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

2019-01-16T15:57:29Z (GMT) by Yu-Hsuan Lee

In recent years, first principles periodic Density Functional Theory (DFT) calculation

has been used to investigate heterogeneous catalytic reactions and examine catalyst

structures as well as adsorption properties in a variety of systems. The increasing

contribution to give detailed understanding of elementary reaction mechanism is critical to

provide fundamental insights into the catalyst design. It is a link to the fundamental

knowledge and a bridge to the practical application. DFT calculations is also a powerful

tool to predict and yield promising catalysts which is time- and cost-saving in the practical

end.

Because of the recent boom in natural shale gas deposit, there is an increasing interest

in developing more efficient ways to transform light alkanes into desired and high-value

chemicals, such as propylene. Propylene is a valuable raw material in the petrochemical

application to make value-added commodities, such as plastics, paints, and fibers, etc. The

conventional cracking, steam cracking (SC) and fluid catalytic cracking (FCC), could not

meet the growing demand of propylene. Thus, it has motivated extensive research of

production technologies. On the other hand, the abundance of light alkanes extracted from

the shale gas makes on-purpose production an appealing method which is economically

competitive. Non-oxidative dehydrogenation of propane (PDH) is a one of ways to make

up the supply and solve the issue.

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According to the current research and industrial work, platinum (Pt) shows promising

performance for the PDH. However, it suffered from some major drawbacks, such as

thermodynamic limitation, rapid deactivation leading to poor catalytic performance and

frequent 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 was

found that the stability and the selectivity of Pt-based catalysts can be improved via

modifying its properties with transition metals as promoters.

In this thesis, DFT calculations were performed for propane dehydrogenation over

two different catalyst systems, bimetallic platinum-zinc alloy and monometallic platinum

catalysts. The work provides insights into the catalyst crystal structures, the adsorption

characteristics of diverse adsorbates as well as the energy profiles regarding to the

selectivity of the propane dehydrogenation. Bulk calculation signifies a stable tetragonal

configuration of the PtZn catalyst which is in accordance with the experimental result. The

thermodynamic stability regarding to the stability of bulk and surface alloys are studied

with the consideration of physical constrains. We have identified the thermodynamic

stability of several PtZn low-index surface facets, (101), (110), (001), (100) flat surfaces

and stepped surface (111), at certain chemical potential environmental conditions through

the surface energy phase diagram. Stoichiometric and symmetric (101) slab is

thermodynamically stable under the region of high Pt chemical potential, and the offstoichiometric

and 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 the

catalytic 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 the

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change of binding site preferences. The divalent groups (1-propenyl, 2-propenyl) change

from Pt top site on PtZn(101) to 3-fold site on Pt(111), which is because of the lack of Pt

3-fold site on alloyed surface. As for propylene, it changes from di-σ site on PtZn to 𝜋 site

on 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 alloyed

surfaces.

Through a complete and classic reaction network analysis, the introduction of Zn

shows an increase in the endothermicity and the energy barrier of each elementary reaction

on the alloy surface. With the consideration of entropy for kinetic under real experimental

condition, the alloying of Zn is found to lower the energy barrier for the propylene product

desorption and increases that for propylene dehydrogenation. Meanwhile, the competition

between desired C-H and undesired C-C cleavages is investigated. It is found that the

cleavage of C-H is energetically favorable than that of C-C. These positive factors

potentially lead to a high selectivity toward propylene production on PtZn(101).

Subsequently, Microkinetic modeling is performed to estimate kinetic parameters

including the reaction order, rate-determining step to build a possible reaction mechanism.

Finally, conclusions brought out about the comparison between bimetallic and

monometallic catalyst, and suggestions for future work are presented.