The production of light alkenes comprises a 250 million ton per year industry due to their extensive use in the production of plastics, rubbers, fuel blending agents, and chemical intermediates. While steam cracking and fluid catalytic cracking of petroleum crude oils are the most common methods for obtaining light alkenes, rising oil prices and low selectivities toward specific alkenes have driven the search for a more economical and efficient process. Catalytic dehydrogenation of light alkanes, obtained from natural gas feedstock, presents an attractive alternative that offers high selectivity and greater flexibility in the alkene pool to address changing demands. Platinum is the most effective metal to catalyze the reaction, but by itself, suffers from catalyst deactivation due to the buildup of carbonaceous deposits. The addition of a secondary metal to form a bimetallic alloy has been of high interest due to its ability to suppress coking and increase selectivity. This dissertation has focused on developing a deeper understanding of promotion effects of various metals and elucidating the mechanism behind coking on Pt catalysts.
The use of Sn as a promoter was first investigated, and the effects of particle size and composition on the ethane dehydrogenation performance were determined using a colloidal method to prepare model catalysts. At high conversions, catalyst deactivation from coke formation was a strong function of particle size and Sn/Pt, in agreement with previous high resolution transmission electron microscopy studies (HRTEM) studies. Deactivation decreased significantly with decreasing particle size and increasing addition of Sn. For a fixed average particle size, the activity and selectivity to ethene increased with increasing content of Sn in the Pt-Sn particle. For Pt and Pt3Sn compositions, the turnover frequency increased with increasing particle size, while the selectivity to ethene was not strongly affected.
For uncovering the mechanism by which Pt catalysts deactivate, carbon formation on MgO-supported Pt nanoparticles was studied by in situ HRTEM in order to obtain time-resolved images of a single nanoparticle during the dynamic coking process. An electron dose rate dependence on the rate of carbon growth was found, and a suitable imaging strategy was adopted in order to minimize beam-induced artifacts. Multi-layer carbon growth around the nanoparticle was investigated, and significant restructuring of the particle was also observed. In particular, step formation was captured in various images, supporting evidence that the nucleation and growth of carbon during coking on Pt catalysts often requires low coordination sites such as step sites. This is in agreement with scanning tunneling microscopy (STM) experiments, which illustrate a slight preference for carbon atoms to nucleate at the step sites on a Pt(111) crystal.
Other promoters for light alkane dehydrogenation were then investigated. The thermal dehydrogenation of n-butane to butene and hydrogen was carried out over Pt nanoparticles supported on calcined hydrotalcite containing indium, Mg(In)(Al)O. The optimal In/Pt ratio was found to be between 0.33 and 0.88, yielding > 95% selectivity to butenes. Hydrogen co-fed with butane was shown to suppress coke formation and catalyst deactivation, with a ratio of H2/C4H10 = 2.5 providing the best catalytic performance. In addition, a Pt-Ir alloy was investigated for ethane and propane dehydrogenation. Following characterization to confirm formation of a bimetallic alloy, intrinsic rate measurements at low feed residence time revealed the following trend in activity: Pt3Sn > Pt3Ir > Pt. DFT calculations carried out on tetrahedral clusters (Pt4, Pt3Ir, Pt3Sn) show that this trend in activity can be attributed to variations in the HOMO-LUMO gap of the cluster.