Molecular Heterogeneous Catalysis, Wiley (2006), 352729662X

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 287

Figure 6.17. Reaction energy diagram for surface oxygen atom-assisted formation of vinyl acetate catalyzed by Pd(111)[39]. Energy is given in kJ/mol.

The primary surface reaction steps which include the activation of ethylene to vinyl and the subsequent coupling of vinyl with acetate surface intermediates have been cited as potential rate-determining steps in the Nakamura and Yasui route. Moiseev and Vargaftik

carried out experiments over giant palladium clusters comprised of 561 atoms and arrived at a similar set of pathways[38]. They suggested, however, that the rate-controlling step for this process involves the shift of ethylene from the π-bound mode to a di-σ-bound mode.

Periodic density functional theoretical calculations were performed by Neurock and Kragten[39] to examine the overall reaction energies and selected activation barriers for the elementary steps for this particular mechanism in both the presence and absence of oxygen. The overall reaction energies for the elementary steps for the oxygen-assisted path are shown in Fig. 6.17. The two potential rate-limiting steps appear to be the reaction of ethylene to vinyl and the coupling of vinyl and acetate to form VAM. The transition states for the proposed limiting steps and their corresponding activation barriers were calculated using ab initio DFT calculations. The results indicate that both the C–H bond activation of ethylene and the coupling of vinyl and acetate have rather high barriers at +120 and +110 kJ/mol, respectively. Atomic oxygen readily dissociates to form atomic oxygen, which is clearly shown to be the stable thermodynamic sink, thus allowing for the formation of water.

The second general mechanism proposed for the vapor VAM synthesis involves the

direct reaction between ethylene and adsorbed acetate as proposed by Samanos and Bountry[40] . Both acetate and oxygen are found to adsorb very strongly to the Pd surface,

thus forming a partially oxidized surface. Ethylene can adsorb in either a π- or a di-σ- configuration or react directly from the vapor phase with adsorbed acetate in a process similar to that suggested for the homogeneous path over Pd3(OAc)6 (see Section 6.3.1). The surface reaction of ethylene with acetate involves the insertion of ethylene into the Pd– OAc bond to form the acetoxyethyl [CH3CO(O)CH2CH2 ] intermediate (labeled C2H4OAc in Fig. 6.16). The CH3CO(O)CH2 CH2 intermediate can subsequently undergo a β-hydride elimination, thus leading to the formation of vinyl acetate, CH3CO(O)CH=CH2 . The hy-

288 Chapter 6

drogen that forms can react with surface oxygen and desorb as water. This mechanism is very similar to that which was proposed by Zaidi[41]. Zaidi, however, suggested that the reaction actually proceeds in the solution phase directly at the reactive Pd–acetate complex that forms in the liquid solution layer. The potential for supported liquid-phase catalysis is described in more detail below.

The primary di erence between the Nakamura ans Yasui and Samanos and Bountry mechanisms specifically involves whether ethylene stays intact before reacting with acetate or dissociates to form the vinyl intermediate. The overall energies calculated from DFT indicate that ethylene insertion into acetate and the β-C–H bond activation of the surface ethyl acetate could be rate limiting reaction steps. The overall energy for the insertion of ethylene into acetate was calculated to be 60 kJ/mol whereas the overall energy for β-C–H activation is exothermic at 30 kJ/mol[39]. The barriers for these steps, however, have not been determined.

6.3.1 Homogeneous Catalyzed Vinyl Acetate Synthesis

The acetoxylation of ethylene to form the vinyl acetate monomer (VAM) can be catalyzed

by homogeneous catalysts comprised of PdCl/CuCl salts and carried out in glacial acetic acid[38]. The reaction is catalyzed by the Pd2+ species which are reduced by the adsorption

of ethylene and subsequently reoxidized by oxygen and copper chloride. The speculated mechanism is as follows

C2H4 + PdX2 + HX −→ [C2H4PdX3]− + H+ [C2H4PdX3]− + AcOH −→ [C2H4PdX2(OAc)]− + HX

[C2H4PdX2(OAc)]− −→ [C2H4PdX(OAc)] + X− [C2H4PdX(OAc)] −→ CH2CH−OAc + Pd + HX

where X is a chloroacetate species[34,35] . Yields of 90% (based on ethylene) have been reported for the processes developed by Hoechst and Bayer[33] .

