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Catalysts 2016, 6(11), 170; https://doi.org/10.3390/catal6110170

Article
An Alumina-Supported Ni-La-Based Catalyst for Producing Synthetic Natural Gas
1
MERLIN Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
2
Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
*
Author to whom correspondence should be addressed.
Academic Editor: Rajendra S. Ghadwal
Received: 21 September 2016 / Accepted: 25 October 2016 / Published: 31 October 2016

Abstract

:
LaNi5, known for its hydrogen storage capability, was adapted to the form of a metal oxide-supported (γ-Al2O3) catalyst and its performance for the Sabatier reaction assessed. The 20 wt % La-Ni/γ-Al2O3 particles were prepared via solution combustion synthesis (SCS) and exhibited good catalytic activity, achieving a CO2 conversion of 75% with a high CH4 selectivity (98%) at 1 atm and 300 °C. Characteristics of the La-Ni/γ-Al2O3 catalyst were identified at various stages of the catalytic process (as-prepared, activated, and post-reaction) and in-situ DRIFTS was used to probe the reaction mechanism. The as-prepared catalyst contained amorphous surface La–Ni spinels with particle sizes <6 nm. The reduction process altered the catalyst make-up where, despite the reducing conditions, Ni2+-based particles with diameters between 4 and 20 nm decorated with LaOx moieties were produced. However, the post-reaction catalyst had particle sizes of 4–9 nm and comprised metallic Ni, with the LaOx decoration reverting to a form akin to the as-prepared catalyst. DRIFTS analysis indicated that formates and adsorbed CO species were present on the catalyst surface during the reaction, implying the reaction proceeded via a H2-assisted and sequential CO2 dissociation to C and O. These were then rapidly hydrogenated into CH4 and H2O.
Keywords:
Sabatier reaction; lanthanum nickel; catalysis

1. Introduction

Fossil fuels such as oil, coal, and natural gas have and continue to be the primary means employed to meet society’s energy needs. A key downside in relying on these carbon-rich compounds has been the accompanying increase in atmospheric CO2 levels, from ~290 ppm in pre-industrial days to ~395 ppm in 2013, which is seen as the major culprit behind climate change [1,2]. Growing awareness of the impact anthropogenic CO2 emissions have on global warming and the detrimental consequences further increases may bring has resulted in a need for reducing/capping these emissions [1,3,4]. The magnitude of the CO2 emissions calls for the application of a portfolio of strategies and technologies to approach the reduction/capping task, from which converting the otherwise wasteful CO2 into added value products including synthetic fuels, chemicals, and raw materials is particularly interesting assuming that the needed energy comes from renewables.
As a mitigation strategy, using CO2 as a feedstock is based on the possibility of storing this compound in manufactured products and therefore preventing it from being released into the atmosphere. Functionality, abundance, renewable nature, and environmental friendliness are some of the features that make CO2 an attractive C1 building block for synthesis products such as paraffin, dimethyl ether, methanol, higher alcohols, and formic acid [3,4]. Nowadays, CO2 is employed in the production of urea, salicylic acid, and polycarbonates at an industrial scale [5], consuming altogether ~10 Mt of CO2 y−1 as of 2007 [6]. However small this rate may seem compared to the ~35 BMt of CO2 y−1 emitted in 2013, it only accounts for those few applications which have commercial status and hence represents a minute portion of its real mitigating potential. One application which could be deployed at a scale that would significantly contribute to the reduction of CO2 emissions is the production of synthetic fuels through direct hydrogenation [3]. Holding great potential among the direct conversion processes is the Sabatier reaction (1):
CO2 + 4H2 → CH4 + H2O
The main advantage behind the Sabatier reaction is its favourable thermodynamics and theoretical capability to reach high conversions at ambient conditions [7]. However, the stability of the fully oxidised CO2 molecule and the eight-electron process its reduction entails set considerable kinetic limitations that hinder the performance of the process at mild operating conditions [3,4,7]. Thus, using a highly efficient catalyst is required to fully exploit the potential of the Sabatier reaction. Most of the research conducted in this field has dealt with the use of Ni-based catalysts [3,4,7,8,9,10,11], although other transition (e.g., Fe and Co) [9,12] and noble (e.g., Rh, Pt, Ru and Pd) [12,13,14,15,16] metal based systems have also proven active.
While more commonly associated with H2 storage applications, bulk LaNi5 has exhibited promising activity/selectivity for the Sabatier reaction [17,18,19]. For instance Ando et al. achieved CO2 conversions of 92% and methane selectivity of 98% (achieved after a 5 h induction period) for bulk LaNi5 (prepared by arc-melting) at 250 °C with a high reactants pressure of 5 MPa [18]. The bulk LaNi5 was far superior to a mixed Ni5LaOx oxide, prepared by co-precipitation, which exhibited 8% CO2 conversion after 5 h under the same reaction conditions.
The work reported here takes LaNi5 and adapts it to a metal oxide-supported (γ-Al2O3) catalyst system where its performance for the Sabatier reaction is assessed. Changes in the La-Ni/γ-Al2O3 catalyst structure invoked by the initial activation step and the ensuing reaction process were examined in detail. In-situ DRIFTS analyses were performed to establish the nature of the surface species generated over the course of the reaction and provide insights into the Sabatier mechanism.

