Design of a promoter to enhance the stability of catalysts for hydrocarbon reactions 2

CHAPTER INTRODUCTION The continuous improvement of computational chemistry algorithms and the everincreasing computational resources have brought realistic first principles studies for industrially relevant catalytic reactions within reach. Theory can be used to help provide a molecular level understanding of the mechanism of the catalytic reaction and elucidate the electronic origin of catalyst promotion. Such understanding can lead to optimization of the industrial process, i.e. selection of improved process conditions, as well as the design of improved catalysts. Nowadays, the combination of density functional theory (DFT) calculations, kinetic measurements, and experimental investigations is being used increasingly for the design of novel/improved catalysts, such as steam reforming and ammonia synthesis catalysts (Hinneman and Nørskov, 2003; Zhang and Hu, 2002). Transition metal catalysts such as Fe, Co and Ni are widely used in hydrocarbon reactions because of the high activity and significantly lower cost in comparison with precious metal-based catalysts (Zonnevylle et al., 1990; Joyner et al., 1988). Especially Ni based catalysts are commonly used in chemical processes of natural gas activation, such as steam reforming (SR) and catalytic partial oxidation (CPO) of natural gas (Twigg, 1996; Ponec and Bond, 1995). Natural gas is gaining importance as an energy source and as a raw material for the petrochemical industry. The decreasing supplies of petroleum and more stringent environmental demands will further strengthen the importance of natural gas. In addition, natural gas conversion into hydrogen for the efficient production of electricity in fuel cells is attracting widespread research attention. Its high hydrogen content makes methane a particularly interesting raw material. Chemisorption and activation of methane on Ni catalysts is of considerable importance because SR and CPO of methane to produce syngas is the first step in several industrially important catalytic processes such as production of ammonia (Haber-Bosch), methanol and higher hydrocarbons (Fischer-Tropsch). However, coke formation on Ni surfaces is an important technological problem. Ni catalyzes SR and CPO of methane reactions, but it also catalyzes the formation of graphitic carbon deposits. Carbon deposition on the catalyst might cause loss of activity, while growth of filamentous carbon nanotubes can lead to reactor blocking, leading to regular shutdowns and production losses (Trimm et al., 1981; Reyniers et al., 1994). Enhancing the stability of Ni based catalysts has therefore been an area of intensive research, and various promoters have been proposed. One of the oldest proposals is to introduce trace amount (2 ppm) of H2S with the feed gas (Rostrup-Nielsen, 1984). This method is industrially implemented in the Sulfur Passivated Reforming (SPARG) Process and was developed by Rostrup-Nielsen. The sulfur selectively poisons the most active sites of the Ni catalyst, believed to be the step sites, leading to a small loss in the reforming activity. However, trace amount of sulfur affects the deactivation rate much more than the reforming rate (Andersen et al., 1987). More recently, promoters such as Au (Besenbacher et al, 1998; Bengaard et al., 2002), K (Rostrup-Nielsen, 1984), Sn (Nikolla et al., 2006) and B (Xu and Saeys, 2006) have been proposed and shown to improve the stability of Ni catalysts. The objective of this thesis is to design a promoter to enhance the stability of Ni-based catalysts. Thermodynamic and kinetic calculations were carried out to quantify the stability of different forms of chemisorbed carbon on a Ni catalyst, and to evaluate the diffusion of carbon atoms from the Ni surface to the first and second subsurface layer and to the Ni bulk. Boron is found to show similar chemisorption characteristics with carbon and is proposed to selectively block sites that initiate catalyst deactivation. Based on this molecular level understanding, boron is proposed as promoter to enhance the stability of Ni catalysts. In this thesis, first principles DFT investigations are combined with experimental validation and optimization to design improved Ni catalysts. This thesis is organized as follows. In chapter 2, an overview is given of the state-ofthe-art in first principles based design of metal catalysts. In chapter 3, the theory and the computational methods used in this work are discussed. In chapter 4, the stability of different forms of carbon that can exist on a Ni catalyst and the kinetics of carbon diffusion are addressed, and the effect of boron as a promoter to improve the coking resistance of Ni catalysts is proposed. A more detailed analysis of the effect of carbon and boron on the activity of a Ni catalyst is presented in chapter 5. In chapter 6, an experimental validation of the proposed effect of boron on the stability of a Ni catalyst during steam methane reforming is presented. Finally, the main conclusions of this work are summarized in Chapter 7. References Andersen, N.T., Topsøe, F., Alstrup, I., and Rostrup-Nielsen, J.R., J. Catal. 104, pp. 454. 1987. Bengaard, H.S., Nørskov, J.K., Sehested, J., Clausen, B.S., Nielsen, L.P., Molenbroek, A.M., and Rostrup-Nielsen, J.R., J. Catal. 209, pp. 365. 2002. Besenbacher, F., Chorkendorff, I., Clausen, B.S., Hammer, B., Molenbroek, A.M., Nørskov, J.K., and Stensgaard, I., Science 279, pp. 1913. 1998. Hinneman, B., and Nørskov, J.K., J. Am. Chem. Soc. 125, pp. 1466. 2003. Joyner, R.W., Darling, G.R., and Pendry, J.B., Surf. Sci. 205, pp. 513. 1988. Nikolla, E., Holewinski, A., Schwank, J., and Linic, S., J. Am. Chem. Sci. 128, pp. 11354. 2006. Ponec, V., and Bond, G.C., Catalysis by Metals and Alloys, Elsevier, New York, 1995. Reyniers, G.C., Froment, G.F., Kopinke, F.D., and Zimmerman, G., Ind. Eng. Chem. Res. 33(11), pp. 2584. 1994. Rostrup-Nielsen, J.R., J. Catal. 85, pp. 31. 1984. Trimm, D.L., Holmen, A., and Lindvag, O., J. Chem. Technol. Biotechnol. 31(6), pp. 311. 1981. Twigg, M.V., Catalyst Handbook, 2nd Ed, Manson Pub., London, 1996. Xu, J., and Saeys, M., J. Catal. 242, pp. 217. 2006. Zhang, C.J., and Hu, P., J. Catal. 116, pp. 4281. 2002. Zonnevylle, M.C., Geerlings, J.J.C., and van Santen, R.A., Surf. Sci. 240, pp. 253. 1990. CHAPTER FIRST PRINCIPLES BASED DESIGN OF METAL CATALYSTS 2.1 Introduction Traditionally, heterogeneous catalysis has largely been an experimental field. While this is still true, the powerful computational resources available today and the continuous improvement of computational chemistry algorithms and software are providing new tools for the study and development of catalytic systems. Using well-chosen and sufficiently accurate quantum chemical calculations, scientists have been able to provide new insights into reaction pathways, to predict properties of catalysts that have not been synthesized, and to bring information for a given system from many different experimental techniques into a coherent picture. Theory can be used to help provide a molecular level understanding of the reaction mechanism and elucidate the electronic origin of catalyst promotion. Such understanding can lead to optimization of industrial processs, i.e. selection of improved process conditions, as well as the design of improved catalysts. It has been demonstrated how first principles calculations can provide a detailed understanding of the elementary steps of catalytic processes. This molecular level insight was used to construct fundamental kinetic models from first principles for industrially important reactions (Saeys et al. 2003; Neurock and van Santen, 2000; Neurock et al., 2000). Recently, in particular the Nørskov group has demonstrated how collaboration between applied catalysis, surface science and theory can lead to the design from first principles of improved catalyst for established processes such as steam reforming and ammonia synthesis (Besenbacher et al., 1998; Honkala et al., 2005). In this chapter, we will review models using first principles computation to represent the surface of catalysts. Here, results from first principles based computation for several notable processes which are metal catalyzed such as hydrogenation of olefins and aromatics, steam reforming, ammonia synthesis and selective catalytic oxidation will be presented. 2.2 Models of catalytic surface The choice of an appropriate model to represent a catalytic surface or active site is important as it can have significant effects on the accuracy of results. Cluster calculations use a limited number of atoms to model the catalytic surface. These models are computationally convenient because they employ atomic or molecular orbital basis sets to satisfy the boundary condition of zero electron density at infinite distance from the cluster. It is also possible to study low coverage and calculate vibrational frequencies with cluster calculations. Unfortunately, sufficiently large clusters need to be used to obtain a reliable representation of the electronic band structure of a catalyst, especially in the case of a metal. To partly overcome this problem, embedded cluster models have been proposed, where the central atoms (the active site) are treated with an accurate computational method and the surrounding atoms are treated at a lower, often semi-empirical level. Figure 2.1. Three approaches and examples for modeling chemisorption and reactivity on surfaces. (Left) cluster approach, maleic anhydride on Pd; (center) embedding scheme: ammonia adsorption in a zeolite cage; (right) periodic slab model: maleic anhydride adsorption on Pd(111). (Neurock, 2003) On the other hand, slab