Particle Size Effects

The influence of Pd particle size on the production of propanal and propenol is evident in Figure 11.1. The 4 nm Pd particles did not produce a significant amount of either propenol or propanal. It is not clear why the 4 nm particles were inactive. There were no species detected on the surface of the 4 nm Pd particles during acrolein hydrogenation in the 220-270 K range, so it is likely that the concentration of Pd atoms is too low to turnover a significant number of acrolein molecules. The amount of Pd in the 4nmPd/Fe₃O₄catalyst is more than an order-of-magnitude less than 7-nm-Pd/Fe₃O₄ and 12-nm-Pd/Fe₃O₄. The activity of the 7 nm and 12 nm Pd particles was similar during pulsed acrolein hydro-genation experiments in the 220-270 K temperature range (see Figure 11.1). However, we will show that there is a significant difference in the selectivity of acrolein hydrogena-tion over 7 nm and 12 nm particles that is not evident in the pulsed reactivity experiments.

In the previous section we showed that propenol production was possible over 12 nm Pd particles, but only in a very narrow temperature range around 250 K. Here we show

11.4 Discussion

that the Pd nanoparticle size also has a significant influence on the selectivity at 250 K.

Figure 11.5(a) shows the gas-phase production of propanal and propenol during acrolein hydrogenation over 7 nm Pd particles at 250 K. Unlike the 12 nm particles, there was no significant production of propenol over 7 nm particles at 250 K. IRAS spectra collected during acrolein hydrogenation over 7 nm particles at 250 K are displayed in Figure 11.5(b), with spectra labeled 1-6 collected during the corresponding regions in Figure 11.5(a).

Similar to the IRAS spectra collected during acrolein hydrogenation over 12 nm particles at 250 K, there are three main bands at 1860, 1760, and 1660 cm⁻¹that are associated with CO on particle facets, non-conjugated C=O stretching, and C=O stretching in acrolein, respectively. It is not clear from IRAS-MS why the 12 nm particles are active for propenol, whereas the 7 nm particles aren’t. It’s possible that the concentration of the surface species which activates the particles for propenol production is much smaller on the 7 nm particles than on the 12 nm particles. It is also possible that larger modified domains are more efficient, or that a minimum domain size is required, for alcohol production.

11.4 Discussion

We have shown that there is a strong structure dependence in the selective partial hydro-genation of acrolein over Pd model catalysts. The change in selectivity from nearly 100%

towards propenol over Pd(111) to nearly 100% towards propanal over Pd nanoparticles (7 or 12 nm), displayed in Figure 11.1, is the most drastic example of the strong influence of the Pd structure on the selective partial hydrogenation of acrolein. The 12 nm Pd parti-cles were capable of producing a small amount of propenol in a narrow temperature range around 250 K, but no propenol production was observed over 7 or 4 nm Pd particles in the 220-270 K temperature range. Therefore, it appears that the selectivity towards propenol production increases with increasing Pd particle size, from 0% over 4 and 7 nm particles up to nearly 100% over the Pd(111) single crystal, which is essentially an infinitely large particle. It is possible that smaller Pd particles have a higher concentration of edge sites than larger particles, and these edge sites catalyze some surface reactions, for example acrolein decomposition, that prevent propenol production.

Using IRAS we were able to gain insight into the surface chemistry that is responsible for the structure-dependent selectivity in acrolein partial hydrogenation. There were sev-eral species observed on the surface of Pd(111) during acrolein hydrogenation at 270 K, including a species which we believe to be an intermediate with a characteristic absorption band near 1120 cm⁻¹, and a spectator species at 1755 cm⁻¹. We believe that the spectator surface species, which has a characteristic infrared absorption band near 1755 cm⁻¹ that is associated with a C=O bond that is not conjugated to a C=C bond (i.e. not acrolein), is responsible for controlling the selectivity. The exact structure of this species is not clear from our results, but we believe that it results from the partial hydrogenation of the C=C bond in acrolein, leaving the C=O bond intact. It is not clear how this surface species activates the Pd(111) surface for propenol production. It’s possible that a dense overlayer of this partially hydrogenated acrolein surface species forces incoming acrolein molecules

observed on the surface of Pd nanoparticles (7 and 12 nm) during acrolein hydrogenation at 270 K because acrolein decomposes to CO, which covers the facets of the particles. There-fore, it is likely that significant propenol production is not observed over Pd nanoparticles because the edges of Pd nanoparticles catalyze the decomposition of acrolein to CO, in-stead of forming the spectator species which activates the surface for propenol production.

