Methane Cracking for Hydrogen Production Using Bi-metallic Zeolites

Abstract

The growing global energy demand and heavy reliance on fossil fuels have raised significant concerns about the finite nature of these resources and their detrimental environmental effects. These challenges underscore the urgent need to adopt sustainable energy alternatives that can meet global demands, ensure energy security, and keep global warming below the 2°C threshold set by the 2015 Paris Agreement. Hydrogen gas stands out as a promising alternative due to its high lower heating value (LHV) of 120 MJ/kg, surpassing that of conventional fuels like diesel, gasoline, and methane. However, current industrial-scale hydrogen production methods have multiple setbacks. Processes such as coal gasification and steam methane reforming generate greenhouse gases and require high operational temperatures, while dry methane reforming similarly demands high temperatures and produces CO as a byproduct, adding to the costs of hydrogen purification. Methane cracking offers a promising approach to hydrogen production without additional hydrocarbons, provided it employs a well-designed catalyst to facilitate the reaction at moderate operating temperatures (500–700°C). Operating at these temperatures reduces costs and creates opportunities to integrate solar energy, enhancing carbon neutrality. The efficiency of such a catalyst relies on the interaction between the active metal, promoters, and support. Therefore, developing a catalyst with balanced metal-support interactions is crucial to forming appropriately sized active metal particulates. This balance ensures high methane conversion rates and catalyst stability while minimizing deactivation. Nickel was selected as the active metal due to its affordability and effectiveness in methane cracking. Various parameters were investigated to optimize catalyst performance, including the choice of support materials, promoters, synthesis methods, and operating conditions. Among the tested supports, silica-alumina zeolites outperformed amorphous silica with promoters and mesostructured siliceous zeolites. Furthermore, mesoporous zeolites with wider pore structures exhibited superior performance, even with nickel loadings as high as 50%. These catalysts demonstrated not only excellent methane conversion rates but also significantly reduced deactivation, highlighting their potential for efficient and stable hydrogen production. After obtaining preliminary results from testing various catalysts, we focused on Ni-Zn/USY due to its notable performance advantages. Zinc-promoted nickel catalysts supported on Ultra-Stable Y (USY) zeolites were evaluated for methane pyrolysis. The unpromoted Ni/USY catalyst exhibited an initial conversion of 65.8%, which declined to 57.3% by the experiment's end. Introducing 5 wt% Zn as a promoter enhanced the conversion to 67.7% and maintained stable performance for 60 hours. XPS analysis suggested electronic interactions between Ni and Zn, while TPR and XRD revealed that Zn reduced the catalyst's metal-support interactions. The 50Ni-5Zn/USY catalyst demonstrated exceptional stability under increased reaction severity, including a temperature of 650°C, 80% methane inlet partial pressure, and a high gas hourly space velocity of 120 L/gcat.h. Spent catalyst analysis identified Ni-Zn carbides, which may have contributed to the improved activity, and significant quantities of highly graphitic, ordered multi-walled carbon nanotubes produced via a tip-growth mechanism, which likely prolonged the catalyst’s operational life. A detailed kinetic model for methane decomposition over a 50Ni–5Zn/USY catalyst was developed and validated. Experiments were conducted in a packed-bed reactor under atmospheric pressure, spanning temperatures of 500°C to 650°C and methane partial pressures from 0.225 to 0.8 bar. Several reaction mechanisms were evaluated, leading to the formulation of multiple kinetic models based on the Langmuir–Hinshelwood framework, with varying assumptions about the rate-determining step. The optimal kinetic parameters were identified using the fmincon algorithm in MATLAB combined with a multi-start search strategy to minimize the impact of initial guesses and avoid convergence to local minima. Constraints were incorporated to ensure zero reaction rate at equilibrium conditions, guaranteeing consistency with thermodynamic principles. Statistical techniques were applied for model selection, and additional validation was achieved using a packed-bed reactor model. The most suitable kinetic model indicates that the reaction proceeds via a dissociative adsorption mechanism, with the first hydrogen abstraction from methane identified as the rate-determining step. The activation energy, calculated as 80.9 kJ/mol, is consistent with previously reported literature values, reinforcing the reliability of the proposed model.