Paula M. Abdala

Cooperativity and Dynamics Increase the Performance of NiFe Dry Reforming Catalysts

The dry reforming of methane (DRM), i.e., the reaction of methane and CO2 to form a synthesis gas, converts two major greenhouse gases into a useful chemical feedstock. In this work, we probe the effect and role of Fe in bimetallic NiFe dry reforming catalysts. To this end, monometallic Ni, Fe, and bimetallic Ni-Fe catalysts supported on a MgxAlyOz matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172-178 m2/gcat), pore volume (0.51-0.66 cm3/gcat), and particle size (5.4-5.8 nm) were kept constant. Bimetallic, Ni4Fe1 with Ni/(Ni + Fe) = 0.8, showed the highest activity and stability, whereas rapid deactivation and a low catalytic activity were observed for monometallic Ni and Fe catalysts, respectively. XRD, Raman, TPO, and TEM analysis confirmed that the deactivation of monometallic Ni catalysts was in large due to the formation of graphitic carbon. The promoting effect of Fe in bimetallic Ni-Fe was elucidated by combining operando XRD and XAS analyses and energy-dispersive X-ray spectroscopy complemented with density functional theory calculations. Under dry reforming conditions, Fe is oxidized partially to FeO leading to a partial dealloying and formation of a Ni-richer NiFe alloy. Fe migrates leading to the formation of FeO preferentially at the surface. Experiments in an inert helium atmosphere confirm that FeO reacts via a redox mechanism with carbon deposits forming CO, whereby the reduced Fe restores the original Ni-Fe alloy. Owing to the high activity of the material and the absence of any XRD signature of FeO, it is very likely that FeO is formed as small domains of a few atom layer thickness covering a fraction of the surface of the Ni-rich particles, ensuring a close proximity of the carbon removal (FeO) and methane activation (Ni) sites.

Scientific opportunities for heterogeneous catalysis research at the SuperXAS and SNBL beam lines.

In this short review, we describe the complementary experimental capabilities for catalysis research at two beam lines available to the Swiss community, SuperXAS at SLS (Swiss Light Source, Villigen) and SNBL (Swiss Norwegian Beam lines) at ESRF (European Synchrotron Radiation Facility, Grenoble). Over the years, these two facilities have been developed to provide powerful techniques for structural studies under in situ and operando conditions. These techniques, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES) in combination with Raman or infrared spectroscopy provide new avenues for structure-performance studies of catalysts. Several exemplary studies are used to demonstrate the capability of these facilities.

Crystal structure and solution species of Ce(III) and Ce(IV) formates: from mononuclear to hexanuclear complexes.

Cerium(III) and cerium(IV) both form formate complexes. However, their species in aqueous solution and the solid-state structures are surprisingly different. The species in aqueous solutions were investigated with Ce K-edge EXAFS spectroscopy. Ce(III) formate shows only mononuclear complexes, which is in agreement with the predicted mononuclear species of Ce(HCOO)(2+) and Ce(HCOO)2(+). In contrast, Ce(IV) formate forms in aqueous solution a stable hexanuclear complex of [Ce6(μ3-O)4(μ3-OH)4(HCOO)x(NO3)y](12-x-y). The structural differences reflect the different influence of hydrolysis, which is weak for Ce(III) and strong for Ce(IV). Hydrolysis of Ce(IV) ions causes initial polymerization while complexation through HCOO(-) results in 12 chelate rings stabilizing the hexanuclear Ce(IV) complex. Crystals were grown from the above-mentioned solutions. Two crystal structures of Ce(IV) formate were determined. Both form a hexanuclear complex with a [Ce6(μ3-O)4(μ3-OH)4](12+) core in aqueous HNO3/HCOOH solution. The pH titration with NaOH resulted in a structure with the composition [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)2(H2O)3]·(H2O)9.5, while the pH adjustment with NH3 resulted in [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)4]·(NO3)3(NH4)5(H2O)5. Furthermore, the crystal structure of Ce(III) formate, Ce(HCOO)3, was determined. The coordination polyhedron is a tricapped trigonal prism which is formed exclusively by nine HCOO(-) ligands. The hexanuclear Ce(IV) formate species from aqueous solution is widely preserved in the crystal structure, whereas the mononuclear solution species of Ce(III) formate undergoes a polymerization during the crystallization process.

