Capturing CO2 by ceria and ceria-zirconia nanomaterials of different origin.

Ceria and ceria-zirconia nanomaterials of different origin were studied in order to elucidate the role of their structural and textural characteristics in controlling the performance towards CO2 capture. Two commercial cerias and two home-prepared samples, CeO2 and CeO2-ZrO2 (75% CeO2) mixed oxide, were investigated. The samples were characterized by a number of analytical techniques including XRD, TEM, N2-adsorption, XPS, H2-TPR, Raman and FTIR spectroscopy. Static and dynamic CO2 adsorption experiments were applied to assess the CO2 capture performance. The type of surface species formed and their thermal stability were evaluated by in situ FTIR spectroscopy and CO2-TPD analysis. The two commercial ceria samples possessed similar structural and textural characteristics, formed the same types of carbonate-like surface species upon CO2 adsorption and, consequently, demonstrated almost identical CO2 capture performance under both static and dynamic conditions. The thermal stability of the adsorbed species increased in the order bidentate (B) carbonates, hydrogen carbonates (HC) and tridentate carbonates (T-III, T-II, T-I). Reduction of CeO2 increased the relative amount of the most strongly bonded T-I tridentate carbonates. Preadsorbed water led to hydroxylation and enhanced formation of hydrogen carbonates. Although the synthesized CeO2 sample had a higher surface area (by 30%) it showed a disadvantageous long mass transfer zone in the CO2-adsorption breakthrough curves. Because of its complex pore structure, this sample probably experiences severe intraparticle CO2 diffusion resistance. Having the same surface area as the synthesized CeO2, the mixed CeO2-ZrO2 oxide exhibited the highest CO2 capture capacity of 136 µmol g-1 under dynamic conditions. This was related to the highest concentration of CO2 adsorption sites (including defects) on this sample. The CeO2-ZrO2 system showed the lowest sensitivity to the presence of water vapor in the gas stream due to the lack of dissociative water adsorption on this material.

Adsorption of CO2 on MIL-53(Al): FTIR evidence of the formation of dimeric CO2 species.

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.

An advanced approach for measuring acidity of hydroxyls in confined space: a FTIR study of low-temperature CO and (15)N2 adsorption on MOF samples from the MIL-53(Al) series.

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.

Hydrogen spillover on Rh/TiO2: the FTIR study of donated electrons, co-adsorbed CO and H/D exchange.

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.

Spectral evidence for hydrogen-induced reversible segregation of CO adsorbed on titania-supported rhodium.

The reduction of a 1.3% Rh/TiO2 sample with carbon monoxide leads to the formation of uniform Rh nanoparticles with a mean diameter of dp ≈ 2.2 nm. Adsorption of CO on the reduced Rh/TiO2 produces linear and bridged carbonyls bound to metallic Rh(0) sites and only a few geminal dicarbonyls of Rh(I). The ν(CO) of linear Rh(0)-CO complexes is strongly coverage dependent: it is observed at 2078 cm(-1) at full coverage and at ca. 2025 cm(-1) at approximated zero coverage. At low coverage, this shift is mainly caused by a dipole-dipole interaction between the adsorbed CO molecules while at high coverage, the chemical shift also becomes important. Hydrogen hardly affects the CO adlayer at high CO coverages. However, on a partially CO-covered surface (θCO ≈ 0.5), the adsorption of H2 at increasing pressure leads to a gradual shift in the band of linear Rh(0)-CO from 2041 to 2062 cm(-1). Subsequent evacuation almost restores the original spectrum, demonstrating the reversibility of the hydrogen effect. Through the use of (12)CO + (13)CO isotopic mixtures, it is established that the addition of hydrogen to the CO-Rh/TiO2 system leads to an increase in the dynamic interaction between the adsorbed CO molecules. This evidences an increase in the density of the adsorbed CO molecules and indicates segregation of the CO and hydrogen adlayers. When CO is adsorbed on a hydrogen-precovered surface, the carbonyl band maximum is practically coverage independent and is observed at 2175-2173 cm(-1). These results are explained by a model according to which CO successively occupies different rhodium nanoparticles.

Silver-catalyzed low-temperature CO isotopic scrambling reaction: 12C16O + 13C18O → 12C18O + 13C16O.

