The effect of CeO2–ZrO2 structural differences on the origin and reactivity of carbon formed during methane dry reforming over NiCo/CeO2–ZrO2 catalysts studied by transient techniques

Abstract

Nickel (1.2 wt%) and cobalt (1.8 wt%) were dispersed over Ce_0.75Zr_0.25O_2−δ solid solution (3NiCo EG) or over a mixture of CeO₂ and ZrO₂ single phases (3NiCo HT) and tested for dry reforming of methane (DRM) at 750 °C. The structural, morphological, textural and redox differences between 3NiCo EG and 3NiCo HT catalysts were probed by powder XRD, HAADF/STEM and SAED, N₂ adsorption/desorption at 77 K, H₂-chemisorption, H₂-TPR and H₂ transient isothermal reduction (H₂-TIR) techniques. The origin, concentration and reactivity of “carbon” deposits formed in DRM (via the CH₄ and CO₂ activation routes) towards gaseous H₂ and O₂ but also towards the support's labile oxygen species in the prepared catalysts were analyzed by a series of various transient and SSITKA experiments (use of ¹⁸O₂ and ¹³CO₂). Regardless of the EG or HT support, the %-contribution of CH₄ and CO₂ to the “carbon” deposition is very similar but the amount and reactivity towards oxidation are largely different. On the other hand, the concentration of active carbon formed in the carbon pathway of the CO₂ activation route is very small (θ_C < 0.16% after 2 h in DRM). Participation of mobile lattice oxygen species in gasification of deposited “carbon” to CO(g) does occur to a large extent on both EG- and HT-supported NiCo catalysts. The catalyst deactivation rate during the first 5 h of DRM was found to depend on the structure of the support (EG vs. HT). The faster deactivation observed with the 3NiCo EG catalyst cannot be linked to the existing differences in the rates of inactive “carbon” formation and depletion or the concentration of active carbon but rather to the different rates of encapsulation of NiCo bimetallic particles in the carbon layers formed (∼30 nm thick) as revealed by HAADF/STEM.