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Hydrogen production from catalytic steamreforming of glycerol over various supported nickelcatalysts

Nurul Huda Zamzuri a, Ramli Mat b, Nor Aishah Saidina Amin b,*,Amin Talebian-Kiakalaieh b

a Process Systems Engineering Centre (PROSPECT), Research Institute on Sustainable Environment (RISE),

Universiti Teknologi Malaysia, 81310 UTM, Johor Bahru, Malaysiab Chemical Reaction Engineering Group (CREG), N01-Faculty of Chemical Engineering, Universiti Teknologi

Malaysia, 81310 UTM, Johor Bahru, Malaysia

a r t i c l e i n f o

Article history:

Received 16 October 2015

Received in revised form

4 May 2016

Accepted 10 May 2016

Available online 30 May 2016

Keywords:

Glycerol to hydrogen

Catalytic conversion

Steam reforming

Optimization

* Corresponding author.E-mail address: [email protected]

http://dx.doi.org/10.1016/j.ijhydene.2016.05.00360-3199/© 2016 Hydrogen Energy Publicati

a b s t r a c t

Supported Ni catalysts have been investigated for hydrogen production from steam

reforming of glycerol. Ni loaded on Al2O3, La2O3, ZrO2, SiO2 and MgO were prepared by the

wet-impregnation method. The catalysts were characterized by nitrogen adsorptionede-

sorption, X-ray diffraction and scanning electron microscopy. The characterization results

revealed that large surface area, high dispersion of active phase on support, and small

crystalline sizes are attributes of active catalyst in steam reforming of glycerol to hydrogen.

Also, higher basicity of catalyst can limit the carbon deposition and enhance the catalyst

stability. Consequently, Ni/Al2O3 exhibited the highest H2 selectivity (71.8%) due to small

Al2O3 crystallites and large surface area. Response Surface Methodology (RSM) could

accurately predict the experimental results with R-square ¼ 0.868 with only 4.5% error. Thehighest H2 selectivity of 86.0% was achieved at optimum conditions: temperature ¼ 692 �C,feed flow rate ¼ 1 ml/min, and water glycerol molar ratio (WGMR) 9.5:1. Also, the optimi-zation results revealed WGMR imparted the greatest effect on H2 selectivity among the

reaction parameters.

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Limited amounts of fossil fuels, especially petroleum, and

concurrent environmental problems associated with green-

house gases have prompted the world to look for clean sus-

tainable resources as alternatives to meet increasing energy

y (N.A. Saidina Amin).84ons LLC. Published by Els

demands [1]. Among the renewable energy, biodiesel is widely

used and appears to be promising and feasible to reduce the

impact on CO2 emissions [2]. One of the main obstacles for

worldwide production of biodiesel is its production costs [3].

Thus, production of value-added chemicals such as hydrogen

from glycerol which is the main by-product of biodiesel pro-

duction process significantly reduces the biodiesel production

evier Ltd. All rights reserved.

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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 9 0 8 7e9 0 9 89088

costs [4]. Hydrogen is considered as a clean fuel of future due

to its excellent energy storage capacity (33 kW h/Kg) [5]. Also,

it can be considered as secondary energy source because it can

be converted to energy (heat or electricity) by combustion or

electrochemical reactions [6].

Hydrogen can be produced from thermo-catalytic decom-

position and single step anaerobic of methane [7e10]. The

most commonly used technology for hydrogen production

from glycerol is the steam reforming process, similar to the

conventional hydrocarbon steam reforming [11,12]. The

reforming reactions of glycerol for hydrogen production are

listed in Equations (1)e(5). A two-step global reaction equation

in which the carbohydrate (glycerol) undergoes thermal

decomposition is presented in the first reaction (Eq. (1)) to

form CO and H2. The CO then reacts with steam (oxidizer) in

the water-gas shift reaction (Eq. (2)) to form CO2 and addi-

tional H2.

C3H8O3��!H2O 3COþ 4H2 (1)

COþH2O4CO2 þH2 (2)Reactions (1) and (2) can be added to obtain eq. (3):

C3H8O3 þ 3H2O/3CO2 þ 7H2 (3)Some H2 is lost via methanation of carbon monoxide (CO)

and carbon dioxide (CO2) as shown in Eqs (4) and (5).

