Sónia A. C. Carabineiro

Graphitic Carbon Nitride: Synthesis, Properties, and Applications in Catalysis

Graphitic carbon nitride, g-C3N4, is a polymeric material consisting of C, N, and some impurity H, connected via tris-triazine-based patterns. Compared with the majority of carbon materials, it has electron-rich properties, basic surface functionalities and H-bonding motifs due to the presence of N and H atoms. It is thus regarded as a potential candidate to complement carbon in material applications. In this review, a brief introduction to g-C3N4 is given, the methods used for synthesizing this material with different textural structures and surface morphologies are described, and its physicochemical properties are referred. In addition, four aspects of the applications of g-C3N4 in catalysis are discussed: (1) as a base metal-free catalyst for NO decomposition, (2) as a reference material in differentiating oxygen activation sites for oxidation reactions over supported catalysts, (3) as a functional material to synthesize nanosized metal particles, and (4) as a metal-free catalyst for photocatalysis. The reasons for the use of g-C3N4 for such applications are also given, and we expect that this paper will inspire readers to search for further new applications for this material in catalysis and in other fields.

Adsorption of ciprofloxacin on surface-modified carbon materials

The adsorption capacity of ciprofloxacin (CPX) was determined on three types of carbon-based materials: activated carbon (commercial sample), carbon nanotubes (commercial multi-walled carbon nanotubes) and carbon xerogel (prepared by the resorcinol/formaldehyde approach at pH 6.0). These materials were used as received/prepared and functionalised through oxidation with nitric acid. The oxidised materials were then heat treated under inert atmosphere (N2) at different temperatures (between 350 and 900°C). The obtained samples were characterised by adsorption of N2 at -196 °C, determination of the point of zero charge and by temperature programmed desorption. High adsorption capacities ranging from approximately 60 to 300 mgCPxgC(-1) were obtained (for oxidised carbon xerogel, and oxidised thermally treated activated carbon Norit ROX 8.0, respectively). In general, it was found that the nitric acid treatment of samples has a detrimental effect in adsorption capacity, whereas thermal treatments, especially at 900 °C after oxidation, enhance adsorption performance. This is due to the positive effect of the surface basicity. The kinetic curves obtained were fitted using 1st or 2nd order models, and the Langmuir and Freundlich models were used to describe the equilibrium isotherms obtained. The 2nd order and the Langmuir models, respectively, were shown to present the best fittings.

Applications of Gold Nanoparticles in Nanomedicine: Recent Advances in Vaccines

Nowadays, gold is used in (nano-)medicine, usually in the form of nanoparticles, due to the solid proofs given of its therapeutic effects on several diseases. Gold also plays an important role in the vaccine field as an adjuvant and a carrier, reducing toxicity, enhancing immunogenic activity, and providing stability in storage. An even brighter golden future is expected for gold applications in this area.

Highly active phosphite gold(I) catalysts for intramolecular hydroalkoxylation, enyne cyclization and furanyne cyclization

New and highly active mononuclear phosphite gold(I) catalysts are described. Turn-over numbers up to 37,000 for the furan-yne reaction and up to 28,000,000 for the two-fold hydroalkoxylation of alkynes are reported.

Heterogenisation of a c-scorpionate feII complex on carbon materials for cyclohexane oxidation with hydrogen peroxide

The hydrotris(pyrazol‐1‐yl)methane iron(II) complex [FeCl2{η3‐HC(pz)3}] (pz=pyrazol‐1‐yl) (1) was immobilized on three different carbon materials (activated carbon, carbon xerogel and multi‐walled carbon nanotubes) with three different surface treatments (original, treated with nitric acid, and treated with nitric acid followed by sodium hydroxide) to produce active, selective and recyclable catalysts. The heterogenisation process was more efficient for carbon nanotubes treated with nitric acid and sodium hydroxide. An outstanding improved catalytic performance of complex 1 upon heterogenisation on carbon nanotubes treated with nitric acid and sodium hydroxide (turnover numbers up to 5.6×103 and overall yield of 21 %), relative to the homogeneous system, was achieved for the single‐pot peroxidative oxidation of cyclohexane to the cyclohexanone and cyclohexanol mixture. The heterogenised systems allowed their easy recovery and reuse, at least for five consecutive cycles, maintaining 96 % of the initial activity and concomitant rather high selectivity to cyclohexanol and cyclohexanone.

Adsorption of small molecules on gold single crystal surfaces

Much more work has been carried out on supported gold catalysts (also called “real” catalysts) than on gold single crystal surfaces. However, for fundamental understanding of catalysis by gold, well-defined gold surfaces and controlled conditions using the surface science approach may provide useful information concerning reaction mechanisms and the nature of active centers. This paper presents a brief overview on the work carried out regarding adsorption of several small molecules on gold surfaces from 2004, date of the last review on gold surface science [R. Meyer, C. Lemire, S.K. Shaikhutdinov and H. Freund,Gold Bull., 2004, 37, 72], until recently. Both experimental and theoretical results are discussed. A large difference between the flat Au(111), by far the most studied surface, and stepped/kinked surfaces, is found, demonstrating the importance of low-coordinated Au atoms for high catalytic activity.

