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.

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.

Aiming at the preparation of catalysts with well-controlled structures, bimetallic PdCu catalysts containing only small amounts of Pd were prepared by two different methods for partial hydrogenation of acetylene. The highly dispersed and stable PdCu catalyst with the Pd:Cu atomic ratio of 1:40 was obtained from a Pd(OH)2/CuMgAl hydrotalcite (HT) precursor by a modified co-precipitation method. The obtained PdCu catalyst possessed uniform PdCu nanoalloys with an average size of 1.8 ± 0.3 nm. As a comparison, the impregnation method was also employed to prepare bimetallic PdCu catalysts and it was confirmed that the Pd-rich(core)–Cu-rich(shell) structure was dominant. Under identical reaction conditions, approximately 100% conversion and 82% selectivity at 100 °C were achieved using the PdCu nanoalloy catalyst which were 23% and 12% higher than those of the Pd-rich(core)–Cu-rich(shell) catalyst. The preferable activity was ascribed to the homogeneous nanoalloy structure and small size effect. The enhanced selectivity was attributed to the strong synergistic effect of PdCu. More significantly, this superior catalytic performance can be retained after 48 h of continuous reaction due to the excellent resistibility against carbon deposition and the confinement effect.

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.

NiTi-layered double hydroxide (NiTi-LDH) with rich defective sites was synthesized and used as the support for the preparation of a novel supported PdAg nanoalloy catalyst for the partial hydrogenation of acetylene. The obtained PdAg/NiTi-LDH catalyst exhibited a remarkable catalytic performance. When the conversion of acetylene reached 90%, the selectivity towards ethene maintained 82%. Superior hydrogenation activity was ascribed to two key factors. Small particle size and high dispersion of PdAg nanoparticles were responsible for boosting the catalytic activity. In addition, Ti3+ defective sites in the support also played an important role in the enhancement of activity. The interface at the Ti3+ species and active metals acting as new active sites enhanced the activation and dissociation of hydrogen and therefore further improved the catalytic activity. Preferable selectivity was assigned to the electronic effect between the NiTi-LDH support and the PdAg nanoalloys. The electron transfer from the Ti3+ species to the Pd resulted in the increase of electron density and the linearly coordinated sites of Pd and therefore facilitated the desorption of ethene. Moreover, due to the reducibility of NiTi-LDH, the selectivity and stability over the reduced PdAg/NiTi-LDH catalyst were further enhanced on account of the strong metal–support interaction.

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 Brønsted 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.

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.

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.

Supported Pd nanowire and cuboctahedron catalysts have been synthesized in an ethylene glycol–poly(vinylpyrrolidone)–KBr system using a precipitation–reduction method. KBr plays a critical role in controlling the morphology of Pd: with a variety of relatively low KBr concentrations, Pd nanowires with different lengths were obtained, but after adding sufficient KBr, shape evolution from nanowires to cuboctahedrons was observed. HRTEM images showed that the twisted Pd nanowires were actually composed of primary cuboctahedrons. Furthermore, lattice distortion was observed at interfacial regions, and the number of crystal boundaries increased with increasing length of the nanowires. The catalytic performance of the Pd materials was investigated in the selective hydrogenation of acetylene. The activities of the Pd nanowire catalysts were significantly higher than those of the cuboctahedron catalyst and gradually increased with the increasing number of crystal boundaries, indicating that the defect sites at crystal boundaries are more active owing to the exposure of larger numbers of Pd atoms. However, higher activity resulted in excessive hydrogenation and a decrease in ethylene selectivity. Therefore, the Pd cuboctahedron catalyst possessed higher selectivity. The relationship between crystal boundaries and catalytic performance was quantified, and the catalytic activity was found to increase linearly with an increasing number of crystal boundaries, whereas the trend in the selectivity was the reverse.Keywords: acetylene-selective hydrogenation; crystal boundary; cuboctahedron; defect sites; nanowire; supported Pd catalyst;

Pd/MgAl-layered double hydroxide (LDH) was synthesized in situ on the surface of spherical Al2O3 to obtain a Pd/MgAl-LDH/Al2O3 catalyst, using hexamine as both a precipitant for LDH and a reductant for Pd2+. After calcination and reduction, another Pd/MgO-Al2O3 catalyst was obtained. As a comparison, the Pd/Al2O3 catalyst was prepared by the conventional impregnation method. Low-temperature N2 adsorption−desorption, NH3 temperature-programmed desorption, and scanning electron microscopy results showed that Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 catalysts possessed larger surface area, lower surface acidity, uniform Pd particle size, and specific Pd shape compared with the Pd/Al2O3 catalyst. The catalytic performances of the catalysts were then studied in the selective hydrogenation of acetylene. In comparison to Pd/Al2O3, Pd/MgAl-LDH-Al2O3 and Pd/MgO-Al2O3 catalysts exhibited not only higher activity due to the uniform size and specific shape of Pd particles which provided more catalytic active sites but also better selectivity because of the lower surface acidity and strong metal/support interaction.

•Pd–Ga/MgO–Al2O3 catalysts were synthesized by an in situ LDH precursor method.•Bimetallic Pd–Ga nanoalloys were highly and stably dispersed.•Ethene selectivity was improved owing to bimetallic synergistic effect.•Due to net trap confinement effect, Pd–Ga catalysts exhibit preferable stability.An effective and versatile synthetic approach is presented to produce highly dispersed bimetallic Pd–Ga catalysts that can be used as hydrogenation catalysts. Mg–Ga–Al-layered double hydroxide (LDH) was synthesized in situ on the surface of spherical alumina to obtain MgGaAl-LDH@Al2O3 precursor, followed by the introduction of PdCl42-. The positive charge of MgGaAl-LDH layer offers an opportunity to realize uniform dispersion of PdCl42-, which facilitates the formation of bimetallic Pd–Ga nanoalloys. Upon thermal reduction of PdCl42-/MgGaAl-LDH@Al2O3 precursor, highly stable dispersed bimetallic Pd–Ga/MgO–Al2O3 catalysts were obtained. Owing to high dispersion and synergistic effect of bimetallic nanoalloys, Pd–Ga/MgO–Al2O3 catalysts exhibited comparable activity and much higher selectivity compared with the monometallic Pd/MgO–Al2O3 in partial hydrogenation of acetylene. More significantly, this good catalytic performance can be totally retained after three times recycling due to the net trap confinement effect, which suppressed the migration and aggregation of bimetallic Pd–Ga nanoalloys.Bimetallic Pd–Ga/MgO–Al2O3 catalysts prepared by in situ LDH precursor route exhibited much preferable selectivity and durability in comparison with the monometallic Pd/MgO–Al2O3 catalyst in partial hydrogenation of acetylene.Download high-res image (174KB)Download full-size image

•PdAu nanoflowers were synthesized by a dropping addition procedure of reductant.•Nanoflowers with the size of 30–55 nm were composed of several small octahedrons.•Crystal defects and synergy among building units were revealed.•Catalytic activity and selectivity were improved due to special structure features.•Catalytic stability was also enhanced due to structure stability of nanoflowers.Three-dimensional PdAu nanoflowers have been designed and effectively synthesized through a procedure of dropping addition of the reductant, in which morphology evolution was recorded by capturing HRTEM images at continuous synthetic stages. HRTEM, positron annihilation spectroscopy, and in situ CO-IR results showed that synthesized flowerlike PdAu nanoalloys with a size of 30–55 nm were composed of several small PdAu building units and have abundant crystal defects as well as synergetic effects. The obtained colloidal PdAu nanoflowers were then immobilized onto MgAl mixed metal oxides as a novel catalyst for partial hydrogenation of acetylene. Due to its complex morphology, rich defective sites, and synergetic effect caused by the cooperation between building units, supported PdAu nanoflower-like catalyst exhibited not only much higher activity and selectivity but also preferable stability compared with the catalyst whose active component was shape- and size-similar to the building units of PdAu nanoflowers.Graphical abstractDownload high-res image (343KB)Download full-size image

