Understanding the Role of Atomic Ordering in the Crystal Structures

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Understanding the Role of Atomic Ordering in the Crystal Structures of NixSny towards efficient Vapor Phase Furfural Hydrogenation Vijaykumar S Marakatti, Nidhi Arora, Sandhya Rai, Saurav Ch. Sarma, and Sebastian C. Peter ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04586 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Sustainable Chemistry & Engineering

Understanding the Role of Atomic Ordering in the Crystal Structures of NixSny towards efficient Vapor Phase Furfural Hydrogenation Vijaykumar S. Marakatti,1 Nidhi Arora,1 Sandhya Rai,2 Saurav Ch. Sarma,1 Sebastian C. Peter1*

1

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India. 2

Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India.

*Corresponding author: [email protected]. Phone: 080-22082998, Fax: 08022082627

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Abstract: In this work, we have synthesized ordered NixSny nanoparticles separated by Al2O3 as highly efficient catalysts for the vapor phase hydrogenation of furfural. The NixSny-Al2O3 composite catalysts with different chemical compositions (Ni3Sn4, Ni3Sn, Ni1.5Sn, Ni3Sn2, Ni) were prepared through layered double hydroxide (LDH) route by annealing at 800 °C in presence of H2. NixSny nanoparticles were well dispersed throughout the Al2O3 matrix indicating the high thermal stability of these materials. The effect of different concentration of Ni, Sn and Al in the formation of NixSny-Al2O3 catalyst was investigated. The XRD, TEM and XAFS analyses indicate the formation of intermetallic phases with spherical particle having average size of 30-160 nm. The nanoparticles with different compositions were tested towards the hydrogenation of furfural and found that crystal structure, atomic ordering and composition play crucial roles in the catalytic activity. DFT calculations indicate the adsorption energy and the geometrical orientation of furfural on the surface of different NixSny-Al2O3 compounds play crucial role in the hydrogenation of furfural. Ni3Sn2-Al2O3 was found to be most efficient catalyst with high furfural conversion (~ 65 %) and selectivity (57-62 %) for furfuryl alcohol.

Keywords: Ni-Sn Intermetallic; Furfural; Hydrogenation, Nano catalyst: DFT calculation.


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Introduction The depletion of petroleum reservoirs, increase in global energy demands and environmental concerns forced humanity to find new energy sources. The research attention is paid for the investigation of new no-fossil carbon resources all over the world.1 In this regard, the production of chemicals and fuels from the renewable biomass is an attractive opportunity for the sustainable development of chemical industry in the future.2-3 One of the most important strategies for biomass conversion is the transformation of the abundant lignocellulosic biomass into fuels and chemicals.4 Furfural is an important lignocellulose derived platform molecule, obtained in a million of tons every year by the hydrolysis and dehydration of xylose containing lignocellulose biomass.5 Based on the catalyst and the reaction conditions, furfural can be converted into various fuel additives and chemicals such as furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA),6 2-methylfuran (2-MF),7 furan,8 2-methyltetrahydrofuran (MTHF),9 tetrahydrofuran (THF), 1,5-pentane diol,10 levulinic acid,11 cyclopentanone11 etc. Among the different products, synthesis of FA has drawn much interest in the industry and academics due to the challenges in the catalyst design and diverse applications.12-14 FA finds huge application in chemical industry as solvent. Apart this, FA is an ingredient for the manufacture of resins, synthetic rubbers, adhesives, and wetting agents etc. 11

FA usually obtained from the selective hydrogenation of furfural in a liquid or vapor phase

reaction. The furfural molecule contains mainly aromatic furanyl ring (C=C) along with the side carbonyl group (C=O). The selective hydrogenation of the aldehyde group keeping C=C of furanyl ring intact is one of the challenges for catalyst design. The catalysts generally used for the furfural hydrogenation are metals because of their ability to dissociate hydrogen and thus making hydrogenation possible.11 The metals with high interaction with the unsaturated furanyl ring can be saturated along with the C=O leading to the different side products. The



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geometric and electronic properties of different metals will affect both the activity and selectivity. Amongst the different metals of the periodic table, the group IB and VIII metals has been widely studied for the furfural hydrogenation.15-19 The group IB metals such as Cu and Ag are more promising for high selectivity of FA with low catalytic activity.20 Conversely, the group VIII metals (Ni, Pd, Pt) shows significant catalytic activity and selectivity below 200 °C, whereas above this temperature forms various side reactions due to the decarbonylation and ring opening reaction.18,

21-22

The major differences in catalytic

activity between both type of metals is their preferential adsorption configuration of the furfural molecule on surface. In case of IB group metals, carbonyl group is directly bonded to metal through oxygen lone pair ƞ1-(O), at the same time the repulsion between the furanyl ring and the IB metal surface makes the C=C hydrogenation difficult leading to high selectivity for FA.23 Whereas, group VIII metals tend to adsorb the aldehyde group in the ƞ2(C, O) configuration, in which the C=O and furanyl ring are oriented parallel to the metal surface leading to formation of side products.19 To change the orientation configuration of furfural to produce selectively FA, group VIII metals are alloyed with other metal such as Cu, In, Sn, Fe.19, 24-27 The alloying with second metal changes both the electronic and geometrical path of the surface for the furfural leading to improved selectivity. Among them ordered intermetallic compounds (IMCs) provide an ideal opportunity to study the correlation between the atomic ordering and the catalytic activity.28 IMCs contain two or more metals arranged in well-ordered structure with directional covalent bonding. Nowadays, in the field of heterogeneous catalysis, much interest has been dedicated to IMCs due to their better control over electronic and surface properties.29-32 However, the catalytic properties of IMCs have been scarcely studied systematically, as many scientists have observed the formation of IM phases coexisting with monometallic, solid solution, or oxide phases.6 The general difficulties encountered in the synthesis are


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incomplete reduction of involved metal ions, non-dispersed nanoparticles, lack of control in composition, morphology, and coagulation due to heat treatment.33-34 Therefore, it is highly essential to develop facile and green strategies for the synthesis of nano IM as efficient catalyst. Synthesis of supported IMCs by Layered Double Hydroxide (LDH) method has drawn enormous attention.6,

