C O M M U N I C A T I O N S
Scheme 2. Pyrrole Hydrogenation Reaction Network
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).14,15 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 hydro-
genation was demonstrated as structure insensitive, while ring
opening to n-butylamine was structure sensitive. Selectivity dif-
ferences 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.
difficulty. Hence, we report that this step is structure insensitive,
which was in agreement with results12-15 that report formation and
scission of C-H bonds as structure insensitive.
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 determin-
ing 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 encap-
sulated 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 hydro-
genation.18 Since n-butylamine is electron-rich, it bonds more
strongly onto surfaces of smaller NPs because the surfaces of
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. DOE
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. DOE 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, charac-
terization results, and additional catalytic data. This material is available
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