Carbon clusters on the Ni(111) surface: a density functional theory study.

To understand the nucleation of carbon atoms to form carbon clusters on transition metal substrates during chemical vapor deposition (CVD) synthesis, the structure, energetics, and mobility of carbon intermediates up to 6 atoms on the Ni(111) surface were investigated using Density Functional Theory (DFT). Carbon clusters were found to be more thermodynamically stable than adsorbed atomic carbon, with linear carbon structures being more stable than branched and ring structures. Carbon chains were also found to have higher mobility than branched configurations. The interaction energy between carbon clusters and the Ni surface shows that branched carbon clusters have stronger interaction with the Ni substrate when compared with the carbon chains, supporting that carbon chains generally have higher mobility than branched clusters. The transition states and energy barriers for the formation of different carbon clusters were also studied. The results show that the formation of the branched configurations is kinetically favored as it presents lower energy barriers than those obtained for carbon chains.

Influence of Surface Properties of Nanostructured Ceria-Based Catalysts on Their Stability Performance.

As the poor cycling stability of CeO2 catalysts has become the major obstacle for applications of diesel particulate filters (DPF), it is necessary to investigate how to reduce their structural and compositional changes during soot oxidation. In this study, different ratios of Samarium (Sm) were doped into the lattice of CeO2 nanoparticles to improve the catalytic performance as well as surface properties. The stability was investigated by recycling the catalyst, mixing it with soot again, and repeating the thermogravimetric analysis (TGA) tests seven times. Consistent observations were expected for more cycles. It was found that doping 5%, 10%, and 20% samarium into the CeO2 lattice can improve the catalyst stability but at the cost of losing some activity. While the catalyst became more stable with the increasing Sm doping, the 10% Sm-doped catalyst showed the best compromise between stability and activity. Ce3+ and Oα were found to play important roles in controlling catalytic soot oxidation activity. These two species were directly related to oxygen vacancies and oxygen storage capacity of the catalyst. Sm-doped catalysts showed a minimized decrease in the Ce3+ and Oα content when the fresh and spent catalysts were compared.

Modeling study of the heat of absorption and solid precipitation for CO2 capture by chilled ammonia.

The contribution of individual reactions to the overall heat of CO2 absorption, as well as conditions for solid NH4HCO3(s) formation in a chilled ammonia process (CAP) were studied using Aspen Plus at temperatures between 2 and 40 °C. The overall heat of absorption in the CAP first decreased and then increased with increasing CO2 loading. The increase in overall heat of absorption at high CO2 loading was found to be caused mostly by the prominent heat release from the formation of NH4HCO3(s). It was found that NH4HCO3(s) precipitation was promoted for conditions of CO2 loading above 0.7 mol CO2/mol NH3 and temperatures less than 20 °C, which at the same time can dramatically increase the heat of CO2 absorption. As such, the CO2 loading is recommended to be around 0.6-0.7 mol CO2/mol NH3 at temperatures below 20 °C, so that the overall absorption heat is at a low state (less than 60 kJ mol-1 CO2). It was also found that the overall heat of CO2 absorption did not change much with temperature when CO2 loading was less than 0.5 mol CO2/mol NH3, while, when the CO2 loading exceeded 0.7 mol CO2/mol NH3, the heat of absorption increased with decreasing temperature.

Real-Time Observation of Carbon Oxidation by Driven Motion of Catalytic Ceria Nanoparticles within Low Pressure Oxygen.

Carbon particulate matter (PM) is an undesirable aerosol pollutant formed from combustors such as power plants, refineries, and engines. The most common and effective method of mitigating PM emission is the capture of particulates using a filter, before particles are released into the atmosphere. In order to develop and improve advanced filtering materials, a better understanding is required of their chemical and mechanical behavior. We report on a novel phenomenon on the mobility and oxidation behavior of catalytic iron doped ceria nanoparticles in contact with mobile carbon black nanoparticles. The process is recorded by real time imaging within an environmental transmission electron microscope. In contrast to observations in previous studies, the separated ceria nanoparticles are found to actively move on the substrate and consume the connecting carbon particles one-by-one. The velocity of particle motion is correlated to the reaction temperature and oxygen pressure, both determining the reaction rate. Modeling using the Density Functional Theory suggests this motion is driven by the chemical bonding between the surface oxygen of the catalyst and the graphite layers of carbon black, initiated through the Van der Waals force between two types of nanoparticles.

Design of a sorbent to enhance reactive adsorption of hydrogen sulfide.

A series of novel zinc oxide-silica composites with three-dimensionally ordered macropores (3DOM) structure were synthesized via colloidal crystal template method and used as sorbents for hydrogen sulfide (H2S) removal at room temperature for the first time. The performances of the prepared sorbents were evaluated by dynamic breakthrough testing. The materials were characterized before and after adsorption using scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that the composite with 3DOM structure exhibited remarkable desulfurization performance at room temperature and the enhancement of reactive adsorption of hydrogen sulfide was attributed to the unique structure features of 3DOM composites; high surface areas, nanocrystalline ZnO and the well-ordered interconnected macroporous with abundant mesopores. The introduction of silica could be conducive to support the 3DOM structure and the high dispersion of zinc oxide. Moisture in the H2S stream plays a crucial role in the removal process. The effects of Zn/Si ratio and the calcination temperature of 3DOM composites on H2S removal were studied. It demonstrated that the highest content of ZnO could reach up to 73 wt % and the optimum calcination temperature was 500 °C. The multiple adsorption/regeneration cycles showed that the 3DOM ZnO-SiO2 sorbent is stable and the sulfur capacity can still reach 67.4% of that of the fresh sorbent at the fifth cycle. These results indicate that 3DOM ZnO-SiO2 composites will be a promising sorbent for H2S removal at room temperature.