Carbon Deposition Assisting the Enhancement of Catalytic Activity with Time-on-Stream in the Dehydrogenation of Isobutane over NiO/Al₂O₃

Abstract

In the transformation reaction of alkanes in alkenes via catalytic dehydrogenation, it is generally accepted that catalytic deactivation will occur. This phenomenon causes a drastic reduction in the catalytic activity with time-on-stream. It is understood that carbon deposits generated during the reaction then covers the surface of the catalyst, leading to a drastic decrease in activity. However, in contradiction, our laboratory reported that the dehydrogenation of isobutane to isobutene on NiO/γ-Al₂O₃ within a specific range of NiO loading with CO₂ improved the yield of isobutene with time-on-stream. Since few such cases have been reported, in this study, isobutane was dehydrogenated with CO₂ over the NiO/α-Al₂O₃ catalyst, with 20% NiO loading, and an improvement was again observed. To investigate the cause of the improvement, both NiO/γ-Al₂O₃ and NiO/α-Al₂O₃ with 20% NiO loading were examined in detail following the reaction. According to transmission electron microscopy (TEM) analysis, both catalysts were covered with a large amount of carbon deposit after the reaction; however, there was a difference in the types. The carbon deposit on NiO/γ-Al₂O₃ exhibited a fibrous nature, while that on NiO/α-Al₂O₃ appeared to be a type of nanowire. Raman spectroscopy revealed that the carbonaceous crystal-growth properties of the two forms differed depending on the support. In particular, a catalytically active species of metallic nickel was formed in a high degree of dispersion in and on the above two forms of carbon deposits during the reaction, and this resulted in a high catalytic activity even when the catalyst was covered with a carbon deposit.