Alloying noble metals with non-noble metals enables high activity while reducing the cost of electrocatalysts in fuel cells. However, under fuel cell operating conditions, state-of-the-art oxygen ...reduction reaction alloy catalysts either feature high atomic percentages of noble metals (>70%) with limited durability or show poor durability when lower percentages of noble metals (<50%) are used. Here, we demonstrate a highly-durable alloy catalyst derived by alloying PtPd (<50%) with 3d-transition metals (Cu, Ni or Co) in ternary compositions. The origin of the high durability is probed by in-situ/operando high-energy synchrotron X-ray diffraction coupled with pair distribution function analysis of atomic phase structures and strains, revealing an important role of realloying in the compressively-strained single-phase alloy state despite the occurrence of dealloying. The implication of the finding, a striking departure from previous perceptions of phase-segregated noble metal skin or complete dealloying of non-noble metals, is the fulfilling of the promise of alloy catalysts for mass commercialization of fuel cells.
The ability to control the surface composition and morphology of alloy catalysts is critical for achieving high activity and durability of catalysts for oxygen reduction reaction (ORR) and fuel ...cells. This report describes an efficient surfactant-free synthesis route for producing a twisty nanowire (TNW) shaped platinum–iron (PtFe) alloy catalyst (denoted as PtFe TNWs) with controllable bimetallic compositions. PtFe TNWs with an optimal initial composition of ∼24% Pt are shown to exhibit the highest mass activity (3.4 A/mgPt, ∼20 times higher than that of commercial Pt catalyst) and the highest durability (<2% loss of activity after 40 000 cycles and <30% loss after 120 000 cycles) among all PtFe-based nanocatalysts under ORR or fuel cell operating conditions reported so far. Using ex situ and in situ synchrotron X-ray diffraction coupled with atomic pair distribution function (PDF) analysis and 3D modeling, the PtFe TNWs are shown to exhibit mixed face-centered cubic (fcc)−body-centered cubic (bcc) alloy structure and a significant lattice strain. A striking finding is that the activity strongly depends on the composition of the as-synthesized catalysts and this dependence remains unchanged despite the evolution of the composition of the different catalysts and their lattice constants under ORR or fuel cell operating conditions. Notably, dealloying under fuel cell operating condition starts at phase-segregated domain sites leading to a final fcc alloy structure with subtle differences in surface morphology. Due to a subsequent realloying and the morphology of TNWs, the surface lattice strain observed with the as-synthesized catalysts is largely preserved. This strain and the particular facets exhibited by the TNWs are believed to be responsible for the observed activity and durability enhancements. These findings provide new insights into the correlation between the structure, activity, and durability of nanoalloy catalysts and are expected to energize the ongoing effort to develop highly active and durable low-Pt-content nanowire catalysts by controlling their alloy structure and morphology.
Abstract
The need for active and stable oxidation catalysts is driven by the demands in production of valuable chemicals, remediation of hydrocarbon pollutants and energy sustainability. Traditional ...approaches focus on oxygen-activating oxides as support which provides the oxygen activation at the catalyst-support peripheral interface. Here we report a new approach to oxidation catalysts for total oxidation of hydrocarbons (e.g., propane) by surface oxygenation of platinum (Pt)-alloyed multicomponent nanoparticles (e.g., platinum-nickel cobalt (Pt–NiCo)). The in-situ/operando time-resolved studies, including high-energy synchrotron X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy, demonstrate the formation of oxygenated Pt–NiOCoO surface layer and disordered ternary alloy core. The results reveal largely-irregular oscillatory kinetics associated with the dynamic lattice expansion/shrinking, ordering/disordering, and formation of surface-oxygenated sites and intermediates. The catalytic synergy is responsible for reduction of the oxidation temperature by ~100 °C and the high stability under 800 °C hydrothermal aging in comparison with Pt, and may represent a paradigm shift in the design of self-supported catalysts.
Understanding the evolution of the composition and atomic structure of nanoalloy catalysts in the ethanol oxidation reaction (EOR) is essential for the design of active and robust catalysts for ...direct ethanol fuel cells. This article describes a study of carbon-supported platinum–ruthenium electrocatalysts (PtRu/C) with different bimetallic compositions and their activities in the EOR, an important anode reaction in direct ethanol fuel cells (DEFCs). The study focused on establishing the relationship between the catalyst’s composition, atomic structure, and catalytic activity for the EOR. Ex situ and in situ synchrotron high-energy X-ray diffraction (HE-XRD) experiments coupled with atomic pair distribution function (PDF) analysis and in situ energy-dispersive X-ray (EDX) analysis were employed to probe the composition and structural evolution of the catalysts during the in situ EOR inside a membrane electrode assembly (MEA) in the fuel cell. The results revealed an intriguing composition–structure–activity relationship for the PtRu electrocatalysts under EOR experimental conditions. In particular, the alloy with a Pt/Ru ratio of ∼50:50 was found to exhibit a maximum EOR activity as a function of the bimetallic composition. This composition–activity relationship coincides with the relationship between the Pt interatomic distances and coordination numbers and the bimetallic composition. Notably, the catalytic activities of the PtRu electrocatalysts showed a significant improvement during the EOR, which can be related to atomic-level structural changes in the nanoalloys occurring during the EOR, as indicated by in situ HE-XRD/PDF/EDX data. The findings shed some new light on the mechanism of the ethanol oxidation reaction over bimetallic alloy nanocatalysts, which is important for the rational design and synthesis of active nanoalloy catalysts for DEFCs.
