Trapping Planetary Noble Gases During the Fischer-Tropsch-Type Synthesis of Organic Materials

Introduction: \Vhen hydrogen, nitrogen and CO arc exposed to amorphous iron silicate surfaces at temperatures between 500 900K, a carbonaceous coating tonns via Fischcr-Tropsch type reactions!, Under normal circumstances such a catalyiic coating would impede or stop further reaction. However, we tind that this coating is a better catalyst than the amorphous iron silicates that initiate these rcactions:u . The formation of a self-perpetuating catalytic coating on grain surfaces could explain the rich deposits of macromolecular carbon found in primitive meteorites and would imply that protostellar nebulae should be rich in organic materiaL Many more experiments are needed to understand this chemical system and its application to protostellar nebulae. Planetary Noble Gases: If FTT reactions fonn macromolecular carbonaceous coatings on grain surfaces in the primitive solar nebula, then such reactions could be responsible for trapping ambient nebular gas, including the Planetary Noble Gas component found in many primitive rneteorites"6. As a corollary, if we can show that the planetary noble gases are trapped in such coatings. and measure the trapping efficiency for this process, then we might be able to use measurements of the noble gas content of meteorites and of samples returned from comets and asteroids to estimate the fraction of solar system organics that were produced via such reactions. \V c have begun to carry out such experiments in our laboratory. Experiment: The experiments reported here are very simple and were designed to test the relative dficiency of various catalysts as a function of temperature. We have slightly modified our procedure to simultaneously study the trapping efficiency of noble gases as the coatings arc produced. A schematic diagram of the closed-cycle apparatus is ShOWll in Figure 1. Following evacuation at room temperature using a liquid-nitrogen-trapped mechanical pump, we fill the system with our gas mixture, begin gas circulation, bring the catalyst up to its working temperature and begin to take our first spectnllll. The initiai heating rate depends on the intended temperature of the run and the time required to achieve a steady state temperature takes somewhat less than the 30-minutes required to obtain the initial FTIR spectrum (recorded at 2 cmresolution). Using the FTIR spectrometer, we monitor the depiction of CO (2044-2225cm"i). and the foonation of methane (2848-3195cm"i), ;..Jtt (959970cm'! 924-936cm"I), CO2 (2285-2388cm'i) and water Figure I. Schematic drawing of the system. The (smoke) catalyst is contained in the bottom finger of a 2-Uter Pyrex bulb that can be heated to controlled temperatures. A Pyrex tube brings reactive gas to the bottom of the finger. The gas then passes through the catalyst into the upper reservoir of the bulb. flows through a copper tube at room temperature to a glass-walled observation cell ( ZnSe windows) in an FTIR spectrometer and a closed-cycle metal bellows pump returns the sample via a second 2-liter bulb and the Pyrex tube to the bottom of the catalyst finger to start the cycle over again. The gas mixture initially consists of 75 torr N" 75 torr CO & 550 torr of H,. Total pressure is reduced as the reaction proceeds, is monitored via a diaphragm gauge placed in-line bet\vcen the two bulbs and has been observed to vary from highs near 750 torr to low pressures near 600 tOIT, To this gas mixture we now add 25 torr of a rare gas mixture consisting of 49°:0 Ne, 49% Ar, 1 % Kr and 1 % Xe.