ABSTRACT
FTIR spectra of (12)CO2 and (12)CO2 + (13)CO2 mixtures adsorbed on MIL-53(Al) reveal the formation of highly symmetric dimeric (CO2)2 species connected to two structural OH groups.
ABSTRACT
Acidity of solids is decisive for their interaction with guest molecules. One of the most used methods for measuring the acidity of surface hydroxyl groups is the hydrogen bond method based on the spectral shift of the OH stretching modes induced by the adsorption of weak bases. However, many materials of practical interest (e.g. metal organic frameworks, zeolites, etc.) are porous and the OH groups are involved in H-bonding with framework basic sites. Here we show that MIL-53(Al) and NH2-MIL-53(Al) samples are characterized by one type of structural hydroxyl but three IR bands are detected at 100 K with these materials (at 3721, 3711 and 3683 cm(-1)). These bands are assigned to structural hydroxyls involved in H-bonding with different strengths. There is no correlation between the acidities of the hydroxyls, as measured by low-temperature CO or (15)N2 adsorption, and the main reason for this is the pre-existing H-bond. A method for the estimation of the intrinsic frequency of the OH groups (i.e. if not participating in H-bonds), based on the analysis of the spectral data obtained with two molecular probes, is proposed. According to this method, the OH stretching frequency of the structural hydroxyls of MIL-53(Al) samples is determined to be 3727 cm(-1). The formation of 1 : 1 adducts between the hydroxyls and strong bases leads to breaking of the pre-existing H-bonds. When the base is weak, bifurcated complexes are formed which slightly affects the spectral shift. The conclusions derived here considerably broaden the applicability of the H-bond method for assessing protonic acidity of materials and systems where the OH groups are preliminarily involved in H-bonding.
ABSTRACT
Hydrogen dissociation and spillover on supported metal nanoparticles have received renewed interest because these chemical processes are closely related to applications in heterogeneous catalysis and hydrogen storage. In heterogeneous catalysis, spillover can control the reaction rate and selectivity of a wide range of reactions, e.g. hydrogenation, synthesis of methanol and hydroisomerization. In this work, we combine three spectroscopic approaches, i.e. the FT-IR spectroscopy of donated electrons, co-adsorbed CO and H/D exchange, to obtain detailed information on the dynamics of hydrogen interaction with a model 1.3% Rh/TiO2 catalyst. Our spectroscopic results helped us to build a physical picture of the processes occurring during the H-spillover on Rh/TiO2. It was found that molecular H2 dissociates on nanocrystalline Rh; H atoms spillover onto the titania thus protonating the semiconductor, while donating electrons to shallow trap (ST) states and the conduction band (CB) of TiO2. These donated electrons are observed by their specific IR features. By simultaneously monitoring the changes in the vibrational modes of CO, and, the infrared absorbance due to transitions involving CB and ST electrons, we found that both CO-reduced and partially re-oxidized Rh nanocrystallites promote the H-spillover and thus the n-doping of TiO2 materials. Upon evacuation, the process reverses: hydrogen atoms spillover back to Rh nanoparticles where they recombine to form H2 molecules that desorb from the surface. These new mechanistic insights into the process of H2 dissociation and spillover on the powder Rh/TiO2 catalyst call for further model surface science studies with model metal nanoparticle-single crystal substrate systems, in which a detailed picture of energetics and spatial distribution of hydrogen and injected electrons could be obtained.
ABSTRACT
In this paper we report on low-temperature CO isotopic scrambling ((12)C(16)O + (13)C(18)O â(12)C(18)O + (13)C(16)O). The reaction proceeds on a commercial silver-exchanged zeolite even at about 100 K and requires an optimal reduction degree of the catalysts.