In the typical NH3-SCR temperature range (100-500 °C), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. Therefore, the study of adsorbed ammonia at high temperature is a key step for the understanding of its role in the NH3-SCR catalytic cycle. We employed different spectroscopic techniques to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, that is, (i) NH3 bonded to copper centers, (ii) NH3 bonded to Brønsted sites, and (iii) NH4(+)·nNH3 associations. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu(+) species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.
Metal–organic
frameworks (MOFs) show great prospect as catalysts
and catalyst support materials. Yet, studies that address their dynamic,
kinetic, and mechanistic role in target reactions are scarce. In this
study, an exceptionally stable MOF catalyst consisting of Pt nanoparticles
(NPs) embedded in a Zr-based UiO-67 MOF was subject to steady-state
and transient kinetic studies involving H/D and 13C/12C exchange, coupled with operando infrared spectroscopy and
density functional theory (DFT) modeling, targeting methanol formation
from CO2/H2 feeds at 170 °C and 1–8
bar pressure. The study revealed that methanol is formed at the interface
between the Pt NPs and defect Zr nodes via formate species attached
to the Zr nodes. Methanol formation is mechanistically separated from
the formation of coproducts CO and methane, except for hydrogen activation
on the Pt NPs. Careful analysis of transient data revealed that the
number of intermediates was higher than the number of open Zr sites
in the MOF lattice around each Pt NP. Hence, additional Zr sites must
be available for formate formation. DFT modeling revealed that Pt
NP growth is sufficiently energetically favored to enable displacement
of linkers and creation of open Zr sites during pretreatment. However,
linker displacement during formate formation is energetically disfavored,
in line with the excellent catalyst stability observed experimentally.
Overall, the study provides firm evidence that methanol is formed
at the interface of Pt NPs and linker-deficient Zr6O8 nodes resting on the Pt NP surface.
This contribution clarifies the overoxidation‐preventing key step in the methane‐to‐methanol (MTM) conversion over copper mordenite zeolites. We followed the methane‐to‐methanol conversion over copper mordenite zeolites by NMR spectroscopy supported by DRIFTS to show that surface methoxy groups (SMGs) located at zeolite Brønsted sites are the key intermediates. The SMGs with chemical shift of 59 ppm are identical to those formed on a copper‐free reference zeolite after reaction with methanol and react with water, methanol, or carbon monoxide to yield methanol, dimethyl ether, and acetate. This reactivity corroborates the location of SMGs at Brønsted sites. We find no evidence for stable SMGs directly at copper sites and explain mechanistically why H‐form mordenites outperform their Na‐form analogues. This finding is of interest for any future process that tries to trap the intermediate methane oxidation product towards methanol.
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