Effect of periodic lean/rich switch on methane conversion over a Pd/Rh-based three way catalyst in the exhausts of natural gas vehicles

The behavior of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst for the abatement of methane from the exhausts of natural gas vehicles (NGVs) is studied in presence of large amounts of water under both stationary conditions and by periodically switching from lean to rich feed. Under stationary conditions with both stoichiometric (λ = 1.00) and lean (λ = 1.02) feed catalyst deactivation is observed after prolonged exposure to the reaction mixture. Periodic rich pulses in a constant lean feed gas result in the stabilization of catalytic performances.A higher methane conversion than those obtained with stoichiometric and lean feed mixtures is observed under rich conditions, during an experiment carried out by performing lean pulses (λ = 1.02) in a constant rich feed gas (λ = 0.98). The analysis of reactants conversion and products distribution suggests that different chemistries are involved under lean and rich conditions. Only reactions of complete oxidation of H2, CO, CH4 and NO occur under excess of oxygen, whereas under rich conditions NO reduction, CH4 steam reforming and water gas shift also occur.The effect of symmetric oscillation of the exhausts composition around stoichiometry is also addressed by periodically switching from slightly rich to slightly lean composition with different oscillation amplitudes (Δλ = \ub10.01, \ub10.02 and \ub10.03). Higher and more stable methane conversion performances are obtained than those observed under constant λ operations. The presence of a more active PdO/Pd0 state is suggested to explain the enhancement of catalytic performances

Effect of periodic lean/rich switch on methane conversion over a Pd/Rh-based three way catalyst in the exhausts of natural gas vehicles

The behavior of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst for the abatement of methane from the exhausts of natural gas vehicles (NGVs) is studied in presence of large amounts of water under both stationary conditions and by periodically switching from lean to rich feed. Under stationary conditions with both stoichiometric (λ = 1.00) and lean (λ = 1.02) feed catalyst deactivation is observed after prolonged exposure to the reaction mixture. Periodic rich pulses in a constant lean feed gas result in the stabilization of catalytic performances.A higher methane conversion than those obtained with stoichiometric and lean feed mixtures is observed under rich conditions, during an experiment carried out by performing lean pulses (λ = 1.02) in a constant rich feed gas (λ = 0.98). The analysis of reactants conversion and products distribution suggests that different chemistries are involved under lean and rich conditions. Only reactions of complete oxidation of H2, CO, CH4 and NO occur under excess of oxygen, whereas under rich conditions NO reduction, CH4 steam reforming and water gas shift also occur.The effect of symmetric oscillation of the exhausts composition around stoichiometry is also addressed by periodically switching from slightly rich to slightly lean composition with different oscillation amplitudes (Δλ = \ub10.01, \ub10.02 and \ub10.03). Higher and more stable methane conversion performances are obtained than those observed under constant λ operations. The presence of a more active PdO/Pd0 state is suggested to explain the enhancement of catalytic performances

SO2 adsorption on silica supported iridium

The interaction of SO2 with Ir/SiO2 was studied by simultaneous in situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, exposing the sample to different SO2 concentrations ranging from 10 to 50 ppm in the temperature interval 200–400 ◦C. Evidences of adsorptionof sulfur species in both absence and presence of oxygen are found. For a pre-reduced sample in the absence of oxygen, SO2 disproportionates such that the iridium surface is rapidly saturated with adsorbed S while minor amounts of formed SO3 may adsorb on SiO2. Adding oxygen to the feed leads to the oxidation of sulfide species that either (i) desorb as SO2 and/or SO3, (ii) remain at metal sites in the form of adsorbed SO2, or (iii) spillover to the oxide support and form sulfates (SO42−). Notably, significant formation of sulfates on silica is possible only in the presence of both SO2 and O2, suggesting that SO2 oxidation to SO3 is a necessary first step in the mechanism of formation of sulfates on silica. During the formation of sulfates, a concomitant removal/rearrangement of surface silanol groups is observed. Finally, the interaction of SO2 with Ir/SiO2 depends primarily on the temperature and type of gas components but only to a minor extent on the inlet SO2 concentration

