Development of a unique Ni^δ+ (0 < δ < 2) in NiMoP/Al₂O₃ catalyst for dry reforming of methane

Abstract

The supported Ni-based catalyst is the most important catalyst system for dry reforming of methane (DRM). In this system, Ni⁰ is usually recognized as an active site for CH₄–CO₂ reaction, while the Ni⁰ can also greatly catalyze coke formation, leading to catalyst deactivation. In this work, we have found a unique Ni^δ+ (0 < δ < 2) in NiMoP/Al₂O₃, and it can show more stable DRM activity than the Ni⁰ in NiMo/Al₂O₃ and some literature reported Ni-based catalyst systems. Furthermore, it was worthy to note that the use of Al₂O₃ supporting NiMoP can greatly improve the catalytic activity, more particularly at lower temperatures (600–800 °C), which might be due to a remarkable increase in surface area and decrease in particle size, as well as a strong interaction between NiMoP and Al₂O₃. The lifetime test showed the catalytic stability of NiMoP/Al₂O₃ was increased with increasing temperature from 650 to 750 °C. This was because the carbon species can not react quickly with the oxidized species at lower temperatures, leading to catalyst oxidation and coking at the same time. It was therefore proposed that besides the matching between the dissociation rates of CH₄ and CO₂ and keeping a close contact of active sites for CH₄ and CO₂ dissociations, choosing an appropriate reaction temperature that can simultaneously remove surface oxygen and carbon species was also important for maintaining the oxidation–reduction cycle of phosphides in DRM.

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1. Introduction

Global warming has become a serious environmental problem due to excessive emissions of greenhouse-gas (CO₂, CH₄, H₂O vapour, NO_x) emissions. In view of the fact that above 6000 trillion cubic feet natural gas has been discovered in its associated and non-associated form at the beginning of 2016,¹ CH₄ has become an imperative energy resource for heat, power and vehicular applications in the future. Therefore, the wastage of valuable hydrocarbon sources and the damage to living environment will be caused by burning of a large amount of CH₄ and associated gases such as CO₂ into the atmosphere. These conditions provide strong motivation for the development of dry reforming of methane (DRM) technology.² DRM not only uses CO₂ and CH₄ as the feedstocks to reduce greenhouse-gas emissions, but also produces synthesis gas with lower H₂/CO ratios, which is a resource for the manufacture of useful value-added products via Fischer–Tropsch reaction.^3–5

In industrial catalytic DRM reaction, Ni-based catalysts offer great promise due to their wide-availability, high performance and low cost. However, they can suffer from metal sintering and coke deposition, leading to catalyst deactivation.^6,7 It was well-known that the modification of Ni-based catalysts with various support materials and second metals can help to reduce carbon deposition and increase the catalyst lifetime.⁸ Up to now, various support materials such as Al₂O₃, SiO₂, TiO₂, ZrO₂, CeO₂ and SBA-15, were used for loading Ni in order to cope with the deactivation problem.^8,9 Among these support materials mentioned above, Al₂O₃ is one of the most widely researched carriers. As summarized by Al-Fatesh et al., the second metals used for the modification of Ni/Al₂O₃ included K, Mg, Ca, Ba, Sr, Y, La, Ce, Ti, Mo, W, Mn, Co, Cu, Zn and Sn.⁸ These second metals can usually make the Ni/Al₂O₃ more stable in DRM by increasing the surface reaction rate, maintaining Ni dispersion and stability or decreasing the methane dissociation rate.^10–14 It was worthy to note that the active component of these Ni-based catalysts was limited to Ni⁰,¹⁵ but the Ni⁰ particles can easily suffer from aggregation in DRM. The aggregated Ni⁰ particles would greatly catalyze coke formation, leading to catalyst deactivation.^16,17 Very recently, our group have developed a new active Ni species, viz. Ni^δ+ (0 < δ < 2) in NiMoP, for DRM.^18–20 This new Ni^δ+ was found to be helpful for improving anti-coking properties of Ni⁰ in DRM.^21,22 Up to date, the NiMoP phase was reported to be active for DRM only at the temperatures above 850 °C. The catalyst particles had to suffer from sintering at such high temperatures.^19,20 In this work, we developed NiMoP catalysts supported on Al₂O₃ as new catalysts for DRM. The NiMoP/Al₂O₃ catalysts can show much higher DRM activity than unsupported NiMoP catalyst at temperatures below 800 °C. Moreover, it can show superior catalytic stability over NiMo/Al₂O₃ and some literature reported Ni-based monometallic and bimetallic catalysts (e.g. Ni and Ni–Co systems). In addition, the conditions for maintaining the surface reaction over NiMoP in DRM can be further completed.

