Methane activation on Ni(1 1 1): Effects of poisons and step defects

doi:10.1016/j.susc.2005.05.057

Surface Science

Volume 590, Issues 2–3, 1 October 2005, Pages 127-137

https://doi.org/10.1016/j.susc.2005.05.057 Get rights and content

Abstract

We have, theoretically and experimentally, investigated the dissociation of methane on the terraces and steps of a Ni(1 1 1) surface. Using Density Functional Theory (DFT) total energy calculations combined with Ultra High Vacuum (UHV) experiments, we find that the steps exhibit a higher activity than the terraces. We have, furthermore, investigated how carbon and sulfur present on the surface will deactivate the steps, leaving only the terraces active. We find the intrinsic sticking probabilities of methane on the steps and terraces at 500 K to be 2.8 × 10⁻⁷ for the steps and 2.1 × 10⁻⁹ for the terraces, in complete agreement with our calculated difference in activation energy of 17 kJ/mol.

Introduction

Methane is the dominating component of natural gas and used to synthesize CO and hydrogen, via the steam reforming process. This is a way to produce cheap hydrogen, and it is the first step in both ammonia and methanol synthesis. The catalyst used for the steam reforming process is nickel based, and hence, methane activation and C–H bond breaking on nickel has received much attention, both theoretically and experimentally [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The majority of the work has been concentrated on surfaces with the lowest Miller indices, and minor attention has been focused on the possible effects of steps or defects on these surfaces. Experimentally methane activation is found to be structure sensitive, with Ni(1 1 1) being the least active and Ni(1 0 0) and Ni(1 1 0) increasingly more active [7], and it is possible that steps could exhibit an even higher activity.

According to Density Functional Theory (DFT) studies performed by Watwe et al., the dissociation of methane on Ni(1 1 1) is activated by 127 kJ/mol [4], but they did not take spin-polarization of the electrons into account, and therefore a slight energy discrepancy of the intermediates were expected. A subsequent DFT study by Bengaard et al. investigated the effect of larger lateral unit cells and magnetic ordering, and found that it had a significant influence on the overall binding and transition state energies. They reported an activation energy on Ni(1 1 1) below 91 kJ/mol [12]. Other theoretical studies by Yang and Whitten, using a configuration interaction approach, have found an activation energy as low as 71 kJ/mol. Bengaard et al. furthermore studied the effect of steps, using a Ni(2 1 1) surface, and found the activation energy to be lowered by 19 kJ/mol relative to Ni(1 1 1).

In the present paper we investigate the methane activation at steps further. By combining theory and experiment, we find that steps are more reactive than the closed-packed (1 1 1) terraces of a Ni surface. Furthermore, we show that both carbon and sulfur prefer the high coordinated step sites and will be present there during our experiments. We demonstrate that both carbon and sulfur present along the steps strongly affects the activation of the first C–H bond in methane, and that beyond a certain carbon or sulfur coverage the dissociation of methane will be dominated entirely by the terrace activity. The suggestion by Bengaard et al. [12], confirmed experimentally by Helveg et al. [13], that step edges on nickel surfaces act as growth centers for graphene is in perfect agreement with our findings.

Section snippets

Theoretical method

We have used the DACAPO code [14] for all total energy calculations. To investigate the influence of steps on Ni(1 1 1) we use a Ni(2 1 1) surface. This system is modeled by a nine layer slab periodically repeated in a super cell geometry. Each metal slab is separated by roughly 11 Å of vacuum. The thermodynamic total energy calculations are done in a 1 × 2 unit cell of Ni(2 1 1). This produces a surface with (1 1 1) terraces three atoms wide and steps with a (1 0 0) symmetry. A single methane molecule

Experimental

Contrary to what one might think, the difficult part, when measuring the importance of steps for a chemical reaction, is not measuring the activity of the steps, but measuring the activity of the flat terraces. It is not possible to create a single crystal surface without steps or defects. A standard single crystal, aligned to within 0.5°, will have about 1% steps or defects on the surface. To be able to measure the activity of the terraces the steps need to be deactivated. One approach to this

Theoretical results

The activation of the first C–H bond in methane, shown in Fig. 2, occurs over the top of a surface Ni atom on Ni(1 1 1). We obtain an energy barrier of 105 kJ/mol. This is in accordance with reported results by Bengaard et al. [12], in which a non zero-point corrected energy barrier of 101 kJ/mol was found. A similar calculation performed on Ni(2 1 1) resulted in a lowering of the activation barrier of 17 kJ/mol. The configuration in which CH₃ is bonded to the bridge step edge site and H to the

Discussion

STM studies of sulfur adsorbed on Ni(1 1 1) by Maurice et al. [34] find that above 625 K sulfur will migrate to the steps, where sulfur islands will nucleate. This is in full agreement with our calculations, which predict sulfur to be 35 kJ/mol more stable on Ni(2 1 1) than on Ni(1 1 1). This provides a strong indication of a preferential decoration of the steps, given a temperature high enough for sulfur to be able to migrate to the steps. The strong effect, we observe upon the adsorption of up to 0.06

