We present a facile, size-controlled and cost-efficient method to synthesize Pd–Au core/shell nanostructures using E. coli
which, in contrast to Desulfovibrio desulfuricans
], can grow to high density at scale, and does not produce H2
S, a catalyst poison. NP synthesis relies on the ability of E. coli
cells to reduce Pd(II) ions enzymatically from a precursor (PdCl42−
salt) using H2
as an electron donor [11
]. We postulated that pre-palladizing cells with a fine layer of Pd(0) would lead to an increase in the rate of Au(III) reduction under H2
(see electronic supplementary material, figure S4) and result in the incorporation of Au atoms into the Pd seeds. The combined use of imaging and bulk/surface probing techniques permits detailed molecular- and atomic-scale structural analysis of the biomass-supported Pd–Au nanostructures. Following the sequential reduction of Pd(II) and Au(III), E. coli
cells exhibited complete coverage of both the cell surface and the periplasmic space, with some cells showing a small number of intracellular NPs (a
). Some large clusters were observed (a
); a bimodal size distribution was reported previously for chemically synthesized Pd–Au NPs [22
] and for bioNPs on D. desulfuricans
]. Bimetallic Pd–Au particles of approximately 16 nm were examined with respect to their Au and Pd distributions as shown by the characteristic X-ray signals from Lα
transitions for Au and Pd atoms. In accordance with other work using high-angle annular dark field analysis [21
], which produces image contrast dependent on atomic number, c
shows that Pd agglomerates (arrowed) decorate an Au-rich NP core region, with some Pd detected also between the NPs (encircled). Analysis by X-ray powder diffraction (electronic supplementary material, figure S5) [23
] showed clearly crystalline Au(0), but not Pd(0), components.
Figure 1. Electron microscopy of metallized cells of E. coli MC4100. (a) TEM of cells of E. coli MC4100 following the sequential reduction of Pd(II) and Au(III) (5%/5% Pd–Au on biomass w/w); untreated cells are shown in inset (b). Scale bars are 500 nm. (more ...)
Characterization of the obtained Pd–Au structures using X-ray absorption spectroscopy analysis shows an important degree of metal–metal coordination. Combined analysis of cyclic voltammetry (CV; ) and EXAFS data obtained at the Pd K- and Au LIII
-edges () is consistent with the development of a core/shell structure where surface-exposed Pd atoms decorate a core of Au atoms. a
shows the voltammetric profile for the bioPd–Au preparation. The first cycle shows the presence of Pd oxide and the absence of any Au oxide, i.e. most surface sites are occupied by Pd. Since XRD analysis (electronic supplementary material, figure S5) showed that Au occupied the bulk sites, the bioPd–Au NPs appear to exhibit an Au–Pd core/shell structure. Pd oxide desorption potentials correspond to those expected from bulk Pd although the full width at half maximum of the peak is somewhat larger than for bulk Pd [24
], indicating that there is a significant perturbation brought about by the gold component of the NPs. Further potential cycling engenders changes to the surface associated with both oxidative cleaning of all Pd sites and Pd dissolution [24
]. Au oxide stripping peaks are now visible and continue to increase in size as a function of potential cycling. This is completely consistent with continual electrochemical dissolution of Pd covering Au sites (after dissolution of a Pd capping layer). Interestingly, this is the reverse configuration of that predicted according to the sequence of reduction of the precursors, i.e. as Pd(0) seeds were used, surface Au-rich Pd NPs immobilized on cells (Pd core/Au shell NPs) were expected. Simple thermodynamic arguments, based on the lower surface energy of Au and stronger Pd–Pd bonding, would also favour the Pdcore
]. This is clearly not the case here, as confirmed by CV (). Similar results have been reported in studies where the sacrificial hydrogen strategy was used to generate the Pd–Au NPs [22
], where the mechanism was attributed to pre-formed Pd particles reducing Au(III) (the respective redox potentials are Au3+
/Au, 1.002 V; Pd2+
/Pd, 0.83V) to generate Pd2+
ions which then relocate around Au NPs and are reduced to Pd(0) via H2
on the Au–NP surface. The two NPs in c
show different stages of this progression.
Figure 2. Surface analysis of bioPd–Au using cyclic voltammetry (CV). (a) Voltammetric profile of bioPd–Au in 0.1 M H2SO4 for the first cycle (solid line), the tenth cycle (dashed line), the last cycle (dotted line) and the glassy carbon support (more ...)
Figure 3. EXAFS analysis of bioPd–Au and reference compounds. K3-weighted EXAFS spectra (left panel) and corresponding FT (right panel) of bioPd–Au sample and reference compound at the (a) Pd K-edge and (b) Au LIII-edge. Blue lines, data; red lines, (more ...)
