**3. Experimental**

## *3.1. Catalyst Synthesis*

A series of Ni-based catalysts were synthesized using an impregnation method. The monometallic catalysts (20 wt% Ni/Al2O3, 5 wt% Cu/Al2O3 and 5 wt% Co/Al2O3), bimetallic catalysts (20 wt% Ni-5 wt% Cu/Al2O3 and 20 wt% Ni-5 wt% Co/Al2O3) and trimetallic catalyst (20 wt% Ni-5 wt% Cu-5 wt% Co/Al2O3) were prepared. Briefly, the Al2O3 support was obtained through the calcination of pseudoboehmite precursor in the air at 550 ◦C for 4 h. The required amount of Ni(NO3)2·6H2O, u(NO3)2·3H2O and Co(NO3)3·6H2O, obtained from Aladdin Industrial Corporation (Shanghai, China), were dissolved in distilled water under ultrasound for 30 min. Then 5 g Al2O3 support was added and mixed for 3 h using an ultrasonic mixer at 35 ◦C. The mixture was then dried at 100 ◦C for 12 h. Next, the sample was heated for calcination from 20 ◦C to 600 ◦C in a furnace at a rate of 1 ◦C/min, held at 600 ◦C for 5 h, then finally cooled to room temperature. Before the experiments, the catalyst was reduced in a 5% H2/95% N2 mixed gas with a flow rate of 50 mL/min at 500 ◦C for 5 h. The prepared Ni-based catalysts are denoted as 20Ni/Al, 5Cu/Al, 5Co/Al, 20Ni-5Co/Al, 20Ni-5Cu/Al, and 20Ni-5Cu-5Co/Al for brevity.
