Combustion Synthesis of oxide materials

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Combustion Synthesis of oxide materials for
Catalytic applications
G. Ranga Rao Indian Institute of Technology
Madras Chennai 600 036
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• Outline
• ? Highlights of combustion synthesis
• ? Potential applications in catalysis
• ? Composite oxide materials and catalysis
• oxides of Cu and CuNi alloy
• Green Cu2(OH)3(NO3)
• Cu-Ce-O
• CeZrO/HPAs
• ? Conclusions

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Combustion synthesis is preferred ? Extremely
simple (reactor set up) and fast (explosion
type) ? Relatively low power requirement ? Low
operating temperature (lt400 oC) and high
combustion temperature (up to 3500 oC) ?
Front propagation velocities ( 25 cm / second )
? Pure product with better catalytic properties
? Very high temperature gradients (105 K cm-1)
and fast reaction rates i.e. elimination of
volatile impurity powders (self-cleaning) ?
Temperature gradients combined with rapid cooling
forms meta-stable phases and unique
structures (impossible by conventional methods)
? Potential for industrial application
Drawback emission of hazardous or polluting NH3
and NOx)
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Combustion Synthesis of CuO, Cu2O, Cu and CuNi
alloy particles
? The effect of the fuel content on the chemical
nature of Cu-based
materials using carbohydrazide as fuel.
? Use of a new organic fuel N-tertiarybutoxy-car
bonylpiperazine
G. Ranga Rao et al, Materials Letters 58 (2004)
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F/O 0.5 Green Cu2(OH)3(NO3) polymeric phase
Carbohydrazide fuel
G. Ranga Rao et al, Materials Letters 58 (2004)
3523
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Cu and CuNi bimetallic alloy particles using
N-tertiarybutoxy-carbonylpiperazine fuel
F/O 1 In a single step combustion
Alloy formation
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Cu-Ce-O composite oxides prepared by Combustion
synthesis (Surface and catalytic properties)
(NH4)2Ce(NO3)6 Cu(NO3)2 /3H2O N2H3CON2H3


(carbohydrazide)
? Combustion synthesis at 350 oC
? Reflections of fluorite phase ? No CuO
reflections up to 40 Cu
content ? Combustion method produces both
CuO and Cu2O phases in the absence of
CeO2 ? Amounts of CuO and Cu2O phases
change with the fuel content ? The increase in
the amount of Cu leads to the CuO phase
separation and bulk CuO particles are
detected by XRD
XRD
a CeO2 b CuO(5) CeO2 c - CuO(10)
CeO2 d - CuO(20) CeO2 e - CuO(40) CeO2 f
- CuO(60) CeO2 g - CuO(80) CeO2 h
CuOCu2O
Ranga Rao et al, Colloids Surf. A 220 (2003) 261
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EPR of 20CuO/CeO2 combustion method at 300K
A isolated Cu2/clusters of CuO O small
particles of CuO K Cu2 dimers
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• Model reaction for characterizing oxide
  catalysts
• - surface composition, oxygen non-stoichiometry,
  redox activity

Kinetic Method used Gasometry
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? Two parallel compensation lines (similar bulk
but different surface properties of the oxides)
? line 1 low copper content (0 20), ?
line 2 high copper content (40 100),
ln kiso and compensation lines demonstrate the
demarcation between highly dispersed and bulk CuO
phases present in the Cu-Ce-O prepared by
combustion.
Consistent with XRD
Ranga Rao et al, Colloids Surf. A 220 (2003) 261
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aqueous NH3 was added drop by drop
Metal nitrates Citric acid (11 mole ratio)
excess NH3
Sol
Gel
dried at 100 C overnight
Dried gel
combusted at 350 C
calcination at 500 C for 2h in air
CeO2-ZrO2 carbon
CexZr1-xO2
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• CeO2 ? (111), (200) (220)
• cubic phase with fluorite structure
• Zirconia ? monoclinic tetragonal phases

porous
Ce0.8Zr0.2O2
(a) CeO2 (b) Ce0.8Zr0.2O2 (c) Ce0.6Zr0.4O2 (d)
Ce0.5Zr0.5O2 (e) Ce0.4Zr0.6O2 (f) Ce0.2Zr0.8O2
(g) ZrO2
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Acidic nature

• CeO2 shows two types of acidic sites at
• 140 C and 230 C
• Total concentration of acidic sites
• increased upon addition of zirconia
• ZrO2 exhibits weak acidity
• Desorption takes place lt 300oC,
• peaking at 170oC

(a) CeO2 (b) Ce0.8Zr0.2O2 (c) Ce0.6Zr0.4O2 (d)
Ce0.5Zr0.5O2 (e) Ce0.4Zr0.6O2 (f) Ce0.2Zr0.8O2
(g) ZrO2
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Application POMs/CeZrO
Polyoxometalates (POMs)
Dispersion/Catalysis
Molecular hybrids with RTIL
Dispersion onto inert supports ? to increase
the surface areas of POMs gt 10 m2/g ? to
immobilize / increase stability (heterogenisation)
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Ce4OZr4OCe4
Metal cations Lewis acid sites Oxygen anions
Lewis base sites -OH groups Bronstead acids /
Lewis base Oxide vacancy Lewis acid
PW12O403- Keggin anion
? Can CexZr1-xO be a good support for POMs? ? Can
the Keggin anions be anchored onto the CeZrO
through WOterminal ? ? Does this interaction
alter the Keggin ion structure in any way? ? How
do we probe it, by simple IR? XPS, EXAFS, 31P
MAS NMR?
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Supports to HPAs (PMOs)

• Surface area of unsupported HPAs are usually low
  (1-10 m2 g-1)

• Acidic or neutral substances
• Silica
• Active carbon
• Acidic ion-exchange resin
• MCM-41
• Zeolite Y
• layered clays
• SBA-15
• ZrO2
• CexZr1-xO2 ??

• Basic support
• MgO tends to decompose HPAs

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POM dispersion onto CeZrO supports
Dispersed PWA (10-50wt) on Ce0.5Zr0.5O2
20wt PWA on CexZr1-xO2
?intrinsic IR features for PWA PO 1080
cm-1 WOterminal 986 cm-1 WOcW
corner-sharing WO6 octahedra 890 cm-1 WOeW
edge-sharing WO6 octahedra 810 cm-1
? IR features for perturbed PWA Intrinsic bands
as above at least two non-intrinsic bands
seen at 1052 (P?O), 958 cm-1(WO) due to
perturbed interfacial Keggin anions.
? Clear evidence for Lewis acid-base interactions
and defect-Keggin anion interactions at the
interface through WO terminal bonds
leading to the additional IR bands.
? Ce4OW and Zr4OW surface bonds between
Keggin molecular species and Ce4/Zr4 ions
G. Ranga Rao and T. Rajkumar, J. Colloid Interf.
Sci. 324(2008)134
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Raman of dispersed PWA (0 50 wt) on
Ce0.5Zr0.5O2
CeO2
Ce0.5Zr0.5O2
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31P MAS NMR of dispersed PWA (0 50 wt) on
Ce0.5Zr0.5O2
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PWA layers
G. Ranga Rao and T. Rajakumar, Catal. Lett. 120
(2008) 261
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Mechanism proposed
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• Conclusions
• The potential application of combustion
  synthesis to produce metal and
• alloy particles as well as Cu2(OH)3(NO3)
  polymeric phase demonstrated.
• Pure oxide composites and CeZrO solid solution
  phases obtained by
• combustion method are suitable materials for
  catalysts and supports.
• The influence of defect chemistry need to be
  explored further.

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Thanks for your attention!