Effects of potassium on Ni-K/Al₂O₃ catalysts in the synthesis of carbon nanofibers by catalytic hydrogenation of CO₂

Commercially available Ni/Al₂O₃ samples containing various concentrations of potassium were used to achieve carbon deposition from CO₂ via catalytic hydrogenation. Experimental results show that K additives can induce the formation of carbon nanofibers or carbon deposition on Ni/Al₂O₃ during the reverse water-gas shift reaction. This work proposes that the formation rate of carbon deposition depends closely on ensemble control, suggesting that the ensemble size necessary to form carbon may be ∼0.5 potassium atoms. The results of CO₂ temperature- programmed desorption provide strong evidence that the new adsorption sites for CO₂ created on Ni-K/Al₂O₃ closely depend upon the synthesis of carbon nanofibers. It is found that some potassium-related active phases obtained by calcination and reduction pretreatments can participate in the carbon deposition reaction. The formation pathway for carbon deposition suggests that the main source of carbon deposition is CO₂ and that the pathway is independent of the reaction products CO and CH ₄ in the reverse water-gas shift reaction.