In situ QXAFS study of the pyrolytic decomposition of nickel formate dihydrate

Article abstract of DOI:10.1021/jp961488m

Quick scanning X-ray absorption fine structure (QXAFS) spectroscopy is applied to an examination of the pyrolytic decomposition of nickel formate dihydrate. Experimental QXAFS data reveal good quality edge and extended XAFS spectra being obtained in ca. 4 and 30 s, respectively. Comparison of data with normal (step/count) techniques reveals no adverse effect on the structural parameters derived using quick scanning. Temperature-programmed and isothermal decomposition studies are consistent with a pyrolysis mechanism involving rapid initial dehydration of the dihydrate followed by subsequent decomposition of the anion, via a nickel oxide intermediate, to yield a finely divided nickel metal (mean particle diameter ca. 12 A). These data confirm the nature of the principle intermediate and final product, both previously uncertain.

Full text of DOI:10.1021/jp961488m

20
J. Phys. Chem. B 1997, 101, 20-26
In Situ QXAFS Study of the Pyrolytic Decomposition of Nickel Formate Dihydrate
A. Bryan Edwards† and C. David Garner
Department of Chemistry, UniVersity of Manchester, Manchester M13 9PL, U.K.
Kevin J. Roberts*
Centre for Molecular and Interface Engineering, Department of Mechanical and Chemical Engineering,
Heriot-Watt UniVersity, Edinburgh, EH14 4AS, U.K., and CCLRC Daresbury Laboratory,
Warrington WA4 4AD, U.K.
ReceiVed: May 22, 1996; In Final Form: September 25, 1996X
Quick scanning X-ray absorption fine structure (QXAFS) spectroscopy is applied to an examination of the
pyrolytic decomposition of nickel formate dihydrate. Experimental QXAFS data reveal good quality edge
and extended XAFS spectra being obtained in ca. 4 and 30 s, respectively. Comparison of data with normal
(step/count) techniques reveals no adverse effect on the structural parameters derived using quick scanning.
Temperature-programmed and isothermal decomposition studies are consistent with a pyrolysis mechanism
involving rapid initial dehydration of the dihydrate followed by subsequent decomposition of the anion, via
a nickel oxide intermediate, to yield a finely divided nickel metal (mean particle diameter ca. 12 Å). These
data confirm the nature of the principle intermediate and final product, both previously uncertain.
Introduction
to-noise ratio. It is therefore important that a suitable counting
regime be established, the mechanics of the monochromator
assembly having been shown to limit the time required to
achieve high-quality data.8 In this paper we demonstrate the
applicability of QXAFS to the structural characterization of
nickel formate dihydrate, Ni(COOH)2‚2H2O, and its subsequent
pyrolytic decomposition through temperature-programmed and
isothermal decomposition experiments. The measurements are
validated, in a quantitative manner, via a comparison of normal
and QXAFS mode data.
X-ray absorption fine structure (XAFS) spectroscopy is a
sensitive probe of local atomic environment in condensed
materials yielding information from the extended fine structure
(EXAFS) on the type, number, and distance to the atoms
surrounding the absorbing atom1 as well as information from
the near-edge structure (XANES) on the site symmetry and
valence state of the absorber.2 The technique is element specific,
averaging over all atoms of a particular type. It does not depend
on the presence of long-range order and can be applied to
samples regardless of their physical state.
Despite the fact that the inherent time scale of XAFS is
essentially that of electronic spectroscopy (ca. 10-15 s3),
experimental considerations have generally limited its applica-
tion to static systems because of the time required to collect
data of sufficiently high quality to allow quantitative analysis.
The normal mode of data collection employs a double-crystal
monochromator to select a single wavelength from a broad
spectrum source, usually synchrotron radiation with the spectrum
being constructed from the collection of discrete measurements
recorded at appropriate intervals over the desired energy range.
