Propylene epoxidation with O2 to propylene oxide is a very valuable reaction but remains as a long-standing challenge due to unavailable efficient catalysts with high selectivity. Herein, we successfully explore 27 nm-sized cubic Cu2O nanocrystals enclosed with {100} faces and {110} edges as a highly selective catalyst for propylene epoxidation with O2, which acquires propylene oxide selectivity of more than 80% at 90–110 °C. Propylene epoxidation with weakly-adsorbed O2 species at the {110} edge sites exhibits a low barrier and is the dominant reaction occurring at low reaction temperatures, leading to the high propylene oxide selectivity. Such a weakly-adsorbed O2 species is not stable at high reaction temperatures, and the surface lattice oxygen species becomes the active oxygen species to participate in propylene epoxidation to propylene oxide and propylene partial oxidation to acrolein at the {110} edge sites and propylene combustion to CO2 at the {100} face sites, which all exhibit high barriers and result in decreased propylene oxide selectivity.
A fundamental
understanding of interactions between catalysts and
gas molecules is essential for the development of efficient heterogeneous
catalysts. In this study, ambient pressure X-ray photoelectron spectroscopy
(APXPS) and density functional theory (DFT) simulation were employed
to investigate the activation of CO2 on Cu surfaces, which
acts as a key step in the catalytic reduction of CO2. APXPS
results show that CO2 is adsorbed as CO2
δ− on the Cu(111) surface under a pressure of
0.01 mbar at 300 K. Adsorbed CO2
δ− gets partially transformed into carbonate with an increase of pressure
to 1 mbar due to the disproportionation reaction between CO2 molecules. Subsequent annealing of the Cu(111) surface in a CO2 atmosphere leads to the dissociation of CO2
δ− and carbonate, and a transformation to a chemisorbed
oxygen covered surface occurred at 400 K and elevated temperatures.
However, on the Cu(110) surface, the CO2
δ− gradually dissociates to CO and chemisorbed oxygen in the presence
of 1 mbar of CO2 at room temperature. The self-deactivation
of CO2 adsorption due to the atomic oxygen generated by
CO2 dissociation is observed on both Cu(111) and Cu(110)
surfaces. Moreover, these experimental results indicate that the Cu(110)
surface is more active than the Cu(111) surface in breaking C–O
bonds, which is consistent with the results of DFT simulations. Our
findings indicate that the activation of CO2 on Cu surfaces
is strongly surface orientation- and pressure-dependent, which is
an important step to clarify CO2 activation mechanisms
on Cu-based catalysts.
Developing efficient electrocatalysts for the oxygen evolution reaction (OER) under neutral conditions is important for microbial electrolysis cells (MECs). However, the OER kinetics in neutral electrolytes at present are extremely sluggish, resulting in high overpotentials that greatly limit the energy conversion efficiencies of MECs. Previous studies failed to probe the adsorbates on surface metal sites of catalysts at the atomic scale and elucidate their influence on the catalytic activities, which has impeded the rational design of efficient neutral OER catalysts with optimal surface structures. Here, using in situ transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy ion scattering studies, we have identified, for the first time, that the electrochemically activated adsorbates on surface metal sites play a critical role in boosting the neutral OER activities of Ru-Ir binary oxide (Ru x Ir y O 2) catalysts. The adsorbate-activated Ru x Ir y O 2 on a glassy carbon electrode achieved a low overpotential of 324 mV at 10 mA cm −2 in neutral electrolyte, with a 36-fold improvement in turnover frequency compared with that of IrO 2 benchmark. Upon application in an MEC system, the resulting full cell showed a decreased voltage of 1.8 V, 200 mV lower than the best value reported to date, facilitating efficient synthesis of poly(3-hydroxybutyrate) from bioelectrochemical CO 2 reduction. Density functional theory (DFT) studies revealed that the enhanced OER activity of Ru x Ir y O 2 catalyst arose from local structural distortion of adjacent adsorbate-covered Ru octa-hedra at the catalyst surface and the consequently decreased adsorption energies of OER intermediates on Ir active center.
Methyl halides are versatile platform molecules, which have been widely adopted as precursors for producing value-added chemicals and fuels. Despite their high importance, the green and economical synthesis of the methyl halides remains challenging. Here we demonstrate sustainable and efficient photocatalytic methane halogenation for methyl halide production over copper-doped titania using alkali halides as a widely available and noncorrosive halogenation agent. This approach affords a methyl halide production rate of up to 0.61 mmol h−1 m−2 for chloromethane or 1.08 mmol h−1 m−2 for bromomethane with a stability of 28 h, which are further proven transformable to methanol and pharmaceutical intermediates. Furthermore, we demonstrate that such a reaction can also operate solely using seawater and methane as resources, showing its high practicability as general technology for offshore methane exploitation. This work opens an avenue for the sustainable utilization of methane from various resources and toward designated applications.
The
structure sensitivity of CO2 activation in the presence
of H2 has been identified by ambient-pressure X-ray photoelectron
spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical
reaction conditions. Based on the APXPS results and computer simulations,
we propose that, around room temperature, the hydrogen-assisted activation
of CO2 is the major reaction path on Ni(111), while the
redox pathway of CO2 prevails on Ni(110). With increasing
temperature, the two activation pathways are activated in parallel.
While the Ni(111) surface is fully reduced to the metallic state at
elevated temperatures, two stable Ni oxide species can be observed
on Ni(110). Turnover frequency measurements indicate that the low-coordinated
sites on Ni(110) promote the activity and selectivity of CO2 hydrogenation to methane. Our findings provide insights into the
role of low-coordinated Ni sites in nanoparticle catalysts for CO2 methanation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.