Zirconium Carbide Mediates Coke-Resistant Methane Dry Reforming on Nickel-Zirconium Catalysts.

Graphitic deposits anti-segregate into Ni0 nanoparticles to provide restored CH4 adsorption sites and near-surface/dissolved C atoms, which migrate to the Ni0 /ZrO2 interface and induce local Zrx Cy formation. The resulting oxygen-deficient carbidic phase boundary sites assist in the kinetically enhanced CO2 activation toward CO(g). This interface carbide mechanism allows for enhanced spillover of carbon to the ZrO2 support, and represents an alternative catalyst regeneration pathway with respect to the reverse oxygen spillover on Ni-CeZrx Oy catalysts. It is therefore rather likely on supports with limited oxygen storage/exchange kinetics but significant carbothermal reducibility.

Increasing Complexity Approach to the Fundamental Surface and Interface Chemistry on SOFC Anode Materials.

ConspectusIn this Account, we demonstrate an increasing complexity approach to gain insight into the principal aspects of the surface and interface chemistry and catalysis of solid oxide fuel cell (SOFC) anode and electrolyte materials based on selected oxide, intermetallic, and metal-oxide systems at different levels of material complexity, as well as into the fundamental microkinetic reaction steps and intermediates at catalytically active surface and interface sites. To dismantle the complexity, we highlight our deconstructing step-by-step approach, which allows one to deduce synergistic properties of complex composite materials from the individual surface catalytic properties of the single constituents, representing the lowest complexity level: pure oxides and pure metallic materials. Upon mixing and doping the latter, directly leading to formation of intermetallic compounds/alloys in the case of metals and oxygen ion conductors/mixed ionic and electronic conductors for oxides, a second complexity level is reached. Finally, the introduction of an (inter)metall(ic)-(mixed) oxide interface leads to the third complexity level. A shell-like model featuring three levels of complexity with the unveiled surface and interface chemistry at its core evolves. As the shift to increased complexity decreases the number of different materials, the interconnections between the studied materials become more convoluted, but the resulting picture of surface chemistry becomes clearer. The materials featured in our investigations are all either already used technologically important or prospective components of SOFCs (such as yttria-stabilized zirconia, perovskites, or Ni-Cu alloys) or their basic constituents (e.g., ZrO2), or they are formed by reactions of other compounds (for instance, pyrochlores are thought to be formed at the YSZ/perovskite phase boundary). We elaborate three representative case studies based on ZrO2, Y2O3, and Y-doped ZrO2 in detail from all three complexity levels. By interconnection of results, we are able to derive common principles of the influence of surface and interface chemistry on the catalytic operation of SOFC anode materials. In situ measurements of the reactivity of water and carbon surface species on ZrO2- and Y2O3-based materials represent levels 1 and 2. The highest degree of complexity at level 3 is exemplified by combined surface science and catalytic studies of metal-oxide systems, oxidatively derived from intermetallic Cu-Zr and Pd-Zr compounds and featuring a large number of phases and interfaces. We show that only by appreciating insight into the basic building blocks of the catalyst materials at lower levels, a full understanding of the catalytic operation of the most complex materials at the highest level is possible.

CO2 Reduction by Hydrogen Pre-Reduced Acceptor-Doped Ceria.

The reactivity of H2 pre-reduced acceptor-doped ceria materials Gd0.10 Ce0.90 O2-δ (GDC10) and Sm0.15 Ce0.85 O2-δ (SDC15) was tested with respect to the reduction of CO2 to CO in the context of the reverse water-gas shift reaction. It was demonstrated that not only oxygen vacancies, but also dissolved hydrogen is a reactive species for the reduction of CO2 . Dissolved hydrogen must be considered upon discussion of the mechanism of the reverse water-gas shift reaction on ceria-derived materials apart from oxygen vacancies and formates. The reduction of CO2 is preceded by the formation of carbonate species of different thermal stability and reactivity. The stability of these carbonates was directly demonstrated by in situ infrared spectroscopy and revealed the largely reversible nature of CO2 ad- and desorption. In comparison to pre-reduced samples, decreased carbonate coverage is obtained after oxidative treatments of GDC10 and SDC15. No significant effect of the sample treatment (O2 oxidation or H2 reduction) on the surface carbonate stability was noticed. Mono-dentate carbonates and carboxylates appear to be more easily formed on pre-reduced (i. e. defective) samples. Ce4+ reduction to Ce3+ (by H2 ) and re-oxidation to Ce4+ (by CO2 ) on GDC10/SDC15 were directly monitored by infrared spectroscopic analysis of a distinct, IR-active electronic transition of Ce3+ . These results show the complex interplay of oxygen vacancy/dissolved hydrogen reactivity and surface chemical aspects in acceptor-doped ceria materials.

