Chemical Engineering Journal 326 (2017) 956–969

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

MMT-supported Ni/TiO2 nanocomposite for low temperature ethanolsteam reforming toward hydrogen production

http://dx.doi.org/10.1016/j.cej.2017.06.0121385-8947/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail addresses: bttahir@yahoo.com, mtahir@cheme.utm.my (M. Tahir).

William Mulewa a,b, Muhammad Tahir a,⇑, Nor Aishah Saidina Amin a

aChemical Reaction Engineering Group (CREG), Faculty of Chemical & Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 UTM Johor Bahru, Johor, Malaysiab Technical University of Mombasa, 90420, Mombasa 80100, Kenya

h i g h l i g h t s

� Ni-incorporated MMT/TiO2

nanocatalyst tested for ethanol steamreforming to hydrogen.

� MMT increased Ni dispersion in Ni/TiO2 composite with improved yieldand selectivity.

� H2 production depends on catalystcomposition, particle size andreaction conditions.

� H2 yield over Ni/MMT-TiO2 compositewas 1.5-fold more than using micro-particles.

� Nanostructured Ni/MMT-TiO2 gaveprolong stability than micro-particlesin ESR.

g r a p h i c a l a b s t r a c t

MMT sheets

TiO2 NPs

Ni-MMT/TiO2 NPs

0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

60

70

80

90

100

Hyd

roge

n Y

ield

(%)

Time on stream (h)

12 % Ni/ 20 % MMT-TiO2 micro-particles12 % Ni/ 20 % MMT-TiO2 nano-composite

Reaction Conditions:Pressure = 1 atm.Temperature = 500 °CS/E = 10:1GHSV = 13200 ml/gcat·h

ESR for Hydrogen Produc�on

a r t i c l e i n f o

Article history:Received 5 January 2017Received in revised form 13 May 2017Accepted 5 June 2017Available online 16 June 2017

Keywords:MontmorilloniteNi/TiO2

Ethanol steam reformingH2 productionStability analysis

a b s t r a c t

Ni/TiO2 nanoparticles dispersed on montmorillonite (MMT) clay with different sizes for selective ethanolsteam reforming with regard to hydrogen production has been investigated. Ni/MMT-TiO2 nano-composite catalysts were prepared by a sol-gel assisted impregnation method. The samples were exten-sively characterized by X-ray diffraction (XRD), N2 adsorption-desorption, Fourier transfer infrared (FTIR)spectroscopy, scanning electron coupled with energy dispersive X-ray (SEM-EDX) spectroscopy and ther-mogravimetric analysis (TGA). While Ni content progressively promoted the activity of TiO2 toward etha-nol conversion and H2 yield, modification with MMT controlled the crystal growth and produced anatasephase of delaminated MMT/TiO2 nanocomposite. Formation of a surface Ni-MMT phase in the modifiedNi/MMT-TiO2 nanocomposite catalyst enhanced Ni-dispersion and reducibility. Various parameters con-cerning the effect of temperature, steam-to-ethanol (S/E) feed ratio, MMT loading and Ni-metal loadingon the catalytic performance, were thoroughly studied. The optimal performance was achieved for 12 wt.% Ni/20 wt. % MMT–TiO2, achieving an ethanol conversion of 89% and a H2 yield of up to 55% at 500 �C. Inaddition, the Ni/MMT-TiO2 nano-composite catalyst possessed the excellent stability at the optimumtemperature, over 20 h reaction time. The relative low cost, good activity and stability of MMT modifiedNi/TiO2 catalyst offers for an economical and feasible route for production of renewable hydrogen fromethanol.

� 2017 Elsevier B.V. All rights reserved.

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 957

1. Introduction

Worldwide interest in renewable energy has been necessitatedby the need to divert from over reliance on fossil fuel based energysources. Combustion of coal, natural gas and oil is a direct source ofgreenhouse gases (GHGs), most prominent being CO2 and otherenvironmental harmful emissions [1]. GHGs emissions contributeto air pollution and induce climate change via global warming.Among different proposed alternatives to fossil fuels, particularattention has recently been addressed on reforming of biofuels toH2.

Catalytic ethanol steam reforming has been widely studied andfound to be a feasible and viable reaction pathway for H2 produc-tion [2–4]. Bio-ethanol produced by the fermentation of sugarcane, corn grains, and other starch-rich materials has severaladvantages compared to other rawmaterials since it is a renewableenergy carrier and only slightly contributes to the greenhouseeffect since CO2 is recycled through photosynthesis [5]. Further-more, ethanol has a relatively high hydrogen content and its usein ethanol steam reforming (ESR) theoretically produces 6 mol ofH2 per mole of ethanol reacted by Eq. (1) [6–10].

C2H5OHþ 3H2O ! 2CO2 þ 6H2; DH298 ¼ 174 kJ=mol ð1ÞThe value of 174 kJ/mol for enthalpy change applies to gas

phase. When considering reactants are in the liquid phase,DH298 ¼ 347:4 kJ=mol is required in ESR reaction [11]. Although,ESR is a promising and mature technology for hydrogen produc-tion, yet high endothermic nature of the ESR reaction constitutesa major drawback with regard to system energy efficiency and pro-motion of side reactions [12,13]. Moreover, the overall reformingprocess consists of a complex network of reactions, such as ethanoldehydrogenation, dehydration, or decomposition, which mayresult in the formation of several by-products such as methane,carbon monoxide, ethylene, acetic acid, acetaldehyde, acetone,diethyl-ether and most important and detrimental; coke [14,15].

Apart from the common issues happening in ESR, to furtherenhance the rate of reaction, different catalytic systems have beeninvestigated for ESR, including both precious/noble metals andtransition metals. Contreras, et al. [16] reviewed catalysts for ESRand from the study; Rh, Ru, Pd, Ir, Ni, Co and Cu were found tobe the most probable metals with high H2 selectivity. Supportmaterials most widely investigated are CeO2, ZnO, MgO, Al2O3,zeolites-Y, TiO2, SiO2, La2O2CO3 and CeO2-ZrO2. Although noblemetal catalysts, such as Rh, Pt, Pd, Ru and Re afford high activityover wide temperature ranges with relatively high space velocities[17–19], high costs and traces availability associated with suchmetals limits their application. Hence, the development of lessexpensive active and stable non-noble metal catalysts for ESR arehighly desirable. Additionally, it has become a common trend tohave bi/multi-metal catalysts, applied to the ESR process [20–25],where metals complement each other for optimum catalyticperformance.

