Copper single-atoms embedded in 2D graphitic carbon nitride for the CO2 reduction

Abstract

We report the study of two-dimensional graphitic carbon nitride (GCN) functionalized with copper single atoms as a catalyst for the reduction of CO2 (CO2RR). The correct GCN structure, as well as the adsorption sites and the coordination of the Cu atoms, was carefully determined by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy, with DFT theoretical calculations. The CO2RR products in KHCO3 and phosphate buffer solutions were determined by rotating ring disk electrode measurements and confirmed by 1H-NMR and gas chromatography. Formate was the only liquid product obtained in bicarbonate solution, whereas only hydrogen was obtained in phosphate solution. Finally, we demonstrated that GCN is a promising substrate able to stabilize metal atoms, since the characterization of the Cu-GCN system after the electrochemical work did not show the aggregation of the copper atoms.

Ab-Initio Spectroscopic Characterization of Melem-Based Graphitic Carbon Nitride Polymorphs

“…Additionally, we convoluted the CLSs with Gaussian functions, with broadening σ = 0.17 eV, for the construction of the XPS spectra. In order to compare the CLS of different two-dimensional supercells and their chemical shift, we included an N 2 molecule sufficiently far from the surface, acting as a common reference: such an approach was proven to be robust in previous works of our group [23,45]. To carry out the calculation of the chemical shift for bulk systems, we optimized slabs with 3 layers, keeping the lowest two fixed, calculated the XPS spectra including the N 2 reference, and only considering the CLSs of the intermediate layer.…”

“…The direct comparison with experimental spectra, where major differences were in the position of XRD peaks at low angles, suggests that, depending on the growth conditions during synthesis, one can obtain either a homogeneous melon [27,28,49,54], a mixture of the two melon structures [23], or igCN [24] structures.…”

“…This could be due to the formation of not-so-pure patches of gCN, but also to the influence of the growth environment, namely, the presence of a supporting surface. Regarding the synthesis of a non-uniform sample, in a recent work by our group about the characterization of Cu-functionalized gCN [23], we suggested that the real sample was composed of a mixture of gCN polymorphs, since the experimental XPS spectrum resembled a sum of the simulated XPS spectra for linear and alternated melon models. In order to give additional insight to this hypothesis, we calculated, for each model, the chemical shift with respect to a common reference, namely, an isolated N 2 molecule inserted into the simulation cell, as reported in Table 1.…”

“…In addition to its aforementioned pristine properties, such a compound has more recently been studied as an ideal platform to host further functionalizing species that improve the chemical activity of gCN: among these, the inclusion of transition metal atoms is one of the most exploited. For example, the inclusion of isolated Fe atoms was reported to improve the α-cyanation of secondary and tertiary amines [21], whereas Cu adatoms were found to benefit the reduction of CO 2 [22,23] or in combination with Ni adatoms, to mediate the methanol electro-oxidation [24].…”

“…However, not all geometric degrees of freedom are considered in those works, such as the proper size of the supercell for igCN or the relative arrangement of the monomers for melon: most frequently, the layer is assumed to be perfectly flat [10,22,26] as performed for experimental characterization, and only Liebig's configuration for a melon is considered [27,28], respectively. Such biased approaches have already been questioned by others, who, for example, studied corrugated igCN models [29,30], discussed melon formation rather than igCN [27,28], or proposed an alternative structure for melon [23]. All of these works suggest that detailed evidence from the prepared samples under study must be carefully reviewed in order to correctly identify the structure of the synthesized polymorph present in the sample.…”