COþ 3H24CH4 þH2O (4)

CO2 þ 4H24CH4 þ 2H2O (5)Steam reforming of glycerol has been investigated over a

wide variety of supported metal catalysts (Ru, Rh, Ni, Ir, Co, Pt,

Pd and Fe) [13e17]. The activity, product distribution and

catalyst stability have been found to be dependent upon the

catalyst composition, support material, catalyst preparation

and pre-treatment technique and reaction conditions. Appli-

cation of noble metal based catalysts registered promising re-

sults (high activity and hydrogen selectivity) in glycerol steam

reforming reaction. However, the main obstacle for industri-

alization of this process is the exorbitant cost of noble metal-

based catalysts synthesis and preparation. Thus, the majority

of researchers have focused on the low cost materials such as

copper, cobalt, and nickel. As a result, nickel based catalysts

have attracted much attention due to their higher activity and

lower costs compared to the other transition metals [17].

Nickel is known to have high capability of breaking the

CeC bonds and promoting the water-gas shift reactions and

thus increasing the hydrogen production [17,18]. For reform-

ing reactions, high-surface area catalysts are used [19]. The

major roles of the supports are to prepare and preserve ther-

mally stable, well-dispersed catalytic phases during the re-

action. The supports are typically porous, high surface area

metal -oxides or carbon [20].

The aim of this work is to investigate the performance of

supported nickel catalysts that could deliver stable perfor-

mance for the steam reforming of glycerol. In an attempt to

achieve high hydrogen selectivity, the effects of operating

conditions and supported catalyst properties are studied. The

catalysts are characterized with BET, XRD and SEM.

Optimization studies have been conducted by applying cen-

tral composite design (CCD) under the response surface

methodology (RSM). RSM is one of the methods to analyse the

significance or the influence of the factors on the response

[21]. This method is useful since it can reduce the number of

experiments and consequently the cost and time consumed.

The variation of process conditions (reaction temperature (T),

feed flow rate (FFR), water glycerol molar ratio (WGMR)) which

could affect the responses (H2 selectivity and glycerol con-

version) were evaluated within the range. Subsequently, the

H2 selectivity at the optimum conditionswas also evaluated in

this study.

Experimental

Catalyst preparation

Calculated quantities of nickel nitrate hexahydrate

[Ni(NO3)2.6H2O] were dissolved in deionisedwater tomake the

precursor solution for a total nickel loading of 10 wt%. The

nitrate solution was added to the supports particles, Al2O3,

La2O3, ZrO2, SiO2 and MgO respectively. The solution was

stirred continuously for 3 h and then dried in the oven at

110 �C overnight before calcination for 5 h at 500 �C.

Catalyst characterization

The structures of the catalysts were determined by using X-

Ray Diffraction (XRD). XRD patterns were measured at Mak-

mal Sains Bahan, UTM Johor Bahru using standard Bragg-

Brentano geometry with Ni-filtered Cu Ka radiation

(l1 ¼ 1.54056 �A). The spectra were collected for a 2q range of10e90� using a step size of 0.05 and a count time of 1 s. Themorphology and elemental analysis of nickel catalysts on

different supports were observed by Scanning Electron Mi-

croscopy and Energy Dispersive X-ray (SEM-EDX). The anal-

ysis was performed in Hitachi S-4000 microscope with a cold

field emission gun and an energy dispersive energy detector.

The surface area of the catalyst samples was determined

using BrunauereEmmeteTeller (BET) method using a Micro-

meritics ASAP 2010.

Catalyst performance testing

The performance of the catalysts was tested in a continuous

flow process using quartz tube in a fixed bed reactor as illus-

trated in Fig. 1. For each reaction test, 0.3 g of the calcined

catalyst was diluted with equal amount of inert silica carbide

(SiC). The catalysts were reduced at 500 �C in 100 ml/min of10% H2/N2 for 1 h. The bed temperature was heated to the

desired reaction temperature, in 100% N2 flow as the carrier

gas. Glycerol and water mixture of 6:1 water to glycerol molar

ratio (WGMR) was introduced into the vaporizer unit using a

high pressure liquid chromatography (HPLC) pump. Themolar

ration of N2 to glycerol was 1:1.6. As the injector was housed in

the furnace, the mixture was heated at 300 �C to vaporize itand was subsequently mixed with diluting N2 stream. The

total molar flow rate of the feed mixture was kept constant at

100 ml/min. The reaction was carried out at atmospheric

http://dx.doi.org/10.1016/j.ijhydene.2016.05.084http://dx.doi.org/10.1016/j.ijhydene.2016.05.084

Fig. 1 e Catalytic testing fixed-bed reactor rig set-up.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 9 0 8 7e9 0 9 8 9089

pressure. The gaseous products were cooled in a condenser

(cold trap consisting of isopropanol and water in series in an

ice bath) and the liquid product was then collected. The

gaseous products continuously flowed into the silica bed and

analysed by the gas chromatography (GC).

