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Photothermal catalytic hydrogenation of CO2 is an intriguing approach to reduce CO2 under mild conditions, which is possible because of photoinduced electron–hole pair generation and an overall increase in the localized temperature under light irradiation. However, the lack of a catalyst with an adequate photothermal conversion efficiency and the ability to generate a sufficient number of photoinduced electrons are the main factors limiting the applicability of this method. H-doped WOy demonstrates surface plasmon resonance (SPR) capabilities, and can be adjusted by changing the dopant (H+) concentration. To improve the potential of the WOy plasmonic effect based on H-doping, we herein report that the Mo-doped Pt/WOy (Pt/MoWOy) substantially increases the dopant (H+) and oxygen vacancy concentration in Pt/HxMoWOy during the H2 reduction process, facilitating photothermal hydrogenation of CO2 to CO. The developed Pt/HxMoWOy exhibits excellent catalytic performance (3.1 mmol h−1 g−1) in the photothermal reverse water-gas shift (RWGS) reaction at 140 °C, outperforming undoped Pt/HxWOy (1.02 mmol h−1 g−1). Experimental and comprehensive analyses, including photoelectrochemical measurements, UV-Vis-NIR diffuse-reflectance spectroscopy, and a model reaction, showed that abundant surface free electrons and oxygen vacancies (VO) in Pt/HxMoWOy are responsible for the efficient CO2 adsorption and transfer of photoinduced electrons to carry out the reduction of CO2 to CO. X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption fine structure (XAFS) measurements revealed a reversible redox event for the Mo and W atoms during the RWGS reaction, confirming that the oxygen vacancies between Mo and W atoms in Pt/HxMoWOy act as active sites and that Pt nanoparticles activate H2 to enable the regeneration of the oxygen vacancies. Moreover, density functional theory (DFT) calculations demonstrated that Mo-doping substantially decreases the energy barrier for oxygen vacancy formation in WOy in the H2 reduction process. We expect that this study will provide an innovative strategy for designing a highly efficient catalyst for photothermal CO2 conversion. [ABSTRACT FROM AUTHOR]

Taking full advantage of solar light to promote CO2 hydrogenation remains a challenge in the catalysis field. Combining a degeneratively-doped metal oxide and a precious metal to obtain hybrids with plasmonic absorption in the visible-light region enables the high-efficiency utilization of solar light in photothermal catalysis. Herein, MoO3 samples with different morphologies were synthesized and combined with Pt to form hybrid materials with a strong plasmonic effect. Among the Pt/MoO3 hybrids with various morphologies, the Pt/HxMoO3−y(Sheet) hybrid displayed the best performance in the photothermal synergistic catalysis of the reverse water-gas shift (RWGS) reaction at 140 °C under visible light irradiation because of the advantages of its nanosheet structure (e.g., a high specific surface area, high proportion of exposed surface atoms, and good optical transparency). The Pt/HxMoO3−y(Sheet) catalyst outperformed Pt/HxMoO3−y hybrids with a bulk, belt, or rod morphology. Thermogravimetry, UV-vis-NIR diffuse reflectance spectroscopy, and model reactions demonstrated that the Pt/HxMoO3−y(Sheet) hybrid contained the largest amount of surface O vacancies and exhibited the strongest plasmonic absorption, which are the primary reasons for its highest catalytic activity among the investigated hybrid catalysts. Photoelectric characterization revealed that the Pt/HxMoO3−y(Sheet) hybrid could generate hot electrons under visible-light irradiation. X-ray photoelectron spectroscopy measurements showed a reversible redox event of the Mo atoms, demonstrating that the O vacancies in Pt/HxMoO3−y(Sheet) acted as active sites and regenerated during the reaction. A possible mechanism is proposed for the photothermal synergistic catalysis in the RWGS reaction. The nanosheet-structured Pt/HxMoO3−y hybrid can exploit solar energy to greatly reduce energy consumption during the CO2 hydrogenation reaction, providing a greener and energy-saving scheme for the conversion of CO2 to CO. [ABSTRACT FROM AUTHOR]

The hydrogenation of CO2 to formate is considered to be a reaction that can achieve the CO2-supported hydrogen energy cycle. However, the design of efficient heterogeneous catalysts possessing high activity for this reaction is challenging because of the high stability of CO2 and harsh reaction conditions. Herein, we design and synthesize ultrafine and well-dispersed PdAg nanoparticles (NPs) with an average diameter of 2.8 nm, which are dispersed uniformly within mesoporous hollow carbon spheres (MHCSs) containing a polymer (polyethylenediamine) with amino functional groups. Due to the limitation of the MHCS cavity spaces, the agglomeration of the alloy NPs could be effectively prevented, and finally the ultrafine PdAg alloy NPs and polymers are dispersed uniformly in MHCSs. The size of MHCSs is about 200–220 nm and the carbon shell is about 30–40 nm, which allows the dispersion of PdAg alloy NPs within MHCSs. Significantly, the PdAg alloy NPs in the vicinity of the aminopolymer dispersed in the hollow structured catalysts reveal enhanced catalytic activity and stability for CO2 hydrogenation to formate compared with control samples, showing a high turnover number (TON) of 2680 at 100 °C after 24 h. This synthetic strategy and the unique mechanism of core–shell catalysts for CO2 hydrogenation to formate will contribute to the CO2-mediated hydrogen energy cycle in the future. [ABSTRACT FROM AUTHOR]

The transformation of CO2 to formic acid (FA, HCOOH) is one of the most promising means of developing economically-viable CO2-mediated energy cycles, owing to the many advantages of FA as a feedstock, hydrogen source and fuel source for fuel cells. In the present study, we synthesized a series of catalysts consisting of PdAg nanoparticles supported on resorcinol-formaldehyde polymers having various amine contents (PdAg/amine-RF). These catalysts exhibited significant activity during CO2 hydrogenation to produce FA, especially when the support with the highest amine concentration was employed. Analyses of active Pd species using X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) demonstrated the formation of Pd–N bonds in these materials, and oxidized nitrogen, phenylmethanimine, phenylimine and phenylamine moieties were identified based on N 1s XPS data. Density functional theory calculations showed that the adsorption energy of bicarbonate ions on the PdAg alloy is much higher in close proximity to phenylamine molecules, while the activation energy of the rate determining step is significantly lower. These predictions are in good agreement with the experimental results, which demonstrate that the reaction order associated with the bicarbonate concentration is decreased with increasing nitrogen concentration in the catalyst. A significant correlation was observed between the turnover number and the nitrogen content (attributed to the presence of phenylamine moieties) estimated from both XPS and elemental analysis, indicating that phenylamine groups made the most significant contribution toward the promotion of this reaction. [ABSTRACT FROM AUTHOR]

A tandem reaction combining direct H2O2 production from H2 and O2 and a subsequent oxidation reaction with H2O2 in a single reaction vessel offers an energy-saving and cost-effective oxidation route for the synthesis of targeted chemicals. Herein, we report the synthesis of hollow titanosilicate nanospheres encapsulating Pd and PdAu alloy nanoparticles (NPs) for the H2O2-mediated one-pot oxidation reaction. The yolk–shell nanostructured catalyst (Pd@Ti-HMSS), composed of a Pd NP core encapsulated within a hollow mesoporous titanosilicate shell having isolated Ti atoms, is fabricated by a facile etching and re-assembly process using SiO2 spheres as a template and two different kinds of Si alkoxides as precursors. The synthesized catalyst shows superior catalytic activity in the one-pot oxidation reaction of sulfide under a co-flow of H2 and O2, and far outperforms the prototype titanosilicate-supported Pd NP catalysts. Detailed structural characterization and kinetic analysis reveal that the increased catalytic efficiency is attributed to efficient mass transfer of reactants and the associated improved reactivity with in situ generated H2O2 over Ti sites. A further enhancement in activity is achievable by encapsulating PdAu alloy NPs, which provide a faster H2O2 production rate than monometallic Pd NPs. Owing to the presence of a protective silicate shell, the catalyst shows good stability and reusability over multiple catalytic cycles without significant loss of activity. [ABSTRACT FROM AUTHOR]

A plasmonic Pd/MoxW1−xO3−y hybrid, having Pd NPs anchored on a plasmonic MoxW1−xO3−y support, is obtained by a solution-processed impregnation-reduction method. The synthesized Pd/MoxW1−xO3−y hybrid exhibits strong visible light absorption with a localized surface plasmon resonance (LSPR) peak at around 700 nm. It is demonstrated that such a Pd/MoxW1−xO3−y hybrid displays dramatic plasmon-enhanced photocatalytic performance in the dehydrogenation of ammonia borane (NH3BH3) solution under visible light irradiation, which mainly results from the synergetic effect between Pd NPs and emerging W5+ ions. More importantly, the performance of the Pd/MoxW1−xO3−y hybrid is almost 5.2 and 5 times higher than those of MoxW1−xO3−y-NaBH4 (MoxW1−xO3−y obtained by NaBH4 reduction) and our previously reported plasmonic Pd/MoO3−x nanosheets. This study may open up the possibility of rationally designing plasmonic metal/semiconductor hybrids for highly efficient heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

