Composition tunable cobalt–nickel and cobalt–iron alloy nanoparticles below 10 nm synthesized using acetonated cobalt carbonyl
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A general organometallic route has been developed to synthesize Co x Ni1−x and Co x Fe1−x alloy nanoparticles with a fully tunable composition and a size of 4–10 nm with high yield. In contrast to previously reported synthesis methods using dicobalt octacarbonyl (Co2(CO)8), here the cobalt–cobalt bond in the carbonyl complex is first broken with anhydrous acetone. The acetonated compound, in the presence of iron carbonyl or nickel acetylacetonate, is necessary to obtain small composition tunable alloys. This new route and insights will provide guidelines for the wet-chemical synthesis of yet unmade bimetallic alloy nanoparticles.
KeywordsSynthesis Nanoparticles Cobalt alloy Carbonyl disproportionation Acetone
Two-component alloy nanoparticles based on Fe, Co, and Ni are of great interest in the catalysis of, for example, the Fischer–Tropsch synthesis or the decomposition of cellulose (Cabet et al. 1998; Zhao et al. 2011; Jia and Schuth 2011). More than the single metals, bimetallic mixtures make it possible to tune carbon deposition and carbide formation rates, which are crucial for catalytic activity and lifetime (Cnossen et al. 1994; Pinheiro and Gadelle 2001) or the adsorbate bond dissociation energies as a function of the metal d-band center as described by the Newns–Anderson model (Nilsson et al. 2008). With bimetallic nanoparticles, catalytic performance is often also enhanced by their superior sintering resistance (Alloyeau et al. 2010; Cao and Veser 2010). Furthermore, an advantage over, for example, Pt, Pd, or Rh is that 3d transition metals are abundant and low priced, and can be used to replace expensive noble metals in catalytic processes (Nørskov et al. 2011; Haynes and Lide 2012).
Ideally, bimetallic catalytic nanoparticles should be prepared with a tunable composition and as small as possible, <10 nm, to maximize their surface-to-volume ratio. Although the preparation of bimetallic nanoparticles has been widely researched (Hyeon 2003; Wang and Li 2011), no general approach has been reported to synthesize Co–Ni or Co–Fe particles <10 nm with a tunable composition. Larger Co x Fe1−x particles in the 10–20 nm range have been prepared by thermal decomposition of organometallic compounds in high-boiling organic solvents, for instance using iron pentacarbonyl (Fe(CO)5) and Co(η3-C8H13)(η4-C8H12) or Co(N(SiMe3)2)2 (Desvaux et al. 2005), or iron(III) and cobalt(II) acetylacetonate (Chaubey et al. 2007). Smaller particles of 5–8 nm were synthesized using bimetallic carbonyl clusters that contain both iron and cobalt, but with a fixed elemental composition of FeCo3 (Robinson et al. 2009). CoNi particles of 30-nm size with a fixed elemental composition were prepared in triethylene glycol with polyvinylpyrrolidone (Hu et al. 2008), and smaller particles were made through a bio-based approach in apoferritine cavities or supported in polymer films (Abes et al. 2003; Gálvez et al. 2010). Monodisperse 8-nm nanoparticles from cobalt and nickel acetate hydrates were also reported but only with a ratio of Co40Ni60 (Murray et al. 2001). Besides wet-chemical techniques, physical evaporation methods have been used to prepare Co–Fe nanoparticles, but this too did not lead to particles <10 nm with a tunable composition (Reetz et al. 1995; Li et al. 2001; Wang et al. 2003).
Here, we report a novel organometallic method to synthesize colloidal nanoparticles of Co–Ni and Co–Fe with a fully tunable composition and a size of 4–10 nm. Our method relies on a straightforward and inexpensive pre-treatment of dicobalt octacarbonyl in dry acetone before it is thermally decomposed together with iron carbonyl or nickel acetylacetonate. First, the importance of the acetonation step will be demonstrated. Second, the tunability of nanoparticle alloy composition will be examined. Finally, it will be shown how the crystal structure of Co–Ni and Co–Fe nanoparticles can be controlled through the choice and concentration of surfactant molecules present during synthesis.
