The formation of methanol from glycerol bio-waste over doped ceria-based catalysts.
ABSTRACT: A series of ceria-based solid solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO2, Pr6O11 and ZrO2. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction, and it was found that an increased defect density or reducibility was beneficial. The space-time yield of methanol normalized to surface area over CeO2 was found to be 0.052 mmolMeOH m-2 h-1, and over CeZrO2 and CePrO2, this was to 0.029 and 0.076 mmolMeOH m-2 h-1, respectively. The inclusion of Pr reduced the surface area; however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue 'Science to enable the circular economy'.
Project description:CO elimination through oxidation over highly active and cost-effective catalysts is a way forward for many processes of industrial and environmental importance. In this study, doped CeO<sub>2</sub> with transition metals (TM = Cu, Co, Mn, Fe, Ni, Zr, and Zn) at a level of 20 at. % was tested for CO oxidation. The oxides were prepared using microwave-assisted sol-gel synthesis to improve catalyst's performance for the reaction of interest. The effect of heteroatoms on the physicochemical properties (structure, morphology, porosity, and reducibility) of the binary oxides M-Ce-O was meticulously investigated and correlated to their CO oxidation activity. It was found that the catalytic activity (per gram basis or TOF, s<sup>-1</sup>) follows the order Cu-Ce-O > Ce-Co-O > Ni-Ce-O > Mn-Ce-O > Fe-Ce-O > Ce-Zn-O > CeO<sub>2</sub>. Participation of mobile lattice oxygen species in the CO/O<sub>2</sub> reaction does occur, the extent of which is heteroatom-dependent. For that, state-of-the-art transient isotopic <sup>18</sup>O-labeled experiments involving <sup>16</sup>O/<sup>18</sup>O exchange followed by step-gas CO/Ar or CO/O<sub>2</sub>/Ar switches were used to quantify the contribution of lattice oxygen to the reaction. SSITKA-DRIFTS studies probed the formation of carbonates while validating the Mars-van Krevelen (MvK) mechanism. Scanning transmission electron microscopy-high-angle annular dark field imaging coupled with energy-dispersive spectroscopy proved that the elemental composition of dopants in the individual nanoparticle of ceria is less than their composition at a larger scale, allowing the assessment of the doping efficacy. Despite the similar structural features of the catalysts, a clear difference in the O<sub>lattice</sub> mobility was also found as well as its participation (as expressed with the α descriptor) in the reaction, following the order α<sub>Cu</sub> > α<sub>Co</sub>> α<sub>Mn</sub> > α<sub>Zn</sub>. Kinetic studies showed that it is rather the pre-exponential (entropic) factor and not the lowering of activation energy that justifies the order of activity of the solids. DFT calculations showed that the adsorption of CO on the Cu-doped CeO<sub>2</sub> surface is more favorable (-16.63 eV), followed by Co, Mn, Zn (-14.46, -4.90, and -4.24 eV, respectively), and pure CeO<sub>2</sub> (-0.63 eV). Also, copper compensates almost three times more charge (0.37<i>e</i><sup>-</sup>) compared to Co and Mn, ca. 0.13<i>e</i><sup>-</sup> and 0.10<i>e</i><sup>-</sup>, respectively, corroborating for its tendency to be reduced. Surface analysis (X-ray photoelectron spectroscopy), apart from the oxidation state of the elements, revealed a heteroatom-ceria surface interaction (O<sub>a</sub> species) of different extents and of different populations of O<sub>a</sub> species.
Project description:A ceria (CeO<sub>2</sub>) promoted Cu-Ni bimetallic catalyst supported on SiO<sub>2</sub> (Cu-Ni/CeO<sub>2</sub>-SiO<sub>2</sub>) was prepared and evaluated for catalytic hydrodeoxygenation (HDO) of vanillin. Silica supported monometallic Cu and Ni catalysts and bimetallic Cu-Ni catalyst (Cu/SiO<sub>2</sub>, Ni/SiO<sub>2</sub>, and Cu-Ni/SiO<sub>2</sub>), without a ceria promoter, were also synthesized and tested for the same application. The highest conversion of vanillin was achieved with the Cu-Ni/CeO<sub>2</sub>-SiO<sub>2</sub> catalyst. Vanillyl alcohol was the sole product in the initial 2 h, followed by the formation of 2-methoxy-4-methylphenol, which was observed. Characterization of the synthesized catalysts revealed the presence of overlapping crystalline phases of CuO, NiO, and CeO<sub>2</sub> on the Cu-Ni/CeO<sub>2</sub>-SiO<sub>2</sub> surface. We extended our study to find out the results of using CeO<sub>2</sub> as the support of the Cu-Ni bimetallic catalyst (Cu-Ni/CeO<sub>2</sub>). Partial incorporation of Cu and Ni cations into the ceria lattice took place, leading to the decrease of specific surface area and a concomitant compromise in the conversion. In the case of the Cu-Ni/CeO<sub>2</sub>-SiO<sub>2</sub> catalyst, the higher conversion was accredited to the facile formation of Cu<sup>+</sup> active centers by the synergistic interaction between Ce<sup>+4</sup>/Ce<sup>+3</sup> and Cu<sup>+2</sup>/Cu<sup>+</sup> redox couples and the incorporation of oxygen vacancies on the catalyst surface.
Project description:The study provides deep insight into the origin of photocatalytic deactivation of Nb<sub>2</sub>O<sub>5</sub> after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO<sub>2</sub>/Nb<sub>2</sub>O<sub>5</sub> heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb<sub>2</sub>O<sub>5</sub> and CeO<sub>2</sub>) and as-prepared CeO<sub>2</sub>/Nb<sub>2</sub>O<sub>5</sub> heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by <i>operando</i>-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO<sub>2</sub>/Nb<sub>2</sub>O<sub>5</sub> heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb<sub>2</sub>O<sub>5</sub> to CeO<sub>2</sub>. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.
Project description:Synthesis of truly monodisperse nanoparticles and their structural characterization to atomic precision are important challenges in nanoscience. Success has recently been achieved for metal nanoparticles, particularly Au, with diameters up to 3?nm, the size regime referred to as nanoclusters. In contrast, families of atomically precise metal oxide nanoparticles are currently lacking, but would have a major impact since metal oxides are of widespread importance for their magnetic, catalytic and other properties. One such material is colloidal CeO<sub>2</sub> (ceria), whose applications include catalysis, new energy technologies, photochemistry, and medicine, among others. Here we report a family of atomically precise ceria nanoclusters with ultra-small dimensions up to ~1.6?nm (~100 core atoms). X-ray crystallography confirms they have the fluorite structure of bulk CeO<sub>2</sub>, and identifies surface features, H<sup>+</sup> binding sites, Ce<sup>3+</sup> locations, and O vacancies on (100) facets. Monodisperse ceria nanoclusters now permit investigation of their properties as a function of exact size, surface morphology, and Ce<sup>3+</sup>:Ce<sup>4+</sup> composition.
Project description:A series of gold catalysts supported on pure CeO<sub>2</sub>, ZrO<sub>2</sub>, and two different Ce-Zr mixed oxides have been prepared and tested in the 5-hydroxymethyl-2-furfural oxidation reaction. All catalysts show high catalytic activity (100% conversion) and important selectivity (27-41%) to the desired product i.e., 2,5-furandicarboxylic acid at low base concentration. Products selectivity changes with the support nature as expected, however, the observed trend cannot be related neither to gold particle size, nor to catalyst reducibility and oxygen mobility. An important relation between the FDCA selectivity and the support textural properties is observed, conducing to the general requirement for optimal pore size for this reaction.
Project description:The widely studied Pt/C catalyst for direct methanol fuel cells (DMFCs) suffers severe carbon corrosion under operation, which undermines the catalytic activity and durability. It is of great importance to develop a carbon-free support with co-catalytic functionality for improving both the activity and durability of Pt-based catalysts. The direct loading of Pt on the smooth surface of oxides may be difficult. Herein, the Cu assisted loading of Pt on CeO<sub>2</sub> is developed. Cu pre-coated CeO<sub>2</sub> was facilely synthesized and Pt was electrochemically deposited to fabricate the carbon-free PtCu/CeO<sub>2</sub> catalyst. The PtCu/CeO<sub>2</sub> catalyst has a mass activity up to 1.84 and 1.57 times higher than Pt/C towards methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR), respectively. Better durability is also confirmed by chronoamperometry and accelerated degradation tests. The strategy in this work would be greatly helpful for developing an efficient carbon-free support of Pt-based catalysts for applications in DMFCs.
