Transcript MS PowerPoint

Mesostructured Hollow Spheres of Graphitic N-Doped
Carbon Nanocast from Spherical Mesoporous Silica
J. Phys. Chem. B 2004, 108, 19293- 19298
Introduction
 Porous carbon - high chemical stability, large surface area and tailorable
structure property.
 High SA and meso str. – more dispersion of metal ions on their surface
 it is difficult to achieve highly dispersed metal catalyst due to the inert
surface of the carbon materials
 Generation functional groups on the carbon surface – by harsh conditions
 Functional groups by in situ generation – N-carbon source
 Varying carbonization temperature – change in N/C ratio
 by adopting CVD method N- OMC were prepared
(SBA-15 hard template, acetonitrile carbon source)
SBA-15
4g P123 + 0.5 g CTAB + 25 ml EtOH + 30 ml H2O + 60 ml HCl (2M)
Stirred at RT
10 g TEOS
Stirred at RT for 1 h
Autoclave at 80 0C for 6 h
Autoclave at 110 0C for 12 h
Washed EtOH and dried Cal at 550 0C
SBA-15
NOMCs (CNx)
0.5 g SBA-15
N2 & sat. with acetonitrile
Carbonized at 900-1000 0C for 3 h under N2 atm.
Washed with 20 % HF at RT and with EtOH
NOMCs
N2- sorption isotherm & SEM of SBA-15
BETSA =1087 m2/g
PD = 6.0 nm
Pvol = 1.13 cc/g
SEM & TEM images OMC
(f)
SEM images of OMCs compaction at 1.0 Gpa for 1 h.
900 0C
1000 0C
900 0C
OMC
composite
1000 0C
XRD of OMC
2θ = 2.11° - d200
basal d100 = 8.4 nm & a0 = 9.7 nm
2θ = 26.2° - d002 = 0.339 nm
N2- sorption isotherm & TEM of OMC
BETSA =779 m2/g
PD = 4.7 nm
Pvol = 0.66 cc/g
Mechanism for the Formation of Mesoporous Carbon Hollow Spheres
Textural properties
XPS of OMC
N1s
N1s
400.8eV – quaternary N atoms
398.9 eV – pyridine - like N atoms
C1s
C1s
284.6 eV – sp2 gC species
D band
G band
Raman spectra of OMC
Conclusion
hollow spheres of structurally wellordered – NOMC may be nanocast using SBA-15
template via a CVD route
The CVD temperature is an important consideration and should be higher than 900 °C
and preferably 1000 °C for successful formation of carbon hollow spheres.
The use of acetonitrile as a carbon precursor results in N-doped (CNx) materials
with a nitrogen content of ca. 6.5 wt %.
The CNx hollow spheres exhibit a high level of graphitization especially for materials
prepared at a CVD temperature of 1000 °C
Preparation of Pt/CMK-3 Anode Catalyst for Methanol
Fuel Cells Using Paraformaldehyde as Reducing Agent
Chinese J Catal, 2007, 28(1): 17-21
Introduction
In recent years, DMFC have attracted significant attention because of their
high energy transfer efficiency and low pollution causing potential.
The disadvantages of the existing catalyst low catalytic activity
The highest electro catalytic activity of two component system is Ru-Pt/C
In this study Pt/CMK-3 anode catalyst for DMFC was prepared by a novel
liquid reduction method using paraformaldehyde has reducing agent.
SBA-15
4g P123 + 22 ml H2O +38 ml HCl (2M)
Stirred at 350C
7 g TEOS
Stirred at 350C for
24 h
Aged at 100 0C for 48 h
Washed EtOH and dried Cal at 550 0C
SBA-15
Msoporus Carbon
1 g SBA-15 + 1.25 g sucrose + 0.14 g H2SO4 + 5 g H2O
Dried at 100 0C for 6
h and 160 0C for 6 h
Black powder
0.8 g sucrose + 0.09 g H2SO4 + 5 g H2O
Dried at 100 0C for 6
h and 160 0C for 6 h
Carbonized at 900 0C for 6 h under N2 atm.
Washed with 5% HF at RT and with EtOH
CMK-3
Preparation of Pt/CMK-3
60 mg CMK-3 + 20 ml H2O +0.039 M H2PtCl6
Ultrasonication 30 min
Heated at 343 K / N2
10 ml Na2CO3 + Paraformaldehyde
Stirred 2.5 h
Filt. Washed & dried
Pt/CMK-3, Pt/C-M
XRD patterns of the Pt/CMK-3, Pt/C-M and Pt/XC-72
Pt/C-M
Pt/XC-72
Pt/CMK-3
Average diameter and relative crystallinity of Pt particles
TEM images of SBA-15 and CMK-3
TEM images of Pt/CMK-3 and Pt/XC-72
Particle size distribution
Pt/CMK-3
Pt/XC-72
The average size of the Pt particles in the Pt/CMK-3 catalyst is 2.8 nm
The average particle size Pt/XC-72 is 3.5 nm
Particles aggregation is observed in Pt/XC-72 more then 10 nm
Electrocatalytic activity of the catalysts for
methanol oxidation
8 mg Pt/CMK-3 + 0.6 ml EtOH +2.4 ml H2O +
0.2 ml Nafion (5 %)
Sonication 30 min
4 µl slurry on GC electrode
Dried at 308 K
0.5 M H2SO4 + 0.5 M MeOH
N2 bubbled to remove O2
50 mV/s at 309 K
Cyclic voltammograms of Pt/CMK-3, Pt/C-M and Pt/XC-72
1.Pt/C-M
2. Pt/XC-72
3.Pt/CMK-3
Positive scan, the potential of oxidation peaks of CH3OH for all 3 catalyst at
0.64 V
Negative scan the potential located at 0.43 V
Methanol is dissociatively adsorbed on the Pt surface
Increasing potential platinum oxides are formed and reaction rate is
enhanced
Current density is increased
The electrocatalytic activity is related to the size of Pt particles. Smaller the
particle size the activity is relatively higher
Catalyst
Peak current
(mA cm-2)
Pt/CMK-3
22.06
Pt/XC-72
19.72
Pt/C-M
14.9
Chronoamperometric Curves
1.Pt/C-M
2. Pt/XC-72
3.Pt/CMK-3
Pt/CMK-3 high catalytic activity
The relative crystalinity and average Pt particle size in
Pt/CMK-3 are lower and smaller.
Catalyst preparation and reduction rate is important factor
for the structure of metal deposition.
Creation of metal atoms from chemical reaction
Formation of crystal cells from atoms
Aggregation of cells to from particles
Coalescence of particles
The primary role of carbon support is to disperse metal
nanoparticles and provide electrical connection among them.
Conclusion
 The nucleation and growth of Pt particles on CMK-3 are influenced by
the experimental conditions such as surface states of carbon,
temperature, time, and concentration of reactants and products.
 Small and uniformly distributed Pt particles can be obtained by
controlling the experimental conditions
 This preparation method is very simple and the electro catalytic
activity of the prepared Pt/CMK-3 catalyst for the methanol oxidation
is high, this method is promising for practical applications.