Catalytic Performance of Cement Clinker Supported Nickel Catalyst in Glycerol Dry Reforming

Journal of Energy Chemistry 23(2014)645–656

Catalytic performance of cement clinker supported nickelcatalyst in glycerol dry reforming

Hua Chyn Leea, Kah Weng Siewa, Maksudur R. Khana, Sim Yee China,b,Jolius Gimbuna,b, Chin Kui Chenga,b∗

a. Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang Kuantan, Pahang Malaysia;b. Centre of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang Kuantan, Pahang Malaysia

[ Manuscript received February 17, 2014; revised April 29, 2014 ]

AbstractThe paper reports the development of cement clinker-supported nickel (with metal loadings of 5 wt%, 10 wt%, 15 wt% and 20 wt%) catalystsfor glycerol dry (CO2) reforming reaction. XRF results showed that CaO constituted 62.0% of cement clinker. The physicochemical character-ization of the catalysts revealed 32-folds increment of BET surface area (SBET) with the addition of nickel metal into the cement clinker, whichwas also corroborated by FESEM images. Significantly, XRD results suggested different types of Ni oxides formation with Ni loading, whilstCa3SiO5 and Ca2Al0.67Mn0.33FeO5 were the main crystallite species for pure cement clinker. Temperature-programmed reduction analysisyielded three domains of H2 reduction peaks, viz. centered at approximately 750 K referred to as type-I peaks, another peaks at 820 K denotedas type-II peaks and the highest reduction peaks, type-III recorded at above 1000 K. 20 wt% Ni was found to be the best loading with the highestXG and H2 yield, whilst the lowest methanation activity. Syngas with lower H2/CO ratios (0.6 to 1.5) were readily produced from glycerol dryreforming at CO2-to-Glycerol feed ratio (CGR) of unity. Nonetheless, carbon deposit comprised of whisker type (Cv) and graphitic-like type(Cc) species were found to be in majority on 20 wt%Ni/CC catalysts.

Key wordscement clinker; dry reforming; glycerol; nickel catalyst; syngas

1. Introduction

The global energy consumption is expected to attain 820quadrillion British thermal units (Btu) by the year 2040 fromthe current 524 quadrillion Btu [1]. If the same consump-tion pattern continues, 80% of the world energy usage willstill depend on the fossil fuel. Unfortunately, fossil fuel com-bustion for transportation and electricity purposes is a ma-jor source of the anthropogenic carbon dioxide (CO2) emis-sion. CO2 concentration in atmosphere has once surpassed400 parts per million (ppm), a level deemed detrimental to cli-mate change [2]. International agreement on limiting warmingto 2 ◦C will only be achieved if two-third of the fossil fuel re-serve remains underground by the year 2050 as reported in the2012 IEA World Energy Outlook report [3]. Significantly, acombination of factors such as fossil fuel depletion and grow-ing environmental concern have largely driven a search formore sustainable and renewable resources. In particular, forbiomass-rich nation like Malaysia, the answer seems to be ob-

vious. The Malaysian government has planned to fully im-plement B5 mandate (5% methyl ester in diesel) nationwidein July 2014. There were 60 biodiesel manufacturing licensesbeing approved by the government anticipating total annualbiodiesel production of 6.5 MT. It is widely known that dur-ing the transesterification, 10 wt% glycerol bio-waste is ob-tained [4−7]. Therefore, an annual production of 0.6 MT ofcrude glycerol is anticipated. High impurity contents in thecrude glycerol necessitate an expensive purification process[8–10]. This has confined the applications of purified glyc-erol to pharmaceutical, cosmetic and food industries. Hencefinding alternative use of crude glycerol is significant. Con-version of glycerol (bio-waste) into biofuel or chemical prod-ucts, i.e. syngas (mixture of H2 and CO), acetone, acrolein,ethers and methanol [11−14], has been touted as a promisingroute to provide a total solution to the biodiesel industries, aswell as energy sustainability.

Syngas is primarily being generated from natural gas, inparticular methane (CH4) reforming. Nonetheless, it is moreenticing to derive syngas from glycerol biowaste instead as

∗ Corresponding author. Tel: +60-9-5492896; Fax: +60-9-5492889; E-mail: [email protected] work was supported by Ministry of Education, Malaysia through MTUN (No. RDU121216).

Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi: 10.1016/S2095-4956(14)60196-0

646 Hua Chyn Lee et al./ Journal of Energy Chemistry Vol. 23 No. 5 2014

glycerol (C3H8O3) possesses more H-elements than CH4.Glycerol steam reforming (Equation 1) has also been exten-sively studied over the past decade [15−17]. However, itproduces high H2/CO ratios (typically more than 10) underexcess steam conditions, as CO has undergone water-gas-shift (WGS) reaction (Equation 2) which is unfavourable forFischer-Tropsch (FT) synthesis [18,19]. Another drawback ofglycerol steam reforming is significant production of CO2.

C3H8 + 6H2O→ 3CO2 + 10H2 (1)

CO + H2O→ CO2 + H2 (2)

To mitigate CO2 emission, dry (CO2) reforming hasgained popularity as an alternative to steam reforming. Dryreforming is highly selective to CO as compared with steamreforming, hence lower H2/CO ratio is more suitable for FTsynthesis and methanol synthesis without syngas condition-ing. Dry reforming has been extensively reported for methane,propane, ethanol and toluene [20–25], yet little works havebeen dedicated to glycerol dry reforming, which are limitedto the thermodynamically study by Wang and his co-workers[26]. The overall glycerol dry reforming concept is expressedas in Equation (3).

