Single-atom catalysts for photocatalytic energy conversion

Direct oxidation of methane to valuable chemicals is a great challenge as catalysts with both high activity and selectivity for the activation of inert C–H bonds are required. Here, we report the highly efficient and selective photo-oxidation of methane to ethane with dioxygen in a flow reactor using a Au nanoparticle (NP) loaded ZnO/TiO2 hybrid. An ethane production rate of over 5,000 μmol g−1 h−1 with 90% selectivity is achieved, which is more than one order of magnitude higher than the state-of-the-art photocatalytic systems. Detailed characterizations and theoretical studies show that the formation of heterojunctions between ZnO and TiO2 leads to enhanced photocatalytic activity, while maintaining high selectivity owing to the weak overoxidation ability of the main component ZnO. Moreover, the Au cocatalyst enables the facile desorption of methyl (CH3) species as •CH3 radicals in the gas phase, thereby facilitating C2H6 formation and inhibiting overoxidation of CH4 to CO2. Photocatalytic oxidation of methane through oxidative coupling presents a route to higher hydrocarbons but has suffered from low activity and uncontrolled product selectivity. Now, Au nanoparticles loaded onto a ZnO/TiO2 heterostructure are shown to deliver high rate production of ethane.

A selective Au-ZnO/TiO2 hybrid photocatalyst for oxidative coupling of methane to ethane with dioxygen

Abstract Methane (CH 4 ) oxidation to high value chemicals under mild conditions through photocatalysis is a sustainable and appealing pathway, nevertheless confronting the critical issues regarding both conversion and selectivity. Herein, under visible irradiation (420 nm), the synergy of palladium (Pd) atom cocatalyst and oxygen vacancies (OVs) on In 2 O 3 nanorods enables superior photocatalytic CH 4 activation by O 2 . The optimized catalyst reaches ca. 100 μmol h −1 of C1 oxygenates, with a selectivity of primary products (CH 3 OH and CH 3 OOH) up to 82.5%. Mechanism investigation elucidates that such superior photocatalysis is induced by the dedicated function of Pd single atoms and oxygen vacancies on boosting hole and electron transfer, respectively. O 2 is proven to be the only oxygen source for CH 3 OH production, while H 2 O acts as the promoter for efficient CH 4 activation through ·OH production and facilitates product desorption as indicated by DFT modeling. This work thus provides new understandings on simultaneous regulation of both activity and selectivity by the synergy of single atom cocatalysts and oxygen vacancies. 

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Synergy of Pd atoms and oxygen vacancies on In2O3 for methane conversion under visible light

Methane hydrate and shale gas are predicted to have substantial reserves, far beyond the sum of other fossil fuels. Using methane instead of crude oil as a building block is, thus, a very attractive strategy for synthesizing valuable chemicals. Because methane is so inert, its direct conversion needs a high activation energy and typically requires harsh reaction conditions or strong oxidants. Photocatalysis, which employs photons operated under very mild conditions, is a promising technology to reduce the thermodynamic barrier in direct methane conversion and to avoid the common issues of overoxidation and catalyst deactivation. In this Review, we cover the development of photocatalysts and co-catalysts, including the use of inorganic materials and polymeric semiconductors, and explain how the use of batch or flow reaction systems affects the reaction kinetics and product selectivity. We also discuss efforts to understand the underlying reaction mechanisms from both a photophysical and a chemical perspective. Finally, we present our view of the challenges facing this field and suggest potential solutions. Methane, as a replacement for crude oil, can be converted into valuable chemicals through photocatalysis. This Review discusses the design of promising photocatalytic systems and strategies to understand their reaction mechanisms and outlines the most pressing questions still facing the field. 