We present results from combined in situ infrared spectroscopy and total X-ray scattering studies on the evolution of catalytically active sites in exemplary binary and ternary Pt-based nanoalloys ...during a sequence of CO oxidation–reactivation–CO oxidation reactions. We find that when within a particular compositional range, the fresh nanoalloys may exhibit high catalytic activity for low-temperature CO oxidation. Using surface-specific atomic pair distribution functions (PDFs) extracted from the in situ total X-ray scattering data, we find that, regardless of their chemical composition and initial catalytic activity, the fresh nanoalloys suffer a significant surface structural disorder during CO oxidation. Upon reactivation in oxygen atmosphere, the surface of used nanoalloy catalysts both partially oxidizes and orders. Remarkably, it largely retains its structural state when the nanoalloys are reused as CO oxidation catalysts. The seemingly inverse structural changes of studied nanoalloy catalysts occurring under CO oxidation and reactivation conditions affect the active sites on their surface significantly. In particular, through different mechanisms, both appear to reduce the CO binding strength to the nanoalloy’s surface and thus increase the catalytic stability of the nanoalloys. The findings provide clues for further optimization of nanoalloy catalysts for the oxidation of carbonaceous species through optimizing their composition, activation, and reactivation. Besides, the findings demonstrate the usefulness of combined in situ infrared spectroscopy and total X-ray scattering coupled to surface-specific atomic PDF analysis to the ongoing effort to produce advanced catalysts for environmentally and technologically important applications.
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•Single BaM hexaferrite structural phases with Cu substations were prepared.•The magnetocrystalline anisotropy decreased with Cu substitution.•The coercivity was significantly ...modified while the magnetization remained high.•Hexaferrites with 0.2–0.4 Cu possess properties suitable for magnetic recording.•Ionic distributions from structural refinement agreed with Mössbauer spectroscopy.
BaFe12−xCuxO19 hexaferrites were prepared using ball milling and sintering at 1100°C. Refinement of the X-ray diffraction patterns was carried out to determine the structural parameters and the ionic distribution over the crystallographic sites. The preferential site occupation and valence state of Cu was consistent with the results obtained from the analysis of Mössbauer spectra. Further, the magnetic parameters of the samples were discussed in light of the structural and Mössbauer analyses. The magnetic phase transition temperature was found to decrease with the level of Cu substitution, in accordance with the reduction of the superexchange interactions. Further, the magnetic softening of the hexaferrite and the significant reduction in magnetocrystalline anisotropy with Cu substitution was consistent with the ionic distribution in the lattice. This study clearly demonstrated the feasibility of using a simple method to fabricate hexaferrites with a modified coercivity, while maintain the saturation magnetization high enough for practical applications.
The present study is concerned with the synthesis and characterization of rare-earth-doped SrM (Sr
1-
x
RE
x
Fe
12
O
19
) hexaferrites (RE = La, Ce and Tb;
x
= 0, and 0.1). The hexaferrite precursor ...powders were prepared by high-energy ball-milling and sol–gel auto-combustion methods, and the powders were sintered at temperatures ≥ 900 °C. The effects of RE substitution and synthesis route on the structural and magnetic properties of SrM hexaferrites were investigated by X-ray diffraction, scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). Single SrM phase was obtained in the sample with
x
= 0, and the sample with La substitution was prepared by ball-milling method. The rest of the samples, however, contained small amounts of minor phases of rare-earth oxides and α-Fe
2
O
3
in addition to the major SrM phase. SEM imaging revealed significant decrease in particle size with RE substitution. Also, VSM measurements revealed small decrease in the saturation magnetization, and a significant increase in the coercivity with RE substitution. Comparable saturation magnetization was observed for the samples prepared by sol–gel method, whereas the coercivity increased significantly. The coercivity of samples prepared by sol–gel method exhibited a large increase with the decrease in sintering temperature, reaching ~ 6.2 kOe for the samples with Ce- and Tb-substituted samples sintered at 900 °C. The relatively high remnant magnetization (~ 30–35 emu/g) and high coercivity make these materials important candidates for permanent magnet applications.
