Nickel Phosphide Catalysts as Efficient Systems for CO₂ Upgrading via Dry Reforming of Methane

Abstract

This work establishes the primordial role played by the support’s nature when aimed at the constitution of Ni₂P active phases for supported catalysts. Thus, carbon dioxide reforming of methane was studied over three novel Ni₂P catalysts supported on Al₂O₃, CeO₂ and SiO₂-Al₂O₃ oxides. The catalytic performance, shown by the catalysts’ series, decreased according to the sequence: Ni₂P/Al₂O₃ > Ni₂P/CeO₂ > Ni₂P/SiO₂-Al₂O₃. The depleted CO₂ conversion rates discerned for the Ni₂P/SiO₂-Al₂O₃ sample were associated to the high sintering rates, large amounts of coke deposits and lower fractions of Ni₂P constituted in the catalyst surface. The strong deactivation issues found for the Ni₂P/CeO₂ catalyst, which also exhibited small amounts of Ni₂P species, were majorly associated to Ni oxidation issues. Along with lower surface areas, oxidation reactions might also affect the catalytic behaviour exhibited by the Ni₂P/CeO₂ sample. With the highest conversion rate and optimal stabilities, the excellent performance depicted by the Ni₂P/Al₂O₃ catalyst was mostly related to the noticeable larger fractions of Ni₂P species established.

1. Introduction

The reduction of atmospheric carbon dioxide, and thus the greenhouse effect, is one of the defining scientific and engineering challenges of our time. Progress is being made in reducing carbon dioxide emissions, and technologies such as carbon capture and storage are available [1]. However, the sequestering of carbon dioxide in oceans or mineral formations is expensive and provides no useful end product. One technology with great potential in this field is the dry reforming of methane (DRM, CO₂ + CH₄ = 2CO + 2H₂) to produce a mixture of carbon monoxide and hydrogen—syngas. Dry reforming was first studied by Fischer and Tropsch as early as 1928 [2], and today attracts ever increasing interest due to the rising importance of CO₂ mitigation. The advantages of the dry reforming of methane are two-fold. Firstly, methane is also a potent greenhouse gas, so the reduction of this pollutant in the atmosphere is highly desirable. Secondly, the product syngas, as well as being a viable fuel for internal combustion [3], is an important feedstock for the production of higher hydrocarbon fuels through the Fischer Tropsch process [4] and for methanol production [5].

The DRM reaction is endothermic, requiring low pressure and high temperatures, above 600 °C according to thermodynamics, to progress at a good rate [6]. A significant amount of research has focused on noble metal catalysts. These catalysts often present far superior resistance to deactivation compared to transition metals. Still, the high cost of noble metals has driven research towards cost-effective formulations, usually based on transition metals such as Cu, Fe and Ni [2]. For traditional supported catalysts, deactivation issues related to carbon deposits, metal sintering and the oxidation of active species have been reported [7]. Compared to Fe and Co systems, relatively higher coking endurances coupled to fair reaction rates have been described for Ni catalysts. Over the past few decades, a substantial body of research has focused on reducing the deactivation of nickel catalysts by employing different metal oxide supports and metal promoters [1,2,6]. On developing effective catalysts, the rapid surface decomposition of methane, coupled with low tendencies towards constituting carbon deposits, potentially blocking the active sites, are generally intended features. On this premise, competitive catalytic systems have been reported. For instance, the Ni-Sn/CeO₂-Al₂O₃ catalyst combined the higher reaction rates depicted by Ni-Sn alloys with the benefits provided by oxygen vacancies of cerium dioxide support [8]. It is known that the support strongly determines the performance displayed by dry reforming catalysts, the metal support interfaces being described as powerful actives sites. Depositing catalytic material on a porous support also increases the metal exposed area whilst the acid-base support promotes CH₄/CO₂ activation, thus favouring the reaction rate [2]. For DRM reactions, it is known that relatively acidic supports such as SiO₂ favour the cracking of methane, which is usually considered as a rate limiting step. On the contrary, basic surfaces promote the activation of CO₂ molecules. In fact, oxygen vacancies present in partially reduced oxides have also been stated as effective sites towards CO₂ dissociative reductions [9]. Nevertheless, the acid base properties could also determine the extent of carbon deposits constituted over the catalyst surface [6].

