Theoretical study of support effect of Au catalyst for glucose oxidation of alkaline fuel cell anode

Highlights

• The catalytic activity of Au in alkaline solution is studied theoretically.

• Carbon and oxide materials are used to estimate support effect for glucose oxidation.

• The glucose oxidation on SnO₂(1 1 0) supported Au catalyst shows high activity.

• The charge transfer from Au catalyst to support materials is dominant.

Abstract

We theoretically analyzed the glucose oxidation reaction mechanism and reaction activity of Au catalyst supported by carbon (graphite(0 0 0 1), ($10\overline{1}0$), and ($11\overline{2}0$)) and oxide (ZrO₂(1 1 1) and SnO₂(1 1 0)) in alkaline solution environment by using density functional theory method. We observed large stabilization of Au catalyst on support materials due to the electron transfer in the case of graphite($11\overline{2}0$) and SnO₂(1 1 0) systems. The catalytic activity for glucose oxidation reaction over Au supported by graphite($10\overline{1}0$) and ($11\overline{2}0$) is calculated to be low in comparison with those of unsupported system. We found that SnO₂(1 1 0) supported Au catalyst shows high activity toward the glucose oxidation. One of the main factors for the observed high catalytic activity is charge transfer from Au catalyst to support materials. When the atomic charge of Au catalyst becomes positive by the support effect, the activity of glucose oxidation reaction on Au catalyst is improved.

Introduction

The selective oxidation of biofuels, such as ethanol and glucose, will be a sustainable, environmentally important process in future low-carbon society [1], [2], [3], [4]. To achieve the high efficiency utilization of biofuels, alkaline fuel cell is one of the promising options [5], [6], [7]. Recently, Au has attracted interests as electrocatalyst for biofuels oxidation in alkaline solution [8], [9], [10], [11]. For example, Koper et al. found the promoting effect of adsorbed CO on the oxidation of alcohols on an Au catalyst [9]. This result suggests that it is possible to avoid CO poisoning, which causes severe deactivation in Pt-based catalyst, by the use of Au catalyst. They also found that the current density for ethanol oxidation drastically increases at high pH region [10]. It is thus expected that the Au catalyst in alkaline solution environment plays an important role for highly efficient oxidation of biofuels.

We focused on the oxidation of glucose on Au catalyst in alkaline solution because the glucose is abundant, cheap, non-toxic, and easy to produce and handle. It is well known that the Au catalyst in alkaline solution shows high activity toward glucose electrooxidation [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. However, the mechanism of glucose oxidation is unclear even the first step of oxidation from glucose to gluconic acid represented by Eq. (1), though there are many experimental studies for Au nanoparticle sizes and support materials.

C₆H₁₂O₆ + 2OH⁻ → C₆H₁₂O₇ + H₂O + 2e⁻

Understanding the glucose oxidation on Au surface in alkaline solution is essential for high active and efficient catalytic material design. Recently, theoretical methods are widely used to clarify the reaction mechanism of heterogeneous catalysis from atomistic point of view [24], [25], [26], [27], [28], [29]. To elucidate the electrooxidation process of glucose in direct alkaline fuel cell, we theoretically analyzed the surface state of Au in alkaline solution and the glucose oxidation reaction by using density functional theory (DFT) method in our preceding study [30]. We have analyzed the glucose oxidation reaction mechanism assuming that the Au surface in alkaline solution is covered by OH⁻ due to the stronger adsorption of OH⁻ onto Au surface than that of H₂O. Through the calculations for various possible reaction pathways from glucose to gluconic acid, we found minimum reaction energy pathway as schematically shown in Fig. 1. The theoretically found catalytic reaction cycle of glucose oxidation on Au catalyst in alkaline solution is as follows. (i) Glucose adsorbs on OH adsorbed on Au surface. (ii) OH⁻ in alkaline solution interacts with CHO group of glucose. (iii) Water is formed by proton transfer from CHO group to OH⁻ in aqueous phase. (iv) Gluconic acid is formed by OH transfer from Au surface. Unique aspects in this unique reaction mechanism we identified are the reaction initiated from the glucose adsorption on OH of Au surface and subsequent attack by OH⁻ in aqueous solution. This is also supported by the study by Pasta et al. proposing the importance of ionic species (for example, OH⁻) for oxidation reaction of glucose [14].

In addition to intrinsic catalytic activity of Au, the choice of support materials of Au catalyst is important for the improvement of catalytic activity [31], [32], [33], [34], [35], [36], [37], [38], [39]. The carbon based materials such as glassy carbon, highly ordered pyrolytic graphite, carbon nanotubes, and graphene are often used as the support material in electrocatalysis [31], [32], [33], [34], [35]. On the other hand, the various oxides have been recognized as the efficient support material for catalysis [36], [37], [38], [39]. A wide range of supports have been tested, such as TiO₂, ZrO₂, SnO₂, and CeO₂. In fact, CO oxidation on Au catalyst was improved by SnO₂ support [39]. One will often observe a certain support facilitates specific reactions while it retards other reactions.

In this study, we analyzed the interaction energy and charge transfer between Au and support materials by DFT calculations. The change of glucose oxidation reactivity on carbon and oxide supported Au catalyst was also investigated.