models use periodic boundary conditions to model extended surfaces. In these models, the surface is represented by a unit cell which is repeated in dimensions. The slab models avoid the electronic structure artifacts that sometimes trouble cluster calculations. Typically, the unit cell consists of a two-dimensional rectangular cluster of about 10 atoms and 3-5 atom layer thickness with a large vacuum layer on top of it, resulting in a model consisting of an infinite 2D slab of 3-5 layer thickness, separated from the next slab by a vacuum layer of 1-2 nanometer. To study low coverage adsorption using periodic slabs, very large unit cells are required. The slab models require the use of periodic basis sets to match the boundary conditions, and often plane waves are used. The convergence can however be slow, depending on the size of the plane wave basis set. The three approaches are illustrated in Figure 2.1. 2.3 Hydrogenation of olefins and aromatics The adsorption of ethene (Fahmi and van Santen, 1996; Shen et al., 1999; Watwe et al., 1998), ethyne (Watwe et al., 1998; Clotet and Pacchioni, 1996; Medlin and Allendorf, 2003), propene (Valcarcel et al., 2002a), propyne (Valcarcel et al., 2002b), cyclohexene (Saeys, 2002), cyclohexadiene (Saeys, 2002; Saeys and Reyniers, 2002), and benzene (Mittendorfer and Hafner 2001; Saeys et al., 2002) on various transition metals (Ni, Pd, Pt) has been studied from first principles. Reaction path studies have been carried out for gas phase hydrogenation of ethene (Pallassana et al., 1999; Pallassana and Neurock, 2000; Neurock and van Santen, 2000; Neurock et al., 2000), ethyne (Sheth et al., 2003), and benzene (Saeys, 2002; Saeys et al., 2003; Morin et al., 2003). Most of the results reported are based on the most thermodynamically stable (111) facet. However, a few studies also employ the (100) and (110) facets. These ideal surfaces are mostly studied with periodic slabs with a few employing cluster models. Ethene hydrogenation has been studied in the greatest detail, in particular by Neurock et al A series of DFT calculations was performed by Neurock et al. to help elucidate the nature of the active sites, understand the kinetics and establish the source of the structure insensitivity. The results indicate that the basic mechanism follows the ideas proposed by Horiuti and Polanyi (1934). At low or moderate coverages, hydrogen adds via a classical homogeneous catalyzed reductive-elimination step that involves hydrogen insertion into a metal carbon bond to subsequently form an ethyl intermediate (Neurock and van Santen, 2000; Neurock et al., 2000). The three-center (Pd-C-H) transition state involves breaking of the metal-hydrogen and metal-carbon bonds. The transition state is early along the reaction path whereby there is still a strong interaction between hydrogen and the metal as well as the carbon and the metal. The C-H bond is still quite long. The resulting transition state structure is consistent with results found for homogeneously catalyzed hydrogenation. The ethyl intermediate that forms reacts in a very similar way to yield ethane. The transition state to ethane is remarkably similar to that for ethyl. In fact, most of the hydrogenation reactions that have been examined in the literature are quite similar. The important point is that the active surface complex involves only one or perhaps two metal atoms. This is likely one of the reasons hydrogenation reactions are structure insensitive. Subsequent calculations over the Pd(100) surfaces have been performed yielding barriers similar to that on Pd(111). Ethene hydrogenation was examined on well-defined model Pd(111) surface. At low coverage, the predicted intrinsic activation barriers for ethene and ethyl hydrogenation are quite similar at 72 and 71 kJ/mol, respectively. The π-bound intermediate (ethene sits atop a single metal atom) is first converted to the di-σ-species (ethene binds parallel to one of the bridge metal-metal forming two σ-metal-carbon bonds) before it reacts with hydrogen. At higher surface coverages, the activation barriers for hydrogenation are reduced by the repulsive interactions between neighboring hydrocarbon and hydrogen intermediates. The higher coverages lead to the population of π-bound ethene states that provide a hydrogenation path which has a lower activation barrier at 36 kJ/mol (Neurock and van Santen, 2000; Neurock et al., 2000). The reaction from the π bound state proceeds through a "slip-type" mechanism that was proposed in the homogeneous literature. The only difference here is that the availability and participation of other surface metal atoms that can assist the reaction on the surface. The transition state takes place over two metal atoms to form a more “five-center like” intermediate. The classical homogeneous slip mechanism takes place over one metal atom to form a “four-center” transition state. Quantum chemical simulations can provide critical information on the nature of the active sites, the bonding, and the activation, reaction energy for individual steps. This is only part of the picture with regard to catalytic performance. A more complete analysis requires elucidating the formation and consumption of all reactants, intermediates, and products along with simulating the full set of possible reaction steps to establish what actual controls the outcome. The Monte Carlo (MC) simulation enables to explicitly include the atomic surface structure and track the dynamics associated with atomic transformations in the adsorbate surface layer including surface diffusion. A representative snapshot of the surface at some instant in time for ethene hydrogenation over Pd is shown in Figure 2.2. The simulation of ethene hydrogenation over Pd in a continuous flow reactor system nicely matches those found experimentally. The apparent activation energy was calculated to be 40 kJ/mol which is within the range of 30-40 kJ/mol reported for fixed-bed experimental systems in the literature (Davis and 10 Figure 6.3. CH4 conversion (a and c) and normalized conversion (b and d) as a function of time on stream. Reaction conditions for Fig. a, b, c and d: T=800ºC, P = atm, CH4:H2O:N2=10:10:1, methane flowrate = 50 Nml/min, catalyst weight w = 20 mg (a and b) and 10 mg (c and d), and GHSV = 330,000 cm3/hr·gcat (a and b) and 660,000 cm3/hr·gcat (c and d). Fitted rate coefficients for a linear deactivation model (eq. 6.4) are given in b and d. Reaction conditions for Fig. e, see Fig. 6.4. 70 wt% B CO2 intensity (a.u.) 60 0.5 wt% B 1.0 wt% B 50 40 30 20 10 750 800 850 900 950 1000 1050 Temperature (K) Figure 6.4. TPO profiles for 15 wt% Ni/ γ-Al2O3 with 0.0, 0.5 and 1.0 wt% boron after reaction. Reaction conditions: T=750ºC, P=1 atm, CH4:H2O:N2=1:1:1, methane flowrate = 50 Nml/min, t=450 min, catalyst weight = 50 mg, and GHSV = 180,000 cm3/hr·gcat. Amount of CO2 evolved: 1.70 mmol/gcat (unpromoted catalyst), 0.31 mmol/gcat (0.5 wt% B), and 0.34 mmol/gcat (1.0 wt% B). 144 for all catalysts, the presence of carbon deposits is illustrated by the SEM images and the TPO profiles. Though SEM does not allow quantifying the amount of deposited carbon, it provides qualitative information. After 450 on stream, the SEM images show significant formation of filamentous carbon on the unpromoted Ni catalysts, while less carbon is observed for the promoted catalysts. The TPO curves indicate that boron reduces the amount of deposited carbon about 5-fold and shifts the TPO curves to slightly higher temperatures. TPO peaks at 925-963 K have been reported earlier, and have been attributed to filamentous carbon (Goula et al., 1996; Wang et al., 2002). The reduction in filamentous carbon deposits by boron promotion is qualitatively confirmed by the SEM images. 145 (a) (b) (c) Figure 6.5. SEM images of 15%Ni/γ-Al2O3 catalysts after steam reforming. (a) unpromoted; (b) 0.5 wt% B; (c) 1.0 wt% B. Reaction conditions: see Fig. 6.4. 146 6.6 Summary Theoretical DFT studies indicate that boron and carbon exhibit similar chemisorption preferences on a Ni catalyst and, therefore, boron can be used to selectively block step and subsurface octahedral sites. This is proposed to enhance the stability of Ni catalysts by reducing the nucleation of graphene islands from steps, and by reducing the diffusion of carbon to the subsurface sites and subsequently to the Ni bulk. To validate the theoretical predictions, 15 wt% Ni/γ-Al2O3 catalysts, promoted with 0.5 and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming. The characterization studies indicate that boron might adsorb on both the Ni particles and the γ-Al2O3 support, and that 1.0 wt% B should be sufficient to block the step and subsurface sites. Experiments at 800 ºC and at a GHSV of 330,000 cm3/hr·gcat demonstrate that promotion with 1.0 wt% B not only reduces the first order deactivation rate coefficient by a factor of 3, but also enhances the initial conversion from 56% to 61%. At a higher GHSV of 660,000 cm3/hr·gcat, 1.0 wt% B reduces the activity loss after 10 hours from 70% to 30%. A TPO and SEM study of the catalysts after 450 minutes of reaction further confirmed that boron assists in preventing carbon buildup. 147 6.7 References Bengaard, H.S., Alstrup, I., Chorkendorff, I., Ullmann, S., Rostrup-Nielsen, J.R., Nørskov, J.K., J. Catal. 187, pp. 238. 1999. Bengaard, H.S., Nørskov, J.K., Sehested, J., Clausen, B.S., Nielsen, L.P., Molenbroek, A.M., Rostrup-Nielsen, J.R., J. Catal. 209, pp. 365. 2002. Besenbacher, F., Chorkendorff, I., Clausen, B.S., Hammer, B., Molenbroek, A.M., Nørskov, J.K., and Stensgaard, I., Science 279, pp. 1913. 