(52c) Thermally Stable Single Atom Pt/m-Al2O3 for CO Oxidation and the Selective Hydrogenation of 1,3-Butadiene

Element sustainability has become a global issue. Maintaining
current consumption rate, there are 22 elements facing depletion within the
coming 50 years, including almost all Platinum Group metals that are crucial
catalyst component^[1]. Despite the critical role of
Pt in catalysis^[2], it is rare, in short supply
in recent years, and has no adequate alternatives. In this regard, single-atom
catalysts (or atomically dispersed catalysts), in which all the metal atoms are
exposed on the support available for catalytic reactions, could help to address
the problem^[3]. The
electronic properties of isolated metal atoms may be distinctly different from
the active sites in bulk materials and nanoparticles, potentially triggering
innovative applications and enabling more effective usage of noble metal
elements^[4].

A challenge in the development of single-atom catalysts is the
increasing difficulty to stabilize single-atom species under drastic reaction
conditions. Many industrially important catalytic processes involving Pt
catalysts, such as reforming of hydrocarbons in petroleum refineries, are
operated at several hundred degrees under oxidative or reductive atmosphere^[5]. In this context, single-atom
catalysts that are able to withstand harsh reaction conditions are highly
desirable. Recent studies highlight the importance of manipulating the
interactions between metal atoms and the host support to achieve high stability
without losing reactivity in single-atom catalysis.

Figure 1 | Schematic illustration of the 0.2Pt/m-Al₂O₃-H₂
synthesis process.

Herein, we report a synthetic strategy for Pt single-atom catalysts
with outstanding stability in a series of reactions under demanding conditions.
The Pt−Al−O system is chosen because Al₂O₃ is a common
support for Pt in industrial and environmental applications. The catalyst was
prepared by a modified sol-gel solvent vaporization self-assembly method^[6], followed by calcination in
air and reduction with H₂ (Fig. 1).The Pt atoms are firmly anchored
in the internal surface of mesoporous Al₂O₃ enriched with
coordinatively unsaturated pentahedral Al³⁺ centers. Activity was
fully maintained in CO oxidation after 60 cycles between 100 ^oC and
400 ^oC over a one-month period without any metal aggregation or
support deterioration. The catalyst also kept its structural integrity and
excellent performance for the selective hydrogenation of 1,3-butadiene after
exposure to a reductive atmosphere at 200 ¡ãC for 24 h. Compared to commercial
Pt nanoparticle catalyst on Al₂O₃ and control samples,
this system also exhibited significantly enhanced stability and performance for
n-hexane hydro-reforming at 550 ¡ãC for 48 h.

[1]          A. J. Hunt,
A. S. Matharu, A. H. King, J. H. Clark, Green. Chem. 2015, 17,
1949-1950.

[2]          Forecast
of Platinum SUPPLY & DEMAND IN 2015, Johnson  Matthey, 2015.

[3]          a) X. F.
Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Acc. Chem. Res. 2013,
46, 1740-1748; b) J. M. Thomas, Nature 2015, 525,
325-326; c) M. Flytzani-Stephanopoulos, B. C. Gates, Annu. Rev. Chem.
Biomol. Eng. 2012, 3, 545-574.

[4]          a) F.
Dvorak, M. Farnesi Camellone, A. Tovt, N. D. Tran, F. R. Negreiros, M.
Vorokhta, T. Skala, I. Matolinova, J. Myslivecek, V. Matolin, S. Fabris, Nat.
Commun. 2016, 7, 10801; b) P. Liu, Y. Zhao, R. Qin, S. Mo, G.
Chen, L. Gu, D. M. Chevrier, P. Zhang, Q. Guo, D. Zang, B. Wu, G. Fu, N. Zheng,
Science 2016, 352, 797-801; c) J. Liu, F. R. Lucci, M.
Yang, S. Lee, M. D. Marcinkowski, A. J. Therrien, C. T. Williams, E. C. Sykes,
M. Flytzani-Stephanopoulos, J. Am. Chem. Soc. 2016, 138,
6396-6399.

[5]          S. H. Joo,
J. Y. Park, C. K. Tsung, Y. Yamada, P. Yang, G. A. Somorjai, Nat. Mater. 2009,
8, 126-131.

[6]          Q. Yuan,
A. X. Yin, C. Luo, L. D. Sun, Y. W. Zhang, W. T. Duan, H. C. Liu, C. H. Yan, J.
Am. Chem. Soc. 2008, 130, 3465-3472.