The surface science of titanium dioxide

Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequent...

Single-atom catalysts: a new frontier in heterogeneous catalysis.

Prebiotics are non-digestible (by the host) food ingredients that have a beneficial effect through their selective metabolism in the intestinal tract. Key to this is the specificity of microbial changes. The present paper reviews the concept in terms of three criteria: (a) resistance to gastric acidity, hydrolysis by mammalian enzymes and gastrointestinal absorption; (b) fermentation by intestinal microflora; (c) selective stimulation of the growth and/or activity of intestinal bacteria associated with health and wellbeing. The conclusion is that prebiotics that currently fulfil these three criteria are fructo-oligosaccharides, galacto-oligosaccharides and lactulose, although promise does exist with several other dietary carbohydrates. Given the range of food vehicles that may be fortified by prebiotics, their ability to confer positive microflora changes and the health aspects that may accrue, it is important that robust technologies to assay functionality are used. This would include a molecular-based approach to determine flora changes. The future use of prebiotics may allow species-level changes in the microbiota, an extrapolation into genera other than the bifidobacteria and lactobacilli, and allow preferential use in disease-prone areas of the body.

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Dietary modulation of the human colonic microbiota: updating the concept of prebiotics

Single-atom catalysis has arguably become the most active new frontier in heterogeneous catalysis. Aided by recent advances in practical synthetic methodologies, characterization techniques and computational modelling, we now have a large number of single-atom catalysts (SACs) that exhibit distinctive performances for a wide variety of chemical reactions. This Perspective summarizes recent experimental and computational efforts aimed at understanding the bonding in SACs and how this relates to catalytic performance. The examples described here illustrate the utility of SACs in a broad scope of industrially important reactions and highlight the advantages these catalysts have over those presently used. SACs have well-defined active centres, such that unique opportunities exist for the rational design of new catalysts with high activities, selectivities and stabilities. Indeed, given a certain practical application, we can often design a suitable SAC; thus, the field has developed very rapidly and afforded promising catalyst leads. Moreover, the control we have over certain SAC structures paves the way for designing base metal catalysts with the activities of noble metal catalysts. It appears that we are entering a new era of heterogeneous catalysis in which we have control over well-dispersed single-atom active sites whose properties we can readily tune. Single-atom catalysts are heterogeneous materials featuring active metals sites atomically dispersed on a surface. This Review describes methods by which we prepare and characterize these materials, as well as how we can tune their catalytic performance in a variety of important reactions.

Heterogeneous single-atom catalysis

The surface and materials science of tin oxide

We studied the cyclotrimerization of acetylene on size-selected Pdn clusters (1 ≤ n ≤ 30) supported on thin MgO(100) films by thermal desorption and Fourier transform infrared spectroscopy. Surprisingly, the production of benzene is already observed on a single palladium atom at low temperature (300 K). Using density functional theory (DFT) calculations we show that free inert Pd atoms are activated by charge transfer from defect sites of the MgO substrate upon deposition. For larger clusters (7 ≤ n ≤ 30) benzene is additionally produced at a temperature of 430 K and our results suggest the existence of a critical ensemble of seven palladium atoms for this high-temperature reaction mechanism.

Acetylene cyclotrimerization on supported size-selected Pd-n clusters (1 <= n <= 30): one atom is enough!

Electronic Structure of F and V Centers on the MgO Surface

Objective: To investigate the effect of a breakfast cereal containing inulin on blood lipids and colonic ecosystem in normolipidemic young men. Setting: Department of Food Science and Microbiology,University of Milan, Italy. Subjects: Twelve healthy male volunteers, age 23.3±0.5 y, body mass index (BMI) 25.7±1.2 kg/m2 (mean±s.e.m.). Interventions: Subjects consumed daily, for three periods of four weeks, 50 g of a rice-based ready-to-eat cereal (placebo) and the same cereal containing 18% inulin (test) in substitution of their habitual breakfast, then returned to the habitual diet (wash-out). They followed no other dietary restrictions. Results: No changes in body weight, dietary habits, faecal and bile acid output, faecal short-chain fatty acid (SCFA) and faecal pH, were observed at the end of each period, whereas plasma total cholesterol and triacylglycerols significantly decreased at the end of test period by 7.9±5.4 (P<0.05) and 21.2±7.8% (P<0.005) respectively. Meal glucose tolerance test (MTT) resulted in the same incremental area under the curve for both cereals (IAUC test 124±35; placebo 118±33 mmol·min/l, ns). Inulin markedly enhanced breath H2 excretion (IAUC test 280±40; placebo 78±26 ppm·h, P<0.005), as well as faecal concentration of L-lactate. Total facultative anaerobes significantly decreased after test, and bifidobacteria increased after correction for total anaerobes (P<0.05). Changes in blood lipids were negatively correlated with bifidobacteria counts and positively with secondary bile acid excretion (P<0.05). Conclusions: Inulin seems to have a lipid lowering potential in normolipidemic men possibly mediated by mechanisms related to colonic fermentation. Sponsorship: National Research Council of Italy, grant number 95.00773.PF41.

Effect of consumption of a ready-to-eat breakfast cereal containing inulin on the intestinal milieu and blood lipids in healthy male volunteers

In this work, by combining FTIR results with periodic DFT calculations, we highlight the role of different anatase surface sites in the CO2 adsorption and in the subsequent reactions. It is shown that CO2 is mainly adsorbed in linear form at the (101) surface, while the formation of a variety of surface carbonates is occurring at the (001) one. The role of coadsorbed water in the formation of surface bicarbonates is also investigated. All the structures identified by FTIR spectroscopy were modeled by DFT, and the adsorption geometries and the energy of formation for the surface species were carefully analyzed. For the most stable structures we also performed the calculation of the vibrational frequencies that were compared with the FTIR data.

CO2 Capture by TiO2 Anatase Surfaces: A Combined DFT and FTIR Study

The interaction of Rb, Pd, and Ag atoms with the surface vacancies of MgO, the Fs and the Vs centers, has been studied by ab initio cluster model wave functions. We have considered the interaction of each atom with Fs, Fs+, Fs2+, Vs, Vs-, and Vs2- sites. These sites correspond to the removal of O, O-, O2-, Mg, Mg+, and Mg2+ atoms or ions, respectively, from the surface. The bond with the metal atoms, which is found to be very weak on the regular surface sites, can be very different depending of the formal charge of the vacancy. Neutral Fs centers are in general rather unreactive as their electronic structure resembles that of the regular surface; Fs+ paramagnetic centers have a relatively large electron affinity and tend to ionize metal atoms with low ionization potentials, such as alkali-metal atoms or to form covalent polar bonds with the adsorbed metal atoms; Fs2+ centers have a very high electron affinity so that all metal atoms are ionized when interacting with these sites. Neutral Vs sites are also ...

Anna Maria Ferrari

Papers

Acetylene cyclotrimerization on supported size-selected Pd-n clusters (1 <= n <= 30): one atom is enough!

Electronic Structure of F and V Centers on the MgO Surface

Effect of consumption of a ready-to-eat breakfast cereal containing inulin on the intestinal milieu and blood lipids in healthy male volunteers

CO2 Capture by TiO2 Anatase Surfaces: A Combined DFT and FTIR Study

Metal Deposition on Oxide Surfaces: A Quantum-Chemical Study of the Interaction of Rb, Pd, and Ag Atoms with the Surface Vacancies of MgO