25, 27, 35

The metal cations will be dispersed in a uniform and

ordered way in bruicite like layers of LDH, which on reduction results in a uniformly dispersed metal/metal oxide composites.27 Taking advantage of the following property, recently Co-Sn, Ni-In, Ni-Ga IMCs were synthesized by LDH route. Ni-In with different chemical compositions supported on Al2O3 and MgO were synthesized and studied for the selective hydrogenation of α-β unsaturated aldehydes.27 Co-Sn IMs with 20 nm particle size supported on Al2ZnO4 by LDH method were reported for the hydrogenation of citral.35 Rodiansono et. al. synthesized NixSny/Al2O3 catalyst through hydrothermal route, which exhibited excellent catalytic activity for furfural hydrogenation in liquid phase.6, 25 However, no much emphasis has been put to synthesize NixSny-Al2O3 catalyst through LDH route and study their physio-chemical properties and exact active sites of catalyst in a reaction. Moreover, for the industrial application and to follow the principles of green chemistry, furfural hydrogenation through vapor phase reactor is more feasible compared to batch reactor due to ease in regeneration of catalyst, absence of solvent and low pressure in the process. In our previous works, NiSb/SBA-15, NiSe, NiSb, CoSb, Ag3In and PdCu IMCs synthesized by various methods and studied as active catalysts for the hydrogenation of pnitrophenol and oxidation of benzylamine, respectively.28,

36-39

Apart from this we also

developed other ordered IMCs PdCu3, Pd2Ge, Pd3Pb, Pt2In3 and PdRE as efficient catalysts for energy conversion in fuel cell. 40-44 In continuation of our investigation for the synthesis of IMCs for organic catalysis, we have successfully synthesized the pure phase of NixSny IMCs



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(Ni1.5Sn, Ni3Sn, Ni3Sn2, Ni3Sn4) well separated by the Al2O3 matrix through LDH route. The synthesized catalysts were characterized by X-ray diffractometer (XRD), inductively coupled plasma (ICP), transmission electron microscope (TEM), X-ray absorption fine structure (XAFS) followed by the density functional theory (DFT) studies. The synthesized catalysts were screened for the vapor phase furfural hydrogenation reaction and the effect of Ni/Sn and Ni/Al ratios was investigated. Furthermore, the role of surface active planes of NixSny IMCs in catalytic activity were correlated with the adsorption energies and geometrical factors of the furfural. The chemical composition and structural properties of NixSny-Al2O3 were correlated with the catalytic activity.

Experimental Details Chemicals The following chemicals were used without further purification: Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 99%, Merck), tin(IV) chloride (Sn(Cl)4, 99%, Sigma-Aldrich), aluminium nitrate (Al(NO3)3, 99%, Merck), sodium carbonate (Na2CO3, 99%, Merck), sodium hydroxide (NaOH, 99%, Merck) and furfural (98%, Spectrochem). THF, MF, Butanol, THFA, Furan, and FA chemicals (99%) were purchased from Sigma-Aldrich. Synthesis of NixSny-Al2O3 The NixSny-Al2O3 with different molar compositions were synthesized by coprecipitation method.45 In a typical synthesis, Ni(NO3)2.6H2O, SnCl4.6H2O and Al(NO3)3.9H2O were dissolved in 100 mL of deionized water to give a green solution with the concentration of 0.2M, 0.2M and 0.08M, respectively. The metal stochiometric ratio was kept as Ni/Sn = 1 and Ni/Al= 2.5. The resulting solution was stirred in an oil bath at 65 ˚C for 3 h followed by titration with an alkaline solution (100 ml) containing NaOH (0.5M) and Na2CO3 (0.1M). Further, titration is continued with NaOH (0.5M) solution till the resulting solution reaches the pH of 10. The slurry was then transferred into a 500 ml polyethylene 6 ACS Paragon Plus Environment

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bottle and heated to 100 ˚C for 24 h. Finally, obtained solid was filtered and washed with deionized water and ethanol several times followed by drying at 120 ˚C for 8 h. The NixSnyAl2O3 with different Ni/Sn ratio (0.75, 1, 1.5, 2, 2.5, 3, 3.5) keeping Ni/Al ratio = 2.5 constant were synthesized by the above procedure. Furthermore, NixSny-Al2O3 (Ni/Sn = 1, 1.5, 3.5) with different Ni/Al ratio of 1.5 and 3.5 were synthesized by the above method. The obtained compounds with different Ni/Sn and Ni/Al ratio were annealed to 800 ˚C in presence of 10% H2 and 90 % He gas for 3 hours resulting in the formation of NixSny-Al2O3. Characterization of NixSny-Al2O3 The NixSny-Al2O3 catalysts were characterized using XRD, ICP-OES, TEM, energy dispersive analysis of X-ray (EDAX) and XAFS. Powder XRD data were collected on a Bruker D8 Discover diffractometer using Cu Kα radiation with scan rate of 0.05/min in the 2θ range of 5−80°. TEM images with EDAX pattern were collected using FEI TITAN3 electron microscope operating at 80−300 kV. The samples for these measurements were prepared by sonicating NixSny powder in ethanol and drop-casting a small volume onto a carbon-coated copper grid. The amount of Ni, Sn and Al in the synthesized catalysts was determined using ICP-OES analysis. Samples were digested in concentrated aqua regia followed by dilution with Millipore water. ICP-OES was performed using PerkinElmer Optima 7000 DV instrument. XPS measurement was performed on an Omicron Nanotechnology spectrometer using an Mg Kα (1253.6 eV) X-ray source with relative composition detection better than 0.1%. The binding energy of 284.8 eV for C1s from surface carbon contaminant was used for the calibrations. Ni and Sn K-edge XAFS measurements of NixSny-Al2O3 nanoparticles were carried out in transmission mode at PETRA III, P65 beamline of DESY, Germany. Pellets for the measurements were made by homogenously mixing the sample with an inert cellulose matrix to have an X-ray absorption edge jump close to one. Standard data analysis procedure was



