The interaction of atomic hydrogen and H 2 O with stoichiometric and partially reduced CeO 2 (111) thin films deposited on a Cu(111) substrate was investigated by temperature programmed desorption and X-ray photoelectron spectroscopy. On stoichiometric CeO 2 (111) surface, the adsorption of atomic H(g) leads to the formation of surface hydroxyl (OH(a)) and H 2 O(a) as well as the reduction of Ce 4+ into Ce 3+ . On reduced CeO 2 (111) surfaces, the stability of OH(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways: one is the associative desorption of OH(a) releasing H 2 O and creating oxygen vacancies (OH(a) + OH(a) → H 2 O(g) + O lattice + O vacancy ), and the other one is to produce H 2 via OH(a) + OH(a) → H 2 (g) + 2O lattice . The presence of oxygen vacancies in CeO 2 favors the reaction pathway of H 2 formation. At 115 K, reversible dissociation and molecular adsorption of H 2 O occur on stoichiometric CeO 2 (111) surface, but irreversible dissociation of H 2 O occurs on reduced CeO 2 (111) surfaces. These results deepen the fundamental understanding of the influence of oxygen vacancies on the reactivity of surface hydroxyls and water on CeO 2 surface.
Metal–organic framework (MOF)-derived composites on the microwave absorption have received extensive attention. However, which kind of organic ligand corresponding MOF derivative has better electromagnetic wave absorption performance is an urgent problem to be solved. In this work, two kinds of Ni@C derived from the Ni-based MOFs with two kinds of organic ligands (dimethylimidazole as a ligand named as Ni-ZIF and trimesic acid as a ligand named as Ni-BTC) were successfully obtained. The compositions, morphologies, and electromagnetic properties of two composites were well controlled. As a result, both kinds of Ni@C exhibited the good microwave absorption properties. Comparatively speaking, the Ni@C derived from Ni-ZIF performs better. The Ni@C-ZIF microspheres with a 40% mass filling ratio exhibited a strong reflection loss of −86.8 dB at 13.2 GHz when the matching thickness was 2.7 mm, and the corresponding effective absorption bandwidth was 7.4 GHz (4–11.4 GHz) with the thickness ranging from 1.5 to 4.0 mm. The impedance matching, multiple reflection, and interfacial polarization among Ni and C were beneficial to the enhancement of microwave attenuation, which N-doping introduced by nitrogen-containing ligands leads to excellent microwave absorption properties. Therefore, this work can give insights into understanding the absorbing mechanism as well as provide a simple and flexible paradigm for the design and synthesis of the absorber with the tunable and high-efficiency performances.
Exploring reactions of methanol on TiO2 surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO2(001)-(1×4) surface. The methanol-to-dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold-coordinated Ti(4+) sites (Ti(4c)), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO2 facets. Surface chemistry of methanol on the anatase TiO2(001)-(1×4) surface is seldom affected by co-chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO2 surfaces but also show that TiO2 with a high density of Ti(4c) sites is a potentially active and selective catalyst for the important methanol-to-DME reaction.
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