Reports: UR553510-UR5: Monitoring Changes in Nanoparticle Electronics with Hammett Studies of Dendrimer Templated Supported Gold Catalysts
Bert D. Chandler, Trinity University
We have
developed 1-hexyne hydrogenation (a model for ore industrially important
acetylene hydrogenation) as a test reaction. We chose hexyne because it
offered easier opportunities to change the reactant composition and was more
easily integrated into our current reactor system. We now have this reaction
up and running, and are using it to compare catalysts. We have tested 1- and
2- hexyne, and see no major differences in reactivity over Au catalysts. To
date, we have largely measured light-off curves with this reaction, rather than
try to measure rates. We believe that this will be a faster and more useful
means of screening catalysts. We are now working to make sure that we
understand alkyne hydrogenation over supported Ni catalysts as control
materials for Au-Ni nanoparticle catalysts. Interestingly, we have found that
Ni/TiO2 catalysts are very selective to alkenes at higher temperatures,
provided the space velocity is sufficiently fast. We are working on understanding
this chemistry, as we believe that some sort of surface Ni-titanate phase may
be prepared in the calcination process.
One
of our primary goals is to develop Hammett studies to probe changes in Au
surface chemistry. We would like to use hydrogenation reactions to complement
our previous work on benzyl alcohol oxidation (a manuscript is in
preparation). Because, there is a fair amount of literature on Au catalyzed
nitrobenzene hydrogenation, we spent a great deal of time working on this
reaction. The reaction is slow at moderate temperatures under one atmosphere
of H2, so our initial studies examined using stronger reducing
agents (e.g. NaBH4). Although these reactions were successful in
that the catalysts were active, the strong dependence on the concentration of
the reducing agent (greater than 2nd order), and complicated
kinetics (at least 1 intermediate was observed) made it clear that this was not
an appropriate reaction for the type of reproducible kinetic tests that we want
to develop.
We therefore
turned to phenylacetylene reduction, which has the advantage of being more
directly comparable to 1-hexyne hydrogenation. After a great deal of testing
with multiple reducing agents and reaction conditions, we settled on using
hydrogen at ambient pressure and roughly 70 °C
. We performed a fairly detailed concentration study to find optimum reaction
conditions (see Figure 1); we are also
working to interpret this data with a Michaelis-Menten analysis and hope to
ultimately compare these results with our Hammett studies.
We
have begun Hammett studies with this reaction, with very promising results. Figure 2 shows our preliminary data using
Au/TiO2 as the initial catalyst. As expected, r
is positive for this reaction, indicating negative charge building on the
benzylic carbon in the transition state, which is consistent with a hydride
transfer from Au to the triple bond in the rate determining step. The r value for PA hydrogenation is substantially
larger than for BA oxidation (1.2 vs. 0.4), suggesting that this reaction will
be more sensitive to electronic effects. There are also some important
differences in the sterics of the reactions, and strong electron withdrawing
groups do not fall on this line. We are currently working to sort out these
details before we begin comparing catalysts.
We
have also been preparing bimetallic Au-M (M=Ni, Co) nanoparticles and catalysts
using PAMAM dendrimer templates. After trying several methods and studying
metal uptake by the dendrimer, we have prepared 1:1 NiAu and Co:Au
nanoparticles (see Figure 3), and deposited
them onto titania and alumina supports. The organic dendrimer was then
thermally removed under flowing H2 at 300 C. The metal content of the
resulting material, determined by AA spectroscopy, was consistent with 1:1
molar ratios for all the catalysts.
We
have since performed 1-hexyne hydrogenation light off curves on these
materials, and appropriately prepared Au catalysts. Comparing bimetallic
catalysts is not always straightforward; because both metals may be active
hydrogenation catalysts, we adjusted the amount of catalyst in the reactor to
keep the total moles of metal relatively constant. Figure 4 shows the data for
Au, Co-Au, and Ni-Au catalysts. TheCo-Au catalyst is slightly more active than
Au, in spite of having less Au. The Ni-Au catalyst is significantly more
active, reaching complete conversion at about 150 C. Further, the reaction is
essentially 100% selective for 1-hexene, suggesting that the reaction occurs on
the Au surface (the lower observed selectivity for the pure Au catalyst is due
to the very low conversion, coupled with a small 1-hexene impurity in the
feed).
We are extremely
excited by these results, and have held off publishing as we consider a
possible patent application. Importantly, we have shown a strong proof of
concept – that heterometals can drastically improve the reactivity of Au
catalysts, while maintaining the desirable selectivity associated with Au and reducing
the total amount of Au in the catalyst. We are now working to more carefully
measure catalytic activities, perform a complete characterization of the
catalysts, and develop preparative routes that will not require the use (and
destruction) of expensive dendrimers.
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