Ivo Alxneit

Structural and chemical changes of Zn-doped CeO2 nanocrystals upon annealing at ultra-high temperatures

The structure of doped ceria plays an important role in its chemical reactivity and catalytic performance. However, for the majority of the dopants studied, whether a monophasic solid solution is formed or not is typically determined only by standard X-ray diffraction (XRD). In-depth structural characterization is lacking. In this paper we have prepared Zn-doped ceria nanocrystals exhibiting twice the oxygen storage capacity (OSC) of pure ceria. X-ray photoelectron spectroscopy (XPS) shows that the material is chemically inhomogeneous with zinc enrichment in the surface layer. X-ray fluorescence (XRF) reveals significant compositional inhomogeneity of the material after annealing in air at 1300 °C for 24 h. The standard structural characterization of this material using room temperature XRD and transmission electron microscopy (TEM) fails to reveal its correct phase composition. Based on clear evidence from in situ high temperature XRD we show that, after calcination at 500 °C, the material is not monophasic: X-ray amorphous ZnO is present within the material. The amorphous ZnO crystallizes at 800 °C and undergoes second-stage incorporation at even higher temperatures. This second-stage incorporation is not complete after annealing and trace amounts of ZnO remain according to synchrotron-based XRD. Our work provides valuable insight into the incorporation mechanism of zinc into the ceria lattice, and in particular, raises some doubts on the phase compositions reported in many previous studies on doped ceria.

First demonstration of direct hydrocarbon fuel production from water and carbon dioxide by solar-driven thermochemical cycles using rhodium–ceria

Solar-driven thermochemical cycles (STCs) are one of the direct pathways to store solar energy in the chemical bonds of energy-rich molecules. By utilizing a redox material like ceria (CeO2) as reactive medium, STCs can produce the chemical fuels hydrogen and carbon monoxide from water and carbon dioxide. The produced syngas, a mixture of hydrogen and carbon monoxide, can be upgraded to hydrocarbon fuels by the Fischer–Tropsch process. Here, we explore a new concept of producing hydrocarbon fuels directly from water and carbon dioxide by incorporating a catalytic process into STCs. To achieve this, a starting material of ceria doped with a catalyst is used as reactive medium. The primary role of the catalyst is to catalyze the formation of hydrocarbon molecules during the reoxidation of ceria by water and carbon dioxide. In this study, nickel-doped ceria and rhodium-doped ceria were investigated for their methane formation activity after being activated by chemical or thermal reduction. Both materials, after being reduced by hydrogen at 600 °C, are active in producing methane during their reoxidation by water and carbon dioxide at 500 °C. After being thermally reduced at extreme temperatures of 1400 °C and 1500 °C, metallic rhodium is formed in rhodium-doped ceria. The activated rhodium–ceria produces methane directly from water and carbon dioxide during reoxidation. The long-term methane formation activity of rhodium–ceria for 59 cycles with thermal reduction is reported. With rhodium–ceria, this study demonstrates for the first time the concept of producing hydrocarbon fuels, i.e. methane, directly from water and carbon dioxide by realistic STCs. In contrast, nickel-doped ceria is not active in producing methane after thermal activation, owing to rapid sintering and loss of nickel at high temperatures. This underlines the importance of evaluating the effect of thermal reduction on the redox material used. The materials physicochemical properties could be rapidly and significantly altered at the extreme temperatures required for the thermal reduction of ceria. Such changes may render a material that is active and stable at low temperatures inactive when used under realistic conditions of STCs.

Spectral Characterization of PSI’s High-Flux Solar Simulator

In this publication, the detailed spectral characterization of the concentrated radiation of PSI’s 50 kW xenon arc lamp based solar simulator (HFSS) is reported. Spectra are presented for the range of 350–1600 nm recorded at different radial distances from the position of maximum concentration, i.e., from the center of the spot. The analysis shows that the relative intensity of the short wavelength region decreases with increasing radial distance from the center of the spot. At the same time, the relative contribution of the xenon emission lines increases. All spectra can be decomposed into a broad background described by a blackbody spectrum with a temperature of T = 6000 ± 200 K and the characteristic line spectrum of xenon.

