Application of Cement Clinker as Ni-Catalyst Support for Glycerol Dry Reforming

The increase in biodiesel production inevitably yield plethora of glycerol. Therefore, glycerol has been touted as the most promising source for bio-syngas (mixture of H2 and CO) production. Significantly, coking on nickel-based catalysts has been identified as a major deactivation factor in reforming technology. Indeed, coke-resistant catalyst development is essential to enhance syngas production. The current work develops cement clinker (comprised of 62.0% calcium oxide)-supported nickel catalyst (with metal loadings of 5, 10, 15 and 20 wt%) for glycerol dry reforming (CO2). Physicochemical characterization of the catalysts was performed using XRD, XRF, BET, TGA and FESEM-EDS techniques. Subsequently, reaction studies were conducted in a 7-mm ID fixed-bed stainless steel reactor at 1023 K with various CO2 partial pressures at constant weight-hourly space velocity (WHSV) of 7.2×104 ml gcat h-1. Gas compositions were determined using Agilent 3000 micro-gas chromatography (GC) and Lancom III gas analyzer. Results obtained showed an increment of BET surface area up to 32-fold with Ni loading which was corroborated by FESEM images. Syngas (H2 and CO) ratios of less than 2 were being produced at 1023 K. A closer scrutiny to the transient profile revealed that the presence of CO2 higher or lower than CGR 1:1 promotes the Boudouard reaction. © 2013 BCREC UNDIP. All rights reserved


Introduction
Post year 2008, in the aftermath of global energy crisis sparked by the sudden surge in the price of petroleum fuel, the search for renewable and sustainable energy source has become imperative.For countries with meager proven petroleum reserves such as Malaysia, a moratorium on petro-leum oil production could spell a disaster to the economy; hence energy security in the context of leveraging on the country's biodiversity is clearly the only way out from the energy malaise.Significantly, syngas (H2 and CO) production from glycerol bio-waste (10 wt% of crude glycerol per kg of biodiesel from the transesterification process) has been touted as one of the most promising route in achieving sustainable energy requirement.Indeed, the H2 combustion value is 122 kJ g -1 , which is 2.75-fold of hydrocarbon fuels [1].In theory, 1 mole of glycerol can decompose to 3 mol of CO and 4 mol of H2 (cf.Equation ( 1)).C3H8O3 (g) ↔ 3CO (g) + 4H2 (g) (1) Production of syngas from glycerol steam reforming has been studied.However, it does not yield favourable H2:CO ratio for Fisher-Tropsch (FT) synthesis.Besides, it also emits CO2 (cf.Equation (2)).C3H8O3 (g) + 3H2O (g) ↔ 3CO2 (g) + 7H2 (g) (2) In particular, glycerol dry reforming as shown in Equation ( 3) is a new area and no prior works have been published on it.Theoretical consideration alone indicates that it is more viable for syngas production intended for FT synthesis as well as utilizing the greenhouse gas, CO2.Wang et al. [2] from their thermodynamic analysis reported that glycerol dry reforming is able to produce 6.4 moles of syngas per mole of glycerol.C3H8O3(g) + CO2(g) ↔ 4CO(g) + 3H2(g) + H2O(g) In reforming technologies, Ni catalyst has been widely used due to its availability and low costs compared to noble metals such as Rh, Pt and Pd [3][4][5].Nonetheless, the main drawback associated with Ni is severe carbon deposition (coking).Significantly, coking onto the catalysts can be reduced by the addition of strong Lewis base oxide such as CaO [6][7][8][9][10] previously reported that cement clinker contains 63.28% and 63.17% CaO, respectively.Importantly, it is an abundant source from cement industry in Malaysia.Endowed by thermal stability, cement clinker hence offers great potential as suitable support for Ni catalyst.
The current work serves to characterize the synthesized cement clinker-supported nickel catalysts using spectroscopic analysis and also to investigate the activity of glycerol dry reforming over the cement clinker-supported Ni catalyst with the aims of producing syngas mixture suitable for FT synthesis and enhancing catalytic stability through carbon lay down reduction.

