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Abstract: Designing CO2 methanation catalysts that meet industrial requirements is still challenging. We report Ni-Fe hydrotalcite-derived catalysts with a wide range of Ni and Mg loadings showing that an optimised composition with Ni0.4 gives a very high CO2 conversion rate of 0.37 mmol/gcat/s at 300°C. This catalyst is studied by in-situ APXPS and NEXAFS spectroscopies and compared with the other synthesised samples to obtain new mechanistic insights on methanation catalysts active for low-temperature (300°C) methanation, which is an industrial requirement. Under methanation conditions, in-situ investigations revealed the presence of metallic Ni sites and low nuclearity Ni-Fe species at (Ni loading) = 21.2 mol%. These sites are oxidised on the low Ni-loaded catalyst ( = 9.2 mol%). The best CO2 conversion rate and CH4 selectivity are shown at intermediate (21.2 mol%), in the presence of Mg. These superior performances are related to the high metallic surface area, dispersion, and optimal density of basic sites. The (turnover frequency of CO2 conversion) increases exponentially with the fractional density of basic to metallic sites ( ) from 1.1 s-1 ( = 29.2 mol%) to 9.1 s-1 ( = 7.6 mol%). It follows the opposite trend of the CO2 conversion rate. In-situ DRIFTS data under methanation conditions evidence that the at high is related to the presence of a formate route which is not predominant at low (high ). A synergistic interplay of basic and metallic sites is present. This contribution provides a rationale for designing industrially competitive CO2 methanation catalysts with high catalytic activity while maintaining low Ni loading.

Open Access

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Abstract: A family of boron nitride (BN)-based photocatalysts for solar fuel syntheses have recently emerged. Studies have shown that oxygen doping, leading to boron oxynitride (BNO), can extend light absorption to the visible range. However, the fundamental question surrounding the origin of enhanced light harvesting and the role of specific chemical states of oxygen in BNO photochemistry remains unanswered. Here, using an integrated experimental and first-principles-based computational approach, we demonstrate that paramagnetic isolated OB3 states are paramount to inducing prominent red-shifted light absorption. Conversely, we highlight the diamagnetic nature of O–B–O states, which are shown to cause undesired larger band gaps and impaired photochemistry. This study elucidates the importance of paramagnetism in BNO semiconductors and provides fundamental insight into its photophysics. The work herein paves the way for tailoring of its optoelectronic and photochemical properties for solar fuel synthesis.

Open Access

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Abstract: Surface oxidation chemistry involves the formation and breaking of metal–oxygen (M–O) bonds. Ideally, the M–O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O2– species within the atomic Cu–O–Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O2– has a lower antibonding orbital energy and thus weaker Cu–O/Fe–O strength. In selective NH3 oxidation, the weak Cu–O/Fe–O bond enables fast Cu redox for NH3 conversion and direct NO adsorption via Cu–O–NO to promote N–N coupling toward N2. As a result, 99% N2 selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O2– in catalysis.

Open Access

Open Access

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Abstract: Photoelectron spectroscopy is a powerful characterisation tool for semiconductor surfaces and interfaces, providing in principle a correlation between the electronic band structure and surface chemistry along with quantitative parameters such as the electron affinity, interface potential, band bending and band offsets. However, measurements are often limited to ultrahigh vacuum and only the top few atomic layers are probed. The technique is seldom applied as an in situ probe of surface processing; information is usually provided before and after processing in a separate environment, leading to a reduction in reproducibility. Advances in instrumentation, in particular electron detection has enabled these limitations to be addressed, for example allowing measurement at near-ambient pressures and the in situ, real-time monitoring of surface processing and interface formation. A further limitation is the influence of the measurement method through irreversible chemical effects such as radiation damage during X-ray exposure and reversible physical effects such as the charging of low conductivity materials. For wide-gap semiconductors such as oxides and carbon-based materials, these effects can be compounded and severe. Here we show how real-time and near-ambient pressure photoelectron spectroscopy can be applied to identify and quantify these effects, using a gold alloy, gallium oxide and semiconducting diamond as examples. A small binding energy change due to thermal expansion is followed in real-time for the alloy while the two semiconductors show larger temperature-induced changes in binding energy that, although superficially similar, are identified as having different and multiple origins, related to surface oxygen bonding, surface band-bending and a room-temperature surface photovoltage. The latter affects the p-type diamond at temperatures up to 400 °C when exposed to X-ray, UV and synchrotron radiation and under UHV and 1 mbar of O2. Real-time monitoring and near-ambient pressure measurement with different excitation sources has been used to identify the mechanisms behind the observed changes in spectral parameters that are different for each of the three materials. Corrected binding energy values aid the completion of the energy band diagrams for these wide-gap semiconductors and provide protocols for surface processing to engineer key surface and interface parameters.