Abstract
Characteristics and catalytic activity in hydrogen production from formic acid of Au catalysts supported on porous N-free (Au/C) and N-doped carbon (Au/N-C) have been compared with those of Au/SiO2 and Au/Al2O3 catalysts. Among the catalysts examined, the Au/N-C catalyst showed the highest Au mass-based catalytic activity. The following trend was found at 448 K: Au/N-C > Au/SiO2 > Au/Al2O3, Au/C. The trend for the selectivity in hydrogen production was different: Au/C (99.5%) > Au/Al2O3 (98.0%) > Au/N-C (96.3%) > Au/SiO2 (83.0%). According to XPS data the Au was present in metallic state in all catalysts after the reaction. TEM analysis revealed that the use of the N-C support allowed obtaining highly dispersed Au nanoparticles with a mean size of about 2 nm, which was close to those for the Au catalysts on the oxide supports. However, it was by a factor of 5 smaller than that for the Au/C catalyst. The difference in dispersion could explain the difference in the catalytic activity for the carbon-based catalysts. Additionally, the high activity of the Au/N-C catalyst could be related to the presence of pyridinic type nitrogen on the N-doped carbon surface, which activates the formic acid molecule forming pyridinium formate species further interacting with Au. This was confirmed by density functional theory (DFT) calculations. The results of this study may assist the development of novel Au catalysts for different catalytic reactions.
Highlights
Supported highly dispersed gold attracts a lot of attention as a catalyst—the properties of which could be better than those of the dispersed Pt-group metals
The surface concentration of gold in the carbon supported catalysts was determined by X-ray photoelectron spectroscopy (XPS) and a good correspondence with the bulk chemical analysis data was obtained
In the present study, using density functional theory (DFT) calculations, we have considered the interaction of the formic acid molecule through the hydrogen atom directed to the pyridinic atom located near the Au atom (Figure S3)
Summary
Supported highly dispersed gold attracts a lot of attention as a catalyst—the properties of which could be better than those of the dispersed Pt-group metals. The activity of supported Au catalysts in formic acid decomposition depends on the type of the support [19,20], Au dispersion [18,21], and doping with alkali metal formates/carbonates [22,23]. The dispersed gold catalysts on carbon might be beneficial for the hydrogen production from formic acid decomposition taking into account the advantages of utilization of carbon support mentioned above. Some Au/C catalysts were tested in the gas-phase [20,24] and in liquid-phase [10] formic acid decomposition The former experiments showed high selectivity for the hydrogen production, even at relatively high temperatures (≥473 K). Density functional theory (DFT) calculations allowed elucidation of the basic steps of the mechanism of the formic acid conversion on the Au/N-C catalyst
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