Effects of P:Ni Ratio on Methanol Steam Reforming on Nickel Phosphide Catalysts

This study investigates the influence of the phosphorus-to-nickel (P:Ni) ratio on methanol steam reforming (MSR) over nickel phosphide catalysts using density functional theory (DFT) calculations. The catalytic behavior of Ni(111) and Ni12P5(001) surfaces was explored and contrasted to our previous results from research on Ni2P(001). The DFT-predicted barriers reveal that Ni(111) predominantly favors the methanol decomposition route, where methanol is converted into carbon monoxide through a stepwise pathway involving CH3OH* → CH3O* → CH2O* → CHO* → CO*. On the other hand, Ni12P5 with a P:Ni atomic ratio of 0.42 (5:12) exhibits a substantial increase in selectivity towards methanol steam reforming (MSR) relative to methanol decomposition. In this pathway, formaldehyde is transformed into CO2 through a sequence of reactions involving CH2O*→ H2COOH* → HCOOH* → HCOO* → CO2. The introduction of phosphorus into the catalyst alters the surface morphology and electronic structure, favoring the MSR pathway. However, with a further increase in the P:Ni atomic ratio to 0.5 (1:2) on Ni2P catalysts, the selectivity towards MSR decreases, resulting in a more balanced competition between methanol decomposition and MSR. These results highlight the significance of tuning the P:Ni atomic ratio in designing efficient catalysts for the selective production of CO2 through the MSR route, offering valuable insights into optimizing nickel phosphide catalysts for desired chemical transformations.


