Extended X-ray Adsorption Fine Structure Research Articles

Probing the Behavior of Composition‐Tunable Ultrathin PtNi Nanowires for CO Oxidation and Small‐Molecule Electrocatalytic Reactions

AbstractWe have successfully synthesized ultrathin nanowires of pure Pt, Pt99Ni1, Pt9Ni1, and Pt7Ni3 using a modified room‐temperature soft‐template method. Analysis of both methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) results found that the Pt7Ni3 samples yielded the best performance with specific activities of 0.36 and 0.34 mA/cm2 respectively. Additionally, formic acid oxidation reaction (FAOR) tests noted that both Pt and PtNi nanowires oxidize small organic molecules (SOMs) via an indirect pathway. CO oxidation data suggests little measurable performance without any pre‐reduction treatment; however, after annealing in H2, we detected significantly improved CO2 formation for both Pt9Ni1 and Pt7Ni3 motifs. These observations highlight the importance of pre‐treating these nanowires under a reducing atmosphere to enhance their performance for CO oxidation. To explain these findings, we collected extended x‐ray adsorption fine structure (EXAFS) spectroscopy data, consistent with the presence of partial alloying with a tendency for Pt and Ni to segregate, thereby implying the formation of a Pt‐rich shell coupled with a Ni‐rich core. We also observed that the degree of alloying within the nanowires increased after annealing in a reducing atmosphere, a finding deduced through analysis of the coordination numbers and calculations of Cowley's short range order parameters.

