Cláudia Morais

Progress in Multiple-Quantum Magic-Angle Spinning NMR Spectroscopy

Recent advances in nuclear magnetic resonance spectroscopy of quadrupolar (I>1/2) nuclei with half-integer spins in solids have been reviewed. The advent of multiple-quantum (MQ) magic-angle spinning (MAS) spectroscopy gave new momentum to the study of quadrupolar nuclei in materials of academic and industrial interest such as minerals, ceramics and glasses, microporous and mesoporous solids and biological materials. It is now possible to record high-resolution solid-state NMR spectra of a range of important nuclei, namely (11)B, (17)O, (23)Na, (27)Al, (71)Ga, (91)Nb. Since its introduction in 1995 MQMAS NMR has evolved considerably and, at present, a range of very useful related techniques are available and have been reviewed, in particular satellite transition (ST) MAS, Inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei, such as cross-polarization MQMAS and MQ heteronuclear correlation spectroscopy (HETCOR) and the recently introduced J-coupling based experiments (such as J-HMQC).

1D to 3D NMR study of microporous alumino-phosphate AlPO4-40

From one‐ to two‐ and three‐dimensional MAS NMR solid‐state experiments involving 31P and 27Al, we show that the structure of microporous alumino‐phosphate AlPO4‐40 contains at least four times more sites than expected, and we attribute two types of AlIV sites. The newly described 27Al‐31P MQ‐HMQC opens new possibilities of describing details of three‐dimensional bounded networks. Copyright

Understanding the high catalytic activity of propylsulfonic acid-functionalized periodic mesoporous benzenesilicas by high-resolution 1H solid-state NMR spectroscopy

Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO3H, 1) has been shown to be exceptional solid catalysts in the acid-catalyzed condensation of indole on benzaldehyde. The reasons for this distinct behavior are so far not completely understood. Here, we present a study involving the combination of advanced high-resolution solid state magic-angle spinning (MAS) NMR characterization with the results of the application of hydrated and dehydrated 1 with different acid loadings in the acid-catalyzed condensation of indole on benzaldehyde attempting an explanation of the higher performance of these materials when compared with the conventional solid catalysts. 1H MAS NMR investigations show the displacement of the propylsulfonic –SO3H protons to higher 1H chemical shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring –SO3H groups. The acid strength of 1 is lowered by the presence of water. At low sulfonic acid loading the catalytic activity is surprisingly high and independent of the water presence. The 2D 1H–1H recoupling MAS NMR experiments indicate that the phenyl rings may protect the acidic sites against water solvation, thus affording a plausible explanation for the negligible effect of water on the catalytic activity of 1 with low acid loading. For a proton loading higher than 0.40 mmol g−1, we observed a linear relationship between the catalyst TOF and the chemical shift value of the –SO3H proton, thus showing that solid 1H NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.

Structure analysis of the novel microporous aluminophosphate IST-1 using synchrotron powder diffraction data and HETCOR MAS NMR

Abstract A combination of advanced powder diffraction and NMR techniques have allowed the structure of the novel microporous aluminophosphate IST-1 ( | (CH3NH2)4(CH3NH3+)4(OH−)4 | [ Al12P12O48 ] ) to be elucidated. The framework structure was determined in the non-centrosymmetric space group Pca21 (a=9.61523(1) A, b=8.67024(1) A, c=16.21957(2) A) from high-resolution synchrotron powder diffraction data using the program FOCUS. Extra framework species were then located on difference electron density maps. A hydroxyl group was found to bridge between two of the framework Al atoms, and one methylamine species, presumably protonated, could be located in the channels where it H-bonds to three framework oxygens. The most unusual feature of the structure is the second methylamine molecule, which bonds directly to a framework Al atom. The structure is entirely consistent with 31P and 27Al MAS NMR studies, which showed there to be three P (all 4-coordinate) and three Al (one 4-, one 5- and one 6-coordinate) sites, and with 13C MAS NMR, which showed there to be two different types of methylamine species in equal amounts. Assignment of the 31P, 27Al and 13C MAS NMR signals could be deduced from the crystallographic data,31P-27Al HETCOR spectra and ab initio calculations.

Size‐Dependent Electrocatalytic Activity of Free Gold Nanoparticles for the Glucose Oxidation Reaction

Understanding the fundamental relationship between the size and the structure of electrode materials is essential to design catalysts and enhance their activity. Therefore, spherical gold nanoparticles (GNSs) with a mean diameter from 4 to 15 nm were synthesized. UV/Vis spectroscopy, transmission electron microscopy, and under-potential deposition of lead (UPDPb ) were used to determine the morphology, size, and surface crystallographic structure of the GNSs. The UPDPb revealed that their crystallographic facets are affected by their size and the growth process. The catalytic properties of these GNSs toward glucose electrooxidation were studied by cyclic voltammetry, taking into account the scan rate and temperature effects. The results clearly show the size-dependent electrocatalytic activity for glucose oxidation reactions that are controlled by diffusion. Small GNSs with an average size of 4.2 nm exhibited high catalytic activity. This drastic increase in activity results from the high specific area and reactivity of the surface electrons induced by their small size. The reaction mechanism was investigated by in situ Fourier transform infrared reflectance spectroscopy. Gluconolactone and gluconate were identified as the intermediate and the final reaction product, respectively, of the glucose electrooxidation.

