(172g) Studying Sub Nano-Meter Ensemble Effects on Selective Hydrogenation Utilizing the ?-Brass Phase Crystal Structure

Maintaining high selectivity is a key concern in many commercial chemical processes, particularly in the petrochemical and fine chemicals industry. Chemoselectivity between two distinct molecules in multi-component feedstocks or functional groups within a single molecule can be controlled to some degree through modification of the active-site ensemble. A popular method to achieve such sub nanometer scale tuning of catalytic active sites is through the use of alloy and intermetallic catalysts. Typically, a relatively inert element such as Zn, Au or Ag is alloyed with an active element such as Pd, Pt or Ni such that the active atoms are segregated into smaller clusters that typically suppresses undesired pathways due to weaker interaction between catalyst and intermediates. Apart from this geometric ensemble effect, alloying may lead to change in the electronic density of state of the active metal which can also affect catalytic behavior (1). In most alloys, the atoms are randomly distributed in different lattice sites so that it is hard to identify any structure-function relations which may potentially lead to better catalyst design. Intermetallics are a special subset of multimetallic catalysts where the atomic site occupation factor on each lattice site is defined such that the nuclearity of each active site can be detemined. Further, the favorable formation enthalpy of intermetallics is thought to safe-guard against severe surface segregation (2). These factors make strategically chosen intermetallic compounds ideal model systems for studying structure-function relations.

In this presentation, we will demonstrate the Pd-Zn γ-brass phase is a model system to test active-site nuclearity effects. This crystal structure has a complex 52 atom unit cell with four symmetrically inequivalent positions: inner tetrahedral (IT), outer tetrahedral (OT), octahedral (OH) and cuboctahedral (CO) sites. Rietveld refinement of XRD patterns and probe reactions (H₂-D₂ exchange, ethylene hydrogenation and acetylene hydrogenation) established Pd₈Zn₄₄ (lower solubility limit of Pd) has completely isolated single-atom Pd sites (due to presence of Pd only on OT site) whereas Pd₉Zn₄₃ has one and Pd₁₀Zn₄₂ two Pd-Pd-Pd trimer sites per unit cell (by Zn replacement on OH site(s)), respectively. Density functional theory (DFT) calculations support the significant body of experimental evidence that trimers are exposed on the surface with increasing Pd amounts.

Apart from changing the nuclearity of Pd active-sites, we found a third metal may be easily incorporated into the Pd-Zn γ-brass phase to form Pd-M-Zn (M=Ni, Pt, Ag, Cu, Au) ternary catalysts. Rietveld refinement along with DFT calculations suggest the preferred position of M atoms is dictated by its electronic structure (i.e. Ni and Pt prefer OT site while Ag, Au, Cu prefer OH site) rather than their physical size. Introduction of a third metal can introduce chemical ensemble effects. As the morphology of the active-site is dependent on the relative site preference (and concentration) of the different metals, it depends on the specific choice of M which should in turn be selected based on the chemistry of interest. Though Pd-(M)-Zn γ-brass phase can be utilized as a model system for any Pd catalyzed chemistry, in our lab we focus on the selective acetylene semi-hydrogenation reaction in presence of excess ethylene and hydrogen (C₂H₂:C₂H₄:H₂=1:30:20). We found among the Pd-Zn materials, only Pd₈Zn₄₄ (having single-atom Pd sites) is selective towards ethylene while Pd trimers unselectively hydrogenate feed ethylene to undesired ethane. We have extended our studies of selective acetylene semi-hydrogenation to Pd-M-Zn γ-brass materials to determine if the addition of M to the active site ensemble enhances selectivity even beyond single-atom Pd. In this case we primarily focus on Pd-Au-Zn and Pd-Ag-Zn because Pd-Au (3) and Pd-Ag (4) random alloy catalysts are selective to ethylene. Since these binary systems are random alloys, no structure-function relations can be determined for these systems and no experimental understanding is available regarding the intrinsic effect of Au and Ag on the Pd active-site.

References

1. J. H. Sinfelt, Catalysis by Alloys and Bimetallic Clusters. Accounts of Chemical Research 10, 15-20 (1977).
2. R. J. Meyer, Q. Zhang, A. Kryczka, C. Gomez, R. Todorovic, Perturbation of Reactivity with Geometry: How Far Can We Go? ACS Catalysis 8, 566-570 (2018).
3. T. V. Choudhary, C. Sivadinarayana, A. K. Datye, D. Kumar, D. W. Goodman, Acetylene Hydrogenation on Au-Based Catalysts. Catalysis Letters 86, 1-8 (2003).
4. G. X. Pei et al., Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene. Acs Catalysis 5, 3717-3725 (2015).