Nickel(II) acetylacetonate (Ni(acac)2; 95 %), cobalt(III) acetylacetonate (Co(acac)3; 99.99 %), trioctylphosphine oxide (TOPO; 99 %), dioctyl ether (99 %), 1,2-dichlorobenzene (anhydrous, 99 %), 2-propanol (anhydrous, 99 %), and cyclohexane (anhydrous, 99.5 %) were purchased from Aldrich. Dicobalt octacarbonyl (Co2(CO)8; hexane stabilized, 95 %), iron pentacarbonyl (Fe(CO)5; 99.5 %), oleic acid (OA; 97 %), acetone (anhydrous, 99.8 %), and toluene (anhydrous, 99.99 %) were obtained from Acros. Benzene (≥99.5 %) was obtained from Fluka. All chemicals were used as received.
Co x Ni1−x nanoparticle synthesis
Co x Ni1−x particles were made by combining literature recipes for the preparation of pure Co or pure Ni nanoparticles and by adding an acetonation step (Murray et al. 2001; Bao et al. 2009). Pure Co nanoparticles were prepared using a Co:OA:TOPO molar ratio of 12.15:2.38:1 (Bao et al. 2009), whereas pure Ni nanoparticles were prepared using a molar ratio nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O) to OA to tributylphosphine to tributylamine of 4:2:1:8 (Murray et al. 2001). Based on this, the following interpolating formulas were used to calculate reactant amounts for a standard synthesis: [OA] = 0.196[Co] + 0.516[Ni] and [TOPO] = 0.0824[Co] + 0.217[Ni], where [i] is the molar concentration of i. First, Co2(CO)8 and Ni(acac)2 were left to dissolve for 30 min in 3 mL of anhydrous acetone in a nitrogen atmosphere glove box, under occasional stirring of the flask by hand. Next, OA and TOPO were simultaneously added to 12 mL dioctyl ether in an adapted round-bottom synthesis flask (see Fig. S1 in Online Resource 1) inside the glove box, and the solution was subsequently heated to 280 °C in a nitrogen Schlenk line outside the glove box. The metal precursor solution was then injected from airtight vials in the hot ligand-containing solvent. Mixtures were refluxed for 30 min, allowed to cool to room temperature, and transferred back to the glove box before further analysis. No amines were used, because we observed that amines destabilize ε-Co nanoparticles (they act as a hard Lewis base forming a strong Co–NH2R bond; see Fig. S2 in Online Resource 1). Synthesis series A1–A4 were made in which the Co-to-Ni metal and/or the metal-to-ligand ((Co + Ni)/(OA + TOPO)) ratios were systematically varied. Essentially, series A1 and A2 keep the amounts of surfactants constant and series A3 and A4 keep the amounts of organometallic precursors constant. Exact amounts of metal precursors and ligands used for all Co x Ni1−x syntheses are given in Table S1 in Online Resource 1. It was verified with duplo syntheses for all syntheses in the manuscript that the results are reproducible.
Co x Fe1−x nanoparticle synthesis
The same procedure as for the Co x Ni1−x nanoparticles was used, but with Ni(acac)2 replaced by Fe(CO)5 and with the following formulas to calculate the amounts of OA and TOPO: [OA] = 0.196[Co] + 0.75[Fe] and [TOPO] = 0.0823[Co] + 0[Fe]. This was based on literature Fe:OA ratios of 1:1 and 1:3 (Murray et al. 2001; Farrell et al. 2003), and the absence of TOPO in reported Fe nanoparticle syntheses (Farrell et al. 2003). Synthesis series A5–A7 aimed to study the Co x Fe1−x composition dependency on the organometallic precursors and organic ligands concentrations. Exact amounts of the chemicals used can be found in Table S2 in Online Resource 1.
Alternating gradient magnetometer (AGM) measurements
A volume of 4 μL of dioctyl ether nanoparticle dispersion was added to airtight glass vials inside the glove box. Magnetization curves were measured using a MicroMag 2900 AGM (Princeton Measurements Corporation). Volume-averaged magnetic dipole moments and magnetic size polydispersities were determined from the curves according to Chantrell et al. (1978). Saturation magnetization values were calculated by dividing the average dipole moment by the average particle volume from transmission electron microscopy.
Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX)
Carbon-coated Formvar Cu-grids (Agar Scientific) were dipped in nanoparticle dispersions and imaged on a Tecnai 12 (FEI) operating at 120 kV, equipped with a SIS CCD camera Megaview II. ITEM software (Olympus) was used to measure size distributions based on at least 200 particles. A Tecnai 20 (FEI) microscope operated at 200 kV, equipped with a field emission gun, Gatan 694 camera, and EDAX spectrometer was used for EDX analysis. For this purpose, raw nanoparticle dispersions were submitted to three washing cycles using 2-propanol to destabilize and cyclohexane to redisperse the particles. At least five different micron-sized spots and up to 20 individual nanoparticles were analyzed in each batch to determine particle composition and to test its uniformity over the batch.
Ultraviolet–visible (UV/Vis) spectroscopy
X-ray powder diffraction (XRD)
XRD diffraction patterns were acquired on a Bruker D8 Advance and a Bruker D2 Phaser diffractometer. Cobalt Kα1,2 X-ray tubes (λ = 1.790 Å) operating at 30 kV were used, with currents of 45 and 10 mA, respectively. Typically, data points were acquired between 40° < 2θ < 100° every 0.2° with 13 s step−1. XRD samples were prepared inside a glove box and enclosed in an airtight and X-ray transparent box to probe the non-oxidized as prepared metal nanoparticles.
First, UV/Vis spectroscopy will be used to demonstrate that Co2(CO)8 reacts with acetone. Next, it will be shown that the acetonation step has a strong effect on the cobalt alloy nanoparticle preparation. The tunability of Co x Ni1−x and Co x Fe1−x particle composition will then be addressed, before revealing the particle magnetic properties. Finally, it is shown how the crystal structure of the nanoparticles is affected by the choice and concentration of the organic ligand molecules present during synthesis. The results will be further interpreted in more general terms in the "Discussion" section.
Acetonation of cobalt carbonyl
Our alloy nanoparticle synthesis approach relies on the pre-treatment of Co2(CO)8 with dry acetone before it is thermally decomposed. In experiments using an analytical balance, mass loss was recorded on Co2(CO)8 dissolution in acetone, corresponding to 3.1 CO molecules per Co2(CO)8. The UV/Vis spectrum of Co2(CO)8 in dioctyl ether is shown in Fig. 1a before and after addition of 100 μL of dry acetone. The initial spectrum is identical to that of Co2(CO)8 in 2-methylpentane (Abrahamson et al. 1977); the peak at 350 nm is assigned to σ → σ* transitions of Co–Co derived molecular orbitals. On addition of 100 μL of dry acetone to the 2.5-mL dioctyl ether solution, a rapid decrease of the 350-nm peak intensity occurs, indicating that Co–Co bonds are broken. Figure 1b zooms in on the part of the spectrum >350 nm, for Co2(CO)8 dissolved directly in dry acetone. A stable species exhibiting two absorption features at 472 and 517 nm is observed, which is assigned to 4T1g → 4T1g(P) transitions in high-spin octahedrally coordinated Co2+ 3d 7 species (Bayliss and McRae 1954; Lever 1984). This is the species from which we start the nanoparticle alloy synthesis. It is different from the species formed when the solution is exposed to air or oxygen, which would exhibit features at 512 and 574 nm because of charge transfer transitions due to O2 adsorption on the octahedrally coordinated Co2+ cations (Semenov et al. 2002). No relevant solvent effects were observed for any of the other metal precursors used in this study (see Fig. S3 in Online Resource 1).
Beneficial effect of cobalt carbonyl acetonation on nanoparticle alloy synthesis
Tunability of nanoparticle alloy composition
Magnetization of the particles
Magnetic properties of Co x Ni1−x and Co x Fe1−x alloy nanoparticles
EDX Co (%)
TEM diameter (nm) polydispersity (%)
Average dipole moment (10−20 A m2)
Polydispersity of magnetic diameter (%)
Nanoparticle magnetization (kA m−1)
Nanocrystalline structural phase analyses
Figure 6b displays a gradual change in nanoparticle crystal structure for series A2, going from fcc Ni, through hcp Ni and fcc CoNi to fcc Co. Pure nickel aggregated nanomaterials exhibited an fcc crystal structure, in contrast to almost Ni pure aggregates in series A1. For the pure cobalt nanoparticles, fcc Co was now observed in contrast to the less dense ε-Co structure for the Co0.96Ni0.04 nanoparticles in series A1. For both synthesis series A3 and A4, mainly fcc CoNi diffractograms were observed as shown in Fig. S7 (in Online Resource 1). Adding more OA (series A3), or changing the relative OA:TOPO ratios with fixed OA + TOPO amounts (series A4), did not result in crystal structure changes. Figure S9 (in Online Resource 1) shows the XRD patterns of the Co x Fe1−x nanoparticles. All materials exhibit a fcc crystal structure, but the noise in the patterns reveals that the particles were amorphous, while the crystallinity increased for Co/(Co + Fe) ratios ≥ 90 %. The Co x Fe1−x nanoparticle phase behavior was found to be much less complex, and less dependent on the ligands used, than that for the Co x Ni1−x nanoparticles.