Silver nanoparticles (AgNPs) encapsulated in porous materials such as mesoporous silica and zeolite exhibit high adsorption toward sulfur compounds owing to their defined porous structure. In this study, we report the synthesis of AgNPs encapsulated in hollow silica spheres, which act as efficient adsorbents for the removal of low-concentrated sulfur compounds. Hollow silica spheres bearing propyl amine groups inside a silica shell (NH2–HSS) were fabricated utilizing an oil-in-water (O/W) microemulsion as a template and a subsequent solvent extraction process. AgNPs were selectively deposited inside the NH2–HSS with the assistance of microwave irradiation. The AgNPs encapsulated within NH2–HSS (Ag@HSS) exhibit adsorption toward several sulfur compounds such as H2S and tert-butylmercaptan (TBM) with low-concentration (45 ppm) at ambient conditions, while prohibiting the adsorption of protein molecules that potentially deactivate AgNPs as inhibitors due to the protective effect of the silica shell. Ag@HSS displayed promising reusability; it retained its initial adsorption performance even after successive high temperature regeneration cycles, suggesting that this material is a promising adsorbent for the efficient and selective removal of volatile sulfur compounds. [ABSTRACT FROM AUTHOR]

Ammonia borane (AB) is a hydrogen storage material which can produce three equivalent moles of H2 gas by a hydrolytic decomposition reaction in the presence of an appropriate catalyst. The photocatalytic H2 generation from AB decomposition was studied by utilizing the Au/TiO2 plasmonic photocatalyst under both visible and UV light conditions. Different amounts of gold (0.5, 1, 2, and 3%) were photo-deposited on TiO2 (anatase) to prepare Au/TiO2 catalysts. Au(1 wt%)/TiO2 showed the highest activity of H2 generation under visible light and a further increase of Au loading reduced the activity. Although LSPR (localized surface plasmon resonance) excitation should increase with gold nanoparticle (Au NP) loading, excessive Au loading also facilitates the recombination of LSPR charge pairs. The time-resolved photoluminescence (TRPL) decay spectra of Au/TiO2 clearly showed that the recombination rate of LSPR charge pairs is accelerated at higher Au loading. As a result, the photocatalytic activity of Au/TiO2 was optimized at 1 wt% Au loading to produce 88 μmol of H2 in 4 h. However, such an optimized level of Au loading was not observed under UV light irradiation conditions and the UV photocatalytic production of H2 increased with the Au loading up to 3% because higher Au loading facilitates the separation of charge pairs and accumulation of electrons in Au NPs through the Schottky barrier. These two different electron transfer behaviors (accumulation and recombination) can be further confirmed by the Cr(vi) reduction experiments since they showed a similar trend of photocatalytic activity with Au loading. This research provides a fundamental understanding for optimizing the photocatalytic activity of various metal–metal oxide heterojunction photocatalysts. [ABSTRACT FROM AUTHOR]

Well-crystallized, high-surface-area plasmonic MoO3−x was synthesized by combining an evaporation induced self-assembly (EISA) process and a subsequent hydrogen reduction at a certain temperature. The intrinsic anisotropic crystal growth process of tiny MoO3 nuclei to MoO3 nanosheets was successfully inhibited. Detailed characterization by means of XRD, TEM, N2 physisorption, and XPS measurements revealed that the synthesized MoO3−x not only showed a strong localized surface plasmon resonance (LSPR) under incident light but also had a relatively large specific surface area. The specific surface area (SBET) of MoO3−x after reduction at 200 °C was 30.0 m2 g−1, which was 22.7 and 9.1 times higher than those of commercially available MoO3 and our previously reported MoO3−x nanosheets, respectively. We also demonstrate that such a semiconductor with a large surface area could be used as a highly efficient catalyst that dramatically enhances the dehydrogenation activity for ammonia borane (NH3BH3; AB) under visible light irradiation. [ABSTRACT FROM AUTHOR]

A variety of physical properties such as optical, fluorescence, and guest absorption capacity integrated within hybrid MOF based materials may show synergetic effects for significantly enhancing the performance of many applications. In this study, new hybrid MOFs, cerium doped chromium based amine-functionalized metal–organic frameworks (CeMIL-101), were successfully synthesized for the first time, via hydrothermal treatment with the aim to enhance the photocatalytic activity of H2 production from ammonia borane (AB) under the assistance of visible light. The results analyzed by X-ray diffraction and X-ray adsorption near-edge structure strongly support the effective doping of Ce ions into the nodes of MIL-101. The catalytic activity of Pd/CeMIL-101 is remarkably higher than that of Pd/MIL-101 without Ce doping under visible light irradiation at ambient temperature. Pd/CeMIL-101 is highly efficient in generating the hydroxyl radicals and photo-excited charge transfer under visible light irradiation. Moreover, the presence of valence fluctuation of Ce3+/Ce4+ was the main factor for reducing the recombination rate of photo-excited charges. This study may bring an effective strategy for the design and development of highly active hybrid materials for achieving high photocatalytic activity. [ABSTRACT FROM AUTHOR]