Acrolein Conversion at 270 K

The time dependence of the propenol formation rate at 270 K illustrated in Figure 10.2 suggests that the course of acrolein conversion can be divided into three steps. First, there is an initial period during which a monolayer of acrolein is adsorbed, but no propenol is formed. Apparently, the Pd(111) surface is getting activated for propenol production. In the second period, high propenol production rates are observed for about 100-200 s. In the final step of the conversion, the activity of the catalyst slowly decreases to zero.

Initial Surface Modification

In order to study the change of the single crystal’s surface in the course of the induction period, the Pd(111) surface has been exposed to both reactants for 24 s at 270 K. Figure 10.3a illustrates the result of a separate experiment clearly showing that after this expo-sure the surface is just activated for propenol production. The Pd(111) surface exposed to acrolein for 24 s has subsequently been exposed to CO in order to probe the availability of pristine Pd sites. The corresponding IR spectrum is displayed with the black line in Figure 10.3b. For comparison, an IR spectrum obtained after exposure of pristine Pd(111) to CO is shown with the gray line. In the latter case, a strong IR absorption peak related to the C=O stretching vibration is observed. After exposure of the modified Pd(111) sur-face to CO, in contrast, no IR absorption feature charateristic for CO is detected, clearly showing that CO does not stick to the surface and thus that no pristine Pd sites are available. Hence, the adsorbed hydrocarbons must be rather homogeneously distributed over the surface than accumulated in islands; otherwise we cannot explain how the expo-sure of one acrolein per four Pd surface atoms could result in blocking of the entire surface.

Surface Species under Reaction Conditions

The chemical structure of the surface species on Pd(111) under reaction conditions at 270 K has been investigated by IRAS. Figure 10.4 shows three IR spectra obtained in the three reactivity regimes on the surface turning over. The second spectrum from top shows the surface during the first 45 s, which include the induction period and the beginning of the propenol formation. The third spectrum has been obtained during high propenol formation rates (45-90 s), and the final spectrum shows the surface with low activity in the final step of the experiment (450-540 s). For comparison, additionally the IR spectrum of a monolayer of acrolein on Pd(111) at 100 K is displayed at the top in Figure 10.4. We distinguish between three spectral regions characteristic for the CH_x stretching vibrations (3200-2700 cm⁻¹), C=O and C=C stretching vibrations (1850-1550 cm⁻¹), and CH_x deformation as well as C–O and C–C stretching vibrations (≤1500 cm⁻¹).

The spectrum of acrolein The uppermost spectrum in Figure 10.4 shows the IR ab-sorption of 3.6·10¹⁴ acrolein molecules/cm² on Pd(111) at 100 K, which is discussed in detail elsewhere [176] (see Chapter 8). IR absorption features appear at 1663 cm⁻¹ and at 1430-1400 cm⁻¹. The pronounced adsorption near 1663 cm⁻¹ is assigned to the stretching

10.3 Results and Discussion

Figure 10.3: Investigation of the Pd(111) surface at the end of the induction period for propenol formation at 270 K: (a) The black and blue curves show the for-mation rates of propenol and propanal. The propenol forfor-mation starts after exposure of both reactants for 24 s. (b) IR spectrum of CO on a pristine Pd(111) single crystal (gray) compared to an IR spectrum after CO adsorp-tion on the Pd(111) crystal exposed to acrolein and H₂for 24 s under reaction conditions (black).

Figure 10.4: IR spectra of a monolayer of moleculary adsorbed acrolein on pristine Pd(111) at 100 K (grey line) and of the surface species on Pd(111) turning over at 270 K during continuous exposure to acrolein and H₂ (black lines). The second spectrum from top has been obtained during the induction period and at the beginning of the propenol formation and mainly shows the IR vibration of spectator S1. The third spectrum has been obtained during high propenol formation rates and shows the additional appearance of the IR vibrations of the reaction intermediate. The fourth spectrum shows spectators S1 and S2 on the inactive surface.

10.3 Results and Discussion

vibration of the carbonyl (C=O) group and the feature at 1430-1400 cm⁻¹ is related to a scissor deformation of the methylene (CH₂) group. Both IR vibrational modes have been discussed in literature before [61, 248–251].