The product gases passed through amoisture trap before it

was analysed using gas chromatograph (GC6890 e Agilent

Technologies) equipped with TCD with Carboxen-1010 capil-

lary column (30 m � 0.5 mm � 0.32 mm). The duration of re-action was 5 h and the products were measured online at

30 min interval. Also, the liquid products in the condensate

were identified using a gas chromatograph-mass spectrom-

eter (GC-MS-QP20105 (Shimadzu), equipped with a column of

rtx-wax (30m� 1 mm� 0.32mm)). The catalysts performanceswere calculated based on the H2 selectivity and glycerol con-

version. Un-reacted water, glycerol and other liquids formed

during the reaction were collected from the condenser and

analysed for determining the glycerol conversion.

H2 Selectivityð%Þ ¼ H2 moles producedTotal C atoms in gaseous products�1RR

� 100

(6)

where RR is H2/CO2 reforming ratio. In the case of glycerol

steam reforming process the ratio is 7/3.

The glycerol conversion in gas phase is calculated by

Equation (7) [16]:

Gas

� phase Glycerol conversion ð%ÞTotal C atoms in gaseous productsC atoms in the feedstock

(7)

The selectivity of species i ¼ CO, CO2, C2H4, C2H6 and CH4are calculated based on equation (8):

Selectivity of i ð%Þ ¼ C atoms in species iTotal C atoms in gaseous products

� 100 (8)

Meanwhile, the yield of hydrogen is calculated by equation

(9):

Yield of H2ð%Þ ¼ Gas� phase Glycerol Conversion� H2 Selectivityð%Þ (9)

Indeed, the main focus of this study is on gas-phase

products including H2, CH4, CO, and CO2 similar to the ma-

jority of previous studies in this field [12,16,17,22].

Experimental design

Two-level full factorial design (23) was applied in this study

with three factors that have significant effects on the

hydrogen selectivity and glycerol conversion from steam

reforming process. The effect of reaction temperature (X1),

feed flow rate, FFR (X2) and water glycerol molar ratio, WGMR

(X3) were investigated at three different levels (low, medium

and high) and coded as (�1, 0, and þ1), respectively as tabu-lated in Table 1.

Statistical analysis of the responses was performed using

Statsoft. Statistica software version 8.0. The mathematical

models for H2 selectivity and glycerol conversion were

established by using the method of least squares as given in

Eq. (9):


Table 1 e Experimental design layout in coded variables.

Factors Symbol Range and levels

�1 0 þ1Reaction temperature,

T (�C)X1 600 650 700

Feed flow rate,

FFR (ml/min)

X2 0.5 1.0 1.5

Water glycerol molar

ratio, WGMR

X3 3 6 9


Yi ¼ b0 þ b1c1 þ b2c2 þ b3c3 þ b12c1c2 þ b13c1c3 þ b23c2c3 þ b11c21þ b22c22 þ b33c33

(10)

where the responses for hydrogen selectivity and glycerol

conversion are Y1 and Y2 respectively. X1, X2 and X3 are the

coded values of the test variables for reaction temperature,

feed flow rate (FFR) and water glycerol molar ratio (WGMR),

respectively. The terms b0 is the offset term; b1, b2, and b3 the

linear terms; b12, b12, and b23 the interaction terms; and b11, b22,and b33 the squared terms.

Results and discussion

Catalyst characterization

X-ray diffraction (XRD)The XRD profiles of Ni/La2O3, Ni/MgO, Ni/ZrO2, Ni/SiO2 and Ni/

Al2O3 catalysts calcined at 500 �C are illustrated in Fig. 2. Eachcatalyst exhibits, besides the respective support peaks, three

peaks at 2q: 37.2�, 43.3� and 63.7� which correspond to thecharacteristic of NiO crystalline phase (JCPDS 44-1159) [17].