In the following, the presented results are systematically dealt with before ending with two general discussions. The requirements for preparing transition bimetal particles are examined in terms of the strengths of the interactions between metal atoms and organic ligands. Finally, it is addressed why these nanoparticles are suitable model systems in the search for non-noble metal based catalysts.
On Co2(CO)8 dissolution in acetone, a mass loss occurred that corresponds to 3.1 CO molecules per Co2(CO)8. This is in fair agreement with the expected value of 2.7 on the basis of the proposed reaction equation. It is also supported by UV/Vis spectroscopy as shown in Fig. 1. After Co2(CO)8 acetonation, absorption due to octahedrally coordinated Co2+ was observed, whereas the tetracarbonyl cobaltate anions are thought to be tetrahedrally coordinated (Bühl et al. 2006) and non-absorbing in the UV/Vis regime (Semenov et al. 2002).
Scheme to synthesize cobalt alloy nanoparticles
Although we observe that synthesis using the proposed [Co2+((CH3)2CO)6][Co−(CO)4]2 complex results in better alloy nanoparticles than when Co2(CO)8 is used, it remains to be revealed what the origin of this effect is. Molecular mechanistic studies of the formation of monometallic Co nanoparticles (Lagunas et al. 2006; Samia et al. 2006; de Silva et al. 2007) showed that Co2(CO)8 decomposition leads rapidly to larger Co4(CO)12 clusters and ligand-substituted analogs. On the basis of UV/Vis, we concluded that the Co–Co bonds are broken because of the acetone, which likely prevents the instantaneous formation of Co4(CO)12 intermediates. We propose that the lack of larger cobalt clusters facilitates the mixing of Co and Ni or Co and Fe atoms in alloy nanoparticles. Also, the presence of an overall neutral complex of ligated cations and carbonylate anions might favor the stabilization of mono-cobalt building blocks, for example, by a facilitated de-protonation of the oleic acid molecules in solution to form bonding oleate complexes. Such intermediates would have slower and similar reaction rates as the Fe(CO)5 or Ni(acac)2 precursors.
The failure to obtain uniform well-mixed Fe x Ni1−x nanoparticles underpins the importance of the proposed disproportionated cobalt complex in the synthesis. The studies of Hieber and others has shown that base-induced disproportionation reactions exist for vanadium (Richmond et al. 1984), manganese (Hieber et al. 1961), iron (Hieber and Kahlen 1958), and nickel (Hieber et al. 1932) carbonyls and further studies might exploit this for the synthesis of other families of alloy nanoparticles.
Structure, composition, and possibility of oxidation of the cobalt alloy nanoparticles
The TEM–EDX results for the Co x Ni1−x and Co x Fe1−x nanoparticles, as shown in Figs. 3 and 4, revealed that the obtained particles contain both metals and that the bulk-determined composition is the same in individual nanoparticles. The powder X-ray diffractograms in Fig. 6 featured only one crystallographic phase per synthesis. In combination with the TEM–EDX results, this indicates that small alloy nanoparticles with one (poly)crystalline phase per synthesis were obtained. Furthermore, because this is not a seeded growth synthesis, core–shell structures are not likely. To determine the atomic distribution within one bimetallic nanoparticle, more advanced characterization methods such as scanning transmission electron microscopy combined with electron energy loss spectroscopy would be needed (van Schooneveld et al. 2010; den Breejen et al. 2011). Nonetheless, series A1 and A2 hinted at important information on the surface composition of the nanoparticles. Small Co x Ni1−x alloy particles were only obtained when increasingly more cobalt was added. This increased the total metal-to-ligand ratio. In a monometallic synthesis, particles normally grow larger when increasing the metal-to-ligand ratio. Here, the opposite behavior is observed and it can be explained by ligand-induced metal segregation (Menning and Chen 2009). For cobalt–nickel alloys, the surface would consist of nickel atoms in vacuum (Menning and Chen 2009). However, for oxygen atoms, it has been predicted that the adsorbate–metal interactions drive cobalt atoms to the particle surface (Menning and Chen 2009). It is also known that cobalt has a higher affinity for oleic acid than nickel. We suggest that the particles were large in order to shield the nickel atoms behind the relatively little amount of cobalt atoms that were forming the surface with the oxygen-containing ligand functional groups. In this view, the particles became smaller on addition of more cobalt, because more cobalt could sit at the particle interface. For bimetallic nanoparticles, these results show that next to the metal-to-ligand ratio, the metal–ligand affinity plays an important role in controlling their size and shape. In this view, single-crystal phase alloy nanoparticles were prepared where the first outer layer consisted of cobalt atoms in case of the Co x Ni1−x nanoparticles.
