CATALYTIC COMPOSITIONS AND METHODS FOR ETHANOL OXIDATION

Abstract

In one aspect, nanoparticles for ethanol oxidation are described herein, which comprise a core including at least one Group IB metal and a shell disposed over the core, wherein the shell comprises islands of alloyed platinum group metals. In another aspect, an electrode is described herein, which in some embodiments, comprises a substrate and electrocatalytic nanoparticles deposited over the substrate. In some embodiments, the electrode described herein further comprises a layer of carbon nanoparticles positioned between the substrate and the electrocatalytic nanoparticles. In yet another aspect, a method of ethanol oxidation is described herein. In some embodiments, such a method comprises (i) providing an electrode comprising a substrate and electrocatalytic nanoparticles deposited over the substrate, (ii) disposing the electrode in an alkaline medium comprising ethanol; and (iii) oxidizing the ethanol with the electrode.

Description

FIELD

The invention is generally related to devices and methods for ethanol oxidation, and, more specifically, devices and methods for complete ethanol oxidation via a C1-12 electron pathway.

BACKGROUND

The hydrogen-rich liquid fuels, such as ethanol, methanol, and ammonia, are attractive alternatives of hydrogen for directly converting chemical energy to electricity in fuel cells due to their high energy density, wide availability, and easiness in storage, distribution, and refuel. Among the three promising liquid fuels, ethanol has the highest energy density. A complete oxidation of ethanol involves transferring 12 electrons (12e) per ethanol molecule: CH₃CH₂OH+3H₂O=2CO₂+12H⁺+12e⁻ with CO₂ as the product in acid. However, incomplete ethanol oxidation reaction (EOR) often occurs with 4e transfer producing acetic acid (CH₃COOH) in acid or acetate (CH₃COO⁻) in base. The two pathways are termed as C1-12e and C2-4e pathways, respectively, referring to the number of carbon atoms in the products and the number of electrons transferred per ethanol molecule. In acidic media, a ternary PtRhSnO₂ catalyst was the first one showing a 12e EOR to CO₂ at low potentials. The optimal composition, PtRh⅓SnO₂, yields a peak current of 1.5 A mg⁻¹ platinum group metal (PGM) and a current of 0.14 A mg⁻¹ PGM after 1 h at 0.45 V. In alkaline media, Pd-based catalysts outperform Pt-based catalysts with the peak current reaching 40 A mg⁻¹ PGM on Au/Ag/Pd alloy aerogels. However, complete oxidation of ethanol at low potentials remains a challenge as energy conversion efficiency is low due to incomplete fuel oxidation and high overpotentials. Thus, there exists a need for improved catalyst performance and understanding of reaction mechanisms.

SUMMARY

In view of these disadvantages, catalysts are described herein that exhibit not only high peak current during ethanol oxidation, but also low onset potential and high selectivity toward the C1-12e pathway.

In one aspect, nanoparticles for ethanol oxidation are described herein. In some embodiments, the nanoparticles comprise a core including at least one Group IB metal and a shell disposed over the core, wherein the shell comprises islands of alloyed platinum group metals. The alloy of the islands can comprise two or more platinum group metals. In some embodiments, the alloyed platinum group metals is platinum-iridium alloy. The ratio of platinum to iridium in the alloy can be greater than 1, in some embodiments. The platinum-iridium alloy, for example, can be of the formula PtIr_x, wherein x ranges from 0.1-0.95 to establish Ir as a fractional value of the Pt content in the alloy. In some embodiments, x ranges from 0.5-0.92 or 0.6-0.7.

The nanoparticles, in some embodiments, have an average diameter of 3-8 nm. Additionally, in some instances, a molar ratio of the alloyed platinum group metals in the shell to the Group IB metal of the core ranges from 0.1 to 0.2. In some embodiments, the core comprises gold or silver. Moreover, the core can be an alloy of Group IB metals, in some embodiments. For example, the core can be an alloy of silver and gold.

In some embodiments, the Group IB metal of the core induces lattice expansion in the islands of the alloyed platinum group metals. Such lattice expansion can induce tensile stress in the islands. Additionally, the Group IB metal of the core, in some instances, has a single crystal structure. The core, for example, can be formed of gold, silver or alloys thereof. It is contemplated that the core can have any desired shape and/or morphology. The core can be spherical, polygonal, elliptical, star-shaped, dendritic, platelet shape or cubed. Shape of the core can be selected according to several considerations, including desired compressive stress or tensile stress condition of the islands.

The islands of alloyed platinum group metals can exhibit monolayer thickness, in some embodiments. For example, the islands can be formed of a single atomic layer. Additionally, in some embodiments, the islands of alloyed platinum group metals form an interface with the core. Alternatively, one or more metal or alloy layers be reside between the core and islands. In some cases, the islands of alloyed platinum group metals comprise platinum-iridium alloy.

