A catalyst is any element or compund that promotes a chemical reaction but is not consumed in the reaction. Palladium has many and varied catalytic properties of research interest..
Below are original article abstracts with the url to complete article:
Title: Effect of longitudinal heat conduction on the catalytic ignition of carbon monoxide in a boundary layer
Author(s): C Treviño, F Méndez, J C Prince and F J Higuera
Publication date: June 2000 Volume: 4 Start page: 173
Publication: Combustion Theory and Modelling
URL: http://stacks.iop.org/1364-7830/4/173
Abstract: The catalytic ignition of dry carbon monoxide and air in a boundary layer flow over a palladium plate is studied in this paper. The heterogeneous reaction mechanism is modelled with the dissociative adsorption of the molecular oxygen and the non-dissociative adsorption of CO, together with a surface reaction of the Langmuir-Hinshelwood type and the desorption reaction of the adsorbed product, CO 2( s ). The critical condition for catalytic ignition, represented by the ignition Damköhler number, has been deduced using high activation energy asymptotics of the desorption kinetics of the most efficiently adsorbed reactant, CO( s ). Longitudinal heat conduction along the plate has been considered and its influence on the ignition temperature has been evaluated. This influence is rather weak, indicating that the flat plate boundary layer flow configuration is a robust device to determine the critical conditions for catalytic ignition.
Title: Electrical characterization of a dc secondary discharge created during plasma sputtering deposition of palladium thin films
Author(s): A L Thomann, C Charles, N Cherradi and P Brault
Publication date: May 2000 Volume: 9 Start page: 176
Publication: Plasma Sources Science and Technology
URL: http://stacks.iop.org/0963-0252/9/176
Abstract: Good quality Pd thin films (for catalysis application) can be obtained by a low-pressure plasma sputtering method. The metal atom source is a helicoidal wire which is negatively biased so that argon ions created in a high frequency plasma are attracted and gain sufficient energy to induce sputtering. Depending on the experimental conditions (gas pressure, wire bias voltage), the sputtering process occurs in different regimes corresponding to either the presence of a simple cathodic sheath or the breakdown of an hollow cathode type discharge inside the helix. Since the flux of metal atoms condensing onto the substrate depends on the sputtering characteristics, it is of particular interest to study the formation of this secondary discharge (breakdown voltage and density profile). This is carried out by measuring the Langmuir saturation current profile inside and around the helix, and correlating the results with the determination by Rutherford backscattering spectroscopy analysis of the deposition rate on a SiO 2 substrate. We also present density profiles of the argon plasma along the main axis of the reactor, which show that the flux of argon ions onto the substrate is not affected by the nature of the helix discharge. Finally, the ability to extend the range of deposition conditions (especially the ion flux to metal atom flux ratio) by using the helix diameter as an additional parameter is discussed.
Title: Enhanced deposition rates in plasma sputter deposition
Author(s): A L Thomann, C Charles, P Brault, C Laure and R Boswell
Publication date: August 1998 Volume: 7 Start page: 245
Publication: Plasma Sources Science and Technology
URL: http://stacks.iop.org/0963-0252/7/245
Abstract: Langmuir probe and emission spectroscopic measurements are performed in a high frequency (100 MHz) argon plasma used for the sputter deposition process of thin films of palladium (dedicated to catalysis applications). The metal source is a helicoidal palladium wire which is negatively biased with respect to the plasma potential. This induces sputtering by the ions present in the plasma. The probe results show that the presence of the helicoidal wire in the chamber does not affect the total ion flux at the substrate location. However, as the bias voltage on the wire and/or the argon pressure are increased, a secondary direct current (DC) discharge is created inside the helicoidal wire which follows a Paschen-like law; the breakdown voltage is lower than in the case of a conventional Ar discharge, probably as a result of the presence of primary electrons generated by the main high frequency (HF) plasma. This second discharge is characterized by a strong flux peak inside the helicoidal wire, which probably arises from a hollow cathode type discharge. From emission spectroscopy and deposition analysis, it is shown that this secondary plasma causes an increase of the sputtered Pd atom number and, consequently, an enhanced deposition rate.
Title: Methane sensing: from sensitive thick films to a reliable selective device
Author(s): H Debéda, P Massok, C Lucat, F Ménil and J L Aucouturier
Publication date: January 1997 Volume: 8 Start page: 99
Publication: Measurement Science and Technology
URL: http://stacks.iop.org/0957-0233/8/99
Abstract: This paper shows the long route between gas-sensitive tin oxide material and a reliable prototype for selective methane sensing. A good reproducibility of fabrication is a prerequisite to any development of a cheap sensing device. Hybrid thick-film technology appears well adapted for this purpose, on the condition that the rheological properties of the pastes are well controlled.
Sensitivity was shown to be modified by the thickness of the tin oxide layer, the addition of palladium, the addition of a glass binder, the nature of the electrode metal and the presence of a coating.
Methane selectivity was achieved by adapting a concept proposed previously, based on the differential catalytic activity of palladium and platinum versus the oxidation of methane at and by combining two sensing principles on the same alumina substrate.
As for long-term stability, much information could be gained by annealing at temperatures higher than that of operation. Metal interdiffusion in the sensor connections (Kirkendall effect) and between the catalysts and the other metallic parts of the sensor was demonstrated to be a major cause of instability. Moreover, the firing and annealing conditions used in the fabrication process of our sensors, including that of the Pt heating resistor, were also shown to influence the reliability.
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