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

In the transformation reaction of alkanes to alkenes via catalytic dehydrogenation, it is generally accepted that the so-called catalytic deactivation behavior will occur. This phenomenon causes a drastic reduction in activity with time-on-stream. It is understood that carbon deposition generated during the reaction then covers the surface of the catalyst, and this leads to a drastic decrease in activity. However, contrary to this common wisdom, our laboratory reported that the dehydrogenation of isobutane to isobutene on NiO/γ-Al2O3 within a specific range of NiO loading in the presence of CO2 actually improved the yield of isobutene with time-on-stream. Since few such cases have been reported, in this study, isobutane was dehydrogenated in the presence of CO2 using NiO/α-Al2O3 as the catalyst with 20% NiO loading and improvement was again observed. In order 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 TEM analysis, both catalysts were covered with a large amount of carbon deposition after the reaction, but there was a difference in the types. The carbon deposition on NiO/γ-Al2O3 had 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 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 deposition during the reaction, and this resulted in high activity even if the catalyst was covered with a carbon deposition.