Among the transition metals, Ni is widely employed in theindustry because it has low cost and exhibit good activity and H2

selectivity in reforming processes owing to the excellent abilityof C–C and C–H cleavage and sufficient oxygen transport capability[26–28]. Several studies have been performed on ESR over varioussupported Ni catalysts such as Ni/Al2O3, Ni/(La2O3, Al2O3, YSZ,MgO), Ni/CexTi1�xO2, and nano-NiO/SiO2 [22,26,29,30]. Similarly,Ni over Zr-Ce and La-modified CaO sorbents has been investigatedfor hydrogen production from steam reforming of biogas [31].However, Ni-based catalysts are susceptible to carbon depositionand sintering at elevated temperature [32]. Promisingly, studieshave shown that highly active and stable Ni-based catalysts canbe obtained through optimization of physicochemical properties

by means of modified synthesis procedures [33] and/or structural[34] and surface promotion [35]. These alterations mainly seek toenhance the dispersion of Ni for improved activity and H2

selectivity.Moreover, suitable supports should be resistant to the high

temperatures applied in ethanol steam reforming while maintain-ing the metal dispersion as high as possible during the reaction.Commonly, the type spinel oxides such as CeO2, SiO2 and ZrO2

are proposed as catalyst supports due to their stability in termsof resistance to coking and sintering [36–38]. Li, et al. [39] used aNi/ZrO2 catalyst for steam reforming of ethanol for H2 production.However, with a Ni/CeO2-ZrO2 catalyst, Ebiad, et al. [40] estab-lished that the addition of CeO2 to ZrO2 increased the surface areafor Ni dispersion, which translated to higher catalyst activity. TiO2

being a similar spinel oxide has added encouraging advantagesincluding relatively low cost, abundance in availability, chemi-cally/thermally and biologically stable and is non-toxic [41]. Previ-ously, TiO2 catalysts have been extensively studied with respect tophoto-catalysis [42–46]. However, from the study of Ye et al. [47],TiO2 addition was found to improve the redox capacity of CeO2

support in ESR. The catalytic activity of TiO2 depends on its crys-talline structure, particle size, surface area and surface hydroxylgroups. On one hand, the dispersion of Ni on TiO2 could beimproved by co-loading TiO2 with CeO2, SiO2 or ZrO2. Alternatively,other efficient, heterogeneous and green materials with porous orlamellar structure such as natural clay minerals could be consid-ered because of their higher surface areas, high metal dispersionwith reduced coke deposition and sintering resistance.

Among the clay minerals, pillared clays constitute a group ofmesoporous materials whose micro-particles can be used as sup-port on which TiO2 nanoparticles can be fixed on the surface of asuitable matrix with particle size in the order of micrometers. Claymaterials are low cost, environment friendly and have high surfacearea. Among the pillared clays, montmorillonite (MMT) is a type ofnatural clays which is multilayered and classified as crystal latticeof 2:1 layered silicates. Its crystal structure consists of layers madeup of inner octahedral sheet sandwiched between two outer tetra-hedral silicon layers. The MMT silicate layers are held together byrelatively weak forces. Water and other relatively polar moleculescan enter between the unit layers, causing its lattice to expand[48]. Therefore, silicate layers could experience dispersion duringintercalation process and also have unique intercalation and exfo-liation characteristics [49].

The application of MMT as a catalyst/support has shownincreased catalytic activity in different heterogeneous processes[50–52]. Besides improving the thermal stability of TiO2 throughintercalation, MMT has been found to manifestly provide high bar-rier properties such as reduced permeability towards water andalcohols. This property improves the hydrothermal stability ofthe catalyst [53]. Recently, NiO loaded over montmorillonite sup-port was tested for hydrogen production from ethanol steamreforming and found best performance for selective H2 productiondue to high NiO dispersion with strong sintering and coke resis-tance effects by MMT support [54]. Similarly, Ni-CaO promotedMMT catalysts tested for ESR and found high activity and stabilitydue to enhanced metal dispersion with minimized sintering andcoke formation effects [55]. Therefore, it is promising to investigateNi as an active metal over MMT-dispersed TiO2 catalyst for selec-tive and enhanced hydrogen production via ESR reaction.

The objective of this study is to design and fabricate montmoril-lonite supported Ni/TiO2 catalyst by a sol-gel assisted impregna-tion technique for steam reforming of ethanol for H2 production.The impact of several reaction parameters, related to reaction tem-perature, time on stream, steam-to-ethanol feed ratio and Ni/MMTloading on activity and stability performance of the catalyst is ana-lyzed. Performance of a Ni/MMT-TiO2 nano-composite catalyst in

958 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

ethanol conversion, H2 yield and product distribution is criticallydiscussed, based on experimental results. Furthermore, an exten-sive characterization study was carried out to ascertain thestructure-activity relationship of the catalyst. The improved dis-persion of Ni on TiO2 support by MMT could improve interactionbetween reactant species and the active sites of catalysts, makingthe Ni/MMT-TiO2 nano-composite a promising catalyst for ethanolsteam reforming.

2. Experimental

2.1. Preparation of Ni/TiO2 catalysts

The chemicals used for the synthesis of Ni/TiO2 nanoparticleswere Titanium (IV) Isopropoxide (TTIPO, 97%, Sigma-Aldrich) as asource of titanium (IV), isopropanol (99.8%, QRëC), acetic acid(99.8%, and QRëC) and Ni(NO3)2�6H2O (Sigma-Aldrich). Initially,TiO2 nanoparticles were prepared using sol-gel method accordingto our previous work [56]. The precursory of titanium solutionwas prepared with molar ratios; Ti (OCH (CH3)2)4:15 C3H7OH:CH3-COOH. Typically, 1 M acetic acid diluted in isopropanol was addeddrop wise into titanium solution of isopropanol for the hydrolysisprocess and stirred vigorously for 24 h at room temperature. Next,specified amount of Ni(NO3)2�6H2O dissolved in isopropanol wasadded to the titanium sol and stirred for another 6 h. Afterwards,sol was dried in an oven at 110 �C for 24 h under air flow and thencalcined in a furnace at a rate of 5 �C/min up to 500 �C and held for5 h. The samples obtained were named Ni/TiO2 nanoparticles (Ni/TNPs).

On the other hand, anatase TiO2 micro-particles (TMPs) (SigmaAldrich) were used to prepare Ni-loaded TMPs via impregnationmethod. Typically, 1 g TiO2 was dispersed in 50 mL deionized(DI) water. Then, appropriate amount of Ni(NO3)2�6H2O dissolvedin water was added to TiO2 solution and stirred for 1 h, then overdried for 24 h at 110 �C under air flow. Finally, samples were cal-cined at 500 �C for 5 h and named NiO/TiO2 micro-particles (NiO/TMPs). The amounts of Ni loading levels in TMPs were 7 wt. %,10 wt. %, 12 wt. % and 15 wt. % and named as 7% Ni/TMPs, 10%Ni/TMPs, 12% Ni/TMPs and 15% Ni/TMPs, respectively.

2.2. Preparation of MMT/TiO2 catalysts

The montmorillonite (Bentonite, Sigma Aldrich) was used forthe preparation of MMT/TiO2 nanocomposite catalysts. Initially,TiO2 nanoparticles were prepared using the sol-gel method as dis-cussed previously. Typically, 30 mL isopropanol was added to10 mL TTIPO solution and hydrolysis process was conducted using1 M acetic acid solution. The acetic acid diluted in isopropanol wasadded drop wise into titanium solution of isopropanol and stirredfor 24 h at room temperature. Next, appropriate amount of MMTdissolved in isopropanol was added drop-wise into the titaniumsol. The process of gelation was continued by stirring the mixtureat room temperature for another 6 h until thick sol was formed.After aging, the slurry was dried in an oven at 110 �C for 24 h underair flow. The cake was finely ground and calcined in a furnace at arate of 5 �C/min up to 500 �C and held at this temperature for 5 hand named as MMT/TiO2 nanocomposites (TNCs).