1998. Boudart, M., and Djéga-Mariadassou (Eds), Kinetics of heterogeneous catalytic reactions, Princeton university Press, Princeton, N.J., 1984. Burke, A.R., Brown, C.R., Bowling, W.C., Glaub, J.E., Kapsch, D., Love, C.M., Whitaker, R.B., and Moddeman, W.E., Surf. Interface Anal. 11, pp. 353. 1988. Chen, L., Lu, Y., Hong, Q., Lin, J., and Dautzenberg, F.M., Appl. Catal. A 292, pp. 295. 2005. Chin, Y.H., King, D.L., Roh, H.S., Wang, Y., and Heald, S.M., J. Catal. 244, pp. 153. 2006. Froment, G.F., and Bischoff, K.B., Eds., Chemical reactor analysis and design, John Wiley & Sons, Second Edition, 1990. Goula, M.A., lemonidou, A.A., and Efstathiou, A.M., J. Catal. 161, pp. 626. 1996. Grim S.O., Matienzo L.J., Swartz W.E., J. Am. Chem. Soc. 94, pp. 5116. 1972. Helveg, S., López-Cartes, C., Sehested, J., Hansen, P.L., Clausen, B.S., Rostrup-Nielsen, J.R., Abild-Pedersen, F., and Nørskov, J.K., Nature 427, pp. 426. 2004. Hillebrecht, F.U., Fuggle, J.C., Bennett, P.A., and Zolnierek, Z., Phys. Rev. B 27, pp. 2179. 1982. 148 Lemaitre, J.L., Menon, P.G., Delannay, F., and Delannay, F. (Ed), Characterization of Heterogeneous Catalysts, Vol. 15, Dekker: New York, 1984. Moulder, J.F., Stickle, W.F., Sobol, P.E., and Bomben, K.D., Handbook of X-ray Photoelectron Spectroscopy, Chastain, J., King Jr., C., Eds., Physical Electronics, Inc.: Eden Prairie, MN, 1995. Nikolla, E., Holewinski, A., Schwank, J., Linic, S., J. Am. Chem. Sci. 128, pp. 11354. 2006. NIST Chemistry WebBook, http://webbook.nist.gov/chemistry/ Rostrup-Nielsen, J.R., Catalytic Steam Reforming. In Catalysis-Science and Technology, Vol5. Springer, Berlin, 1984a. Rostrup-Nielsen, J.R., J. Catal. 85, pp. 31. 1984b. Rostrup-Nielsen, J.R., in “Catalysis Science and Technology” (J.R. Andersson and M. Boudart, Eds), Vol.5 Chapter 1.Springer-Verlag, Berlin, 1984c. Rostrup-Nielsen, J.R., Sehested, J., Nørskov, J.K., Hydrogen and Syngas by Steam Reforming. In Advances in Catalysis, Volume 47 (B.C. Gates, H. Knözinger, Eds.) Elsevier Science, 2002. Sarantaridis, D., and Atkinson, A., Fuel Cells 7(3), pp. 246. 2007. Schreifels, J.A., Maybury, P.C., and Swartz, W.E., J. Catal. 65, pp. 195. 1980. Stranick, M.A., Houalla, M., and Hercules, D.M., J. Catal. 104, pp. 396. 1987. Tamaki, J., Takagaki, H., and Imanaka, T., J. Catal. 108, pp. 256. 1987. Trimm, D.L., Catal. Today 49, pp. 3. 1999. Wang, P., Tanebe, E., Ito, K., Jia, J., Morioka, H., Shishido, T., and Takehira, K., Appl. Catal. A 231, pp. 35. 2002. 149 Wei, J., and Iglesia, E., J. Catal. 224, pp. 370. 2004. 150 CHAPTER CONCLUSIONS AND FUTURE SUGGESTIONS In this thesis, density functional calculations were used to develop a molecular level understanding of the deactivation of Ni catalysts by carbon deposition. Based on the molecular level understanding, a boron promoter was proposed to enhance the stability of Ni catalysts. To validate the theoretical predictions, a series of boron promoted supported Ni catalysts were synthesized and tested during methane steam reforming. The main findings of this study can be summarized as follows. Chemisorbed graphene is the most stable form of carbon on a Ni catalyst, with a carbon binding energy of –760 kJ/mol. On-surface carbon atoms are relatively unstable with binding energies of around –660 kJ/mol. Hence, the driving force to form graphene is significant. Graphene formation is generally unwanted, as graphene sheets cover active sites and lead to a loss in activity. However, the formation of graphene islands resembles a crystallization process and small islands are thermodynamically unstable. Step sites provide the most stable adsorption sites for carbon atoms and might act as nucleation sites for the formation of graphene islands. Subsurface octahedral sites are also more stable than on-surface sites, and subsurface carbon easily builds up under typical steam reforming conditions. It was shown that carbon diffusion to the octahedral sites of the first subsurface layer is thermodynamically favorable until about 75% of the subsurface sites are occupied. 151 Activation barriers for diffusion to the subsurface octahedral sites decrease with onsurface carbon coverage, and typical barriers of less than 110 kJ/mol were calculated for moderate to high surface coverages. Diffusion from the subsurface sites to the Ni bulk was found to be thermodynamically favorable if more than 50 % of the subsurface octahedral sites are occupied. Such diffusion is however kinetically limited with typical barriers above 150 kJ/mol. For carbon concentrations above 50 % in the first subsurface layer, calculations indicate that the Ni lattice is likely to expand. This expansion decreases the diffusion barriers to below 100 kJ/mol and opens up a diffusion path. Hence, carbon diffusion into Ni catalysts is proposed to be a gradual process, first partially filling the subsurface octahedral sites, before diffusing to the bulk. Subsurface carbon increases the barrier for methane activation from 91 kJ/mol to 