Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO₃ Nanocrystals

To gain an in-depth understanding of the surface properties relevant for catalysis using ternary oxides, we report the acid–base pair reactivity of shape-controlled SrTiO₃ (STO) nanocrystals for the dehydrogenation of ethanol. Cubes, truncated cubes, dodecahedra, and etched cubes of STO with varying ratios of (001) and (110) crystal facets were synthesized using a hydrothermal method. Low-energy ion scattering (LEIS) analysis revealed that the (001) surface on cubes of STO is enriched with SrO due to surface reconstruction, resulting in a high ratio of strong base sites. Chemical treatment with dilute nitric acid to form etched cubes of STO resulted in a surface enriched with Ti cations and strong acidity. Furthermore, the strength and distribution of surface acidic sites increase with the ratio of (110) facet from cubes to truncated cubes to dodecahedra for STO. Kinetic, isotopic, and spectroscopy methods show that the dehydrogenation of ethanol proceeds through the facile dissociation of the alcohol group, followed by the cleavage of the C_α–H bond, which is the rate-determining step. Co-feeding of various probe molecules during catalysis, such as NH₃, 2,6-di-<i>tert</i>-butylpyridine, CO₂, and SO₂, reveals that a pair of Lewis acid site and basic surface oxygen atom is involved in the dehydrogenation reaction. The surface density of acid–base site pairs was measured using acetic acid as a probe molecule, allowing initial acetaldehyde formation turnover rates to be obtained. Comparison among various catalysts reveals no simple correlation between ethanol turnover rate and the percentage of either surface facet ((001) or (110)) of the STO nanocrystals. Instead, the reaction rate is found to increase with the strength of acid sites but reversely with the strength of base sites. The acid–base property is directly related to the surface composition as a result from different surface reconstruction behaviors of the shaped STO nanocrystals. The finding in this work underscores the importance of characterizing the top surface compositions and sites properties when assessing the catalytic performance of shape-controlled complex oxides such as perovskites

Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia

The role of Lewis and Brønsted sites in the dehydration of glycerol on niobium oxide and Na⁺-exchanged niobium oxide is investigated using FTIR spectroscopy supported by DFT calculations. Glycerol is impregnated on the catalysts at room temperature using an <i>ex-situ</i> method. Under high vacuum conditions, glycerol forms a stable multidentate alkoxy species through its primary hydroxyl groups with the Lewis sites. When coordinated this way, the primary C–O bonds are polarized, favoring dehydration in this position to form hydroxyacetone. In contrast, dehydration of the secondary alcohol group is kinetically favored over Brønsted acid sites in the absence of steric constraints. The primary product of this reaction, 1,3-propenediol, is further dehydrated to acrolein. When more than a monolayer of glycerol is impregnated on niobia, monoaromatic compounds are also formed on the surface upon heating

Important Roles of Enthalpic and Entropic Contributions to CO₂ Capture from Simulated Flue Gas and Ambient Air Using Mesoporous Silica Grafted Amines

The measurement of isosteric heats of adsorption of silica supported amine materials in the low pressure range (0–0.1 bar) is critical for understanding the interactions between CO₂ and amine sites at low coverage and hence to the development of efficient amine adsorbents for CO₂ capture from flue gas and ambient air. Heats of adsorption for an array of silica-supported amine materials are experimentally measured at low coverage using a Calvet calorimeter equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized silica materials, higher amine densities resulted in higher isosteric heats of adsorption, clearly showing that the density/proximity of amine sites can influence the amine efficiency of adsorbents. In a series of materials with fixed amine loading but different amine types, strongly basic primary and secondary amine materials are shown to have essentially identical heats of adsorption near 90 kJ/mol. However, the adsorption uptakes vary substantially as a function of CO₂ partial pressure for different primary and secondary amines, demonstrating that entropic contributions to adsorption may play a key role in adsorption at secondary amine sites, making adsorption at these sites less efficient at the low coverages that are important to the direct capture of CO₂ from ambient air. Thus, while primary amines are confirmed to be the most effective amine types for CO₂ capture from ambient air, this is not due to enhanced enthalpic contributions associated with primary amines over secondary amines, but may be due to unfavorable entropic factors associated with organization of the second alkyl chain on the secondary amine during CO₂ adsorption. Given this hypothesis, favorable entropic factors may be the main reason primary amine based adsorbents are more effective under air capture conditions

Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO₂ nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO₂. The two TiO₂ nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO₂ surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C–H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD₃OH and CD₃OD), confirming that the cleavage of the C–H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO₂ nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO₂ is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface

Acid–Base Reactivity of Perovskite Catalysts Probed via Conversion of 2‑Propanol over Titanates and Zirconates

Although perovskite catalysts are well-known for their excellent redox property, their acid–base reactivity remains largely unknown. To explore the potential of perovskites in acid–base catalysis, we made a comprehensive investigation in this work on the acid–base properties and reactivity of a series of selected perovskites, SrTiO₃, BaTiO₃, SrZrO₃, and BaZrO₃, via a combination of various approaches including adsorption microcalorimetry, in situ FTIR spectroscopy, steady state kinetic measurements, and density functional theory (DFT) modeling. The perovskite surfaces are shown to be dominated with intermediate and strong basic sites with the presence of some weak Lewis acid sites, due to the preferred exposure of SrO/BaO on the perovskite surfaces as evidenced by low energy ion scattering (LEIS) measurements. Using the conversion of 2-propanol as a probe reaction, we found that the reaction is more selective to dehydrogenation over dehydration due to the dominant surface basicity of the perovskites. Furthermore, the adsorption energy of 2-propanol (Δ<i>H</i>_{<i>ads,</i>2<i>–propanol</i>}) is found to be related to both a bulk property (tolerance factor) and the synergy between surface acid and base sites. The results from in situ FTIR and DFT calculations suggest that both dehydration and dehydrogenation reactions occur mainly through the E_1cB pathway, which involves the deprotonation of the alcohol group to form a common alkoxy intermediate on the perovskite surfaces. The results obtained in this work pave a path for further exploration and understanding of acid–base catalysis over perovskite catalysts

