Influence of Alkali (Cesium, Sodium, Lithium) on Platinum/Zirconia Catalysts in the Production of Hydrogen for Potential Applications in Healthcare
This dissertation aims to investigate the impact of alkali-promoted-Pt/m-ZrO2 catalysts to accelerate or improve the H2 selectivity of hydrogen production and purification reactions and explore the potential effect of hydrogen gas on UCP1 gene expression which is associated with the pathogenesis of obesity. The main phase of the study involved examining the impact of Cs, Li as alkalis, on the performance of Pt/m-ZrO2 catalysts in hydrogen production reactions including low-temperature water-gas shift (LT-WGS), ethanol steam reforming (ESR) and impact of Na as alkali in formaldehyde steam reforming (FSR) reaction. In the present study, we determined the near optimal Cs, Li, and Na loadings for improving catalyst performance parameters (e.g., activity, selectivity, and stability) and analyzed the effect of alkali on the catalyst in terms of various factors: (1) its ability to facilitate formate dehydrogenation (LT-WGS, FSR) or acetate demethanation (ESR), (2) its ability to release CO2 due to its basicity, (3) the potential for electron transfer from alkali to Pt as an explanation for the catalytic action, and (4) geometric arguments such as the proximity of alkali to Pt as well as Pt cluster size. Our results showed that adding alkali within a certain loading range resulted in acceleration of the forward decomposition rate of the formate intermediate (or acetate intermediate in ESR), producing H2 (or methane in ESR) and adsorbed CO2. Our results showed that formate / acetate decomposition increased with alkali basicity (Cs > Na > Li) and that a shift in formate ν(CH) band position (LT-WGS and FSR) to lower wavenumbers (i.e., indicative of bond weakening) is more significant with the addition of the more basic alkalis. On the other hand, carbonate stability significantly increased with alkali basicity. At optimized dopant levels, the trend in the LT-WGS rate was Na > Li > unpromoted > Cs. The results of FSR revealed that formaldehyde is adsorbed at reduced defect sites on zirconia, where it is converted to formate species through the addition of labile bridging OH species. In ESR, ethanol is likewise adsorbed at reduced defect sites, in this case forming ethoxy species. These ethoxy species undergo oxidative dehydrogenation to acetate intermediates. In both ESR and FSR, DRIFTS, temperature-programmed reaction of steam reforming and fixed bed catalyst testing showed that the addition of alkali was able to stave off decarbonylation almost completely by promoting forward decomposition of formate intermediates in FSR (or acetate intermediates in the case of ESR) and by attenuating the metallic function. In the case of ESR, although alkali addition decreases the ethanol conversion rate, the improved selectivity to CH4 could be useful for a process involving pre-reforming of ethanol to CO2, H2, and CH4, with subsequent further conversion to H2 via methane steam reforming through commercial processes such as autothermal reforming (ATR). Because decarbonylation activity (CO production) results in decreased H2 selectivity for an overall ESR process (i.e., including downstream ATR of CH4), adding the alkali is beneficial for tuning catalyst surface basicity and boosting the H2 selectivity. At the end, we investigated the potential therapeutic effect of hydrogen, a novel medical gas, on beige adipocytes, which could serve as target cells. Beige cells were placed in a bioreactor and exposed to various concentrations of hydrogen gas in a suitable cell survival environment. The results demonstrated that hydrogen gas had minimal cytotoxicity on the cells. Furthermore, Real- Time Polymerase Chain Reaction (RT-PCR) analysis demonstrated a significant increase in the expression of UCP1 gene expression as thermogenic genes, which are associated with a reduction in obesity. The exact targets and molecular mechanisms of hydrogen remain unknown, and further studies are needed outside of this scope of work.