143 kJ/mol, resulting in a loss in catalyst activity. Carbon at the step sites also increases the activation barrier at these sites, from 57 kJ/mol to 93 kJ/mol. Boron was found to enhance the stability of Ni catalysts by selectively blocking subsurface and step sites. Boron has a strong preference for subsurface octahedral sites and causes a reconstruction of the Ni(111) surface, creating a step-like surface. This reconstruction lowers the activation energy for methane activation to 64 kJ/mol. Boron adsorption at the step sites increases the barrier for methane activation from 57 to 70 kJ/mol. The variations in activity are well described by the d-band model. Ni catalysts promoted with 0.5 wt% and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming, to evaluate the effect of boron on the 152 deactivation behavior. Characterization of the unreduced catalyst indicates that boron distributes between the Al2O3 support and the Ni catalyst particles, and that 1.0 wt% B might be sufficient to occupy all the step and subsurface sites. Catalytic studies at 800 ºC, atm, a methane stoichiometric to steam ratio, and a high GHSV of 660,000 cm3/hr·gcat show that promotion with 1.0 wt% B significantly enhances the residual activity after 10 hours on stream from 30% of the initial activity for the unpromoted catalyst to 70%. %. In addition, promotion with 1.0 wt% B increases the initial methane conversion from 56% for the unpromoted catalyst to 61 % at a lower space velocities of 330,000 cm3/hr·gcat. Temperature Programmed Oxidation and Scanning Electron Microscopy studies confirm the formation of coke and showed that 1.0 wt% boron reduces the amount of deposited carbon by 80%. Suggestions for future work In this thesis, methane activation following a classical, non-assisted, reductive elimination mechanism was studied, and CH4 activation was assumed to be the ratelimiting step in methane steam reforming (SR) and methane catalytic partial oxidation (CPO), following careful experimental studies. However, an alternative, oxygenassisted CH4 activation mechanism could be envisioned when the oxygen surface coverage is sufficiently high, and other steps in the conversion of CH4 to CO could become rate controlling, e.g. the activation of CH. In order to construct detailed microkinetic models for methane SR and CPO these additional steps would have to be considered. Secondly, in this thesis we used molecular modeling to gain insight into key steps in catalyst deactivation, and to propose boron as a possible promoter. These ideas 153 were then tested with real-world catalysts under industrially relevant conditions. To further help bridge the gap between these two types of investigations, a surface science approach, possibly using model catalyst systems, would be extremely useful. Effect of the oxygen surface coverage on the dissociative adsorption of methane. Dissociative adsorption of alkanes is a key step in many hydrocarbon processes. Dissociative adsorption of methane has been proposed as the rate limiting step in both SR and CPO. However, molecular details of the dissociative adsorption of alkanes on oxygen covered surfaces are not yet well understood. Recent studies indeed indicate that oxygen might affect the turn-over frequency for methane activation. Two mechanisms have been proposed for dissociative methane adsorption on an oxygen covered catalyst: (a) A classical, non-assisted reductive elimination step A metal atom is inserted in a methane C-H-bond to form a hydrogen atom and a methyl species. This mechanism is identical to the mechanism on a clean surface and has been studied in detail for different surfaces. In this mechanism, dissociation of methane is expected to be the rate limiting step. The presence of oxygen might however change the electronic structure of the metal surface and affect the kinetics of methane activation. (b) Oxygen-assisted dissociative adsorption In this mechanism, the chemisorbed oxygen atoms participate actively and abstract a hydrogen atom from the approaching methane to form a hydroxyl group and adsorbed methyl species. In this mechanism, the role of surface hydroxyl groups is clearly important. OH surface species are known to influence catalytic activity [Zaera, 2003], 154 but the details of this effect are not well understood. It has been reported that surface hydroxyl groups play an important role in the partial oxidation of propane, in particular for reactions involving 2-propyl and 2-propanol [Zaera et al., 1999]. Alternatively, it is possible that the O2 precursor, rather than the chemisorbed oxygen atom, abstracts a hydrogen atom from the approaching CH4, since the O2 precursor state is stable on a Ni catalyst. Theory could be used to distinguish these mechanisms. To evaluate the relative kinetics, methane chemisorption on clean Ni surfaces, chemisorption near oxygen