www.sciencemag.org/cgi/content/full/335/6073/1209/DC1

Supporting Online Material for

Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations

Georgios Kyriakou, Matthew B. Boucher, April D. Jewell, Emily A. Lewis, Timothy J. Lawton, Ashleigh E. Baber, Heather L. Tierney, Maria Flytzani-Stephanopoulos, E.

Charles H. Sykes*

*To whom correspondence should be addressed. E-mail: charles.sykes@tufts.edu

Published 9 March 2012, Science 335, 1209 (2012)

DOI: 10.1126/science.1215864

This PDF file includes:

Materials and Methods Figs. S1 to S4 References

1 

 

Supporting Online Material for

Isolated Metal Atom Geometries as a Strategy for Selective

Heterogeneous Hydrogenations

Georgios Kyriakou, Matthew B. Boucher, April D. Jewell, Emily A. Lewis,

Timothy J. Lawton, Ashleigh E. Baber, Heather L. Tierney,

Maria Flytzani-Stephanopoulos, and E. Charles H. Sykes*

*To whom correspondence should be addressed. E-mail: charles.sykes@tufts.edu

 

Experimental

Experiments were performed in two ultra high vacuum (UHV) instruments. An Omicron

Nanotechnology LT-UHV STM was used for high-resolution imaging of the alloy samples.

The base pressure in the low temperature chamber was <5 x 10-11 mbar. The instrument

incorporated a preparation chamber operated at a base pressure of 2 x 10-10 mbar, in which

cleaning, annealing and Pd deposition were performed. Hydrogen (99.999%, AirGas)

adsorption on Pd/Cu(111) was performed in the LT chamber with the sample held at 80 K.

The sample was then cooled to 5 K for imaging. Typical STM imaging conditions for bare

metal and alloy surfaces were -1.0 < V < +1.0 V and tunneling currents < 3 nA. Surface-

bound H atoms were typically imaged with conditions -0.05 < V < +0.05 V and tunneling

currents < 100 pA. The Cu(111) sample (MaTeck) was cleaned by cycles of Ar+ sputtering

(1.5 keV, 20 μA) followed by annealing to 800 K until no impurities were detected by STM.

The pressure in this instrument was monitored using hot cathode ion gauges (Arun

Microelectronics Limited, NGC2).

2 

 

The second UHV system was operated at a pressure of <1 x 10-10 mbar and incorporated

facilities for Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and

a quadrupole mass spectrometer used for temperature programmed desorption/reaction

(TPD/TPR) measurements. The pressure was monitored using a hot cathode ion gauge (VG

Scienta, IGC3). The sample could be resistively heated to 1000 K and cooled to 85 K.

Cleaning of the Cu(111) single crystal (MaTeck) was achieved by cycles of Ar+ sputtering

(1.5 keV, 20 μA) followed by annealing to 800 K until no impurities were detected by AES

and LEED. TPR measurements were performed with the mass spectrometer collimator

positioned 3 mm from the center of the front face of the Cu(111) crystal. High purity styrene

(99.9%, Aldrich) and acetylene (99.6%, Airgas) were dosed on the sample by backfilling the

chamber to the required pressure. Hydrogen was dosed on the sample by means of a 6 mm

diameter collimator tube pointed directly at the sample. Exposures, after correction for the

collimator gain factor, are quoted in Langmuir ((L) 1 L = 1x10-6 torr·s). Pressures were

corrected for ion gauge sensitivity to each molecule using each molecule’s ionization cross

section. The hydrogenation TPR spectra of acetylene presented in the manuscript and the

supporting information were recorded with a heating ramp of 1 Ks-1 following the parent ion

of acetylene at m/z 26 and the parent ion of ethene at m/z 28. The fragment for ethene at m/z

26 was subtracted to yield the pure desorption profile of acetylene. Styrene and ethlylbenzene

yields were recorded with a heating ramp of 1 Ks-1 by tracking their parent ions m/z 104 and

m/z 106, respectively. TPR data were corrected for the mass spectrometer’s sensitivity after

calibrating the mass dependent instrumental response function. Hydrogen coverages were

calculated based on a saturation coverage of unity when hydrogen was adsorbed on a 4

monolayer (ML) Pd film, assuming that the film terminates as Pd(111) (33). Acetylene (and

styrene) relative coverage is expressed in ML, defined as 1 ML being the saturation of the

3 

 

monolayer TPD peak. The desorption profiles of the two molecules from clean Cu(111) are

shown in figure S1.

