CO and CO2 Detection for Catalysis Diagnosis by Means of PLIF

Abstract: In order to fully understand the underlying processes of catalysis, it is vital to combine measurements of the catalyst surface, where the reaction occurs, with measurements of the gas concentration close to the catalyst. We have developed a convenient setup, which uses a single laser system to visualise the gas surrounding a working catalyst in situ at realistic operating conditions. Planar Laser Induced Flourescence (PLIF) is a technique in which the gas to be detected is excited by a planar laser sheet. The subsequent fluorescence is then detected by a camera. With this technique, we are able to create accurate two-dimensional representations of CO and CO2 concentrations, however, we think it will be possible to extend the list of gases in the future. For CO2 detection, we use an Nd:YAG laser to pump an OPO which had its idler output tuned to 2.7 µm. This beam was used to vibrationally excite the CO2 molecules, after which they deexcite via a path that entails fluorescence at 4.26 µm. A focal plane array was subsequently used to image the fluorescence. To detect CO, the same Nd:YAG laser is frequency doubled and used to pump a dye laser, the output of which is frequency doubled again and mixed with the residual beam left after the frequency doubling process. By tuning the dye laser, an output beam of 230.1 nm is obtained, which is used to excite CO using a two-photon transition. The fluorescence, which is in the visible range between 400 nm and 700 nm, is then deteced by an ICCD camera. We have performed proof-of-concept measurements where the setup was used to image the gas surrounding a working Pd(110) single crystal catalyst. In these measurements, the catalyst temperature was ramped from 200 °C to 350 °C. CO and CO2 were measured consecutively using similar temperature ramps. We found that we achieve detection limits of below 0.5 mbar for CO and below 0.1 mbar for CO2 at temporal resolutions of 100 ms and spatial resolutions of 40 µm. The results of these measurements have resulted in a manuscript now submitted to Applied Physics B. In addition, we have shown that it is possible and feasible to use the PLIF setup in conjunction with SXRD to perform simultaneous measurements of the catalyst surface structure and the surrounding gas. Clear correlations between a change in surface structure and changes in gas phase reaction products were found. Furthermore, we think this is the first time PLIF and SXRD have been used simultaneously.