Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

Polymeric carbon nitride-hydrogen peroxide adduct (PCN-H2O2) has been prepared, thoroughly characterised and its application for selective photocatalytic conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxaldehyde (FDC) in aqueous suspension has been studied. The PCN-H2O2 adduct is stable in aqueous suspension under UV and solar irradiation up to 100 °C. It is also stable up to 200 °C if heated in air, while at temperatures close to 300 °C its decomposition takes place. Based on the obtained characterisation data it has been proposed that H2O2 attaches to the non-polymerised carbon nitride species and to the heptazine nitrogen atoms, thus producing strong hydrogen bonding within the PCN-H2O2 adduct. The blockage of the surface amino-groups in PCN-H2O2 by H2O2 hinders the interaction of HMF with these sites, which are responsible for unselective substrate conversion. PCN-H2O2 possesses a superior selectivity in natural solar light assisted oxidation of HMF to FDC reaching 80% with respect to its thermally etched PCN counterpart, which gives rise to a 40-50% selectivity. We believe that the exceptional performance of the applied photocatalyst in the selective photocatalytic conversion of HMF to a high added value FDC in a green solvent under natural illumination makes a significant contribution to the development of environmentally friendly technologies for biomass valorisation.

Although spectroscopic investigation of surface chemisorbed CO 2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO 2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO 2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13 C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times ( T 1 ). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO 2 signals, which contributed to an empirical model of CO 2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T 1 values confirm the presence of CO 2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO 2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13 C-labeled CO 2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO 2 species, showing that 45% of chemisorbed CO 2 versus 55% of physisorbed CO 2 is formed from the overall confined CO 2 in amine-modified hybrid silicas. A total of six distinct CO 2 environments were identified from which three physisorbed CO 2 were discriminated, coined here as “gas, liquid, and solid-like” CO 2 species. The complex nature of physisorbed CO 2 in the presence and absence of chemisorbed CO 2 species is revealed, shedding light on what fractions of weakly interacting CO 2 are affected upon pore functionalization. This work extends the current knowledge on CO 2 sorption mechanisms providing new clues toward CO 2 sorbent optimization.

This work reports a study on the reactivity of pristine and H2O2 treated carbon nitride samples for sunlight photocatalytic selective reactions. The characterization of these materials was reported in a previous paper where the reactivity versus the partial oxidation of 5hydroxymethylfurfural (HMF) was studied by using two different laboratory scale photoreactors; one irradiated by UV lamps and the other one by natural sunlight. In the present study it has been confirmed the effectiveness of these C3N4 based materials for the selective partial oxidation of HMF to FDC (2,5-furandicarboxaldehyde) in aqueous medium in a pilot plant photoreactor irradiated by natural solar light. The reactivity results and, in particular, the selectivity to FDC formation have been very encouraging, mostly by considering that the reaction was carried out in water. Moreover, they are comparable with those obtained in the laboratory scale photoreactor irradiated by both UV artificial lamps and natural sunlight. Interestingly, the pristine C3N4 sample has shown a higher HMF conversion with respect to that of the C3N4-H2O2 adduct, but the last one is more selective to the FDC formation. A kinetic study indicates that, the pseudo-2 first-order rate constant for HMF oxidation is higher in the case of bare photocatalyst and that the equilibrium adsorption constants of HMF are higher in the case of C3N4-H2O2 adduct catalyst. Finally, the partial oxidation of two aromatic alcohols, i.e. benzyl alcohol (BA) and 4-methoxy benzyl alcohol (4-MBA) to benzaldehyde (BAL) and 4-methoxy benzaldehyde (4-MBAL), respectively, has also been studied. It has been found that the inductive and delocalization effects as well as the ortho-para orienting ability of the methoxy group with respect to the hydroxyl one affects the conversion of aromatic alcohol and the selectivity towards the corresponding aldehyde.

Extended X-ray Absorption Fine Structure (EXAFS) investigation of the oxygen-rich titania formed via the thermal treatment of N-doped TiO 2 has revealed that the removal of N-dopants is responsible for the creation of defect sites in the titanium environment, thus triggering at high temperatures (500-800 o C) the capture of atmospheric oxygen followed by its diffusion towards the vacant sites and formation of interstitial oxygen species. The effect of the dopants on Ti coordination number and Ti-O int and TiN int bond distances has been estimated. The photocatalytic p-cresol degradation tests have demonstrated that the interband states formed by the N-dopants contribute to a greater extent to the visible-light activity than the oxygen interstitials do. However, under the UV irradiation the oxygen-rich titania shows higher efficiency in the pollutant degradation, while the N-dopants in N-TiO 2 play the role of recombination sites. On the other hand, the presence of the surface nitrogen species in TiO 2 is highly beneficial for the application in partial photooxidation reactions, where N-TiO 2 demonstrates a superior selectivity of 5-hydroxymethyl furfural (HMF) oxidation to 2,5furandicarboxaldehyde (FDC). Thus, this work underlines the importance of a rational design of non-metal doped titania for photocatalytic degradation and partial oxidation applications, and it establishes the role of bulk defects and surface dopants on the TiO 2 photooxidation performance.

Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

A new solvent-exchange method for preparation of undoped anatase TiO2 nanoparticles with unprecedented thermal stability has been presented.

Four different C3N4 specimens have been prepared, a bulk one (MCN), a thermally etched (MCN-TE), a solid prepared by hydrothermally treating MCN with H2O2 (MCN-H2O2) and a polymeric carbon nitride-hydrogen peroxide adduct (MCN-TE-H2O2). Photoelectrochemical studies revealed that MCN-TE represented the best material in terms of band gap energy and photoconductivity, 2 whereas MCN-H2O2 was defective and evidenced a poor mobility of carriers. EPR studies showed a maximum generation of reactive oxygen species irradiating the MCN-TE sample. The photocatalytic activity of these materials in the selective oxidation of three different alcohols to the corresponding aldehydes, both under UV and natural solar light, showed that the highest conversion was obtained in the presence of the MCN-TE sample, whereas the most selective was MCN-TE-H2O2. Under solar light irradiation the performances of the powders were generally better than those carried under UV light. The characterization of the C3N4-based materials well justified their photocatalytic activity, where the pristine C3N4 materials were more active but less selective than those prepared in the presence of H2O2.