First Principles Calculations of Propane Dehydrogeanation on PtZn and Pt Catalyst Surfaces

Abstract

In recent years, first principles periodic Density Functional Theory (DFT) calculationhas been used to investigate heterogeneous catalytic reactions and examine catalyststructures as well as adsorption properties in a variety of systems. The increasingcontribution to give detailed understanding of elementary reaction mechanism is critical toprovide fundamental insights into the catalyst design. It is a link to the fundamentalknowledge and a bridge to the practical application. DFT calculations is also a powerfultool to predict and yield promising catalysts which is time- and cost-saving in the practicalend.Because of the recent boom in natural shale gas deposit, there is an increasing interestin developing more efficient ways to transform light alkanes into desired and high-valuechemicals, such as propylene. Propylene is a valuable raw material in the petrochemicalapplication to make value-added commodities, such as plastics, paints, and fibers, etc. Theconventional cracking, steam cracking (SC) and fluid catalytic cracking (FCC), could notmeet the growing demand of propylene. Thus, it has motivated extensive research ofproduction technologies. On the other hand, the abundance of light alkanes extracted fromthe shale gas makes on-purpose production an appealing method which is economicallycompetitive. Non-oxidative dehydrogenation of propane (PDH) is a one of ways to makeup the supply and solve the issue.xiiiAccording to the current research and industrial work, platinum (Pt) shows promisingperformance for the PDH. However, it suffered from some major drawbacks, such asthermodynamic limitation, rapid deactivation leading to poor catalytic performance andfrequent regeneration. In addition, it is a relatively high cost noble metal. Consequently,many efforts have been devoted to the enhancement of the catalytic performance. It wasfound that the stability and the selectivity of Pt-based catalysts can be improved viamodifying its properties with transition metals as promoters.In this thesis, DFT calculations were performed for propane dehydrogenation overtwo different catalyst systems, bimetallic platinum-zinc alloy and monometallic platinumcatalysts. The work provides insights into the catalyst crystal structures, the adsorptioncharacteristics of diverse adsorbates as well as the energy profiles regarding to theselectivity of the propane dehydrogenation. Bulk calculation signifies a stable tetragonalconfiguration of the PtZn catalyst which is in accordance with the experimental result. Thethermodynamic stability regarding to the stability of bulk and surface alloys are studiedwith the consideration of physical constrains. We have identified the thermodynamicstability of several PtZn low-index surface facets, (101), (110), (001), (100) flat surfacesand stepped surface (111), at certain chemical potential environmental conditions throughthe surface energy phase diagram. Stoichiometric and symmetric (101) slab isthermodynamically stable under the region of high Pt chemical potential, and the offstoichiometricand symmetric (100 Zn-rich) slab under the low Pt chemical potential.In this work, PtZn(101) is used as a model surface to demonstrate the effect on thecatalytic performance with zinc promotion of platinum. In comparison with Pt(111) surface,an elimination of 3-fold Pt hollow site on PtZn(101) is of important and it leads to thexivchange of binding site preferences. The divalent groups (1-propenyl, 2-propenyl) changefrom Pt top site on PtZn(101) to 3-fold site on Pt(111), which is because of the lack of Pt3-fold site on alloyed surface. As for propylene, it changes from di-σ site on PtZn to 𝜋 siteon Pt. The surface reaction intermediates are found to bond more weakly on PtZn(101)than on the Pt surface. Especially, the binding energy of propylene reduces from -1.09 to -0.16 eV. The weaker binding strength facilitates the activity of propylene on alloyedsurfaces.Through a complete and classic reaction network analysis, the introduction of Znshows an increase in the endothermicity and the energy barrier of each elementary reactionon the alloy surface. With the consideration of entropy for kinetic under real experimentalcondition, the alloying of Zn is found to lower the energy barrier for the propylene productdesorption and increases that for propylene dehydrogenation. Meanwhile, the competitionbetween desired C-H and undesired C-C cleavages is investigated. It is found that thecleavage of C-H is energetically favorable than that of C-C. These positive factorspotentially lead to a high selectivity toward propylene production on PtZn(101).Subsequently, Microkinetic modeling is performed to estimate kinetic parametersincluding the reaction order, rate-determining step to build a possible reaction mechanism.Finally, conclusions brought out about the comparison between bimetallic andmonometallic catalyst, and suggestions for future work are presented.