SECCIONADOR – FUSIBLE PARA OPERACIÓN CON CARGA.

This technique is concerned with the magnetic properties o f atomic nuclei Only

nuclei that have the spin quantum number I>0 are NMR active. Under the influence o f an external magnetic field the magnetic moment, p, o f the NMR active nucleus aligns itself with the external field. The resonance produced for a solid is very broad mainly due to chemical shift anisotropy i.e. NMR interactions depend on the orientation o f the nuclei related to the magnetic field and in a solid different molecules have different orientations, thus the anisotropy is not averaged to zero (unlike in a liquid). In this situation, averaging the isotropic value may narrow the broad peaks. The averaging is carried out by quick spinning the sample around an axis so as to modify the shift anisotropy by a factor o f (3cos^0-l). Thus, if the sample is spun at the so-called magic angle o f 54°44 this term becomes zero.

MAS NMR

^^Si NMR is a very important tool for the study o f zeolites, since it allows us to determine the ordering o f the silicon and aluminium in the framework, this influences the catalytic activity High resolution solid-state ^^Si NMR is able to distinguish between the five possible Si (wAl) structure blocks, where n («=0-4) is the number o f Al atoms connected to the central Si via oxygen bridges. Each Al tetrahedron adjacent to a Si tetrahedron shifts the ^^Si signal by 5-6 ppm down-field. As a result five distinct chemical shifts ranges are created between -85 and -115 ppm. The signals corresponding to Si(OAl), Si(lA l), Si(2Al), Si(3Al) and Si(4Al) approximately appear at: -112 ± 3, -104.9, -98.4, -92.0 and -85 ppm, respectively

27

Al M AS NM R

The Si(lA l), Si(2Al), Si(3Al) and Si(4Al) environments are difficult to probe for a zeolite with a high Si/Al ratio, consequently it becomes less useful to ascertain the tetrahedral Al inside the framework using ^^Si NMR, thus ^^Al NMR spectroscopy is used. The ^^Al NMR spectra which has a broad absorption at the chemical shift o f ~55 ppm indicates that the aluminium has tetrahedral coordination however, a band between

0 and - 6 ppm corresponds to octahedrally coordinated aluminium

NM R o f silicate solutions

^^Si NMR spectroscopy can also be used to study solutions and can give detailed information on the structural entities in silicate solutions For the presentation o f structure o f building units the Q" notation is adopted. Here, Q represents the Si atom bonded to four oxygen atoms (Si0 4 tetrahedron), and n is the number o f atoms attached to the Si0 4 tetrahedron e.g. Q° is related to the monomeric orthosilicate tetrahedron Si0 4. More notations o f building units and silicate ions along with the ^^Si chemical shift ranges are shown in table 2.4.

Table 2.4: Notations o f building units and silicate ions in silicate solutions

notation

_Q“

_Q'

_Q"

_Q'

Monomer End group Middle

group Branching group Cross- linking group Chemical shifts/ 0 ppm -65 to -75 -78 to -85 - 8 8 to -92 -95 to -102 -104 t o -120 O O Si 0 O 0 0 Si OSi O O' SiOSi OSi Q- OSi SiO SI OSi O' OSi SiO Si OSi OSi 2.5 M ossbauer spectroscopy

Like NMR spectroscopy, Mossbauer spectroscopy measures the transitions inside atomic nuclei Analysis gives information on the local structure such as oxidation states and coordination numbers. Unfortunately, only a limited number o f atoms can be used to produce y-rays suitable for use in Mossbauer spectroscopy work, the most widely used isotopes being ^^Fe*29 or * ^^Sn*so. Here, the sample under investigation

Experimental techniques and analysis

is irradiated by a highly monochromatic heam o f y-rays, where the energy is varied. The y-rays are associated with a change o f population o f energy levels o f nuclei and not a change with atomic mass or number The sample absorbs the y-rays and the spectrum obtained is a plot o f absorption vs energy (see figure 2.5 (i)).

(b)

(a )

s a m p k

V

-V • +V

Figure 2.5 (i) Schem atic illustration sh o w in g (a) the M o ssb a u e r effect; th e energ y o f the y-rays is a d ju sted by m o vin g the em itter source, (b) Typical sin g le lin e spectrum

o b ta in e d w hen so u rce a n d sa m p le are id en tic a l V is velo city o f m odulation.

The Mossbauer effect represents a phenomenon o f recoil free emission, where all the energy changes in the nuclei are transmitted to the emitted y-rays which gives rise to a highly monochromatic beam o f radiation. This radiation, in turn, is absorbed by the sample which must contain identical or similar atoms to those responsible for the emission. The nuclear energy levels of the absorbing atoms vary depending on the oxidation state or the coordination number o f the element. Therefore, some manipulation o f the y-rays is needed since resonant absorption occurs only when ^^Fe29*-Fe separations in emitter and absorber are matched The y-ray source is moved away or towards the sample (by a few millimetres per second) and this has the effect o f either increasing or decreasing the energy o f the y-rays incident on the sample. In this way, the y-ray absorption spectrum o f the sample is determined. When the emitter and the sample are not identical, the absorption peak is shifted. This chemical shift, Ô, arises because the nuclear energy levels have been modified by changes in the extra-nuclear density distribution in the atoms. Thus, chemical shifts are connected to the density at the nucleus o f the outer S shell electrons and hence reveal the oxidation state, coordination number and bonding properties. In nuclei that have a nuclear quantum spin I>I/2, the distribution o f positive charges inside the nucleus is non- spherical and a quadrupole momentum, Q arises. When we have a quadrupole

moment the nuclear energy levels split and the peaks o f the spectrum split. The separation o f the peaks (doublets are seen for ^^Fe and ^^^Sn) known as the quadrupole splitting, A, is sensitive to the local structure and oxidation state o f the sample.

In this thesis, Mossbauer spectroscopy was used to investigate the oxidation state and coordination geometry of the Fe ions incorporated in two different AIPO4 structures.