New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd−Au Catalysts

Guan, Erjia; Foucher, Alexandre C.; Marcella, Nicholas; Shirman, Tanya; Luneau, Mathilde; Head, Ashley R.; Verbart, David M. A.; Aizenberg, Joanna; Friend, Cynthia M.; Stacchiola, Darı́o; Stach, Eric A.; Frenkel, Anatoly I.

doi:10.1002/cctc.201901847

Cited by 15 publications

(25 citation statements)

References 25 publications

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“…Formation of Pd hydride would require three-dimensional Pd islands and is therefore deemed unlikely. Indeed, previous work shows that hydride formation in the PdAu alloy nanoparticle requires relatively high Pd concentrations (≥25 atm% Pd) and large Pd ensembles in Au. , Hydride formation generally leads to overhydrogenation and a poor selectivity in alkyne and alkene hydrogenation . Hence, dilute alloy catalysts are an effective design in preventing the formation of subsurface hydrogen without compromising the hydrogen dissociation activity.…”

Section: Discussionmentioning

confidence: 99%

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

et al. 2021

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Dilute Pd-in-Au alloy catalysts are promising materials for selective hydrogenation catalysis. Extensive surface science studies have contributed mechanistic insight on the energetic aspect of hydrogen dissociation, migration and recombination on dilute alloy systems. Yet, translating these fundamental concepts to the kinetics and free energy of hydrogen dissociation on nanoparticle catalysts operating at ambient pressures and temperatures remains challenging. Here, the effect of the Pd concentration and Pd ensemble size on the catalytic activity, apparent activation energy and rate limiting process is addressed by combining experiment and theory. Experiments in a flow reactor show that a compositional change from 4 to 8 atm% Pd of the Pd-in-Au alloy catalyst leads to strong increase in activity, even exceeding the activity per Pd atom of monometallic Pd under the same conditions, albeit with an increase in apparent activation energy. First-principles calculations show that the rate and apparent activation enthalpy for HD exchange increase when increasing the Pd ensemble size from single Pd atoms to Pd trimers in a Au surface, suggesting that the ensemble size distribution shifts from mainly single Pd atoms on the 4 atm% Pd alloy to larger Pd ensembles of at least three atoms for the 8 atm% Pd/Au catalyst. The DFT studies also indicated that the rate-controlling process is different: H 2 (D 2 ) dissociation determines the rate for single atoms whereas recombination of adsorbed H and D determines the rate on Pd trimers, similar to bulk Pd. 2Both experiment and theory suggest that the increased reaction rate with increasing Pd content and ensemble size stems from an entropic driving force. Finally, our results support hydrogen migration between Pd sites via Au and indicate that the dilute alloy design prevents the formation of subsurface hydrogen, which is crucial in achieving high selectivity in hydrogenation catalysis.

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Section: Discussionmentioning

confidence: 99%

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

et al. 2021

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“…The preferential isomerization of the hexyl intermediates on dilute alloys is mainly due to the larger energy barrier for hydrogenation induced by the high energy barrier for H 2 dissociation and H spillover. The formation of the Pd-hydride is likely prevented in the dilute alloy design because three-dimensional Pd clusters are needed to accommodate such a phase, 31,32 which are unlikely to exist in a Pd 4 Au 96 system. 21 Interestingly, the isotope-exchange experiments do not show obvious indications that hydride formation plays an important role in the selectivity of the Pd catalyst either: no large increase in the deuterium incorporation is observed compared to Pd 4 Au 96 (Figures S4 and S5), which would be the case if a large excess of dissociated deuterium was stored in the Pd subsurface.…”

Section: ■ Discussionmentioning

confidence: 99%

Unraveling 1-Hexene Hydrogenation over Dilute Pd-in-Au Alloys

et al. 2022

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Dilute Pd-in-Au alloys are valuable materials for selectively hydrogenating alkynes and isomerizing alkenes. By diluting Pd in a Au host, the selectivity toward semihydrogenated alkene isomers can be significantly enhanced and the unfavorable overhydrogenation to alkanes is suppressed. However, a detailed mechanistic study on the origin of the enhanced alkene selectivity over dilute alloy catalysts is still missing. Here, we combine experiment and theory to unravel the reaction mechanism, identifying rate-limiting and selectivity-controlling steps in 1-hexene hydrogenation over dilute Pd-in-Au catalysts. Using isotope-exchange hydrogenation experiments, we show that 1-hexene and hydrogen over a bimetallic Pd4Au96 in silica catalyst preferentially form 1-hexene isomers, (trans and cis) 2- and 3-hexene and only a small amounts of hexane. The reaction is consistent with a Horiuti–Polanyi mechanism, similar to a monometallic Pd nanoparticle catalyst. Computation of the free-energy profiles for 1-hexene hydrogenation and isomerization over a single Pd atom in a Au surface using first principles calculations indicated that the isomerization of 1-hexene to 2-hexene is energetically favorable due to the relatively large barrier for H2 dissociation preventing hydrogenation to n-hexane. Microkinetic modeling established that H2 dissociation on the single-atom Pd sites and H spillover from these sites onto the Au host are rate-limiting and key in steering the selectivity of dilute Pd-in-Au alloys toward the hexene isomers. The mechanistic insights from this study contribute to the rational design of optimized dilute alloy catalysts for selective alkene isomerization.

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“…The long-term stability of the beamline energy calibration was characterized based on the reproducibility of metal foil spectra obtained during standard user operations involving continuous data recording over a period of 24 h. Fig. 10 shows the data recorded at lower energies, on cobalt foil at the Co Kedge at 7709 eV, as well as at high energies, on palladium foil at the Pd K-edge at 24 350 eV, recorded during user experiments described in the literature (Liu et al, 2021;Guan et al, 2020). We observe that the spectra are reproducible throughout the time frame.…”

Section: Beamline Performancementioning

confidence: 99%

The Inner Shell Spectroscopy beamline at NSLS-II: a facility for in situ and operando X-ray absorption spectroscopy for materials research

Leshchev¹,

Rakitin²,

Luvizotto³

et al. 2022

J Synchrotron Rad

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The Inner Shell Spectroscopy (ISS) beamline on the 8-ID station at the National Synchrotron Light Source II (NSLS-II), Upton, NY, USA, is a high-throughput X-ray absorption spectroscopy beamline designed for in situ, operando, and time-resolved material characterization using high monochromatic flux and scanning speed. This contribution discusses the technical specifications of the beamline in terms of optics, heat load management, monochromator motion control, and data acquisition and processing. Results of the beamline tests demonstrating the quality of the data obtainable on the instrument, possible energy scanning speeds, as well as long-term beamline stability are shown. The ability to directly control the monochromator trajectory to define the acquisition time for each spectral region is highlighted. Examples of studies performed on the beamline are presented. The paper is concluded with a brief outlook for future developments.

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New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd−Au Catalysts

Cited by 15 publications

References 25 publications

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

Unraveling 1-Hexene Hydrogenation over Dilute Pd-in-Au Alloys

The Inner Shell Spectroscopy beamline at NSLS-II: a facility for in situ and operando X-ray absorption spectroscopy for materials research

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