2.3. Preparation of Ni/MMT-TiO2 catalysts

The Ni-loaded/MMT-TiO2 nano-composite catalysts were pre-pared using impregnation method. Typically, 1 g of MMT/TiO2

nanocomposite was dispersed in 50 mL DI water and stirred for1 h. Next, an appropriate amount of Ni(NO3)2�6H2O dissolved inwater was added to mixture and stirred for 4 h. Finally, samples

were dried in an oven at 110 �C for 24 h under airflow and calcinedat 500 �C for 5. The samples were named as Ni-MMT/TiO2

nanocomposites (Ni/TNCs). Similarly, TiO2 micro-particles (TMPs)were used to prepare Ni-MMT/TiO2 micro particles samples andnamed as Ni-MMT/TMPs.

2.4. Catalyst characterization

All the catalyst samples were characterized using relevantphysico-chemical methods. Crystalline structure and phase trans-formation analysis of samples were undertaken by X-ray diffrac-tion (XRD), performed on Bruker D 8 advance diffractometer (Cu-ka radiation, k = 1.54 Å, operated at 40 kV and 40 mA) at a rate of1� min�1 from 2h = 1.5�80�. BET (Brunauer-Emmett-Teller) surfacearea, average pore volume and pore diameters were determined byN2 adsorption examined at �196 �C. All the samples were degasi-fied at 250 �C for four hours under vacuum conditions. BET analysiswas carried out by micrometrics 3Flex Version 3.01. FTIR spec-troscopy was used to follow the evolution of catalyst surface spe-cies during temperature programmed desorption of ethanol. Thetechnique obtained an infrared spectrum of absorption of the solidcatalyst pellets. This was done by IRAffinity-1S equipment fromShimadzu Corporation. The microstructures, surface morphologyand quantitative elemental analysis of the catalyst were investi-gated using scanning electron microscopy (SEM) coupled with anX-ray energy dispersive spectrometer (EDX). Carl Zeiss Supra 35VP SEM and Oxford instruments were used respectively. To moni-tor the extent of coke deposition on catalyst, raw and used catalystparticle sample were subjected to thermo gravimetric analysis(TGA), performed by a TGA 8000 system from PerkinElmerinstruments.

2.5. Experimental procedure

Experimental set up consisted of a feed, reaction and analysissection as shown in Fig. 1. The feed to the reactor was a gas mix-ture of water, ethanol and nitrogen. Water-ethanol mixture wasfed to an evaporator, operated at 150 �C through a syringe pump(KDS-100), operated at 2 mL/h. N2 was introduced as a carriergas at a flow rate of 40 mL/min, except for tests of space velocity.The evaporator was connected to the reactor through stainlesssteel tubes and connectors, maintained at 130 �C by use of a heat-ing tape to prevent condensation. The ESR reactions were carriedout isothermally in a borosilicate glass tube. The catalytic bed, real-ized by sandwiching the powder catalyst (0.5 g, diluted in quartz,with catalyst to inert weight ratio of 1:1) between quartz wool,was located in the annular section of the reactor (8 mm i.d. and29 cm length). The reactor was placed in a three zone horizontaltube furnace (CARBOLITE, MTF 10/15/130) which was monitoredby a programmable temperature controller. ‘‘K type” thermocou-ples were used to measure the temperature of the catalytic bedinside the reactor; in particular, in correspondence of the inlet,the middle and the outlet of the catalytic bed.

The analysis section consisted of an online GC (Agilent 6890Nnetwork GC system) for product gas samples and an offline GC(Agilent 7820A GC system) for product liquid samples. For onlineGC, the FID detector was connected with an HP-PLOT Q capillarycolumn (Agilent, length 30 m. ID 0.53 mm, film 40 lm) for separa-tion of C1–C6 hydrocarbons, alcohols and oxygenated compounds.The TCD detector was connected to UCW982, DC-200, Porapak Qand Mol Sieve 13 X columns for detection of C1–C5 compoundsand light gases (H2, O2, N2, CO).

The catalytic performance was evaluated through the conver-sion of ethanol (X ethanol), selectivity (S) and yield towards the mainproducts (H2, CO2, CH4 and CO) and minor/negligible products

MFC

MFC

MFC

MFC

TC

TemperatureDisplay

Online GasChromatograph

FumeHood /

ExhaustAr

Air

Syringe Pump

Mixer

Evaporator

Tubular FurnacePBR

Condenser/ Ice Trap

HeatedFeedLine

PC

Condenser

Silica GelFlowMeter

Offline GC(Liquid

Samples)

(GasProduct)

(LiquidProduct)

H2

N2

Thermocouples

Fig. 1. Schematic of experimental apparatus for ESR towards hydrogen production.

TiO 2

MMTNi/TiO micro-particles

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 959

(C2H4, C2H6, C3H6, C3H8, CH3CHO, CH3COOH, (C2H5)2O and (CH3)2-CO), as listed in Table 1.

*

Inte

nsity

(a.u

)

2

Ni/20% MMT-TiO 2 micro-particlesNi/20% MMT-TiO 2 nano-composite

NiO

*

TiO 2

001

101 004 200 105 211 204

3. Results and discussion

3.1. Characterization of calcined catalyst samples

The XRD patterns of TiO2, MMT, 12% Ni/TiO2 micro-particles,12% Ni/20% MMT-TiO2 micro-particles and 12% Ni/20% MMT-TiO2

nano-composite, calcined at 500 �C for 5 h are exhibited in Fig. 2.The TiO2 micro-particles were in pure anatase phase with 101 peak

Table 1Equations used to compute various parameters used in analysis of the ESR process.

Parameter Equation

Ethanol Conversion Xethanolð%Þ ¼ F inethanol�Foutethanolð ÞFinethanol

� 100 (i)

Hydrogen Yield YH2 ð%Þ ¼ FoutH2

6�F inethanol� 100 (ii)

Yield of C-Containing Product i Yið%Þ ¼ #C�Fouti

2�F inethanol� 100 (iii)

Product Selectivity Sið%Þ ¼ FoutiPjFoutj

� 100 (iv)

CO2 Ratio CO2COx

¼ FoutCO2FoutCO2

þFoutCO

(vi)

where (F) indicates the molar flowrates of reactants and products in mol/h, (#C)represents the number of carbon atoms in a particular compound. Fouti refers to themolar flowrates of reaction products in the gaseous products, while Foutj depicts themolar flow rate of the component j coming out from the reactor. Selectivity valuesare reported as the molar percentage of the products obtained. It should also benoted that selectivity is calculated based on product distribution, whereas, Rj (Fjout)excludes, unconverted ethanol and water, plus nitrogen (carrier gas) [57,58].

