P‐type Cu2O has been long considered as an attractive photocatalyst for photocatalytic water reduction, but few successful examples has been reported. Here, we report the synthesis of TiO2 ...(core)/Cu2O (ultrathin film shell) nanorods by a redox reaction between Cu2+ and in‐situ generated Ti3+ when Cu2+‐exchanged H‐titanate nanotubes are calcined in air. Owing to the strong TiO2‐Cu2O interfacial interaction, TiO2 (core)/Cu2O (ultrathin film shell) nanorods are highly active and stable in photocatalytic water reduction. The TiO2 core and Cu2O ultrathin film shell respectively act as the photosensitizer and cocatalyst, and both the photoexcited electrons in the conduction band and the holes in the valence band of TiO2 respectively transfer to the conduction band and valence band of the Cu2O ultrathin film shell. Our results unambiguously show that Cu2O itself can act as the highly active and stable cocatalyst for photocatalytic water reduction.
TiO2/Cu2O core/ultrathin shell nanorods result in a strong TiO2–Cu2O interfacial interaction that facilitates the interfacial charge transfer process and stabilizes the Cu2O shell, leading to an efficient photocatalyst for water reduction.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
Supported metal nanoparticles are of universal importance in many industrial catalytic processes. Unfortunately, deactivation of supported metal catalysts via thermally induced sintering is ...a major concern especially for high-temperature reactions. Here, we demonstrate that the particle distance as an inherent parameter plays a pivotal role in catalyst sintering. We employ carbon black supported platinum for the model study, in which the particle distance is well controlled by changing platinum loading and carbon black supports with varied surface areas. Accordingly, we quantify a critical particle distance of platinum nanoparticles on carbon supports, over which the sintering can be mitigated greatly up to 900 °C. Based on in-situ aberration-corrected high-angle annular dark-field scanning transmission electron and theoretical studies, we find that enlarging particle distance to over the critical distance suppress the particle coalescence, and the critical particle distance itself depends sensitively on the strength of metal-support interactions.
Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2‐ and H2‐rich reformates, but suffer from continuous deactivation during reaction. This limitation ...can be overcome through the fabrication of highly active and non‐deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2‐{100} and Ru/TiO2‐{101} are very stable, while Ru/TiO2‐{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal–support interactions (MSIs). The stronger MSIs on the defect‐rich TiO2‐{100} and TiO2‐{101} supports stabilize flat Ru nanoparticles, while on TiO2‐{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2‐{001}.
Keeping it flat: Morphology‐engineered TiO2‐{100} and TiO2‐{101} nanocrystal supports can stabilize flat Ru nanoparticles, resulting in a very stable activity of the Ru/TiO2 catalysts for the selective CO methanation. Weaker metal–support interactions on the TiO2‐{001} nanocrystals result in a shape change of the Ru nanoparticles, from flat to hemispherical, together with continuous deactivation.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Pd nanocrystals were prepared by the reduction of a H2PdCl4 aqueous solution with C2H4 in the presence of different amounts of poly(N-vinyl-2-pyrrolidone) (PVP). Their average size decreases ...monotonically as the PVP monomer/Pd molar ratio increases up to 1.0 and then does not vary much at higher PVP monomer/Pd molar ratios. Infrared spectroscopy and X-ray photoelectron spectroscopy results reveal the interesting size-dependent interaction of PVP molecules with Pd nanocrystals. For fine Pd nanocrystals capped with a large number of PVP molecules, each PVP molecule chemisorbs with its oxygen atom in the ring; for large Pd nanocrystals capped by a small number of PVP molecules, each PVP molecule chemisorbs with both the oxygen atom and nitrogen atom in the ring, which obviously affects the structure of chemisorbed PVP molecules and even results in the breaking of involved C–N bonds of some chemisorbed PVP molecules. Charge transfer always occurs from a chemisorbed PVP ligand to Pd nanocrystals. These results provide novel insights into the PVP–metal nanocrystal interaction, which are of great importance in the fundamental understanding of surface-mediated properties of PVP-capped metal nanocrystals.
