Nanoengineered chiral Pt-Ir alloys for high-performance enantioselective electrosynthesis

Abstract

The design of efficient chiral catalysts is of crucial importance since it allows generating enantiomerically pure compounds. Tremendous efforts have been made over the past decades regarding the development of materials with enantioselective properties for various potential applications ranging from sensing to catalysis and separation. Recently, chiral features have been generated in mesoporous metals. Although these monometallic matrices show interesting enantioselectivity, they suffer from rather low stability, constituting an important roadblock for applications. Here, a straightforward strategy to circumvent this limitation by using nanostructured platinum-iridium alloys is presented. These materials can be successfully encoded with chiral information by co-electrodeposition from Pt and Ir salts in the simultaneous presence of a chiral compound and a lyotropic liquid crystal as asymmetric template and mesoporogen, respectively. The alloys enable a remarkable discrimination between chiral compounds and greatly improved enantioselectivity when used for asymmetric electrosynthesis (>95 %ee), combined with high electrochemical stability.

Heterogeneous Enantioselective Catalysis with Chiral Encoded Mesoporous Pt−Ir Films Supported on Ni Foam

“…The characteristic XRD pattern of Pt in the alloy appears at 2 θ of 40.0° and 46.5° (Figure S4b) originating from its (111) and (200) planes, respectively. The values are shifted to slightly higher 2 θ angles compared to a pure Pt film on Ni foam due to the presence of Ir [14d,23] . Although the characteristic Ir peak cannot be seen directly in the case of the Pt−Ir sample, due to the very low amount of Ir with respect to Pt (10% of Ir see Table S2), the slight shift of the Pt peak can be attributed to a partial substitution of Pt by Ir, which is consistent with previous studies [14d] .…”

“…The values are shifted to slightly higher 2 θ angles compared to a pure Pt film on Ni foam due to the presence of Ir [14d,23] . Although the characteristic Ir peak cannot be seen directly in the case of the Pt−Ir sample, due to the very low amount of Ir with respect to Pt (10% of Ir see Table S2), the slight shift of the Pt peak can be attributed to a partial substitution of Pt by Ir, which is consistent with previous studies [14d] . The composition and alloy structure of the chiral imprinted Pt−Ir film has also been studied by X‐ray photoelectron spectroscopy (XPS) in comparison to monometallic Pt, deposited on nickel foam.…”

“…The XPS spectra of Pt and Ir in monometallic as well as bimetallic layers obtained by using various deposition charge densities are shown in Figure 2c and 2d. The binding energies of 69.7 eV and 73.1 eV for Pt 4 f 7/2 and 58.8 eV and 61.8 eV for Ir 4 f 7/2 are consistent with the standard cards of these elements [14d,23–24] . Remarkably, the characteristic peaks of Pt and Ir of the bimetallic Pt−Ir samples are slightly shifted to a higher binding energy due to the charge transfer between Pt and Ir [25] .…”

“…However, after several consecutive washing cycles, the amplitude of this characteristic peak dramatically decreases. In particular, no more chiral molecules can be detected in the rinsing solution after washing for 30 h [14b–d] . It is noteworthy that even when using one type of chiral molecule (PE) for generating the recognition sites, it allows the stereoselective discrimination of another type of chiral molecule (DOPA) if they have similar stereocentre configurations [11b] .…”

“…Concerning the latter aspect, CIMMs have been employed as working electrodes, yielding initially an only moderate enantiomeric excess (%ee) [14a] . However, by fine‐tuning the conditions of the electroreduction, a very high %ee (>95%) could be achieved, especially when using a pulsed electrosynthesis strategy with imprinted bimetallic Pt−Ir alloys as electrode material [14b–d] …”