On the Limited Role of Electronic Support Effects in Selective Alkyne Hydrogenation: A Kinetic Study of Au/MO_x Catalysts Prepared from Oleylamine‐Capped Colloidal Nanoparticles

Abstract

We report a quantitative kinetic evaluation and study of support effects for partial alkyne hydrogenation using oleylaminecapped Au colloids as catalyst precursors. The amine capping agents can be removed under reducing conditions, generating supported Au nanoparticles of~2.5 nm in diameter. The catalysts showed high alkene selectivity (> 90 %) at all conversions during alkyne partial hydrogenation. Catalytic activity, observed rate constants, and apparent activation energies (25-40 kJ/mol) were similar for all Au catalysts, indicating support effects are relatively small. Alkyne adsorption, probed with FTIR and DFT, showed adsorption on the support was associated with hydrogen-bonding interactions. DFT calculations indicate strong alkyne adsorption on Au sites, with the strongest adsorption sites at the metal-support interface (MSI). The catalysts had similar hydrogen reaction orders (0.7-0.9), and 1octyne reaction orders (~À 0.2), suggesting a common mechanism. The reaction kinetics are most consistent with a mechanism involving the non-competitive activated adsorption of H 2 on an alkyne-covered Au surface.[a] Dr.

Water Poisons H2 Activation at the Au–TiO2 Interface by Slowing Proton and Electron Transfer between Au and Titania

“…Adsorbate-modified catalysts may similarly have lower energy reaction pathways available that have not been previously considered. This may be particularly useful for similar reactions over d 10 metals where the metal–support interface is known to be important, such as alkyne partial hydrogenation over Au − or Ag, CO 2 hydrogenation over Cu based catalysts, − or methanol synthesis catalysts…”

Hydrogen Adsorption at the Au/TiO2 Interface: Quantitative Determination and Spectroscopic Signature of the Reactive Interface Hydroxyl Groups at the Active Site

“…Since H 2 adsorption involves both Au atoms and MSI TiOH sites, the number of active sites cannot be determined simply from the number of perimeter sites on Au nanoparticles (Au per ). − ,− The local structure of the support around Au particles must also play a role, as active sites require hydroxyl groups that are (i) physically close enough to Au per for H 2 to bridge the two components of the site and (ii) sufficiently basic to accept a proton during H 2 activation. The surface terminations of anatase and rutile structures typically have a combination of terminal (basic) and bridging (acidic) hydroxyls.…”

“…Even though Au catalysts often have highly desirable selectivity for important industrial reactions, Au-catalyzed hydrogenations have garnered less attention than well studied oxidations. Some examples include semihydrogenations, − selective hydrogenations, , nitro-aromatic reduction, ,, hydrodechlorination, and biomass conversion. − The primary reason these reactions have not been employed industrially is the well-known low hydrogenation activity of Au, which is orders of magnitude lower than Pt, Pd, or Ni . At the same time, this presents an important opportunity: if the fundamental hydrogenation chemistry can be understood and improved, there are numerous potential applications for Au selective hydrogenation catalysts.…”

“…Inelastic neutron scattering and Fourier transform infrared spectroscopy (FTIR) experiments provided clear spectroscopic evidence for both Au–H and protons on the support after H 2 adsorption . Rossi’s group has also shown that Au-catalyzed hydrogenations are considerably faster in the presence of a base. , Our own work has provided an abundance of kinetic evidence for the importance of heterolytic H 2 activation in hydrogenation reactions ,− and allowed us to quantify H 2 adsorption with in situ FTIR spectroscopy …”

C, N Co-Decorated Alumina-Supported Au Nanoparticles: Enhanced Catalytic Performance for Selective Hydrogenation of Acetylene

“…Even though palladium-based catalysts exhibit high acetylene conversion, they also show low selectivity to ethylene simultaneously [10,11]. Compared with Pd-based catalysts, Au-based catalysts show excellent selectivity of ethylene during the hydrogenation process [12][13][14][15][16]. Tamaru and coworkers reported that an Au/Al 2 O 3 catalyst showed 100% selectivity to ethylene in the selective hydrogenation of acetylene in the temperature range between 313K and 523K [14].…”