Highly Selective Oxidation of Carbohydrates in an Efficient Electrochemical Energy Converter: Cogenerating Organic Electrosynthesis

The selective electrochemical conversion of highly functionalized organic molecules into electricity, heat, and added-value chemicals for fine chemistry requires the development of highly selective, durable, and low-cost catalysts. Here, we propose an approach to make catalysts that can convert carbohydrates into chemicals selectively and produce electrical power and recoverable heat. A 100% Faradaic yield was achieved for the selective oxidation of the anomeric carbon of glucose and its related carbohydrates (C1-position) without any function protection. Furthermore, the direct glucose fuel cell (DGFC) enables an open-circuit voltage of 1.1 V in 0.5 m NaOH to be reached, a record. The optimized DGFC delivers an outstanding output power Pmax =2 mW cm(-2) with the selective conversion of 0.3 m glucose, which is of great interest for cogeneration. The purified reaction product will serve as a raw material in various industries, which thereby reduces the cost of the whole sustainable process.

Novel nuclear magnetic resonance techniques for the study of quadrupolar nuclei in clays and other layered materials

Abstract The main developments taking place in Nuclear Magnetic Resonance Spectroscopy (NMR) of quadrupolar (spin I > 1/2) nuclei with half integer spins in solids, particularly clays and other layered materials, have been reviewed. The advent of Multiple-Quantum (MQ) Magic-Angle Spinning (MAS) NMR spectroscopy has been a step-change development to the studies of quadrupolar nuclei in solids. It is now possible to record high-resolution spectra of important nuclei, such as 11B, 17O, 23Na, 27Al and 69,71Ga, in synthetic and natural clays. Since its introduction in 1995 MQMAS NMR has evolved considerably and, at present, a range of very useful related techniques are available and have been reviewed, as has the current situation with regard to applications to clays and other layered materials.

Three dimensionally ordered mesoporous hydroxylated NixCo3−xO4 spinels for the oxygen evolution reaction: on the hydroxyl-induced surface restructuring effect

Surface restructuration upon potential cycling of three dimensionally ordered NixCo3−xO4 spinels for the oxygen evolution reaction (OER) in an alkaline medium is studied using structural, spectroscopic and electrochemical techniques. It was shown that the intrinsic activity of different catalysts depends on the incorporated amount of nickel and surprisingly correlates with the CoIII/CoIV peak potential. The electrochemical activity of the OER is amazingly improved upon potential cycling. It was observed that potential cycling induces an increase of active sites up to 45% on the most effective electrocatalyst. This unexpected increase in activity is very pronounced and becomes stable after 30 voltammetric cycles. Such a phenomenon is explained by the formation of a layered mixed nickel/cobalt oxyhydroxide active site whose oxidation potential is related to the nickel amount in the catalyst. The formation of this layer is promoted by the surface hydroxylation degree of non-cycled catalysts. In these catalysts, nickel modulates the electronic properties of the active site, which modifies the adsorption energies of key oxygenated intermediates. The synthesis route proposed herein allows an efficient way for obtaining high specific surface areas as well as highly hydroxylated surfaces, the latter being the key factor in the enhancement of the electrocatalytic activity of nickel cobaltites.

Identification of chemicals resulted in selective glycerol conversion as sustainable fuel on Pd-based anode nanocatalysts

Palladium-based nanoparticles were prepared using mild microwave-assisted heating. The activity of carbon supported PdM (M = Mn and Fe) toward glycerol oxidation in alkaline medium was studied by coupling electrochemical, analytical and in situ spectroscopic techniques. The complementary findings showed that glycerol was converted into oxalate, tartronate, glycerate, glycolate and formate. The ex situ analytical methods (liquid chromatography and mass spectrometry) were helpful to reveal glycerate as the major reaction product on PdM/C anodes, while from in situ infrared spectroscopy measurements no irreversible adsorbed poisoning species was detected in glycerol or intermediate oxidation to carbonate at the prepared electrodes. The correlation of the analytical and physicochemical (XRD, EDX and TEM) results concerned the shift of the onset potential toward lower values and the high Faradaic currents due to electronic structures provided by the Mn and Fe contents to the Pd based materials. Accordingly, glycerol is a sustainable raw material, which can be used in cogeneration processes for renewable energy sources and selective production of added-value molecules.

Electronic modification of Pt via Ti and Se as tolerant cathodes in air-breathing methanol microfluidic fuel cells

We reported herein on the use of tolerant cathode catalysts such as carbon supported Pt(x)Ti(y) and/or Pt(x)Se(y) nanomaterials in an air-breathing methanol microfluidic fuel cell. In order to show the improvement of mixed-reactant fuel cell (MRFC) performances obtained with the developed tolerant catalysts, a classical Pt/C nanomaterial was used for comparison. Using 5 M methanol concentration in a situation where the fuel crossover is 100% (MRFC-mixed reactant fuel cell application), the maximum power density of the fuel cell with a Pt/C cathodic catalyst decreased by 80% in comparison with what is observed in the laminar flow fuel cell (LFFC) configuration. With Pt(x)Ti(y)/C and Pt(x)Se(y)/C cathode nanomaterials, the performance loss was only 55% and 20%, respectively. The evaluation of the tolerant cathode catalysts in an air-breathing microfluidic fuel cell suggests the development of a novel nanometric system that will not be size restricted. These interesting results are the consequence of the high methanol tolerance of these advanced electrocatalysts via surface electronic modification of Pt. Herein we used X-ray photoelectron and in situ FTIR spectroscopies to investigate the origin of the high methanol tolerance on modified Pt catalysts.