*3.2. Experimental Section*

#### 3.2.1. Preparation of Catalyst

Transition metal-doped CaO, MgO, and ZnO catalysts were synthesized in nanoparticle form by following a modified incipient-wetness impregnation method [35]. In a typical preparation, metal oxide (CaO or MgO or ZnO) slurry (10 mg/40 mL ethanol) was sonicated for 1 h, and then 10 mL of transition metal (Fe or Co or Cu or Zn or Cd) solution in ethanol of a desired concentration was added dropwise into the metal oxide slurry and stirred moderately for 3 h at 25 ◦C. The resulting mixture was then dried and calcined at varying temperatures from 100 to 800 ◦C for 12 h. The solid then obtained was characterized by BET surface area measurement ((ASAP 2010, Micromeritics, USA)), a Hammett indicator test, powder-XRD, DLS (Brookhaven Instruments Corporation, Austin, TX, United States), FESEM, and TEM.
