www.sciencemag.org/cgi/content/full/320/5872/86/DC1
Supporting Online Material for
The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed
Alkyne Hydrogenation
Detre Teschner,* János Borsodi, Attila Wootsch, Zsolt Révay, Michael Hävecker, Axel
Knop-Gericke, S. David Jackson, Robert Schlögl
*To whom correspondence should be addressed. E-mail: [email protected]
Published 4 April 2008, Science 320, 86 (2008)
DOI: 10.1126/science.1155200
This PDF file includes:
Materials and Methods
Figs. S1 to S5
References
Supporting Online Material for
The roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed
Alkyne Hydrogenation
Detre Teschner*, János Borsodi, Attila Wootsch, Zsolt Révay, Michael Hävecker,
Axel Knop-Gericke, S. David Jackson, Robert Schlögl
*To whom correspondence should be addressed,
E-mail: [email protected]
Materials and Methods
In situ (high-pressure) x-ray photoelectron spectroscopy (XPS) experiments were
performed at beamlines U49/2-PGM1 and PGM2 at BESSY, Berlin, with a setup
schematically shown in Figure S5. Samples were transferred from a pre-chamber and were
mounted inside the reaction cell, 2 mm away from an aperture that is the entrance to the
differentially pumped stages of the lens system of the hemispherical analyzer. The
application of differential pumping allows minimization of the travel path of photoelectrons
in the gas phase. The sample can be heated from the back side using an infrared laser
system. Gas flows into the reaction cell were regulated by leak valves and mass flow
controllers. The gas phase composition, to monitor the catalytic activity, can be recorded
on-line by a mass spectrometer connected directly to the outlet of the reaction chamber.
The undulator beamline provides a high intensity synchrotron light, essential for us to allow
recording spectra under high mbar pressure conditions. Since the aim of these experiments
was to record Pd 3d spectra at pressures as high as possible, we have used pure palladium
samples: Pd foil (Goodfellow) and Pd black (Goodfellow, BET surface area 2.06 m2g-1).
The samples were cleaned with oxygen and hydrogen treatments prior to experiments, in
situ. Pd 3d core levels were recorded at normal emission with 720 eV excitation,
corresponding to an inelastic mean free path of approximately 9 Å (SR1). The metallic Pd
3d spectra were fitted using Gauss-Lorentz functions with an exponential tail, while the
PdC component was approximated by a simple Gauss-Lorentz function as indicated by the
symmetric character of the peak.
In situ Prompt Gamma Activation Analysis (PGAA) was carried out at the cold
neutron beam of the Budapest Neutron Centre, Budapest, Hungary. The characteristic
prompt gamma radiation was collected by a Compton-suppressed High-Purity Germanium
detector; the spectra were acquired using a multi-channel analyzer. For the in situ PGAA
experiments we placed a continuous flow reactor (Al2O3 ceramic tube with inner diameter
of 2 mm) into the neutron beam, which was almost perfectly transparent to the cold
neutrons. Gas flow was supplied by mass flow controllers (Bronkhorst) and the reaction
was monitored by an online micro gas chromatograph (Varian CP-4900, CP-Sil 5 CB). 1Pentyne (1.6 cm3min-1) was introduced into the hydrogen stream (flow rate 2–64 cm3min-1)
via N2 flow going through a saturator held at 273 K. For the study here, 7 mg Pd black
(Goodfellow) was mixed with 100 mg of SiC and loaded into the reactor. Hydrogenation
was carried out adiabatically at near room temperature, without any regenerative treatment
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between catalytic runs. Since the typical timescale for recording a spectrum (0-12000 keV)
is 1-2 hours, measurements were done after allowing an equilibration period of 20-30 min
prior to acquisition. Molar ratios (H/Pd) were determined from the characteristic peak areas
corrected by the detector efficiency and the nuclear data of the observed elements (SR2,
SR3). Since the spectrum of hydrogen contains extra contribution from gas phase hydrogen
in the feed and moisture in the “viewing angle” of the detector, reference experiments were
carried out without Pd in the reactor but otherwise with identical conditions as during
hydrogenation. This background hydrogen was subtracted from the hydrogen content of
palladium during data evaluation. The uncertainty values (shown in Table 1 of the
manuscript) involve all deviation in calculating H/Pd ratios (e.g. the uncertainty in
determining the PGAA peak areas; uncertainty due to background correction) and not only
the deviation of H/Pd values during averaging the individual experiments; hence the law of
propagation of errors was applied. Since the agreement of H/Pd in the fresh sample to the
measured phase diagram is much better than estimated by the error bars, we are confident
that the uncertainty of the relative changes between experiments are an order of magnitude
lower than indicated by the error bars.