Temperature also has a significant influence on the selectivity in partial hydrogenation over 12 nm Pd particles. At 270 K acrolein decarbonlyates on 7 and 12 nm Pd particles, producing CO which covers the facets. At lower temperature, acrolein decomposes to a lesser extent and other surface species are observed. At 220 K, the spectator species which we believe is responsible for activating the Pd(111) surface for propenol production, with a characteristic absorption band near 1755 cm⁻¹, is observed on the surface of 7 and 12 nm Pd particles; however, 220 K is too low for significant production of propenol.

At 250 K, the temperature is high enough for propenol production, but not so high that acrolein decomposes to CO, and a small amount of propenol production is observed on 12 nm Pd particles. These results improve our understanding of the structure-dependence in selective hydrogenation of a model multi-unsaturated compound acrolein.

11.5 Conclusions

Selective hydrogenation of either the C=C or the C=O bond in acrolein is strongly de-pendent on temperature and the structure of the Pd catalyst. We believe that selective hydrogenation of the C=O bond in acrolein is related to a spectator species that is formed on the Pd surface during the beginning of the reaction, and the formation of this ac-tivating spectator species is strongly dependent on temperature and Pd structure. Pd nanoparticles in general are much less active for C=O bond hydrogenation than single crystal Pd(111). The largest Pd nanoparticles in this study (12 nm) produced a signifi-cant amount of propenol from selective C=O bond hydrogenation, but only in a narrow temperature range around 250 K. At higher temperature (270 K), acrolein decarbonylates on Pd nanoparticles producing CO which covers the facets and prevents the formation of the activating spectator species. At lower temperature (220 K), the activating spectator species is formed on the surface of Pd nanoparticles, but the temperature is too low for hydrogenation of the C=O bond, even over Pd(111).

12 Insights into the Origin of Selectivity in Acrolein Conversion over Pd/Fe ₃ O ₄

Karl-Heinz Dostert¹, Swetlana Schauermann^1,2, and Hans-Joachim Freund¹

1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

2Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 1, 24118 Kiel, Germany

to be submitted

Abstract

Atomic-level understanding of kinetic effects that govern the selectivity in partial hydro-genation ofα,β-unsaturated aldehydes is of pivotal importance for the rational design of new catalytic materials with the desired selectivity towards C=C or C=O bond conver-sion. However, in previous studies, the reason for the high selectivity towards C=C bond hydrogenation over Fe₃O₄-supported Pd particles remained unclear. In the present study, the binding of acrolein to an Fe₃O₄ film and to Fe₃O₄-supported Pd particles is studied by infrared reflection-absorption spectroscopy (IRAS) at 120 K and temperature-programmed desorption (TPD) experiments to obtain detailed information on the activation of chemical bonds by means of Fe₃O₄ and Pd. IRAS studies show strongly inclined acrolein molecules on the Fe₃O₄ support with heavily perturbed chemical bonds. Especially the C=O bond appears significantly weakened in Fe₃O₄-adsorbed acrolein. Nevertheless, in TPD exper-iments, acrolein molecules stay intact on the Fe₃O₄ film until desorption; on Pd/Fe₃O₄, however, decomposition as well as conversion to propanal occurs. Our results indicate that the Fe₃O₄ support promotes the conversion of the C=C bond by activation of the β-C atom. The polarized form of the C=O bond (C=O↔C⁺–O⁻) seems to be stabilized by a Lewis acid–base complex between the oxygen atom and electron accepting sites of the support; the electrophilic character of the carbonyl-C atom is transferred along the conjugatedπ system to the β-C atom.

12.1 Introduction

Figure 12.1: Reverse selectivity in hydrogenation of acrolein over Pd/Fe₃O₄ model cata-lysts and Pd(111): Over Pd/Fe₃O₄, acrolein is converted to propanal with nearly 100% selectivity; while over a Pd(111) single crystal, unsaturated al-cohols are formed with nearly 100% selectivity.

12.1 Introduction

The catalytic hydrogenation of α,β-unsaturated aldehydes is of broad interest for fun-damental understanding as well as for industrial applications [30]. The primary hydro-genation product is either a saturated aldehyde or an unsaturated alcohol. Thermody-namically, the hydrogenation of the C=C bond to the saturated aldehyde is favored [28].

