Structure Sensitivity of Carbon-Nitrogen Ring Opening: Impact of Platinum Particle Size from below 1 to 5 nm upon Pyrrole Hydrogenation Product Selectivity over Monodisperse Platinum Nanoparticles Loaded onto Mesoporous Silica Page: 2 of 3
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for pyrrolidine formation pyrrole were - 70 ks-1 except for the
smallest Pt size. The lower activity was again attributed to
reduction difficulty. Hence, we report that this step is structure
insensitive, which was in agreement with results 12-15 that report
formation and scission of C-H bonds as structure insensitive.
Scheme 2. Pyrrole hydrogenation reaction network.
\N> .2H c - I-L HN/V *II2 / +NH.
n-butylamine Butane and
Pyrrole Pyrrolidine ammonia
b>utane and ammonia
0 1 2 3 4 5
Pt size (nm)
Figure 2. Pyrrole hydrogenation selectivity (color coordinated to Scheme
2) as a function of Pt NP size (T = 413 K and 11 2% conversion). Feed
was 4 torr of pyrrole and 400 torr of H2 with a He balance.
For n-butylamine formation, TOFs increased as the Pt NP size
increased. This structure sensitivity for the ring opening led to
differences in product selectivity (Figure 2). This finding is
important because ring opening was identified as the rate
determining step for pyridine HDN over Pt.17 For NPs smaller
than 2 nm, selectivity was a strong function of size as pyrrolidine
formation occurred more easily over smaller sizes. As NP size
increased above 2 nm, behavior became independent of size with
n-butylamine selectivity approaching 100%. Since the largest
dendrimer encapsulated NP (2.0 nm) was larger than the smallest
PVP capped NP (1.5 nm), differences in capping agent and
activation protocols did not appear to influence the catalytic
behavior. TOFs for the cracked products (butane and ammonia)
also increased as the NP size increased. It is difficult to comment
on the structure sensitivity of this step because this amount was
proportional to n-butylamine formation.
The reaction results demonstrated that the ring opening was
more facile over larger Pt NPs, leading to almost entirely n-
butylamine, compared to smaller ones, which formed both
pyrrolidine and n-butylamine. We believe that these findings are
caused by n-butylamine product poisoning. The N of n-
butylamine is more electron-rich than its counterparts in pyrrole
(lone electron pair shared with ring) and pyrrolidine (fewer N-H
bonds than n-butylamine) and thus can form stronger bonds with
the surface and consequently inhibit turnover. The same principle
has been observed with butane formation occurring more easily
over Pt supported catalysts for pyrrolidine hydrogenation than n-
butylamine hydrogenation.18 Since n-butylamine is electron-rich,
it bonds more strongly onto surfaces of smaller NPs because the
surfaces of smaller NPs contain more unsaturated surface sites
compared to surfaces of larger ones. The higher degree of
unsaturated bonds leads to rougher surfaces (more steps and kinks
compared to smoother surfaces of larger NPs) and/or electronic
effects (decreased metallic character).1 Either factor can
explain product poisoning by n-butylamine.
To conclude, we studied pyrrole hydrogenation over
mesoporous SBA-15 supported Pt NPs between 0.8 and 5.0 nm.
Ring hydrogenation was demonstrated as structure insensitive
while ring opening to n-butylamine was structure sensitive.
Selectivity differences were believed to occur because the N of n-
butylamine is more electron rich than its counterpart in
pyrrolidine and pyrrole and can therefore form stronger adsorbate-
surface interactions. These interactions became stronger over
smaller NPs, which possess more unsaturated surface bonds. Our
observations of different behavior as a function of Pt size indicate
new chemistry is achievable for ultrasmall NPs. The effects of
temperature and support upon selectivity are under investigation.
ACKNOWLEDGMENT. We acknowledge support from the
Director, Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geological and Biosciences of the
U.S. D.O.E. under Contract DE-AC03-76SF00098 and the
Director, Office of Science, Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering of the U.S.
D.O.E. under Contract No. DE-AC02-05CH11231. Additional
support from Chevron is also appreciated. We also thank the
Molecular Foundry of the LBNL and Prof. A. Paul Alivisatos for
use of facilities. Y.W.Z. thanks the Huaxin Distinguished Scholar
Award from Peking University Education Foundation of China.
Supporting Information Available: Experimental details,
characterization results, and additional catalytic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Kuhn, John N.; Huang, Wenyu; Tsung, Chia-Kuang; Zhang, Yawen & Somorjai, Gabor A. Structure Sensitivity of Carbon-Nitrogen Ring Opening: Impact of Platinum Particle Size from below 1 to 5 nm upon Pyrrole Hydrogenation Product Selectivity over Monodisperse Platinum Nanoparticles Loaded onto Mesoporous Silica, article, July 1, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc935134/m1/2/: accessed February 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.