•MgxTi1−xOy with tunable acidity/basicity is prepared as new supports for acetylene hydrogenation.•PdAg/Mg0.5Ti0.5Oy shows excellent performance with 83.8% selectivity at >99% conversion.•Moderate acidity promotes hydrogen-spillover effect, which favors hydrogen dissociation.•Negatively-charged Pd caused by basic sites and Ti3+ favors desorption of ethene.•High alloying degree of PdAg/Mg0.5Ti0.5Oy also contributes to the improved selectivity.A series of reducible Mg-Ti mixed oxides supported PdAg catalysts with tunable acidity/basicity were synthesized for selective acetylene hydrogenation, to investigate the implications of acid-base property on the nature of active component. Catalytic performance of PdAg/MgxTi1−xOy varied with Mg/Ti ratio increasing as volcano curve, which corresponded well with amount of medium acid and weak basic sites. PdAg/Mg0.5Ti0.5Oy exhibited >99% conversion and 83.8% selectivity at 70 °C. Enhanced activity was attributed to the promoted hydrogen-spillover effect by moderate acidic sites of Mg0.5Ti0.5Oy support, which facilitated hydrogen activation/dissociation. Preferred selectivity was reasonably owing to the significant geometric effect resulting from high alloying degree of Pd-Ag, which increased the number of Pd linearly coordinated sites, and therefore facilitated the desorption of ethene. Additionally, the increased Pd electronic density caused by the electron transfer from the basic sites and Ti3+ species of Mg0.5Ti0.5Oy support also contributed to the improvement of selectivity.Download high-res image (134KB)Download full-size image

NiTi-layered double hydroxide (NiTi-LDH) with rich defective sites was synthesized and used as the support for the preparation of a novel supported PdAg nanoalloy catalyst for the partial hydrogenation of acetylene. The obtained PdAg/NiTi-LDH catalyst exhibited a remarkable catalytic performance. When the conversion of acetylene reached 90%, the selectivity towards ethene maintained 82%. Superior hydrogenation activity was ascribed to two key factors. Small particle size and high dispersion of PdAg nanoparticles were responsible for boosting the catalytic activity. In addition, Ti3+ defective sites in the support also played an important role in the enhancement of activity. The interface at the Ti3+ species and active metals acting as new active sites enhanced the activation and dissociation of hydrogen and therefore further improved the catalytic activity. Preferable selectivity was assigned to the electronic effect between the NiTi-LDH support and the PdAg nanoalloys. The electron transfer from the Ti3+ species to the Pd resulted in the increase of electron density and the linearly coordinated sites of Pd and therefore facilitated the desorption of ethene. Moreover, due to the reducibility of NiTi-LDH, the selectivity and stability over the reduced PdAg/NiTi-LDH catalyst were further enhanced on account of the strong metal–support interaction.

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 Brønsted 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.

Aiming at the preparation of catalysts with well-controlled structures, bimetallic PdCu catalysts containing only small amounts of Pd were prepared by two different methods for partial hydrogenation of acetylene. The highly dispersed and stable PdCu catalyst with the Pd:Cu atomic ratio of 1:40 was obtained from a Pd(OH)2/CuMgAl hydrotalcite (HT) precursor by a modified co-precipitation method. The obtained PdCu catalyst possessed uniform PdCu nanoalloys with an average size of 1.8 ± 0.3 nm. As a comparison, the impregnation method was also employed to prepare bimetallic PdCu catalysts and it was confirmed that the Pd-rich(core)–Cu-rich(shell) structure was dominant. Under identical reaction conditions, approximately 100% conversion and 82% selectivity at 100 °C were achieved using the PdCu nanoalloy catalyst which were 23% and 12% higher than those of the Pd-rich(core)–Cu-rich(shell) catalyst. The preferable activity was ascribed to the homogeneous nanoalloy structure and small size effect. The enhanced selectivity was attributed to the strong synergistic effect of PdCu. More significantly, this superior catalytic performance can be retained after 48 h of continuous reaction due to the excellent resistibility against carbon deposition and the confinement effect.