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.

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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.

We report an in situ, temperature and H 2 pressuredependent, characterization of (2.6 ± 0.4) nm palladium nanoparticles supported on active carbon during the process of hydride phase formation. For the first time the core−shell structure is highlighted in the single-component particles on the basis of a different atomic structure and electronic configurations in the inner "core" and surface "shell" regions. The atomic structure of these particles is examined by combined X-ray powder diffraction (XRPD), which is sensitive to the crystalline core region of the nanoparticles, and by first shell analysis of extended Xray absorption fine structure (EXAFS) spectra, which reflects the averaged structure of both the core and the more disordered shell. In the whole temperature range (0−85 °C), XRPD analysis confirms the existence of two well-separated αand β-hydride phases with the characteristic flat plateau in the phase transition region of the pressure-lattice parameter isotherms. In contrast, first shell interatomic distances obtained from EXAFS exhibit a slope in the phase transition region, typical for nanostructured palladium. Such difference is explained by distinct properties of bulk "core" which has crystalline structure and sharp phase transition, and surface "shell" which is amorphous and absorbs hydrogen gradually without forming distinguishable αand β-phases. Combining EXAFS and XRPD we extract, for the first time, the Pd−Pd first-shell distance in the amorphous shell of the nanoparticles, that is significantly shorter than in the bulk core and relevant in catalysis. The core/shell model is supported by the EXAFS analysis of the higher shells, in the frame of the multiple scattering theory, showing that the evolution of the third shell distance (ΔR 3 /R 3 ) is comparable to the evolution of (Δa/a) obtained from XRPD since amorphous PdH x shell gives a negligible contribution in this range of distances. This operando structural information is relevant for the understanding of structure-sensitive reactions. Additionally, we demonstrate the differences in the evolution of the thermal parameters obtained from EXAFS and XRPD along the hydride phase formation.

Activated carbons and related Pd-based catalysts are investigated with a multi-techniques approach, which allows correlating structure and performance.

CO 2 hydrogenation was carried out over Pt-containing UiO-67 Zr-MOFs at T = 220−280 °C and ambient pressure, with H 2 / CO 2 = 0.2−9 and contact times, τ = 0.004−0.01 g cat •min•mL −1 . The catalysts were characterized by XRD, N 2 adsorption, FESEM, TEM and HRTEM, Pt L 3 -edge XANES and EXAFS, dissolution-NMR, CO chemisorption, IR spectroscopy, and TGA. A positive correlation was observed between the degree of Pt reduction and CO 2 conversion. Contact time variation experiments showed that CO is a primary product of reaction, while CH 4 is a secondary product. Testing of catalyst crystals with 0.15 and 2.0 μm crystal size, respectively, revealed no influence of diffusion on the reaction rate. Comparison to a conventional Pt/SiO 2 catalyst showed very similar activation energy, with E app = 50 ± 3 kJ• mol −1 . However, the turnover frequency over Pt/SiO 2 was significantly lower, and Pt/SiO 2 did not yield methane as a product. The Pt-containing UiO-67 Zr-MOF catalyst showed stable activity during 60 h of testing.

We report a complete experimental characterization of the surface Pthydride species on an industrial 5 wt % Pt/Al 2 O 3 catalyst (average particle size of 1.4 ± 0.4 nm) under different hydrogenation/dehydrogenation conditions. By combining inelastic neutron scattering, FT-IR spectroscopy, and synchronous DRIFT/XAS/MS, we identified n-fold coordinated Pt-hydrides and four different types of linear Pthydrides characterized by different adsorption strength, whose relative proportion depends on the experimental conditions. In particular, we observed that the n-fold coordinated hydrides convert into linear ones upon decreasing hydrogen coverage, and vice versa, and we traced this phenomenon to a morphological and electronic reconstruction of the Pt nanoparticles. Although only a fraction of the surface Pthydrides are directly involved in the hydrogenation of toluene, all the others play an indirect but fundamental role, maintaining the Pt nanoparticles electronically and morphologically stable during the reaction and hence avoiding the occurrence of deactivation processes. Our results, which are in good agreement with the theoretical predictions reported in the literature, offer a comprehensive picture of the dynamics of Pt nanoparticles in hydrogenation conditions. This always involves a change in the relative proportion of the Pt-hydride species, and only in some cases an electronic and morphological reconstruction of the Pt particles.