1kW imaging furnace with in situ measurement of surface temperature

This article describes the development and characterization of a 1kW imaging furnace that allows to investigate materials such as sulfides at ultrahigh temperatures under controlled atmosphere. Peak flux densities up to (15.37±0.66)×106Wm−2 corresponding to a maximum stagnation temperature of 3090K can be reached in the center of the heating zone of 3mm diameter (full width at half height). Individual sample holders can be mounted on a generic sample stage that is aligned in three axes. Together they define an experiment. Experiments can thus be easily interchanged without requiring any realignment. The use of a specific sample holder is reported where the sample rests on a water-cooled tip to avoid contamination by crucible material and where a protective glass dome can be mounted to allow the study of samples releasing condensable or corrosive gases. With the dome in place the peak flux density decreases to a value of (13.59±0.45)×106Wm−2 (Tstag=2980K). The surface temperature of the sample and the aver...

A compact setup to study homogeneous nucleation and condensation.

An experiment is presented to study homogeneous nucleation and the subsequent droplet growth at high temperatures and high pressures in a compact setup that does not use moving parts. Nucleation and condensation are induced in an adiabatic, stationary expansion of the vapor and an inert carrier gas through a Laval nozzle. The adiabatic expansion is driven against atmospheric pressure by pressurized inert gas its mass flow carefully controlled. This allows us to avoid large pumps or vacuum storage tanks. Because we eventually want to study the homogeneous nucleation and condensation of zinc, the use of carefully chosen materials is required that can withstand pressures of up to 10(6) Pa resulting from mass flow rates of up to 600 l(N) min(-1) and temperatures up to 1200 K in the presence of highly corrosive zinc vapor. To observe the formation of droplets a laser beam propagates along the axis of the nozzle and the light scattered by the droplets is detected perpendicularly to the nozzle axis. An ICCD camera allows to record the scattered light through fused silica windows in the diverging part of the nozzle spatially resolved and to detect nucleation and condensation coherently in a single exposure. For the data analysis, a model is needed to describe the isentropic core part of the flow along the nozzle axis. The model must incorporate the laws of fluid dynamics, the nucleation and condensation process, and has to predict the size distribution of the particles created (PSD) at every position along the nozzle axis. Assuming Rayleigh scattering, the intensity of the scattered light can then be calculated from the second moment of the PSD.

Zn-modified ceria as a redox material for thermochemical H2O and CO2 splitting: effect of a secondary ZnO phase on its thermochemical activity

Two-step thermochemical cycles based on ceria (CeO2) are a promising way to store dilute and intermittent solar energy by producing chemical fuels (solar fuels) such as H2 and CO from H2O and CO2 with concentrated solar radiation. Many studies have shown that the fuel yield per cycle can be enhanced by introducing certain heterocations (dopants) into the ceria lattice. In this study, dual-phase Zn-modified ceria synthesized by coprecipitation was investigated as a redox material for thermochemical H2O and CO2 splitting. Surprisingly, the material exhibits significant increase of the H2 and CO productivities during the first few cycles, in contrast to an anticipated decrease of activity due to sintering. Data suggests that the materials solar fuel productivity eventually stabilizes and exceeds that of native ceria. To elucidate the cause of this observation, changes of the materials physicochemical properties during the initial cycles were analysed in detail. The chemical compositions and elemental distribution were probed using X-ray fluorescence (XRF) spectroscopy with lab and synchrotron X-ray sources. The observed increase of productivity during the initial cycles is correlated with a significant loss of zinc through ZnO sublimation, suggesting a negative effect of the secondary ZnO phase in Zn-modified ceria for its thermochemical activity. Structural changes were revealed by synchrotron micro X-ray powder diffraction (μ-XRD) and X-ray absorption spectroscopy (XAS). The zinc that remains incorporated in the ceria lattice after the initial cycles is likely to be responsible for its higher thermochemical activity in comparison to native ceria.