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ) thin film and bulk model systems during iron exsolution.

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.

On the structural stability of crystalline ceria phases in undoped and acceptor-doped ceria materials under in situ reduction conditions.

The reduction of pure and Sm-doped ceria in hydrogen has been studied by synchrotron-based in situ X-ray diffraction to eventually prove or disprove the presence of crystalline cerium hydride (CeH x ) phases and the succession of potential structural phase (trans)formations of reduced cerium oxide phases during heating-cooling cycles up to 1273 K. Despite a recent report on the existence of bulk and surface CeH x phases during reductive treatment of pure CeO2 in H2, structural analysis by Rietveld refinement as well as additional 1H-NMR spectroscopy did not reveal the presence of any crystalline CeH x phase. Rather, a sequence of phase transformations during the re-cooling process in H2 has been observed. In both samples, the reduced/defective fluorite lattice undergoes at first a transformation into a bixbyite-type lattice with a formal stoichiometry Ce0.58 3+Ce0.42 4+O1.71 and Sm0.15 3+Ce0.39 3+Ce0.46 4+O1.73, before a transformation into rhombohedral Ce7O12 takes place in pure CeO2. This phase is clearly absent for the Sm-doped material. Finally, a triclinic Ce11O20 phase appears for both materials, which can be recovered to room temperature, and on which a phase mixture of bixbyite Ce0.66 3+Ce0.34 4+O1.67, rh-Ce0.60 3+Ce0.40 4+O1.70 and tri-Ce0.48 3+Ce0.52 4+O1.76 (for pure CeO2) or bixbyite Sm0.15 3+Ce0.47 3+Ce0.38 4+O1.69 and tri-Sm0.15 3+Ce0.31 3+Ce0.54 4+O1.77 (for Sm-doped CeO2) prevails. The absence of the rhombohedral phase indicates that Sm doping leads to the stabilization of the bixbyite phase over the rhombohedral one at this particular oxygen vacancy concentration. It is worth noting that recent work proves that hydrogen is indeed incorporated within the structures during the heat treatments, but under the chosen experimental conditions it has apparently no effect on the salient structural principles during reduction.

Reactive metal-support interaction in the Cu-In2O3 system: intermetallic compound formation and its consequences for CO2-selective methanol steam reforming.

The reactive metal-support interaction in the Cu-In2O3 system and its implications on the CO2 selectivity in methanol steam reforming (MSR) have been assessed using nanosized Cu particles on a powdered cubic In2O3 support. Reduction in hydrogen at 300 °C resulted in the formation of metallic Cu particles on In2O3. This system already represents a highly CO2-selective MSR catalyst with ~93% selectivity, but only 56% methanol conversion and a maximum H2 formation rate of 1.3 µmol gCu -1 s-1. After reduction at 400 °C, the system enters an In2O3-supported intermetallic compound state with Cu2In as the majority phase. Cu2In exhibits markedly different self-activating properties at equally pronounced CO2 selectivities between 92% and 94%. A methanol conversion improvement from roughly 64% to 84% accompanied by an increase in the maximum hydrogen formation rate from 1.8 to 3.8 µmol gCu -1 s-1 has been observed from the first to the fourth consecutive runs. The presented results directly show the prospective properties of a new class of Cu-based intermetallic materials, beneficially combining the MSR properties of the catalyst's constituents Cu and In2O3. In essence, the results also open up the pathway to in-depth development of potentially CO2-selective bulk intermetallic Cu-In compounds with well-defined stoichiometry in MSR.

CO2 Reduction on the Pre-reduced Mixed Ionic-Electronic Conducting Perovskites La0.6 Sr-0.4 FeO3-δ and SrTi0.7 Fe0.3 O3-δ.

