Fenglin Liao

Nanojunction‐Mediated Photocatalytic Enhancement in Heterostructured CdS/ZnO, CdSe/ZnO, and CdTe/ZnO Nanocrystals

A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis.

Recent Developments in Palladium‐Based Bimetallic Catalysts

As one of the alternative metals to platinum, which supports a wide range of applications in chemistry and catalysis in industry, palladium increasingly receives attention because of its similar physicochemical properties. However, Pd is generally less expensive than Pt and has a richer natural reserve. Herein, some recently developed techniques for the preparation and characterization of Pd‐based bimetallic catalysts are reviewed. The impact on catalytic reactions of interest, including hydrogenation, dehydrogenation, hydrogenolysis, reforming, the oxygen reduction reaction, and hydrodesulfurization are also discussed. It is shown that the catalytic performance of Pd‐based bimetallic catalysts is strongly dependent on the geometric and electronic states of Pd, which can be significantly affected by blended foreign element(s). Rationalization of the structure–activity relationship can provide useful guidelines to the fine tuning of these important catalytic reactions.

Palladium on iron oxide nanoparticles: the morphological effect of the support in glycerol hydrogenolysis

Palladium supported on different shaped iron oxides has been synthesized and tested in glycerol hydrogenolysis in order to investigate the morphological effect of the support on catalytic performance. The plate-like iron oxide support exhibits a much higher activity due to the presence of more basic sites (oxygen vacancies) on its predominant polar surface. Thus, by tailoring the shape of the crystallized support, we show that bifunctional metal-oxide catalysis at the interface, like in the present example, can be significantly manipulated.

Temperature and solvent-dependent morphological sol gel transformation: an in situ microscopic observation.

A thermoreversible self-assemble process from gel (fiber) to sol (vesicle) state in the system alkylamine-ethylene glycol is for the first time monitored by in situ polarized optical microscopy, XRD, (1)H NMR, SEM, SAXRD, FTIR and drop shape analysis. It is found that the solvent molecules are intercalated with alkylamine molecules to form the organogel and vesicle structures. A model based on structural transformation with respect to these alkylamine gelator-solvent assembles is therefore proposed.

PdFe nanoparticles as selective catalysts for C–C cleavage in hydrogenolysis of vicinal diol units in biomass-derived chemicals

A series of supported PdFe bimetallic nanoparticles with variable compositions are successfully synthesized through a careful reduction of a PdO/FeOx complex with a tunable support structure, which are demonstrated to achieve ultra-selective C–C bond cleavage over C–O bond of vicinal diol units in the hydrogenolysis of biomass-derived molecules: a gradual increase in the Fe(0) content of the bimetallic nanoparticles, as suggested by EXAFS analysis and DFT calculations, is shown to afford more enhanced selectivity of C–C cleavage, reaching a maximum value of 95%.

A promising low pressure methanol synthesis route from CO2 hydrogenation over Pd@Zn core–shell catalysts

At present, there is no low pressure methanol synthesis from CO2/H2 with high yield despite the presence of an upstream process of aqueous phase reforming (APR) of biomass derivatives on an industrial scale for CO2/H2 production at ca. 2 MPa. This is due to the intrinsic thermodynamics of the system which leads to particularly high CO levels at low pressure through reversed water gas shift reaction (RWGS) for most studied catalysts. Here we report a new Pd@Zn core–shell catalyst that offers a significantly higher kinetic barrier to CO/H2O formation in CO2 hydrogenation to reduce the CO levels but facilitates CH3OH formation at or below 2 MPa with CH3OH selectivity maintained at ca. 70% compared to ca. 10% over industrial Cu catalysts. The corresponding methanol yield at 2 MPa reaches 6.1 gmethanol gactive metal−1 h−1, which is comparable to the best reported value among a wide variety of catalysts under 5 MPa. It is thus believed that this active Pd based catalyst opens up a promising possibility for low pressure and temperature methanol production using a renewable biomass resource for fossil-fuel-starved countries.

A New Class of Tunable Heterojunction by using Two Support Materials for the Synthesis of Supported Bimetallic Catalysts

Finely dispersed Rh or Pd nanoparticles are decorated with a small quantity of Fe atoms, originating from the controlled reduction of two supports, the mixed oxides of ZnFe2O4–Fe2O3, to form supported RhFe or PdFe bimetallic nanoparticles without significant change in particle size. The selectivity of ethylene glycol hydrogenolysis can be manipulated by adjusting the compositions of the catalysts. This reveals the important underlying principle for rational design and synthesis of new supported bimetallic nanoparticles by using two mixed oxides with a tailored heterojunction that exerts tuning catalytic properties.

Tunability of catalytic properties of Pd-based catalysts by rational control of strong metal and support interaction (SMSI) for selective hydrogenolyic C–C and C–O bond cleavage of ethylene glycol units in biomass molecules

It is shown that the catalytic properties of Pd can be fine-tuned by rationally varying the metal–support interaction through the formation of bimetallic nanoparticles with a support after hydrogen reduction; in the hydrogenolysis of a –CHOHCHOH– vicinal diol unit, the ability to break the C–C bond over the C–O bond is found to increase significantly with the decrease in d-band filling, while the ability to break the C–O bond is enhanced by the upshift of the d-band center of modified Pd.

The remarkable activity and stability of a highly dispersive beta-brass Cu-Zn catalyst for the production of ethylene glycol.

Incorporation of Zn atoms into a nanosize Cu lattice is known to alter the electronic properties of Cu, improving catalytic performance in a number of industrially important reactions. However the structural influence of Zn on the Cu phase is not well studied. Here, we show that Cu nano-clusters modified with increasing concentration of Zn, derived from ZnO support doped with Ga3+, can dramatically enhance their stability against metal sintering. As a result, the hydrogenation of dimethyl oxalate (DMO) to ethylene glycol, an important reaction well known for deactivation from copper nanoparticle sintering, can show greatly enhanced activity and stability with the CuZn alloy catalysts due to no noticeable sintering. HRTEM, nano-diffraction and EXAFS characterization reveal the presence of a small beta-brass CuZn alloy phase (body-centred cubic, bcc) which appears to greatly stabilise Cu atoms from aggregation in accelerated deactivation tests. DFT calculations also indicate that the small bcc CuZn phase is more stable against Cu adatom migration than the fcc CuZn phase with the ability to maintain a higher Cu dispersion on its surface.

Pd@Zn core–shell nanoparticles of controllable shell thickness for catalytic methanol production

It is reported that well-dispersed core–shell Pd@Zn nanoparticles of controllable shell thickness can be synthesized from a Pd/CdSe–ZnO precursor in H2 without using a surfactant: the Pd ensembles on the material surface with electronic modulation by Zn atoms give a methanol turnover frequency (TOF) and selectivity of 3.3 × 10−1 s−1 and 80%, respectively, the yield of which is 2-fold the best reported value over other Pd-based catalysts.