When seeking novel catalyst formulations, transition metal carbides and phosphides have been proposed as appealing catalytic materials for a number of catalytic reactions, such as WGS and CO hydrogenation [10,11]. Among them, Mo₂C catalysts are suggested as appealing alternatives, even being described as Pt-like materials [12,13]. In this context, Yao et al. [14] proposed molybdenum phosphide as a highly active and stable catalyst for dry reforming. Likewise, Guharoy et al. [15] proposed, via Density Functional Theory (DFT) calculations, nickel phosphide structures as active sites for Reverse Water Gas Shift (RWGS) reaction on the basis of the potential surface energies estimated for different reaction intermediates. Although scarcely analysed for CO₂ reduction processes, nickel phosphide clusters have proven successful as a catalyst for the hydrotreating of species such as thiophene [11]. For Ni₂P surfaces, the strong oxygen interaction has been also related to promoted Water Gas Shift reaction rates [16] and hindered deactivation by coking in dry reforming applications [17]. Furthermore, the low barrier energy towards H₂ dissociation, along with the thermodynamic stability of H species over Ni₂P surfaces under hydrogen rich environments, suggest Ni₂P as an active phase for DRM reactions [18].

Considering the promising prospect envisaged for Ni₂P active sites, this work aims to establish the influence of the support on Ni₂P catalysts for DRM reactions. For that purpose, and in order to cover a wide range of chemical properties, 20% Ni₂P was impregnated over Al₂O₃, SiO₂-Al₂O₃ and CeO₂ supports. From the obtained characterisation and activity outcomes, Ni₂P/Al₂O₃ catalysts could lead to a viable industrial CO₂ recycling process, with an impact on the pressing problem of rising atmospheric CO₂ levels.

2. Results and Discussion

2.1. Structural Characterization of the Samples

The textural properties of the as-prepared mesoporous samples were evaluated by N₂-physisorption. Figure 1 displays the N₂ adsorption desorption hysteresis exhibited by the catalysts’ series. According to IUPAC (International Union of Pure and Applied Chemistry) classification, the Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples presented a type H4 hysteresis shape characteristic of mesoporous materials. Considerably different hysteresis was noticed for the Ni₂P/CeO₂ system, which presented type H3 isotherm shapes usually associated to aggregated particles.

Table 1. Textural properties of the prepared catalysts.

Support	Surface Area (m2/g)	Dpore (nm)	Vpore (cm3/g)
Al2O3	202	7.4	0.51
SiO2-Al2O3	420	8.5	0.80
Ni2P/Al2O3	161	6.8	0.41
Ni2P/CeO2	16	2.2	0.09
Ni2P/SiO2-Al2O3	226	6.8	0.58

summarises the results of the BET and BJH analysis. Compared to Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ catalysts, significantly lower surface areas were discerned for the Ni/CeO₂ catalyst. Besides, the metal incorporation resulted in decreased surface areas and pore volumes for all the catalysts’ series. The lower surface and pore volumes observed with respect to the supports are most likely associated with the presence of non-porous Ni particles which, at the same time, might be partially blocking the surface pores [19].

Figure 2 displays the diffractograms obtained for the as-prepared samples where γ-Al₂O₃ support (JCPDS No. 29-0063) is also included for clarity. For the Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples, no diffraction peaks associated to AlPO₄ or NiAl₂O₄ phases were observed. The constitution of NiAl₂O₄ phases could be restricted due to the relatively lower calcination temperatures (500 °C). Concerning the Ni₂P/SiO₂-Al₂O₃ sample, the relatively lower signal to noise ratio showed should be ascribed to the amorphous structure of silica present in the support (20% SiO₂) [20]. The characteristic peaks of fluorite CeO₂ (JCPDS No. 34-0394) were clearly observed for the Ni₂P/CeO₂ system.

Besides, no clear diffraction peaks associated to Ni phases were observed for any of the samples. Omar et al. [21] argued that nickel phosphate calcined up to 600 °C results in amorphous structures, whilst increasing the calcination temperature up to 900 °C permits the constitution of highly crystalline Ni₂P phases. Thus, the lack of Ni₂P peaks in XRD data should relate to the amorphous character of nickel phosphate deposits because of the employed calcination temperature.

