Study On Ni-Ga Bimetallic Catalysts For Selective Hydrogenation Of Acetylene

Selective hydrogenation is an effective means to remove a small amount of acetylene in ethylene,so it is very theoretically and practically significant for developing the catalysts for the selective hydrogenation of acetylene.SiO₂ and CeO₂supported Ni-Ga bimetallic catalysts were prepared by two different methods,and their physical and chemical properties were characterized by means of H₂-TPR,XRD,XPS,H₂ chemisorption,C₂H₄-TPD,NH₃-TPD,N₂ physisorption-desorption and TG-DTA.The catalyst performance for the selective hydrogenation of acetylene was tested on a fixed-bed reactor.The relationship between the catalyst structure and reactivity was discussed.First,Ni/SiO₂ and Ni_xGa/SiO₂ bimetallic catalysts with different Ni/Ga molar ratios were prepared by the incipient impregnation method.In Ni_xGa/SiO₂ catalysts,Ni-Ga alloys were formed when the Ni/Ga atomic ratio was?4,while Ni₃Ga intermetallic compound?IMC?was formed when the Ni/Ga ratios were 3 and 2.Charge transfer from Ga to Ni appear in Ni-Ga alloy and Ni₃Ga IMC.Compared with Ni/SiO₂,the Ni_xGa/SiO₂ catalyst has stronger acidity,weaker ethylene adsorption capacity,and lower H₂ adsorption amount,which is closely related to the geometric and electronic effects of Ga in Ni-Ga alloy and Ni₃Ga IMC.In the selective hydrogenation of acetylene,the formation of Ni-Ga alloy and Ni₃Ga the improved the selectivity to ethylene.With the decrease of Ni/Ga atomic ratio,the activity and stability of the Ni_x Ga/SiO₂ catalysts increased first and then decreased,while the ethylene selectivity tended to increase.Ni₅Ga/SiO₂ exhibited the best performance.Under the conditions of180 ^oC,0.1 MPa,and a reactant?1.0 vol%acetylene,5.0 vol%H₂ and 94 vol%N₂?with the space velocity of 36,000 mL/?h·g?,the acetylene conversion maintained at 100%on Ni₅Ga/SiO₂ during 120 h time on stream and the selectivity to ethylene was 75%⁸1%after reaction for 68 h.The catalyst deactivation study shows that the formation of Ni-Ga alloy and Ni₃Ga suppressed the incorporation of carbon to form NiC_x,subsequently enhancing the catalyst stability.However,as the Ni/Ga atomic ratio decreased,the increase of catalyst acidity promoted the carbon deposition and so the catalyst deactivation.Then,Ni/CeO₂ and Ni_xGa/CeO₂ catalysts were prepared by the citric acid thermal decomposition followed by the temperature-programmed reduction method.Similarly,in Ni_xGa/CeO₂ catalysts,Ni-Ga alloy was formed when the Ni/Ga atomic ratio was?5,and Ni₃Ga was formed at the Ni/Ga atomic ratio of 3.Due to the geometric and electronic effects of Ga in Ni-Ga alloys and Ni₃Ga,Ni_xGa/CeO₂ had lower ability for the adsorptions of ethylene and H₂ than Ni/CeO₂,so Ni_xGa/CeO₂ showed higher selectivity to ethylene.With the decrease of Ni/Ga atomic ratio,the stability and ethylene selectivity of Ni_xGa/CeO₂ catalysts first increased and then decreased.Among Ni_xGa/CeO₂,Ni₅Ga/CeO₂ reduced at 650 ^oC had the best performance,and it gave higher selectivity to ethylene than Ni₅Ga/SiO₂.