Pellistor poisoning reduces the reaction of the sensor to a target gas due to the presence of different types of gases/vapours. They react with the surface of the catalyst and decrease its capacity of oxidizing the target gas, it is also referred to as its catalytic activity. They can be categorised based on the degree of reversibility of the effect.
Poisons can be classfied as compounds with irreversible effects. Basic poisons are organic silicon compounds, organic phosphate esters, and organo-metallic compounds. Pellistor sensors are not designed for complete resistance to poisons as they are usually non-porous, tend to lose over 90% of their reaction to methane within minutes of being exposed to even 10ppm of an organic silicon compound.
Compounds are classified as inhibitors if there is a recovery of the pellistor response on elimination of the compund. Usual inhibitors are halogen-containing hydrocarbons. Compounds with sulfur, e.g. H2S, can display both tendencies based on the concentration and the exposure time. The rate of recovery and the degree of poison resistance reveal that there is an increase with the operating temperature of the sensor.
Mechanism of Poisoning
Two compound types can be differentiated by the manner in which they adsorb on the catalyst surface and what compounds are subsequently generated.
For both compound types, the catalytic activity is decreased as the poison or inhibitor adsorbs more strongly onto the catalyst surface than the target gas. This adsorption stage is crucial to the overall reaction. Therefore the number of available catalytic sites are decreased or covered, thereby resulting in decrease of the rate of the reaction with the target gas.
In the case of the inhibitor, the process reaches equilibrium, which is based on an operation of the temperature and the concentration of inhibitor. Once the inhibitor is eliminated, the molecules of inhibitor adsorbed on the surface are desorbed and the sites are then accessible for reaction with the target gas and the catalytic activity and signal are re-established.
In the case of a poison, there is no set equilibrium and the response with the catalytic site cannot be reversed. There is an increase in site coverage and theoretically, the reaction can be decreased to zero despite poisoning species concentration. The time taken for this to occur is based on the concentration.
The degree of signal loss is also based on the target gas. For instance with methane, the oxidation reaction occurs only on the catalytic sites with the most activity, thus it is the methane signal that is most affected by poisoning and inhibition.
More specifically organic silicon compounds react on these sites to form a silica-type compound layer. The effect of the presence of poisons on the response to gases that are easier to oxidise, e.g. butane, is much less than that for methane. Pellistors have lost nearly all of their response to methane due to silicon poisoning but have not reacted to butane.
Methods to Reduce Poisoning Effects
Limitation of Diffusion Rate
The poisoning rate and the diffusion rate of the poisoning species to the detecting element is related, therefore decrease reduction in the diffusion rate causes a reduction in the effect. This can be realized by fitting a fine mesh or sinter in front of the detecting element. It can also be achieved by fixing a can with a small hole above the detecting element. This will enhance the degree of resistance to poisons, however the target gas signal will drop as its rate of diffusion to the detecting element is minimized. Nevertheless, any improvement will not be sizeable.
There are many commercial filter models available for the purpose of removing pellistor poisons. Many are activated carbon types. These compounds possess a large absorption capacity however have a slight disadvantage that in case of the structure being too fine, there is a decrease of the gas flow into the sensor and the response time to the target gas will increase.
Open-weave carbon cloths provide a balance between response time and absorption capacity. One major problem with this type of filter is that hydrocarbons above C4 are partially or totally absorbed, thus limiting the application of the sensor in areas such as coal mining, where these types of compounds are not mostly not present.
Detecting Bead Design
A key technique to improving poison resistance is to raise the bead’s intrinsic catalytic by increasing the number of sites. This is most easily performed by just increasing the size of the detecting element.
ven in the case of pellistors that are not designed to be poison-resistant, larger beads a higher resistance than smaller beads. However, just increasing the bead size is not a solution for low power sensors of portable tools. In this case the increase in the number of catalytic sites is caused by creating porous bead and diffusing the catalyst site all over the pores.
The total size and power needs of the small bead are maintained as it is. The porous structure has another property restricting the contact of high molecular weight poisons to the catalytic sites and at the same time allowing the reactant and product gases to diffuse in and out of the pores.
Low power pellistors made using these techniques aid in improving the time taken to lose 10 % of the sensor output in 20ppm of silicon from several seconds to about an hour. In the case of larger high power sensors, this can be increased to over 10hrs.
Portable gas detection tools use a number of pellistor sensors. While using this sensor it must be located in very close contact with parts of the tool. It is important to carefully choose the materials used in the instrument, ensuring that compounds which can desorb and behave as pellistor poisons are not used. Problem areas include parts such as conformal coatings, seals, and displays. It is suggested that all parts are tested before the tool is used in production.
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This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
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