In another aspect, an electrode is described herein, which comprises a substrate and electrocatalytic nanoparticles deposited over the substrate. Any nanoparticle described herein can be used in the electrode construction. In some cases, the electrocatalytic nanoparticles comprise a core-shell architecture, wherein the core comprises at least one Group IB metal, and the shell comprises islands of alloyed platinum group metals. The electrocatalytic nanoparticles, in some instances, have an average size of 3-8 nm. The islands of alloyed platinum group metals can exhibit monolayer thickness and/or comprise a platinum-iridium alloy. Moreover, the Group IB metal of the core, in some embodiments, induces lattice expansion in the islands of alloyed platinum group metals. In some embodiments, the electrode described herein further comprises a layer of carbon nanoparticles positioned between the substrate and the electrocatalytic nanoparticles.

In some embodiments, the electrocatalytic nanoparticles of electrodes described herein can provide one or more features or characteristics of the electrode. For example, in some cases, the electrocatalytic nanoparticles can provide peak current of at least 50 A/mg of platinum group metal during ethanol oxidation in alkaline media. The electrocatalytic nanoparticles, in some cases, can also provide an onset potential of 0.4 V to 0.5 V for ethanol oxidation. Furthermore, the electrocatalytic nanoparticles can be selective to a C1-12 electron pathway for ethanol oxidation, and the C1-12 electron pathway can account for greater than 50 percent of current generated during ethanol oxidation.

In yet another aspect, a method of ethanol oxidation is described herein. In some embodiments, such a method comprises (i) providing an electrode comprising a substrate and electrocatalytic nanoparticles deposited over the substrate, (ii) disposing the electrode in an alkaline medium comprising ethanol; and (iii) oxidizing the ethanol with the electrode. Any nanoparticle and/or electrode described herein can be used in any one or more methods of ethanol oxidation. In some embodiments, the electrocatalytic nanoparticles comprise a core-shell architecture, wherein the core comprises at least one Group IB metal, and the shell comprises islands of alloyed platinum group metals.

These and other embodiments are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an X-ray diffraction profile of Au@PtIr/C compared to PtIr/C showing Au-induced lattice expansion.

FIG. 1B show a TEM image of well dispersed Au@PtIr particles on carbon support with narrow particles size distribution. The insert shows atomic spacing of 2.35 Å consistent with Au{111} facets.

FIG. 1C shows a HAADF-STEM image with line profiles of EDS elemental distributions of Au, Pt, and Ir across a single particle.

FIG. 1D-G shows two-dimensional mapping of Au-Lα, Pt-Lα, and Ir-Lα EDS intensities.

FIG. 2A illustrates the PGM mass activities of three Au core catalysts: Au@PtIr/C Au@Pt/C, and Au@Ir/C.

FIG. 2B illustrates metal mass activities of the three core-shell catalysts Au@PtIr/C Au@Pt/C, and Au@Ir/C) and a PtIr (1:1 molar ratio) alloy catalyst.

FIG. 2C illustrates PGM mass activities of Au@PtIr without and with functional carbon nanotube (f-CNT) on gas diffusion electrodes.

FIG. 2D illustrates the durability measured by chronoamperometry at 0.45 V versus RHE.

FIG. 3A-3B illustrates the in-situ IRRAS spectra for the products (FIG. 3A) and adsorbed intermediates (FIG. 3B) of ethanol oxidation in base by Au@PtIr/C.

FIG. 3C-3D illustrates the in-situ IRRAS spectra for the products (FIG. 3C) and adsorbed intermediates (FIG. 3D) of ethanol oxidation in base by PtIr/C.

FIG. 3E-3F illustrates the in-situ IRRAS spectra for the products (FIG. 3E) and adsorbed intermediates (FIG. 3F) of ethanol oxidation in base by Au@P/Ct.

FIG. 4A illustrates the integrated absorbances of EOR products (Left axis) and molar ratio of carbonate to acetate (Right axis) for Au@Pt/C and PtIr/C.

FIG. 4B illustrates the integrated absorbances of EOR products (Left axis) and molar ratio of carbonate to acetate (Right axis) for Au@PtIr/C.

FIG. 4C is a schematic of the proposed direct C1-12e, C2-4e, and indirect C1-12e pathways of ethanol oxidation.

FIG. 5A illustrates voltammetry curves normalized to metal surface areas calculated from average metal densities and diameter of particles, using mass-specific surface area=6000/diameter/density.

FIG. 5B illustrates area-specific peak current versus peak potential (bars) for EOR in 1 M KOH and 1 M ethanol on Au@Ir/C, PtIr/C, Au@Pt/C, and Au@PtIr/C catalysts. The curved line is an EOR polarization curve for Au@PtIr/C. Values for unary Ir(111), Pt(111), and Au(111) are referred to the results on (111) surfaces reported in literature. The insert is a schematic illustration of atoms at edges of a 2.3-nm sphere-like nanoparticle.

FIG. 5C shows schematic illustrations of atomic models for surface layers with Pt, Ir, and a core of Au. Lateral expansion of Pt spacing is illustrated by black arrows and atomic steps are shown by gray lines.

FIG. 6A illustrates EOR performance of Au@PtIr_x catalysts in 1 M KOH and 1 M ethanol at 20 mV/s measured from 0.05-1.0 V vs RHE.

FIG. 6B is a magnified view of the boxed area of FIG. 6A.

FIG. 7 illustrates PGM mass activity of the Au@PtIr_x electrocatalysts at 0.45 V, 0.6 V and peak potential.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by reference to the following detailed description and examples. Methods, devices, and features described herein, however, are not limited to the specific embodiments presented in the detailed description and examples. It should be recognized that these embodiments are merely illustrative of the principles of the present disclosure. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the disclosure.

In addition, all ranges disclosed herein are to be understood to encompass any and all subranges subsumed therein. For example, a stated range of “1.0 to 10.0” should be considered to include any and all subranges beginning with a minimum value of 1.0 and ending with a maximum value of 10.0 or less, e.g. 1.0 to 5.3, or 4.7 to 10.0, or 3.6 to 7.9.

All ranges disclosed herein are also to be considered to include the end points of the range, unless expressly stated otherwise. For example, a range of “between 5 and 10,” “from 5 to 10,” or “5-10” should generally be considered to include the endpoints 5 and 10.

Further, when the phrase “up to” is used in connection with an amount or quantity, it is to be understood that the amount is at least a detectable amount or quantity. For example a material present in an amount “up to” a specified amount can be present from a detectable amount and up to and including the specified amount.

EXAMPLES

The following nanoparticle compositions were fabricated and characterized as examples of the compositions and methods described herein. Accordingly, the present disclosure is not limited to the specific compositions and methods described in the examples.

PtIr on carbon supported Au (Au/C) nanoparticles were synthesized by simultaneously reducing Pt and Ir precursors onto Au nanoparticles in an aqueous solution at a temperature near to boiling. This method was chosen to promote the formation of a well-mixed atomic layer of Pt and Ir on Au nanoparticles. The molar ratio of metals was 10:1:1 for Au:Pt:Ir based on an estimated one monolayer coverage of Pt and Ir on 5-nm Au particles. As shown in FIG. 1a, the resulting Au@PtIr core-shell nanoparticles exhibit X-ray diffraction peaks at the positions close to those for face-centered-cubic Au, indicating that the Pt and Ir atoms are in registry with the underlying Au lattice (a_Au=0.408 nm). The tensile strains calculated from the peak positions are 3.8% relative to Pt (a_Pt=0.392 nm) and 6.1% relative to Ir (a_Ir=0.386 nm). These values are close to the lattice mismatches of 4.1% and 6.3%, respectively. In contrast, the lattice spacing of PtIr alloy particles is between those of Pt and Ir, resulting in tensile strained Ir but compressively strained Pt.

The transmission electron microscopy (TEM) images shown in FIG. 1b verifies a narrow particle size distribution of ±1.8 nm and a lattice spacing of Au {111} facets. Note that the surface to core atomic ratio for 5.8±1.8 nm particles calculated using a cuboctahedra model is 0.19 and the molar ratio of 10:1:0.7 for Au:Pt:Ir determined by x-ray absorption spectroscopy yields a 0.17 (Pt+Ir) to Au ratio. Thus, the PtIr shell is essentially in the form of monoatomic islands. FIGS. 1c-g show the elemental distribution of a representative Au@PtIr particle. The images of energy dispersive spectroscopy mapping show that signals for Pt (blue) and Ir (green) spread to a slightly larger circle than that for Au (pink) in the two-dimensional projections, suggesting a uniformly mixed PtIr atomic layer on the Au particle.

FIG. 2a shows an EOR peak current of 58 A mg⁻¹ PGM obtained using the Au@PtIr/C catalyst in 1 M ethanol and 1 M KOH solution, which is 8 and 38 times of the peak currents on Au@Pt/C and Au@Ir/C, respectively. On (111) single crystal surfaces, the EOR peak current on Pt is about fivefold of that on Ir, while the EOR onset potential is lower on Ir than on Pt by about 0.25 V.16 This trend persists for the Au@Pt/C and Au@Ir/C catalysts as shown by the blue and green curves in FIG. 2a. It is remarkable that the combination of Pt and Ir on Au results in an eightfold increase in peak current and a 0.25 V reduction of onset potential compared to Au@Pt/C. The EOR currents normalized to the mass of all metals are shown in FIG. 2b. At 0.6 V, Au@PtIr/C exhibited 0.52 A mg⁻¹, which is 4.3 times of that for a commercial PtIr/C catalyst (1:1 Pt:Ir alloy). These measurements were carried out using gas diffusion electrodes, focusing on the EOR performance below 700 mV where the activity matters the most for fuel cell applications.