In this paper we report on low-temperature CO isotopic scrambling ((12)C(16)O + (13)C(18)O →(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.

New types of nonclassical iridium carbonyls formed in Ir-ZSM-5: a Fourier transform infrared spectroscopy investigation.

In this work we report some new nonclassical carbonyls of iridium formed after CO adsorption on Ir-ZSM-5 (Ir-MFI). Mainly Ir+ cations were found on sample activated at 523 K and reduced by CO at the same temperature. With CO they formed Ir+(CO)2 gem-dicarbonyls (2104 and 2033 cm(-1)) that decomposed at 673 K without leaving a measurable fraction of monocarbonyls. The dicarbonyl structure was established by 12CO-13CO coadsortpion experiments. In the presence of CO in the gaseous phase and at ambient temperature the Ir+(CO)2 dicarbonyls were converted into Ir+(CO)3 species (2182, 2099, and 2074 cm(-1)). At 100 K these complexes are able to accommodate a fourth CO molecule thus producing tetracarbonyls (2155, 2145, 2125, and 2105 cm(-1)). The results are explained by the high coordinative unsaturation of the Ir+ cations in the ZSM-5 matrix. This is also the reason for the formation of mixed Ir+(H2O)(CO)2 species after CO-H2O coadsorption (2087 and 2015 cm(-1)). Evacuation of the sample at 673 K, followed by treatment with CO at 523 K, generates Ir2+ cations. With CO these cations form another kind of geminal complex, namely, Ir2+(CO)2 species (2173 and 2129 cm(-1)). Here again, the structure was confirmed by 12CO-13CO coadsortpion experiments. These dicarbonyls are decomposed at 573 K (again without producing monocarbonyls) and are able to accommodate additionally neither CO nor water molecules. The results are explained by the smaller cationic radius of Ir2+ (as compared to Ir+), which is associated with a decrease of the number of ligands required for coordinative saturation.

FTIR spectroscopic study of low temperature NO adsorption and NO + O2 coadsorption on H-ZSM-5.

Adsorption of NO and coadsorption of NO and O2 on H-ZSM-5 have been studied at low and room temperature by means of FTIR spectroscopy. For better interpretation of the spectra, experiments involving isotopic labeled molecules have been performed. Low temperature adsorption of NO on H-ZSM-5 results initially in formation of NO which is H-bonded to the zeolite acidic hydroxyls. A second NO molecule is inserted into the OH-NO species at higher coverages, thus forming OH(NO)2 complexes. Different kinds of NO dimers are also formed. Negligible amounts of oxygenated compounds have been detected. In the presence of oxygen, the (di)nitrosyl species are oxidized very fast even at 100 K to N2O3, NO+, NO2, and N2O4. Different kinds of adsorbed N2O3 species have been evidenced. With increasing temperature, NO+ migrates and occupies cationic positions. The latter species interacts with NO at low temperature to give an [ONNO]+ complex. This reaction is used to prove that the different bands in the 2206-2180 cm(-1) region are also due to NO+ species.

Fourier transform infrared study of low-temperature CO adsorption on CuMgAl-hydrotalcite.

Single-phase CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized by coprecipitation. Thermoanalytical studies of this sample showed transformations in three stages in the temperature range up to ca. 900 K yielding mainly CuO phase. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements showed the presence of carbonates even after calcination of the sample at 973 K. The genesis of Cu+ sites during thermal treatment in vacuo at different temperatures for this sample was followed by IR spectroscopy of CO adsorbed at low temperature. Essentially no Cu+ sites are present on a sample calcined at 723 K, consistent with X-ray photoelectron spectroscopic (XPS) data. However, sample subjected to activation (1 h of O2 treatment at 723 K followed by 1 h of evacuation at the same temperature) upon CO adsorption at 85 K unambiguously showed the presence of Cu+ sites. 12CO-13CO coadsorption studies confirmed the presence of dicarbonyls, which are converted to linear Cu+-CO species during evacuation at 85 K. Concentration of the accessible Cu+ sites increased with the increase in activation temperature up to 873 K and decreased with a further temperature rise. The copper sites on the sample are heterogeneously distributed and their distribution depends on the activation temperature. Two routes of reduction of Cu2+ to Cu+ are proposed: (i) autoreduction during evacuation and (ii) reduction by CO.