The reaction can also be carried out by starting out directly with palladium acetate complexes. Alkali metal salts are typically used as promoters to help increase both the activity and the selectivity. In the presence of an alkali metal promoter such as Na, palladium acetate can exist as either a trimer (Pd3 OAc6), a dimer (Na2Pd2OAc6 ) or a monomer (Na2PdOAc4 ). These three di erent cluster forms are in equilibrium and can be interchanged by changing the concentration of alkali metal. The rate appears to be controlled by the β-hydrogen transfer from the bound β-acetoxyethyl intermediate to

form VAM. The apparent activation energy for VAM formation was reported to be 70 kJ/mol[42].

In the presence of water, Wacker chemistry tends to predominate, whereby ethylene reacts with water rather than acetic acid and forms acetaldehyde as the primary product. Henry and Pandey[44] showed that the addition of alkali metal acetates can help to shift the product spectrum in order to form vinyl acetate in higher yields. The reaction is thought to involve the nucleophillic attack of ethylene by acetate to form a C2H4–OAc– Pd complex which subsequently undergoes β-C–H activation to form VAM. Acetic acid or HCl can desorb from the complex to form Pd(0), which is reoxidized back to copper chloride to regenerate Pd2+ .

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 289

Zaidi[41] and Samanos[40] suggested that a similar path, which is shown below, may give rise to catalysis in the supported liquid layer for heterogeneous Pd catalysts:

Pd is first oxidized to form a homogeneous Pd2+ cluster. Ethylene can then react via an inneror outer-sphere mechanism to form the acetoxyethyl intermediate (see next section). The acetoxyethyl intermediate then undergoes a β-hydride elimination to form VAM. DFT calculations were performed in order to understand the potential presence of the liquid layer on a Pd surface and the energetics for the proposed homogeneous pathways.

6.3.2 Elementary Reaction Steps of Vinyl Acetate in the Liquid Phase

In order to understand the influence of a liquid layer, Desai et al.[3a]. carried out periodic DFT calculations on the dissociative adsorption of acetic acid in both the presence and absence of multilayers of water over Pd. Water was used as a simple model to mimic the solution phase. The calculations were carried out by placing enough water molecules throughout the unit cell in order to minimize the energy and approach the bulk density of liquid water. Various reaction channels for acetic acid were subsequently explored. The results indicate that the dissociative adsorption of acetic acid over Pd(111) is 28 kJ/mol endothermic in the gas phase. This reaction proceeds via a homolytic process, resulting in products that are radical-like. The dissociation of acetic acid in the presence of an aqueous medium, however, proceeds via a heterolytic process, resulting in products that take on ionic characteristics. The dissociation energy of acetic acid in aqueous water over Pd(111) to produce adsorbed acetate and a hydronium ion in the aqueous phase is +37 kJ/mol. The products form a double layer at the metal–solution interface, as shown in Fig. 6.18. The acetate anions are adsorbed at the surface whereas the protons that form are removed from the surface by one solvation shell. Although this structure is thermodynamically stable at the interface, it is not the lowest energy structure that can form. The acetate anion would also prefer to be located in solution. The water molecules in solution are more e ective in solvating the anion than the metal surface is. The energy di erence between adsorbed acetate and free acetate anions in solution is exothermic at –57 kJ/mol, which indicates that the acetate anions prefer to reside in solution. The ability to access this state, however, requires energy to surmount the activation barrier associated with desorption and solvent reorganization. At higher temperatures, the proton from the solution phase recombines with the surface acetate species to form acetic acid. Water subsequently displaces acetic acid in a concerted e ort. Acetic acid ultimately redissociates in solution to form an acetate anion and a proton. This process requires the energy necessary to overcome the barrier associated with solvent reorganization. Similarly, Campbell[43] found experimentally that the presence of an ethanol solution phase gave rise to a significant increase in the activation barrier for the adsorption and desorption of di erent alkanethiols on to the Au substrate. There is no measurable barrier for alkanethiol to adsorb on to Au under UHV conditions. In solution, the alkanethiol must first displace

290 Chapter 6

Figure 6.18. The adsorption of liquid water on Pd(111). The Pd–water binding energy on Pd(111) in the vapor phase is 30 kJ/mol. The Pd–water binding energy for liquid water is reduced to –2.5 kJ/mol.

The overall binding energy, however, increases owing to the formation of hydrogen bonds with the aqueous phase[3a].

the ethanol before it can adsorb. This solution reorganization energy ultimately leads to the measured barrier.

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various PdxOAcy clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)6 clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)6 cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Ethylene subsequently inserts into a Pd–acetate bond. The cation is then reduced by the reaction to form the neutral Pd0. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd2+ to form acetaldehyde. Pd0 is subsequently reoxidized by oxygen to form Pd2+ [35,36,44].