Further enhancement of EOR current at low potentials was obtained by adding acid-treated functional carbon nanotube (f-CNT) on gas diffusion layer before loading Au@PtIr/C catalyst. FIG. 2c shows that the presence of f-CNT lowers the onset potential for EOR and increases the EOR currents at potentials below 0.55 V. At 0.45 V, the EOR current doubles that in the absence of f-CNT. The same activity increase was found by the chronoamperometry measurements at 0.45 V. As shown in FIG. 2d, the currents at 15 minutes on the Au@PtIr catalysts with and without f-CNT are 0.38 and 0.18 A mg⁻¹, respectively, which are ten times of those for Pd/C (˜0.03 A mg⁻¹) and Pt/C (˜0.01 A mg⁻¹).17 With the f-CNT, the currents on Au@PtIr at 30 and 60 minutes are 0.34 and 0.30 A mg⁻¹, respectively, which are about twice of those (0.17 and 0.14 A mg⁻¹) on the best catalyst, PtRh1/3/SnO₂, for EOR in acid.

Table 1 summaries the PGM activities of the Au@PtIr core-shell catalyst and the most active EOR catalysts of various types previously reported. The peak current in forward potential sweep given in the fifth column is commonly used for comparing catalyst performance. In most cases, the peak potentials are above 0.7 V, out of the overpotential range for an anode in fuel cells. Thus, activities at 0.45 and 0.6 V are listed in the third and fourth columns, respectively. The second column provides the EOR activity after 30 min at 0.45 V. As the list shows, Pd-based and PdPt-based catalysts performed better than Pt-based catalysts prior to this work for EOR in alkaline solutions. Differing from EOR in acid, where Rh and Sn oxide greatly promoted the activity on Pt, Au is involved in the most active Pt-based (this work) and Pd-based catalysts for EOR in alkaline solutions. Atomic structure at the catalyst surface plays a very important role in determining the EOR activity; one example is the AuPtIr alloy particles being orders of magnitude less active than the Au@PtIr core-shell catalysts.

To gain insights in how metal components and atomic structure at nanoparticle surfaces affect EOR selectivity toward to C1-12e pathway, onset potential, and peak current, in-situ IRRAS studies of EOR were performed in 1 M KOH on the ternary Au@PtIr core-shell catalyst and its two binary counterparts: PtIr alloy and Au@Pt core-shell catalysts. For EOR in acid, the appearance of the 2343 cm⁻¹ CO₂ band indicates the formation of C1-12e product. In alkaline solutions, CO₂ reacts with OH— to form carbonate and the CO₂ band appears only when pH is ≤13. FIG. 3a shows the spectra acquired during a positive potential sweep for Au@PtIr/C, in which the 2343 cm⁻¹ CO₂ band emerges at potentials above 0.8 V. This feature indicates that the C1-12e pathway is active on the ternary catalyst and that the pH in the thin layer solution between electrode and optical window can be lowered significantly by EOR during the time for a potential scan at 1 mV s−1 from 0.05 V to 0.8 V.

To evaluate selectivity between C1-12e and C2-4e pathways at low potentials, the characteristic bands for carbonate and acetate were analyzed. In transmission spectra, a single band at 1390 cm⁻¹ was observed for Na₂CO₃ while CH₃COONa exhibited two bands at 1550 and 1415 cm⁻¹ plus a narrow and weak band at 1348 cm⁻¹. As illustrated in FIG. 3b, fitting the top spectrum (908 mV) with three Lorentzian peaks deconvolutes the overlapped bands in the region from 1430 to 1260 cm⁻¹. The areas under the 1408 and 1394 peaks are taken as the integrated absorbance for CH₃COO— and HCO₃—/CO₃²⁻, respectively. The third fitted peak at 1346 cm⁻¹ is broad, differing from the narrow 1348 cm⁻¹ band from acetate (ignored in fitting), and is likely due to the formation of small amount of HCOO⁻. This is supported by the assignment of 1340 cm⁻¹ to adsorbed formate on Pt(111).26 Other characteristic bands include a small band at 923 cm⁻¹ for acetaldehyde, a bipolar band at 1044 cm⁻¹ due to adsorbed ethanol, and a band that shifts from 1956 to 1985 cm⁻¹ with increasing potential (FIG. 3c), which is assigned to linearly adsorbed CO.

Integrated absorbances for EOR products are plotted as a function of potential for Au@Pt and PtIr in FIG. 4a and for Au@PtIr in FIG. 4b. The rising absorbances for acetate (circles) and carbonate (triangles) reflect the concentration increases for C2-4e and C1-12e products, respectively. After reaching maximal values around 0.7 V, these absorbances slightly decrease due to depletion of reactants in the thin solution layer and products diffusing away. To determine the selectivity toward C1-12e pathway at potentials below 0.7 V, the ratio for charges generated from forming carbonate and acetate were calculated from the ratio of absorbances using C_Carbonate/C_Acetate=6A1394/4A1408/2.2, in which 6 and 4 are the electron transfer numbers per carbonate and acetate, respectively; 2.2 is the absorbance coefficient ratio determined by measuring absorbance ratio for a solution containing equal molar amounts of carbonate and acetate. As shown in FIG. 4b (right axis), the C_Carbonate/C_Acetate ratio is nearly a constant of 1.3 between 0.3 and 0.7 V for Au@PtIr/C. Since the band at 1348 cm⁻¹ for HCOO⁻ (a C1-10e product) and the 923 cm⁻¹ band CH₃CHO (a C2-2e product) are weak, the percentage of current generated by C1 pathway can be estimated by counting the charges associated with the two major products, i.e., C_Carbonate/(C_Carbonate+C_Acetate)=1.3/(1.3+1)=0.57. Thus, about 57% EOR current is generated via a C1-12e pathway on Au@PtIr/C.

