Effects Of In And K On Performance Of Ni/SiO₂ Catalyst For The Selective Hydrogenation Of Acetylene

Ni/SiO₂,bimetallic N ix In/SiO₂ with different N i/In molar ratios?x?and Ni₂In??K?/SiO₂ with different potassium mass contents???were prepared by the incipient wetness method.Their physicochemical properties be fore and after the reaction were characterized by means of H₂-TPR,XRD,TEM,XPS,H₂ chemisorption,CO chemisorption,C₂H₄-TPD,NH₃^-TPD,FT-IR of adsorbed pyridine,N2-sorption,TG/DTA and Raman.Their performances for the selective hydrogenation of acetylene were tested on an atmospheric fixed-bed reactor.The relationship between the catalyst structure and performance was discussed.It was found that Nix In/SiO₂ with a suitable Ni/In ratio gave much higher acetylene conversion,ethylene selectivity and catalyst stability than Ni/SiO₂.This is ascribed to the geometrical isolation of the reactive N i atoms with inert In ones and the charge transfer from the In atoms to N i ones.As a result,the formation of multi-?-adsorbed intermediate species was restrained,and the adsorption strength of ethylene and acetylene was weakened.Both geometrical and electronic effects of In are favorable for enhancing selectivity to ethylene and restraining the C-C hydrogenolysis and polymerization.O n the whole,N i6In/SiO₂ and N i10In/SiO₂ had better stability than Ni/SiO₂ and other Nix In/SiO₂.Nevertheless,with increasing In content,the catalyst acidity increased,subsequently aggravating the polymerization of acetylene and ethylene.With respect to the catalyst deactivation,the N i/SiO₂ catalyst deactivation is mainly attributed to the transformation of metallic N i to nickel carbide.The introduction of In inhibited the formation of nickel carbide.However,as the In content increased,the main reason for N ix In/SiO₂ catalyst deactivation changed from the formation of nickel carbide to the carbonaceous deposit.The introduction of potassium into N i2In/SiO₂ increased the selectivity to ethylene and inhibited the C-C hydrogenolysis and the polymerization.This is probably due to the geometrical isolation of the reactive N i sites by potassium and the charge transfer from potassium to N i.Moreover,potassium also weakened the acid strength and reduced the acid amount,subsequently inhibiting the polymerization of acetylene and ethylene and decreasing the amount of carbonaceous deposit.The larger the K content in N i2In??K?/SiO₂ was,the less the amount of carbonaceous deposit.The low K content??1 wt.%?enhanced the Ni₂In/SiO₂ stability.However,the Ni₂In??K?/SiO₂ catalysts with the K content ?1.5% had inferior stability to Ni₂In/SiO₂ since the excessive potassium covered the Ni sites.