Catalytic upgrading of a biogas model mixture via low temperature DRM using multicomponent catalysts

Cameron Alexander Hurd Price, William Arnold, Laura Pastor-Pérez, Bahman Amini-Horri, Tomas R. Reina*

Department of Chemical and Process Engineering, University of Surrey, GU2 7XH, UK

Corresponding author: t. ramirez reina@surrey.ac.uk

AbstractThe catalytic performance of a series of bimetallic Ni-Co/CeO2-Al2O3 catalysts were evaluated within the dry reforming of methane (DRM) reaction, commonly used for upgrading biogas. The study focused on the variation of CeO2 weight loadings between 0, 10, 20 and 30%. It was found that the addition of CeO2 promoted CH4 and CO2 conversion across the temperature range and increased H2/CO ratio for the “low temperature” DRM. X-Ray Diffraction (XRD), H2-Temperature Programmed Reduction (H2-TPR) and X-Ray Photoelectron Spectroscopy (XPS) analysis revealed the formation of Ce4+ during activation of the 30% sample, resulted in excessive carbon deposition during reaction. The lowest CeO2 weight loadings exhibited softer carbon formation and limited increased chemical stability during reaction at the expense of activity. Of the tested weight loadings, 20 wt% CeO2 exhibited the best balance of catalytic activity, chemical stability and deactivation resistance in the DRM reaction. Hence this catalyst can be considered a promising system for syngas production from biogas at relatively low temperatures evidencing the pivotal role of catalysts design to develop economically viable processes for bioresources valorisation.

Keywords: CO2 utilisation, Biogas Upgrading, Ni-Co catalysts, Syngas, Ceria

1. Introduction

Given the increasing trend in the concentration of atmospheric greenhouse gases and the associated accelerating impacts of climate change, processes to remove and recycle greenhouse gases have been receiving significant attention from the scientific community [1]. One process, the upgrading of biogas via the dry reforming of methane reaction (DRM, Equation 1), is of particular interest [2–6]. Biogas is a well-regarded renewable energy source, produced from the digestion of varied biomass anaerobically and is frequently used as an in-situ fuel source for combined heat and power plants [7].

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Owing to the high concentration of CO2 present in biogas, it requires upgrading to reduce the CO2

content before it can be considered a replacement for natural gas (biomethane), which requires a composition of at least 90% methane [8]; biogas reforming and upgrading to biomethane are popular fields of research [9–12] (Fig. 1).

The reforming of biogas offers an interesting and more economically favourable alternative to simply burning it to produce heat and power. This reforming process mainly concerns the dry reforming of methane (DRM) (eqn. 1) to produce hydrogen from two otherwise environmentally harmful greenhouse gases [12]. Although hydrogen is the more valuable single component, the DRM produces a 1:1 ratio syngas. This syngas is a highly sought feedstock for numerous forms of chemical or fuel synthesis, which becomes increasingly valuable if produced from renewable sources, further increasing the attraction towards biogas reforming [12]. However, if the biogas does not contain the appropriate methane content to be considered biomethane, it must then be enriched to meet the requirements before it can be used in existing DRM processes [13].

However, the DRM reaction is highly endothermic and requires temperatures more than 800oC to achieve complete conversion. There are also a number of competing side reactions that are favourable at high temperatures, such as reverse water gas shift (RWGS) and the decomposition of methane (Eqns 2 and 3), while low temperature reforming competes with the Boudouard, methanation and reduction of CO2 reactions (Eqns 4-6) [14].

DRM: C O2+C H 4⇌ 2CO+2 H 2 ( Δ H298=+247 kJ mol−1 )(eqn .1)

RWGS: C O2+H 2⇌CO+H 2O (Δ H 298=+41 kJ mol−1 )(eqn . 2)

Methane Decomposition: CH 4⇌C+2 H2 ( Δ H298=+75 kJ mol−1 )(eqn . 3)

Boudouard: 2 CO+H2⇌C O2+C ( Δ H 298=−172 kJ mol−1 )(eqn . 4)

Methanation: C O2+4 H2⇌CH 4+2H 2 O ( Δ H 298=−164 kJ mol−1 )(eqn .5)

Fig. 1 Schematic representation of biogas generation that is then used in a combined heat and power (blue box) or syngas generation (red box)

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CO2 Reduction: C O2+2 H 2⇌C+2 H 2O ( Δ H 298=−90 kJ mol−1 )(eqn .6)

Noble metals, such as Ru, Rh and Pt, possess both excellent catalytic activity with respect to the DRM reaction [15] and considerable resistance to coking and sintering. However, their rarity makes them uneconomical for industrial-scale applications [16]. Transition elements, particularly Ni, Fe and Co, are also catalytically active in DRM. In particular, Ni has demonstrated comparable activity to noble metals [17, 18]; this activity comes at a cost, as Ni is also prone to deactivation by solid carbon deposition [19]. In order to overcome the problems associated with monometallic catalysts, the alloying or doping of active metals have been investigated, with the most widely studied combinations for the DRM being those between Ni and other metals [20–24]. Such catalysts demonstrate superior catalytic activity and carbon deposition resistance over monometallic Ni catalysts [25]. For example, bimetallic Ni-Co catalysts have been found to experience reduced active site agglomeration (sintering) while promoting formation of more defective, and therefore more easily oxidised, carbon deposits compared to monometallic Ni catalysts [26].

The addition of promoters to the catalyst support phase is another effective method of reducing sintering and hard carbon deposition. Cerium oxide (CeO2) is a popular research choice due to the excellent redox properties of surface Ce4+/Ce3+ species [27], for Ni-based catalysts, particularly as a co-support with γ-Al2O3. Al2O3-CeO2 is especially effective as a support for reforming reactions due to these redox properties, high oxygen storage capacity (OSC) and high surface area, all of which significantly enhance the ability of the catalyst to oxidise solid carbon deposits and resist sintering [28]. The integration of Ce into support γ-alumina spinels also modifies the Ni-support interaction through enhanced oxygen mobility, thus preventing the formation of inactive NiO species [29]. Ce has also been found to integrate into Ni-Co clusters, improving CH4 and CO2 conversion and reducing carbon deposition by reducing cluster size and improving dispersion [30].

Under these premises, this paper investigates the effect of varying levels of ceria doping on Ni-Co/γ alumina catalysts with a view to establish an upper limit for efficient use as a co-support in such catalysts as the high value of ceria makes high loadings uneconomical, for application within the low temperature (500-700oC) DRM. We have selected a biogas model mixture (1:1 ratio CH4/CO2) in order to explore the applicability of our multicomponent catalysts for bioresources upgrading.

2. Experimental

All chemicals were sourced from Sigma-Aldrich and used as received and without further purification, unless otherwise stated.

2.1 Catalyst Synthesis

All catalysts and supports were prepared using a wet impregnation method. The 10, 20 and 30% ceria-alumina supports were prepared first, using cerium nitrate hexahydrate [Ce(NO 3)3 6H‧ 2O] (Sigma-Aldrich) and γ-alumina powder (Sigma-Aldrich). The required amount of cerium precursor for each ceria loading, 10, 20 or 30 wt%, was dissolved in ethanol and then mixed with a rotary evaporator for 1 hour. The solution was then gradually placed under vacuum to evaporate the solvent and dried overnight at 80oC. The dried Ce-Al support was calcined at 400oC at 10oC/min for 4 hours.

Nickel nitrate hexahydrate [Ni(NO3)2 6H‧ 2O] (Sigma-Aldrich) and cobalt nitrate hexahydrate [Co(NO3)2 6H‧ 2O] were used as precursors for the active component of the catalyst. These materials were simultaneously impregnated on the calcined Ce-Al support, dried and calcined at 400oC at

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10oC/min for 4 hours. Appropriate amounts of these precursors were used to produce weight loadings of 10% w/w Ni and 5% w/w Co in the NiCo/Ce-Al, Ni-Co/Al and 15% for w/w Ni in the Ni/Al catalysts, where appropriate.

2.2 Catalyst Characterisation

The X-Ray diffraction (XRD) patterns of the fresh, active and post-reaction catalysts were recorded using a X’Pert Powder X-Ray generator (PANalytical). Diffractograms were recorded between Bragg angles (2ϴ) of 10o - 90o using Cu Kα radiation (λ=1.54 Å) at 30mA and 40kV. The results were analysed using X’Pert Highscore Plus (PANalytical) and Origin software packages, with crystallite sizes estimated using the Scherrer equation [31].

X-ray Photoelectron Spectroscopy was performed using a K-ALPHATM Thermo ScientificTM device, utilising Al- K-α radiation (1486.6 eV) and a twin crystal monochromator to produce a focussed x-ray spot at 3 mA x 12 kV (400 µm major axis length of the elliptical shape). The alpha hemispherical analyser was operated at the constant energy mode with survey scan pass energies of 200 eV to measure the whole energy band and 50 eV in a narrow scan to selectively measure the particular elements. Charge compensation was achieved with the system flood gun that provides low energy electrons and low energy argon ions from a single source. C 1s core level is used as reference binding energy and is located at 284.2 eV. Prior to the spectral acquisition, the samples were pre-reduced simulation the activation treatment. The data was then processed using the Avantage software package.

The textural properties of the fresh catalyst samples were determined using nitrogen adsorption-desorption characterisation using a 3Flex device (Micrometrics). The samples were degassed at 350 oC for 4 hours before each measurement. Surface area and pore volume were calculated using the Brunauer-Emmett-Teller (BET) equation and the Barrett-Joyner-Halenda (BJH) method, respectively. The data presented is an average of three measurements taken for each sample.

Raman spectroscopy measurements were used to determine the nature of carbon deposits on the spent catalysts and was performed using an XploRA Plus MULTILINE confocal Raman microscope (Horiba) using a green laser (λ = 532nm) with a strength of 0.1 %.

Thermogravimetric analysis (TGA) was conducted on spent samples using a Q500 V6.7 TGA machine (TA Instruments) with a temperature ramp rate of 10oC/min from room temperature to 900oC, under air.

The reducibility of fresh catalyst samples was studied using hydrogen temperature programmed reduction (H2-TPR), utilising a total flow rate of 50 ml/min (10% H2, 90% Ar). 50mg of catalyst was loaded into a quartz fixed-bed reactor in a 10mm (internal diameter) quartz tube and heated using a Carbolite furnace from room temperature to 900oC at 10oC/min. H2 consumption was measured using an Omni-Star gas analysis system and data logged using the Quadera software package.

2.3 Catalytic Behaviour

DRM temperature screening experiments were conducted using 100mg catalyst, supported on quartz wool and loaded into a 10mm (internal diameter) quartz tube, over a temperature range of 500-850 oC and a reactant flow rate of 50 ml/min (CO2:CH4:N2 ratio = 1:1:3). The product composition was monitored for 30 minutes at 50oC intervals using an online AO2020 continuous gas analyser (ABB) equipped with a thermal conductivity detector (TCD) and infrared (IR) detector at a weight-hourly space velocity of 30,000 ml/(gcat h). Prior to reaction, catalysts were reduced in situ at 850 C, 10⁰ ⁰ C/min, under a 100 ml/min flow of 20%:80% H2:N2 at atmospheric pressure for 1 hour.

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Catalyst stability for the Ni-Co/20Ce-Al and Ni-Al samples were measured at 550oC for 72 h using the same catalyst loading and feed specifications as was used for the activity tests.

Catalyst performance was determined by the CO2 and CH4 conversions (Xi) and the H2/CO product ratio, calculated using equations 6-8.

XCH 4=100 ×

[CH 4]¿−[CH 4]out

[CH 4]¿(eqn .6)

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[CO2]¿−[CO2]out

[CO2]¿(eqn . 7)

H 2

CO=

[H 2 ]out

[ CO ]out¿

3. Results and Discussion

3.1 Textural Properties

Following BET analysis, all catalysts were found to be mesoporous materials, with their surface area, pore volume and pore diameter given in Table 1. Textural properties are governed by the primary support, γ-alumina. It can be seen that the addition of cobalt and ceria incrementally reduces the textural properties of the catalysts, relative to the Ni-Al material. This trend suggests that CeO 2

nanoparticles may be covering and filling pores in the γ-alumina support, which is consistent with literature data [32, 33].

Table 1: Textural PropertiesCatalyst SBET

(m2 g-1)Pore Volume(cm3g-1)×10-3

Pore Diameternm

Ni-Co/30Ce-Al 129 0.226 7.01Ni-Co/20Ce-Al 154 0.291 6.87Ni-Co/10Ce-Al 167 0.338 8.12

Ni-Co/Al 186 0.405 8.70Ni-Al 225 0.490 8.71

3.2 Reducibility: H2-TPR The H2-TPR profiles for each catalyst found in Fig. 2 show different complex reduction zones. There are several reduction zones present between 200 – 350oC, which are attributed to the multi-stage and overlapping reductions of cobalt and nickel oxide particles with comparatively weak interaction with the support phase [19, 34, 35]. This includes the multistep reduction of Co3O4 between 200-500oC [36]. The peaks seen between 400-600oC are indicative of the reduction of Ni and Co that possess stronger interaction with the CeO2-Al2O3 support as this is also the region in which Ce4+ surface particles begin to reduce [37]. Finally, between 600-800oC, there is a small feature for all except the Ni/Al catalyst plot, which appears centred at 750oC. These zones are attributed to Ni, Ni/Co-Alumina spinels (i.e. CoAl2O4, NiAl2O4, NiCo2O4) and Ni/Co species with strong support interactions being reduced [33, 36–38]. A small zone first seen ca. 650oC, with the inclusion of 10 wt% Ce, and seen again at decreased temperature for the 20wt% CeO2, (ca. 600oC) and then again ca. 650oC in the 30 wt

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% Ce catalyst. This suggests that Co and CeO2 serve to make the Ni species more reducible, possibly through promoting better dispersion, which is also supported by surface composition data presented in section 3.2.

