Experimental and modeling studies of acetylene detection in hydrogen/acetylene mixtures on PdM bimetallic metal–insulator–semiconductor devices

The effect of the composition of the metal gate on acetylene response in hydrogen/acetylene mixtures was tested for a number of metal–insulator–semiconductor (MIS) devices with bimetallic gates. The motivation for this study was that bimetallic catalysts are employed in the commercial acetylene hydrogenation process because of superior performance compared to the pure metals. A variety of metal compositions and operating temperatures were tested, with the largest reproducible acetylene response observed for a 15% Ag/Pd sensor at 398 K. Kinetic modeling of the relevant surface reactions on Pd and bimetallic PdAg provided insights into how temperature, feed concentration, and Ag content affected response. The model predicted that significant coverages of carbon species formed on the surface, mainly CH(s) and C(s) on Pd and CCH(s) on PdAg, and that these species strongly influenced sensor responses. The dynamic changes in the surface coverages of carbonaceous intermediates after acetylene introduction were correlated with a response overshoot seen experimentally. The model indicated that the superior performance of Ag/Pd sensors relative to Pd sensors could be explained in terms of a higher hydrogen consumption rate. These results indicate a strong connection between the high reaction rates observed for industrial Ag/Pd acetylene hydrogenation catalysts and acetylene response in Ag/Pd MIS sensors.