Mechanistic Insight into the Interaction Between a Titanium Dioxide Photocatalyst and Pd Cocatalyst for Improved Photocatalytic Performance

Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+cations in aqueous environment. Implication on water splitting

Ce4+cations are commonly used as electron acceptors during the water oxidation to O2reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+cations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO2(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350-900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ce4+resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2(ca. 400 nm) with a time constantt= 4-5 ps. This indicated that the Ce4+cations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450-550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+at the surface of TiO2(110) and positively charged trapped holes. It was also found that at the very short time scale (<2-3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Ce4+cations. Results point out that the presence of Ce4+cations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to O2in the presence of Ce4+cations.

Highly selective oxidation of benzene to phenol with air at room temperature promoted by water

Phenol is one of the most important fine chemical intermediates in the synthesis of plastics and drugs with a market size of ca. $30b¹ and the commercial production is via a two-step selective oxidation of benzene, requiring high energy input (high temperature and high pressure) in the presence of a corrosive acidic medium, and causing serious environmental issues^2-5. Here we present a four-phase interface strategy with well-designed Pd@Cu nanoarchitecture decorated TiO₂ as a catalyst in a suspension system. The optimised catalyst leads to a turnover number of 16,000-100,000 for phenol generation with respect to the active sites and an excellent selectivity of ca. 93%. Such unprecedented results are attributed to the efficient activation of benzene by the atomically Cu coated Pd nanoarchitecture, enhanced charge separation, and an oxidant-lean environment. The rational design of catalyst and reaction system provides a green pathway for the selective conversion of symmetric organic molecules.

Enabling High Loading in Single-Atom Catalysts on Bare Substrate with Chemical Scissors by Saturating the Anchoring Sites

Atomically dispersed metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low to avoid the formation of metal nanoparticles, making it difficult to improve the overall activity. Diverse strategies based on creating more anchoring sites (ASs) have been adopted to elevate the loading density. One problem of such traditional methods is that the single atoms always gather together before the saturation of all ASs. Here, a chemical scissors strategy is developed by selectively removing unwanted metallic materials after excessive loading. Different from traditional ways, the chemical scissors strategy places more emphasis on the accurate matching between the strength of etching agent and the bond energies of metal-metal/metal-substrate, thus enabling a higher loading up to 2.02 wt% even on bare substrate without any pre-treatment (the bare substrate without any pre-treatment generally only has a few ASs for single atom loading). It can be inferred that by combining with other traditional methods which can create more ASs, the loading could be further increased by saturating ASs. When used for CH₃ OH generation via photocatalytic CO₂ reduction, the as-made single-atom catalyst exhibits impressive catalytic activity of 597.8 ± 144.6 µmol h^-1 g^-1 and selectivity of 81.3 ± 3.8%.

Photocatalytic hydrogen evolution over Pt–Pd dual atom sites anchored on TiO2 nanosheets

The defective TiO2 nanosheets (Vo-TiO2) supported dual atomic catalyst (Pt–Pd SAs/Vo-TiO2) to product hydrogen.

Edge-Rich Bicrystalline 1T/2H-MoS2 Cocatalyst-Decorated {110} Terminated CeO2 Nanorods for Photocatalytic Hydrogen Evolution

Developing all-solid-state Z-scheme systems with highly active photocatalysts are of huge interest in realizing long-term solar-to-fuel conversion. Here we reported an innovative hybrid of {110}-oriented CeO₂ nanorods with edge-enriched bicrystalline 1T/2H-MoS₂ coupling as efficient photocatalysts for water splitting. In the composites, the metallic 1T phase acts as an excellent solid state electron mediator in the Z-scheme, while the 2H phase and CeO₂ are the adsorption sites of the photosensitizer and reactant (H₂O), respectively. Through optimal structure and phase engineering, 1T/2H-MoS₂@CeO₂ heterojunctions simultaneously achieve high charge separation efficiency, proliferated density of exposed active sites, and excellent affinity to reactant molecules, reaching a superior hydrogen evolution rate of 73.1 μmol/h with an apparent quantum yield of 8.2% at 420 nm. Furthermore, density functional theory calculations show that 1T/2H-MoS₂@CeO₂ possesses the advantages of intensive electronic interaction from the built-in electric field (negative MoS₂ and positive charged CeO₂) and reduced H₂O adsorption/dissociation energies. This work sheds light on the design of on-demand noble-metal-free Z-scheme heterostructures for solar energy conversion.

Influence of Photo-Deposited Pt and Pd onto Chromium Doped TiO2 Nanotubes in Photo-Electrochemical Water Splitting for Hydrogen Generation

Hydrogen (H2) is considered as an ideal fuel for the future. The photo-electrochemical (PEC) water splitting employing semiconducting materials and induced irradiation, preferably of solar spectrum, presents a viable route for H2 production. In this work, self-ordered chromium-doped TiO2 nanotube (CT) was fabricated using in-situ electro-anodization. CT surface modification was then performed by photo-deposition of Pt and Pd particles, producing Pt-CT and Pd-CT catalysts, respectively. Their morphological features, crystallinity, surface composition, and optical absorption have been inspected by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), Raman, and UV–vis absorption spectroscopy. Linear sweep voltammetry, chronoamperometry, and open circuit potential methods have been applied to study PEC activities of Pt-CT and Pd-CT catalysts in a form of electrodes. It was found that Pt-CT and Pd-CT electrodes possess excellent photo-generated electron/hole (e−/h+) separation and transport properties. The enhanced photocurrent responses of 4 and 3 times more than that of CT are revealed for Pt-CT and Pd-CT, respectively. The activity of as-prepared Pt-CT and Pd-CT catalysts was then tested for H2 generation. The maximum amount of the evolved H2 followed decreasing order: 1.08 > 0.65 > 0.26 mL cm−2 h−1 for Pt-CT, Pd-CT, and CT electrodes, respectively, clearly showing the positive contribution of photo-deposited (nano)particles onto CT surface.

Synergistic Pd Single Atoms, Clusters, and Oxygen Vacancies on TiO2 for Photocatalytic Hydrogen Evolution Coupled with Selective Organic Oxidation

Developing efficient photocatalysts for synchronously producing H₂ and high value-added chemicals holds great promise to enhance solar energy conversion. Herein, a facile strategy of simultaneously engineering Pd cocatalyst and oxygen vacancies (V_O s) on TiO₂ to promote H₂ production coupled with selective oxidation of benzylamine is demonstrated. The optimized Pd_SA+C /TiO₂ -V_O photocatalyst containing Pd single atoms (SAs), clusters (C), and V_O s exhibits much superior performance to those of TiO₂ -V_O and Pd_SA /TiO₂ -V_O counterparts. The production rates of H₂ and N-benzylidenebenzylamine over Pd_SA+C /TiO₂ -V_O are 52.7 and 1.5 times those over TiO₂ -V_O , respectively. Both experimental and theoretical studies have elucidated the synergistic effect of Pd SAs, clusters, and V_O s on TiO₂ in boosting the photocatalytic reaction. The presence of Pd SAs facilitates the generation and stabilization of abundant V_O s by the formation of PdOTi³⁺ atomic interface, while Pd clusters promote the photogenerated charge separation and afford the optimum active sites for H₂ evolution. Surface V_O s of TiO₂ guarantee the efficient adsorption and dissociation/activation of reactant molecules. This study reveals the effect of active-site engineering on the photocatalysis and is expected to shed substantial light on future structure design and modulation of semiconductor photocatalysts.

Nanoscale Spatial Distribution of Supported Nanoparticles Controls Activity and Stability in Powder Catalysts for CO Oxidation and Photocatalytic H2 Evolution

Supported metal nanoparticles are essential components of high-performing catalysts, and their structures are intensely researched. In comparison, nanoparticle spatial distribution in powder catalysts is conventionally not quantified, and the influence of this collective property on catalyst performance remains poorly investigated. Here, we demonstrate a general colloidal self-assembly method to control uniformity of nanoparticle spatial distribution on common industrial powder supports. We quantify distributions on the nanoscale using image statistics and show that the type of nanospatial distribution determines not only the stability, but also the activity of heterogeneous catalysts. Widely investigated systems (Au-TiO2 for CO oxidation thermocatalysis and Pd-TiO2 for H2 evolution photocatalysis) were used to showcase the universal importance of nanoparticle spatial organization. Spatially and temporally resolved microkinetic modeling revealed that nonuniformly distributed Au nanoparticles suffer from local depletion of surface oxygen, and therefore lower CO oxidation activity, as compared to uniformly distributed nanoparticles. Nanoparticle spatial distribution also determines the stability of Pd-TiO2 photocatalysts, because nonuniformly distributed nanoparticles sinter while uniformly distributed nanoparticles do not. This work introduces new tools to evaluate and understand catalyst collective (ensemble) properties in powder catalysts, which thereby pave the way to more active and stable heterogeneous catalysts.

Porous Polymer-Titanium Dioxide/Copper Composite with Improved Photocatalytic Activity toward Degradation of Organic Pollutants in Wastewater: Fabrication and Characterization as Well as Photocatalytic Activity Evaluation

Titanium dioxide (TiO2) and TiO2/copper (denoted as TC) composite were prepared via hydrothermal process. In the meantime, divinylbenzene (DVB) and bismaleimide (BMI) monomers were allowed to participate in in-situ radical polymerization in the presence of azobisisobutyronitrile (AIBN) initiator to afford porous polymers (abridged as PP). The as-obtained PP were mixed together with tetrabutyl titanate (TBT) and CuSO4·5H2O in vacuum to obtain PP/TC composite (denoted as PPTC) containing incorporated TC composite in the pores of PP. The as-prepared TiO2, TC, and PPTC were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, fluorescence spectrometry, and electron spin resonance spectrometry, and so on. Furthermore, their photocatalytic activity for the degradation of N,N-dimethylformamide, methyl orange, phenol, and methylene blue under the irradiation of simulated sunlight (Xe lamp light) and natural sunlight were investigated. Findings indicated that, whether under simulated sunlight or nature sunlight irradiation, PPTC exhibited much better photocatalytic performance than TiO2 and TC for the degradation of the tested organic pollutants. Particularly, it allowed N,N-dimethylformamide (DMF) to be degraded by a rate of 73.7% under simulated sunlight irradiation and it retained photocatalytic activity even after six cycles of reuse, exhibiting promising potential for the removal of organic pollutants in wastewater (including industrial water, aquaculture wastewater, and domestic sewage). The desired photocatalytic performance of the as-prepared PPTC is attributed to two aspects. Namely, the incorporation of Cu2+ into the fine structure of TiO2 contributes to increasing photocatalyst activity and producing more free radical while the embedding of TC composite into the PP pores improves to the contact area between the photocatalyst and organic pollutants, and both are beneficial for improving the adsorption capacity and activity of the photocatalyst, thereby enhancing the degradation of the organic pollutants.

Selective Oxidation of Benzyl Alcohol by Ag/Pd/m-BiVO4 Microspheres under Visible Light Irradiation

A series of Ag/Pd/m-BiVO4 (monoclinic) bimetallic photocatalytic materials with different loading amounts and different mass ratios of Ag and Pd were synthesized by a hydrothermal method and an NaBH4 reduction method. The Ag/Pd/m-BiVO4 photocatalyst with a total Ag and Pd loading of 2 wt% and an Ag-to-Pd mass ratio of 2:1 can selectively oxidize benzyl alcohol to benzaldehyde under visible light irradiation, the conversion rate was up to 89.9%, and the selectivity was greater than 99%. The conversion rate on Ag/Pd/m-BiVO4 was higher than those on Ag/m-BiVO4 and Pd/m-BiVO4. The photocatalysts were characterized by X-ray powder diffraction (XRD), ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, N2 adsorption-desorption isothermal curves (BET) and other means. The effects of different light wavelengths and light intensities were compared. Then, the effects of different alcohol derivatives on the reactions were explored. The cycle experiments proved that the Ag/Pd/m-BiVO4 photocatalyst had good light stability and thermal stability. In addition, the capturing experiment of active species shows that the selective oxidation of benzyl alcohol is mainly accomplished through the synergistic action of h+, e−, •OH and •O2−.

Mixed Ligand Shell Formation upon Catechol Ligand Adsorption on Hydrophobic TiO2 Nanoparticles

Modifying the surfaces of metal oxide nanoparticles (NPs) with monolayers of ligands provides a simple and direct method to generate multifunctional coatings by altering their surface properties. This works best if the composition of the monolayers can be controlled. Mussel-inspired, noninnocent catecholates stand out from other ligands like carboxylates and amines because they are redox-active and allow for highly efficient surface binding and enhanced electron transfer to the surface. However, a comprehensive understanding of their surface chemistry, including surface coverage and displacement of the native ligand, is still lacking. Here, we unravel the displacement of oleate (OA) ligands on hydrophobic, OA-stabilized TiO₂ NPs by catecholate ligands using a combination of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy techniques. Conclusive pictures of the ligand shells before and after surface modification with catecholate were obtained by ¹H and ¹³C NMR spectroscopy (the ¹³C chemical shift being more sensitive and with a broader range). The data could be explained using a Langmuir-type approach. Gradual formation of a mixed ligand shell was observed, and the surface processes of catecholate adsorption and OA desorption were quantified. Contrary to the prevailing view, catecholate displaces only a minor fraction (∼20%) of the native OA ligand shell. At the same time, the total ligand density more than doubled from 2.3 nm^-2 at native oleate coverage to 4.8 nm^-2 at maximum catecholate loading. We conclude that the catecholate ligand adsorbs preferably to unoccupied Ti surface sites rather than replacing native OA ligands. This unexpected behavior, reminiscent of the Vroman effect for protein corona formation, appears to be a fundamental feature in the widely used surface modification of hydrophobic metal oxide NPs with catecholate ligands. Moreover, our findings show that ligand displacement on OA-capped TiO₂ NPs is not suited for a full ligand shell refunctionalization because it produces only mixed ligand shells. Therefore, our results contribute to a better understanding and performance of photocatalytic applications based on catecholate ligand-sensitized TiO₂ NPs.

Ultrafine CoO nanoparticles as an efficient cocatalyst for enhanced photocatalytic hydrogen evolution

In order to further enhance the performance of photocatalysts, cocatalysts are used to accelerate the photocatalytic reactions. Herein, ultrafine cobalt oxide (CoO) nanoparticles are synthesized through a novel bottom-up strategy and explored as an efficient non-noble cocatalyst to dramatically promote the photocatalytic hydrogen evolution rate of CdS nanorods. CdS/CoO heterostructures, consisting of highly dispersed 3-5 nm CoO nanoparticles anchored on the CdS nanorods, can provide a high photocatalytic hydrogen evolution rate of 6.45 mmol g^-1 h^-1 (∼36 times higher than that of bare CdS nanorods) in the visible-light region (>420 nm). Combined X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy analyses suggest Co-S bond formation between CoO and CdS, which guarantees efficient migration and separation of photogenerated charge carriers. This work provides a new avenue for adopting CoO as an effective cocatalyst for enhanced photocatalytic hydrogen production in the visible-light region.

Self-Assembled 3D Hierarchical Copper Hydroxyphosphate Modified by the Oxidation of Copper Foil as a Recyclable, Wide Wavelength Photocatalyst

In this work, three-dimensional flower-like and petal-like copper hydroxyphosphate Cu₅(OH)₄(PO₄)₂ (CHP) based on the self-assembly of numerous nanosheets has been successfully fabricated on a copper foil by a mild one-pot wet-chemical method without ligand assistance. This research contributes to the development of the method to change the morphology of the CHP active material by varying the degree of substrate oxidation. The two different CHP architectures were used to photocatalytically degrade rhodamine 6G (Rh 6G) under solar light, which can absorb wide-range light wavelength from the UV to the near-infrared region. They all exhibit high photocatalytic activity and good durability, which are potential candidates for high performance and recyclable wide wavelength photocatalysis.

Enhanced visible-light photocatalytic H2-generation activity of carbon/g-C3N4 nanocomposites prepared by two-step thermal treatment

Brown carbon/g-C₃N₄ nanocomposites synthesized by two-step calcination exhibited a wide visible light response range and improved photocatalytic H₂-generation performance.

Emerging trends in photodegradation of petrochemical wastes: a review

Various human activities like mining and extraction of mineral oils have been used for the modernization of society and well-beings. However, the by-products such as petrochemical wastes generated from such industries are carcinogenic and toxic, which had increased environmental pollution and risks to human health several folds. Various methods such as physical, chemical and biological methods have been used to degrade these pollutants from wastewater. Advance oxidation processes (AOPs) are evolving techniques for efficient sequestration of chemically stable and less biodegradable organic pollutants. In the present review, photocatalytic degradation of petrochemical wastes containing monoaromatic and poly-aromatic hydrocarbons has been studied using various heterogeneous photocatalysts (such as TiO₂, ZnO and CdS. The present article seeks to offer a scientific and technical overview of the current trend in the use of the photocatalyst for remediation and degradation of petrochemical waste depending upon the recent advances in photodegradation of petrochemical research using bibliometric analysis. We further outlined the effect of various heterogeneous catalysts and their ecotoxicity, various degradation pathways of petrochemical wastes, the key regulatory parameters and the reactors used. A critical analysis of the available literature revealed that TiO₂ is widely reported in the degradation processes along with other semiconductors/nanomaterials in visible and UV light irradiation. Further, various degradation studies have been carried out at laboratory scale in the presence of UV light. However, further elaborative research is needed for successful application of the laboratory scale techniques to pilot-scale operation and to develop environmental friendly catalysts which support the sustainable treatment technology with the "zero concept" of industrial wastewater. Nevertheless, there is a need to develop more effective methods which consume less energy and are more efficient in pilot scale for the demineralization of pollutant.