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used to extract the extended x-ray absorption fine structure (EXAFS) signal from the measured absorption spectra. Background subtraction, normalization, and alignment of the XAFS data were performed by ATHENA software. Theoretical EXAFS models were constructed and fitted to the experimental data in ARTEMIS. The EXAFS fitting of NixSny was done by taking the respective crystal structure of each compound in NixSny. EXAFS data of the catalyst materials were fitted simultaneously with k-weights of 1, 2, and 3. The So2 values was fixed to 1 prior to the fitting procedure. The EXAFS data were Fourier transformed in the range of 4−13.8 Å−1. Data were fitted in R-space between 1 and 3 Å for all the samples. The fitting parameters consist of bond length change between atoms (∆R), change in energy scale between data and theory (∆E0), and mean-square displacement of the bond length (σ2). The coordination numbers (CNs) were taken from the respective crystal structures and varied until low R-factor was achieved.28 The carbon present on the catalyst after reaction was determined by the CHNS analysis using Thermo Finnigan FLASH EA 1112 CHNS analyzer. Catalytic activity The furfural hydrogenation was carried out in a fixed bed down-flow quartz reactor in vapor phase.46 In a typical procedure, catalyst was activated in hydrogen gas for 4 h at 500 ˚C prior to the reaction at 280 ˚C. The mixture of furfural and H2 in the mole ratio of 1:5 was fed through a pre-heater connected to a quartz reactor containing 1 g of catalyst with weight hour space velocity (WHSV) = 0.5/hr. The gaseous products coming from the reactor were cooled in a condenser at 2 ˚C. The liquid products were analysed through a gas chromatograph (Shimadzu-2014, FID detector) equipped with a capillary column (Stabil wax, 30 m× 0.25 mm×0.25 mm). The components were identified and quantified using standards and confirmed by GCMS.


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Electronic Structure Calculation All the calculations were performed with the periodic density functional theory (DFT) method using the Quantum Espresso v.5.12 package.47 The structure models for Ni3Sn4 (04004-4077), Ni1.5Sn (04-002-1277), Ni3Sn (04-002-1277), and Ni3Sn2 (04-010-3106) were built based on the ICSD standard cards, in which the crystal structures are in accordance with experimental results. The exchange and correlation energies were calculated using the Perdew, Burke, and Ernzerhof (PBE) functional with the generalized gradient approximation (GGA).48-49 The electron–ion interactions were described by ultrasoft pseudopotentials given by Rappe, Rabe, Kaxiras and Joannopoulos.50 The Brillouin-zone of the p(4 x 4) lateral supercell was sampled at 3 x 3 x 1 k-points using the Monkhorst–Pack scheme.51 First-order Mouthfeel-Paxton smearing of 0.2 eV was employed in the integration to speed up the convergence.52 The convergence criteria for structure optimization and energy calculation were set to the tolerance for SCF, energy, maximum force, and maximum displacement of 2.0 x 10-6 eV per atom, 2.0 x 10-5 eV per atom, 0.05 eV Å-1, and 2.0 x10-3 Å, respectively. The surface calculations were done by using a 4 x 4 slab of the constituent alloy considering 4 layers in each case. A symmetrical vacuum of 7 Å was applied on the either side of the slab. The surfaces were chosen as per the minimum energy obtained for each of the surface. The bottom two layers were frozen at the bulk lattice constant, allowing the rest of the slab to relax. Furfural was initially placed 4.5 Å above the slab in different possible confirmations (5 confirmations in each case were considered). The system could relax using the same protocol as used for the slab. The adsorption energy, Eads, is defined as

Eads = Etotal - (Eslab + Efurfural)

(1)

where Etotal is the total energy of the catalyst–reactant complex, Eslab is the energy of the catalyst and Efurfural is the energy of furfural molecule. All the calculations were performed considering the system to be in vacuum. 9 ACS Paragon Plus Environment


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Results and discussion Generally, large quantity of Al was observed in Al2O3 supported NixMy (M = metal) synthesized by LDH method. This enhances the alloy-support interaction, which is very crucial for any catalytic reaction. Recently Co-Sn/Al2ZnO4 was also synthesized by the LDH method with very high Al content having the particle size of 20 nm.35 In case of NixIny IMCs supported on Al2O3, the Ni/Al ratio was found to be as low as 0.5.27 Moreover, the synthesized particle size was 5.1 nm supported on Al2O3. Rodiansono et. al. synthesized different Ni-Sn alloys supported on Al2O3 though hydrothermal route with very high Al (Ni/Al ratio of 0.9-1.0) having surface area above 90 m2/g.6, 25 The synthesized NixSny/Al2O3 compounds were found to contain multiple phases along with unreacted tin metal by annealing at 600 °C. However, not much emphasis has been put on the particle size distribution and the morphology of the Ni-Sn-alloy particles. In most of the reports, very less Al+3 ions of LDH are replaced easily by the In3+, Ga3+ due to the similar oxidation state, whereas substitution with M4+ metal ion such as Sn4+ is difficult task. Therefore, substitution of large amount of M3+ ion in LDH by the M4+ ion, (lower Al) followed by the formation of IMCs is found to be challenging and fascinating, as results there are no reports on such synthesis. Considering the above facts, we put an effort to synthesize NixSny-Al2O3 with lower Al content in such way that it is sufficient to form well separated IM nanoparticles (IMNP) with different Ni/Sn composition. At this lower Al content, Al2O3 can act as a separator so that agglomeration between the IMNP is hindered and keeps them well dispersed, rather than acting as a support.53 The catalytic activity of the catalyst will be mainly contributed by well separated NixSny IMCs. To tune the Al content, Ni/Al ratio was varied from 1.5 to 3.5 in each composition of NixSny and correlated with the catalytic activity. The percentage of NixSny to Al was found to be 70 to 90 % as tabulated in Table1. 10 ACS Paragon Plus Environment