2. Results and Discussion

2.1. As-Prepared Catalyst (P_LaNi/Al)

ICP–OES gave the La-Ni loading and La/Ni molar ratio in P_LaNi/Al to be 18.3 and 4.5 wt %, respectively. The XRD pattern of P_LaNi/Al, as shown in Figure 1, exhibited reflections similar to those found in the pristine support, which corroborated the presence of poorly crystalline γ-Al2O3 (JCPDS 086-1410) and revealed a signal (centred at 2θ = 43.6°) corresponding to the stainless-steel (JCPDS 033-0397) sample holders used in the analyses. No additional features were observed in the XRD pattern of the P_LaNi/Al sample, where the absence of La- and Ni-related reflections suggested that these components were present either as amorphous phases or as small crystalline structures indiscernible by this technique.
Considering the aforementioned, La was assumed to be present as highly-dispersed, amorphous La2O3 at the surface of the P_LaNi/Al. In contrast, nickel in P_LaNi/Al was presumed to have formed Ni-Al surface spinels since the occurrence of such species in Ni/Al systems is widely known [19,20,21,22,23,24]. According to Li et al. the absence of XRD lines corresponding to these surface spinels results from their short-range three-dimensional order, provided that Ni ions only diffused into the first few outer layers of the Al2O3 support during their 450 °C calcination stage [22].
SEM imaging of P_LaNi/Al (Figure 2a) revealed an irregular surface comprising superimposed slab-like features and sub-micrometric aggregates (ca. 200–600 nm) which were evenly distributed across the micron-sized catalyst particles. TEM analysis of P_LaNi/Al (Figure 2b) revealed the presence of small, evenly distributed particles with a diameter ranging from 1 to 6 nm (Table 1), which were discernible from the prevailing γ-Al2O3 support by their darker tone. These particles likely correspond to La- and Ni-containing deposits, but their exact make-up could not be defined.
The H2-TPR profile of P_LaNi/Al (Figure 3) revealed the presence of three reduction bands within two major reduction zones at temperatures across the range of 290–750 °C. The low-temperature zone between 290 and 425 °C contained a symmetrical band centred at 320 °C attributed to the reduction of amorphous NiO on the catalyst surface [22,25,26,27] since no nitrates from the SCS were expected to remain after calcination at 500 °C [28]. The peak at 583 °C was ascribed to octahedrally coordinated Ni2+ species (i.e., Ni O C 2 + ) embedded in the crystalline structure of the γ-Al2O3 support [24,27,28], which concurs with the formation of surface Ni-Al spinels. The small shoulder at 695 °C was assigned to the reduction of Ni2+ species, either tetrahedrally coordinated (i.e., Ni T 2 + ) in the crystalline structure of the support as per previous reports [22,27], or in La–Ni–Al containing species derived from the strong interaction between La and Ni ions [26]. The former interpretation stems from the widely accepted perception that Ni T 2 + species are more difficult to reduce than Ni O C 2 + ones [23,27], while the latter coincides with experimental observations from studies dealing with the promotion effects of La on Ni-based catalysts [26].
Additional H2-TPR analyses of Ni/Al (12.5 wt %) and La/Al (6.4 wt %) were conducted to assist with interpretation of the P_LaNi/Al profile (Figure 3). No reducible phases were observed in the La/Al sample, agreeing with the known irreducible nature of La oxides below 1000 °C [26,29]. However, the Ni/Al sample exhibited two reduction peaks including a broad shoulder-like feature at 365 °C and a peak centred at 583 °C. Similar to P_LaNi/Al, the low-temperature feature was attributed to surface NiO while the higher temperature peak was attributed to Ni O C 2 + species. The absence of a higher temperature shoulder, in conjunction with the absence of La in Ni/Al, infers that the 695 °C shoulder in the P_LaNi/Al H2-TPR profile originates from the reduction of La–Ni–Al species and not from Ni T 2 + species. Additionally, the more prominent bulk NiO peak exhibited by the P_LaNi/Al sample indicates that La promoted the formation of the bulk-like NiO on the γ-Al2O3 support.
The XPS spectrum of the P_LaNi/Al sample was examined at the overlapping Ni2p/La3d, C1s, Al2p, and O1s regions to establish the surface chemical characteristics of the as-synthesised material. Analysis of the C1s region of the P_LaNi/Al spectra gave a C/Al atomic ratio of 0.11 (Supplementary Materials, Figure S1; Table S1), indicating that no substantial residual carbon remained on the catalyst surface after synthesis.
Peaks relating to the Ni and La species were extracted from the de-convoluted spectra (Supplementary Materials, Figure S2) and are shown in Figure 4a,b, respectively, with values for the relative amounts of each species provided in Table 2. Within Figure 4a the peaks at 856.49 and 862.72 eV for P_LaNi/Al correspond to the primary and shake-up lines of octahedrally coordinated Ni ( Ni O C 2 + ) in nickel-alumina spinels, respectively [21,23,27,30]. According to Poncelet et al., enhanced reducibility in Ni/Al observed at high Ni loadings is associated with octahedral nickel ( Ni O C 2 + ) species which resemble NiO [23]. These species would account for the low temperature reduction peak in the P_LaNi/Al TPR profile, provided that no bulk NiO signals (i.e., ca. 854.24 eV) [26] were observed from the XPS spectrum. No signals attributable to Ni T 2 + (i.e., at 858.1 eV) [23] were observed in the P_LaNi/Al XPS spectra, corroborating the H2-TPR findings which suggested suppression of these species in the presence of La. The signals observed at 835.86 and 839.43 eV are attributed to the main and satellite peaks of La3+ from La2O3 dispersed on γ-Al2O3 [18,26,31].
The Al2p and O1s regions of the P_LaNi/Al XPS spectrum (Supplementary Materials, Figure S3) exhibited single peaks centred at 75.17 and 531.75 eV, respectively, which corresponded to the reported binding energies for Al2O3 [26,32]. These results diverge from the 74.0 eV (Al2p) and 830.8 eV (O1s) expected for bulk NiAl2O4 [33], implying the support surface remained essentially as alumina.

