Ni Facets Research Articles

Nanostructure of Indium-driven nickel catalysts break CO2 hydrogenation preference

Identifying the key factors governing the selectivity of CO2 hydrogenation catalysts remains a formidable challenge. Herein, we elucidate the complex interaction between active sites nanostructure and selectivity in CO2 hydrogenation through in-depth characterization and theoretical simulations. NiIn alloy formed on the Ni facets with hydrogen reduction, constructing a core-shell-like structure. Ni-mediated methane formation was suppressed almost entirely upon alloying with indium, allowing the reaction pathway shifted from methanation into the reverse-water-gas-shift (RWGS). On the contact surface, In restricted CO adsorption on Ni, thereby preventing methane generation from CO via successive hydrogenation. DFT calculations confirmed that Ni and CO are negatively charged and repel each other due to electron transfer from indium, resulting in a decrease in adsorption energy, which is crucial for the pathway shift. Understanding the regulation of Ni-In structure in product distribution is expect to provide valuable insights into the precise design of catalysts with controllable product selectivity.

Understanding the transformation mechanism of carbon species on different Nickle facets in CO2 reforming reaction

As human industry continues to develop, the concentration of greenhouse gases in the atmosphere continues to rise, causing certain impacts on human daily production and life. The dry reforming of methane (DRM) reaction can simultaneously utilize both CH4 and CO2 greenhouse gases, thus achieving the goal of turning waste into treasure. This study uses density functional theory (DFT) to simulate the transformation of carbon species on Ni(111), Ni(100), Ni(211), and Ni(311) facets from a kinetic perspective. These transformations include the carbon growth paths of CH*→C*+H* and C*+C*→C2, as well as the carbon elimination paths of CH*+O*→CHO*, C*+O*→CO*, and C*+CO2*→2CO*. Combining the energy barrier of different reaction paths, it is found that the Ni(111) facet exhibits the best ability to inhibit carbon growth and elimination, while the Ni(100) facet has weaker carbon elimination capability. Carbon is prone to deposition and difficult to be eliminated on step facets of Ni(211) and Ni(311). Furthermore, the present study investigates the binding relationship between different Ni facets and O*, finding that Ni(111) and Ni(311) facets have strong interaction ability with O*, which further explains the reason that the lower energy barrier of Ni(111) and Ni(311) facets in both CH*+O* and C*+O* reaction paths.

First-Principles Calculation and Multi-Scale Observation for a Carbon Deposition on a SOFC Anode Near the Interface

Carbon deposition site in the Ni/YSZ was studied through microscale observation and first-principles calculation. Early stage of carbon deposition was appropriate reaction to determine the active site because the carbon worked as a visible marker. SEM observation showed that the carbon deposition was progressed not in the overall Ni surface but in the specific sites such as the Ni/YSZ interfaces at low temperature. STEM-HAADF showed that the graphite layer was not found on the Ni(111) surface but on the Ni(112) surface, indicating that the unsaturated step facets such as Ni(112) enhanced the carbon deposition. Similar trends were found in the first-principles calculation. The CH adsorption energy in the Ni(112) facet was higher than that in the Ni(111) facet. In addition, it was suggested that the YSZ structure also influenced the carbon deposition as well as the Ni facet.

Theoretical Investigation of Solvent Effects on the Hydrodeoxygenation of Propionic Acid over a Ni(111) Catalyst Model

The effect of two solvents, liquid water and 1,4-dioxane, has been studied from first-principles on the hydrodeoxygenation of propionic acid over a Ni(111) catalyst surface model. A mean-field microkinetic model was developed to investigate these effects at a temperature of 473 K. Under all reaction conditions, a decarbonylation mechanism is favored significantly over a decarboxylation pathway. Although no significant solvent effects were observed on the decarbonylation rate, a substantial solvent stabilization of two key surface intermediates in the decarboxylation mechanism, CH3CCOO and CH3CHCOO, leads to a notable increase of the decarboxylation rate by 2 orders of magnitude in liquid water and by 1 order of magnitude in liquid 1,4-dioxane. Furthermore, a significant solvent stabilization of the transition state of C–H bond cleavage of the α-carbon of CH3CHCO, relative to the stabilization of the C–C bond cleavage of the α-carbon of CH3CHCO, leads to a change in dominant pathway in the liquid phase environments. Finally, a sensitivity analysis shows that the C–OH bond cleavage of propionic acid and C–C bond cleavage of the α-carbon of CH3CHCO are the most rate controlling states in the gas phase. In contrast, in solvents the dehydrogenation of CH3CHCO becomes the most influential step. This shift in rate controlling state is attributed to the solvent effect on the dehydrogenation of CH3CHCO, which is facilitated in the aqueous phase. Overall, it is likely that the investigated (111) facet of Ni is not active for the hydrodeoxygenation of propionic acid in either the gas or the liquid phase and other Ni facets or phases must be responsible for the experimentally observed kinetics.

Effects of structure and size of Ni nanocatalysts on hydrogen selectivity via water-gas-shift reaction—A first-principles-based kinetic study

The effects of structure and size of nickel nanocatalysts on hydrogen production via water-gas shift reaction (WGSR) were investigated using a first-principles-based kinetic model. Using periodic density functional theory and statistical calculations, thermochemistry and kinetics of the WGSR and competing methanation was calculated on Ni(111), Ni(100), and Ni(211) facets. The kinetics of the elementary reactions involving CH, OH, and CO bond was found to fit to a general Brønsted–Evans–Polanyi (BEP) type linear relationship on all Ni facets considered. A mechanism describing the competition between the hydrogen and methane formation routes is constructed for further microkinetic modeling. The hydrogen production turnover frequency (TOF) via the WGSR route suggests the preference to the low-coordinated surface sites with the reaction activities following the order of Ni(211)>Ni(100)>Ni(111) using a simulated feed gas with a molar ratio of CO:H2O=1:2. Due to the methanation, the TOF of methane production follows the same trend of hydrogen production. Consequently, the TOF of hydrogen production decreases with increasing particle diameters, due to the decreasing fractions of low-coordinated surface nickel atoms. It is also found that the presence of H2 in feed gas can largely enhance the methanation reaction.

Self-assembly of faceted Ni nanodots on Si(111)

We report the formation of Ni nanodots on Si(111). Island density is varied by annealing temperature and time and is studied using atomic force microscopy (AFM) and magnetic force microscopy. Activation energies of 0.09±0.02 and 0.31±0.05eV are observed for the formation of these islands. These are associated with Ni surface self-diffusion across the (111) and (110) Ni facets, respectively. For brief 500°C anneals, regular nanodots are observed with self-limiting sizes of height ∼16nm and area 180nm×260nm, while density exhibits a power-law time dependence with exponent 1.13±0.12. AFM analysis reveals a “truncated hut” shape consistent with (110) top and (111) sidewall surfaces.