<p>In recent years, first principles periodic Density Functional Theory (DFT) calculation</p><p>has been used to investigate heterogeneous catalytic reactions and examine catalyst</p><p>structures as well as adsorption properties in a variety of systems. The increasing</p><p>contribution to give detailed understanding of elementary reaction mechanism is critical to</p><p>provide fundamental insights into the catalyst design. It is a link to the fundamental</p><p>knowledge and a bridge to the practical application. DFT calculations is also a powerful</p><p>tool to predict and yield promising catalysts which is time- and cost-saving in the practical</p><p>end.</p><p>Because of the recent boom in natural shale gas deposit, there is an increasing interest</p><p>in developing more efficient ways to transform light alkanes into desired and high-value</p><p>chemicals, such as propylene. Propylene is a valuable raw material in the petrochemical</p><p>application to make value-added commodities, such as plastics, paints, and fibers, etc. The</p><p>conventional cracking, steam cracking (SC) and fluid catalytic cracking (FCC), could not</p><p>meet the growing demand of propylene. Thus, it has motivated extensive research of</p><p>production technologies. On the other hand, the abundance of light alkanes extracted from</p><p>the shale gas makes on-purpose production an appealing method which is economically</p><p>competitive. Non-oxidative dehydrogenation of propane (PDH) is a one of ways to make</p><p>up the supply and solve the issue.</p><p>xiii</p><p>According to the current research and industrial work, platinum (Pt) shows promising</p><p>performance for the PDH. However, it suffered from some major drawbacks, such as</p><p>thermodynamic limitation, rapid deactivation leading to poor catalytic performance and</p><p>frequent regeneration. In addition, it is a relatively high cost noble metal. Consequently,</p><p>many efforts have been devoted to the enhancement of the catalytic performance. It was</p><p>found that the stability and the selectivity of Pt-based catalysts can be improved via</p><p>modifying its properties with transition metals as promoters.</p><p>In this thesis, DFT calculations were performed for propane dehydrogenation over</p><p>two different catalyst systems, bimetallic platinum-zinc alloy and monometallic platinum</p><p>catalysts. The work provides insights into the catalyst crystal structures, the adsorption</p><p>characteristics of diverse adsorbates as well as the energy profiles regarding to the</p><p>selectivity of the propane dehydrogenation. Bulk calculation signifies a stable tetragonal</p><p>configuration of the PtZn catalyst which is in accordance with the experimental result. The</p><p>thermodynamic stability regarding to the stability of bulk and surface alloys are studied</p><p>with the consideration of physical constrains. We have identified the thermodynamic</p><p>stability of several PtZn low-index surface facets, (101), (110), (001), (100) flat surfaces</p><p>and stepped surface (111), at certain chemical potential environmental conditions through</p><p>the surface energy phase diagram. Stoichiometric and symmetric (101) slab is</p><p>thermodynamically stable under the region of high Pt chemical potential, and the offstoichiometric</p><p>and symmetric (100 Zn-rich) slab under the low Pt chemical potential.</p><p>In this work, PtZn(101) is used as a model surface to demonstrate the effect on the</p><p>catalytic performance with zinc promotion of platinum. In comparison with Pt(111) surface,</p><p>an elimination of 3-fold Pt hollow site on PtZn(101) is of important and it leads to the</p><p>xiv</p><p>change of binding site preferences. The divalent groups (1-propenyl, 2-propenyl) change</p><p>from Pt top site on PtZn(101) to 3-fold site on Pt(111), which is because of the lack of Pt</p><p>3-fold site on alloyed surface. As for propylene, it changes from di-σ site on PtZn to 𝜋 site</p><p>on Pt. The surface reaction intermediates are found to bond more weakly on PtZn(101)</p><p>than on the Pt surface. Especially, the binding energy of propylene reduces from -1.09 to -</p><p>0.16 eV. The weaker binding strength facilitates the activity of propylene on alloyed</p><p>surfaces.</p><p>Through a complete and classic reaction network analysis, the introduction of Zn</p><p>shows an increase in the endothermicity and the energy barrier of each elementary reaction</p><p>on the alloy surface. With the consideration of entropy for kinetic under real experimental</p><p>condition, the alloying of Zn is found to lower the energy barrier for the propylene product</p><p>desorption and increases that for propylene dehydrogenation. Meanwhile, the competition</p><p>between desired C-H and undesired C-C cleavages is investigated. It is found that the</p><p>cleavage of C-H is energetically favorable than that of C-C. These positive factors</p><p>potentially lead to a high selectivity toward propylene production on PtZn(101).</p><p>Subsequently, Microkinetic modeling is performed to estimate kinetic parameters</p><p>including the reaction order, rate-determining step to build a possible reaction mechanism.</p><p>Finally, conclusions brought out about the comparison between bimetallic and</p><p>monometallic catalyst, and suggestions for future work are presented.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7439063 |
| Date | 16 January 2019 |
| Creators | Yu-Hsuan Lee (5930717) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/First_Principles_Calculations_of_Propane_Dehydrogeanation_on_PtZn_and_Pt_Catalyst_Surfaces/7439063 |
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