2.2. Surface Characterization of the Samples: XPS Analysis

Figure 3A shows the Ni 2p_3/2 spectra obtained for reduced samples where Ni^δ+ and Ni²⁺ species along with their corresponding satellites are identified. For the Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples, the band contribution located at 852.7 eV was associated to Ni^δ+ species. Slightly lower binding energies (852.5 eV) were observed for Ni^δ+ species in the case of the Ni/CeO₂ catalyst. For Ni-supported ceria systems, peak shifts towards lower binding energies are usually related to the higher electron densities attained by Ni species in contact with partially reduced ceria support [22].

Besides, all samples exhibited peaks attributed to oxidised nickel species. Still, the support nature significantly influenced the electronic structure displayed by surface Ni²⁺ species. Typically, bands located at 854.7 and 856 eV are assigned to NiO and Ni(OH)₂ species, respectively. For the reduced Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples, XPS data showed a single Ni²⁺ band located at 854.7 and 855.0 eV, correspondingly. The differences observed for the Ni²⁺ binding energies between the Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples underline the higher electronic density achieved for the Ni₂P/Al₂O₃ sample. Instead, two different peaks located at 854.7 and 856.8 eV were observed for Ni/CeO₂ catalyst, underlining two different Ni²⁺ species interacting with partially reduced cerium dioxide.

In the P2p region (Figure 3B), bands associated to slightly negatively charged phosphorous species (P^δ−) were discerned for all samples. Considering the Ni^δ+ species noticed in the Ni 2p_3/2 spectra, the presence of Ni₂P surface clusters with the consequent electrons transfer from Ni to P species could be advocated [23]. Once again, the support nature influenced the electron properties exhibited by P^δ− surface species. Thus, while the Ni₂P/Al₂O₃ sample exhibited bands located at 129.2 and 130.4 eV, associated to P^δ− species, the larger binding energies noticed for P^δ− species in the Ni₂P/SiO₂-Al₂O₃ sample (129.4 and 130.6 eV) suggest that P^δ− species presented lower electron densities. This suggests that stronger Ni–P interactions and enhanced electron donations from Ni to P were achieved for the Ni₂P/Al₂O₃ sample. For the Ni₂P/CeO₂ system, P^δ− species appeared (129.5 and 133.0 eV) due to, in agreement with previous outcomes, efficient charge redistribution processes at the Ni₂P-CeO₂ interface [24].

Furthermore, the bands showing higher binding energies for all samples are usually ascribed to highly oxidized P species. In this sense, the bands located at 134.6 and 135.0 eV noticed for the Ni₂P/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples were ascribed to P⁵⁺ species. For the Ni₂P/CeO₂ samples, P⁵⁺ species appeared at significantly lower binding energies. For the latter systems, bands placed at 135.8 and 136.4 eV were ascribed to the constitution of AlPO₄ [25].

Therefore, all reduced catalysts displayed both Ni²⁺ and Ni slightly oxidized (Ni^δ+) species constituting Ni₂P surface clusters, in agreement with the Ni 2p_3/2 and P 2p spectra. The larger electron density displayed by P^δ− for the Ni₂P/Al₂O₃ catalyst suggested strongly interacting nickel phosphide structures. Besides, Ni particles interacting with cerium dioxide result in the evolution of: (i) rich-electron density Ni^δ+ species and (ii) two different Ni²⁺ species with relatively low electronic densities. The dispersion of the catalyst material on the supports was approximated by the atomic ratio of nickel species to support material on the catalysts’ surfaces (

Table 2. Surface composition of the catalysts as determined by XPS analysis, the atomic ratios of nickel to support material.

Catalyst	Ni/Support	Ni Coverage (%)	Niδ+/Ni2+	Pδ−/[P5++P-O]
Ni2P/Al2O3	0.08	1.52	0.67	0.48
Ni2P/SiO2-Al2O3	0.09	2.25	0.90	0.13
Ni2P/CeO2	0.28	6.24	0.78	0.22