The fact that Au@Pt/C also has a high C_Carbonate/C_Acetate ratio of ˜1.2 but there is no CO₂ band (FIG. 3e) can be explained by the high onset potential (˜0.5 V) and the low rate in producing carbonate and acetate (FIG. 4a), which means insufficient OH consumption to lower the local pH for CO2 band to emerge. On the other hand, there is no measurable absorbance for carbonate in the spectra obtained with PtIr/C as one can see that the peaks centered at 1411 cm⁻¹ in FIG. 3d are symmetric without a shoulder at 1394 cm⁻¹. Yet, a weak CO₂ band emerges in the spectra above 0.8 V for PtIr/C (FIG. 3e). These distinct behaviors can be rationalized by two types of C1-12e pathways as illustrated in FIG. 4c.

The direct C1 pathway has the C—C bond split occurring via potential-independent ethanol dissociation, followed by dehydrogenation and oxidation steps without strongly adsorbed intermediates. It competes with non-dissociative adsorption of ethanol, which can be dehydrogenated without or with C—C bond split. The latter is termed as indirect C1-12e pathway for the C—C bond splits after partial dehydrogenation and has adsorbed CO as a major intermediate. The high and constant C_Carbonate/C_Acetate ratio at potentials from onset to peak, as well as the absence of *CO and other strongly adsorbed intermediates in the spectra for EOR on Au@PtIr and Au@Pt catalysts, indicate that the direct C1-12e pathway is activated on these core-shell catalysts. In contrast, PtIr alloy nanoparticles provide an example for the indirect C1-12e pathway. The appearance and growth of the 1970 cm⁻¹ band for adsorbed *CO (cross signs in FIG. 4a) concurs with the lack of carbonate, indicating that oxidation of strongly adsorbed CO is the rate-limiting step for C1-12e pathway below 0.8 V on PtIr/C.

The EOR pathway diagram in FIG. 4c indicates that formation of strongly adsorbed *CO is most likely when C—C bond cleavage occurs at partially dehydrogenated intermediates, while incompletely dehydrogenated C1 intermediates, CH₂O and CHO, are not site-blocking adsorbates. Therefore, high coverage of adsorbed ethanol means low rate of ethanol dissociation and low selectivity toward direct C1-12e pathway. The rationale is supported by higher coverage of non-dissociated ethanol on PtIr/C than on Au@PtIr/C and Au@Pt/C, especially at low potentials. The evidence is the larger amplitude of the bipolar band near 1046 cm⁻¹ in FIG. 3d than in FIGS. 3b and 3f. A bipolar band arises from the peak position difference between the spectrum obtained at a high potential and the reference spectrum. The downward lobe envelop shows decreasing coverage of adsorbed ethanol with increasing potential from the reference potential of 0.05 V and a small upward lobe emerges due to a potential-induced shift of the band center. For the spectra at the highest potential in FIGS. 3b and 3d, the bipolar band can be replicated by the orange curve obtained by subtracting the black curve, A(_50mV) from the green curve A(_E). The integrated absorbances (given in the inserts) are higher at both low and high potentials for PtIr/C than those for Au@PtIr/C.

The competition between C2 and indirect C1 pathways is largely determined by the barriers for oxidation of acetaldehyde and C—C bond cleavage of partially dehydrogenated EOR intermediates. Density functional theory (DFT) calculations have found that among the (100) surfaces of Pd, Pt, and Ir, Ir(100) has the highest barrier for forming CH₃COOH from CH₃CO+OH and the lowest barrier for C—C bond cleavage in CHCO to form CH+CO species. This theoretical result is consistent with the present disclosure of stronger CO adsorption band at ˜1970 cm⁻¹ for EOR on PtIr alloy nanoparticles than on Pt nanoparticles (barely visible in FIG. 6A of a previous publication).

FIGS. 5a and 5b summarize the voltammetry and EOR peak currents normalized to the calculated metal surface area. Hydrogen adsorption is significantly stronger on PtIr/C than the core-shell catalysts with Au as the cores. The bars in FIG. 5b show EOR peak currents versus peak potential. Results for EOR on (111) single crystals of Ir, Pt and Au are included to show the property of metals without nanostructure-induced effect. Nanometer-sized particles are rich of low-coordination sites at edges, which can be highly active for many reactions. For example, a 4.5 nm cuboctahedron Au particle has about 10 atoms at each of 24 edges, which account for ˜20% surface atoms. While there is no EOR current below 0.9 V on Au(111) single crystal surface, EOR current was observed on Au nanoparticles at potentials comparable to that of Pt nanoparticles. For EOR in acid, the CO₂ band at 2343 cm⁻¹ was observed for Pt monolayer and sub-monolayer on Au nanoparticles, but not for Pt monolayer on Au(111) single crystals. Thus, ethanol dissociation is likely facile at the monoatomic steps of Pt or PtIr islands, especially those near the edges of Au nanoparticles.

