Chemical Engineering Science 62 (2007) 5039–5041www.elsevier.com/locate/ces

First principles study of the coking resistance and the activity of a boronpromoted Ni catalyst

Jing Xu, Mark Saeys∗

Department of Chemical and Biomolecular Engineering, 4 Engineering Drive 4, National University of Singapore, Singapore 117576, Singapore

Received 16 June 2006; accepted 25 November 2006Available online 8 December 2006

Abstract

Based on first principles density functional theory calculations, boron is proposed as a promoter to improve the coking resistance of Ni-basedcatalysts. Three types of chemisorbed carbon are distinguished on the Ni(1 1 1) surface: on-surface carbon is an important reaction intermediate,while both bulk carbon and graphene islands are unavailable for reaction and might lead to catalyst deactivation. Promotion by small amountsof boron was found to inhibit the formation of bulk carbon and weaken the on-surface carbon binding energy, possibly slowing down theformation of graphene islands. To confirm the activity of the boron promoted Ni catalyst, the activation energy for methane activation wascalculated. A modest increase by 12 kJ/mol was found upon boron promotion.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Catalysis; Computational chemistry; Coking; Ni catalysts; Boron promoter; Rational catalyst design

1. Introduction

First principles-based modeling is rapidly becoming an im-portant tool for the rational design of novel catalysts (Jacobsenet al., 2001; Xu and Saeys , 2006) and for kinetic modeling ofcatalytic processes (Honkala et al., 2005; Saeys et al., 2005). Itallows quantifying, on a molecular level, the influence of sur-face structure, electronic properties and promoters on the re-action mechanism. Though first principles-based modeling hasnot yet reached chemical accuracy for heterogeneous catalyticreactions, it begins to provide important leads for the design ofheterogeneous catalysts.

Natural gas will become increasingly important as a feed-stock for the chemical industry in the near future, as a resultof tightening supplies of liquid hydrocarbons and more strin-gent environmental demands (Venkatamaran et al., 1998). Sincemethane is a relatively unreactive molecule, many studies onthe utilization of natural gas focus on methane activation. Be-cause of the relatively low price as compared to Pt, Pd and Rh

∗ Corresponding author. Tel.:+65 6516 5826; fax: +65 6779 1936.E-mail address: chesm@nus.edu.sg (M. Saeys).

0009-2509/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2006.11.050

and the good catalytic activity, Ni-based catalysts have receivedconsiderable attention as a catalyst for methane activation pro-cesses such as stream reforming and catalytic partial oxidationof methane (Rostrup-Nielsen, 2000).

Unfortunately, catalyst deactivation by coking is especiallysevere for Ni-based catalysts. Three forms of chemisorbed car-bon can be distinguished on the Ni catalyst: on-surface carbon,carbide-like bulk carbon and graphene islands (Abild-Pedersenet al., 2006; Xu and Saeys , 2006). On-surface carbon is an im-portant reaction intermediate, whereas both graphene islandsand bulk carbon might lead to catalyst deactivation and shouldbe prevented. Studies have indicated that step sites on the Nicatalyst are nucleation sites for graphene formation (Bengaardet al., 2002; Abild-Pedersen et al., 2006). Promoters like potas-sium, sulfur, and gold selectively block the step sites and havebeen proposed to improve coking resistance (Bengaard et al.,2002). To prevent the formation of bulk carbon, boron was re-cently proposed as a promoter (Xu and Saeys , 2006). It wasfound that boron selectively blocks subsurface sites, therebypreventing carbon diffusion into the Ni particle. Recently, Chenet al. (Chen et al., 2005) have experimentally tested the effectof boron promotion of Ni-based catalysts during the catalyticpartial oxidation of methane and observed an improved cokingresistance.

5040 J. Xu, M. Saeys / Chemical Engineering Science 62 (2007) 5039–5041

In this work, we have applied first principles modeling tostudy the different forms of carbon and their interconversionon an Ni(1 1 1) surface. Based on a molecular level under-standing of the coking mechanism, the addition of subsurfaceboron is proposed to improve the coking resistance of Ni-basedcatalysts. Additional calculations were performed to confirmthe activity of the boron promoted Ni catalyst for methaneactivation.

2. Computational methods

Carbon and boron chemisorption energies were calculatedusing periodic spin-polarized density functional theory (DFT)with the Perdew-Wang 91 (Perdew et al., 1992) functionalas implemented in the Vienna Ab initio Simulation Pack-age (VASP) (Kresse and Hafner, 1993, 1994; Kresse andFurthmüller, 1996a,b). The calculations were performed usinga plane wave basis, with a cut-off kinetic energy of 400 eV.Projector-augmented-wave (PAW) (Blöchl, 1994; Kresse andJoubert, 1999) pseudopotentials were used to describe the innershell electrons. The Ni catalyst was modeled as a four-layeredslab where the topmost two layers are allowed to relax and theremaining layers are fixed at their bulk positions.

The nudged elastic band (NEB) method (Jónsson et al.,1998) was used to determine minimum energy paths (MEP) formethane activation and the corresponding activation energies.Nine images were used to describe the reaction path betweenreactants and products.

