Paula S. Cappellari*,
Germán J. Soldano and
Marcelo M. Mariscal
INFIQC, CONICET, Departamento de Qumíca Teórica y Computacional, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba (XUA5000), Córdoba, Argentina. E-mail: marcelo.mariscal@unc.edu.ar; pcappellari@exa.unrc.edu.ar
First published on 14th March 2018
IrAu nanoalloys have been proven to have remarkable reactivity for several reactions. In this work, mixed IrAu nanoalloys of 8, 27, 48 and 64 total atoms were studied in different atomic compositions (IrmAun) using Density Functional Theory (DFT). A notable segregation tendency is observed, where Ir atoms are located in the inner part and Au atoms in the outermost region of the nanostructure. We found that IrAu nanoalloys present a distinctive synergistic effect with respect to reactivity. In addition, the projected density of electronic states (PDOS) energies were analyzed by examining the d-band shift to estimate the reactivity of various IrAu nanoalloys. Furthermore, the adsorption energies for the CO molecule in the domains of the Ir–Au interface were evaluated. In this sense, the addition of Au atoms to Ir clusters increases the reactivity of Ir by generating unoccupied orbitals near the Fermi level as indicated by the PDOS study.
The morphology of a NA is specified when both its geometric structure and its chemical ordering pattern are specified.5 The chemical ordering pattern is the way in which the two elements are arranged within the geometric structure. Besides the size, shape and chemical composition, the spatial distribution and the stability of the elements constitute an excellent opportunity for the rational design of NAs.6,7 In fact, both the geometric shape and the energetic stability may drastically change with size.8
The great variety of compositions, structures, and features of metallic alloys has led to a wide range of new applications in electronics, engineering, and catalysis.9–12 With regard to the area of catalysis, several bimetallic systems have been reported and studied in the last few years.13–16 Bimetallic clusters offer fascinating prospects for the design of new catalysts. NA catalysts containing Pt with Ir or Re have found extensive use in the reforming of petrochemicals.17 Experimental reports show that the addition of the second metal improved the gold catalytic activity.18,19 On the other hand, gold can influence the activity and selectivity of some metals, such as palladium, platinum and ruthenium. Gold-containing bimetallic catalysts have found important industrial applications and represent an active research area. For the particular case of Pd and Pt, it was reported that the presence of gold can modulate the catalytic properties of H2 dissociation.20
Nilekar et al.21 have described that the presence of Ir and other transition metals in Pt NPs improves the preferential CO oxidation in hydrogen dramatically. Specifically regarding the IrAu system, it was shown that the addition of Au enhanced the chemisorptive and catalytic properties of bimetallic Ir–Au/γ-Al2O3 catalysts compared to the pure iridium samples in methylcyclopentane (MCP) hydrogenolysis.22 It has also been reported that IrAu NAs displayed enhanced activity in ethanol oxidation to acetaldehyde, outperforming their monometallic counterparts.18 In this respect, the effects of gold addition to iridium catalysts for citral hydrogenation over Ir–Au/TiO2 catalysts were investigated by Rojas et al.23 Additionally, the improvement of the catalytic activity for the oxidation of CO of Ir–Au/rutile catalysts, which could be due to a synergetic effect caused by the combination of gold and iridium in small particles, has been reported by Bokhimi et al.24
Despite the experimental evidence on the synergic effect between iridium and gold in nanoalloys, a systematic first principles study of such systems and in addition, a fundamental understanding of the physical picture behind this phenomenon, is still missing. Jiménez-Díaz et al.25 studied the reactivity of CO and O2 on 20-atom IrAu NAs by DFT. The nanoclusters on which the adsorption was tested were relaxed using the Gupta potential; consequently, the metal atoms are highly coordinated. In contrast, DFT studies suggest that such clusters are characterized by low coordinated atoms forming cubic building blocks.26–29 Clearly, a new approach on more realistic clusters is needed.
Theory can also shed light into an experimental blindspot, which is the spatial distribution of species. Previous works25,29,30 have attempted to do so, although the cluster sizes investigated were always smaller than the experimental ones (1–2 nm).15,23,24 Therefore, the present study represents the first theoretical approximation to “real” synthesized nanoalloys. In this report, IrAu NAs of different sizes (8, 27, 48 and 64 total atoms) and compositions are studied using DFT calculations in order to understand the effects of chemical ordering on the stability and chemical reactivity of these nanostructures. The latter was evaluated using CO as a probe molecule, comparing its adsorption energy on IrAu NAs and the corresponding pure metal clusters. The DFT results presented here reveal an improvement in the reactivity of Ir by Au atoms already evidenced experimentally and subsequently, by means of the study of PDOS, the reason for such improvement could be understood.