A number of methods have been employed to extend XAFS to
the time domain including freeze-quench techniques4 and time-
resolved laser flash photolysis.5 A recent method is quick
scanning XAFS, or QXAFS,6 in which the monochromator is
swept across an angular range in a continuous movement and
data collected by time-integrating absorption measurements
recorded while the monochromator remains in motion. This
approach reduces the energy resolution, since the measurements
are no longer made at a single wavelength. However, by the
choice of an appropriately short integration time relative to the
speed of the monochromator, this can be maintained within
acceptable limits. The energy resolution for EXAFS and
XANES is ca. 5 and 1 eV, respectively.7 For a given
monochromator speed the integration time will determine not
only the energy resolution but also the sampling rate and signal-
Experimental Details
Materials and Measurements. XAFS data were recorded
on station 7.1 of the Daresbury synchrotron radiation source
(SRS) operating in multibunch mode with a beam energy of 2
GeV and average circulating current of 170 mA. Data were
collected in transmission mode using gas-filled ionization
chambers and a Si(111) order sorting double-crystal monochro-
mator. Reference data were collected between 300 eV below
and k ) 15 Å-1 above the Ni K-edge with the monochromator
detuned to 50% transmission to achieve a suitable level of
harmonic rejection. The postedge data were collected at
energies corresponding to increments of 0.04 in k-space with
k2 time weighting and final point integration time of 8000 ms.
QXAFS measurements, made using the FLYPAST option
available within the Daresbury TOPOGRAPHY data acquisition
program,9 were collected over a range of monochromator
scanning speeds and data integration times in order to define
optimal data acquisition conditions. A monochromator angle-
scanning range 14 250-12 587 mdeg, corresponding to an
energy range approximately equivalent to that used for the
conventional step/count mode, was used with the monochro-
mator detuned to about 40% transmission at the midpoint of
the EXAFS region during data collection. The data were
truncated at k ) 12 Å-1 prior to analysis owing to the existence
of a reproducible discontinuity in the spectra when the mono-
chromator was operated in QXAFS mode.
† Now at LSU College of Higher Education, The Avenue, Southampton
SO9 5HB, U.K.
X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S1089-5647(96)01488-5 CCC: $14.00 © 1997 American Chemical Society

Nickel Formate Dihydrate
J. Phys. Chem. B, Vol. 101, No. 1, 1997 21
The nickel metal reference spectrum was recorded from a 5
µm rolled metal foil measuring some 4 cm × 4 cm. Other data
were recorded from AnalaR grade samples, ground into a fine
powder and evenly diluted with boron nitride before being
pressed into cylindrical pellets 10 mm in diameter and typically
1-2 mm thick in a high-pressure hydraulic press. Only those
with an absorption step of 0.8-1.5 at the Ni K-edge were
retained. These pellets were secured in a specially constructed
electrically heated microfurnace10 for data acquisition. The
temperature was measured using a previously calibrated ther-
mocouple and quoted with an estimated error not exceeding
(0.2 °C.
The thermal behavior of nickel formate dihydrate was
recorded using a DuPont 9900 differential scanning calorimeter
from 6 mg of fresh nickel formate dihydrate in a sealed
aluminum sample ampule heated at a constant rate of 2 °C
min-1
.
Data Reduction and Analysis. Calibration and background
subtraction of the raw absorption data were performed using
unmodified versions of the Daresbury Laboratory programs
EXCALIB and EXBACK, respectively. The isolated EXAFS
data were analyzed within the framework of the curved wave
theory11 using the program EXCURV92.12 Multiple scattering13
from collinear atomic arrangements was included in all simula-
tions where appropriate. Phase shifts were derived from ab
initio calculations using Hedin-Lundqvist exchange potentials
and von Barth and Hedin ground states14 taken within a 1s core
hole excited-state approximation. During analysis, the bond
lengths (R), coordination numbers (N), and Debye-Waller-like
disorder (2σ) were optimized using curve-fitting techniques in
which the difference between the experimental and simulated
spectra was minimized. However, the shell occupation number
and disorder parameters, being highly correlated, typically
prevented the coordination number from being determined to a
precision better than (1. The error in the bond length is
estimated to be of the order of (0.03 Å for the nearest-neighbor
shell.
Figure 1. Comparison of K-edge X-ray absorption profiles for nickel
metal recorded in normal step/count (a) and QXAFS (b-d) modes,
the latter recorded with a constant integration time of 0.01 s and
monochromator speeds of 20 mdeg s-1 (b), 50 mdeg s-1 (c), and 100
mdeg s-1 (d). The inset shows an expanded view of the pre-edge and
XANES profiles of the same data.
