Gold-silver catalysts based on ZnO in propene oxidation – the effect of silver dopant and hydrogen treatment on catalytic properties Iveta Kaskow 1,2* , C-M. Yang 2 , I. Sobczak 1 , M. Ziolek 1 (1) Adam Mickiewicz University in Poznan, Faculty of Chemistry, Umultowska 89b, Poznan, Poland (2) Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Catalysis by gold has been continuously developing and recently one of the main subjects of interest in this area has been bimetallic catalysts containing gold [1]. This study is focused on gold-silver catalysts which have not been frequently studied. It was expected that the addition of silver to gold would increase the activity of the catalyst in oxidation processes because oxygen is effectively chemisorbed on silver species. The choice of zinc oxide as a support for metals was made taking into account strong metal-support interaction properties of ZnO which allows modification of electronic and catalytic properties of metals, and thus permits tuning their activity. The AuAg-ZnO catalysts were used in propene oxidation. The aim of this study was on the effect of silver and the treatment with hydrogen on the surface and catalytic properties of these bimetallic systems. Two kinds of ZnO were used as supports for gold and silver – commercial ZnO(C) (Aldrich – BET surface area = 10 m 2 /g) and ZnO synthesized according to [2] (BET surface area = 20 m 2 /g). Both supports were modified in two steps: i) by grafting with 3-mercaptopropyl-trimethoxysilane on ZnO and next ii) Au (2.2 wt.%) and/or Ag (0.7 wt.%) introduction according to [3]. The materials obtained were reduced with NaBH 4 and next calcined at 723 K. The physicochemical properties of the catalysts were investigated by XRD, N 2 ads./des., TEM, UV-Vis and XPS. Catalytic activity of the catalysts was studied in propene oxidation at 473, 523 and 573 K. Negatively charged metallic gold particles (Au 0 ) δ- were identified on the surface of all (mono-and bimetallic) gold catalysts (Au4f 7/2 BE = ca 83.2 eV – XP spectra). On the AuAg-ZnO surface silver Ag + (Ag 2 O) species dominated (Ag3d BE = 367.3 eV – XP spectra). Moreover, it was found that the temperature treatment of AuAg-ZnO systems in the presence of hydrogen at 673 K led to the AuAg alloy formation. A shift of the band in the UV-Vis spectra to lower wavelengths in comparison to its position in the calcined AuAg-ZnO spectra was observed (from 520 to 500 nm for AuAg-ZnO and from 521 to 496 nm for AuAg-ZnO(C)). These studies allowed the identification of bimetallic structures in calcined and reduced catalysts. Silver oxide (Ag 2 O), located in the vicinity of metallic gold particles, was present on the surface of calcined AuAg-ZnO. After reduction with hydrogen both metals existed in the form of AuAg alloy. The reaction of propene oxidation with oxygen was used to evaluate the activity of the materials studied. It was found that calcined monometallic Au-ZnO systems exhibited very low activity (< 1% of propene conversion), but after their modification with silver the activity increased. Moreover, the type of bimetallic structures has an important impact on the activity. The reduction treatment of AuAg-ZnO systems resulted in AuAg alloy formation and led to a decrease in propene conversion, indicating that the active centers in propene oxidation are cationic Ag + species. It was stated that metallic gold increased the mobility of oxygen in ZnO and it was responsible for the catalyst selectivity to acrolein. The selectivity to acrolein (measured at a similar level of propene conversion) increased from 2% on ZnO and Ag-ZnO to 15% (573 K) on Au-ZnO. The addition of silver to gold catalyst increased the selectivity from 15% (573 K) on Au-ZnO to 35% (473 K) on AuAg-ZnO. To sum up, the presented results of propene oxidation on calcined and reduced bimetallic gold-silver catalysts supported on ZnO showed better catalytic performance of calcined AuAg-ZnO than monometallic Au-ZnO catalyst and AuAg-ZnO after reduction with hydrogen. The calcined AuAg-ZnO was also found to be the most selective to acrolein. References 1- A .Wang, X.Y. Liu, Ch-Y. Mou, T. Zhang, J. Catal. 308, 258 (2013) 2- L. Wolski, J. E. Whitten, I. Sobczak, M. Ziolek, Mater. Res. Bulletin, 85, 35 (2017) 3- X. Liu, A. Wang, X. Yang, T. Zhang, Ch-Y. Mou, D-S. Su, J. Li, Chem. Mater. 21, 410 (2009) Acknowledgements National Science Center in Poland (Grant No. 2014/15/B/ST5/00167) and “EURASIACAT: Advanced Education European-Asiatic Exchange Programme in Materials Science and Catalysis" are acknowledged for the financial support Corresponding author email: iveta.kaskow@gmail.com