Undoped CeO2 Research Articles

Preparation of La-Dy-CeO2 ternary compound: Examination of photocatalytic and photoluminescence properties

This study investigated the photochemical changes of cerium oxide (CeO2) with the contribution of rare earth ions. Lanthanum (La) and Dysprosium (Dy) were added to the CeO2 crystal structure simultaneously via citric acid sol-gel synthesis. Following synthesis, the sintering temperatures were chosen as 800 and 1000 °C. Results of X-ray diffraction (XRD) analysis showed that the doped and undoped CeO2 compounds exhibited a cubic phase. The surface morphology of the samples was examined by scanning electron microscopy (SEM). The particle sizes varied in the range of 90−150 nm and the composite elements were observed to maintain the stoichiometric ratio. Compound impurity control was performed using energy dispersive X-ray (EDX). Lattice changes in the compounds were revealed via Raman spectroscopy, which indicated that the La and Dy had caused changes in the CeO2 symmetry. The photocatalytic behavior of the compounds was investigated using methylene blue degradation. Optical band gaps were calculated and the La and Dy were found to have narrow band spacing. The band gap of the Ce0.85La0.10Dy0.05O2 compound sintered at 1000 °C was 1.63 eV, the rate constant was 9.4 * 10−3 min−1, and the degradation efficiency at 100 min was calculated as 60 %. The photoluminescence spectra exhibited four bands centered at 435, 532, 565, and 610 nm. The emission band at 565 nm was attributed to the transition from the 4F9/2 to the 6H13/2state, whereas the emission bands peaking at 435 nm, 532 nm, and 610 nm can be explained by the intrinsic luminescence of the host lattice.

Characterization of Gd doped CeO2 thin films grown by ultrasonic spray pyrolysis

Abstract Gd concentration effect on the optical, structural, surface and electrical properties of Gd doped CeO2 thin films are coated on glass substrates by spray pyrolysis technique with precursors containing cerium nitrate hexahydrate [Ce(NO3)3·6H2O] and gadalinium nitrate hexahydrate [Gd(NO3)3·6H2O] as a source material. The structural, morphologicaly and elemental characterizations of the films have been carried out using x-ray diffraction, field emission scanning electron microscopy and energy dispersive x-ray analysis, respectively. An optical energy band gap is calculated in the range 3.3–3.8 eV from UV–Vis transmittance spectra. Surface morphologies and roughness values of the films have been investigated by atomic force microscopy. Thicknesses have been determined using spectroscopic ellipsometry technique and Cauchy–Urbach model. A four-probe setup has been used to determine the electrical resistivity values. All samples showed x-ray diffraction peaks corresponding to a cubic fluorite structure. The results indicate that the deposited films have a preferential growth in the (0 0 2) direction. The observed transmittance in the visible region has been situated between 60% and 70%. Photoluminescence spectra showed an ultraviolet emission band around 365–400 nm and the visible emission peaks between 469 and 590 nm. Raman active peaks for undoped and Gd doped CeO2 films were determined at ~465 cm−1. Finally, it has been concluded that Gd doped CeO2 films have value to work on and may be promising materials for optoelectronic applications.

Homogenously Rare-Earth-Ion-Doped Nanoceria Synthesis in KOH-NaOH Molten Flux: Characterization and Photocatalytic Property

Homogeneously rare-earth (RE=La, Sm and Y) ions doped CeO2 nanoparticles (NPs) with enhanced photocatalytic performance were synthesized by a facile KOH-NaOH molten flux method. XRD, TEM, EDS mapping and XPS results demonstrate that RE cations have been successfully doped into the CeO2 lattice, which leads to the lattice expansion/shrinking, and more oxygen vacancies are generated after RE ions doping. The UV-Vis DRS and PL spectra indicate that RE-doped CeO2 samples have slightly narrower band gaps and significantly reduced photo-generated electron-hole recombination rates. 5% RE ions doped CeO2 NPs show excellent photocatalytic performances and the photodegradation ratio of methylene blue (MB) increases from 71.63% of undoped CeO2 to 76.35%, 77.51% and 77.37% for 5%-La, 5%-Sm and 5%-Y-doped CeO2 NPs, respectively. The obviously reduced recombination rate of e−/h+ pairs after RE ions doping may be the critical factor for the enhanced photocatalytic performance.

Assessment of positrons for defect studies in CeO2 materials

Disposal of spent nuclear fuel poses significant challenges, as the UO2 and fission products are under constant irradiation and must be safely stored for millennia. CeO2 is a non-radioactive analog for UO2 for studying microstructure and its evolution. Many techniques have been applied to uranium and cerium oxides to investigate point defects. Positron annihilation spectroscopies (PAS) are sensitive to neutral and negatively charged vacancy-like point defects and impurity vacancy complexes. PAS has been applied previously to UO2+x to investigate nuclear fuels, but virtually no PAS work exists on CeO2. Here, the basics of positron annihilation spectroscopy is reviewed, and preliminary work on undoped and doped CeO2 is shown and compared to the literature results from UO2. To simulate fission product incorporation in spent nuclear fuels, CeO2 samples were doped at different concentration with yttrium. Select samples were irradiated with heavy ions at different doses. Doping and irradiation are shown to give rise to different defect characteristics.

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

Methane, discovered in 1766 by Alessandro Volta, is an attractive energy source because of its high heat of combustion per mole of carbon dioxide. However, methane is the most abundant hydrocarbon in the atmosphere and is an important greenhouse gas, with a 21-fold greater relative radiative effectiveness than CO2 on a per-molecule basis. To avoid or limit the formation of pollutants that are dangerous for both human health and the atmospheric environment, the catalytic combustion of methane appears to be one of the most promising alternatives to thermal combustion. Total oxidation of methane, which is environmentally friendly at much lower temperatures, is believed to be an efficient and economically feasible way to eliminate pollutants. This work presents a literature review, a statu quo, on catalytic methane oxidation on transition metal oxide-modified ceria catalysts (MOx/CeO2). Methane was used for this study since it is of great interest as a model compound for understanding the mechanisms of oxidation and catalytic combustion on metal oxides. The objective was to evaluate the conceptual ideas of oxygen vacancy formation through doping to increase the catalytic activity for methane oxidation over CeO2. Oxygen vacancies were created through the formation of solid solutions, and their catalytic activities were compared to the catalytic activity of an undoped CeO2 sample. The reaction conditions, the type of catalysts, the morphology and crystallographic facets exposing the role of oxygen vacancies, the deactivation mechanism, the stability of the catalysts, the reaction mechanism and kinetic characteristics are summarized.

Defects mediated enhanced catalytic and humidity sensing performance in ceria nanorods

The content of defects and manipulation of oxygen vacancies in ceria (CeO2) nanomaterials through doping have been widely used as effective strategies to rationally control its reactivity for diverse applications. In this work, a facile and simple fabrication method of CeO2 nanorod crystals is demonstrated where; defects content and oxygen vacancies were tailored by doping with transition metal (Iron, Fe) ions. The effect of Fe-doping level (Ce0.95Fe0.05O2 and %, Ce0.9Fe0.1O2) on the structural, morphological, and optical properties of ceria nanorods was explored in detail. Undoped and Fe-doped CeO2 nanorods were further explored for their use in photocatalytic activity and humidity sensing applications. The results revealed superior photocatalytic activity of Ce0.9Fe0.1O2 by showing about 90% degradation of organic dye within 80 min of reaction time. Further, the same sample demonstrated excellent humidity sensing capabilities with faster response (~19 s) and recovery rate (~ 49 s) at 100% humidity conditions than other investigated samples. This work presents a systematic study on the fluencies of Fe doping in CeO2 nanorod crystals to better understand the role of oxygen vacancies for tailoring their structural and optical properties for proficient environmental and sensing applications.

Core and dopant effects toward hydrogen gas sensing activity using Pd@N-CeO2 core–shell nanoflatforms

Developing efficient and stable hydrogen gas sensors may be of urgent demand for its safety uses. Herein, Pd@N-CeO2 core–shell nanoflatforms (CSNFs) are fabricated and utilized for this purpose. The resulting Pd@N-CeO2 CSNFs offer small particle sizes with high Brunauer–Emmett–Teller (BET) surface area and porous nanostructures. The core–shell sensors establish high hydrogen sensing response and fast response and recovery times at a lower optimal working temperature compared to undoped and doped CeO2 ones. In addition, it further demonstrates high selectivity and stability toward hydrogen gas among interfering different target gases. The hydrogen gas sensing betterment is synergistically assigned to Pd core, N dopant, and high BET surface area effects, which decidedly modulate the electrical resistance of core–shell sensors to improve overall gas sensing performance accordingly. Our finding provides an efficient way to design and fabricate versatile hydrogen gas sensors based on metal@nitrogen doped-semiconductor oxide core–shell nanostructures.

Physical and in-vitro evaluation of pure and substituted MxCe1-xO2 (M = Co, Fe or Ti and x = 0.05) magnetic nanoparticles

High resolution TEM studies revealed the spherical morphology of MxCe1-xO2 (M = Co, Fe or Ti, x = 0.05) particles with their size in the range of 8–11 nm. Raman, UV and PL spectroscopy analyses evidenced that oxygen vacancy concentration modified with the type of dopants. The concentration of vacancies in the Co0.05Ce0.95O2 sample was relatively higher and hence it had optimum magnetization value. The in-vitro cytotoxicity study for MxCe1-xO2 (M = Co, Fe or Ti, x = 0.05) nanoparticles against human lung adenocarcinoma (A549 cells) was conducted. The results suggested that at a 10 μg/mL concentration, the undoped CeO2 nanoparticles have shown cell viability up to 99%. In contrast, at the same concentration, the doped CeO2 such as Co0.05Ce0.95O2, Fe0.05Ce0.95O2 and Ti0.05Ce0.95O2 nanoparticles demonstrated the cell viability of ~97%. Furthermore, the samples displayed reliable biocompatibility up to 1000 μg/mL concentration. Interestingly, Co-doped CeO2 nanoparticles exhibited relatively higher biocompatibility against A549 cells at all concentrations. Further, the higher amount of vacancies might have improved the free radical scavenging effect and so the biocompatibility for the samples.

Water-Mediated ElectroHydrogenation of CO2 at Near-Equilibrium Potential by Carbon Nanotubes/Cerium Dioxide Nanohybrids

The combination of multiwalled carbon nanotubes (MWCNTs) with undoped CeO2 nanoparticles (NPs) is effective for the direct electrocatalytic reduction of CO2 to formic acid (FA) at acidic pH (0.1 M ...

Tuning of structural, optical, and impedance properties of CeO2 thin films by incorporation palladium ions

Thin films of pure and Pd-doped CeO2 with different doping concentrations deposited via sol gel spin coating technique. The influence of Pd doping level (0, 2, 4 and 8%) on the physical characteristics was investigated of nanocrystalline CeO2 thin films. From X-ray diffraction (XRD) pattern, the (111) preferred oriented CeO2 cubic structure was observed in all thin films. Crystallite size decreased with increasing Pd doping from 67.41 to 41.67 nm, whereas the crystallinity of thin film increased with doping level. The field emission scanning electron microscopy (FESEM) analyses showed the uniform distribution of nanospherical grains, decreased in size and agglomerated of nanocrystallite with increasing Pd doping. Energy-dispersive X-ray (EDX) results revealed that the weight percentage of un-doped and Pd-doped CeO2 thin films was confirmed and very close to that in mixed precursor. The energy bandgap values of prepared thin films were measured as a function of Pd doping centration using UV–Vis spectroscopy. Energy bandgap as well as the transmission of CeO2 were increased with Pd doping level. The PL spectra show noticeable variations in intensity and red shift of the emission peaks upon Pd doping. These effects have been associated with structural modifications and defect states Pd-doped CeO2 thin films. The Nyquist plots suggest that the ionic conductivity through the grains is responsible in the conduction mechanism of the thin films.

Raman Study of Nanocrystalline-Doped Ceria Oxide Thin Films

Samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) thin films were formed by e-beam vapor deposition on SiO2 substrate, changing the deposition rate and substrate temperature during the deposition. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-Ray spectrometry (EDS) were employed in order to investigate the structure ad morphology of the films. A single Raman peak describing the structure of undoped CeO2 was observed at a frequency of 466 cm−1. Doping of cerium oxide with rare-earth elements shifted the peak to lower frequencies (for Sm—462 cm−1). This shift occurs due to the increased number of oxygen vacancies in doped cerium oxide and it depends on the size and concentration factor of the dopant. It was found that wavenumbers and their intensity differed for the investigated samples, even though the peaks resembled each other in shape. The indicated bands for doped ceria originated as a result of the Raman regime (F2g) of fluorite dioxide associated with the space group (Fm3m). The observed peak‘s position shifting to a lower frequency range demonstrates the symmetric vibrations of oxygen ions around Ce4+ ions in octahedra CeO8. Raman shift to the lower frequencies for the doped samples has two reasons: an increase in oxygen vacancies caused by doping cerium oxide with rare-earth materials and the size factor, i.e., the change in frequency Δω associated with the change in the lattice constant Δa.

Enhanced third-order optical susceptibility in Ag-doped CeO2 nanostructures under pulsed Nd-YVO4 laser

In this research, cerium dioxide nanoparticles with different Ag doping levels were prepared through a one-pot hydrothermal synthesis route. The as-prepared samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrophotometry, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM). The linear optical characteristics were analyzed by photoluminescence (PL) emission technique at an excitation wavelength of 250 nm. A red shift towards longer wavelengths was detected in NBE-related energies of Ag-doped samples as compared to Un-doped CeO2 NPs. The incorporation of Ag in CeO2 lattice was also justified using fluorescence quenching. The third-order nonlinear optical parameters were investigated using single-beam Z-scan technique under a 1064 nm Nd-YVO4 laser beam with 50 ns pulse width and 10 KHz repetition rate as an excitation source. All samples presented negligible nonlinear absorptions and self-defocusing responses due to negative lens effects in the near infra-red region. The Z-scan results indicated stronger third-order nonlinear susceptibilities for Ag-doped CeO2 samples as compared to Un-doped NPs. The highest third-order nonlinear susceptibility was found to be about 22.19×10-6esu belonging to 0.75Ag-doped CeO2 sample with an enhancement about 4.5 times higher than that of Un-doped CeO2.

Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zirconium Doping

The effects of Zr doping on the stability of the CeO2(111) surface as a function of the dopant concentration and distribution, as well as on the relative stability of surface and subsurface oxygen vacancies, were studied by means of density functional theory (DFT+U) calculations. For a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O-Ce-O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. For the undoped CeO2(111) surface, the preference of subsurface vacancies with next-nearest neighbor (NNN) Ce3+ configuration has earlier been predicted. For the Zr-doped surface, the formation of vacancies was studied using a surface unit cell with 2x2 periodicity, and it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with NNN Ce3+, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO2(111) surface. Moreover, when Zr is located in TL2 the subsurface vacancy becomes by 0.10 eV less stable, compared to Zr in the TL1. The Ce3+ preference for the next-nearest neighbor cationic sites to both surface and subsurface vacancies at CeO2 (111), becomes more pronounced upon Zr doping. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects.

High ionic conductivity in CeO2 SOFC solid electrolytes; effect of Dy doping on their electrical properties

Ionic conductors composed of lanthanide-doped ceria with general formula DyyCe1-yO2-δ (y = 0.05, 0.1 and 0.15) were synthesized by mechanochemistry (mechanical milling), and their electrical properties analyzed to be used as solid electrolytes in low-temperature SOFC. Starting oxide reagents were milled at different times in a planetary mill and the evolution of their structures and phases with milling time and temperature (up to 1500 °C) was followed by XRD. Just milled powders were also uniaxially pressed and sintered at different temperatures (1200, 1350 and 1500 °C), and analyzed by FE-SEM, to explore their morphologies as a function of temperature and Dy content. The electrical properties of these materials and undoped commercial CeO2 were analyzed by impedance spectroscopy at different temperatures (200–650 °C) and frequencies (100 Hz - 1 MHz). Results showed that mechanochemistry is a suitable method to obtain the DyyCe1-yO2-δ systems after 20 h of milling, since XRD patterns of these milled powders reveal the formation of fluorite-type cubic solid solutions for all studied compositions. Increasing of temperature generates a higher crystallinity in these materials while the absence of phase transitions in them is corroborated at 1200 °C. Analysis of electrical properties of samples sintered a 1200 °C corroborates the viability of these systems to be used as solid electrolytes in the SOFC technology, being that high dc conductivities (σdc) were obtained for all doped samples, especially for the composition Dy0.1Ce0·9O2-δ, which showed a σdc = 1 × 10−1.91 S cm−1 at 650 °C. This value represents an increase of almost three orders of magnitude for this composition with respect to the undoped CeO2 sample (y = 0, σdc = 1 × 10 −4.83 Scm−1).

Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zirconium Doping

The effects of Zr doping on the stability of the CeO₂(111) surface as a function of the dopant concentration and distribution, as well as on the relative stability of surface and subsurface oxygen vacancies, were studied by means of density functional theory (DFT+U) calculations. For a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O–Ce–O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. For the undoped CeO₂(111) surface, the preference of subsurface vacancies with next-nearest neighbor (NNN) Ce³⁺ configuration has earlier been predicted. For the Zr-doped surface, the formation of vacancies was studied using a surface unit cell with 2 × 2 periodicity, and it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with NNN Ce³⁺, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO₂(111) surface. Moreover, when Zr is located in TL2 the subsurface vacancy becomes by 0.10 eV less stable, compared to Zr in the TL1. The Ce³⁺ preference for the next-nearest neighbor cationic sites to both surface and subsurface vacancies at CeO₂ (111), becomes more pronounced upon Zr doping. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects.

Investigation of Zr, Gd/Zr, and Pr/Zr – doped ceria for the redox splitting of water

There is a renewed interest in CeO2 for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO2 is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (ZrxCe1-xO2, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M0.1Zr0.25Ce0.65O2, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O2 cycle capacity and H2 production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H2 production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H2O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO2. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H2 production, in agreement with experimental work. The calculated surface H2 formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.

RF Sputtered CeO2Thin Films-Based Oxygen Sensors

In this paper, we report the scalable, high sensitivity, fast response, and low operating temperature Cerium oxide (CeO2) thin film-based oxygen sensors by optimizing CeO2 film thickness. CeO2 thin films of thickness ranging from 90 to 340 nm have been deposited at 400°C using radio frequency (RF) magnetron sputtering on Al2O3 substrates. Ellipsometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) have been used to characterize the CeO2 films for their thickness, structural, compositional/chemical, and surface morphology properties. Gas sensors have been fabricated using CeO2 film as a sensing material and tested in an oxygen gas environment. CeO2 film with an optimum thickness of 260 nm has shown high sensitivity (12.6) and fast response time (≤10 s) along with fast recovery time (15 s) at a low operating temperature of 400°C. To the best of our knowledge, these are the best values reported till date for undoped CeO2 thin film-based oxygen sensors. Furthermore, from the sensor’s response, it was observed that there was no drifting from the baseline. This superior performance of CeO2 thin film-based oxygen sensor may be attributed to the combination of three factors, i.e., 1) high surface energy and reactivity due to the presence of (200) oriented CeO2 plane; 2) low resistance due to better crystallinity; and 3) perfect stoichiometry with required roughness.

