H2_2 [Pt(C2_2O4_4)2_2] as a Tailor‐made Halide‐free Precursor for the Preparation of Diesel Oxidation Catalysts: Nanoparticles Formation, Thermal Stability and Catalytic Performance

The aim of this study was to investigate a tailor-made metal precursor and its chemical properties to tune the properties of supported metal nanoparticles (NPs) and their catalytic performance when used as Diesel Oxidation Catalyst (DOC). The formation of extremely small Pt NPs from a new halide-free Pt complex was investigated, namely bis(oxalato)platinate, H$_2$ [Pt(C$_2$O$_4$)$_2$]. The size evolution of the supported NPs, from the formation upon the Pt precursor decomposition on γ-alumina to the sintering of the NPs at high temperatures, was followed by thermogravimetric analysis coupled with mass spectrometry (TG-MS) and differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. A correlation between the NPs’ size of the catalyst and the performance for the CO, C$_3$H$_6$, C$_3$H$_8$ and NO oxidation reactions pointed out a retained activity during test cycles, showing low sensitivity to the test conditions applied (i. e., temperature and gas composition). The overall catalytic performance was better in the fresh catalysts compared to the reference catalyst prepared from platinum nitrate, Pt(NO$_3$)$_4$. In particular, the different dispersion of the Pt NPs over the support obtained from the two precursors was identified as the reason for the different catalytic performance, retaining small NPs size after the tests cycles

Towards FIB-SEM Based Simulation of Pore-Scale Diffusion in SCR Catalyst Layers

The diffusivity in the upper Cu-Chabazite layer of a dual layer ammonia oxidation catalyst with a lower Pt layer was investigated. In a first step, the pore structure of the upper Cu-Chabazite catalyst layer was determined by Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) slice&view tomography. From the FIB-SEM data the 3D pore structure of the catalyst was reconstructed and diffusion simulations were performed on the reconstructed pore geometry, resulting in an estimated effective diffusivity of D$_{eff}$/D$_{gas}$ = 0.31. To validate the FIB-SEM derived estimates of the diffusivity, measurements of CO oxidation on the dual layer catalyst were performed, where the CO was oxidized in the lower Pt-layer while the upper SCR layer served as an inactive diffusion barrier. In this way, the effective diffusivity can be determined from the measured CO conversion. An effective diffusion coefficient of D$_{eff}$/D$_{gas}$ = 0.11 was obtained from the CO oxidation measurements, three times lower than the value obtained from the FIB-SEM data, but in line with previous literature data for the effective diffusivity in monolith washcoat layers. Additional NH$_{3}$ oxidation experiments were performed on the dual layer catalyst. The results were well reproduced by a reactor model applying the effective diffusion coefficient obtained by the CO oxidation experiments. The origin of this apparent inconsistency is currently not understood and requires further investigation

Ageing Effects on Exhaust Gas Catalysts: Microscopic Changes Captured by X-Ray Tomography

In this work we examine the different aspects of catalyst ageing with effects ranging from the nano to the macro scale. Underlining the general importance of combining different characterisation techniques, like transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) for the nanoscale, we focus on the application of X-ray absorption micro-computed tomography (micro-CT) to capture macroscopic changes in the um to mm scale. Two series of tomographic measurements were carried out: (i) investigation of three differently treated samples by collecting one channel from a fresh, a conditioned and an aged monolith and (ii) examination of one single coated honeycomb channel with 4 wt% Pt/γ-Al2O3 and for comparison one with pure γ-Al2O3 washcoat, which have been measured in a non-destructive ex situ manner at the same position after each ageing treatment.Main observations of the tomographic study are: (1) coating inhomogeneities between different channels taken from the same honeycomb and between different honeycombs, (2) formation of cracks in the washcoat material and (3) formation of macroscopic Pt particles in the case of 4 wt% Pt/γ-Al2O3 washcoat. Particularly valuable is the non-destructive ex situ investigation after different ageing steps on the same channel using X-ray tomography

Ignition and Oxidation of Dilute Silane-Oxidizer Mixtures Behind Reflected Shock Waves

High-temperature experiments were performed behind reflected shock waves using SiH4, SiH4/O2, H2/O2, SiH4/H2/O2, CH4/O2, and SiH4/CH4/O2 mixtures highly diluted in argon. Reflected-shock temperatures ranged from 1050-2250 K at a pressure near 1 atm. Reaction progress was monitored by observing the time histories of several species, including OH*SiH4, SiH2, and CH*using emission and laser absorption techniques. The oxidation and ignition data are reported in the form of species concentration profiles and plots of characteristic times as a function of temperature. The species time histories exhibit a dual oxidation behavior for most of the fuel/O2/SiH4 experiments, indicating the silane reacts first followed by reaction of the fuel with the remaining oxygen. As expected, the presence of silane in the fuel/O2 mixtures markedly reduced the ignition delay time, usually by a factor of two or more even at molar SiH4 concentrations as low as 1% of the fuel concentration. The database presented herein can be used to validate chemical kinetics models of hightemperature silane oxidation for propulsion and materials processing applications. © 2002 by The Aerospace Corporation

Density and production of NH and NH2 in an Ar-NH3 expanding plasma jet

The densities of NH and N H2 radicals in an Ar-N H3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and N H3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the A ¿3 ¿ X ¿-3 transition for NH and the (0,9,0)-(0,0,0) band of the A~ A12 ¿ X~ B12 transition for N H2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100 K were found, respectively. The measurements revealed typical densities of 2.5× 1012 cm-3 for the NH radical and 3.5× 1012 cm-3 for the N H2 radical. From the combination of the data with ion density and N H3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and N H2 are discussed. © 2005 American Institute of Physics