Redox properties and VOC oxidation activity of Cu catalysts supported on Ce1−xSmxOδ mixed oxides

A series of Cu catalysts supported on Ce1-xSmxOδ mixed oxides with different molar contents (x=0, 0.25, 0.5, 0.75 and 1), was prepared by wet impregnation and evaluated for volatile organic compounds (VOC) abatement, employing ethyl acetate as model molecule. An extensive characterization study was undertaken in order to correlate the morphological, structural and surface properties of catalysts with their oxidation activity. The optimum performance was obtained with Cu/CeO2 catalyst, which offers complete conversion of ethyl acetate into CO2 at temperatures as low as 260°C. The catalytic performance of Cu/Ce1-xSmxOδ was interpreted on the basis of characterization studies, showing that incorporation of samarium in ceria has a detrimental effect on the textural characteristics and reducibility of catalysts. Moreover, high Sm/Ce atomic ratios (from 1 to 3) resulted in a more reduced copper species, compared to CeO2-rich supports, suggesting the inability of these species to take part in the redox mechanism of VOC abatement. Sm/Ce surface atomic ratios are always much higher than the nominal ratios indicating an impoverishment of catalyst surface in cerium oxide, which is detrimental for VOC activity.

Nanostructured iron oxide catalysts with gold for the oxidation of carbon monoxide

A commercial iron oxide support is compared with Fe2O3 samples prepared by decomposition of iron nitrate, at 300 °C and 500 °C and heating times varying from 30 min to 96 h in N2. Different methods were used for gold deposition, namely double impregnation (DIM), liquid phase reductive deposition (LPRD) and ultrasonication (US). Samples were characterised by N2 adsorption at −196 °C, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction. CO oxidation was used as a test reaction to compare the catalytic activities. The best results were obtained for the sample produced by decomposition of nitrate at 300 °C for 1 h, which showed the largest surface area and highest amount of hydroxylated iron species. Increasing the calcination time and/or the temperature produced less active samples. Although LPRD materials showed the smallest gold nanoparticle sizes (1–12 nm), the best catalytic results were obtained for the DIM materials. This is most likely related to the oxidation state of gold (Au+) found in the DIM catalysts, in contrast with LPRD and US materials, where metallic gold (Au0) is present. Other samples showed lower activities. Nevertheless, all samples prepared by DIM had better catalytic activity than the Au/Fe2O3 World Gold Council reference catalyst.

Effect of preparation method on the solid state properties and the deN2O performance of CuO–CeO2 oxides

The present work aims at investigating the catalytic decomposition of N2O over CuO–CeO2 single or mixed oxides prepared by different synthesis routes, i.e., impregnation, precipitation and exotemplating. To gain insight into the particular role of CeO2 as well as of CuO–CeO2 interactions, three different types of materials were prepared and tested for N2O decomposition both in the absence and presence of excess O2: (i) bare CeO2 prepared by precipitation and exotemplating, (ii) CuO/CeO2 oxides synthesized by the impregnation of CeO2 samples prepared in (i) with CuO, and iii) single stage CuO–CeO2 mixed oxides synthesized employing the co-precipitation and exotemplating methods. The corresponding commercial samples were also examined for comparison purposes. All materials were characterized by N2 adsorption at −196 °C, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy (micro-Raman) and scanning electron microscopy (SEM). The results demonstrated the key role of the preparation procedure on the direct catalytic decomposition of N2O. Among the bare CeO2 samples, the best performance was obtained with the samples prepared by the precipitation method, followed by exotemplating, while commercial CeO2 showed the lowest performance. All bare oxides demonstrated low N2O conversion, never exceeding 40% at 600 °C. Amongst the CuO–CeO2 oxides, the optimum performance was observed for those prepared by co-precipitation, which achieved complete N2O conversion at 550 °C. In the presence of excess oxygen in the feed stream, a slight degradation is observed, with the sequence of deN2O performance remaining unchanged. The superiority of the Cu–Ce mixed oxides prepared by precipitation compared to all of the other materials can be mainly ascribed to their excellent redox properties, linked to Ce4+/Ce3+ and Cu2+/Cu+ redox pairs. A redox mechanism for the N2O catalytic decomposition is proposed, involving N2O adsorption on Cu+ sites and their regeneration through Cu–ceria interactions.

Facile one-pot synthesis of Pt nanoparticles /SBA-15: an active and stable material for catalytic applications

Pt/SBA-15 with an enhanced surface area but unchanged pore diameter (compared to pure SBA-15) and a Pt average particle size of ∼9 nm shows a high and stable activity for both gas-phase CO oxidation and liquid-phase cyclooctadiene hydrogenation. No intrinsic change in the structure of the catalyst occurs after several reaction cycles, suggesting that the Pt/SBA-15 presented here is an active and stable catalyst.