C3H8O3 + CO2 → 4CO+ 3H2 + H2O (3)

Nickel-based catalysts have widely replaced the noblecatalysts such as Rh, Pt and Pd [27−29] in various reform-ing technologies due to their availability and lower costs. Ni-based catalysts possess high activity in hydrogenation anddehydrogenation, which means the bond cleavages betweenO−H, –CH2–, C–C and –CH3– are easily accomplished [30].Nevertheless, it is well-known that Ni-based catalysts sufferedfrom the deactivation attributed to coke deposition. Cokedeposition is influenced by the types of support employed.Modification of catalyst support by addition of strong Lewisbase oxide such as CaO and MgO has improved carbon deacti-vation resistance during reforming [31−33]. Previous studies[34,35] reported that cement clinker (CC) contains 63.28%and 63.17% CaO respectively, hence exhibiting propertiessuitable as catalyst synthesis. CC is an intermediate prod-uct of the cement industries. Halmann and Steinfeld [36]have performed the thermo-chemical calculations for the co-production of limestone (CaO) and syngas via CH4 dry re-forming, using CO2 released from lime-kiln. Gimbun et al.[37] have activated CC for biodiesel production and glycerolwas the by-product. Some of the researchers also reported thatcarbon nanotubes deposited on Ni catalyst could mechanical-strengthened the cementitious material. Therefore, it is fea-sible to conduct reforming of glycerol (from the biodiesel in-dustry) over the CC utilizing the emitted CO2 from the ce-ment kiln. This will provide an utmost solution to environ-ment, cement and biodiesel industry. The current work servesto characterize the catalysts using spectroscopic analysis andalso to investigate the activity of glycerol dry reforming overCC-supported Ni catalysts with the aims of producing syngasand understanding the carbon laydown behaviour.

2. Experimental

2.1. Materials and catalyst preparation

Analytical grade glycerol was supplied by Sigma Aldrichand the CC was obtained from the Pahang Cement Sdn. Bhd.CC was ground with mortar and sieved for the particle rangeof 100−200 μm before wet-impregnated with 5 wt%, 10 wt%,15 wt% and 20 wt% nickel (Ni)-metal, respectively, usingNi(NO3)2·6H2O as precursor in 50 mL ultrapure water. Sub-sequently, the resulting slurry was stirred for 3 h at room tem-perature before oven-dried for 24 h at 403 K. The catalystswere then air-calcined at 1073 K for 6 h at 5 K·min−1. Post-calcination, the catalysts were ground and sieved again to thesize of 90−200 μm for reaction studies.

2.2. Catalyst characterization

N2 physisorption isotherms of the catalysts were per-formed at 77 K using thermo scientific surface gas adsorp-tion porosimeter. Before physisorption, the catalysts were de-gassed overnight at 573 K and 1×10−4 Torr. The cumulativepore size was determined using Barett-Joyner-Halenda (BJH)equation. The oxide composition of the catalysts was deter-mined by wavelength X-ray fluorescence (WD-XRF) spec-trometry (Bruker, S8 Tiger Model, Germany). The crys-talline behaviour of the catalysts was measured by powderXRD measurement (Rigaku Miniflex II) with Cu Kα radia-tion, λ = 1.5418 A at 30 kV and 15 mA, from 2θ of 10o to 80o

with a step size of 0.02o and a step time of 1 s. The oxidecrystalline size of the catalysts was determined using Scherrerequation, d = 0.94λ/(βd cosθ), where d is the crystallite size,λ is the wavelength of the radiation, βd is the full-width at halfmaximum (FWHM) of the diffraction peak and θ is the halfof the diffraction angle.

The surface structure of the catalysts was capturedby field emission electron microscopy (FESEM) modelJOEL/JSM-7800F, equipped with energy microscopy disper-sive spectroscopy (EDS). Qualitatively, Fourier transform in-frared (FTIR) spectra of the catalysts were recorded from 600to 4000 cm−1 with a Perkin Elmer (Model Spectrum 100)FTIR spectrophotometer using KBr disks technique. Ther-mogravimetric analysis (TGA) was performed to observe thegas-solid catalyst interaction in 60 mL·min−1 of air with40 mL·min−1 of N2 carrier up to 1173 K employing rampingrate of 10 K·min−1.

Temperature-programmed reduction (TPR) was per-formed using Thermo Finnigan TPD/R/O 1100 series unitequipped with TCD detector to gauge the reducibility of thecatalysts. An approximately 0.1 g catalyst (with different Niloadings) was reduced with 20 mL·min−1 5% H2 in N2 carriergas at ramping 20 K·min−1 up to 1173 K with holding periodof 30 min prior to cooling to room temperature.

After the reaction, temperature-programmed oxidation(TPO) and FESEM were also performed on the used cata-lysts to examine the carbon deposition phenomenon and types

Journal of Energy Chemistry Vol. 23 No. 5 2014 647

of solid carbon deposited onto the catalysts. TPO analysiswas accomplished via using 60 mL·min−1 oxygen (O2) with40 mL·min−1 N2 carrier.

2.3. Catalyst activity

Figure 1 shows the experimental rig for the catalysts test-ing which comprised of stainless steel 316 fixed-bed reactor(ID: 9.42 mm; OD: 12.7 mm; length: 400 mm). The catalyticevaluation was carried out over 0.20 g catalyst sandwichedbetween two layers of quartz wool. The catalyst was acti-vated and reduced by 50% H2/N2 gas (50 mL·min−1 STP)for 2 h and held at 1073 K. Glycerol was pumped into thereactor by high performance liquid chromatography (HPLC)pump (LabAlliance Series 1, Max Pressure: 3000 psi andflowrate from 0.01 to 10.0 mL·min−1) while the flowrates ofreactant and carrier gas (as diluent) were controlled by Ali-cat Scientific electronic mass flow controller (Model: MC-500SCCM-D). The performance of each catalysts was eval-uated at 1023 K with carbon dioxide-to-glycerol feed ra-tio (CGR) of unity and constant gas-hourly space velocity(GHSV) of 3.6×104 mL·g−1

cat ·h−1 at STP. Reactor outlet gases

were passed through a cold trap for liquid product captureand then over a drierite (CaSO4 & CoCl2) bed (8 mesh).The gaseous product was collected into 1-L Tedlar gas sam-pling bag and the flowrate of the exit gas was measured usingbubble meter. The composition of produced syngas was de-termined using Agilent gas chromatography (GC) with TCDcapillary columns, HP-MOLSIV (Model No. Agilent 19095P,30.0 m×530 μm×50.0 μm) and HP-Plot/Q column (ModelNo. Agilent 19095-Q04, 30.0 m×530 μm×40.0 μm). The to-tal amount of gas entering the sample loop was 25 μL. Theglycerol conversion to H2, CO and CH4 gaseous products wascalculated based on H-atom balance as in Equation (4). Theyields of hydrogen (YH2) and carbon containing species (Yi)were calculated according to Equations (5) and (6), respec-tively, whilst the rate of gaseous product formation was calcu-lated based on Equation (7).

XG =2×FH2 + 4×FCH4

8×FC3H8O3

×100% (4)

YH2 =2×FH2

8×FC3H8O3

×100% (5)

Yi =Fi

3×FC3H8O3,in×100%, where i = CO and CH4

(6)

ri

(mol

gcat.s

)=

yi×Fi

W, where i = H2, CO and CH4

(7)

Figure 1. Experimental rig for glycerol dry reforming studies

3. Results and discussion

3.1. Catalyst characterization

3.1.1. N2 physisorption, FESEM and XRF analyses

Table 1 shows that the BET specific surface area (SBET)and the cumulative pore volume of the catalysts increased withthe increasing Ni metal loading. SBET has increased 32-foldfrom 0.55 g·cm−3 to 17.83 g·cm−3 with Ni loading of 20 wt%.The low surface area of CC was attributed to the sinteringeffect from calcination at 1723 K in the cement kiln [38]. Thedensity of the catalysts also increased with Ni loading, viz.from 2.99 to 3.22 g·cm−3, which was consistent with the re-sult of Taylor [39].

Figure 2 depicts the N2 physisorption isotherms of thepure cement clinker (CC) and 20 wt% Ni/CC catalysts (asa representative). The pore size of CC fell in the range ofmacroporous (Figure 2a) and shifted towards mesoporous re-gion (Figure 2b) upon Ni-loadings, indicating the formation ofnew compound attributed to the alteration of the surface struc-ture of the catalysts. This explained the increment of SBET ofthe catalyst with Ni loading. As CC was a macroporous mate-rial, Ni dopant was unlikely to cover the pores of the support.

Table 1. Properties of fresh solid catalysts

SBET Density Pore volume Oxide composition from XRF analysis (wt%)Catalysts

(m2·g−1) (g·cm−3) (cm3·g−1) CaO NiO SiO2 Al2O3 Fe2O3 MgOPure CC 0.55 2.99 0 61.98 62a 17.21 3.90 3.53 0.55

5 wt% Ni/CC 7.73 3.03 0.0022 53.05 18.67 11.49 2.66 2.64 0.4110 wt% Ni/CC 15.81 3.12 0.0053 57.02 14.14 8.44 3.07 3.16 0.5015 wt% Ni/CC 17.30 3.12 0.0054 44.11 27.63 9.28 1.75 2.38 0.3220 wt% Ni/CC 17.83 3.22 0.0057 38.66 34.83 6.77 1.36 2.07 0.25a The data presented was in wt% except this label in ppm


Hence, more Ni loading has resulted in more NiO formationand therefore larger SBET.

Figure 3 shows the FESEM images of the catalysts. Thesurface of pure CC (Figure 3a) was very smooth and less-porous, which was in agreement with SBET. The morphologyof the surface became rugged and bulkier with more Ni dop-ing. Table 2 shows the element composition of the catalysts,

which exhibited similar trend to XRF results summarized inTable 1.

