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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Maximizing Active Site Concentrations at Ni-Substituted WS Edges for Hydrogenation of Aromatic Molecules 2

Wanqiu Luo, Hui Shi, Manuel F. Wagenhofer, Oliver Y. Gutiérrez, and Johannes A. Lercher J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02203 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry Letters

Maximizing Active Site Concentrations at NiSubstituted WS2 Edges for Hydrogenation of Aromatic Molecules Wanqiu Luo,1 Hui Shi,1,* Manuel Wagenhofer,1 Oliver Gutiérrez,1,§ Johannes Lercher1,*

AUTHOR ADDRESS 1

Technische Universität München, Department of Chemistry, Catalysis Research Center,

Lichtenbergstraße 4, 85748 Garching (Germany) §

Present address: Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O.

Box 999, Richland, WA 99352 (USA) Corresponding Authors * [email protected], [email protected] Tel. 0049 89 28913540, Fax 0049 89 28913544

1 Environment ACS Paragon Plus

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Transition metal sulfide (TMS) catalysts are widely applied in hydrotreating.1-4 TMS-based materials have also found versatile chemical applications in biomass conversion,5 electrochemical H2 evolution,6,7 and, more recently, electrocatalytic CO2 reduction.8,9 Monometallic MoS2 and WS2 are extensively studied members of TMS, while in many cases, transition metal ions, Fe, Co, and Ni in particular, can significantly improve the activities of MoS2- or WS2-based catalysts.2,6,7,10,11 The promotion by Ni and Co in hydrotreating TMS catalysts has been attributed to the formation of a ternary structure, i.e., the so-called “Co(Ni)-Mo(W)-S” phase, in which Co(Ni) atoms substitute Mo(W) atoms in MoS2 or WS2 slabs at the edges.12-14 Segregation of Ni sulfides (NiSx) from the active slab edge, however, is thermodynamically favored at relevant hydrotreating conditions,15,16 and has been reported to be rather pronounced in Ni-promoted Mo(W)S2 catalysts after long periods of utilization.17-19 Large NiSx crystals, in particular, can be located in the pores and cavities of the alumina support, thus, decreasing the surface area, pore volume, and catalytic activity.17 Their presence is often detrimental in bulk TMS-based hydrotreating catalysts as well.17,20 A number of studies consistently reported that a maximum promotional effect of Ni occurred at atomic Ni/metal ratios of 0.3-0.4,21-24 likely as a result of (at least in part) the more significant formation of NiSx phases at higher Ni loadings. The present communication, therefore, reports on a post-synthetic treatment with concentrated HCl to remove inactive NiSx phases without doing harm to the active domains. We show that the rates of phenanthrene hydrogenation started to decrease at Ni-to-metal ratios of 0.2-0.3 for parent Ni-WS2/Al2O3 catalysts; in contrast, the catalytic rates increased monotonically with the Ni loading for acid-treated catalysts up to a Ni-to-metal ratio of about 0.4. Quantitative evaluations of active sites by infrared spectroscopy with probe molecules demonstrate that acid treatment



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unblocks the active slab edge and markedly increases the fraction of exposed Ni at the edge, leading to a five-fold rate enhancement for the best case. The Ni and W loading in the parent Ni-WS2/Al2O3 samples was 1.1-11.1 and 17.2-21.6 wt%, respectively. Compared to the parent sulfides (see Table S1, data taken from Ref.25), Ni concentration was 33-62% lower in the acid-treated and re-sulfided materials (at pH = -1 and 1) without appreciable losses of W, indicating a selective removal of Ni by aqueous HCl at room temperature (Table 1). Specifically, the Ni loss was 39-62% after treatment with concentrated HCl (pH = -1), while being less pronounced at 33-39% after treatment with diluted HCl (pH = 1). The Cl contents measured after HCl treatment (but before re-sulfidation) were somewhat higher in the series that was subjected to treatment with concentrated HCl (suffix “-a”) than in the diluted HCltreated series (0.45-0.60 vs. 0.27-0.35 wt.%; Table S2). After re-sulfidation, however, Cl contents in all samples fell below the detection threshold (equivalent to 0.1 wt.% of a solid sample). Changes in the Ni content did not lead to significant variations in the slab diameter and stacking degree of the main WS2 phase (for representative TEM micrographs, see Figure S1). The crystalline structure of WS2 was retained after treatment with either concentrated or dilute HCl. The peaks from crystalline Ni3S2 were largely eliminated in both cases (see XRD in Figure S2).

Table 1. Atomic Ni/(Ni+W) ratio and Ni loss (compared to the parent) for the leached materials.a

WS2AB

Catalyst

Ni/(Ni+W), mol/mol

Ni loss, %

2O3-a

0

-


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Ni(1.1)WS2AB

2O3-a

0.11

39

Ni(3.2)WS2/B-Al2O3-a

0.19

63

Ni(4.5)WS2AB

0.27

51

0.42

62

2O3-b

0.25

39

Ni(4.5)WS2/B-Al2O3-b

0.33

39

Ni(11.1)WS2/B-Al2O3-b

0.61

33

Ni(11.1)WS2AB Ni(3.2)WS2AB

a

2O3-a

2O3-a

The suffix –a denotes samples obtained after treatment in 11.6 M HCl while the suffix –b denotes samples obtained

after treatment in 1 M HCl; see details in the Experimental in the SI). The number in parentheses refers to the measured Ni loading (in wt.%) in the corresponding parent sample.

The catalytic activities of the (Ni)WS2/Al2O3 samples treated with concentrated and dilute HCl were evaluated for hydrogenation of phenanthrene (denoted as Phe hereafter), a molecule representative of the polycyclic aromatic hydrocarbons to be hydrogenated during hydrotreating. In this connection, we note that Ni-W sulfide catalysts are more efficient than the Co- and Mobased counterparts in (poly)aromatics hydrogenation.25-27 Figure 1 shows the reaction rate of Phe hydrogenation as a function of the measured atomic Ni/(Ni+W) ratio (bulk) for the parent and the acid-treated series of materials. The variation of the Phe conversion with residence time is shown in Figure S3 for the acid-treated (Ni)WS2AB

2O3 catalysts (plots for the parent catalysts are shown

elsewhere25), and the corresponding reaction rates are compiled in Table S3. Hydrogenation of



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Phe produces 9,10-dihydrophenanthrene (DiHPhe) and 1,2,3,4-tetrahydrophenanthrene (TetHPhe) as

primary

products,

while

1,2,3,4,5,6,7,8-octahydrophenanthrene

(symOHPhe)

and

1,2,3,4,4a,9,10,10a-octahydrophenanthrene (asymOHPhe) are secondary products. A simplified reaction network under these conditions comprises two parallel routes,23,25 i.e., initial hydrogenation of the middle ring (Phe O DiHPhe O asymOHPhe) and initial hydrogenation of a lateral ring (Phe O TetHPhe O symOHPhe), and the rates of these two routes are denoted as r1 and r2, respectively. The ratio of r1 to r2 was 4:1 on WS2AB Ni(x)WS2AB

2O3-a

2O3-a,

while being 1:1 to 1:1.7 on

and -b catalysts (Table S3). Products from ring opening and multiple

hydrogenolysis reactions were not detected. These observations are in agreement with those reported for (Ni)MoS2AB

23

2O3

and parent (Ni)WS2AB

25 2O3.

Figure 1. Reaction rates of phenanthrene hydrogenation as a function of the measured Ni/(Ni+W) molar ratio, parent (Ni)WS2/B-Al2O3 (data points labeled with black marker P3) sulfides treated by concentrated HCl (Ni)WS2/B-Al2O3-a (blue Q3) and sulfides treated at pH = 1 (Ni)WS2/B-Al2O3-b (green R38 The data of parent (Ni)WS2/B-Al2O3 are taken from Ref.25


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The yields and selectivities of the products as a function of Phe conversion are shown in Figures S4 and S5. On WS2AB

2O3-a,

secondary products were not detected at

Maximizing Active Site Concentrations at Ni ... - ACS Publications

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