545493 Atomically Resolved in Situ investigation of Cu2o Reduction with Methanol

Monday, June 3, 2019: 1:54 PM
Republic ABC (Grand Hyatt San Antonio)
Hao Chi1, Meng Li1, Matthew Curnan2, Christopher M. Andolina1, Judith Yang3, Götz Veser1 and Stephen House4, (1)Chemical Engineering, University of Pittsburgh, Pittsburgh, PA, (2)Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, PA, Korea, Republic of (South), (3)University of Pittsburgh, Pittsburgh, PA, (4)Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

Catalytic gas-surface reactions are dynamic processes for which changes in structure and composition of the catalysts under working conditions play a pivotal role. Post-reaction examinations of a catalyst often do not accurately represent its active state at reaction conditions [1]. For example, morphology and surface structure of metal particles can change upon cooling and differ from those under reaction conditions [2], and catalytic surfaces are known to reconstruct – often reversibly – upon exposure to adsorbates [3].

Direct in situ or operando observation of the micro-structural evolution and active sites of a catalyst under reaction conditions aims to address these issues and yield much deeper and more robust insights into catalytic reaction mechanisms [2, 4-6]. In this context, environmental transmission electron microscopy (ETEM) has emerged as a powerful in situ technique that allows gathering direct real-time dynamic information on the nanoscale. Using in situ TEM techniques, nanoscale morphological evolution and chemical modifications of a catalyst can be visualized directly under gas exposure, providing essential insights into catalytically active phases, transient states, and reaction mechanisms.

Recent in situ studies have provided a significant understanding of atomic scale Cu oxidation, especially during early stage oxidation [7,8]. In contrast, the reduction of metal oxides, such as Cu2O, has been studied much less extensively. However, such reductions play an important role in many technologies, such as catalysis, electrochemistry, and environmental degradation of materials [9]. Here, we present an in-situ investigation of the dynamics of the reduction of Cu2O islands supported on Cu(100) during exposure to methanol (MeOH) using ETEM. Cu2O nano-islands supported on Cu facets were created via controlled in situ oxidation of copper films, followed by reduction in MeOH vapor.

Cu(001) thin films with a thickness of 600 Å were produced by e-beam evaporation of 99.999% pure Cu pellets on NaCl(001) substrates. The films were grown in a UHV evaporator system with a base pressure of 1×10-9 Torr at a deposition rate of 1.2 Å/s and a substrate temperature of 300 °C. Native oxides (Cu2O) were removed by injection of H2 (pH2 = 7.6×10-6 Torr) at a temperature of 550 °C, followed by annealing at elevated temperature to forma faceted holes in the Cu films. Cu2O nano-islands were then created along Cu(100) facets via controlled in situ oxidation of the Cu films (pO2 = 1.5x10-4 Torr, 350 °C). This procedure results in the formation of Cu2O islands with the epitaxial relation Cu2O(200)//Cu(200) and typical sizes of ~2-3nm in height and 5-6 nm in diameter. These islands were then exposed to MeOH (pMeOH = 1x10-2 Torr, 250°C) and the resulting reduction of the Cu2O islands was imaged using an ETEM (Hitachi H9500) equipped with a specially designed double-tilt heating holder and gas injection system [10]. The obtained ETEM images and videos were evaluated using Fiji-imageJ.

Our results reveal a complex dynamic of the Cu2O reduction process, characterized by two distinct temporal phases: During the initial phase of the reduction (~15 s), the Cu2O islands reduction proceeds strongly anisotropically, i.e. the island radius decreases rapidly while its height remains almost unchanged. Interestingly, once the island radius roughly matches its height, the reduction switches from anisotropic to isotropic island shrinkage as both island height and radius simultaneously decrease until the whole island is completely removed after ~25 s. This shrinking behavior was observed uniformly for a total of eight Cu2O islands studied on Cu(100), i.e. it appears to be representative of the reduction dynamics of Cu2O islands on Cu(100) in an MeOH atmosphere. We propose that this behavior is caused by a preferential reaction between MeOH and stepped surfaces on the sides of the Cu2O islands (e.g. Cu(110) with atomic steps) which show substantially higher reactivity than the island top terrace (Cu(100)). To support this hypothesis, we conducted DFT calculations for MeOH adsorption energies on Cu(110) and Cu(100) surfaces, which confirm the preferential adsorption of MeOH on the stepped surface.

The combination of in situ observations and computational studies in this work hence revealed deeper atomistic insights into the interactions of methanol with different crystallographic facets of Cu2O surfaces. More broadly, this work aims to contribute to the understanding and manipulation of supported metal oxide systems widely used in heterogeneous catalysis.

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Reactive Environments. Science, 2011. 331: p. 171.

[2] Gai, P.L. and E.D. Boyers, Electron Microscopy in Heterogeneous Catalysts. 2003, London: Institute of Physics.

[3] Ertl, G., Reactions on Solid Surfaces. 2 ed. 2009: Wiley.

[4] Sharma, R., et al., In Situ Nanoscale Observations of the Mg(OH)2 Dehydroxylation and Rehydroxylation Mechanisms. Phil. Mag., 2004. 84: p. 2711-2729.

[5] Gai, P.L., Dynamic studies of metal oxide catalysts: MoO3. Phil. Mag., 1981. 43(4): p. 841-855.

[6] Gai, P.L., J.C. Bart, and E.D. Boyes, Electron microscopy of industrial oxidation catalysts. Phil. Mag. A, 1982. 45: p. 531-547.

[7] Zhou, G., Luo, L., Li, L., Ciston, J., Stach, E.A. and Yang, J.C., Physical Review Letters, 109 (2012) 35502.

[8] Zhou, G. and Yang, J.C., Journal of Materials Research, 20 (2005) 1684-1694.

[9] Zhou, G. and Yang, J.C., Phys Rev Lett, 93 (2004) 226101.

[10] Chi, H., Bonifacio, C., Andolina, C., Stach, E.A., Veser, G., and Yang, J.C. Microscopy and Microanalysis, (2017) 23(S1), 2100-2101.


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