Direct and precise determination of volumetric mass transfer coefficient of carbon monoxide for miniaturized gas-liquid reactors via sensitive probing of raman transitions

Here, we report the direct and precise determination of volumetric mass transfer coefficient (k(L)a) of Carbon monoxide (CO) in miniaturized gas-liquid reactors via sensitive probing of the Raman transitions of aqueous trace CO without sampling. We confirm that weak Raman signals of aqueous CO can be significantly amplified by using Au@Pd core-shell nanoparticle monolayer via surface-enhanced Raman scattering and the amplified Raman intensity is linearly proportional to the concentration of aqueous CO in the range of 20 to 200 mu M that is indirectly estimated by gas chromatography. We successfully determine k(L)a of CO in a miniaturized column-type gas-liquid reactor with a working volume of 2 ml which is not determined by sampling and subsequent gas chromatography measurement for gaseous CO due to small headspace volume. We find that the k(L)a values in miniaturized column-type and plate-type reactors are up to two orders of magnitude lower than those in large-scale reactors, presumably due to the absence of mechanical stirring. The lower k(L)a revealed by our method is also validated by fluid dynamic simulation. In addition, our method allows to determine k(L)a in miniaturized reactors, either with internal baffles or at different gas flow rates and temperatures. The k(L)a values in the miniaturized reactors are found to be more sensitive to changes in the temperature and flow rate than in the large-scale reactors, which is likely attributable to both improved mixing due to higher flow rates and faster heating rate due to smaller working volume. We believe that our method has a huge impact on CO gas conversion ranging from design and scale-up of gas-liquid reactors, process monitoring and optimization to high-throughput screening of strains or catalysts.