Search

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can ...

Successful synthesis of some perovskite oxyhydrides and their unique catalytic properties have recently attracted researchers’ attention. However, their surface structure remains unclear. Here we i...

Hydrogen adsorption on transition-metal oxides can lead to surface metallization, but it is unclear how this change of surface property impacts the subsequent surface chemistry and catalysis. Herein, we investigate from the first-principles density functional theory how hydrogen-induced metallization affects the adsorption of common intermediates during the conversion of oxygenates and alkanes (such as alkoxy and alkyl groups) on the SrTiO3(001) surface. We find that alkoxy adsorption is greatly enhanced by hydrogen-induced metallization and the strength increases with the hydrogen coverage. This is because hydrogen adsorption leads to electron donation to the Ti site, which then forms a bond with the alkoxy group. The enhancement is found to be nonlocal. The same chemistry is also responsible for the enhanced adsorption of alkyl groups on the Ti site, which renders a switch in the site preference from the O site on pristine SrTiO3(001) to the Ti site on hydrogen-functionalized SrTiO3(001). These insights...

As petroleum-related products dominate the global fuel market, acid gas resistant materials are in demand. ZnO is one of the widely used materials as SO2 sorbent and catalyst/catalyst support. To u...

The mixed-ligand metal–organic framework (MOF) DMOF-1 has been successfully synthesized via ligand: 2,3,5,6-tetramethylterephthalic acid (TM), 9,10-anthracenedicarboxylic acid (ADC), 1,4-naphthalen...

Hydrogen interaction with the TiO2 surfaces is important in many catalytic and photocatalytic reactions. However, the mechanisms of H2 activation on TiO2 surfaces remain unclear. Here, we study H2 ...

Perovskites are interesting materials for catalysis due to their great tunability. However, the correlation of many reaction processes to the termination of a perovskite surface is still unclear. I...

Propylene epoxidation with molecular oxygen has been proposed as a green and alternative process to produce propylene oxide (PO). In order to develop catalysts with high selectivity, high conversion, and long stability for the direct propylene epoxidation with molecular oxygen, understanding of catalyst structure and reactivity relationships is needed. Here, we combined atomic layer deposition and deposition precipitation to synthesize series of well-defined Au-based catalysts to study the catalyst structure and reactivity relationships for propylene epoxidation at 373 K. We showed that by decorating TiO2 on gold surface the inverse TiO2/Au/SiO2 catalysts maintained ∼90% selectivity to PO regardless of the weight loading of the TiO2. The inverse TiO2/Au/SiO2 catalysts exhibited improved regeneration compared to Au/TiO2/SiO2. The inverse TiO2/Au/SiO2 catalysts can be regenerated in 10% oxygen at 373 K, while the Au/TiO2/SiO2 catalysts failed to regenerate at as high as 473 K. Combined characterizations of ...

The surface properties of CeO2, Pr-CeO2, and 5% and 15% Cu-doped Pr-CeO2 were investigated using methanol as a probe molecule through adsorption and desorption studies carried out using in situ DRIFTS. It was revealed that the surfaces of the 5% and 15% Cu materials were dominated by reduced cations/vacancies and that the 15% Cu material contained the highest concentration of these active species. The high oxygen storage capacity (OSC) of the 15% Cu material, as determined using TGA, reflects the available vacant sites for oxygen adsorption. Formates were formed on all materials, with those formed on the Cu-doped materials present at temperatures as low as 25 °C, hence showing their superior reactivity toward methoxy oxidation. During formate dehydrogenation, H2, CO, CO2, and H2O evolved as the surface cations were simultaneously reduced. It was also observed that, for the Cu-containing materials, H2 was not formed and the high surface mobility determined through isotopic exchange simultaneously generated...

The behavior of metal–organic frameworks (MOFs) in the presence of acid gases may be decisive in their suitability for industrial applications. In this study, MIL-125 and MIL-125-NH2 were investigated with SO2 exposure in dry, humid, and aqueous environments. MIL-125 was found to be unstable in both humid and aqueous acidic environments, while MIL-125-NH2 was stable under these exposure conditions, showing no change in textural properties or visual degradation, as observed through SEM. Both materials were stable in the presence of water and dry SO2, suggesting that the reaction of these molecules to form an acidic species is likely a key factor in the degradation of MIL-125. In situ IR experiments confirmed the presence of sulfite species, supporting the hypothesis that the presence of an acidic sulfur species likely leads to the degradation of the MIL-125 structure. Computational investigation of several potential reaction mechanisms in MIL-125 indicated reactions involving the bisulfite ion are favored ...

The effects of the surface structure of ceria (CeO2) on the nature, strength, and amount of species resulting from SO2 adsorption were studied using in situ IR and Raman spectroscopies coupled with mass spectrometry, along with first-principles calculations based on density functional theory (DFT). CeO2 nanocrystals with different morphologies, namely, rods (representing a defective structure), cubes (100 facet), and octahedra (111 facet), were used to represent different CeO2 surface structures. IR and Raman spectroscopic studies showed that the structure and binding strength of adsorbed species from SO2 depend on the shape of the CeO2 nanocrystals. SO2 adsorbs mainly as surface sulfites and sulfates at room temperature on CeO2 rods, cubes, and octahedra that were either oxidatively or reductively pretreated. The formation of sulfites is more evident on CeO2 octahedra, whereas surface sulfates are more prominent on CeO2 rods and cubes. This is explained by the increasing reducibility of the surface oxyge...

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 bas...

A series of new heterobimetallic zeolites has been synthesized by incorporating a secondary metal cation M (Sc3+, Fe3+, In3+, and La3+) in Cu-exchanged ZSM-5, zeolite-β, and SSZ-13 zeolites under c...

The structure of vanadium oxide species supported on ceria (VOx/CeO2) was investigated under various conditions by in situ multiwavelength Raman spectroscopy, IR spectroscopy, isotopic labeling, and temperature-programmed reduction (TPR). For the first time, the detailed structure of dehydrated VOx species was revealed on the polycrystalline ceria support. VOx species can coexist on ceria surface in the structure of monomer, dimer, trimer, polymer, crystalline V2O5, and CeVO4 as a function of VOx loading. These species interact strongly with both the defect sites and labile surface oxygen of ceria, passivating the redox property of ceria. Under ambient condition, the dispersed VOx species are hydrated into polyvanadate species that can be reversibly dehydrated back to the original structure forms. The ceria support with defect sites facilitates the interaction between water (H218O) and V16Ox species, leading to very facile isotopic oxygen exchange between the two even at room temperature. During H2 reduct...

Herein, we utilize surface enhanced Raman spectroscopy (SERS) for the in situ analyses of catalyst structure while operating at elevated temperatures in various atmospheres. In order to accomplish this, robust SERS substrates were generated by depositing an ultrathin protective coating of alumina on top of silver nanowires (NWs) via atomic layer deposition (ALD). In situ studies were then conducted by analyzing the effects of heating a solid acid, phosphotungstic acid (PTA), on the alumina surface in either an oxygen or hydrogen environment at temperatures up to 400 °C. Interestingly, the distance-dependent decay of the enhancement factor of the SERS signal from the underlying NWs allowed us to probe with great detail the interfacial region between the PTA and the alumina surface. The ability to analyze the area closest to the alumina surface was further confirmed by assembling vanadia onto the substrate and monitoring the intensity differences between the V−O−Al and outer V ═ O bonds.

The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V═O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitat...

The nature and role of different Au species on a Au/SiO2 catalyst in room temperature (rt) CO oxidation have been studied by operando diffuse reflectance infrared spectroscopy (DRIFT) coupled with quadruple mass spectrometry (QMS). It has shown that different pretreatments (oxidative and reductive) of Au/SiO2 have a significant effect on the nature of Au species and thus the CO oxidation performance. High temperature (500 °C) O2-treatment leads to cationic Au species which is inactive for rt CO oxidation. Reductive treatment (either H2 or CO) results in metallic Au species that are immediately active for rt CO oxidation. Furthermore, CO oxidation activity is found in good correlation with the reduction degree of Au species, a clear indication of the essential role of metallic Au species played in rt CO oxidation. The accompanying slight deactivation with the oxidation of metallic Au species on reductively treated Au/SiO2 in CO oxidation suggests that cationic Au species may play a negative role in rt CO o...

Two interesting topics about supported vanadia catalysts were studied using in situ UV and visible Raman and UV-vis diffuse reflectance spectroscopy (DRS): The quantification of different surface vanadia species and the hydrogen reduction of these vanadia species. Using the diffuse reflectance value as an external standard, we could correct the Raman intensity measurements of V/{theta}-Al{sub 2}O{sub 3} for the self-absorption effect. On the basis of the ability to selectively detect monovanadate (UV-excited), polyvanadate (visible-excited), and V{sub 2}O{sub 5} (visible-excited) in the Raman measurements, the distribution of monovanadate, polyvanadate, and V{sub 2}O{sub 5} present on dehydrated V/{theta}-Al{sub 2}O{sub 3} samples was successfully quantified as a function of surface VO{sub x} density. It is shown that monovanadate species are present at all surface VO{sub x} densities studied but are the dominant species at low surface VO{sub x} density. Polyvanadate and V{sub 2}O{sub 5} are also present and predominate on the surface at intermediate and high surface VO{sub x} density. The UV- and visible-excited Raman studies of the V/{theta}-Al{sub 2}O{sub 3} samples reduced in hydrogen show that polyvanadate and V{sub 2}O{sub 5} are more easily reduced than monovanadate species. UV Raman is better able to obtain information on reduced vanadia species than visiblemore » Raman, mainly because of a decrease in self-absorption and resonance enhancement in the UV region. Comparison of the UV Raman spectra from reduced V/{theta}-Al{sub 2}O{sub 3} with bulk vanadium oxide compounds suggests that reduced VO{sub x} species can assume a V{sub 2}O{sub 3}-like form. The reduced VO{sub x} species redisperse on the support surface upon reoxidation.« less