The activity of the pre-reduced perovskites La0.6 Sr0.4 FeO3-δ (LSF64) and SrTi0.7 Fe0.3 O3-δ (STF73) for the CO2 reduction to CO was investigated with special focus on the reactivity of oxide-dissolved hydrogen. This is of particular interest in hydrogen solid-oxide electrolysis cell (H-SOEC) technology, where proton-conducting ceramics are used and the reaction 2e- +2H+ +CO2 →CO+H2 O is of central importance. To clarify if hydrogen dissolved in LSF64 and STF73 partakes in the CO2 reduction, temperature-programmed reduction (TPR) in H2 , followed by temperature-programmed reoxidation (TPO) in CO2 and, moreover, temperature-programmed desorption (TPD) of ad- and absorbed species were utilized. The experiments reveal that 50 mol % of the CO2 is converted by hydrogen dissolved in STF73 and reacts quantitatively. On the other hand, LSF64 converts less than 20 mol % of CO2 via dissolved hydrogen and a residual of bulk OH is still detectable after CO2 -TPO.

H2 reduction of Gd- and Sm-doped ceria compared to pure CeO2 at high temperatures: effect on structure, oxygen nonstoichiometry, hydrogen solubility and hydroxyl chemistry.

The effect of Gd- and Sm-doping on pure CeO2 with respect to crystal structure, oxygen nonstoichiometry, hydrogen solubility and hydroxyl chemistry in a hydrogen atmosphere at elevated temperatures was studied using a combination of powder X-ray diffraction (XRD), temperature-programmed methods (such as reduction, desorption and oxidation), and Fourier-Transform Infrared Spectroscopy (FT-IR). In particular, Gd0.1Ce0.9O2-δ (GDC10) and Sm0.15Ce0.85O2-δ (SDC15) were compared to pure CeO2. After H2 reduction of GDC10/SDC15/CeO2 at 900 °C, two distinct phases form, which differ from each other in terms of oxygen nonstoichiometry. One phase is only slightly reduced and maintains a cubic fluorite unit cell. The other phase is strongly oxygen depleted and changes its lattice to triclinic. Enrichment or depletion of the dopants in the two phases upon reduction was not observed. No evidence for a long-range ordered cerium hydride could be found, despite the fact that all samples clearly incorporate hydrogen during the reduction procedure. Generally, the treatment of all three samples with flowing H2 at 700 °C, 800 °C and 900 °C causes the oxygen deficiency and the amount of bound hydrogen to increase with reduction temperature. Acceptor doping, thus, promotes hydrogen incorporation, but it at the same time decreases the amount of reactive oxygen. In conclusion, the study of hydroxyl chemistry shows that doping CeO2 with Gd or Sm creates binding sites for reactive hydroxyl groups that are not observed for pure CeO2. The distinct infrared absorption peak at ca. 2127 cm-1 - which originates from an electronic transition of Ce3+ (2F5/2 → 2F7/2) - is a viable indicator for the reduction degree of all three specimens.

Zirconium-Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming.

C-saturated Pd0 nanoparticles with an extended phase boundary to ZrO2 evolve from a Pd0 Zr0 precatalyst under CH4 dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO2 is efficiently activated at oxidic phase boundary sites and Pdx C provides fast supply of C-atoms toward the latter.

Structural and chemical degradation mechanisms of pure YSZ and its components ZrO2 and Y2O3 in carbon-rich fuel gases.

Structural and chemical degradation mechanisms of metal-free yttria stabilized zirconia (YSZ-8, 8 mol% Y2O3 in ZrO2) in comparison to its pure oxidic components ZrO2 and Y2O3 have been studied in carbon-rich fuel gases with respect to coking/graphitization and (oxy)carbide formation. By combining operando electrochemical impedance spectroscopy (EIS), operando Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS), the removal and suppression of CH4- and CO-induced carbon deposits and of those generated in more realistic fuel gas mixtures (syngas, mixtures of CH4 or CO with CO2 and H2O) was examined under SOFC-relevant conditions up to 1273 K and ambient pressures. Surface-near carbidization is a major problem already on the "isolated" (i.e. Nickel-free) cermet components, leading to irreversible changes of the conduction properties. Graphitic carbon deposition takes place already on the "isolated" oxides under sufficiently fuel-rich conditions, most pronounced in the pure gases CH4 and CO, but also significantly in fuel gas mixtures containing H2O and CO2. For YSZ, a comparative quantification of the total amount of deposited carbon in all gases and mixtures is provided and thus yields favorable and detrimental experimental approaches to suppress the carbon formation. In addition, the effectivity and reversibility of removal of the coke/graphite layers was comparably studied in the pure oxidants O2, CO2 and H2O and their effective contribution upon addition to the pure fuel gases CO and CH4 verified.