There have been rare experimental researches studied the CO oxidation in the recent years. For the theoretical investigations of CO oxidation, Cheng
et al. [
1] demonstrated that the Mo
2CO
2 MXene could serve as an excellent substrate material of the SACs, as shown in
Fig. 2. The CO oxidation on the Pd SAC supported on the defective Mo
2CO
2 monolayer with an oxygen vacancy (Pd/O
vMo
2CO
2) preferred to occur along the TER mechanism and the energy barrier was 0.49 eV. The CO oxidation catalytic activity of Pd/O
v-Mo
2CO
2 was better than that of the Pd(111) [
75] catalyst and approached to that of the Pt(111) [
76] catalyst at low CO coverage. Then the authors proceeded to solve the CO poisoning problems and the stability problems of the commercial Pt/C catalysts. The study investigated by Cheng
et al. [
77] demonstrated that the small Cu
3 clusters supported on the Mo
2CO
2 substrate possessed good stability and high catalytic activity for CO oxidation, in which the Cu
3 cluster acted as an electron reservoir to regulate the gaining and losing of electrons, thus promoted the entire reaction, as seen in
Fig. 3. The rate-limiting energy barrier was 0.72 eV, which was lower than those of Pt(111) (1.05 eV) [
78], Pd(111) (0.93 eV) [
79] and Pd(110) (0.78 eV) [
79], indicating the higher reaction activity. Further, the authors continued to explore the common nature of the catalytic materials based on the former investigation [
1] and provided a useful guide for fabricating efficient SACs based on MXenes. The eleven metal atoms from VIII~IIB groups were doped in the Mo
2CO
2-δ monolayer to form the SACs and three criterias were proposed to screen their stability, anti-oxidation properties, and resistance properties of CO poisoning at low temperature. It was concluded that the Zn-doped Mo
2CO
2-δ is a promising SAC candidate for CO oxidation, which may occur along the ER mechanism with the energy barrier of 0.15 eV [
80]. The reaction barrier of Zn-doped Mo
2CO
2-δ was much lower than those of the Pt(111) catalyst (1.05 eV) [
78], and Pd(111) catalyst (0.93 eV) [
79], indicating the higher catalytic performance of Zn-doped Mo
2CO
2-δ. In addition, the authors [
81] also found that the composite with the Ag monolayer supported on Mo
2C MXene not only possessed high ORR activity, but also exhibited high resistance for CO poisoning. The large surface electron perturbations caused by the strong metal-support interactions and the moderate binding strength of CO could accelerate the speed of CO oxidation at higher CO concentration, thus purified the hydrogen fuel. Besides, the CO oxidation occurred on the Ti anchored Ti
2CO
2 MXene (Ti/Ti
2CO
2) also exhibited outstanding performance along the ER mechanism as that investigated by Zhang
et al. [
33]. The reaction barrier of CO was 0.25 eV, which was lower than that of Pt/FeO
x [
82], exhibiting the high catalytic activity for Ti/Ti
2CO
2 in low-temperature CO oxidation, as shown in Fig. S2 (Supporting information). Therefore, the MXenes can serve as the support with outstanding catalytic performance for CO oxidation for the single atom, single cluster or the monolayer of transition metals.