Three primary amine materials functionalized onto mesoporous silica with low, medium, and high surface amine coverages are prepared and evaluated for binary CO2/H2O adsorption under dilute conditions. Enhancement of amine efficiency due to humid adsorption is most pronounced for low surface amine coverage materials. In situ FT-IR spectra of adsorbed CO2 on these materials suggest this enhancement may be associated with the formation of bicarbonate species during adsorption on materials with low surface amine coverage, though such species are not observed on high surface coverage materials. On the materials with the lowest amine loading, bicarbonate is observed on longer time scales of adsorption, but only after spectral contributions from rapidly forming alkylammonium carbamate species are removed. This is the first time that direct evidence for bicarbonate formation, which is known to occur in liquid aqueous amine solutions, has been presented for CO2 adsorption on solid amine adsorbents.

Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce-H species upon H dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce-H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved.

The measurement of isosteric heats of adsorption of silica supported amine materials in the low pressure range (0-0.1 bar) is critical for understanding the interactions between CO2 and amine sites at low coverage and hence to the development of efficient amine adsorbents for CO2 capture from flue gas and ambient air. Heats of adsorption for an array of silica-supported amine materials are experimentally measured at low coverage using a Calvet calorimeter equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized silica materials, higher amine densities resulted in higher isosteric heats of adsorption, clearly showing that the density/proximity of amine sites can influence the amine efficiency of adsorbents. In a series of materials with fixed amine loading but different amine types, strongly basic primary and secondary amine materials are shown to have essentially identical heats of adsorption near 90 kJ/mol. However, the adsorption uptakes vary substantially as a function of CO2 partial pressure for different primary and secondary amines, demonstrating that entropic contributions to adsorption may play a key role in adsorption at secondary amine sites, making adsorption at these sites less efficient at the low coverages that are important to the direct capture of CO2 from ambient air. Thus, while primary amines are confirmed to be the most effective amine types for CO2 capture from ambient air, this is not due to enhanced enthalpic contributions associated with primary amines over secondary amines, but may be due to unfavorable entropic factors associated with organization of the second alkyl chain on the secondary amine during CO2 adsorption. Given this hypothesis, favorable entropic factors may be the main reason primary amine based adsorbents are more effective under air capture conditions.

The nature of the surface species formed through the adsorption of CO on amine-grafted mesoporous silica is investigated through in situ FTIR spectroscopy with the aid of N dynamic nuclear polarization (DNP) and C NMR spectroscopy. Primary, secondary, and tertiary amines are functionalized onto a mesoporous SBA-15 silica. Both isotopically labeled CO and natural-abundance CO are used for accurate FTIR peak assignments, which are compared with assignments reported previously. The results support the formation of monomeric and dimeric carbamic acid species on secondary amines that are stabilized differently to the monocarbamic acid species on primary amines. Furthermore, the results from isotopically labelled CO experiments suggest the existence of two carbamate species on primary amines, whereas only one species is observed predominantly on secondary amines. The analysis of the IR peak intensities and frequencies indicate that the second carbamate species on primary amines is probably more asymmetric in nature and forms in a relatively smaller amount. Only the formation of bicarbonate ions at a low concentration is observed on tertiary amines; therefore, physisorbed water on the surface plays a role in the hydrolysis of CO even if water is not added intentionally and dry gases are used. This suggests that a small amount of bicarbonate ions could be expected to form on primary and secondary amines, which are more hydrophilic than tertiary amines, and these low concentration species are difficult to observe on such samples.

Taming interfacial electronic effects on Pt nanoparticles modulated by their concomitants has emerged as an intriguing approach to optimize Pt catalytic performance. Here, we report Pt nanoparticles assembled on vacancy-abundant hexagonal boron nitride nanosheets and their use as a model catalyst to embrace an interfacial electronic effect on Pt induced by the nanosheets with N-vacancies and B-vacancies for superior CO oxidation catalysis. Experimental results indicate that strong interaction exists between Pt and the vacancies. Bader charge analysis shows that with Pt on B-vacancies, the nanosheets serve as a Lewis acid to accept electrons from Pt, and on the contrary, when Pt sits on N-vacancies, the nanosheets act as a Lewis base for donating electrons to Pt. The overall-electronic effect demonstrates an electron-rich feature of Pt after assembling on hexagonal boron nitride nanosheets. Such an interfacial electronic effect makes Pt favour the adsorption of O2, alleviating CO poisoning and promoting the catalysis.