Palladium dispersion effects on wet methane oxidation kinetics

The catalytic activity for dry and wet methane oxidation over a series of palladium–alumina catalysts with palladium loadings from 0.23 to 3.6 wt% Pd and systematically varied PdO dispersions from 8.1 to 39% was evaluated by flow reactor measurements and compared with multiscale simulations. The catalysts were prepared by industrially relevant incipient wetness impregnation followed by controlled calcination to provide similar active surface area with a realistic contact between active PdO nanoparticles and the alumina support. Kinetic analysis reveals that in wet conditions, the apparent activation energy for methane oxidation decreases as the PdO particle size increases as opposed to dry conditions where it increases. Active sites at the rim of the PdO particles in contact with the alumina support seem to contribute more to the overall activity under dry conditions but are more sensitive to wet conditions than PdO sites farther away from the rim. This sensitivity is likely due to more severe blocking by hydroxyl groups formed by water dissociation and reversed spillover. Simulations support that PdO bound hydroxyls well may form under the present reaction conditions. It is envisaged that the design of palladium–alumina catalysts for high methane turn-over frequency should target high but not too high PdO dispersion, i.e., the PdO particles should not be smaller than about 2 nm, as to balance water tolerance and palladium utilisation

Hampered PdO Redox Dynamics by Water Suppresses Lean Methane Oxidation over Realistic Palladium Catalysts

By use of operando spectroscopies under cycling reaction conditions, water is shown to hamper the redox dynamics of realistic palladium oxide nanoparticles dispersed onto alumina and hydrophobic zeolite supports thereby lowering the activity for total oxidation of methane. Water adsorption forms hydroxyl ad-species that block the methane and oxygen dissociation and seem to prevent lattice oxygen to take part in the methane oxidation. The main catalytic action is thus proposed to shift from the Mars-van Krevelen mechanism in dry conditions to a slower route that relies on Langmuir-Hinshelwood type of steps in wet conditions. This key finding has clear implications on catalyst design for low-temperature gas combustion emission control

NOx storage in barium-containing catalysts

The effect of key parameters on the characteristics of barium oxide-based NOx storage catalysts was systematically investigated. Model Pt/BaO/Al2O3, BaO/Al2O3, Pt–Rh/Al2O3, and Pt–Rh/BaO/Al2O3 catalysts were prepared and evaluated with respect to NOx storage capacity using transient flow reactor studies, temperature-programmed desorption studies (TPD), and in situ Fourier transform infrared (FTIR) absorption spectroscopy. The influence of temperature, storage and regeneration times, NOx source (NO or NO2), oxygen concentration, reducing agent (C3H6, C3H8, CO, or H2), and carbon dioxide concentration onNOx storage capacity was studied. Significant amounts of NOx were found to be stored in the catalysts containing both barium oxide and noble metals. For these catalysts the following observations were made: (1) maximum NOx storage was observed at about 380C; (2) around this temperature no significant differences between NO and NO2 on NOx storage capacity could be observed; (3) a slow increase in stored NOx could be observed with increasing oxygen concentration during the lean phase; (4) significant NOx desorption peaks, mainly of NO, were observed immediately after the switch from lean to rich conditions; and (5) at about 380±C the in situ FTIR spectra show characteristic nitrate peaks in the region 1300–1400 cm¡1 when NOx was stored under lean conditions and isocyanate peaks around 2230 cm-1 when the catalysts were regenerated under rich conditions in the presence of hydrocarbons. The step leading to stored NOx is believed to involve NO2 and the presence of atomic oxygen. During the rich period, the noble metal surfaces are probably reduced, leading to breakthrough peaks when NO desorbs

Deceleration of SO2 poisoning on PtPd/Al2O3 catalyst during complete methane oxidation

The inhibiting effect of SO2 on the catalytic activity of the monometallic Pt/Al2O3 and Pd/Al2O3, as well as on bimetallic PtPd/Al2O3 catalyst for the complete oxidation of methane under lean conditions has been studied. Flow reactor experiments, in-situ DRIFT spectroscopy and characterization with XPS, STEM-EDX were performed. It was found that the addition of Pt to the Pd/Al2O3 resulted in a catalyst that was more robust towards sulfur poisoning. XPS results revealed residual sulfates on catalyst surface after regeneration. This was confirmed with EDX analysis, which demonstrated that sulfur was accumulated in noble metal particles and especially in the region of the particle rich in Pt. Although the catalyst has been deactivated in the presence of both SO2 and H2O, an additional presence of NO in the gas mixture of reactants resulted in an increased lifetime of the sample under reaction conditions. This NO effect strongly depends on the temperature of experiments and is most intense at a temperature close to 550 degrees C A postponed inhibition caused by the addition of NO may be explained by the DRIFTS results, which demonstrated that the presence of NO lowers the sulfate formation and mostly surface sulfites are observed that increase the lifetime of the catalyst during SO2 exposure

The effect of water on methane oxidation over Pd/Al2O3 under lean, stoichiometric and rich conditions

In this study, the effect of oxygen concentration and the presence of water on methane oxidation were examined over a Pd/Al2O3 catalyst. The physicochemical properties of the catalyst were investigated in detail using BET, XRD, STEM, O-2-TPO and CH4-TPR. Ramping experiments from 150 to 700 degrees C were conducted using rich, stoichiometric and lean gas mixtures in the absence and presence of water. It was found that increasing the oxygen concentration in a dry atmosphere resulted in higher methane oxidation activity, which can be connected to the facilitation of palladium oxide formation. The TPO data showed that only minor amounts of PdO up to 700 degrees C were decomposed; however, in the stoichiometric and rich reaction mixture, PdO was still decomposed because of the oxygen limitation. This fact resulted in a "negative activation" during cooling, with increased activity because of palladium re-oxidation. Moreover, methane steam reforming and water gas shift reactions were important reactions under rich conditions over the metallic palladium sites. A significant inhibiting effect of water on the Pd-catalyst with loss of methane activity was found. Interestingly, the inhibition effect was much greater using high oxygen concentration in the gas mixture (500 ppm CH4, 8% O-2, 5% H2O) than that at lower oxygen levels (800-1200 ppm) and we propose that the hydroxyl species formation, which blocks the active sites, are facilitated by a large oxygen excess. In addition, the re-oxidation of palladium occurring during the cooling ramp in dry feed using rich and stoichiometric gas mixtures was also significantly suppressed in the presence of a large amount of water. Thus, water impedes the oxidation of palladium, which significantly deactivates the Pd catalyst

Sulphur dioxide interaction with NOx storage catalysts

The effect of SO2 on the NOx storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al2O3 NOx storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO2 to a synthetic lean exhaust gas caused deactivation of the NOx storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NOx storage capacity was found to be proportional to the total SO2 dose that the catalyst had been exposed to. SO2 was found to be accumulated in the catalyst as sulphate