Project description:The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO<sub>2</sub> ) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO<sub>2</sub> (111) thin films and CeO<sub>2</sub> powders, and theoretical calculations of CeO<sub>2</sub> (111) surfaces with oxygen vacancies (O<sub>v</sub> ) at the surface and in the bulk. We show that, on a stoichiometric CeO<sub>2</sub> (111) surface, H<sub>2</sub> dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO<sub>2-x</sub> samples, both films and powders, hydroxyls and hydrides (Ce-H) are formed on the surface as well as in the bulk, accompanied by the Ce<sup>3+</sup> ↔ Ce<sup>4+</sup> redox reaction. As the O<sub>v</sub> concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H<sub>2</sub> /CeO<sub>2</sub> interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.
Project description:Atomically dispersed precious metals on oxide supports have recently become increasingly interesting catalytic materials. Nonetheless, their non-trivial preparation and limited thermal and environmental stability constitutes an issue for their potential applications. Here we demonstrate that an oxygen plasma pre-treatment of the ceria (CeO<sub>2</sub> ) surface serves to anchor Pt single atoms, making them active and resistant towards sintering in the CO oxidation reaction. Through a combination of experimental results obtained on well-defined CeO<sub>2</sub> films and theory, we show that the O<sub>2</sub> plasma causes surface nanostructuring and the formation of surface peroxo (O<sub>2</sub> <sup>2-</sup> ) species, favoring the uniform and dense distribution of isolated strongly bonded Pt<sup>2+</sup> atoms. The promotional effect of the plasma treatment was further demonstrated on powder Pt/CeO<sub>2</sub> catalysts. We believe that plasma functionalization can be applied to other metal/oxide systems to achieve tunable and stable catalysts with a high density of active sites.
Project description:Methanol dehydrogenation is an efficient way to produce syngas with high quality. The current efficiency of sunlight-driven methanol dehydrogenation is poor, which is limited by the lack of excellent catalysts and effective methods to convert sunlight into chemicals. Here, we show that atomically substitutional Pt-doped in CeO<sub>2</sub> nanosheets (Pt<sub>s</sub>-CeO<sub>2</sub>) exhibit excellent methanol dehydrogenation activity with 500-hr level catalytic stability, 11 times higher than that of Pt nanoparticles/CeO<sub>2</sub>. Further, we introduce a photothermal conversion device to heat Pt<sub>s</sub>-CeO<sub>2</sub> up to 299°C under 1 sun irradiation owning to efficient full sunlight absorption and low heat dissipation, thus achieving an extraordinarily high methanol dehydrogenation performance with a 481.1 mmol g<sup>-1</sup> h<sup>-1</sup> of H<sub>2</sub> production rate and a high solar-to-hydrogen (STH) efficiency of 32.9%. Our method represents another progress for ambient sunlight-driven stable and active methanol dehydrogenation technology.
Project description:Single-atom catalysts (SACs) have attracted considerable attention in the catalysis community. However, fabricating intrinsically stable SACs on traditional supports (N-doped carbon, metal oxides, etc.) remains a formidable challenge, especially under high-temperature conditions. Here, we report a novel entropy-driven strategy to stabilize Pd single-atom on the high-entropy fluorite oxides (CeZrHfTiLa)O<sub>x</sub> (HEFO) as the support by a combination of mechanical milling with calcination at 900?°C. Characterization results reveal that single Pd atoms are incorporated into HEFO (Pd<sub>1</sub>@HEFO) sublattice by forming stable Pd-O-M bonds (M?=?Ce/Zr/La). Compared to the traditional support stabilized catalysts such as Pd@CeO<sub>2</sub>, Pd<sub>1</sub>@HEFO affords the improved reducibility of lattice oxygen and the existence of stable Pd-O-M species, thus exhibiting not only higher low-temperature CO oxidation activity but also outstanding resistance to thermal and hydrothermal degradation. This work therefore exemplifies the superiority of high-entropy materials for the preparation of SACs.