). This calculation revealed that the ceria supported catalyst had the greatest proportion of its surface occupied by nickel species. Although strong Ni-ceria interactions and high metal dispersions are usually reported, this result can also be significantly influenced by the lower specific surface area of the ceria support. When comparing the alumina and silica-alumina supported catalysts, the higher Ni surface coverage found for the Ni₂P/SiO₂-Al₂O₃ sample suggests that the SiO₂-Al₂O₃ dispersing matrix favours, at least a priori, the Ni metal dispersion. For Ni supported catalysts, the optimal Ni loading over the Al₂O₃ and ZrO₂ dispersing matrix was found at 20 and 30%, respectively [26]. In agreement, the obtained outcomes suggest that silica-based supports lead to greater dispersion of the nickel species over the surface. From XPS data, the Ni/Ni^δ+ surface ratio displayed by the catalyst series was also estimated. Qualitatively, the higher Ni^δ+/Ni²⁺ ratios found for the Ni₂P/SiO₂-Al₂O₃ sample might be associated to the higher dispersions. Besides, the significantly higher P^δ−/[P⁵⁺+P-O] ratios found over the Ni₂P/Al₂O₃ sample corroborated the favourable constitutions of Ni₂P phases already intuited from the higher P^δ− electron densities noticed in the P 2p spectra.

2.3. Catalytic Activity

Figure 4 shows the catalyst performance exhibited by the samples at 30 L/g·h and 700 °C. For all reaction tests, the CH₄ conversion was lower than the CO₂ conversion. This is congruent with the current literature on the kinetics and equilibrium of dry reforming of methane where the activation of CH₄ on the catalysts’ surface is often cited as the rate limiting step for DRM [27]. Hence, the equilibrium conversion of CO₂ should be ca. 10% higher than for CH₄ [2]. Initially, all systems achieved close to 1 H₂/CO ratios and, in the case of the Ni₂P/CeO₂ sample, decreased proportionally to the conversion rate.

The reaction rate observed for the catalysts’ series decreased according to the sequence: Ni₂P/Al₂O₃ > Ni₂P/CeO₂ > Ni₂P/SiO₂-Al₂O₃. Thus, the highest CO₂ conversion, combined with good catalyst stabilities, revealed the Ni₂P/Al₂O₃ system as the best-performing catalyst. On the contrary, the worst catalyst behaviour of the series was exhibited by the Ni₂P/SiO₂_Al₂O₃ catalyst. The lower conversion rates might be associated to important cooking processes over such acidic surfaces. Rather unexpected poor catalytic behaviours were displayed by Ni₂P/CeO₂ catalysts. Thus, even though relatively optimal catalyst performances were initially depicted, the Ni₂P/CeO₂ catalyst depicted an important loss of catalytic activity, with the CO₂ conversion dropping from around 80 to 50% and the CH₄ conversion dropping from around 70 to 35% during the catalytic test. For this sample, the significantly lower catalyst surface area could be related to the deprived catalyst performance.

2.4. XRD of the Spent Samples

Considering the importance of deactivation issues related to sintering or coking processes described for DRM catalysts [27], a glimpse into the catalyst structural variations attained over the different samples under reaction conditions was attempted through XRD analysis. Figure 5 shows the diffractograms obtained for the post-reacted samples. For Ni/CeO₂ catalysts, no diffraction lines associated to Ni phases were discerned. In agreement with XPS data, ceria support seems, indeed, to favour higher Ni metal dispersions [6]. For Ni/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ catalysts, diffraction lines associated to the Ni⁰ phase were clearly evidenced. Still, despite the good dispersions observed for the reduced sample, the spent Ni₂P/SiO₂-Al₂O₃ sample showed prominent Ni⁰ diffraction peaks, indicating that significantly larger Ni crystal sizes were constituted by the Ni₂P/SiO₂-Al₂O₃ catalyst under reactions conditions. The higher Ni particles observed for the SiO₂-Al₂O₃ catalyst suggest that, despite the relatively high metal dispersions obtained with silica-based catalysts, poor resistances against sintering issues are presented. Moreover, the Ni/Al₂O₃ sample exhibited clear diffraction peaks at 41.5°, ascribed to Ni₂P phases (JCPDS No. 01-074-1385), whilst a small Ni₂P contribution could also be intuited for the Ni₂P/SiO₂-Al₂O₃ catalyst sample. The larger concentration of Ni₂P species, constituted for the Ni/Al₂O₃ catalyst, agrees with the findings provided by XPS data.

In respect to carbon deposits, both the Ni/Al₂O₃ and Ni₂P/SiO₂-Al₂O₃ samples exhibited evident diffraction lines placed at 26.5° associated to structured carbon phases (JCPDS No. 75-1621), evidencing the constitution of coke deposits under reaction atmospheres. Comparatively, significantly larger amounts of C deposits were constituted over the silica supported catalyst. Given the higher surface acidity of silica-doped alumina [27], promoted deactivation processes related to coking phenomena are, indeed, expected [28,29]. In any case, carbon-forming reactions are, indeed, thermodynamically favoured for large nickel crystallites [30,31]. Therefore, the larger Ni particle sizes, combined with the intrinsic acidity of silica surfaces, account for the significantly higher amounts of carbon deposits constituted and for the observed conversion drop during the catalytic test. On the contrary, the absence of C related peaks for the Ni/CeO₂ catalyst suggest the active role of ceria support against the constitution of carbon deposits.