X-ray absorption near-edge structure analysis of the bioPd–Au sample at both Au and Pd edges shows that, while Au is mainly present as Au(0), Pd is present as a mixture of Pd(0) and Pd(II), with a significant dominance of the ionic part (electronic supplementary material, figure S6). The decrease of whiteline intensity observed at the Au LIII
-edge of the experimental sample in comparison with the Au foil sample is indicative of the decrease in the density of unoccupied sites of the Au 5d orbital in the bioPd–Au sample relative to the Au bulk, which is typical of Pd–Au alloy formation [26
]. To confirm Pd–Au alloy formation and to discern different alloying motifs (random or core/shell-like non-random), EXAFS spectroscopy was used. shows the EXAFS spectra of bioPd–Au NPs and reference compounds (Au, Pd foils) at both the Pd K- and Au LIII
(i)) along with their corresponding Fourier transforms (FTs; a
(ii)). Pd K and Au LIII
structural parameters including the coordination number of the different paths (Au–Au, Au–Pd, Pd–Pd, Pd–Au and Au–M; Pd–M (where M is Au or Pd)) of Au, Pd foils and bioPd–Au NPs are summarized in . The first shell Pd–metal coordination number (NPd−M
) was calculated to be 3.1 ± 0.4 () and is much smaller (by a factor of 3) than that of Au–metal (NAu−M
) calculated to be 10.7 ± 0.6. According to Teng et al.
], the fact that NPd−M
indicates that a larger number of Pd atoms segregate to the surface of the NPs and Au atoms are present in the core, since atoms on the surface have fewer neighbours than those in the core. In addition, the environment of Au atoms is highly ordered in the bioPd–Au sample, presumably owing to their preferential bonding in the core since no lattice expansion was observed in this sample as the RAu−Au
in Au foil and bioPd–Au were similar within the experimental errors. We conclude that the EXAFS fitting results are completely in accordance with a Pd shell and Au core structure as N
(Pd–M) (). Additional evidence for the formation of a non-random alloy with a core/shell structure is the fact that the Au–Pd bond length (2.75 ± 0.02 Å) is smaller than those observed for Pd–Pd (2.76 ± 0.02 Å) and Au–Au (2.84 ± 0.02 Å). As suggested previously [28
], Au–Pd bonds possibly formed very stable bridges between two sub-lattices at the interface of the ordered Au core and disordered Pd shell structure. The disorder in the Pd shell is due to the bonding of Pd atoms with O, as was demonstrated by EXAFS spectroscopy, which indicates that the Pd surface atoms are exposed/coordinated to oxygen and/or nitrogen donor atoms as significant contribution to the EXAFS signals arose from those of Pd–O or Pd–N bonds. EXAFS spectroscopy cannot distinguish between the Pd–O and Pd–N contribution; therefore, they are both modelled as Pd–O for simplicity. Thus, at the Pd edge, the first three peaks of the FT correspond to Pd–O1
and Pd–Pd bonds, respectively. The distances were identified using the Pd–O and Pd–Pd backscattering phase and amplitude functions obtained from atomic coordinates of PdO using the FEFF 8 program. From the Pd–O coordination numbers, the fraction of oxidized atoms was estimated to be about 65% (1.2/1.9).
Best-fit results obtained by EXAFS analysis of Pd foil, Au foil and bioPd–Al bimetallic sample.
The size of the bimetallic NPs, estimated by means of the determination of the average metal coordination number, NM−M
, where NM−M
is 7.8 ± 0.6, corresponds to a particle size of 1.5–2.5 nm using a previously reported correlation between the coordination number and the particle size [29
This is not consistent with the particle size estimation obtained from the XRD spectrum (estimated particle size of about 4.5 nm; see electronic supplementary material, figure S5), owing probably to the enhanced surface disorder (significant relaxation under the influence of ligands, e.g. oxygen donor atoms), which may result, according to Yevick & Frenkel [30
], in the underestimation of particle size of metal NPs in the size range under 5 nm.
Finally, we showed the catalytic activity of the bioPd–Au NP material. A mass metal loading of 5% is commonly used for chemical catalysts; the NPs (2.5% Pd/2.5% Au) were reacted against benzyl alcohol. The biogenic catalyst in air compares well with the chemically prepared catalyst in O2
] (; entry 3) at a similar catalyst loading. Much higher turnover frequencies (TOFs) can be achieved with TiO2
-supported catalysts than thus far observed for the bioPd–Au catalyst, but these high activities appear to be favoured by much lower catalyst loadings (approx. 6 × 10−5
in entries 4–8, cf. approx. 130–180 × 10−5
; entries 2 and 3). Also, entries 9 and 10 show a 20-fold enhancement in TOF over a 40°C increase in temperature, which suggests a high potential of the biomaterial as this has an activity comparable to that of entry 9 at a 30°C lower temperature.
Comparison of the catalytic activity of the biocatalyst for benzyl alcohol oxidation with data from the literature.