Reduction kinetics of ceria surface by hydrogen

Abstract

Powdered cerium dioxide (ceria, CeO 2 ) as compressed, sintered pellets, of porosity 16.4% and density 5.99 g cm −3 , were treated in hydrogen flow at 1 atm and various temperatures to effect reduction. The electrical conductivity was measured in situ during the reduction process. The conductivity increased continuously during the hydrogen treatment because of the creation of anion vacancies and accompanying small polaron electrons. The conductivitytime relationship exhibits three distinct regions indicated as I, II, and III. For each of steps I and II, the conductivity increases exponentially with the reduction. It is suggested from the kinetic analysis of the data that region I is due to desorption of adsorbed oxygen states. Region II appears to be the reduction of surface lattice oxygen. The kinetics of the reactions in both regions I and II obey first-order rate laws with similar activation energies of 86 and 115 kJ mol −1 , respectively. Thermogravimetric experiments were used to determine the time needed to remove one monolayer of adsorbed oxygen from the surface. This could be used to estimate the activation energy of the desorption process at 95 kJ mol −1 -close to the value measured by conductivity measurements. After completing the surface reduction the electrical conductivity subsequently increased slowly during region III. This step is assigned to a diffusion-controlled process during which the bulk of the pellets are reduced. 1 H MASNMR and in situ PXRD experiments confirmed the chemical nature of each of the three steps. C

Synthesis, Structure and Electrical Properties of Mo‐doped CeO2–Materials for SOFCs

“…Chemical compatibility with 10 mol-% Y 2 O 3 stabilised ZrO 2 (YSZ) and Ce 0.9 Gd 0.1 O 1.9 (CGO) electrolytes in wet H 2 were evaluated in the temperature range of 800-1,000°C using PXRD and EDX. The area specific polarisation resistance of the investigated CMO was found to be higher than those of other ceria and zirconia anodes, and ASPR activation energy was found to be comparable to that of other electrodes reported in the literature [53][54][55][56][57][58][59]. and Tools (RTI) (cat.…”

“…Table 1 shows the lattice parameters of as-prepared and wet H 2 treated CMO samples. The increase in lattice parameter for reduced ceria is well known, and is believed to be due to oxygen vacancy formation [54][55][56][57][58] and simultaneous formation of polarons [56,59]. However, in the present study, we found no significant change in the lattice parameters in air and wet H 2 at 800°C which may be due to loss of interstitial oxygen atoms in CMO.…”

Raman Spectroscopy as a Probe of Temperature and Oxidation State for Gadolinium-Doped Ceria Used in Solid Oxide Fuel Cells

“…The thermodynamic calculations show that such a process is expected within the CGO and, supported by the thermodynamic analysis, the three regions shown in Figure are attributed to the three phases of ceria. These results compare well with measurements of the electrical conductivity of cerium dioxide during reduction made by Al-Madfa et al for which three regions were observed . These first results are certainly encouraging as they show that the real time method, combined with accurate measurement of the surface temperature using the peak position, could be employed widely to understand the dynamics of phase stability and formation of unwanted phase under different experimental conditions highly complementary to thermodynamic calculations.…”

Field and frequency modulated sub-THz electron spin resonance spectrometer

“…1 spectroscopy in order to explore for example the spin properties of novel oxide interfaces, 2-7 or of spin functional surfaces, [8][9][10][11][12][13][14][15][16] or to unravel details of the reactions taking place at the surface of catalysts [17][18][19][20][21][22][23][24][25][26][27] requires ever greater resolution and sensitivity. This implies the use of ever greater applied fields, which require in turn sample irradiation frequencies in the sub-THz range.…”