Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal-Organic Framework.

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported by a UiO-66 metal-organic framework (Cu/UiO-66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density-functional-theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2 , by reaction of O2,ad with COad , leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as the rate limiting step. This is removed in a second activated step.

Crystal structures of tetra-methyl-ammonium (2,2'-bi-pyridine)-tetra-cyanidoferrate(III) trihydrate and poly[[(2,2'-bi-pyridine-κ(2) N,N')di-µ2-cyanido-dicyanido(µ-ethyl-enedi-amine)(ethyl-enedi-amine-κ(2) N,N')-cadmium(II)iron(II)] monohydrate].

The crystal structures of the building block tetra-methyl-ammonium (2,2'-bi-pyridine-κ(2) N,N')tetra-cyanidoferrate(III) trihydrate, [N(CH3)4][Fe(CN)4(C10H8N2)]·3H2O, (I), and a new two-dimensional cyanide-bridged bimetallic coordination polymer, poly[[(2,2'-bi-pyridine-κ(2) N,N')di-µ2-cyanido-dicyanido(µ-ethyl-enedi-amine-κ(2) N:N')(ethyl-enedi-amine-κ(2) N,N')cadmium(II)iron(II)] monohydrate], [CdFe(CN)4(C10H8N2)(C2H8N2)2]·H2O, (II), are reported. In the crystal of (I), pairs of [Fe(2,2'-bipy)(CN)4](-) units (2,2'-bipy is 2,2'-bi-pyri-dine) are linked together through π-π stacking between the pyridyl rings of the 2,2'-bipy ligands to form a graphite-like structure parallel to the ab plane. The three independent water mol-ecules are hydrogen-bonded alternately with each other, forming a ladder chain structure with R 4 (4)(8) and R 6 (6)(12) graph-set ring motifs, while the disordered [N(CH3)4](+) cations lie above and below the water chains, and the packing is stabilized by weak C-H⋯O hydrogen bonds. The water chains are further linked with adjacent sheets into a three-dimensional network via O-H⋯O hydrogen bonds involving the lattice water mol-ecules and the N atoms of terminal cyanide groups of the [Fe(2,2'-bipy)(CN)4](-) building blocks, forming an R 4 (4)(12) ring motif. Compound (II) features a two-dimensional {[Fe(2,2'-bipy)(CN)4Cd(en)2]} n layer structure (en is ethyl-enedi-amine) extending parallel to (010) and constructed from {[Fe(2,2'-bipy)(CN)4Cd(en)]} n chains inter-linked by bridging en ligands at the Cd atoms. Classical O-H⋯N and N-H⋯O hydrogen bonds involving the lattice water mol-ecule and N atoms of terminal cyanide groups and the N-H groups of the en ligands are observed within the layers. The layers are further connected via π-π stacking inter-actions between adjacent pyridine rings of the 2,2'-bipy ligands, completing a three-dimensional supra-molecular structure.