In both pathways, except the first adsorption step the rests are PCET steps. The intermediates investigated that can couple with 𝑆𝑂 or 𝐻𝑆𝑂 are CO, COH, CHO, CHOH, CH2O and CH2OH. In Rx7, the intermediates following COH were not considered since formation of COH is unfavourable at 0.6 eV on Cu (100), as shown in Fig. . However, COH coupling with SO3 2- was still considered since COH is believed to be an intermediate to methane production. For Rx8, CHO, CHOH, CH2O and CH2OH were coupled with 𝑆𝑂 to create the C-S bond. The final product considered in DFT calculations was HMS as seen in Fig. as it is the primary C-S product in the electrolysis experiments. In solution, 𝑆𝑂 or 𝐻𝑆𝑂 can exist depending on the pH of the electrolyte. Thus, both compounds are tested for the nucleophilic attack. Furthermore, at any point during protonation, one of the oxygens attached to the sulfur can be protonated to yield -RSO2OH. All of the aforementioned pathways are considered and this is illustrated in Fig. ) reveal that: (i) 𝑆𝑂 is adsorbed more favourably than 𝐻𝑆𝑂 . This is mostly because 𝑆𝑂 species has 2 negative charges and is relatively smaller than 𝐻𝑆𝑂 , making it easier to bond with copper atoms (Fig. ); (ii) Both S species are adsorbed more strongly on the (110) facet, followed by ( ) then ( ) facets, as seen on Table . Accordingly, the rest of the simulations to calculate the reaction energy diagram are performed only on (110) and (100) facets. The (110) facet might perform the best as it interacts more strongly with S species compared to the other two surfaces. However, according to the Sabatier principle, the middle adsorption by (100) facet could render it the best performing as the adsorption is neither too week (not able to hold the species on the surface for enough time to react with other reaction intermediates) nor too strong (to avoid further interactions with other reaction intermediates). The reaction energy diagram is shown for the (100) facet on Fig. , and that for the (110) facet is demonstrated on Fig. .