3. Results and discussion

To gain a better understanding of the carbon deposition onan Ni(1 1 1) surface, the relative stability of the different typesof chemisorbed carbon is discussed first. Next, the effect ofsubsurface boron on the stability of chemisorbed carbon andon the catalyst activity is addressed.

3.1. Carbon chemisorption on the Ni(1 1 1) surface

Three types of chemisorbed carbon were distinguished onthe Ni(1 1 1) surface. On-surface carbon was found to be rel-atively unstable. Diffusion to the octahedral sites of the Nibulk is thermodynamically preferred by 50–120 kJ/mol and thecorresponding activation energy rather low at about 70 kJ/mol(Xu and Saeys , 2006). Extended graphene islands are evenmore stable than bulk carbon, but only form for high carboncoverages because of the high energy cost associated with theformation of small graphene islands. Step sites might help tostabilize the initial graphene islands and facilitate grapheneformation (Bengaard et al., 2002). Bulk carbon and extendedgraphene islands reduce the activity of the Ni catalyst andshould be prevented. We propose to selectively block the sub-surface sites to force carbon to stay on the surface available forreaction.

Fig. 1. Illustration of the boron promoted Ni(1 1 1) surface. The boron atomsoccupy all the octahedral sites of the first subsurface layer. The larger, greyballs indicate the Ni atoms and the smaller, black balls indicate the boronatoms.

3.2. Effect of boron on the coking resistance

Different elements were considered to selectively block thesubsurface sites. Since carbon prefers the subsurface sites, ele-ments close to carbon in the periodic table, i.e., Be, B, N and P,were considered. Ab initio calculations indicate that boron be-haves rather similar to carbon and strongly prefers to adsorb inthe octahedral sites of the first subsurface layer of the Ni(1 1 1)surface (Xu and Saeys , 2006). Be, N and P prefer to adsorb on-surface. In addition, the boron promoter in the first subsurfacelayer destabilizes on-surface carbon atoms and might hencelower carbon coverages. This might in turn lower the nucleationrate of graphene islands. Since boron prefers the octahedral sitesin the first subsurface layer over the sites in the Ni bulk, a rela-tively small number of boron atoms are required to occupy allthe octahedral sites of the first subsurface layer. Such a catalystsurface is illustrated in Fig. 1. The first principles calculationssuggest that blocking of the subsurface sites is responsible forthe improved coking resistance observed experimentally for theboron promoted Ni catalysts (Chen et al., 2005).

3.3. Effect of subsurface boron on methane activation

Although the calculations indicate that subsurface boronmight be a promising promoter to improve the coking resis-tance of a Ni catalyst, the effect of boron promotion on CH4activation has to be considered as well. To test the effect ofboron promotion on the catalytic activity, activation energieswere calculated for the rate-determining first dehydrogenationstep in methane activation.

Different methane dehydrogenation mechanisms were con-sidered on the Ni(1 1 1) and the promoted NiB(1 1 1) surface.The classical non-assisted reductive elimination mechanismwhere a metal atom is inserted in a C–H bond to form a hy-drogen atom and a methyl species in opposite hollow siteswas found to be the preferred mechanism on both the Ni(1 1 1)and the NiB(1 1 1) surface. The potential energy surfaces forCH4 dissociation on Ni(1 1 1) and NiB(1 1 1) are reported inFig. 2. The calculated activation energy for methane dissocia-tion on the Ni(1 1 1) surface, 101 kJ/mol, is in agreement with

J. Xu, M. Saeys / Chemical Engineering Science 62 (2007) 5039–5041 5041

0

50

100

150

Reaction coordinate

Energ

y (

kJ/m

ol)

Fig. 2. Potential energy profile along the reaction path for methane dissociationover the Ni(1 1 1) surface and over the boron promoted Ni(1 1 1) surface. Allthe energies are relative to methane in the gas phase. The structures illustratethe transition state geometry for methane dissociation on the Ni(1 1 1) andon the boron promoted Ni(1 1 1) surface.

experimental data and other DFT values (Kratzer et al., 1996;Abild-Pedersen et al., 2005). Although the activation energyon the boron promoted Ni(1 1 1) surface increases slightly by12 kJ/mol, it is likely that the boron promoted catalyst is still ac-tive for CH4 activation. The reaction is also more endothermicby 47 kJ/mol on the promoted catalyst. The higher endother-micity of the reaction might possibly influence the further re-action mechanism.

The higher activation barrier for the boron promoted catalystcan be understood by analyzing the projected density of state(DOS) (Hammer and NZrskov, 2000). For the boron promotedcatalyst, the center of the 3d states of the Ni surface layer isshifted down by 0.26 eV. This will decrease the interaction ofthe antibonding C–H orbital of the CH4 molecule with the Ni3d states, leading to a higher activation barrier.

