Gas adsorption and desorption effects on cylinders and their importance for long-term gas records

Abstract

Abstract. It is well known that gases adsorb on many surfaces, in particular metal surfaces. There are two main forms responsible for these effects (i) physisorption and (ii) chemisorption. Physisorption is associated with lower binding energies in the order of 1–10 kJ mol−1 compared to chemisorption ranging from 100 to 1000 kJ mol−1. Furthermore, chemisorption forms only monolayers, contrasting physisorption that can form multilayer adsorption. The reverse process is called desorption and follows similar mathematical laws, however, it can be influenced by hysteresis effects. In the present experiment we investigated the adsorption/desorption phenomena on three steel and three aluminium cylinders containing compressed air in our laboratory and under controlled conditions in a climate chamber, respectively. We proved the pressure effect on physisorption for CO2, CH4 and H2O by decanting one steel and two aluminium cylinders completely. The CO2 results for both cylinders are in excellent agreement with the pressure dependence of a monolayer adsorption model. However, adsorption on aluminium (< 0.05 and 0 ppm for CO2 and H2O) was about 10 times less than on steel (< 0.41 ppm and about < 2.5 ppm, respectively). The CO2 amounts adsorbed (5.8 × 1019 CO2 molecules) corresponds to about the five-fold monolayer adsorption indicating that the effective surface exposed for adsorption is significantly larger than the geometric surface area. Adsorption/desorption effects were minimal for CH4 and for CO. However, the latter dependence requires further attention since it was only studied on one aluminium cylinder with a very low mole fraction. In the climate chamber the cylinders were exposed to temperatures between −10 and +50 °C to determine the corresponding temperature coefficients of adsorption. Again, we found distinctly different values for CO2 ranging from 0.0014 to 0.0184 ppm °C−1 for steel cylinders and −0.0002 to −0.0003 ppm °C−1 for aluminium cylinders. The reversed temperature dependence for aluminium cylinders point to significantly lower desorption energies than for steel cylinders and might at least partly be due to temperature and gas consumption induced pressure changes. Temperature coefficients for CH4, CO and H2O adsorption were, within their error bands, insignificant. These results do indicate the need for careful selection and usage of gas cylinders for high precision calibration purposes such as requested in trace gas applications.