Introduction
Proton-exchange membrane (PEM) fuel cells, water splitting, new-generation batteries, and CO 2 conversion hold great promise as cutting-edge energy technologies for the future [1][2][3][4][5][6]. However, PEM fuel cells face challenges with hydrogen storage [7][8][9]. Methanol steam reforming (MSR) shows great potential as a viable solution for addressing this challenge given its ability to generate hydrogen from liquid methanol, which offers advantages in terms of storage and handling compared to hydrogen gas [10][11][12]. MSR is a relatively simple process that can be integrated with PEM fuel cells, making it an attractive option for on-board hydrogen production [13,14]. Methanol can be used to produce hydrogen through two main reactions: methanol decomposition and steam reforming. In methanol decomposition, methanol is broken down into hydrogen and carbon monoxide via: To prevent CO poisoning in Pt-based fuel cells and further enhance hydrogen production, the water-gas shift (WGS) reaction is often used to further convert carbon monoxide to carbon dioxide via: CO + H 2 O → H 2 + CO 2 (2) Steam reforming provides a more efficient route to produce hydrogen directly from methanol: CH 3 OH + H 2 O → 3H 2 + CO 2 (3) thus eliminating CO production while producing a high yield of hydrogen [13,[15][16][17][18][19][20][21][22]. Cu-based catalysts have been considered the traditional catalysts for MSR [9], but they face challenges related to rapid deactivation and inadequate thermal stability [22][23][24]. Recent studies have investigated other metal-based catalysts, such as Pd/ZnO and PdZn alloys [18,19], that have shown higher selectivity towards CO 2 but lower catalytic activity [25][26][27][28]. The primary goal of catalyst performance enhancement for MSR is to increase the rate of hydrogen production and improve CO 2 selectivity while using a thermally stable material. Transition metal phosphides (TMPs) have emerged recently as a highly versatile class of catalysts which can be used for a wide range of applications due to their unique selectivity and resistance to coke formation [29][30][31][32][33]. Among those TMPs, nickel phosphides (Ni x P y ) have been previously examined for C-O bond cleavage in biomassderived molecules [34,35]. These studies show that the addition of P atoms to Ni promotes cleaving the tertiary bond ( 3 C-O) in 2-methyltetrahydrofuran relative to pure Ni in which cleaving the secondary bond ( 2 C-O) is much more facile. Increasing the P:Ni atomic ratio (i.e., Ni → Ni 12 P 5 → Ni 2 P) increases the selectivity towards 3 C-O bond cleavage. The geometric or electronic effects caused by the incorporation of P atoms with Ni appear to play a role in this unique selectivity to activate hindered C-O bonds [36].
In our previous study, we explored the reaction network shown in Scheme 1 over the Ni 2 P(001) surface using density functional theory (DFT) calculations [32]. Our DFTderived activation barriers indicate that methanol decomposition on the Ni 2 P(001) surface occurs through CH 3 OH* → CH 3 O* → CH 2 O* → CHO* → CO*. In the MSR pathway, formaldehyde (CH 2 O*) reacts with a hydroxyl (OH*) originating from a water splitting reaction to yield H 2 COOH*, which ultimately produces CO 2 via H 2 COOH*→ HCOOH* → HCOO* → CO 2 . The activation barriers for both routes are comparable (a difference of only 5 kJ mol −1 ) whereas, on transition metal catalysts, methanol decomposition is more likely to dominate. The role of phosphorus and the effects of the P:Ni atomic ratio on selectivity, however, remain unclear.
To prevent CO poisoning in Pt-based fuel cells and further enhance hydrogen production, the water-gas shift (WGS) reaction is often used to further convert carbon monoxide to carbon dioxide via: Steam reforming provides a more efficient route to produce hydrogen directly from methanol: thus eliminating CO production while producing a high yield of hydrogen [13,[15][16][17][18][19][20][21][22]. Cu-based catalysts have been considered the traditional catalysts for MSR [9], but they face challenges related to rapid deactivation and inadequate thermal stability [22][23][24]. Recent studies have investigated other metal-based catalysts, such as Pd/ZnO and PdZn alloys [18,19], that have shown higher selectivity towards CO2 but lower catalytic activity [25][26][27][28]. The primary goal of catalyst performance enhancement for MSR is to increase the rate of hydrogen production and improve CO2 selectivity while using a thermally stable material. Transition metal phosphides (TMPs) have emerged recently as a highly versatile class of catalysts which can be used for a wide range of applications due to their unique selectivity and resistance to coke formation [29][30][31][32][33]. Among those TMPs, nickel phosphides (NixPy) have been previously examined for C-O bond cleavage in biomass-derived molecules [34,35]. These studies show that the addition of P atoms to Ni promotes cleaving the tertiary bond ( 3 C-O) in 2-methyltetrahydrofuran relative to pure Ni in which cleaving the secondary bond ( 2 C-O) is much more facile. Increasing the P:Ni atomic ratio (i.e., Ni → Ni12P5 → Ni2P) increases the selectivity towards 3 C-O bond cleavage. The geometric or electronic effects caused by the incorporation of P atoms with Ni appear to play a role in this unique selectivity to activate hindered C-O bonds [36].
In our previous study, we explored the reaction network shown in Scheme 1 over the Ni2P(001) surface using density functional theory (DFT) calculations [32]. Our DFT-derived activation barriers indicate that methanol decomposition on the Ni2P(001) surface occurs through CH3OH* → CH3O* → CH2O* → CHO* → CO*. In the MSR pathway, formaldehyde (CH2O*) reacts with a hydroxyl (OH*) originating from a water splitting reaction to yield H2COOH*, which ultimately produces CO2 via H2COOH*→ HCOOH* → HCOO* → CO2. The activation barriers for both routes are comparable (a difference of only 5 kJ mol −1 ) whereas, on transition metal catalysts, methanol decomposition is more likely to dominate. The role of phosphorus and the effects of the P:Ni atomic ratio on selectivity, however, remain unclear. Scheme 1. Reactions network to form CO and CO2 examined in our prior and current studies over Ni, Ni12P5, and Ni2P catalysts. Asterisk (*) denotes a surface site. Reprinted from reference [32]. Scheme 1. Reactions network to form CO and CO 2 examined in our prior and current studies over Ni, Ni 12 P 5 , and Ni 2 P catalysts. Asterisk (*) denotes a surface site. Reprinted from reference [32].
In this paper, we build upon our previous study by incorporating Ni and Ni 12 P 5 catalysts to illustrate the notable impact of the P:Ni atomic ratio (y:x in Ni x P y ) on the selectivity and reactivity of MSR. The results demonstrate that the choice of P:Ni atomic ratio plays a crucial role in shaping catalytic performance. The moderate P:Ni of 0.42 for Ni 12 P 5 (5:12) enhances the selectivity of MSR relative to methanol decomposition. The insights gained from these findings can guide the design of efficient catalysts tailored for use in MSR applications.

