Naphthalene on Ni(111): Experimental and Theoretical Insights into

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Naphthalene on Ni(111): Experimental and Theoretical Insights Into Adsorption, Dehydrogenation and Carbon Passivation Milad Ghadami Yazdi, Pouya H. Moud, Kess Marks, Witold Piskorz, Henrik Öström, Tony Hansson, Andrzej Kotarba, Klas Engvall, and Mats Göthelid J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07757 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation and Carbon Passivation Milad Ghadami Yazdi1*, Pouya. H. Moud2*, Kess Marks4, Witold Piskorz3, Henrik Öström4, Tony Hansson4, Andrzej Kotarba3, Klas Engvall2 and Mats Göthelid1, +

1 2

Material Physics, SCI, KTH Royal Institute of Technology, 16440 Kista, Sweden

Department of Chemical Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden 3

Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland 4

Department of Physics, Fysikum, Stockholm University, 106 91 Stockholm, Sweden

* The authors have equally contributed to the paper + corresponding author: Mats Göthelid, [email protected]

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Abstract An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit in use for long time in fossil feedstock catalytic steam reforming, the understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature programmed desorption) and STM (scanning tunneling microscopy) in ultra-high vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the di-bridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may co-exist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 K and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.

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Introduction Biomass gasification is one of the routes for converting renewable feedstock into useful energy carriers. However, to utilize the produced gas in, for instance, syngas applications, certain levels of unwanted hydrocarbons, where heavier hydrocarbons generally are denoted as tar, need to be removed. The biomass gasification gas composition varies depending on the type of feedstock, technology, and the operating conditions of the gasifier. As the temperature in the gasifier increases, the chemical composition of tar adjusts to the actual conditions. Water-soluble tars are produced at lower temperatures while tars containing heavy poly-aromatic hydrocarbons (PAH) such as naphthalene, are produced at higher temperatures 1. An attractive solution to mitigate the tars and also to decompose lighter hydrocarbons is secondary catalytic reforming, converting hydrocarbons to useful permanent gases 2. Nickel-based catalysts are commonly used in industry for catalytic steam reforming of fossil hydrocarbon feedstock, and are also foreseen as technically and economically feasible in biomass gasification 3. Albeit in use for long time in fossil feedstock steam reforming, the complexity of the biomass tar and presence of impurities remains a challenge for understanding the catalytic processes, as previously shown by Moud et al. 4. Naphthalene is typically present in the biomass gasification gas and has been identified as one of the most difficult molecules to decompose 2, and therefore frequently used as a model molecule for catalyst activity testing and design. Since naphthalene is also an intermediate in the decomposition mechanisms of higher poly-aromatic hydrocarbons to syngas molecules 3, it is in this perspective important to understand the elementary steps of its transformation such as primary adsorption and dehydrogenation, as well as possible surface carbon passivation mechanisms, caused by naphthalene. Experimental and theoretical studies of naphthalene adsorption and dehydrogenation on Ni(111) are scarcely reported in the literature. However, other metal surfaces have previously been studied. Gland and Somorjai studied naphthalene monolayers on Pt(111) using low energy electron diffraction (LEED) 5. While room temperature adsorption did not result in any observable ordering, mild heating to 420 K resulted in a (6x6) structure. This observation was later corrected to (6x3) in a study by Dahlgren and Hemminger 6. In a subsequent study 7, it was claimed that the ordering of neighboring naphthalene molecules appears to be governed by intermolecular forces rather than molecule-Pt interactions for naphthalene overlayers. STM 3



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(scanning tunneling microscopy) studies were done on the Pt(111)-naphthalene system by Hallmark et al. 8, 9, 10

, which agreed on the disordered molecular layer at room temperature. The adsorption geometry was

flat lying, but the exact site could not be determined. A DFT (density functional theory) study by Santarossa et al. 11 of naphthalene adsorption on Pt(111), Rh(111) and Pd(111) revealed that on all three surfaces the preferred geometry is flat in a so-called di-bridge-7 site. These findings were later confirmed in a DFT study by Jenkins 12. Similarly to Pt(111), earlier experiments on Rh(111) also revealed a well ordered monolayer with lying naphthalene in di-bridge[7] adsorption state

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, in agreement with an early LEED study

14

. On

Cu(111) 15, naphthalene forms very well ordered monolayer structures at low temperature. In that study naphthalene was suggested to lie down with the central carbon atoms located in a bridge position and the carbon rings above hollow sites, however, no structure determination method was applied. Dehydrogenation of naphthalene on Pt(111) and Rh(111) was experimentally investigated by Dahlgren and Hemminger 7 and Lin et al. 14, respectively. A complete dehydrogenation with an onset at around 473 K was proposed as a potential fate of the naphthalene molecules adsorbed on Pt(111) 7. In the case of Rh(111), TPD studies revealed a first H2 desorption peak at about 578 K, interpreted as originating from H2 abstraction from naphthalene 14. Dehydrogenation using other aromatic model compounds is reported in 16, 17

. Friend et al.

