CNT and H2 Production during CH4 Decomposition over Ni/CeZrO2. II. Catalyst Performance and its Regeneration in a Fluidized Bed

In this work, a ceria-zirconia supported nickel catalyst (Ni/CeZrO2) was for the first time used in a fluidized bed reactor in order to obtain carbon nanotubes (CNTs) and H2 in the reaction of the decomposition of CH4. The same catalyst was afterward regenerated with H2O, which was accompanied with the production of H2. The impact of catalyst granulation, temperature, and gas hourly space velocity (GHSV) on the amount and type of carbon deposits was determined using thermogravimetric analysis (TGA) and scanning and transmission electron microscopy (SEM and TEM). The presence of randomly oriented and curved CNTs with an outer diameter of up to 64 nm was proved. The Ni/CeZrO2 particles were loosely covered with CNTs, freely dispersed over CNTs, and strongly attached to the external CNT walls. TEM proved the presence of a Ni/CeZrO2@CNT hybrid material that can be further used as catalyst, e.g., in WGS or DRM reactions. The impact of GHSV on hydrogen production during catalyst regeneration was determined. The catalyst was subjected to cyclic tests of CH4 decomposition and regeneration. According to the obtained results, Ni/CeZrO2 can be used in CH4 conversion to CNTs and H2 (instead of CH4 combustion), e.g., in the vicinity of installations that require methane utilization.

Table S1.Mass increase of Ni/CeZrO2 (0.125-0.2 mm) after tests of CH4 decomposition at various T, t and GHSV.Table S2.Mass increase of Ni-MgO after tests of CH4 decomposition at various T, t and GHSV.Regeneration of Ni/CeZrO2 was carried out in flowing 4.15 vol.%H2O/Ar under GHSV of 5000 h −1 .The samples used for regeneration were obtained after CH4 decomposition at 700 °C carried out for 1, 3 and 5 h in GHSV of 5000 and 20,000 h −1 .Yields of H2, CO and CO2 as a function of temperature during regeneration tests are presented in Figure S9.Conversion of H2O is shown in Figure S10.In each case small amounts of H2 are observed from ca. 400 °C, while at 500-550 °C H2O conversion usually exceeds 90%.The H2O dissociates on catalyst active sites (*) being oxygen vacancies in CeZrO2 or Ni 0 (Equation (S1)).Hydrogen desorbs from catalyst surface whereas oxygen (O*) that is still adsorbed on the active site can oxidize nearby carbon deposit to CO2 (at lower temperatures) and to CO (at higher temperatures) (Equations ( S2)-( S4)).Amorphous carbon deposits are oxidized at lower temperatures and structural carbon (CNTs) are oxidized at higher temperatures.According to TGA of catalyst samples after separation of deposit, the amorphous carbon prevailed.Rapid decrease in H2 formation in the case of samples obtained after CH4 decomposition for 1 hour is due to lower amount of carbon deposits on their surface.The rate of carbon deposits oxidation is determined by temperature, whereas the duration of regeneration is determined by the amount of deposits on catalyst surface.

Figure S2 .
Figure S2.SEM picture, Ni, Ce, Zr and O mapping and composition of Ni/CeZrO2 before CH4 decomposition.

Figure S4 .
Figure S4.SEM picture, Ni, Ce, Zr and O mapping for Ni/CeZrO2 after separation of carbon deposit.

Figure S5 .
Figure S5.EDS spectra for carbon fraction after separation from Ni/CeZrO2.

Figure S6 .
Figure S6.SEM picture and Ni, Ce, Zr and O mapping for carbon fraction after separation from Ni/CeZrO2.

Table S3 .
CH4 consumption and H2 production per 1 mol of Ni in Ni/CeZrO2 and Ni-MgO during CH4 decomposition tests.
Regeneration ofNi/CeZrO2 in H2O in Temperature Programmed Conditions in Micro-Reactor.