Kinetic Study of Zirconia-Alumina-Supported Ni-Fe Catalyst for Dry Reforming of Methane: Impact of Partial Pressure and Reaction Temperature

Abstract

A better understanding of the reaction mechanism and kinetics of dry reforming of methane (DRM) remains challenging, necessitating additional research to develop robust catalytic systems with high catalytic performance, low cost, and high stability. Herein, we prepared a zirconia-alumina-supported Ni-Fe catalyst and used it for DRM. Different partial pressures and temperatures are used to test the dry reforming of methane reaction as a detailed kinetic study. The optimal reaction conditions for DRM catalysis are 800°C reaction temperature, 43.42 kPa CO₂ partial pressure, and 57.9 kPa CH₄ partial pressure. At these optimal reaction conditions, the catalyst shows a 0.436 kPa² equilibrium constant, a 0.7725 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_Cat/h rate of CH₄ consumption, a 0.00651 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/m²/h arial rate of CH₄ consumption, a 1.6515 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_Cat/h rate of H₂ formation, a 1.4386 mol_CO/g_Cat/h rate of CO formation. This study’s findings will inspire the cost-effective production of robust catalytic systems and a better understanding of the DRM reaction’s kinetics.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Dry reforming of methane (DRM) has received worldwide interest in terms of its ability to reduce the concentration of greenhouse gases (CH₄ and CO₂) and its efficiency in producing an important synthetic feedstock known as “syngas” (CO + H₂) (reaction 1). Dry reforming of methane is a highly endothermic reaction, and it is feasible between 600 and 800°C reaction temperature [1]. The reverse water gas shift reaction (RWGS) is the most significant competitive reaction (reaction 2) with DRM. As RWGS consumes H₂, the presence of this reaction may affect the H₂ yield of the reaction. It is the responsibility of the DRM research community to optimize the reaction parameters, such as the partial pressure of each feed gas component and reaction temperature, to maximize the catalytic activity of the DRM reaction while minimizing the other parallel side reactions (such as RWGS).$$\begin{array}{c}\text{(1)}& {\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\rightleftharpoons 2{\mathrm{H}}_2+2\mathrm{C}\mathrm{O}∆\mathrm{H}˚=\frac{247.34\mathrm{K}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}{\mathrm{H}}_2+{\mathrm{C}\mathrm{O}}_2⟶\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}∆\mathrm{H}˚=\frac{41.145\mathrm{K}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}\text{.}\end{array}$$

The DRM reaction proceeds in two steps: the dissociation of CH₄ over catalytically active sites followed by the oxidation of dissociated CH_x species by CO₂. Slow dissociation of CH₄ results in low activity, whereas delayed oxidation leads to coke deposition over catalytic active sites, ultimately affecting the catalytic activity of the catalyst used. This necessitates that DRM scientists address both aspects, namely how to increase the catalytic active site and provide instant oxidation of carbon deposits. Metal centres such as Ni, Co, Rh, Pd, and Pt are catalytically active sites for CH₄ dissociation, with Ni having numerous advantages over the others. Ni offers low-cost preparation and has 25 times more CH₄ interaction energy than Co and less CH₄ dissociation energy than Pd and Pt [2, 3]. However, Ni sintering at high temperatures remains problematic, leading to the size growth of metallic Ni to the point of inactivation. For CH₄ decomposition over silica-supported Fe, the catalyst was inactive, whereas redox metal oxides such as ZrO₂-supported Fe catalysts were active [4, 5]. The redox property of ZrO₂ enables the support to release instant lattice oxygen during the surface reaction, thus leaving a vacancy behind. Furthermore, ZrO₂ is supposed to enhance CO₂ dissociation, forming oxygen [6, 7]. ZrO₂ may be channel oxygen flow for a surface oxidation reaction under DRM conditions of high reaction temperatures. Overall, it can be stated that Fe dispersed over ZrO₂ is responsible for CH₄ dissociation, and “CO₂ along with ZrO₂” channelizes the oxygen flow for instant oxidation of carbon deposits during the DRM reaction. ZrO₂-supported Fe catalysts achieved ∼20% CH₄ conversion up to 4 h TOS [4, 5].