Optimized Adsorbates and Their Binding Energies
In this section, we study the geometries and energies of all the intermediates that form during methanol decomposition and MSR. We aim to identify the most stable configuration by exploring various adsorption modes on examined surfaces, as depicted in Figures S1 and S2 (Supplementary Materials). The binding energies presented in Table 1 were calculated using Equation (4). Methanol (CH 3 OH) binds weakly to the atop site (M 1 ) of the Ni(111) surface through its oxygen atom (Figure S1e), with a binding energy of −11 kJ mol −1 (Table 1). This observation aligns with those found in previous theoretical and experimental studies, which also demonstrate the weak adsorption of methanol onto Ni and other transition metals [37][38][39][40]. Similarly, other physisorbed species such as water and formic acid also exhibit low binding energy to the atop site (−9 and −13 kJ mol −1 , respectively). The interaction between these three intermediates and the surface metal atom likely involves the donation of the oxygen atom's lone pair of electrons, contributing to their adsorption. Formaldehyde (CH 2 O; Figure S1h), on the other hand, displays a slightly stronger binding energy (−32 kJ mol −1 ) and preferentially binds to the 3-fold site (M 3 ) in a top-bridge configuration. While it can also bind to the bridging site (M 2 ) in a di-σ mode, this configuration is less favored by 18 kJ mol −1 . The remaining unsaturated intermediates demonstrate stronger binding energies ranging from −134 kJ mol −1 for CH 2 OH* to −506 kJ mol −1 for O* ( Table 1). The majority of these intermediates (H*, OH*, O*, CH 3 O*, CHO*, CO*, H 2 COOH*, and H 2 COO*) prefer to adsorb on the 3-fold site (M 3 ) except CH 2 OH*, COOH*, and HCOO* which favorably bind to the bridging site (M 2 ). The trends in binding energies observed here on Ni(111) closely resemble those seen in our previous study on the Ni 2 P(001) surface ( Figure 1).
Compared to the Ni(111) and Ni 2 P(001) surfaces, the Ni 12 P 5 (001) surface exhibits an overall increase in binding strength for all surface intermediates ( Figure 1). While the differences in binding energies among different species remain generally similar, the average binding energy on Ni 12 P 5 (001) is notably higher, with an average binding energy of approximately −230 kJ mol −1 on Ni 12 P 5 , compared to −170 kJ mol −1 on both Ni(111) and Ni 2 P(001). For example, methanol, water, and formic acid also bind to the metal atop site (M 1 ), but with stronger affinities relative to Ni and Ni 2 P surfaces. Formaldehyde, a crucial intermediate in the MSR pathway, exhibits a binding energy of −126 kJ mol −1 when interacting with the metal 4-fold site on Ni 12 P 5 (001), which is over 100 kJ mol −1 stronger than its binding energy on Ni 2 P(001). This is consistent with the differences in calculated binding energies on Ni, Ni 12 P 5 , and Ni 2 P reported in previous studies for the intermediates involved in the hydrodeoxygenation reaction of oxygenated compounds [34]. Ni 12 P 5 (001) has two unique 4-fold sites denoted by M 4a and M 4b (discussed in Section 3). Notably, the M 4b site displays higher reactivity compared to M 4a , as evidenced by the preference of most intermediates examined here, with the exception of H 2 COOH*. For instance, formaldehyde (CH 2 O) binds more strongly to the M 4b site by around 75 kJ mol −1 compared to the M 4a site. These findings collectively suggest that Ni 12 P 5 exhibits higher reactivity compared to Ni and Ni 2 P surfaces.

Figure 1. Trends in binding energies
ΔEads (kJ mol −1 ) for all surface intermediates examined in this study over Ni (green) and Ni12P5 (red) surfaces compared to Ni2P (yellow) examined in our previous study [32]. Dashed lines are provided as visual guides. Asterisk (*) denotes a surface site.
Compared to the Ni(111) and Ni2P(001) surfaces, the Ni12P5(001) surface exhibits an overall increase in binding strength for all surface intermediates ( Figure 1). While the differences in binding energies among different species remain generally similar, the average binding energy on Ni12P5(001) is notably higher, with an average binding energy of approximately −230 kJ mol −1 on Ni12P5, compared to −170 kJ mol −1 on both Ni(111) and Ni2P(001). For example, methanol, water, and formic acid also bind to the metal atop site (M1), but with stronger affinities relative to Ni and Ni2P surfaces. Formaldehyde, a crucial intermediate in the MSR pathway, exhibits a binding energy of −126 kJ mol −1 when interacting with the metal 4-fold site on Ni12P5(001), which is over 100 kJ mol −1 stronger than its binding energy on Ni2P(001). This is consistent with the differences in calculated binding energies on Ni, Ni12P5, and Ni2P reported in previous studies for the intermediates involved in the hydrodeoxygenation reaction of oxygenated compounds [34]. Ni12P5(001) has two unique 4-fold sites denoted by M4a and M4b (discussed in Section 3). Notably, the M4b site displays higher reactivity compared to M4a, as evidenced by the preference of most intermediates examined here, with the exception of H2COOH*. For instance, formaldehyde (CH2O) binds more strongly to the M4b site by around 75 kJ mol −1 compared to the M4a site. These findings collectively suggest that Ni12P5 exhibits higher reactivity compared to Ni and Ni2P surfaces.