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investigated benzene and toluene interacting with various Ni single crystal surfaces

observing a sharply different surface chemistry comparing benzene and toluene. The aliphatic C-H bond is first decomposed at a much lower temperature before the aromatic C-H bond initiated at temperatures above 433 K. The presence of a non-graphite carbon overlayer on the surface, produced by thermal decomposition of benzene, did not affect the spatial arrangement or the physical and chemical properties upon benzene and toluene chemisorption on any of the various Ni surfaces. In a study investigating dehydrogenation of coronene on Ir(111)

17

, it was shown that the coronene molecules initially to some

extent tilt upward with respect to the surface, keeping their planar configuration. As the subsequent dehydrogenation proceeds, the molecules experience a progressive increase in the average interatomic distance and gradually settle to form dome-shaped nanographene flakes. Formation of graphitic carbon species on transition metal catalysts has been comprehensively studied for its role in catalyst deactivation, during hydrocarbon reforming 18, and for metal-catalyzed growth of carbon 4


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nanotubes

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and planar graphene

20

. The study by Wintterlin et al.

20

revealed that graphene and/or

graphite growth on Ni(111) occurs by surface segregation and deposition of carbon, originating from mixed Ni-carbon phases, initially formed by carbon diffusing into Ni. Lahiri et al. 21 later identified a mechanism for temperatures below 533 K, where graphene grows by transformation of surface confined Ni-carbide phases along a one-dimensional phase-boundary. In the study, applying CVD (Chemical Vapour Deposition) technique exposing Ni(111) to ethene, it was shown that for clean Ni(111), below 773 K, the formation of an intermediate structural surface Ni-carbide is most probable, subsequently converted into epitaxial graphene. At temperatures above 773 K, graphene preferably grows directly on the Ni surface via replacement mechanisms, resulting in embedded epitaxial and/or rotated graphene domains 22. Reactive molecular dynamics simulations

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of graphene growth on Ni(111) using naphthalene/fluorene in CVD

synthesis revealed a series of steps, including an initial surface-assisted dehydrogenation reaction, followed by coalescence reaction of active molecular species. This differs from the nucleation and growth mechanisms in the conventional graphene CVD process. In a recent study by Wang et al. 24, theoretical and experimental methods where combined to investigate graphene formation on Rh(111) from ethene, through a reaction scheme where adsorbed ethene upon heating evolves to form segmented onedimensional polyaromatic hydrocarbon (1D-PAH) chains. Additional heating leads to a dimensionality crossover and a dynamical restructuring processes at the PAH chain ends with subsequent activated detachment of size-selective carbon clusters. A rate-limiting diffusional coalescence of these dynamically self-evolved precursors culminates (≤ 1000 K) in a condensation into graphene. In the present study we use a combined experimental and theoretical approach to understand the catalytic reaction pathways from naphthalene to formation of extended graphene on a Ni(111) surface.

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Experimental and Methodology Materials Pure naphthalene was purchased from Sigma-Aldrich and dosed onto the surface through a precision leak valve at 5×10-8 Torr. Doses are given in L (Langmuir) where 1 L equals 10-6 Torr × sec and it takes ca 3.5 L to form a saturated monolayer at room temperature. Additional dosing at room temperature does not increase the surface coverage. The Ni(111) crystal was purchased from Surface Preparation Laboratory (SPL) in the Netherlands. The sample was polished and aligned to within less than 0.1° from the (111) plane. The sample was prepared by repeated cycles of Ar-sputtering (1 kV) and annealing at 1123 K, until a sharp (1x1) LEED (low energy electron diffraction) pattern was observed with low background and STM showed a smooth surface with large flat terraces.