Interestingly, alumina-silicate (Al/Si = 80 : 20) supported Fe catalyst and alumina-supported Ni catalyst showed <5% CH₄ conversion after 4 h, >40% CH₄ conversion after 5 h, and >80% CH₄ conversion up to 5 h, respectively [4]. With increasing alumina content, acidity has increased, and high and stable CH₄ conversion has been observed. Apart from this, Fe dispersion over basic support MgO was also found to be moderately active for CH₄ decomposition [4, 8] (∼60% CH₄ conversion, ∼50% H₂ yield).

Fe dispersion over Al₂O₃ was highly active for CH₄ decomposition, but this catalyst system is inactive in DRM. In the presence of CO₂, the dissociation of CH₄ may be inhibited by an iron-based catalyst, as Fe is oxidized into FeO [9], which is incapable of dissociating CH₄. At the same time, in the presence of CO₂, the Ni-supported catalyst was found to be quite active for CH₄ decomposition. However, it is rapidly rendered inactive due to coke deposition. Optimizing Ni and Fe content was beneficial regarding catalytic activity and stability. Ni-Fe alloy is stable at extremely high temperatures; therefore, catalytically active metallic Ni is maintained under DRM conditions [10]. Al₂O₃-supported NiFe (3 : 1) catalyst had a specific alloy composition (Ni₃Fe), resulting in improved catalytic activity (13% CH₄ conversion after 3 h TOS) than Al₂O₃-supported Ni catalyst (8% CH₄ conversion after 3 h) at 600°C [11]. In low-temperature DRM, evaporation-induced self-assembly (EISA)-prepared NiFe catalysts supported on alumina have garnered interest. It showed 26.6% CH₄ conversion, 37.8% CO₂ conversion, and 0.67 H₂/CO ratio at 550°C [12]. Li et al. prepared a mesoporous alumina-supported NiFe (Ni/Fe = 10/7) catalyst using the EISA method and found that the catalyst was deactivated after 24 hours due to the dealloying of FeNi₃ [13]. Gunduz-Meric et al. prepared a coke and sinter-resistant “Ni-iron core (ratio 4 : 1) and silica sphere” catalyst [14]. Silica shell formed SiC and protects carbon decomposition in the presence of catalytically active Ni, as well as limiting Ni dispersion. It showed more than 70% CH₄ conversion and a 0.70 H₂/CO ratio for up to three cycles. Fe-modified MgAl₂O₄-supported Ni catalysts and MgAl₂O₄ supported NiFe catalysts were also tested for DRM [10, 15]. At Ni/Fe = 1.4, loaded over MgAl₂O₄ support, the role of iron was found to be crucial in the decoking at the metal centre [10]. Here, some of the metallic Fe is oxidized to FeO_x by CO₂. Besides, the lattice oxygen of FeO_x is superior for decoking, producing CO and metallic Fe, where the latter restores the original Fe-Ni alloy [16]. The second pathway for coke oxidation involves the dissociation of CO₂ over metallic Ni into CO and O, followed by subsequent coke oxidation by surface oxygen [9]. Ni-Al (3 : 1) supported on Mg(Al)O demonstrated >1.5 mol_CH4 mol_metal⁻¹ s⁻¹ CH₄ conversion for up to 30 h time on stream. The effective decoking at the metal centres was due to the reaction of FeO with surface carbon [17]. Mayenite (Ca₁₂Al₁₄O₃₃) support Ni catalyst is prone to deactivation in DRM, but adding 2 wt.% Fe [18] effectively suppressed coke. The support may facilitate the transfer of oxygen species from FeO_x to Ni sites, thereby promoting carbon deposit oxidation.

We expect that by incorporating ZrO₂ into Al₂O₃, lattice oxygen endowing capacity will be enhanced and that by introducing Fe with Ni, effective decoking will occur. Herein, we prepare a zirconia-alumina-supported Ni-Fe catalyst. Different partial pressures and temperatures are used to test the dry reforming of methane in a detailed kinetic study. To the authors’ best knowledge, this is the first detailed study to optimize the partial pressure of each feed gas component over a 700–800°C reaction temperature to maximize the DRM catalytic activity and minimize side reactions like RWGS.