Pressure dependent catalytic experiments were carried out in a closed-loop
apparatus (71 cm3) equipped with GC analytic (50-m CP-Sil capillary column, FID). This
setup permits to investigate reactions in the “low-mbar” to “sub-atmospheric” pressure
range. The catalytic samples (Pd black, as above; 1% Pd/Al2O3, Johnson Matthey) were
allowed at each condition to come to steady state by repeated experiments without
regeneration. The applied partial pressure and temperature conditions are detailed at each
corresponding Figure.
Supporting Figures
Figure S1. A, 1-pentyne hydrogenation experiments over 1% Pd/Al2O3 with 10 Torr 1-pentyne and
100 Torr H2 at 305 K in the closed-loop reactor. The catalyst was treated in hydrogen at 358 K prior
to the first experiment. The reaction mixture was analyzed after 5 minutes circulation through the
catalyst bed. The figure shows repeated experiments indicating an activation period, and
stabilization only after 3-5 experiments (15-25 minutes on stream). B, 1-pentyne hydrogenation
experiments over Pd black at atmospheric condition using 4 cm3ml-1 H2, 1.6 cm3ml-1 1-pentyne (in 8
2
cm3ml-1 N2) at 292 K, during in situ PGAA experiments. The catalyst was pretreated in hydrogen
before hydrogenation.
Figure S2. 1-pentyne hydrogenation experiments over 1% Pd/Al2O3 with (A) 10 Torr and (B) 0.33
Torr 1-pentyne as a function of H2 partial pressure (Torr). The reaction rate is expressed as moles of
1-pentyne converted pro surface Pd (in moles). Experiments were carried out at 305 K in the closedloop reactor; sampling time 5 minutes. At both “low mbar” and “sub-atmospheric” conditions
hydrogenation is characterized by two distinct regimes: one with first order for hydrogen (and
zeroth or slightly negative order for 1-pentyne; not shown here), and the other with 0th order for
hydrogen (and 1st order for pentyne). Hydrogenation is generally selective in the regime of 1st order
for hydrogen, yielding 1-pentene. A similar dependence was observed for Pd black, as well.
Figure S3. The effect of reaction temperature on the formation of PdC. The in situ Pd 3d spectra
clearly indicate that at ~ 7 mbar, above the decomposition temperature of β-hydride, formation of
PdC was significantly enhanced.
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Figure S4. 1-pentyne conversion (A, C) and corresponding H/Pd (B, D) values during in situ PGAA
experiments. Experiments 1-4 were carried out using 4 cm3min-1 hydrogen and Exp. 5,6 with 6
cm3min-1 hydrogen. 1-Pentyne flow was kept constant at 1.6 cm3min-1. Experiments marked by 1-4
have not been performed directly after each other; therefore the prehistory of the sample was always
different. The same holds for Experiments 5 and 6, as well, at which both the initial and the final
(after ~2 h time on stream) activities are included. The increasing conversion was due to the
activation effect and partly due to slow temperature rise of the adiabatic catalyst bed. During all
these experiments hydrogenation was selective toward 1-pentene. The comparison clearly indicates
that there is absolutely no correlation between activity in the regime of selective hydrogenation and
the amount of dissolved hydrogen in palladium. Since the reaction rate is limited by hydrogen (see
Figure S2), the equilibrium of hydrogen between bulk and surface should be necessarily disturbed.
Figure S5. Schematic representation of the in situ XPS setup
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Supporting References
SR1, S. Tanuma, C. J. Powell, D. R. Penn, Surf. Interface Anal. 17, 911 (1991).
SR2, Zs. Révay, T. Belgya, Zs. Kasztovszky, J. L. Weil, G. L. Molnár, Nucl. Instrum. and Meth. B 213 385 (2004).
SR3, G.L. Molnár (Ed.) Handbook of Prompt Gamma Activation Analysis with Neutron Beams, Kluwer Academic
Publishers, Dordrecht/London/Boston, 2004.
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