Stability of Ag2Cu2O3 Catalyst under Reaction Conditions
IV International Scientific School-Conference for Young Scientists in Memory of Professor L.N. Kurina «Catalysis: from Science to Industry»
23-27 Oct 2016
conference_type.international conference, Tomsk
|| Kardash Tatyana Yurievna
, Svintsitskiy Dmitry Antonovich
, Stonkus Olga Aleksandrovna
, Koshcheev Sergei Valentinovich
, Boronin Andrei Ivanovich
Boreskov Institute of Catalysis SB RAS
Novosibirsk State University
Mixed silver-copper oxide Ag2Cu2O3 has the tetragonal crystal structure of paramelaconite type (ISCD, CC=87609) with the next features: (a) two non-equivalent lattice oxygen species; (b) rhomboid distortion of square planar CuO4 fragments; (c) the enhanced Ag-O bond length in comparison with individual silver oxides. The great interest to Ag2Cu2O3 in heterogeneous catalysis field relates to the catalytic properties in low-temperature oxidation of CO  and selective alcohol oxidation . However, paramelaconite structure is known to be metastable . So, it is necessary to investigate the stability of Ag2Cu2O3 under reaction conditions.
In this study, mixed oxide Ag2Cu2O3 was prepared by co-precipitation in alkaline solution. The combination of XRD, TEM and XPS methods was applied to obtained data about the structure and surface composition of Ag2Cu2O3. Based on XRD data the as-prepared sample contained of particles with paramelaconite structure only. Such particles with brick-like morphology were found to be elongated along c axis. Air storage of Ag2Cu2O3 during 4 months resulted in the appearance of metallic silver traces (up to 2 wt.%). No any crystallized copper-containing phases were detected by XRD for the stored Ag2Cu2O3. In accordance with EFTEM data the initial Ag2Cu2O3 bricks after long air exposure were covered by round-like silver-containing 10-20 nm particles and thin copper-rich layer. The small thickness of copper-rich layer can be a reason for the absence of any XRD signal for CuOx structures. It should be noted that the exfoliation of copper-rich layers was observed by EFTEM. Using in situ XRD the mixed oxide was studied under different reaction conditions (air, He, CO, CO+O2) during calcination up to 500°C. It was observed that the initial decomposition of Ag2Cu2O3 structure at temperature 100-150°C was accompanied by the appearance of Ag0 traces only. The temperature of full decomposition into Ag0 and CuOx was varied in 250-350°C range in the dependence on Red-Ox potential of reaction medium. Also prolonged treatment of Ag2Cu2O3 by catalytic CO+O2 mixture at 150°C during more than 3 hours resulted in the appearance of metallic silver. To study the initial stage of Ag2Cu2O3 decomposition the prolonged and powerful exposure by electron beam was performed in microscope chamber immediately. Analysis of electron microdiffraction images showed that silver particles were epitaxially grown relatively to the initial Ag2Cu2O3 particles. In accordance with in situ XRD data the decomposition of Ag2Cu2O3 under catalytic conditions was accompanied by the significant decrease of crystal size along a direction only, while no significant changes was found for crystal size along c axis. The evident growth of a lattice parameter was additionally observed. Using XPS data the Cu/Ag and O/(Cu+Ag) surface ratios were monitored during the calcination of Ag2Cu2O3 under UHV conditions. The observed changes were discussed in comparison with in situ XRD and TEM results.
Based on all obtained data the overall scheme of Ag2Cu2O3 decomposition under reaction conditions was proposed. The possible reasons of Ag2Cu2O3 instability during air storage were also discussed.