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Open AccessCCS ChemistryRESEARCH ARTICLE24 May 2022Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis Gaofeng Chen, Olga A. Syzgantseva, Maria A. Syzgantseva, Shuliang Yang, Guihua Yan, Li Peng, Changyan Cao, Wenxing Chen, Zhiwei Wang, Fengjuan Qin, Tingzhou Lei, Xianhai Zeng, Lu Lin, Weiguo Song and Buxing Han Gaofeng Chen College of Energy, Xiamen University, Xiamen 361102 Google Scholar More articles by this author , Olga A. Syzgantseva Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 Google Scholar More articles by this author , Maria A. Syzgantseva Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 Google Scholar More articles by this author , Shuliang Yang *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Energy, Xiamen University, Xiamen 361102 Fujian Engineering and Research Center of Clean and High-Valued Technologies for Biomass; Xiamen Key Laboratory of Clean and High-Valued Utilization for Biomass, Xiamen 361102 Google Scholar More articles by this author , Guihua Yan College of Energy, Xiamen University, Xiamen 361102 Google Scholar More articles by this author , Li Peng College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Google Scholar More articles by this author , Changyan Cao Beijing National Laboratory for Molecular Science, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Wenxing Chen *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Energy & Catalysis Center, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081 Google Scholar More articles by this author , Zhiwei Wang School of Environmental Engineering, Henan University of Technology, Zhengzhou 450001 Google Scholar More articles by this author , Fengjuan Qin Energy & Catalysis Center, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081 Google Scholar More articles by this author , Tingzhou Lei Institute of Urban and Rural Mining, Changzhou University, Changzhou 213164 Google Scholar More articles by this author , Xianhai Zeng *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Energy, Xiamen University, Xiamen 361102 Fujian Engineering and Research Center of Clean and High-Valued Technologies for Biomass; Xiamen Key Laboratory of Clean and High-Valued Utilization for Biomass, Xiamen 361102 Google Scholar More articles by this author , Lu Lin College of Energy, Xiamen University, Xiamen 361102 Fujian Engineering and Research Center of Clean and High-Valued Technologies for Biomass; Xiamen Key Laboratory of Clean and High-Valued Utilization for Biomass, Xiamen 361102 Google Scholar More articles by this author , Weiguo Song Beijing National Laboratory for Molecular Science, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author and Buxing Han Beijing National Laboratory for Molecular Science, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201930 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Higher alcohol synthesis (HAS) from syngas could efficiently alleviate the dependence on the traditional fossil resources. However, it is still challenging to construct high-performance HAS catalysts with satisfying selectivity, space–time yield (STY), and stability. Herein, we designed a diatomic catalyst by anchoring Co and Cu sites onto a hierarchical porous N-doped carbon matrix (Co/Cu–N–C). The Co/Cu–N–C is efficient for HAS and is among the best catalysts reported. With a CO conversion of 81.7%, C2+OH selectivity could reach 58.5% with an outstanding C2+OH STY of 851.8 mg/g·h. We found that the N4–Co1 and Cu1–N4 showed an excellent synergistic effect. The adsorption of CO occurred on the Co site, and the surrounding nitrogen sites served as a hydrogen reservoir for the CO reduction reactions to form CHxCo. Meanwhile, the Cu sites stabilized a CHOCu species to interact with CHxCo, facilitating a barrier-free formation of C2 species, which is responsible for the high selectivity of higher alcohols. Download figure Download PowerPoint Introduction Higher alcohol (C2+OH) synthesis from direct thermocatalytic conversion of syngas (CO and H2) is a promising strategy to alleviate the intensified energy crisis and global environmental issues because the fossil-based syngas route could be replaced by alternative green approaches such as biomass gasification and the electrochemical CO2 reduction reaction.1–4 In our daily life, higher alcohols (HAs) have been applied widely in different fields such as pharmaceuticals, solvents, fuel additives, hydrogen carriers, and so on.1,3,5 In the past decades, significant advances have been made in higher alcohol synthesis (HAS) from syngas with Rh-based,6,7 Mo-based,8,9 modified methanol/Fischer–Tropsch (F–T) synthesis catalysts, and so on.4,10–12 Among them, modified F–T Co–Cu-based catalysts have been considered one of the most attractive candidates owing to its low feedstock cost, abundant reserves, benign reaction condition, and high CO hydrogenation activity.13 The HAS reaction requires “dual-active sites” in close proximity for the Co–Cu catalyst, where Co active sites are active for CO dissociation, alkyl group (CxHy*) formation, and carbon chain propagation, and the Cu species are responsible for the insertion of non-dissociated CO molecule, further facilitating alcohol formation.14,15 Thus, it is crucial to enhance the intimacy of dual-active sites to make full use of the synergistic catalytic effect to improve the yield of HAs. Even though great efforts have been dedicated to fabricate adjacent Co–Cu dual-active-center catalysts derived from oxalate co-precipitation oxides,16 perovskite structure oxides,17 and layered double hydroxides,18 catalyst stability issues, such as the agglomeration and phase separation of active particles, are often inevitable due to the collapse of the catalyst skeleton under the relatively harsh reaction conditions.16,17 For example, recently in 2022, CoFe alloy or α-Fe phases in the CoFe-300 catalyst carburized into ε-carbide and χ-carbide during the HAS reaction process.19 At the same time, the HAS catalysts often suffer decreased selectivity at a relative higher conversion. Thus, it is very hard to obtain high selectivity for alcohol (ROH, such as methanol, ethanol, etc.) in HAS at high conversion,10,16,20–25 which results in low efficiency for HA production with a relatively low space-time yield (STY) of HAs. Therefore, developing novel strategies to construct stable and highly efficient catalysts for HAS is highly demanded. Compared with the bulk catalysts and catalysts with big particles, the atomic-scale single-atom catalysts (SACs) could in theory endow the catalysts with ultrahigh atom utilization efficiency and abundant synergistic sites.26–34 However, it is still problematic for SACs, with only one kind of active site, to catalyze the complicated reactions involving multiple intermediates.35 In contrast to SACs, the diatomic site catalysts (DASCs) possess more flexible and sophisticated active sites, a substantially different coordination environment, and quantum size effects, thereby offering more chances to obtain multiple active sites for a multistep catalytical process.36–39 Particularly, the electronic interactions between adjacent heteroatomic metal species could be tailored to make the catalytic environment suitable for the reaction route involving multiple intermediates.40–43 Moreover, DASCs have enormous potential to improve the intrinsic activity as a result of the multi-functional activity centers with more than one kind of adsorption site. Therefore, designing DASCs with proper metal sites could offer new opportunities for the development of HAS catalysts considering that the interfacial synergy of different active metal sites is vital for HAS to obtain high alcohol selectivity. Herein, we have successfully designed a dual-metal single-atom Co/Cu–N–C catalyst consisting of high-density Co and Cu atoms anchored onto a N-doped porous carbon matrix. The preparation process of Co/Cu–N–C is simple and cost-efficient without using expensive metal precursors. Gratifyingly, Co/Cu–N–C displayed extraordinary thermocatalytic performance for syngas-to-higher alcohols (CO conversion of 81.7%, C2+OH selectivity of 58.5%, C2+OH STY of 851.8 mg/g·h) and excellent stability over 200 h, greatly outperforming most of the reported catalysts for HAS ( Supporting Information Table S8). The in-depth electronic structure analysis revealed that the Co/Cu–N–C possesses high density of states (DOS) centered on both Co and Cu metals near the Fermi level. This enabled an easy electron transfer between these sites, a strong interaction with the adsorbates, a facile accommodation of reduction electrons from adsorbed hydrogen, and a series of barrier-free HAS reaction steps, including a C–C coupling state, thus resulting in good catalytic activity and selectivity of Co/Cu–N–C. This work provides critical insights into the catalyst rational design, structure–activity relationship, and application of hetero-DASCs. To the best of our knowledge, there is no previous report on the application of Co/Cu–N–C DASCs for HAS. We anticipate that the results reported here could move the industrialization process of the HAS reaction forward a step. Experimental Methods Preparation of Co/Cu–N–C In a typical process, 4.0 g sucrose and 40.0 g urea were dissolved in 270 mL 70% (v:v) ethanol–aqueous solution. Cobalt(II) acetate tetrahydrate (0.17 g) and 0.23 g copper(II) acetate monohydrate were dissolved in 30 mL 70% (v:v) ethanol–aqueous solution and added to the above solution, followed by stirring for 2 h. The as-obtained solution was rotary evaporated at 65 °C until the solvent was completely removed. The as-prepared sol-gel mixture was further vacuum dried at room temperature for 24 h. After drying, the sample was thoroughly ground. Subsequently, the powder was transferred into a tube furnace and heated to 300 °C for 1 h, and then pyrolyzed at 700 °C for 2 h with a heating rate of 2 °C/min under a flowing argon atmosphere. The collected black powders were denoted as Co/Cu–N–C and used directly without further treatment. Catalytic reaction experiments The dual-metal single-atom Co/Cu–N–C catalyst was investigated for HAS from syngas in a Microactivity Effi (Micromeritics Instrument Ltd., Norcross, GA, United States) with a tubular fixed-bed reactor. The catalyst was loaded into the middle of the reaction tube. Then, the catalyst was evaluated under 260 °C, 3.0 MPa syngas (60 vol % H2, 30 vol % CO, and 10 vol % N2) with gas hourly space velocity (GHSV) of 6000 h−1. Here, N2 was used as the internal standard. The tail gas containing CO, H2, N2, CO2, CH4, and C2+H was analyzed using an Agilent GC 7890B gas chromatograph equipped with a thermal conductivity detector, a hydrogen flame ionization detector (FID), and a TDX-01 column and HayeSep Q packed column using Ar as the carrier gas. The liquid products collected from a high-precision gas-liquid separator ( Supporting Information Figure S1) were analyzed offline over an Agilent GC 7890B with an HP-5 capillary column and FID using 2-butyl alcohol as the internal standard with Ar as the carrier gas. CO conversion (XCO), carbon-based product selectivity (Si), STY, and Anderson–Schulz–Flory (ASF) distribution of HAs were calculated using the following equations. The carbon/mass balance obtained throughout the performed studies was within 95–99%. X CO = n CO , in − n CO , out n CO , in × 100 % (1) S CO 2 = n CO 2 n CO , in − n CO , out × 100 % (2) S i = N i × n i ∑ ( N i × n i ) × 100 % (3) STY = weight of alcohols product ( g ) weight of catalysts ( g ) × h (4) W n n = ( 1 − α ) 2 a n − 1 → ln V n n = n ln α + c (5) C = M R × N R A + ∑ ( N i × n i ) M R × N R B × 100 % (6)where n CO , in and n CO , out were the moles of CO at the inlet and outlet, respectively. MR was the number of carbon atoms in the reactant; NRB and NRA were the moles of the reactant before and after the reaction. The N i and n i denote the moles and carbon number of products, respectively. Computational details All calculations were performed with density functional theory (DFT) applying the Perdew–Burke–Ernzerhof density functional,44 plane waves with 40 Ry and 320 Py kinetic energy cutoffs for wave function and charge density, respectively, as a basis set and ultrasoft pseudopotentials45,46 for the description of the core region, as implemented in the Quantum ESPRESSO software package.47 Spin polarization was accounted for during the geometry optimizations and electronic structure calculations. A 4 × 4 × 1 Monkhorst–Pack k-point grid was used to sample the reciprocal space. The Co/Cu–N–C atomic structures were constructed from a pristine graphene 6 × 6 unit cell by corresponding substitutions followed by a full relaxation of both cell parameters and atomic positions. A vacuum thickness of at least 10 Å was used in all calculations. The structures were visualized using Visual Molecular Dynamics (VMD)48 software. Results and Discussion Synthesis and structural characterization of Co/Cu–N–C The one-pot complexation annealing strategy for preparation of Co/Cu–N–C DASCs is schematically depicted in Figure 1. The Co/Cu–N–C catalyst was obtained by homogeneously mixing precursors of copper(II) acetate monohydrate, cobalt(II) acetate tetrahydrate, sucrose, and urea followed by calcination under Ar atmosphere at 700 °C for 2 h. The Co2+/Cu2+ ions coordinated with the abundant –OH and –NH2 groups of sucrose and urea through a simultaneous complexation process,49 which was beneficial for the formation of highly dispersed Co and Cu and will further be