Au/ZnO as catalyst for methanol synthesis: The role of oxygen vacancies
Graphical abstract
Gold catalysts supported on zinc oxide with Au loadings of 1, 2, and 3 wt% were prepared by the colloidal deposition method and characterized by TEM, XRD, and N2O frontal chromatography. The presence of the Au particles is assumed to enhance the number of exposed oxygen vacancies in ZnO, presumably located at the interface region. It is concluded that oxygen vacancies in ZnO are also the active sites in methanol synthesis over Au/ZnO.
Introduction
Although bulk gold is essentially chemically inert, gold can be a catalyst for many reactions, such as CO oxidation, selective oxidations of hydrocarbons, alcohols and aldehydes, hydrogenation reactions or the water gas shift reaction (WGSR, Eq. (1)), when it is applied as nanoscaled supported particles [1]. It has been shown that gold can also be an active catalyst in methanol synthesis (Eqs. (2), (3)) [2], [3], [4], [5]. Catalysts such as Au/TiO2, Au/Al2O3, Au/Fe2O3, and Au/ZnO were investigated [3], [4], and all of these systems were found to exhibit catalytic activity for the WGSR and for the hydrogenation of carbon oxides to methanol and methane [3], [4], [6], [7].Compared to the industrial Cu/ZnO/Al2O3 catalyst, which has been extensively characterized [8], [9], these Au catalysts are still barely understood. ZnO as support material for Au was shown to be essential to obtain a high selectivity towards methanol and to minimize methane formation [3], [4], [10]. Nevertheless, the role of ZnO has not been elucidated in detail yet [10].
Generally, the catalytic activity of supported Au particles depends on three key features: the size of the Au particles, the type of the support, and the structure of the interface, suggesting that the catalytic performance is significantly influenced by the interaction between the Au particles and the support [1]. It was claimed that the active sites are located at the interface between the Au particles and the support at the perimeter line of the particles [3], [4], [5], [7], [10], [11]. This concept is also believed to be valid for the performance of Au catalysts both in methanol synthesis [3], [4] and in the WGSR [7]. Smaller Au particles are thus desired to increase the total length of the perimeter at constant Au loading [3], [4], [5], [7], [10]. The reason for the interface being the active part of the catalyst is still under debate. For Cu/ZnO Spencer and coworkers [12], [13] suggested hydrogen dissociation to take place on ZnO followed by reverse spillover to Cu. King and Nix [14] found experimentally that the presence of the metal enhanced the reducibility of ZnO in the system Cu/ZnO. This may also be true for the presence of nanoscaled metallic Au. Frost [15] proposed already in 1988 that the presence of small clusters of any of the Ib group metals (Cu, Ag, Au) perturbs the oxide defect equilibria in semiconducting oxides by metal/oxide junctions enhancing the surface concentration of oxygen vacancies. It is discussed in Refs. [3], [4], whether these oxygen vacancies in the support act as active sites for methanol synthesis in oxide-supported Au catalysts.
It has been proposed that on pure ZnO oxygen vacancies on the polar oxygen-terminated ZnO surface are the active sites for methanol synthesis. Reaction pathways starting from CO [16], [17] and from CO2 [18], [19] were investigated. Experimentally, a higher activity in methanol synthesis on ZnO was observed, when CO2 was absent [16], [17]. In a combined kinetic and theoretical study, an energetically preferable reaction pathway at the oxygen vacancy site was identified starting from CO instead of CO2 [16]. It was proposed that in presence of CO2 a change in the reaction mechanism occurred. From CO2, a very stable formate species can be formed in the defect site, which can be hydrogenated to methanol. Due to its high stability, this rate is supposed to be much lower than methanol formation from CO [16]. Experimental evidence for the fast hydrogenation of CO over oxygen vacancies was obtained by investigating highly oxygen-deficient ZnO samples with electron paramagnetic resonance (EPR) spectroscopy. The samples exhibited a remarkably high catalytic activity in CO/H2, which scaled with the integrated EPR signal intensity and which was quenched in the presence of CO2 [17].