10 20 30 40 50 60 702-theta (degree)

Fig. 2. XRD patterns of TiO2, MMT, Ni/TiO2 and Ni/MMT-TiO2 catalyst samples.

located at 2-theta of 25.62�. Ni loaded TiO2 micro-particles demon-strated a similar peak but of a lower intensity. The MMT XRD pat-tern showed basal (001) reflection around 2h = 3.70�, due to inter-layer of clays. In case of 20% MMT loading on Ni/TiO2, the diffrac-tion peaks were wider and weaker than those of catalysts preparedwithout MMT. Similarly, Ni and TiO2 containing MMT samples haddissimilar XRD patterns compared to the pure MMT. MMT peak(001) is present in the pure MMT sample but disappears in theNi/MMT-TiO2 samples due to efficient dispersion MMT on TiO2

particles. Furthermore, this could be an indication that the original

960 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

layers of MMTwere destroyed and thinly deposited on TiO2. For Ni/MMT-TiO2 micro-particles and Ni/MMT-TiO2 nano-compositesamples, diffraction peaks are almost equivalent, exhibiting 2hpeaks at 25.318�, 37.811�, 48.053�, 53.916�, 55.057� and 62.716�,which are consistent with (101), (004), (200), (105), (211) and(204) planes associated with tetragonal anatase. A NiO peak is pre-sent in Ni/TiO2, Ni/MMT-TiO2 micro-particles and Ni/MMT-TiO2

nano-composite samples but absent in bare TiO2 and MMT sam-ples. The slight broadening of the TiO2 peak (101) for Ni/MMT-TiO2 nano-composite is an indication of a smaller crystallite sizeas compared to the Ni/MMT-TiO2 micro-particles sample. Averagecrystallite sizes were calculated using Scherer’s equation, repre-sented by Eq. (2).

Dp ¼ Kkb1=2 cos h

ð2Þ

where Dp is crystallite diameter (nm), K is shape constant (0.94 forthis study). k is the X-ray source wavelength (1.5406 Å), b1/2 is peakFWHM (degrees) and h is Bragg’s angle of the 2h peak. The crystalsize of TiO2 NPs was 19 nm, reduced to 11 nm in Ni/MMT-TiO2

nano-composite.The morphology and microscopic structure of catalysts was

investigated using SEM as illustrated in Fig. 3(a–e). Spherical par-ticles with almost uniform size for TiO2 micro-particles are shownin Fig. 3(a). For TiO2 nano-particles, while the spherical shape ismaintained, the size of the particles is reduced as depicted inFig. 3(b). The SEM images in Fig. 3(c–d) show MMT sheets and lay-ers being almost smooth, lamellar and uniform shape, respectively.Fig. 3(e) illustrates the SEM images of the Ni/MMT-TiO2 nano-

Fig. 3. SEM micrographs of different catalyst samples; (a) TiO2 micro-particles, (b)TiO2 nano-particles, (c) MMT sheets, (d) MMT layers, (e) Ni/MMT-TiO2 nano-composite.

composite catalyst. The catalyst shows both MMT sheets andTiO2 nano-particles, thus creating a nano-composite. TiO2 particlesare almost spherical in shape and uniform in size. Evidently, MMTlayers are destroyed and produced delaminated MMT structure,over which Ni and TiO2 nanoparticles distributed inside MMT gal-leries and over the surface. The Ni/MMT-TiO2 nano-compositesample can be classified as heterogeneous because the compositeis a cluster of particles and sheets, both on a nano-scale. However,it is impossible to identify Ni due to its small weight percentageand efficient dispersion on MMT/TiO2 nanocomposite.

The N2 adsorption and desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore size distribution of the calcined Ni/TiO2 nano-particles and Ni/MMT-TiO2 nano-composite are exhib-ited in Fig. 4(a–c). From Fig. 4(a), the isotherms of the samplesare similar to type IV curves with hysteresis loops, correspondingto mesoporous materials [59]. Both samples exhibit increasedabsorption beyond P/P0 = 0.9. Curves are essentially identical, indi-cating minimal distortion of the TiO2 crystal lattice by MMT addi-tion. Fig. 4(b), shows the relation of pore size distribution and porevolume of both the samples. Ni/TiO2 nano-particles show volumeof pores in the pore diameter range of 5.54–183.77 nmwith centralvalue 29.8 nm at which highest pore volume detected. However,Ni/MMT-TiO2 nano-composite has a different distribution withmost of the pores being around 5.45–277.38 nm with central value17.5 nm. Similar trends of pore size distribution could be seen inpore areas as depicted in Fig. 4(c). Evidently, both pore volumeand pore areas are more concentrated towards the low pore diam-eter vicinity of the graph for Ni/MMT-TiO2 nano-composite samplethan Ni/TiO2 nano-particles. This suggests that the impregnationand structural modification of TiO2 results in small pores, probablydue to the presence of MMT. In addition, smaller pores aid ininhibiting agglomeration, resulting in a higher Ni dispersion forthe Ni/MMT-TiO2 composite catalyst. Similar observations hasbeen reported previously [60].

The BET surface area, pore volume and pore size of Ni/TiO2

nanoparticles and Ni/MMT-TiO2 nano-composite are summarizedin Table 2. The BET surface area of Ni/TiO2 was 15.46 m2/g, whileMMT loading on Ni/TiO2 nano-particles barely affected the BETsurface area of the composite catalyst (15.384 m2/g). BJH surfaceof Ni/TiO2 (11.68 m2/g) was higher than the Ni/MMT-TiO2

(8.77 m2/g) composite, whereas, t-plot micro-pore of 10.47 m2/gwas observed in Ni/MMT-TiO2 sample, somewhat, higher thanthe Ni/TiO2 (7.94 m2/g) sample. This revealed that Ni/TiO2 samplehas more mesopores than the Ni/MMT-TiO2 nano-composite, evi-dently due to microporous structure of MMT. Furthermore, MMTaddition could be assigned to the partial insertion of MMT parti-cles, partly filling the structural and textural defects of the TiO2

support [61]. This revealed that BET surface area is not an influen-tial factor to enhance the catalytic activity for hydrogen produc-tion. This can be attributed to the fact that MMT is a layeredstructure which could provide good interaction between MMT/TiO2 composite, which results in higher Ni-dispersion [54,55]. Fur-thermore, BJH pore volume of Ni/TiO2 sample is higher than Ni/MMT-TiO2 composite, while t-plot micro-pores have oppositereflections. In general, t-plot micro-pores volume is much lowerthan the BJH pore volume in both the samples, attributed to moremesopores than the micro-pores in both the samples. The BJH porediameter of Ni/MMT-TiO2 is comparatively increased on MMTaddition, probably due to MMT sheets being deposited on TiO2

pores.

3.2. Catalysts screening for H2 production

The metal loading and nature of support is known to influencethe performance of supported metal catalysts. Additionally, thestructure of the support dictates the dispersion of the active metal,

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

20

40

60

80

100

Vol

ume

Ads

orbe

d (c

m3 /g

@ S

TP)

Relative Pressure (P/P 0)

12 % Ni/TiO 2 nano-particles 12 % Ni/20 % MMT-TiO 2 nano-composite

(a)

0 500 1000 1500 2000 2500 30000.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

0.00014

0 500 1000 1500 2000 2500 30000.000

0.005

0.010

0.015

0.020

0.025

0.030dA

/dD

Por

e A

rea

(cm

2 /g·Å)

Pore Diameter (Å)

dV/d

D P

ore

Vol

ume

(cm

3 /gÅ

)

Pore Diameter (Å)

12 % Ni/TiO 2 nano-particles12 % Ni/20 % MMT-TiO 2 nano-composite

(b) (c)

Fig. 4. (a) N2 adsorption-desorption isotherms; BJH pore size distribution: (b) pore volume, (c) pore area.

