Assessing the concept of structure sensitivity or insensitivity for sub-nanometer catalyst materials

Abstract

The nature of the nano-catalyzed hydrogenation of ethylene, yielding benchmark information pertaining to the concept of structure sensitivity/insensitivity and its applicability at the bottom of the catalyst particle size-range, is explored with experiments on size-selected Ptn (n=7-40) clusters softlanded on MgO, in conjunction with first-principles simulations. As in the case of larger particles both the direct ethylene hydrogenation channel and the parallel hydrogenation-dehydrogenation ethylidyne-producing route must be considered, with the fundamental uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to the structure insensitivity found for larger particles. In this size-regime, the chemical properties can be modulated and tuned by a single atom, reflected by the onset of low temperature hydrogenation at T >150 K catalyzed by Pt n (n≥10) clusters, with maximum room temperature reactivity observed for Pt 13 using a pulsed molecular beam technique. Structure insensitive behavior, inherent for specific cluster sizes at ambient temperatures, can be induced in the more active sizes, e.g. Pt 13 , by a temperature increase, up to 400 K, which opens dehydrogenation channels leading to ethylidyne formation. This reaction channel was, however found to be attenuated on Pt 20 , as catalyst activity remained elevated after the 400 K step. Pt 30 displayed behavior which can be understood from extrapolating bulk properties to this size range; in particular the calculated d-band center. In the non-scalable subnanometer size regime, however, precise control of particle size may be used for atom-by-atom tuning and manipulation of catalyzed hydrogenation activity and selectivity.

Controlling Ethylene Hydrogenation Reactivity on Pt13 Clusters by Varying the Stoichiometry of the Amorphous Silica Support

“…This reflects the inapplicability for metal clusters of less than about 20 atoms (that is at the bottom of the nanometer size-scale) of d-band model theories, [39] as well as other concepts formulated originally for particles of larger sizes or extended metal surfaces;s ee ar ecent reassessment of the validity of the concept of structure sensitivity/insensitivity to sub-nanometer platinum catalysts. [30,31] In this model, as hift in the d c away from the Fermi level is found to be correlated with lowering of the activation barrier for ethylene hydrogenation, as well as predicting under such circumstances an increase in the activation barrier for ethylene dehydrogenation. In the opposite situation, as hift of the d-band center towards the Fermi level is associated with alowered activation barrier for ethylene dehydrogenation and an increased barrier for hydrogenation.…”

“…However,arecent reassessment has concluded against the applicability of such classification for platinum catalyst particles of sub-nanometer dimensions.This was shown in ajoint theoretical and experimental approach to coincide with the suppression of ethylene dehydrogenation on Pt 13 ,which induces structure insensitivity,atlow temperature (< 400 K). [30,31] Consequently,w ef ocus in this work on ethylene hydrogenation turnover frequencies (TOF) on as ingle cluster size (Pt 13 )s upported on the aforementioned a-SiO 2 supports grown on aPt(111) single crystal and isolate the effect of support stoichiometry on the reaction activity,as well as demonstrate that the stoichiometry can be used to tune the activity of the reaction.…”

Spectroscopy for model heterogeneous asymmetric catalysis

“…Our understanding of heterogeneous catalysis in general (not only for asymmetric catalysis) is still lagging behind compared with homogeneous catalysis. However, the latest studies on model heterogeneous catalysts under controlled conditions have been gaining enormous momentum [48][49][50][51][52] and have led to speeding advances in the field even under more realistic conditions. [53][54][55][56] This progress has only been possible as a result of the collaboration between theoretical researchers and experimentalists.…”

Supported Single Atom and Pseudo‐Single Atom of Metals as Sustainable Heterogeneous Nanocatalysts

“…Crampton and co‐workers dwelled into the insights of ethylene hydrogenation catalyzed by Pt supported on MgO, yielding information related to concept of the structure sensitivity/insensitivity and its applicability at the bottom of the catalyst particle size range . Experiments, supported by first‐principles simulations, were performed with size‐selected Pt clusters soft‐landed on MgO.…”

“…Crampton andc o-workersd welled into the insights of ethylene hydrogenation catalyzed by Pt supported on MgO, yielding information relatedt oc oncept of the structure sensitivity/ insensitivity and its applicability at the bottom of the catalyst particles ize range. [73] Experiments, supported by first-principles simulations, were performed with size-selectedP tc lusters softlandedo nM gO. In this technique, aN d:YAGl aser was focused onto ar otating platinum target,w ith each pulse being thermalized and extracted into the vacuum with ad elayed pulse of helium gas.E lectrostatic lenses guided the clusters to a bender from which the cationic species were transferred into a quadrupolem ass spectrometer.T he clusterb eam was then ChemCatChem 2018, 10,881 -906 www.chemcatchem.org size-selected and as ingle cluster size was deposited onto the MgO support.…”