Reaction Pathways
Next, we present a thorough analysis of the reaction pathways shown in Scheme 1 on Ni and Ni12P5. We compare these results with our prior work on Ni2P to elucidate the Trends in binding energies ∆E ads (kJ mol −1 ) for all surface intermediates examined in this study over Ni (green) and Ni 12 P 5 (red) surfaces compared to Ni 2 P (yellow) examined in our previous study [32]. Dashed lines are provided as visual guides. Asterisk (*) denotes a surface site.

Reaction Pathways
Next, we present a thorough analysis of the reaction pathways shown in Scheme 1 on Ni and Ni 12 P 5 . We compare these results with our prior work on Ni 2 P to elucidate the effects of the P:Ni atomic ratio on selectivity and reactivity [32]. The reaction network, as illustrated in Scheme 1, serves as the basis for our investigation. To ensure that the most stable transition states are identified, we extensively explored various configurations on distinct surface sites for each elementary step. Table 2 displays the respective forward activation barriers and reaction energies. These values do not encompass the diffusion steps, as these steps are typically assumed to be relatively fast and have negligible influence on the overall reaction pathways.

H 2 O Dissociation
On the Ni(111) surface, the direct water dissociation (reaction 17; Figure S3) has an enthalpic activation barrier (∆H act ) of 74 kJ mol −1 and is an exothermic reaction with a reaction energy (∆H rxn ) of −25 kJ mol −1 (reaction 17; Table 2). Further dehydrogenation of hydroxyl (OH*) adsorbed onto the M 3 site takes place with a higher activation barrier of 97 kJ mol −1 and a reaction energy of −18 kJ mol −1 . These values agree with previously reported data on the Ni(111) surface [41], and they are consistent with our earlier findings on  Table 2) by 6 kJ mol −1 . The structures of the reactants, transition states, and products for all reactions examined in this study are shown in Figures S3 and S4 (Supplementary Materials).  Transition state structures were identified using the combination tic band (NEB) method and the dimer method [56][57][58]. Electronic ene within 10 −6 eV, and forces converged to less than 0.05 eVÅ −1 . Frequenc performed, with all catalysts' atoms constrained, to estimate enthalpies ing energy (ΔEads) is defined as: ∆ / and effective enthalpy barriers (ΔH҂) are calculated using:

∆ ҂ ҂
where λ is the number of H2 molecules desorbed from the surface as a gen removal steps. Section S1 in the Supplementary Materials provid garding the computational methods.

Conclusions
The effects of the P:Ni ratio on selectivity towards the methanol s action were investigated using periodic density functional theory calc culations showed that Ni(111) surface (P:Ni = 0) predominantly dec into carbon monoxide through: CH3OH* → CH3O* → CH2O* → CHO sults are in agreement with those of prior studies on various transit  Transition state structures were identified using the combination of the nudged elastic band (NEB) method and the dimer method [56][57][58]. Electronic energies converged to within 10 −6 eV, and forces converged to less than 0.05 eVÅ −1 . Frequency calculations were performed, with all catalysts' atoms constrained, to estimate enthalpies at 573 K. The binding energy (ΔEads) is defined as: ∆ / (4) and effective enthalpy barriers (ΔH҂) are calculated using: where λ is the number of H2 molecules desorbed from the surface as a result of the hydrogen removal steps. Section S1 in the Supplementary Materials provides more details regarding the computational methods.

Conclusions
The effects of the P:Ni ratio on selectivity towards the methanol steam reforming reaction were investigated using periodic density functional theory calculations. These calculations showed that Ni(111) surface (P:Ni = 0) predominantly decomposes methanol into carbon monoxide through: CH3OH* → CH3O* → CH2O* → CHO* → CO*. These re-values are referenced to the energies of gas-phase methanol and water, which is particularly relevant when the reaction involves water.
On the Ni (111)   Transition state structures were identified using the combination of the nudg tic band (NEB) method and the dimer method [56][57][58]. Electronic energies conv within 10 −6 eV, and forces converged to less than 0.05 eVÅ −1 . Frequency calculatio performed, with all catalysts' atoms constrained, to estimate enthalpies at 573 K. T ing energy (ΔEads) is defined as: ∆ / and effective enthalpy barriers (ΔH҂) are calculated using:

∆ ҂ ҂
where λ is the number of H2 molecules desorbed from the surface as a result of th gen removal steps. Section S1 in the Supplementary Materials provides more de garding the computational methods.

Conclusions
The effects of the P:Ni ratio on selectivity towards the methanol steam refor  Figure 2a. This suggests that CH 3 O* is more likely the dominant species on Ni(111) as reported in previous experimental studies [42,43]. Subsequent dehydrogenation of CH 2 O* leads to the formation of CHO* and finally CO* with activation barriers of 76 and 60 kJ mol −1 , respectively. For Ni 12 P 5 (001), the methanol decomposition steps followed to form CH 3 O* and then CH 2 O* are both facile, with relatively similar activation barriers (22-25 kJ mol −1 ; Figure 2b). Once CH 2 O* is formed, it can undergo C-H bond activation to form CHO* with a barrier of 59 kJ mol −1 , which eventually forms CO* with a barrier of only 2 kJ mol −1 . Both CH 3 O* and CH 2 O* are stable intermediates on Ni 12 P 5 , in contrast to Ni and Ni 2 P, where CH 3 O* is more dominant.  Table 2) on the Ni(111) surface. However, the activation barrier of the C-H bond to form CH2OH* (141 kJ mol −1 ) is twice that of O-H bond activation required to form CH3O* (70 kJ mol −1 ), suggesting that the methoxy formation route is more favorable. Similarly, on the Ni12P5(001) surface, O-H activation in methanol is more favorable by 33 kJ mol −1 . In order to assess the relative preference of the various reaction routes shown in Scheme 1, we next analyze the effective enthalpy barriers (ΔH҂; Equation (5)) as shown in Figure 2. These ΔH҂ values are referenced to the energies of gas-phase methanol and water, which is particularly relevant when the reaction involves water.  Figure 2a. This suggests that CH3O* is more likely the dominant species on Ni(111) as reported in previous experimental studies [42,43]. Subsequent dehydrogenation of CH2O* leads to the formation of CHO* and finally CO* with activation barriers of 76 and 60 kJ mol −1 , respectively. For Ni12P5(001), the methanol decomposition steps followed to form CH3O* and then CH2O* are both facile, with relatively similar activation barriers (22-25 kJ mol −1 ; Figure 2b). Once CH2O* is formed, it can undergo C-H bond activation to form CHO* with a barrier of 59 kJ mol −1 , which eventually forms CO* with a barrier of only 2 kJ mol −1 . Both CH3O* and CH2O* are stable intermediates on Ni12P5, in contrast to Ni and Ni2P, where CH3O* is more dominant.

Methanol Steam Reforming
Here, we examine the possibility of reacting formaldehyde (CH2O*) with co-adsorbed OH* to form H2COOH*. On the Ni(111) surface, this reaction requires an effective activation barrier of 116 kJ mol −1 (Figure 2a), which is much larger than the activation barrier of further decomposition to CHO* (76 kJ mol −1 ). This is consistent with the results of the previous studies that demonstrated the preference of transition metal catalysts to decompose methanol into CO, except for Cu, which shows a distinct selectivity towards H2COOH* formation via MSR. In contrast, Ni12P5(001) can form H2COOH* from CH2O* and OH* with a barrier of 37 kJ mol −1 (Figure 2b), which is 22 kJ mol −1 lower than that of CH2O* decomposition. In our previous study, we found that the barriers of CH2O*  Figure S5 (Supplementary Materials). Asterisk (*) denotes a surface site. Black and red represent methane decomposition pathway, green represents MSR pathway, and blue represents WGS reaction pathway.

Methanol Steam Reforming
Here, we examine the possibility of reacting formaldehyde (CH 2 O*) with co-adsorbed OH* to form H 2 COOH*. On the Ni(111) surface, this reaction requires an effective activation barrier of 116 kJ mol −1 (Figure 2a), which is much larger than the activation barrier of further decomposition to CHO* (76 kJ mol −1 ). This is consistent with the results of the previous studies that demonstrated the preference of transition metal catalysts to decompose methanol into CO, except for Cu, which shows a distinct selectivity towards H 2 COOH* formation via MSR. In contrast, Ni 12 P 5 (001) can form H 2 COOH* from CH 2 O* and OH* with a barrier of 37 kJ mol −1 (Figure 2b), which is 22 kJ mol −1 lower than that of CH 2 O* decomposition. In our previous study, we found that the barriers of CH 2 O* decomposition to CHO* and CH 2 O* reaction with OH* are within~5 kJ mol −1 on the Ni 2 P(001) surface ( Figure S5), indicating that both pathways are kinetically relevant. Figure 3 shows the reactant, transition state, and product structures for reaction 7 on Ni(111) and Ni 12 P 5 (001) surfaces. On both surfaces, the carbon atom in CH 2 O* needs to undergo partial desorption from the surface before reacting with the adjacent OH* to form H 2 COOH*.
Once H 2 COOH* is formed on the Ni 12 P 5 (001) surface, it can either cleave its C-H bond or its O-H bond, but C-H bond cleavage with an activation barrier of 66 kJ mol −1 is slightly more favorable (Figure 2b). Formic acid (HCOOH*) then forms HCOO*, which eventually forms CO 2 *, the product of the MSR reaction. Taken together, these findings indicate that the P:Ni atomic ratio can dictate the selectivity towards CO 2 formation via MSR relative to methanol decomposition into CO. The moderate P:Ni atomic ratio in Ni 12 P 5 (P:Ni = 0.42) favors the MSR pathway compared to Ni 2 P (P:Ni = 0.5), in which both methanol decomposition and MSR are competitive. Ni (P:Ni = 0), on the other hand, predominantly decomposes methanol to carbon monoxide. decomposition to CHO* and CH2O* reaction with OH* are within ~5 kJ mol −1 on the Ni2P(001) surface ( Figure S5), indicating that both pathways are kinetically relevant. Figure 3 shows the reactant, transition state, and product structures for reaction 7 on Ni(111) and Ni12P5(001) surfaces. On both surfaces, the carbon atom in CH2O* needs to undergo partial desorption from the surface before reacting with the adjacent OH* to form H2COOH*. Once H2COOH* is formed on the Ni12P5(001) surface, it can either cleave its C-H bond or its O-H bond, but C-H bond cleavage with an activation barrier of 66 kJ mol −1 is slightly more favorable (Figure 2b). Formic acid (HCOOH*) then forms HCOO*, which eventually forms CO2*, the product of the MSR reaction. Taken together, these findings indicate that the P:Ni atomic ratio can dictate the selectivity towards CO2 formation via MSR relative to methanol decomposition into CO. The moderate P:Ni atomic ratio in Ni12P5 (P:Ni = 0.42) favors the MSR pathway compared to Ni2P (P:Ni = 0.5), in which both methanol decomposition and MSR are competitive. Ni (P:Ni = 0), on the other hand, predominantly decomposes methanol to carbon monoxide.
The impact of the P:Ni atomic ratio on the differences in selectivity stems from a complex interplay between geometric and electronic factors, each contributing to the observed effects. For example, the Ni atoms exposed on the Ni2P(001) surface form 3-fold sites similar to those found in Ni(111), only with larger Ni-Ni bond distances (3.16 Å versus 2.49 Å). This may explain the similarities in the reaction coordinate diagram between Ni ( Figure 2a) and Ni2P ( Figure S5), except that both MSR and methanol decomposition pathways start to compete on Ni2P, whereas methanol decomposition is more dominant on Ni. The Ni12P5(001) surface, however, exposes unique metal 4-fold sites (M4) that predominantly favor MSR. Another factor to consider here is the electronic effects. We have previously performed charge analysis for these three surfaces and we have shown that the Ni atoms become more positively charged with an increasing P:Ni atomic ratio [35]. This may indicate that the moderate Lewis acidity on Ni12P5 enhances the selectivity of MSR relative to methanol decomposition.

