Authors:D. Amari, J. M. Lopez Cuesta, N. P. Nguyen, R. Jerrentrup, and J. L. Ginoux
Calorimetric measurements of the heat of adsorption of CO2 on zeolites with variable content of mono- and divalent cations lead to common conclusions. High initial heats (up to 120 kJ·mol−1 for NaA), generally associated with a slow and activated rate of adsorption, are found for high contents of Na+, Li+ or Ca2+. They are attributed to a limited number of chemisorption sites (0.3 per α cage in NaA).
Authors:L. Li, X. Wang, J. Shen, L. Zhou, and T. Zhang
Adsorption microcalorimetry has been employed to study the interaction of ethylene with the reduced and oxidized Pt-Ag/SiO2catalysts with different Ag contents to elucidate the modified effect of Ag towards the hydrocarbon processing on platinum
catalysts. In addition, microcalorimetric adsorption of H2, O2, CO and FTIR of CO adsorption were conducted to investigate the influence of Ag on the surface structure of Pt catalyst.
It is found from the microcalorimetric results of H2and O2adsorption that the addition of Ag to Pt/SiO2leads to the enrichment of Ag on the catalyst surface which decreases the size of Pt surface ensembles of Pt-Ag/SiO2catalysts. The microcalorimetry and FTIR of CO adsorption indicates that there still exist sites for linear and bridged CO
adsorption on the surface of platinum catalysts simultaneously although Ag was incorporated into Pt/SiO2. The ethylene microcalorimetric results show that the decrease of ensemble size of Pt surface sites suppresses the formation
of dissociative species (ethylidyne) upon the chemisorption of C2H4on Pt-Ag/SiO2. The differential heat vs. uptake plots for C2H4adsorption on the oxygen-preadsorbed Pt/SiO2and Pt-Ag/SiO2catalysts suggest that the incorporation of Ag to Pt/SiO2could decrease the ability for the oxidation of C2H4.
Authors:José Ortiz-Landeros, Tatiana L. Ávalos-Rendón, Carlos Gómez-Yáñez, and Heriberto Pfeiffer
CO 2 at relatively high temperatures (400–600 °C). Since then, several works have studied the CO 2 chemisorption of different lithium ceramics [ 16 – 62 ]. The ceramics that have been mainly studied are lithium zirconates (Li 2 ZrO 3 , Li 6 Zr 2 O 7
activities? The homogeneity or heterogeneity is considered as a fundamental feature of crystalline solids rather than as an individual feature of some samples.
In the context of the science on chemisorption and catalysis, the following definitions
Authors:Alfredo Román-Tejeda and Heriberto Pfeiffer
earth-alkaline ceramics have been proposed as possible CO 2 captors because of the chemisorption process occurring between CO 2 and the alkaline or the earth-alkaline atoms present in the ceramic, such as Li 4 SiO 4 , Li 2 ZrO 3 , Na 2 ZrO 3 , Li 5 AlO
Authors:P. Rossi, M. Bassoli, G. Oliveri, and F. Guzzo
We have studied by means of differential microcalorimetry the adsorption process of 1-propanol on α-Al2O3 at the temperatures of 25, 50, 100, 150 and 200°C, respectively. Both amounts of adsorbed alcohol and heats released decrease
as the temperature of adsorption increases. At very low coverage, the high value of differential heat shows a strong irreversible
chemisorption on active sites (Lewis acid sites) (qdiff>200 kJ·mol−1). Moreover, we carried out some thermokinetic investigations on heat emission peaks at different coverage degree (different
equilibrium pressure of 1-propanol vapour as a function of time) and at different temperatures of adsorption, at same coverage.
A differential microcalorimeter (E. Calvet) was used to study the processes of adsorption of five aliphatic alcohols (C1-C5) on α-Al2O3 at 25, 50, 100, 150 and 200°C. In particular, the importance of the thermokinetic study of the chemisorption of such alcohols
at different experimental temperatures was demonstrated, with regard to the variations in the thermokinetic parameters (tmax, t1/2 and t0) and the evolution of the alcohol vapor pressure on the adsorbent during the adsorption process. It was concluded that:
all the heat emission peaks of alcohol adsorption have the same rounded shape at 25°C;
on passing from methanol to 1-pentanol, the area of the adsorption peaks increases as the chain length or molecular weight
on passing from 25 to 200°C, the shape of the adsorption peaks becomes more pointed.
The mechanism of adsorption and desorption of ethyl cellulose (EC) on and from silver powder was studied. After analysis with DSC, XPS and dynamic FT-IR, which could be heated by a program-controlled heater, applied to observe the states of pure EC and of EC adsorbed on silver powder, and also the thermal decomposition of EC from silver powder, the chemisorption bonding bridge between the oxygen atoms of EC and the silver atoms investigated. The differences in thermal decomposition between pure EC and EC adsorbed in silver powder were also studied. It was concluded that the chemisorption bonding between the oxygen atoms of EC and the silver atoms mainly involved the lone-pair electrons of the oxygen atoms on the EC chain and the outer empty 5 sp3 hybrid orbital of the silver. Because of the formation of this bridge, the bonds between neighboring carbon and oxygen atoms are weakened. As a result of this effect, the C-O bonds in the EC chain are broken more easily than the C-H bonds, which leads to the observation of -C-H- fragments in the upper space when EC adsorbed on silver is heated. When the same experiments were carried out on pure EC, almost all kinds of broken fragments of the EC molecule, including -C-H-, -C=O- and -C-O- appeared simultaneously.
Catalysis of mixed oxide LaMnO3 was studied for the decomposition of hydrogen peroxide (H2O2). The catalyst was -irradiated in open petri dishes, vacuum, dry oxygen and moist oxygen. LaMnO3 irradiated in moist oxygen showed highest catalytic activity. X-ray photoelectron spectroscopic (XPS) studies were carried out to investigate the surface modifications occurred during -irradiaiton of LaMnO3. No significant change in the surface was noticed in LaMnO3 irradiated in vacuum and dry oxygen. However, LaMnO3 irradiated in moist oxygen and in open petri dishes showed the reduction of transition metal (MN3+ to Mn2+) which in turn leads to the formation of chemisorbed superoxide ions (O
) and surface carbonate species (CO
). The latter processes decreases the electrical conductivity by trapping the charge carriers. The hydrated electron generated by the radiolysis of moisture reduces the transition metal. A qualitative molecular orbital model has been proposed for the chemisorption of O
on the reduced transition metal centers (Mn2+).
Chemisorbed oxygen can be determined quantitatively by the measurement of gaseous N2/N2O liberated by treatment with hydrazine sulphate/hydroxylamine hydrochloride. The amount of chemisorbed oxygen depends on the degree of dispersion during irradiation and also the -dose. The chemisorption is enhanced in the presence of moisture. The partial reduction of the transition metal ion favours the formation of chemisorbed oxygen.