Development of MgAl2O4-stabilized, Cu-doped, Fe2O3-based oxygen carriers for thermochemical water-splitting

The commercially dominating technology for hydrogen production (i.e. steam methane reforming) emits large quantities of CO2 into the atmosphere. On the other hand, emerging thermochemical water splitting cycles allow the production of a pure stream of H2 while simultaneously capturing CO2. From a thermodynamic point of view, the Fe2O3–Fe couple is arguably the most attractive candidate for thermochemical water splitting owing to its high H2 yield and H2 equilibrium partial pressure. However, the low reactivity of Fe2O3 with methane and the high activity of Fe for methane decomposition (leading to carbon deposition and in turn COx poisoning of the H2 stream) are major drawbacks. Here, we report the development of MgAl2O4-stabilized, Cu-modified, Fe2O3-based redox materials for thermochemical water-splitting that show a high reactivity towards CH4 and low rates of carbon deposition. To elucidate the effect of Cu doping on reducing significantly the rate of carbon deposition (while not affecting negatively the high redox activity of the material) extended X-ray absorption fine structure spectroscopy and energy dispersive X-ray spectroscopy was employed.

CuO promoted Mn2O3-based materials for solid fuel combustion with inherent CO2 capture

We experimentally demonstrate the promising redox and oxygen release characteristics of a novel bimetallic Cu–Mn oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) based CO2 capture. The new material was prepared via a co-precipitation technique and showed a higher oxygen partial pressure than pure CuO and a higher oxygen carrying capacity than Mn2O3, thus, synergistically combining the advantages of the individual metal oxides. The promising CLOU characteristics of the new material were demonstrated further by combusting charcoal fully in a fluidized bed and producing a pure stream of CO2.

A large-area CMOS detector for high-energy synchrotron powder diffraction and total scattering experiments

A complementary metal-oxide semiconductor (CMOS) detector with an active area of 290.8 × 229.8 mm has been evaluated for X-ray scattering experiments at energies between 20 and 50 keV. Detector calibration and integration procedures are discussed in addition to the determination of the linearity, angular resolution and energy response of the detector in the context of its envisaged use. Data on reference compounds and samples with different crystallinity were collected and analysed with classical Rietveld and pair distribution function refinements. Comparisons with literature and high-resolution data from the same beamline demonstrate that the presented detector is suitable for crystallographic and total scattering experiments.

Synthesis and Theoretical Investigations of the Solid Solution CeRu1–xNixAl (x = 0.1–0.95) Showing Cerium Valence Fluctuations

Members of the solid solution series of CeRu(1-x)Ni(x)Al can be obtained directly by arc melting of the elements. The presented compounds with 0.1 ≤ x ≤ 0.85 crystallize in the orthorhombic space group Pnma (No. 62) in the LaNiAl structure type, while for 0.9 ≤ x ≤ 1, the hexagonal ZrNiAl-type structure is found. The orthorhombic members exhibit an anomaly in the trend of the lattice parameters as well as an interesting behavior of the magnetic susceptibility, suggesting that the cerium cations exhibit no local moment. Besides the mixed-valent nature of the cerium cations, valence fluctuations along with a change in the cerium oxidation state depending on the nickel content have been found. The oxidation state has been determined from the magnetic data and additionally by XANES. Density functional theory calculations have identified the shortest Ce-Ru interaction as decisive for the stability of the orthorhombic solid solution.