However, fundamental understanding of the parameters governing the selectivity is nec-essary to avoid the formation of undesired products and thereby an often difficult and cost-intensive separation process.

It is generally believed that the adsorption geometry of the reactant on the catalyst surface is an important factor governing the selectivity of the hydrogenation reaction.

The adsorption geometry of anα,β-unsaturated aldehyde or ketone can be manipulated by adding bulky substituents [27, 31, 32]. However, also the structure of the catalyst can have a decisive influence on the selectivity. Enhanced conversion of polar functional groups, such as carbonyl groups, carbon monoxide, and carbon dioxide, over Pt group metals was achieved upon addition of promoters. Studies on the promoting effect of various metal oxides show that the activity of a catalyst for conversion of the C=O bond scales with their Lewis acidity, indicating a critical role of charge transfer between the C=O group and cationic sites of the metal oxide [297, 298]. Particularly TiO₂ supports were found to increase the selectivity of C=O bond hydrogenation in unsaturated aldehydes and ketones over supported Pt catalysts [41, 52, 54].

Figure 12.1 summarizes the previously observed strong dependence of the selectivity in partial hydrogenation of acrolein on the structure of the Pd catalyst. Highly selective for-mation of unsaturated aclohols was observed over a Pd(111) single crystal, while propanal formation occurred with≈100% selectivity over Fe₃O₄-supported Pd nanoparticles. The formation of unsaturated alcohols was found to critically depend on the presence of an overlayer of spectator species formed on Pd(111) at the initial stages of the reaction. The origin of the high selectivity towards propanal formation by C=C bond hydrogenation over Pd/Fe₃O₄, however, remained unclear [185–187] (Chapters 9, 10 and 11).

In a previous publication we reported the molecular structures of acrolein, propanal and

C=O and C=C bonds parallel to the Pd(111) surface [176] (Chapter 8).

In order to explore the origin of the selectivity in acrolein hydrogenation over Pd/Fe₃O₄ model catalysts, we here report a detailed study on the binding of acrolein to an Fe₃O₄ film and Fe₃O₄-supported Pd particles with a diameter of 7 nm by coverage-dependent infrared reflection-absorption spectroscopy (IRAS) and temperature-programmed desorp-tion (TPD) experiments. We particularly focus on the molecular structure of acrolein in the low-coverage limit. The results indicate that the Fe₃O₄ support promotes the activa-tion of theβ-C atom and thus the selective conversion of the C=C bond over Pd/Fe3O₄ model catalyst.

12.2 Experimental Details

All experiments were performed at the Fritz-Haber-Institut, Berlin, in an ultrahigh vac-uum (UHV) apparatus that has been described in detail before [100]. Acrolein was dosed onto the sample through a doubly differentially pumped multi-channel array source con-trolled by valves and shutters. The source was operated at room temperature, and the beam diameter was chosen to exceed the sample size. The method for preparing the Pd/Fe₃O₄/Pt(111) model catalysts has been described in detail previously [153]. A well-ordered 10 nm thick Fe₃O₄ film was grown on a Pt(111) substrate followed by Pd de-position onto the freshly prepared Fe₃O₄ film at 120 K by physical vapor deposition of 4 ˚A Pd (Goodfellow,≥99.9%) using a commercial electron-beam evaporator (Focus EFM 3). After depositing Pd, the sample was annealed at 600 K and the Pd particles were stabilized by repeated cycles of oxidation in 1·10⁻⁶ mbar O₂ for 15 min and reduction in 1·10⁻⁶ mbar CO for 45 min at 500 K (see [152]). The stabilization procedure was also used to clean the particles after each experiment. The quality of the particles was checked by infrared reflection-absorption spectroscopy (IRAS) of adsorbed CO prior to every experiment. Shortly before each experiment, the sample was flashed to 600 K before cooling to 120 K.

IRAS data were acquired using a vacuum Fourier-Transform infrared (FT-IR) spec-trometer (Bruker IFS 66v/S) with a spectral resolution of 2 cm⁻¹, a mid-infrared (MIR) polarizer and p-polarized IR light. Temperature-programmed desorption (TPD) experi-ments were carried out in the same UHV system by using an automated quadrupole mass spectrometer (QMS) system (Hiden Analytics). In TPD experiments the following masses were detected: 2, 28, 31, 56, 57, 58, and 60.

12.3 Results and Discussion