2.2. Reduced Catalyst (A_LaNi/Al)

The XRD pattern of the reduced catalyst (Figure 1) exhibited diffraction lines corresponding to the γ-Al2O3 support and the stainless-steel sample holder used for the analysis. Typical metallic nickel (JCPDS 001–1260) diffraction peaks were also observed with the Ni crystallite size estimated to be of 5 nm from the Scherrer equation (Table 1). No distinct LaNi5 diffraction lines were observed for the A_LaNi/Al material as LaNi5 formation from the corresponding metallic oxides has been reported to occur at temperatures >1000 °C [29,34,35].
SEM imaging of the A_LaNi/Al revealed a morphology akin to that of P_LaNi/Al. Clearly distinguishable nanoparticles were observable by TEM (Figure 2c), which were uniformly scattered across the γ-Al2O3 support and had diameters ranging between 4 and 20 nm. The majority of the deposits were observed to have diameters at the lower end of the size range, coinciding with the average Ni crystallite size determined from XRD. EDS elemental mapping (Supplementary Materials Figure S4) revealed even La and Ni dispersions across the catalyst surface, which suggests that the deposits comprise both Ni and La.
From the H2-TPR profiles (Figure 3), the conditions used to activate the catalyst (700 °C under pure H2) were expected to reduce all the nickel species present on Ni/Al and the majority of the nickel species on P_LaNi/Al. However, a minor portion of the nickel in the form of the La–Ni–Al component on P_LaNi/Al may have remained in an oxidised state. The average crystallite size of Ni and dispersion from the pulsed H2 adsorption experiments for the reduced catalysts are given in Table 1. Both the A_LaNi/Al and Ni/Al samples exhibited a relatively low degree of Ni dispersion (<2.6%) as can occur for catalysts with a high Ni content [26]. A slightly higher Ni dispersion and concomitant lower crystallite size were found for the A_LaNi/Al sample compared to the Ni/Al sample, providing additional evidence of the La having some impact in the interaction between the Ni and the support.
Considerable disagreement was found between the average crystallite size estimated via pulsed H2 chemisorption and the value derived from XRD (Table 1). The wide variation in average crystallite size cannot be solely attributed to uncertainties within the estimation techniques. A possible explanation for the difference may be via the occurrence of a Strong Metal Support Interaction-like (SMSI-like) effect between the dispersed La and Ni particles. That is, partially reduced LaOx species, originating from the well-dispersed La2O3 on the support, may begin to decorate the surface of metallic Ni crystallites during the reduction at 700 °C, hindering H2 adsorption at the blocked surface Ni sites and distorting the dispersion calculations. A depiction of the proposed SMSI-like effect is shown in Scheme 1a. An analogous SMSI-like effect was proposed by Zhang et al. who reported similar discrepancies in Ni crystallite size estimation for a Ni/La2O3 catalyst reduced under H2 at 500 °C [36]. Sánchez-Sánchez et al. also hinted at the decoration of metallic particles by dispersed species upon reducing a La-promoted Ni/Al catalyst at 700 °C [26].
Deconvoluted peaks for the Ni2p and La3d region of the A_LaNi/Al XPS spectrum are depicted in Figure 4a,b, respectively, with related information summarised in Table 2. Both the main and satellite Ni2p peaks were present at 854.24 and 860.95 eV, indicating the presence Ni2+ despite the catalyst undergoing a prior reduction step [21,26,27,30]. The presence of Ni2+ may derive from either: (i) La–O–Ni species at the interface between Ni crystallites and the LaOx decoration (potentially formed during the reduction process); or (ii) surface NiO formed during re-oxidation of the metallic Ni during sample transfer to the XPS equipment.
The La3d signal for A_LaNi/Al appeared at 835.53 eV, representing a small shift towards lower binding energies (compared to 835.9 eV for P_LaNi/Al) but remaining within the range acceptable for surface La2O3. The 0.33 eV shift towards lower binding energies supports the notion of LaOx decorating the Ni deposits as a 0.2 eV shift was reported by Fleisch et al. in a Pd/La2O3 system exhibiting SMSI effects [37]. Additionally, the increase in the La3d5/2 satellite-split between the P_LaNi/Al and A_LaNi/Al samples (i.e., 3.57 to 3.73 eV) may endorse the proposed SMSI-like effect as they observed an analogous increase (i.e., from 3.3 to 3.8 eV) after reduction at 500 °C [37].
As shown in Table 2, a decrease in relative Ni/Al atomic ratio between the A_LaNi/Al and P_LaNi/Al samples indicates a loss in exposed Ni during the reduction at 700 °C. In contrast, no variation was seen for the relative La/Al ratio between the two materials signifying that La remained exposed during the reduction step. This loss of exposed Ni and the retention of exposed La further support the perception that La is decorating the Ni deposits during the reduction process.