Generally, the Ni on the support (La2O3, ZrO2, Al2O3, and MgO)

is highly dispersed except for SiO2 which displays sharp peak

of bunsenite NiO. In particular, the XRD spectrum attributed

to the Ni/La2O3 catalyst did not show clear and large peak of Ni

due to the high dispersion of 10 wt% Ni on La2O3 support.

Indeed, a high concentration of Ni is required to saturate the

surface of the support. Similarly, Cui et al. [23] reported that

only >15 wt% Ni loading can saturate the La2O3 surface anddetected by XRD. However, Song and co-workers [24]

mentioned that no lanthanum nickelate was observed in the

XRD characterization because the interaction between Ni and

the support was only limited to near the surface catalyst and

did not form a bulk compound. Intensity peaks at 37�, 43� and63.4� correspond to MgO while those at 74.9� and 78.8� corre-spond to MgNiO2. It was difficult to differentiate NiO peaks

from MgO because the XRD patterns of NiO and MgO are very

close to each other and the NiO concentration was much

lower thanMgO [25]. The spectrum for support ZrO2 highlights

the main peaks characteristic of the monoclinic phase at 2q:

28.2�, 31.5�, 49.3� and 50.2�. Finally, the diffraction peak relatedto the bulk Al2O3 is identified to be at 18.75�, 36.9�, 44.2�, and67.3�. Similar to the Ni/La2O3 sample, Ni/Al2O3 also exhibited ahigh dispersion of Ni on the support. Li and Chen [26] reported

that the diffusion of nickel ions during calcination into the

alumina lattice sites is limited to the first few outer layers of

the support, resulting in a material without three-

dimensional long-range order.

In addition, the crystalline size of Ni in supported cata-

lysts is calculated using Scherrer's equation [27], DP ¼ 0:94:lb:Cosq,where Dp, l, b, and q are average crystalline size, X-ray

wavelength, line broadening (Peak half-width) in radius, and

diffraction angle, respectively. From the results, Ni/MgO

with 29.7 nm and Ni/Al2O3 with 3.8 nm possessed the largest

and smallest crystalline size, respectively while the other

samples registered almost similar crystalline size

(14.9e15.6 nm).

BET surface areaThe textural characteristics of the supported Ni catalysts,

derived from nitrogen physisorption isotherms and crys-

tallite size from Scherrer equation are presented in Table 2.

The highest BET surface area (169.8 m2/g) belongs to Ni/SiO2.

The surface area of the prepared catalysts increased in the

following order: Ni/La2O3 < Ni/ZrO2 < Ni/MgO < Ni/Al2O3 < Ni/SiO2. In general, the higher the surface area of acatalyst, the better is the reactivity since higher surface area

provides larger contact area for the reactant gas [28]. How-

ever, it is not the only factor affecting the catalyst reactivity.

This suggests that even though the catalyst has low overall

surface area it may have the highest amount of exposed

active Ni [29].

Scanning electron microscopy (SEM)The scanning electron microscopy (SEM) images in Fig. 3 were

taken to observe the dispersion of the metals on the supports.

Small dispersion of Ni on the supports was observed in Fig. 3a,

c and d. Large crystallite of Ni/MgO is evident in Fig. 3b.

Generally, metals exhibit aggregated particles with small and

uniform sizes. Also, the images exhibited well developed

microstructure with uniform colour density. The bright parts

were identified as Ni-rich area. In SEM images, Ni species

appears as roughly spherical particles embedded in the sur-

rounding complex matrix. In studying this interaction, we

observed in Fig 3e that alumina has a greater capacity for

nickel ions than the other supports and that alumina interacts

more strongly with nickel ions. Therefore, the quasi-meshy

structure may be attributed to the nature of the nickel sup-

ported on alumina by impregnation technique [30]. This is

because the strong interactions of nickel nitrate with alumina

on the support surface form such a new structure. In addition,

agglomerated nickel particles might contribute stable perfor-

mance for hydrogen production by glycerol steam reforming

[31].

Catalyst screening performance

In the present work, Ni catalyst impregnated with five

different supports (La2O3, Al2O3, ZrO2, SiO2 and MgO) were

screened at 650 �C, FFR ¼ 1 ml/min and WGMR ¼ 6. Fig. 4adepicts the H2 selectivity of all the catalysts tested. Ni/Al2O3registered the highest H2 selectivity (71.8%) at 650 �C in 5 hreaction time. The catalyst overall activity increased in the

following order: SiO2 < MgO < ZrO2 < La2O3 < Al2O3.


Fig. 2 e XRD patterns of a) Ni/La2O3, b) Ni/MgO, c) Ni/ZrO2 d) Ni/SiO2 and e) Ni/Al2O3.


Furthermore, the small crystallite size of Ni/Al2O3 (3.8 nm)

increased the catalyst activity for steam reforming of glycerol

compared to the other supports. This observation is related to

the dispersion of relatively large crystallites of nickel oxides

Table 2e BET surface area and crystallite size of catalysts.

Catalyst BET surface area (m2/g)a Crystallite size (nm)b

Ni/La2O3 20.3 15.5

Ni/ZrO2 31.0 15.6

Ni/MgO 67.8 29.7

Ni/Al2O3 123.4 3.8

Ni/SiO2 169.8 14.9

a Obtained from BET method.b Obtained from XRD analysis.

responsible for the coke precursor adsorption [32]. Ni supported

on Al2O3 catalyst has higher weight and atomic percentage

with good selectivity of hydrogen for the reforming process [33].

The smaller particle size of the catalyst led to higher catalyst

dispersion and large surface area of the supported nickel -oxide

based catalyst. These results agree with previous finding by

Cheng et al. [34], inferring the most efficient ways to improve

the reactivity for glycerol steam reforming is to usemetalswith

high dispersion and large surface area. However, the sample

with the largest surface area,Ni/SiO2 registered the lowest ac-

tivity among the supported samples. This can be due to the low

dispersion of Ni on support (SiO2) and large crystallite size as

confirmed by the XRD results.

Fig. 4b illustrates mixed results on glycerol conversion.

This is because the reaction pathway is complex for each


Fig. 3 e Scanning electron microscopy of a) Ni/La2O3, b) Ni/MgO, c) Ni/ZrO2, d) Ni/SiO2 and e) Ni/Al2O3.


support and a number of undesirable side reactions occurred

thus, producing several carbonaceous by-products [35].

Furthermore, Dou et al. [36] explained high glycerol conver-

sion with Ni/Al2O3 may be caused by the active support

catalyst. The glycerol conversion for the reaction at 650 �Cwasin the order:

SiO2 < ZrO2 < La2O3 < MgO < Al2O3.Table 3 lists the mean compositions of the products from

these reactions. Ethylene and ethane gases were not observed

over Ni supported on La2O3 and Al2O3. Methane (CH4) selec-

tivitywas sensitive to the particle size of supported Ni, and the

smaller nickel particles favoured lower amount of CH4 [37].

Although the crystallite size of the Ni/La2O3 in this study is

relatively large, it is evident its other characteristics attributed

to the increased activity. Dehydration of glycerol to ethylene

which usually contributed to rapid deactivation of the cata-

lysts through coke formation was observed over Ni supported

on ZrO2, SiO2 and MgO within the range of the operating

conditions studied.

Thermodynamic analysis of the water-glycerol system in-

dicates that at equilibrium the only additional reaction prod-

uct in the gas phase is methane, the formation of which is due

to the hydrogenation of CO or also called methanation reac-

tion (Eq. (4)). Since the decomposition of glycerol to CH4 is

highly favourable during the reforming process, the Ni sup-

ported on La2O3 and Al2O3 catalysts must have sufficient ca-

pacity for reforming the produced CH4 into hydrogen and

carbonmonoxide (reversed of Eqs. (4) and (5)) and the catalyst

also must facilitate the water gas shift reaction to convert CO

into CO2. Some of the major products in the liquid phase were


Fig. 4 e a) Hydrogen selectivity; b) Glycerol conversion (Reaction conditions: WGMR ¼ 6:1, FFR ¼ 1.0 ml/min, T ¼ 650 �C).


formaldehyde, acetaldehyde, ethanol, acetic acid, acetol, and

propylene glycol (data not shown). Based on the H2 selectivity

and glycerol conversion, Ni/Al2O3 was the best performing

catalyst in this study.

Large surface area, high dispersion of active phase on

support, and small crystalline sizes are attributes of active

catalyst in steam reforming of glycerol to hydrogen. How-

ever, some researchers reported that support basicity is the

other important factor in catalyst activity. The basicity of

bulk supports La2O3, MgO, TiO2, and SiO2 were 31, 9.8, 6.2,

and 3.2 mmol/g.cat, respectively and ZrO2 and Al2O3 only

exhibited trace of basicity [38]. Bulk ZrO2 and Al2O3 possessed

more acidic sites than basic sites. In fact, ZrO2 and Al2O3possessed 0.2 and 1.6 mmol/g.cat acidity. Viinikainen et al.

[39] and Fleyset al. [40] reported that higher basicity of cata-

lyst can limit the carbon deposition and enhance the catalyst

stability. The significant effect of basicity can clearly be seen

in the activity of Ni/La2O3 sample with 68.3% hydrogen

selectivity. In fact, the Ni/La2O3 registered the smallest sur-

face area (20.3 m2/g) and the lowest weight percentage (4.7%)

of Ni on the support. However, it performed the second

highest hydrogen selectivity among the tested catalysts in

this study. The La2O3 basicity is essential for better Ni dis-

tribution on the support surface and to facilitate carbon

gasification. Also, Mazumder and de Lasa [41] revealed that

increasing the Ni loading from 10 to 20 wt% decreased the

total basicity due to the partial blocking of basic sites by the

impregnated nickel.

Table 3 e Mean composition of gaseous products (%) of glycero

Catalyst Glycerol con (%)

H2 CH4

Ni/La2O3 70 68.3 0.5

Ni/Al2O3 80 75.1 4.0

Ni/ZrO2 57 60.7 1.1

Ni/SiO2 48 45.9 16.8

Ni/MgO 73 55.4 12.8

a Reaction conditions: T ¼ 650 �C; WGMR ¼ 6; TOS ¼ 5 h.

Ni/La2O3, Ni/ZrO2and Ni/SiO2 were stable throughout the

reaction period, whereas Ni/Al2O3, Ni/MgO unveiled catalyst

deactivation with time on stream (Fig. 4a). The deactivation

may be attributed to the increased crystallinity of support,

sintering of metal particles, oxidation of metal sites and car-

bon deposition as have been reported elsewhere [42e44]. For

instance, Bartholomew et al. [42] reported carbon deposition

caused Ni/Al2O3 catalyst deactivation. Also, Sad et al. [12]

confirmed that the catalyst deactivation is mostly due to

blockage of the active sites by coke precursors formed on

surface acid sites thereby decreasing the rate of H2 produc-

tion. Indeed, as we reported in Eqs. (4) and (5), production of H2reduced and CH4 increased via methanation of CO and CO2.

Thus, based on the results reported in this section, Ni/Al2O3catalyst has been selected in the optimization of the process

by Response Surface Methodology (RSM).

Model analysis

Response surface methodology was employed to analyse the

interaction or relationship between the responses and the

variables. The matrix for the experimental and predicted re-

sults of glycerol conversion and hydrogen selectivity are given

in Table 4. The H2 selectivity, Y1 and glycerol conversion, Y2are the responses for the tested variables in coded units: re-

action temperature (x1), FFR (x2) and WGMR (x3). The second

order polynomial models for H2 selectivity, Y1 and glycerol

conversion, Y2 are as follows:

l steam reforming over supported Ni catalystsa.

Gaseous products (%)

CO CO2 C2H4 C2H6

3.5 27.7 e e

8.3 12.6 e e

4.3 26.1 5.2 2.6

8.9 16.2 9.3 2.9

5.6 16.8 4.6 4.8


Table 4 e The 23 factorial central composite design matrix in real and coded units and the experimental response values.

NO X1 X2 X3 Experimental results RSM predicted results

T FFR WGMR Glycerolconversion,

Y2 (%)

Hydrogenselectivity,

Y1 (%)

HydrogenYield (%)

Glycerolconversion,

Y2 (%)

Hydrogenselectivity,

Y1 (%)