A facilitated formation of oxygen vacancies at the Au/ZnO interface, according to the enhanced reducibility of the oxide as suggested by Frost [15], would result in a larger amount of highly active sites in methanol synthesis. A better catalytic performance compared to pure ZnO should then be observed, especially in CO2-free synthesis gas. Yet, if oxygen vacancies in ZnO are also the active sites in Au/ZnO, the catalytic behavior should still resemble that of pure ZnO [16], [17], [20].
As the particle size of Au is a crucial parameter for the catalytic performance, the colloidal deposition route was applied to obtain samples with constant high quality [21]. A method to derive the number of surface oxygen vacancies in Au/ZnO would be desirable, but is not yet available. In Cu catalysts, the reactive frontal chromatography with N2O (N2O RFC) is used to titrate the exposed Cu surface [22]. For Cu/ZnO, it was proposed that a quantification of surface oxygen vacancies can also be performed by N2O RFC [14]. This method was therefore applied to the Au/ZnO catalysts. It was shown recently that microcalorimetry provides a method for a quantification of the amount of removable lattice oxygen: by applying reduction–oxidation steps using subsequent dosing sequences with CO and O2, the amount of removable lattice oxygen in different Au/oxide samples was determined including Au/ZnO [23]. Comparing the amount of removable lattice oxygen to N2O RFC results, the potential of this method for a quantification of the oxygen vacancies is evaluated. Furthermore, the comparison of the N2O RFC results with the estimated values of Au surface and perimeter atoms should help to identify the location of the removable oxygen atoms.
Section snippets
Catalysts preparation method
Generally, the colloidal deposition route was applied according to Ref. [21] with slight modifications. The colloidal Au solutions were prepared using polyvinyl alcohol ( 10,000 from Aldrich, 80% hydrolyzed). The protecting agent was added (Au:PVA = 1.5:1 mg mg−1) to 100 ml of an aqueous Au solution with a Au concentration of 100 mg l−1 (HAuCl4, Alfa-Aesar, 99,99%) at room temperature under vigorous stirring. The obtained solution was then left under stirring for 10 min. A following rapid injection of
TEM and XRD measurements
The TEM analysis was performed for two samples containing 1 wt% Au and for one sample each with 2 and 3 wt% Au. One micrograph for each Au content is displayed in Fig. 1. The particle size distributions of the corresponding samples are shown in Fig. 2. It can be seen that the distributions differed only slightly before and after methanol synthesis. The maximum of the distributions shifted to only slightly higher particle sizes, and a few larger particles (7 nm) were found. Thus, the particles
Discussion
From the TEM characterization it can be concluded that the samples are very suitable for systematic investigations: the Au particles are quite uniform in size with a mean diameter of approximately 3 1 nm, and the size distributions are quite stable even under strongly reducing methanol synthesis conditions. Only a minor growth of the particles and a slight broadening of the size distribution is observed. Obtaining such a good reproducibility of the sample preparation is not easily possible with
Conclusions
Au/ZnO samples obtained by the colloidal deposition route were found to exhibit Au particles with a very uniform size distribution and negligible particle growth under the strongly reducing methanol synthesis conditions. N2O frontal chromatography was applied to the Au/ZnO samples after methanol synthesis in CO + H2 and CO + CO2 + H2. A higher N2O consumption was found for samples with higher Au content and after methanol synthesis without CO2, that is, the amount of N2O consumed is assumed to be
Acknowledgements
The authors thank Bernd Meyer, Yuemin Wang, and Christof Wöll for fruitful discussions and the Deutsche Forschungsgemeinschaft (DFG) for financial support within the Collaborative Research Center (SFB 558) “Metal–Substrate Interactions in Heterogeneous Catalysis”.
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