Calcination temperature affects the existing types of NiO, and the influence of the three NiO types on the catalytic activity of samples is bound type ≫ free type > combined type.

Three Ni-Al 2 O 3 catalysts were prepared, in planetary ball-milling machine, by the mechanochemical method with Al(NO 3 ) 3 ·9H 2 O as the aluminum precursor, (NH 4 ) 2 CO 3 as the precipitant, and Ni(NO 3 ) 2 ·6H 2 O, NiCl 2 ·6H 2 O, and Ni(CH 3 COO) 2 ·4H 2 O as nickel precursors (the corresponding catalysts were labeled as Ni-NO, Ni-Cl, and Ni-Ac). The prepared catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (H 2 -TPR), and N 2 adsorption–desorption technologies, and CO methanation performance evaluation was carried out for the catalysts. Results showed that the catalyst with Ni(NO 3 ) 2 ·6H 2 O as the precursor presented good Ni dispersibility and a small Ni grain size of 6.80 nm. CO conversion, CH 4 selectivity, and yield of the catalyst were as high as 78.8, 87.9, and 69.8%, respectively. Carbon deposition analysis from temperature-programmed hydrogenation (TPH) characterization showed that the H 2 consumption peak area of the three samples followed the order: Ni-NO (2886.66 au) < Ni-Cl (4389.97 au) < Ni-Ac (5721.65 au), indicating that the Ni-NO catalyst showed higher resistance to carbon deposition, which might be due to its small Ni grain size.

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A series of Ni/MgO-Al 2 O 3 catalysts with different MgO abundance were prepared by mechanochemical method for CO methanation based-on slurry-bed reactor. The prepared catalysts were characterized by X-ray diffraction (XRD), H 2 temperature-programmed reduction (H 2 -TPR), N 2 adsorption-desorption, thermogravimetric analysis (TG-DTG) and temperatureprogrammed hydrogenation (TPH) techniques, and CO methanation performance evaluation was carried out for the samples. Results showed that the catalyst with MgO loading of 2 wt.% (Ni/MgO (2) -Al 2 O 3 ) presented smaller Ni grain size of 9.64 nm and larger specific surface area of 329 m 2 ⋅ g À 1 , indicating that a well dispersion of active component on the catalyst. Evaluation results showed that Ni/MgO (2) -Al 2 O 3 could improve the activity of catalyst for CO methanation. Carbon deposition analysis, from TG-DTG characterization, found that MgO could significantly inhibit the formation of carbon deposition in the reaction process. The amount of carbon deposition on Ni/ MgO (2) -Al 2 O 3 was the least, only 6.5 %, and the deposited carbon was consisted with the results obtained from TPH analysis. Kinetic analysis showed that the carbon removal reaction mainly occurred at the stage of 250-550 °C. And the activation energy from carbon removal reaction for Ni/MgO (2) -Al 2 O 3 was lower to 13.12 kJ/mol, demonstrating that the catalyst with MgO as additive is more beneficial to anti-carbon ability.