Stability of Pt/γ-Al₂O₃ Catalysts in Lignin and Lignin Model Compound Solutions under Liquid Phase Reforming Reaction Conditions

The stability of a 1 wt % Pt/γ-Al₂O₃ catalyst was tested in an ethanol/water mixture at 225 °C and autogenic pressure, conditions at which it is possible to dissolve and depolymerize various kinds of lignin, and structural changes to the catalysts were studied by means of X-ray diffraction (XRD), ²⁷Al MAS NMR, N₂ physisorption, transmission electron microscopy (TEM), H₂ chemisorption, elemental analysis, thermogravimetric analysis-mass spectrometry (TGA-MS), and IR. In the absence of reactants the alumina support is found to transform into boehmite within 4 h, leading to a reduction in support surface area, sintering of the supported Pt nanoparticles, and a reduction of active metal surface area. Addition of aromatic oxygenates to mimic the compounds typically obtained by lignin depolymerization leads to a slower transformation of the support oxide. These compounds, however, were not able to slow down the decrease in dispersion of the Pt nanoparticles. Vanillin and guaiacol stabilize the aluminum oxide more than phenol, anisole, and benzaldehyde because of the larger number of oxygen functionalities that can interact with the alumina. Interestingly, catalyst samples treated in the presence of lignin showed almost no formation of boehmite, no reduction in support or active metal surface area, and no Pt nanoparticle sintering. Furthermore, in the absence of lignin-derived aromatic oxygenates, ethanol forms a coke-like layer on the catalyst, while oxygenates prevent this by adsorption on the support by coordination via the oxygen functionalities

Direct Neutron Spectroscopy Observation of Cerium Hydride Species on a Cerium Oxide Catalyst

Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H₂, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce–H species upon H₂ dissociation over ceria via <i>in situ</i> inelastic neutron scattering spectroscopy. Combined with <i>in situ</i> ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H₂ over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce–H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved

Quantitative Analysis of the Morphology of {101} and {001} Faceted Anatase TiO₂ Nanocrystals and Its Implication on Photocatalytic Activity

The atomistic structure and morphology (shape and size) of nanomaterials have strong influences on their physical and chemical properties. However, many characterization techniques focus exclusively on one length-scale regime or another when developing quantitative morphology/structural models. In this article, we demonstrate that powder X-ray diffraction and neutron pair distribution function (PDF) can be used to obtain accurate average morphology and atomistic structure of {001} and {101} faceted anatase TiO₂ nanocrystals based on differential evolution refinements using Debye scattering equation calculations. It is also demonstrated that the morphology polydispersity of TiO₂ nanocrystals can be effectively obtained from the diffraction data via a numerical refinement routine. The morphology refinement results are in good agreement with those from transmission electron microscopy and the modeling of small angle neutron scattering data. This method is successfully used to quantify the facet-specified photocatalytic hydrogen evolution activity of anatase TiO₂ nanocrystals with different {001} to {101} ratios. It is found that the sample with an intermediate amount of both {001} and {101} facets shows the best photocatalytic hydrogen evolution reaction (HER) activity. It is expected that the simultaneous structure and morphology refinement technique can be generally used to study the relationship between morphology and functionality of nanomaterials

Hydrolysis of Cellobiose over Selective and Stable Sulfonated Activated Carbon Catalysts

Activated carbon is functionalized by different treatments with sulfuric acid and hot liquid water and used as catalyst for the hydrolysis of cellobiose in a continuously operated fixed bed reactor. Characterization results reveal that the chemically treated materials are more disordered with a lower degree of graphitization, while adsorption isotherms demonstrate that van der Waals forces dominate the interaction between carbohydrates and the surface of catalysts. All catalysts are stable during the hydrolysis of cellobiose under flow conditions. Carbon catalysts with a limited fraction of sulfonic acid groups exhibit moderate cellobiose conversion but a higher and sustained glucose selectivity. The high selectivity is attributed to a higher fraction of weak acid sites, where degradation of glucose only occurs to a limited extent due to less accessibility and competitive adsorption with cellobiose. Furthermore, the strong sulfonic acid groups are more accessible for degradation reactions to occur. In contrast, the catalyst with a higher fraction of sulfonic acid groups shows increased cellobiose conversion but decreased glucose selectivity because glucose monomers can be converted to degradation products at these sites

Toward the Design of a Hierarchical Perovskite Support: Ultra-Sintering-Resistant Gold Nanocatalysts for CO Oxidation

An ultrastable Au nanocatalyst based on a heterostructured perovskite support with high surface area and uniform LaFeO₃ nanocoatings was successfully synthesized and tested for CO oxidation. Strikingly, small Au nanoparticles (4–6 nm) are obtained after calcination in air at 700 °C and under reaction conditions. The designed Au catalyst not only possessed extreme sintering resistance but also showed high catalytic activity and stability because of the strong interfacial interaction between Au and the heterostructured perovskite support