Finally, it is noted that the examined particles are unlikely to be oxidized. They were prepared in a nitrogen atmosphere Schlenk line and stored in a nitrogen atmosphere glove box. The XRD and AGM measurements were done under exclusion from air. No oxidation-related peaks were observed in XRD, and the nanoparticle magnetic properties are indicative of the highly magnetic metals as compared with the somewhat less magnetic metal oxides. The inferior magnetic properties of the Co x Fe1−x with respect to the Co x Ni1−x particles might, however, be due to a slight degree of iron oxidation, undetectable by XRD. In case of oxidation, iron is likely oxidized first in Co x Fe1−x , while the oxidation of cobalt is expected to occur first in Co x Ni1−x nanoparticles as a result of their respective oxidation potentials (Haynes and Lide 2012).
Magnetic properties of the alloy nanoparticles
The saturation magnetization values for pure Fe, Co, and Ni solids at room temperature are 1,711, 1,424, and 485 kA m−1, respectively (Chikazumi 1964). In bulk alloys of iron, cobalt, and nickel, saturation magnetizations are found to be intermediate between their components (Crangle and Hallam 1963). For the bimetallic nanoparticles prepared here, the saturation magnetization values were of the same order of magnitude, indicating a good magnetic quality. They are at least as magnetic as magnetite or maghemite nanoparticles of the same size, because the latter are usually less magnetic than expected from the magnetization of about 450 kA m−1 for bulk magnetite or maghemite (Chikazumi 1964). The saturation magnetization values and average magnetic dipole moments of the Co x Fe1−x particles were, however, lower than those of the Co x Ni1−x particles. Possibly, a minor degree of iron oxidation resulted in loss of magnetization. Alternatively, it could be due to the lower degree of Co x Fe1−x crystallinity, since crystalline defects are known to have a detrimental effect on the magnetic properties of nanoparticles (Luigjes et al. 2011). Overall, it is important to note that the effective magnetic properties for alloy nanoparticles cannot be solely predicted on the basis of the bulk magnetic properties, but effects such as crystallinity and the ease and degree of metal oxidation should be taken into account. Here, an example is reported where Co x Ni1−x particles display superior magnetic properties to their Co x Fe1−x analogs, while the opposite is expected based on bulk properties alone.
Ligand tunability of the crystal structure
General energetic considerations of preparing alloy nanoparticles
The previous discussion raises the question what the general requirements are with respect to the strength of the metal–metal, ligand–ligand, and metal–ligand interactions. In designing an alloy nanoparticle synthesis method, one can first consider metal–metal interactions. The bulk alloy phase diagrams indicated that Co/Ni and Co/Fe are miscible over a large range of elemental ratios at the synthesis temperature of 280 °C (Baker 1992). Furthermore, the enthalpies of formation were favorable. For Co x Fe1−x bulk systems, experimental and calculated enthalpies of formation for ordered or interstitial alloys were reported to be respectively −10 to −1 kJ mol−1 and −22 to −1 kJ mol−1 for all x (de Boer et al. 1989). For Co x Ni1−x bulk systems, these were found to be respectively 0 and −13 to +3 kJ mol−1 for all x (de Boer et al. 1989).
