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A. Kitla, R. Rameshan, B. Klötzer, K. Föttinger:
"Surface Segregation during Methane Decomposition of Nickel and Copper promoted Zirconia catalysts";
Poster: EuropaCat XI, Lyon, France; 01.09.2013 - 06.09.2013; in: "EuropaCat XI 2013", (2013), S. 182.

Surface Segregation during Methane Decomposition of Nickel and Copper promoted Zirconia catalysts
A. Kitla*,1, , R. Rameshan2, B. Klötzer2, K. Föttinger1
1 Vienna University of Technology, Institute of Materials Chemistry, Getreidemarkt 9 BC 01, 1060 Vienna, Austria
2 University of Innsbruck, Institute of Physical Chemistry, Innrain 52a, 6020 Innsbruck, Austria
(*) corresponding author: @astrid.kitla@tuwien.ac.at

Keywords: CuNi alloy, bimetallic catalyst, methane decomposition, coke, surface segregation, XPS-Spectroscopy

1 Introduction
Ni/ZrO2 is used as catalyst for methane reforming reactions, which are key processes for hydrogen production in industry and in solid oxide fuel cells. Nickel shows a good activity but is rapidly deactivated by coke formation, which is a major problem. The addition of copper and formation of a CuNi alloy are expected to reduce coke for-mation [1].
In this contribution we have characterized zirconia based copper, nickel and bimetallic copper/nickel catalysts and explored the formation of the CuNi alloy, their catalytic properties for methane decomposition and the surface composition in the reaction atmosphere to get insights into segregation processes occurring as a consequence of the reaction with methane.
2 Experimental
The samples were prepared by impregnation of ZrO2 with copper and nickel nitrate and calcined at 450 °C. The catalysts contain 5 % w/w metal.
Characterization techniques used include in situ X-Ray Photoelectron Spectroscopy, IR spectroscopy of CO as a probe molecule as well as applied in situ during methane decompositon and oxidation, X-Ray Absorption Near Edge Structure (XANES), transmission electron microscopy (TEM), H2 chemisorption and temperature programmed reaction (TPReaction), oxidation (TPO) and reduction (TPRed). These methods provided information on Cu and Ni oxidation states, reduction temperature, catalytic performance, morphology and metal distribution on the surface.
3 Results and Discussion
Formation of a copper-nickel alloy during reduction was confirmed by various techniques. X-Ray Absorption Spectroscopy and Temperature Programmed Reduction showed a decrease in the reduction temperature of nickel by copper addition. In TEM images of catalysts reduced prior to the microscopy particles of about 20 nm size containing both copper and nickel were found. These alloy particles were enriched in nickel. Additionally, much smaller particles containing mainly or only copper were found very finely distributed over the zirconium oxide sup-port. Figure 1 shows an elemental map of CuNi-ZrO2.

Fig. 1. Elemental Map of 1:1 CuNi-ZrO2
In order to get information about the Cu:Ni ratio on the surface, chemisorption of hydrogen as adsorbing agent was performed. Hydrogen was chosen because it adsorbs irreversibly on nickel but not on copper. The results of these experiments showed, that the surface was enriched incopper, in agreement with FTIR spectroscopy of CO adsorption [2].
TPReaction with MS detection showed that the methane decompositon rate to hydrogen in absence of oxygen was much higher on nickel than on the copper catalyst. The bimetallic Cu/Ni catalysts showed low activity until a sudden strong increase of the H2 formation rate was detected at a certain temperature between 360 and 460 °C depending on the Cu:Ni ratio. Figure 2 shows TPReaction of methane decomposition on the nickel rich sample 1:3CuNi-ZrO2. By repeating the TPReaction measurements H2 production was already observed at lower temperature and was comparable with the monometallic Ni catalyst. The sudden and irreversible change in H2 formation activity is explained by a change of the surface composition of the bimetallic particles. The catalsyt surface, which was initially enriched in copper resulting in low activity, changes to a Ni-enriched state because of Ni segregation to the surface. This results in a strong increase in activity. A likely driving force for the Ni surface segregation is the interaction with either methane or carbon, which is formed during methane decomposition.

Fig. 2. Hydrogen and carbon dioxide production during three subsequent heating cycles in methane on 1:3CuNi-ZrO2
In order geht more information about surface segregation processes during methane decompositon, in situ XPS in 0.25 mbar methane was applied at different temperatures. These measurements allowed to quantify the Cu:Ni ratio in the surface-near region.
Temperature programmed oxidation after the methane decomposition was applied to determine coke formation. On the monometallic Cu hardly any coke was formed, while on the monometallic Ni catalyst a considerable amount of CO and CO2 was produced during oxidation. On the bimetallic cataysts CO2 was also formed, but to a lower extent than on monometallic Ni indicating reduced coke formation on bimetallic CuNi catalysts.

4 Conclusions
CuNi-ZrO2 catalysts were found to form an alloy during reduction decreasing the nickel reduction temperature. After reduction the surface was enriched in copper. In methane decomposition reaction, the alloy catalysts show rather low activity in the beginning but after a surface segregation process of nickel, about the same activity for hydrogen formation could be achieved as on monometallic Ni-ZrO2. The segregation of nickel to the surface is assumed to be a consequence of interaction with methane or carbon. The amount of coke formed on the bimetallic catalysts was much lower and the carbonaceous deposits could be burnt off at lower temperatures com-pared to Ni-ZrO2.
Acknowledgements
In situ XPS experiments were performed at the ISIS-PGM beamline (BESSY II, Berlin). Thanks to the BESSY team tor the support.
XANES experiments were carried out at the SuperXAS beamline (Swiss Light Source, PSI Villigen, Switzerland). Thanks to Dr. Olga Safonova for supporting the XAS measurements.
This project is financially supported by the FWF (SFB F45 "Functional Oxide Surfaces and Interfaces FOXSI").
References
[1] S. McIntosh, R. J. Gorte, Chem. Rev., 104 (4845-4865) 2004.
[2] A. Kitla, O. Safonova, K. Föttinger, submitted.