The reductive elimination of vinyl acetate can then proceed through either an inneror an outer-sphere reaction channel. The inner-sphere mechanism involves the reaction between two ligands that are both already coordinated to the metal center. The outersphere mechanism involves the reaction between ethylene which is coordinated to the metal center and an acetate anion which resides in solution. A sketch of both the innerand outer-sphere reactions is given in Fig. 6.19.

Kragten et al.[36] carried out DFT calculations to determine the reaction energies and the activation barriers for a sequence of elementary steps that make up both the innerand outer-sphere mechanisms. The e ects of solution are included via the explicit introduction of one or two acetic acid molecules along with an overall reaction field. The solute is modeled by a cluster in which the charge is balanced by the coordination of protons to

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 291

Figure 6.19. Proposed catalytic cycles for the innerand outer-sphere Wacker-like mechanisms for the acetoxylation of ethylene to vinyl acetate[36].

the cluster. The field is modeled via an overall continuum comprised of a specific dielectric constant.

The overall potential energy profiles were calculated for both the innerand outersphere mechanisms for the homogeneous acetoxylation of ethylene to vinyl acetate. The results, which include the e ect of solvation, are shown in Fig. 6.20.

Both mechanisms initially proceed by the direct coordination of ethylene to a Pd2+ cation vacancy site. In the inner-sphere path, the coordinated ethylene species reacts with an adjacent acetate ligand. This results in the formation of the acetoxy ethyl intermediate coordinated to the Pd2+ center via the formation of a six-membeed ring structure. The acetoxy ethyl intermediate subsequently undergoes β-CH bond activation over the palladium ion and forms vinyl acetate along with a hydride ligand (Pd–H). The calculated barrier for C–H activation is 68 kJ/mol. Vinyl acetate subsequently desorbs, thus regenerating a free site. The resaturation of this vacancy with a ligand costs 85 kJ/mol. The desorption of VAM therefore appears to compete with β-C-H transfer as the rate-limiting process.

In the outer-sphere mechanism, the coordinated ethylene reacts with an acetate anion in the solution phase to form the acetoxy ethyl intermediate at the Pd center. Acetoxy ethyl subsequently undergoes a β-C–H transfer step which involves the coordinative transfer of a proton from ethyl acetate to an adjacent acetate anion bound to the Pd2+ center. This step liberates vinyl acetate along with acetic acid.

Figure 6.20 shows quite clearly that the outer-sphere mechanism is much less endothermic than the inner-sphere mechanism and hence the more likely path. β-Hydrogen transfer appears to be the limiting step for both mechanisms. Although the presence of solution helps to stabilize the transition state for this step in the outer-sphere process, the reaction is still fairly endothermic. In the outer-sphere mechanism, hydrogen does not have to transfer to a vacant site but can undergo a ligand-to-ligand transfer. This coupling reaction is clearly much more favored in the outer-sphere path. The reaction is negative order in acetate. This suggests that an acetate vacancy is necessary to carry out the reaction.

292 Chapter 6

Figure 6.20. Energy diagram of the innerand outer-sphere mechanisms, including solvent e ects.

Activation barriers are indicated by the small arcs; a barrier as low as the reaction energy is designated by “no act.” The structure number refers to models shown in Fig. 6.19[36].

The results are consistent with the literature, which indicates that VAM synthesis proceeds over homogeneous palladium acetate complexes via either an inneror an outersphere mechanism[35,44]. The reaction process is quite similar to that for Wacker chemistry, which is thought to proceed via an outer-sphere process. The rate-limiting step for both the innerand outer-sphere mechanisms appears to be the β-hydrogen transfer. This is consistent with experimental studies for vinyl acetate synthesis over homogeneous Pd acetate and with other theoretical studies carried out on oxidized Pd clusters. The activation barrier for the β-hydrogen transfer was calculated by DFT to be 67 kJ/mol, which is in good agreement with the experimental value of 70 kJ/mol found by Tamura and Yasui[44] for VAM synthesis over homogeneous Pd acetate. These comparisons, should be made very carefully, however, since the results are highly dependent on the actual reaction conditions.

Solvent e ects were found to be quite important in stabilizing charged complexes, especially for the outer-sphere mechanism. In general, outer-sphere reactions are found to have lower activation barriers than those for inner-sphere reactions. This is due to the enhanced charge stabilization due to the presence of solution. Solvent e ects play a small role in altering the energy landscape. The influence of solvation has been included and has been estimated using the reaction field theory expressions. Solvation corrections may vary between –1 and –12 kJ/mol.

