Adsorption of Propanal

Figure 8.2(a) illustrates the coverage dependent evolution of IR spectra of propanal on Pd(111) from the sub-monolayer to the multilayer regime at 120 K. Coverage-dependent changes of IR absorption features suggest three coverage regimes. The sub-monolayer range is indicates up to 2.7·10¹⁴propanal molecules per cm², the formation of a complete monolayer seems to require approximately an exposure of 3.6·10¹⁴to 5.4·10¹⁴ molecules per cm², and the multilayer regime is observed when the surface coverage exceeds 5.4·10¹⁴ molecules per cm². Table 8.2 gives an overview on all IR vibrational peaks at low, inter-mediate, and high propanal coverages and compares them to values reported in literature.

Table 8.2: Assignment of IR vibrational modes of propanal on Pd(111) at 120 K.

mode IRAS vibrations / cm⁻¹ references / cm⁻¹

sub-monolayer near monolayer multilayer

ν(C=O) 1560 1663 1728/1695 1569^c [261], 1580 [262]

1595, 1562 [263]

1565 [264], 1750 [254]

1740-1720 [109]

1754-1729 [253]

δa(CH3) 1455-1468 1455-1468 1455-1468 1475-1450 [109]

1467-1451 [253]

1460,1451 [254]

δ(CH₂) 1413 1413 1413 1425 [256], 1420 [254]

1423-1413 [253]

δ(CH) 1370 1370 1392 1395 [254], 1385 [258]

1381-1374 [253]

ρ(CH3) 1070 1070 1095 1098-1093 [253]

νa(CCC) 990 1001-993 [253]

ν: stretching,ν_a: asymmetric stretching,ν_s: symmetric stretching,δ: bending, δ_s: bend-ing,ω: wagging, ^b: Fermi resonance, ^cin HREELS measurement

8.3 Results

propanal at low coverage

A variety of IR absorption features are identified at low propanal concentration on the Pd(111) surface. At surface coverages ranging from 9·10¹³to 2.7·10¹⁴molecules/cm², pro-nounced IR absorption peaks are observed in the region of the C–H stretching vibrations.

The IR vibrations at 2960 cm⁻¹, 2943 cm⁻¹, and 2876 cm⁻¹are assign to CH₃asymmetric stretching, CH₂asymmetric stretching, and CH₃ symmetric stretching modes. Previously, CH₃ asymmetric and symmetric stretching vibrations in propanal were observed at 2992-2980 cm⁻¹ [253–255] and 2905-2880 cm⁻¹ [253, 254]. The CH₂ asymmetric stretching was found at 2914-2899 cm⁻¹ [253, 255]. However, none of the literature data refers to propanal on a metal surface.

In the region of typical C=O stretching vibrations, no IR absorption features can be identified. At 1560 cm⁻¹, however, an IR vibration is observed, which is present neither in propanal nor in other aldehydes and can therefore not be related to any prominent vibration of intact propanal molecules. Nevertheless, similar vibration frequencies have been observed before. Murilloet al. observed a vibration at 1569 cm⁻¹in HREELS exper-iments on acrolein adsorbed on Pt-Ni-Pt(111) and Pt-Co-Pt(111) surfaces and assigned it to a C–O stretching vibration of acrolein adsorbed in a di-σ-C–O configuration [261].

Furthermore, the COO-asymmetric stretching vibration of a propanoate species on oxide surfaces were found at 1595 cm⁻¹ [263] and 1565 cm⁻¹ [264], and the C=O stretching frequencies ofβ-diketone complexes with Cu were reported to appear down to 1524 cm⁻¹ [262]. Hence, this IR absorption feature could point to a severe weakening of the carbonyl group because of the tendency of the oxygen to attract electrons C=O ↔ C⁺–O⁻. The polarized form could gain in importance, if the charge is stabilized by the Pd surface.

However, a di-σ configuration as observed by Murilloet al. for acrolein on Pt-Ni-Pt(111) and Pt-Co-Pt(111), seems unlikely here since a di-σ-bounded C–O group would be parallel to the Pd surface and thus not detectable by IRAS.

