Photodriven
nonoxidative coupling of CH4 (NOCM) is a
potential alternative approach to clean hydrogen and hydrocarbon production.
Herein, a Mott–Schottky photocatalyst for NOCM is fabricated
by loading Pt nanoclusters on a Ga-doped hierarchical porous TiO2–SiO2 microarray with an anatase framework,
which exhibits a CH4 conversion rate of 3.48 μmol
g–1 h–1 with 90% selectivity toward
C2H6. This activity is 13 times higher than
those from microarrays without Pt and Ga. Moreover, a continuous H2 production (36 μmol g–1) with a high
CH4 conversion rate of ∼28% can be achieved through
a longtime irradiation (32 h). The influence of Ga on the chemical
state of a surface oxygen vacancy (Vo) and deposited Pt is investigated
through a combination of experimental analysis and first-principles
density functional theory calculations. Ga substitutes for the five-coordinated
Ti next to Vo, which tends to stabilize the single-electron trapped
Vo and reduce the electron transfer from Vo to the adsorbed Pt, resulting
in the formation of a higher amount of cationic Pt. The cationic Pt
and electron-enriched metallic Pt form a cationic–anionic active
pair, which is more efficient for the dissociation of C–H bonds.
However, the presence of too much cationic Pt results in more C2+ product with a decrease in the CH4 conversion
rate due to the reduced charge-carrier separation efficiency. This
study provides deep insight into the effect of the doping/loading
strategy on the photocatalytic NOCM reaction and is expected to shed
substantial light on future structural design and modulation.
Photodriven nonoxidative coupling of CH4 (NOCM) is an attractive potential way to use abundant methane resources. Herein, an n‐type doped photocatalyst for NOCM is created by doping single‐atom Nb into hierarchical porous TiO2–SiO2 (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) plane and forms shallow electron‐trapped surface polarons along [0 1 0] direction and the comparison of different models proves that the electron localization caused by the n‐type doping is beneficial to both methane activation and ethane desorption. The positive effect of n‐type dopant on CH4 conversion is further verified on Mo‐, W‐ and Ta‐doped composites. In contrast, the doping of p‐type dopant (Ga, Cu, Fe) shows a less active influence.
Synergistic
nitrogen reduction and water oxidation process is significant
to the photocatalytic fixation of nitrogen. However, the coupling
mechanism remains ambiguous and lacks study. Herein, we report enhanced
photocatalytic nitrogen fixation on single-atom Fe-modified macro-/mesoporous
TiO2-SiO2 (Fe-T-S), with a high ammonia generation
rate of 32 μmol g–1 h–1 without
any sacrificial agent and cocatalysts. Experimental and theoretic
calculation studies confirmed the formation of a photoinduced hole-trapping
polaron on the Fe dopant, resulting in the high-valent Fe(IV) species.
The single-atom Fe(IV) site is responsible for water oxidation and
helps promote N2 hydrogenation on neighboring oxygen vacancy.
This study explicitly unravels the key to achieve the coupling between
photocatalytic N2 hydrogenation and water oxidation through
a doping strategy and provides significant guidance for the rational
design of photocatalysts for ammonia synthesis.
Surface-enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single-molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble-metal-free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble-metal-free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene-based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble-metal-free materials for enhancing detection sensitivity can further fuel the development of SERS-related applications.
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