Restructuring of titanium oxide overlayers over nickel nanoparticles during catalysis.

Reducible supports can affect the performance of metal catalysts by the formation of suboxide overlayers upon reduction, a process referred to as the strong metal-support interaction (SMSI). A combination of operando electron microscopy and vibrational spectroscopy revealed that thin TiOx overlayers formed on nickel/titanium dioxide catalysts during 400°C reduction were completely removed under carbon dioxide hydrogenation conditions. Conversely, after 600°C reduction, exposure to carbon dioxide hydrogenation reaction conditions led to only partial reexposure of nickel, forming interfacial sites in contact with TiOx and favoring carbon-carbon coupling by providing a carbon species reservoir. Our findings challenge the conventional understanding of SMSIs and call for more-detailed operando investigations of nanocatalysts at the single-particle level to revisit static models of structure-activity relationships.

The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks.

Capturing and converting CO2 through artificial photosynthesis using photoactive, porous materials is a promising approach for addressing increasing CO2 concentrations. Porphyrinic Zr-based metal-organic frameworks (MOFs) are of particular interest as they incorporate a photosensitizer in the porous structure. Herein, the initial step of the artificial photosynthesis is studied: CO2 sorption and activation in the presence of water. A combined vibrational and visible spectroscopic approach was used to monitor the adsorption of CO2 into PCN-222 and PCN-223 MOFs, and the photophysical changes of the porphyrinic linker as a function of water concentration. A shift in CO2 sorption site and bending of the porphyrin macrocycle in response to humidity was observed, and CO2/H2O competition experiments revealed that the exchange of CO2 with H2O is pore-size dependent. Therefore, humidity and pore-size can be used to tune CO2 sorption, CO2 capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO2 photoreduction.

Structure Sensitivity of CO2 Conversion over Nickel Metal Nanoparticles Explained by Micro-Kinetics Simulations.

Nickel metal nanoparticles are intensively researched for the catalytic conversion of carbon dioxide. They are commercially explored in the so-called power-to-methane application in which renewably resourced H2 reacts with CO2 to produce CH4, which is better known as the Sabatier reaction. Previous work has shown that this reaction is structure-sensitive. For instance, Ni/SiO2 catalysts reveal a maximum performance when nickel metal nanoparticles of ∼2-3 nm are used. Particularly important to a better understanding of the structure sensitivity of the Sabatier reaction over nickel-based catalysts is to understand all relevant elementary reaction steps over various nickel metal facets because this will tell as to which type of nickel facets and which elementary reaction steps are crucial for designing an efficient nickel-based methanation catalyst. In this work, we have determined by density functional theory (DFT) calculations and micro-kinetics modeling (MKM) simulations that the two terrace facets Ni(111) and Ni(100) and the stepped facet Ni(211) barely show any activity in CO2 methanation. The stepped facet Ni(110) turned out to be the most effective in CO2 methanation. Herein, it was found that the dominant kinetic route corresponds to a combination of the carbide and formate reaction pathways. It was found that the dissociation of H2CO* toward CH2* and O* is the most critical elementary reaction step on this Ni(110) facet. The calculated activity of a range of Wulff-constructed nickel metal nanoparticles, accounting for varying ratios of the different facets and undercoordinated atoms exposed, reveals the same trend of activity-versus-nanoparticle size, as was observed in previous experimental work from our research group, thereby providing an explanation for the structure-sensitive nature of the Sabatier reaction.

In situ observations of an active MoS2 model hydrodesulfurization catalyst.

The hydrodesulfurization process is one of the cornerstones of the chemical industry, removing harmful sulfur from oil to produce clean hydrocarbons. The reaction is catalyzed by the edges of MoS2 nanoislands and is operated in hydrogen-oil mixtures at 5-160 bar and 260-380 °C. Until now, it has remained unclear how these harsh conditions affect the structure of the catalyst. Using a special-purpose high-pressure scanning tunneling microscope, we provide direct observations of an active MoS2 model catalyst under reaction conditions. We show that the active edge sites adapt their sulfur, hydrogen, and hydrocarbon coverages depending on the gas environment. By comparing these observations to density functional theory calculations, we propose that the dominant edge structure during the desulfurization of CH3SH contains a mixture of adsorbed sulfur and CH3SH.

Computational assessment of the environmental fate, bioaccumulation, and toxicity potential of brominated benzylpolystyrene.

Brominated benzylpolystyrene (BrBPS) is a fire safety polymer, which imparts flame retardancy in a variety of styrenic polymers and resins. In this study, the individual components of BrBPS were evaluated using a novel screening process to inform various endpoints relevant for assessing risks to the environment and human health. For this purpose, representative components were created using the B3LYP density functional method to generate the energetically optimal bromine substitution patterns of the parent molecules toluene, ethylbenzene, and cumene. Representative low energy conformations of the components were identified by using repetitive short bursts of Molecular Dynamics, followed by Molecular Mechanics minimization. The resulting structures were energy minimized at the quantum mechanical Becke-Perdew GGA density functional level. Thereafter, octanol:water partition coefficients, maximum molecular lengths, average maximum diameters, and cross-sectional diameters were calculated to inform parameters on environmental fate, bioaccumulation, and mammalian and ecological toxicity. The relevance of these data for informing risk assessment is discussed.