Shape Effect of Pd-Promoted Ga₂O₃ Nanocatalysts for Methanol Synthesis by CO₂ Hydrogenation

In this paper, we present a new approach to investigate metal–support interaction in catalysis. First, we have carried out a controlled growth of two semiconductive Ga₂O₃ nanocrystals in distinctive shapes, namely, plate and rod with the majority of their surfaces covered with polar and nonpolar facets, respectively. We have then placed the same contents of Pd on these nanocrystals and carried out a systematic testing and characterization for methanol synthesis from CO₂ hydrogenation under industrial applicable conditions. It is found that a low indexed (002) polar Ga₂O₃ surface is highly unstable, which gives oxygen defects and mobile electrons in the conduction band more readily than those nonpolar (111) and (110) surfaces. A significantly strong metal–support interaction between the (002) polar Ga₂O₃ surface and Pd was determined, and it gave rise to higher metal dispersion and facilitated electron transfer between them, leading to the formation of PdGa_<i>x</i>. This renders such composite nanocatalysts active for methanol production

Dramatic Effects of Gallium Promotion on Methanol Steam Reforming Cu–ZnO Catalyst for Hydrogen Production: Formation of 5 Å Copper Clusters from Cu–ZnGaO_<i>x</i>

A new class of copper, zinc, and gallium mixed oxides (CuZnGaO_<i>x</i>) with different chemical compositions obtained by a coprecipitation technique is identified as a highly active catalyst for the low-temperature, direct steam reforming of methanol to supply hydrogen gas to portable fuel cell devices. Their catalytic activity and selectivity are found to be critically dependent on the copper surface area, catalyst structure, and metal–support interaction, etc. As a result, temperature-programmed reduction has been used to investigate the copper ion reducibility and resulting copper speciation; N₂O chemisorption and advanced microscopies to determine specific copper surface area, dispersion, and particle size; XRD to investigate the catalyst structure; EPR spectroscopy to probe the environment of Cu²⁺ species; and AC impedance spectroscopy to probe the mobility of trapped ions in solids. It is proposed that Ga incorporation into Cu–Zn oxide leads to the formation of a nonstoichiometric cubic spinel phase containing interstitial Cu⁺ ions, which can produce in situ a high population of extremely small 5 Å copper clusters at high dispersion on a defective ZnGa₂O₄ surface for effective catalysis

Morphology-Controlled Synthesis of Au/Cu₂FeSnS₄ Core–Shell Nanostructures for Plasmon-Enhanced Photocatalytic Hydrogen Generation

Copper-based chalcogenides of earth-abundant elements have recently arisen as an alternate material for solar energy conversion. Cu₂FeSnS₄ (CITS), a quaternary chalcogenide that has received relatively little attention, has the potential to be developed into a low-cost and environmentlly friendly material for photovoltaics and photocatalysis. Herein, we report, for the first time, the synthesis, characterization, and growth mechanism of novel Au/CITS core–shell nanostructures with controllable morphology. Precise manipulations in the core–shell dimensions are demonstrated to yield two distinct heterostructures with spherical and multipod gold nanoparticle (NP) cores (Au_sp/CITS and Au_mp/CITS). In photocatalytic hydrogen generation with as-synthesized Au/CITS NPs, the presence of Au cores inside the CITS shell resulted in higher hydrogen generation rates, which can be attributed to the surface plasmon resonance (SPR) effect. The Au_sp/CITS and Au_mp/CITS core–shell NPs enhanced the photocatalytic hydrogen generation by about 125% and 240%, respectively, compared to bare CITS NPs

Direct Catalytic Conversion of Biomass-Derived Furan and Ethanol to Ethylbenzene

Herein, we report a synthetic strategy to convert biomass-derived unsubstituted furan to aromatics at high selectivity, especially to ethylbenzene via alkylation/Diels–Alder cycloaddition using ethanol, while greatly reducing the formation of the main side product, benzofuran, over zeolite catalysts. Using synchrotron X-ray powder diffraction and first-principles calculations, it is shown that the above methodology favors the formation of aromatic products due to ready alkylation of furan by the first ethanol molecule, followed by Diels–Alder cycloaddition with ethylene derived from the second ethanol molecule on a Brønsted acid site in a one-pot synthesis. This gives a double-promoting effect: an alkyl substituent(s) on furan creates steric hindrance to inhibit self-coupling to benzofuran while an alkylated furan (diene) undergoes a Diels–Alder reaction more favorably due to higher HOMO energy