Conclusion

We have calculated the barriers for methane dissociation along a step on Ni(1 1 1) in the presence of carbon and sulfur, and found that both adsorbates will deactivate the steps. We have verified this experimentally, used this knowledge to measure the activity of a Ni(1 1 1) surface free from steps, and compared the activity of steps and terraces on Ni(1 1 1). The intrinsic sticking probabilities we find at 500 K are 2.1 × 10⁻⁹ for the terraces and 2.8 × 10⁻⁷ for the steps. This corresponds to an

Acknowledgements

We gratefully acknowledge financial support from the Danish Research Agency through the Centre of Excellence “Towards a Hydrogen-based Society” grant no. 2052-01-0054.

The Center for Atomic-scale Materials Physics (CAMP) is sponsored by the Danish National Research Foundation.

References (39)

H.S. Bengaard et al.
J. Catal.
(1999)
R. Watwe et al.
J. Catal.
(2000)
L. Hanley et al.
Surf. Sci. Lett.
(1991)
H.S. Bengaard et al.
J. Catal.
(2002)
G. Mills et al.
Surf. Sci.
(1995)
I. Chorkendorff et al.
Surf. Sci.
(1990)
M. Perdereau et al.
Surf. Sci.
(1970)
M. Valden et al.
Chem. Phys. Lett.
(1996)
J.B. Benziger et al.
Surf. Sci.
(1984)
G. Held et al.
Surf. Sci.
(1998)

H. Nakano et al.

Surf. Sci.

(2002)

C. Klink et al.

Surf. Sci.

(1995)

H. Robota et al.

Surf. Sci.

(1984)

B. Hammer et al.

Surf. Sci.

(1995)

J.J. Mortensen et al.

Surf. Sci.

(1998)

V. Pallassana et al.

J. Catal.

(2000)

R.C. Egeberg et al.

Surf. Sci.

(2002)

V. Maurice et al.

Surf. Sci.

(1997)

J.R. Rostrup-Nielsen

J. Catal.

(1984)

Cited by (225)