Secondly, the metal–ligand interactions should be considered. In the synthesis of nanoparticles, extensive use is made of a few ligands that include phosphines (R3P), phosphites (R3PO), acids (RCOOH), alcohols (ROH), amines (RNH2), and thiols (RSH) (Donega 2011). An assumption in this study was that particle stability would be favored if the average dissociation energies for M x –M y ~ M x –M x ~ M y –M y ≥ M x/y –La, and M x/y –Lb. Although solvation and surface energy arguments are neglected here, the idea of the requirement is that metal atoms would be prone to leaching from the nanoparticle surface by strongly binding ligands. Literature values of dissociation energies of cationic species Fe+–Fe and Co+–Fe are ca. 260 kJ mol−1 (Haynes and Lide 2012), between M+–S with M = Fe, Co, Ni they are 250–260 kJ mol−1 (Marks 1990), and between M+–NH2 they are 232–235 kJ mol−1 for Ni, 247–260 kJ mol−1 for Co and 280 kJ mol−1 for Fe (Marks 1990; Haynes and Lide 2012). Based on this, it was decided not to use thiols or amines in the synthesis of Co x Ni1−x and Co x Fe1−x nanoparticles. Although amines are usually applied in nickel nanoparticle synthesis, we verified by ligand-exchange tests on pure ε-Co nanoparticles, prepared by the Puntes method (Puntes et al. 2001), that these aggregated and even dissolved on post-synthesis addition of dodecylamine and 1-dodecanethiol, respectively (see Fig. S2 in Online Resource 1). Kitaev (2008) also noted the (partial) dissolution of cobalt nanoparticles on thiol addition. Instead, OA and TOPO molecules that both act as soft Lewis bases on the hard Lewis acid transition metals were chosen for their mild binding energies with cobalt and iron.
The third consideration concerns the dilemma between bond strength and amount of ligands used in the bimetal nanoparticle synthesis. Ligands, here OA and TOPO, which are just right to form cobalt and iron nanoparticles cannot prevent nickel from aggregating when used in equally low concentrations. On the other hand, ligands, such as the amines that bind strongly with nickel, dissolve the cobalt and iron into molecular complexes. The preparation of composition tunable transition metal nanoparticles, from metals with seemingly incompatible ligand affinity, can be realized by the use of one of the ligands at higher concentrations, at the expense of product yield. For example, the low binding strength of OA and TOPO initially prevented the formation of Co x Ni1−x particles with low cobalt content in series A1 and A2, but by adding more ligands, such particles were obtained in series A3, albeit together with cobalt molecular complexes and thus incomplete conversion.
Model systems for non-noble metal based catalysis
Solution prepared nanoparticles are capped with ligands to prevent them from aggregation. These ligands might seriously lower their activity in a catalytic reaction that occurs at the particle surface. In this respect, it is more useful to prepare nanoparticles on a support material through classic preparation routes used in heterogeneous catalysis. The advantage of using colloidal nanoparticles is, however, that the size and composition of all particles is readily controlled, as, for example, shown in this study. These particles, when coated with different ligands and consisting of different metal atoms, are then suitable model systems to study the interactions of alloys with the chemical intermediates of catalyzed reactions. Especially, the Newns–Anderson model predicts the adsorbate bond dissociation energies and adsorbate-induced metal segregation in bimetallic systems, as a function of the metal d-band center, providing a predictive framework for active non-noble metal catalysts (Nilsson et al. 2008; Menning and Chen 2009; Nørskov et al. 2011).
A generally applicable organometallic synthesis route, based on the reaction of Co2(CO)8 with acetone, is reported for the synthesis of 4–10 nm Co x Ni1−x and Co x Fe1−x nanoparticles with tunable elemental compositions. Based on the results of seven series of syntheses where the metal precursor concentrations and ligand type and concentrations were varied, insights intrinsic to the size, composition, and phase behavior of stable bimetallic alloy nanoparticles has been obtained. These basic insights will provide guidelines for the wet-chemical synthesis of yet unmade bimetallic alloy nanoparticles. We further envisage that the well-defined Co x Ni1−x and Co x Fe1−x nanoparticles are suitable prototypes to test the Newns–Anderson model as used in catalysis.
MMvS and JvR thank the Netherlands Organization for Scientific Research (NWO-CW and NWO-FOM) for financial support. We thank C. de Mello Donegá and B. M. Weckhuysen for discussions.
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