helpful for the construction of dual atomic catalyst. During the preparation process, sucrose and urea not only acted as the sources of carbon and nitrogen, but also formed the hierarchical porous nanosheet to support the atomically isolated Co and Cu atoms. Figure 1 | Schematic illustration of the synthesis of Co/Cu–N–C dual atomic catalyst. Download figure Download PowerPoint The powder X-ray diffraction (PXRD) patterns of the Co/Cu–N–C exhibit two broad peaks located at ∼26° and ∼44°, which were assigned to the (002) and (101) diffractions of graphitic carbon, respectively ( Supporting Information Figure S2).50 These peaks indicate the presence of amorphous carbon matrix. Moreover, no X-ray diffraction (XRD) peaks of Co and Cu related to metal clusters, nanoparticles, alloys, oxides, or carbides were observed, indicating that the metal species were highly dispersed in the carbon matrix.41,42,51 As revealed by field emission scanning electron microscopy (SEM) (Figure 2a and Supporting Information Figure S3) and transmission electron microscopy (TEM) (Figure 2b), a hierarchical porous nanosheet was obtained. The detailed morphology feature was further identified by scanning TEM (STEM) (Figure 2c). It revealed that the Co/Cu–N–C was composed of N-doped porous carbon without detectable nanoparticles, indicating that the Co and Cu existed in an atomic dispersion state. These results were consistent with the above PXRD analysis result. High-resolution TEM images of Co/Cu–N–C in Figure 2d show an expanded interlayer space of 0.39 nm compared to 0.34 nm for typical graphite, which is due to the doping of metal sites and carbon defects.29 The metal species (single atom or nanoparticle) were further uncovered by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 2e). The atomically dispersed Co and Cu on the N-doped porous carbon was confirmed by the appearance of bright spots in Co/Cu–N–C based on the different Z-contrast; no clusters or nanocrystals were observed (Figure 2e). The elemental distributions in Co/Cu–N–C were further determined by HAADF-STEM imaging and the corresponding energy dispersive X-ray spectroscopy (EDS) maps. As shown in Figures 2f–2i, the Co, Cu, C, and N elements coexisted on the carbon matrix, and Co and Cu single atoms distributed uniformly on the N-doped porous carbon nanosheets. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis revealed that the Co and Cu contents in Co/Cu–N–C were 1.39 and 2.60 wt %, respectively ( Supporting Information Table S1). The Raman spectrum of the Co/Cu–N–C showed the characteristic D band (∼1340 cm−1) and G band (∼1584 cm−1) resulting from carbon lattice defects and sp2-hybridized carbon, respectively ( Supporting Information Figure S4). The ratio values of ID/IG for Co/Cu–N–C was 0.96, indicating high structural defect density introduced by dual-metal single-atom Co/Cu on the N-doped porous carbon nanosheet. Additionally, the N2 adsorption–desorption test at 77 K showed that the Co/Cu–N–C had a Brunauer–Emmet–Teller surface area of 246 m2/g ( Supporting Information Figure S5a). The elemental composition and surface chemical states were further characterized by X-ray photoelectron spectroscopy (XPS). The survey XPS spectra of the Co/Cu–N–C confirmed the presence of Co, Cu, C, N, and O ( Supporting Information Figure S6 and Table S2). The high-resolution C 1s spectrum of the Co/Cu–N–C was deconvoluted into C=C (284.4 eV), C=N (285.5 eV), C–N/C–O (287.3 eV), and C=O (290.2 eV), respectively ( Supporting Information Figure S7a). The N 1s spectra of the Co/Cu–N–C showed the existence of pyridinic N (398.1 eV, 41.3%), Co–N/Cu–N (399.1 eV, 19.7%), pyrrolic N (400.2 eV, 19.7%), graphitic N (401.4 eV, 14.2%), and oxidized N (404.1 eV, 5.1%) species, respectively (Figure 3h, Supporting Information Figure S8a and Table S3).52 Accordingly, the high-resolution Co 2p spectrum of Co/Cu–N–C in Supporting Information Figure S9a exhibited two peaks at 780.5 eV (2p3/2) and 795.9 eV (2p1/2), which could be attributed to the oxidation state of a Co single atom. The peaks of Cu 2p3/2 at 931.6 eV and Cu 2p1/2 at 951.4 eV in the Cu 2p spectrum corresponded to the oxidation state of a Cu single atom ( Supporting Information Figure S9b). Figure 2 | Structural characterizations. (a) SEM, (b) dark-field TEM, (c) STEM and (d) (d) HRTEM images of Co/Cu–N–C (inset is the lattice distance of carbon layer in the selected area). (e) HAADF-STEM image of Co/Cu–N–C (isolated bright dots marked with yellow circles are Co or Cu single atoms). (f) HAADF-STEM image of Co/Cu–N–C and (g) the corresponding EDS elemental mapping. (h) HAADF-STEM images of Co/Cu–N–C and (i) its corresponding EDS elemental mapping with higher magnification at 5 nm scale. Download figure Download PowerPoint For comparison, single metal Co–N–C and Cu–N–C were also prepared following a similar method, but with only cobalt(II) acetate tetrahydrate or copper(II) acetate monohydrate as the metal precursor, respectively. Detailed synthetic procedures are shown in the Supporting Information. HAADF-STEM and EDS elemental mapping analysis of both samples confirmed the similar atomically dispersed features with that of Co/Cu–N–C ( Supporting Information Figures S10 and S11). Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis revealed the Co–N–C had 1.33 wt % Co loading and Cu–N–C had 2.54 wt % ( Supporting Information Table S1). In contrast, the ratio values of ID/IG for Co–N–C and Cu–N–C are 0.91 and 0.87, indicating lower structural defect density when compared with that of Co/Cu–N–C ( Supporting Information Figure S4). In addition, the N2 adsorption–desorption test at 77 K reported the Co–N–C and Cu–N–C with surface areas of 184 and 168 m2/g, which are lower than that of the Co/Cu–N–C surface area of 246 m2/g ( Supporting Information Figure S5). Figure 3 | Detailed coordinated structure analysis of Co/Cu–N–C, Co–N–C, and Cu–N–C catalysts. (a) Co K-edge XANES spectra of Co/Cu–N–C, Co–N–C, Co foil, CoO, and Co3O4. (b) FT k3-weighted EXAFS spectra for Co K-edge of Co/Cu–N–C, Co–N–C, Co foil, CoO, and Co3O4. (c) WT k3-weighted EXAFS spectra of Co–N–C, Cu–N–C, and Co/Cu–N–C. (d) Cu K-edge XANES spectra of Co/Cu–N–C, Cu–N–C, Cu foil, Cu2O, and CuO. (e) FT k3-weighted EXAFS spectra for Cu K-edge of Co/Cu–N–C, Cu–N–C, Cu foil, Cu2O, and CuO. (f) Corresponding Co K-edge EXAFS fitting curves of Co/Cu–N–C in R space (inset: model of N4–Co–Cu–N4 sites). (g) Corresponding Cu K-edge EXAFS fitting curve of Co/Cu–N–C in R space (inset: model of N4–Co–Cu–N4 sites). (h) High-resolution N 1s XPS spectra of Co/Cu–N–C. Download figure Download PowerPoint Compared with the metal-N content in Co–N–C (13.4%) and Cu–N–C (11.9%), the content of Co-N/Cu-N species of Co/Cu–N–C are higher (19.7%), which is consistent with the ICP-OES result of Co/Cu–N–C with a higher total metal content of 3.99 wt % ( Supporting Information Figure S8 and Table S3). However, the binding energy of Co 3d in Co/Cu–N–C shifted toward a higher valence state than that of the Co–N–C. In contrast, the binding energy of Cu 2p in Co/Cu–N–C shifted to a lower binding energy compared to that of Cu–N–C. These results firmly indicate the presence of a strong electronic interaction between the Co and Cu atoms assisted by N in Co/Cu–N–C. Synchrotron-radiation-based X-ray adsorption fine structure (XAFS) analysis, including X-ray adsorption near-edge spectroscopy (XANES) and extended X-ray adsorption fine structure (EXAFS) analyses, were conducted to interpret the detailed electronic structure and coordination environment of Co/Cu–N–C.40 In the Co K-edge XANES spectra (Figure 3a), the Co/Cu–N–C, Co–N–C, and standard Co species (e.g., Co foil, CoO, and Co3O4) were employed as the control samples for X-ray adsorption spectroscopy (XAS) investigation. It is well known that the adsorption threshold can be applied to monitor the electronic state of the Co. The near-edge adsorption energy of Co sites in Co/Cu–N–C is located between that of the Co foil and CoO. These tendencies disclose that the electronic state of Co in Co/Cu–N–C is between Co0 and Co2+, suggesting that the electronic state of Co atoms in Co/Cu–N–C is close to +1.53 The energy level of the pre-edge peak for Co/Cu–N–C decreased compared with Co–N–C (inset of Figure 3a), implying a small distortion of D4h symmetry for Co atoms in Co/Cu–N–C.54 As shown in the Fourier transform (FT) k3-weighted EXAFS spectra, the prominent peak of Co/Cu–N–C is located at about 1.50 Å stemming from the first shell scattering of the Co–N1 path, without the appearance of the Co–Co scattering at 2.21 Å, confirming the atomically dispersed Co in nitrogen coordination environments (Figure 3b). Interestingly, a shoulder peak located at ∼2.15 Å was observed, which is ascribed to the Co–N2 scattering. Also, the Co–N–C displayed a peak at 1.65 Å (attributed to the Co–N), confirming the existence of the atomically dispersed Co in Co–N–C (Figure 3b). The wavelet transform (WT) plots (Figure 3c and Supporting Information Figure S12) were conducted to further analyze the Co K-edge EXAFS oscillation. The only intensity maximum at about 2.4 Å−1 is ascribed to the Co–N bonds in the Co WT contour plots of Co/Cu–N–C, a little more positive than that of Co–N–C (1.8 Å−1), and the Co–Co signals at about 6.2 Å−1 are not observed (compared to the Co foil, CoO, and Co3O4) in Supporting Information Figure S12, implying the absence of Co–Co bonds. By fitting the EXAFS spectra of Co/Cu–N–C in R space, the local structure of Co was identified. The EXAFS fitting result showed that coordination number is ca. 3.8 and the average bond length of the Co–N bond is ∼2.00 Å in Co/Cu–N–C ( Supporting Information Table S4), indicating that each Co atom in Co/Cu–N–C is coordinated with four N atoms (Figure 3f). Moreover, the Co in Co–N–C is coordinated with four N atoms, as shown in Supporting Information Figure S13a. The corresponding Co K-edge EXAFS k space fitting curves of Co/Cu–N–C and Co–N–C are shown in Supporting Information Figure S14. XAFS measurements were also performed to investigate the coordination environments of Cu in Co/Cu–N–C and Cu–N–C. Several standard Cu species (e.g., Cu foil, Cu2O, and CuO) were employed as references. The Cu K-edge of Co/Cu–N–C is situated at Cu2O and CuO, revealing the electronic state of Cu species is between Cu1+ and Cu2+ (Figure 3d). However, the value is slightly more negative than that obtained for Cu in Cu–N–C (inset of Figure 3d), which is consistent with the XPS results. As shown in the FT k3-weighted EXAFS spectra, one main peak at about 1.50 Å ascribed to Cu–N shell (Cu–N1) scattering could be observed in Co/Cu–N–C, and there is no peak corresponding to the Cu–Cu scattering pathway, indicating a single Cu atomic coordination structure. Additionally, a significant signal at about 2.09 Å just like that of the Cu–N2 scattering was monitored at the Cu K-edge EXAFS (Figure 3e). Only the peak at ∼1.50 Å could be observed in the Cu–N–C, which was ascribed to the Cu-N scattering pathway (Figure 3e). The WT was also conducted to analyze the Cu (Figure 3c and Supporting Information Figure S15) K-edge EXAFS oscillation. The only intensity maximum at about 2.2 Å−1 was assigned to the Cu–N pair in Co/Cu–N–C and Cu–N–C. The Cu–Cu signal at ∼3.4 Å−1, as referenced by the WT of Cu foil, Cu2O, and CuO, was not detected ( Supporting Information Figure S15), suggesting a negligible number of Cu–Cu bonds. The EXAFS fitting result showed that coordination number is ca. 4.0 and the average bond length of Cu–N is ca. 2.00 Å in Co/Cu–N–C and 1.93 Å in Cu–N–C ( Supporting Information Table S5), indicating that each Cu atom in Co/Cu–N–C and Cu–N–C is coordinated with four N atoms (Figure 3g and Supporting Information Figure S13b). The corresponding Cu K-edge EXAFS k space fitting curves of Co/Cu–N–C and Cu–N–C are shown in Supporting Information Figure S16. Based on these results, we conclude that the Co and Cu in Co/Cu–N–C dominantly coexist as hetero-DASCs sites (N4–Co–Cu–N4), separately anchored on the N-doped porous carbon support. Catalytic performance in HAS The catalytic performance of the Co/Cu–N–C DASCs catalyst for CO hydrogenation was evaluated on a Microactivity Effi with a tubular fixed-bed reactor. The parameters including temperature, GHSV, and pressure were screened to explore the optimum reaction conditions. First, the reaction temperature was optimized from 240 to 270 °C with a step of 10 °C. As CO conversion increased with the increasing reaction temperature, the product selectivity of CH4 and CO2 improved concurrently, accompanied by a decreased ROH selectivity (Figure 4a). The yield of ROH reached the highest value at 260 °C. The lower alcohol selectivity at higher temperature may be because of the exothermic reaction of the alcohol production.20 Regarding the variation of the total reaction pressure (Figure 4b), CO conversion increased with increasing pressure, and this was also the case for the alcohol fraction. However, the selectivity for C2+H as well as CH4 and CO2 decreased within the range of total reaction pressure from 1 to 4 MPa, which should be due to the volumetric contraction for HAs synthesis under increasing reaction pressure based on the Le Chatelier’s principle.55 Increasing the GHSV within the range from 4000 h−1 to 10,000 h−1 in steps of 2000 h−1 led to a decrease in CO conversion (Figure 4c). This is ascribed to the shorter residence reaction time with the higher GHSV. As for the product distribution, an increase in the GHSV resulted in a decrease in C1 by-product (CH4 and CO2),