XRF analysis (Table 1) shows that the locally-sourced CCwas a mixture of oxide metals with CaO as the major ingre-dient of 61.98%, corresponding to the earlier studies [34,35].Besides CaO, silicates (SiO2), aluminate (Al2O3) and ferrite(Fe2O3) were the major compounds in CC. The percentage of

Figure 2. N2 physisorption isotherms of pure cement clinker (a) and 20 wt% Ni/CC catalysts (b) as a representative

Table 2. Element composition estimated from EDS analysis

Elements composition (wt%)Catalysts

C Al Si S K Ca Fe Ni OPure CC 3.35 0.54 10.65 − − 45.69 − − 39.77

5 wt% Ni/CC 5.48 2.23 3.37 0.54 0.81 36.97 2.81 8.52 39.2710 wt% Ni/CC 5.91 0.44 5.83 − 0.45 35.52 − 11.65 40.2115 wt% Ni/CC 6.88 0.18 2.50 − − 16.55 − 36.10 37.7820 wt% Ni/CC 7.76 0.62 3.20 0.55 0.67 8.95 0.45 37.93 39.87

Figure 3. FESEM images of different catalysts. (a) Fresh cement clinker support, (b) 5 wt% Ni/CC catalysts, (c) 10 wt% Ni/CC catalysts, (d) 15 wt% Ni/CCcatalysts, (e) 20 wt% Ni/CC catalysts


CaO and SiO2 in catalysts decreased from 61.98% to 38.66%and 17.21% to 6.77%, respectively, as Ni loadings were in-creased. Indeed, the doping of Ni has increased the amountof NiO species (originally also present in CC support), con-tributed to a hike from 62 ppm to 34.83%. This result wasalso qualitatively captured by the subsequent XRD diffractionpatterns.

3.1.2. X-ray dif fraction analysis

Figure 4 shows the XRD patterns of both undoped and Ni-doped CC catalysts. The main mineralogical phase presentingin pure CC was alite (Ca3SiO5), judging from the peaks shownin Table 3. The average crystalline size of alite was 33.65 nm.

Figure 4. XRD patterns of different catalysts. 1—Bunsenite, NiO;2—Calcium nickel catena silicate, CaNiSi2O6; 3—Calcium nickelsilicate, CaNiSi4O10; 4—Nickel magnesium manganese oxide,(Ni0.9MgO0.1)6MnO8; 5—Calcium aluminum iron manganese oxide,Ca2Al0.67Mn0.33FeO5; 6—Alite, Ca3SiO5

The alite species in CC had monoclinic crystals system (asopposed to pure alite that possessed triclinic structure) dueto the presence of some impurities [40]. Besides alite, an-other major phase presenting in CC was calcium aluminumiron manganese oxide (Ca2Al0.67Mn0.33FeO5) in CC, poss-esing orthorhombic system with crystalline size of 25.65 nm.Upon Ni doping onto CC, some peaks of alite, i.e. 38.79o;45.79o and 50o disappeared, whilst some additional peaks atapproximately 2θ = 37.5o, 43.3o and 62.9o were found relatedto Ni oxides formations. Table 3 shows that different percent-age of Ni doping resulted in the formation of different Ni ox-ides species. For 5 wt% Ni loading, Ni was bonded with freeMg and MnO to form nickel magnesium manganese oxide(Ni0.9MgO0.1)6MnO8, with crystalline size of 12.84 nm. Cal-cium nickel catena-disilicate (Nickeldiopside, CaNi(Si2O6))and calcium nickel silicate (CaNiSi4O10), were found inmonoclinic and tetragonal systems, respectively, for 10 wt%Ni/CC catalyst. The average crystalline size for CaNi(Si2O6)and CaNiSi4O10 were 29.08 nm and 27.45 nm, respectively.The presence of both CaNi(Si2O6) and CaNiSi4O10 were at-tributed to the reactions in Equations (8) and (9) during thecalcination stage [41].

Ca3SiO5 + 5SiO2 + 3NiO→ 3CaNiSi2O6 (8)

Ca3SiO5 + 11SiO2 + 3NiO→ 3CaNiSi4O10 (9)

A pure NiO was only found for the catalysts with Nidoping more than 10 wt%. By comparing both 15 wt% and20 wt% Ni doping, XRD peaks shifted to higher angle ofdiffraction with higher Ni doping, indicating the substitutionof Ni2+ ions into the support lattice [42]. The pure NiO forma-tion was confirmed by peaks at 37.4o, 43.3o and 62.9o for both15 wt% and 20 wt% Ni/CC catalysts with plane reflection of(111), (200) and (220). The mean crystalline size of NiOin 15 wt% and 20 wt% Ni/CC catalysts were 16.67 nm and16.53 nm, respectively, hence no significant difference.

Table 3. Average crystallite size, dcrystallite of Ni oxides calculated from XRD patterns

Catalysts Oxides/Ni oxides presents 2θ Average dcrystallite (nm)

Pure CC Ca3SiO5 29.45o, 30.17o, 32.21o, 32.54o, 33.92o, 34.41o, 38.79o, 41.31o, 41.63o, 45.79o, 33.6546.84o, 50.00o, 51.90o, 56.37o, 59.96o, 62.28o, 63.52o

Ca2Al0.67Mn0.33FeO5 29.45o, 32.21o, 33.92o, 34.41o, 41.63o, 47.38o, 59.96o, 63.52o 25.655 wt% Ni/CC (Ni0.9MgO0.1)6MnO8 18.2o, 37.51o, 43.43o, 46.91o, 62.35o 12.84

