Showing papers by "Luca Lietti published in 2011"

Detailed Kinetics of the Fischer–Tropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

Abstract: A complete mechanistic kinetic model of the Fischer–Tropsch synthesis (FTS) over a Co/Al2O3 state-of-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H2 consumption and n-paraffins, α-olefins and H2O formation are derived. Such expressions are functions of the molar fraction of CO, H2 and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by non-linear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8–25 bar, T = 210–235 °C, H2/CO feed ratio = 1.8–2.7 mol mol−1, GHSV = 2,000–7,000 cm3(STP) h−1 g cat −1 ). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer–Tropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.

Basic catalysis and catalysis assisted by basicity: FT-IR and TPD characterization of potassium-doped alumina

Abstract: Adsorbents and catalyst supports produced by loading different amounts of potassium acetate on alumina followed by calcination are characterized using IR spectroscopy and TPD. CO, CO2 and NOx adsorption is studied. Two different ranges of potassium loading on alumina are found, namely light and heavy doping corresponding to 1% and more than 3% K wt/wt, respectively. Light doping results in the weak adsorption of CO2 as bicarbonate species and NO2 as bidentate nitrates. Heavy doping results in the adsorption of CO2 as bidentate carbonates and NO2 as polydentate nitrate species. Three families of exposed K+ ions are observed at increasing loading.

The NOx reduction by CO on a Pt-K/Al2O3 Lean NOx trap catalyst

Abstract: The reduction by CO under dry conditions of NOx species stored at 350 °C onto a Pt−K/Al2O3 lean NOx trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FT-IR spectroscopy. The results show that the pathway for the reduction of stored NOx by CO under dry and near isothermal conditions (and in the absence of CO2) is the same as that proposed by some of us in a previous work for a Pt−Ba/Al2O3 catalyst. In particular, (i) the reduction of stored NOx by CO occurs according to a Pt-catalyzed surface pathway that does not involve, as a first step, the release of NOx in the gas phase, is effective already at low temperature, and leads to nitrogen; (ii) the reaction scheme implies the formation of surface isocyanate species, followed by the reaction of these species with residual NOx to give nitrogen; and (iii) the reaction of NCO species with nitrates to give nitrogen is slightly slower than the reduction of nitrates to give NCO species; but on t...