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NixSny-Al2O3 with different Ni/Sn ratio ranging from 0.75 to 3.5 (Ni/Al =2) were synthesized by co-precipitation method (Table S1). The XRD patterns shown in Figure S1, can be indexed as a rhombohedral structure with the typical (003), (006), (110) and (113) reflections at 2θ 11.95°, 23.70°, 58.61° and 60.05° for LDH materials, respectively.45 The additional planes (110), (101) and (211) reflections observed at 2θ 26.6°, 33.9° and 51.8° respectively correspond to the tetragonal Sn0.6(OH)1.6O0.4 phase.54 The presence of Sn0.6(OH)1.6O0.4 phase along with low crystalline LDH phase can be explained as the difficulty in substituting high concentration tetravalent Sn4+ ion with Al3+ site of LDH due to which large amount of tin remains unreacted in the form of tin oxyhydroxide. Furthermore, the intensity of LDH phase decrease with the increase of tin content due to the formation of tin oxyhydroxide and similar kind of trend was also observed in the synthesis of Ni-In-Al LDH.27 The TEM images of NixSny-Al2O3 indicate the formation of irregular amorphous hydroxide lumps having average size of 1-5 µm (Figure S2). The synthesized LDH materials were annealed in presence of 10% H2 in He gas at high temperature of 700-800 °C resulting in the formation of NixSny-Al2O3. The TEM images showed spherical shaped particles with average size ranging from 20-160 nm (discussed later). The aluminum hydroxide phase of LDH with high reduction potential cannot be reduced at this temperature and as a result it forms Al2O3. The annealing of sample (Ni/Sn = 3.5) at 700 ˚C resulted in Ni3Sn IMC along with unreacted elemental Ni as shown in Figure S3. Further annealing of sample at high temperature of 800˚C resulted in the formation of pure Ni3Sn. In another case, the formation of IM phase Ni3Sn2 or Ni1.5Sn was not clear at 700 °C due to incomplete reaction between the Ni and Sn, whereas annealing at 800 °C resulted in clear formation of Ni3Sn2 IM phase (Figure S4). However, the formation of Ni1.5Sn phase is more probable at high temperature than the Ni3Sn2 phase as indicated by phase diagram.55 This indicates that high temperature of 800 ˚C is required for the complete reduction and



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diffusion of Ni and Sn to form an ordered IMC. The XRD patterns of all the synthesized NixSny-Al2O3 annealed at 800 °C were given in Figure S5. The elemental compositions of all the synthesized NixSny-Al2O3 were analyzed using ICP-OES. The minimal amount of metal used for synthesis and actual present in the material after synthesis were comparable as tabulated in Table 1. Moreover, for an ease of understanding the nominal chemical composition was mentioned throughout the manuscript. For the synthetic composition of Ni/Sn ratio of 1 and 0.75, monoclinic system Ni3Sn4 IM phase (space group of C2/m) was formed along with small amount of NiSn phase (space group P63/mmc). Single phase of Ni3Sn2 crystallizing in the orthorhombic system having Pnma space group was obtained for Ni/Sn ratios 1.5 and 2. The hexagonal system Ni3Sn (space group of P63/mmc) in pure form was obtained for Ni/Sn ratio of 3.5, whereas mixed phases (Ni3Sn and NiSn) were obtained for Ni/Sn ratios 2.5 and 3. For the comparative studies, Ni-Al2O3 was also synthesized by the above method and annealed at 800 °C. The XRD pattern confirms the formation of Ni phase with Fm͞3m space group (Figures S6). The TEM analysis indicates the average particle size of Ni to be around 20-30 nm as shown in Figure S7. In all the XRD patterns, diffraction peaks correspond to Al2O3 were not observed because of the amorphous nature and low concentration of Al2O3 compared to IMCs. To check the crystalline nature of Al2O3 present in the catalyst, alumina was synthesized by similar procedure as described earlier without Ni and Sn precursors. The XRD analysis of this synthesized Al2O3 annealed at 800 °C indicates the major α-phase along with minor θphase (Figure S8).56-57 The synthesized NixSny-Al2O3 catalysts were screened for the vapor phase furfural hydrogenation reaction and results are tabulated in Table S1. Ni3Sn exhibited higher conversion (70-80 %) with very low selectivity (less than 30%) for FA and Ni3Sn4 exhibited negligible catalytic activity. The Ni3Sn2 catalyst exhibited the furfural conversion in the range


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of 55-70 % with high selectivity of 68 % for FA. Pure Al2O3 did not show any conversion indicating the IMCs are the active sites for the hydrogenation of furfural. Ni-Al2O3 showed complete conversion of furfural with high selectivity for furan and THFA as indicated in Table 1 and S2. One may assume that the catalytic activity can be changed with the introduction of defects in the crystal structures. The small shift of the peaks in XRD of compounds Ni3Sn4, Ni3Sn, Ni1.5Sn suggest presence of minute either Ni or Sn defects in their corresponding structures. However, the difference in the activity between the exact composition and excess composition containing IMCs was negligible hence ruled out the role of defects in catalytic activity (Table S1). To further study the role of Al2O3 on the physio-chemical properties and subsequent effect of catalyst on furfural hydrogenation, we synthesized NixSny-Al2O3 with Ni/Sn = 1, 1.5, 3.5 (chosen due to the formation of pure phase) having different Ni/Al (0, 1.5, 2.5, 3.5) ratio and will be dealt in detail in the subsequent sections. Characterization and catalytic activity of Ni3Sn4 with different Al content The XRD patterns of all the compounds with Ni/Al ratio ranges from 0 to 3.5 and keeping the Ni/Sn ratio equal to 1 indicate Ni3Sn4 as major phase along with minor NiSn phase (Figure1), which is in agreement with the known phase diagram.55 The TEM image (Figure 2) with different Al2O3 content showed the formation of spherical nanoparticles with average particle size ranging from 60 to 160 nm. An enhancement in the Ni/Al ratio has increased the particle size due to the agglomeration of small particles. But, this kind of agglomeration was hindered at high composition of Al2O3 (Ni/Al = 1.5) with particle size of 80 nm. The average particle size increases from 80 to 140 nm with an increase of Ni/Al ratio from 1.5 to 3.5, respectively. Another effect of Ni/Al ratio is seen on the distribution of IM particles with Al2O3 matrix. In case of lower Ni/Al ratios (1.5 and 2.5), the particles are inter connected through alumina matrix as inferred from the TEM images (Figure 2). A continuous



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matrix holding these particles in between can be seen, whereas at high Ni/Al (3.5) the particles are well separated/dispersed from one another with destruction of continues Al2O3 matrix. The IMC particles synthesized without addition of Al results in the formation of bulk Ni3Sn4 compound with average particle size of 8-12 µm. The TEM images confirm that alumina is playing a crucial role in the distribution and size of nanoparticles. Elemental mapping shows the uniform distribution of Ni and Sn in ordered Ni3Sn4 IM structure (Figure S9). The EDAX pattern further confirms the Ni and Sn are in the atomic percentage of 3:4 and as supported by ICP analysis (Figure S10, Table S6). Interestingly all the Ni3Sn4 catalyst showed lower furfural conversion values below 5%. Similar kind of lower catalytic activity was also observed for Ni3Sn4 catalyst in liquid phase furfural hydrogenation.6,