2.3. Spent Catalyst (S_LaNi/Al)

The XRD pattern of the S_LaNi/Al (Figure 1) exhibited the reflections associated with the catalyst alumina support and the stainless steel sample holders. Metallic Ni reflections were also observed in the S_LaNi/Al XRD pattern, with a small relative increase in signal intensity being discernible compared to the A_LaNi/Al counterpart. The increase in Ni diffraction peaks intensity may correspond to further reduction of remaining NiO during the catalytic process.
SEM imaging of the S_LaNi/Al sample (Figure 2d) revealed a surface mainly composed of aggregates (150–600 nm) and flake-like components evenly distributed across the catalyst particles. TEM micrographs (Figure 2e) revealed the presence of clearly discernible La/Ni particles with diameters between 4 and 9 nm (Table 1), uniformly distributed across the γ-Al2O3 support. None of the larger Ni deposit diameters (in the order of 20 nm) observed for A_LaNi/Al (Figure 2c) were readily observed for S_LaNi/Al suggesting a redistribution of the larger Ni deposits occurred during the reaction. EDS mapping (Figure 2f) confirmed that both La and Ni remained well-dispersed across the catalyst particle following reaction. No carbonaceous deposits were observed by TEM/SEM and associated EDS elemental mapping.
The impact of the Sabatier reaction on the Ni and La surface species, from XPS, is provided in Figure 4a,b, respectively, with associated details provided in Table 2. The Ni2p primary and satellite peaks for S_LaNi/Al occurred at 853.04 and 859.27 eV, respectively, which represented a shift towards lower binding energies compared to A_LaNi/Al and revealed the presence of surface metallic Ni [21,26]. No peaks corresponding to Ni2+ species were found for S_LaNi/Al, indicating that in-situ reduction of these species occurred during the course of the reaction. Abelló et al. reported a similar in-situ reduction in their study of a mixed-oxide NiO/Al2O3 catalyst for the Sabatier reaction [38]. They concluded that the Ni2+ that was reduced during reaction corresponded to NiO which had remained unreduced after the H2 activation stage owing a certain level of sintering. In the case of S_LaNi/Al, though, the Ni2+ reduced in-situ is suspected to either comes from the NiO surface layer on the deposits and/or the possible La–O–Ni species observed in A_LaNi/Al. Reduction of surface NiO is supported by the small increase in intensity observed by XRD (Figure 1). The alternate hypothesis of La–O–Ni reduction is supported by a shift in the La3d peaks to 835.86 and 839.50 eV (Figure 4b), reverting to values similar to those observed for the P_LaNi/Al. This includes the satellite split which returned to a value of 3.57 eV. Accordingly, these results indicate that La is present as La2O3 after the reaction and provide evidence towards the disintegration of the proposed LaOx decorations during reaction in agreement with the re-dispersion of the larger Ni particles observed by TEM (Figure 2b,c,e). A small decrease in the La/Al atomic ratio is also noted for S_LaNi/Al when compared to A_LaNi/Al (0.022 to 0.020, Table 2) and this also corroborates with the hypothesis of a disintegration of the LaOx decorations, as previously hypothesized. No major changes were observed in the binding energies of the S_LaNi/Al XPS C1s components compared to those observed in P_LaNi/Al and A_LaNi/Al, (Supplementary Materials, Table S1). A depiction of the SMSI-like effect breakup leading to the re-dispersion of La and Ni at the surface of γ-Al2O3 is shown in Scheme 1a.

2.4. Catalyst Performance

Performance of the A_LaNi/Al catalyst for the Sabatier reaction over the range 200–400 °C is shown in Figure 5a in terms of CO2 conversion and CH4 selectivity. CO2 conversion increased from <3% at 200 °C to ~80% at 350 °C. Selectivity towards CH4 remained constant across the entire temperature range at values >98%. CO was the only secondary product detected from the hydrogenation reaction. Stability tests on A_LaNi/Al were conducted at 275 °C (Figure 5b) whereby, once stabilised, the CO2 conversion remained steady at 65% with 98% selectivity towards CH4 (pressure = 1 atm, H2:CO2 ratio = 4:1, GHSV = 7.2 × 104 mL·h−1·g−1 cat., 20 wt % Ni loading). By comparison: (i) Ando et al. reported a 56% CO2 conversion with 98% CH4 selectivity at 300 °C for a neat LaNi5 catalyst prepared by arc melting (pressure = 1 atm, H2:CO2 ratio = 4:1, WHSV = 3000 mL·h−1·g−1 cat.) [18]; (ii) Liu et al. reported a 45% CO2 conversion with 99% CH4 selectivity at 300 °C for a Ni/Al2O3 catalyst prepared by impregnation (pressure = 1 atm, H2:CO2 ratio = 4:1, WHSV = 15,000 mL·h−1·g−1 cat., 15 wt % Ni loading) [39]; and (iii) Abelló et al. reported a 8% CO2 conversion with 95% CH4 selectivity at 300 °C for a Ni6Al (reduced) catalyst prepared by co-precipitation (pressure = 5 atm, H2:CO2 ratio = 4:1, WHSV = 26.88 × 105 mL·h−1·g−1 cat., 69.1 wt % Ni loading) [38]. The stability profile also showed that an initial induction period (~90 min) was needed prior to the catalyst reaching a stable and maximum activity. Although no direct evidence is available at this time, the induction period may account for the transformation of the LaOx-decorated NiO into the reduced Ni/La2O3 form, which could be responsible for the higher catalyst activity. Irrespective of this and to our best knowledge, the P_LaNi/Al catalyst shows good performances as compared to other Ni-Al-based catalysts at a moderate temperature and ambient pressure.
The performance of the La-Ni/γ-Al2O3 catalyst prepared via SCS is postulated to derive from a combination of the SMSI-like effect and the physical-chemical promotional impact of La itself. According to the former, the formation of LaOx decorations provided the catalytic system with La–O–Ni at the interface with the metallic Ni crystallites, which have been proposed to be highly active for the Sabatier reaction [40]. On the other hand, the promoting effects of La would be responsible for quicker/improved CO2 dissociative adsorption due to its higher basicity [17,41,42,43] and better dispersion/stabilisation of the active Ni phase during preparation and reaction [27,44,45]. Interestingly, the post-reaction state of Ni and La in the catalyst differed to the activated.