Fig. S1. Desorption profiles of saturation monolayer coverages of acetylene (m/z 26) and

styrene (m/z 104) from Cu(111).

Pd deposition was performed in both systems via Omicron Nanotechnology Focus EFM 3

electron beam evaporators. These sources allowed the very clean deposition of Pd on Cu(111)

with accurate and reproducible control of metal flux, identical in the two UHV systems. The

source/sample geometry and Pd flux was kept the same in both instruments. The Pd coverage

was calibrated using AES and quoted in ML. Figure S2 shows the peak-to-peak values for the

Pd and Cu signals and raw Cu and Pd spectra as a function of deposition time. A linear

decrease of the Cu signal and a linear increase of the Pd signal were observed. The break

point in the curves indicating the onset of the second layer formation was used to calibrate the

surface coverage of Pd. A second set of calibrations were performed to verify Pd coverage by

means of CO titration on the sub-monolayer Pd/Cu(111) systems.

4 

 

Fig. S2 (a) Uptake curve and raw AES data for (b) Cu (MVV) and (c) Pd (MNN) during Pd

deposition on Cu(111).

The desorption profile of styrene from a 0.01 ML Pd/Cu(111) alloy (manuscript Fig. 4A)

remains essentially unchanged from what is observed on clean Cu(111). Figure S3 shows

representative TPD spectra of styrene from clean Cu(111). The spectra are characterized by

three distinct desorption maxima (α1, α2 and α3) in agreement to the findings of Wei et al. for

styrene adsorption on Ag(111) (34). Using reflection absorption infrared spectroscopy

(RAIRS) the authors assigned α1 and α2 adsorption states to styrene in direct contact with the

substrate. The α1 species has the aromatic ring parallel to the surface and α2 nearly parallel.

The α3 species corresponds to the formation of multilayers on the surface.

5 

 

Fig. S3. TPD spectra of styrene from clean Cu(111) as function of exposure.

Figure S4 shows that acetylene desorption and ethene formation on the 0.01 ML Pd/Cu(111)

was accompanied by the formation of small amounts of benzene due to self-coupling of

acetylene. In agreement with earlier observations, this conversion of acetylene to benzene

was estimated to be ~2 % (27).

Fig. S4. Benzene (m/z 78) and acetylene (m/z 26) desorption after exposing the 0.01 ML

Pd/Cu(111) sample to H2 and acetylene as described in Fig. 4 in the manuscript.

6 

 

Conversion/Selectivity

The conversion and selectivity for styrene and acetylene hydrogenation was calculated using

the integrated intensity, I, of the desorption peaks of interest, after correcting for mass

spectrometer sensitivity, adjacent point averaging of the desorption trace, and taking into

account the stoichiometry of each reaction as described in equations 1 – 5. In the 1 ML

Pd/Cu(111) experiments the high temperature desorption of hydrogen was used in order to

estimate the amount of styrene (or acetylene) decomposed on the surface during the TPR

sweep.

Styrene:

Styrene decomposition: C8H8(a) 8C(a) + 4H2(g) (1)

Styrene hydrogenation: C8H8(a) + 2H(a) C8H10(g) (2)

Acetylene:

Acetylene decomposition: C2H2(a) 2C(a) + H2(g) (3)

Acetylene hydrogenation: C2H2(a) + 2H(a) C2H4(g) (4)

Acetylene trimerization: 3C2H2(a) C6H6(g) (5)

100 x/4 ︶︵II

I%Sel , 100 x

/4 ︶︵III/4 ︶︵II

%Conv22

2

deccomp.Hneethylbenze

neethylbenzeneethylbenze

decomp.Hneethylbenzestyrene

decomp.Hneethylbenzestyrene

100 x3 ︶*︵I︶︵II

I%Sel , 100 x3 ︶*︵I︶︵III

3 ︶*︵I ︶︵II%Conv

benzenedecomp.Hethene

etheneethene

benzenedecomp.Hetheneacetylene

benzenedecomp.Hetheneacetylene

22

2

7 

 

STM imaging of H atoms on Cu(111)

The appearance of H atoms and islands as depressions in STM images stems from an

electronic effect. It is only possible to image H atoms on metal surfaces in a narrow range of

voltages around the Fermi level (typically < ±0.1 V), as at higher biases inelastic tunneling

causes rapid motion. The density of states of the H-metal complex at this energy leads to a

lower tunneling probability than the bare metal, and the appearance of H atoms and islands in

STM images as depressions typically between 10-30 pm in depth. This effect has been

reported for hydrogen on a variety of metals including Cu (14,15,35-37).

References

 

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