Charge transfer boosts up methane adsorption and activation on three-coordinated metal sites
2024, Molecular Catalysis
The activation and conversion of CH₄ has attracted great interest due to its mitigation impact on the greenhouse gas and contribution to the high value-added chemicals. However, the activation of CH₄ is the rate-limiting step due to the high symmetry of the molecule and its chemical inertness. In this study, we constructed active sites with specific geometry for methane activation based on the Nickel and other post-transition metals, where the interactions were significantly enhanced by 0.2∼0.6 eV varying with different VIII group metal types. With advanced electronic structure analysis methods, the origin of this interaction was unraveled, that is the Coulomb attraction induced by charge transfer from hydrogen of methane to three-coordinated metal Ni sites, rather than the covalent bonding or dispersion interaction. This enhancement not only increases the collision possibility by several orders of magnitude, but also lowers the barrier of CH bond breaking. These findings provide deep insight on the methane activation on metal catalysts and shed the light to design more efficient methane conversion catalysts.
Addressing complexity in catalyst design: From volcanos and scaling to more sophisticated design strategies
2023, Surface Science Reports
Volcano plots and scaling relations are commonly used to design catalysts and understand catalytic behavior. These plots are a useful tool due to their robust and simple analysis of catalysis; however, catalysts that follow the volcano plot paradigm have an inherent limit to their performance. Scaling and Brønsted-Evans-Polanyi (BEP) relations, which are linear correlations in reaction energetics, force tradeoffs when optimizing catalysts, which leads to this limit on performance. Therefore, materials and design strategies that are not limited by volcano plots and scaling relations are of high interest, and this is the focus of this Report. We first give an overview of volcano plots and scaling relations. Deviations from scaling relations and the volcano plot and their causes are discussed in more detail. Finally, design strategies that do not rely on the volcano plot paradigm are reviewed.
Carbon nanofiber growth from methane over carbon-supported NiCu catalysts: Two temperature regimes
2023, Catalysis Today
An alternative route towards CO_x-free production of hydrogen is the thermal catalytic decomposition of methane. In addition to hydrogen, valuable carbon nanomaterials can be formed. Carbon nanofiber formation from methane decomposition over carbon supported NiCu catalysts was studied in-situ using a Thermogravimetric analyzer. We especially investigated how the carbon yield is influenced by reaction parameters. Based on experiments with varying temperature (450–600 °C), two distinct temperature regimes were identified. Different kinetic parameters were derived for the two regimes. Activation energies of 86 and 45 kJ/mol, and reaction orders in methane of close to 1 and 1.5, were found in the low and high temperature regimes, respectively. We postulate that at lower temperature the methane dissociation is rate limiting, while at higher temperature the carbon formation plays a more critical role. At low temperatures mostly full fishbone fibers are formed, whereas at higher temperatures mainly hollow fibers are formed. The maximum carbon yield is obtained at the transition between the two regimes, when the carbon supply and carbon nanostructure formation are balanced.
A review on bi/polymetallic catalysts for steam methane reforming
2023, International Journal of Hydrogen Energy
Blue hydrogen production by steam methane reforming (SMR) with carbon capture is by far the most commercialised production method, and with the addition of a simultaneous in-situ CO₂ adsorption process, sorption-enhanced steam methane reforming (SESMR) can further decrease the cost of H₂ production. Ni-based catalysts have been extensively used for SMR because of their excellent activity and relatively low price, but carbon deposition, sulphation, and sintering can lead to catalyst deactivation. One effective solution is to introduce additional metal element(s) to improve the overall performance. This review summarizes recent developments on bi/polymetallic catalysts for SMR, including promoted nickel-based catalysts and other transition metal-based bi/polymetallic materials. The review mainly focuses on experimental studies, but also includes results from simulations to evaluate the synergistic effects of selected metals from an atomic point of view. An outlook is provided for the future development of bi/polymetallic SMR catalysts.
Conversion of carbon dioxide to carbon monoxide: Two-step chemical looping dry reforming using Ca<inf>2</inf>Fe<inf>2</inf>O<inf>5</inf>–Zr<inf>0.5</inf>Ce<inf>0.5</inf>O<inf>2</inf> composite oxygen carriers
2022, Fuel
In this study, Zr_0.5Ce_0.5O₂ (ZC) was used to improve the reactivity of Ca₂Fe₂O₅ (CF) oxygen carriers (OCs) and alleviate sintering. The reactivity of modified OCs in Chemical looping dry reforming (CLDR) was explored using a thermogravimetric analyzer and a fixed-bed reactor, and the fresh and reacted OCs were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results revealed that CF6 (60 wt% CF + 40 wt% ZC) had the best reactivity. Owing to the addition of ZC, the carbon monoxide (CO) yield of CF6 was twice that of CF during carbon dioxide (CO₂) oxidation, increasing from 72.3 mL/g to 138 mL/g. In the cyclic experiments, the CO yield from CO₂ oxidation decreased from 138 mL/g in the first cycle to 34.3 mL/g in the fifth cycle and subsequently stabilized over the next 15 cycles. The decrease in cyclic performance is related to the sintering of the sample and the formation of a new phase, CaZrO₃, which hindered the redox reaction in the sample. Although CF6 had a decreased cyclic performance, it still had a better reactivity than CF, which had a CO yield that was about 20% lower than that of CF6. The reason for the improvement is that ZC improved the dispersibility of CF. After 20 cycles, the generation of CO was facilitated by extending the reduction time of the OC, and the CO yield increased from 35 mL/g to 60 mL/g when the reduction time was doubled.
CH<inf>4</inf> valorisation reactions: A comparative thermodynamic analysis and their limitations
2022, Fuel
In this study, we report on a first comparative thermodynamic analysis of various CH₄ valorisation processes, taking into account the formation of byproducts through a network of side reactions. The analysis is carried out to assess the influence of a wide range of reaction parameters on the performance of each valorisation process in terms of CH₄ conversion and desired selectivity metrics. The Gibbs free energy minimization method is used for the equilibrium study of nine CH₄ conversion processes, including ten side reactions. The ΔG calculations revealed that the oxidative coupling of methane (OCM) and the methane dehydro-aromatization (MDA) reactions are thermodynamically the most and least feasible reactions, respectively. At P = 1 bar and T = 700 °C, the highest and lowest CH₄ conversions were obtained by the dry reforming of methane (DRM) (90%) and oxidative dimerization (18%), respectively. The DRM and steam reforming of methane (SRM) were highly sensitive to changes in operation parameters, while the partial oxidation of methane (POM) and OCM were the least affected. The increasing addition of water in reforming reactions reduced the coke formation to zero at temperatures above 675 °C. From the comparison of the thermodynamic results with the documented experimental data, the carbon deposition was found to decrease both the selectivity and stability of catalysts. Investigations into the thermodynamic and kinetic limitations together with the catalyst design will help in the development of a commercial CH₄ valorisation process.

View all citing articles on Scopus

View full text

Methane activation on Ni(1 1 1): Effects of poisons and step defects

Abstract

Introduction

Section snippets

Theoretical method

Experimental

Theoretical results

Discussion

Conclusion

Acknowledgements

J. Catal.

J. Catal.

Surf. Sci. Lett.

J. Catal.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Chem. Phys. Lett.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Surf. Sci.

J. Catal.

Surf. Sci.

Surf. Sci.

J. Catal.