Morphological, Structural and Chemical Properties of Thermally Stable

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C: Physical Processes in Nanomaterials and Nanostructures

Morphological, Structural and Chemical Properties of Thermally Stable Ni-Nb2O5 for Catalytic Applications Glauco Ferro Leal, Dean Howard Barrett, Heloise Carrer, Erico Teixeira-Neto, Santiago JA Figueroa, Antonio A.S. Curvelo, and Cristiane Barbieri Rodella J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09177 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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The Journal of Physical Chemistry

Morphological, Structural and Chemical Properties of Thermally Stable Ni-Nb2O5 for Catalytic Applications Glauco F. Leal a,b, Dean H. Barrett b, c, Heloise Carrer b, Santiago J. A. Figueroa b, Erico Teixeira-Neto d, Antonio Aprigio S. Curvelo a, Cristiane B. Rodellab* a

Department of Physical Chemistry, Institute of Chemistry of São Carlos, University of

São Paulo, Av. Trabalhador São Carlense, 400, São Carlos, SP, 13566-590, Brazil. b

Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in

Energy and Materials (CNPEM), Campinas, SP, 13083-970, Brazil. c

School of Chemistry, University of the Witwatersrand, Jorissen Street, Braamfontein,

Johannesburg, South Africa. d

Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for

Research in Energy and Materials (CNPEM), Campinas, SP, 13083-970, Brazil. *E-mail: [email protected]

Abstract Structural stability is a pivotal property required for Nb2O5 to be applied as a solid-acid catalyst in heterogeneous catalytic reactions. When combined with Ni, Nb2O5 produces cheap and active hydrogenation catalysts. Ni-Nb2O5 operates as a bifunctional catalyst and is being widely explored for various catalytic applications without, however, exploring its structural stability and its effects on catalytic activity and durability. Herein we studied two forms of niobia, one with non-uniform morphology and another comprising a nanorod morphology. Various selected Ni loadings were dispersed on the two supports via a deposition-precipitation method. Physical and chemical characterization revealed that morphological control in combination with a highly efficient Ni deposition method is key in producing a structurally stable Ni-Nb2O5 catalyst. High surface area and porosity as exhibited by the Nb2O5 nanorods, in the pseudohexagonal phase, combined with small, well dispersed Ni particles provides a 1 ACS Paragon Plus Environment

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structurally stable material up to 500oC, with high acidity (Lewis and Brønsted acid sites). Moreover, the local and long-range order, characterized by in situ (XANES and XRD) determined the temperature limits for the optimization of metallic Ni particles in relation to the Nb2O5 structure.

Introduction Niobium oxide (Nb2O5) and mixed oxides containing niobia have been shown to be active catalysts in a number of industrially relevant reactions due to their inherent acidic properties, as well as high activity towards selective oxidation and photosensitive catalytic reactions1–6. Recently, niobium oxide has gained attention in reactions pertaining to the catalytic conversion of biomass, where its high catalytic activity is attributed to its solid-acid and water-tolerant Lewis acid sites which are active for lignocellulosic

biomass

and

derivate

conversions.7,8,17,18,9–16

Besides

catalytic

applications, Nb2O5 is found in chemical sensors, piezoelectric materials, and optical filters19-23. It is well known that Nb2O5 exists as a polymorphic material. It may occur in an amorphous form or in one of its many different crystalline phases, which can be obtained depending on the synthesis method employed to access different chemical and physical properties1,2,20,21,24–29,3–6,10–12,19. The polymorphs are classified, traditionally, according to the temperature at which they crystallize starting with the: TT-phase (monoclinic or pseudo-hexagonal), T-phase (orthorhombic), which crystallizes at lowtemperature (320 – 700oC), B-phase (monoclinic) and M-phase (monoclinic), form at intermediate temperatures (700 – 1000oC) and finally H-phase (monoclinic) forming above 1000oC

20–23.

Figure 1 summarizes the phase transition of Nb2O5 related to the

thermal treatment temperature 22. The TT-phase has been shown to be a type of poorly crystalized T phase which is only stabilized by impurities, such as OH- or Cl-, or vacancies

23.

The TT phase may be obtained by hydrothermal synthesis using

amorphous niobic acid or heating the sulfates or chlorides of niobic acid in air. The Hpolymorph is the most thermodynamically stable Nb2O5 crystalline phase 23.

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Figure 1. Scheme of the phase transition exhibited by Nb2O5 as a function of temperature (extracted from reference 22,30).

Nb2O5 crystals are comprised of distorted octahedra (NbO6) connected via edge and/or corner sharing atoms, with the degree of distortion dependent on the polymorph structure 21–23. The TT and T-phases are both characterized by the presence of distorted octahedra, as well as pentagonal and hexagonal bipyramidal polyhedra, i.e. NbO6, NbO7 and NbO8 polyhedra, are isostructural when compared to amorphous Nb2O5 23,31. Although structural stability increases for polymorphs produced at higher temperatures (B, M, H phase), the contrary occurs with acid site concentration on the surface of Nb2O5 decreasing at higher temperatures. A reduction in the acidic sites directly affects the materials catalytic properties for acid catalyzed reactions 30. The highly distorted octahedral (NbO6) units exhibit Nb=O bonds, which act as Lewis acid sites. However, the slightly distorted NbO6, as well as NbO7 and NbO8 groups, only have Nb-O single bonds producing Brønsted acid sites

7,25,31.

As the Nb5+

cation is very large it finds difficulty inserting into a tetrahedral geometry (NbO4) within the niobium pentoxide30. However, Hara et al.25 have shown that group 5 metal oxides, including TiO2, ZrO2, and hydrated Nb2O5 after drying are characterized by highly distorted tetrahedra (MO4) present on the surface. The Nb-O bonds are highly polarized in these distorted polyhedra (NbO6 and NbO4), allowing some of the surface OH groups to function as Brønsted acid sites, while NbO4 tetrahedra function as Lewis acid sites. Importantly, it was shown that these Lewis acid sites remain active in water despite coordination with water, showing Nb2O5 to be a water-resistant solid-acid catalyst

30–32.

The ability to perform catalytic reactions in aqueous media bring 3 ACS Paragon Plus Environment


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enormous benefits such as non-toxicity, reduced separation, and purification costs as well as greener catalytic processes 33,34. Metals such a nickel as well as platinum group metals supported on Nb2O5 exhibit catalytic activity in important industrial catalytic reactions such as hydrogenation, hydrotreating, hydrogenolysis of hydrocarbons, methanation and steamreforming35. For example, in Fischer-Tropsch synthesis, Rh and Co deposited on niobia are highly selective for C4-C5 hydrocarbon formation and diesel production, respectively

3,35.

Another example is the use of Ni-Nb2O5 for the hydrocracking of

natural gas and oil to produce high octane gasoline with high selectivity compared NiSiO2 and Ni-Al2O3 catalysts3. Research has shown that conversion and product selectivity are strongly influenced by the synthesis method employed, crystallite size and reduction treatment of Ni-Nb2O5

1,3–5.

interactions

1,2

The Ni-Nb2O5 system is known to give rise to strong metal-support with a number of nickel species present on the Nb2O5 surface due to

differences in the synthesis methods and variable strength of the metal-support interactions. Consequently, the reducibility, surface or hydrogenation properties change depending on the synthesis method employed as well as the metal deposition method, metal loading, and particle size of the active phase and activation procedure used 2. At low nickel loadings (1-6 wt.%), temperature programmed reduction (TPR) analysis showed that a large percentage of metal atoms are in close contact with support leading to high reduction temperatures (above 400 oC)

1,5

or non-reducible species 2. Samples

with higher nickel loadings (6-15 wt.%) lead to a decrease in the nickel reduction temperatures (around 300 oC) due to larger nickel particles dispersed over the Nb2O5 surface with hydrogen consumption only detected at higher temperatures relating to the reduction of smaller nickel particles 5. In spite of successful catalytic applications, detailed structural information and the well-established acidic properties for numerous variants of niobium oxide their respective structural stability are still largely unknown. Further studies are vital as their catalytic performance and reusability are directly correlated to these chemical and physical properties. For new applications of niobia-based catalysts such as lignocellulosic biomass conversation, the hydrothermal stability of the catalyst is crucial to ensure a highly active and recyclable catalyst 36. Thus, in this work, we demonstrate 4 ACS Paragon Plus Environment

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the correlation among the synthesis methods of niobium oxide and nickel loadings with structural, electronic and textural stability.

Experimental Nb2O5 synthesis Basic hydrolysis of ammonium oxalate (BH) The Nb2O5 samples were prepared by basic hydrolysis of the ammonium niobium oxalate precursor (ANO) using ammonium hydroxide37. The niobium precursor (2.0 g; ~100%, CBMM) was dissolved in distilled water (20 mL) under magnetic stirring. The pH of the solution was then adjusted to 9.0 by adding NH4OH(aq) (26-30%, Sigma-Aldrich) and the system was kept under stirring for 3 h. The final white solid was centrifuged and washed to neutrality with deionized water and oven dried at 120 oC overnight. The equation below describes the hydrolysis reaction of the niobium precursor. 2𝑁𝐻4[𝑁𝑏𝑂(𝐶2𝑂4)2(𝐻2𝑂)2].(𝐻2𝑂)𝑛𝑁𝐻4𝑂𝐻/𝐻2𝑂𝑁𝑏2𝑂5 + 4𝐶2𝑂4(𝑁𝐻4)2 Hydrothermal synthesis (HT) Samples of Nb2O5 in the form of nanorods were prepared using the ANO precursor obtained commercially from CBMM. The niobium precursor (5.22 g; ~100%, CBMM) was dissolved in 65.3 mL distilled water and left under magnetic stirring until complete solubilization. Following this, about 14.7 ml of hydrogen peroxide (30%) is added in a ratio of 10 M: H2O2/1 M: Nb, resulting in a clear yellowish solution, which indicates the formation of a colloidal solution of niobium peroxide complex (NPC) 38-40. The hydrothermal treatment for nanorod formation was performed with the NPC solution at 175 oC, for 15 hours. The resulting white precipitate (Nb2O5), was washed with distilled water and centrifuged for multiple times to remove impurities and then dried in an oven at 60 °C. The equation below describes the synthesis reaction of Nb2O5; 2𝑁𝐻4[𝑁𝑏𝑂(𝐶2𝑂4)2(𝐻2𝑂)2].(𝐻2𝑂)𝑛𝐻2𝑂2/𝐻2𝑂𝑁𝑏2𝑂5 + 4𝐶2𝑂4(𝑁𝐻4)2 The samples of Nb2O5 are named Nb2O5-BH, representing the basic hydrolysis, and Nb2O5 -HT for the hydrothermal synthesis.



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Nickel deposition onto Nb2O5 The resulting Nb2O5 supports were calcined at 400 oC for 2 h under air. Selected supports then had 5, 10, 15 and 25% Ni loadings by mass deposited onto them. The procedure for nickel deposition is described below. Deposition-precipitation assisted by ammonia (DPNH4) 1.0 g of niobium oxide, 1.98 g (NH4)2CO3 (Sigma-Aldrich) were used in 19 mL water and NiCO3.2Ni(OH)2.xH2O (99.9%, Sigma-Aldrich) sufficient to obtain 5, 10, 15 and 25 wt. % Ni loadings. Using an adapted method, 19 mL of NH4OH(aq) (26-30%, Sigma-Aldrich) was added to the solution to raise the pH to 11.5 at the start of the synthesis

41,42.

The system was kept under stirring for 3 h at 90 °C under a nitrogen

atmosphere. The resulting solid was then washed to neutrality, separated by centrifugation and oven dried overnight at 120 °C. In situ X-ray Diffraction (XRD) analysis during TPR process In situ XRD measurements were carried out at the XPD beamline at the Brazilian Synchrotron Light Laboratory (LNLS). The experiment consisted of thermal treatment using a sample furnace from room temperature up to 700 oC, with a heating rate of 5 oC.min-1 and under a flow of 50.0 mL.min-1 of synthetic air or 5% H2/He. The beamline energy was set at 8.0 keV, corresponding to a wavelength of 1.5498 Å. The Xray diffractograms were obtained using a Dectris Mythen 1K linear detector installed 1 m from the sample position. The ICSD (Inorganic Crystal Structure Database) was used to obtain the relevant CIF files. In situ Absorption Edge X-ray Absorption Spectroscopy (XANES) analysis during TPR process In situ XANES measurements at the Ni K-edge (8333 eV) were completed at the DXAS beamline at the LNLS synchrotron43. The samples were analyzed in the form of pellets inside a tubular reactor. The heat treatment consisted of a heating ramp of 5°C.min-1 to 600 °C under a flow of 50 mL min-1 of a 5% H2/He gas mixture. Isothermal studies of 1h each were also carried out on selected samples at 280, 300, 320 and 360 °C. The XANES spectrum of Ni foil and Ni reference materials were obtained at room temperature. Data analysis was conducted using conventional methods for background extraction and normalization using Prestoponto44 and Iffefit packages45. Principal component Analysis (PCA)46,47 was performed using Prestopronto. 6 ACS Paragon Plus Environment

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N2 physisorption N2 physisorption analysis at -196 oC was used to investigate the surface area and the porosity of the materials. The samples were heat treated under vacuum at 120 °C overnight to remove water and/or adsorbed volatile compounds. The surface area, pore volume, and pore diameter were then determined by adsorption-desorption isotherms of N2 at 77 K (-196 oC) in an Autosorb 1C (Quantachrome) instrument at the LNLS. The surface area was determined by the BET method. The pore volume was calculated as w / w0 = 0.95 and the pore size distribution was obtained from the desorption isotherm and calculated by the BJH method 48. HAADF-STEM and EDX-SI Catalysts were investigated by the acquisition of high angle annular dark-field (HAADF-STEM) images in an aberration corrected scanning transmission electron microscope (FEI Titan Themis 60-300 operating at 300 kV). The samples were prepared by directly applying the dry catalyst powder onto standard 400 mesh TEM cooper grids. Compositional maps were acquired by EDX spectrum imaging (EDX-SI) using a SuperX detector. Complete EDX-spectra were acquired at each SI in the 0-20 keV energy range. ATR-FTIR of Pyridine Adsorbed on Ni/Nb2O5 The acidic properties of niobium oxide support were evaluated by attenuated total reflection Fourier Transform infrared spectroscopy (ATR-FTIR) of pyridine (Py) adsorbed on Ni/Nb2O5. FTIR spectra were collected on a Spectrum Two FT-IR spectrometer from Perkin Elmer in the range of 4000 to 400 cm-1 at a resolution of 0.5 cm-1. 32 scans were collected for signal accumulation. Initially, the samples in powder form were dried at 150 oC for 1h, under N2 flow, in a three-neck rounded bottle flask (50 mL) equipped with a heating mantle. N2 carried Py vapour was introduced into the flask for the adsorption of pyridine on Ni/Nb2O5 and Nb2O5 samples. The sample was exposed to Py vapour at 150 oC for 1 h. Then, the flask was purged with N2 at 120 oC, for 30 min, to remove physisorbed Py. After cooling to room temperature, the sample was transferred to an Eppendorf and the FTIR spectra were collected with the ATR probe. The spectrum of adsorbed Py was obtained by subtracting the spectrum of the sample with chemisorbed Py from that of the treated sample without pyridine.



Results To explore the influence of the synthesis method on the structural characteristics of the Nb2O5 produced, in-situ XRD was carried out during the calcination process. Figure 2 shows the diffractograms depicting the structural evolution of Nb2O5-BH. The temperature at which the amorphous phase crystalizes into the TT-phase is highlighted in red, with blue representing the initial point of the TT to T-phase transition.

(101)

(100)

(201)

(200)

(181)

(180)

(001)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

637 C 0

450 C 20

22

24

26

28

30

32

34

36

38

40

o

2 ( )

Figure 2. In situ synchrotron XRD of Nb2O5-BH showing the evolution of the diffraction peaks between 2θ = 29.5o and 36.9o. Crystallization of the TT phase (pseudohexagonal) shown in red and the phase transition to the T (orthorhombic) phase shown in blue. JCPDS 7-61 and JCPDS 30-873 The diffraction data shows that niobia synthesized via basic hydrolysis is initially amorphous with crystallization occurring via the formation of pseudohexagonal (TT-phase) at 450 oC. At higher temperatures, the phase transition to the orthorhombic structure (T-phase) takes place at 637oC. The TT-phase is characterized by the (001), (100) and (101) peaks in the angular range as shown in Fig. 2. The transformation to Tphase is characterized by the peak split at 32o and 40o 2θ attributed to the (180) - (200) and (181) – (201) reflections, respectively. The XRD patterns of niobia obtained via hydrothermal synthesis (Nb2O5-HT) exhibits peaks relating to TT (pseudohexagonal) phase as shown in Figure 3. Diffraction peaks resulting from the (001) reflections are observed to be more intense relative to the (100) peak intensities in the Nb2O5-HT sample due to anisotropic growth in the (001) direction as shown in Figure 3. This reveals a high aspect ratio nanorod morphology 8 ACS Paragon Plus Environment

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49,50.

However, above 390 oC the intensity of the (100) peak starts increasing,

characterizing a more isotropic crystallite. This implies that the nanorod morphology is lost when the calcination temperature increases above 390 oC. At 430 oC the phase transition from TT to T phase occurs.

TT to T Transition Isomorphic Transition

o

550 C o

500 C

Intensity (a.u.)

o

460 C o

444 C o

430 C o

414 C o

o

300 C

(101)

(100)

391 C

(001)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

25 C 20

25

30

35

40

o

2 ( )

Figure 3. In situ synchrotron XRD characterization during the calcination process of Nb2O5-HT obtained by hydrothermal synthesis. The pseudohexagonal phase (TT) presents peaks in hachured area in red, related to (001), (100) and (101) peaks. The hachuread are in blue shows the orthorhombic phase (T) which peaks are (001), (180), (200), (181) and (201). The textural properties of the samples were investigated by N2 adsorptiondesorption isotherm profiles (Figure S1) with Table 1 summarizing these properties. Table 1. Textural properties of the Nb2O5 samples after calcination at 400oC. Sample

Surface Area

Pore

Average Pore



(m2.g-1)

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Volume

Diameter

(cm3.g-1)

(nm)

Nb2O5-BH

46

0.096

9

Nb2O5-HT

196

0.276

6

Nb2O5-BH presents type IV(a) isotherms and H2(b) hysteresis, suggesting the presence of non-structured mesoporosity, in addition to structural ink-bottle-shaped mesopores. By contrast, Nb2O5-HT exhibits a mixed type I and II isotherm and a mixed H3 and H4 hysteresis, indicating the presence of macro and micropores.51 Markedly, Nb2O5-HT showed a surface area that is ≈ 4 times larger than that of Nb2O5-BH. Furthermore, the pore volume of Nb2O5-HT is twice as large as that of Nb2O5-BH. These textural properties are reflected in high aspect ratio nanoparticles of niobia in a nanorod morphology

50.

Both niobia samples, BH and HT, exhibit a wide pore-size

distribution (Fig. S2) in the mesoporous region. However, for Nb2O5-HT the pore-size distribution increases in the micropore limit of the curve which is in accordance with the high surface area obtained by these samples. STEM analysis was performed to investigate the morphology of the Nb2O5 samples (Fig. 4). The STEM images obtained in dark field show the Nb2O5-HB is comprised of porous niobia particles with no defined shape, whereas the Nb2O5-HT shows randomly oriented, anisotropic and crystalline Nb2O5 nanorods, in agreement with the synthesis methodology and XRD results (Fig. 2 and 3). B)


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A)

B)

C)

D)

Figure 4. STEM micrographs of A) and B) Nb2O5-BH and C) and D) Nb2O5-HT.

Deposition of Ni onto Nb2O5 UV-VIS spectra obtained from the deposition solutions after synthesis are shown in Figure S3. In all cases, loadings efficiencies were quantified above 94 %, irrespective of the metal loading, implying a high loading yield using the DPNH4 method as shown in Table 2. Table 2. Results of nickel loading and yield derived from UV-VIS spectroscopy. Sample

Ni deposited

Loading Efficiency

(± 0.3%)

(± 3 %)

5% Ni/Nb2O5-HB

4.7

94

10% Ni/Nb2O5-HB

9.9

99

15% Ni/Nb2O5-HB

14.7

98

25% Ni/Nb2O5-HB

24.9

99

5% Ni/Nb2O5-HT

4.7

94 11

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10% Ni/Nb2O5-HT

9.7

97

15% Ni/Nb2O5-HT

14.8

99

25% Ni/Nb2O5-HT

24.1

96

TPR experiments were performed to characterize the reduction events of Ni/Nb2O5 as shown in Figure 5. b)

I

25% Ni/Nb2O5-BH

II

15% Ni/Nb2O5-BH

10% Ni/Nb2O5-BH

H2 Consumption (a.u.)

a)

H2 Consumption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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I

25% Ni/Nb2O5-HT

II

15% Ni/Nb2O5-HT

10% Ni/Nb2O5-HT

5% Ni/Nb2O5-BH 5% Ni/Nb2O5-HT

Nb2O5-BH

Nb2O5-HT

100 200 300 400 500 600 700 800 900 1000

100 200 300 400 500 600 700 800 900 1000 o

Temperature ( C)

o

Temperature ( C)

Figure 5. TPR profile of Ni/Nb2O5-HB and Ni-Nb2O5-HT (temperature error ± 5oC). The hachured area shows the two reduction events. Both Ni/Nb2O5-HB and HT show two reduction events labeled as I and II in Figure 6. The first, from approximately 260 oC to 410 oC is assigned to the reduction of Ni(II) species to metallic Ni. These broad and asymmetric reduction peaks reflect a convolution of reduction events related to a variety of Ni species supported on niobia with differing metal-support interaction strengths as well as different Ni particle sizes1. The reduction events are seen to shift to lower temperatures as the Ni loadings are increased. The peak position of the first TPR event shifted slightly to higher temperatures in the 15 and 25% Ni-Nb2O5-BH (Fig. 5 a)) samples in comparison with samples with the same Ni loadings but supported on Nb2O5-HT (20oC). Due to a 12 ACS Paragon Plus Environment

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diverse range of Ni species interacting with the niobia in both samples, a broad TPR peak in both sets of samples (Fig 5 a and b) is noted. The second peak is assigned to the reduction of the niobium support above 800 oC 52. To further understand structural changes occurring in the catalysts during the activation, TPR experiments were performed using in situ synchrotron XRD. Samples containing 5 - 25% Ni loadings supported on both niobia (HT and BH) were measured. The results are shown in Figures 6 a) and b) and Figure S4-S11 (supporting information). TT to T Transition

a)

Nb2O5 Crystalization 0

Ni Crystalization

o

607 C

o

574 C

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

509 C

o

455 C o

382 C o

372 C o

301 C o

253 C o

198 C o

25 C 30

35

40

45

50

55

o

2 ( )



TT to T Transition Nb2O5 Crystalization

b)

0

Ni Crystalization

o

610 C

o

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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570 C o

533 C o

484 C o

387 C o

293 C o

190 C o

25 C 30

35

40

45

50

55

o

2( )

Figure 6. In situ XRD of the Ni activation process of a) 10% Ni/Nb2O5-BH and b) 10% Ni/Nb2O5-HT. The initial detection of the nickel phase is shown in green, the niobia crystallization to TT phase is highlighted in red, and the transition from TT to T phase is highlighted in blue. The diffractograms show that in both HT and BH samples, the structural evolution of Nb2O5 is the same as those without Ni (Fig. 2 and 3). Moreover, the initial Ni species (oxide/hydroxide/nickel carbonate) diffraction peaks were not detected (5-25 wt.%) at room temperature. Table 3 reports the initial temperature of the phase transitions of Ni and Nb2O5 phases determined from in situ XRD. Ni reduction and crystallization occur within temperature ranges similar to those determined from TPR analysis (Fig. 5).


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Table 3. Ni reduction temperature and phase transition temperatures derived from insitu XRD. Sample

Phase transition temperature (oC) (±10oC) Ni

Nb2O5 (TT)

Nb2O5 (T)

Nb2O5-BH

---

450

637

5% Ni/Nb2O5-BH

299

387

572

10% Ni/Nb2O5-BH

265

382

543

15% Ni/Nb2O5-BH

300

447

602

25% Ni/Nb2O5-BH

272

439

623

Nb2O5-HT

---

396 *

430

5% Ni/Nb2O5-HT

305

530

562

10% Ni/Nb2O5-HT

293

511

580

15% Ni/Nb2O5-HT

315

550

577

25% Ni/Nb2O5-HT

268

548

579

* Grey column shows the temperature where Nb2O5 changes from nanorod to isotropic morphology. Examination of the Ni/Nb2O5-BH results shown in Table 3 show the structural transformation of Nb2O5 from amorphous to TT-phase and subsequently to the T-phase is not affected by higher Ni loadings (15 and 25%). The crystallization temperature, as well as the transformation from TT to T phase of these Ni-supported samples, are similar to the pure Nb2O5 (Table 1). However, in the case of samples with 5 and 10% Ni loadings, their respective crystallisation and phase transition temperatures are quite different to that of pure Nb2O5-BH. Possibly, lower Ni concentrations interact with niobia to a higher degree which slows Nb mobility and inhibits crystallization and phase transformations 1,21. In the case of Ni/Nb2O5-HT, the column highlighted in grey in Table 3, represents the temperature at which the anisotropic morphology of the nanorods is lost. This is derived from peak analysis of the (001) reflection as its peak intensity increases but the structure remains as TT-phase. It is further noted that after losing the nanorod 15 ACS Paragon Plus Environment


morphology, the transition from TT to T-phase occurs a few degrees celsius above this point. The same structural modification was characterized in pure Nb2O5-HT, whereby the crystal morphology becomes isotropic at 396 oC with the phase transition to Tphase occurring around 440 oC. Importantly, however, the loss of the nanorod morphology occurs in all cases, above 500 oC when Ni is present providing good thermal phase and morphological stability. From the collected diffractograms the formation and evolution of nickel crystallites as a function of reduction were calculated from the Ni (111) peak. The average crystal size and the area (integrated intensity) of the Ni (111) peak were determined for all diffractograms and plotted as a function of the reduction temperature. The structural evolution determined from mean crystallite size and integrated intensity are presented in Figures 7 and 8. 75

65

10

60 55

9

50

8

45

7

40 o

6

572 C

35

50

22 20

40

18

30

16

20

o

543 C

o

382 C

14

30

250

300

350

400

450

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550

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600

250

300

o

55

40

75

35

15%Ni-Nb2O5-BH

30 25

25 o

447 C

20

0 500

600

550

600

105

90

150

Area (a.u.)

45

450

550

175

Average crystal size (nm)

100

400

500

200

50

350

450

Temperature ( C)

125

300

400

o

Temperature ( C)

50

350

125

75

100 60

75

25%Ni-Nb2O5-BH

50

45

25 0 250

o

439 C 300

o

Temperature ( C)

350

400


o

387 C

60

24

Area (a.u.)

70

11

10%Ni-Nb2O5-BH

26


12

Area (a.u.)

70

80

5%Ni-Nb2O5 -BH


13

Area (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 36

30

450

500

550

600

o

Temperature ( C)

Figure 7. Integrated intensity and average crystallite size of the Ni (111) peak obtained from in situ synchrotron XRD during TPR analysis of Ni/Nb2O5-BH. Vertical dashed lines indicate the temperature at which Nb2O5-BH forms the TT-phase and then to Tphase at higher temperature.


120

5%Ni-Nb2O5-HT

35

30

20

25

18 20

16 14

o

530 C

15

12

50 45

80

40

60

35 30

40

25

o

o

580 C

511 C

20

20

o

562 C

15

10 350

400

450

500

550

600

650

300

700

350

400

450

500

550

600

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o

Temperature ( C)

o

Temperature ( C) 200

80

40

45

60

15%Ni-Nb2O5-HT 35

40 30

30

o

577 C

o

550 C

20 350

400

450

500

550

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25

650

700

35 150

Area (a.u.)

40

175

Avarege crystal size (nm)

70

50

55

30

25%Ni-Nb2O5-HT

125

25

100

20

75

15 o

579 C

o

10

548 C

50 300

350

400

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500


Area (a.u.)

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60

10%Ni-Nb2O5-HT

100

Area (a.u.)

24

Avarege crystal size (nm)

26

Area (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 17 of 36

550

600

650

700

o

Temperature ( C)

o

Temperature ( C)

Figure 8. Integrated intensity and average crystallite size of the Ni (111) peak obtained from in situ synchrotron XRD during TPR analysis of the Ni/Nb2O5-(HT) samples. Dashed lines indicate the temperature at which Nb2O5-HT changes from nanorod to an isotropic morphology in the TT-phase and then to T-phase at higher temperature.

To further explore the structural changes in the local order of the nickel species during Ni activation, in-situ dispersive X-ray adsorption (XANES) measurements at the Ni K-edge (8333 eV) were performed. XANES data were collected continuously from room temperature up to 600 oC, the results are shown in the supporting information Figures. S14 and S15. Figure 9 shows the normalized XANES spectra obtained during the reduction of the Ni/Nb2O5 samples for selected temperatures for clarification. The spectrum of Ni metal foil is also shown.

Prior to thermal treatment, the XANES spectra (Fig. 9 as-prepared) exhibit a low-intensity pre-edge (1s  3d transition) and high-intensity main edge (1s  4s transition) typical of Ni2+ in octahedral geometry from the oxide/hydroxide/nickel carbonate35.



a) 5%Ni-Nb2O5-BH

Normalized xE)

as-prepared 280°C 300°C 320°C 360°C 400°C 600°C Ni foil

8320

8330

8340

8350

8360

8370

8380

Energy (eV)

b) 10%Ni-Nb2O5-HT

as-prepared 280°C 300°C 320°C 360°C 400°C 600°C Ni foil

Normalized xE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

8320

8330

8340

8350

8360

8370

8380

Energy (eV) Figure 9. Ni K-edge in-situ XANES spectra from (a) 5% Ni/Nb2O5-BH and (b) 10% Ni/Nb2O5-HT obtained at the Ni K-edge during the Ni activation process. It is observed in Figures 9, S14 and S15 that the XANES spectrum of the samples shows a pre-edge feature gradually forming as the intensity of the main edge decreases as the reduction temperature increases. This is indicative of the reduction of nickel oxide to metallic nickel. Interestingly, the XANES profiles at the Ni K-edge for all studied Ni-Nb2O5 samples (Figures. 9, S14 and S15) are similar but not identical to the Ni foil above 400 oC up to 18 ACS Paragon Plus Environment

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600 oC. This indicates that Ni species in the samples have on average a short-range structure which differs from that of metallic Ni. Due to the variety of Ni species that may be formed during the TPR treatment, PCA was performed using the normalized in situ XANES spectra of the 5%Ni-Nb2O5-BH (Fig. S21) and 10%Ni-Nb2O5-HT (not shown) samples. The results of PCA determined that at least 3 different Ni species contribute to the XANES signal in the TPR experiment (Figure S21). The first is related to the initial chemical state of Ni in the samples, present as Ni+2 in an octahedral geometry. The second specie is Ni in the metallic state. Finally, the third Ni specie is formed during the thermal treatment and is present up to 600oC in combination with metallic Ni. XANES simulation of the references (Figure S22) indicates that a small proportion of NiNb2O6 is likely to form during the TPR treatment due to the nickelniobia SMSI`s interaction 1. The textural properties of the Ni/Nb2O5 were investigated by N2 physisorption, (Figure S17-20), with Table 4 summarizing the results obtained. Ni/Nb2O5-BH showed type IV(a) isotherms and H2(b) hysteresis, suggesting the presence of nonstructural mesoporosity, in addition to structural ink-bottle-shaped mesopores. In contrast, Ni/Nb2O5-HT exhibits a mixed type I and II isotherm with a mixed H3 and H4 hysteresis, indicating the presence of macro and micropores51. This textural characteristic is compatible with a rod-shape morphology obtained from the hydrothermal synthesis. The nanorods also provide high surface area, approximately 4 times that of Ni/Nb2O5-BH and microporosity originating from the morphology of each nanorod. Furthermore, the pore volume of Ni/Nb2O5-HT is twice that of Ni/Nb2O5-BH. Both materials Ni/Nb2O5-BH and Ni/Nb2O5-HT exhibit a wide pore-size distribution (Figure S18 and S20, Table 4) in the mesoporous region. However, for Ni/Nb2O5-BH, the distribution extends over into the mesoporous range (1-100 nm). On the other hand, for Ni/Nb2O5-HT, the pore-size distribution increases in the microporous range of the curve, which agrees with the high surface area obtained. Ni deposition did not restrict surface area and porosity of Ni/Nb2O5-HT, except at 25 wt.% Ni loadings, where it causes a decrease in the textural properties of the catalyst. For Ni/Nb2O5-BH, only Ni loadings above 15 wt.% showed smaller mesopores being blocked by Ni which is confirmed by the pore-size distribution curve. 19 ACS Paragon Plus Environment


Page 20 of 36

Table 4 – Textural properties of the Nb2O5 samples with different Ni loadings. Surface

Pore

Average Pore

Area

Volume

Diameter (nm)

(m2.g-1)

(cm3.g-1)

5% Ni-Nb2O5-BH

44

0.104

9.4

10% Ni-Nb2O5-BH

48

0.120

10.1

15% Ni-Nb2O5-BH

51

0.151

11.8

25% Ni-Nb2O5-BH

36

0.106

12.6

5% Ni-Nb2O5-HT

219

0.284

5.2

10% Ni-Nb2O5-HT

181

0.251

5.5

15% Ni-Nb2O5-HT

180

0.279

6.2

25% Ni-Nb2O5-HT

141

0.220

6.2

Sample

To investigate the morphology of the samples, STEM analysis was undertaken on 15% Ni/Nb2O5-HB and 15% Ni/Nb2O5-HT. Figures 10 and 11 show the resulting STEM images, respectively.


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A)

B)

b)

b)

b)

b)

b)

b)

C)

D)

b)

b)

b)

b)

b)

b)

Figure 10 - Micrographs of A) 15% Ni/Nb2O5-HB; B) Ni and Nb mapping; C) Ni mapping on 15% Ni/Nb2O5-HB and D) Nb mapping on 15% Ni/Nb2O5-HB.



A)

B)

b)

b)

b)

b)

b)

b)

C)

D)

b)

b)

b)

b)

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Page 22 of 36

Figure 11 - Micrographs of A) 15% Ni/Nb2O5-HT; B) Ni and Nb mapping; C) Ni mapping of 15% Ni/Nb2O5-HT and D) Nb mapping of 15% Ni/Nb2O5-HT.

Figures 10 and 11 show that even after nickel deposition and activation, niobia retains the same initial morphology (Fig. 5). The non-defined and porous morphology of the Nb2O5-HB is preserved as well as the nanorod shape of Nb2O5-HT. Ni forms approximately spherical particles and presents non-uniform particle size distribution from 5-25 nm. Small Ni particles 5 nm are dispersed on the surface of the Nb2O5, which indicates strong interactions established between Ni and Nb2O5. Moreover, bigger Ni particles (5 nm) are separated from each other which may prevent the sintering of the Ni phase. 22 ACS Paragon Plus Environment

Page 23 of 36

To assess the nature of acidic sites of the catalysts especially after Ni deposition onto the niobia, ATR-FTIR adsorption spectra of pyridine adsorbed on Ni/Nb2O5-BH and Ni/Nb2O5-HT were collected (Figure 12 a and b). The vibrational modes of Py affected by the interaction with solid acid sites are ring stretching modes 19b and 8a 53, which occur normally at 1439 and 1583 cm-1, respectively.54 Considering the Py adsorbed on Nb2O5, bands in the wavenumber regions of 1610, 1480 and 1440 cm-1 are related to Py coordinated to Lewis acid sites (LAS), whereas 1540 cm-1 is related to pyridinium ion formation on Brønsted acid sites (BAS). 55,56 L

1445

1540

B

L

L+B 1489

1607

a)

1639

B

Absorbance (a.u.)

25% Ni/Nb2O5-BH

15% Ni/Nb2O5-BH 10% Ni/Nb2O5-BH 5% Ni/Nb2O5-BH Nb2O5-BH

1700

1650

1600

1550

1500

1450

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1350

-1

Wavenumber (cm )

L

L L+B

1446

1540

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1489

1606

1639

b)

B

25% Ni/Nb2O5-HT

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15% Ni/Nb2O5-HT

10% Ni/Nb2O5-HT

5% Ni/Nb2O5-HT

Nb2O5-HT 1700

1650

1600

1550

1500

1450

1400

1350

-1

Wavenumber (cm )



Page 24 of 36

Figure 12. The ATR-FTIR spectrum of pyridine absorbed on a) Ni/Nb2O5-BH; and b) Ni/Nb2O5-HT. Ni/Nb2O5-HT shows both LAS and BAS (Fig. 12 b), whereas Ni/Nb2O5-HB (Fig. 12 a) shows only LAS. Moreover, according to the quantitative results in Table S1, Nb2O5-HT showed superior acidity when compared to Nb2O5-BH.

Discussion Two methods were applied for the synthesis of two differing morphological structures of Nb2O5. One via basic hydrolysis (BH) method, similar to the most commonly employed method used for heterogeneous catalysis applications

37,57.

This

method produced amorphous Nb2O5 with no defined shape, and relatively low surface area (46 m2/g) and porosity (0.096 cm3/g), (Table 1 and Figs. S1 and S2). The second method employed a hydrothermal synthesis (HT) to obtain niobia in a nanorod morphology with high surface area (196 m2/g) due to the high aspect ratio with larger porosity (0.276 cm3/g) 58. Moreover, the HT method resulted in a nanocrystalline niobia in TT-phase. Concerning the acid properties of the samples (Fig. 12, Table S1), the HT method resulted in a niobia presenting Lewis and Brønsted acid sites. The amorphous Nb2O5-BH showed primarily Lewis acidity with fewer Brønsted acid sites, both less intense than in the nanorod (HT) niobia. Textural and acid properties are fundamental characteristics for a solid-acid and Ni supported catalyst for catalytic applications 59–63. As shown in Figure 1, increasing calcination temperatures result in structural transformations within the Nb2O5. However, this process also leads to decreases in the surface area and porosity and consequently, reduces the acidic properties of the Nb2O 15. Surface areas of amorphous Nb2O5 are typically around 130 m2.g-1 with pore volumes around 0.11 cm3.g-1. These values decrease to 65 and 12 m2.g-1 and 0.10 and 0.08 cm3.g -1 for TT and T-phase respectively. Surface area values below 1 m2.g-1 and < 0.01 cm3.g-1 are common for the H-phase 7. According to the STEM analysis (Fig. 4), the hydrothermal synthesis yielded Nb2O5 nanorods with approximately 8-25 nm length and 3-4 nm width. The crystal growth in the hydrothermal method follow a oriented attachment mechanism,58 producing smaller nanorod than using a surfactant as a shape directing agent that provides a slower crystal growth and produces bigger nanorods.37 24 ACS Paragon Plus Environment

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The enhanced acid properties of Nb2O5 (HT) nanorods obtained herein reside in the combination of a pseudohexagonal structure combined with the high aspect ratio morphology. TT-phase is formed by distorted octahedral (NbO6) units exhibit Nb=O bonds (related to Lewis acid sites) and slightly distorted NbO6, NbO7 and NbO8 groups presenting Nb-O single bonds (related to Brønsted acid sites)

7,25,31

.

These surficial

groups were numerically amplified due to the nanorod morphology, resulting in a more acidic niobia when compared to the amorphous niobia. However, the TT structure is only stabilized by impurities, such as OH- and oxygen vacancies

23

. The presence of non-coordinated surface atoms is likely to act to

increase the surface energy of the nanorods. The nanoscale stability of TT-phase is then determined by the competition between the surface and bulk energy of the Nb2O5 nanorod

64-66.

Compared to amorphous Nb2O5-BH, the nanorod Nb2O5-HT is

structurally more unstable. In situ XRD results (Table 3) showed the nanorod morphology is lost at 390 oC and the transition to T-phase occurs approximately at 440 oC. On the other hand, amorphous Nb2O5-BH crystallizes to TT-phase only at 450 oC and the transition to T-phase started at 637 oC. Effective deposition of the nickel species using precipitation-deposition method requires adjusting the pH during synthesis which in turn needs the isoelectric points (IEP) of the supports to be determined.67 IEP`s between 2 and 4 were determined for both supports. A starting pH of 10.5 was therefore selected via the addition of NH4(aq), the pH of the solution is chosen to be higher than that of the IEP of the Nb2O5 surface ensuring strong electrostatic interaction between the Nb2O5 surface and the nickel cationic species. According to the quantification shown in Table 3, the amount of Ni deposited was very close to that of the nominal quantity for all samples, showing an efficient deposition of the metal onto the niobia surface. TPR analysis showed broad and asymmetric reduction peaks for all Ni loadings supported on Nb2O5 prepared by basic hydrolysis as well as for the niobia nanorods. These results reflect the convolution of reduction events related to a variety of Ni species supported on niobia with different metal-support interaction strengths as well as differing Ni particle sizes1,3–5,67. STEM analysis (Fig. 10 and 11) are in agreement with these findings. STEM results with Ni mapping revealed that in both niobia types, (BH and HT), Ni is well dispersed forming 25 ACS Paragon Plus Environment


Page 26 of 36

two particles size distributions, one composed of very small particles (5 nm), below that of the mapping pixel resolution of the analysis and a second group formed by larger nanoparticles (10 nm). R. Wojcieszak et al.1 studied the strong metal-support interaction in catalysts containing 1 and 5 wt.% Ni supported on Nb2O5. TEM analysis of the 5 wt.% Ni/Nb2O5 sample showed Ni particles of 30 nm on average and very small particles 1 nm, below the resolution of the microscope used which are similar to the results obtained here. The authors detected that nickel reduction occurred only in ions weakly attached to the support. This happened after successive Ni depositions onto the support from 1wt. to 5wt.% of Ni loading. According to the authors findings, Ni strongly interacting with the niobia surface was not active for benzene hydrogenation reactions. From the STEM images shown, mapping of Ni revealed that the Ni nanoparticles agglomerate to a higher degree in 15% Ni/Nb2O5-BH than in the 15% Ni/Nb2O5-HT sample. Furthermore, is evident there are far higher numbers of very small Ni particles dispersed on the support than in the 15% Ni/Nb2O5-BH sample. These results are in accordance with BET, and in situ XRD results. High surface area and porosity of Nb2O5 nanorods in association with the Ni deposition methodology provides a better Ni dispersion on the Nb2O5-HT surface. Interestingly, the presence of Ni on the Nb2O5-HT nanorods stabilizes the pseudo-hexagonal structure (Table 3) at higher temperature when compared to the thermal stability of the pure Nb2O5-HT. With Ni chemically bound to the surface via the deposition-precipitation method and strong-metal-support-interaction, a stabilization of the surface is realized leading to higher thermal stability when compared to samples void of nickel. In agreement with this, ATR-FTIR spectrum of absorbed pyridine (Figure 12) showed a small reduction in both Lewis and Brønsted acid sites when Ni was deposited onto the nanorods. Figure 12 shows the intensity of the BAS band decreasing as nickel loading increases. Positively charged nickel species are likely to replace the BAS of surface hydroxyl groups. However, even with 25 wt.% Ni, the niobia still shows BAS characterized by an FTIR band at 1639 cm-1. LAS character (bands at around 1606 cm-1 and 1446 cm-1) are preserved after nickel deposition with small decreases in LAS as Ni loading increases. This shows that Ni interacts with the vacant d-orbitals of the Nb exposed due to oxygen vacancies of TT-phase23. These


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interactions work to decrease the surface energy of the nanorods resulting in a more thermally stable pseudohexagonal phase niobia. The integrated intensity of the Ni (111) peak, shown in Fig. 7 and 8, provides information regarding the abundance of crystalline Ni on the sample during the reduction process.68 The area increases rapidly once the initial crystallization of Ni begins and is independent of the Ni loading. Small decreases in the abundance of crystalline Ni are observed near to the phase change of Nb2O5 from the TT to T phase in the Ni/Nb2O5-BH samples as well as during the loss of the nanorod structure for the Ni/Nb2O5-HT samples. The dashed lines in Fig. 7 indicate the temperature where the niobia phase transition takes place. In the case of Fig. 8, the first dashed line marks the temperature where niobia changes from nanorod to isomorphic. These observations likely result from the formation of a nickel niobate compound (NiNb2O6) at the interface of the nickel-niobia due to the strong metal support interactions1. Moreover, these events are followed by a reduction in the surface area and porosity of the Nb2O5 (Table 6). This is followed by a constant increase in the average crystallite size (Fig. 7 and 8 blue curves), especially above the niobia phase transition temperature resulting in modification of the morphology. This, in turn, is related to Ni sintering and crystallization in the samples. At higher temperatures, the quantity of crystalline Ni increases again, indicating that Ni is likely extracted from the nickel niobate and reconstituted to form metallic Ni once again. This structural evolution is evident in both Ni/Nb2O5-HT and Ni/Nb2O5-HB with 5 and 10 wt.% Ni. At higher Ni loadings (15 and 25 wt.%) the NiNb2O6 formation is less pronounced. The global effect of Ni, which strongly interacts with the niobia surface, is greater in the samples with low Ni loadings (5 and 10% Ni) than those with higher loadings (15 and 25% Ni). At high loadings of Ni, the average crystal size and the crystalline quantities are dictated by larger nickel crystallites which are more prevalent in these samples. This result is in agreement with TPR analysis which showed a variety of Ni interaction strengths with niobia (Fig. 5). Ni reduction commenced at higher temperatures in samples with low nickel loadings due to a higher proportion of strongly interacting Ni particles directly in contact with the Nb2O5 surface.



Page 28 of 36

Investigating the local order of Ni (in situ XANES analysis) during the same reduction process revealed that Ni is never completely reduced (Fig. 9). The data showed that while the profiles of the XANES spectra approach that of the Ni reference foil they still differ even after heating to 400 oC and 600 oC, where the complete reduction of nickel would be expected according to the TPR analysis (Fig. 5). This is caused by SMSI`s which hinder the reduction of nickel due to strong interactions with the Nb2O5 surface. XANES simulations

69

of the Ni0 and NiNb2O6 references and the

combination of the different proportion between them (Figure S22) revealed the average of the 90-95% Ni foil and 10-5% NiNb2O6 structures can reproduce the main features observed at white line of the measured samples above 400 0C. This indicates that a small proportion of NiNb2O6 is likely to form during the TPR treatment in our samples due to the nickel-niobia strong interaction.

Conclusion The physical, chemical and morphological characterization techniques applied here show that the thermal stability of Ni/Nb2O5 was optimized which opens new possibilities for its catalytic application. The combination of high aspect ratio niobia nanorods with Ni deposited via a deposition-precipitation method are able to produce a structurally stable material up to 500 oC while retaining high concentrations of both Lewis and Brønsted acid sites. Furthermore, in situ XRD and XANES studies during the reduction process revealed that Ni is not completely reduced even at a temperature of 600 oC, where high temperatures cause niobia phase transitions. A nickel niobate is likely to be formed at the interface of the nickel-niobia due to strong metal support interactions.

Acknowledgements The authors are grateful to LNLS/CNPEM for the infrastructure (XPD and DXAS beamlines and chemistry laboratory) and staff, LNNano for the STEM infrastructure, CNPq for Glauco F. Leal`s PhD scholarship (process no 165106/2014-0), Capes for Glauco F. Leal PDSE scholarship (process no 88881.132245/2016-01) and FAPESP for Heloise Carrer scholarship (2015/22711-1). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. Finally, the authors are thankful to CBMM for the niobium samples. 28 ACS Paragon Plus Environment

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Supporting information N2 Physisorption results of Nb2O5 samples (Figure S1 and S2); UV-Vis spectra of the standard solutions to quantify Ni in the catalysts (Figure S3); In situ XRD of the Ni/Nb2O5 samples during TPR process (Figures from S4 to S13); In situ XANES of the Ni/Nb2O5 samples during TPR process (Figures S14 and S15; Temperature Programmed Oxidation by Thermogravimetric Analysis – results and discussion (Figure S16); N2 Physisorption results of Ni/Nb2O5 samples Figures from S17 to S20); Principal Component Analysis (PCA) of the in situ XANES of 5% Ni-Nb2O5-BH during the TPR treatment- - results and discussion (Figure S21) and XANES simulation results using FEFF9. Corresponding author: Cristiane B. Rodella* E-mail: [email protected] References (1)

Wojcieszak, R.; Jasik, A.; Monteverdi, S.; Ziolek, M.; Bettahar, M. M. Nickel niobia interaction in non-classical Ni/Nb2O5 catalysts. J. Mol. Catal. A Chem. 2006, 256, 225–233.

(2)

Jasik, A.; Wojcieszak, R.; Monteverdi, S.; M. Ziolek, Bettahar, M. M. Study of nickel catalysts supported on Al2O3, SiO2 or Nb2O5 oxides. J. Mol. Catal. A Chem. 2005, 242, 81–90.

(3)

Tanabe, K.; Okazaki, S. Various reactions catalyzed by niobium compounds and materials. Appl. Catal. A, Gen. 1995, 133, 191–218.

(4)

Ko, E. I.; Hupp, J. M.; Wagner, N. J. Ethane hydrogenolysis and carbon monoxide hydrogenation over niobia-supported nickel catalysts: A hierarchy to rank strong metal-support interaction. J. Catal. 1984, 86, 315–327.

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Chary, K. V. R.; Lakshmi, K. S.; Murthy, M.R.V.S.; Rao, K.S.R.; Papadaki, M. Hydrodechlorination of 1,2,4-trichlorobenzene over niobia supported nickel catalysts. Catal. Commun. 2003, 4, 531–535.

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Lopes, O. F.; Paris, E. C.; Ribeiro, C. Synthesis of Nb2O5 nanoparticles through 29 ACS Paragon Plus Environment


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the oxidant peroxide method applied to organic pollutant photodegradation: A mechanistic study. Appl. Catal. B Environ. 2014, 144, 800–808. (7)

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