The preparation of a highly pure X-type hexaferrite phase is challenging, and critically dependent on the preparation method and the adopted experimental conditions, as it normally coexists at ...equilibrium with M and W phases at the formation temperature in the range 1250–1400 °C. In this article, we report the effects of partial rare-earth (RE) and Zn co-substitution on the structural and magnetic properties of Co2X hexaferrites (Ba2-xRExCo2ZnxFe28-xO46; RE: La, Nd, Pr). The RE-Zn substituted samples were prepared by ball milling and sintering at 1300 °C. Analysis of the XRD patterns indicated the coexistence of X, W, and M phases in all samples, with the X-type being the majority phase in all RE-Zn substituted samples. The weight fractions of the phases changed depending on the type and concentration (x) of RE-Zn substitution, and yields of X-type phase as high as 68 ± 3% and 67 ± 3% were observed for La-Zn (x = 0.1) and Pr-Zn (x = 0.2) substitutions, respectively. Also, the saturation magnetization varied slightly with the substitution, and the highest value of 76.8 emu/g (1 emu/g = 1 Am2/kg) was observed for the Pr-Zn substituted sample. All samples exhibited low coercivity in the range of 32–53 Oe (1 Oe = 79.58 A/m), and magnetocrystalline anisotropy field in the range 5.07–7.78 kOe, and the coercivity exhibited an increasing tendency with the increase of the anisotropy field. The magnetic phases in each sample were further confirmed by thermomagnetic measurements, and the corresponding Curie temperatures were determined. The thermomagnetic behavior indicated an enhancement of the superexchange interactions, and a corresponding increase of the Curie temperature of the W and X phases with the La-Zn and Nd-Zn substitutions, whereas the Pr-Zn substitution did not exhibit such enhancements in the W phase.
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•X-type hexaferrites were synthesized by the standard ceramic method.•The RE-Zn substitution increased the fraction of the X phase.•The saturation magnetization improved with RE-Zn substitution.•The magnetocrystalline anisotropy and coercivity decreased with RE-Zn substitution.•The RE-Zn substitution enhanced the strength of superexchange interactions.
The lattice structure is known to influence interfacial reactivities of nanoscale alloy catalysts, but little is known about how the lattice strain can be sustainably controlled by the nanoscale ...morphology under electrocatalytic reaction conditions. Herein, a previously unknown self-regulated stability of lattice strains is demonstrated by engineering highly active platinum–copper alloy nanowires with two distinctive types of morphology. The dendritic alloy nanowires exhibit the best performance for oxygen reduction reaction among the reported platinum–copper alloy catalysts. In comparison with the initial difference of compressive lattice strains between smooth and dendritic nanowires, the strains are shown to be controllable, which coincides with the highly durable electrocatalytic performance throughout the duration of oxygen reduction reaction despite the occurrence of dealloying. By thorough characterizations of the nanowire morphologies, compositions, and lattice strains, the self-regulated stability of lattice strains is revealed to originate from the operation of a combination of morphology-tuned compressive strain and realloying in the dendritic nanowires for the enhanced electrocatalytic activity and durability. These findings have significant implications for the design of high-durability alloy catalysts in heterogeneous catalysis.
The present study is concerned with the synthesis and characterization of rare-earth-doped SrM (Sr1-xRExFe12O19) hexaferrites (RE = La, Ce and Tb; x = 0, and 0.1). The hexaferrite precursor powders ...were prepared by high-energy ball-milling and sol–gel auto-combustion methods, and the powders were sintered at temperatures ≥ 900 °C. The effects of RE substitution and synthesis route on the structural and magnetic properties of SrM hexaferrites were investigated by X-ray diffraction, scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). Single SrM phase was obtained in the sample with x = 0, and the sample with La substitution was prepared by ball-milling method. The rest of the samples, however, contained small amounts of minor phases of rare-earth oxides and α-Fe2O3 in addition to the major SrM phase. SEM imaging revealed significant decrease in particle size with RE substitution. Also, VSM measurements revealed small decrease in the saturation magnetization, and a significant increase in the coercivity with RE substitution. Comparable saturation magnetization was observed for the samples prepared by sol–gel method, whereas the coercivity increased significantly. The coercivity of samples prepared by sol–gel method exhibited a large increase with the decrease in sintering temperature, reaching ~ 6.2 kOe for the samples with Ce- and Tb-substituted samples sintered at 900 °C. The relatively high remnant magnetization (~ 30–35 emu/g) and high coercivity make these materials important candidates for permanent magnet applications.
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