Not only the interaction between CO and ceria but also the reactivity and mobility of lattice oxygen in ceria are greatly influenced by the surface structure of ceria, leading to the strong variation ...in CO oxidation over ceria nanoshapes. Display omitted
► Surface structure dependence of CO oxidation over ceria rods, cubes, and octahedra. ► Carbonates strongly bonded on ceria surfaces of {110} and {100} than on {111}. ► Surface structure-dependent contribution of four channels in CO-TPR on ceria. ► Structure dependence of ceria lattice oxygen reactivity and mobility. ► Most carbonates observed under CO oxidation conditions are reaction spectators.
CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({110}+{100}), cubes ({100}), and octahedra ({111}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods>cubes>octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO2, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.
We have performed computations to better understand how surface structure affects selectivity in dehydrogenation and dehydration reactions of alcohols. Ethanol reactions on the (111) and (100) ceria ...surfaces were studied starting from the dominant surface species, ethoxy. We used DFT (PBE+U) to explore reaction pathways leading to ethylene and acetaldehyde and calculated estimates of rate constants employing transition state theory. To assess pathway contributions, we carried out kinetic analysis. Our results show that intermediate and transition state structures are stabilized on the (100) surface compared to the (111) surface. Formation of acetaldehyde over ethylene is kinetically and thermodynamically preferred on both surfaces. Our results are consistent with temperature-programmed surface reaction and steady-state experiments, where acetaldehyde was found as the main product and evidence was presented that ethylene formation at higher temperature originates from changes in adsorbate and surface structure.
In addition to their well-known redox character, the acid–base property is another interesting aspect of ceria-based catalysts. Herein, the effect of surface structure on the acid–base property of ...ceria was studied in detail by utilizing ceria nanocrystals with different morphologies (cubes, octahedra, and rods) that exhibit crystallographically well-defined surface facets. The nature, type, strength, and amount of acid and base sites on these ceria nanoshapes were investigated via in situ IR spectroscopy combined with various probe molecules. Pyridine adsorption shows the presence of Lewis acid sites (Ce cations) on the ceria nanoshapes. These Lewis acid sites are relatively weak and similar in strength among the three nanoshapes according to the probing by both pyridine and acetonitrile. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the ceria nanoshapes, as probed by CO2 adsorption. CO2 and chloroform adsorption indicate that the strength and amount of the Lewis base sites are shape dependent: rods > cubes > octahedra. The weak and strong surface dependence of the acid and base sites, respectively, are a result of interplay between the surface structure dependent coordination unsaturation status of the Ce cations and O anions and the amount of defect sites on the three ceria nanoshapes. Furthermore, it was found that the nature of the acid–base sites of ceria can be impacted by impurities, such as Na and P residues that result from their use as structure-directing reagent in the hydrothermal synthesis of the ceria nanocrystals. This observation calls for precaution in interpreting the catalytic behavior of nanoshaped ceria where trace impurities may be present.
Au15(SR)13 is the smallest stable thiolated gold nanocluster experimentally identified so far, and its elusive structure may hold the key to the origin of the nucleus in the formation of thiolated ...gold nanoclusters. By an extensive exploration of possible isomers by density functional theory, we arrive at a novel structure for Au15(SR)13 with high stability and whose optical absorption characteristics match those of the experiment. Different from the previous structures and the prevailing working hypothesis about the construction of thiolated gold nanoclusters, the Au15(SR)13 model features a cyclic Au(I)-SR pentamer interlocked with one staple trimer motif protecting the tetrahedral Au4 nucleus, together with another trimer motif. This structure suggests that Au15(SR)13 is a transitional composition from an Au(I)-SR x polymer such as Au10(SR)10 to larger Au n (SR) m (n > m) clusters that have only the staple motifs and that the nucleation process starts from the Au4 core.
Methanol has been considered as a “smart” molecule in studying the surface sites of metal oxide catalysts. In this work, methanol was utilized to probe the nature of surface sites of ceria ...nanocrystals with defined surface planes (nanoshapes), including rods (containing {110}), cubes ({100}), and octahedra ({111}). The adsorption and desorption of methanol were followed by in situ IR and Raman spectroscopy as well as mass spectrometry. Upon methanol adsorption at room temperature, on-top, bridging and three-coordinate methoxy species are formed on the surface of rods and cubes, whereas only on-top methoxy is present on the octahedra surface. The distribution of the methoxy species is believed to be determined not only by the coordination status of surface Ce cations but also by the number of defect sites on the three nanoshapes. During the desorption process, the methoxy species are gradually dehydrogenated into H2 and CO via formate species as intermediates on the three ceria surfaces. A second intermediate, formyl species is also evident on the rods’ surface. The methoxy species are more reactive and less stable on the rods’ surface, which results in desorption of H2 and CO at lower temperature (<583 K) than on cubes and octahedra. A higher than stoichiometric H/CO ratio is observed in the methanol-TPD products, attributed to the retention of some amount of formate and carbonate species on the ceria nanoshapes, as revealed by in situ IR. A small amount of methanol and formaldehyde desorbs at low temperature (<423 K) on the three surfaces as a result of the disproportionation reaction of the methoxy species. The UV Raman and IR results indicate that the ceria nanoshapes are slightly reduced at room temperature upon methanol adsorption and become more reduced at higher temperatures during methanol desorption. The degree of reduction is found to be dependent on the surface structure of the ceria nanoshapes.
Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman ...spectroscopy and O2 adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on one-electron defect sites and peroxide on two-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphology. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which is of significance for ceria-catalyzed oxidation reactions.
We report an efficient electrochemical conversion of CO2 to CO on surface-activated bismuth nanoparticles (NPs) in acetonitrile (MeCN) under ambient conditions, with the assistance of ...1-butyl-3-methylimidazolium trifluoromethanesulfonate (bmimOTf). Through the comparison between electrodeposited Bi films (Bi-ED) and different types of Bi NPs, we, for the first time, demonstrate the effects of catalyst’s size and surface condition on organic phase electrochemical CO2 reduction. Our study reveals that the surface inhibiting layer (hydrophobic surfactants and Bi3+ species) formed during the synthesis and purification process hinders the CO2 reduction, leading to a 20% drop in Faradaic efficiency for CO evolution (FECO). Bi particle size showed a significant effect on FECO when the surface of Bi was air-oxidized, but this effect of size on FECO became negligible on surface-activated Bi NPs. After the surface activation (hydrazine treatment) that effectively removed the native inhibiting layer, activated 36-nm Bi NPs exhibited an almost-quantitative conversion of CO2 to CO (96.1% FECO), and a mass activity for CO evolution (MACO) of 15.6 mA mg–1, which is three-fold higher than the conventional Bi-ED, at −2.0 V (vs Ag/AgCl). This work elucidates the importance of the surface activation for an efficient electrochemical CO2 conversion on metal NPs and paves the way for understanding the CO2 electrochemical reduction mechanism in nonaqueous media.
CeO2 cubes with {100} facets, octahedra with {111} facets, and wires with highly defective structures were utilized to probe the structure-dependent reactivity of acetaldehyde. Using ...temperature-programmed desorption (TPD), temperature-programmed surface reactions (TPSR), and in situ infrared spectroscopy, it was determined that acetaldehyde desorbs unreacted or undergoes reduction, coupling, or C–C bond scission reactions, depending on the surface structure of CeO2. Room-temperature FTIR indicates that acetaldehyde binds primarily as η1-acetaldehyde on the octahedra, in a variety of conformations on the cubes, including coupling products and acetate and enolate species, and primarily as coupling products on the wires. The percent consumption of acetaldehyde ranks in the following order: wires > cubes > octahedra. All the nanoshapes produce the coupling product crotonaldehyde; however, the selectivity to produce ethanol ranks in the following order: wires ≈ cubes ≫ octahedra. The selectivity and other differences can be attributed to the variation in the basicity of the surfaces, defects densities, coordination numbers of surface atoms, and the reducibility of the nanoshapes.
This study reports and compares the adsorption and dissociation of water on oxidized and reduced CeO2(100) and CeO2(111) thin films. Water adsorbs dissociatively on both surfaces. On fully oxidized ...CeO2(100) the resulting surface hydroxyls are relatively stable and recombine and desorb as water over a range from 200 to 600 K. The hydroxyls are much less stable on oxidized CeO2(111), recombining and desorbing between 200 and 300 K. Water produces 30% more hydroxyls on reduced CeO1.7(100) than on oxidized CeO2(100). The hydroxyl concentration increases by 160% on reduced CeO1.7(111) compared to oxidized CeO2(111). On reduced CeO1.7(100) most of the hydroxyls still recombine and desorb as water between 200 and 750 K. Most of the hydroxyls on reduced CeO1.7(111) react to produce H2 at 560 K, leaving O on the surface. A relatively small amount of H2 is produced from reduced CeO1.7(100) between 450 and 730 K. The differences in the adsorption and reaction of water on CeO X (100) and CeO X (111) are attributed to different adsorption sites on the two surfaces. The adsorption site on CeO2(100) is a bridging site between two Ce cations. This adsorption site does not change when the ceria is reduced. The adsorption site on CeO2(111) is atop a single Ce cation, and the proton is transferred to a surface O in a site between three Ce cations. When the CeO X (111) is reduced, vacancy sites are produced which allows the water to adsorb and dissociate on the 3-fold Ce cation sites.
The temperature-dependent adsorption and reaction of acetaldehyde (CH3CHO) on a fully oxidized and a highly reduced thin-film CeO2(111) surface have been investigated using a combination of ...reflection–absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT+U) calculations. On the fully oxidized surface, acetaldehyde adsorbs weakly through its carbonyl O interacting with a lattice Ce4+ cation in the η1-O configuration. This state desorbs at 210 K without reaction. On the highly reduced surface, new vibrational signatures appear below 220 K. They are identified by RAIRS and DFT as a dimer state formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules. This dimer state remains up to 400 K before decomposing to produce another distinct set of vibrational signatures, which are identified as the enolate form of acetaldehyde (CH2CHO¯). Furthermore, the calculated activation barriers for the coupling of acetaldehyde, the decomposition of the dimer state, and the recombinative desorption of enolate and H as acetaldehyde are in good agreement with previously reported TPD results for acetaldehyde adsorbed on reduced CeO2(111) Chen et al. J. Phys. Chem. C 2011, 115, 3385. The present findings demonstrate that surface oxygen vacancies alter the reactivity of the CeO2(111) surface and play a crucial role in stabilizing and activating acetaldehyde for coupling reactions.
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