Electrochemical Catalytic Activities of Pseudomorphic Pt Monolayers Electrodeposited on Au(111) for Various Reactions

Introduction. 

    Ultra-thin metal layers on the foreign metal substrates have been attracting interests because of their unique physical and chemical properties, particularly their high electro-catalytic activities. Such special catalytic activity is due to their unique surface atomic arrangements and their induced surface electronic energy.

     Because Pt is one of the best catalysts for various chemical reactions, Pt is one of the most precious and expensive metals, and Pt resources are limited, Pt ultrathin layer formed on the foreign metal substrates is greatly expected that catalytic activity of Pt becomes higher and that loading amount of Pt becomes less. Recently, we succeeded in electrochemically construction of a pseudomorphic Pt monolayer on a single crystal Au(111) surface and its atomic arrangement was precisely confirmed to be pseudomorphic by resonance surface X-ray scattering (RSXS) measurements [1,2]. Moreover, this pseudomorphic Pt monolayer on the Au(111) surface has higher electro-catalytic activity than Pt(111) [3,4]. In this report, electro-catalytic activities of the electrochemically prepared pseudomorphic Pt monolayers on the Au(111) surface for hydrogen oxidation reaction (HOR) and methanol oxidation reaction (MOR) were investigated.

Experimentals. 

     Pseudomorphic Pt monolayer on Au(111) (Pt/Au(111)) was electrochemically deposited by the same procedures as previously reported [1-4]. HOR measurements were carried out in a hydrogen saturated 0.1 M HClO₄ with a scan rate of 50 mV s^-1 at several rotation rates by using rotation disk electrode (RDE) system. MOR measurements were performed in a nitrogen saturated 1.0 M CH₃OH + 0.1 M HClO₄ with a scan rate of 50 mV s^-1 from 0.20 V to 1.15 V (vs. RHE). Same measurements of Au(111) and Pt(111) single crystal electrodes were carried out and the results were compared and examined.

Results and Discussion.

    As a limitation of the space, the results of MOR were shown here. Cyclic volutammograms (CVs) for MOR of the Pt/Au(111), Pt(111) and Au(111) electrode show as follows. Oxidation current at Au(111) did not flow at all.  On the other hand, oxidation current at Pt/Au(111) and  Pt(111) started to flow from ca. 0.5 V (vs. RHE). As compared the result of Pt/Au(111) with that of Pt(111), the starting potential to flow oxidation current and the peak current density at Pt/Au(111) were more negative and higher than those at Pt (111), respectively. Thus, we can conclude that the electro-catalytic activity of the pseudomorphic Pt monolayer formed on Au(111) for MOR is higher than those of Pt(111) and Au(111).

     MOR can be simply indicated as the equation (1).

 

          CH₃OH  +  H₂O  →CO₂  +  6H⁺  +  6e^- (1)

However, as soon as a methanol molecule is adsorbed on the Pt surface at relatively negative potential, the reaction (2) takes place.

          CH₃OH  →  CO_ad  +  4H⁺  +  4e^-         (2)

This phenomenon is so-called CO poisoning [5], which causes the loss of electro-catalytic activity of Pt. When the potential is positively made, adsorbed CO is oxidized and then desorbed as equation (3).

          CO_ad  +  H₂O  →  CO₂  +  2H⁺  +  2e^- (3)

Thus, the above results of higher electro-catalytic activity of Pt/Au(111) for MOR were caused by the decrease of adsorption energy of CO, because of the changes of atomic arrangement and surface energy [6].

Dependence of the thickness of the pseudomorphic Pt layers on the electro-catalytic activity is now under investigation.

 

Acknowledgement.

This work was supported by New Energy and Industrial Technology Development Organization (NEDO).

References.

[1]    T. Kondo, M. Shibata, N. Hayashi, H. Fukumitsu, T. Masuda, S. Takakusagi, and K. Uosaki, Electrochim. Acta, 55, 8302 (2010).

[2]    M. Shibata, N. Hayashi, T. Sakurai, A. Kurokawa, H. Fukumitsu, T. Masuda, K. Uosaki, and T. Kondo, J. Phys. Chem. C, 116, 26464 (2012).

[3]    T. Kondo, C. Song, N. Hayashi, T. Sakurai, M. Shibata, H. Notsu, and I. Yagi, Chem. Lett., 40, 1235 (2011).

[4]    M. Ishizaki, M. Kawabuchi, I. Yagi, and T. Kondo, 224th ECS Meeting, San Francisco, #43 (2013).

[5]    B. Beden, C. Lamy, A. Bewick, K. Kunimatsu, J. Electroanal. Chem. 121 (1981) 343.

[6]    T. E. Shubina, M. T. M. Koper, Electrochem. Acta 47 (2002) 3621.