Oxide-supported single-atom
catalysts are commonly modeled as a
metal atom substituting surface cation sites in a low-index surface.
Adatoms with dangling bonds will inevitably coordinate molecules from
the gas phase, and adsorbates such as water can affect both stability
and catalytic activity. Herein, we use scanning tunneling microscopy
(STM), noncontact atomic force microscopy (ncAFM), and X-ray photoelectron
spectroscopy (XPS) to show that high densities of single Rh adatoms
are stabilized on α-Fe
2
O
3
(11̅02)
in the presence of 2 × 10
–8
mbar of water at room temperature, in marked contrast to the rapid sintering
observed under UHV conditions. Annealing to 50 °C in UHV desorbs
all water from the substrate leaving only the OH groups coordinated
to Rh, and high-resolution ncAFM images provide a direct view into
the internal structure. We provide direct evidence of the importance
of OH ligands in the stability of single atoms and argue that their
presence should be assumed when modeling single-atom catalysis systems.
A series of neutral bis- and cationic tris-carbonyl complexes of the types cis-[M(κ3P,N,P-PNP)(CO)2Y] and [M(κ3P,N,P-PNP)(CO)3]+ was prepared by reacting [M(CO)5Y] (M = Mn, Re; Y = Cl or Br) with PNP pincer ligands derived from the 2,6-diaminopyridine, 2,6-dihydroxypyridine, and 2,6-lutidine scaffolds. With the most bulky ligand PNPNH-tBu, the cationic square-pyramidal 16e bis-carbonyl complex [Mn(PNPNH-tBu)(CO)2]+ was obtained. In contrast, in the case of rhenium, the 18e complex [Re(PNPNH-tBu)(CO)3]+ was formed. The dissociation of CO was studied by means of DFT calculation revealing in agreement with experimental findings that CO release from [M(κ3P,N,P-PNP)(CO)3]+ is in general endergonic, while for [Mn(κ3P,N,P-PNPNH-tBu)(CO)3]+, this process is thermodynamically favored. X-ray structures of representative complexes are provided.Graphical abstract
Electronic supplementary materialThe online version of this article (10.1007/s00706-018-2307-7) contains supplementary material, which is available to authorized users.
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