Overall, the behaviour observed for the catalysts’ series evidences Al₂O₃ support as the right choice for Ni₂P catalysts in DRM reactions. For CO₂ methanation, higher concentrations of Ni₂P were also related to improved reaction rates over Al₂O₃ supported systems [24]. Along with promoted CO₂ activations, the perturbed electronic structure achieved within Ni₂P has been proposed to increase the resistance against deactivation effects related to Ni oxidation [32] and carbon deposits [33]. Thus, the optimal activity and stability depicted by the Ni₂P/Al₂O₃ catalyst could be associated to the significant highest fractions of Ni₂P species present on the catalyst surface, coupled with fair and relatively stable Ni dispersions. In a similar manner, the lower catalytic behaviour observed for the Ni₂P/SiO₂-Al₂O₃ system should be associated with larger Ni agglomerates along with the lower Ni₂P concentrations observed over the catalyst surface. Besides, the larger coke deposits found on the Ni₂P/SiO₂-Al₂O₃ catalyst surface might also deplete its catalyst performance and propensity towards the development of carbon deposits due to the large Ni agglomerates dispersed over highly acidic surfaces. Moreover, the lower DRM performance observed for the Ni/CeO₂ catalyst might be strongly affected by the lower surface areas depicted by this sample. For the ceria supported sample, the strong interaction established between Ni and CeO₂ supports might hinder the effective constitution of Ni₂P phosphide species. In any case, Ni-O-Ce sites are suggested to be active for the total oxidation of methane [34], with the evolution of CO₂ molecules formed through the reaction between hydroxyls groups constituted over Ce³⁺ sites and surface methyl groups [35]. The relatively lower catalytic performances displayed by the Ni/CeO₂ sample could also be associated with favoured methane oxidation reactions. The absence of carbon deposits and Ni sintering effects evidenced in the XRD analysis of the post-reacted samples suggest the constitution of Ni²⁺ species, inactive for CO₂ activation, as the main deactivation cause.

3. Materials and Methods

3.1. Catalyst Synthesis

All three catalysts were prepared by impregnation of the relevant support with a nickel phosphate solution. For that purpose, γ-Al₂O₃ and 20% SiO₂-80% Al₂O₃ (Puralox SCFa-230 and Siralox 20/380, Sasol, Johannesburg, South Africa) commercial supports were employed. A third home-made ceria support was also employed. Then, CeO₂ support was synthesised by the reaction of urea and cerium nitrate. Thus, adequate amounts were dissolved in 100 mL of deionised water. After being stirred at 85 °C for 24 h, the solution was then filtered to collect the precipitate, which was dried overnight at 80 °C. The obtained solid was calcined in air at 400 °C for 4 h to produce a CeO₂ powder.

The nickel phosphide catalysts were prepared by a procedure similar to that seen in the literature [36,37]. Thus, the intended nominal contents were 20%Ni and had a Nickel:Phosphorous molar ratio of 3:2. For that purpose, a nickel phosphate solution was prepared by mixing solutions of Ni(NO₃).6H₂O and (NH₄)₂HPO₄ in deionised water, producing a nickel phosphate precipitate. The nickel phosphate solution was impregnated over the chosen supports and the excess of solvent was removed by rotary vacuum evaporation. The obtained samples were in an oven at 80 °C and calcined in air at 500 °C for 4 h. Over Puralox SCFa-230, Siralox 20/380 and CeO₂ supports, the obtained catalysts were labelled Ni₂P/Al₂O₃, Ni₂P/SiO₂-Al₂O₃ and Ni₂P/CeO₂, correspondingly. In agreement with previous works [36,37], Ni₂P structures were constituted after subsequent reductions in H₂ presence.

3.2. Catalyst Characterisation

The textural properties of the samples were analysed using N₂ adsorption at −196 °C on an Autosorb iQ Station 2 instrument (Anton Paar QuantaTec Inc., Boynton Beach, FL, USA). The weight-specific surface area of the catalysts was determined using the Brunauer–Emmett–Teller equation [11]; the average pore size and weight-specific pore volume were calculated using the Barett–Joyner–Halenda method [22].