In addition to introducing the concepts of innerand outer-sphere mechanisms, we have also described proton transfer between ligands without direct interference with the Pd2+ cation. This coordinative proton transfer helps to lower the activation barrier for β-CH transfer in the outer-sphere mechanism, thus providing a lower energy path than that available for the inner-sphere reaction route. Vinyl acetate desorption then becomes easier in the outer-sphere route with a desorption energy of only 45 kJ/mol. The outersphere mechanism is therefore preferred. The rate-controlling step in the outer-sphere mechanism appears to be that for β-CH transfer. The activation energy for the ethylene– acetate coupling reaction is significantly lower than that found in the inner-sphere. This is because the presence of solution stabilizes the anion, thus opening up a low-energy nucleophillic attack by the anion.

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 293

6.3.3 VAM Synthesis: Homogeneous or Heterogeneous?

It is clear that there is now evidence that VAM synthesis can occur via both homogeneous and heterogeneous pathways. Which of these mechanisms predominates is still actively

debated. Elegant reviews comparing the two mechanisms were given by Kragten[45] and by Reilly and Lerou[34] . Mosieev and Vargaftik[38] indicated that the active sites are the Pd0

species and that these sites lead selectively to vinyl acetate with essentially no production of acetaldehyde. Nakamura and Yasui[37] and Debellefontine and Besombes–Vailhe[46] et al., on the other hand, suggest that Pd1+ species in the form of Pd–OAc are the active species that catalyze the reaction. There is evidence that a supported liquid layer can form

under reaction conditions[40] and that VAM synthesis predominantly occurs over the Pd2+ species that form in this layer. Augustine and Blitz[47] showed that Pd(OAc)2 can form

over Pd crystallites. Crathorne et al.[48] suggested that a liquid layer which is 3 ML of acetate/acetic acid thick can form and that the presence of alkali metal acetates increases the absorption of AcOH. KOAc is speculated to be essential for VAM formation. KOAc is thought to immobilize acetic acid in the form of a KOAc melt, which can solubilize homogenous Pd2+ -acetate complexes that then carry out the chemistry. The presence of potassium and acetic acid help to limit the production of acetaldehyde. In addition, the liquid layer that forms can block the metal surface to ethylene exposure and thus suppresses ethylene combustion. Others, however, have indicated that the Pd2+ species that form in the liquid layer are not active and can actually lead to agglomeration and sintering since they are more mobile.

Recent in situ spectroscopic studies on the reactions of ethylene and acetic acid over homogeneous complexes and reactions carried out over single-crystal Pd(111) surfaces tend to suggest that the dominant path is over the metal surface. In situ ultraviolet– visible, Raman, and infrared experiments carried out on the homogeneous Pd acetate clusters by Kragten et al.[36b] show that although vinyl acetate is formed, the primary product is actually acetaldehyde, which occurs via Wacker chemistry. Water, which is the product from VAM synthesis, readily reacts with the homogeneous complexes to form acetaldehyde, thus significantly decreasing the overall selectivity. The thought is that although these homogeneous complexes may form, they are unlikely to catalyze the primary route to VAM.

Recently, Tysoe and Stacchiola[49a], showed that VAM can be readily formed under UHV conditions by the addition of ethylene over an acetate-covered Pd(111) surface. The acetate intermediates were formed by the dissociative adsorption of acetic acid at low temperature on the surface. The subsequent addition of ethylene leads to the titration of acetate from the surface. The primary product identified by mass spectrometry was vinyl acetate. The reaction was carried out over a narrow temperature range indicating an activation barrier of about 65 kJ/mol. More recently, the same group[49b] used in situ IR spectroscopy to show that reactions with labeled ethylene resulted in a shift of the IR band at 1788 cm−1, which is indicative of VAM, to 1718 cm−1. DFT calculations indicate that the band at 1788 cm−1 is from adsorbed VAM whereas the band at 1718 cm−1 is much more likely the adsorbed acetoxyethyl intermediate. This suggests that the Samanos mechanism, which proceeds via the direct coupling of ethylene with acetate over the Pd surface, is the dominant path rather than that which proceeds through the formation of surface vinyl intermediates and their coupling to form VAM.

The in situ Raman and UHV studies indicate that it is more likely that the metal surface, and not solution-phase homogeneous complexes, catalyzes the acetoxylation of ethylene to form vinyl acetate.