Binding and transport of Cr(III) by clay minerals during the Great Oxidation Event

A spike in chromium (Cr) abundance above crustal background in iron formations post-dating the Great Oxidation Event (ca. 2.5-2.3 billion years ago) has been suggested to reflect the evolution of terrestrial aerobic pyrite and siderite oxidation and the initiation of widespread acid rock drainage that would enhance Cr transport from terrestrial weathering environments to the oceans. However, it remains unclear whether Cr was transported in a soluble form (as Cr(III) or Cr(VI)), or bound to particulate surfaces. Here, we experimentally investigate the binding mechanisms of Cr(III) – the typical oxidation state associated with primary igneous minerals such as chromite – to three common soil clay minerals (kaolinite, illite, and montmorillonite) using extended X-ray adsorption fine structure (EXAFS) spectroscopy. Our results demonstrate that Cr(III) precipitates on the clay surfaces over a pH range of 6 to 8 as guyanaite (β-CrOOH) due to the replacement of surface Al-octahedra by paired Cr(III) octahedra. Bidentate bonding with a Cr-Cr interatomic distance of 3.43-3.51 Å indicates the incorporation of Cr(III) into the clay structure. A comparison between Cr(III) adsorption/precipitation onto the three clays and the desorption of Cr(III) from Cr(III)-spiked clays shows that kaolinite has the highest retention capacity for Cr(III), a predictable result given that an entire Al-octahedral sheet is exposed for Cr(III) binding, while illite and montmorillonite only have Al-edge sites. Moreover, Cr(III) was essentially immobilized in our experiments except under very acidic conditions (pH<2). Extending our results to the interpretation of the Cr record in iron formations, we suggest that under intense chemical weathering conditions, not only did acidity promote the solubilization of Cr(III) from primary Cr-bearing minerals, but that parent rocks were more systematically weathered to an advanced state dominated by kaolinite – creating ideal conditions for Cr adsorption. Erosion of regolith that scavenged mobilized Cr(III) could then facilitate transport of Cr(III)-bearing kaolinite to coastal environments where it contributed to the super-crustal Cr abundances above detrital background preserved in ca. 2.5-2.0 Ga iron formations.

Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and non-paddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-microaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

Effects of Co doping on the structure and physicochemical properties of hausmannite (Mn3O4) and its transformation during aging

Hausmannite is a common low-valence manganese oxide mineral with a distorted spinel structure in surficial sediments. Although natural Mn oxides often contain various transition metal (TM) impurities, few studies have addressed the substitution mechanisms of TMs, such as cobalt (Co), with hausmannite and induced changes in the mineral physicochemical properties. In this study, Co-doped hausmannites with final Co/Mn molar ratios of 0.05 and 0.11 were synthesized and characterized by wet chemical analysis, powder X-ray diffraction (XRD), Fourier transformed infrared spectra (FTIR), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS). Co-doped samples have larger crystallite sizes and smaller specific surface areas than those of the hausmannite without Co. The Co cations exist in a mixed-valence of +3 and + 2. Powder XRD and acid dissolution experiments indicate that Co incorporates into the mineral structures, and increases the mineral structural stability. Co K-edge extended X-ray adsorption fine structure (EXAFS) analysis demonstrates that Co likely substitutes for lattice Mn(III). After aging at room temperature for three years, 14.1 ± 1.9% of pure hausmannite is transformed to manganite while only 2.6 ± 0.7% of manganite appears in the aged Co-doped sample with an initial Co/Mn molar ratio of 0.10. These results provide new insights into the incorporation mechanisms of TMs into low valence Mn oxides, and the stability of these minerals in natural environments.

