J. Silvestre-Albero et al. / Journal of Catalysis 223 (2004) 179–190
189
implies that loss of surface Pt atoms was accompanied by
formation of other active centers which promoted the hydro-
genation of the unsaturated aldehyde. IR spectra of adsorbed
CO (Figs. 3 and 4) reveal subtle differences between the
mono and bimetallic catalysts that might be attributed to the
influence of zinc even after reduction at 473 K, with the
influence increasing after reducing at higher temperatures.
A role of Znδ+ sites at the Pt–support interface cannot be
ruled out.
Zn interaction. The combined effects of the presence of
zinc and the high temperature reduction have detrimental
effects on the catalytic activity for toluene hydrogenation.
After reduction at low temperature, the bimetallic catalyst
was somewhat less active in the vapor-phase hydrogenation
of crotonaldehyde and also less selective toward crotyl al-
cohol than Pt/TiO2. This behavior was drastically modified
after reduction at high temperature. While the monometal-
lic catalyst was essentially deactivated, the initial activity of
the PtZn/TiO2 sample was greatly improved. Furthermore,
its selectivity toward crotyl alcohol remains stable at about
30% during time on stream, despite the strong deactivation
during the first hour of reaction. The combined effects of the
partially reduced titania and the presence of zinc are sug-
gested as being the origin of this behavior.
The high initial selectivity to crotyl alcohol for Pt/TiO2
reduced at 473 K must be assigned to the characteristics of
the platinum if it is assumed that reduction of the support
had not commenced. This high selectivity has been related to
the abundance of Pt(111) surfaces, which optimize configu-
ration for crotonaldehyde adsorption via the carbonyl bond
[8,42]. Surface Zn atoms would disrupt that surface geome-
try, modifying the adsorption mode for the crotonaldehyde.
The effect would be to decrease the selectivity to crotyl al-
cohol, although this would be compensated by any positive
effect on selectivity due to the newly formed active sites.
The reduction temperature has a strong influence on the
catalytic behavior. The monometallic catalyst was almost in-
active for toluene and crotonaldehyde hydrogenation after
reduction at 773 K. Reduction at 773 K for 10 h caused par-
tial reduction of the support, leaving few remaining exposed
platinum centers as indicated by the hydrogen uptake and
the IR of adsorbed CO. Reduction at 773 K for only 4 h
yielded a Pt/TiO2 catalyst that was still relatively active and
showed a high initial selectivity toward crotyl alcohol [15].
Reduction of the PtZn/TiO2 catalyst at 773 K for 10 h, on the
other hand, produced a drastic increase in the initial activity
compared to the same catalyst reduced at low temperature.
In this case, the reduction treatment causes two main phe-
nomena: first, enhanced reduction of zinc, which increased
the interaction with platinum; and second, the onset of the
SMSI effect [43,44]. The combined effects are the origin of
the observed catalytic behavior. It also seems clear that dec-
oration of Pt by partially reduced titania is not as extended as
in the absence of zinc, although it is sufficiently important to
render the catalyst inactive for toluene hydrogenation. The
possibility of some interaction between the support and the
zinc species which could stabilize the titania surface is being
considered.
Acknowledgments
Financial support by the Comisión Interministerial de
Ciencia y Tecnología (Project BQU 2000-0467) is gratefully
acknowledged. We also thank the M.E.C. (Spain) for a travel
grant (to J.S.-A.). We also acknowledge the contribution
made by Dr. F. Coloma in the XPS and AES measurements.
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