Gold cores induce a lateral lattice expansion of Pt and Ir monolayers. The tensile strain has been shown highly effective in enhancing EOR current in acid, especially at high potentials. Relative to Pt(111), Pt monolayer on Au(111) exhibits a fourfold peak current, while Pt monolayers on Pd(111), Ir(111), Rh(111), and Ru(0001) are compressed and thus result in lower peak currents than that on Pt(111). The effect on peak current is even higher for methanol oxidation reaction as a sevenfold enhancement is seen on Pt monolayer/Au(111) relative to Pt(111). Thus, oxidation of either C1 or C2 intermediates are benefited from the Au-induced tensile strain that enhances water and OH adsorption in acid and base, respectively. In this study, the six orders of magnitude higher EOR activity on Au@PtIr core-shell catalysts than on AuPtIr alloy catalysts demonstrates the role of both the atomic steps for promoting direct C1 pathway and the tensile strain for improving oxidation kinetics. This discovery can guide catalyst design and synthesis optimization.

Iridium, on the other hand, acts as a promotor for dehydrogenation, which lowers the onset potential. Ammonia oxidation reaction, 2NH₃+6OH⁻=N₂+6H₂O+6e⁻, involves dehydrogenation but not oxidation process. DFT calculations have found Ir as the best metal for dehydrogenation of ammonia, and experimental studies have shown the importance of Ir for lowering the onset potential for ammonia oxidation. For EOR in base, the onset potential for producing carbonate/acetate is in the order of Au@PtIr<PtIr<Au@Pt (FIGS. 4a and 4b), supporting dehydrogenation as the rate limiting step at low potentials for EOR and Ir being a dehydrogenation promotor. The eightfold activity enhancement by adding Ir to Au@Pt demonstrates the impact of the synergy generated from monatomic steps for ethanol dissociation, Ir for dehydrogenation, and tensile strain for oxidation.

In conclusion, the ternary core-shell catalyst, Au@PtIr/C, exhibits extraordinary EOR performance in alkaline solution—a high peak current of 58 A mg⁻¹ (PGM) and 8.3 A g⁻¹ (all metals), a low onset potential of 0.3 V, and a high percentage C1-12e current of 57%. All three measures together significantly improved the efficiency of ethanol electrooxidation. This is achieved by activating a direct C1-12e pathway, in which the C—C bond splits via ethanol dissociation and site-blocking CO intermediate is circumvented. In addition, cleavage of the C—C bond prior to electrochemical dehydrogenation of ethanol provides C1 fragments at low potentials. These intermediates are less stable than ethanol, and thus, offer opportunities for reducing the EOR onset potential. Adding functional carbon nanotubes lowered the EOR onset potential to 0.2 V, which bodes well for future study along this direction. Comparison of peak currents normalized to metal surface areas for the ternary Au@PtIr core-shell catalyst and its binary and unary counterparts show remarkable sensitivity of EOR kinetics on three-dimensional atomic structure at metal surfaces. Monoatomic steps of Pt or PtIr islands on Au cores activate the direct C1-12e pathway, while smooth surface of PtIr alloy particles results in the indirect C1-12e pathway that has the oxidation of strongly adsorbed *CO as the rate-limiting step. Ir and Au are inactive for EOR due to too strong and too weak adsorption of reaction intermediates, respectively. Placing Ir at the surface and Au in the core allows them to play complementary roles in promoting dehydrogenation and oxidation of both C1 and C2 intermediates. These results and new insights are encouraging for advancing direct liquid fuel cells and point directions for future studies of EOR catalysts and other electrocatalytic reactions.

In a separate study, the shell composition ratio in the Au(core)-PtIr(shell) nanoparticles for EOR was varied. It was found that shell composition ratios of Pt:Ir>1 performed better than shell composition ratios of Pt:Ir<1. This result suggests that Pt is the major player responsible for the EOR electrocatalysis, while Ir is the promoter for dehydrogenation to lower the overpotential of the reaction. Further refining the ratio between PtIr0.59 to PtIr0.95 indicates that the ratio of PtIr0.65 exhibited the highest performance. EOR performance and mass activity of the Au core-PtIr_x shell electrocatalysts are provided in FIGS. 6A and 6B, respectively. A summary is provided in Table I.

