Density Functional Study Of Propane Dehydrogenation On Pt Catalyst

Ethylene and propylene are important raw material in petrochemical industry. In recent years, the production capacity can not feed the ever-increasing demand for propylene because of the large demand of olefin derivatives. Therefore, extensive efforts have to be devoted to improve the technology to increase the production of propylene.Direct dehydrogenation (DDH) of propane is an inexpensive way to increase the production and of great application potentials in industry. In order to increase the dehydrogenation activity and selectivity to meet industry application targets, the reaction mechanism of propane dehydrogenation has been studied in this thesis. First principals calculations based on density functional theory have been performed to investigate the C-H and C-C bond activation of propane on Pt and PtSn alloy surfaces. Different Pt surfaces show different activity for propane dehydrogenation and selectivity to propylene, which help us to understand size effect, shape effect and alloy effect of Pt catalyst. The coadsorption effects of H and C on the dehydrogenation barrier and propylene adsorption are also investigated so as to elucidate the effect of hydrogen partial pressure and coke formation on the reactivity of Pt catalyst, respectively. Moreover, on the basis of the structure of core-shell catalyst, several Pt-shell core-shell structures are built. Among these core-shells, we are able to find out a catalyst with higher selectivity for H oxidation. The main achievements in this thesis are as follows:1. The dehydrogenation of propane involves a large number of side reactions including the deep dehydrogenation and hydrogenolysis of propane which lead to numerous fragments ranging from C1 to C3 species adsorbed on the catalyst surface. All the carbonaceous species are found to be adsorbed on Pt(111) with C atoms sp3 -hybridized. Propane is a saturated molecule which does not favor any particular sites on Pt surfaces and the adsorption energy is close to zero. For propylene, two adsorption mode are identified, the Bridge and Atop adsorption, namely the di-σmode and% mode, respectively. On the basis of the adsorption energy, density of states (DOS), bader charge and electron density analysis, the di-σmode is found to be more stable than theπmode. Combining the adsorption and reaction heat, propylidyne (CH3CH2C) is suggested to be the most stable C3 intermediates on Pt(111) and propyne is predicted to be the most likely starting point for the C-C scission. Furthermore, the calculated adsorption energies of C1 species, ranked in descending order, are as follows, C> CH> CH2> CH3, and the similar trend has been observed with respect to the chemisorption of C2 species.2. The energy barriers for the dehydrogenation of propane to form propylene are calculated to be in the region of 0.65-0.75 eV and 0.25-0.35 eV on flat and stepped surfaces, respectively. According to the Arrhenius equation, a variation of 0.40 eV in the activation energy will change the rate constant by 200 times. Therefore, the stepped surface is more active than the flat surface. The selectivity towards propylene is determined by the competition between propylene desorption and other side reactions, such as deep dehydrogenation and cracking. Taking the activation energy difference between propylene dehydrogenation and propylene desorption as the descriptor, we find that while step sites play a crucial role in the activation of propane dehydrogenation, the selectivity towards propylene is substantially lowered in the presence of the unsaturated surface Pt atoms. These findings provide a rational interpretation of the recent experimental observations that smaller Pt particles containing more step sites are much more active but less selective than larger particles in propane dehydrogenation.3. The octahedral and cubic Pt particles are surrounded by Pt(111) and Pt(100) surfaces, respectively. For large particles, the activity of the catalyst is dominated by the main surfaces. On the basis of our calculations, the Pt(100) surface shows higher activity to propane dehydrogenation but low selectivity towards propylene, which elucidate the experimental observations that the cubic particles show higher conversion of propane and higher production of cracking products than the octahedral particles.4. In the realistic dehydrogenation system, the Pt surface may adsorb many H or C atoms. To simulate the effect of hydrogen and coke on propane dehydrogenation, the Pt surfaces are pre-adsorbed with several H or C atoms. As the relationship between coadsorbed H and C3 species is repulsive, the adsorption of propylene is weakened with the increase of the number of H atoms. Therefore, the desorption of propylene is promoted, which leads to higher the selectivity. The adsorption of C not only weakens the adsorption of other species, but also increases the dehydrogenation barrier, leading to a lower surface activity.5. Comparing with pure Pt surface, the activity of PtSn surfaces are significantly lowered because the dehydrogenation barriers are increased by 0.30-0.50 eV. This is because the electronegativity of Sn is lower than Pt, the alloying of Sn with Pt will lead electron transformation from Sn to Pt. And the filling of d-band will down shift the d-band center, which indicates a decline of dehydrogenation activity and bonding ability of PtSn alloys. With the increase of the Sn content, the surface activity and adsorption energies will be further decreased, which leads to higher selectivity towards propylene.6. The oxidation mechanisms of H and propylene on Pt surfaces are investigated. For H oxidation, the OH formation, H*+O*→OH*, is the rate-determining step representing the activity; and for propylene oxidation, the further dehydrogenation of propylene is the key step leading to the formation of the precursor for oxidation. Though the activity for H oxidation on Pt is high, there is still part of propylene oxidized by oxygen. To further increase the selectivity of H oxidation, several core-shell structures are built. Compared with pure Pt surface, the core-shell surfaces with lower d-band show higher activity towards H oxidation and lower activity towards propylene dehydrogenation, which indicate higher selectivity towards H oxidation.