C.N. Costa

Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/CeO2 catalyst

A mathematical model has been developed for the first time to study the oxygen storage capacity (OSC) phenomenon by the CO pulse injection technique over a 1 wt% Pd/CeO2 model catalyst in the 500–700 ◦ C range. A two-step reaction path that involves the reaction of gaseous CO with the oxygen species of PdO (pre-oxidized supported palladium particles in the 500–700 ◦ C range) and of the backspillover of the oxygen process from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the outlet CO pulse transient response and the experimentally measured quantity of OSC (µatoms of O/g) obtained in a CSTR microreactor. With the proposed mathematical model, the transient rates of the CO oxidation reaction and of the back-spillover of the oxygen process can be calculated. In the 500–700 ◦ C range, the transient rate of CO oxidation was always greater than that of the back-spillover of oxygen. The ratio, ρ ,o f the maximum CO oxidation rate to the maximum back-spillover of the oxygen rate was found to decrease with increasing reaction temperature in the 500–700 ◦ C range. In particular, at 500 and 700 ◦ Ct he value ofρ was found to be 1.6 and 1.2, respectively. The present mathematical model allows also the calculation of the intrinsic rate constant k1 (s −1 ) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 9.2 kJ/mol was estimated for this reaction step. In addition, an apparent rate constant k app (s −1 ) was estimated for the process of back-spillover of oxygen. The ratio of the two rate constants (k1/k app ) was found to be greater than 100 in the 500–700 ◦ C range. A Langmuir–Hinshelwood surface elementary reaction step of adsorbed CO with atomic oxygen of PdO failed to describe the experimental transient kinetics of CO oxidation in the 500–700 ◦ C range. The results of the present work provide the means for a better understanding of the effects of various additives and contaminants present in a three-way commercial catalytic converter and other related model catalysts on their OSC kinetic behavior. In addition, intrinsic effects of a given regeneration method for a commercial three-way catalyst on the OSC phenomenon could better be studied by making use of the results of the present mathematical model.

Catalytic behavior of La-Sr-Ce-Fe-O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La 0.5 Sr x Ce y FeO z were prepared by using the nitrate method and characterized by XRD and Mossbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3- and oxides α-Fe 2 O 3 and CeO 2 depending on x and y values, The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4 +O 2 (lean-NO, ) reactions in the temperature range 250-550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5 Sr x Ce y FeO z materials to reduce NO by CO or by CH 4 under lean-NO x conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s -1 ) at 300 C (based on NO chemisorption) revealed values comparable to Rh/α-Al 2 O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5 Sr x Ce y FeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5 Sr 0.2 Ce 0.3 FeO z and La 0.5 Sr 0.5 FeO z solids.

The mechanism of reduction of NO with H2 in strongly oxidizing conditions (H2-SCR) on a novel Pt/MgO-CeO2 catalyst: Effects of reaction temperature

Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments using on-line Mass Spectrometry (MS) and in situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) have been performed to study essential mechanistic aspects of the Selective Catalytic Reduction of NO by H2 under strongly oxidizing conditions (H2-SCR) in the 120–300°C range over a novel 0.1 wt % Pt/MgO-CeO2 catalyst. The N-path of reaction from NO to the N2 gas product was probed by following the 14NO/H2O2 → 15NO/H2/O2 switch (SSITKA-MS and SSITKA-DRIFTS) at 1 bar total pressure. It was found that the N-pathway of reaction involves the formation of two active NOx species different in structure, one present on MgO and the other one on the CeO2 support surface. Inactive adsorbed NOx species were also found on both the MgO-CeO2 support and the Pt metal surfaces. The concentration (mol/g cat) of active NOx leading to N2 was found to change only slightly with reaction temperature in the 120–300°C range. This leads to the conclusion that other intrinsic kinetic reasons are responsible for the volcano-type conversion of NO versus the reaction temperature profile observed.

The CH4/NO/O2 “Lean-deNOx” Reaction on Mesoporous Mn-Based Mixed Oxides

High surface area Mn-based porous oxides (MANPO) containing additives like Ce, Sr and La were found to be very active and selective materials under 0.67% CH4/0.2% NO/5% O2 “lean-deNOx” conditions in the 200–300°C low-temperature range. These materials perform also impressively in the presence of 4% H2O in the feed stream, where a N2 selectivity of 98% and an excellent stability over 24 h on stream have been observed. The MANPO materials can be considered serious competitors of noble metals for low-temperature “lean-deNOx” applications.