2. Experimental

2.1 Sample preparation

The alumina-supported oxide precursors with a molar ratio of Ni : Mo = 1 : 1 were prepared by the incipient wetness method via stirring γ-Al₂O₃ in aqueous solutions of (NH₄)₆Mo₇O₂₄·4H₂O and Ni(CH₃COO)₂·4H₂O. The Ni concentration was regulated for a Ni loading of 0.62–9.74 wt%, respectively. The resulting precursors were dried at 110 °C for 12 h and calcined at 500 °C for 3 h. The alumina-supported NiMoP phosphide catalysts were prepared from alumina-supported oxide precursors via H₂ reduction method. The precursor was heated in a quartz reactor under a flow of pure H₂ (150 ml min⁻¹). The temperature was increased from room temperature (RT) to 300 °C at a rate of 10 °C min⁻¹ followed by a rise in temperature from 300 to 850 °C at a rate of 1 °C min⁻¹ and then kept at the temperature for 1 h before cooling to RT in a H₂ flow. Finally, the material was passivated in 1%O₂/Ar for 12 h before it was exposed to air. An unsupported NiMoP catalyst was also prepared by the same method and procedure mentioned above. The obtained alumina-supported NiMoP phosphide catalysts with Ni loadings of 0.62, 1.86, 4.40 and 9.74 wt% were designated as xNiMoP (x = 0.62, 1.86, 4.40 and 9.74). And the unsupported NiMoP catalyst was denoted as NiMoP.

2.2 Sample characterizations

X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (X'Pert Pro MPD) equipped with a Cu Kα source. BET surface areas were obtained from a surface area analyzer (NOVA4200). X-ray photoelectron spectroscopy (XPS) investigation was carried out using a Kratos Axis ultra (DLD) equipped with Al Kα X-ray source. TEM and EDS mapping images were acquired using a transmission electron microscopy (Philips Tecnal 10). H₂-TPR experiments were carried out in a quartz tube micro-reactor. The effluent gases were monitored by means of a GC thermal conductivity detector. Prior to the H₂-TPR process, the alumina-supported (100 mg) and unsupported (20 mg) oxide precursors were heated from RT to 300 °C in Ar and maintained at this temperature for 30 min. After being cooled to RT, the samples were heated from 30 °C to 900 °C at a rate of 4 °C min⁻¹ in 10%H₂/Ar (30 ml min⁻¹). Thermo-gravimetric analysis (TGA) was performed on a SDT Q600 analyzer. The catalyst samples were heated from RT to 900 °C at a rate of 10 °C min⁻¹ under air flow (100 ml min⁻¹). CO₂ and CH₄ dissociation studies were performed using a flow of 2%CO₂(CH₄)/Ar (50 ml min⁻¹). Prior to the dissociation reaction, the sample was preheated in H₂ at 850 °C for 0.5 h and then the gas mixtures were allowed to pass through the sample. The CO₂ and CH₄ concentrations during reaction were monitored using gas chromatography (GC).

2.3 Catalyst performance tests

Catalytic activities of these NiMoP phosphide catalysts for DRM were evaluated in a micro-reactor with an inner diameter of 8 mm at atmospheric pressure. Prior to the reaction, the sample was preheated in H₂ at 850 °C for 0.5 h, and then further cooled or heated to a given reaction temperature (600–900 °C) under Ar flow. A stoichiometric mixture (CH₄ : CO₂ = 1 : 1) was allowed to pass through the catalyst (60–80 meshes) with a WHSV of 24 000 cm³ g⁻¹ h⁻¹. The gas-phase products were analyzed by on-line GC. The conversions of CH₄ and CO₂, and H₂/CO were defined as follows (n_i,in = the initial molar fraction of component i in the feed, n_i,out = the final molar fraction of component i in the gaseous effluent):

3. Results and discussion

3.1 Characterizations of fresh NiMoP catalysts

Fig. 1 shows the XRD patterns of fresh Al₂O₃-supported and unsupported NiMoP catalysts obtained in this study. At Ni loadings above 4.40 wt%, the XRD pattern showed main diffraction peaks at 39.2, 43.2 and 47.3° corresponding to (111), (201) and (210) reflections, respectively, of bulk NiMoP (PDF 71-0202). The other peaks were diffraction lines of Al₂O₃ (PDF 77-0396). At 0.62 and 1.86 wt% Ni loadings, only the diffraction peaks of Al₂O₃ can be observed, probably due to the fact that the NiMoP phase was well dispersed on Al₂O₃ support.

Fig. 1 XRD patterns of fresh (a) 0.62NiMoP, (b) 1.86NiMoP, (c) 4.40NiMoP, (d) 9.74NiMoP and (e) NiMoP (PDF file references are also included).

Subsequent XPS analysis was used to further confirm the formation of NiMoP on Al₂O₃. Fig. 2 shows the XPS spectra in the Mo 3d, Ni 2p and P 2p regions for the Al₂O₃-supported NiMoP samples. Since as-prepared phosphide samples were usually passivated prior to exposure to air, the surface regions should be dominated by oxidized species and underlying reduced species related to phosphide phases.^23–25 In the case of 9.74NiMoP sample, the binding energy peaks of oxidized Mo (229.9 eV), Mo (232.8 eV), Ni (856.6 eV) and P (133.9 eV) were accordant with assignments by others to Mo⁴⁺, Mo⁶⁺, Ni²⁺ and P⁵⁺ species,^25,26 respectively. And the binding energy peaks at 228.2, 853.3 and 129.5 eV were attributed to Mo^σ+ (Mo–P), Ni^δ+ (Ni–P) and P^α− (Mo(Ni)–P) species,^27–29 respectively. The binding energy of Ni 2p_3/2 at 853.3 eV was higher than that assigned to Ni⁰ (852.6 eV)³⁰ but lower than that assigned to Ni²⁺ (856.6 eV). Clearly, the sequence of 0 < δ < 2 was accepted. In the case of 4.40NiMoP, 1.86NiMoP and 0.62NiMoP samples, it was reasonable that the intensities of Mo 3d, Ni 2p and P 2p levels would decrease with the decrease in Ni loading and even the peaks were too weak to be observed. Table 1 lists the BET surface area of the NiMoP samples obtained in this study. It was clear that the BET surface areas of the NiMoP/Al₂O₃ catalysts decreased monotonically with increase in Ni loading, and they were much higher than that of bulk NiMoP.

Fig. 2 XPS spectra of Mo 3d, Ni 2p and P 2p (a) 0.62NiMoP, (b) 1.86NiMoP, (c) 4.40NiMoP and (d) 9.74NiMoP.

Table 1 BET surface area of the NiMoP catalysts obtained in this study

Sample	Ni loading (wt%)	BET surface area (m^2 g^−1)
0.62NiMoP	0.62	198.5
1.86NiMoP	1.86	190.9
4.40NiMoP	4.40	153.8
9.74NiMoP	9.74	127.5
NiMoP	—	5.4

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Fig. 3 shows the TEM images of 0.62NiMoP, 1.86NiMoP, 4.40NiMoP, 9.74NiMoP and NiMoP samples. In the case of the supported NiMoP samples, Fig. 3a–d showed the small and dispersed nanoparticles on Al₂O₃. In contrast, the unsupported NiMoP was composed of big size nanoparticles with serious conglomeration (see Fig. 3e). The insets clearly showed the (201) crystal plane of NiMoP, which proved the existence of NiMoP in the samples. The size distributions of the NiMoP nanoparticles determined by measuring particle size directly from TEM images can also be shown in these insets. It can be observed that the average particle size of supported NiMoP increased with the Ni loading. It was 8.6 nm in 0.62NiMoP, 9.1 nm in 1.86NiMoP, 10.8 nm in 4.40NiMoP and 12.3 nm in 9.74NiMoP, respectively. Note that the average particle sizes of the supported NiMoP were much smaller than that of unsupported one (150.2 nm). The results were in agreement with the results of BET (Table 1), because the decrease in agglomeration was usually responsible for the increase in the surface area of phosphide, as suggested before.^31,32 In addition, the EDS-mapping images (Fig. S1†) confirmed the presence of Ni, Mo and P elements in the 1.86NiMoP, in agreement with the XPS results (Fig. 2).

Fig. 3 TEM images of (a) 0.62NiMoP, (b) 1.86NiMoP, (c) 4.40NiMoP, (d) 9.74NiMoP and (e) NiMoP. The insets show the particle size distribution and crystal lattice of NiMoP.

It was believed that the interaction between particles and Al₂O₃ support was an important factor for the stabilizing of DRM catalysts.^33,34 In order to give insights into the interaction between NiMoP and Al₂O₃, the reduction processes of oxide precursors were investigated by H₂-TPR (see Fig. 4). As for the unsupported oxide precursor, the main reduction peak for the formation of NiMoP was observed at 660 °C. In the case of Al₂O₃-supported oxide precursors, the reduction temperatures shifted to a higher temperature region (665–710 °C) and they increased with the decrease in Ni loading. The results clearly indicated that there was an interaction between NiMoP and Al₂O₃, and the degree of interaction increased with the decrease in Ni loading.

Fig. 4 H₂-TPR profiles for the synthesis of (a) 0.62NiMoP, (b) 1.86NiMoP, (c) 4.40NiMoP, (d) 9.74NiMoP and (e) NiMoP from oxide precursors.

3.2 Catalytic performance

Fig. 5 shows the dependence of catalytic performance on temperature over these NiMoP catalysts. It was clear from Fig. 5 that compared to unsupported NiMoP catalyst, alumina-supported NiMoP catalysts showed higher activity for DRM at the entire reaction temperature range. Particularly, they were significantly more active at lower temperatures (≤800 °C). It was well known that the surface area and particle size were responsible for the catalytic performance of catalyst. The BET and TEM results (Table 1 and Fig. 3) indicated that the alumina-supported NiMoP catalysts can show much higher surface area and smaller particle size than unsupported NiMoP catalyst, which can account for their higher catalytic activity. Amongst this series of alumina-supported NiMoP catalysts, the 1.86NiMoP was found to exhibit the highest catalytic performance for DRM, which was probably attributed to its appropriate Ni loading and stronger interaction between NiMoP and Al₂O₃ suggested by H₂-TPR (Fig. 4).

Fig. 5 Dependence of catalytic performance on temperature over 0.62NiMoP, 1.86NiMoP, 4.40NiMoP, 9.74NiMoP and NiMoP catalysts. Reaction conditions: CH₄ : CO₂ = 1 : 1, WHSV = 24 000 cm³ g⁻¹ h⁻¹, reaction pressure = 1 atm, reaction time = 1 h.

Fig. 6 shows the time dependence of catalytic performance at a given temperature over 1.86NiMoP. It can be seen that the durability of 1.86NiMoP was strongly dependent on reaction temperature. At 650 °C, there was a rapid decline of performance within 3 h. The catalytic stability was slightly enhanced when the temperature was increased to 700 °C: the DRM performance decreased from X_CH4 = 73%, X_CH4X_CO2 = 82%, and H₂/CO = 0.74 to X_CH4 = 40%, X_CO2 = 42%, and H₂/CO = 0.56 during 10 h. Note that at 750 °C the catalytic activity (X_CH4 = 74% and X_CO2 = 83%) was relatively stable through out the test period of 30 h. As proved by XPS (Fig. 2), the reduced Ni species in NiMoP/Al₂O₃ was considered to be Ni^δ+ (0 < δ < 2). For the sake of comparison, the DRM performance of NiMo/Al₂O₃ with Ni loading of 1.86 wt% (hereafter denoted as 1.86NiMo) was also investigated under similar reaction conditions (see Fig. S2†). Although the 1.86NiMo showed a slightly higher activity (X_CH4 = 85% and X_CO2 = 94%) than 1.86NiMoP (X_CH4 = 84% and X_CO2 = 92%) in the initial reaction period, the activity after 30 h for the former (X_CH4 = 57% and X_CO2 = 63%) was lower than that for the latter (X_CH4 = 74% and X_CO2 = 83%). In addition, the excellent performance of 1.86NiMoP showed great advantages over some literature reported Ni⁰ based catalysts with lower Ni loadings (see Table 2).

Fig. 6 Time dependence of catalytic performance over 1.86NiMoP catalyst at various reaction temperatures. Reaction conditions: CH₄ : CO₂ = 1 : 1, WHSV = 24 000 cm³ g⁻¹ h⁻¹, reaction pressure = 1 atm.

Table 2 Comparison of CH₄ conversion rate for the as-prepared and literature reported catalysts

Catalyst	Ni loading	Temperature (°C)	Feed composition	WHSV (cm^3 g^−1 h^−1)	Time on stream (h)	CH4 conversion rate (mol gcat^−1 h^−1)		Ref.
						1 h	20 h
1.86NiMoP	1.86	750	CH4 : CO2 = 1 : 1	24 000	30	0.45	0.40	This work
1.86NiMo	1.86	750	CH4 : CO2 = 1 : 1	24 000	30	0.46	0.37
Ni/SBA-15	2	800	CH4 : CO2 : N2 = 1 : 1 : 1	15 000	24	0.17	0.14	33
NiCo/CeZr	1.2	750	CH4 : CO2 = 1 : 1	12 000	20	0.19	0.19	34
NiCe/HAP	2	750	CH4 : CO2 : He = 1 : 1 : 3	60 000	110	0.47	0.45	35
NiCo/HAP	2.4	750	CH4 : CO2 : Ar = 2 : 2 : 5	15 882	160	0.13	0.12	36
Ni/DFSBA-15	3	800	CH4 : CO2 = 1 : 1	15 000	30	0.28	0.25	37

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3.3 Characterizations of used NiMoP catalysts

In this study, the used 1.86NiMoP catalysts functioned in DRM at different temperatures (650, 700 and 750 °C) were hereafter denoted as 1.86NiMoP-T (T = 650, 700 and 800). Fig. S3† shows the XPS spectra of Mo 3d, Ni 2p and P 2p for the 1.86NiMoP-650, 1.86NiMoP-700 and 1.86NiMoP-750 samples. On the basis of curve fitting, the Mo 3d_5/2, Ni 2p_3/2 and P 2p_3/2 binding energies and the distribution of these corresponding species as well as the contents of Mo^σ+ and Ni^δ+ are listed in Table 3. For the sake of comparison, the XPS results for fresh 1.86NiMoP from Fig. 2 are also included. Compared with 1.86NiMoP sample, the 1.86NiMoP-650 and 1.86NiMoP-700 samples showed two Mo species (Mo⁴⁺ and Mo⁶⁺) and one Ni species (Ni²⁺) but no Mo^σ+ and Ni^δ+. This was probably because the Mo^σ+ and Ni^δ+ species were so low in concentration that their corresponding peak areas were overshadowed by Mo⁴⁺ and Ni²⁺, respectively. However, the Mo^σ+ and Ni^δ+ contents (12.9 and 14.1%) for 1.86NiMoP-750 sample were very close to those (13.1 and 13.8%) for 1.86NiMoP sample. The results indicated that the 1.86NiMoP catalyst can remain its surface structure relatively stable at 750 °C in DRM but it suffered from surface oxidation at lower temperatures (650 and 700 °C).

Table 3 XPS results for fresh and used NiMoP catalysts

Sample	Binding energy (eV)							Mo^σ+ content (%)	Ni^δ+ content (%)
	Mo 3d5/2			Ni 2p3/2		P 2p3/2
	Mo^σ+ (Mo–P)	Mo^4+ (Mo–O)	Mo^6+ (Mo–O)	Ni^δ+ (Ni–P)	Ni^2+ (Ni–O)	P^α− (Mo(Ni)–P)	P^5+ (P–O)
Mo^σ+ content = Mo^σ+/(Mo^σ+ + Mo^4+ + Mo^6+); Ni^δ+ content = Ni^δ+/(Ni^δ+ + Ni^2+).
1.86NiMoP	228.4	229.9	232.7	853.3	856.4	N/A	133.7	13.1	13.8
1.86NiMoP-650	N/A	229.9	232.6	N/A	856.1	N/A	133.9	0	0
1.86NiMoP-700	N/A	229.9	232.6	N/A	856.1	N/A	133.9	0	0
1.86NiMoP-750	228.3	229.8	232.8	853.1	856.6	N/A	134.0	12.9	14.1

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In order to quantify the carbon deposition on the catalyst surfaces, the 1.86NiMoP-650, 1.86NiMoP-700 and 1.86NiMoP-750 samples were characterized by TGA (see Fig. 7). It had been widely accepted that the weight loss at the lower temperature (below 400 °C) was due to thermal desorption of H₂O and removal of easily oxidizable amorphous carbon species.^38,39 And the weight loss at the higher temperature (400–750 °C) was fully attributable to carbon combustion.^40,41 By comparing the TGA curve of fresh catalyst with used catalysts, the weight loss below 400 °C was mainly due to thermal desorption of H₂O on the surface and inside catalyst. The weight losses of 1.86NiMoP-650, 1.86NiMoP-700 and 1.86NiMoP-750 in the high temperature range were 1.3, 2.6 and 2.3%, respectively. Correspondingly, their carbon deposition rates estimated by TGA were 4.3, 2.6 and 0.8 mg g_cat⁻¹ h⁻¹, respectively, indicating that the low reaction temperature was more favourable to the formation of carbon deposition. Correlating the XPS and TGA results with catalytic stability data, it can be deduced that the rapid deactivation of 1.86NiMoP at 650 and 700 °C should be attributed to both surface oxidation and carbon deposition.

Fig. 7 TGA curves of 1.86NiMoP, 1.86NiMoP-650, 1.86NiMoP-700 and 1.86NiMoP-750 samples. The inset shows their carbon deposition rates in DRM.

3.4 CH₄ and CO₂ dissociations

It was well known that the DRM reaction mechanism on phosphide catalysts can be proposed as two competing mechanisms (noble metal type mechanism and oxidation–reduction type mechanism).²⁵ The noble metal type mechanism can be represented as:

CH₄ → C(♦) + 2H₂, (1)

CO₂ → O(♦) + CO, (2)

C(♦) + O(♦) → CO + 2(♦), (3)

where (♦) represented a vacancy near the surface of the phosphide. The oxidation–reduction cycle for phosphides (e.g. MoP) usually included two reactions, viz. oxidation reaction (CO₂ dissociation, reaction (4)) and carbothermal reduction reaction (reactions (5)):^42–45

(x + y)CO₂ + MoP → MoO_x + PO_y + (x + y)CO, (4)

MoO_x + PO_y + (x + y)C → MoP + (x + y)CO (5)

The coke formed was mainly considered in CH₄ cracking reaction (reaction (1)) because the rate of Boudouard reaction (2CO → C + CO₂) was much lower than that of CH₄ cracking reaction when the temperature was above 650 °C.⁴⁶ It was suggested that the matching between CH₄ dissociation rate and CO₂ dissociation rate was the first condition for maintaining the oxidation–reduction cycle.^{19,20,42–45} If the CO₂ dissociation was more favorable, the phosphides would suffer from oxidation. Conversely, the excessive dissociation of CH₄ would facilitate the formation of coke. As shown in Fig. 8, it was clear that the R_CH4 was higher than R_CO2 on the 1.86NiMoP catalyst in the whole temperature range from 650 to 850 °C, which suggested that coking on the catalyst was unavoidable in DRM. In addition, the R_CH4/R_CO2 ratio was found to decrease linearly with the temperature. This suggested that the carbon deposition rate was also increased with temperature. The result was in good agreement with the result of TGA (Fig. 7). Interestingly, the 1.86NiMoP suffered not only from carbon deposition but also from surface oxidation in DRM at 650 and 700 °C (see Table 3). Indeed, it was possible that phosphides/carbides would simultaneously undergo carbon deposition and surface oxidation during DRM.^19,47 This was probably because high-temperature sintering of catalyst can result in the segregation of the active sites for CO₂ dissociation and CH₄ dissociation, which hindered the process of reaction (5). In other words, the reaction between carbon species and oxidized species viareaction (5) can not be completed quickly enough, which was the main reason for the coexistence of coking and oxidation of catalyst. Therefore, the second condition mentioned before for maintaining the oxidation–reduction cycle was that the active sites for CO₂ dissociation and CH₄ dissociation over phosphides must locate very close to each other.¹⁹ However, it was found that the catalytic stability of 1.86NiMoP was greatly enhanced by increasing temperature from 650 to 750 °C (Fig. 6). Thus, the current coexistence of coking and oxidation of catalyst after DRM might not be due to high-temperature sintering. In view of the fact that the catalytic stability was greatly dependent on temperature, it was reasonable to deduce that a high enough temperature was essential for a fast achievement of reaction (5). The H₂-TPR result (Fig. 4) clearly indicated the oxidized species on the 1.86NiMoP surface can not be reduced completely until 710 °C. It was suggested that the carbon species can not react quickly with the oxidized species at 650 and 700 °C, leading to catalyst oxidation and coking at the same time. When the reaction temperature was at 750 °C, the rate for reaction (5) was enhanced and the oxidation–reduction cycle can be well established. This was the reason why the lifetime of 1.86NiMoP can be greatly extended in DRM at 750 °C. Therefore, we proposed that choosing an appropriate reaction temperature that can simultaneously remove surface oxygen and carbon species was another condition for maintaining the oxidation–reduction cycle in DRM.

Fig. 8 CH₄ conversion rate (R_CH4), CO₂ conversion rate (R_CO2) and R_CH4/R_CO2 over 1.86NiMoP at different temperatures. Reaction conditions: CH₄ : Ar = 1 : 49, CO₂ : Ar = 1 : 49, WHSV = 30 000 cm³ g⁻¹ h⁻¹, reaction time = 10 min.

Conclusions

A series of Al₂O₃-supported NiMoP phosphides with 0.62–9.47 wt% Ni loadings were synthesized and used as catalysts for DRM. These supported NiMoP catalysts showed much higher performance than unsupported one, more particularly at low temperatures (600–800 °C), which might be attributed to their optimal microstructures (high surface area and small particle size) and strong interaction between NiMoP and Al₂O₃. Note that the active Ni species in the NiMoP/Al₂O₃ was regarded as Ni^δ+ (0 < δ < 2), which can show more stable DRM activity than the Ni⁰ in NiMo/Al₂O₃ and some literature reported Ni-based catalyst systems. It was found that a high enough temperature was essential for maintaining the oxidation–reduction cycle over phosphides in DRM. Therefore, besides the two conditions: matching between the dissociation rates of CH₄ and CO₂ and keeping a close contact of active sites for the two reactants, choosing an appropriate reaction temperature that can simultaneously remove surface oxygen and carbon species was proposed to be another condition for maintaining the oxidation–reduction cycle of phosphides in DRM.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (No. 21978125), the Liaoning Revitalization Talents Program (No. XLYC1907003), the Fushun Revitalization Talents Program (No. FSYC202101001), the Project of Liaoning Province Department of Education (No. LJKZ0376) and Liaoning BaiQianWan Talents Program.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cy01595a

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