Surface basicity controls C-C coupling rates during carbon dioxide-assisted methane coupling over bifunctional Ca/ZnO...

Area of Science:

• Catalysis
• Materials Science
• Green Chemistry

Background:

• Methane upgrading via carbon dioxide (CO2) coupling offers a route to convert greenhouse gases and natural gas into valuable chemicals like ethylene and syngas.
• Catalyst discovery has focused on bifunctional oxides, with reducible and basic oxides showing promise.
• Detailed characterization of these oxide systems is needed to understand active sites and reaction mechanisms.

Purpose of the Study:

• To investigate the impact of calcium (Ca) modification on zinc oxide (ZnO) properties and catalytic performance in CO2-assisted methane coupling.
• To elucidate the relationship between catalyst structure, surface basicity, and activity for C2 production.
• To establish a fundamental understanding of the active sites at the interface of reducible and basic oxide phases.

Main Methods:

• X-ray photoelectron and absorption spectroscopies to study electronic properties.
• Electron microscopy for structural characterization.
• Infrared spectroscopic temperature-programmed desorption (IR-TPD) to assess surface basicity and CO2 adsorption.

Main Results:

• Introduction of just 0.6 mol% Ca onto ZnO significantly improved C2 selectivity from 5% to 58% at similar methane conversions.
• The enhanced selectivity is attributed to the formation of an interface between basic CaO and reducible ZnO phases.
• Catalyst activity showed a volcano-type relationship with surface basicity, indicating optimal CO2 adsorption strength for methane coupling.

Conclusions:

• Ca-modified ZnO catalysts exhibit significantly enhanced performance for CO2-assisted methane coupling.
• The interface between CaO and ZnO is crucial for high C2 selectivity.
• Surface basicity is a key descriptor for optimizing catalyst design, with moderate CO2 adsorption being optimal.