25

The

selectivity for FA was 75% along with THFA (16.1%) as side product. Characterization and catalytic activity of Ni1.5Sn with different Al content The variation of Ni/Al keeping Ni/Sn ratio 1.5 mainly results in the formation of Ni1.5Sn and Ni3Sn2 phases. Ni1.5Sn formed at lower Al2O3 (Ni/Al = 3.5) content could be due to aggregation of the particles. Whereas at higher Al2O3 content, Ni3Sn2 phase is formed as shown in Figures 3 and S11. The difference between Ni1.5Sn and Ni3Sn2 can be interpreted by the presence of extra diffraction peaks at 2θ of 27.3°, 35.3°, 37.3°, 44.4° correspond to the (201) (103) (013) (004) planes, respectively. The TEM images at higher Al2O3 content shows the Ni3Sn2 IM embedded in the matrix of Al2O3 and at lower Al2O3 IM particles are completely isolated from the support as seen in the case of Ni3Sn4. Further TEM images confirm complete isolation of Ni3Sn4 IM particles as shown in Figure S12. The TEM images show the average particle size ranging from 30-120 nm as given in Figure 4 and the particle size increased with increase of Ni/Al ratio. Elemental mapping shows the uniform distribution of Ni and Sn in ordered Ni3Sn2 IM structure (Figure 5). The EDAX pattern further confirms the Ni and Sn are in the atomic percentage of 3:2 (Figure S.13, Table S6)).


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Interestingly, the Ni3Sn2 phase showed good furfural conversion (~ 65 %) along with high selectivity (~57-62 %) for FA as compared to any other NixSny-Al2O3 catalyst. The Ni1.5Sn catalyst exhibited less conversion (23.4 %) with FA selectivity of 73.5 %. The other major side product observed was furan with ~15 % selectivity. The bulk Ni1.5Sn synthesized without Al2O3 exhibited almost no catalytic activity due to the large particle size (2-8 µm) with lower surface area. Characterization and catalytic activity of Ni3Sn with different Al content The variation of Ni/Al ratio keeping Ni/Sn ratio 3.5 mainly resulted in the Ni3Sn phase. The low concentration of Al (Ni/Al = 3.5) resulted in the formation of major Ni3Sn phase along with minor NiSn phase (Figure 6). This could be due to the easy aggregation of particles leading to different composition. Figure 7 represents the TEM images of all the compounds with different Ni/Al ratio. Interestingly the particle size did not increase with increase of Ni/Al ratio, in fact remained almost independent of Al2O3 with average particle size ranging from 20-80 nm. This could be due to the lower concentration of tin, which favors the formation of tin oxyhydroxide. Moreover, at high concentration of Al (Ni/Al = 1.5) the particles are well dispersed within the Al2O3 matrix, at medium concentration of Al (Ni/Al =2.5) they are well dispersed without any intact Al2O3 matrix and at low concentration (Ni/Al =3.5) are more densely arranged without undergoing any agglomeration. Elemental mapping shows the uniform distribution of Ni and Sn in ordered Ni3Sn IM structure (Figure S14). The EDAX pattern at different regions further confirms the Ni and Sn are in the atomic percentage of 3:1 (Figure S15, Table S6). The NixSny IMC without addition of Al results in the formation of bulk Ni3Sn compound with particle size of 8-12 µm. All the Ni3Sn compounds with different Al content were studied for the hydrogenation reaction of furfural and the results are tabulated in Table 1. The conversion of furfural was around ~ 75 % with very low FA selectivity of ~ 25 %. The other major side products obtained over Ni3Sn



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catalyst formed by the hydrogenation of intermediate compounds and their ring opening to the alcohols. The side products mainly contain furan, THF, butanol and 2-MF. The change in the particle size did not have any effect on the catalytic activity of furfural hydrogenation. The variation of Al content in each NixSny IMCs (Ni/Sn = 1, 1.5, 3.5) did not change the catalytic activity and selectivity considerably, this could be due to the wide range of particle size distribution. It is interesting to note that the average particle size decrease with increase of Ni/Sn ratio as shown in the Table 1. The average particle size was 60-160 nm for Ni/Sn =1 and it decreased to 30-120 nm for Ni/Sn = 1.5 and moreover for Ni/Sn = 3.5 particle size further decreased to 20-80 nm. This decrease in particle size with decrease in tin content could be due to the lower formation of tin oxyhydroxide and increase in the formation of ordered LDH as inferred from the XRD (Figure S1). To have insights into the oxidation state and nature of bonding of the compounds, we performed XPS studies on the synthesized Ni3Sn, Ni3Sn2 and Ni3Sn4 compounds (Ni/Al = 2.5). The Ni 2p3/2 and Sn 3d5/2 XPS spectra and peak assignment of the as-prepared Ni-Sn IMCs are shown in Figure 8. In the Ni core level XPS spectra, the peak corresponding to the Ni 2p3/2 is observed at ∼851.7 eV in all the NixSny compounds, which closely match with the literature value for Ni0 (2p3/2 binding energy (BE) 852.4 eV). This small shift of peak towards the lower binding energy indicates the electron transfer from Sn atom to Ni. In addition, all the spectra exhibit a broad peak at ∼855.2eV, which is related to the Ni2+ in the Ni−O species. (2p3/2 BE = 855.5 eV. We could also find the satellite and auger peaks of Ni 2p3/2 decomposed at higher binding energy and are overlapped with the peaks of Ni0 and Ni2+.58 These peaks make tough in the quantification of the exact Ni amount in the IMCs. In the Sn core level XPS spectra, the peak corresponding to the Sn 3d5/2 is observed at ∼ 484.3 eV in all the IMCs, related to the values for Sn0 (3d5/2, BE = 484.3 eV). The additional peak observed at higher binding energy of 486.2 eV is due to the Sn2+ or Sn4+ species. (3d5/2, BE =