atoms, and oxygen-assisted chemisorption should be studied. Kinetics of CH activation and effect of CHx species on graphene formation In this thesis, CH4 dissociation is taken to be the rate-limiting step for methane SR and CPO, and only fully dehydrogenated surface carbon species Cx are considered in carbon deposition. Surface carbon atoms are generally accepted as key reaction intermediates in the conversion of hydrocarbons to syngas. However, other carbon species, such as CHx, might be envisioned to participate in graphene formation and Ni catalyst deactivation. Again, such alternative mechanisms leading to graphene would be interesting to study with DFT. Recently, C-H activation for surface CH species was proposed as a rate controlling step for methane CPO on Rh(111). The stability and high concentration of surface CH species could then lead to aldehyde-type surface intermediates, CHO. This step was 155 shown to be kinetically and thermodynamically more favorable than C-H activation [Inderwildi et al., 2007]. This alternative pathway might also play a role in CH4 activation on Ni catalysts. Ab initio reaction path analysis and microkinetic modeling of methane SR and CPO. The first principles data obtained from the previous studies could be used to perform an ab initio reaction path analysis and construct microkinetic models for methane SR and CPO. This could provide further insight in the reaction pathways and the rate controlling steps under different reaction conditions. Important questions such as the presence of a most abundant reaction intermediate (MARI), the relative importance of the classical dehydrogenation and oxygen assisted dehydrogenation pathways, and the location of possible rate controlling steps could be studied. Finally, the kinetic and thermodynamic parameters can be implemented into a microkinetic model and possibly tuned to simulate experimental data. Application of surface science techniques to bridge the gap between DFT and reactor studies. DFT is a very powerful technique to evaluate proposals about stability, reactivity, mechanism, nature and structure of active sites in catalysis on a molecular level. Such molecular level information is hard to determine experimentally. DFT is often argued to be a zero-temperature, zero-pressure technique, however, catalysts such as the boron promoted supported Ni catalysts proposed in this work, are 156 tested and operated at very different conditions, e.g. 800 °C and to 30 atm for methane SR. In addition, DFT studies are typically performed for extended, periodic and ideal surfaces (e.g., the Ni(111) and Ni(211) surface in this thesis), though defects can be considered as well. Industrial catalysts are rarely this simple, and typically exhibit various surface facets, steps and defects. To provide a better link between the DFT studies and industrial catalysis, surface science studies on idealized single crystals could serve as a bridge. To experimentally evaluate DFT predictions, such as the location of carbon and boron atoms on a Ni catalyst, the adsorption and decomposition of carbon and boron precursors on single crystal surfaces, such as Ni(111) and Ni(211), could be studied under ultrahigh vacuum (UHV) conditions using techniques such as scanning tunneling microscopy (STM), low energy electron diffraction (LEED). In this thesis, XPS was used to determine the chemical and electronic nature of the catalyst surface elements, B and Ni, for the calcined and reduced catalysts, but surface science techniques such as STM and LEED were not used. Today, the need for a molecular level understanding of heterogeneous catalyst is greater than ever. Breakthrough advances in catalyst selectivity, activity, and stability will only occur if we can understand and design the active sites at the molecular level. Molecular modeling not only provides information and insights that may be difficult to obtain experimentally, it can begin to replace expensive experiments in some cases. Although DFT can begin to help understand the origin of catalyst activity and promotion, there are large gaps between the DFT simulations and industrial application. A close 157 interaction between modeling and experiment is therefore vital. In this thesis, it was demonstrated how collaboration between DFT simulations and experimental investigations can lead to the design of improved Ni catalysts. This approach is not limited to metal catalysis, but has the potential to be extended to a large number of industrially important catalyst systems, such as metal sulfides, metal oxides and zeolites. The design of new catalytic materials using DFT simulations, followed by experimental validation and optimization is expected to become a valuable tool for the design of improved catalysts. 158 Reference Inderwildi, O. R., Jenkins S. J., King, D. A., J. Am. Chem. Sci., 129 (6), pp. 1751. 