2.5. Reaction Routes

An example of in-situ DRIFTS analysis profile for the La-Ni/γ-Al2O3 catalyst immediately following a H2 pulse is provided in Figure 6a. The spectrum displays multiple bands between 1050 and 3010 cm−1. The different species found at the surface of the catalyst during reaction include: La2O2CO3 (1050 cm−1) [46], unidentate formates on the Ni surface (1305 and 1607 cm−1) [47], formates on the La2O3 surface (1350 and 1500 cm−1) [40], bidentate formates on the γ-Al2O3 support (1375, 1395, 1590, and 3010 cm−1) [47], adsorbed CO on Ni in bridge configuration (1860 and 1910 cm−1) [47,48], adsorbed CO on Ni in linear configuration (2030 cm−1) [47,48], and adsorbed/gaseous CO2 (2320 and 2360 cm−1). Conversion of CO2 into CH4 during reaction was corroborated by the presence of the characteristic multi-peak bands observed at ca. 3100 and 1400 cm−1 with a strong peak at 3020 cm−1 corresponding to the Q-band of gaseous CH4, while similar signals between 1200 and 2200 cm−1 indicated the presence of water.
The evolution of surface species during the Sabatier reaction (400 °C and 0.1 MPa) is displayed by the consecutive in-situ DRIFTS spectra in Figure 6b. The monitored reflectance bands were those corresponding to unidentate formates over Ni, adsorbed CO on Ni in bridged configuration, adsorbed/gaseous CO2, and bidentate formates on the γ-Al2O3 support. Results showed that gaseous/adsorbed CO2 bands appeared right after the injection of H2, indicating that H2 preferentially adsorbed onto the catalyst and released part of the pre-adsorbed CO2. Bands corresponding to water, methane, and formates on both Ni and the support were detected early within the reaction process (i.e., at t1 = 3 min). As reaction time elapsed, the higher-amplitude signal corresponding to CO adsorbed on Ni in bridge configuration (1860 cm−1) and the formates on the γ-Al2O3 support (3010 cm−1) gradually disappeared. The Ni-bound formates disappeared quicker than those bound to the support, and were indiscernible after 9 min (t3). The bands corresponding to the Ni adsorbed linear CO (2030 cm−1) and the lower-amplitude bridged CO (1910 cm−1), which were apparent 3 min after the first H2 pulse (t1), were not present after 6 min (t2). After 15 min (t5), some CO2-related species remained adsorbed on the catalyst since the band at 2360 cm−1 was still present. After a second H2 injection, the whole cycle was repeated and the presence of CO2, remaining since the first H2 injection, was corroborated.
On studying the hydrogenation of CO2 into CH4 over Ni/Al using FTIR and TPR, Fujita et al. identified different types of CO adsorbed on Ni in bridged position, proposing that the higher-amplitude band (i.e., 1860 cm−1) corresponded to a strongly bound CO species while the lower-amplitude band (i.e., 1910 cm−1) was related to weakly bound CO [47]. Additionally, formates present on the support and adsorbed CO species were presumed to undergo hydrogenation into CH4 through a mechanism which involved further dissociation into adsorbed carbon, allowing for the amounts of adsorbed carbon and linear CO species to be negligible during reaction.
The results from in-situ DRIFTS appear to corroborate the presence and hydrogenation of formates and adsorbed CO strongly bound in a bridged configuration. Based on the findings by Fujita et al. [47] and accounting for the possible effects of LaOx decorations on the catalyst as found in this work, a mechanism is proposed for the Sabatier reaction as follows: (a) dissociative adsorption of H2 into Hatomic on Ni sites; (b) adsorption and hydrogen assisted cleavage of CO2 on Ni sites; (c) dissociative adsorption of CO2 onto LaOx decorations; (d) cleavage of strongly adsorbed CO into adsorbed O and C; (e) hydrogenation of LaOx-adsorbed CO over highly active La–O–Ni at the interface with the Ni crystallites; and (f) instantaneous hydrogenation of adsorbed C and O from adsorbed CO and formates into CH4 and H2O, respectively. The final step is consistent with the negligible carbon deposition on the S_LaNi/Al sample from XPS and TEM/SEM. A depiction of the previous reaction steps and the catalytic system is provided in Scheme 1b, where Ni- and γ-Al2O3-bound formates are represented since in-situ DRIFTS revealed their presence.

