550798 Subsurface Effects on Supported 2 Nm Pt3mn Nanoparticles for Catalytic Propane Dehydrogenation

Monday, June 3, 2019: 10:39 AM
Republic ABC (Grand Hyatt San Antonio)
Zhenwei Wu1, Brandon C. Bukowski2, Zhe Li3, Cory Milligan1, Lin Zhou4, Tao Ma4, Yue Wu3, Yang Ren5, Fabio H. Ribeiro2, W. Nicholas Delgass1, Jeffrey Greeley2, Guanghui Zhang1 and Jeffrey T. Miller1, (1)Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, (2)Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, (3)Chemical and Biological Engineering, Iowa State University, Ames, IA, (4)Department of Materials Science, Ames Laboratory, Ames, IA, (5)X-Ray Science Division, Argonne National Laboratory, Argonne, IL

Subsurface Effects on Supported 2 nm Pt3Mn Nanoparticles for Catalytic Propane Dehydrogenation

Zhenwei Wu1 Brandon C. Bukowski1, Zhe Li2, Cory Milligan1, Lin Zhou3, Tao Ma3, Yue Wu2, Yang Ren4, Fabio H. Ribeiro1, W. Nicholas Delgass1, Jeffrey Greeley1, Guanghui Zhang1, *, Jeffrey T. Miller1, *

1Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 (USA)

2Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011 (USA)

3Department of Materials Science, Ames Laboratory, Ames, IA 50012 (USA)

 4 X-Ray Science Division, Argonne National Laboratory, Argonne, IL 60439 (USA)

 *gzhang@dlut.edu.cn, mill1194@purdue.edu

Introduction

Supported multimetallic nanoparticles (NPs) are highly tunable by control of their composition and structure. The geometric and electronic effects related to the NP surface have been well studied since the early work of Sinfelt. [1, 2] It has been, however, very difficult to identify any subsurface effects on NPs, even while changing the subsurface metal atoms has been observed to have a large impact on single crystal model catalysts [3]

The challenges to study the effect of subsurface layers on NPs lie in the difficulties in preparing and characterizing atomically precise and high temperature stable NPs with the same surface structure but different subsurface phase. Recently it was shown that stable core-shell NPs can be achieved by intermetallic compound phases which feature precisely ordered arrays of different metals with strong heteroatomic bonds. An intermetallic surface or subsurface maintain ordering even at high temperatures. [4] Herein, we investigate the effect of subsurface composition on supported 2 nm Pt3Mn NPs for catalytic selectivity of C-H vs C-C bond activation during propane dehydrogenation at 550 ¡C.

Materials and Methods

Catalyst synthesis was conducted through diffusion-controlled Pt3Mn intermetallic compound NPs formation. The positively charged metal precursors was electrostatically adsorbed onto the deprotonated SiO2 support surface under controlled pH, leading to uniform and small NP size. The catalyst structure was characterized by in situ synchrotron X-ray diffraction (in situ sXRD), in situ X-ray absorption spectroscopy (in situ XAS), STEM imaging and EDS analysis. The catalytic performance was evaluated for light alkane dehydrogenation at 550 ¡C in a plug-flow tube reactor. The heats of CO adsorption were measured experimental on a Micromeritics Autochem 2920 automated catalyst characterization system. All DFT calculations are performed using VASP with PBE exchange-correlation function.

Results and Discussion

Two different Pt-Mn NP catalysts were prepared, namely Pt3Mn-s (core-shell NPs with Pt3Mn shell on Pt core), and Pt3Mn (full-body intermetallic NPs with Pt3Mn shell on Pt3Mn core). Formation of partial and full Pt3Mn phase were confirmed from in situ synchrotron XRD by different peak shifts compared to Pt and in situ EXAFS fits. Ordering in the Pt3Mn intermetallic phase is identified through the superlattice diffraction. For Pt3Mn-s, its Pt core Ð Pt3Mn shell structure is discerned from a physical mixture of the two phases according to EDS and a difference EXAFS analysis showing that the surface Mn concentration is consistent with Pt3Mn phase while the core is Pt.

Figure 1.  A schematic of the three catalysts investigated: Pt, Pt3Mn-s and Pt3Mn, and their propylene selectivity during propane dehydrogenation vs experimental heats of CO adsorption

The effect of NP subsurface composition on selectivity of C-H over C-C bond activation was evaluated for propane dehydrogenation at 550 ¡C. At 20 % conversion with co-feeding hydrogen, the propylene selectivity is improved from 35 % for Pt, to 82 % for Pt3Mn-s and further to 98 % for Pt3Mn, showing separately the impact of surface phase vs subsurface phase on these supported 2 nm NPs. The improvement is correlated with significant reduction in reactants adsorption strength as shown with the experimental heats of CO adsorption. DFT derived binding energies of CO, H and CxHy show consistent trends for the three samples. Calculations on different model structures suggest that the decrease in binding energy is mainly due to the Pt3Mn phase in the subsurface. Weaker binding of olefins promotes their desorption compared to further dehydrogenation or C-C bond breaking, improving their selectivity.

Significance

This work highlights the importance of the subsurface composition to the surface chemistry for industry relevant supported NP catalysts. The subsurface electronic effect can be precisely controlled via a diffusion limited synthesis of (surface) intermetallic phases with a wide range of possible compositions, promising versatile catalyst design for various catalytic processes. The Pt3Mn NP catalyzes stable dehydrogenation with high selectivity as well as conversion even at 750 ¡C.

References

1.      Bond, G. C.: Supported metal catalysts: some unsolved problems. Chem. Soc. Rev. 1991, 20, 441-475

2.      Sinfelt, J. H.: Catalysis by alloys and bimetallic clusters. Acc.Chem.Res 1977, 10, 15-20.

3.      Greeley, J.; Mavrikakis, M.: Alloy catalysts designed from first principles. Nat. Mater. 2004, 3, 810

4.      Gallagher, J. R.; Childers, D. J.; Zhao, H.; Winans, R. E.; Meyer, R. J.; Miller, J. T.: Structural evolution of an intermetallic Pd-Zn catalyst selective for propane dehydrogenation. Phys. Chem. Chem. Phys. 2015, 17, 28144-28153.


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