Quantitative Differences in Sulfur Poisoning Phenomena over Ruthenium and Palladium: An Attempt To Deconvolute Geometric and Electronic Poisoning Effects Using Model Catalysts

Sulfur poisoning over noble-metal catalysts has traditionally been regarded as very complex and precluding from easy rational understanding, because of the problems of interference from using different supports, inability of controlling coverage due to nonuniform metal particle size, intrinsic size/shape effect of metal component, etc. Here, high-quality polyvinylpyrrolidone (PVP) polymer-supported ruthenium and palladium model nanocatalysts using without solid support are equivalently modified with preadsorbed mercaptoethanol over a range of surface concentrations in order to compare sulfur poisoning effects on the two important noble metals commonly used in industry. A typical consecutive hydrogenation reactions of alkyne to alkene and then to alkane is studied under mild reaction conditions in the liquid phase. The first stage alkyne hydrogenation is well-known to be <i>surface insensitive</i>, because of strong adsorption of alkyne on both metals. However, the second stage, <i>surface-sensitive</i> hydrogenation/isomerization of weakly adsorbed alkenes, is highly influenced by perturbations in metal surface electronic states induced by sulfur adsorbates. Using a combination of ¹³C NMR, Fourier transform infrared (FTIR) measurements of chemisorbed CO, kinetic products analysis and density functional theory (DFT) calculations, the electronic and geometric components of sulfur poisoning can be assigned in an almost-quantitative manner for the first time, over these two metal nanocatalysts. It is found that this sulfur adsorbate dwells preferentially on terrace sites for both metals at high coverage, causing <i>deactivation by surface site blockage</i> for the alkyne hydrogenation. The adsorbate can also deplete electron density from the metal surface (mixing with higher vacant band states of sulfur). As a result, reduction in adsorption strength for alkenes in the second-stage hydrogenation, leading to <i>deactivation by electronic effects</i>, is observed. This component is shown to contribute more significantly to the total deactivation for palladium (electron-rich metal) than ruthenium (electron-poor metal). At 60% sulfur coverage on Pd, the electronic contribution to surface adsorption can be totally cancelled out. This work clearly shows that the differing nature of metals can result in very different degrees of geometric and electronic deactivation upon sulfur adsorption over a size range of 2–3 nm without any interference from solid support, particle size/shape variations, giving important insights to developing more sulfur-tolerant catalysts in the future

Entrapped Single Tungstate Site in Zeolite for Cooperative Catalysis of Olefin Metathesis with Brønsted Acid Site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS–MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and <i>trans</i>-2-butene to propene, under mild reaction conditions, the propene production rate over WO_<i>x</i>/USY is ca. 7300 times that over the industrial WO₃/SiO₂-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO_<i>x</i>/USY for a wide range of reaction conditions

CO₂ Hydrogenation to Methanol over Catalysts Derived from Single Cationic Layer CuZnGa LDH Precursors

Ultrathin (1–3 cationic-layers) (CuZn)_1–<i>x</i>Ga_<i>x</i>-CO₃ layered double hydroxide (LDH) nanosheets were synthesized following the aqueous miscible organic solvent treatment (AMOST) method and applied as catalyst precursors for methanol production from CO₂ hydrogenation. It is found that, upon reduction, the aqueous miscible organic solvent treated LDH (AMO-LDH) samples above a critical Ga³⁺ composition give consistently and significantly higher Cu surface areas and dispersions than the catalysts prepared from conventional hydroxyl-carbonate phases. Owing to the distinctive local steric and electrostatic stabilization of the ultrathin LDH structure, the newly formed active Cu­(Zn) metal atoms can be stably embedded in the cationic layers, exerting an enhancement to the catalytic reaction. The best catalyst in this study displayed methanol productivity with a space-time yield of 0.6 g_MeOH·g_cat^–1 h^–1 under typical reaction conditions, which, as far as we are aware, is higher than most reported Cu-based catalysts in the literature