Silver Nanoparticles for Olefin Production: New Insights into the Mechanistic Description of Propyne Hydrogenation

Abstract

The gas‐phase partial hydrogenation of propyne was investigated over supported Ag nanoparticles (2–20 nm in diameter) prepared by using different deposition methods, activation conditions, loadings, and carriers. The excellent selectivities to propene attained over the catalysts, exceeding 90 %, are independent of the particle size but the activity is maximal over approximately 4.5 nm Ag particles. Certain kinetic fingerprints of Ag, such as the positive dependence on the alkyne pressure, the relatively low reaction order in H2, and the low apparent activation energy, deviate from those of conventional hydrogenation metals such as Pd and Ni, questioning the applicability of the classical Horiuti–Polanyi scheme. Periodic dispersion‐corrected density functional theory (DFT‐D) calculations and microkinetic analysis demonstrate the occurrence of an associative mechanism, which features the activation of H2 on the adsorbed propyne at structural step sites. By using the atomistic Wulff model, the number of B5 sites available on the Ag nanoparticles was estimated to be maximal in the size range of 3.5–4.7 nm. The rate of propene production correlates with the density of B5 sites, which suggests that the latter are potential active centers for the reaction. This alternative pathway broadens the mechanistic diversity of hydrogenation reactions over metal surfaces and opens new directions for understanding metals that do not readily activate H2.

Recent advances in selective acetylene hydrogenation using palladium containing catalysts

“…A catalyst which offers thermodynamic selectivity involves one where there is an energy barrier which hinders ethylene adsorption relative to acetylene adsorption meaning that over-hydrogenation is limited. This is not the main driver for palladium based catalysts but is the most controlling factor for moderate-high alkene selectivity over metals such as Cu, Ni, Au and Ag [6][7][8]. Such metals offer high alkene selectivity but typically require activation/use at elevated temperatures which limit widespread industrial use [7][8][9][10][11][12][13][14].…”

“…This is not the main driver for palladium based catalysts but is the most controlling factor for moderate-high alkene selectivity over metals such as Cu, Ni, Au and Ag [6][7][8]. Such metals offer high alkene selectivity but typically require activation/use at elevated temperatures which limit widespread industrial use [7][8][9][10][11][12][13][14]. Selectivity over Pd based catalysts is believed to involve control over hydride formation which results in the formation of reactive subsurface hydrogen which is prone to hydrogenate ethylene to ethane upon migration back to the surface [15][16][17][18][19][20].…”

CO induced surface segregation as a means of improving surface composition and enhancing performance of CuPd bimetallic catalysts

“…The role of Ag is to dilute Pd ensemble size [23] and limit palladium hydride formation [26] which has been shown to be detrimental for achieving high alkene selectivity [27][28][29][30][31][32], although a similar effect may be achieved by using organic sulfur [33][34][35][36] and phosphorous modifiers [33,37]. Several other monometallic [38][39][40], bimetallic [41][42][43][44], trimetallic [45] and metal-free catalysts [46,47] have been shown to offer high alkene selectivity, although they typically require activation and/or use at elevated temperatures. This is in contrast to CuPd catalysts which can be activated and used at much more moderate temperatures [21,22].…”

Shaping the selectivity in heterogeneous hydrogenation by using molecular modification strategies: Experiment and theory

“…Fundamentally, different exposed facets might have different atom arrangement and density, causing different adsorption and activation energies for various catalytic substrates, thus resulting in a big difference in the selectivity and activity. Many recent works indicated that the situation is more complex than expected [41,42]. During hydrogenation, the high coverage H atoms on the Pt and Pd surface would strongly alter the adsorption behaviors of reactants and intermediates, or even change the hydrogenation mechanism.…”