Photocatalyst Turns Methane and CO2 into Green Methanol

By Muhammad OsamaReviewed by Laura ThomsonSep 27 2024

In a recent paper published in Nature Communications, researchers from McGill University, Canada, presented an innovative technique for converting methane (CH₄) and carbon dioxide (CO₂) into valuable chemicals. This process utilizes gold-palladium (AuPd) nanoparticles supported on gallium nitride (GaN) semiconductors to address the growing issue of greenhouse gas emissions by transforming these gases into green methanol (CH₃OH) and carbon monoxide (CO) through a photo-driven chemical reaction.

The study demonstrated a highly efficient method for converting greenhouse gases into valuable chemical feedstocks under mild conditions. This could potentially revolutionize industrial CH₃OH production and support climate change mitigation efforts.

Global Challenge of Greenhouse Gas Emissions

The increasing global energy demands and the urgent need to address climate change have led to significant advancements in photocatalysis. CH₄ and CO₂, which account for over 90% of global emissions, are the primary anthropogenic greenhouse gasses. Traditionally, these gases have been treated as C1 feedstocks for producing bulk chemicals and fuels.

Recent progress has been made in reducing CO₂ with hydrogen sources and oxidizing CH₄ into high-value products. However, while most efforts have focused on lowering CO₂ and oxidizing CH₄ separately, developing a direct co-reforming process to convert both gases simultaneously into valuable chemicals has remained a significant challenge.

Innovative Photocatalytic System for Methanol Synthesis

In this paper, a photo-driven chemical process, or photocatalytic system, was developed to directly synthesize green CH₃OH from CH₄ and CO₂. The authors proposed that a heterogeneous photocatalyst, specifically AuPd nanoparticles supported on a GaN semiconductor, could activate both CH₄ and CO₂ on its surface, facilitating the formation of CH₃OH through an oxygen-atom-grafting process.

This AuPd/GaN catalyst enables the conversion under light illumination at room temperature and ambient pressure, significantly improving over traditional high-temperature, high-pressure methods.

The catalyst was prepared using a meticulous soft colloid-immobilization process, resulting in well-dispersed metal nanoparticles on the GaN surface. Advanced characterization techniques, such as transmission electron microscopy (TEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were used to analyze the structural and electronic properties of the catalyst.

The experimental setup employed various heterogeneous photocatalysts in a custom-designed quartz chamber illuminated by full-spectrum light (300 W xenon lamp). Control experiments confirmed that CH₄ and CO₂ did not produce detectable CH₃OH without light or catalysts at room temperature. Using commercial GaN powder as a catalyst revealed its exceptional potential for the efficient photocatalytic conversion of CH₄ and CO₂ into CH₃OH.

To improve reaction efficiency, air-stable single-metal particles such as iridium (Ir), ruthenium (Ru), platinum (Pt), rhodium (Rh), Pd, and Au were deposited on the GaN support using a chemical-reduction method. Monometallic interfaces featuring Au and Pd nanoparticles significantly increased photocatalytic activity.

The catalyst was optimized using a soft colloid-immobilization process, resulting in uniformly distributed metal nanoparticles on the GaN surface. The optimized catalyst, AuPd/GaN, exhibited superior photocatalytic performance compared to other tested samples.

To further understand the catalyst's mode of action, density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation Package (VASP). These computational studies provided valuable insights into the adsorption and activation of reactant molecules on the catalyst surface, complementing the experimental findings.

Key Findings

The outcomes indicated that the AuPd/GaN catalyst significantly improved the photocatalytic conversion of CH₄ and CO₂ into CH₃OH, achieving a productivity rate of up to 1405 μmol g⁻¹ h⁻¹, 3.8 times higher than existing co-reforming processes for syngas synthesis.

Detailed analyses confirmed that CH₃OH and CO were the primary products, with minimal formation of hydrogen gas (H₂) and other hydrocarbons. Isotopic labeling experiments also traced the carbon and oxygen sources in the products, confirming that CH₄ was the primary carbon source and CO₂ was the oxygen source for CH₃OH.

The study also explored the effects of different CH₄/CO₂ ratios on CH₃OH production, finding that a 1:1 ratio was optimal. The catalyst showed strong performance under ambient pressure, achieving a total CH₃OH yield of 13.66 mmol g⁻¹ over 10 hours with a turnover number (TON) of 941. The researchers proposed a mechanism in which oxygen-atom grafting from CO₂ to CH₄ occurs, facilitated by the photoexcited AuPd/GaN interface.

Applications

This research has significant implications for sustainable chemical conversions and climate change mitigation. The direct and highly selective synthesis of CH₃OH from CH₄ and CO₂ under mild conditions offers a scalable and efficient method for producing green CH₃OH. This process can be adapted for continuous flow operations, potentially transforming industrial CH₃OH production. The study emphasizes the potential of interfacial metal/semiconductor photocatalysts in advancing sustainable energy technologies.

Conclusion

In summary, the novel method effectively converted CH₄ and CO₂ into green CH₃OH using a photo-driven chemical process. The AuPd/GaN catalyst was crucial in facilitating this transformation, achieving high selectivity and productivity under mild conditions.

The authors provided a strong scientific foundation for extending the oxygen-atom-grafting process to the visible light region, enhancing its applicability in solar-driven chemical conversions. Future work could optimize the catalyst's stability and explore its potential in other photocatalytic applications, contributing to developing sustainable and efficient chemical processes for a greener future.

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