Carbon Nanofiber Supported Ni catalysts: Effects of nanostructure of
supports and catalyst preparation
Esther Ochoa-Fernández, De Chen*, Magnus Rønning and Anders Holmen
Department of Chemical Engineering, Norwegian University of Science and Technology
(NTNU), Sem Sælandsvei 4, N-7491 Trondheim, Norway.
To whom correspondence should be addressed:
E-mail: chen@chemeng.ntnu.no*
Fax: +47 73593149
Introduction
Carbon nanofibers (CNFs) have many unique properties, resulting in a wide range of
applications such as catalyst supports and catalysts [1,2]. The present work deals with the
preparation and characterization of CNF supported Ni catalyst. CNFs have been used as
templates for manipulating the properties of the Ni metal particles.
Experimental
Four types of CNF support materials have been prepared in our laboratory: Platelet (Pl),
carbon filament (CF), herring-bone (HB) and irregular CNF (IrCNF). The 12.5wt% Ni
catalysts were prepared by incipient wetness impregnation (IW) and deposition-precipitation
(DP) of nickel nitrate (Ni(NO3)26H2O) onto the four types of nanofibers. The CNFs were
previously purified and some of them were oxidised by HNO3. The catalysts were calcined in
air at 250 C for 60 minutes and reduced for 150 minutes at 300 C in a hydrogen atmosphere
(N2/H2, 1:1). The prepared catalysts have been characterised by different techniques, such as
N2 adsorption (BET), H2-chemisorption, temperature-programmed oxidation (TPO) and
reduction (TPR), XRD and TEM. Ethane hydrogenolysis has been used as a probe reaction in
the present work to test the catalytic activity of the Ni nanoparticles. The reaction was carried
out by temperature programmed scanning in a vertical flow reactor system connected to a
mass spectrometer.
Results
TPO experiments have been found to be an efficient tool to evaluate the CNF supported
catalyst and can provide information on Ni loading and relative activity of NiO for CNF
oxidation. The Ni loading measured by TPO on most of the catalysts are close to the nominal
value 12.5 wt.%. The dispersion measured by chemisorption ranges from 1% to 2.5%
depending on the nanostructure of CNFs and catalyst preparation. However, the particle size
measured by XRD is in the range of 1.5 - 6.7 nm, which is significantly smaller.
The results clearly indicate
-5
that both the CNF structure
have strong effect on the
catalyst
properties.
Fig.
1
shows that the rate of the
hydrogenolysis of ethane on
-5,5
ln r [mol ethane/g cat.h]
and the preparation method
Carbon filamental DP
Carbon filamental IW
Herring Bone oxid DP
Herring Bone oxid IW
-6
-6,5
-7
the catalyst prepared by DP is
larger than on the catalyst
prepared
by
IW.
Catalyst
based on the CF support display
-7,5
0,00171
0,00173
0,00175 1/T 0,00177
0,00179
0,00181
Fig. 1 Arrhenius plot for the hydrogenolysis of ethane. Effect of
the CNF nanostructure and preparation method on Nickel activity.
higher activity than the catalyst on HB. However, the difference is reversed in the case of
turnover frequencies (TOF), and the difference is explained by the microstrain of the Ni
particles. A relation between activity detected by means of TPO and TOF for the
hydrogenolysis of ethane has also been observed.
An example of a TEM image for well-dispersed Ni nanoparticles deposited on oxidised
CNFs is presented in Fig. 2. Fig. 3 shows that the diameter of the CNFs plays a significant
role in determining Ni crystal size. Small Ni crystals with narrow distribution can be obtained
on the CNFs with small diameter. As a result, the average metal particle size and its
distribution can be controlled by means of the CNF diameter.
100 nm
25
150 nm
50 nm
20
frequency [%]
200 nm
15
10
5
0
0
Fig. 2 TEM image of Ni/HB ox. IW CNF
10
20
30
40
diameter [nm]
50
60
Fig. 3 Dependence of Ni crystal size on CNF diameter
References:
1. de Jong, K.P. and Geus, J.W., Catal. Rev.-Sci. Eng. 42 (2001) 481
2. Salman, F., Park, C. and Baker, R.T.K. Catalysis Today 53 (1999) 38
70