Results and Discussion
Validation of the QXAFS Implementation against Con-
ventional Step/Counting Data Acquisition. The X-ray ab-
sorption spectrum of Ni metal was recorded at ambient
temperature in QXAFS mode using a fixed integration time of
0.01 s at a variety of monochromator speeds. A comparison
of the calibrated and normalized pre-edge and XANES absorp-
tion profiles with reference data recorded in normal mode
(Figure 1) reveals that the resolution is insufficient, even at the
highest sampling rate, to resolve the splitting in the first postedge
feature. The pre-edge feature is similarly poorly reproduced
in the QXAFS data with increasing monochromator speeds.
Figure 2. Comparison of isolated k3-weighted Ni-edge QXAFS data
of nickel foil recorded at room temperature in normal step/count (a)
and QXAFS (b-d) modes (s) and their associated Fourier transforms
together with simulations based on the parameters listed in Table 1
(- - -). The QXAFS data were recorded with a constant integration time
of 0.01 s and monochromator speeds of 20 mdeg s-1 (b), 50 mdeg s-1
(c), and 100 mdeg s-1 (d).
The energy dependent sampling frequency evident in the
isolated EXAFS data (Figure 2) is due to the nonlinear
relationship between the monochromator angle and photoelec-
tron wave vector (k), the measurements having been made at
equal time intervals with a monochromator moving with
constant angular velocity. The trend in both energy resolution
and counting statistics is unavoidable given the unsophisticated
nature of this implementation. An inevitable consequence is
to discriminate against accurate curve fitting of the contribution
from low-Z neighbors whose scattering amplitude decreases
more rapidly with increasing energy than heavy atom scatterers,
concentrating the contribution low-Z neighbors make to the total
EXAFS signal in the low k region. Similarly, the fitting of
low- rather than high-temperature data is favored because of
the damping effect of increased thermal disorder. Furthermore,
relatively infrequent sampling of the low k data makes quan-
tification of multiple scattering and other secondary processes
unreliable and exaggerates the need for reliable phase shifts,
which may differ considerably in this region depending on the
computational methodology.
The similarity of the higher energy EXAFS data used for

22 J. Phys. Chem. B, Vol. 101, No. 1, 1997
Edwards et al.
quantitative analysis (i.e., beyond k ≈ 2.5 Å-1) was found to
TABLE 1: Optimized Parameters Derived from Simulation
of Ni K-Edge EXAFS of Nickel Foil Recorded in Normal
and QXAFS Modes at Ambient Temperaturea
be generally good up to 50 mdeg s-1
. The effect of the
systematic degradation in data quality with increasing mono-
chromator speeds was evident in the Fourier transforms of these
data. The most obvious manifestation is the failure to convinc-
ingly reproduce shells beyond the second in the fastest scan. A
similar, though less dramatic, change can be seen in the more
distant shells (beyond ca. 6.5 Å) on moving from 20 to 50 mdeg
s-1, although in this case the differences occur beyond the
maximum range normally included in quantitative analysis.
In Normal Mode
scan time
shell
type
N
R (Å)
2σ (Å2)
(min:s)
1
2
3
4
Ni
Ni
Ni
Ni
12
6
24
12
2.47
3.49
4.31
4.95
0.010
0.017
0.016
0.014
32:15
The total EXAFS above an absorption edge is the sum of
damped sinusoidal contributions, one from each shell of atoms
surrounding the absorber. The frequency of each contribution
is proportional to the mean absorber-scatterer distance within
that shell. Sampling theory dictates that such a sine wave must
be sampled at a minimum of two points per cycle (i.e., twice
the frequency of the wave) and a complex function at twice the
frequency of the highest frequency component if it is to be
accurately characterized by discrete sampling. This is com-
monly called the Nyquist condition.15 Thus, the sampling rate
imposes a limit on the highest frequency of contribution that
can be reliably isolated and, in turn, the maximum distance from
the absorbed that can be probed. Increasing the monochromator
scan speed without decreasing the integration time reduces the
sampling rate to the extent that it fails to satisfy the Nyquist
condition for ever closer shells as the monochromator speed
increases. This could in principle be accommodated by reducing
the integration time, since this would increase both the number
of data points and their energy resolution. However, to do so
would incur a penalty of poor counting statistics. The increasing
dissimilarity between the QXAFS and reference data with
increasing monochromator speed is attributed to sampling effects
and demonstrates the need to validate an experimental regime
against a previously characterized system before its application
to an unknown.
