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An integrated catalysis-surface science-theory approach to understand selectivity in the hydrogenation of 1hexyne to 1-hexene on Pd-Au single-atom alloy catalysts Jilei Liu, Matthew B Uhlman, Matthew M. Montemore, Antonios Trimpalis, Georgios Giannakakis, Junjun Shan, Sufeng Cao, Ryan T Hannagan, E. Charles H. Sykes, and Maria Flytzani-Stephanopoulos ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00491 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019
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ACS Catalysis
An integrated catalysis-surface science-theory approach to understand selectivity in the hydrogenation of 1-hexyne to 1hexene on Pd-Au single-atom alloy catalysts Jilei Liu1‡, Matthew B. Uhlman2‡, Matthew M. Montemore3‡, Antonios Trimpalis1, Georgios Giannakakis1, Junjun Shan1, Sufeng Cao1, Ryan T. Hannagan2, E. Charles H. Sykes2 and Maria Flytzani-Stephanopoulos1* 1Department
of Chemical and Biological Engineering, Tufts University, Medford, MA 02155, USA of Chemistry, Tufts University, Medford, MA 02155, USA 3Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States KEYWORDS: Single-atom alloy, selective hydrogenation, catalysts, Pd, Au, 1-hexyne 2Department
ABSTRACT: The selective hydrogenation of alkynes to alkenes is an important industrial process. However, achieving high selectivity and reducing the usage of precious platinum group metals is still challenging for the conventional hydrogenation catalysts. With atomically dispersed active metal atoms in the surface of a host metal, single-atom alloys have shown excellent hydrogenation selectivity and activity, but their hydrogenation mechanism is not fully understood. This work reports on the selective hydrogenation of 1-hexyne to 1-hexene on PdAu single-atom alloy catalysts. Au is a highly selective hydrogenation catalyst, but it is not active at low temperatures. Through measurements of reaction kinetics and in operando spectroscopy studies, we follow the much more facile activation of PdAu single-atom alloy catalysts and demonstrate the different hydrogenation chemistry of single Pd atoms and Pd nanoparticles. We further investigate the role of Pd atoms and the mechanism behind the improved hydrogenation selectivity through surface science and density functional theory. These studies indicate that the difference in reactivity stems from the relative energy barrier heights for over-hydrogenating the terminal C atom. The complementary catalysis-surface science-theory investigation described here is powerful and a general approach for understanding and controlling nanoparticle performance. The selective hydrogenation on PdAu single-atom alloys is demonstrated and understood fundamentally and serves as a guide for future designs of this type of catalyst. Single-atom supported metal catalysts are currently under intense investigation.1–4 They offer the possibility of 100% efficient metal utilization as well as high selectivity. To date, several such systems have been reported with outstanding performance in catalytic reactions that include CO oxidation5–8, water-gas shift9–14, methanol steam reforming15–17, oxidation of methane18, and hydrogenation reactions19–23. In these catalysts, the isolated active metal atoms show higher or exclusive reactivity and selectivity for different reactions compared to the corresponding metal clusters and nanoparticles10–12,20. Similarly, single-atom alloys (SAAs), an alloy architecture with isolated metal atoms embedded in a different host metal, also enable efficient utilization of expensive metals and improved selectivity.4,19,21,22,24–30 The Sykes and FlytzaniStephanopoulos groups have investigated the selective hydrogenation of styrene, acetylene, and phenylacetylene on Pd-Cu(111) and PdCu nanoparticle (NP) SAAs,17,23 and the selective hydrogenation of 1,3-butadiene on PtCu(111) and PtCu NP SAAs.20,24 The fundamental understanding of these catalytic systems in which the active sites are wellcharacterized and amenable to modeling with theory is of
great significance for the rational design of highly efficient catalysts.31 Recently, several groups have demonstrated the highly selective nature of supported atomic Pd catalytic centers in hydrogenation and made efforts to understand the selective hydrogenation chemistry that occurs on these catalysts. It has been demonstrated that Pd can be used as the minority component in copper, silver, or gold, to prepare SAAs that catalyze the partial hydrogenation of alkynes and dienes with high selectivity and only a small loss of activity24,25,29,32– 34. Pérez-Ramírez and coworkers conducted density functional theory (DFT) calculations to elucidate the reaction pathway of acetylene hydrogenation over Pd/mpgC3N4 catalysts and found the reactively formed ethylene molecules are adsorbed in a non-active configuration on the electron-lean Pd centers, which results in the high catalyst selectivity.35 Zhang and coworkers demonstrated that single atom Pd-doped Cu, Ag and Au catalysts have high selectivity and remarkable reactivity in the selective hydrogenation of acetylene. They proposed that both the isolation of Pd atoms and the electron transfer between the
