Yiyun Du

Au–Pd nanoalloys supported on Mg–Al mixed metal oxides as a multifunctional catalyst for solvent-free oxidation of benzyl alcohol

Au-Pd nanoalloys supported on Mg-Al mixed metal oxides prepared using sol-immobilisation are found to be highly efficient and reusable catalysts for the solvent-free oxidation of benzyl alcohol using molecular oxygen under low pressure. When using this support alloying Pd with Au resulted in an increase in both activity and selectivity to benzaldehyde and moreover an improved resistance to catalyst deactivation compared with the monometallic Pd and Au catalysts. The turnover number for the Au/Pd 1:1 molar ratio catalyst achieved 13,000 after 240 min and the selectivity to benzaldehyde was maintained at 93%; this high catalytic activity can be retained in full after three successive uses. The ensemble and electronic effect of Au-Pd nanoalloys were studied by IR spectroscopy using CO chemisorption, XPS and HRTEM. Moreover, the bifunctional nature of the acid-base MgAl-MMO support was found to be important as the acid sites are considered to be responsible for the improvement of catalytic activity; while, the basic sites gave rise to high selectivity. A possible mechanism with Au-Pd nanoparticles as the active sites has been proposed, illustrating that the oxidation of benzyl alcohol can proceed through the cooperation between the Au-Pd nanoalloys and the base/acid sites on the surface of the support.

Flower-like Au/Ni–Al hydrotalcite with hierarchical pore structure as a multifunctional catalyst for catalytic oxidation of alcohol

Flower-like hierarchical Au/NiAl-LDH catalysts were synthesized for selective oxidation of alcohols. The abundant hydrogen vacancies at the edge of the flowers as nucleation centers contributed to the uniform dispersion of Au NPs. The confinement effect of the hierarchical pores promoted 60% higher activity than the common Au/NiAl-LDH nanoparticle catalyst in the oxidation of benzyl alcohol by heightening the effective collisions between substrates and active sites. The evolution process of the hierarchical pores in the support was further proposed. Moreover, the reaction mechanism of the cooperation among Bronsted base sites, NiIII coordinatively unsaturated metal sites and isolated gold cations was concretely proved. In the oxidation of other typical alcoholic substrates, the flower-like catalyst showed higher activity than the common nanoparticle one except for linear alcohols, which could be attributed to the shape selectivity of straight macropores.

Hybrid Ni–Al layered double hydroxide/graphene composite supported gold nanoparticles for aerobic selective oxidation of benzyl alcohol

In this work, a Ni–Al layered double hydroxide/graphene (NiAl-LDH/RGO) nanocomposite which was synthesized by introducing NiAl-LDH on the surface of graphene oxide (GO) and simultaneously reducing graphene oxide without any additional reducing agents was utilized as the support for Au nanoparticles. Raman spectroscopy and XPS analysis revealed that the NiAl-LDH/RGO composite had both defect sites and oxygenic functional groups in RGO to control the directional growth of Au nanoparticles and lead to a small particle size. Compared to an Au catalyst supported on single GO and RGO or NiAl-LDH, this composite-supported Au catalyst (Au/NiAl-LDH/RGO) exhibited superior catalytic activity and stability in the selective oxidation of benzyl alcohol using molecular oxygen under low pressure. Improved activity was mainly ascribed to the small Au particle size effect caused by RGO and the contribution of basic sites in NiAl-LDH. Moreover, the preferable catalytic stability of the Au/NiAl-LDH/RGO catalyst was attributed to the defect sites and oxygenic functional groups in RGO which anchored the Au NPs and prevented the agglomeration, meanwhile, the agglomeration of RGO was inhibited by the introduction of NiAl-LDH.

Facile and surfactant-free synthesis of supported Pd nanoparticles on hydrotalcite for oxidation of benzyl alcohol

We report a facile modified deposition–precipitation method that permits reproducible preparation of a supported Pd catalyst with small particle size and narrow size distribution but without the protection of a surfactant and any additional treatment. The pH value in this technique plays a key role in controlling the size of the Pd nanoparticles as well as the electronic environment of the surface Pd atoms. With the increasing pH (4.0–12.0), the average size of the Pd nanoparticles decreases gradually, meanwhile, the peak area ratio for CO adsorbed on bridge-bonded Pd to that adsorbed on threefold-coordinate Pd increases. Stronger support–metal interaction (electron transfer from Pd0 to support) is observed at pH values of 7.0 and 10.0. Both the small particle size and the electron-deficient surface metallic Pd contribute to enhancement in the activity for the solvent-free oxidation of benzyl alcohol. Therefore, compared with supported Pd catalysts prepared by sol immobilization, impregnation and deposition–precipitation methods, Pd/hydrotalcite synthesized by this modified deposition–precipitation approach shows a higher TOF value (5330 h−1). This enhanced catalytic performance can also be maintained in five cycles. Under the considerations of green chemistry, a number of Pd catalysts were then prepared on alternative supports using this method without the addition of alkali in the preparation process.

The role of various oxygen species in Mn-based layered double hydroxide catalysts in selective alcohol oxidation

Precise identification of oxygen species in LDH-based catalysts was investigated for the first time for alcohol oxidation. Assisted by surface O2−, O2 molecules were activated in various oxidation states. Labile oxygen species could react with alcohol except for the O22− immobilized by unsaturated metals, while other oxygen species only transferred oxygen species as mediators.

Facile synthesis of supported RuO2·xH2O nanoparticles on Co–Al hydrotalcite for the catalytic oxidation of alcohol: effect of temperature pretreatment

RuO2·xH2O supported on a CoAl-LDH catalyst was synthesized by the co-precipitation (CP) method and the deposition–precipitation (DP) method for the selective oxidation of alcohols. The catalyst prepared by the CP method exhibited higher activity compared with that obtained by the DP method due to stronger interaction between RuO2 and the CoAl-LDH support as well as the slightly smaller particle size of the RuO2 nanoparticles. The influence of the temperature pretreatment on catalytic performance was then investigated. Among the catalysts pretreated at different temperature, RuO2/CoAl-LDH treated at 200 °C showed the highest activity with a TOF of 142 h−1, which was nearly 55% higher than that of the untreated catalyst. It could be related to not only the suitable amount of RuO2·xH2O for β-H cleavage, but also the presence of Co3+ species for the activation of O2 molecules and storage of the resulting active O* species. Furthermore, the strong interaction between RuO2 and the support was revealed to promote the adsorption and activation of benzyl alcohol and thus enhance the catalytic performance. Significantly, RuO2/CoAl-LDH treated at 200 °C was found to selectively oxidize various alcohols to the corresponding aldehydes and ketones with respectable activity and had greater advantage comparable to that of some Ru catalysts.