(675f) Dynamics of Carbon Deposition on Alumina Supported Nickel Catalysts during Dry Reforming Reaction - an Important Role of Carbon Species on the Nickel Surfaces -

Abstract for AIChE2014 annual meeting

Title: Dynamics of Carbon Deposition on Alumina Supported Nickel Catalysts during Dry Reforming Reaction - An
Important Role of Carbon Species on the Nickel Surfaces -
Authors: Yuki Kitano, Naoya Mihara, Atsushi Okemoto, Keita Taniya, Yuichi Ichihashi, and Satoru Nishiyama Affiliation: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan
Abstract
Methane dry reforming reaction (CH4 + CO2 â?? 2CO + 2H2) is one of the most promising processes for reduction of CO2 emission accompanying H2 production. In comparison with the steam reforming reaction, the reaction atmosphere of the dry reforming is significantly more reducing. The carbon deposition from hydrocarbon reactants would be more crucial during the dry reforming than the steam reforming. Carbon deposition which brings about catalyst deactivation usually proceeds via a side reaction pathway. In the methane dry reforming reaction, the carbon deposition process is included in main reaction pathway [1]. Dehydrogenation of methane forming with carbon species on the nickel surfaces is a first step of the reaction, then the formed carbon species will react with the oxygen species which is generated by dissociative adsorption of CO2 on the nickel surfaces [1]. In order to suppress the deactivation, the rate of carbon deposition should be reduced. But in the dry reforming reaction, the suppression of the carbon deposition process will decrease the reaction rate of the dry reforming. The balance of the each elementary reaction step would be very important to control the total process of the dry reforming reaction.
In this paper, we will discuss about the detail of the reaction mechanism of the methane dry reforming and the behavior of carbon deposition. An effective procedure to optimize the activity and stability of the reaction will be proposed for the methane dry reforming reaction on the supported nickel catalysts.
The alumina supported nickel catalysts, Ni/Al2O3, were prepared by a conventional impregnation method using one of the reference catalysts supplying from The Catalysis Society of Japan, ALO-6 and nickel nitrate in an aqueous solution. The loading of nickel was 15 wt%. The carbon deposition rate was measured in a DTA-TG apparatus (DTG60, Shimadzu, Japan) under the dry reforming condition, at 873 K in the flowing gas mixture of CH4, N2, and/or CO2. Characterization of the deactivated catalysts was carried out by using X-ray diffraction powder pattern, Raman spectroscopy, and scanning electron microscopy.
Methane dry reforming reaction proceeds via the following reaction pathway [1]: CH4 â?? C + 2H2 (1)
CO2 â?? CO + O (2)
C + O â?? CO (3)
On the Ni surfaces of Ni/Al2O3 catalysts at 873 K, CH4 can be readily activated by dehydrogenation forming carbon species and molecular hydrogen. Carbon dioxide is also dissociatively adsorbed on the Ni surface and the produced oxygen atoms can react with the carbon species from CH4, generating CO. SEM observation and Raman spectroscopy studies strongly suggest that two kind of carbon species exist. One was graphite like carbon which was assignable to G band species in Raman spectra and whisker structure was observed in SEM images. The other was amorphous carbon which was also assignable to D band in Raman spectra and no specific morphology was found
on the surface of the used catalysts. When the Ni catalysts was exposed by CH4 and N2 mixture, significant amount of carbon deposition was observed. The amount of deposited carbon was gradually increased with time on stream. After that, the stream was switched to the gas mixture of CO2 and N2. Major part of the accumulated carbon was removed by the reaction with CO2. The SEM images of the carbon deposited Ni catalyst after the CO2 exposure indicate that the whisker structure completely disappeared. Some portion of carbon was remained on the Ni catalysts after CO2-oxidation step. These results suggest that the carbon species, indicating whisker structure, can be easily removed by the reaction with CO2 and a small portion of amorphous carbon is hardly removed from the Ni catalysts. The carbon which is hardly removed plays a crucial role in the catalyst deactivation for the dry reforming reaction. We have also studied the kinetics of the carbon deposition with CH4 and the removal with CO2. The rate of carbon deposition in CH4 atmosphere was much faster than the removal in CO2 atmosphere. The catalyst deactivation of the supported Ni catalysts was controlled by 1) the balance of carbon deposition ad removal by CO2, and 2) the kind of deposited carbons, removable or unremovable.
Finally, we propose surface dynamics on Ni/Al2O3 catalysts for the methane dry reforming reaction as follows:

CH4 + Ï? (active site) â?? C (amorphous) - Ï? + 2H2 refractory carbon

C (whisker) - Ï? + 2H2 intermediate carbon

CO2 + Ï? (active site) â?? CO + O- Ï?


C (whisker) - Ï? + O- Ï? â?? CO + Ï? removable
{ Ï? (active site): metallic Ni site }
The activated adsorption of CO2 on the Ni catalysts was confirmed by the change of color of the Ni catalyst from black (metallic) to green (oxide, Ni2+) at the elevated temperature.
Reference
[1] Y. Ikeda, T. Horie, K. Taniya, Y. Ichihashi, N. Ohmura, and S. Nishiyama, Kagakukogaku Ronbunshu, 2011, 37, pp. 128-133.