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First Principles Based Promoter Design for Heterogeneous Catalysis

Jing Xu1, Kong Fei Tan1, Armando Borgna2, and Mark Saeys1. (1) Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore, (2) Institute of Chemical and Engineering Sciences, Singapore

Introduction

First principles based modelling is rapidly becoming an important tool for the rational design of novel catalysts [1,2] and for kinetic modelling of catalytic processes [3]. It allows to quantify, on a molecular level, the influence of surface structure, electronic properties and promoters on the reaction kinetics. Though first principles based modelling has not yet reached chemical accuracy for heterogeneous catalytic reactions, it can provide important leads for the design of novel heterogeneous catalysts. These concepts can then be verified experimentally.

In this work, we have applied first principles modelling to elucidate the mechanism of carbon deposition on Ni and Co catalysts. Ni is an important catalyst for hydrogenation/dehydrogenation processes and steam reforming, while Co is used in Fischer-Tropsch synthesis. However, both Ni and Co catalysts suffer from deactivation by carbon deposition, coking, in these processes. Based on our molecular level understanding of the coking mechanism, boron is proposed as a potential promoter to improve the coking resistance of these catalysts. Additional calculations were done to confirm that the B promoted catalysts maintains its catalytic activity. Finally, experimental coking experiments were performed using Thermal Gravimetric Analysis (TGA) to confirm the effect of the promoter.

 

Results and Discussion

The relative stability of three types of chemisorbed carbon, on-surface chemisorbed carbon, bulk carbon and extended graphene islands, was calculated for the Ni(111) [2] and the Co(0001) surface. On-surface carbon was found to be relatively unstable. Diffusion to octahedral sites of the first subsurface layer is thermodynamically preferred by 50-120 kJ/mol and the corresponding activation energy rather low. Extended graphene islands are even more stable than bulk carbon, but only form for high carbon coverages.

A number from the same row as carbon (N, B, P, Be) were considered as potential promoters to reduce deactivation by coking. Ab initio calculations indicate that boron behaves rather similar to carbon and prefers to adsorb in the octahedral sites of the first subsurface layer for both Ni and Co. These boron atoms might block the subsurface sites and prevent carbon diffusion into the bulk, forcing carbon atoms to stay on the surface available for reaction. In addition, the boron promoter in the first subsurface layer reduces the on-surface carbon binding energy and might hence lower carbon coverages. This might in turn lower the nucleation rate of graphene islands [3].

To ensure that the Ni and Co catalysts maintain their activity after promotion with boron, activation energies for CH4 activation on promoted Ni (Table 1) and CO dissociation on promoted Co were calculated. Though B promotion has a clear influence on the calculated activation energies for methane activation, it is interesting to note that the highest barrier along the reaction path from methane to adsorbed carbon and hydrogen is actually lower for the promoted Ni(111) catalyst. Though the relative stability of the reaction intermediates is different on the promoted Ni(111) catalyst, it is likely that it still maintains the high activity of the Ni(111) catalyst.

Next, the effect of B promotion on the coking resistance of Ni and Co catalysts was tested experimentally using Thermal Gravimetric Analysis. A series of Ni and Co catalysts containing 0, 1 and 5 wt% boron was prepared by the incipient wetness impregnation. Propane gas was used to simulate coking. Even at 1 wt% B promotion of the Co catalyst, a clear effect on the coking rate was observed as depicted in Figure 1. Further studies will be performed to study the effect of B promotion under reaction conditions.

 

Conclusions

First principles based modeling was used to provide insight into the mechanism of carbon deposition on Ni and Co catalysts. Based on this analysis, boron is proposed as a promoter to reduce deactivation of Ni and Co-based catalysts by coking. Calculations indicate that boron promotion does not decrease the catalyst's activity. The effect of boron promotion was verified experimentally by TGA.

 

Table 1. Effect of boron promotion on the activation energies for methane decomposition.

 

Step

Ea (kJ/mol)

Ni(111)

Ea (kJ/mol)

B promoted Ni(111)

CH4 -> CH3* + H*

101

113

CH3* -> CH2* + H*

73

120

CH2* -> CH* + H*

27

76

CH* -> C* + H*

131

107

 

Figure 1. Effect of B promotion on the rate of coking for a Co catalyst

 

References

[1] F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. NÝrskov, I. Stensgaard, Science 279 (1998) 1913.

[2] J. Xu and M. Saeys, J. Catal. (submitted)

[3] M. Saeys, M. F. Reyniers, J. W. Thybaut, M. Neurock, G. B. Marin, J. Catal. 236 (2005) 129.