TABLE I
Summary of PGM mass activity (A mgPGM−1) of the
Au@PtIrx electrocatalysts at 0.45 V, 0.6 V and peak potential
	0.45 V	0.6 V	peak
	Au@Pt	0.82	5.66	17.70
	Au@PtIr0.59	0.84	6.80	33.12
	Au@PtIr0.65	1.01	11.86	53.19
	Au@PtIr0.81	0.86	8.35	39.67
	Au@PtIr0.91	0.50	7.94	37.61

Methods

Chemicals and Materials.

All metal precursors, HAuCl₃.H₂O, IrC₃.H₂O, and K₂PtCl₄ purchased from Sigma-Aldrich were used without further purifications. Vulcan 72R was used as the carbon support.

Synthesis of 20 wt. % Au/C.

The 20 wt. % Au/C was synthesized by the reduction of Au precursor using sodium borohydride, followed by the loading to the carbon support. In a typical synthesis, 2 mL of 1 wt. % HAuCl₃.H₂O aqueous solution and 2 mL of 1 wt. % trisodium citrate aqueous solution were added to 200 mL of water in a round bottom flask. One minute after the solution mixing, 2 mL of 0.075 wt. % sodium borohydride in 1 wt. % trisodium citrate solution was added to the reaction solution under vigorous stirring. After another 5 min, 40 mg of carbon black were added to the solution. The reaction proceeded under vigorous stirring for additional 2 h. The product was purified by water twice and ethanol once, collected by vacuum filtration, and dried under vacuum overnight for further use.

Synthesis of Ir on 20 wt. % Au/C.

The Ir on 20 wt. % Au/C was synthesized by the reduction of Ir precursor using ascorbic acid at in the presence of 20 wt. % Au/C. In a typical synthesis, 60 mg of ascorbic acid and 50 mg of Au/C (20 wt. %) was added to 10 mL of water in a 100 mL round bottom flask. After the mixture in water was heated boiled, 6 mL of 0.5 mg/mL IrCl₃.H₂O aqueous solution was added to the mixture at a rate a rate of 2.0 mL/h. After the addition of Ir precursor, the reaction proceeded under vigorous stirring for additional 2 h to overnight. The product was purified by water twice and ethanol once, collected by vacuum filtration, and dried under vacuum overnight for further use.

Synthesis of Pt and Ir on 20 wt. % Au/C in Water.

The Pt—Ir shell on 20 wt. % Au/C core was synthesized by the reduction of Pt and Ir precursors using ascorbic acid at in the presence of 20 wt. % Au/C. In a typical synthesis, 60 mg of ascorbic acid and 50 mg of Au/C was added to 10 mL of water in a 100 mL round bottom flask. After the mixture in water was heated to boil, 3 mL of 0.5 mg/mL IrCl₃.H₂O and 3 mL of 0.7 mg/mL K₂PtCl₄ aqueous solutions were simultaneously added to the mixture at a rate of 2.0 mL/h. After the addition of Pt and Ir precursors, the reaction proceeded under vigorous stirring for additional 2 h to overnight. The product was purified by water twice and ethanol once, collected by vacuum filtration, and dried under vacuum. The molar ratio determined by XAS for Au:Pt:Ir is 10:1:0.7 (within <5% error), close to that in the precursors.

Characterization.

X-ray diffraction (XRD) profiles of catalyst samples were collected using Cu Kα radiation (λ=1.5418 Å). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-ARM200CF operated at 200 kV.

Catalyst Ink and Electrode Preparation.

Catalyst inks were made with a mixed solvent of deionized water, ethanol, and isopropanol (volume ratio 1:1:2) containing Nafion with 0.1 to 0.3 weight ratio to the carbon weight in the catalyst. After sonicating in ice water bath for more than 20 min, 5 to 15 μL of catalyst ink was dropped onto glassy carbon electrode to have a catalyst loading of 0.1 mg cm⁻². The total metal loadings were about 0.02 mg cm⁻². For gas diffusion electrode samples, a calculated amount of ink was placed over 1 cm² area at one end of a 1.4 cm wide and 3 cm long gas diffusion layer (Sigracet 25 BC) to get total metal loading about 0.1 mg cm⁻².

Electrochemical Measurements.

A Voltalab PGZ 402 potentiostat was used for electrochemical measurement with a Hg/HgO electrode as the reference electrode and a Pt-flag as the counter electrode in 1 M KOH solution. The zero potential versus reversible hydrogen electrode (RHE) in 1 M KOH was determined by the open circuit potential on Pt in hydrogen saturated solution. The iR-free potential was obtained by subtracting the product of measured currents and the high-frequency resistance determined from electrochemical impedance spectra acquired at 500 mV versus RHE.

In-Situ Infrared Reflection Absorption Spectroscopy (IRRAS).

