Porous Silicon Oxycarbonitride Ceramics with Palladium and Pd2Si Nanoparticles for Dry Reforming of Methane.

Pd-containing precursor has been synthesized from palladium acetate and poly(vinly)silazane (Durazane 1800) in an ice bath under an argon atmosphere. The results of ATR-FTIR and NMR characterizations reveal the chemical reaction between palladium acetate and vinyl groups in poly(vinyl)silazane and the hydrolyzation reaction between -Si-H and -Si-CH=CH2 groups in poly(vinyl)silazane. The palladium nanoparticles are in situ formed in the synthesized precursors as confirmed by XRD, XPS, and TEM. Pd- and Pd2Si-containing SiOCN ceramic nanocomposites are obtained by pyrolysis of the synthesized precursors at 700 °C, 900 °C-1100 °C in an argon atmosphere. The pyrolyzed nanocomposites display good catalytic activity towards the dry reforming of methane. The sample pyrolyzed at 700 °C possesses the best catalytic performance, which can be attributed to the in situ formed palladium nanoparticles and high BET surface area of about 233 m2 g-1.

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.

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.

Modeling and optimization of V2O5/TiO2 nanocatalysts for NH3-Selective catalytic reduction (SCR) of NOx by RSM and ANN techniques.

In the present study, two statistical methods including the response surface method (RSM) and artificial neural network (ANN), were employed for modeling and optimization of selective catalytic reduction of NOx with NH3 (NH3-SCR) over V2O5/TiO2 nanocatalysts. The relationship between catalyst preparation variables, such as metal loading, impregnation temperature, and calcination temperature on NO conversion were investigated. The R2 value of 0.9898 was obtained for quadratic a RSM model, which proves the high agreement of the model with the experimental data. The results of Pareto analysis revealed that three factors including calcination temperature, V loading, and impregnation temperature have a considerable impact on the response. Deduced from the established RSM model, the order of influence on the NO conversion was as follows: calcination followed by V loading and impregnation temperature. The optimum condition of catalyst preparation for maximum NO conversion over V2O5/TiO2 nanocatalysts was predicted to be at 0.0051 mol of V loading, an impregnation temperature of 50 °C and a calcination temperature of 491 °C. Moreover, an ANN model was created by a feed-forward back propagation network (with the topology 4, 12 and 1) to model the relation between the selected catalyst preparation variables and NH3-SCR process temperature. The R2 values for training, validation as well as test sets, were 0.99, 0.9810 and 0.9733. These high values proved the accuracy of the AAN model in modeling and estimating the NO conversion over V2O5/TiO2 nanocatalysts. According to the ANN model, the relative significance of each variable on NO conversion is calcination temperature, process temperature loading, and impregnation temperature from high to low importance, respectively, corroborating the obtained results from RSM.

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.

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.

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.

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.

Structural investigations of La0.6Sr0.4FeO3-δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena.

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ , LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (R3̄c to Pm3̄m) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

Evidence for dissolved hydrogen in the mixed ionic-electronic conducting perovskites La0.6Sr0.4FeO3-δ and SrTi0.7Fe0.3O3-δ.

Two mixed ionic-electronic conducting, Fe-containing perovskites were investigated regarding their reducibility in dry H2, namely lanthanum strontium ferrite (LSF4, La0.6Sr0.4FeO3-δ) and strontium titanium ferrite (STF3, SrTi0.7Fe0.3O3-δ). Upon treatment under comparable reduction conditions, LSF4 is by far more affected by reduction and is reduced more deeply than STF3. Thermal treatments of fully oxidized or slightly reduced LSF4/STF3 at decreased O2 partial pressure lead to spontaneous desorption of O2. Temperature-programmed desorption (TPD) spectra of H2 reveal distinct differences in H2 and H2O desorption. A simple mass balance of H2 reveals that oxygen vacancies formed on STF3 are more resilient towards O2 re-oxidation compared to those on LSF4. The results also imply that substantial amounts of hydrogen are dissolved in the bulk of LSF4 or STF3. 4.9 × 10-2 mol H2 per mol LSF4 and 1.6 × 10-2 mol H2 per mol STF3 are incorporated if the specimens are heated in flowing/dry H2 up to 550 °C and 612 °C, respectively. For LSF4 this equals about 13 hypothetical ML of H2 and for STF3 about 20 hypothetical ML of H2. This conclusion is also supported by Fourier-transform infrared spectroscopy (FT-IR). FT-IR reveals water formation during static H2 treatment of LSF4/STF3, which indicates perovskite reduction. Furthermore, both samples behave extraordinarily hydrophobic and no chemistry involving surface hydroxy groups was observed.