Materials and Catalyst preparation
Cement clinker (CC) was obtained from the Pahang Cement Sdn.Bhd. after the cement kiln which undergone calcination at 1673 to 1723 K.All the chemicals such as nickel (II) nitrate hexahydrate (Ni(NO3)2•6H2O) and pure glycerol were analytical grade purchased from Sigma-Aldrich.Catalysts were prepared via wet-impregnation of CC with 5 wt%, 10 wt%, 15 wt% and 20 wt % nickel (Ni)-metal respectively using Ni(NO3)2.6H2Osolution prepared from 50 ml ultrapure water (Milipore Elix 5-UV) as precursors.The slurry was then stirred for 3 h at room temperature before ovendried at 403 K for 24 h.Subsequently, it was calcined at 1073 K for 6 h.Post-calcination, the catalysts were ground and sieved to the size < 200 μm for reaction studies.

Characterization of The Catalysts
The freshly-calcined catalysts were subjected to a series of characterization, viz.thermogravimetric analysis (TGA), x-ray fluorescence (XRF), x-ray diffraction (XRD), field emission scanning electron microscopy-energy dispersive x-ray spectrometry (FESEM-EDS) and also specific surface area measurement by liquid N2 adsorption analysis.The XRD measurement employed radiation, λ= 1.5418 Ǻ at 30 kV and 15 mA, in 2θ from 10 o to 80 o with a step size of 0.02 o and step time of 1 s.The crystalline size of the catalysts was determined from the Scherrer equation, d=0.94λ/(βdcosθ),where d is the crystallite size, λ is the wavelength of the radiation, βd is the full-width at half maximum (FWHM) of the diffraction peak and θ is the half of the diffraction angle.The surface structure of the catalysts was captured by FESEM unit JOEL/JSM-7800F model at 3 kV with ×10,000 magnification.Specific surface area was determined by BET model.The catalysts were degassed overnight at 573 K prior to specific surface area measurement at 77 K.The cumulative pore volumes of the catalysts were determined by using Barrett-Joyner-Halenda (BJH) analysis.The chemical composition of the catalysts was determined by both EDS and XRF.TGA analysis was performed to obtain the phase transformation of the catalyst in N2 atmosphere up to 1173 K employing the ramping rates of 10 K min -1 .

Catalyst Testing
Figure 1 shows the experimental rig for the catalyst testing.The catalytic evaluation was carried out by placing 0.10 g of catalyst into the stainless steel 316 fixed-bed reactor (ID: 7 mm) supported by two layers of quartz wool.The catalyst was reduced by 50% H2/Ar gas (50 ml min-1 STP) for 2 h and held at 1073 K. Glycerol was pumped into the reactor by HPLC pump while the flowrate of carrier gas (as diluent) was controlled using flowmeter controller.CO2 to glycerol ratio (CGR) was adjusted to determine the partial pressure of the reactants at constant weight-hourly space velocity (WHSV) of 7.2 × 10 4 ml gcat -1 h -1 .The reaction temperature was set at 1023 K and ex-

Power Law Model
Power law equation (cf.Equation ( 4)) was being used to approximate the kinetics of the glycerol dry reforming reactions and also its rate exponential. ( where r is rate of reaction (mol gcat -1 s -1 ), k is the rate constant, Pgly is the partial pressure of glycerol (kPa), PCO2 is the partial pressure of carbon dioxide (kPa) while a and b are the order of reaction unique to the catalyst system.The reaction rate constant, k was expressed in Arrhenius form (cf. Equation ( 5)) to estimate the activation energy.
where A is pre-exponential constant factor, s -1 ; Ea is activation energy, kJ mol -1 ; T is reaction temperature (K) and R is the gas constant, 8.314 J mol - 1 K -1 .

Brunauer-Emmett-Teller (BET)
Table 1 shows the density, BET surface area and cumulative pore volume of the catalysts with increasing Ni metal loading.Density measurement results generally agreed with Taylor [11] who reported that the density of cement clinker (CC) was between 3.15 to 3.2 g cm -3 .BET surface area of the 100% CC was relatively low (0.55 m 2 g -1 ) in agree-ment with prior work by Chitra [12].Low surface area of CC apparently related to the sintering effect of calcination at 1723 K in the cement kiln [13].Significantly, Ni-doping has increased the BET specific surface area of the catalysts.This is a further testament of the successful deposition of Ni on the CC surface.The pore size of the 100% CC falls in the range of macroporous as observed from the physisorption isotherms (not shown).The isotherms displayed a shift towards mesoporous range with metal loadings.Therefore, this indicates the formation of new compound attributed to the alteration of the surface structure of the catalysts, which was further confirmed by XRF analysis tabulated in Table 2.