The initially formed spectator (S1) The second spectrum in Figure 10.4 has been col-lected during the first 45 s of acrolein conversion on Pd(111) at 270 K, which corresponds to an exposure of 6.8·10¹⁴ molecules/cm². It thus includes the induction period and the beginning of the propenol appearance in the gas phase. The obtained IR spectrum is clearly different from that of molecularly adsorbed acrolein. A pronounced IR vibra-tional mode appears at 1755 cm⁻¹ and a second one near 1120 cm⁻¹. The vibration at 1755 cm⁻¹strongly indicates a C=O stretching vibration that is not conjugated to a C=C group anymore. Vibrations near 1755 cm⁻¹ are typical for the carbonyl stretching mode in aldehydes and ketones [109, 240, 244].

The appearance of this IR vibration under reaction conditions points to the formation of an oxopropyl surface species, resulting from partial hydrogenation of acrolein with only one H atom at the C=C group. Our data do not allow to make a more precise conclusion on whether acrolein has been hydrogenated on theα- or β-C atom to form this species;

both products would be consistent with IR vibration at 1755 cm⁻¹. We will refer to this initially formed adsorbate as spectator 1 (S1). A possible structure of S1 is illustrated on the right hand side of in Figure 10.4. The IR absorption feature at 1120 cm⁻¹ starts to appear during the first 45 s, however, it becomes very pronounced in the spectrum obtained during high propenol formation rates and will therefore be discussed in the following paragraph.

The reaction intermediate (I_Pd(111)) The third IR spectrum in Figure 10.4 illustrates the surface composition in the period of high propenol formation rates (45 s to 90 s). It clearly shows the formation of a further surface species in the presence of S1 with strong IR absorption features. The newly formed species exhibits pronounced IR vibrations at 1090 cm⁻¹, 1120 cm⁻¹, and 1463-1650 cm⁻¹. Weak IR absorption is detected in the C–H stretching region near 2966 cm⁻¹ and 2980 cm⁻¹. In the C=O stretching region, however, no IR absorption feature can be assigned to the newly formed species.

The intense IR absorption features at 1090 cm⁻¹ and 1120 cm⁻¹ are present neither in adsorbed molecular acrolein on Pd nor in acrolein ice and therefore cannot be related to any distinctive vibration of intact acrolein molecules. Instead, the IR vibration at 1120 cm⁻¹ is assigned to a stretching mode of a saturated C–O bond. Previously, C–O stretching vibrations have been observed in the range from 1120 cm⁻¹ to 1200 cm⁻¹ when the oxygen is coordinated to a metal surface [61, 267, 275, 281–284]. The IR absorption at 1090 cm⁻¹ is assigned to a stretching vibration of a saturated C–C bond. In literature C–C bond vibrations are reported in the range from about 1000 cm⁻¹ to 1130 cm⁻¹, de-pending on their coordination to the surface [61, 180, 250, 284, 285]. The IR absorption at 1450 cm⁻¹ to 1463 cm⁻¹ appear in a typical range for CH₂ and CH₃ bending vibrations.

Tentatively, we assign it to CH₃ asymmetric bending modes, which have been reported

−1

1420-1430 cm⁻¹ [61, 248–251]. The vibrations at 2966 cm⁻¹ and 2980 cm⁻¹ clearly show C–H stretching vibrations. The vibration near 2980 cm⁻¹ indicates a C–H bond with a C atom that is part of an unsaturated C=C bond [109].

As no IR vibration characteristic for an alcohol group (O–H) is detected, it is most likely not the final hydrogenation product that is visible in the IR spectrum. However, a new C–H bond vibration appears, which can clearly be assigned to a surface species only formed during high propenol formation rates. We therefore relate this surface species to a the half-hydrogenated reaction intermediate (I_Pd(111)). According to the Horiuti-Polanyi mechanism, first a half-hydrogenated intermediate is formed on the surface by reversible addition of one hydrogen atom to an unsaturated bond. In the following step, the final product is produced by irreversible addition of the second hydrogen atom before it desorbs [286]. Theoretical calculations show that the barrier for addition of an H atom at a C atom is smaller than for addition at the O atom and should therefore occur preferentially [280].