Table 2Summary of physico-chemical characteristics of catalyst samples.

Property Catalyst Sample

Ni/TiO2 nano-particles

Ni/MMT-TiO2

nano-composite

BET surface area (m2/g) 15.46 15.38BJH surface area (m2/g) 11.68 8.77t-plot, micro-pore area (m2/g) 7.94 10.47BJH pore volume (cm3/g) 0.139 0.112t-plot micro-pore volume (cm3/g) 0.0033 0.0043BJH pore width (nm) 4.77 5.12

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 961

consequently influencing catalytic activity. For this reason, prelim-inary catalytic performance tests were conducted for hydrogenproduction during ESR reaction over Ni/TiO2 micro-particles, Ni/MMT-TiO2 micro-particles, Ni/TiO2 nano-particles and Ni/TiO2

nano-composite under the same operating conditions. Ni/TiO2

MPs and Ni/MMT-TiO2 micro-particles were prepared via impreg-nation method, while Ni/TiO2 nano-particles and Ni/MMT-TiO2

nano-composite were prepared via a combination of the sol-geland impregnation methods.

The different samples were prepared, calcined at 500 �C for 5 hand tested for catalytic activity in ethanol steam reforming. Nickelloading was varied from (7 to 15 wt. %), while the influence ofMMT addition to TiO2 support was evaluated at 10 and 20 wt. %.The summary of the yield rates and conversion of ethanol over dif-ferent types of catalysts has been presented in Table 3. Evidently,in homogeneous system, without using catalyst, both ethanol con-version and as a result, H2 yield were extremely low at 0.55 and0.34%, respectively. This revealed, in the absence of a catalyst inthe reactor affected the reaction through the decrease in space-time. Mildly improved results would be expected for a pure TiO2

sample, since, as a support, TiO2 would have minimal catalyticeffect but would marginally increase the space-time for the reac-tion. Loading TiO2 micro-particles with 7 wt. % Ni, significantlyraised the values of ethanol conversion and H2 yield to 17.62 and11.96%, respectively. Ethanol conversion and H2 yield wereobserved to be progressive up to 12% but regressed at 15% Niloading. Ni contents and its dispersion dictate the activity of a Ni

Table 3Catalyst Screening: steady state catalytic activity results for ESR by different catalysts.

Catalyst EtOH Conv. (%) Product Yield (%)

H2 CO2 CO CH4

No catalyst (Homogeneous) 0.55 0.34 0 0 0.117% Ni/TiO2 MPs 17.62 11.96 3.65 0.97 0.4510% Ni/TiO2 MPs 44.99 31.49 10.55 0.79 2.1512% Ni/TiO2 MPs 53.77 37.48 12.80 0.87 2.6515% Ni/TiO2 MPs 47.72 33.48 11.46 0.97 1.8112% Ni/10% MMT–TiO2 MPs 75.05 47.94 17.61 2.23 7.1612% Ni/20% MMT–TiO2 MPs 83.78 51.68 20.21 1.29 10.5612% Ni/TiO2 NPs 71.55 47.67 16.84 1.29 5.7112% Ni/20% MMT-TiO2 NPs 84.41 55.54 21.45 1.98 5.44

Reaction Conditions; 1 atm., 500 �C, S/E = 10:1, GHSV = 13,200 ml/gcat�h.

0 1 2 3 4 5 60

10

20

30

40

50

60

70

80

H2

Sele

ctiv

ity (%

)

Time on stream (h)

Homogeneous 7% Ni/TiO 2 micro-particles 10% Ni/TiO 2 micro-particles 12% Ni/TiO 2 micro-particles 15% Ni/TiO 2 micro-particles 12% Ni/ 10% MMT-TiO 2 micro-particles 12% Ni/ 20% MMT-TiO 2 micro-particles 12% Ni/TiO 2 nano-particles 12% Ni/MMT-TiO 2 nano-composite

(a)

23.75 24.1125.42

1.62 1.552.34

4.91

12.96

6.44

12% Ni/TMPs 12% Ni/ 20 % MMT-TMPs 12% Ni/TNC0

5

10

15

20

25

Prod

uct S

elec

tivity

(%)

Catalyst

CO2

CO CH4

(b)

Fig. 5. Catalytic performance; (a) H2 selectivity, (b) Product selectivity. Reactionconditions: 1 atm., 500 �C, S/E = 10:1, GHSV = 13200 ml/gcat�h, 6 h reaction time.

962 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

promoted catalyst; activity increases with Ni contents due to anincrease in the number of active sites, whereas Ni dispersiondecreases, hence limiting the same activity [62]. Generally, Ni con-tent of conventional catalysts applied in ESR does not exceed12 wt. % to avoid aggregation and sintering of particles. Both Roy,et al. [63] and Vicente, et al. [64] applied a 10 wt. % Ni loading onan Al2O3 and SiO2 support, respectively in ESR for H2 production.Aggregation of Ni species could be inhibited through increased dis-persion of Ni metal on the catalyst support.

Having established 12 wt. % as the optimum Ni loading on TiO2

micro-particles, the effect of MMT addition on catalyst activity wasinvestigated. Increasing the MMT amount from 10 to 20 wt. %increased ethanol conversion by 11.6% and H2 yield by 7.2%. Thiscan be attributed to MMT increasing dispersion of Ni particles onthe TiO2 micro-particles. Interestingly, the increase in MMTamount from 10 to 20 wt. %, suppressed CO yield by almost 50%.However, attempts to further add to the MMT-loading above20 wt. %, resulted in the clay/TiO2 slurry converting into hardenedcake upon calcination. Therefore, optimal of 20 wt. % MMT-loadedinto TiO2 is selected for further investigation of Ni-incorporation.

The effect of structural modification was further evaluated bycomparing the performance of micro-particles, nanoparticles andnanocomposites materials. Using 12 wt. % Ni/TiO2 micro-particles,the conversion of ethanol achieved was 53.77%, increased to71.55% over 12% Ni/TiO2 nanoparticles. Similarly, efficiency wasfurther increased in 12 wt. % Ni/MMT-TiO2 nano-composites, whileconversion reached to 84.41%. Similarly, the yield rate of H2 wasincreased with the change in support structure from micro parti-cles to nano-particles of TiO2, whereas, the highest yield of ethanolobtained was 55.54% with 12% Ni-MMT-TiO2 nanocomposite cata-lyst. This revealed that support has significant effect on the conver-sion and yield rate during ESR process. This is because more activesites and efficient Ni dispersion can be achieved using nanostruc-tures than the micro-particles. The addition of MMT furtherimprove the Ni dispersion, results in enhanced process efficiencyand similar observations have been reported previously [54].Nano-particles and nano-composites exhibit similarities in theirsize range (nano-scale) but differ in terms of regularity of theirstructures. The irregular structure of the nano-composite sampleproved to offer a higher dispersion for Ni, resulting in high ethanolconversion and hydrogen yield rates. Previously, low temperatureethanol steam reforming for hydrogen production was conductedby Lee at al. [65] over Sn-incorporated SBA-15 catalysts, while Ros-setti et al. [66] investigated the use of Ni, Co and Cu metals incor-porated over SiO2 and ZrO2 supports in ethanol steam reformingfor hydrogen production. In both reports, higher conversion effi-ciency has been reported compared to current study using Ni/MMT-TiO2 nano-composite catalyst. The lower conversion effi-ciency in current study compared to previously reported valuesis probably due to different operating parameters. Furthermore,

MMT is a green and environment friendly materials with relativelylow cost, good metal dispersion and improved stability [54]. It canbe explored further by combining with supports (e.g., SBA-15,ZrO2) to get improved activity at reduced cost in ethanol steamreforming process for hydrogen production.

