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In the deep-sea methane seepage environment, hydrate formation on the methane bubbles is a crucial pathway for methane transformation, and plays an important role in determining whether seepage methane can enter the surface ocean. While how the pressure determines the hydrate formation process in different water depths remains unclear. In this study, we investigated the formation kinetic characteristics and microstructure evolution of methane hydrate on suspended gas bubbles within varied pressure conditions. The pressure range (4–14 MPa) was in the deep-sea methane seepage environment. The results indicated that pressure obviously affected the hydrate film formation characteristics. As pressure increased, the lateral growth rate of the hydrate film accelerated, the hydrate crystal particles became smaller, and the initial hydrate film surface became smoother. Raman spectroscopy results showed that pressure changes did not obviously alter the microporous evolution on the hydrate film surface, that the gas-phase/water-phase Raman peak area ratio exhibited similar trends. The dissolved methane concentration under different pressure conditions was not the primary factor affecting hydrate thickening, rather, it depended mainly on the driving force. This study is of great significance in revealing the characteristics of methane hydrate formation and methane transfer in the oceanic methane seepage environment. • The increased pressure induces the initial formation of finer single hydrate crystals. • The elevated initial formation pressure enhances the collapse resistance of the hydrate shell. • Disparities in mass transfer driving force and microscopic film morphology are the primary reasons for varied hydrate formation characteristics. [ABSTRACT FROM AUTHOR]

Abstract Au nanoparticles and fine Ta 2 O 5 nanocrystallines were deposited on nonaggregated, spherical, monodisperse SiO 2 micorspheres to form a kind of sandwiched core-shell structured SiO 2 Au Ta 2 O 5 composite plasmonic photocatalyst by an in-situ reduction process integrated with a sol-gel process. The prepared SiO 2 Au Ta 2 O 5 composite plasmonic photocatalyst was nitrided in ammonia flow at an appropriate temperature to obtain SiO 2 Au Ta 3 N 5 composite plasmonic photocatalyst. Benefitting from Au nanoparticles' surface plasmon resonance effect, the obtained SiO 2 Au Ta 2 O 5 composite plasmonic photocatalyst presents a novel photochemical activity of H 2 production from H 2 O with visible light response, and the prepared SiO 2 Au Ta 3 N 5 composite plasmonic photocatalyst exhibits a higher photochemical activity of H 2 production from H 2 O with visible light response than that of blank Ta 3 N 5 nanocrystalline. Furthermore, the excellent morphologies of SiO 2 Au Ta 2 O 5 and SiO 2 Au Ta 3 N 5 composites, e.g., sandwiched core-shell structure, monodisperse, and spherical, drastically improve their dispersion in solution, light harvesting, charge separation and transportation, which might do some contributions to improve their photocatalytic activities accordingly. This investigation might set a good example for designing highly efficient plasmonic photocatalysts for photochemical solar H 2 production with visible light response. Graphical abstract Benefitting from the surface plasmon resonance effect, the monodisperse, submicron spherical and sandwiched core-shell structure SiO 2 Au Ta 2 O 5 and SiO 2 Au Ta 3 N 5 plasmonic photocatalysts show a novel photocatalytic activity and a significantly enhanced photocatalytic activity for hydrogen evolution from water with visible light response, respectively. Image 1 Highlights • The SiO 2 template-synthesis individualizes composites with excellent morphologies. • The excellent morphologies improve the photocatalytic efficiency of composites. • The SPR effect of Nano Au updates the photocatalytic efficiency of composites. [ABSTRACT FROM AUTHOR]

The kinetic characteristics governing methane conversion into solid hydrates within seabed sediments influence hydrate accumulation and resource distribution, which is significant for the evaluation of global natural gas hydrate reserves and the implication of hydrate development. However, the influence of sediment particle size on methane hydrate formation kinetics remains unclear. In this study, simulating in situ conditions of the seafloor, the kinetics behaviors of CH 4 hydrate formation within saline porous sediments were investigated. The experimental results indicate that the methane hydrate formation rate does not exhibit a strictly unidirectional change with changed quartz sand particle size. The unidirectionality and magnitude of hydrate formation rate change are simultaneously influenced by both quartz sand particle size and water saturation levels. The different effective gas-liquid contact area is the primary factor underlying the occurrence of these phenomena. However, for more porous clay sediments with greater specific surface area, the theory of effective gas-liquid contact area becomes inapplicable. The addition of clay to quartz sand sediment inversely inhibits the methane hydrate formation, which is caused by water absorption. This work can advance the comprehension of the intricate interplay between hydrate growth kinetics and the attributes of sedimentary hosts. • CH 4 hydrate formation rate does not exhibit a strictly unidirectional change with varying quartz sand particle size. • The unidirectionality of hydrate formation rate change are simultaneously influenced by particle size and water saturation. • Effective gas-liquid contact area determines hydrate formation rate rather than quartz sand particle size. • The inherent water-absorbing quality of porous clay instead results into reduced CH 4 hydrate formation kinetics. [ABSTRACT FROM AUTHOR]

Submarine CO 2 sequestration via solid hydrate has been considered a promising strategy for controlling greenhouse gas emissions. The simultaneous replacement of CH 4 from subsea natural gas hydrates through buried CO 2 offers a win-win approach. Utilizing H 2 as an enhancer to promote CH 4 –CO 2 hydrate replacement, coupled with integrating the replacement products into CH 4 steam reforming for cyclic H 2 production, presents a potential pathway for CO 2 geological storage and eventual H 2 recovery. However, the enhancement mechanism of H 2 in the hydrate replacement process remains unclear. Focusing on the critical replacement condition, this study investigates the effects of gas-phase partial pressures on hydrate replacement in CH 4 -rich systems to elucidate the H 2 influence mechanism. Remarkably, the results indicate, for the first time, that H 2 induces premature CH 4 hydrate dissociation under conditions exceeding the corresponding CH 4 gas equilibrium pressure in CO 2 /CH 4 /H 2 ternary gas systems. The critical condition for hydrate replacement depends on the total system pressure, rather than CO 2 gas partial pressure. Additionally, in the CH 4 -rich systems, the driving force for CH 4 hydrate decomposition takes precedence over that for hydrate reformation in controlling replacement. The replacement ratio increases from 16.7 % to 24.5 % as the CH 4 partial pressure decreases from 2.01 MPa to 1.11 MPa at 274.15 K, despite simultaneous declines in both CO 2 partial pressure and total system pressure from 1.70 MPa to 0.97 MPa and from 6.10 MPa to 3.39 MPa, respectively. These findings can provide insights into the improvement of CH 4 –CO 2 /H 2 hydrate replacement efficiency and the development of oceanic CO 2 sequestration. [Display omitted] • H 2 promotes the premature dissociation of CH 4 hydrates above their phase equilibrium pressure. • Critical condition for CO 2 hydrate replacement only depends on the total system pressure. • Replacement ratio from 16.7 % to 24.5 % with total pressure decreasing from 6.10 to 3.39 MPa at 274.15 K. • Combination of non-replacement gas sweep and reservoir restored could be an effective approach. [ABSTRACT FROM AUTHOR]