2. Experimental

2.1. Catalyst Preparation

The catalyst 5Ni2Fe/ZrAl is synthesized by impregnating the required amounts of Ni(NO₃)₂.6H₂O (99%; Alfa Aesar) aqueous solution (equivalent to 5 wt% NiO) and Fe(NO₃)₃.9H₂O (403.99 g/mol; 99.99%; Alfa Aesar) (equivalent to 2 wt% of Fe₂O₃) with commercially available 10 wt% zirconium oxide–90 wt% alumina support. The solution was heated under stirring until a slurry was formed. It was further dried at 120°C and calcined at 700°C with a heating rate of 3°C/min for 5 hrs. The catalyst is abbreviated as 5Ni2Fe/ZrAl.

2.2. Catalyst Reaction

DRM reaction is carried out in a stainless tubular reactor (diameter 0.91 cm and length 30 cm) made by PID Eng. & Tech. Micro. Activity company. 100 mg catalyst is packed in the reactor and reduced under reductive pretreatment with H₂ (flow rate 20 ml/min) at 600°C for 1 h. The gas feed is composed of CH₄, CO₂ and N_2, which are allowed to pass through the catalyst bed at different flow rates (total flow rate = 70 ml/min) at three different reaction temperatures (700, 750, and 800°C). The products are analyzed by a gas chromatograph equipped with a TCD detector.

The expressions for mole fraction, partial pressure, and specific feed rate of each gas are shown in supporting information S1. In this manuscript, we have studied different activity terms (shown below) at a different partial pressure of gas feed at 700°C, 750°C, and 800°C reaction temperatures. ${\mathrm{C}\mathrm{H}}_4\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}$ conversion, ${\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}$, and H₂/CO ratio are expressed by equations (4)–(6). The specific feed rate of a gas is defined as the flow rate of a gas per gram weight of catalyst. The rate of CH₄ consumption and rate of CO₂ consumption are shown by equations (6) and (7) , respectively. The details of expressions for rate of H₂ formation, rate of CO formation, and rate of H₂O formation are derived in supporting information S2 and the final expressions are shown in equations (9)–(11). The arial rate of gas consumption is defined as the rate of gas consumption per unit surface area per gram weight of catalyst. The expression for the arial rate of CH₄ consumption and the arial rate of CO₂ consumption are shown in equations (10) and (11), respectively.$$\begin{array}{}\text{(2)}& {\mathrm{C}\mathrm{H}}_4\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}-{\mathrm{C}\mathrm{H}}_{4,\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}}\times 100\%\text{,}\text{(3)}& {\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{{\mathrm{C}\mathrm{O}}_{2,\mathrm{i}\mathrm{n}}-{\mathrm{C}\mathrm{O}}_{2,\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{C}\mathrm{O}}_{2,\mathrm{i}\mathrm{n}}}\times 100\%\text{,}\text{(4)}& \frac{{\mathrm{H}}_2}{\mathrm{C}\mathrm{O}}=\frac{\mathrm{M}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{f}{\mathrm{H}}_2\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}}{\mathrm{M}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{C}\mathrm{O}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}}\text{,}\text{(5)}& \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{C}\mathrm{H}}_4\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=-{\mathrm{R}}_{{\mathrm{C}\mathrm{H}}_4}=\frac{{\mathrm{C}\mathrm{H}}_4\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{x}{\mathrm{C}\mathrm{H}}_4\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{100}\text{,}\text{(6)}& \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=-{\mathrm{R}}_{{\mathrm{C}\mathrm{O}}_2}=\frac{{\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{x}{\mathrm{C}\mathrm{O}}_2\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{100}\text{,}\text{(7)}& \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{H}}_2\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}={\mathrm{R}}_{{\mathrm{H}}_2}={\mathrm{R}}_{{\mathrm{C}\mathrm{O}}_2}-3{\mathrm{R}}_{{\mathrm{C}\mathrm{H}}_4,}\text{(8)}& \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}CO\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=R_{CO}=-R_{{CH}_4}+R_{{CO}_2}\text{,}\text{(9)}& \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{H}}_2\mathrm{O}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}={\mathrm{R}}_{{\mathrm{H}}_2\mathrm{O}}={\mathrm{R}}_{{\mathrm{C}\mathrm{H}}_4}{-\mathrm{R}}_{{\mathrm{C}\mathrm{O}}_2}\text{,}\text{(10)}& \mathrm{A}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{C}\mathrm{H}}_4\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=-{\mathrm{R}\mathrm{A}}_{{\mathrm{C}\mathrm{H}}_4}=\frac{{\mathrm{R}}_{{\mathrm{C}\mathrm{H}}_4}}{\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{o}\mathrm{f}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}}\text{,}\text{(11)}& \mathrm{A}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{o}\mathrm{f}{\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=-{\mathrm{R}\mathrm{A}}_{{\mathrm{C}\mathrm{O}}_2}=\frac{{\mathrm{R}}_{{\mathrm{C}\mathrm{O}}_2}}{\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{o}\mathrm{f}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}}\text{.}\end{array}$$

3. Results and Discussion

3.1. Characterization Results

The catalyst had a 118.6 m²/g BET surface area and type IV adsorption/desorption isotherms with an H1 hysteresis loop (Figure 1(a)). The sharp inflection between 0.6 and 0.75 relative pressure regions in the isotherm indicates capillary condensation, an indication of uniformity of pore distribution in the mesoporous material [19, 20]. Pore size distribution over the catalyst surface is shown by the dV/d log W vs. W plot (where V is volume and W is the pore width) in Figure 1(a) (inset). The majority of the pores on the surface of the catalyst are 27 nm in size. Based on the BJH pore size measurement, the average pore size is determined to be 15.5 nm. The fresh 5Ni2Fe/ZrAl catalyst has a thermally stable tetragonal ZrO₂ phase (at 2θ = 30, 50, 60°; JCPDS reference number: 00-024-1164), rhombohedral Al₂O₃ phase (at 2θ = 36.8, 45.5, 60, 66°; JCPDS reference number: 01-077-2135), and cubic NiAl₂O₄ phases (at 2θ = 30, 36.8, 45.5, 50, 60, 66°; JCPDS reference number: 01-071-0965) (Figure 1(b)). Previously, metallic Ni derived from thermally stable NiAl₂O₄ (during reduction) was claimed prominent active site for high catalytic activity in DRM [21–23]. The IR spectra reveals the presence of physically adsorbed CO₂ at approximately 2349 cm⁻¹ [24], carbonate species at 1384 cm⁻¹, and formate at 2850 and 2925 cm⁻¹ [7] (Figures 1(c) and 1(d)). The IR peak at 1631 cm⁻¹ and 3444 cm⁻¹ indicates the bending and stretching vibrations of O-H, respectively [25]. Figures 1(e) and 1(f) show an HR-TEM image of a fresh and spent catalyst. The spent catalyst has carbon nanotubes with a variable diameter.

[figure(s) omitted; refer to PDF]

3.2. Catalytic Activity Result and Discussion

The catalyst 5Ni2Fe/ZrAl has uniform mesopores with an average size of 15.5 nm. It possessed thermally stable support (as tetragonal ZrO₂ phase and rhombohedral Al₂O₃ phase), metallic Ni (derived from thermally stable NiAl₂O₄ upon reduction), catalytic active sites, and various types of “CO₂ interacting species (physically adsorbed CO₂, carbonate or formate)” over the catalyst surface [23, 24, 26]. The role of Fe-Ni was previously claimed in coke suppression [18]. A uniform mesoporous, thermally stable, metallic Ni dispersed (derived from NiAl₂O₄ after reduction), and CO₂-interacting catalyst surface seems efficient for the DRM and RWGS reaction. Table 1 shows the catalytic activity results of the 5Ni2Fe/ZrAl catalyst in terms of an equilibrium constant, CH₄ conversion, CO₂ conversion, and H₂/CO ratio. At low CH₄ partial pressure and high CO₂ partial pressure, CH₄ conversion is very high (Table 1, Entry 5-6, 14-15, and 23-24) due to the instant oxidation of the substrate (CH₄) by a large number of oxidants (CO₂). Similarly, at low CO₂ partial pressure and high CH₄ partial pressure, CO₂ conversion is very high (Table 1, Entry 1-2, 10-11, 19-20) due to the instant utilization of the oxidating agent (CO₂) by a large number of substrates (CH₄). From a catalytic standpoint, the “mole of CH₄ or CO₂ consumption per gram mass of catalyst per hour,” “mole of CH₄ or CO₂ consumption per gram mass of catalyst per hour per surface area,” and “mole of product formation per gram mass of catalyst per hour” are exact presentations of catalytic activity. Thus, furthermore, Table 1 contains catalytic activity data regarding the rate of CH₄ consumption, areal rate of CH₄ consumption, the rate of CO₂ consumption, the areal rate of CO₂ consumption, the rate of H₂ formation, the rate of CO formation, and the rate of H₂O formation at a different partial pressures of feed gas during 700, 750, and 800°C reaction temperatures.