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group IB metal and Pd contributed to the improved selectivity.33 The mechanism of selective hydrogenation over SAA catalysts has been studied under ultrahigh vacuum (UHV) conditions.4,31,36 Through complementary scanning tunneling microscope (STM) and temperature programmed desorption (TPD) experiments, relationships between atomic scale structure and reactivity can be understood and used to inform the rational design of new materials.31 This approach has been successfully applied to the design of PtCu SAA catalysts that exhibit highly selective hydrogenation of butadiene to butenes in the presence of propene under realistic reaction conditions.20 The model studies revealed that molecular hydrogen is dissociated at isolated Pt sites and the hydrogen atoms spill over onto the Cu host metal where the butadiene reacts to form butene, which then desorbs prior to complete hydrogenation, demonstrating the bifunctional nature of this catalyst.19,20 Hydrogenation reactions on Au(111) under UHV conditions have been studied; however, these studies often make use of an atomic hydrogen source as Au(111) is incapable of dissociating molecular hydrogen under UHV conditions.37 To aid in overcoming this limitation, addition of small amounts of Pd to form a PdAu(111) SAA has been shown to activate molecular hydrogen, although the dissociated hydrogen does not spill over onto the Au host.38 The hydrogen spillover to copper and the highly selective nature of copper for hydrogenation effectively explains the high selectivity of catalysts like Pd-Cu and Pt-Cu SAAs. However, the origin of the high selectivity of catalysts like Pd-Au SAAs has not been fully understood. Due to the very weak H-Au(111) adsorption, spontaneous hydrogen spillover to Au(111) has not been observed under UHV conditions. The much stronger adsorption strength of alkynes on palladium compared to the gold surface makes the Pd single atoms significantly more favorable adsorption sites and likely the active sites. The Pd-Au SAA has been shown to be highly selective for the partial hydrogenation of 1-hexyne.29 This paper aims to investigate why Pd single atoms surrounded by Au atoms show such an improvement in selectivity compared to contiguous Pd. We report an integrated catalysis, surface science, and theory approach to address mechanistic and structural effects in the selective hydrogenation of 1-hexyne to 1-hexene over PdAu SAA catalysts.
Experimental Methods Catalysis study The PdAu SAA catalysts were prepared by a sequential reduction method reported previously29. Accordingly, Au NPs were formed with 0.3 g HAuCl4·3H2O, 0.3 g NaHCO3 and 1.2 g poly(vinylpyrrolidinone) (PVP, MW= 58,000) in 50 mL ethylene glycol. With nitrogen flowing through the flask, the solution was heated to 90 °C and held for half an hour. Then the solution was cooled to ambient temperature and the desired amount of Pd(NO3)2·xH2O was added. The solution was heated to 90 °C with vigorous stirring and held for 8 h. The obtained PdAu NPs were mixed with fumed silica (heat treated in air at 650 °C for 3 h) in water and stirred overnight. The slurry carrying the supported NPs was
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centrifuged and dried in vacuum for 24 h before being calcined in air at 300 °C. The unsupported Pd NPs used in ATR-IR studies were prepared by the same method with the Pd(NO3)2 precursor. The Pd-NP samples (5wt% Pd on silica) used in the batch reactor tests were prepared by incipient wetness impregnation using an aqueous solution of Pd(NO3)2. The catalytic performance was studied in a batch reactor.29,39 A stainless-steel Parr reactor was used for the hydrogenation of 1-hexyne. The desired amount of sample was added to 20-80 mL ethanol. The mixture was stirred for half an hour and sonicated for 15 minutes before being transferred into the reactor. Pure hydrogen was bubbled through the solution while stirring for 1 h. 1-hexyne was injected into the solution to reach the desired concentration in ethanol. The reactor was then pressurized with pure hydrogen to the desired pressure under stirring at 600 rpm. Liquid-phase samples were taken from the reactor at different time points. Fourier-transform infrared spectroscopy (FTIR) was performed in both attenuated total reflection (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reaction cells with a Thermo Nicolet i550 instrument. TPD All TPD experiments were performed in a UHV chamber with a base pressure of