In QXAS Mode
monochromator
scan time
(min:s)
speed (mdeg s-1
20
)
shell type
N
R (Å) 2σ (Å2)
1
2
3
4
1
2
3
4
1
2
3
4
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
12 2.47
3.49
24 4.32
12 4.97
12 2.47
0.010
0.017
0.017
0.015
0.010
0.017
0.016
0.015
0.013
0.017
0.026
0.024
01:23
00:33
00:16
6
50
6
3.50
24 4.32
12 4.97
12 2.43
100
6
3.42
24 4.27
12 4.89
a The shell occupation number (N) has been fixed at crystallographic
value.
Quantitative analysis of the data was performed by simulating
scattering from the four nearest-neighbor shells. The shell
occupation numbers and distances were initially set at the
crystallographic values. The distances and their associated
Debye-Waller parameters (which includes both structural and
thermal disorder) and energy correction terms were subsequently
refined. Multiple scattering from the collinear first and fourth
shells was included (the scattering angle set to 180° and not
refined during the fitting process). The optimized parameters
(Table 1) confirm the observed qualitative agreement between
the QXAFS and normal mode data. Despite the very significant
reduction in data collection times, derived parameters for each
of the four shells are identical within experimental error to the
reference data for all but the fastest (100 mdeg s-1) scan.
Figure 3. Differential scanning calorimetry trace for Ni(COOH)2‚
2H20 in a sealed ampule. The data reveal three main regions: (A)
dehydration, (B) conversion from anhydrate to oxide, and (C) reduction
of oxide to metal. The inset shows an enlargement of region A.
Relatively few generalizations regarding points of similarity or
trends within these comparable systems have been accorded
widespread acceptance.
Temperature-Programmed Study. Differential scanning
calorimetry (DSC) of nickel formate dihydrate (Figure 3)
provides evidence of changes at ca. 175, 225, and 270 °C with
no evidence for further changes to 350 °C. A sample of nickel
formate dihydrate was heated from ambient temperature to 310
QXAFS Examination of the Decomposition Dynamics of
Nickel Formate Dihydrate. There has been significant interest
in the thermal decomposition of transition metal carboxylates
as potential sources of high surface area solids for catalysis,
which demands particles in the range 10-30 Å for maximum
efficiency16 and whose properties are strongly size dependent
in this range.17 More recently, there has been a resurgence of
interest in microclusters with the discovery of a shell structure,
similar to that of an individual atom or nucleus, dependent upon
the number of atoms present.18 The pyrolysis of nickel formate
and related carboxylates has been explored under a wide range
of conditions using a variety of techniques for both bulk
samples19-23 and well-defined surfaces.24-38 Collectively, these
have aided an understanding of the factors affecting such
reactions, although there are significant qualitative and quantita-
tive differences reported for the mechanisms and energetics.
°C, without interruption, at a constant rate of 10 °C min-1
.
Absorption spectra were recorded at 30 °C steps to 180 °C,
then at 240 °C and finally at 310 °C. The pyrolysis products
were immediately cooled to room temperature and the absorp-
tion spectrum rerecorded.
The XANES (Figure 4) and EXAFS data (Figure 5) recorded
in the temperature step experiment suggest negligible differences
in the nickel coordination environment to 180 °C, confirmed
by the Fourier transforms of the EXAFS data. Further heating
to 240 °C precipitates only a slight change in the feature at k ≈
6 Å-1 and slightly greater than expected drop in the magnitude
of the feature at R ≈ 4 Å based on the trend established at

Nickel Formate Dihydrate
J. Phys. Chem. B, Vol. 101, No. 1, 1997 23
TABLE 2: Optimized Parameters Derived from Simulation
of QXAFS Data from the Temperature-Programmed
Decomposition Experiments
temperature (°C)
type
N
R (Å)
2σ (Å2)
30
60
90
120
150
180
240
O
O
O
O
O
O
O
5.6
5.1
5.1
5.0
4.9
4.5
3.9
2.04
2.04
2.04
2.04
2.04
2.04
2.02
0.015
0.013
0.014
0.015
0.014
0.012
0.012
constant below 180 °C above which there is evidence of
decreasing nearest-neighbor shell occupation.