Fig. 2 H2-TPR profiles of each catalyst

When considering the reducibility of CeO2 on Al2O3, literature most often describes it occurring in a stepwise, an “outer-to-inner” process: low temperature (500-600oC) is thought to be the reduction of small Ce4+ particles on the surface, medium temperature (600-800oC) corresponds to the reduction of larger, bulk ceria and high temperature (>800oC) is related to the Ce4+ Ce3+ reduction that occurs during the formation of CeAlO3. The exact occurrence of these zones is dependent on numerous other factors: CeO2 loading, strength of interaction with Al2O3 and overall particle size [39–41]. The clear region above 800oC is representative of the reduction of nickel and/or cobalt alumina spinel structures, as well as the formation of CeAlO3 [37]. As this zone is only seen on the Ni-Co/10Ce-Al and NiCo/20Ce-Al catalysts, and not seen in the Ni-Co/30Ce-Al, this suggests that the latter catalyst has larger CeO2 particles that require higher temperatures to fully reduce to CeAlO3 [42].

3.2 XPS Characterisation

The surface chemistry of each sample was assessed using XPS, prior to use in the DRM. Fig. 3 displays the 2p3/2 regions for both Ni and Co, in addition to the Ce 3d region for all prepared catalysts. Table 2 details a summary of the peaks found in Fig. 3 and their assignments. Before analysis, all samples were reduced under the same conditions used before reaction (H2, 850oC, 1 hour). Both Fig. 3a and Table 2 detail the electronic states of Ni that exist in the reduced materials, following spectral deconvolution. As shown in Fig. 3a, the Ni 2p3/2 spectra observes several different species. While the bands ca. 852 eV are attributed to Ni0, the bands ca. 854-857 eV are typical of two forms of Ni2+ species with increasing intensity of interaction with Al2O3 and/or CeO2, or even as NiAl2O4 [43, 44], as the binding energy increases [45, 46]. Furthermore, it has been found that NiAl2O4 displays its Ni2+ species at BEs ca. 856 eV in the presence of a CeO2-Al2O3, comparable to BEs reported here, supported by the TPR [47]. Additionally, these binding energies are also similar to those displayed by NiCo2O4 spinel structures [48]. This would suggest that the Ni species are most strongly bound to the support material in the Ni-Co/30Ce-Al material due to displaying higher binding energies (BEs) and reduction temperatures. These higher BEs, and associated stronger support interactions present in all but the 10 and 20 wt% ceria catalysts, also suggest that these nickel species are much more resistant to reduction

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due to the formation of various spinel structures (i.e. NiAl2O4, NiCo2O4) [43, 44, 48–50]. The presence of the Ni2+ is further supported by the observance of a higher band at approx. 858 eV, which is the shake-up satellite peak associated with the Ni2+ species [45, 51, 52]. The Co 2p3/2 region depicted in Fig. 3b shows three main bands for the Ni-Co/10Ce-Al and Ni-Co/20Ce-Al catalysts: the first at ca. 777-778 eV, the second ca. 779 eV and the third at ca. 782 eV representing Co0 [53], Co3+ (octahedral) and Co2+ (tetrahedral), respectively [48, 54–56]. These assignments are further verified by the splitting energy (spin-orbit coupling) between the 2p 3/2 and 2p1/2 orbitals is approximately 15 eV, confirming the presence of both of these species coexisting [54]. The Ni-Co/Al and Ni-Co/30Ce-Al catalysts report only two Co species: the first at ca. 778 eV and a second at ca. 781 eV, assigned to Co0 and Co2+, respectively. The remaining peak present in all samples is attributed to being a shake-up satellite attributed to Co2+ [57, 58]. These assignments, and their respective intensities and BE shift, suggest that although a great deal of the Co exists as metallic Co, there also exist some surface Co that is strongly interacting with the support to remain as Co3+ and Co2+ species either as CoAl2O4 [59] or as Co3O4 [59, 60] or NiCo2O4 [48], which also explains the peak shifting and agrees with the complex TPR profiles.

Fig. 3c details the entire Ce 3d orbital of each Ce containing material. The peak assignments for Ce are complex, owing to the overlap of Ce 4f levels with O 2p states, in addition to overlap of the Ce 3d5/2 spin-orbital with the Ni 2p1/2 spin-orbital. However, after careful consideration of reported XPS data for this region in literature, the following assignments and assessment are made with a reasonable degree of certainty. The peaks described in fig. 3c concern the 3d 5/2 spin-orbital and confirm the presence of both Ce4+ and Ce3+, with the feature at ca. 917 eV (fig. 3c) being used as further evidence of Ce4+ [61, 62].

