In Situ Methane Pyrolysis with Catalyst Regeneration via Reverse Boudouard

Abstract

Catalytic methane pyrolysis (CMP) is gaining attention as a low-carbon route for hydrogen production, but carbon deposition still limits catalyst stability and long-term performance. This work investigates cyclic CMP over Ni/USY zeolite in a fixed-bed reactor with in situ CO2 regeneration through the reverse Boudouard reaction (RBR). The selected operating conditions were 650 °C with 40 vol.% CH4 for 10 min during CMP and 800 °C with 80 vol.% CO2 during RBR until no CO was detected, using N2 as the balance gas at a gas hourly space velocity of 120 L g-1 h-1. Among the parent catalysts, 35Ni/USY was selected as the base catalyst. For the promoters, 5 wt.% Ce was the most effective promoter with cyclic performance comparable to that of 50Ni/USY while replacing 15 wt.% Ni. Co did not improve cyclic stability, whereas Zn showed a delayed effect during cracking. The trimetallic 35Ni-5Ce-2.5Zn/USY catalyst showed only a slight improvement. Also, solvent variation affected performance where the substitution of a small amount of solvent with ethylene glycol improved cyclic behavior and gave the smallest crystallite size. The best overall performance was obtained with 35Ni-5Ce/USY prepared at pH = 10 by successive impregnation. This agrees with the literature, where alkaline preparation promotes surface hydroxyl groups that improve metal anchoring and strengthen metal-support interaction (MSI). This catalyst remained active for 25 cycles corresponding to about 33 h of total time on stream. More importantly, during this period CH4 conversion gradually decreased from about 72% to 38%, while CO2 conversion decreased from about 85% to 48%. Fresh catalyst characterization by XRD, BET, SEM, and H2-TPR linked this behavior to smaller particles, higher pore accessibility, uniform morphology, and stronger MSI. Spent catalyst characterization by TGA, Raman, and SEM confirmed high carbon deposition/effective regeneration, more defective carbon, and mainly filamentous carbon and carbon nanofibers. As this technique moves toward commercialization, this dual benefit reinforces the use of industrially relevant feeds to support scale up and circular carbon economy principles.