3. Materials and Methods

3.1. Catalyst Preparation

The Ni-La/γ-Al2O3 catalyst was prepared via solution combustion synthesis (SCS) using the corresponding metallic nitrates as oxidisers and glycine as the fuel/complexing agent [40]. A nickel nitrate (Ni(NO3)2·6H2O, Ajax Finechem Pty. Ltd., Sydney, Australia), lanthanum nitrate (La(NO3)3·6H2O, Merck KGaA, Sydney, Australia), and glycine (C2H5O2N, Ajax Finechem Pty. Ltd., Sydney, Australia) solution was prepared at a fixed 1:5:6 molar ratio using deionised water as the solvent and keeping the overall combusting mixture under 500 mg. Once the nitrates and glycine had been dissolved, sufficient γ-Al2O3 (particle size 45–53 μm, Saint-Gobain NorPro, Japan) was added to the solution to give a nominal 20 wt % Ni-La loading on the final catalyst. The resulting slurry was stirred for 30 min and allowed to settle for a further 30 min after which it was transferred into a ceramic crucible and placed in a temperature controlled (±1 °C, FP21 Shimade Co., Ltd., Tokyo, Japan) muffle furnace. The SCS mixture was heated in the presence of air at 5 °C·min−1 from room temperature to 500 °C, calcined for 1 h, and then allowed to cool to room temperature. Control samples comprising 6.4 wt % La on γ-Al2O3 (La/Al) and 12.5 wt % Ni on γ-Al2O3 (Ni/Al) were also prepared using SCS.

3.2. Catalyst Characterisation

The La-Ni/γ-Al2O3 catalyst was characterised following preparation (P_LaNi/Al), activation (A_LaNi/Al) and post-reaction (i.e., spent following the Sabatier reaction at 400 °C for 3 h, S_LaNi/Al). P_LaNi/Al composition was determined by ICP-OES on a Perkin Elmer OPTIMA 7300 (Perkin Elmer, Sydney, Australia). Prior to analysis a small amount of the material was digested in concentrated nitric acid (70 vol % HNO3, Ajax Chem, Sydney, Australia) for 8 h at 80 °C and then diluted with de-ionised water.
Crystallinity of P_LaNi/Al, A_LaNi/Al and S_LaNi/Al was studied by X-ray Diffraction (XRD), using a Philips PANalytical X’pert Multipurpose XRD System (Phillips, Sydney, Australia) equipped with a monochromated Cu-Kα source (λ = 1.5418 Å), and operating at 45 kV and 40 mA.
Hydrogen temperature programmed reduction (H2-TPR) analyses were performed on a Micromeritics Autochem II 2920 (Micromeritics, Sydney, Australia) to assess the reducibility of P_LaNi/Al catalyst as well as the control materials. Materials were pre-treated by flowing argon through the bed at 120 °C for 30 min and then cooled to 50 °C. Then, 10 vol % H2/Ar was passed over the catalyst bed while the temperature was increased at 5 °C·min−1 from 50 to 800 °C.
Pulsed H2 chemisorption assays were conducted to estimate the Ni dispersion and crystallite size for P_LaNi/Al and Ni/Al using a Micromeritics Autochem II 2920. The materials were initially pre-treated by flowing argon through the bed at 120 °C for 30 min after which they were cooled to room temperature. Pure hydrogen was passed through the sample bed as the temperature was increased at 10 °C·min−1 up to 700 °C where it was held for 5 h. The system was subsequently flushed with argon for 2 h at 700 °C and cooled to room temperature. Consecutive volumetric pulses (0.4664 mL) of 10 vol % H2/Ar were then dosed to the sample every 1.5 min. Ni dispersion and crystallite size were estimated assuming an atomic ratio of 1 between adsorbed atomic hydrogen and surface Ni at room temperature [20,36,49] and by following the procedure proposed by Bartholomew et al. [50].
Scanning Electron Microscopy (SEM) analyses were carried out using a FEI Nova Nano-SEM 230 FESEM instrument (FEI, Sydney, Australia), while Transmission Electron Microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) studies were performed on a Philips CM200 FEG TEM (Phillips, Sydney, Australia) equipped with an interfaced EDAX system (Phillips, Sydney, Australia) and operated at 200 keV.
Chemical characteristics and relative atomic compositions of the P_LaNi/Al, A_LaNi/Al and S_LaNi/Al catalyst surfaces were studied by XPS using a Thermo ESCALAB250i XPS instrument (Thermo Scientific, Sydney, Australia) equipped with a monochromated Al-Kα X-ray source and powered at 13 kV/12 mA. Adventitious hydrocarbon, C1s (285.0 eV), was used as the binding energy (BE) reference. Air sensitive samples were prepared in a glove box (LC Technology, Salisbury, USA) under high purity argon immediately prior to XPS analysis to minimise the potential of oxidation.