294 Chapter 6

6.4 Low-Temperature Ammonia Oxidation

In a second example, we examine reactions that relate to the Ostwald oxidation process, where NH3 is converted to NO with high selectivity. The reaction is typically run at high temperatures of around 1100 K over Pt/Rh alloy catalysts. The NO that forms is subsequently converted into nitric acid via a series of consecutive reaction steps. At lower temperatures, ammonia reacts to form N2 and N2O instead. The low-temperature conversion of ammonia to N2 would be much more desirable in that it would lower energy costs and, in addition, replace NO, an atmospheric pollutant, with N2, which is environmentally benign. We will describe here the low-temperature catalytic conversion of ammonia to form N2. For a review of high-temperature oxidation, in which coupling with gas-phase radical chemistry plays an important role, we refer to Ref. [50].

Here, we compare three di erent catalytic systems used to carry out the catalytic oxidation of ammonia: gas-phase oxidation over transition-metal heterogeneous catalysts, gas-phase oxidation within zeolites and electrocatalytic oxidation of ammonia.

Figure 6.21. Low-temperature oxidation of NH3 using transition-metal catalysts.

In this first section, we focus on heterogeneous transition-metal catalysis. Two di erent reaction paths can be distinguished in the activation of NH3 (see Fig. 6.21). The first distinguishes the direct path through the dissociation of ammonia and subsequent recombination of Nads atoms. The second reaction path involves the reaction of NH3, NO and O to form N2 and H2O (see also Chapter 5, Section 5.6.10).

Ammonia adsorbed on the transition-metal surface can react to form di erent NHx species. Both the presence of adsorbed oxygen and hydroxyl adsorbed on the metal surface can help promote this reaction (see Chapter 3, Section 3.8).

Adsorbed oxygen readily acts to promote the activation of NH3 to NH2. Surface hydroxyl intermediates, however, were found to be even more reactive than adsorbed oxygen and lower the barriers of all NHx decomposition reactions. At high temperatures all of the NHx intermediates dissociate to form nitrogen atoms, which can then recombine and desorb as N2 or react with adsorbed O and desorb as NO. In Table 6.1 we present a comparison of DFT-calculated energies found in the literature for some of the elementary reaction steps over di erent Pt surfaces. The results indicate that there is a very high sensitivity of the activation energies to surface structure. The reactivity of the (100) surface is dramatically increased over that of the (111) surface. As was discussed in Chapter 3, this increase in the rate is due to the removal of metal atom sharing between the fragments that form in the transition state.

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 295

Table 6.1. Computed activation energies in kJ/mol for elementary surface reactions relevant to ammonia oxidation.

Surface

————————————————————————–

M. Neurock, S.A. Wasileski, D. Mei, Chem. Eng. Sci. 59, 4703 (2004)

W. O ermans, R.A. van Santen, to be published.

a R. Burch, S. T. Daniells, P. Hu, J. Chem. Phys. 117, 2902 (2002) b A. Bogicevic, K. C. Hass, Surf. Sci. 506, L237 (2002)

c A. Eichler, F. Mittendorfer, J. Hafner, Phys. Rev. B 62, 4744 (2000) d A. Michaelides, P. Hu, J. Am. Chem. Soc. 122, 9866 (2000)

eM. Neurock, From First Principles To Catalytic Performance: Tracking Molecular Transformations, Presentation at Schloss Ringberg Conference, Tegernsee 11

september 2003

f Q. Ge, M. Neurock, J. Am. Chem. Soc. 126, 1551 (2004)

g E.H.G. Backus, A. Eichler, M.L. Grecea, A.W. Kleyn, M. Bonn, J. Chem. Phys. 121,

7946 (2004)

h H. Wang, R.G. Tobin, C.L. DiMaggio, G.B. Fisher, D.K. Lambert, J. Chem. Phys. 107,

9569 (1997)

i A. Eichler, J. Hafner, Chem. Phys. Lett. 343, 383 (2001)

The activation of ammonia with oxygen to produce NO is endothermic, whereas the activation of ammonia to form N2 in the gas phase is exothermic. The selectivity towards NO will, therefore, increase as the temperature is increased. However, the activation barrier to form adsorbed NO from adsorbed N and O is comparable to that for the recombination of two nitrogen adatoms to form N2. The only data that can be directly compared are those for the (111) surface, where there is a large enough dataset and the methodology applied is similar. These reactions preferentially occur at step edges. NO strongly adsorbs to the surface, so that at low temperatures NO likely does not appear as a product. Only N2 and N2O are formed as products at low temperature. As is seen from Table 6.1, N2O formation readily occurs at low temperature due to the recombination of adsorbed NO. N2O is weakly adsorbed and therefore desorbs once it is formed.

As was just mentioned, in addition to its reaction with ammonia, NO can react with itself or with a surface nitrogen adatom to form N2O: