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

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.


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 (CH 3 OH) 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 CH 3 OH 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 indepth 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 CH 3 OH and the formation of CO; then, CO is oxidized to CO 2 via the water-gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) [22,23].The second mechanism (M2) includes the reactions of intermediates with adsorbed OH, which is generated from water decomposition (H 2 O → OH + H), to yield CH 2 OO, CHOOH, CHOO and, finally, H 2 and CO 2 [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 CH 2 O to CO is favored rather than CH 2 OOH 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 CH 3 OH 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 CH 3 OH occurs first, followed by CH 2 O formation; then, CH 2 O combines with OH, which produces a CH 2 OOH intermediate.Finally, CH 2 OOH is further dehydrogenated to yield CO 2 .CH 3 O 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 CH 3 OH yield CH 2 O.Then, CH 2 OOH is formed through the combination of CH 2 O and OH, which is preferred over the decomposition path of CH 2 O 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 CH 3 OH 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, CH 3 OH preferentially undergoes O-H bond scission to form CH 3 O 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 x Sn catalysts with different Sn contents, Pt 3 Sn has been proven to have the best performance for the oxidation of methanol and CO in DMFCs [17,18].Thus, Pt 3 Sn(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.

Computational Methods
DFT calculations were conducted using the DMol 3 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 Pt 3 Sn was calculated to be 4.01 Å, in good agreement with the experimental value of 4.00 Å [40].The Pt 3 Sn(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 Pt 3 Sn(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.
The uppermost two layers of the slab were relaxed with adsorbates, whil layers were fixed at bulk positions.High-symmetry sites on Pt3Sn(111) are presented in Figure 1.The a gies (Eads) were calculated as follows: [42,43] where Eadsorbate/slab is the energy of the adsorbate/slab adsorption system, a Eslab are the energies of the free adsorbate and the clean slab, respectively tion, stable adsorption will have a positive adsorption energy.Transition state (TS) searches were performed at the same theoretica complete linear synchronous transit/quadratic synchronous transit (LST [22][23][24]44].In this method, an LST maximization was performed, followe minimization in directions conjugating to the reaction pathway to obtain a TS.The approximated TS was used to perform a QST maximization, an conjugated gradient minimization was performed.This cycle was repeated ary point was located.The convergence criterion for the TS searches was tree/Å for the root mean square of the atomic forces.The energy barrier mined as the energy difference between the corresponding TS and the i and the reaction energy (Er) was defined as the energy difference betwee (FS) and the IS.

Adsorption Structures and Energies
Figure 2 shows the most stable adsorption geometries of intermedia Table 1 shows the corresponding adsorption energies (Eads) and geometric clarity, the geometries and energies of the sub-stable adsorptions of the inv diates are presented in Figure S1 and Table S1 of the Supporting Informat vious study of methanol decomposition on Pt3Sn(111) [43], several interme culated in detail, and the most stable adsorption sites along with Eads can as follows CH3OH at T Sn (0.47 eV), CH3O at T Sn (1.71 eV), CH2OH at T Pt (1.F 2PtSn (0.38 eV), CHOH at B 2Pt (3.14 eV), CHO at T Pt (2.28 eV), COH at H 3Pt ( ⃝~8 ⃝ represent the top of the Pt site (T Pt ), the top of the Sn site (T Sn ), the Pt-Pt bridge site (B 2Pt ), the Pt-Sn bridge site (B PtSn ), the fcc/hcp site consisting of three surface nearest-neighboring Pt atoms (F 3Pt /H 3Pt ) and the fcc/hcp site consisting of one Sn and two Pt atoms (F 2PtSn /H 2PtSn ), respectively.
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.