In the region below 1500 cm⁻¹, several pronounced vibrations are detected at sub-monolayer coverages. IR absorption features at 1468-1455 cm⁻¹ and 1413 cm⁻¹ are as-signed to CH₃ asymmetric bend and CH₂ scissor vibrations. Previously, CH₃ asymmetric bending modes were found in the ranges from 1475-1451 cm⁻¹ and CH₂ scissor defor-mation vibrations were detected at 1425-1413 cm⁻¹ [109, 253, 254]. The vibrations at 1370 cm⁻¹and 1358 cm⁻¹strongly indicate CH bending or CH₃umbrella bending modes;

a clear assignment to one of them is difficult. In literature, CH bending and CH₃ sym-metric bending modes were reported in overlapping frequency ranges at 1410-1374 cm⁻¹ [109, 253, 254, 258] and 1395-1338 cm⁻¹ [109, 253–255]. With higher certainty, we relate the peaks at 1270 cm⁻¹, 1070 cm⁻¹, and 990 cm⁻¹ to CH₂ twist, CH₃ rock, and CCC antisymmetric stretching modes. Previously, CH₂ twist vibrations were observed at 1261-1250 cm⁻¹, CH₃ rocking was found at 1098-1093 cm⁻¹, and CCC antisymmetric stretching modes were reported at 1001-993 cm⁻¹ [253].

propanal at intermediate coverage

Pronounced changes of IR vibration frequencies are observed at intermediate surface cov-erages ranging from 3.6·10¹⁴to 5.4·10¹⁴molecules/cm². The CH₃asymmetric stretching is observed at 2970 cm⁻¹, which is slightly higher than at low coverage and closer to the literature values for non-adsorbed molecule. The same trend is observed for the more ten-tatively assigned CH₃ symmetric bending, which appears at 1385 cm⁻¹. The vibrations of the CH₂ and CH groups as well as the CH₃ asymmetric bending appear at identical wavenumbers as observed at lower coverages.

In the C=O stretching region a clearly observable peak saturates at 1663 cm⁻¹. Previous studies on rather unperturbed propanal molecules showed C=O stretching vibrations in the range of 1754-1720 cm⁻¹and thus at significantly higher wavenumbers [109, 253, 254, 256].

The strong decrease of the C=O stretching frequency in our studies points to a significant weakening of the C=O bond and hence to strong interaction of this group with the Pd surface. Finally it should be notes that the C=O stretching vibration appears at identical frequency as that of acrolein type C on Pd(111).

propanal at high coverage

At high propanal coverages, several further IR vibration features are observed. The fre-quencies of the CH₃ stretching vibrations appear at values closer to that of molecules in the gas-phase. CH₃ asymmetric and symmetric stretching modes are assigned to IR absorption features at 2987 cm⁻¹ and 2884 cm⁻¹. CH₂ asymmetric stretching is observed at 2943 cm⁻¹ and thus at the same wavenumber as observed for lower propanal coverages.

The CH₂ symmetric stretching mode, which could not be detected at lower coverages, is observed at 2904 cm⁻¹. Previously, this vibration was observed at 2914-2899 cm⁻¹ [253, 255]. The features at 2868 cm⁻¹ and 2752 cm⁻¹ are both assigned to the aldehyde-C–H group. As discussed for acrolein, these two peaks are known to result from strong Fermi resonance between the first overtone of the CH bending and CH stretching funda-mental [253, 254, 256, 257].

In the C=O stretching region, we observe two features growing simultaneously at 1728 cm⁻¹ and 1695 cm⁻¹. The vibration at 1728 cm⁻¹ is close to the frequency reported for propanal in the gas phase and thus points to a mainly unperturbed C=O group. The vibration at 1695 cm⁻¹, however, indicates the simultaneous formation of a species with a slightly weakened C=O bond.

CH₃asymmetric bending and CH₂scissor bending modes appear at identical frequencies as observed for lower coverages. However, the CH₃ symmetric bend and/or CH bend as well as CH₃ rock vibrations are identified at slightly higher frequencies. The former one is observed at 1392 cm⁻¹ and the latter one at 1095 cm⁻¹. Moreover, IR absorption is observed at 1344 cm⁻¹, which we assign to the CH₂ wag mode. Previously, this vibration was observed at 1340 cm⁻¹ [254].

8.3 Results

Figure 8.3: a) Coverage-dependent IR spectra of allyl alcohol on Pd(111) recorded at 120 K. b) TPD after deposition of about six layers of allyl alcohol on Pd(111).

TPD of propanal at multilayer coverage

Figure 8.2b illustrates a TPD experiment of approximately six layers of propanal on Pd(111). Desorption of acrolein, allyl alcohol, propanal and hydrogen has been stud-ied. Neither acrolein nor allyl alcohol are observed in the gas phase. Propanal, however, appears in a strong and sharp desorption peak at 145 K and in a weak feature at 180 K.

Hydrogen desorption is observed near 345 K and 470 K. The two hydrogen peaks point to sequential decomposition resulting in a desorption-limited and a reaction-limited for-mation of H₂. The two propanal desorption peaks indicate a relatively large amount of weakly bound molecules, which desorb near 145 K, and a smaller fraction of molecules from a more strongly bounded state desorbing near 180 K.