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IJS, KILJ, NUK, PNG, UL, UM
We have prepared a series of Au–Pd/SiO₂ catalysts by the routine deposition–precipitation method followed by calcination at 200°C in air for 4h. The structures of these catalysts have been ...characterized with powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and H₂-temperature-programmed reduction. Their catalytic activity in CO oxidation has also been studied. We found that the preparation procedure leads to the formation of highly dispersed PdO supported on SiO₂ in Pd/SiO₂, but to the formation of both highly dispersed PdO and AuₓPd alloy nanoparticles in Au–Pd/SiO₂ catalysts. The fraction of metallic Pd in Pd species increases with the increase of Au:Pd molar ratio in Au–Pd/SiO₂ catalysts. We proposed that the alloying of Au and Pd can promote the thermal decomposition of PdO supported on SiO₂. Because of the existence of metallic Pd, Au–Pd/SiO₂ catalysts are more active in CO oxidation than Pd/SiO₂. Au–Pd/SiO₂-0.5 exhibits the same activity as Pd/SiO₂-H₂ in CO oxidation. Our results provide some novel information on the fabrication of supported AuₓPd alloy nanoparticles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Catalysts for CO oxidation reaction are mainly based on oxide/hydroxide materials with multicomponent active sites. Here, we report a nonoxide/hydroxide material, atomically dispersed dual-metal ...single sites (Fe–Co sites) on N-doped carbon support, as a highly active catalyst for CO oxidation. It can greatly lower the temperature for complete CO conversion as low as −73 °C with a turnover frequency of 0.096 s–1. X-ray absorption near-edge structure spectra, pulse-adsorption microcalorimetry, and density functional theory studies show that the Fe–Co sites synergistically catalyze CO oxidation facilely following the Langmuir–Hinshelwood (L-H) mechanism with CO preferentially adsorbing at the Co sites and O2 adsorbing at the Fe sites. These results, for the first time, reveal that the dual-metal single site on N-doped carbon can efficiency catalyze low-temperature CO oxidation reaction without the involvement of supports, such as oxygen vacancies and surface hydroxyl groups.
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Abstract
H
2
O
2
is widely used as an oxidant for photocatalytic methane conversion to value-added chemicals over oxide-based photocatalysts under mild conditions, but suffers from low utilization ...efficiencies. Herein, we report that O
2
is an efficient molecular additive to enhance the utilization efficiency of H
2
O
2
by suppressing H
2
O
2
adsorption on oxides and consequent photogenerated holes-mediated H
2
O
2
dissociation into O
2
. In photocatalytic methane conversion over an anatase TiO
2
nanocrystals predominantly enclosed by the {001} facets (denoted as TiO
2
{001})-C
3
N
4
composite photocatalyst at room temperature and ambient pressure, O
2
additive significantly enhances the utilization efficiency of H
2
O
2
up to 93.3%, giving formic acid and liquid-phase oxygenates selectivities respectively of 69.8% and 97% and a formic acid yield of 486 μmol
HCOOH
·g
catalyst
−1
·h
−1
. Efficient charge separation within TiO
2
{001}-C
3
N
4
heterojunctions, photogenerated holes-mediated activation of CH
4
into ·CH
3
radicals on TiO
2
{001} and photogenerated electrons-mediated activation of H
2
O
2
into ·OOH radicals on C
3
N
4
, and preferential dissociative adsorption of methanol on TiO
2
{001} are responsible for the active and selective photocatalytic conversion of methane to formic acid over TiO
2
{001}-C
3
N
4
composite photocatalyst.
