Electrocatalytic CO2 Reduction over Cu3P Nanoparticles Generated via a Molecular Precursor Route

The design of nanoparticles (NPs) with tailored morphologies and finely tuned electronic and physical properties has become a key strategy for controlling selectivity and improving conversion efficiency in a variety of important electrocatalytic transformations. Transition metal phosphide NPs, in particular, have emerged as a versatile class of catalytic materials due to their multifunctional active sites and composition- and phase-dependent properties. Access to targeted transition metal phosphide NPs with controlled features is necessary to tune the catalytic activity. To this end, we have established a solution-synthesis route utilizing a molecular precursor containing M–P bonds to generate solid metal phosphide NPs with controlled stoichiometry and morphology. We expand here the application of molecular precursors in metal phosphide NP synthesis to include the preparation of phase-pure Cu3P NPs from the thermal decomposition of [Cu(H)(PPh3)]6. The mechanism of [Cu(H)(PPh3)]6 decomposition and subsequent formation of Cu3P was investigated through modification of the reaction parameters. Identification and optimization of the critical reaction parameters (i.e., time, temperature, and oleylamine concentration) enabled the synthesis of phase-pure 9–11 nm Cu3P NPs. To probe the multifunctionality of this materials system, Cu3P NPs were investigated as an electrocatalyst for CO2 reduction. At low overpotential (−0.30 V versus RHE) in 0.1 M KHCO3 electrolyte, Cu3P-modified carbon paper electrodes produced formate (HCOO−) at a maximum Faradaic efficiency of 8%.


Figure S1 .
Figure S1.TEM images of Cu3P synthesized with 15 mmol OAm following a 30 min hold at 250 °C and then a (a) 15 min hold or (b) 30 min hold at 320 °C.

Figure S3 .
Figure S3.XRD pattern of reaction aliquot removed after 30 min at 250 °C with 15 mmol OAm.Reference pattern for Cu is shown below, and the dotted line on the experimental pattern indicates the highest intensity peak for Cu.

Figure S6 .
Figure S6.Expansion of the Cu region of the XRD patterns after 5 min, 15 min, 30 min, 1 h, and 2 h reaction at 300 °C with an oleylamine concentration of 15 mmol.Reference pattern for Cu is shown below, and the dotted line on the experimental patterns indicates the highest intensity peak for Cu.

Figure S7 .
Figure S7.TEM images of Cu3P synthesized with 20 mmol OAm following 30 min temperature hold at 250 °C and 15 min of heating at 320 °C.

Figure S8 .
Figure S8.Comparison of XRD patterns after reaction at 300 °C with 15 mmol OAm and 20 mmol OAm for 1 h and 2 h.Reference patterns for Cu and Cu3P are shown below, and the dotted lines on the experimental patterns indicate the highest intensity peak for Cu and Cu3P.

Figure S9 .
Figure S9.XRD patterns of Cu3P NPs synthesized with 2 equivalents of PPh3 and 15 mmol OAm and held at 320 °C for 15 and 30 min.

Figure S10 .
Figure S10.TEM images of Cu3P synthesized with 15 mmol OAm and 2 equivalents of PPh3 at 320 °C for (a) 15 min and (b) 30 min.

Figure S11 .
Figure S11.(a) XRD patterns of products formed with 4 equivalents of PPh3 after 1 h and 2 h reaction at 300 °C.(b) TEM image of the reaction mixture following synthesis with 15 mmol OAm and 4 equivalents of PPh3 at 300 °C for 2 h.

Figure S12 .
Figure S12.(a) FTIR spectra of as-synthesized Cu3P NPs and neat OAm.(b) FTIR spectra of assynthesized Cu3P on FTO before electrolysis and following 3 h of electrolysis in CO2-saturated 0.1 M KHCO3 at -0.40 V and -0.50 V versus RHE.

Figure S14 .
Figure S14.Capacitive current at the open circuit potential (OCP) of OAm-Cu3P/CP and R-Cu3P/C in CO2-saturated 0.1 M KHCO3 at a scan rate of 100 mV/s.

Figure S20 .
Figure S20.Quality of fits for P-Cu3P/C (a) OAm-P-Cu3P/C, (b) after reduction at 450 °C in 5% H2/He, and (c) after passivation with 1% O2/He at 20 °C, where the black traces represent the FT magnitudes and red represents the imaginary components with dashed lines for the fitted curves.

Table S4 .
Fits of the k 2 -weighted EXAFS for P-Cu3P/C in He at room temperature, after reduction at 450 °C in 5% H2/He, and after 1 h passivation in 1% O2/He at 20 °C.