10 wt% Ni/CC CaNi(Si2O6) 29.50o, 34.09o, 34.51o, 37.54o, 41.33o, 43.48o, 47.18o, 51.97o, 54.03o, 60.01o, 62.86o 29.08CaNiSi4O10 29.50o, 34.51o, 41.33o, 43.48o, 47.18o, 51.97o, 54.03o, 62.86o 27.54

15 wt% Ni/CC NiO 37.38o, 43.29o, 62.90o 16.6720 wt% Ni/CC NiO 37.41o, 43.36o, 62.94 wto, 75.37o 16.53

3.1.3. Fourier transform infrared spectroscopy (FTIR)

FTIR spectra of calcined samples as depicted in Fig-ure 5 shows razor-sharp peaks around 3642 cm−1 which rep-resents the free O−H stretch for each catalyst that ascendedwith Ni loading. This result mirrored the previous works

[43,44], which mentioned that O−H stretching at frequency of3645 cm−1 was due to calcium hydroxide (Ca(OH)2). Broadpeaks at 3436 to 3451 cm−1 represented the hydrogen-bondedO−H band (symmetric and asymmetric stretching (v1 and v3)vibration of water) [45] that appeared widened and peak inten-sity increased with Ni loading; its increases can be attributed


to the hydrophilic-nature of Ni. Besides, the bands centered at1634 cm−1 referred to the vibrations of H2O (v2) moleculesadsorbed on the surface of CC [43,46,47]. Moreover, thebands from 1410 to 1425 cm−1 can be assigned to the vibra-tion of carbonate ion on the catalysts (v3 CO2−

3 ) [48,49] orthe presence of CO2 [50] attributed to atmospheric CO2 ad-sorption, forming CaCO3 compound. In addition, the bandat frequencies ranging from 990 to 910 cm−1 indicated thepresence of SiO4−

4 in C3SiO5 after the hydration of Si–O (v3)functional group in CC, consistent with the previous findings[43,51,52]. Ylmen et al. [47] described that the dissolutionof C3S phase in clinker could also occur at the spectra rangeof 800 to 970 cm−1. The band in the region for the frequencyless than 1000 cm−1 can be referred to NiO arising from inter-atomic vibrations [53,54].

Figure 5. FTIR spectra of pure CC and Ni/CC catalysts with different Niloadings

3.1.4. Temperature-programmed calcination prof iles

The synthesized catalysts (dried only, non-calcined), theCC support, and the metal precursor itself viz. nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) were subjected totemperature-programmed calcination studies to obtain the in-dividual calcination characteristic. Figure 6 shows the ther-mal profile of Ni(NO3)2·6H2O in air blanket. Significantly,the weight reduction profile shows numerous inflection pointssymptomatic of metal-salt decomposition/oxidation, hencesolid structural phases evolution. The decomposition of purenickel nitrate salt under the air blanket consistently depictedthree major peak segments (Figure 6), viz. below 420 K,from 420 to 520 K and a sharp peak centered at 582 K.The peaks below 420 K can be assigned to the hydrationwater elimination, from Ni(NO3)2·6H2O to Ni(NO3)2·4H2Ospecies at 334 K and 366 K [55−59]. The two peaks cen-tered at about 465 and 504 K in the second segment may be

linked to the formation of Ni(NO3)2·2H2O after water elimi-nation and also Ni2O3, respectively. Previous studies [58,60]mentioned that anhydrous Ni(NO3)2 cannot be thermally-obtained and remained in basic nickel nitrate intermediate,[Ni(NO3)2·Ni(OH)2]. The release of NO2, NO, O2 and H2Ofrom 2[Ni(NO3)2; Ni(OH)2] contributed to the formation ofNi2O3 (Equation 10). The sharp peak in the third segment(570−620 K) corresponded to further oxide decomposition ofNi2O3→2NiO+1/2O2. Beyond 620 K, no formation of physi-cally meaningful peaks was recorded.

2[Ni(NO3)2 ·Ni(OH)2]→

2Ni2O3 + 2NO2 + 2NO+ O2 + 2H2O(10)

Figure 6. Derivative weight profile of Nickel (II) nitrate hexahydrate at10 K·min−1 ramping rate under air blanket

Significantly, Figure 7 shows that CC support had greaterthermal stability with only two decomposition peaks at 720 Kand 917 K. The peak at 720 K can be attributed to the decom-position of calcium hydroxide (portlandite, Ca(OH)2) as wa-ter was readily adsorbed on the surface of CC support, whichcomprised mainly of CaO (Equation 11). Another possibil-ity causing the formation of portlandite is alite (Ca3SiO5) inthe support which has been hydrated (Equation 12). Thisfinding was validated with the FTIR results which recordedO−H stretching at 3645 cm−1 (Figure 5).

CaO+ H2O→ Ca(OH)2 (11)

2Ca3SiO5 + 7H2O→ 3CaO ·2(SiO2) ·4(H2O)+ 3Ca(OH)2

(12)The peak at 917 K of CC support in Figure 7 could be at-

tributed to the loss of CO2 from CaCO3 contained in the sup-port as CaO presents a promising CO2 capture agent (Equa-tion 13) at room temperature [61]. Once CaCO3 was formedfrom CaO, it required high temperature to release the CO2.This finding was substantiated by the FTIR results at bandranging from 1410 to 1425 cm−1.