We have investigated the geometric and electronic structures of the cerium oxide (CeO2)-titanium dioxide (TiO2) mixed oxides with various Ce/TiO2 weight ratios prepared by the sol-gel method in ...detail by means of X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS), Raman spectroscopy excited by 325 and 514.5nm lasers, and scanning electron microscope (SEM). Existence of cerium effectively inhibits the phase transition of TiO2 from the anatase phase to the rutile phase. XRD peaks of TiO2 anatase attenuate continuously with the increasing amount of CeO2 in the mixed oxide, but the XRD peaks of cubic CeO2 appear only after the weight ratio of Ce/TiO2 reaches 0.50. The average crystalline sizes of TiO2 anatase and cubic CeO2 in CeO2-TiO2 mixed oxides are smaller than those in the corresponding individual TiO2 anatase and cubic CeO2. Raman spectroscopy excited by the 514.5nm laser detects CeO2 after the weight ratio of Ce/TiO2 reaches 0.70 whereas Raman spectroscopy excited by the 325nm laser detects CeO2 after the weight ratio of Ce/TiO2 reaches 0.90. XPS results demonstrate that Ti exists in the form of Ti4+ in the CeO2-TiO2 mixed oxide. Ce is completely in the form of Ce3+ in the mixed oxides with a 0.05 weight ratio of Ce/TiO2. With the increasing weight ratio of Ce/TiO2, Ce4+ dominates. On basis of these results, we proposed that CeO2 initially nucleates at the defects (oxygen vacancies) within TiO2 anatase, forming an interface bridged with oxygen between CeO2 and TiO2 anatase. At the interface, Ce species cannot substitute Ti4+ in the lattice of TiO2 anatase whereas Ti4+ can substitute Ce4+ in the lattice of cubic CeO2. The decreasing concentration of oxygen vacancies, the Ti-O-Ce interface, and the decreasing average crystalline size of TiO2 anatase act to inhibit the phase transformation of TiO2 anatase. With the increasing amounts of CeO2, the CeO2 clusters continuously grow and form cubic CeO2 nanocrystals. Spectroscopic results strongly demonstrate that the surface region of CeO2-TiO2 mixed oxide is enriched with TiO2.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Superhydrophilicity of TiO2 thin film under natural conditions is of great importance for antifogging and self-cleaning applications of glass products. Transparent superhydrophilic porous ZnO/TiO2 ...composite films were prepared on glass substrates using a template-free sol–gel method. The results indicated that the porous ZnO (10%)/TiO2 composite film possesses superhydrophilicity with water contact angle less than 5° without UV irradiation. Surface chemical composition of the films was characterized by X-ray photoelectron spectroscopy (XPS). The morphologies of the porous films were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The porous morphology, crystal structure and surface chemical composition were significantly affected by ZnO addition. The natural superhydrophilic performance was attributed to the synergistic effect of the porous structure and surface hydroxyl groups. Photocatalytic activities of the ZnO/TiO2 composites were also evaluated using methyl orange (MO) as a model pollutant, with 78.1% of MO degraded by ZnO (10%)/TiO2, which was higher than 49.3% obtained from pure TiO2.
•Macroporous ZnO/TiO2 films were synthesized using a template-free sol-gel method.•Surface wetting behavior was tested without UV irradiation.•The film with superhydrophilic and highly transparent.•Synergistic effects of porous structure and surface hydroxyl on superhydrophilicity.•Photocatalytic property was also studied under solar simulator irradiation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
It is well-known that water has a detrimental effect on the low-temperature methane combustion activity of palladium catalysts. However, when the transient activity (i.e., light-off or ...ignition–extinction experiments) of methane combustion catalysts is compared, the effects of water adsorption–desorption phenomena are seldom directly considered. While these effects are important to keep in mind when studying support-dependent methane combustion activity, they are crucial when selecting a catalyst diluent. In many cases, the water adsorption–desorption properties of “inert” reactor diluents may dominate the transient methane combustion activity of a Pd catalyst. In this contribution, we show how different catalyst pretreatments on various Pd catalysts can influence the presence of water and hydroxyl groups on the surface of catalyst supports, reactor diluents, and active phase, and how this process dramatically affects observed methane combustion activity. Transiently, alumina (both support and diluent), which strongly binds water that is produced in the reaction, can keep the PdO phase active and water-free. However, after alumina surfaces become saturated with water, the PdO surface also becomes hydroxylated, which decreases the catalyst’s methane combustion activity. Due to this time-dependent surface titration, care must be taken when comparing transient experiments between catalysts on different supports; comparable data for methane combustion must be collected while carefully checking for water adsorption on the surface of the catalyst and diluent. Finally, we propose that a channel for sustainable high combustion rates is possible if water is prevented from adsorbing on the highly active PdO surfaces.
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