Interestingly, the Ni-Co/20Ce-Al catalyst shows a significantly reduced peak at 917 eV compared to the Ni-Co/30Ce-Al catalyst, which suggests a considerably lower abundance of Ce4+ in the former catalyst [62]. Furthermore, the peaks at 885.8 eV and 904 eV (Ce3+) for the Ni-Co/10Ce-Al & Ni-Co/20Ce-Al have markedly higher intensities compared to the same peaks present in the Ni-Co/30Ce-Al material. These peaks appear as a result of a greater influence from Co and are a typical feature of Ce3+ [61, 63–66]. Previous XPS studies concerning Co and Ce materials have found that cobalt promotes the formation of Ce3+, which in turn enhances the Ce4+ ↔ Ce3+ redox system [57, 61, 66]. The Ce3+ species, existing CeAlO3 structures as suggested by TPR, has been found to resist carbon formation through a redox reaction with CO2, to form CeO2 (Ce4+) that goes on to react with CHx species to reform the CeAlO3 while producing CO and H2 [41, 49]. Calculation of the amount of surface Ce4+ in each of these samples reveals that the Ni-Co/30Ce-Al catalyst has approximately 59% surface Ce4+, a substantially higher content than the approximate values of 35% and 40% Ce4+ found at the surface of the Ni-Co/20Ce-Al & Ni-Co/10Ce-Al, respectively, further supporting a greater formation of CeAlO3 species at the surface of the Ni-Co/20Ce-Al catalyst [41]. Table 2 also shows a trend in surface concentration of the elements, in that the Ni-Co/10&20Ce-Al catalysts report small increases in the surface presence of each element, suggesting the similar amounts of dispersion. This sharply increases with the Ni-Co/30Ce-Al catalyst, due to the highest reported at% values for each element, which suggests the worst dispersion of the three Ce-Al supported catalysts and supports the fact that the Ni-Co/30Ce-Al material will produce more coke deposition, due to the known relation between Ni crystallite size and carbon formation.

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Table 2. XPS peak assignments for all produced catalysts for the Ni and Co, with the elemental composition of each Ni, Co and Ce

CatalystNi 2p3/2 (eV) Co 2p3/2 (eV)

Elemental surface Composition

(at%)

Ni0 Ni2+ Co0 Co3+ Co2+ Ni Co CeNi/Al 852.4 854.9, 857.3 - - - 0.65 - -

Ni-Co/Al 852.2 854.8, 857.9 778.4 - 781.1 0.83 0.53 -

Ni-Co/10CeAl 852.4 854.4, 856.6 777.6 779.4 781.9 0.95 0.49 1.3

Ni-Co/20CeAl 852.2 854.1, 856.5 777.6 779.2 781.2 1.5 0.85 3.09

Ni-Co/30CeAl 852.2 854.8, 857.6 777.9 - 780.5 3.17 0.91 7.49

3.3. Temperature ScreeningThe catalytic performance of each sample in the DRM reaction can be seen in Fig. 4. Ni-Co/Al and ceria-doped catalysts demonstrate superior CH4 and CO2 conversion compared with the unimproved Ni/Al catalyst, with noteworthy increases in conversion at lower temperatures. At temperatures above 700°C, there is negligible increase in catalytic activity of the improved catalysts over the monometallic one (Ni/Al). Interestingly, the Ni-Co/30Ce-Al displays higher CH4 and lower CO2

conversions than the Ni/Al, while all other catalysts demonstrate negligible deviation from the reference sample conversion at these temperatures. Hence, it may be inferred that the effects of Co and CeO2 promotion on catalysts on that improve the activity for the DRM reaction become less significant when approaching thermodynamic equilibrium.

Fig. 3 XPS analysis results for the 2p3/2 region of a) Ni and b) Co, c) the 3d region of Ce

(c)

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Below 650oC, the doped catalysts demonstrate clear improvements in reactivity over the unpromoted catalyst. The addition of Co improves both CH4 and CO2 conversion, as well as significantly increasing the H2/CO ratio. This is due to many factors that include promoting hydrogen and oxygen spillover transport between the active metal and support that prevents the build-up of carbon species, while enhancing hydrogen selectivity [67]. Cobalt has also been noted to have a useful affinity for oxygen species and when used in a suitable ratio, is instrumental in preventing coke formation [68].