Error Analysis of the Radiative Power Determined From Flux Distributions Measured With a Camera in a Xe Arc Lamp-Based Solar Simulator

The CCD camera-based flux measurement at Paul Scherrer Institute’s (PSI) high flux solar simulator (HFSS) is influenced by a spatially variable spectrum of the concentrated radiation characteristic for arc lamp-based solar simulators. This results in a substantial error in the radiative power determined by integration of the flux distribution. This systematic error is assessed by numerically modeling the response of the CCD camera in use. Measured spectra of concentrated radiation obtained at different points in the flux distribution, measured transmission characteristics of all optical elements, and published data for the spectral sensitivity of the CCD chip are applied in the model. The response of a water calorimeter is used as baseline case. It is shown that the magnitude of the error depends strongly on the region analyzed, i.e., on aperture size, on the wavelength band analyzed, and, unfortunately, also on the number of lamps in operation. A relative error in the range of 10–30% is observed if an aperture with 1 cm in diameter covering the region of peak concentration is considered. It will be shown that the error arises due to the fact that a photon counter (CCD camera) is used to determine the thermal power.

Modeling the Formation and Chemical Composition of Partially Oxidized Zn/ZnO Particles Formed by Rapid Cooling of a Mixture of Zn(g) and O2

A model and its numerical implementation are presented which describe the formation of mixed Zn/ZnO droplets by rapid cooling of a mixture of zinc vapor and oxygen. The model incorporates a nucleation step producing pure metal droplets of a fixed size and three condensation-like processes determining the further growth of the droplets by adding metal atoms and oxygen. Properties to characterize the resulting droplet population are obtained from the model. Examples such as the number density of particles, their average chemical composition and surface area, or the particle mass distribution are presented. The model is verified on a qualitative level by comparing with experimental data the influence of the quench rate and the initial partial pressures of the reactants on the average chemical composition of the product. The model predicts, conforming qualitatively with experimental findings, that increasing the quench rate has little influence on the chemical composition of the products but that decreasing the initial partial pressures of zinc results in less oxidized products, thus, a higher average zinc content.

Double modulation pyrometry: A radiometric method to measure surface temperatures of directly irradiated samples

The design, implementation, calibration, and assessment of double modulation pyrometry to measure surface temperatures of radiatively heated samples in our 1 kW imaging furnace is presented. The method requires that the intensity of the external radiation can be modulated. This was achieved by a rotating blade mounted parallel to the optical axis of the imaging furnace. Double modulation pyrometry independently measures the external radiation reflected by the sample as well as the sum of thermal and reflected radiation and extracts the thermal emission as the difference of these signals. Thus a two-step calibration is required: First, the relative gains of the measured signals are equalized and then a temperature calibration is performed. For the latter, we transfer the calibration from a calibrated solar blind pyrometer that operates at a different wavelength. We demonstrate that the worst case systematic error associated with this procedure is about 300 K but becomes negligible if a reasonable estimate of the samples emissivity is used. An analysis of the influence of the uncertainties in the calibration coefficients reveals that one (out of the five) coefficient contributes almost 50% to the final temperature error. On a low emission sample like platinum, the lower detection limit is around 1700 K and the accuracy typically about 20 K. Note that these moderate specifications are specific for the use of double modulation pyrometry at the imaging furnace. It is mainly caused by the difficulty to achieve and maintain good overlap of the hot zone with a diameter of about 3 mm Full Width at Half Height and the measurement spot both of which are of similar size.

Thermochemical Conversion of CO2 into Solar Fuels Using Ferrite Nanomaterials

This paper reports the synthesis of Ni x Fe 3-x O 4 nanoparticles via sol-gel method. For Ni x Fe 3-x O 4 synthesis, the Ni and Fe precursor salts were dissolved in ethanol and propylene oxide (PO) was added dropwise to the well mixed solution achieve gel formation. As-prepared gels were aged, dried and subsequently calcined upto 600°C in air. The calcined powders were characterized by powder x-ray diffractometer (XRD), BET surface area, as well as scanning (SEM) and transmission (TEM) electron microscopy. The derived Ni x Fe 3-x O 4 nanoparticles were further examined towards thermochemical conversion of CO 2 into solar fuels by performing several reduction/re-oxidation cycles using a thermogravimetric analyzer (TGA).