2 research outputs found
Structure of Selective and Nonselective Dicopper (II) Sites in CuMFI for Methane Oxidation to Methanol
The
one-step valorization of natural gas remains a challenge. Methane
conversion to methanol via chemical looping over
copper-containing zeolites is a promising route, and CuMFI is among
the earliest successfully applied. However, the structure of the active
sites in CuMFI, as well as the effect of copper loading and Si/Al
ratio on the copper speciation, are yet to be understood. We found
that for CuMFI, the Cu/Al ratio determines the selectivity of methane
conversion by governing the structure of the active dicopper sites.
At a Cu/Al ratio below 0.3, copper-containing MFI materials host dimeric
centers with a Cu–Cu separation of 2.9 Å and a UV/vis
absorption band at 27 200 cm–1 capable of
selective oxidation of methane to methanol in a wide temperature range
(450–550 K). A higher Cu/Al ratio leads to the formation of
mono-μ-oxo dicopper sites with Cu–Cu = 3.2 Å, which
exhibit a characteristic band at 21 900 cm–1 and react with methane at lower temperatures (<450 K), yielding
overoxidation products. Identifying distinctions in the structure
of selective and nonselective copper sites will aid in the design
of better-performing materials
The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide
Hydrogen spillover from metal nanoparticles to oxides
is an essential
process in hydrogenation catalysis and other applications such as
hydrogen storage. It is important to understand how far this process
is reaching over the surface of the oxide. Here, we present a combination
of advanced sample fabrication of a model system and in situ X-ray
photoelectron spectroscopy to disentangle local and far-reaching effects
of hydrogen spillover in a platinum–ceria catalyst. At low
temperatures (25–100 °C and 1 mbar H2) surface
O–H formed by hydrogen spillover on the whole ceria surface
extending microns away from the platinum, leading to a reduction of
Ce4+ to Ce3+. This process and structures were
strongly temperature dependent. At temperatures above 150 °C
(at 1 mbar H2), O–H partially disappeared from the
surface due to its decreasing thermodynamic stability. This resulted
in a ceria reoxidation. Higher hydrogen pressures are likely to shift
these transition temperatures upward due to the increasing chemical
potential. The findings reveal that on a catalyst containing a structure
capable to promote spillover, hydrogen can affect the whole catalyst
surface and be involved in catalysis and restructuring