3.3. Catalytic Assessment

Catalytic activity tests for the Sabatier reaction were conducted in a continuous-flow fixed-bed reactor (6 mm ID quartz tube, (CGB, Sydney, Australia)) equipped with both temperature and mass flow controllers (H2 and N2: Brooks Sydney, Australia; CO2: Aalborg, Sydney, Australia). Prior to each catalytic test, 0.05 g of the catalyst was reduced in-situ for 3 h using 50% hydrogen in nitrogen at 60 mL·min−1 and 700 °C. A stoichiometric mixture of H2/CO2 (i.e., 4:1 molar ratio) was fed into the system at a 7.2 × 104 mL·h−1·g−1 cat. GHSV, with hydrogen and carbon dioxide (Coregas, Sydney, Australia) flowing at 40 and 10 mL·min−1·g−1 cat. STP, respectively. Reaction temperatures were varied between 150 and 450 °C. The product gas was analysed with an on-line gas chromatograph (Shimadzu GC-2010 Plus, Shimadzu, Sydney, Australia) equipped with a TCD and Agilent CarboPlot P7 column (Agilent, Sydney, Australia), and an FID and methaniser, and used argon (Coregas, Sydney, Australia) as the carrier gas. The reaction was then stopped, the reactor cooled under flowing H2/CO2, and the spent catalyst samples collected/stored in sealed Ar-filled vials for further analysis. Catalyst stability was also assessed by running the reaction under the conditions described above at 275 °C for 5 h.
The evolution of surface species during CO2 hydrogenation was followed using in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) (Harrick, New York, NY, USA). Prior to measurements, the catalyst was activated in-situ as per conditions above. Spectra were collected using a Brüker VERTEX 70v FTIR spectrometer (Bruker, Sydney, Australia) and equipped with a commercial in-situ DRIFTS cell (Harrick’s Scientific, Ithaca, NY, USA). For each analysis, approximately 20 mg of the activated catalyst (A_LaNi/Al) was diluted with synthetic mono-crystalline diamond powder (~1 μm, Sigma-Aldrich, Sydney, Australia). Operating conditions within the sample cell were set at 400 °C and 0.1 MPa. Prior to analysis, the cell was flushed with argon for 30 min at 150 °C. Carbon dioxide was then fed for 30 min into the reaction chamber at 400 °C and allowed to adsorb onto the activated catalyst. The pre-adsorbed CO2 was reacted with H2 by dosing ~20 mL injections every 3 min and the evolution of surface species during reaction was followed.

4. Conclusions

A LaNi/γ-Al2O3 catalyst, synthesised using SCS, exhibited good activity for the Sabatier reaction at atmospheric pressure, reaching CO2 conversions of ~80% at temperatures of 350 °C and beyond and a CH4 selectivity consistently >98%. Characterisation of the LaNi/γ-Al2O3 catalyst at various stages of the reaction process (i.e., as-prepared, activated and post-reaction) demonstrated the La and Ni existed in different forms at each stage. La and Ni in the as-prepared catalyst were observed to be present as amorphous surface La–Ni spinels. The activation process did not reduce the Ni as anticipated, rather it appeared to promote the formation of oxidised Ni particles (i.e., Ni2+) decorated with LaOx moieties. The reaction conditions led to the in-situ reduction of the Ni (to give metallic Ni deposits) with the LaOx species returning to an oxidation state (La2O3) similar to that observed for the as-prepared catalyst. At the interface between LaOx decorations and the Ni crystallites, La–O–Ni species may be present which could potentially be accountable for the catalyst activity, although the change in catalyst characteristics during the reaction in conjunction with the stable activity of the catalyst imply that the activated catalyst may be an intermediary state of the ultimately active material. In-situ DRIFTS analyses revealed the presence of adsorbed formates and adsorbed CO species during reaction at 400 °C. The results suggested that the reaction advanced through a CO-based mechanism involving CO2 hydrogenation, although the pathway involving the hydrogenation of formates cannot be excluded. The findings demonstrate the capacity for hydrogen storage based materials, such as LaNi alloys, to behave as active catalysts for the Sabatier reaction in this instance, and potentially for other hydrogenation reactions in general, when loaded on a metal oxide support.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/6/11/170/s1, Figure S1: X-ray Photoelectron (XPS) spectra and curve fitting of P_LaNi/Al, A_LaNi/Al and S_LaNi/Al at the C1s region, Figure S2: X-ray photoelectron spectra and curve fitting of P_LaNi/Al, A_LaNi/Al and S_LaNi/Al at the Ni2p and La3d region, Figure S3: X-ray photoelectron spectra of the P_LaNi/Al at the (a) O1s and (b) Al2p regions, Figure S4. TEM images and (f) associated elemental mapping of A_LaNi/Al, Table S1: Carbon (C1s) binding energies (BEs) of core electrons, surface species and relative Ci/Al atomic ratio for the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al.

Acknowledgments

The authors would also like to acknowledge the use of facilities within the UNSW Mark Wainwright Analytical Centre and Bill Bin Gong for assistance with the XPS analyses, also located within the UNSW Mark Wainwright Analytical Centre.