Scanning Tunneling Microscopy (STM) The STM experiments were done in a RHK UHV 3500 SPM system with a base pressure of 3×10-11 Torr. The sample was prepared in a preparation chamber connected to the STM chamber via a gate valve. The preparation chamber is equipped with LEED optics, an Ar-ion sputter gun and sample heating. The sample was mounted on a Mo sample holder. Sample heating was done by electron bombardment, and the temperature was measured with a pyrometer at temperatures above 573 K. For lower temperatures than that we estimated the temperature from the heating power. STM was done using etched W and Au tips. The bias was applied to the tip; negative bias thus corresponds to imaging empty states and positive bias images the filled states on the surface.

Temperature Programmed Desorption (TPD) Temperature programmed desorption measurements were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10-10 Torr. The system is equipped with a quadrupole mass spectrometer (QMS), LEED and a sputter gun. The polished Ni(111) sample was mounted on a sample holder and connected to a thermocouple for direct temperature monitoring. A filament was secured to the 6


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sample holder for radiative heating and through a transfer line liquid nitrogen cooling could be applied. The sample was cleaned through cycles of Ar+ sputtering at room temperature (1 kV, 10 mA for 5 mins) and sample annealing up to 1100 K. The quality of the surface was determined using LEED. The sample temperature was set to 278 K using a combination of liquid nitrogen cooling and radiative heating. Naphthalene and hydrogen were dosed into the UHV chamber and onto the sample using leak valves. The ramping rate for heating was set to 0.83 K/s. During the measurements the desorption fluxes of both naphthalene and hydrogen were continuously monitored with the QMS lines with m/z = 128 and 2).

Density Functional Theory (DFT) The adsorption structure of naphthalene on Ni(111) was optimized using the DFT PBE+D 25 level of theory implemented in the VASP code 26, 27. The energy cutoff was 400 eV, the IBZ sampling was done according to the Monkhorst-Pack scheme 28 with k-points density of 3×3×1. The tight convergence criteria (10-7 eV in the electronic structure optimization and 10-5 eV/Å for ionic relaxation) were used. The cell parameter of the bulk phase was calculated as a minimum of the Birch-Murnaghan equation of state fitting

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. As a

convergence accelerator the Methfessel-Paxton 30 smearing was used with the width parameter σ = 0.1 eV. The Tersoff-Hamann 31 approximation was utilized for the STM image simulations. The atomic charges were calculated according to the Bader population analysis 32, 33, 34 and the bond orders were obtained via DDEC6 method 35, 36. As a surface model a Ni(111) slab of the 12.46×12.46×27.52 Å3 unit cell size (5×5×5 Ni atoms with 2 bottom layers fixed) was used. The vacuum layer was 19.5 Å thick.

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Results and Discussion Adsorption of naphthalene Adsorption of naphthalene at room temperature on Ni(111) gives a molecular monolayer, as shown by the STM images presented in figure 1a, which is a 20x20 nm2 filled state image showing a flat terrace with a smooth, partially ordered monolayer. The separation between protrusions is of the order of 1 nm, but there is no long-range order. A closer look at the protrusions reveals an elongated shape, suggesting that they represent one naphthalene molecule each. The STM image reveals that naphthalene molecules are oriented in three directions, following the substrate symmetry, as indicated in figure 1b. This is in line with either of the two structures proposed for naphthalene adsorbed on Cu(111)

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and Rh(111) 13. Figure 1b,

which is a 40x40 nm2 empty state STM image recorded after naphthalene deposition at ca 373 K. Here the ordered patches have grown to 10-15 nm, in which most molecules exhibit the same orientation. This rotational flexibility is similar to that observed on Pt(111)

8, 9, 10

. On Cu(111), on the other hand,

naphthalene desorbed from the surface already well below room temperature 37, and the structural order of the naphthalene molecules on the surface clearly showed preferred mutual alignment 11.

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Figure 1. STM images from naphthalene adsorbed on Ni(111). a) 20x20 nm (-2.8 V, 80 pA) and b) 40x40 nm (4.0 V, 391 pA) high-resolution images after deposition at room temperature and 373 K, respectively. Three high symmetry directions have been indicated in figure 1b.

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Figure 2. Three different adsorption geometries for naphthalene on Ni(111), obtained from DFT simulations, with structures are the same as proposed by Santarossa et al. 11. a) Di-bridge[7], b) Di-bridge[6] and Di-hollow(30)[5]. Side view of an adsorbed naphthalene molecule with Di-bridge[7] geometry is shown in d).

Figure 2 depicts three different geometries for naphthalene on Ni(111) obtained from the DFT simulations. Their respective adsorption energies are listed in Table 1 together with the corresponding values for Pt(111) and Rh(111) calculated by Santarossa et al. 11. The adsorption energies for the two noble metals were calculated using periodic cluster modeling, where chemisorption bonds are formed between the organic molecules and the metal, neglecting the dispersion forces. We used fully periodic mode modeling for Ni, as described above, including also dispersion effects. This is a more accurate way of calculating the adsorption energies as previously shown by Godlewski et al. 38. In all cases, a flat lying down geometry is preferred, but the potential energy surface is rather flat and allows for a variety of slightly different co-ordinations to the surface, suggestively explaining the lack of long range order. The obtained higher adsorption energies for naphthalene on Ni(111) indicate much stronger adsorption in comparison to the noble metals for all three geometries, with a significant difference towards Pt and a more moderate in the case of Rh.

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Table 1. Adsorption energies (eV) for different adsorptions structures naphthalene on Ni(111), Pt(111) and Rh(111). Geometry

Ni(111)

Pt(111)*

Rh(111)*

Di-bridge[7]

4.42

1.51

2.86

Di-bridge[6]

4.46

0.72

2.75

Di-hollow(30)[5]

4.28

0.51

2.00

* ref 11

Table 2 shows the total bond orders and partial charges for the calculated adsorption states of naphthalene on Ni(111). The adsorption of naphthalene on the nickel surface results in a geometric distortion and redistribution of the electronic density. The changes observed in the aromatic ring revealed that it undergoes a pronounced dearomatization. This can be readily inferred from the decrease in the sum of C-C bond orders from 17.05 (in the gas phase) to 13.6-13.7 (in the adsorbed state). The deformation of the aromatic ring toward a twisted structure is reflected by the ring planarity distortion gauged by the change in dihedral angles from 180° to about 140° (see the side view in figure 1d). The changes in geometry are accompanied by the redistribution of electron density. While the total negative charge on naphthalene carbon atoms decreases from -0.78 to -0.5, the hydrogen atoms become more positively charged (charge on H atoms changes from 0.78 to 1.06 upon adsorption). The total charge on the naphthalene molecule changes from 0 in the gas phase to 0.5 revealing that the electron density shifts from the adsorbed molecule into the nickel surface. The obtained results clearly reveal that the naphthalene adsorbed on nickel is strongly activated, which is reflected not only by the high adsorption energy but also by the aromatic ring distortion. This distortion has important practical implications because the bent molecule is much more prone to the catalytic modification, in particular, to dehydrogenation.

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Table 2. Total bond orders and partial charges for the calculated adsorption states of naphthalene on Ni(111). C-C

C-H

Naphthalen e-Ni(111)

Partial charge on carbon atoms

Partial charge on hydrogen atoms

Total charge of naphthalene molecule

Gas phase molecule

17.05

7.49

-

-0.78

0.78

0

Di_Bridge[7]

13.69

6.69

6.51

-1.30

0.87

-0.43

Di_Bridge[6]

13.76

6.67

6.36

-1.44

1.02

-0.42

Di-Hollow[5]

14.68

6.33

5.71

-1.38

1.01

-0.37

Di-Hollow(30)[5]

13.72

6.69

6.34

-1.64

1.18

-0.46

Naphthalene state

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Dehydrogenation and passivation In order to gain insight into the mechanism of naphthalene dissociation on Ni(111), TPD was employed in the T range of 110-800 K. Naphthalene was adsorbed on the surface at T= 110 K with four different doses: 0.5 L, 1 L, 2 L, and 3 L. Figure 3a shows the TPD spectra of naphthalene, measuring 128 m/e (naphthalene molecule) at different coverages, for a dosing temperature of 110 K. There is a clear maximum in the rate of desorption occurring at 250 K for the highest coverage (3 L) of naphthalene, which is less significant for lower coverage (500 K for Pt(111) and Rh(111), respectively. Both of these temperatures are significantly higher than the 380 K obtained for Ni(111), indicating that Ni is a better CH-bond cleavage catalyst. The similar desorption spectra for 2 L and 3 L suggests that the surface is saturated with a monolayer of naphthalene at 2 L at 110 K. For lower coverage (

Naphthalene on Ni(111): Experimental and Theoretical Insights into

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