Crystallographically characterized formate species are rare,
making a direct comparison of the EXAFS-derived results
difficult. However, a number of analogous covalently coordi-
nated structural comparators do exist, e.g., catena-(tetraaquatet-
rakis(µ2-formato-O,O′)dinickel(II))39 has two distorted octahe-
drally coordinated nickel sites with a mean Ni-O distance of
2.063 Å. This is similar to the local atomic environment about
nickel in diaquabis(acetylacetonato)nickel(II)40 and bis(acetyl-
acetonato)nickel(II) bis(ethanol)41 with mean Ni-O distances
of 2.058 and 2.054Å , respectively. In contrast, the 4-fold
coordination of bis(2-hydroxy-5-methylacetophenato)nickel(II)42
has a reduced mean Ni-O distance of 1.84 Å. The structural
parameters derived for nickel formate dihydrate at 30 °C (5.6
oxygen atoms at a mean distance of 2.04 Å) are thus chemically
reasonable.
Figure 4. Temperature-programmed QXAFS data (XANES region)
in the temperature region between 250 and 300 °C recorded with a
data acquisition time of ca. 4 s, revealing a reduction of white line
intensity associated with the thermally induced reduction of NiO to
metallic Ni at around 265 °C.
The DSC data, given in Figure 3, accords with a previous
comparison of hydrated and anhydrous nickel formate salts,23
with only the high-temperature feature common to both (the
lower two absent in the anhydrous sample). Both EXAFS and
DSC data are consistent with the first stage in the thermal
decomposition of nickel formate dihydrate being elimination
of water to yield anhydrous nickel formate. Changes in Ni-O
distance are on the order of the statistical error associated with
determination of this parameter and cannot be interpreted as
definitive evidence of contraction of the primary coordination
sphere with decreasing occupation, although they would be
consistent with it. Indeed, the possible intermediate status of
the anhydrous salt and the larger bond length of the Ni-O
contact of nickel oxide (akin to that of the hydrated salt) into
which it is converted (discussed subsequently) conspire against
its identification by EXAFS, which yields data averaged over
all atoms of a particular type. Dehydration could account for
the reduction in R ≈ 4 Å peak at 240 °C (Figure 5) by reducing
any multiple scattering enhancement of the contribution from
these more distant atoms by removing an intermediate oxygen
atom. However, lattice reorganization to yield a new crystalline
phase or amorphous material following initial dehydration prior
to anion decomposition, observed in similar systems,43 cannot
be excluded on the basis of these data alone and for which there
is limited evidence in the case of nickel formate dihydrate.20
Although impossible to define the precise onset of dehydration,
these data suggest that the hydrated phase persists to higher
temperatures than sometimes reported, the onset of dehydration
having been reported to be as low as 40 °C in vacuum.20
Analysis of Decomposition Products. The data recorded
from the cooled decomposition products were modeled as nickel
metal assuming retention of the FCC structure. The bond
lengths and disorder parameters for all four shells used in the
simulation were initially set to those of the bulk metal.
Scattering amplitudes were initially adjusted by varying the shell
occupation numbers with simultaneous refinement of the
interatomic scattering path lengths. The constant Debye-Waller
constraint was subsequently relaxed and all parameters refined
Figure 5. In situ temperature-programmed temperature dependent
k3-weighted EXAFS data recorded in QXAFS mode with a fixed
integration time of 0.01 s and monochromator speed of 20 mdeg s-1
(s) and associated Fourier transforms together with simulations based
on the parameters listed in Table 2 (- - -).
lower temperatures. The data recorded at 310 °C are quite
unlike that preceding it. Qualitatively similar to that of bulk
metal, particularly at low k and R, the relative and absolute
magnitudes of each Fourier transform feature are unlike those
of the model system.
To avoid over parametrization of the simulation, the isolated
EXAFS data to 240 °C were simulated assuming a single shell
of oxygen backscatterers whose distance was initially set to 2
Å. Each of the structural parameters was allowed to float during
the optimization, the amplitude being fitted by simultaneous
refinement of both the shell occupation number and the Debye-
Waller parameter. The optimized parameters (Table 2) support
the view that the local atomic environment remains essentially

24 J. Phys. Chem. B, Vol. 101, No. 1, 1997
Edwards et al.
Figure 6. k3-weighted EXAFS data of the decomposition products as
recorded in QXAFS mode with a fixed integration time of 0.01 s and
monochromator speed of 20 mdeg s-1 (s) and associated Fourier
transforms together with simulations based on the parameters listed in
Table 3 (- - -).
Figure 7. k3-weighted QXAFS data, together with associated Fourier
transforms, showing a comparison between the pyrolyzed material
produced in the temperature-programmed experiment (a) and data
recorded from a reference nickel foil (b). Particle size estimates based
on the analyzed data of the decomposed material, given in Table 3,
are given in Table 4.
TABLE 3: Optimized Parameters Derived from Simulation
of QXAFS Data Recorded from the Decomposition Products
Following Cooling Back to Ambient Temperature
shell
type
N
R (Å)
2σ (Å2)
1
2
3
4
Ni
Ni
Ni
Ni
8.2
3.7
11.9
5.0
2.47
3.50
4.31
4.96
0.014
0.019
0.016
0.014
effectively instantaneous on the experimental time scale acces-
sible using the current implementation.
The isolated, k3-weighted isothermal EXAFS data and their
Fourier transforms (Figure 8) are suggestive of continuous
change in the Ni environment over the time scale of the
experiment. This is particularly apparent in the Fourier
transforms with a reduction in the height of the nearest-neighbor
shell accompanied by the emergence of a strong scattering from
a shell centered at R ≈ 3 Å. The data were analyzed using a
simple two-shell model: a nearest-neighbor shell of oxygen
atoms and more distant shell of Ni atoms at ca. 2 and 3 Å,
respectively. The fitting procedure followed that outlined
previously. The nickel shell was excluded from the simulation
if it failed to offer a statistically significant improvement to the
fit.51 The model, although simple, reproduces the principle
features of the EXAFS and their Fourier transforms. The
optimized results, collated in Table 5, suggest the creation of
an oxide phase, the observed metal-metal distance of ca. 2.98
Å comparing favorably with the crystallographic distance of
2.954 Å.52 Other identifiable Ni-Ni contacts whose distances
are consistent with those expected for NiO reinforce this
assignment but were excluded from the final fit because of the
high {σ, N} correlation, making meaningful amplitude quanti-
fication impossible. The derived structural parameters are
incompatible with a NiCO3 intermediate proposed elsewhere.53
This oxide intermediate is presumably reduced on further heating
by CO, H2 (both of which have been detected experimentally38),
and possibly other reactive intermediates generated in the course
of the reaction but inhibited from diffusing from the reaction
zone. It is probable that behavior in this region is ill-suited to
description by balanced stoichiometric equations, the composi-
simultaneously. The best simulation and optimized parameters
are presented in Figure 6 and Table 3, respectively. The refined
parameters with those of the bulk FCC Ni (Table 1) reveals
reduced coordination numbers for the reaction product associated
with a particle size diameter (Table 4) of ca. 12 Å. Some
caution is needed in directly comparing this with other measure-
ments, since it is known that particle size estimation based on
EXAFS coordination numbers can tend to slightly underestimate
the particle size44-46 owing to the generation of asymmetric pair
distribution functions by the very anharmonic motion of the
atoms47,48 at even modest temperatures. Previously reported
size estimates for this material range from 200 to 500 Å from
electron microscopy21 to ca. 190 Å using X-ray diffraction22
and the integral breadth method.49 However, direct comparison
of the particle size with previous work is not very realistic, since
the reaction is very sensitive to sample preparation and
conditions that are not well documented in the literature. Our
smaller particle size is probably consistent with temperature
cycling rather than with products being maintained at elevated
temperatures as in the other experiments.
Isothermal Study. A fresh nickel formate dihydrate sample
with no prior thermal history was subsequently loaded into the
furnace and the temperature raised as rapidly as possible to 180
°C and stabilized. Absorption spectra were recorded in QXAFS
mode at regular intervals. Attempts to repeat the isothermal
experiment at higher temperatures corresponding to onset of
the transition at 270 °C were unsuccessful, the reaction being
TABLE 4: Estimation of Particle Size for the Decomposed Nickel Formate Dihydrate Based on the Experimental Data Given
in Table 3 and Theoretical Calculations of the Coordination Numbers versus Particle Size
no. atoms in
cluster
ave first shell
occupation number
ave second shell
occupation number
ave third shell
occupation number
ave fourth shell
occupation number
particle radius (Å)
4
6
8
10
15
19
79
177
369
1205
6.3
8.5
9.1
9.8
10.5
8.2
1.9
3.2
3.9
4.4
4.9
3.7
5.1
11.5
14.4
16.3
18.7
11.9
1.9
5.0
6.6
7.6
9.0
5.0
QXAFS data (Table 3, column 3)

Nickel Formate Dihydrate
J. Phys. Chem. B, Vol. 101, No. 1, 1997 25
Figure 9. Time dependent evolution of the oxygen (top) and nickel
(bottom) shell occupation numbers (estimated uncertainty is (1)
determined from the isothermal QXAFS experiment. The integration
Figure 8. k3-weighted time dependent QXAFS data and associated
Fourier transforms recorded under isothermal conditions (180 °C) with
a fixed integration time of 0.01 s and monochromator speed of 20 mdeg
s-1 (s) together with simulations based on the parameters listed in
Table 5 (- - -).
time is 0.01 s, and the monochromator speed is 20 mdeg s-1
.
previously reported temperature dependent induction period
preceding a brief acceleratory period in the decomposition of
the anion, which is subsequently retarded by the accumulation
of reaction products.21
TABLE 5: Optimized Parameters Derived from Simulation
of EXAFS Data from the Isothermal Decomposition
Experiments
oxygen
nickel
R (Å)
time (min)
N
R (Å)
2σ (Å2)
N
2σ (Å2)
Conclusions
0
3
8
15
20
25
30
35
40
45
4.8
4.9
4.9
4.4
4.1
3.7
3.8
3.6
3.9
3.8
2.01
2.02
2.02
2.01
2.00
2.01
2.01
2.02
2.02
2.02
0.016
0.018
0.020
0.019
0.018
0.017
0.020
0.017
0.022
0.020
The application of QXAFS to the structural characterization
of materials has been quantitatively demonstrated through a
careful examination of the structure, and subsequent thermal
decomposition, of nickel formate dihydrate. The study reveals
that data acquisition times can be reduced by nearly 2 orders
of magnitude when compared with normal (step/count) tech-
niques.
1.5
3.1
4.7
6.4
6.5
6.3
6.9
2.96
2.98
2.98
2.99
2.99
2.98
2.99
0.021
0.024
0.027
0.027
0.027
0.026
0.026
Although currently unable to offer the very high time
resolution of dispersive EXAFS,54 hardware developments,
including high-quality detectors,55 are narrowing the gap.
Similar experiments carried out on a third generation synchro-
tron radiation source, such as the ALS (USA), ESRF (Europe)
or Spring 8 (Japan), would provide data acquisition times for
XANES and EXAFS of ca. 30 and 300 ms, respectively, thus
yielding some considerable scope for further developments of
the QXAFS method and its application to in situ structurokinetic
studies.56-60
tion of each particle at any instant depending in a complex way
on its immediate environment. Identification of mechanisms
and kinetics for reactions of this type will therefore continue to
present a particular challenge.
In general, any parameter that can be related to the fractional
extent of a reaction can be used as a kinetic probe if the
measurements can be completed on a time scale short relative
to the process. No attempt has made to quantify rate constants
because of the known sensitivity of the kinetics to factors such
as reactant particle subdivision, agglomeration, and ambient
atmosphere20 beyond the control of the experiment. Despite
this, some qualitative statements are possible. The oxygen
coordination number at the beginning of the isothermal experi-
ment (Figure 9) is consistent with partial dehydration having
occurred during the temperature elevation phase, implying that
this step in the pyrolysis is a rapid one. Similarly, the nonlinear
variation in the Ni shell occupation number accords with a
Acknowledgment. We thank the CCLRC Daresbury Labo-
ratory for the provision of synchrotron beam time on the SRS,
D. Cunningham for recording the DSC data, E. Pantos and N.
Harris for their help in implementing the data acquisition
software, and EPSRC for the current support of a senior
fellowship (K.J.R.).

26 J. Phys. Chem. B, Vol. 101, No. 1, 1997
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