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486.0 eV). Approximately 0.2 to 0.7 eV energy shifts of the Ni 2p and Sn 3d core level was observed in all the NixSny compounds. This shift is attributed to the strong interaction between Ni and Sn atoms. These XPS results suggest that as-synthesized nano particles consists of primarily Ni and Sn and the major catalytic sites are the intermetallic surfaces. Some amount of Ni and Sn at the surface of the Nano catalyst are oxidized due to exposure to air. It is also interesting to note that, with increase of Sn content in NixSny, the surface SnO and NiO increases in the following order: Ni3Sn < Ni3Sn2 Ni3Sn > Ni3Sn2 > Ni1.5Sn > Ni3Sn4. Ni showed the maximum adsorption energy of 65.2 KJ/mol and it dropped below 30 KJ /mol for all NixSny IMCs. Therefore, the Ni-Al2O3 catalyst reached complete conversion (100%) and it decreased to lower values for NixSny based on the different structure and composition. Ni1.5Sn and Ni3Sn4 with lower adsorption energy of 18.7 and 16.1 KJ/mol showed lower conversion values. Though the adsorption energy for Ni3Sn2 is higher than the Ni3Sn, the observed reverse trend in conversion values could be due to the geometrical effects. (see later) This clearly indicates that the adsorption energy and geometrical effects of furfural determines the catalytic activity and selectivity in the furfural hydrogenation. The diverse selectivity of products observed over different NixSny could be related to the geometrical orientation of furfural on the catalytic surface. The Ni/Al2O3 catalyst selectively produces furan (52.8 %), THFA (17.3%) and FA (8.11%). In case of Ni the furfural molecule is adsorbed parallel to



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the surface through ƞ2- (C, O) bonding as seen from the DFT calculations. (Figure11) This kind of adsorption overlaps of d orbitals of metal with the π* orbitals of carbonyl, resulting in back donation of electron from the metal to stronger metal-aldehyde bonding. This interaction results in the formation of stable acyl surface species, followed by the decarbonylation to form furan. Furthermore, the furan and FA compounds may undergo hydrogenation and ring opening reaction to produce THF, 2-MF, 2-MTHF, BUL, THFA and pentanediol. The Ni3Sn2 catalyst showed high FA selectivity of 57-62 % with furan selectivity around 15-20 %. In case of Ni3Sn the selectivity was well distributed over all the products as given in Table S2 and no major selectivity for any one component was observed. It is observed that furfural is more parallel (26.7°) to the active surface plane compared to the Ni3Sn2 surface (29.6°) and as a result more active for the ring opening and hydrogenation reaction leads to more side products and higher conversion. In case of Ni1.5Sn and Ni3Sn4 the furanyl ring is far away from the surface as a result, low conversion and high selectivity was observed (Figure 11 and Table 1). Similar kind of geometrical effect was observed for the Pd and PdCu systems for the furfural hydrogenation.19 In summary, the surface of NixSny plays a crucial role in the activity and selectivity of the furfural hydrogenation reaction. Based on the different product distribution over NixSny in vapor phase furfural hydrogenation, the possible reaction mechanism is shown in the Scheme 1. The furfural adsorbed on the NixSny IM surface in η2 (C-O) form further undergoes various intermediate steps to produce different products 1) The Ni rich Ni3Sn (100) plane could be active for the hydrogenolysis of furfural i.e hydrogenation of adsorbed furfural to an alkoxy intermediate that will subsequently deoxygenate and hydrogenate to MF. 2) The decarbonylation of furfural is a thermodynamically favorable process and forms furan as the product at high temperature. The adsorption of furfural is stronger over Ni rich catalysts, i. e. presence of high dense Ni atoms in Ni/Al2O3 and Ni3Sn IMC makes this reaction favorable. 3) The


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Ni3Sn2 (112), Ni3Sn4 (40͞1) and Ni3Sn (101) with active site isolation Ni atoms by Sn resulted in higher selectivity for the FAL. Though the (112) plane of Ni3Sn2 contains completely Ni atoms, the high selectivity can be explained by the isolation effect of Ni atoms in (112) plane as seen in Figure 10b. Therefore, hydrogenation of carbonyl group of the adsorbed furfural to generate a hydroxyalkyl intermediate, followed by hydrogenation of carbon bearing hydroxyl group to form furfural alcohol was observed over these catalysts. The furfural may undergo a series of internal reaction pathways including hydrogenation, dehydrogenation and decarbonylation to form a different product. Which will be difficult to find the exact path for the formation of each product. Wang et.al. studied the detailed adsorption of furfural, FA, furan. 2MF and reaction barriers for the interconversion by DFT on the furfural hydrogenation.62-63 They supported the furan observed in the reaction could be formed by the furfural, MF and furfural alcohol. The formation of furan from the furfural is more thermodynamically favorable due to the lower energy barrier. Similarly, the formation of MF from the furfural alcohol is more favorable than the direct conversion from the furfural. The furfural conversion to furfural alcohol is having lower reaction barrier and can be produced kinetically depending on the H2 coverage, temperature and pressure. Thus, the molecules formed from these routes undergo further C=C hydrogenation to form a cyclic compound such as THF, 2-THFA,2-MF, 2-MTHF. Furthermore, at high temperature these cyclic compounds will undergo ring opening reaction to produce various alcohols such as butanol, pentanediols, pentanol etc. The formation of wide variety of products could be due to the different reactivity and adsorption rate of intermediate molecules. Except for the hydrogenation of FA, all other side reactions are catalyzed by the Ni3Sn IMC with MF, FA, furan and THF as major products. Ni3Sn4 and bulk NixSny IMCs result in the formation of FA and further hydrogenation of FA to THFA, however no other side products were observed for these catalysts. Ni3Sn2 mainly produces the FA and furan,