Resonant Raman Scattering in Undoped and Lanthanide-Doped CeO2

CeO2 has a narrow, empty band of Ce 4f states that lies between an O 2p-based valence band and a Ce 5d-based conduction band. The O 2p–Ce 4f optical band gap is positioned at ∼3.2 eV with an absorption band centered at ∼3.8 eV. We investigated the Raman scattering of bulk CeO2 in the excitation energy range of 1.96–3.81 eV. The resonant enhancement profile of the longitudinal optical (LO) phonon at ∼590 cm–1 closely follows that of the 2LO band and both profiles track the optical absorption of the O 2p–Ce 4f electronic transition. Multi-LO phonon bands were found to appear up to the sixth order, pointing to an electron–phonon Frohlich interaction as the source of the resonant enhancement. The ∼600 cm–1 off-resonant D2 band (denoted as MO8-type complex in ceria doped with M aliovalent ions) is overshadowed under resonant conditions by the resonant LO phonon scattering. Hence, spectral analysis of defect bands under resonant conditions has to be distinct from that applied under off-resonant conditions and c...

Understanding the Role of Surface Oxygen in Hg Removal on Un-Doped and Mn/Fe-Doped CeO2 (111).

Effects of surface-adsorbed O and lattice O for the CeO2 (111) surface on Hg removal has been researched. In this work, periodic calculations based on density functional theory (DFT) were performed with the on-site Coulomb interaction. Hg is oxidized to HgO via the surface-adsorbed O by overcoming a Gibbs free energy barrier of 114.1 kJ·mol-1 on the CeO2 (111) surface. Mn and Fe doping reduce the activation Gibbs free energy for the Hg oxidation, and energies of 70.7 and 49.6 kJ·mol-1 are needed on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. Additionally, lattice O also plays an important role in Hg removal. Hg cannot be oxidized leading to the formation of HgO on the un-doped CeO2 (111) surface owing to the inertness of lattice O, which can be easily oxidized to HgO on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. It can be seen that both surface-adsorbed O and lattice O play important roles in removing Hg. The present study will shed light on understanding and developing Hg removal technology on un-doped and Mn/Fe-doped CeO2 (111) catalysts. © 2019 Wiley Periodicals, Inc.

On the mechanism of alkyne hydrogenation catalyzed by Ga-doped ceria

The hydrogenation of acetylene by Ga-doped ceria is investigated using density functional theory (DFT). Different from a previously proposed mechanism that contains an unattainable barrier, our models are based on two Ga-doped ceria surfaces with oxygen vacancies, representing respectively low and high Ga dopant concentrations. The doping of Ga is shown to promote the formation of surface oxygen vacancies, which create a template containing frustrated Lewis pairs to facilitate heterolytic H2 dissociative chemisorption. This mechanism is similar in spirit to our recently proposed mechanism for bare and Ni-doped CeO2 catalysts (J. Am. Chem. Soc., 2018, 140, 12964–12973) and calls for the formation of GaH hydride species along with OH ones on the ceria surface. The former has recently been experimentally observed, thus providing strong support for this mechanism. The rate-limiting step in doped CeO2(1 1 1) with low Ga concentrations has a barrier that is 0.18 eV for the Ga/O FLP and 0.05 eV for the Ce/O FLP higher than that of undoped CeO2(1 1 1), while that for high Ga concentrations is 0.21 eV lower, consistent with the experimental observed concentration dependence of catalytic activity. However, different from the Ni dopant, which was found to be a promoter, the Ga dopant is not only a promoter but also an active participant in the catalysis.