Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles

Abstract

Platinum nanoparticles (Pt NPs) are one of the most efficient cocatalysts in photocatalysis, and their size determines the activity and the selectivity of the catalytic reaction. Nevertheless, an in-depth understanding of the platinum's size effect in the carbon dioxide photocatalytic reduction is still lacking. Through analyses of the geometric features and electronic properties with variable-sized Pt NPs, here we show a prominent size effect of Pt NPs in both the activity and selectivity of carbon dioxide photocatalytic reduction. Decreasing the size of Pt NPs promotes the charge transfer efficiency, and thus enhances both the carbon dioxide photocatalytic reduction and hydrogen evolution reaction (HER) activity, but leads to higher selectivity towards hydrogen over methane. Combining experimental results and theoretical calculations, in Pt NPs, the terrace sites are revealed as the active sites for methane generation; meanwhile, the low-coordinated sites are more favorable in the competing HER.

Fig. 1

Preparation and structure characterization of the Pt nanoparticle distribution. a Schematic illustration of the acid–base-mediated alcohol reduction method for manipulation of the size of Pt NPs. TEM and corresponding HR-TEM images of b, f, j 1.8PHTSO, c, g, k 3.4PHTSO, d, h, l 4.3PHTSO, and e, i, m 7.0PHTSO; the red squares indicate the stepwise magnifications of the local sites. HR-TEM images of n 1.8 nm Pt/C₃N₄; o 3.4 nm Pt/C₃N₄; p 4.3 nm Pt/C₃N₄; and q 7.0 nm Pt/C₃N₄. The scale bars are 50 nm in b–e; 10 nm in f; 20 nm in g–i; n–q; and 2 nm in j–m

Fig. 2

Crystallinity, surface morphology and optical properties of xPHTSO. a XRD patterns and the inset image show the local magnification from 2θ at the 35–45° range. b UV–Vis DRS spectra and the corresponding samples’ photos (inset). c HR-TEM image of 7.0PHTSO, the scale bar is 2 nm; the inset image with the marked facets of Pt NPs shows the characteristic truncated-octahedron shape of this Pt particle; the inset image of the corresponding fast Fourier transform (FFT) pattern of the Pt particle. d Proportions of the corner, edge, Pt (111) and Pt (100) surface sites based on the standard truncated octahedron model of Pt NPs with different sizes

Fig. 3

Electronic properties of xPHTSO. High-resolution Pt 4f XPS spectra of a 1.8PHTSO, b 3.4PHTSO, c 4.3PHTSO, and d 7.0PHTSO. e The kinetics of the characteristic transient absorption band observed at 350 nm in the fs-TA spectra after 310 nm excitation of samples 1.8PHTSO and 4.3PHTSO. The solid lines were the curves fitted by a two exponential. f Transient photocurrent response of xPHTSO (x = 1.8, 3.4, 4.3, and 7.0), where a 300 W Xe lamp is used as the light source and a 0.5 M Na₂SO₄ solution is used as the electrolyte

Fig. 4

Investigation of the effect of carbon impurities in the CO2PR. a CH₄ yield comparison of xPHTSO (x = 1.8, 3.4, 4.3, and 7.0) in different atmospheres, where blue bars denote the yield of CH₄ generated from CO₂/H₂O atmosphere and gray bars denote the yield of CH₄ generated from Ar/H₂O atmosphere. b TEM images of 1.7Pt@PVP/HTSO and 1.7 nm Pt NPs colloidal (inset) the scale bars are 20 and 10 nm (inset), and c comparison of CO₂ photoreduction evaluation over 1.7Pt@PVP/HTSO and 1.8PHTSO

Fig. 5

Size-dependent activity and selectivity of Pt NPs in CO2PR. a Correlations between the selectivity for CH₄ and surface site proportion as functions of the size of Pt NPs in xPHTSO (x = 1.8, 3.4, 4.3, and 7.0). b Free energy diagrams for CO₂ reduction to CH₄ by the thermochemical model on Pt(111) surface and Pt55. c Scheme illustration of partial CO-modified 1.8PHTSO through stepwise adsorption and desorption of CO. d CO-TPD results of 1.8PHTSO after CO pulse adsorption and stepwise CO pulse adsorption and He flow desorption at 280 °C. e Performance comparisons of CO2PR between 1.8PHTSO and CO-1.8PHTSO, the RE denotes as the reacted electrons, and CH₄ S denotes as the CH₄ selectivity