Effect of Ni-Fe Segregation on the Deactivation Behavior of Co-Precipitated NiFeAlOx Catalysts in the CO2 Methanation Reaction

Motivation

The CO₂ methanation reaction is one of the key steps in the power-to-gas process chain. For potential commercialization, efficient and cost-effective catalyst materials need to be developed that feature high intrinsic activity to achieve high CH₄ yields at low temperatures. In addition, the catalyst should be able to withstand high reaction temperatures that may evolve due to hotspot formation in this highly exothermal reaction. Catalyst deactivation by temperature-induced effects may further be aggravated by the high partial pressure of steam at high CO₂ conversion levels. Fe was shown to be an efficient promoter for significantly enhancing the activity of Ni-Al-based catalysts ^[1-3]. However, comprehensive and time-resolved studies on the catalyst deactivation behavior featuring post-characterization studies to draw conclusions on the structure-activity relationship are lacking so far and therefore are addressed in this work.

Experimental

A NiAlO_x and a NiFeAlO_x (Ni/Al = 1, Ni/Fe = 5.5) catalyst were prepared by co-precipitation at pH 9 and 30 °C. After aging for 18 h, the precipitate was dried at 80 °C and calcined in flowing synthetic air at 450 °C for 6 h. After in situ reduction at 485 °C for 5 h (5 % H₂ in Ar), the catalysts were aged in thermodynamic equilibrium for 72 h at 8 bar, 36 NL (g_cat h)^-1 (Ar/H₂/CO₂ = 5/4/1) and temperatures between 350 and 450 °C. In between, the catalysts were periodically cooled down to 230 °C to evaluate their activity under differential conditions (4 bar, 600 NL (g_cat h)^-1, Ar/H₂/CO₂ = 75/4/1, 230 °C) and to track deactivation away from thermodynamic equilibrium. After definite aging times, the catalysts were thoroughly characterized without exposure to air.

Results and discussion

Material characterization prior to catalyst testing

The catalysts feature a hydrotalcite-derived mixed oxide structure with mesoporous structure. The BET surface area amounts to 293 m² g_cat^-1 for the benchmark NiAlO_x (37 % Ni) and 266 m² g_cat^-1 for NiFeAlO_x (35 % Ni, 5 % Fe). Catalyst activation leads to partial reduction of Ni in NiAlO_x (60 %) and Fe as well as Ni in NiFeAlO_x. For the Fe-promoted catalysts, in situ X-ray powder diffraction analysis reveals the presence of Ni-Fe alloy particles by the characteristic shifts of the Ni reflexes to higher diffraction angles. Ferromagnetic measurements show a clear increase of all relevant magnetic characteristics for NiFeAlO_x, indicating that Fe substantially contributes to the magnetic properties of Ni. Since Fe is known to be a weak adsorbent for H₂ at room temperature ^[4], a decrease of the metal surface area from 30.1 m² g_cat^-1 for the benchmark NiAlO_x to 20.1 m² g_cat^-1 is observed in static H₂ chemisorption, indicating that significant amounts of Fe are present on the Ni surface after reduction.

Catalyst testing and material characterization after definite aging times

As illustrated in Figure 1A, the initial CH₄ weight time yield over NiFeAlO_x is 1.5 times higher compared to NiAlO_x. Despite, the initial apparent activation energy is 76.2±1.4 kJ mol^-1 for NiAlOx and 89.5±1.4 kJ mol^-1 for NiFeAlO_x, which clearly shows that Fe modifies the active Ni centers and indicates a compensation effect ^[5] for the methane production rate over NiFeAlO_x.

Aging of NiAlO_x at 450 °C leads to a strong decrease of the catalytic activity. During the first 6 h of aging, already 28 % of the initial activity are lost. After 40 h of aging the catalytic activity reaches a stable level at 49 %. The Ni surface area decreases to 15.3 % during 6 h of aging (BET surface area 207 m² g_cat^-1), after 40 h it stays constant at approx. 12 m² g_cat^-1 (BET surface area 131 m² g_cat^-1)_. Crystallite size estimation from XRD indicates crystallite growth from 3.2 to 4.8 nm during 32 h of aging. Similarly, the CO₂ uptake declines from 199 to 123 µmol g_cat^-1 during 6 h and to 84 µmol g_cat^-1 during 40 h, indicating that besides sintering effects on the Ni particles also the basic site density decreases. Interestingly, all sorption parameters as well as the Ni crystallite size remain constant once the constant activity level has been achieved. However, no direct correlation of any of the sorption parameters to the catalytic activity can be found. It is therefore hypothesized, that a complex interplay of the total surface area, the Ni surface area and the basic site density is responsible for changes in the catalytic activity of the NiAlO_x catalyst. Throughout the aging treatment, the apparent activation energy remains constant, which indicates that the type of the active sites does not change.

In contrast, the activity for NiFeAlO_x rises within the first 6 h, with a maximum value of 122 % of the initial one before apparent catalyst deactivation starts. After 72 h of aging, the activity is still at 90 %. The performance of the NiFeAlO_x aged at different temperatures is illustrated in Figure 1B. When aged at 350 °C, a stable activity level is obtained after 20 h of aging (119 %) and no apparent deactivation takes place. When aged at 400 °C, the maximum activity is reached at 129 % after 16 h of aging, from where on slow deactivation is observed. It is therefore hypothesized that two countervailing aging effects take place on NiFeAlO_x. Besides deactivation phenomena similar to NiAlO_x (all relevant sorption properties of NiFeAlO_x are affected similarly to NiAlO_x, cf. Table 1), X-ray diffraction analysis indicates that the previous Ni-Fe alloy gradually segregates during aging. Accordingly, the magnetization of the NiFeAlO_x catalyst first rises but then decreases to finally reach the characteristics of the NiAlO_x catalyst after 72 h of aging. This agrees well with the higher loss of metal surface area for NiFeAlO_x (73 %) compared to NiAlO_x (60%), where the segregated Fe species might block H₂ adsorption sites on Ni. The results indicate that the composition of the Ni-Fe particles is dynamically modified under aging conditions, which leads to a temporal improvement of the CH₄ production rate. The impact of Fe on the active sites is further corroborated by a clear change of the activation energy to 104.8±3.8 kJ mol^-1 already after 6 h of aging. Again, the opposing enhanced activity indicates a compensation effect ^[5].

Figure 1A: WTY(CH4) as a function of aging time for NiAlOx and NiFeAlOx, aging temperature 450 °C.	Figure 1B: Normalized WTY(CH4) as a function of aging time for NiFeAlOx, aging carried out at different temperatures.

Table 1: Catalyst sorption data and activation energies as a function of aging time and temperature.

	ta	Ta	SNi	DNi	dPa	SBET	U(CO2)	EA
	h	°C	m2 gcat-1	%	nm	m2 gcat-1	µmol gcat-1	kJ mol-1
NiAlOx	0	-	30.1	12.2	3.2	293	198.7	76.2±1.4
	6	450	15.3	6.2	4.2	207	123.1	73.9±3.6
	40	450	12.2	5.0	4.8	131	83.8	75.0±2.5
	72	450	12.1	4.9	4.8	132	81.5	73.7±2.1
NiFeAlOx	0	-	20.1	8.6	3.2	266	218.7	89.5±1.4
	6	450	13.4	5.8	3.7	130	121.0	104.8±3.8
	40	450	6.8	2.9	4.4	110	86.9	104.2±2.7
	72	450	5.4	2.3	5.3	108	83.5	101.9±1.2
	72	350	9.0	3.9	4.0	191	139.8	99.7±1.9
	72	400	8.1	3.5	4.7	131	108.7	102.3±0.9

^a determined by XRD, Ni(Fe) reflex at 2θ = 51.3–51.7 °.

Conclusion

It is shown that the high stability of co-precipitated NiFeAlO_x catalysts in the CO₂ methanation reaction is caused by a temporal increase of the catalytic activity, that is linked to (partial) segregation of the Ni-Fe alloy particles, induced by hydrothermal aging. For isothermal low-temperature methanation, the catalytic activity of NiFeAlO_x catalysts therefore can be strongly enhanced by an aging pretreatment. Ambient XPS and high-resolution synchrotron X-ray data might contribute to derive a deeper understanding of the underlying segregation mechanisms and the Ni-Fe particle compositions desirable to maximize the catalytic activity of Ni-Fe-based CO₂ methanation catalysts.

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