The most likely surface species related to the observed IR absorption features is a pro-penoxy-group, in which the C–O entity is attached to the Pd through the O atom in anη¹-(O) configuration (CH₂=CH–CH₍₂₎–O–Pd). A C–O stretching vibration at similar frequency has been observed in an η¹-(O) configuration of prenal on a Pt-Sn(111) alloy surface [275]. The high intensity of the 1120 cm⁻¹ band, exceeding even the most intense C=O vibrational band in acrolein and in the oxopropyl species, additionally supports the formation of a C–O bond exhibiting a large dynamic dipole moment and, hence, explains the very high IR intensity. Furthermore, the IR vibration near 2980 cm⁻¹, which indi-cates a C–H stretching vibration at a C=C group, suggests that the reaction intermediate I_Pd(111) preserves a C=C group. Finally, it should be noted the strong IR absorption re-lated to the C–C stretching at 1090 cm⁻¹appears relatively intense although the dynamic dipole moment of a C–C bond is rather small. According to the metal surface selection rule (MSSR), the projection of vibrations of the dynamic dipole moment perpendicular to the surface are visible in IRA spectra, while the vibrations parallel to the surface are strongly attenuated due to formation of an image dipole moment in the underlying metal substrate. Based on the MSSR, the high intensity of the C–O and C–C stretching vibra-tion indicates a strongly inclined C–C–C–O–Pd entity.

It is important to note that the surface reaction intermediate is formed not on the pristine Pd(111) surface, but on the surface strongly modified with S1 (oxopropyl species).

Indeed, about one acrolein molecule per four Pd surface atoms is accumulated on Pd(111) to form the spectator prior to the onset of the propenol formation. Microscopically, this corresponds to a situation when every fourth Pd atom is covered by S1, forming a dense spectator overlayer structure. Most likely, such strong geometrical confinement of an adsorption site for acrolein on the S1-covered surface prevents the competing pathway of C=C bond hydrogenation and allows acrolein to adsorb only via the O atom to activate the C=O group. Obviously, the clean Pd(111) surface is not capable of activating the C=O group towards selective hydrogenation and the strong modification of the surface by S1 is required to trigger the desired selective chemistry.

10.3 Results and Discussion

The slowly formed spectator (S2) The fourth IR spectrum in Figure 10.4 (bottom) has been collected after the formation rate of gas-phase propenol has decreased almost to zero (450 s to 540 s). The features assigned to the half-hydrogenated intermediate I_Pd(111)have disappeared. Instead vibrations are observed at 1330 cm⁻¹, 1375 cm⁻¹, in the range from 2883 cm⁻¹ to 2892 cm⁻¹, and at 2942 cm⁻¹. All these IR absorption features strongly point to vibrations of C–H bonds. The sharp peak at 1330 cm⁻¹ is very characteristic for the umbrella bending mode of the –CH₃ group in ethylidyne or an ethylidyne-like species, which has been observed in previous studies on Pd(111) and Pt(111) before [287, 288].

This result indicates that a fraction of the acrolein molecules decomposes in a decar-bonylation reaction yielding a C₂ fragment and a carbonyl group which eventually poison the surface. Decarbonylation of acrolein and similarα, β-unsaturated aldehydes on metal surfaces is a well-known phenomenon [181, 289–292]. The decomposition yields a carbonyl fragment and a C₂ group, such as ethylene. Ethylene formation from acrolein on Pd has been observed in previous studies [181, 289–291]. Moreover, ethylene is known to convert to ethylidyne on metal surfaces such as Pd and Pt [287, 288, 293]. Identification of the carbonyl fragment is most likely not possible, since the C=O stretching vibration of this species may strongly overlap with the C=O vibration of S1. In literature, C=O stretching vibrations of various aldehydes have all been reported near 1790-1750 cm⁻¹ [109, 244].

Evolution of the Reaction Intermediate and the Final Product

IRAS studies with higher time resolution have been performed in order to compare the formation of the reaction intermediate on the surface to the appearance of propenol in the gas phase in more detail. Figure 10.5a illustrates a series of IR spectra obtained on the Pd(111) surface turning over at 270 K. During the collection of each spectrum the surface has been exposed to 1.2·10¹³ acrolein molecules/cm², which corresponds to 8 s in the QMS measurement in Figure 10.5b. Note that after the 6^th spectrum only every fourth spectrum is illustrated in Figure 10.5a. Approximately in the 2^nd or 3^rd spectrum, the vibrations indicating I_Pd(111) start to appear. The intensities of the peaks grow until about the 7^th or 8^th spectrum. Finally, all features pointing to I_Pd(111) disappear again.

Figure 10.5b shows the gas-phase formation rate of propenol (grey line) together with the integral intensity of the most intense IR vibration band of I_Pd(111) at 1120 cm⁻¹ (black squares). We assume that the intensity of the band at 1120 cm⁻¹ approximately reflects the concentration of I_Pd(111) on the surface. Thus, the propenol formation rate detected in the gas phase clearly follows the concentration of I_Pd(111) on the surface. The observed strong correlation unambiguously shows that the corresponding surface species is a reaction intermediate that is directly involved in the selective hydrogenation of acrolein to the propenol.