PtSnRh ternary catalysts have been proved to be useful for ethanol oxidization reaction (EOR).¹ We demonstrated that when Rh was deposited on Pt adjacent to the SnO₂ on SnO₂–modified Pt substrate, the selectivity of CO₂ was greatly improved,² suggesting that these three elements play an essential role for the complete EOR to produce CO₂. In this study, to confirm this conclusion with nanoparticles, we prepared two kinds of Pt/Rh/SnO₂ nanoparticle catalysts, in which the position of the Rh atoms was controlled by the partial CO stripping. The resultant Pt/Rh/SnO₂ nanoparticle catalysts were evaluated the EOR activity as well as the CO₂ selectivity by electrochemical and spectroscopic methods.

Experimental

Pt/SnO₂(3:1) nanoparticle-loaded carbon black was prepared according to ref. 3. The Rh deposition on the Pt/SnO₂/C nanoparticle catalyst was conducted in a three-electrode cell with a Pt plate (2x3 cm) as the counter electrode (CE), reversible hydrogen electrode (RHE) as reference electrode, and a Pt mesh as the working electrode (WE). The Pt/SnO₂/C powder was sonicated in an Ar-saturated 0.5 M H₂SO₄ solution at 30 ^oC for 1 h to get the well-dispersed suspension.

In the first method, Rh was directly deposited on the Pt/SnO₂ nanoparticles. Hydrogen was bubbled in the Pt/SnO₂/C suspension containing 0.1 mM RhCl₃ with stirring to deposit Rh on the Pt surface of the nanoparticles. The ternary catalyst was obtained after filtering, washing, and evaporating, which is named Ter-1.

In the second method, the Rh deposition site was controlled by the partial CO stripping. CO was bubbled in the Pt/SnO₂/C suspension to adsorb CO on the Pt surface of the Pt/SnO₂ nanoparticles. Then the adsorbed CO on Pt neighboring to Sn was selectively desorbed while the potential was set at 0.5V vs. RHE for 15 min. After that, Rh was deposited on the exposed Pt surface of the Pt/SnO₂ nanoparticles by using the same process as in the first method, which is named Ter-2.

For electrochemical measurements, each ternary catalyst was casted on a glassy carbon (GC) electrode as described in ref. 3. Linear sweep voltammetry (LSV) in 1 M C₂H₅OH/0.1 M HClO₄ was performed to evaluate EOR activity. Products and intermediates of EOR were identified by infrared reflection absorption spectroscopy (IRRAS).

Results and discussion

From X-ray diffraction spectroscopy, the size of Pt/SnO₂ and ternary nanoparticles was estimated as 2.5 nm and 2.8 nm, respectively. Energy-dispersive X-ray spectroscopy revealed the similar metal composition for the two ternary catalysts, in which the Rh content was ca. 5%. Figure 1 illustrates that Ter-2 has the lowest onset potential and the highest current density toward EOR, whereas the current density of Ter-1 is even lower than Pt/SnO₂/C. Figure 2 shows the IRRAS spectra of the Pt/SnO₂, Ter-1 and Ter-2. In comparison with Pt/SnO₂/C, Ter-2 greatly facilitated the formation of CO₂ (2343 cm^-1) as opposed to acetic acid (1277 cm^-1). In contrast, Ter-1 exhibited the similar product distribution for EOR to Pt/SnO₂/C.

References

1. A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel and R.R. Adzic. Nature Materials, 8, 325 (2009).
2. M. P. Tu, M. Chiku, E. Higuchi and H. Inoue, ECS Transactions, 69 (17) 675-681 (2015)
3. E. Higuchi, K. Miyata, T. Takase, H. Inoue, J. of Power Sources, 196 (2001), 1730 – 1737.