Further analysis of Fig. 4b shows that increasing ceria content in the Ni-Co catalysts between 0 – 20 wt%, promotes CO2 conversion across the temperature range, most likely due to the redox reaction between CeAlO3 ↔ CeO2, which consumes CO2 to occur [41].Fig. 4 Performance of five catalysts under dry reforming conditions from 500 to 850oC with a WHSV

of 30,000 ml/(g-cat h), feed composition of 20/20/60 CH4/CO2/N2, a) CH4 conversion, b) CO2

conversion c) H2/CO ratio. The inset bar graphs in each figure are the corresponding data points at 500oC.

Fig. 4a reveals that CH4 conversion does not increase proportionally with increased ceria loading, as can be seen in the case of CO2 conversion. Instead, the Ni-Co/20Ce-Al catalyst demonstrates methane conversions between those of the Ni-Co/10Ce-Al catalyst and Ni-Co/Al catalysts below 650°C and exhibits no improvement over the standard Ni/Al catalyst above this temperature. This trend can be linked to the H2/CO ratio data seen in Fig. 4c to suggest that the non-ceria incorporated catalysts and

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especially the Ni-Co/30Ce-Al catalyst are more susceptible to the methane cracking reaction, which is known to produce C deposits along with high levels of H2.

The H2/CO ratio was slightly affected by ceria doping after the addition of cobalt, however the highest level of CeO2 doping further increased the H2/CO ratio, particularly between 700 and 800oC. This increase may be attributed to formation of cerium aluminate (CeAlO 3) during activation. However, above this temperature the H2/CO ratio rises above unity, which may be attributed to several contributing factors; competitive side reactions that occur with the production of water vapour, i.e bi- or steam reforming of methane, or this could be the result of CeAlO3 formation that promotes the formation of hydrogen [41] and the oxidation of CO to CO2 [37, 49]. This in conjunction with high levels of methane cracking explain the higher CH4 conversion, while also explaining the higher-than-unity H2/CO ratio of the Ni-Co/30Ce-Al, Ni/Al and Ni-Co/Al catalysts from 700-850oC. Evidence of this can be seen in Fig. 4a, where despite the fresh Ni-Co/30Ce-Al catalyst demonstrating the lowest surface area, this sample reported the highest methane conversions of all tested catalysts. Below 700oC however, the increase in H2/CO ratio indicates that the addition of cobalt improves the selectivity of the catalyst for DRM at lower temperatures, achieving an ideal syngas composition of 1:1 H2:CO at 600oC.

Post-reaction visual inspection of the Ni-Co/30Ce-Al catalyst revealed a large accumulation of solid carbon inside the reactor, clearly indicating the coke formation over this catalyst, most likely due to methane cracking reaction. This evidence suggests that the redox reaction between CeAlO3 ↔ CeO2

does not occur as prevalently over the Ni-Co/30Ce-Al material as over the Ni-Co/20&10Ce-Al catalysts further supporting the XPS conclusions that high Ce4+ presence does not serve to inhibit coke formation as effectively as Ce3+. Furthermore, without the CeAlO3 species, more Al3+ species remain unbound to ceria and thus increase the amount of Lewis acid sites that can catalyse cracking and polymerisation reactions among carbon species, resulting in harder, less easily oxidised carbon species [69].

3.4 Stability Test

Two catalysts, the Ni-Co/20Ce-Al and the reference Ni/Al, were each subjected to 72-hour reaction tests under DRM conditions at 550oC to evaluate their long-term stability (Fig. 5). At this temperature, thermodynamic analysis indicates that the exothermic Boudouard and methanation reactions (Eqns. 3-4) are thermodynamically favourable, which will lead to catalyst deactivation by coking as well reducing hydrogen production; reducing the H2/CO ratio [14] and catalytic performance.

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Fig. 5 Performance of improved Ni-Co/20Ce-Al (Red) and reference Ni-Al (Blue) catalysts over 70 h period at 550oC, with a WHSV of 30,000 ml/(gcat h) and feed composition of 1/1/3 CH4/CO2/N2

While the Ni/Al catalyst lost 10% conversion of both CO2 and CH4, the Ce-Al catalyst experienced no net change over the test duration. This clearly demonstrated the benefits of Co and ceria addition, as the Ni-Co/20Ce-Al catalyst successfully resisted deactivation effects caused by coking and sintering. Furthermore, the H2/CO ratio of the Ce-Al catalyst was consistently closer to unity than that of the Ni/Al catalyst, further evidencing the conclusions surrounding the involvement of CeAlO 3 species enhancing H2 production. Compared to a recently developed Ni-Sn/Al2O3-CeO2 catalyst for DRM with a similar level of ceria doping present in the support [33], our Nickel-Cobalt/Ceria-Alumina sample demonstrated far superior methane conversion stability and H2/CO ratio after 10 hours of reaction, while operating at a lower temperature. This proves that the combination of Co and Ce is a viable method to improve the long-term stability and selectivity of a nickel catalyst, especially at lower temperatures. The improved catalytic activity of the Ni-Co/20Ce-Al catalyst can also be linked to the predominant presence of Ce3+ within the structure, as discussed at length earlier in section 3.2, where the beneficial redox behaviour of Ce imbues the catalyst with anti-coking and enhanced CO 2