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Methane transformation by photocatalysis

Nonoxidative coupling of methane (NOCM) is a highly important process to simultaneously produce multicarbons and hydrogen. Although oxide-based photocatalysis opens opportunities for NOCM at mild condition, it suffers from unsatisfying selectivity and durability, due to overoxidation of CH4 with lattice oxygen. Here, we propose a heteroatom engineering strategy for highly active, selective and durable photocatalytic NOCM. Demonstrated by commonly used TiO2 photocatalyst, construction of Pd-O4 in surface reduces contribution of O sites to valence band, overcoming the limitations. In contrast to state of the art, 94.3% selectivity is achieved for C2H6 production at 0.91 mmol g-1 h-1 along with stoichiometric H2 production, approaching the level of thermocatalysis at relatively mild condition. As a benchmark, apparent quantum efficiency reaches 3.05% at 350 nm. Further elemental doping can elevate durability over 24 h by stabilizing lattice oxygen. This work provides new insights for high-performance photocatalytic NOCM by atomic engineering. 

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High-performance photocatalytic nonoxidative conversion of methane to ethane and hydrogen by heteroatoms-engineered TiO2

The high inertness of the C–H bond makes the photocatalytic methane conversion a significant challenge. The platinum nanoparticle is a promising cocatalyst for CH4 activation, while the study of it...

Exploring the Size Effect of Pt Nanoparticles on the Photocatalytic Nonoxidative Coupling of Methane

Photodriven nonoxidative coupling of CH 4 (NOCM) is an attractive potential way to use abundant methane resources. Herein, a n-type doped photocatalyst for NOCM is created by doping single-atom Nb into hierarchical porous TiO 2 -SiO 2 (TS) microarray, which exhibits a high conversion rate of 3.57 μmol g -1 h -1 with good recyclability. The Nb dopant replaces the 6-coordinated titanium on the (1 0 1) surface and forms shallow electron-trapped surface polarons along [0 1 0] direction as verified by the experimental analyses and the first-principle density functional theory calculation. The comparison of different models proves that the electron localization caused by the n-type doping is beneficial to both the methane activation and ethane desorption. The positive effect of n-type dopant on CH 4 conversion is further verified on Mo-, W- and Ta-doped composites. In contrast, the doping of p-type dopant (Ga, Cu, Fe) shows less active influence. This study provides an in-depth insight into the effect of dopant type and is expected to guide the smart photocatalyst design for CH 4 conversion.

Non‐oxidative Coupling of Methane: N‐type Doping of Niobium Single Atoms in TiO2–SiO2 Induces Electron Localization

Although it is well-known that the size can influence the surface plasmon resonance property of coinage metals and the electronic state of the Mott-Schottky junction formed at the metal/semiconductor interface, insights into how the size can be exploited to optimize the photocatalytic activity and selectivity of metal/semiconductor composites are lacking. Here we utilize operando SERS spectroscopy to identify the size effect on the electron-transfer dynamics and the direction at the Au/TiO2 interface. This effect was characterized by the photocatalytic reduction sites of p-nitrothiophenol, which were self-tracked with the SERS spectra from Au nanoparticle and inverse-opal structured TiO2, respectively. The size-dependent unidirectional/bidirectional transfer of photoinduced electrons at the Au/TiO2 interface was revealed by operando SERS spectroscopy, which enables the rational tuning of the reduction selectivity.

Unidirectional/Bidirectional Electron Transfer at the Au/TiO2 Interface Operando Tracked by SERS Spectra from Au and TiO2

Amorphous MoO3-x nanosheets were fabricated by the room-temperature oxidation of molybdenum powder with H2 O2 , followed by light-irradiation reduction in methanol. When applied as a substrate for surface-enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO3-x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H2 O2 -based Fenton-like reaction. More importantly, the non-invasive SERS was unprecedentedly used for the self-diagnosis of amorphous MoO3-x as a more selective photocatalyst than its crystalline counterpart.

Jiayu Ma

Papers

Exploring the Size Effect of Pt Nanoparticles on the Photocatalytic Nonoxidative Coupling of Methane

Non‐oxidative Coupling of Methane: N‐type Doping of Niobium Single Atoms in TiO2–SiO2 Induces Electron Localization

Unidirectional/Bidirectional Electron Transfer at the Au/TiO2 Interface Operando Tracked by SERS Spectra from Au and TiO2

Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment