Progress in Nuclear Magnetic Resonance Spectroscopy. Volume 14

69 Pages·1980·1.404 MB·English

by  J. W. Emsley, J Feeney and L H Sutcliffe (Eds.)

Volume 14 Part 1
Progress in nUElear
magnetiE HesananEe
5peEtrasEapy
Editors: J. W. Emsley, J. Feeney and L. H. Sutcliffe
(Southampton) (London) (Liverpool)
An International Review Journal
Nuclear Magnetic Resonance Studies of
Molecules Physisorbed on Homogeneous
Surfaces
J.Tabony 1
Digitisation and Data Processing in Fourier
Transform NMR
J. C. Lindon and A. G. Ferrige 27
Subject Index
ISSN 0079-6565"
PNMRAT 14 (1) 1-66 (1980)
Pergamon Press
Oxford • New York
Paris • Frankfurt
PROGRESS IN NUCLEAR MAGNETIC RESONANCE
SPECTROSCOPY
Edited by:
J. W. EMSLEY, Department of Chemistry, The University, Southampton, S09 5NH,
England
J. FEENEY, National Institute for Medical Research, Mill Hill, London NW7 1AA,
England
and
L. H. SUTCLIFFE, Donnan Laboratories, The University, Liverpool, England.
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NUCLEAR MAGNETIC RESONANCE STUDIES OF
MOLECULES PHYSISORBED ON HOMOGENEOUS
SURFACES
J. TABONY
Institut Laue-Langevin, 156X, 38042, Grenoble Cedex, France
(Received 25 October 1979)
CONTENTS
1. INTRODUCTION intermolecular forces, whilst in chemisorption they are
large and more comparable with chemical bonds. The
1.1. General
division between the two is arbitrary and many real
Adsorption is an important phenomenon in physics, systems cannot be classified as either one or the other.
chemistry and biology. It is an important concept in It is not uncommon for the first few molecules to be
disciplines ranging from semi-conductor physics to strongly adsorbed (chemisorption) on a few surface
heterogeneous catalysis and membrane biophysics, sites of high energy followed by weak adsorption
and has many technical applications. (physisorption) on the rest of the surface.
Adsorption is normally divided into physisorp- Most conventional techniques when applied to
tjo dn,2) a nj( chemisorption. In physisorpfion the sur- surface studies have much lower signal-to-noise ratios
face adsorbate forces are weak and comparable with than experiments in bulk materials and this has
IPNMRS 14: 1--A
2 J. TABONY
0.5 1.0 0.5 1.0 0.5 1.0
Relative pressure
0.5 1.0 0.5
Relative pressure
(5)
FIG. 1. Brunauer's five types of adsorption isotherm.
influenced the development of the subject. Of the well-defined layers, whereas in type III isotherms,
different possible interfaces (gas-solid, liquid-solid, adsorption consists mainly of the formation and
liquid-liquid etc.) the gas-solid interface is the easiest growth of islands of multilayers. In 1969 Thorny
67)
to study since microscopic observation of the inter- and Duval* ' (using a specially prepared exfoliated
facial layer is aided by the fact that one of the bulk graphite) found type II isotherms showing clearly
phases present is a relatively dilute gas. Here there defined steps and substeps characteristic of two-
have been two areas of investigation, one has been the dimensional (2D) condensation.
study of isolated single crystal surfaces under ultra- Experimental techniques had meanwhile improved
high vacuum conditions whilst the other has been the sufficiently to make microscopic measurements
development of very fine powders having large possible and in the last few years a number of neutron
surface-to-mass ratios. Since the early work on physi- scattering,*8'9* X-ray,(1) 0Mössbauer(1'112) and NMR
sorption, the preparation of solid surfaces has im- experiments have characterized a variety of these two-
proved considerably and a variety of materials are dimensional adsorbed phases. At the moment two-
available having large specific surface areas and good dimensional gases, liquids and solids are known to
homogeneity. Nuclear magnetic resonance is not yet exist*2'9'13) as well as epitaxial phases where the
sensitive enough for single crystal work but has substrate structure impresses some of its own regu-
sufficient sensitivity for high surface area powder larity on the structure of the film.
studies.
For some time the lack of sensitivity prevented
1.2. Microscopic Methods
microscopic measurements, resulting in classification
by thermodynamic measurements such as the adsorp- Nuclear magnetic resonance has been successfully
tion isotherm. This is a measure of the mass of material applied to the study of the structure and molecular
adsorbed as a function of the equilibrium vapour dynamics of bulk materials, but there has been com-
pressure at a fixed temperature. Although a set of paratively little work on surface problems. Only a
isotherms at different temperatures can yield detailed handful of reports concerning physisorption of gases
340
thermodynamic data* ' they give little information at on homogeneous well-characterized solid surfaces
142 -5)
the molecular level. In particular, they give no direct have appeared.* There have been several reviews
263 - υ
information about the molecular dynamics of the of the application of NMR to surface problems,*
adsorbed species. but they are primarily concerned with molecules either
(5)
Brunauer, Emmett and Teller (B.E.T.) charac- trapped in zeolites, or adsorbed upon highly hetero-
terized physisorption on powders by the five types of geneous substrates. The cage structure of zeolites
isotherm shown in Fig. 1, with type II and type III prevents two-dimensional condensation. For mole-
isotherms representing extremes. In type II isotherms, cules adsorbed upon heterogeneous substrates the
the adsorbed molecules tend to wet the surface forming large number of variables together with the lack of
Nuclear magnetic resonance studies of molecules physisorbed on homogeneous surfaces 3
complementary measurements makes interpretation NMR experiments only the adsorbate gives a spect-
difficult. This article aims to provide an introduction rum and thus difference spectra (with the uncertainties
and to establish the methodology of the NMR from which are involved) are unnecessary.
physisorbed molecules as it relates to the new physics
of physisorption which has developed since the orig- 1.3. Homogeneous Substrates
inal work of Thorny and Duval. Only measurements
Of the different powder substrates available, graph-
made using homogeneous substrates will be con-
ite is considered as providing the closest approach to
sidered. As only a few examples have been studied in
a clean, flat surface and adsorption upon its surface is
depth they will be referred to continuously. (3)4
thought to be almost uniform. It has the advantages
Comparison of NMR results with those from other
of being non-porous, easy to handle and is readily
microscopic methods, principally neutron scatter-
( 3 2)3 3 cleaned by outgassing at moderate temperatures.
ing should allow NMR to be applied where
Most graphite surfaces are either exfoliated graphite or
neutron scattering is insensitive or where facilities are
graphitised carbon blacks.
lacking.
Exfoliated graphites are made by intercalating
NMR and neutron scattering are complementary
molecules such as FeCl , CoCl etc. into graphite and
and overlapping techniques in that they both give 3 3
heating. When heated, the guest molecules force apart
structural and dynamic information on the adsorbed
the graphite planes to expose large areas of freshly
layer. For ordered solid adsorbed layers the neutron (3)5
cleaved basal planes. Graphitised carbon blacks are
diffraction technique gives superior structural inform-
produced by the thermal treatment of carbon
ation to that from NMR. In disordered solid or fluid (3)4
blacks. Amorphous carbon black powders when
adsorbed phases, NMR probably gives as much
heated for long periods at high temperature ( ~ 3000°C)
structural data as neutrons but more quickly. One
graphitise to form polyhedral particles of several tens
particular advantage of NMR is that it gives the
of nanometres diameter, with each polyhedral face
orientation of the molecules with respect to the
exposing the basal plane of the graphite structure. The
surface: information which is not readily obtainable
dimensions of these faces, as well as the homogeneity
from neutron scattering experiments, especially when
and the surface area available for adsorption, depend
the adsorbate is fluid.
upon the origin and thermal treatment of the carbon
As concerns molecular dynamics, both NMR and
black used. Graphitised carbon blacks are slightly less
neutron scattering give data on rotational and trans-
homogeneous than exfoliated graphites, but tend to
lational motions. The main difference between the two
have larger surface areas and densities which leads to
techniques is the time scale. NMR spin-lattice relax-
increased adsorption capacity and thus a better signal-
ation times (Ti) are sensitive to motions close to the
9 to-noise ratio for the interfacial layer.
Larmor frequency (10" sec). Spin-lattice relaxation
A schematic diagram of the graphite structure is
times in the rotating frame (T ) and dipolar relaxation
lp 2 shown in Fig. 2. The high chemical and thermal
times (T) are sensitive to even slower motions (10~ -
6 D stability of graphite means that its adsorption
10" sec). Since neutron scattering has a time scale of
9 13 properties are readily reproduced and the potential
10" -10" sec, a combination of neutron scattering
barriers to molecular displacement across the basal
and NMR is capable of probing molecular motion (3)4
plane are known to be small.
over more than ten decades of frequency.
Lamellar halides such as Pbl or NiCl also have
Compared with neutron scattering, there have been 2 2
only one exposed basal plane and show very homo-
few recent NMR publications of molecules physi- (3)6
geneous adsorption. Like graphite they are made
sorbed on homogeneous surfaces. This is disappoint-
by exfoliation but differ in that the surface potential
ing since both methods give similar information and
wells are larger. Other suitable substrates which are
NMR has several practical advantages. At present
less homogeneous are BN, NaCl, KCl. Titanium
NMR is at least ten times more sensitive than neutron
dioxide is heterogeneous because it has several dif-
scattering. Using Fourier transform methods a proton 37)
ferent crystal faces for adsorption/ It has some uses
NMR spectrum of a monolayer of material may be
as a catalyst and understanding of adsorption on its
obtained in a few minutes, whereas a comparable
moderately heterogeneous surface could help bridge
neutron spectrum from a much larger sample would
the gap between the understanding of adsorption on
take several hours counting. Another advantage of
homogeneous surfaces and heterogeneous catalysis.
NMR is availability. Neutron scattering experiments
on physisorbed layers require the highest possible
1.4. Sensitivity
neutron fluxes, which are only available at a few
specialised centres : this is a severe practical constraint. One reason for the scarcity of NMR adsorption
The third practical advantage of NMR is that in studies using homogeneous substrates has been the
neutron scattering experiments both the adsorbate difficulty in observing spectra from the small quantities
and substrate scatter neutrons. Since the spectrum of materials adsorbed. Heterogeneous substrates nor-
from the substrate with the adsorbate is only slightly mally have larger surface areas than homogenous
stronger than the substrate spectrum, the adsorbate materials and have thus been more common. After the
spectrum is normally obtained by subtraction. In most technical progress of the last few years, sensitivity is
4 J. TABONY
3.35 A
1.415 A
FIG. 2. The normal structure of graphite showing the basal plane on which adsorption occurs.
13 1 5
now sufficient and good proton spectra can be ob- ratios. C and N are more difficult to observe
tained easily. because of their low natural abundance. Figure 4
13
Most common substrates have surface areas varying shows the natural abundance C spectrum of one
2 1 2 - 1
between 1 m g" and 100 m g , with the more
homogenous substrates being grouped around the
lower end of the range. For a typical graphitized
2
carbon black having a specific surface area of ~ 60 m (a)
-1 3
g and a density of 1 g cm" , one monolayer of a
3
relatively low molecular weight material on 1 cm of
21
sample correspond to ~ 10 mg or ~10 protons.
Figure 3 shows examples of proton spectra for a
variety of different adsorbates and substrates. The
signal to noise is good and each spectrum was obtained
in less than ten minutes. They show that proton NMR
is at present capable of detecting resonances from
materials of quite low surface area.
(b)
Although the exfoliated graphite used by Thorny
(6,)7
and Duval in their original work has a surface area
2 1 - 3
of ~20 m g" , the density is ~0.05 g cm , and one
3
monolayer on 1 cm of substrate corresponds to only
19
~ 0.1 mg or 10 protons. The low density of exfoliated
graphite may be increased by mechanically pressing it
into sheets, and under the trade names of Grafoil or
(c)
Papyex this has been used in a variety of experiments. 400 Hz
It is inconvenient for NMR studies, because the large
conductivity of graphite causes radiofrequency ab-
sorption in the graphite sheets with consequent loss of
signal. This does not occur in the very finely powdered
graphitized carbon blacks. Recompressed exfoliated
graphite does have the property of being partially
oriented and in some situations this may be worth the
loss in sensitivity. (d)
There have not been any reports of spectra obtained
from the uncompressed exfoliated graphite of Thorny
FIG. 3. 60 MHz proton spectra of monolayer films of some
and Duval, but with larger sample tubes and longer
typical adsorbed liquids,
(2)4
time averaging it should be possible to detect proton (a) Neopentane adsorbed upon Graphon. Average of 100
2 _1
signals. transients. Surface (2a)r5ea 90 m g . (b) Benzene adsorbed
There should be no difficulty in observing spectra upon S2tirlin-g1 F.T. Average of 100 transients. Surface area
11 m g . (c) Neopentane adsorbed upon titanium
from other spin 1/2 nuclei of high natural abundance (5)1 2 _1
such as 1F9 or 3P1, although the sensitivity will be dadiosoxribdeed. u p1o ntr asnilsvieern ti. oSduidrfea/c5e υ aArevae ra6g7e mof 4g00. t(rda)n sWieanttes.r
2 _ 1
reduced as a result of their smaller gyromagnetic Surface area 2m g . High fields are to the right.
Nuclear magnetic resonance studies of molecules physisorbed on homogeneous surfaces 5
Ho
500 Hz
I 1
J\l
1 3
FIG. 4. Natural abundance 25 MHz C spectrum of one monolayer of neopentane adsorbed on titanium
5 υ
dioxide/ Average of 300 transients.
monolayer of neopentane upon titanium dioxide after 1.5. Adsorption Isotherms
1 5
averaging 300 triansients. For N the sensitivity is
As stated above, physisorption is characterized by
probably so low as to make direct observation im- (6,)7
the adsorption isotherm. In type II isotherms such
practicable. The availability of fully deuterated mol-
2 as found for the rare gases upon graphite or methane
ecules should make Η measurements relatively easy.
upon graphite, the enthalpy of adsorption is greater
All the spectra shown have been of adsorbed
than the enthalpy of condensation of the adsorbate
materials in the liquid rather than the solid phase.
into its bulk phase at the temperature of measurement.
Figure 5 shows the proton spectrum of solid hydrogen
(1)9 The adsorbed molecules or atoms "wet" the surface of
on graphite obtained by Kubik and Hardy. The
the adsorbate, forming uniform monolayers and
signal to noise is good for a cw spectrum and indicates
multilayers. As a result of the reduction in dimension-
that spectra of adsorbed solid materials can be ob-
ality from three to two the resulting layer is qualitat-
tained without any more difficulty than that for
ively different from bulk matter and at a molecular
adsorbed liquids.
level needs to be interpreted in terms of these new two
dimensional phases. Depending upon the relative
dimensions of the adsorbed molecules and the lateral
repeat distance of the substrate, either registered or
non-registered layers may be produced.
(3)8
In contrast, type III isotherms such as water on
(3940)
graphite or ammonia on graphite ' occur when the
enthalpy of adsorption is rather less than the enthalpy
of condensation of the adsorbate into its bulk phase at
the temperature of measurement. Adsorption only
occurs because of a favourable entropy term. The
surface can then be thought of as hydrophobic and the
tendency in adsorption is to form either islands of
multilayers or nucleated crystallites.
Figure 6 presents the step isotherm for methane on
(67)
graphite measured by Thorny and Duval ' at 77.3 K.
The almost vertical steps correspond to the adsorption
of successive layers of methane upon the graphite
FIG. 5. Proton spectrum of solid hydrogen adsorbed upon surface with increasing pressure. Thorny and Duval
exfoliated graphite at 4.2 K.(19) attributed the presence of this step structure to the
6 J. TABONY
c, >
P/Po
01 06
FIG. 6. Adsorption isotherm at 77.3 Κ of methane on exfoliated graphite(6,)7 (p = 9.4 torr). The inset shows
0
the low pressure region (dotted line is enlarged).
increased homogeneity of their exfoliated graphite boundaries of these new phases and then determine the
compared with other substrates. If the surface is non- disposition and motion of the adsorbed molecules.
uniform so that it contains regions of different binding As previously stated measurements made only on
energies, then the characteristic pressures of layer homogeneous substrates will be considered, here there
completion are different and the total sample is seen as have been few NMR measurements of this type and all
a superposition of the various isotherms. This causes have used graphite as the adsorbate. Of the adsorbates
blurring of the step features to give the ogival shape of used the work on helium 06'18'42-4*8 will be omitted
the normal BET curve. since it is mainly concerned with the physics of helium.
The formation of the first layer shows additional
details indicating the presence of more than one phase.
Figure 7 shows the variation of the first part of the 2. PHASE-BOUNDARIES
adsorption isotherm of methane on graphite with
2.1. S olid-Liquid Phase Transitions
temperature. Between 0.1 and 0.7 monolayers the
isotherms gradually develop a point of inflexion as the Phase changes in the adsorbed layer may be de-
temperature is raised, which is attributed to the critical tected by following the variation in the NMR para-
point for the adsorbed layer. Between 0.8 and 1.0 meters with temperature and coverage. Because solids
monolayer an additional sub-step appears which is and liquids give broad and narrow resonances respect-
interpreted as a liquid-solid phase transition. ively, NMR may easily distinguish between the two. By
A phase diagram for methane upon graphite (41) as using a spectrometer having a receiver dead time
determined by different methods is shown schema- longer than the free induction decay from the solid,
tically in Fig. 8. only the signal from the mobile fraction is observed.
The aim of magnetic resonance studies of physisor- The modification of this signal with either temperature
bed molecules is first to identify and delineate the or coverage may be used to construct solid liquid
Nuclear magnetic resonance studies of molecules physisorbed on homogeneous surfaces 7
V/V (CH 77.3)
B 4i
Pressure (lO"3Torr
FIG. 7. Adsorption isotherms of methane on exfoliated graphite.(6,)7 Formation of the first layer. (1) 77.3 Κ ;
(2) 80.0 Κ ; (3) 80.9 Κ ; (4) 82.3 Κ ; (5) 83.5 Κ ; (6) 90.1 Κ. 77 < Τ < 90.1 Κ for first layer.
phase diagrams. The most common NMR method of This method was used by Rollefson (42) for helium
tracing out phase transitions has been to plot the adsorbed upon graphite, and gave melting tempera-
appearance of the liquid factors with increasing tem- tures in agreement with values found from thermody-
perature for different fixed coverages. This will be namic measurements.
discussed in more detail in the next section/ 20,21,-2245) Phase changes may also be detected by varying the
Since the onset of melting narrows the broad solid coverage whilst keeping the temperature fixed. The
resonance to a sharp liquid resonance, an alternative experimental arrangements are more complicated
way is by monitoring the variation in the linewidth. since at constant temperature the coverage may be
"Coverage
i_
solid
+8οβι£. τ-Γ;","'
LCL ΐliquid
\
solid
+ gas hyper,
critical
fluid
liquid
+ gas
-—J. las T T(K)
c
• >-
50 T 70
t
FIG. 8. Schematic phase diagram of the first methane monolayer adsorbed upon graphite.(41)
8 J. TABONY
liquid. At this particular temperature, the fact that the
signal area is linearly related to the mass adsorbed and
passes through the origin shows all the neopentane to
be adsorbed as a liquid. Isotherms obtained using the
NMR signal area as a measure of the mass adsorbed
are shown in Fig. 10 and 11.
When working at a temperature where all the
adsorbed material is fluid, varying the equilibrium
pressure to keep the NMR signal area constant at
different temperatures may be used to calculate the
heat of adsorption. Heats of adsorption determined in
this way are in agreement with those from classical
measurements/20,21,)2 4
, ,
A
£
2.2. Melting in Physisorbed Layers
0 1 2
Compared with bulk materials, physisorbed layers
N MR SIGNAL AREA
often have significant melting point depression, with
FIG. 9. Mass adsorbed, determined volumetrically against
solid and liquid co-existing over a considerable tem-
NMR signal area for neopentane on graphite at 256 K.(21)
The linearity and intersect show that all the neopentane is perature range. In type II adsorption isotherms
adsorbed as a mobile fluid. (neopentane on graphite), the adsorbed two-
dimensional liquid freezes to form a two-dimensional
changed only by varying the equilibrium pressure of solid. In type III systems (ammonia on graphite), the
the adsorbate. This necessitates working with a gas line adsorbed liquid freezes to form three-dimensional
directly attached to the sample in the spectrometer crystallites. Benzene on graphite is intermediate be-
probe. tween the two with both 2D and 3D solids being
Solid-liquid transitions have not been detected this formed on freezing. We shall now discuss the melting
way, but the procedure has been used to show the of all three.
absence of a solid phase.(24) One of the advantages of 2.2.1. Ammonia on Graphite (BET III). Figure 12
the method is that since both the volume of gas shows the melting curves, as determined by the
adsorbed and the equilibrium pressure may be appearance of the narrow liquid signal with increasing
measured, the adsorption isotherm may be determined temperature for ammonia upon graphite of several
in situ.{20>21) different fixed coverages.(20) The curves show no
Figure 9 shows a plot of the mass of neopentane hysteresis on heating and cooling and therefore ap-
adsorbed upon graphite at 253 Κ determined volu- proximate to thermodynamic equilibrium. They are
metrically against the signal area of the adsorbed coverage dependent with the bulk behaviour being
12 3^
O
2 - / 1 r
< χ /
LeUr 7
<
0.5 h /
S2 1 J
er
Ζ
J_
50 100 150 200 250
PRESSURE JORR
FIG. 10. Adsorption isotherm determined by NMR for neopentane upon graphite at 256 K.(24)