4. Conclusions

Based on a first principles analysis of the different types ofadsorbed carbon, boron is proposed as a promoter to improvethe coking resistance of Ni catalysts. The calculations indicatethat boron effectively blocks the subsurface sites and preventscarbon diffusion into the bulk, forcing carbon atoms to stay onthe surface. In addition, the boron promoter reduces the on-surface carbon binding energy and might reduce graphene for-mation. Additional calculations indicate that boron promotionslightly increases the activation energy for the first methanedehydrogenation step by 12 kJ/mol.

Acknowledgment

It is our pleasure to acknowledge stimulating discussionswith Tan Kong Fei and Armando Borgna.

References

Abild-Pedersen, F., Lytken, O., Engbæk, J., Nielsen, G., Chorkendorff, I.,NZrskov, J.K., 2005. Methane activation on Ni(1 1 1): effects of poisonsand step defects. Surface Science 590, 127–137.

Abild-Pedersen, F., NZrskov, J.K., Rostrup-Nielsen, J.R., Sehested, J., Helveg,S., 2006. Mechanisms for catalytic carbon nanofiber growth studied by abinitio density functional theory calculations. Physical Review B: CondensedMatter and Materials Physics 73, 115419.

Bengaard, H.S., NZrskov, J.K., Sehested, J., Clausen, B.S., Nielsen, L.P.,Molenbroek, A.M., Rostrup-Nielsen, J.R., 2002. Steam reforming andgraphite formation on Ni catalysts. Journal of Catalysis 209, 365–384.

Blöchl, P.E., 1994. Projector augmented-wave method. Physical Review B:Condensed Matter and Materials Physics 50, 17953–17979.

Chen, L., Lu, Y., Hong, Q., Lin, J., Dautzenberg, F.M., 2005. Catalytic partialoxidation of methane to syngas over Ca-decorated-Al2O3-supported Niand NiB catalysts. Applied Catalysis A: General 292, 295–304.

Hammer, B., NZrskov, J.K., 2000. Theoretical surface science andcatalysis—Calculations and concepts. Advances in Catalysis 45, 71–129.

Honkala, K., Hellman, A., Remediakis, I.N., Logadottir, A., Carlsson, A.,Dahl, S., Christensen, C.H., NZrskov, J.K., 2005. Ammonia synthesis fromfirst-principles calculations. Science 307, 555–558.

Jacobsen, C.J.H., Dahl, S., Clausen, B.S., Bahn, S., Logadottir, A., NZrskov,J.K., 2001. Catalyst design by interpolation in the periodic table: bimetallicammonia synthesis catalysts. Journal of the American Chemical Society123, 8404–8405.

Jónsson, H., Mills, G., Jacobsen, K.W., 1998. Nudged elastic band methodfor finding minimum energy paths of transitions. In: Berne, B.J., Ciccotti,G., Coker, D.F. (Eds.), Classical and Quantum Dynamics in CondensedPhase Simulations. World Scientific, Singapore, p. 385.

Kratzer, P., Hammer, B., NZrskov, J.K., 1996. A theoretical study ofCH4 dissociation on pure and gold-alloyed Ni(1 1 1) surface. Journal ofChemical Physics 105 (13), 5595–5604.

Kresse, G., Furthmüller, J., 1996a. Efficient iterative schemes for ab initiototal-energy calculations using a plane-wave basis set. Physical Review B:Condensed Matter and Materials Physics 54, 11169–11186.

Kresse, G., Furthmüller, J., 1996b. Efficiency of ab-initio total energycalculations for metals and semiconductors using a plane-wave basis set.Computational Materials Science 6, 15–50.

Kresse, G., Hafner, J., 1993. Ab initio molecular dynamics for liquid metals.Physical Review B: Condensed Matter and Materials Physics 47, 558–561.

Kresse, G., Hafner, J., 1994. Ab initio molecular-dynamics simulation of theliquid-metal–amorphous-semiconductor transition in germanium. PhysicalReview B: Condensed Matter and Materials Physics 49, 14251–14269.

Kresse, G., Joubert, D., 1999. From ultrasoft pseudopotentials to the projectoraugmented-wave method. Physical Review B: Condensed Matter andMaterials Physics 59, 1758–1775.

Perdew, J.P., Chevary, J.A., Vosko, S.H., Jackson, K.A., Pedersen, M.R.,Singh, D.J., Fiolhais, C., 1992. Atoms, molecules, solids, and surfaces:applications of the generalized gradient approximation for exchange andcorrelation. Physical Review B: Condensed Matter and Materials Physics46, 6671–6687.

Rostrup-Nielsen, J.R., 2000. New aspects of syngas production and use.Catalysis Today 63, 159–164.

Saeys, M., Reyniers, M.F., Thybaut, J.W., Neurock, M., Marin, G.B., 2005.First-principles based kinetic model for the hydrogenation of toluene.Journal of Catalysis 236, 129–138.

Venkatamaran, V.K., Guthrie, H.D., Avellanet, R.A., Driscoll, D.J., 1998.Overview of US DOE’s natural gas-to-liquid RD&D program andcommercialization strategy. Studies in Surface Science and Catalysis 119,913–918.

Xu, J., Saeys, M., 2006. Improving the coking resistance of a Ni-based catalystby promotion with subsurface boron. Journal of Catalysis 242, 217–226.