorption energies (E_ads) and geometric parameters. For clarity, the geometries and energies of the sub-stable adsorptions of the involved intermediates are presented in Figure S1 and Table S1 of the Supporting Information. In our previous study of methanol decomposition on Pt₃Sn(111) [43], several intermediates were calculated in detail, and the most stable adsorption sites along with E_ads can be summarized as follows CH₃OH at T^Sn (0.47 eV), CH₃O at T^Sn (1.71 eV), CH₂OH at T^Pt (1.94 eV), CH₂O at F^2PtSn (0.38 eV), CHOH at B^2Pt (3.14 eV), CHO at T^Pt (2.28 eV), COH at H^3Pt (4.05 eV), CO₂ at B^PtSn (0.11 eV), OH at B^2Pt (2.51 eV) and O at F^2PtSn (4.12 eV). In this work, we focus on the reformation process, especially the OH-involved paths. Accordingly, the reaction intermediates of MSR are described in detail below.

Carboxymethyl (CH₂OOH) is formed through the combination of CH₂O and an OH group and preferentially adsorbs at a bridge site via the η¹(O)-η¹(O) mode, which is different from the unidentate η³(O) modes at the hollow sites of Cu(111) [25], PdZn(111) [26] and Co(111) [27]. The E_ads values of CH₂OOH are 1.89 eV (B^PtSn) and 1.65 (B^2Pt), respectively. At the B^PtSn site (Figure 2), two C–O bond lengths are 1.35 and 1.52 Å, and the O–Sn and O–Pt distances are 2.15 and 2.31 Å, respectively. Dioxomethylene (CH₂OO) was reported to adsorb at a bridge site in a bidentate η¹(O)-η¹(O) mode on Cu(111), PdZn(111) and Co(111) surfaces [25,26,27]. However, we found that CH₂OO has two adsorption modes on Pt₃Sn(111) which are the η²(O)-η¹(O) mode at the F^2PtSn site and the η¹(O)-η¹(O) mode at the B^PtSn site. As listed in Table 1, the η²(O)-η¹(O) mode (Figure 2) is more stable with an E_ads of 3.24 eV, and the two O–Pt and O–Sn distances are 2.09, 2.26 and 2.27 Å, respectively. The E_ads of the η¹(O)-η¹(O) mode (Figure S1) was calculated to be 3.08 eV, consistent with the previous DFT result for CH₂OO adsorption via the same η¹(O)-η¹(O) mode on Cu(111) [25]. For Formic acid (CHOOH), the most stable adsorption site is T^Sn, and the corresponding E_ads is 0.49 eV. The molecule plane of CHOOH is almost vertical with the OH group pointing down toward the surface (Figure 2). The two C–O bond lengths are 1.23 and 1.32 Å, respectively. The CHOOH at the T^Pt has a similar adsorption configuration (Figure S1) with a lower E_ads of 0.38 eV. The other four adsorption geometries of CHOOH at F^2PtSn, F^3Pt, H^2PtSn and H^3Pt involve molecule planes almost parallel to Pt₃Sn(111) with ~3.70 Å above the surface (Figure S1). Formate (CHOO) can adsorb at B^2Pt and B^PtSn with the η¹(O)-η¹(O) mode, and the B^PtSn site is preferred. At the B^PtSn site, the molecular plane is perpendicular to the Pt₃Sn(111), with an E_ads of 2.52 eV; the O–Pt and O–Sn distances are 2.17 and 2.29 Å, respectively (Figure 2). At the B^2Pt site, the E_ads decreases to 2.08 eV. Carboxyl (COOH) has two isomers which are cis- and trans-COOH, respectively [25]. The cis-COOH can adsorb at the T^Pt, T^Sn and T^2Pt sites with corresponding E_ads values of 2.48, 1.26 and 2.37 eV, respectively. The T^Pt site can thus be identified as the most stable binding site for cis-COOH; the molecular plane is nearly perpendicular to Pt₃Sn(111), with C–Pt and two C–O bond lengths of 2.03, 1.22 and 1.37 Å, respectively (Figure 2). For trans-COOH, the E_ads values are 2.35 (T^Pt), 1.09 (T^Sn), 2.39 (T^2Pt) and 2.41 (T^PtSn) eV. The cis isomer binds slightly more strongly to Pt₃Sn(111) than its trans counterpart (2.48 vs. 2.41 eV), similar to COOH adsorption on Cu(111) [25]. CO₂ adsorbs weakly above the B^PtSn and B^2Pt sites with the same E_ads value of 0.11 eV. At bridge sites, this linear molecule lies almost parallel to the Pt₃Sn(111) at a distance of ~4.00 Å above the surface (Figure 2). These results are consistent with those of previous DFT studies of weak CO₂ adsorptions over Cu(111) [25], Co(0001) [45] and Co(111) [27]. H₂O adsorbs above the T^Sn site via the O–Sn bond, and the two O–H axes are parallel to the Pt₃Sn(111) surface (Figure 2) with bond lengths of 0.98 Å. The binding strength of H₂O is very weak, mirrored by a low E_ads of 0.01 eV, which is also consistent with weak H₂O adsorption on Cu(111) [25], Co(111) [27] and Co(0001) [45]. The most stable sites and E_ads values for intermediates via η(O) can be summarized as followed: CH₂OOH at B^PtSn (1.89 eV), CH₂OO at F^2PtSn (3.24 eV), HCOOH at T^Sn (0.49 eV), CHOO at B^PtSn (2.52 eV), cis-COOH at T^Pt (2.48 eV), trans-COOH at B^PtSn (2.41 eV) and H₂O at T^Sn (0.01 eV). Taking into account the adsorption properties of other intermediates (CH₃OH, CH₂OH, CH₃O, CHOH, CH₂O, COH, CHO, etc.) [43], Sn strengthens the binding of these intermediates to the Pt₃Sn(111) surface via η(O).

3.2. Elementary Reaction Steps

The decomposition reactions of CH₃OH, CH₂OH, CH₃O, CH₂O and CHO via O−H, C−H and C−O bond scissions were calculated in our previous study [43]. We found that CH₃OH decomposition began with O−H bond scission, followed by C−H bond cleavages, that is, CH₃OH → CH₃O → CH₂O → CHO → CO. To identify the optimal MSR pathway, multiple reactions were further investigated in this work, including H₂O dissociation into OH and H and subsequent OH-involving reactions with CH₃OH and its dehydrogenated intermediates. The configurations of the involved IS, TS and FS are presented in Figure 3 and Figure 4. Sixteen reactions (R1–R16) were considered in total with their thermodynamic and kinetic parameters.

H₂O Activation. In the IS, H₂O adsorbs weakly above the T^Sn site. For the reaction R1, the O–H bond is ruptured, with the H atom migrating toward the adjacent Pt atom. The O–H distance of H₂O is elongated from 0.98 Å in the IS to 1.62 Å in TS1, as shown in Figure 3. Finally, the OH binds at the B^2Pt site, and the H sits at the H^3Pt site. This reaction is exothermic by 0.47 eV, with an energy barrier of 0.97 eV. For comparison, the E_a of H₂O decomposition on Pt₃Sn is much lower than that on Cu(111) (1.11 eV) [25].

CH₃OH + OH. In reaction R2, CH₃OH and OH adsorb at the T^Sn and T^Pt sites in the IS, respectively, and in the FS, CH₃O and H₂O locate at the same sites as in the IS. In TS2 (Figure 3), the distance of the breaking O−H bond in CH₃OH is 1.24 Å, smaller than that in the direct dehydrogenation of CH₃OH (0.97 Å) [43] This step is slightly exothermic by 0.04 eV, and the E_a is only 0.02 eV, which is 0.97 eV lower than direct methanol dehydrogenation by O−H bond cleavage at the same site of the T^Sn [43].

CH_xO + OH (x = 0–3). In reaction R3 of CH₃O with OH, the energy barrier is 0.84 eV with a reaction energy of −0.87 eV. In TS3, the distance of the breaking O−H bond is 1.27 Å. In reaction R4, the CH₂O fragment is weakly bound at the F^2PtSn, while the OH fragment stays at T^Pt site, yielding CH₂OOH at the B^2Pt site. In TS4, two fragments move to the T^Pt site, and the distance of the cleaved O−H bond is 2.09 Å. This step is exothermic by 0.45 eV and has an activation barrier of 0.43 eV, lower than that of 0.75 eV for CH₂O → CHO [25]. A similar process also occurs on Cu(111) [25] and PdZn(111) [26] For reaction R5, co-adsorbed CHO at the H^3Pt site and OH at the T^Sn site are taken as the IS, and the HCOOH at the H^3Pt site is the FS. The distance between C and O atoms is shortened from 3.67 Å in the IS to 1.92 Å in TS5 and to 1.36 Å in the FS. This reaction has an activation barrier of 0.63 eV with an exothermicity of 0.70 eV. For reaction R6, the IS is the co-adsorption of OH at the T^Sn site and CO at the T^Pt site, and the FS is COOH at the T^Pt site. In TS6, the distance of the forming C−O bond is 1.91 Å. This step is exothermic by 0.25 eV, with an activation barrier of 0.39 eV.

CH₂OOH dehydrogenation. Two reaction pathways exist for CH₂OOH dehydrogenation. The first is C–H bond scission (R7, CH₂OOH → CHOOH + H), producing a CHOOH fragment above the B^PtSn site with H at the H^3Pt site. For TS7 (Figure 3), the C−H distance of the breaking C−H bond is 1.53 Å, which stretches from 1.11 Å in the IS to 3.95 Å in the FS. This reaction has an activation barrier of 0.40 eV with a reaction energy of −0.74 eV. The second is O–H bond scission (R8, CH₂OOH → CH₂OO + H), which starts with the CH₂OOH at the B^2Pt site and ends with a co-adsorbed CH₂OO fragment at the B^2Pt site and H at the T^Pt site. In TS8 (Figure 3), the O–H distance of the breaking O–H bond is 1.49 Å. This reaction has a higher activation barrier of 1.64 eV and is endothermic by 0.91 eV. Based on thermodynamic and kinetic viewpoints, CH₂OOH dehydrogenation on Pt₃Sn(111) tends to yield CHOOH rather than CH₂OO, that is, the C–H bond scission of reaction R7 is more competitive than the O–H bond scission of reaction R8.

CH₂OO and CHOOH dehydrogenation. CH₂OO dehydrogenation, denoted as reaction R9, yields bidentate CHOO binding at the B^PtSn site and H at the T^Pt site (Figure 4). This step has a low activation barrier of 0.37 eV and a high exothermicity of 1.55 eV. For TS9, H moves down and locates above the B^2Pt site, while CHOO remains at the B^PtSn site; the C–H distance of the breaking C–H bond is 1.11 Å. CHOOH dehydrogenation includes C–H bond scission (reaction R10) and O–H bond cleavage (reaction R11). The C–H bond cleavage of CHOOH yields a B^PtSn-site-adsorbed COOH fragment and a T^Pt-site-adsorbed H atom (Figure 4). This reaction involves an energy barrier of 0.44 eV and an endothermicity of 0.01 eV. For TS10, the breaking C–H bond is elongated to 2.07 Å. The O–H bond cleavage of CHOOH is slightly exothermic by 0.02 eV, and the activation barrier is 0.78 eV. For TS11, the O–H bond is elongated by 1.35 Å, and the leaving H adsorbs at the B^2Pt site. In the FS, CHOO binds to the Pt₃Sn(111) surface in a bidentate configuration, and the detached H locates at the T^Pt site.

CHOO and COOH dehydrogenation. The CHOO is produced from CH₂OO dehydrogenation or CHOOH dehydrogenation via the O–H bond cleavage. The further dehydrogenation of CHOO generates CO₂ and an H atom, which is denoted as reaction R12 (Figure 4). In TS12, the C–H distance of the breaking C–H bond is 2.40 Å. After the C–H bond scission, the detached H atom adsorbs at the T^Pt site, while the CO₂ adsorbs above the B^PtSn site. This reaction is exothermic by 0.31 eV, and the activation barrier is 1.06 eV. The dehydrogenation process of CHOO could also be accomplished with assistance from an adsorbed OH group (reaction R13). This step starts with co-adsorbed CHOO at the B^PtSn site and OH at the T^Pt site and ends with weakly bonded CO₂ and H₂O on the surface. The activation barrier of this step is 1.53 eV, and the reaction energy is −1.10 eV. Compared with the direct dehydrogenation of CHOO (R12), the OH-assisted reaction of CHOO with OH to H₂O and CO₂ (R13) has a relatively higher energy barrier, suggesting that R12 is more favorable than R13. The isomerization of cis-COOH to form trans-COOH (R14) is necessary for COOH dehydrogenation because the O–H bond of the adsorbed COOH points away from the surface in the cis-mode but swings toward the surface in the trans-mode, which is helpful for O–H bond activation. This isomerization step involves an energy barrier of 0.53 eV. Subsequently, CO₂ is produced by removing the H atom from trans-COOH (R15), which accounts for an activation barrier of 1.04 eV and a reaction energy of −0.23 eV. For TS15, the O–H distance of the breaking O–H bond is 1.38 Å, and the CO₂ is above the B^PtSn site and an H atom locates at the T^Pt site. Similar to CHOO, trans-COOH can also react with OH to generate H₂O and CO₂ (R16). This step starts with co-adsorbed trans-COOH at the T^Pt site and OH at the B^PtSn site and ends with CO₂ above the B^PtSn site and H₂O above the T^Pt site. This OH-assisted step is exothermic by 0.74 eV, with a lower activation barrier of 0.11 eV.

3.3. MSR Mechanisms

Based on the calculated results, the potential energy surfaces of MSR on Pt₃Sn(111) are presented in Figure 5. CH₃OH decomposition with the assistance of OH to form CH₃O + H₂O and CH₃OH dehydrogenation via O–H bond scission to form CH₃O + H involve activation barriers of 0.02 and 1.01 eV, respectively. Compared with the direct dehydrogenation of CH₃OH to CH₃O on Pt₃Sn(111), the involvement of the OH group greatly promotes this dehydrogenation step. For the intermediate CH₃O, however, the OH group is not helpful for C–H bond cleavage because the direct dehydrogenation of CH₃O to CH₂O only needs to overcome an activation barrier of 0.42 eV compared with the case of CH₃O + OH → CH₂O + H₂O (E_a = 0.84 eV). For the intermediate CH₂O, the transition state of the C–H bond activation with the participation of the OH group was not found in spite of an elaborate search. CH₂O has two competitive paths: the direct dehydrogenation, CH₂O → CHO (E_a = 0.75 eV), and a combination with the OH group, CH₂O + OH → CH₂OOH (E_a = 0.43 eV). Therefore, the combination of CH₂O with OH is more favorable. The further dehydrogenation of the newly formed CH₂OOH has two possibilities, which are O–H and C–H bond activations. We found that the C–H bond scission of CH₂OOH → CHOOH + H (E_a = 0.40 eV) is more competitive than the O–H bond cleavage of CH₂OOH → CH₂OO + H (E_a = 1.64 eV). Similar to CH₂OOH, the intermediate CHOOH also tends to break the C–H bond (E_a = 0.44 eV) rather than the O–H bond (E_a = 0.78 eV). CHOOH dehydrogenation yields cis-COOH, followed by an isomerization step toward trans-COOH. Compared with cis-COOH, the adsorption geometry of trans-COOH is favored for O–H bond activation: trans-COOH → CO₂ + H (E_a = 1.04 eV). The participation of the OH group substantially reduces the dehydrogenation barrier of trans-COOH via trans-COOH + OH → CO₂ + H₂O (E_a = 0.11 eV), indicating the promotion effect of the OH group.

Figure 6 summarizes the MSR reaction network based on the direct decomposition of methanol in our previous work [43] and the results calculated in this study. The most favorable pathway follows the M2 mechanism, in which important intermediates were identified as follows:

CH₃OH + OH → CH₃O + H₂O

(2)

CH₃O → CH₂O + H

(3)

CH₂O + OH → CH₂OOH

(4)

CH₂OOH → CHOOH + H

(5)

CHOOH → COOH + H

(6)

COOH + OH → CO₂ + H₂O

(7)

H₂ production originates from H₂O decomposition and the dehydrogenation of important intermediates (CH₃O, CH₂OOH and CHOOH). The promotion effect of the surface OH group on the conversions of CH₃OH, CH₂O and trans-COOH is remarkable. In particular, the energy barriers of the O–H bond activation (e.g., CH₃OH → CH₃O and trans-COOH → CO₂) decrease substantially by ~1 eV due to the involvement of the surface OH group, while OH fails to facilitate C–H bond activation. The above results are consistent with previous DFT calculations of CH₃OH decomposition by Huang et al. [32] in which the presence of a surface OH group on PdZn(111) impeded the C–H bond scission of CH₃OH but substantially decreased the O–H bond-activation barrier. For comparison, Jin et al. [46] found that the OH group on Pt(111) could also be beneficial to MSR reactions, such as CH₃OH → CH₃O and CH₂O + OH → CH₂OOH. However, it is relatively difficult to dissociate water and generate the OH group on Pt(111) compared with the direct dehydrogenation of CH₃OH. The OH group is only available when the difference in the energy barrier between H₂O decomposition and CH₃OH dehydrogenation is comparable. Thus, the MSR process on Pt(111) still follows the M1 mechanism, which is stepwise CH₃OH decomposition to CO followed by WGS reactions: CH₃OH → CH₂OH → CHOH → CHO → CO → CO + OH → COOH. In this study, the Pt₃Sn(111) surface reduced the difference in the E_a between H₂O → H + OH (E_a = 0.97 eV) and CH₃OH → CH₃O + H (E_a = 1.01 eV)/CH₃OH → CH₂OH + H (E_a = 1.09 eV). The relatively lower E_a of H₂O decomposition indicates the availability of the OH group, which facilitates the MSR process. The initial H₂O decomposition to the OH group involves the highest activation barrier of 0.97 eV through the main reaction pathway. Thus, H₂O decomposition could be identified as the rate-determining step for MSR on Pt₃Sn(111) rather than the commonly accepted C–H bond-cleavage steps such as CH₃OH → CH₂OH on Pt(111) [46] and CH₃O → CH₂O on both Cu(111) [25] and PdZn(111) [26]. Compared with the dehydrogenation reactions of CH₃OH, the initial H₂O → OH step involves relatively higher selectivity on Pt₃Sn, which accounts for the improved CO tolerance of PtSn alloys over pure Pt.

4. Conclusions

DFT calculations were performed to investigate possible intermediates and MSR reaction pathways on Pt₃Sn(111). The MSR network was mapped out. The most favorable pathway was identified as follows: CH₃OH + OH → CH₃O → CH₂O → CH₂O + OH → CH₂OOH → CHOOH → COOH → COOH + OH → CO₂ + H₂O. Along this main reaction pathway, the adsorption strengths of CH₃OH, CH₂O, CHOOH, H₂O and CO₂ are relatively weak (E_ads < 0.5 eV), while other intermediates are strongly adsorbed at the T^Sn site for CH₃O (E_ads = 1.71 eV), at the T^Pt site for cis-COOH (E_ads = 2.48 eV) and at the B^PtSn site for CH₂OOH (E_ads = 1.89 eV) and trans-COOH (E_ads = 2.41 eV). H₂ production originates from H₂O decomposition and the dehydrogenation of important intermediates (CH₃O, CH₂OOH and CHOOH). H₂O decomposition into OH involves an activation barrier of 0.97 eV and was identified as the rate-determining step for the MSR process on Pt₃Sn(111). The promotion effect of the surface OH group on the conversions of CH₃OH, CH₂O and trans-COOH is remarkable. In particular, the energy barriers of the O–H bond activation (e.g., CH₃OH → CH₃O and trans-COOH → CO₂) decrease substantially by ~1 eV due to the involvement of the surface OH group. Compared with the case on Pt(111), the formation of a surface OH group from H₂O decomposition is more competitive on Pt₃Sn(111), and the presence of abundant OH facilitates the combination of CO with OH to generate COOH, which accounts for the improved CO tolerance of PtSn alloys over pure Pt.