CaO+ CO2 → CaCO3 (13)


Figure 7. Derivative weight profiles at 10 K·min−1 ramping rate under airblanket

The doping of Ni (5 wt% to 20 wt%) onto CC supporthas resulted in a near-similar decomposition trend for calci-nation under air blanket (Figure 7). The peaks observed atbelow 500 K was most likely attributed to the elimination ofwater. Indeed, only one peak was noticeable, centered at ap-proximately 570 K which can be linked to NiO formation at582 K due to the total metal precursor (nickel (II) nitrate hex-ahydrate) decomposition (Figure 6). The calcination profilescorroborated the XRD patterns which showed the presenceof NiO. The formation of shoulder peak at 673 K indicateda continuous thermal decomposition of complex mixture ofhydrated silicate and aluminate-type compounds [62]. Thepeaks formed at 740 K could be due to the release of hydrox-ide group from the hydrated calcium silicate compounds inthe catalyst. Moreover, the right-shifting of the decomposi-tion peaks as depicted in Figure 7 may be linked to the incor-poration of Ni metal into the matrix of CC, consequently en-dowing it with higher capacity to capture water and forming

complex hydrated calcium silicate compounds. In addition,the small peak around 925 K can be attributed to the releaseof CO2 from CaCO3 formed in the support matrix. The pres-ence of carbonate species was consistent with the FTIR resultsin Figure 5.

3.1.5. Temperature-programmed reduction (TPR) prof iles

The reducibility of Ni/CC catalyst was determined bytemperature-programmed reduction (TPR) profiling. TheTPR profiles of the fresh catalysts (with 5 wt% to 20 wt%Ni loadings) are depicted in Figure 8 while the summaries ofH2 uptake at each peak are presented in Table 4. The TPRcurves of the metal-impregnated catalysts showed multiplemajor reduction peaks indicative of different Ni-support in-teraction degree [63].

Figure 8. Temperature-programmed reduction (TPR) profiles of freshly-calcined catalysts

Table 4. Reduction peak temperatures and H2 uptake estimated from H2-TPR profiles

Ni loading Peak reduction temperature (K) and H2 uptake (μmol·g−1cat ) Total

on CC Peak I H2 uptake Peak II H2 uptake Peak III H2 uptake metal surface(wt%) (K) (μmol·g−1

cat ) (K) (μmol·g−1cat ) (K) (μmol·g−1

cat ) (m2)0 (pure CC) − − − − 982; 1036 205.89 0

5 752 519.11 − − 1006 615.12 6.9610 745 630.63 804 619.89 995 720.02 8.9015 768 648.20 839 624.47 1012 957.31 14.1020 749, 777 596.03 844 708.22 1031 912.34 13.27

The profiles, in general, illustrated three domains of H2

reduction peaks, viz. centered at approximately 750 K re-ferred to as type-I peaks, another peaks at 820 K denotedas type-II peaks and the highest reduction peaks, type-IIIrecorded at above 1000 K. The low temperature type-I peakswere generally due to H2 reduction of metal particles whichindicated the exposed fraction of Ni atoms/free Ni compoundas well as reduction of Ni-metal weakly interacted with the

support (primarily comprised of CaO) [25,64,65]. This wasalso echoed by Richardson et al. [66]. Through their inves-tigation, they have found that the reduction peaks occurredbetween 635 K to 746 K with CaO as the additive to NiO cat-alyst. In addition, Choudhary et al. [67] also found via theirTPR studies that peak maximum temperatures for NiO-CaOcatalysts were 763 K and 1073 K (small hump). Overall, itseemed that the H2 reduction peaks shifted to higher reduc-


tion temperature with the increasing Ni loading. This wasprobably due to the stronger interaction between nickel andmatrices of CC support.

The reduction peaks from 995 to 1036 K (type-III) couldbe due to the very strong interaction between NiO and thecompounds such as Al2O3 and MgO present in CC [68–70]. Itcan therefore be surmised that different elements on CC haveinteracted with NiO species, contributing to different degreereduction peaks on TPR profiles. Likewise, TPR profile ofpure CC also yielded two reduction peaks (Figure 8) at 982and 1036 K symptomatic of H2 desorption from subsurfacelayers and/or spillover H2. As shown in Table 4, 15 wt%Ni/CC catalyst had higher metal surface than 20 wt% Ni/CCcatalyst. This may be due to the lower dispersion or agglom-erations of Ni on the surface of 20 wt% Ni catalyst. As NiOreduction still occurred at 1031 K, therefore 1073 K was se-lected as the in-situ reduction temperature for the catalystsprior to reaction studies.

3.2. Glycerol dry reforming studies

The catalysts with different Ni doping levels werescreened over continuous 4 h reaction at 1023 K (the high-est reforming temperature employed in the current work) andalso CO2-to-C3H8O3-ratio (CGR) of unity with weight hourlyspace velocity (WHSV) of 3.6×104 mL·g−1

cat ·h−1. Blank tests

using the same feed with either an empty reactor or calcinedCC bed yielded negligible glycerol conversion. This suggeststhat neither homogeneous gas phase glycerol dry reformingnor reaction over sites on the support occurred at sufficientrates.

