## 2 Computational Details  
Systematic ab initio theoretical calculations were performed to study the equilibrium bond lengths, adsorption reactions, and work functions of carbon monoxide on the surfaces of the clean and Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111). The Vienna ab initio simulation package [16,17] was applied at the generalized gradient approximation (GGA) with the Perdew-Wang (PW91) correction [18,19] in all cases. The ground state structure of bulk ZnGa\({}_{2}\)O\({}_{4}\) is cubic _Fd-3m_ and performed using 8 Zn, 16 Ga, and 32 O atoms per unit cell, as shown in Figure 1. The Zn\({}^{2+}\) and Ga\({}^{3+}\) cations are distributed in tetrahedral and octahedral lattice sites, respectively. The cutoff energy and the self-consistent total energy criterion were set to 500 eV and 10\({}^{-5}\) eV/unit cell, respectively, and the equilibrium lattice parameter of bulk ZnGa\({}_{2}\)O\({}_{4}\) can be obtained as 8.334 A. To simulate the work function of the clean and Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111) with and without adsorbed CO molecules, we have employed a repeated slab geometry with a 112-atom ZnGa\({}_{2}\)O\({}_{4}\) with an in-plane lattice constants of 11.85 A \(\times\) 11.85 A, which is sufficient to decouple the interactions between CO molecules, separated by a vacuum region equivalent to 20 A vacuum, which decouples top and bottom interactions. Our slabs are terminated by the Ga-Zn-O surface with a low surface energy of 0.10 eV/A\({}^{2}\), proposed earlier study by [PERSON] et al. [20]. The top and side views for the selected adsorption sites of Ga-Zn-O-terminated ZnGa\({}_{2}\)O\({}_{4}\)(111) are displayed in Figure 1. A gamma-centered 3 \(\times\) 3 \(\times\) 1 Monkhorst-Pack grid was used for the density of state integration. This supercell was fully relaxed until the force acting on each atom was less than 0.001 eV/A. To determine the preferred adsorption sites of the CO molecules on the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface, we labeled the surface atoms as Ga\({}_{3c}\), Zn\({}_{3c}\), O\({}_{3c}\), and O\({}_{4c}\) in the top and side views of Figure 1. To gain further insight into the CO adsorption on the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface, we consider three following types. One type plotted in the upper panel of Figure 2 is that a carbon atom from a CO molecule perpendicular to the surface is adsorbed on Ga\({}_{3c}\), Zn\({}_{3c}\), O\({}_{3c}\), and O\({}_{4c}\) sites, labeled as CO-C1, CO-C2, CO-C3, and CO-C4, respectively, or CO-C\(i\) denoted by model number \(i\) (where \(i\) = 1-4). The second type, plotted in the middle panel of Figure 2, is an oxygen atom from a CO molecule perpendicular to the surface is adsorbed on Ga\({}_{3c}\), Zn\({}_{3c}\), O\({}_{3c}\), and O\({}_{4c}\) sites, labeled as CO-O1, CO-O2, CO-O3, and CO-O4, respectively, or CO-O\(i\) denoted by model number \(i\) (where \(i\) = 1-4). Thirdly, four adsorption models, CO-CO\(i\), denoted by model number \(i\) (where \(i\) = 1-4), with both the O and C atoms from a CO molecule parallel to the surface interacting with the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface were constructed as shown in the bottom panel of Figure 2. In model CO-CO1 (CO-CO2), a carbon (an oxygen) atom from a CO molecule is adsorbed on the Ga\({}_{3c}\) site. In model CO-CO3 (CO-CO4), a carbon (an oxygen) atom from a CO molecule is adsorbed on the Zn\({}_{3c}\) site. On the other hand, we also constructed four adsorption models of Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111), denoted as Pd-ZGO\(i\) (where \(i\) = 1-4), as shown in the upper panel of Figure 3. Here, the subscripts 1, 2, 3, and 4 in Pd-ZGO, respectively, indicate the positions of the initial adsorbed sites Ga\({}_{3c}\), Zn\({}_{3c}\), O\({}_{3c}\), and O\({}_{4c}\) on the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface.