Adsorption of Small Hydrocarbons on the Three-Fold PdGa Surfaces: The Road to Selective Hydrogenation

Abstract

Intermetallic compounds are a promising class of materials as stable and selective heterogeneous catalysts. Here, the (111) and (-1-1-1) single crystal surfaces of the PdGa intermetallic compound were studied as model catalysts with regard to the selective hydrogenation of acetylene (C2H2) to ethylene (C2H4). The distinct atomic surface structures exhibit isolated active centers of single atomic and three atomic Pd ensembles, respectively. For the two prototypal model catalyst surfaces, the adsorption sites and configurations for hydrogen (H2), acetylene, and ethylene were investigated by combining scanning tunneling microscopy, temperature-programmed desorption, and ab initio modeling. The topmost Pd surface atoms provide the preferred adsorption sites for all studied molecules. The structural difference of the Pd ensembles has a significant influence on the adsorption energy and configuration of C2H2, while the influence of the ensemble structure is weak for C2H4 and H2 adsorption. To approach the question of catalytic performance, we simulated the reaction pathways for the heterogeneous catalytic hydrogenation of acetylene on the two surfaces by means of density functional theory. Due to the geometrical separation of the Pd sites on the surfaces, the steric approach of the reactants (H and C2Hx) was found to be of importance to the energetics of the reaction. The presented study gives a direct comparison of binding properties of catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore of major relevance to the knowledge-based design of highly selective hydrogenation catalysts.

Selective hydrogenation of acetylene on SiO2 supported Ni-In bimetallic catalysts: Promotional effect of In

“…Since the formation of the multi-σ-bonded surface intermediates requires the Pd ensembles with three contiguous adsorption sites, a common approach to improve the ethylene selectivity is to block these ensembles through the modification with an inert component, so-called "active site isolation concept" [44,45,46]. According to this concept, the isolation of the surface Pd atoms inhibits the formation of multi-σ-adsorbed species and subsequently the formations of ethane, oligomers as well as carbonaceous deposit, while the selectivity to ethylene is enhanced.…”

The Electronic Structure and Bonding of Acetylene on PdGa(110)

“…Prinz et al 7 recently studied the adsorption sites of the 3-fold-symmetric surfaces of the PdGa (111) and 1 1 1 ( ) surfaces, in a combined experimental and computational approach using CO as a 21 analyzed the activity and selectivity of these two surfaces for the semihydrogenation of acetylene to ethylene using ab-initio DFT based methods. Prinz et al 22 performed a similar study on the same PdGa surfaces, by combining scanning tunneling microscopy, temperature-programmed desorption, and ab-initio catalysis.…”

Highly Enantioselective Adsorption of Small Prochiral Molecules on a Chiral Intermetallic Compound

“…[22] Owing to its noncentrosymmetric bulk structure of the space group P2 1 3, PdGa exists in two enantiomeric forms Aa nd B, which are denoted as PdGa:A and PdGa:B,respectively. [23][24][25] Single crystals grown by the Czochralski method are available in cm 3 -sized monoliths [26] and various low-index surfaces can be prepared to show large atomic terraces by standard ultrahigh vacuum (UHV) preparation using sputtering and annealing cycles. [24,25] As no significant surfaces reconstructions occur, [24,25] the bulk chirality is maintained at all of these surfaces,t hus each surface unit cell exhibits achiral atomic arrangement, as shown in Figure 1.…”