Through micromachining, it has become possible to realize mechanical structures of small dimensions with high reproducibility. There are numerous fields of application for such devices. The miniaturised structures have not only been used to improve existing techniques, but they have provided us with totally new possibilities. In this work, gas flow and chemical reactions were investigated in shallow (~ 100 nm) micromachined channel, with an extreme length-to-depth ratio (~500000). The flow through the channels is molecular, also at atmospheric pressures. Besides, the number of wall collisions suffered by the transmitted molecules is of the order of(L/d)2 .(~ 2.5 x 1011) Any surface induced phenomena, which occurs upon one wall collision, will thus be enhanced by the numerous wall interactions. Especially, a chemical reaction, which occurs with extremely small probability when a molecule strikes the wall once, may occur with unit probability for the transmitted molecules. In short, the channel is an instrument, which through its geometry makes certain surface phenomena observable. Several findings are presented, which contributes to the understanding of surface induced effects, such as adsorption, reflection and chemical reactions. Besides, the characterizations of inert and reactive flows in the micromachined channels meet an increased need to understand the physics inside of submicron structures.
Two different kinds of channel structure model catalysts were used, and the thesis contains descriptions of their respective design and fabrication. The channels were occasionally made catalytically active through evaporation of a 10 Å thick Pt filmalong the bottom. The experiments were performed with a channel structure inserted as a leak between a gas cell and an ultra-high vacuum (UHV) mass spectrometer chamber. When gas is flown into the gas cell, molecules from the gas impinge on the channel orifice and a few of them diffuse through the channel into the UHV chamber. The transmitted molecules were recorded with mass spectrometer.
Gas transport at atmospheric pressures under Knudsen-like conditions in Si-quartz glass channels was thoroughly characterized and modelled.
The water forming reaction from oxygen and hydrogen/deuterium on Pt was investigated in SiO2/Pt-quartz glass channel structures. The channel walls were found to act as a sink for water molecules. Furthermore, it was found found that oxygen adsorbates more easily block for hydrogen adsorption/dissociation than vice versa. Data is presented, which confirm that the water forming reaction in the channel proceeds until, and even after, the channel is obstructed with water and thus leak tight. When the channel is drained through spontaneous desorption, water is released at an almost constant rate from the moment the channel has opened until all water has left the channel.
Hydrogenation of ethylene was investigated in a SiO2/Pt-quartz glass channel and evidence for a process, which involves hydrogen adsorption, possibly induced by ethylene deposits, was found. This process is suggested to promote the continuous hydrogenation reaction.
Hydrogen induced C02 formation from ethylene deposits on Pt during consecutive H2 and 02 exposures was observed in a SiO2/Pt-quartz glass channel. Frequent switching of the H2/02 exposure pulses was found to increase the efficiency of the oxidation of the carbonaceous deposits markedly. This result may, for instance, have implications for conditioning of various catalysts.