Figure 9 shows the highest gaseous product output

Figure 9. Output flowrate of the gaseous product for Ni/CC catalysts withdifferent Ni loadings

flowrate was obtained over 20 wt% Ni/CC catalyst, with anaverage of 190 mL·min−1. At the 4th hour, the gaseous outputflowrate of 5 wt%, 10 wt% and 15 wt% Ni/CC catalysts werealmost similar to each other, approximately, 160 mL·min−1.The gaseous product comprised primarily of H2, CO, CO2 andCH4. Figure 10 represents the gaseous product output (vol%)at the 4th hour when steady state condition was attained. Sig-nificantly, this indicates that H2 and CO were the productsfrom the primary reaction (Equation 3) whilst CH4 which oc-cupied the least percentage of no more than 5 wt% and de-creased when Ni loading was increased, was a by-product thatmost likely originated from the competing methanation pro-cess. The outlet composition of H2 gaseous increased with Niloading, whilst CO decreased. Besides, less output of gaseousCO2 over higher Ni doped catalysts was observed, indicatingmost of CO2 were being consumed for glycerol dry reforming.

Figure 10. H2, CO, CH4 and CO2 product output volume (%), glycerol con-version (XG) at CGR of unity, T = 1023 K at 4th-h reaction. The WHSV wasset at of 3.6×104 mL·g−1

cat ·h−1

Table 5 summarizes the transient yield of gaseous prod-uct (%), glycerol conversion (XG) and the H2/CO ratio ob-tained for the 1st and 4th hour of screening tests. The cat-alytic performance at the initial stage of reaction (1st hour)was ranked in the order of 10 wt%>20 wt%>15 wt%>5 wt%in terms of glycerol conversion (XG). After 4 h reaction, XG

attained range from 64% to 76% for both 15 wt% Ni and20 wt% Ni catalysts. XG over 10 wt% catalysts decreasedto the lowest one at the 4th hour, which was 46.14% whilst5 wt% catalyst showed an increase of XG with margin of29%. 5 wt% Ni/CC possessed higher stability over 4 h reac-tion than 10 wt% Ni/CC catalysts, which might be attributedto the smaller Ni oxide crystallize size (Table 3). 15 wt% and20 wt% Ni/CC catalysts showed higher conversion; 5 wt% and10 wt% Ni/CC catalysts showed improved catalytic stability

Table 5. Glycerol conversion and yield of gases products

Glycerol conversion, XG (%) Yield of H2, YH2 (%) Yield of CO, YCO (%) Yield of CH4, YCH4 (%)Catalysts

1 h 4 h 1 h 4 h 1 h 4 h 1 h 4 h5 wt% Ni/CC 24.7 53.4 13.6 28.6 32.7 69.4 7.4 16.610 wt% Ni/CC 77.0 46.1 53.7 24.2 89.9 59.6 15.5 14.615 wt% Ni/CC 65.3 63.8 56.4 50.4 51.3 47.7 5.9 8.920 wt% Ni/CC 76.7 75.6 69.3 66.1 55.0 59.3 4.9 6.3


which was due to higher number of Ni active sites (Table 4)that involving scission of C–H and C–C bonds of glycerol.

In terms of YH2 (Table 5), 5 wt% and 10 wt% Ni/CCcatalysts showed the similar yield at the 4th hour reaction,in the range of 24% to 28% while achieving steady state.On the other hand, both 15 wt% and 20 wt% Ni/CC cata-lyst showed an insignificant decrease of YH2 over 4 h reac-tion. As depicted in Figure 11, the rate of H2 production (rH2 )over 4 h reaction for 15 wt% and 20 wt% Ni/CC catalystswere 1.5×10−4 mol·g−1

cat ·s−1 and 1.14×10−4 mol·g−1

cat ·s−1, re-

spectively, double from 5 wt% and 10 wt% Ni/CC catalysts.This can be attributed to the higher SBET of 15 wt% and20 wt% Ni catalyst, which means more reactants was in con-tact with the active metals, resulting in higher H2 productionrate. Moreover, stronger Ni-support interaction of 15 wt%and 20 wt% Ni/CC observed from TPR analysis has con-tributed to the insignificant deterioration in catalytic activity,hence better catalyst performance was obtained. The rates ofCO and CH4 production formation (rCO and rCH4 ) in gen-eral reduced with higher Ni dopant levels. Figure 12 illus-trates that H2/CO ratios obtained at CGR of unity over thecatalysts were in the range of 0.5 to 1.50, which were sub-stantially lower than those obtained from glycerol steam re-forming (ratios averaging 10.0). H2/CO and CO/CH4 prod-uct ratios generally increased with Ni dopant amounts. Over-all, 15 wt% and 20 wt% Ni/CC catalysts yielded H2/CO ra-tios between 1.5 to 2.0 which is desirable for Fischer-Tropsch(FT) synthesis. Among all the evaluated catalysts, 20 wt%Ni/CC catalyst exhibited the best performance and stabilitywith the highest XG and YH2, which could be attributed tothe stronger support-metal interaction and total metal surfacesfrom TPR analysis. Besides, 20% Ni/CC catalysts yieldedthe lowest methanation activity, satisfying H2/CO productratio ≈ 1.5.

Figure 11. Rate of product formation at 4 h on stream

Figure 12. Product ratio of different catalysts (data were taken at 4 h on stream)

3.3. Used catalyst characterization

3.3.1. Temperature-programmed oxidation (TPO)

Dry reforming process normally suffered from catalystdeactivation via coke deposition [71]. The used catalyst wassubjected to temperature-programmed oxidation (TPO) anal-ysis to study the chemical nature of coke deposit. The useof pure O2 during the oxidation ensured complete conversionof coke into CO2 gas. Table 6 and Figure 13 list the totalamount of carbon deposits and its gasifying temperature byoxidation. Coke deposition showed an increasing trend withmore Ni loading. The amount of coke deposit on 15 wt%and 20 wt% Ni/CC catalysts showed no significant difference(difference of 0.01 gcoke·g

−1cat ), as was also between 5 wt% and

10 wt% Ni/CC catalyst (difference of 0.03 gcoke·g−1cat ). It can

therefore inferred that higher Ni loading produced higher ac-tivity in terms of catalytic cracking of C–C bond, leading tohigher coke deposition. There were three gasifying peaks tem-perature regions, viz. Peak I, 884 to 900 K; Peak II, 907 to941 K and Peak III, 960 to 1000 K, which can be categorizedinto different types of carbon formed. The presence of thosepeaks indicates the laydown of carbon deposits with dissimi-lar reactivity towards O2 species. Fundamentally, this positsto entirely different chemical species of carbons, viz. PeakI temperature region assigned to the fibers or whiskers typecoke (Cv) can be easily removed at lower gasification, whilstPeaks II and III temperature regions falls to the graphitic-alike(Cc) carbon species [72].

Table 6. Total amount of coke deposited on catalystsafter 4 h on stream, at 1 atm, CGR of unity and 1023 K

Total carbon Carbon gasification temperature (K)Catalysts

(gcoke·g−1cat ) Peak I Peak II Peak III

5 wt% Ni/CC 0.46 890 941 −

10 wt% Ni/CC 0.49 885, 890 946 100015 wt% Ni/CC 1.78 884 907 96020 wt% Ni/CC 1.79 900 921 960


Figure 13. TPO profiles of various catalysts with different Ni loadings

3.3.2. FESEM images

As aforementioned, 20 wt% Ni/CC catalyst suffered fromthe highest coke deposition, therefore it was further charac-terized with FESEM. By comparing Figure 14(a) with 14(b),it apparently showed some spherical deposits, indicating car-bon clusters encapsulation occurred on the surface of the usedcement clinker support. This corresponded to the previouswork [73] onto the gasoline fueled exhaust catalyst which

underwent morphology transformation that looks similar toFigure 14(a). There were several short hollows tube was ob-served on the encapsulated Ni surface, it was believed to be thestarting of carbon whiskers formation. Obviously, there weredifferent types of carbon, which appeared to whiskers type ofcarbon in the case of used 20 wt% Ni/CC catalyst (Figure 14b)as opposed to the fresh catalyst (Figure 3e). This findingwas consistent with Papadopoulou et al. [74] whereby theydescribed that in most Ni based catalysts, carbon whiskers

Figure 14. FESEM images of used cement clinker support (a) and used 20 wt% Ni/CC catalysts (b)


predominate. Zooming into the circles zone of Figure 14(b),the hollow formations at the tip of carbon filaments (nan-otubes) were clearly seen. Ni particles on the surface of thesupport were pushed away by the growing carbon whiskersand Ni metals were eventually dropped onto the open dissem-ination over the support surface, leaving the hollows.

4. Conclusions

Cement clinker catalysts with Ni dopant were synthesizedfor glycerol dry reforming reaction. The glycerol conver-sion obtained over the screening of the catalysts was in therange of 46% to 75% over 4 h on stream test. Therefore, ce-ment clinker could be the potential support of glycerol dry re-forming. Physicochemical characterization has revealed thatcement clinker was a complex mixture of oxide compoundswith CaO and SiO accounted for more than half (62% and17%, respectively). Although cement clinker was macrop-orous, the addition of Ni has significantly improved the BETspecific surface area (from 0.55 to 17.83 m3·g−1). XRD ex-amination of the calcined catalysts showed complex Ni ox-ide phase formation at different Ni loading. The screeningof the catalysts has determined that 20 wt% Ni is the bestdoping percentage, as 20 wt% Ni/CC catalysts has the high-est activity and stability in terms of glycerol conversion re-gardless the effect of coke deposition. Glycerol dry reform-ing activity over 20 wt% Ni/CC catalysts also yielded H2/CO≈ 1.5 at 1023 K, which is desirable for Fischer-Tropsch syn-thesis compared with glycerol steam reforming. In post dryreforming, used catalysts characterization revealed the phe-nomenon of carbon deposition which was confirmed by thewhisker-type carbon laydown captured by FESEM technique.Significantly, temperature-programmed oxidation studies in-dicated that the carbon deposit can be completely gasifiedwith O2 and deduced the possibility of catalyst recyclability.

AcknowledgementsAuthors would like to acknowledge the financial support from

Ministry of Education, Malaysia through MTUN (No. RDU121216).Both Hua Chyn Lee and Kah Weng Siew appreciate the UniversitiMalaysia Pahang for a provision of GRS120377 scholarship.

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Catalytic Performance of Cement Clinker Supported Nickel Catalyst in Glycerol Dry Reforming

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