X-ray diffraction analysis was carried out on Panalytical X’Pert Pro Powder equipment (Marvern Panalytical) using Cu Kα radiation at 30 mA and 40 kV. X-ray photoelectron spectra were collected in order to determine the composition of the catalysts’ surfaces using a K-Alpha Thermo Scientific instrument. The XPS analysis was carried out on freshly reduced catalysts. The spectra were recorded using Al-K radiation at 1486.6 eV, which was monochromatised by a twin crystal monochromator to produce a focused elliptical X-ray spot with a major axis length of 400 µm. The initial survey was carried out from a binding energy of 1350 eV to 0 eV, with higher resolution detailed scans being taken of the most relevant sections.

3.3. Catalyst Reaction Testing

The catalysts were tested at atmospheric pressure in a 10 mm internal diameter quartz tube reactor. In these tests, the reactor was loaded with 0.1 g of catalyst and fed with 100 mL/min of 50 vol.% N₂, 25 vol.% CO₂ and 25 vol.% CH₄; neglecting the inert nitrogen, the Weight Hourly Space Velocity (WHSV) was 30 L·g⁻1·h⁻¹. Prior to reaction testing, the catalysts were reduced in a 10 vol.% hydrogen stream with nitrogen as an inert carrier at 800 °C for 1 h. The gas mixture compositions were evaluated with an ABB continuous gas analyser (model ABB AO2020). The CO₂ and CH₄ conversions were estimated according to Equations (1) and (2), respectively.

$${\mathrm{Conversion}\mathrm{of}\mathrm{CO}}_2\left(\%\right)=100\times \frac{{\left[{\mathrm{CO}}_2\right]}_{\mathrm{in}}-{\left[{\mathrm{CO}}_2\right]}_{\mathrm{out}}}{{\left[{\mathrm{CO}}_2\right]}_{\mathrm{in}}}$$

(1)

$${\mathrm{CH}}_4\mathrm{Conversion}\left(\%\right)=100\times \frac{{\left[{\mathrm{CH}}_4\right]}_{\mathrm{in}}-{\left[{\mathrm{CH}}_4\right]}_{\mathrm{out}}}{{\left[{\mathrm{CH}}_4\right]}_{\mathrm{in}}}$$

(2)

4. Conclusions

In this work, the catalytic performance of supported nickel phosphide catalysts was investigated for dry reforming of methane. By analysing Al₂O₃, SiO₂-Al₂O₃ and CeO₂ supports, the prominent impact of the support nature on the catalytic behaviour of Ni₂P systems was clearly established. The reaction rate observed for the catalysts’ series decreased, following the trend Ni₂P/Al₂O₃ > Ni₂P/CeO₂ > Ni₂P/SiO₂-Al₂O₃. The observed catalytic trend should be explained considering different factors. Thus, lower conversion rates, depicted by the Ni₂P/SiO₂-Al₂O₃ system, were related to the higher Ni sintering and coking rates, coupled with hindered constitutions of Ni₂P species. For the Ni₂P/CeO₂ system, the lower surface areas coupled with low concentrations of Ni₂P constituted over the catalyst surface might account for the exhibited catalytic performance. In any case, the strong deactivation issues displayed by the Ni₂P/CeO₂ catalyst under reaction conditions should most likely relate to the partial oxidation of Ni species, since no significant C deposits nor Ni sintering effects were discerned for the post-reacted sample. For the ceria supported catalysts, the oxidation of methane and C intermediate species might also play a role.

Therefore, the obtained outcomes suggest the unsuitability of Ni₂P/CeO₂ and Ni/SiO₂-Al₂O₃ catalysts for DRM processes. Within the analysed series, the Ni₂P/Al₂O₃ catalyst showed the highest catalyst performance as well as excellent stabilities. At 700 °C, the alumina supported catalyst consistently produced stable CO₂ conversions only 12% below equilibrium and a favourable H₂/CO ratio of 1. The observed catalyst behaviour could be associated to the larger concentrations of Ni₂P phases and fair Ni dispersions. Overall, the Ni₂P/Al₂O₃ catalyst has been proven to be an effective and easily synthesised novel catalyst with remarkable activity and stability for DRM reaction.