Transcript Energetics of Nanomaterials and Zeolites Alexandra Navrotsky UC Davis Control of Polymorphism at the Nanoscale  Competition between polymorphism and surface energy  Free energy crossovers as function.

Energetics of Nanomaterials
and Zeolites
Alexandra Navrotsky
UC Davis
Control of Polymorphism
at the Nanoscale

Competition between polymorphism and
surface energy
 Free energy crossovers as function of size
 More metastable polymorphs have lower
surface energies in general
Surface area (m2 /g)
0
50
100
150
Enthalpy (kJ/mol)
16
rutile
12
brookite
8
4
anatase
0
0
4000
8000
12000
Surface area (m2 /mol)
Enthalpy of titania polymorphs as a function of surface area (8).
Energetics of Nanocrystalline
Zirconia
H w.r.t. bulk m-ZrO2(kJ/mol)
140
monoclinic
120
100
tetragonal
80
60
amorphous
40
20
0
0
10000
20000
30000
40000
Surface area measured (m2/mol)
50000
ZEOLITES: NANOMATERIALS
WITH INTERNAL SURFACES
• Many different framework types, all of enthalpy
8 - 14 kJ/mol above quartz
• Molar volume changes by a factor of two because
of large internal pores and channels
• Internal surfaces generated by pores, can be
modeled using Cerius2 software
• Can one define a physically meaningful surface
energy from slope of trend between enthalpy
and internal surface area?
16
MEI
13.9
0
 Htrans (kJ/mol)
12
CHA
11.4
0
AST
10.9
0
MTW
8.70
MFI
6.80
6.60
FER
10.5
0EMT
9.30
BEA
MEL
8.20
8
13.6
0FAU
7.20
AFI
4
y = 0.6992x 2
17.5
R =
0.7147
0
0.00
quart 25
z
30
35
40
Molar Volume (cm 3/mol)
45
50
Surface Energy of 40 nm Particle
Material
Enthalpy relative to bulk(kJ/mol)
__________________________________________
Silicalite
0.5
Corundum
10
g-alumina
6.5
Rutile
6.2
Brookite
3.1
Anatase
1.2
Low value of surface energy (internal and external) may be what
allows many open polymorphs, the manganese oxides may be a
test case.
Enthalpy of formation relative to quartz (kJ/mol)
32
28
24
20
16
12
8
4
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Pore size (nm)
Enthalpies of formation of pure-silica mesoporous materials relative to
quartz as a function of pore size.  represents SBA-15 and MCM-41
materials (Trofymluk et al. 2005);  - MCM-48 and SBA-16 materials;
- MCM-41 (Navrotsky et al. 1995)  - MCM-41 materials from Lee, B.
MS thesis 2003, UC Davis
Challenges
of Hydration
 Detailed structural rearrangements
at surface and in frameworks related
to degree of hydration
 Energetics
 Is hydration a major driving force or
a by-product? Which is the tail,
which the dog?
Hydration control of growth
• High energy surface sites have highest
heats of hydration, hold on to water
• Hydrated surface layers for enhanced
reactivity, less hydration and more order
as particle grows, e.g. apatite
• Hydrophilic-hydrophobic competition
• Control of shape
Scanning heat flow
curves of a zeolite
synthesis mixture
(5.15Na2O-1.00Al2O33.28SiO2- 165H2O at a
constant heating rate of
0.10 ºC/min in a
Setaram C-80 heat flux
microcalorimeter.
Repeated in situ
experiments were
performed and stopped
at the selected
temperatures denoted
by capital letters.
Apparent peaks below
30 oC are artifacts.
Peaks between 40 and
70 oC represent several
steps of gel formation
Crystal Growth from Nanoclusters
• Attachment of nanoclusters, rather
than atoms or molecules, to growing
crystal
• Elimination of surface area and
eentually of surface-adsorbed species
• Classical nucleation and growth not
applicable
• Ostwald step rule rationalized
Insight into Zeolite Growth Mechanisms
Alexandra Navrotsky
University of California at Davis, DMR-01-01391
Teflon liner
Endo
Framework structure of MFI zeolite
Calorimetric curve
Time
Exo
Stainless steel vessel
A
F
B
E
C
D
Synthesis mixture
Vessel set used in
in situ calorimetry
Increase of pH in solution
Increase of surface charge density
Schematic representation of zeolite crystal growth by
aggregation of the pre-assembled nano-precursor
particles from exothermic stage to endothermic stage.
Zeolites are widely used in ion exchange,
Catalysis and separation because of their
Uniform cages and channels of nanometer
Dimension. Design of zeolite materials for
Applications demands a detailed underStanding of zeolite formation mechanisms.
Here we demonstrate that in situ
calorimetry reveals a two-stage
crystallization process for MFI-type
zeolite
Chem. Mater. 14, 2803 (2002)
critical nucleus or cluster for assembly
Free energy (schematic)
nanoclusters
nanoparticles
.
bulk phases
polymorph
species in solution
or melt
metastable
polymorph
stable
polymorph
Particle radius
Nanoparticles and
Biomineralization
• Control of polymorphism
• Selection of hydrous precursors with low
surface energy
• Storage, transport and attachment of
nanoparticles rather than of individual ions
• Specific surface-protein interactions
• Non-classical reinterpretation of nucleation,
growth, Ostwald step rule
Other Possible Advantages of
Nanoparticles
• Efficient concentration and
storage of precursors, including
sparingly soluble materials
• Tethering of particles to active
sites
• Membrane transport
• Detox
Synthesis of Silver Thiolates
R-SH (sol)+Ag NO3(sol) → R-S Ag (solid)+HNO3(sol)
AgS
CH3
Self-assembled monolayers
Atul Parikh et al 1999
Structure of silver thiolates. Phase
transitions.Temperature-dependent XRD
interlayer d-spacing
35
a, Å
30
25
20
6
8
10
12
14
16
18
20
number of carbons
interlayer d-spacing, Å
55
a
50
45
40
d = 8.14+1.21*(2N)
35
30
25
16
20
24
28
32
36
2(N-1)
Micellar (columnar) mesophase
Enthalpy, kJ/mol
Enthalpy, kJ/mol
Phase Transitions in Silver Thiolates. DSC data
80
60
40
60
40
hydrocarbons
20
20
0
50
0
8
10
12
14
16
18
100
150
200
Entropy, J/K mol
20
Entropy, J/K mol
Number of carbons
160
120
80
40
8
10
12
14
16
18
20
Number of carbons
n
T, °C
9
H, kJ/mol
S, J/K mol
130.5±0.5
35.3±0.5
86.2±1.0
11
131.1±0.3
39.3±2.7
97.2±6.2
15
131.0±0.5
53.7±1.2
132.9±3.8
17
131.1±0.3
58.8±2.2
145.5±7.5
Enthalpy, kJ/mol
Heat flow, ar.un.
5
5
4
2
R = 0.98
-200
-150
3
4
2
-100
1
3
0
-1
2
0
-50
10 20 30 40 50 60 70
0
2
4
6
8
10 12 14 16 18 20
Number of carbons
1
0
0
20
40
60
80
100 120 140
Time, min
Enthalpy, kJ/mol
-250
-100
-80
solution in toluene
-60
melting
-40
-20
0
6
8
10
12
14
16
18
20
Number of carbons
Conclusions

Silver thiolates and zeolites both explore
spatial confinement
 The former show much stronger
“tethering”
 Both show enthalpy-entropy
compensation