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whereas Ni selectively produce furan and THFA due to the parallel geometric orientation and high adsorption energy. The specific activity was determined by the amount of furfural(g) converted per gram of Ni metal in NixSny per hour. Ni with too high specific activity led to lower selectivity, whereas Ni3Sn2 with balanced amount of Ni and Sn showed the higher specific activity with high selectivity for furfural alcohol. Tin rich compounds showed lower specific activity due to the presence of high concentration of inactive Sn in the compound (Table 1). Optimization of reaction conditions and Stability Test To evaluate the distribution of products in furfural hydrogenation effect of WHSV and H2/Furfural ratio were carried out. The WHSV was varied from 0.2 to 1.1 h-1, the activity and selectivity for the different products are as shown in Figure S17a. With increasing the WHSV the conversion of furfural decreased gradually with increase in selectivity for furfural alcohol. The formation of more side products such as furan and its decomposition to alcohols are observed at lower WHSV, due to the high contact time between the catalyst and the furfural. The high furfural alcohol selectivity of 72.1% was observed at WHSV of 1.1 h-1. Further optimization of H2/furfural ratio was varied from 1 to 15 and the results are as shown in Figure S17b. The high amount of furan and its decomposed alcohols are formed in large quantity at lower H2/furfural ratio of 1. This could be due to the lower amount of H2 availability for reaction and high retention of furfural molecules on catalyst as result the decarbonylation of furfural and further decomposition is predominant reaction. Similar kind of observation was made by the Vlachos group, lower hydrogen coverage will lead to the formation of furan and in reverse at high coverage of H2, the selectivity for furfural alcohol will increase.63 From the Figure S17, it is clear that at H2/Furfural = 5 the H2 coverage is sufficient to form furfural alcohol, and further increase in the H2, could not reduce the


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formation of furan to great extent. As the H2/ Furfural ratio increases from 1 to 15 the furfural conversion decreases and selectivity for furfural alcohol increases form the 28.2 to 70.2%. The stability of catalyst was also studied for 10 h reaction time as shown in the Figure S18. The conversion for Ni/Al2O3 and Ni3Sn catalyst dropped drastically after 3 h of reaction whereas the Ni3Sn2 catalyst showed slight less decreases in the activity. The conversion for Ni3Sn2 dropped from 74% to 50% in the first 7 hours time and become steady later. The selectivity for furfural alcohol increased slightly with increase of reaction time, this could be due to the blockage of some of the active sites of catalyst due to the formation of coke with time. Among all the other catalyst Ni3Sn2 exhibited a good stability with high conversion and selectivity in furfural hydrogenation as discussed earlier. Since the reaction involves lots of side reactions such as decarbonylation, decomposition and ring opening reaction, it is significant to determine the carbon balance. The amount of carbon balance of NixSny catalyst was determined in the reactant, products and catalyst (Table 1). The analysis showed the carbon loss was below 8 % for all the catalyst whereas for Ni/Al2O3carbon loss was higher due to the formation furan and its derived products by decarbonylation reaction. With increase of Sn content in IMC, the carbon loss has been decreased as shown form the Table 1. The presence of Sn in the IM structure improved selectivity, intern reduced the formation of coke in the NixSny IMCs. There are very limited number of catalyst studied for the vapor phase furfural hydrogenation reaction as indicated in the Table S6. Among them, high activity and selectivity was observed for the Cu metal along with Cr, Ca, Co based additive catalyst.64-67 The Cu-Cr, Cu-Ca and PtSn@mSiO2 exhibits the high furfural alcohol yield of 99% and 98%, respectively.64-67,53 However, Cu based catalysts usually undergo deactivation due to the formation of coke, poisoning by adsorption of furan or other reaction products, change in the oxidation of Cu species and finally the sintering of the cu particles during the catalytic



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process.68-69 The high cost of Pt materials makes it difficult for the industrial practice. Other catalyst such as Cu/SiO2, Co-Cu/SiO2, Cu-Ni/Mg-Al-oxide and PdCu/Zeolite-Y showed the comparable activity with NixSny-Al2O3 catalyst.70-71 However, the NixSny-Al2O3 are unsupported catalyst with lower surface area (>10 m2/g) in comparison to the supported catalysts in Table S6. Therefore, the specific activity of NixSny-Al2O3 compounds will be high in comparison with the reported catalysts. This shows the potential application of NixSny-Al2O3 intermetallic compounds in the catalysis. We are further working on these intermetallic systems by further addition of metal and modification to get uniform nanoparticles with smaller size. Which will eventually help in improving the intermetallic catalyst for the furfural hydrogenation.

Conclusions It is always challenging chemically and structurally to design thermally stable catalysts with enhanced activity and durability for any industrially important chemical reaction. Towards this direction plenty of successful research attempts such as controlling the morphology and size of the catalysts, introducing supports, engineering the heterostructures, etc., were performed, but synthesis of an efficient and thermally stable material having ordered structure remains a herculean task. In this work, we have synthesized NixSny-Al2O3 catalyst with lower Al content in such way that, it is sufficient to form well-separated intermetallic nanoparticles through the LDH route. The NixSny IMP are well separated and dispersed even after annealing at high temperature of 800 °C which indicates their high stability. Thus, the synthesized NixSny-Al2O3 catalysts show excellent catalytic activity for furfural hydrogenation. The Ni3Sn4 and Ni1.5Sn catalyst with very low adsorption energy for furfural and furanyl ring being away from the catalyst surface resulted in lower catalytic activity and high selectivity. Ni3Sn2 with intermediate adsorption and geometrical effect of furfural showed the high conversion and selectivity compared to Ni and Ni3Sn compounds. 24 ACS Paragon Plus Environment

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This clearly indicates the geometrical factors, atomic ordering, composition of active sites and surface of catalysts play crucial role in the catalysis.

Acknowledgement We thank Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Grant No. 01 (2787)/14/EMR-11) for financial support. V. M. thanks Science & Engineering Research Board (SERB) for National Post-Doctoral Fellowship (Grant No. PDF/2016/001118). Parts of this research were also carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF) and we thank Dr. Edmund Welter for assistance in using PETRA III beamline P65 at DESY, Germany. We are grateful to Prof. C. N. R. Rao for his constant support and encouragement.

Supporting Information: XRD pattern of the NixSny-Al2O3 intermetallic compounds, TEM images, colour mapping and EDAX analysis, XANES Spectra, determination of exposed plane, optimization of reaction conditions and stability.

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TABLES Table 1. Physico-chemical and catalytic properties of NixSny-Al2O3 catalysts. Nominal Amounta Ni/Sn

Ni /Al

Actual Amountb Ni/Sn

Chemical composition

Ni/Al

Crystallogra

Particle Size

Conversion of

Selectivity for

Specific

phic Phase

(nm)

furfural

furfural alcohol

Activity d

(%)

(%)

1.0

1.5

0.91

1.43

Ni3.26Sn3.58-(Al2O3)1.14

Ni3Sn4

60-100

2.33

74.1

0.215

1.0

2.5

0.90

2.48

Ni3.21Sn3.60--(Al2O3)0.64

Ni3Sn4

60-120

3.48

72.0

0.302

1.0

3.5

0.92

2.90

Ni3.34Sn3.65--(Al2O3)0.57

Ni3Sn4

60-140

4.50

76.2

0.382

1.0

0

0.85

--

Ni3.38Sn3.98

Ni3Sn4

2-8c

0.71

71.4

0.052

1.5

1.5

1.41

1.51

Ni2.92Sn2.08-(Al2O3)0.97

Ni3Sn2

30-60

61.2

57.1

3.992

1.5

2.5

1.43

2.63

Ni3.08Sn2.14-(Al2O3)0.59

Ni3Sn2

40-90

67.8

61.2

3.931

1.5

3.5

1.47

3.41

Ni1.47Sn1.01-(Al2O3)0.22

Ni1.5Sn

40-120

23.2

83.5

1.267

1.5

0

1.8

--

Ni1.8Sn

Ni1.5Sn

2-8c

0.82

72.5

0.038

3.5

1.5

3.39

1.46

Ni3.05Sn0.90-(Al2O3)1.04

Ni3Sn

20-80

70.3

24.0

3.056

3.5

2.5

3.32

2.45

Ni3.10Sn0.93-(Al2O3)0.63

Ni3Sn

20-70

78.2

20.7

3.272

3.5

3.5

3.53

3.3

Ni3.25Sn0.93-(Al2O3)0.48

Ni3Sn

20-80

78.0

26.5

3.146

c

3.5

0

3.4

--

Ni3.4Sn

Ni3Sn

2-10

1.70

71.7

0.056

0.0

--

--

--

--

Ni

20-30

100.0

8.11

5.000

Reaction conditions: Catalyst amount = 1 g; H2: furfural = 5:1; WHSV =0.5 /h; Temperature =280 °C, Pressure = 1 atm; Product was analysed during 3 h of reaction. a-Synthesis composition; b-Composition of synthesized material as determined by ICP-OES (standard deviation = ±0.1); c-Particle size measured in µm. Specific activity determined by the amount of furfural(g) converted per gram of Ni metal in NixSny Intermetallic per hour.

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Table .2. The exposed pane NixSny IMCs and their geometric, energetic details with furfural. Intermetallic Catalyst Ni

Exposed Surface plane (111)

rsurf-fur* (Å) 2.43

Eads (kJ/mol) 65.2

Angle between the IM surface and furfural plane (°) 0

Ni3Sn2

(112)

2.97

27.4

29.6

Ni3Sn

(100)

3.04

22.2

27.4

(101)

3.09

22.7

26.7

Ni1.5Sn

(101)

3.21

18.7

46.3

Ni3Sn4

(40-1)

2.81

16.1

79.2

* rsurf - fur is the distance along the z axis between the surface and the center of furfural.


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FIGURES

Figure 1. The comparison between powder XRD patterns of Ni3Sn4 with different Ni/Al.


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Page 36 of 47

Figure 2. TEM Images of Ni3Sn4 with different Ni/Al ratio of a) 1.5 b) 2.5 c) 3.5 d) 0. The particle size distribution of each composition is shown below of each one.

36


(123)

(302) (004)

(122)

(013)

(020) (103)

Ni/Al = 1.5

(110)

(101)

(102)

Ni/Al =2.5

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


(112)

Page 37 of 47

Ni/Al =3.5 Ni/Al = 0

Sim-Ni1.5Sn Ni3Sn2

30

40 2 Theta (°)

50

60

Figure 3. The comparison between powder XRD patterns of Ni1.5Sn with different Ni/Al.



Page 38 of 47

Figure 4. TEM Images of Ni1.5Sn with different Ni/Al ratio of a) 1.5 b) 2.5 c) 3.5 d) 0. The particle size distribution of each composition is shown below of each one.

38


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Figure 5. DRIFT, STEM, and elemental color mapping of Ni3Sn2 separated by Al2O3. The spherical particles show the homogeneous distribution of Ni and Sn indicating the formation of IMCs. The well separated Al2O3 matrix from the particles can also be observed.


(202)

(201)

(002)

(200)

(110)

Page 40 of 47

* NiSn

Ni/Al- 1.5

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

(101)


Ni/Al -2.5

*

*

*

Ni/Al-3.5

Ni/Al = 0

Sim-Ni3Sn-P63/mmc

30

40

50

60

70

2 theta (°) Figure 6. The comparison between powder XRD patterns of Ni3Sn with different Ni/Al.


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Figure 7. TEM Images of Ni3Sn with different Ni/Al ratio of a) 1.5 b) 2.5 c) 3.5 d) 0. The particle size distribution of each composition is shown below of each one

41



Page 42 of 47

Figure 8. XPS spectra (a, b, c) Ni 2p3/2 and (d, e, f) Sn 3d5/2 of NixSny-Al2O3(Ni/Al=2.5) IMCs. The green solid curve is the fit by the Levenberg−Marquardt method. 42


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Figure 9. Fourier transform magnitudes of the Ni K-edge EXAFS oscillations in Ni3Sn4, Ni3Sn2, Ni1.5Sn and Ni3Sn. Insets are the corresponding EXAFS oscillations. 43



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Figure 10. High resolution TEM images of exposed plane (shown below the corresponnding TEM image) of NixSny catalyst. Red ball indicates the Ni atoms and blue ball indicates the tin atoms and green colour indicates the respective imaginary plane. 44


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Figure 11. Adsorption state of furfural on the (a) Ni (111), (b) Ni1.5Sn (101), (c) Ni3Sn (100), (d) (101), (e) Ni3Sn2 (112) and (f) Ni3Sn4 (40͞1) facets. Colour code: Blue is Ni, orange is Sn, grey is C, red is O and white is H.

45



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Scheme 1. Plausible reaction mechanism and product distribution over different NixSny Al2O3 catalyst in vapor phase furfural hydrogenation. 46 ACS Paragon Plus Environment

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TOC

Graphical Abstract Well-separated NixSny intermetallic nanoparticles on Al2O3 synthesized through LDH route showed excellent catalytic activity for furfural hydrogenation. Geometrical factors derived from different metal composition play crucial role in catalysis.


Understanding the Role of Atomic Ordering in the Crystal Structures

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