Methane, a primary component of shale gas and methane hydrates, is an abundant resource. Transforming methane into valuable liquid chemicals and fuels via low-carbon processes is crucial for maximizing fossil feedstock economic value and significantly reducing greenhouse gas emissions. Formaldehyde (HCHO), a high-volume methane derivative, is a vital precursor in various industries, including resins, plastics, textiles, and pharmaceuticals. The current industrial HCHO production is indirect, involving high-temperature and high-pressure steam reforming of methane to produce synthesis gas, which then converts to methanol, ultimately oxidized to HCHO at around 350 °C. This multi-step process is energy-intensive and economically viable only at large scales, limiting its applicability in localized or on-site applications. Direct methane oxidation to HCHO would be a more efficient and eco-friendly alternative. However, the high C-H bond energy (439 kJ mol⁻¹) and chemical inertness of methane necessitate high temperatures (above 500 °C) in thermal catalysis, leading to over-oxidation to CO and CO₂, and HCHO selectivity below 65%. Photocatalysis offers a pathway to activate methane under milder conditions. Recent research has demonstrated methane conversion to HCHO using various metal oxide semiconductors, achieving ~90% selectivity, yet the HCHO production rate remains low (~20 µmol h⁻¹), with prolonged reaction time leading to over-oxidation. To enhance efficiency, the introduction of phonon energy is proposed to improve reaction kinetics and product desorption. The ideal approach involves a cascade process: photocatalysis initiates C-H dissociation, followed by phonon energy to convert the intermediate selectively to the desired product. This study combines photocatalysis and phonon-driven transformation to achieve efficient and selective HCHO production from methane using a Ru-modified ZnO catalyst.

The current industrial method for formaldehyde production is an indirect route involving steam reforming of methane to produce synthesis gas, followed by methanol synthesis and subsequent oxidation. This multi-step process is energy-intensive and large-scale oriented. Direct oxidation of methane to formaldehyde offers significant advantages in terms of efficiency and environmental impact. However, previous attempts using thermal catalysis have been hindered by the high activation energy of the C-H bond and the tendency for over-oxidation at high temperatures. Photocatalysis has shown some promise in activating methane under milder conditions using various metal oxide semiconductors, but production rates have remained low. This study builds upon these previous efforts by integrating photocatalysis and phonon-driven decomposition to overcome the limitations of both individual approaches. Several studies reported on using photocatalysis for methane oxidation to formaldehyde with varying degrees of success. While some achieved high selectivity, the production rate was often limited. This research aims to address this limitation by combining photocatalysis and thermal decomposition.

ZnO nanoparticles were synthesized via a precipitation method using zinc nitrate and oxalic acid, followed by calcination. Ruthenium (and other metals for comparison) was deposited onto ZnO using a chemical reduction method with NaBH₄. The resulting catalysts were characterized using various techniques including XRD, UV-Vis DRS, XPS, TEM (including HAADF-STEM), and XAS (XANES and EXAFS). Electrochemical measurements (oxygen reduction reaction and transient photocurrent density) were performed to investigate charge transfer processes. Photoluminescence (PL) and in situ photo-induced absorption (PIA) spectroscopy were used to study charge recombination and transfer. In situ electron paramagnetic resonance (EPR) spin-trapping with DMPO was employed to detect •OOH and •OH radicals. Nitroblue tetrazolium (NBT) and coumarin were used to quantify superoxide and hydroxyl radicals, respectively. Oxygen isotopic experiments (using ¹⁸O₂ and H₂¹⁸O) were conducted to determine the oxygen source in the products. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to monitor the reaction intermediates. Density functional theory (DFT) calculations were performed to understand the reaction pathways. Photocatalytic methane oxidation reactions were carried out in a batch reactor under controlled conditions of temperature, pressure, and light intensity. Products (HCHO, CH₃OOH, CH₃OH, CO, CO₂) were quantified using various analytical methods (GC, ¹H NMR, colorimetric method).

The key finding is the demonstration of a highly efficient and selective method for methane oxidation to formaldehyde using a photon-phonon cascade catalysis approach. A ZnO catalyst modified with single Ru atoms achieved a formaldehyde production rate of 401.5 µmol h⁻¹ (40,150 µmol g⁻¹ h⁻¹) with a selectivity of 90.4% at 150 °C. This represents a significant improvement compared to previous photocatalytic approaches, which yielded substantially lower rates. The optimized catalyst (0.5 wt% Ru on ZnO) showed superior performance compared to those with other metal co-catalysts (Pd, Ag, Au, Pt) or different Ru loadings. Characterization revealed that single Ru atoms are crucial for efficient charge separation and oxygen reduction. Oxygen isotopic experiments confirmed that the oxygen in formaldehyde originates primarily from water, while that in methanol comes from O₂. In situ DRIFTS studies showed the presence of key intermediates, such as CH₃ and CH₃OOH. DFT calculations provided insights into the reaction mechanisms, showing that the reaction proceeds through a two-step process: photocatalytic formation of CH₃OOH followed by thermal decomposition to formaldehyde. The catalyst exhibited excellent stability over multiple reaction cycles.

The high efficiency and selectivity achieved in this work demonstrate the potential of photon-phonon cascade catalysis for the sustainable conversion of methane. The two-step reaction pathway effectively utilizes the advantages of both photocatalysis (selective activation of methane) and thermal catalysis (efficient conversion of intermediates). The use of single-atom Ru catalysts minimizes energy consumption and avoids the formation of undesired byproducts. The findings address the long-standing challenge of directly converting methane to valuable chemicals under mild conditions, offering a potentially economically viable and environmentally friendly alternative to the current industrial process. This approach holds promise for applications in distributed or on-site methane valorization, reducing reliance on large-scale centralized facilities and mitigating methane flaring.

This research successfully demonstrates a highly efficient and selective method for converting methane to formaldehyde using a photon-phonon cascade catalysis strategy. The use of a ZnO catalyst modified with single Ru atoms achieves unprecedented productivity and selectivity under mild conditions. This work offers a promising pathway for sustainable methane transformation and potentially reduces the environmental impact associated with formaldehyde production. Future research could explore other single-atom catalysts and optimization of reaction parameters to further enhance efficiency and scalability.

The study was conducted using a batch reactor, which may not directly translate to continuous flow systems. The scalability of the process needs further investigation. The long-term stability of the catalyst under industrial conditions requires further evaluation. The current study focused on optimizing the reaction conditions for formaldehyde production; further investigations could explore the potential for producing other valuable oxygenates from methane using this methodology.