X-ray Fluorescence (XRF)
Table 2 shows that pure CC was in fact a mixture of oxide metals with CaO as the major ingredient at 61.98%.This also corresponds to the earlier studies by Kurdowski [9] and Tsakiridris et al. [10].Further inspection of Table 2 also revealed that the percentage of CaO and SiO2 in catalysts decreased from 61.98 % to 38.66 % and 17.21% to 6.77 % respectively as Ni metal loading being increased.Indeed, the doping of Ni has increased the amount of NiO that originally also presents in the pure CC, contributed to a spike from 62 ppm to 34.83%.This observation was also confirmed by XRD analysis (cf. Figure 2).

X-ray Diffraction (XRD)
XRD analysis was carried out for both CC (Reference) and Ni-loaded catalysts.Diffractograms were shown in Figure 2. The main mineralogical phase presents in the CC was alite (tricalcium silicate, Ca3SiO5) in agreement with Shih et al. [14] judging from 11 peaks shown in Figure 2. Alite found in CC presents as monoclinic crystals due to the impurity contents [15], differed from pure alite that is triclinic.XRD also confirmed the presence of calcium aluminum iron manganese oxide (Ca2Al0.67Mn0.33FeO5) in CC with orthorhombic shape.
For 5% Ni loading, clinker phase was essentially the same with presence of a substituted alite in nature named Hatrurite (Ca3O(SiO4)) [11].Hatrurite has hexagonal crystals and X-ray lines were overlapped by those of larnite at 32.This corroborated with Shui et al. [16] who reported that alite peaks ceases in fly ash and slowly replaced by Ca54MgAl2Si16O90 (flue gas desulfurater) after reactions.The crystal, Ca54MgAl2Si16O90 which acts as flue gas desulfurater was estimated to reduce the poisoning effect of catalyst during the reactions testing and enhance the catalytic performance.
Ca3SiO5 + 11 SiO2 + 3 NiO → 3CaNiSi4O10 (7) For Ni loading more than 10%, the promotion of alite to larnite (β-belite, Ca2SiO4) was being observed.Similar result was reported before by Sinyoung et al. [18].They have investigated the effect of chromium metal loading in CC.Increasing peak intensities across 2θ of 37 o and 43 o signified the in-
In contrast, with Ni loading of up to 15%, Bunsenite (NiO), in cubic system and Ca2SiO4, in monoclinic were found to be the main components.The XRD results detected the crystalline in the form of Ca2SiO4 instead of Shannonite (-belite) which can be attributed to the presence of impurity ions and also slow rate of cooling after the calcination [10].The results demonstrated that the Ni metal has inhibited the transformation of Ca2SiO4 back to Ca3SiO5 and there was no alite corresponding to results by Sinyoung et al. [18].High Ni loading > 10% seems to assist the Ca3SiO5 polymorph to decompose and transform to Ca2SiO4, and NiO species as illustrated in Figure 2, as also obtained by Shih et al. [14].
With the loading of Ni metals, a slight shift of 2θ to a lower degree value can be observed from a closer scrutiny towards peaks occurs at 32 o and 34 o in Figure 3. Significant reduction of peaks intensities between 32-36 o and left shifting effects indicates structural transformation due to Ni loading.Subsequently, the peak intensities especially the larnite compounds at peak 43 o increases indicating an increase of crystalline size (cf.Figure 4 and Table 3).The XRD results (shows the occurrence of Bunsenite, NiO) are comparable with the results in XRF indicating a decreasing amount of CaO and SiO2 for NiO formation.

FESEM Studies
The surface morphology obtained by FESEM imaging shows that the surface of pure CC (cf. Figure 5(a)) was very smooth and appeared nonporous.Interestingly, at the loading of 15% Ni (representative sample), the surface of the resulting catalyst has become rougher and bulkier with creation of more finely dispersed crystallites (cf.Fig. 5(b)).This may have explained the higher BET surface area obtained (cf.Table 1).