Vibrational Study of SOx Adsorption on Pt/SiO2

The formation of ad-SOx species on Pt/SiO2 upon exposure to SO2 in concentrations rang- ing from 10 to 50 ppm at between 200 and 400◦C has been studied by in situ diffuse reflectance infrared Fourier transformed spectroscopy. In parallel, first-principles calculations have been carried out to consolidate the experimental interpretations. It was found that sulfate species form on the silica surface with a concomitant removal/ rearrangement of silanol groups. For- mation of ad-SOx species occurs only after SO2 oxidation to SO3 on the platinum surface. Thus SO2 oxidation to SO3 is the first step in the SOx adsorption process, followed by spillover of SO3 to the oxide and, finally, the formation of sulfate species on the hydroxyl positions on the oxide. The sulfate formation is influenced by both temperature and SO2 concentration. Furthermore, exposure to hydrogen is shown to be sufficiently efficient as to remove ad-SOx species from the silica surface

Mechanisms behind sulfur promoted oxidation of methane

The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied using a flow-reactor, in situ infrared spectroscopy and in situ high-energy X-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in the absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SOx species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporarily enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed

Vibrational Study of SO_<i>x</i> Adsorption on Pt/SiO₂

The formation of ad-SO_<i>x</i> species on Pt/SiO₂ upon exposure to SO₂ in concentrations ranging from 10 to 50 ppm at between 200 and 400 °C has been studied by in situ diffuse reflectance infrared Fourier transformed spectroscopy. In parallel, first-principles calculations have been carried out to consolidate the experimental interpretations. It was found that sulfate species form on the silica surface with a concomitant removal/rearrangement of silanol groups. Formation of ad-SO_<i>x</i> species occurs only after SO₂ oxidation to SO₃ on the platinum surface. Thus, SO₂ oxidation to SO₃ is the first step in the SO_<i>x</i> adsorption process, followed by spillover of SO₃ to the oxide, and finally, the formation of sulfate species on the hydroxyl positions on the oxide. The sulfate formation is influenced by both temperature and SO₂ concentration. Furthermore, exposure to hydrogen is shown to be sufficiently efficient as to remove ad-SO_<i>x</i> species from the silica surface

Investigation of Low-Temperature OHC and RHC in NH₃–SCR over Cu-CHA Catalysts: Effects of H₂O and SAR

A transient kinetic approach was applied to independently investigate the oxidation half-cycle (OHC) and reduction half-cycle (RHC) of NH3–selective catalytic reduction (SCR) at low temperature (150–200 °C). Three model Cu-exchanged chabazite (Cu-CHA) samples with fixed Cu loading (∼1.8% w/w) and different silica-to-alumina ratios (SARs = 10-17-25) were investigated under dry and wet conditions. We confirmed the following: (i) OHC proceeds via second- and first-order kinetics in CuI and O2, respectively, with O2 (+H2O) alone able to completely reoxidize CuI sites; (ii) RHC proceeds via second- and first-order kinetics in CuII and NO, respectively, according to a Cu/NO = 1:1 stoichiometry. Notably, coupling RHC and OHC rates resulted in an accurate prediction of the steady-state standard SCR conditions, providing the consistent closure of the SCR redox chemistry. Unexpectedly, we revealed the impact of H2O to vary depending on the catalyst formulation. At high SAR, water inhibits the RHC and promotes the OHC. As a result, a limited impact was observed on steady-state deNOx activity, while the Cu-oxidation state was significantly enhanced by H2O. With decreasing SAR, however, the H2O effect on RHC gradually shifts from inhibition to promotion, while the OHC is always promoted. At fixed water content, we revealed the RHC rate to decrease with decreasing Al density, with minor influence observed on the OHC; as a result, a lower deNOx activity was observed upon increasing SAR. Remarkably, the application of transient kinetic analysis to decouple RHC and the OHC greatly facilitated the identification of complex H2O and SAR effects on the global SCR redox chemistry