Table 1

The catalytic activity results at a different partial pressure of feed gas during 700, 750, and 800°C reaction temperatures.

Entries	Temp. (°C)	$P_{{\mathrm{C}\mathrm{H}}_4}$	$P_{{\mathrm{C}\mathrm{O}}_2}$	$P_{{\mathrm{N}}_2}$	K(×10−3)	$C_{{\mathrm{C}\mathrm{H}}_4}$%	$C_{{\mathrm{C}\mathrm{O}}_2}$%	H2/CO	${-R}_{{\mathrm{C}\mathrm{H}}_4}$(×10−3)	${-RA}_{{\mathrm{C}\mathrm{H}}_4}$(×10−3)	${-R}_{{\mathrm{C}\mathrm{O}}_2}$(×10−3)	${-RA}_{{\mathrm{C}\mathrm{O}}_2}$(×10−3)	$R_{{\mathrm{H}}_2}$(×10−3)	$R_{\mathrm{C}\mathrm{O}}$(×10−3)	$R_{{\mathrm{H}}_2\mathrm{O}}$(×10−3)
1	700	43.43	14.48	43.43	9.5	39.21	82.24	1.4	288.5	2.43	201.7	1.7	663.8	490.2	—
2		43.43	21.71	36.19	4.7	36.12	70.83	0.94	265.8	2.24	260.6	2.2	536.7	526.3	—
3		43.43	28.95	28.95	3.5	36.75	62.18	0.82	270.4	2.28	305.0	2.6	506.2	575.4	34.6
4		43.43	57.9	0	2.6	43.44	46.57	0.86	319.6	2.69	456.8	3.9	502.0	776.5	137.2
5		14.48	43.43	43.43	2.6	80.06	34.54	0.72	196.3	1.65	254.1	2.1	334.9	450.5	57.8
6		21.71	43.43	36.19	4.2	69.75	44.84	0.93	256.6	2.16	329.9	2.8	439.9	586.5	73.3
7		28.95	43.43	28.95	3.6	55.78	48.43	0.79	273.6	2.31	356.3	3.0	464.5	629.9	82.7
8		57.9	43.43	0	6.9	43.08	60.28	0.94	422.6	3.56	443.5	3.7	824.3	866.1	20.9
9		50.66	50.66	0	4.8	43.34	52.51	0.82	380.6	3.21	450.7	3.8	691.1	831.3	70.1
10	750	43.43	14.48	43.43	17.7	46.325	95.1	1.86	340.8	2.87	233.2	2.0	789.3	574.1	—
11		43.43	21.71	36.19	15.1	49.95	89.62	1.41	367.5	3.10	329.7	2.8	772.8	697.2	—
12		43.43	28.95	28.95	13.3	52.765	87.44	1.2	388.2	3.27	428.9	3.6	735.8	817.1	40.7
13		43.43	57.9	0	10.9	65.785	53.41	0.8	484.0	4.08	523.9	4.4	928.1	1008.0	39.9
14		14.48	43.43	43.43	6.2	98.31	35.95	0.85	241.1	2.03	264.5	2.2	458.8	505.6	23.4
15		21.71	43.43	36.19	14.7	96.63	55.18	1.13	355.5	3.00	406.0	3.4	660.4	761.5	50.5
16		28.95	43.43	28.95	20.8	89.81	69.31	1.3	440.5	3.71	509.9	4.3	811.6	950.5	69.4
17		57.9	43.43	0	22.3	61.29	87.58	1.23	601.3	5.07	644.4	5.4	1159.4	1245.6	43.1
18		50.66	50.66	0	18.7	67.03	74.55	1.07	575.4	4.85	639.9	5.4	1086.2	1215.3	64.5
19	800	43.43	14.48	43.43	7.9	35.65	95.66	1.2	262.3	2.21	234.6	2.0	552.3	496.9	—
20		43.43	21.71	36.19	11.3	45.2	93.13	1.24	332.6	2.80	342.6	2.9	655.1	675.2	10.0
21		43.43	28.95	28.95	16.2	55.78	91.55	0.99	410.4	3.46	449.0	3.8	782.1	859.4	38.7
22		43.43	57.9	0	3.4	55.775	91.55	0.99	410.4	3.46	898.1	7.6	333.0	1308.5	487.7
23		14.48	43.43	43.43	6.2	98.5	43.815	0.62	241.6	2.04	322.4	2.7	402.3	563.9	80.8
24		21.71	43.43	36.19	15.7	98.19	60.08	0.86	361.2	3.04	442.0	3.7	641.6	803.2	80.8
25		28.95	43.43	28.95	25.9	95.295	74.615	1.1	467.4	3.94	549.0	4.6	853.3	1016.4	81.6
26		57.9	43.43	0	43.6	78.75	90.53	1.16	772.5	6.51	666.1	5.6	1651.5	1438.6	—
27		50.66	50.66	0	23.7	71.59	89.01	1.12	614.5	5.18	764.0	6.4	1079.5	1378.5	149.5

Abbreviation and unit: (1) P: pressure; kPa (2) K: equilibrium constant of DRM; (kPa)² (3) C: conversion (4) $-$R: rate of consumption; mol/g_cat/h (5) $-$RA: arial rate; mol/m²/h (6) R: rate of formation; mol/g_cat/h.

In the dry reforming of methane, CH₄ is the substrate and CO₂ is the oxidant. Upon increasing the partial pressure of CH₄ (substrate) or CO₂ (oxidant), the rate of CH₄ and CO₂ consumption increases at reaction temperatures of 700, 750, and 800°C (Figures 2(a)–2(d)). This suggests that the presence of an increasing amount of substrate (CH₄) over a fixed oxidant (CO₂) or the presence of an increasing amount of oxidant (CO₂) over a fixed substrate (CH₄) gives rise to more collision, and a higher rate of conversion at a given temperature. At 43.43 kPa partial pressure of CH₄ and 14.48 kPa partial pressure of CO₂, the H₂/CO ratio is found to be 1.4, 1.86, and 1.2 at 700, 750, and 800°C, respectively (Table 1 Entry 1, 10, and 19). The high H₂/CO ratio at these partial pressures is due to the availability of a high concentration of CH₄ (which is primarily responsible for H₂ generation) as well as a low concentration of CO₂ (which is mainly responsible for CO generation) over the catalyst surface.

[figure(s) omitted; refer to PDF]

At constant CO₂ partial pressure (43.425 kPa) and increasing CH₄ partial pressure (from 14.475 to 57.9 kPa) at 800°C, the rate of CH₄ consumption is significantly increasing (Table 1 Entry 23–26, Figure 2 A). At constant 43.425 kPa CH₄ partial pressure at 800°C, the rate of CH₄ consumption increases sharply (0.2623 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h to 0.4104 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h) up to 29 kPa CO₂ partial pressure, after which it remains constant (Table 1 Entry 19–22, Figure 2(c)). This finding needs to be explained in more detail. Upon doubling the partial pressure of CO₂ (from 28.95 kPa to 57.9 kPa), the rate of CH₄ consumption remains constant at 800°C reaction temperature, whereas the rate of CO₂ consumption doubles (0.4490 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h at 28.97 kPa CO₂ partial pressure to 0.8981 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h at 58 kPa CO₂ partial pressure) (Figure 2(d)). It indicates that as CO₂ partial pressure increases from 29 kPa to 58 kPa, CO₂ remains converted but does not oxidize CH₄ (as in conventional DRM reaction). In this partial pressure range, it may oxidize the H₂ or carbon deposit on the catalyst surface. Under the same conditions, the rate of hydrogen formation decreases by more than 50% (0.7821 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h at 28.97 kPa CO₂ partial pressure to 0.3330 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h at 58 kPa CO₂ partial pressure) (Figure 3(c)), the rate of CO formation increases by up to 1.5 times (0.8594 mol_CO/g_cat/h at 28.97 kPa CO₂ partial pressure to 1.3085 mol_CO/g_cat/h at 58 kPa CO₂ partial pressure) (Figure 3(d)) and rate of H₂O formation increases by up to 12 times (0.0387 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2\mathrm{O}}$/g_cat/h at 28.97 kPa CO₂ partial pressure to 0.4877 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2\mathrm{O}}$/g_cat/h at 58 kPa CO₂ partial pressure) (Figure 3(f)). This observation suggests that the reverse water gas shift reaction is accelerating in the 29 kPa to 58 kPa CO₂ partial pressure range at 800°C reaction temperature, whereas the dry reforming of methane is just continuing at the same rate.

[figure(s) omitted; refer to PDF]

At 750°C reaction temperature and constant CH₄ partial pressure (43.42 kPa), with a rise of CO₂ partial pressure from 14.475 kPa to 28.95 kPa, the rate of CH₄ formation constantly increases (from 0.3408 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h to 0.3772 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h), but the rate of H₂ formation decreases from 0.7893 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h to 0.7358 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h, and the rate of water formation becomes significant. This observation indicates the presence of a reverse water gas shift reaction (Table 1 Entry 10–12, Figures 2(c), 3(c), and 3(f)). However, on the further increase of CO₂ partial pressure up to 57.9 kPa; a rise of rate CH₄ consumption (0.3882 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h to 0.4840 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h), a rise in the rate of CO₂ consumption (0.4289 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h to 0.5239 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h), a rise in the rate of H₂ formation (0.7358 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h to 0.9281 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h), and a rise in the rate of CO formation (0.8171 mol_CO/g_cat/h to 1.0081 mol_CO/g_cat/h) were achieved without affecting the rate of H₂O formation much (Table 1 Entry 12-13, Figures 2(c), 3(c), and 3(f)). Thus, RWGS is competent up to 28.95 kPa CO₂ partial pressure, but at 57.9 kPa CO₂ partial pressure, RWGS product formation rates are not significantly affected, whereas DRM product formation rates are significantly affected. At constant CO₂ partial pressure (43.425 kPa) and increasing CH₄ partial pressure (from 14.475 kPa to 57.9 kPa) at 750°C, the rate of CH₄ consumption is increased to about ∼2.5 times (0.2411 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h to 0.6013 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_cat/h), rate of CO₂ consumption is increased to about ∼2.5 times (0.2645 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h to 0.6444 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{O}}_2}$/g_cat/h), the rate of H₂ formation is again increased to 2.5 times (0.4588 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h to 1.1594 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h), and the rate of CO formation is again increased to about ∼2.5 times (0.5056 mol_CO/g_cat/h to 1.2456 mol_CO/g_cat/h) (Table 1 Entry 14–17, Figures 2(a), 2(b), 3(a), and 3(b)). No such progressive correlation with the rate of H₂O formation is found (Figure 3(e)), but it remains significant. It demonstrates that at 750°C reaction temperature, with constant CO₂ partial pressure and increasing CH₄ partial pressure, the dry reforming of methane reaction progressed continuously.

At a low reaction temperature of 700°C, the least activity is noticed. On constant CH₄ partial pressure (43.42 kPa) and increased partial pressure of CO₂ from 14.47 kPa to 57.9 kPa, the rate of H₂ production decreases continuously (0.6638 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h to 0.5020 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_cat/h), the rate of CO formation increases continuously (0.4902 mol_CO/g_cat/h to 0.7765 mol_CO/g_cat/h), and the rate of H₂O formation is found to a maximum of 0.1372 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2\mathrm{O}}$/g_cat/h at 57.9 kPa CO₂ partial pressure (Table 1 Entry 1–4; Figures 3(c)–3(f)). It indicates that at a constant CH₄ partial pressure and a rising partial pressure of CO₂ at 700°C, the RWGS product formation rate influences the DRM product formation rate. However, at constant CO₂ partial pressure (43.42 kPa) and rise of CH₄ partial pressure (14.47 kPa to 57.9 kPa), the rate of H₂ formation and rate of CO formation are increased, but no such correlation is found with the rate of water formation (Table 1 Entry 5–8; Figures 3(a), 3(b), and 3(e)). It indicates that a higher rate of H₂ and CO formation may be caused by a pronounced DRM reaction in which a portion of H₂ participates in the RWGS reaction, but this does not affect the high rate of H₂ production.

Comparing the catalytic activity results at all temperatures reveals that at 14.47 kPa CO₂ partial pressure, 43.42 kPa CH₄ partial pressure, and 43.42 kPa N₂ partial pressure, the RWGS reaction does not occur at 700°C, 750°C, and 800°C reaction temperatures. In these instances, DRM activity is, however, low. At 800°C reaction temperature, the 43.42 kPa CO₂ partial pressure and 57.9 kPa CH₄ partial pressure (with diluent N₂), the equilibrium constant of DRM is maximum (0.436 kPa²). In this reaction condition, the RWGS reaction is not exiting, and the rate of CH₄ consumption (0.7725 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_Cat/h), the arial rate of CH₄ consumption (0.00651 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/m²/h), the rate of H₂ formation (1.6515 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_Cat/h), and the CO formation (1.4386 mol_CO/g_Cat/h) are the maximum among all tested conditions. In the mean of the maximum rate of CO₂ consumption, 57.9 kPa CO₂ partial pressure, 43.42 kPa CH₄ partial pressure and 800°C reaction temperature are found appropriate. The equal partial pressure of CH₄ and CO₂ (50.66 kPa) at 750°C, 0.6145 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_Cat/h rate of CH₄ conversion, 1.0795 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_Cat/h rate of H₂ formation, and 1.3785 mol_CO/g_Cat/h CO formation were noticed.

The apparent activation energy for CH₄ dissociation, CO₂ dissociation, H₂ formation, and CO formation is shown in Figure 4. It is interesting to note down that with an increasing flow rate of CH₄ from 10 ml/min to 40 ml/min and a fixed CO₂ flow rate of 30 ml/min, the apparent activation energies for CH₄ dissociation, CO₂ dissociation, and H₂ formation are increasing (Figure 4). Upon increasing the flow rate of CO₂ and fixing CH₄ flow rate at 30 ml/min, no such correlation is found (Figure S3). It indicates that the C-H dissociation of CH₄ is a rate-determining step (Figure 4 and Figure S3). The Mears criterion (for external diffusion concerning CH₄ and CO₂) and Weisz-Prater criterion (for internal diffusion for CH₄ and CO₂) values for the 5Ni2Fe/ZrAl catalyst were found < 0.1 and < 1 (in the Supporting Information S4), respectively, in every case [27]. The absence of both external and internal mass transfer limitations is found in the 5Ni2Fe/ZrAl catalyst at various gas feed rates.

[figure(s) omitted; refer to PDF]

4. Conclusion

The thermally stable catalytic active sites NiAl₂O₄ as well as the CO₂-interacting mesoporous surface of the 5Ni2Fe/ZrAl catalyst were found to be effective in the DRM reaction and a competing RWGS reaction. C-H dissociation of CH₄ is the rate-determining step. Upon different flow rates of gas feed over the 5Ni2Fe/ZrAl catalyst, external and internal mass transfer limitations are absent. At 800°C reaction temperature, the constant 43.43 kPa CH₄ partial pressure, and 28.95 to 57.9 kPa CO₂ partial pressure range, the reverse water gas shift reaction is accelerated over a zirconia-alumina-supported Ni-Fe catalyst. Again, at 750°C reaction temperature, at a constant 43.43 kPa CH₄ partial pressure and 28.95 kPa CO₂ partial pressure, RWGS is noticed. At 750°C, 43.425 kPa CO₂ partial pressure and 14.475 kPa to 57.9 kPa CH₄ partial pressure, the rate of DRM reaction is increased to about 2.5 times. At a reaction temperature of 700°C, the catalyst’s performance is diminished. The optimal reaction conditions for DRM catalysis are 800°C reaction temperature, 43.42 kPa CO₂ partial pressure, and 57.9 kPa CH₄ partial pressure. At these optimal reaction conditions, the catalyst shows a 0.436 kPa² equilibrium constant, a 0.7725 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/g_Cat/h rate of CH₄ consumption, a 0.00651 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{C}\mathrm{H}}_4}$/m²/h arial rate of CH₄ consumption, a 1.6515 ${\mathrm{m}\mathrm{o}\mathrm{l}}_{{\mathrm{H}}_2}$/g_Cat/h rate of H₂ formation, a 1.4386 mol_CO/g_Cat/h rate of CO formation. On increasing the flow rate of CH₄, the apparent activation energy for CH₄ dissociation, CO₂ dissociation and H₂ formation are increasing which indicates that C-H dissociation of CH₄ is a rate-determining step in DRM over a 5Ni2Fe/ZrAl catalyst. The mass transfer limitation is absent over this catalyst.

Disclosure

The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB).

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