Promotional role of Ni-Cu alloy in catalytic performance and carbon properties for CO2-free H2 production from thermocatalytic decomposition of methane

While water electrolysis (H₂O_(l) → H₂ + ½ O₂; ΔH° = 286 kJ/mol H₂ ) holds promise for the generation of CO₂-free hydrogen (H₂), the high energy intensity and cost of electricity hinder their widespread utilization for H₂ generation. However, thermocatalytic decomposition (TCD) of methane (CH_4(g) → C_(s) + 2H₂; ΔH° = 37.5 kJ/mol H₂) is a more energy efficient process for generating carbon-free H₂ sources because it has a lower energy demand compared to electrolysis (green H₂). Additionally, it uses the already stablished natural gas (NG) infrastructure and generates a valuable solid carbon co-product (MWCNTs) instead of CO₂ that can be sold to meet the current H₂ Earthshot of $1/kg.
In this study, a series of Ni-Cu/CNT catalysts, prepared with varying Ni/Cu metal ratios and synthesis methods, were evaluated for methane TCD performance at various temperatures. Catalysts before and after reaction, and properties of the carbon product, were characterized to identify activity-structure relationships. At 550 °C, a 10 wt% Ni/CNT catalyst was active; however, it deactivated within 1 h of reaction at >600 °C. The addition of Cu increased its stability. At temperatures above 650 °C, only the catalyst with Cu loadings above 10 wt% remained active and stable. Catalyst characterization revealed that changes in i) Ni/Cu ratio, ii) metal particle size, and iii) operating temperature are key factors for TCD activity, stability, and carbon coproduct morphology. The carbon co-product is mainly composed of multiwalled carbon nanotubes (MWCNTs) whose morphology changes with Ni/Cu ratio and affects the rate of catalyst deactivation, Figure 1.