Water-Gas Shift Reaction
CO* generated through the methanol decomposition route can be transformed into CO2 via the water-gas shift reaction. It can either react with a co-adsorbed OH* to form COOH* in the carboxyl mechanism, which subsequently dehydrogenates to produce CO2 (CO* + OH* → COOH* → CO2* + H*), or it can react with O* to directly produce CO2 via The impact of the P:Ni atomic ratio on the differences in selectivity stems from a complex interplay between geometric and electronic factors, each contributing to the observed effects. For example, the Ni atoms exposed on the Ni 2 P(001) surface form 3-fold sites similar to those found in Ni(111), only with larger Ni-Ni bond distances (3.16 Å versus 2.49 Å). This may explain the similarities in the reaction coordinate diagram between Ni (Figure 2a) and Ni 2 P ( Figure S5), except that both MSR and methanol decomposition pathways start to compete on Ni 2 P, whereas methanol decomposition is more dominant on Ni. The Ni 12 P 5 (001) surface, however, exposes unique metal 4-fold sites (M 4 ) that predominantly favor MSR. Another factor to consider here is the electronic effects. We have previously performed charge analysis for these three surfaces and we have shown that the Ni atoms become more positively charged with an increasing P:Ni atomic ratio [35]. This may indicate that the moderate Lewis acidity on Ni 12 P 5 enhances the selectivity of MSR relative to methanol decomposition.

Water-Gas Shift Reaction
CO* generated through the methanol decomposition route can be transformed into CO 2 via the water-gas shift reaction. It can either react with a co-adsorbed OH* to form COOH* in the carboxyl mechanism, which subsequently dehydrogenates to produce CO 2 (CO* + OH* → COOH* → CO 2 * + H*), or it can react with O* to directly produce CO 2 via the redox mechanism (CO* + O* → CO 2 *), as shown in Scheme 1. The carboxyl mechanism was found to be more favorable on Ni 2 P than the redox mechanism, which is similar to what we found here on the Ni(111) surface (Table 2). However, for Ni 12 P 5 (001), we could not find a stable transition state for the carboxyl mechanism. Instead, Ni 12 P 5 exclusively forms CO 2 via the redox mechanism with a large effective barrier of 89 kJ mol −1 (Figure 2b). WGR reaction pathways involve substantially higher barriers than those observed for the MSR pathway across all catalysts studied here. Thus, CO 2 formation via the WGS reaction can be ruled out.
Available crystallographic data were used to obtain the bulk unit cells of Ni and Ni 12 P 5 , which were then optimized using DFT to determine their lattice parameters, as reported in more detail in our previous work [34]. Spin polarization was used for all Ni calculations, whereas Ni 12 P 5 does not exhibit ferromagnetic properties. A 3 × 3 periodic lattice was used to model the Ni(111) surface (Figure 4a), with four atomic layers and 10 Å of vacuum orthogonal to the surface (Figure 4b). For geometric convergence, a k-point mesh of 3 × 3 × 1 was employed [54,55]. This was followed by a single-point calculation with a k-point mesh of 6 × 6 × 1 to compute the electronic energy. For the Ni 12 P 5 catalyst, the Ni-terminated (001) surface was used (Figure 4c) due to its favorable surface formation energy, as shown previously [34], with two repeating units (8 atomic layers) and 10 Å of vacuum orthogonal to the surface (Figure 4d). This termination does not expose any phosphorus atoms on the surface. It has atop sites (M 1 ), bridging sites (M 2 ), 3-fold sites (M 3 ), and two unique 4-fold sites (M 4a and M 4b ). The M 4a site has a P atom directly beneath it (in the second layer), whereas the P atom beneath M 4b is in the fourth layer. A k-point mesh of 3 × 3 × 1 was used for Ni 12 P 5 calculations. The bottom half of each catalyst model was constrained during optimizations.  Transition state structures were identified using the combination of the nudged elastic band (NEB) method and the dimer method [56][57][58]. Electronic energies converged to within 10 −6 eV, and forces converged to less than 0.05 eVÅ −1 . Frequency calculations were performed, with all catalysts' atoms constrained, to estimate enthalpies at 573 K. The binding energy (ΔEads) is defined as: and effective enthalpy barriers (ΔH҂) are calculated using: where λ is the number of H2 molecules desorbed from the surface as a result of the hydro- Transition state structures were identified using the combination of the nudged elastic band (NEB) method and the dimer method [56][57][58]. Electronic energies converged to within 10 −6 eV, and forces converged to less than 0.05 eVÅ −1 . Frequency calculations were performed, with all catalysts' atoms constrained, to estimate enthalpies at 573 K. The binding energy (∆E ads ) is defined as: