2 research outputs found
Direct Observation of Reactant, Intermediate, and Product Species for Nitrogen Oxide-Selective Catalytic Reduction on Cu-SSZ-13 Using <i>In Situ</i> Soft X‑ray Spectroscopy
Catalytic processes
have supported the development of
myriad beneficial
technologies, yet our fundamental understanding of the complex interactions
between reaction intermediates and catalyst surfaces is still largely
undefined for many reactions. Experimental analyses have generally
been limited to investigation of catalyst materials or a subset of
functional groups as indirect probes of the critical surface-bound
intermediate species and reaction mechanisms. A more direct approach
is to probe the intermediate species themselves, but this requires
direct study of the local chemical environment of light elements.
In this work, we use soft X-ray emission spectroscopy (XES) and a
custom-designed in situ reactor cell to directly
observe and characterize the electronic structure of reactant, intermediate,
and product species under reaction conditions. Specifically, we employ
N K XES to probe the interaction of various nitrogen species with
a Cu-SSZ-13 catalyst during selective catalytic reduction of nitrogen
oxides (NO and NO2) by ammonia (NH3-SCR), a
reaction that is critical for the removal of NOx pollutants
formed in combustion reactions. This work reveals a novel spectral
feature for all spectra measured with flowing NO gas present, which
we attribute to the interaction of NO with the catalyst. We find that
introducing both NO and O2 gases (compared to only NO)
increases the interaction of NO with Cu-SSZ-13. Adsorption of NH3 leads to a more pronounced spectral signal compared to NO
adsorption. For the standard NH3-SCR reaction, we observe
a strong N2 signal, comprising 30% of the total spectral
intensity. These results demonstrate the vast potential of this technique
to provide direct, novel insights into the complex interactions between
reaction intermediates and the active sites of catalysts, which may
guide advanced knowledge-based optimization of these processes
Formation of a Kî—¸Inî—¸Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se<sub>2</sub> Thin-Film Solar Cell Absorbers
A NaF/KF
postdeposition treatment (PDT) has recently been employed to achieve
new record efficiencies of CuÂ(In,Ga)ÂSe<sub>2</sub> (CIGSe) thin film
solar cells. We have used a combination of depth-dependent soft and
hard X-ray photoelectron spectroscopy as well as soft X-ray absorption
and emission spectroscopy to gain detailed insight into the chemical
structure of the CIGSe surface and how it is changed by different
PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers
grown by low-temperature coevaporation have been interrogated. We
find that the alkali-free and NaF-PDT CIGSe surfaces both display
the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT,
however, leads to the formation of bilayer structure in which a Kî—¸Inî—¸Se
species covers the CIGSe compound that in composition is identical
to the chalcopyrite structure of the alkali-free and NaF-PDT absorber