Na+ doping induced changes in the reduction and charge transport characteristics of Al2O3-stabilized, CuO-based materials for CO2 capture

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are emerging CO2 capture technologies that could reduce appreciably the costs associated with the capture of CO2. In CLC and CLOU, the oxygen required to combust a hydrocarbon is provided by a solid oxygen carrier. Among the transition metal oxides typically considered for CLC and CLOU, copper oxide (CuO) stands out owing to its high oxygen carrying capacity, exothermic reduction reactions and fast reduction kinetics. However, the low Tammann (sintering) temperature of CuO is a serious drawback. In this context, it has been proposed to support CuO on high Tammann temperature and low cost alumina (Al2O3), thus, reducing the morphological changes occurring over multiple CLC or CLOU redox cycles and stabilizing, in turn, the high activity of CuO. However, in CuO-Al2O3 systems, phase stabilization and avoiding the formation of the CuAl2O4 spinel is key to obtaining a material with a high redox stability and activity. Here, we report a Na(+) doping strategy to phase stabilize Al2O3-supported CuO, yielding in turn an inexpensive material with a high redox stability and CO2 capture efficiency. We also demonstrate that doping CuO-Al2O3 with Na(+) improves the oxygen uncoupling characteristics and coke resistance of the oxygen carriers. Utilizing in situ and ex situ X-ray absorption spectroscopy (XAS), the local structure of Cu and the reduction pathways of CuO were determined as a function of the Na(+) content and cycle number. Finally, using 4-point conductivity measurements, we confirm that doping of Al2O3-supported CuO with Na(+) lowers the activation energy for charge transport explaining conclusively the improved redox characteristics of the new oxygen carriers developed.

The solid solutions CeRu1–xPdxSn and CeRh1–xPdxSn – Applicability of the ICF model to determine intermediate cerium valencies by comparison with XANES data

Abstract Several samples of the solid solutions CeRu1–xPdxSn and CeRh1–xPdxSn have been synthesized by arc-melting and characterized by X-ray powder diffraction. Guinier powder patterns prove that the ZrNiAl-type structure is the dominating one, besides the CeRuSn and TiNiSi type structures. The structures of CeRu0.28Pd0.72Sn (ZrNiAl type, P6̅2m, a = 751.95(3), c = 418.70(2) pm, wR2 = 0.0274, 332 F2 values, 14 variables) and CeRh0.66Pd0.34Sn (ZrNiAl type, P6̅2m, a = 750.26(3), c = 411.59(2) pm, wR2 = 0.0533, 358 F2 values, 14 variables) were refined from single crystal diffractometer data. Magnetic measurements in combination with XANES (X-Ray Absorption Near Edge Structure) clearly proved intermediate cerium valencies for most compounds and revealed the best fitting parameters for those with the ICF model (Interconfiguration fluctuation). The electrical resistivity is also influenced by the substitutions. At low and high valence electron counts (VECs) metallic character is present, while around the VEC of CeRhSn the typical resistivity behavior for valence fluctuating compounds is observed.

Atomic layer deposition of a film of Al2O3 on electrodeposited copper foams to yield highly effective oxygen carriers for chemical looping combustion-based CO2 capture

A rapid electrochemical deposition protocol is reported to synthesize highly porous Cu foams serving as model oxygen carriers for chemical looping, a promising technology to reduce anthropogenic CO2 emission. To overcome the sintering-induced decay in the oxygen carrying capacity of unsupported Cu foams, Al2O3 films of different thicknesses (0.1-25 nm) are deposited onto the Cu foams via atomic layer deposition (ALD). An ALD-grown Al2O3 overcoat of 20 nm thickness (∼4 wt % Al2O3) is shown to be sufficient to ensure excellent redox cyclic stability. Al2O3-coated Cu foams exhibit a capacity retention of 96% over 10 redox cycles, outperforming their coprecipitated counterpart (equal Al2O3 content). The structural evolution of the stabilized foams is probed in detail and compared to benchmark materials to elucidate the stabilizing role of the Al2O3 overcoat. Upon heat treatment, the initially conformal Al2O3 overcoat induces a fragmentation of large Cu(O) branches into small particles. After multiple redox cycles, the Al2O3 overcoat transforms into sub-micrometer-sized grains of aluminum-containing phases (δ-Al2O3, CuAl2O4, and CuAlO2) that are dispersed homogeneously within the CuO matrix. Finally, the diffusion of Cu through an Al2O3 layer upon heat treatment in an oxidizing atmosphere is probed in model thin films.