Zeolite catalysts were prepared by carrying out an ion-exchange process of
transition metals and impregnation to incipient-wetness method of metal catalyst
using a chlorine free gold precursor, KAu(CN)2. The instability of Au/Y
(3.74wt%Au) resulted in low CO oxidation activity (~ 18 % conversion at 450
0C), suggesting that the reduced gold metal atoms are bound to the zeolite by a
weak interaction. This is subject to migration within the passages of the zeolite
during use. The presence of proton stabilized most of Au clusters (electron
deficient species) within the HY zeolite, resulting in small amounts of gold
species migrating to the outer surface. Interestingly the CO oxidation activity of
Au/HY is half that of Au/Y, which clearly indicate that the presence of metallic
gold plays a significant role during CO oxidation.
The loading of Au/M-Y (M = Ni2+, Fe3+, Co2+ or Cr3+) were varied from 1.67-
7.48wt%Au and from 1.76-5.45wt%M. Modification of this Y structure with
transition metals has been found to be beneficial for both activity and stability of
smaller gold clusters, by strengthening the interaction between gold and zeolite
exchange sites and by large magnitude in maintaining the dispersion of gold. This
suggests that the unreduced chromium ions function as a chemical anchors for
reduced Au metal and that the reduced atoms of gold may form small clusters
with the anchoring metal. TPR profile has confirmed that the introduction of
1.67wt%Au on Fe-Y (1.88wt%Fe) increased the stability of Fe ions as stabilizer
metal. However, as the gold loading of Au/Fe-Y catalyst increases the TPR profile
shows that the stability of Fe ions decreases and hence the activity of catalysts. An
increase in transition metal content, above 1.88wt%Fe was found to lower the CO
oxidation activity. A TPR profile has confirmed that as the reduction potential
became more negative, the activity of supported Au increases following the
sequence: Ni2+, 0.23 << Fe3+, -0.41 < Cr3+, -0.56. The estimated particle sizes of
gold by X-ray diffraction were found to be ~ 12 nm for Ni2+, ~ 7 nm for Fe3+, and
~ 5 nm for Cr3+ stabilized metal.
Samples of Au/HY (3.77wt%Au) have been prepared by an ion-exchange method
using Au(III) ethylenediamine complex-ions, [Au(en)2]3+. Following a pretreatment
in an O2 atmosphere, the catalyst showed the existence of an induction
period before reaching a steady state activity; suggesting the need for activating
gold prior to catalyzing CO oxidation reaction. As-prepared catalyst contained
85% of gold in the Au3+ valence state as confirmed by Mössbauer spectroscopy.
The catalyst was treated with various reducing agents (such as NaBH4); to yield
stable and active smaller gold clusters (< 2 nm) inside the HY cavities, as revealed
by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Uv-
Vis Spectrophotometer. DRIFTS revealed that electron-deficient particles (Auó+-
CO species) of gold clusters, inside the HY framework and in contact with
protons are active species for CO oxidation. CO activity and formation of smaller
gold clusters depends on the nature and molar ratio of reducing agents, and the
source of gold. The induction period observed for unreduced Au catalyst is a slow
step in the activation of gold active sites. Treatment of Au/Y (3.46wt%Au) with
sodium borohydride enhanced the activation of gold active species and hence
improves the catalytic activity. The NaBH4 treated Au/Y (3.73wt%Au) catalyst
has shown, for the first time, activity of approximately 28% CO conversion. The
catalyst showed almost the same activity and induction period as that of the
untreated Au/HY (3.77wt%Au) catalyst, which leaves much to be investigated
about the behaviour of Au on Y zeolite upon treatment with a proper reducing
agent. The protons have been found to stabilize the smaller Au nanoparticles
within the zeolite cavities.
The modification of zeolite-Y was carried out by treatment with different alkali
metal nitrates such as LiNO3, NaNO3 and KNO3 before introducing gold from
different sources, (i.e. gold ethylenediamine complex ion, Au(en)2Cl3; chloroauric
acid, HAuCl4; or potassium dicyano aurate, KAu(CN)2 complex). The CO
oxidation activity of the catalysts was found to depend on the nature of the gold
source and on the type of alkali metal nitrate used. The order of activity was as
follows: HAuCl4 >> KAu(CN)2 > Au(en)2Cl3. It was found that the activity of
catalysts prepared by deposition of Au from an aqueous solution of chloroauric
acid on Na-modified zeolites-Y, increased as a result of an increase in the amount
of Au deposited as confirmed X-ray fluorescence spectroscopy (XRF). The Kmodified
zeolite-Y had a smaller amount of Au deposited (i.e. Au/KY,
0.454wt%Au; Au/NaY, 0.772wt%Au and Au/LiY, 0.212wt%Au) and hence the
CO oxidation activity was lower than that of Na-modified zeolites-Y. Thus, the
order of the catalytic activity is as follows: Na > K > Li. The XRD studies have
revealed that metallic gold particles sizes do not depend on the nature of alkali
metal nitrates used to modify the zeolite-Y surface and the zeolite-Y crystallinity
has been maintained.
Monometallic Au/NaY (0.772wt%Au, treated with NaNO3) was found to be
active in ethylene hydrogenation with ~5% conversion. Treatment of catalysts
with NaBH4 was found to lower the catalytic activity of the catalysts, contrary to
activities observed on CO oxidation and these concluded that cationic gold are
responsible for the observed activity. The activity was found to depend on the
source of Au used, and the order is as follows; HAuCl4 >> KAu(CN)2 >
Au(en)Cl3. Bimetallic catalysts of Au/M-Y (where Au represent gold from
KAu(CN)2, and M = Ni2+, Fe3+, or Cr3+) were found to be more active compared
to monometallic catalysts due to promotional effect of transition metal. The order
of activity of the bimetallic system at 260 0C was as follows; Ni2+ >> Fe3+ > Cr3+,
and at 150 0C, was Ni2+ >> Cr3+ > Fe3+, contradicting the order of activity
observed on CO oxidation. Formation of carbonaceous deposits on the surface of
the catalyst at temperature higher than 260 0C has been confirmed.
Cu modified Au/TiO2 (anatase, 200m2/g) has been prepared by incipient-wetness
method by either introducing the modifier, before or after Au loading. Such
catalysts were found to give high and stable activity for the water-gas shift (WGS)
reaction, when compared to unmodified Au/TiO2 catalysts. It has been suggested
that an increase in activity on modified Au/TiO2, is mainly due to the existence of
a synergetic interaction between Cu and Au, since the activity of both Cu/TiO2
and Au/TiO2 is lower than that of bimetallic system. The presence of nitrates on Cuc-Au/TiO2 (c Cu precursor is Cu(NO3)2*2.5H2O) has been found to be
detrimental to the activity of Au on TiO2; due to the poisoning of Au active sites
and enhancement of Au agglomeration by NO2
- formed during the reaction. An
increase in Cu loading lowers the activity of Au. A XANES spectrum has
confirmed that gold exists as either Au+/Au0 during WGS reaction and Cu exists
as copper ions (Cu+/Cu2+) before and during WGS reaction. Formation of
bimetallic particles was not detected by EXAFS data analysis. The observed
effects are interpreted as a mutual influence of gold and copper ions and reduced
species of gold and copper due to their competing for ion exchange sites. Cu has
no promotional effect on low temperature CO oxidation and on preferential CO
oxidation in excess of hydrogen.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/4638 |
| Date | 11 March 2008 |
| Creators | Magadu, Takalani |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 2786665 bytes, application/pdf, application/pdf |
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