Hydrodeoxygenation of benzofuran on novel CoPdP catalysts supported on potassium ion exchanged ultra-stable Y-zeolites

The synthesis of bimetallic CoPdP phosphides supported on potassium ion exchanged ultra-stable Y (KUSY) zeolites was studied and catalytic activity tests on the hydrodeoxygenation of a model pyrolysis oil compound, benzofuran, were conducted. Temperature programmed reduction (TPR) profiles revealed that Pd aided the reduction of the Co, lowering both the onset and peak temperatures for reduction. The catalyst structure was probed by X-ray diffraction (XRD) and extended X-ray adsorption fine structure (EXAFS), which showed the formation of CoPdP alloys. Fourier-transform infrared spectroscopy was used to probe the Co and Pd sites on the surface. Several Co:Pd ratios were tested in the range of 225–350 °C at a medium pressure of 0.5 MPa. Synergistic effects were observed between Co and Pd, with the alloys showing higher activity than the single metal Pd or Co phosphide catalysts. High selectivity to unsaturated high carbon number products were obtained for the bimetallic phosphide catalysts.

Co-Induced Electronic Optimization of Hierarchical NiFe LDH for Oxygen Evolution.

Developing efficient and stable non-noble electrocatalysts for the oxygen evolution reaction (OER) remains challenging for practical applications. While nickel-iron layered double hydroxides (NiFe-LDH) are emerging as prominent candidates with promising OER activity, their catalytic performance is still restricted by the limited active sites, poor conductivity and durability. Herein, hierarchical nickel-iron-cobalt LDH nanosheets/carbon fibers (NiFeCo-LDH/CF) are synthesized through solvent-thermal treatment of ZIF-67/CF. Extended X-ray adsorption fine structure analyses reveal that the Co substitution can stabilize the Fe local coordination environment and facilitate the π-symmetry bonding orbital in NiFeCo-LDH/CF, thus modifying the electronic structures. Coupling with the structural advantages, including the largely exposed active surface sites and facilitated charge transfer pathway ensured by CF, the resultant NiFeCo-LDH/CF exhibits excellent OER activity with an overpotential of 249mV at 10mA cm-1 as well as robust stability over 20 h.

Design of Hybrid Zeolitic Imidazolate Framework-Derived Material with C-Mo-S Triatomic Coordination for Electrochemical Oxygen Reduction.

The emergence of Mo-based hybrid zeolitic imidazolate frameworks (HZIFs) with MoO4 units brings substantial advantages to design and synthesize complex Mo-based electrocatalyst that are not expected in their conventional synthesis path. Herein, as a newly proposed concept, a facile temperature-induced on-site conversion approach (TOCA) is developed to realize the transformation of MoO4 units to C-Mo-S triatomic coordination in hierarchical hollow architecture. The optimized hybrid (denoted as MoCSx 1000) shows accelerating oxygen reduction reaction (ORR) kinetics and excellent stability, which are superior to the most reported Mo-based catalysts. Extended X-ray adsorption fine structure (EXAFS) analysis and computational studies reveal that the near-range electronic steering at C-Mo-S triatomic-coordinated nanointerface guarantees moderate ORR intermediates adsorption and thus is responsible for the boosted ORR activity. This work sheds light on exploring the intrinsic activity of catalysts by interfacial electronic steering.

Mechanism of arsenate coprecipitation at the solid/liquid interface of ferrihydrite: A perspective review

Arsenate (As(V)) is a toxic element in acid mine drainage and has to be removed during the neutralization process. Coprecipitation with ferrihydrite is the main mechanism for As(V) removal from acid mine drainage. To improve treatment efficiency, a quantitative understanding of the coprecipitation mechanism is required. Coprecipitation can incorporate more As(V) into ferrihydrite than adsorption. The results of XRD (X-ray Diffraction) and XANES (X-ray Adsorption Near Edge Structure) analysis confirmed that the formation of poorly crystalline ferric arsenate increased when the initial As/Fe molar ratio increased in the coprecipitation with ferrihydrite. EXAFS (Extended X-ray Adsorption Fine Structure) analysis at the iron K-edge showed that the proportion of octahedral structures in ferrihydrite increased when the initial As/Fe molar ratio increased. Moreover, EXAFS analysis at the arsenic K-edge, assuming three kinds of surface complexes for the AsFe bond, revealed that the coordination number for AsFe with an atomic distance of 2.85 × 10−10 m increased and that for As-Fe with an atomic distance of 3.24 × 10−10 m decreased as the initial As/Fe molar ratio increased. Thus, for more efficient wastewater treatment, active control of coprecipitation phenomena according to mechanistic details is essential.

Combined EXAFS and ab initio study of copper complex geometries adsorbed on natural illite

The adsorption of copper on the 2:1 clay mineral illite (0.4 to 20μm in size) was studied using a combination of extended X-ray adsorption fine structure (EXAFS) and hybrid-Density Functional Theory (DFT) modelling. The study evaluates the effect of varying pH and copper concentration on the mechanisms of copper adsorption in solutions at background electrolyte concentration typical of natural surface continental freshwaters in granitic environments. The EXAFS spectra revealed both the elongated square pyramidal and Jahn-Teller octahedral coordinated copper clusters as feasible with the former providing better fits using spertiniite (crystalline copper hydroxide) as model compound. Additionally, ab initio calculations also predicted the square pyramidal geometry to be more stable. Copper ions have four Oeq at an average distance of 1.95(1)Å and two independent Oax at average distances of 2.32(16)Å and 3.06(9)Å, with the latter decreasing to 2.97(2)Å as copper concentration and pH are increased. This may reveal different mechanism by which copper adsorbs on illite, as a weakly bound complex at low pH likely at exchange and edge sites and changing towards more strongly bound complexes at high affinity edge sites at higher pH and copper loads. Above 1% Cu model fits suggest formation of copper oligomers with average Cu-Cu distance of 3.10(2)Å. These occur at pH>6, where the correlation between Cu-Cu and Al-Al distances in the illite edge surfaces supports the formation of surface precipitates.

En-situ EXAFS investigation of zeolite supported Pt electrocatalyst structure

Experimental investigation was carried out for Pt electrochemical performance and Pt particle size using 1.5 and 5 wt% Pt loading on zeolite electrocatalysts made by Pt(NH3)4(NO3)2 or Pt(NH3)4(NO3)2/NH4NO3 salt with ion exchanged method and calcined at 350 °C and reduced at 400 °C or direct reduced at 400 °C, respectively. Cyclic voltammetry measurement indicated that the hydrogen energy binding level on Pt surfaces is higher for electrocatalyst under direct reduction process than those made by calcination and reduction process. The extended X-ray adsorption fine structure measurement revealed that Pt size for electrocatalyst made by calcination and reduction method is smaller than those made by direct reduced method. Furthermore, Pt size for electrocatalysts with 1.5 wt% Pt loading on zeolite is smaller compared to those with 5 wt% Pt loading electrocatalysts. Aforementioned electrochemical performance of Pt zeolite electrocatalysts as depicted by a hypothesis of hydrogen spillover and surface conductance pathway.

Synthesis of phytic acid-decorated titanate nanotubes for high efficient and high selective removal of U(VI)

In this study, the inorganic-organic hybrid of PA/TNTs was synthesized and characterized by using multiple analysis techniques. Batch experiments were conducted to investigate the removal performance of PA/TNTs towards U(VI). The sorption kinetics procedure achieved equilibrium within 6h. The maximum sorption capacity at 293K and pH 5.0 was calculated to be 1.16×10−3mol/g. In addition, the PA/TNTs composite showed a higher sorption selectivity towards U(VI) over many other metal ions. The ionic strength experiments, desorption results, X-ray diffraction (XRD), solid fluorescence, extended X-ray adsorption fine structure (EXAFS) and Fourier transform infrared spectroscopy (FTIR) analysis suggested the predominant sorption mechanism was inner-sphere complexation rather than ion exchange or precipitation. Overall, the synthesized PA/TNTs composite could be utilized as a cost-effective material for the removal of U(VI) from wastewater.

Basaluminite Structure and its Environmental Implications

Basaluminite is a nanocrystalline aluminum oxyhydrosulfate of important environmental implications. It is present in areas affected by acid mine drainage and acid sulfate soils, where potential toxic elements present in solution, such as Cu and As, can be retained by co-precipitation or adsorption onto it. Basaluminite has been described as a nanomineral variety of felsöbányaite. In the present study, high-energy X-ray diffraction (HEXD) and extended X-ray adsorption fine structure (EXAFS) experiments were performed to determine the local order of basaluminite nanoparticles. Pair distribution function (PDF) analyses showed that both synthetic and natural basaluminite have identical short-range order, with 1nm coherent domain size. PDFs also show strong similarities between the local order of basaluminite and felsobanyaite. On the other hand, S K-edge EXAFS showed different structural coordination between natural and synthetic basaluminite, where sulfate in the natural phase were coordinated in outer-sphere positions whereas inner-sphere sulfate was observed in the synthetic samples. Preliminary results indicated that basaluminite is a highly defective felsobanyaite mineral nanoparticle. This nanocrystalline character has therefore important implication in terms of stability in natural condition and contaminant mobility in stream affected by acid sulfate water.

Investigation of zeolite supported platinum electrocatalyst for electrochemical oxidation of small organic species

Zeolite supported Pt electrocatalysts, made by ion exchange method using Pt/Y type zeolite, have been investigated to determine Pt electrochemical activity of HCOOH and CH3OH oxidation using the cyclic voltammetry (CV) and the extended X-ray adsorption fine structure (EXAFS) techniques. The study reveals that the introduction of excess H+ ions during electrocatalyst pre-treatment could enhance electrochemical reaction on Pt surface due to higher Pt dispersion, regardless of zeolite being a direct current electronic conducting insulator. Two possible conduction pathways might contribute to the electrocatalytic reaction on Pt surface with Pt particle size and loading: (1) hydrogen atoms/H+ ions spillover through zeolite framework and at the electrode and solution interface; (2) surface mobility of adsorbed species on electrode surface. The water may act as a carrier in assisting the migration of the H+ ions throughout zeolite channels to facilitate the charger and electron transfer in such an electrical system.

La1−xCaxAl1−yNiyO3 perovskites used as precursors of nickel based catalysts for ethanol steam reforming

On the search of new design procedures for high performance catalysts we have obtained a very precisely defined surface architecture starting from a particular host structure. In this case, high purity La1−xCaxAl1−yNiyO3 (x = 0, 0.1; y = 0.1, 0.2, 0.3) perovskites were chosen, prepared by the citrate method and evaluated in the ethanol steam reforming reaction. The samples were characterized by X-ray diffraction (XRD), specific surface area measurements, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) and extended X-ray adsorption fine structure (EXAFS), temperature-programmed reduction (TPR) and Hydrogen Chemisorption. All the samples presented an excellent catalytic activity and stability. Exceptionally high H2 yield values and low CO and CH4 selectivities were observed. Perovskite without the addition of Ca and with a value of y = 0.2 presented the highest stability during 12 h on stream.

Experimental study of characteristics of bimetallic Pt–Fe nano-particle fuel cell electrocatalyst

The characteristics of 1.5 wt% Platinum (Pt) loading on Fe incorporated Y zeolite (Pt–Fe/Y zeolite) nano-electrocatalysts have been experimentally studied by the extended X-ray adsorption fine structure (EXAFS) and cyclic voltammetry (CV) techniques using Nafion@ bound electrode to determine Pt electrocatalytic performance in direct methanol fuel cell. The Pt particle size was found to be small in electrochemical environment (0.7 nm with 55 atoms). Study implies that the Pt electrocatalytic performance can be affected by the Pt cluster electron deficiency, due to the change of Pt particle size associated with the lattice strain energy. The CV measurement in the hydride region indicated higher Pt dispersion for Pt–Fe/Y zeolite electrocatalyst chemically reduced in H2 at 400 °C (15PtFeancr4), compared to that of Pt/Y zeolite reduced at 400 °C (15Ptancr4) and Pt–Fe/Y zeolite electrocatalysts reduced at 300 °C (15PtFeancr3), respectively. This provided further implication that the chemical reduction temperature would be important for achieving a higher Pt dispersion. The present study has revealed two possible electron transfer pathways that might contribute to the Pt electronic conduction: (1) the surface mobility of adsorbed species; (2) the hydrogen atoms/H+ ion spillover through the zeolite framework and on the electrode surface, despite the DC insulator nature of zeolite.

