Carbon materials-supported Ru catalysts are some of the most efficient catalysts for ammonia synthesis. Usually they are prepared by impregnation for the dispersion of the Ru metal. In the present study, we report that a strong interaction of ruthenium species (with triruthenium dodecacarbonyl as the precursor) with a graphite structure enables the self-dispersion of Ru under solvent-free conditions. Via simple mixing and heat treatment, sub-nano Ru particles over high surface area graphite (HSAG) are obtained with uniform distribution. The ammonia yields or ammonia synthesis rate is almost 50% higher than the catalysts prepared via impregnation whether the support is activated carbon or HSAG. During catalyst preparation, Ru3(CO)12 is adsorbed on the surface of HSAG via CO groups in Ru3(CO)12 and surface oxygen groups on HSAG. Subsequently, Ru3(CO)12 undergoes decarbonylation reaction at very low temperatures. Then, CO has a dismutation reaction over the Ru surface leading to the formation of RuO2 at temperatures between 180 and 190 °C. During heat treatment, RuO2 was partially reduced by carbon at elevated temperatures, and the resulting Ru has strong interaction with HSAG, which further leads to the dispersion and stabilization of Ru nanoparticles.Keywords: Ammonia synthesis; Ball milling; Graphite; Interaction between metal and support; Ruthenium; Self-dispersion;

•γ-Al2O3 with 3/4 sphere and cabbage-like structures were prepared.•They are formed by the loose assembly of nano-sheets.•Cabbage-like alumina shows higher activity for Cl/F exchange reaction.•Cabbage-like structure tends to form more chlorofluoride structure.γ-Al2O3 with 3/4 sphere and cabbage-like structures were prepared via hydrothermal synthesis. Then they were evaluated as catalysts for the dismutation of HCFC-22 as a model reaction for Cl/F exchange reactions and compared with a commercial γ-Al2O3. The results indicate that both hydrothermal samples are formed by the loose assembly of nano-sheets. For cabbage-like alumina, hollow structure is detected. Aluminum chlorofluoride (ACF) structure with strong Lewis acidity is beneficial for dismutation of HCFC-22. Following pre-fluorination by HCFC-22, the high activity of cabbage-like γ-Al2O3 is attributed to the high content of ACF. It indicates that cabbage-like structure tends to form more ACF structure.Download high-res image (183KB)Download full-size image

Hydrofluorocarbons (HFCs) which are usually potent greenhouse gases are regulated by the Montreal Protocol and its amendments, especially the recent Kigali Amendment. Dehydrofluorination of HFCs is an efficient route for the conversion of these greenhouse gases to value added and environmentally benign chemicals. Although AlF3 with strong Lewis acidity catalyzes dehydrofluorination, it also favors carbon deposition. MgF2 with weak acidity inhibits coking significantly. Unfortunately, MgF2 sinters dramatically at temperatures below 300 °C leading to the low activity for dehydrofluorination. In the present work, we report that sub-nano MgF2 embedded in carbon fibers and electrospun MgF2 fibers prevent sintering of MgF2 during dehydrofluorination reaction. Via simple and one-step electrospinning and calcination in a N2 (for embedded MgF2) or air (for MgF2 fibers) atmosphere, embedded MgF2 with particle sizes between 3–6 nm and pure MgF2 fibers with diameters of around 100 nm were fabricated. No sintering was observed following reaction at 450 °C. The fine MgF2 particles and MgF2 fibers facilitate the formation of under coordinated Mg species in MgF2 which are the weak acid sites. By embedding MgF2 or fabrication of MgF2 fibers, weak acid sites are increased significantly, while strong acid sites remain almost unchanged. Hence, they show significantly higher reaction rates than MgF2 prepared by precipitation of Mg(CH3COO)2·4H2O with NH4F for the dehydrofluorination of 1,1,1-trifluoroethane (HFC-143a) to VDF (CH2CF2) at 450 °C.

A series of high surface area graphitic carbon materials (HSGCs) were prepared by ball-milling method. Effect of the graphitic degree of HSGCs on the catalytic performance of Ba-Ru-K/HSGC-x (x is the ball-milling time in hour) catalysts was studied using ammonia synthesis as a probe reaction. The graphitic degree and pore structure of HSGC-x supports could be successfully tuned via the variation of ball-milling time. Ru nanoparticles of different Ba-Ru-K/HSGC-x catalysts are homogeneously distributed on the supports with the particle sizes ranging from 1.6 to 2.0 nm. The graphitic degree of the support is closely related to its facile electron transfer capability and so plays an important role in improving the intrinsic catalytic performance of Ba-Ru-K/HSGC-x catalyst.The graphitic degree of high surface area graphitic carbons plays an important role in improving the intrinsic catalytic performance of HSGCs supported ruthenium catalysts for ammonia synthesis.Download full-size image