activation through acid-base interaction [70]. In any case the excellent stability and high conversion levels reached by the Ni-Co/20Ce-Al catalyst are highly commendable considering we are running DRM at very low temperature which is an enormous advantages from the process economy perspective.

3.5 XRD

Following XRD analysis (Fig. 6), it can be seen that as the number of compositional components and subsequent ceria loadings increase, the more complex the diffraction patterns become. In the calcined materials, the most common peaks throughout are those typical peaks of Al 2O3 (JCPDS No. 00-010-0173, 00-001-1303). The presence of Co-/Ni-Al2O4 spinel peaks can be clearly seen, confirmed by XPS, TPR, XRD reference cards (JCPDS No. 00-010-0339, 00-003-0896) and literature [30, 69] , throughout each sample, which serves to confirm the coexistence of this composite with Al2O3, owing to the Ni loading of our materials being well below the 33wt% threshold that would fully convert the Al2O3 into the spinels [71]. As can be expected for the CeO2 loaded samples (fig. 6c-e), the XRD clearly indicates the presence of the Ce fluorite phase (JCPDS No. 00-001-0800) [72].

It is important to study the changes in a material brought about by reduction, if such a treatment process is used before reaction, to provide significant insight into the surface chemistry. Following reduction, each material undergoes substantial changes to its surface structure. The most significant changes can be seen in the fig. 6c-e, where the CeO2 phase completely disappears from the Ni-Co/10Ce-Al material, decreases in intensity in the Ni-Co/20Ce-Al catalyst and finally, remains largely unchanged in the Ni-Co/30Ce-Al catalyst. The latter material however displays the formation of a new phase belonging to CeAlO3 (JCPDS No. 00-028-0260), which is in good agreement with the XPS and TPR data, as discussed earlier. That these peaks are not noted in the other two ceria containing materials perhaps suggests better dispersion or incorporation of this phase over or into these materials, as compared to the Ni-Co/30Ce-Al sample, which is also supported by the surface composition data presented in table 2. The reduced catalysts also detailed peaks characteristic of metallic face-centred cubic (fcc) Ni and Co (JCPDS No. 00-004-0850, 00-005-0727), confirming the presence of Ni and Co clusters in the reduced and spent catalysts. Following catalytic testing, the approximate crystallinity of these materials remained unchanged, indicating their resistance to sintering under DRM conditions.

325326327328329330331332333334335336337338339340341342343344345346

347348349350351352353354355356357358359360361362363364365366367368369370371372

Following reaction, all catalyst samples underwent post-mortem evaluation. Most notably, all materials have developed crystalline carbon as shown by the peak at ca. 26o, the approximate crystallite sizes for which can be found in table 3. The other notable development post-reaction again concerns the ceria loaded materials. While the CeAlO3 peaks remain undeveloped in the Ni-Co/10Ce-Al sample, the Ni-Co/20Ce-Al sample clearly demonstrates the peaks associated with this phase, suggesting the complete reduction and subsequent crystallisation of Ce (IV) to Ce (III) over the course of the reaction. The approximate crystallite sizes (Table 3) clearly show one of the benefits of cobalt inclusion, with the immediate reduction in carbon crystal size between the Ni/Al and Ni-Co/Al catalysts, due to the spillover mechanisms and oxygen affinity of cobalt discussed earlier. This is then compounded with the further benefit of ceria incorporation up to 20wt% and the effect that CeAlO 3

has on combating carbon formation, with the carbon crystal size increasing with ceria loading. The approximate crystal sizes of CeO2 are also useful to show the link between CeAlO3 formation and coking prevention. While it can be seen that Ceria containing catalysts shared a similar size of CeO 2

following calcination, this was found to sharply increase in the Ni-Co/30Ce-Al catalyst after reduction and reaction, most likely due to sintering of the material during reduction [73]. The subsequent reduction in crystallite size for this material following reaction (and the other two following reduction) can be attributed to the development of the CeAlO3 and the incorporation of Ce into the alumina structure.

373374375376377378379380381382383384385386387388389390391392393

Fig. 6 XRD patterns of the calcined, reduced and spent a) Ni/Al, b) Ni-Co/Al, c) Ni-Co/10Ce-Al, d) Ni-Co/20Ce-Al and e) Ni-Co/30Ce-Al

Table 3: Values for the CeO2 crystal size in calcined, reduced and spent CeO2 containing samples, carbon crystal size in all the spent catalysts as calculated using the Scherrer equation. Also included

are the Id/Ig ratios calculated from Raman spectroscopy data of the carbon region of each catalyst.

SampleCeO2 crystal diameter

(nm)Carbon crystal diameter

(nm)Carbon ID/IG ratio

Calcined Reduced SpentNi/Al - - - 13 1.12

Ni-Co/Al - - - 4 0.96Ni-Co/10Ce-Al 8 n/a n/a 4 1.16Ni-Co/20Ce-Al 9 7 n/a 3 0.83Ni-Co/30Ce-Al 9 45 39 7 0.42

394395396397398399

400

3.6 Raman SpectroscopyRaman spectroscopy was used to further analyse the nature of the carbon deposits evidenced by the XRD analysis. The Raman spectra (Fig. 7) reveal the presence of several different forms of carbon in the catalysts and confirm the differing nature of the deposits at different ceria doping levels.

Fig. 7 Raman spectra of improved catalysts post reaction

The most relevant trend in this diagram is that in the relative heights of the D and G band peaks at 1350 and 1585 cm-1, which are attributed to sp2-bonded carbon in graphitic structures. The D peak is associated with the defect density [74, 75] and the G peak indicating tangential C-C bond vibrations within crystalline carbon structures [76, 77]. Hence, the ratio of peak intensities, ID/IG is used as an approximate measure of the graphitic nature of these carbon deposits [33, 78].

From Table 3 it is apparent that the addition of ceria to the Ni-Co catalyst increases the crystallinity of the carbon deposits at either end of the range we investigated, whereas the carbonaceous deposits are comparatively “softer” with the median CeO2 loading (20%). This “softening” can be attributed to oxygen being more readily available on the surface and the active-support phase interaction sites to enable the oxidative effect oxygen has on graphitic carbon [79], the availability of which is greatly enhanced by Ce and Co and their redox chemistry. This data can also indicate an optimum surface oxygen availability at 10 wt% CeO2 concentration. Higher CeO2 doping levels significantly increases carbon crystallinity, with the 30 wt% Ce-Al catalyst showing very hard carbon with an ID/IG ratio of 0.42.

4. ConclusionsThe addition of CeO2 up to 20 wt% has been found to be an excellent strategy to boost the performance of a Ni-Co/Al2O3 catalysts for model biogas mixtures upgrading via DRM. High levels of CO2 and CH4 conversions are reached at low temperatures also maintaining an H2/CO ratio close to unity for long durations. When compared to standard Ni/Al catalysts the multicomponent systems containing Cobalt and Ceria present superior behaviour in terms of stability and activity – particularly

Intensity (a.u.)

Raman Shift (cm-1)

401402403404

405406407408409410411412413414415416417418419420421422423

424425426427428429

in the low temperature range. This is in fact one of the key findings of this paper, our Ni-Co/CeO 2-Al2O3 overcome the temperature limitations of traditional reforming catalysts displaying relatively high conversion levels in the so called “low-temperature” DRM range. This is a remarkable result which indicate the possibility of lowering the energy requirements for syngas production and CO 2

recycling through this route.

The formation of large CeAlO3 crystallites at CeO2 loadings above 20 wt% indicates an optimal loading of CeO2 within the support lattice; exceeding this limit appears to result in a low ratio of CeAlO3:CeO2, where a higher ratio has been demonstrated to be much efficacious. Although the Ni-Co/30Ce-Al catalyst details a marginal increase in CO2 conversion, which is thought to be linked to the formation of CeAlO3, this comes at the cost of excessive formation of hard carbon deposits on the catalyst and a shorter catalytic lifetime.

From the catalysts design perspective, this work has provided a first step towards the investigation of CeO2 loadings and the associated effects had upon not only catalytic performance, but also the chemical structure of the material itself. From the process side, the results presented in this paper showcase the possibility to conduct biogas upgrading in a very efficient manner using relatively low temperatures by implementing carefully engineered multicomponent catalysts in dry reforming units. The later may represent an appealing “waste to added value products” approach which showcases the essential role played by catalysis to facilitate the transition towards a low-carbon society.

Acknowledgements

Financial support for this work was provided by the Department of Chemical and Process Engineering at the University of Surrey and the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. LPP acknowledge Comunitat Valenciana for her APOSTD2017 fellowship. This work was also partially sponsored by the CO2Chem through the EPSRC grant EP/P026435/1.

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