(1) |
The energy of chemisorption of a CO molecule onto a metal cluster is defined as
(2) |
The relative CO adsorption energy is defined as the difference between the adsorption energy on pure Ir and that on the IrAu NA at analogous sites.
(3) |
Fig. 1 8-atom IrAu NAs. The green spheres represent atoms of Au and the blue spheres atoms of Ir. Under each structure the formation energy ΔE is reported. |
It is important to mention that gold and iridium are impossible to distinguish from one another using the Z-contrast annular dark-field images in HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy) due to the proximity of their atomic number (77 and 79, respectively).24 In this regard, the study by means of DFT of IrAu NAs with high atomic numbers would generate a complete approach for the experimental field as well as assist in interpreting the different environments of Ir and Au in STEM images. The high energy of the more mixed clusters (not shown) reveals clear segregation phenomena among the two metals, as described in a study on first principles calculations of transition metal binary bulk alloys by Aspera et al.44
In addition, it is found that Ir atoms are more unstable at the undercoordinated sites than Au. This effect causes Au atoms to occupy locations on the periphery of the nanostructure, whereas Ir atoms tend to locate in the innermost region. This last trend was clearly recorded in the IrAu structures with N = 8, as well as 2D nanostructures with N = 9 (see Fig. S1, ESI†). This phenomenon has been reported for different metals.7,45,46 In particular, this type of performance has been recorded in IrAu NA structures previously by Davis et al.29 and Jiménez-Díaz et al.25
The same trends were found for larger clusters, as shown in Fig. 2–4. In the case of 27-atom IrAu NAs, four different isomers were tested for each type of atomic distribution. The formation energy differences between them oscillate in the range of 0.1–0.2 eV (see Fig. S2 ESI†). Fig. 2 shows the most stable isomers found here for mixtures of different atomic compositions for clusters of 27 atoms.
Fig. 2 27-atom IrAu NAs. The green spheres represent atoms of Au and the blue spheres atoms of Ir. Under each structure the formation energy ΔE is reported. |
For the IrAu NAs with N = 48 and 64 two possible compositions for each atomic distribution were studied, which are shown in Fig. 3 and 4. Isomers containing “house” blocks were found to be more stable than those with cubic shapes. If the formation energy of Ir32Au16.01 is compared with that of Ir32Au16.02 (Fig. 3) there is a stabilization of 0.11 eV in the latter. The repetition of “house” blocks is clearly perceived for the insertion of Ir atoms in Ir32Au16.02, while the Au environment remains amorphous. The spin multiplicity is in agreement with what has been described by Davis et al.,33 where mixed structures are analyzed starting from Ir cubes. In cases where the structure is maintained, the spin multiplicity is low (1–3), similar to the trend seen for IrAu NA, in Table 1.
Fig. 3 48-atom IrAu NAs. The green spheres represent atoms of Au and the blue spheres atoms of Ir. Under each structure the formation energy ΔE is reported. |
Fig. 4 64-atom IrAu NAs. The green spheres represent atoms of Au and the blue spheres atoms of Ir. Under each structure the formation energy ΔE is reported. |
IrAu BNA | ΔE/eV | (2S + 1) |
---|---|---|
Ir27 | −5.11 | 2 |
Ir23Au4 | −4.68 | 2 |
Ir21Au6 | −4.45 | 2 |
Ir19Au8 | −4.29 | 2 |
Ir14Au13 | −3.71 | 3 |
Ir14Au13 | −3.71 | 3 |
Ir48 | −5.35 | 3 |
Ir32Au16.01 | −4.24 | 3 |
Ir32Au16.02 | −4.35 | 2 |
Ir24Au24.01 | −3.80 | 1 |
Ir24Au24.02 | −3.93 | 2 |
Ir64 | −5.45 | 2 |
Ir48Au16 | −4.72 | 2 |
Ir32Au32.01 | −3.83 | 2 |
Ir32Au32.02 | −3.92 | 3 |
Ir32Au32.03 | −3.93 | 2 |
We remark that the geometries are fully relaxed at 0 K as this is straightforward in DFT calculations, however it should be taken into account that in “real” experimental systems these materials are subjected to high temperatures, where a large number of isomers can “live” at the same time.