Simulation of the isotropic EXAFS spectra for the S2 and S3 structures of the oxygen evolving complex in photosystem II

Most of the main features of water oxidation in photosystem II are now well understood, including the mechanism for O-O bond formation. For the intermediate S2 and S3 structures there is also nearly complete agreement between quantum chemical modeling and experiments. Given the present high degree of consensus for these structures, it is of high interest to go back to previous suggestions concerning what happens in the S2-S3 transition. Analyses of extended X-ray adsorption fine structure (EXAFS) experiments have indicated relatively large structural changes in this transition, with changes of distances sometimes larger than 0.3 Å and a change of topology. In contrast, our previous density functional theory (DFT)(B3LYP) calculations on a cluster model showed very small changes, less than 0.1 Å. It is here found that the DFT structures are also consistent with the EXAFS spectra for the S2 and S3 states within normal errors of DFT. The analysis suggests that there are severe problems in interpreting EXAFS spectra for these complicated systems.

Support Effects of Ni2P Catalysts on the Hydrodeoxygenation of Guaiacol: In Situ XAFS Studies

The hydrodeoxygenation (HDO) of guaiacol was investigated over SiO2, ZrO2, and active carbon (AC) supported Ni2P catalysts. The physical properties of the catalysts were analyzed by temperature-programmed reduction (H2-TPR), CO-uptake chemisorption, and N2 physisorption. X-ray diffraction and extended X-ray adsorption fine structure spectroscopy were used to obtain structural properties for the supported Ni2P catalysts. The HDO was tested in a batch reactor at 573 K and 30 atm. The Ni2P/SiO2 catalyst underwent a decrease in the HDO conversion from 87 to 30 % for the first and second run of reaction. However, the Ni2P/ZrO2 and Ni2P/AC catalysts showed a little low but stable HDO conversions of 72 and 46 %, respectively. The in situ XAFS analysis revealed that differently from the cases of Ni2P/ZrO2 or and Ni2P/AC catalysts, the local structure of the Ni2P on SiO2 support underwent an oxidation to form nickel phosphate during the reaction, demonstrating that the SiO2 based Ni2P was vulnerable to the water or hydroxyl group of the reactant due to the hydrophilic nature of SiO2 support.

Investigations on Pt-Pd Aerogels for Oxygen Reduction Reaction

Carbon supported Pt nanoparticles are widely used as electrocatalysts for polymer electrolyte fuel cells (PEFCs). However, the carbon corrosion associated with particle detachment leads to a strong loss of electrochemically active surface area (ECSA) under operating fuel cell conditions. In order to increase the long-term performance, a promising strategy is the utilization of unsupported high surface area metallic electrocatalysts. For instance, nanostructured thin films (NSTF) showed significantly improved activity and durability for the sluggish kinetics of oxygen reduction reaction (ORR). [1] The challenge, therefore, is the development of unsupported high surface area metallic electrocatalysts for the ORR. Recently, we reported that multimetallic aerogels show considerable catalytic performance and durability over the time. They consist of a three-dimensional network of well-connected nanoparticles, having high porosity, high conductivity and high surface area of up to 180 m2 g-1. [2-3] The benefits for ORR activity and durability have recently been published. [4 - 5]In this work, we show two approaches for the preparation of Pt-Pd aerogel: the spontaneous one-step gelation route and the two-step gelation route. Both methods lead to the formation of the aerogel structure. However, the spatial distribution of the dissimilar metals and the homogeneity within the aerogels are expected to be strongly different. Ex-situ Extended X-ray Adsorption Fine Structure (EXAFS) spectroscopy was performed to provide chemical and structural information about homogeneity and distribution for the differently synthesized Pt-Pd aerogel materials. Afterwards, electrochemical investigations on the differently synthesized Pt-Pd aerogels were performed by using RDE technique. An electrochemical protocol was developed to establish the ECSA for the highly complex Pt-Pd aerogels. Based on the previous work, the durability of the Pt-Pd aerogels were traced out by monitoring of the ECSA and ORR activity over time combined with the changes of particle size, network structure and chemical composition.Our data show that the chemical and structural homogeneity results from the two differently used synthetic approaches and have strong influences on the catalytic performance and long-term stability. The one-step gelation process forms almost alloy particles, while non-alloyed connected particles are generated by using two-step gelation process. The homogeneously alloyed Pt-Pd aerogels exhibit higher activity and durability compared to non-alloyed systems. In-situ electrochemical XAS studies were performed to point out the changes of structure and composition during the electrochemical cycling.Our work provides new insights into the relationship between structure, composition, reactivity and robustness for unsupported high surface area bimetallic aerogels.[1]Debe, M..K., Schmoeckel, A. K., Vernstrom, G. D., Atanasoski R. J. Power Sources. 2006, 161(2), 1002-11.[2] Liu, W. et al., Angew. Chem. Int. Ed. 2012, 51, 5743-5747.[3] Herrmann, A.-K. et al., Chem. Mat., 2014, 26(2), 1074–1083.[4] Liu W. et al., Angew. Chem. Int. Ed. 2013, 52, 9849–985.[5] Schmidt, T. J. et al.; European Patent Application EP 12177908, 2012

Uniaxial magnetic anisotropy in Pd/Fe bilayers on Al2O3 (0001) induced by oblique deposition

This study reports the preparation of self-organized 1-dimensional magnetic structures of Fe on Al2O3 (0001) by oblique deposition. The x-ray diffraction (XRD) results in this study show the preferred (110) texture of the Fe films. XRD and extended x-ray adsorption fine structure measurements indicate larger oblique deposition angle (65°) leads to more disorder in the Fe crystalline structure. After capping with a Pd overlayer, the Pd/Fe/Al2O3 (0001) still exhibits uniaxial magnetic anisotropy induced by the underlying 1-dimensional Fe nanostructure. This uniaxial magnetic anisotropy changes with the variation in Fe thickness and oblique deposition angle. These results clearly indicate the feasibility of manipulating uniaxial magnetic anisotropy and crystalline order through the oblique deposition of magnetic materials.