Nanometer-Sized MoS₂ Clusters on Graphene Flakes for Catalytic Formic Acid Decomposition

MoS₂ was deposited on graphene flakes via decomposition of MoS₃ in vacuum at different temperatures (500–800 °C). The materials obtained were tested for catalytic formic acid decomposition, giving mainly hydrogen and carbon dioxide. According to atom-resolved transmission electron microscopy study, a considerable amount of MoS₂ clusters with a mean size of 1 nm was formed on the graphene surface at 500 °C. Simulation of the structure of a cluster revealed the presence of Mo-edge atoms. Raising the preparation temperature up to 800 °C led to agglomeration of MoS₂ clusters and formation of thin crystalline MoS₂ particles 20–30 nm in size. The sample enriched with the MoS₂ clusters showed 6 times higher catalytic activity at 160 °C than the sample with the crystalline MoS₂ particles. This demonstrates that the observed nanometer-sized MoS₂ clusters are responsible for catalysis

Support effect for nanosized Au catalysts in hydrogen production from formic acid decomposition

Catalysts with about 2.5 wt% of gold supported on Al2O3, ZrO2, CeO2, La2O3 and MgO oxides and with the same mean metal particle sizes of 2.4-3.0 nm have been studied in hydrogen production via formic acid decomposition. A strong volcano-type relation of the catalytic activity on the electronegativity of the support's cation was demonstrated with the Au/Al2O3 catalyst on the top. This indicated that the activity is affected by the acid-base properties of the support. A study of the most active Au/Al2O3 catalyst with aberration-corrected HAADF/STEM, XPS and EXAFS proved that gold is in metallic state. The content of single supported gold atoms/cations was negligible. Therefore, the mechanism of the reaction was related to the activation of formic acid on the catalyst's support followed by further decomposition of the formed reaction intermediate on the Au/support interface

Single isolated Pd2+ cations supported on N-Doped carbon as active sites for hydrogen production from formic acid decomposition

Single-site heterogeneous catalysis with isolated Pd atoms was reported earlier, mainly for oxidation reactions and for Pd catalysts supported on oxide surfaces. In the present work, we show that single Pd atoms on nitrogen-functionalized mesoporous carbon, observed by aberration-corrected scanning transmission electron microscopy (ac STEM), contribute significantly to the catalytic activity for hydrogen production from vapor-phase formic acid decomposition, providing an increase by 2-3 times in comparison to Pd catalysts supported on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity was also achieved. According to X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) studies after ex situ reduction in hydrogen at 573 K, these species exist in a Pd2+ state coordinated by nitrogen species of the support. Extended density functional theory (DFT) calculations confirm that an isolated Pd atom can be the active site for the reaction, giving decomposition of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl fragment, but only if it is coordinated by a pair of pyridinic-type nitrogen atoms located on the open edge of the graphene sheet. Hence, the role of the N-doping of the carbon support is the formation and stabilization of the new active Pd sites. A long-term experiment performed for more than 30 h on stream indicated an excellent stability of these Pd species in the reaction