In order to evaluate the degree of crystallinity of these systems, the pair correlation functions g(r) were analyzed for the most relevant structures, shown in ESI Fig. S3.† This analysis confirms that Ir atoms present a high degree of crystal ordering while Au atoms are rather disordered. This result contributes to the hypothesis that the highly ordered and randomly distributed atoms in the TEM images correspond to the Ir and Au atoms, respectively.24,34
Since we are interested in the electronic properties and even more so in the reactivity of these IrAu NAs, the projected density of states (PDOS) for the most stable structures was calculated. In Fig. 5, the PDOS is shown for the case of (IrAu)48 with different compositions (other cases are shown in the ESI, Fig. S4†).
We focus our analysis on the d orbitals, which are known to actively participate in catalytic reactions.47 In fact, it has been shown that the reactivity of transition metals is directly related to the d-band shape, width, and position.48,49 Fig. 5a and b show the PDOS of the d-band for the Ir and Au atoms in the mixture, respectively. In each case they are compared with the PDOS of the pure clusters. An upward shift of the Ir d-band center is revealed, as shown in Table 2.
IrAu nanoalloy | Ir d-band center | Au d-band center |
---|---|---|
Ir48 | −2.54 | — |
Au48 | — | −3.14 |
Ir32Au16 | −2.48 | −3.21 |
Ir24Au24 | −2.20 | −3.12 |
In contrast, for Au atoms, a slight downward shift upon alloying is observed. These d-band center displacements induced by gold have also been observed for RhAu nanostructures.50 An upward displacement of the d-band center is usually associated with an increase of reactivity: the d-band tends to form bonding and antibonding states at energies below and above its original value, respectively. If the d-band of the pure metal is above the Fermi level, the upward shift induced by the other metal tends to empty these antibonding states, resulting in a stronger reactant–metal bond.
The adsorption energy on the terrace atoms in Ir48 is between 0.1 and 0.9 eV higher than that on the low coordinated atoms. CO adsorption on Au was also calculated and found to be at least 1 eV higher in energy than that on the Ir NPs. Other data to consider is that the average distance of Ir–C for all the adsorption sites and in the different structures was in the range of 1.88–1.90 Å, while the bond distance of CO under vacuum is 1.15 Å, and after the allocation of CO in the sites, this was maintained between values of 1.16 and 1.19 Å.
The same procedure was applied for the other IrAu NAs (N = 8, 27, and 64). In all the cases studied, the same behavior for the adsorption of the CO molecule was found. The adsorption of CO in the pure Au clusters was investigated. CO adsorbs more strongly on Ir48 than Au48, with adsorption energies of −2.25 eV and −1.03 eV, respectively, for the best atomic adsorption site (see Fig. S5 in the ESI†).
A direct comparison between the adsorption energies of CO on the pure and mixture clusters can be misleading since gold often induces changes in the coordination and geometry of Ir ensembles. The changes in the adsorption energy could be attributed to the change in coordination rather than the changes occurring as a consequence of the d-band shift. For this reason, only cases with similar chemical environments were compared. Numbers 1–6 are assigned to each corresponding position in Ir48 in Fig. 7 and 8. In this way, sites homologous to Ir48 were found but with different chemical environments due to the presence of Au. As in the case shown in Fig. 7, where two 4 sites were recorded, the first has double coordination with Au and has lower energy than the second. On the other hand, the adsorption energies of CO on the Au atoms for the IrAu NAs are always smaller in absolute value, with more than a 1–1.6 eV difference with respect to the adsorption on the Ir atoms. In the IrAu NA with N = 48 the values of the CO adsorption energies remain negative and even in some positions more negative than those in Ir48. Relative CO adsorption energies (ΔE) are defined as the difference between the adsorption energy of CO on pure Ir with respect to that on an analogous site on the IrAu NA. Fig. 9 shows ΔE for two IrAu NA compositions. The CO adsorption on Ir atoms close to Au atoms is 0.62 eV more stable than that on Ir atoms far from the Au atom, as in the case of positions 4 and 5 for Ir32Au16 shown in Fig. 9. At position 6 for Ir32Au16 the Au atoms are located farther than in position 5, where the Ir atom is adjacent to a Au atom, as shown in Fig. 7. As shown in Fig. 9, in almost all the CO adsorption Ir positions for Ir32Au16 and Ir24Au24 the energies are positive, indicating that CO adsorption on these positions is favorable.
Summarizing this section, the presence of Au atoms in the IrAu mixture enhances the adsorption of CO on Ir atoms and this has also been evidenced experimentally.34
PDOS reveals an up-shift of the d-band of Ir induced by Au. According to the d-band model, this fact allows us to find a possible interpretation for the improvement in the reactivity of Ir in the presence of Au, which was confirmed using CO as a probe molecule. These findings open up new possibilities to explore the catalytic potential of mixed IrAu systems, which are of great technological interest.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra13347b |
This journal is © The Royal Society of Chemistry 2018 |