Author Contributions

Daniel E. Rivero-Mendoza carried out all the experimental work which was conceived and designed with Jason Scott and Kondo-François Aguey-Zinsou. Jessica N. G. Stanley, conducted the stability test. All contributed to the writing of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of the pristine γ-Al2O3 and the La-Ni/γ-Al2O3 catalyst as-prepared via SCS (solution combustion synthesis) (P_LaNi/Al), reduced under H2 at 700 °C (A_LaNi/Al), and after 3 h of CO2 hydrogenation at 400 °C (S_LaNi/Al).
Figure 1. XRD patterns of the pristine γ-Al2O3 and the La-Ni/γ-Al2O3 catalyst as-prepared via SCS (solution combustion synthesis) (P_LaNi/Al), reduced under H2 at 700 °C (A_LaNi/Al), and after 3 h of CO2 hydrogenation at 400 °C (S_LaNi/Al).
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Figure 2. (a) SEM; and (b) TEM images of the La-Ni/γ-Al2O3 catalyst as-prepared via SCS (P_LaNi/Al); (c) reduced under H2 at 700 °C (A_LaNi/Al); and (d) SEM; (e) TEM images and (f) associated elemental mapping after 3 h of CO2 hydrogenation at 400 °C (S_LaNi/Al). Black arrows indicate La- and/or Ni particles.
Figure 2. (a) SEM; and (b) TEM images of the La-Ni/γ-Al2O3 catalyst as-prepared via SCS (P_LaNi/Al); (c) reduced under H2 at 700 °C (A_LaNi/Al); and (d) SEM; (e) TEM images and (f) associated elemental mapping after 3 h of CO2 hydrogenation at 400 °C (S_LaNi/Al). Black arrows indicate La- and/or Ni particles.
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Figure 3. H2-TPR profiles of La deposited on γ-Al2O3 (La/Al), Ni deposited on γ-Al2O3 (Ni/Al), and the as-prepared La-Ni/γ-Al2O3 (P_LaNi/Al).
Figure 3. H2-TPR profiles of La deposited on γ-Al2O3 (La/Al), Ni deposited on γ-Al2O3 (Ni/Al), and the as-prepared La-Ni/γ-Al2O3 (P_LaNi/Al).
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Figure 4. Deconvoluted XPS spectra for (a) Ni and (b) La species on the surface of the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
Figure 4. Deconvoluted XPS spectra for (a) Ni and (b) La species on the surface of the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
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Scheme 1. Representation of the: (a) evolution of the La-Ni/γ-Al2O3 catalyst structure driven by SMSI-like (Strong Metal Support Interaction-like) effects promoted by La2O3; and (b) proposed catalytic reaction mechanism for CO2 hydrogenation into CH4.
Scheme 1. Representation of the: (a) evolution of the La-Ni/γ-Al2O3 catalyst structure driven by SMSI-like (Strong Metal Support Interaction-like) effects promoted by La2O3; and (b) proposed catalytic reaction mechanism for CO2 hydrogenation into CH4.
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Figure 5. CO2 conversion and CH4 selectivity for the Sabatier reaction of A_LaNi/Al at: (a) temperatures ranging between 200 and 400 °C; and (b) at a constant temperature of 275 °C for 5 h.
Figure 5. CO2 conversion and CH4 selectivity for the Sabatier reaction of A_LaNi/Al at: (a) temperatures ranging between 200 and 400 °C; and (b) at a constant temperature of 275 °C for 5 h.
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Figure 6. In-situ DRIFTS spectra of the A_LaNi/Al catalyst immediately following pulsed H2 injection at 400 °C and 0.1 MPa over pre-adsorbed CO2: (a) detailed account of the surface species present. LaC corresponds to La2O2CO3, COOHU,Ni to unidentate formate on Ni, COOHLa to formate on La2O3, COOHB,Sup to bidentate formate on the γ-Al2O3 support, COa,Br-Ni to adsorbed CO2 on Ni in a bridged configuration, and COa,L-Ni to adsorbed CO2 on Ni in a linear configuration; and (b) evolution of surface species as a function of time. The spectra correspond to the raw data and the “negative” features, e.g., at 15 min, are related to the slight differences between the diamond powder background and the spectrum of the catalyst diluted in diamond powder.
Figure 6. In-situ DRIFTS spectra of the A_LaNi/Al catalyst immediately following pulsed H2 injection at 400 °C and 0.1 MPa over pre-adsorbed CO2: (a) detailed account of the surface species present. LaC corresponds to La2O2CO3, COOHU,Ni to unidentate formate on Ni, COOHLa to formate on La2O3, COOHB,Sup to bidentate formate on the γ-Al2O3 support, COa,Br-Ni to adsorbed CO2 on Ni in a bridged configuration, and COa,L-Ni to adsorbed CO2 on Ni in a linear configuration; and (b) evolution of surface species as a function of time. The spectra correspond to the raw data and the “negative” features, e.g., at 15 min, are related to the slight differences between the diamond powder background and the spectrum of the catalyst diluted in diamond powder.
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Table 1. Average particle and Ni crystallite sizes (including degree of Ni dispersion) as estimated from TEM, XRD and pulsed H2 chemisorption for the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
Table 1. Average particle and Ni crystallite sizes (including degree of Ni dispersion) as estimated from TEM, XRD and pulsed H2 chemisorption for the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
MaterialsParticle Size (nm)Ni Crystallite Size (nm)
TEMXRDH2 Pulse (% Disp.)
P_LaNi/Al1–6--
A_LaNi/Al4–20537 (2.58)
S_LaNi/Al4–96-
Ni/Al--41 (2.36)
Table 2. Binding energies (BE) of core electrons, surface species and relative atomic ratios for Ni and La in the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
Table 2. Binding energies (BE) of core electrons, surface species and relative atomic ratios for Ni and La in the P_LaNi/Al, A_LaNi/Al, and S_LaNi/Al materials.
MaterialsNi2p3/2 (Sat)La3p5/2 (Sat)
BE (eV)SpeciesNi/AlBE (eV)SpeciesLa/Al
P_LaNi/Al856.49Ni-Al surface spinels0.081835.86La2O30.022
862.72839.43
A_LaNi/Al854.24Ni2+0.028835.53LaOx0.022
860.95839.26
S_LaNi/Al853.04Metallic0.029835.93La2O30.020
859.27Ni839.50
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