Besides screening the catalysts performance on the basis ofethanol conversion and H2 yield, product selectivity was alsoevaluated. H2 and CO2 selectivity were almost constant for both

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 963

homogeneous and heterogeneous reaction conditions as depictedin Fig. 5(a). This could be attributed to the temperature dependentnature of ethanol steam reforming. At low temperatures, below350 �C, reactions such as ethanol dehydrogenation, aldehydedecomposition and methanation dominate. However, at the giventemperature of 500 �C, ESR and coke formation are most effective[67]. From Fig. 5(b), on addition of MMT to Ni/TiO2 micro-particles, selectivity of valuable CH4 increased significantly whilethat towards CO dropped marginally. The drop in both H2 and COselectivity may be due to promotion of the methanation reaction(Eq. (3)), which converts both CO and H2 [68].

COþ 3H2 $ CH4 þH2O ð3ÞMethanation reaction is an endothermic reaction and thus is

favored by the high temperature. Furthermore, MMT additioncould have altered selectivity with preference towards methaneformation. 12% Ni/MMT-TiO2 nano-composite exhibited higherselectivity for all products analyzed as compared to 12% Ni/TiO2

NPs. This demonstrated that both MMT addition and modificationof particle size to nano-structure influenced the activity of the cat-alyst. Hence, a nano-composite catalyst composed of 12% Ni/20%MMT-TiO2 was adjudged to be of optimum formulation andstructure.

3.3. Effect of operating parameters on catalytic activity and selectivity

In almost all the cases reported in literature with regard toethanol steam reforming to H2, the catalysts are reduced in hydro-gen before reforming reaction. This treatment at high temperaturescould lead to sintering of Ni and consequently increase the ten-dency to carbon formation. In this study, the Ni/MMT-TiO2 catalystwas further used without reduction with hydrogen to investigatethe effect of process parameters, namely operating temperatures,feed molar ratio and space time.

3.3.1. Effect of temperatureESR was carried out over Ni/MMT-TiO2 nano-composite at

300 �C, 400 �C and 500 �C. The conversion of ethanol and productyield at different temperatures and at a feed molar ratio (S/E = 10:1) was compared and the results are presented in Table 4.Ethanol conversion was very low at 300 �C but increased progres-sively with the increase in temperatures, reaching a high of 88.85%at 500 �C. These results agree well with previously reported work[8,9,69]. These observations have confirmed that ESR is anendothermic reaction, thus favored at high temperatures.

High ethanol conversion at 500 �C is further supported by thecomparative high yields of both H2 and CO2. Compounds such asC2H4 and C2H6 had higher yields than CH4, CO and CO2 at temper-atures below 500 �C. This occurrence can be attributed to side reac-tions such as ethanol dehydration (Eq. (4)), which is exothermicand thus favored by lower temperatures. Thermodynamicallyprobable compounds such as CH3COOH, CH3CHO, C2H5OC2H5 andCH3COCH3 were not detected, most likely because they could have

Table 4Ethanol conversion and product distribution as functions of temperature.

Ethanol conversion (%)

Product yield (%) CH4

CO2

H2

COC2H4

C2H6

been undergoing rapid reforming upon formation. The concentra-tion of CO increased monotonously as expected, from the water-gas-shift (WGS) reaction (Eq. (5)), shift in equilibrium to the left,with the increased in temperature.

C2H5OH $ C2H4 þH2O ð4Þ

COþH2O $ CO2 þH2 ð5ÞAt 500 �C, ethanol approached full conversion while C2H4 and

C2H6 species disappeared, probably since the C2 intermediateswere decomposed into fragments desorbing as H2, CO and CH4.This study achieved a high ethanol conversion for the Ni/MMT-TiO2 nano-composite catalyst at 500 �C. Consequently, the increasein ethanol conversion resulted in a H2 yield of 54.54%. While etha-nol steam reforming is an endothermic reaction, both the high con-version and yield could be attributed to the increase intemperature in addition of high activity of the catalyst. While H2

yield increased, selectivity dropped with an increase in tempera-ture as shown in Fig. 6. This can be attributed to the fact that,besides high temperatures favouring the main ESR reaction, otherside reactions such as acetaldehyde decomposition (Eq. (6)) andmethanation/reverse water gas shift reaction (Eq. (7)), which areendothermic, were also promoted.

CH3CHO $ CH4 þ CO ð6Þ

CO2 þ 4H2 $ CH4 þ 2H2O ð7ÞBoth these reactions result in production of valuable CH4 but in

acetaldehyde decomposition, CO is also formed. CO formation atincreasing temperatures is further emphasized by evaluating theCO2/COx ratio. At 400 �C, CO2 dominates but CO starts to form withincreasing temperature. At 500 �C, CO2/CO = 9:1, showing the com-mencement of CO formation. Therefore, it can be concluded thatlower temperature is favourable for higher H2 selectivity, yethigher temperature promoted ethanol conversion.

3.3.2. Effect of feed molar ratio (S/E)Fig. 7 shows ethanol conversion, H2 selectivity and product

yield as a functions of feed molar ratio. From Fig. 7(a), it can beobserved that ethanol conversion decreases with an increase infeed molar ratio. This indicates that water is preferentiallyadsorbed on the catalyst. In other words, water competes withethanol for similar ‘‘active sites” on the catalyst surface. The pres-ence of large amounts of water at high S/E feed molar ratio (10:1)hinders the adsorption of ethanol, while shifting the equilibriumfor ethanol dehydrative coupling reaction to the left, hencedecreasing the ethanol conversion. Ethanol dehydrative coupling,an endothermic reaction (Eq. (8)), is plausible, feasible and favoredat 500 �C.

C2H5OH $ 1=2C2H5OC2H5 þ 1=2H2O ð8ÞWhile high feed molar ratios were synonymous with lower ethanolconversion, the high amount of water and low amount of ethanol

Temperature (�C)

300 400 500

0.63 7.55 88.85

0.025 0.081 12.640 0.529 21.390.534 5.747 54.540 0 2.450.072 0.798 0.000.003 0.142 0.04

0 1 2 3 4 5

0

20

40

60

80

100

H2 S

elec

tivity

(%)

Time on Stream (h)

300

400

500500

300

400

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CO

2/CO

x

Fig. 6. Effect of temperature on H2 selectivity and CO2/COx ratio. Reactionconditions: 1 atm., 500 �C, S/E = 10:1, GHSV = 13200 ml/gcat�h, 5 h reaction time.

