Elucidating the Reactivity of Oxygenates on Single-Atom Alloy Catalysts

Doping isolated transition metal atoms into the surface of coinage-metal hosts to form single-atom alloys (SAAs) can significantly improve the catalytic activity and selectivity of their monometallic counterparts. These atomically dispersed dopant metals on the SAA surface act as highly active sites for various bond coupling and activation reactions. In this study, we investigate the catalytic properties of SAAs with different bimetallic combinations [Ni-, Pd-, Pt-, and Rh-doped Cu(111), Ag(111), and Au(111)] for chemistries involving oxygenates relevant to biomass reforming. Density functional theory is employed to calculate and compare the formation energies of species such as methoxy (CH3O), methanol (CH3OH), and hydroxymethyl (CH2OH), thereby understanding the stability of these adsorbates on SAAs. Activation energies and reaction energies of C–O coupling, C–H activation, and O–H activation on these oxygenates are then computed. Analysis of the data in terms of thermochemical linear scaling and Bro̷nsted–Evans–Polanyi relationship shows that some SAAs have the potential to combine weak binding with low activation energies, thereby exhibiting enhanced catalytic behavior over their monometallic counterparts for key elementary steps of oxygenate conversion. This work contributes to the discovery and development of SAA catalysts toward greener technologies, having potential applications in the transition from fossil to renewable fuels and chemicals.

and SAA (red) surfaces.The "preferable pathway" in this table is defined as the pathway with the lower absolute energy of the transition state.Energy differences of ΔE IS Tot and ΔE TS Tot are calculated by the activation energy of h-pathway minus that of the d-pathway and the reaction energy of h-CH3OH pathway minus that of the d-CH3OH pathway, respectively.Double dash means that we were not able to find a transition state for the corresponding pathway.

Comparing Apparent Activation Energies among SAAs
As two different pathways are investigated for each step on SAAs (d-and h-pathways), the apparent activation energies on these materials are calculated to properly compare activities among SAAs (Figure S4).Regarding the CH3OH C-H activation, Rh-doped SAAs have the lowest energy barriers compared to other transition metal doped counterparts.The smallest apparent activation energy among them is observed on Rh/Ag(111), 0.59 eV, which is approximately half of the maximum energy of 1.11 eV, exhibited on Pd/Cu(111), as shown in Figure S4a.Moreover, Pd-doped SAAs exhibit the highest activation energies for CH3OH C-H scission in SAAs.On the other hand, the opposite trend is found for the CH2OH O-H activation.In particular, SAAs doped with Pd exhibit lower energy barriers for this dehydrogenation reaction, while Rhdoped SAAs exhibit higher energy barriers compared to other SAAs, except for Rh/Cu(111) (Figure S4b).The data also indicates that Ag-based SAAs generally have the highest activation energies, followed by Au-and Cu-based SAAs.Continuing our discussion with the CH3OH O-H activations, our results indicate that these are less likely to occur on Ag-and Au-based SAAs and are most likely to occur on Cu-based SAAs (Figure S4c).Pd/Au(111) exhibits the highest apparent activation energy (1.58 eV), which is more than twice the lowest energy on Pt/Cu(111) (0.72 eV).
As for the CH3O C-H activation, Pd/Cu(111) and Pt/Cu(111) have large energy barriers at 1.22 and 1.15 eV, respectively, while the lowest energy barrier was observed on Rh/Ag(111) (Figure S4d).
These apparent activation energies can be used to determine the path through which formaldehyde is generated.As Rh/Ag(111) has quite a large energy barrier for the O-H activation of CH2OH (0.44 and 0.98 eV higher than CH3OH O-H activation, and CH3O C-H activation, respectively), the CH2O species is more likely to be produced through the "CH3O-path" (Figure S4).This result is also true for Ni/Ag  Moving on to C-O coupling between CH3 and CH3O, all SAAs can be divided into two groups; dopant atoms in Group 1 SAAs, which include Pd/Au, Pd/Ag, Pd/Cu, and Pt/Cu, produce CH3OCH3 from both host-based and dopant-based initial states.For Group 2 SAAs, the dopant atoms have the potential to activate the C-H bonds in CH3 along the h-pathway to produce CH3OCH2 and H adatoms.Among Group 2 SAAs, only Ni/Au(111) has a reduced apparent activation energy for CH3OCH3 formation, which is 0.17 eV lower than the C-O coupling step towards CH3OCH2 (Figure S5).To avoid forming CH3OCH2, Pt/Cu(111) and Pd-doped SAAs could be used, which are predicted to be the most promising catalysts for CH3OCH3 synthesis.Among these SAAs, Pd/Au(111) has the lowest apparent activation energy, showing the best activity compared to other SAAs.As a "second choice" alternative, Pd/Ag(111) can be considered.Although the apparent activation energy on Pd/Ag(111) is 0.30 eV higher than that of gold, using silver as the host metal can significantly reduce costs.

Figure S1 .
Figure S1.DFT-optimized structures for (a) the CH3O intermediate on Ag(111), (b) the CH2OH intermediate on Pd(111), (c) the CH3OCH3 intermediate on Cu(111), and (d) the CH2OCH3 intermediate on Au(111).Pd in green, Cu in orange, Au in gold, Ag in silver, C in black, O in red, and H in white in the figure.

Figure S2 .
Figure S2.Front views of DFT-optimized structures for the CH2O intermediates on (a) Au(111) and (b) Pd(111).Top views of DFT-optimized structures for the CH2O intermediates on (c) Au(111) and (d) Pd(111).Pd in green, Au in gold, C in black, O in red, and H in white in the figure.

Figure S4 .
Figure S4.Heatmap charts of the apparent activation energies (  ) for (a) CH3OH C-H activation, (b) CH2OH O-H activation, (c) CH3OH O-H activation, and (d) CH3O C-H activation for Cu-, Agand Au-based SAAs.For each SAA,   is calculated as the formation energy of the lowerenergy transition state (between the d-and h-pathways) minus the energy of the most stable initial state.
(111) and Pt/Ag(111).On the other hand, because of the high energy barrier of CH3OH O-H activation on Pd/Au(111), this alloy can preferably generate CH2OH through the "CH2OH-path".Although Pd/Cu(111) and Pt/Cu(111) also have high energy barriers for CH3O C-H activation, similar activation energies are also found in CH3OH C-H and CH2OH O-H activations.In conclusion, Ag-based SAAs (except for Pt/Ag(111)), and Pd/Au(111) have the potential to catalyze CH3OH dehydrogenation to CH2O via CH3Oor CH2OH-path, respectively.

Figure S5 .
Figure S5.Heatmap charts of the apparent activation energies (  ) for (a) CH3OCH2 formation and (b) CH3OCH3 formation for Cu-, Ag-and Au-based SAAs.For each SAA,   is calculated as the formation energy of the lower-energy transition state (between the d-and h-pathways) minus the energy of the most stable initial state.

Table S2 .
Formation energies (EF) of fragments adsorbed to the most favored (lowest energy) adsorption site on pure metal and SAA (111) surfaces, as computed by Results of Student's T-test (to determine if the slopes and intercepts of pure metal and SAA TCS relations are different) are given for 90 % and 99 % confidence intervals as "Yes" or "No" for different and not different, respectively.Details of calculation methods can be found in the work ofDarby et TableS3.Thermo-chemical scaling relations (TCS) for species 1 (x-axis) and species 2 (y-axis) adsorbed in their most favored adsorption sites on pure metals and SAAs.Linear Regression parameters (slope, a and intercept, b in eV) are given in addition to coefficients of determination (R 2 ) and mean absolute errors in eV (MAE).

Table S5 .
Activation energies (Ea) and reaction energies (ΔERxn) for the h-CH3OH and the d-CH3OH O-H dissociation reaction pathways on pure metal (blue)

Table S6 .
Activation energies (Ea) and reaction energies (ΔERxn) for the h-CH3O and the d-CH3O C-H dissociation reaction pathways on pure metal (blue) and SAA (red) surfaces.Preferable pathway in the table is defined as the pathway with lower absolute energy of the transition state.Energy differences ofΔE ISTot and ΔE TS Tot are calculated by the activation energy of h-pathway minus that of the d-pathway and the reaction energy of h-CH3OH pathway minus that of the d-CH3OH pathway, respectively.Double dash means that we were not able to find a transition state for the corresponding pathway.

Table S7 .
Activation energies (Ea) and reaction energies (ΔERxn) for the h-CH2OH and the d-CH2OH O-H dissociation reaction pathways on pure metal (blue) and SAA (red) surfaces.Preferable pathway in the table is defined as the pathway with lower absolute energy of the transition state.Energy differences ofΔE ISTot and ΔE TS Tot are calculated by the activation energy of h-pathway minus that of the d-pathway and the reaction energy of h-CH3OH pathway minus that of the d-CH3OH pathway, respectively.Double dash means that we were not able to find a transition state for the corresponding pathway.CH3OH pathway minus that of the d-CH3OH pathway, respectively.Double dash means that we were not able to find a transition state for the corresponding pathway.
TableS8.Activation energies (Ea) and reaction energies (ΔERxn) for the h-CH3O and the d-CH3O C-O coupling reaction pathways on Group 1 pure metal (blue) and SAA (red) surfaces.Preferable pathway in the table is defined as the pathway with lower absolute energy of the transition state.Energy differences of ΔE IS Tot and ΔE TS Tot are calculated by the activation energy of h-pathway minus that of the d-pathway and the reaction energy of h-

Table S9 .
Activation energies (Ea) and reaction energies (ΔERxn) for the h-CH3O and the d-CH3O C-O coupling reaction pathways on Group 2 pure metal (blue) and SAA (red) surfaces.Preferable pathway in the table is defined as the pathway with lower absolute energy of the transition state.Energy differences of ΔE IS Tot and ΔE TS Tot are calculated by the activation energy of h-pathway minus that of the d-pathway and the reaction energy of h-CH3OH pathway minus that of the d-CH3OH pathway, respectively.Double dash means that we were not able to find a transition state for the corresponding pathway.

Table S10 .
Formation energies of initial states (EF,ini), transition states (EF,tra), and final states (EF,fin) of C-O coupling on pure metal and SAA (111) surfaces, as computed by DFT.Values of EF are relative to CH4(g), H2(g), and CO2(g) as given by equation (1) in the main text.Double dash means that we were not able to find a transition state for the corresponding pathway.