Adsorption Structures and Energies
Figure 2 shows the most stable adsorption geometries of intermediates in MSR, and Table 1 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 Pt 3 Sn(111) [43], several intermediates were calculated in detail, and the most stable adsorption sites along with E ads can be summarized as follows CH 3 OH at T Sn (0.47 eV), CH 3 O at T Sn (1.71 eV), CH 2 OH at T Pt (1.94 eV), CH 2 O 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), CO 2 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 (CH 2 OOH) is formed through the combination of CH 2 O and an OH group and preferentially adsorbs at a bridge site via the η 1 (O)-η 1 (O) mode, which is different from the unidentate η 3 (O) modes at the hollow sites of Cu(111) [25], PdZn(111) [26] and Co(111) [27].The E ads values of CH 2 OOH 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 (CH 2 OO) was reported to adsorb at a bridge site in a bidentate η 1 (O)-η 1 (O) mode on Cu(111), PdZn(111) and Co(111) surfaces [25][26][27].However, we found that CH 2 OO has two adsorption modes on Pt 3 Sn(111) which are the η 2 (O)-η 1 (O) mode at the F 2PtSn site and the η 1 (O)-η 1 (O) mode at the B PtSn site.As listed in Table 1, the η 2 (O)-η 1 (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 η 1 (O)-η 1 (O) mode (Figure S1) was calculated to be 3.08 eV, consistent with the previous DFT result for CH 2 OO adsorption via the same η 1 (O)-η 1 (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 Pt 3 Sn(111) with ~3.70 Å above the surface (Figure S1).Formate (CHOO) can adsorb at B 2Pt and B PtSn with the η 1 (O)-η 1 (O) mode, and the B PtSn site is preferred.At the B PtSn site, the molecular plane is perpendicular to the Pt 3 Sn(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 cisand 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 Pt 3 Sn(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 Pt 3 Sn(111) than its trans counterpart (2.48 vs. 2.41 eV), similar to COOH adsorption on Cu(111) [25].CO 2 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 Pt 3 Sn(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 CO 2 adsorptions over Cu(111) [25], Co(0001) [45] and Co(111) [27].H 2 O adsorbs above the T Sn site via the O-Sn bond, and the two O-H axes are parallel to the Pt 3 Sn(111) surface (Figure 2) with bond lengths of 0.98 Å.The binding strength of H 2 O is very weak, mirrored by a low E ads of 0.01 eV, which is also consistent with weak H 2 O 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: CH 2 OOH at B PtSn (1.89 eV), CH 2 OO 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 H 2 O at T Sn (0.01 eV).Taking into account the adsorption properties of other intermediates (CH 3 OH, CH 2 OH, CH 3 O, CHOH, CH 2 O, COH, CHO, etc.) [43], Sn strengthens the binding of these intermediates to the Pt 3 Sn(111) surface via η(O).

Elementary Reaction Steps
The decomposition reactions of CH 3 OH, CH 2 OH, CH 3 O, CH 2 O and CHO via O−H, C−H and C−O bond scissions were calculated in our previous study [43].We found that CH 3 OH decomposition began with O−H bond scission, followed by C−H bond cleavages, that is, CH 3 OH → CH 3 O → CH 2 O → CHO → CO.To identify the optimal MSR pathway, multiple reactions were further investigated in this work, including H 2 O dissociation into OH and H and subsequent OH-involving reactions with CH 3 OH and its dehydrogenated intermediates.The configurations of the involved IS, TS and FS are presented in Figures 3 and 4. Sixteen reactions (R1-R16) were considered in total with their thermodynamic and kinetic parameters.
H 2 O Activation.In the IS, H 2 O 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 H 2 O 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 H 2 O decomposition on Pt 3 Sn is much lower than that on Cu(111) (1.11 eV) [25].
CH 3 OH + OH.In reaction R2, CH 3 OH and OH adsorb at the T Sn and T Pt sites in the IS, respectively, and in the FS, CH 3 O and H 2 O locate at the same sites as in the IS.In TS2 (Figure 3), the distance of the breaking O−H bond in CH 3 OH is 1.24 Å, smaller than that in the direct dehydrogenation of CH 3 OH (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 x O + OH (x = 0-3).In reaction R3 of CH 3 O 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 CH 2 O fragment is weakly bound at the F 2PtSn , while the OH fragment stays at T Pt site, yielding CH 2 OOH 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 CH 2 O → 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.
CH 2 OOH dehydrogenation.Two reaction pathways exist for CH 2 OOH dehydrogenation.The first is C-H bond scission (R7, CH 2 OOH → 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, CH 2 OOH → CH 2 OO + H), which starts with the CH 2 OOH at the B 2Pt site and ends with a co-adsorbed CH 2 OO 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, CH 2 OOH dehydrogenation on Pt 3 Sn(111) tends to yield CHOOH rather than CH 2 OO, that is, the C-H bond scission of reaction R7 is more competitive than the O-H bond scission of reaction R8.
The decomposition reactions of CH3OH, CH2OH, CH3O, CH2O and C C−H and C−O bond scissions were calculated in our previous study [43].W CH3OH decomposition began with O−H bond scission, followed by C−H bo that is, CH3OH → CH3O → CH2O → CHO → CO.To identify the optimal M multiple reactions were further investigated in this work, including H2O dis OH and H and subsequent OH-involving reactions with CH3OH and its deh intermediates.The configurations of the involved IS, TS and FS are presented and 4. Sixteen reactions (R1-R16) were considered in total with their thermo kinetic parameters.H2O Activation.In the IS, H2O adsorbs weakly above the T Sn site.For th the O-H bond is ruptured, with the H atom migrating toward the adjacent O-H distance of H2O is elongated from 0.98 Å in the IS to 1.62 Å in TS1, as sho 3. Finally, the OH binds at the B 2Pt site, and the H sits at the H 3Pt site.This re thermic by 0.47 eV, with an energy barrier of 0.97 eV.For comparison, the composition 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 IS, respectively, and in the FS, CH3O and H2O locate at the same sites as in CH 2 OO and CHOOH dehydrogenation.CH 2 OO 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 cleav-age 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 Pt 3 Sn(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 CH 2 OO dehydrogenation or CHOOH dehydrogenation via the O-H bond cleavage.The further dehydrogenation of CHOO generates CO 2 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 CO 2 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 CO 2 and H 2 O 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 H 2 O and CO 2 (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, CO 2 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 CO 2 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 H 2 O and CO 2 (R16).This step starts with co-adsorbed trans-COOH at the T Pt site and OH at the B PtSn site and ends with CO 2 above the B PtSn site and H 2 O above the T Pt site.This OH-assisted step is exothermic by 0.74 eV, with a lower activation barrier of 0.11 eV.

MSR Mechanisms
Based on the calculated results, the potential energy surfaces of MSR on Pt 3 Sn(111) are presented in Figure 5   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:

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 (Eads < 0.5 eV), while other intermediates are strongly adsorbed at the T Sn site for CH3O (Eads = 1.71 eV), at the T Pt site for cis-COOH (Eads = 2.48 eV) and at the B PtSn site for CH2OOH (Eads = 1.89 eV) and trans-COOH (Eads = 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.

Figure 1 .
Figure 1.High-symmetry adsorption sites on Pt3Sn(111).The labels ①~⑧ repres Pt site (T Pt ), the top of the Sn site (T Sn ), the Pt-Pt bridge site (B 2Pt ), the Pt-Sn brid fcc/hcp site consisting of three surface nearest-neighboring Pt atoms (F 3Pt /H 3Pt ) an consisting of one Sn and two Pt atoms (F 2PtSn /H 2PtSn ), respectively.

Figure 1 .
Figure 1.High-symmetry adsorption sites on Pt 3 Sn(111).The labels 1⃝~8 ⃝ represent the top of the Pt site (T Pt ), the top of the Sn site (T Sn ), the Pt-Pt bridge site (B 2Pt ), the Pt-Sn bridge site (B PtSn ), the fcc/hcp site consisting of three surface nearest-neighboring Pt atoms (F 3Pt /H 3Pt ) and the fcc/hcp site consisting of one Sn and two Pt atoms (F 2PtSn /H 2PtSn ), respectively.

Figure 2 .
Figure 2. Stable adsorption structures of MSR intermediates on Pt3Sn(111).The C, H, O, Pt a atoms are denoted as gray, white, red, blue and gray balls, respectively.

Figure 2 .
Figure 2. Stable adsorption structures of MSR intermediates on Pt 3 Sn(111).The C, H, O, Pt and Sn atoms are denoted as gray, white, red, blue and gray balls, respectively.
. CH 3 OH decomposition with the assistance of OH to form CH 3 O + H 2 O and CH 3 OH dehydrogenation via O-H bond scission to form CH 3 O + H involve activation barriers of 0.02 and 1.01 eV, respectively.Compared with the direct dehydrogenation of CH 3 OH to CH 3 O on Pt 3 Sn(111), the involvement of the OH group greatly promotes this dehydrogenation step.For the intermediate CH 3 O, however, the OH group is not helpful for C-H bond cleavage because the direct dehydrogenation of CH 3 O to CH 2 O only needs to overcome an activation barrier of 0.42 eV compared with the case of CH 3 O + OH → CH 2 O + H 2 O (E a = 0.84 eV).For the intermediate CH 2 O, 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.CH 2 O has two competitive paths: the direct dehydrogenation, CH 2 O → CHO (E a = 0.75 eV), and a combination with the OH group, CH 2 O + OH → CH 2 OOH (E a = 0.43 eV).Therefore, the combination of CH 2 O with OH is more favorable.The further dehydrogenation of the newly formed CH 2 OOH has two possibilities, which are O-H and C-H bond activations.We found that the C-H bond scission of CH 2 OOH → CHOOH + H (E a = 0.40 eV) is more competitive than the O-H bond cleavage of CH 2 OOH → CH 2 OO + H (E a = 1.64 eV).Similar to CH 2 OOH, 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 → CO 2 + H (E a = 1.04 eV).The participation of the OH group substantially reduces the dehydrogenation barrier of trans-COOH via trans-COOH + OH → CO 2 + H 2 O (E a = 0.11 eV), indicating the promotion effect of the OH group.C-H bond activations.We found that the C-H bond scission of CH2OOH → CHOOH + H (Ea = 0.40 eV) is more competitive than the O-H bond cleavage of CH2OOH → CH2OO + H (Ea = 1.64 eV).Similar to CH2OOH, the intermediate CHOOH also tends to break the C-H bond (Ea = 0.44 eV) rather than the O-H bond (Ea = 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 (Ea = 1.04 eV).The participation of the OH group substantially reduces the dehydrogenation barrier of trans-COOH via trans-COOH + OH → CO2 + H2O (Ea = 0.11 eV), indicating the promotion effect of the OH group.

Figure 5 .
Figure 5. Potential energy surface (PES) of MSR on Pt3Sn(111).The detailed reaction pathways of the M1 and M2 mechanisms are shown in red and blue colors, respectively.Data on the direct decomposition of methanol (CH3OH → CH3O → CH2O → CHO → CO) were taken from our precious work [43].

Figure 5 .
Figure 5. Potential energy surface (PES) of MSR on Pt 3 Sn(111).The detailed reaction pathways of the M1 and M2 mechanisms are shown in red and blue colors, respectively.Data on the direct decomposition of methanol (CH 3 OH → CH 3 O → CH 2 O → CHO → CO) were taken from our precious work [43].

Figure 6 .
Figure 6.Proposed detailed MSR pathways on Pt3Sn(111).The M1 and M2 mechanisms are denoted by red and blue boxes, respectively.The OH-assisted steps are marked with green lines.The generated H and H2O are omitted for clarity.

Table 1 .
The most stable adsorption sites, geometric parameters (in Å) and energies (in eV) for MSR intermediates on Pt 3 Sn(111).
[46]et al.[46]found that the OH group on Pt(111) could also be beneficial to MSR reactions, such as CH 3 OH → CH 3 O and CH 2 O + OH → CH 2 OOH.However, it is relatively difficult to dissociate water and generate the OH group on Pt(111) compared with the direct dehydrogenation of CH 3 OH.The OH group is only available when the difference in the energy barrier between H 2 O decomposition and CH 3 OH dehydrogenation is comparable.Thus, the MSR process on Pt(111) still follows the M1 mechanism, which is stepwise [32]and trans-COOH is remarkable.In particular, the energy barriers of the O-H bond activation (e.g., CH 3 OH → CH 3 O and trans-COOH → CO 2 ) 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 CH 3 OH 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 CH 3 OH but substantially decreased the O-H bond-activation barrier.For comparison,