1 600 0.5 3 58.6 74.2 43.5 62.8 74.8

2 700 0.5 3 60.3 76.1 45.9 61.2 74.9

3 600 1.5 3 52.9 70.6 37.4 55.8 69.9

4 700 1.5 3 56.9 71.8 40.9 60.9 72.3

5 600 0.5 9 74.1 85.5 63.4 73.8 83.7

6 700 0.5 9 79.6 89.4 71.2 80.4 88.8

7 600 1.5 9 57.4 72.9 41.8 60.2 72.8

8 700 1.5 9 73.7 82.1 60.5 73.2 80.2

9 565.9 1 6 68.5 75.7 51.9 64.6 76.3

10 734.1 1 6 75.6 81.2 61.4 74.3 82.5

11 650 0.16 6 74.1 80.6 59.7 72.6 81.8

12 650 1.84 6 64.4 69.8 45.0 60.7 70.5

13 650 1 0.95 58.6 69.6 40.8 53.2 69.5

14 650 1 11.1 72.8 81.6 59.4 72.9 83.6

15 650 1 6 72.2 80.4 58.1 72.4 80.4

16 650 1 6 72.2 80.4 58.1 72.4 80.4

17 650 1 6 72.2 80.4 58.1 72.4 80.4

18 650 1 6 72.2 80.4 58.1 72.4 80.4

19 650 1 6 72.2 80.4 58.1 72.4 80.4

20 650 1 6 72.2 80.4 58.1 72.4 80.4


Y1 ¼ �44:0905þ 0:3546X1 � 28:6535X2 � 1:8333X3 þ 0:0655X1X2þ 0:0134X1X3 � 1:1250X2X3 � 0:0003X21 � 7:0177X22� 0:3217X23

(11)

Y2 ¼ 6:8789þ 0:2093X1 � 14:1614X2 þ 0:1187X3 þ 0:0380X1X2þ 0:0058X1X3 þ 0:7500X2X3 � 0:0002X21 � 6:1025X22� 0:1552X23

(12)

These models signified the adequacy between the

observed and predicted results where the coefficient of

Fig. 5 e Parity plot for a) hydrogen sele

determination (R2) value for both H2 selectivity and glycerol

conversion were closer to 1. The values of R2 were 0.868

and 0.966 for H2 selectivity and glycerol conversion as

shown in Fig. 5 indicating that 86.8% and 96.6% of the

variability in the responses can be explained by the models.

The empirical model is adequate to explain most of the

variability in the assay reading which should be at least

0.75 [45].

The analysis of variance (ANOVA) as tabulated in Table 5

was used to check the F-values by comparing it with the

tabulated F-value. The tabulated F-value was used at high

confidence level (95%) in order to obtain a good prediction

model. The F-values for H2 selectivity and glycerol conversion

are 4.39 and 18.93; respectively, higher than the tabulated F-

ctivity and b) glycerol conversion.


Table 5 e Analysis of variance (ANOVA) for quadratic model.

Sources Sum of squares(SS)

Degree of freedom(DF)

Mean squares(MS)

F-value F0.05

H2 selectivity model

Regression (SSR) 915.13 9 101.68 4.39 >4.1Residual 139.02 6 23.17

Total (SST) 1054.15 15

Glycerol conversion

model

Regression (SSR) 444.62 9 49.40 18.93 >4.1Residual 15.66 6 2.61

Total (SST) 460.27 15

Fig. 6 e Pareto chart of a) hydrogen selectivity and b) glycerol conversion.


value (F0.05, 9, 6 ¼ 4.1) by rejecting the null hypothesis at 0.05significant level.

The significance of each coefficient is determined by Pareto

chart (Fig. 6). The greater the magnitude of the t-value and the

smaller the p-value, the more significant is the corresponding

coefficient. As illustrated, the largest effect on hydrogen

selectivity and glycerol conversion is the linear term ofWGMR

(X3), with the largest t-value (4.4702, 9.0880) and smallest p-

value (0.0042, 0.0001) at approximately 99% significant level,

respectively. Linear term of FFR (X2) could also be regarded as

a significant variable in H2 selectivity at 98% significant level

since p-value < 0.05. Meanwhile, the linear term of FFR (X2), T(X1), quadratic term of FFR (X2

2), and WGMR (X32) are significant

in glycerol conversion at 98% and 97% significant level,

respectively.

Variables effects on the responses

The empirical models were plotted as a three-dimensional

surface representing the responses of H2 selectivity, Y1 and

glycerol conversion, Y2 as a function of two factors within

the experimental range considered. In the presence of 6:1

WGMR, the elliptical nature of H2 selectivity (Fig. 7a) and

glycerol conversion (Fig. 7b) can be observed. The contour

plots indicated the interaction of FFR and reaction

temperature was significant on H2 selectivity and glycerol

conversion. Higher H2 selectivity and glycerol conversion

could be attained at higher reaction temperature and lower

feed flow rate implying endothermic reaction [46]. Fig. 7c and

d illustrate the interaction of WGMR and FFR at temperature

650 �C on H2 selectivity and glycerol conversion, respectively.Increment of H2 selectivity and glycerol conversion with

increasing WGMR and decreasing FFR can be observed from

the figures. Meanwhile, H2 selectivity and glycerol conver-

sion increased with increasing WGMR and higher reaction

temperature at FFR ¼ 1.0 ml/min as depicted by Fig. 7e and f.With increasing WGMR and higher reaction temperature, the

production of CH4 was completely inhibited due to water gas

shift reaction [47]. Generally, high temperature, low pres-

sure, and high water/glycerol ratio favour hydrogen pro-

duction. Indeed, methane was decreased and carbon

formation was thermodynamically inhibited under these

conditions [12].

