Pedro Castro-Fernández

Colloidal solutions of gallia‐alumina (Ga,Al)₂O_3(x:y) solid‐solution nanoparticles with nominal atomic Ga : Al (x:y) ratios of 1 : 6, 1 : 3, 3 : 1, and 1:0 were used to prepare silica‐supported catalysts for propane dehydrogenation (PDH). A comparison of the unsupported and silica‐supported catalysts reveals that the dispersion on silica increases the Ga‐normalized PDH rates for all catalysts, albeit with a notably lower propene selectivity for (Ga,Al)₂O_3(1:6)/SiO₂. Fourier transform infrared (FTIR) spectroscopy allows contrasting the H₂ dissociation sites in the calcined and H₂‐treated (Ga,Al)₂O_3(x:y)/SiO₂, indicating a transformation of Ga³⁺ surface sites with Al (mainly) and Ga atoms in the second coordination sphere (Ga_(Al/Ga) sites) in the calcined (Ga,Al)₂O_3(1:6)/SiO₂ to predominantly Ga_(Ga/Si) surface sites in the H₂‐treated material. The resulting sites are similarly unselective as in amorphous gallia on silica. H₂ produced during the PDH reaction can cause a similar transformation as H₂ pretreatment in (Ga,Al)₂O_3(1:6)/SiO₂, rapidly resulting in a notably lowered selectivity. The stable and selective Ga_(Al/Ga) surface sites in (Ga,Al)₂O_3(1:3)/SiO₂ yield a Ga−H band at ca. 1990 cm⁻¹ under H₂ dissociation conditions while the less selective surface sites, observed for the other Ga : Al ratios, give Ga−H bands at ca. 2040 and 2060 cm⁻¹.

For enhancing hydrogen evolution, solution-processed Mo-BiVO₄ exhibits a distinct atomic phenomenon with a high photocharging current density.

We explore how H₂ pretreatment changes the structure of two gallia-based propane dehydrogenation catalysts, viz. crystalline γ/β-Ga₂O₃ and amorphous Ga₂O₃ supported on silica, and how it affects their activity, selectivity and stability on stream.

Gallia–alumina (Ga,Al)₂O_3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as ²⁷Al and ⁷¹Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning ¹⁵N{²⁷Al} and ¹⁵N{⁷¹Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using ¹⁵N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)₂O₃ oxides. Notably, we demonstrate that (Ga,Al)₂O₃ catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)₂O₃ catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance.

The addition of molten alkali metal salts drastically accelerates the kinetics of CO2 capture by MgO through the formation of MgCO3. However, the growth mechanism, the nature of MgCO3 formation and the exact role of the molten alkali metal salts on the CO2 capture process remains elusive, holding back the development of more effective MgO-based CO2 sorbents. Here, we unveil the growth mechanism of MgCO3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO3. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO2, a non-crystalline surface carbonate layer of ca. 7 Å thickness forms. In contrast, when MgO(100) is coated with NaNO3 MgCO3 crystals nucleate and growth. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO3. MgCO3 grows epitaxially with respect to MgO(100) and the lattice mismatch between MgCO3 and MgO is relaxed through lattice misfit dislocations. Pyramid shaped pits on the surface of MgO, in the proximity and below the MgCO3 crystals, point to the etching of surface MgO, providing dissolved [Mg2+…O2–] ionic pairs for MgCO3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.