Proton exchange membrane fuel cell (PEMFC) has been reported as clean and efficient energy technology from conversion of H2. However, one of the main challenges remains the storage and transport of hydrogen. The promising alternative is to produce H2 on site by a reformer using a H2-dense liquid as a fuel, a technology known as fuel processing. Methanol is an attractive source of H2 compared to other fuels as it presents several advantages, i.e. it is obtained sulphur-free, has a high H to C ratio and therefore produces a H2-rich reformate, can be reformed at low temperatures (200 - 300°C) and is a liquid at ambient conditions so that it can be easily handled. Typically, Cu-based catalysts are used for steam reforming of methanol due to their high activity (i.e. H2 production) and high selectivity towards CO2. As CO poisons anodic catalyst of PEMFC, high selectivity towards CO2 is crucial so as to eliminate or at least minimize CO removal load downstream a fuel processor. However, Cubased catalysts are thermally unstable and suffer deactivation due to sintering at high temperatures (> 250°C). Moreover, Cu-based catalysts are pyrophoric and therefore difficult to handle. Recent studies show that PdZn catalysts are very promising as they exhibit comparable activity and selectivity to Cu-based ones. Furthermore, PdZn catalysts are thermally stable in the typically methanol steam reforming temperature range (200 - 300°C). Most literature attributes high CO2 selectivity of PdZn catalysts to formation of PdZn alloy. It is generally agreed that PdZn alloy is formed when PdZn catalysts are reduced in H2 at high temperatures (> 250°C). In this work, a Pd/ZnO catalyst aimed at 2.5 wt% Pd was successfully prepared via incipient wetness impregnation and the duplicate preparation of the catalyst was successful. Both impregnation catalysts were confirmed by ICP-OES to contain similar weight Pd loadings i.e. 2.8 and 2.7 wt%, respectively. The actual Pd loading (ICP-OES) was slightly higher than the target loading (2.5 wt%) due to Pd content of Pd salt underestimated during catalyst preparation. Furthermore, crystallite size distribution, i.e. PdO crystallites on ZnO support, was similar (i.e. 6.7 ± 2.4 nm and 6.3 ± 1.9 nm) for both impregnation catalysts. The TPR analysis of the catalyst showed two peaks, i.e. a narrow peak at 87°C and a broader peak starting at approximately 260°C with a maximum at 348°C and ending around 385°C. The peak which occurred at the lower temperature was due to reduction of PdO to metallic Pd. The H2-TPR analysis of pure ZnO shows that ZnO (in the absence of Pd) did not reduce below 600°C. Therefore the peak which occurred at the higher temperature in the case of the impregnation catalyst was due to reduction of ZnO and this was facilitated by H2 spillover from the metallic Pd. To confirm the difference in selectivity towards CO2 between the ‘only PdO reduced’ and the ‘PdO and ZnO’ reduced catalysts, the impregnation catalyst was reduced at different temperurates (i.e. 120, 180 and 450°C) prior to catalyst performance tests. However, selectivity towards CO2 remained > 99% for all conditions. To not influence the reduction by in-situ reduction due to H2 produced by the methanol steam reforming reaction, reforming was carried out at low temperatures. Consequently, CH3OH conversion was low (< 30%). Since the feed molar steam to carbon ratio was 1.1, sligthly higher than the stoichiometric ratio, the low CH3OH conversions (< 30%) resulted in excess steam (compared to CO and CO2) in the reactor and this condition drove the water-gas shift reaction to high or equilibrium selectivity towards CO2 ( ~ 99%). The co-precipitation preparation method was not successful.i.e. the co-precipitation catalyst was aimed at 3 wt% but was confirmed by ICP-OES to be only 1 wt%. A significant fraction of Pd was lost as it did not precipitate during catalyst preparation. The uncontrollable precipitation of Pd makes the method irreproducible. Hence less focus was paid on the coprecipitation catalyst.

show more