Optimization of hydrogen selectivity

Based on the full quadratic model, the H2 selectivity was

predicted at optimum conditions in order to obtain high H2selectivity in glycerol steam reforming. The optimization

using CCDwas conducted based on the variables in the range


Fig. 7 e The response surface plot of hydrogen selectivity and glycerol conversion as a function of aeb) FFR and temperature,

ced) WGMR and FFR, eef) temperature and WGMR.


of experimental design. The optimum conditions predicted

at temperature ¼ 692 �C, FFR ¼ 1 ml/min, and WGMR 9.5:1,respectively corresponded to 86.0% of predicted H2 selec-

tivity. Triplicate experiments were carried out at these op-

timum conditions to confirm the accuracy and validation of

the regression model. The H2 selectivity for the observed

value was 82.3% indicating a 4.5% error between the

observed and predicted values. The error is considered small

as the observed value was within the 5% of significance level.

The results in Table 6 confirmed that 86% hydrogen selec-

tivity obtained in the current study is better than the ma-

jority of previous results. Sad et al. [12] reported 100%

hydrogen yield in a double bed reactor by application two

catalysts in series, but in a single bed reactor the hydrogen

yield was only 78.8%.

Conclusion

Glycerol steam reforming for hydrogen production was con-

ducted over Ni supported on La2O3, Al2O3, ZrO2, SiO2, andMgO

catalysts. Ni/Al2O3 was found to be the best catalyst with

maximumhydrogen selectivity (71.5%) was obtained at 650 �C,FFR ¼ 1 ml/min and WGMR ¼ 6. Large surface area (123.4 m2/g), small crystallite size (3.8 nm) and high dispersion of Ni on

the support were the main reason which increased the ac-

tivity of Ni/Al2O3 sample for steam reforming of glycerol

compared to the other supported catalysts.

The optimization results revealed that the water glycerol

molar ratio (WGMR) has the greatest effect on the hydrogen

selectivity and glycerol conversion compared to reaction


Table 6 e Comparison of the current study with previously reported results.

Catalyst Tem (�C) WGMR Glycerol con (%) H2 Ref

Ni/Al2O3 650 6:1 80 Selectivity 86% This Study

Ni/(Ca/Al2O3) 550 3:1 54.1 Concentration 85% [22]

LaNi0.9Cu0.1O3 650 3:1 73 Selectivity 67% [17]

Pt/SiO2 350 e 100 Yield 78.8% [12]

0.5Pt/SiO20.5Pt/TiO2(Double bed Reactor)

350 3:1 100 Yield 100% [12]

NieFeeCe/Al2O3 450 e 94.06 Selectivity 64.04% [16]

Ni/La2O3eSiO2 (P þ I) 600 e 79 Yield 3.8 mol/mol [48]


temperature and feed flow rate (FFR). Also, the optimization

results revealed that steam reforming of glycerol produced

86.0%maximum hydrogen selectivity at the optimum reaction

temperature 692 �C, 1 ml/min feed flow rate, andWGMR 9.5:1.

Acknowledgements

The authors would like to express their gratitude to the Min-

istry of Higher Education for supporting this project under the

Fundamental Research Grant Scheme (FRGS) vote number

78422 and RACE grant vote number 00M32. Furthermore, one

of the authors (NHZ) is thankful to the Ministry of Science,

Technology and Innovation for the National Science Fellow-

ship (NSF) scheme.

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Hydrogen production from catalytic steam reforming of glycerol over various supported nickel catalystsIntroductionExperimentalCatalyst preparationCatalyst characterizationCatalyst performance testingExperimental design

Results and discussionCatalyst characterizationX-ray diffraction (XRD)BET surface areaScanning electron microscopy (SEM)

Catalyst screening performanceModel analysisVariables effects on the responsesOptimization of hydrogen selectivity

ConclusionAcknowledgementsReferences

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