In-situ IRRAS measurements were carried out with a Nicolet iS50 FT-IR spectrometer equipped with an A-type MCT detector cooled with liquid nitrogen. The working electrodes were made via casting appropriate amounts of catalyst inks on a gold disk electrode. The loadings of carbon supported catalysts are ˜0.05 mg cm⁻² total metals. A Hg/HgO electrode was used as the reference electrode and Pt was the count electrode. During in situ IRRAS measurements, the working electrode was pressed against a ZnSe hemisphere. The spectral resolution was set at 4 cm⁻¹ and the reference reflectivity (R_ref) was taken at 50 mV vs. RHE. Absorbance spectra, −log(R/R_ref), are presented. Each of potential-dependent spectra was acquired by integrating 288 interferograms collected in 50 s. The potential for each spectrum is the average potential between that at the beginning and end of data collection while the potential was continuously increased at 1 mV s⁻¹.

Many modifications and other embodiments of the subject matter will come to mind to one skilled in the art to which the subject matter pertains having the benefits of the teachings presented in the foregoing descriptions and the associated drawings. For example, although specific configurations of nanoparticles are described above and depicted in the figures, numerous other nanoparticles selective to a C1-12 electron pathway for ethanol oxidation and/or configured to provide a peak current of at least 50 A/mg of platinum group metal during ethanol oxidation in alkaline media may benefit from embodiments of the subject matter described herein. Therefore, it is to be understood that the subject matter is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific tetras are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Various implementations of devices and methods have been described, and exemplary embodiments are described below in fulfillment of various objectives of this disclosure. It should be recognized that these implementations are merely illustrative of the principles of this disclosure. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of this disclosure. For example, individual steps of methods described herein can be carried out in any manner not inconsistent with the objectives of this disclosure, and various configurations or adaptations of devices described herein may be used.

Claims

1. A nanoparticle comprising: a core including at least one Group IB metal; anda shell disposed over the core, the shell comprising islands of alloyed platinum group metals.

2. The nanoparticle composition of claim 1, wherein the islands of alloyed platinum group metals exhibit monolayer thickness.

3. The nanoparticle composition of claim 1, wherein the islands comprising platinum-iridium alloy.

4. The nanoparticle composition of claim 3, wherein a ratio of platinum to iridium in the alloy is greater than 1.

5. The nanoparticle composition of claim 3, wherein the platinum-iridium alloy is of the formula PtItx with x being 0.6-0.7.

6. The nanoparticle composition of claim 1, wherein the core comprises gold, silver or an alloy of gold and silver.

7. The nanoparticle composition of claim 1, having a diameter of 3-8 nm.

8. The nanoparticle of claim 1, wherein the Group IB metal of the core induces lattice expansion in the islands of alloyed platinum group metals.

9. The nanoparticle composition of claim 8, wherein the islands of alloyed platinum group metals exhibit tensile stress.

10. The nanoparticle composition of claim 1, wherein the islands of alloyed platinum group metals form an interface with the core.

11. The nanoparticle of claim 1, wherein at least one Group IB metal of the core has a single crystal structure.

12. The nanoparticle of claim 1, wherein a molar ratio of the alloyed platinum group metals in the shell to the Group IB metal of the core ranges from 0.1 to 0.2.

13. The nanoparticle of claim 1, wherein the core is spherical or elliptical.

14. The nanoparticle of claim 1, wherein the core is polygonal.

15. An electrode comprising: a substrate; andelectrocatalytic nanoparticles deposited over the substrate, the electrocatalytic nanoparticles comprising a core-shell architecture, wherein the core comprises at least one Group IB metal, and the shell comprises islands of alloyed platinum group metals.

16. The electrode of claim 15 further comprising a layer of carbon nanoparticles positioned between the substrate and the electrocatalytic nanoparticles.

17. The electrode of claim 15, wherein the electrocatalytic nanoparticles have an average size of 3-8 nm.

18. The electrode of claim 15, wherein the electrocatalytic nanoparticles provide a peak current of at least 50 A/mg of platinum group metal during ethanol oxidation in alkaline media.

19. The electrode of claim 18, wherein the electrocatalytic nanoparticles provide a peak current of at least 50-60 A/mg of platinum group metal during the ethanol oxidation.

20. The electrode of claim 15, wherein the electrocatalytic nanoparticles are selective to a C1-12 electron pathway for ethanol oxidation.

21. The electrode of claim 15, wherein the electrocatalytic nanoparticles provide an onset potential of 0.4 V to 0.5 V for ethanol oxidation.

22. The electrode of claim 20, wherein the C1-12 electron pathway accounts for greater than 50 percent of current generated during ethanol oxidation.

23. The electrode of claim 15, wherein the islands of alloyed platinum group metals exhibit monolayer thickness.

24. The electrode of claim 15, wherein the islands comprise platinum-iridium alloy.

25. The electrode of claim 15, wherein the Group IB metal of the core induces lattice expansion in the islands of alloyed platinum group metals.

26. A method of ethanol oxidation comprising: providing an electrode comprising a substrate and electrocatalytic nanoparticles deposited over the substrate, the electrocatalytic nanoparticles comprising a core-shell architecture, wherein the core comprises at least one Group IB metal, and the shell comprises islands of alloyed platinum group metals;disposing the electrode in an alkaline medium comprising ethanol; andoxidizing the ethanol with the electrode.