Preparation of isolated Co3O4 and fcc-Co crystallites in the nanometre range employing exfoliated graphite as novel support material

Thermodynamic assessment of the stability of bulk and nanoparticulate cobalt and nickel during dry and steam reforming of methane

The high reaction temperatures during steam and dry reforming of methane inevitably entail catalyst deactivation. Evaluation of the feasibility or potentially relevant mechanisms at play is of utmost importance to develop highly active and stable catalysts. Herein, various oxidation reactions of bulk-sized nickel and cobalt to the corresponding metal oxide or in the presence of a metal oxide carrier are evaluated thermodynamically and linked to approximated conditions during methane reforming. In particular cobalt aluminate, as well as cobalt or nickel titanates are likely to form. As oxidation to bulk-sized metal oxide is unlikely, a thermodynamic analysis of metallic nanoparticles was performed to calculate the size dependent stability against oxidation to nickel oxide or cobalt oxide in water and carbon dioxide-rich environments. The calculations indicate that nickel nanoparticles >3 nm and cobalt nanoparticles >10 nm are expected to withstand oxidation during steam and dry reforming of methane with stoichiometric feed compositions and methane conversion levels >10% at temperatures up to 1100 and 900 °C, respectively. Lastly, the reduced thermal stability of nanoparticles due to melting point suppression was assessed, leading to similar recommendations concerning minimum particle sizes.

Thermodynamic assessment of oxidation and sintering of Co or Ni as well as the size dependent oxidation of nanoparticles to the corresponding oxide are presented considering the prevailing conditions during steam and dry reforming of methane.

Supported FexNiy catalysts for the co-activation of CO2 and small alkanes

The effect of both the Fe : Ni ratio (5 to 1 : 1) and the relative Lewis acidity of a metal oxide support on catalytic activity, selectivity and stability was investigated in the CO₂ mediated oxidative dehydrogenation of ethane (CO₂-ODH). To avoid effects of varying pore sizes, shapes and volumes of the supports, chromia and zirconia overlayers were coated onto a common γ-Al₂O₃ carrier (CrO_x@Al₂O₃ and ZrO_x@Al₂O₃). Separately, oxidic Fe_xNi_y alloy precursor nanoparticles were prepared using a nonaqueous surfactant-free method and deposited by sonication onto the carrier. In comparison to previous studies in the field, this synthesis technique yields closely associated iron and nickel increasing the chances for alloy formation. During reduction, a mixture of a bcc and a fcc alloy phase was formed, with the content of bcc increasing with increasing iron content as predicted by the bulk phase diagram. Upon exposure to carbon dioxide at elevated temperatures, the bcc metallic phase is selectively oxidised to an inverse spinel structure via the dissociation of CO₂. When exposed to CO₂-ODH conditions, the bare ZrO_x@Al₂O₃ support shows no activity. The presence of FeNi phases increases the conversion of ethane and CO₂ marginally (<2%) but forms ethylene at high selectivity (S_C2H4 > 80%). The CrO_x@Al₂O₃ support shows some initial activity (X_C2H6 < 5%) at very high ethylene selectivity (S_C2H4 > 90%) but deactivates with time on stream. Comparison of the ethane and carbon dioxide conversions suggests that direct dehydrogenation rather than the oxidative pathway is taking place. When FeNi particles with the highest Fe content are added, the ethane conversion behavior hardly changes, but the CO₂ conversion is increased now supporting the stoichiometric CO₂-ODH reaction (S_C2H4 > 95%). It is therefore evident that a tandem catalyst system between a reducible oxide carrier and the FeNi species is required. Increasing the Ni content results in an increase in activity and stability while changing the dominant reaction pathway to a combination of dry reforming, CO₂-ODH and possibly the reverse Boudouard reaction, with the latter countering catalyst deactivation through carbon deposition.

Activation of NLRP3 inflammasome in hepatocytes after exposure to cobalt nanoparticles: The role of oxidative stress

With the increased use of nanomaterials and increased exposure of humans to various nanomaterials, the potential health effects of nanomaterials cannot be ignored. The hepatotoxicity of cobalt nanoparticles (Nano-Co) is largely unknown and the underlying mechanisms remain obscure. The purpose of this study was to exam the hepatotoxicity induced by Nano-Co and its potential mechanisms. Our results showed that exposure of human fetal hepatocytes L02 to Nano-Co caused a dose- and a time-dependent cytotoxicity. Besides the generation of reactive oxygen species (ROS) and mitochondrial reactive oxygen species (mtROS), exposure to Nano-Co also caused activation of NOD-like receptor protein 3 (NLRP3) inflammasome in hepatocytes. After silencing NLRP3, one component of NLRP3 inflammasome, expression by siRNA strategy, we found that upregulation of NLRP3-related proteins was abolished in hepatocytes exposed to Nano-Co. Using antioxidants to scavenge ROS and mtROS, we demonstrated that Nano-Co-induced mtROS generation was related to Nano-Co-induced NLRP3 inflammasome activation. Our findings demonstrated that Nano-Co exposure may promote intracellular oxidative stress damage, and mtROS may mediate the activation of NLRP3 inflammasome in hepatocytes exposed to Nano-Co, suggesting an important role of ROS/NLRP3 pathway in Nano-Co-induced hepatotoxicity. These results provide scientific insights into the hepatotoxicity of Nano-Co and a basis for the prevention and treatment of Nano-Co-induced cytotoxicity.