TGA-Calcination Studies
Calcination profiles of both 15% and 20% Ni doping in N2 atmosphere was plotted as in Figure 6.Calcination profiles obtained from thermogravimetric analysis (TGA) shown in Figure 6 for pure 100% CC shows a thermal stability with no obvious decomposition.Calcination profiles for 15% Ni-85% CC and 20% Ni-80% CC catalysts (as representative samples) showed approximately similar trend of decomposition in N2 atmosphere.Peaks formation before 500 K were probably due to the water vapour entrainment in the catalysts.
For both 15% and 20% Ni catalysts in Figure 6, there were formations of two peaks around 504 to 620 K and 620 to 794 K (with a shoulder peak around 673 K) respectively.The peaks for 20% Ni catalyst were shifted to the right as compared to 15% Ni catalyst.The first peak (504 to 620 K) was due to the transformation of Ni3(NO3)2(OH)4 to NiO in consensus with Estellé et al. [19].It was also proven by the formation of highest peak that occurs at 580 K (green line) which indicates the full decomposition of nitrate in of nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O)(cf. Figure 6).Ni3(NO3)2(OH)4 species was most likely obtained from the subsequent decomposition of Ni(NO3)2.6H2Oduring the drying of wetimpregnated catalyst at 403 K whilst second peak formed (620 to 794 K) was an indicative of NiO formation due to the total metal precursor (nickel (II) nitrate hexahydrate) decomposition in agreement with Loaiza-Gil et al. [20].The calcination profiles corroborated with the XRD patterns which showed the presence of NiO.The formation of the shoulder peak at 673 K indicates the continuous thermal decomposition of a complex mixture of hydrated sili-cate and aluminate-type compounds as described by Gabrovšek et al. [21].The decomposition of silicate described the formation of Ca2SiO4 crystalline present in 15% Ni-85% CC and 20% Ni-80% CC catalysts.

Reaction Studies
Figure 7 shows the hydrogen product ratio profiles for glycerol dry reforming carried out at 1023 K over 15%Ni-85% CC catalyst.Overall, H2, CO and CH4 were produced.Blank tests using the same feed with either an empty reactor or 100% CC bed yielded negligible glycerol conversion.This suggests that neither homogeneous gas phase glycerol dry reforming nor reaction over sites on the support occurred at detectable rates.The slight de-crease of H2:CO ratio with PCO2 is most likely an attribute of the reverse-water-gas-shift reaction.Nonetheless, the H2:CO ratio remained > 1.0.In addition, the formation behaviour of both CH4 and CO also forms the focal point of the current study as these two species are well-known carbon precursors via methane cracking and Boudouard reaction respectively.Hence, transient product ratio profiles of CO:CH4 were plotted and shown in Figure 8 for 15% Ni-85% CC catalyst at different partial   .For PCO2 at 42 kPa (CGR 3:1), the CO:CH4 ratio increased in the first 30 min of reaction time and thereafter gradually dropped.For PCO2 at 14.0 kPa (CGR 1:1), the CO:CH4 ratio gradually increases up to 6.5, an indication of the decrease in CH4.Indeed, higher CGR and higher temperature favour the formation of CO.However, Figure 8 depicts that the CH4 formation was also increased with higher CGR.This could be explained by Equation (8), where the excess CO2 has reacted with H2 to form methane, CH4.This was further proven by the methane production rate illustrated in Figure 9.The average methane production rate for 15% Ni-85% CC catalysts at PCO2 , 14.0 kPa (CGR 1:1) was 1.46E-5 mol gcat -1 s -1 while for the PCO2 of 42.0 kPa, with CGR 3:1, the average methane production rate was 4.9E-5 mol gcat -1 s -1 .CO2 + 4H2 ↔ CH4 + 2H2O (8) Interestingly, excess CO2 (culminate in CGR 3:1) did not promote Boudouard reaction (cf.Equation (9) to form CO as the CO to CH4 ratio remained constant at circa 4.6 for CGR 3:1 (PCO2 = 42.0 kPa).Most likely, reaction as in Eq. ( 10) also has taken place.

Conclusion
Cement clinker catalyst with Ni dopants could be a potential catalyst for the purpose of producing syngas from glycerol dry reforming.Physicochemical characterization has revealed that cement clinker was a complex mixture of oxide compounds with CaO and SiO accounted for more than half (62% and 17% respectively).Although cement clinker was non-porous, addition of Ni has improved significantly the BET surface area with at least 32-folds increment.XRD examination showed formation of complex oxide phases depending on the Ni loading.Interestingly, glycerol dry reforming reaction yield H2:CO < 2.0, suitable for Fischer-Tropsch synthesis.Nonetheless, the sidereactions such as methanation and hydrogenation of CO2 have majorly affected the amount of syngas produced.

Table 1 .
Composition, BET specific surface area and density of the catalysts

Table 2 .
XRF results of the catalysts The data presented was in wt % except for * in ppm.

Table 3 .
Crystalline size of larnite, Ca2SiO4 at 2θ around 43 o with Ni content