964 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

favored H2 selectivity and results are presented in Fig. 7(b). H2

selectivity is mainly affected by the increase in production of side

88.85 90.4 91.7 92.1598.89

10:1 5:1 1:1 1:5 1:100

20

40

60

80

100

Etha

nol C

onve

rsio

n (%

)

Feed Molar Ratio (S/E)

(a)

12.6

4 17.1

8

31.5

1

21.4 22

.87

19.4

8

54.5

5

55.6

1

2.45 3.31

10:1 5:1 1:10

10

20

30

40

50

60

Prod

uct Y

ield

(%)

Feed Molar Ra

(c)

Fig. 7. Feed molar ratio influence on; (a) ethanol conversion, (b) H2 selectivity, (c) prodtime.

products. Lowering the S/E feed molar ratio from 10:1 to 1:10resulted in an approximate 50% drop in H2 selectivity. This can beattributed to the fact that an increase of ethanol fraction in feedcomposition at 500 �C promotes endothermic side reaction of etha-nol decomposition (Eq. (9)), which promotes yields of side productsCH4 and CO, thus reducing overall H2 selectivity. Water inhibits theethanol decomposition reaction by channeling ethanol reactivitytowards the main ESR reaction. This point is supported by the highCO2/COx ratio (�0.9) at high S/E (10:1) feed molar ratio. CO2 domi-nates at high S/E ratio (10:1) while CO dominates at low S/E feedmolar ratio (1:10) [70].

C2H5OH $ COþ CH4 þH2 ð9ÞFig. 7(c) shows the yield of significant products; H2, CO2, CH4,

and CO in relation to the feed molar ratio. H2 and CO2 yield pro-gressively decreased as the feed molar ratio (S/E) was reduced from10:1 to 1:10. However, yield of valuable CH4 and undesirable COincreased. This observation can be linked to the promotion ofmethanation and ethanol decomposition reactions by higher etha-nol fraction in feed. It further demonstrates that the high ethanolconversion exhibited by low feed molar ratio (S/E = 1:10) did nottranslate to higher H2 yield but resulted in production of moreCH4 and CO.

0 1 2 3 4 520

25

30

35

40

45

50

55

60

65

7010:1

H2 S

elec

tivity

(%)

Time on Stream (h)

10:1

(b)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CO

2/CO

x

37.8

2 42.0

2

14.6

2

13.3

35.1

1

29.2

1

28.7

6

5.51

10.5

2 14.2

8

1:5 1:10tio (S/E)

CH 4

CO 2

H2

CO

1:10

1:5

1:1

5:1

1:1

5:1

1:5

1:10

uct yield. Reaction conditions: 1 atm., 500 �C, GHSV = 13200 ml/gcat�h, 5 h reaction

3850 3300 2750 2200 1650 1100 550

Abs

orba

nce

(a.u

)

Wavenumber (cm-1)

Ni/MMT-TiO2 nano-composite (Fresh)Ni/MMT-TiO2 nano-composite (Spent)

3631

1640

1042 918

Fig. 9. FTIR spectra of fresh and spent Ni/MMT-TiO2 composite catalyst samples.

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 965

3.3.3. Effect of space timeIn order to investigate the influence of space-time (W/F) on the

conversion of ethanol and products distribution, catalytic testswere performed in the space time range of 1.639–8.197 kg catalysth/kmol EtOH fed at 500 �C with a S/E feed molar ratio of 10:1. Asseen in Fig. 8, the products selectivity of CO, CH4 and CO2 are close,indicating the simultaneous occurrence of ethanol decompositionand WGS reactions. However, it can be noted that selectivity ofall products is minimally affected by changes in space-time. Thiscan be attributed to contact time between reactants and activesites of catalysts. High contact times would result in high ethanolconversions and subsequent promotion of all plausible and feasiblereactions and vice versa.

An increase in space-time from 1.639 to 4.098 kg catalyst h/kmol EtOH fed resulted in increased H2 yield. This is due to the factthat enough contact time allowed for conversion of both reactantsand intermediate products into H2. All products yield except CO2

tended to reach a maximum after which yield is no longer affectedby increase in space-time. For CO2, yield dropped when the space-time was increased from 4.098 to 8.197 kg catalyst h/ kmol EtOHfed. This could be an indication that the reverse water-gas-shiftreaction was not only dictated by temperature, but the equilibriumshift was also favored by low contact time.

3.4. Characterization of spent catalysts

To examine possible carbon deposition and catalyst particlesstructural changes during ethanol steam reforming, post-reactioncharacterization of 12% Ni/20% MMT-TiO2 nano-composite catalystsample was performed. Fig. 9 shows the FTIR spectra obtained fortwo different catalyst specimens; fresh and used catalysts. Clearly,the samples exhibited almost parallel spectra features, indicatingsimilarity in the surface chemical composition of both samples.This confirmed that the spent sample had only mild carbon resi-due. Peak at 3631 cm�1 corresponds to the stretching vibration ofthe hydroxyl AOH bond, which is probably due to the fact thatthe spectra were recorded in situ and some re-adsorption of waterfrom the ambient atmosphere has occurred [71]. Peak observed at1640 cm�1 corresponds to the C@O vibrations [72]. The sharp peakat 1042 cm�1 and the peak observed at 918 cm�1 may be related tohydroxyl (bending) groups of molecular water [73]. Small peaksobserved as the curve tends towards 550 cm�1 correspond to Ti-O vibrations. No evidence about the type of carbon presented overthe Ni/MMT-TiO2 composite detected in FTIR analysis.

1 2 3 4 5 6 7 8 90

10

20

30

40

50

60

CO

CO2

CH4

H2

Prod

uct S

elec

tivity

(%)

Space time, W/F (kg catalyst h/ kmol ethanol fed)

H2

CH4

CO

CO2

0

10

20

30

40

50

60

Prod

uct Y

ield

(%)

Fig. 8. Effect of space-time on product selectivity and yield. Reaction conditions:500 �C, S/E = 10:1, 5 h reaction time.

TGA curves for fresh catalyst sample are shown in Fig. 10(a). Thedegradation has occurred in three steps. The first degradation from50 to 150 �C is attributed to the removal of water from the surface(weight loss of about 1%). The second degradation is from 500 to700 �C with �0.75 weight loss %, due to dehydration and removalof organic residues. The third degradation occurs beyond 900 �C.Total weight loss for the calcined catalyst was approximately2.5%. Similarly, for the reduced sample in Fig. 10(b), it can beobserved that the sample experiences an overall weight loss ofaround 12% from 50 to 1000 �C. Low weight loss is an indicatorof the thermal stability for the catalyst. However, the comparativeextensive weight loss and exothermicity is indicative of loss of car-bon from the catalyst surface at temperatures above 500 �C. How-ever, both samples show single peaks for the% carbon curve. Thesingle peak is indicative of only one species of carbon being formedon the surface. Furthermore, the carbon peak for the reduced sam-ple is sharper and steeper, occurring at temperatures above 700 �C,illustrating formation of a more stable carbon species after thereaction in the spent catalyst.

Fig. 11 shows the SEM micrographs of spent Ni/MMT-TiO2

nano-composite catalyst. The micrographs in Fig. 11(a and b) sug-gestive of coke deposition over the catalyst surface due to alter-ation of shape. The presence of MMT, TiO2 and carbon speciescould be seen in both the images, yet less coke was deposited overthe MMT surface compared to TiO2. This revealed, MMT hinderedcoke formation during ESR reaction and provides stability to cata-lyst. Fig. 11(c) shows the presence of coke over the support struc-ture, evidencing the in particular the presence of carbon nanotubes(CNTs) during ESR reaction for hydrogen production. This confirmsthat main coking mechanism is the formation of CNTs. Similarobservation has been reported previously [66]. However, the deac-tivation was not so severe indicating that an important Ni fractionremained exposed at the reactive mixture on the top of filamentsand the carbon species which were not in contact with metallicparticles were partially removed by gasification or reversed Bou-douard reaction in Eq. (10) [37].

Cþ CO2 $ 2CO ð10ÞEnergy dispersive X-ray spectrometry (EDX) analysis of reduced

catalyst at 500 �C shows peaks for Ti, O, C, Ni and Al with Fe as theonly impurity within the detection limit as presented in Fig. 11(d).Si and Al are both constituents of montmorillonite (Al2O3�4(SiO2)H2O). The effectiveness of MMT is the formation of layers on theTiO2 support that create sites for active Ni deposition. Hence, the

0 200 400 600 800 100097.5

98.0

98.5

99.0

99.5

100.0

Wei

ght %

Temperature ( °C)

617.9 °C

(a)

-0.02

0.00

0.02

0.04

0.06

0.08

Der

iv. W

eigh

t (%

/°C

)

0 200 400 600 800 100086

88

90

92

94

96

98

100

Wei

ght %

Temperature (°C)

750.18 °C

(b)

-0.05

0.00

0.05

0.10

0.15

0.20

Der

iv. W

eigh

t (%

/ °C)

Fig. 10. TGA plots of Ni/MMT-TiO2 composite catalyst; (a) fresh catalyst, (b) spent catalyst.

966 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

presence of these species can be a further verification of MMTstructure. The occurrence of C-species could be attributed to accu-mulation of coke in the form of CNTs, which may be due to ethanoldehydration to ethylene, polymerizing at low temperature if notefficiently gasified as explained in Eq. (11).

C2H4 ! polymers ! 2Cþ 2H2 ð11Þ

3.5. Catalyst stability tests

The development of stable catalyst is one of the most importantissues in the production of H2 from steam reforming of ethanol,particularly since there is no oxygen (partial oxidation) availableto remove carbon deposits that are detrimental to the catalytic sta-bility. Results of stability test for ethanol conversion, H2 selectivityand yield are shown in Fig. 12(a). H2 selectivity approached steadystate before both ethanol conversion and H2 yield because of itstemperature dependent nature in ESR. Over 20 h of reaction time,minimal variations in overall activity were observed while highvalues were maintained. Ethanol conversion, H2 selectivity andyield were at an average of 88, 62 and 55% respectively. Further-

more, during time on stream, the curves for conversion and yieldrate take longer time to stabilize (reached to equilibrium), proba-bly due to using low temperature and because of specific charac-teristics of catalysts. Similar observations have been reportedpreviously [65].

Comparatively, when Ni/MMT-TiO2 micro-particles and Ni/MMT-TiO2 nano-composite catalysts were kept on stream at500 �C, the micro-particles sample showed depreciating activityafter four hours. The depreciating activity was accompanied bypressure build up in the reactor, suggesting significant carbondeposition over this catalyst. The nano-composite catalyst on theother hand showed no activity loss over 20 h of reaction, whileattaining a near steady state condition after 3 h, as shown inFig. 12(b). These results supports the findings that over the Ni/MMT-TiO2 nano-composite catalyst, ethylene is not formed at500 �C and thus, the ethylene polymerization reaction that con-tributes to coke formation, does not occur. Furthermore, since Ni-based catalysts are known to experience coke deposition and sin-tering, minimal coke formation in this catalyst can be attributedto the influence of MMT in improving dispersion of active Ni spe-cies on catalyst surface.

wt. % σTi 36.3 0.2 O 31.9 0.3 C 23.2 0.2 Ni 5.7 0.1 Si 1.7 0.0 Al 1.0 0.0 Fe 0.2 0.0

(a) (b)

(c) (d)

MMT

CNT

TiO2

TiO2

CNT

TiO2

TiO2

CNT

MMT

CNT

TiO2

MMT

CNT

CNT

Fig. 11. SEM micrographs of Ni/MMT-TiO2 nanocomposite: (a-c) SEM images of spent samples, (d) EDX analysis of sepnt catalyst.

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

(%)

Time on stream (h)

EtOH ConversionH2 SelectivityH2 Yield

0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

60

70

80

90

100

Hyd

roge

n Y

ield

(%)

Time on stream (h)

12 % Ni/ 20 % MMT-TiO 2 micro-particles12 % Ni/ 20 % MMT-TiO 2 nano-composite

Reaction Conditions:Pressure = 1 atm.Temperature = 500 °CS/E = 10:1GHSV = 13200 ml/g cat·h

(a) (b)

Fig. 12. (a) Activity performance stability of Ni/MMT-TiO2 nano-composite catalyst. Reaction conditions; 1 atm., 500 �C, S/E = 10:1, 20 h reaction time, (b) Results of stabilitycomparison between micro-particles and nano-composite with regard to H2 yield.

W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969 967

4. Conclusions

The performance of a mono-metallic nano-structured catalyticsystem, based on Ni (12%) and supported on MMT (20%) andTiO2, were investigated for ethanol steam reforming at atmo-spheric pressure and from 300 to 500 �C. The Ni/MMT-TiO2 nano-composite catalyst revealed a good ability in the CAC bond raptureof the ethanol molecule. This can be ascribed to the well-knownfunction of nickel, which is directly available on the catalyst sur-face, enhanced by improved dispersion due to MMT addition toTiO2 nano-particles. The nano-scale particle size of the support

leads to superior activity for ethanol dehydration due to a combi-nation of factors, including; increased reducibility, improved metaldispersion and an increased abundance of redox sites on the cata-lyst surface. Hydrogen is the principle product at all temperatures,feed molar ratios and space-time. Furthermore, high ethanol con-version, H2 selectivity and yield are favored at high temperaturedue to a combination of ethanol decomposition, ethanol dehydra-tion and ESR reactions. There is significant carbon deposition(CNTs) as indicated by TGA results and EDX analysis, but this haslittle effect on catalyst activity over 20 h of reaction time. The highS/E feed molar ratio (10:1) contributes more of H and O rather than

968 W. Mulewa et al. / Chemical Engineering Journal 326 (2017) 956–969

C element, hence minimizing coke formation. In general, the cata-lyst sample demonstrated promising results for activity, selectivityand appreciable stability.

Acknowledgements

The authors would like to extend their deepest appreciation toMOHE (Ministry of Higher Education), Malaysia for financial sup-port of this research project under FRGS (Fundamental ResearchGrant Scheme, Vote 4F876).

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