2007. Zaera, F., Catal. Today 81, pp. 149. 2003. Zaera, F., Gleason, N. R., Klingenberg, B., Ali, A. H., J. Mol. Catal. A 146, pp. 13 1999. 159 [...]... periodic DFT calculations His group's research, recently in collaboration with Haldor Topsøe A/ S, has led to a detailed understanding of the elementary steps and the origin of catalyst promotion, as well as to the design of a new, improved catalyst The starting point to the design of an ammonia catalyst is the volcano-shaped relation between the ammonia synthesis activity of different catalysts and their nitrogen... 2. 3) A reaction path analysis based on DFT calculations indicates that there is a dominant reaction path, along which the activation energy of every elementary step is at least 15 kJ/mol lower than along an alternative path (Saeys, 20 02; Saeys et al 20 03) The dominant reaction path is shown in bold in Figure 2. 3 Along the dominant reaction path, the addition of the fifth hydrogen to benzene to form the. .. surfaces They also calculated that the N2 dissociation energy on this alloy is intermediate between the dissociation energies on the pure metal components Finally, they used a Co3Mo3N catalyst to demonstrate experimentally that this alloy has an ammonia synthesis activity comparable to that of the best industrial catalysts (Jacobsen et al., 20 02; Boisen et al., 20 02) 17 The maximum of the volcano curve... the TOF for ethene hydrogenation was changed less than a factor of 2 by increasing Au to 40% in the Pd/Au alloy The two important points in this system are that the reaction occurs over one to two metal atom centers and that the reaction environment near the active site is important Benzene adsorption and hydrogenation was studied in detail by Saeys et al (20 02; 20 03) A fundamental kinetic model was... (Besenbacher et al., 1998; Bengaard et al., 20 02) As a possible solution to this problem, Besenbacher et al (1998) considered a gold-doped nickel catalyst Au was found to substantially improve stability of Ni catalysts under n-butane steam reforming (Figure 2. 7) They also performed theoretical calculations for methane steam reforming They calculated the energy barriers of methane activation, when 0 .25 ... states R i are connected by springs The total force on an atom is however not just the sum of the true and the spring force Instead, the total force is the sum of the spring force along the local tangent and the true force perpendicular to the local tangent It turns out that the method is stable over many orders of magnitude of the spring constant The main challenge in using the NEB method is to obtain... graphite formation 2. 6 Selective catalytic oxidation A microkinetic model for ethene epoxidation was derived by Linic and Barteau (20 0 3a and 20 03b) using surface science experiments and DFT calculations Theory was used to study the various possible reaction paths It was found ethene adsorbs on top of an oxygen atom to form a surface oxametallacycle intermediate This intermediate reacts to form ethene... regressed to accurately model laboratory scale data for the hydrogenation of toluene over a Pt catalyst The optimized hydrogen adsorption enthalpy of – 62 kJ/mol is consistent with the value from experimental and theoretical studies of high coverage hydrogen adsorption 14 2. 4 Ammonia synthesis The synthesis of NH3 is probably the most studied reaction in heterogeneous catalysis The best elementary metal catalysts, ... decrease in the surface coverage of hydrogen will act to 11 lower the rate On the other hand, the presence of gold weakens the metal-hydrogen and metal-carbon bonds to increase the rate of reaction These two factors tend to compensate each other and the activity remains essentially the same (Mei et al., 20 03) This is consistent with the experimental results by Davis and Boudart (1991), who show that the. .. information to be provided are the atomic numbers and positions of the atoms within the system In contrast, empirical or semiempirical approaches require a model for the interactions between the atoms to be supplied The parameters of these models are usually derived by fitting the outcome of simulations to experimental data 3.1.1 Fundamentals The ultimate goal in most quantum chemical calculations is to solve . (Nikolla et al., 20 06) and B (Xu and Saeys, 20 06) have been proposed and shown to 2 improve the stability of Ni catalysts. The objective of this thesis is to design a promoter to enhance the stability. stability of Ni-based catalysts. Thermodynamic and kinetic calculations were carried out to quantify the stability of different forms of chemisorbed carbon on a Ni catalyst, and to evaluate the. led to a detailed understanding of the elementary steps and the origin of catalyst promotion, as well as to the design of a new, improved catalyst. The starting point to the design of an ammonia