Study of the main reactions involved in reforming of exhaust gas recirculation (REGR) in gasoline engines

Abstract

In a previous paper, it was demonstrated that a 1 wt% Rh catalyst supported on modified ZrO₂ exhibits a high activity and stability in REGR conditions. The purpose of the present study was to investigate the main reactions involved in REGR conditions in the presence of this catalyst, namely steam reforming (SR), dry reforming (DR), and the reactions producing methane, i.e. methanation, decomposition and hydrogenolysis. Isooctane (C₈H₁₈) was used as a molecule representative of gasoline, and a mixture of N₂, H₂O, CO₂ and O₂ as model exhaust gas. A reaction scheme was proposed for REGR conditions. Based on the study of individual reactions (SR, DR and methane formation) at 450 °C, 520 °C and 580 °C, the complex system can be described following three consecutive steps: reforming reactions, yielding H₂ and CO₂ as primary products; reverse water gas shift (RWGS), yielding CO as secondary product; and finally methanation reactions, specially with CO, yielding CH₄ as tertiary product.

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1. Introduction

The exhaust gas recirculation (EGR) technique was developed in the 1970's in order to reduce NO_x emissions^1–3 by decreasing (i) the flame temperature and (ii) the oxygen concentration, by dilution effect, in the combustion chamber. Presently, EGR systems are used in a majority of Diesel engines but their use is limited to 15 to 20% of EGR, which are the maximum admitted in gasoline engines. A higher dilution is damaging for the quality of the combustion and even causes problems of ignition.^2,3 The Reformed Exhaust Gas Recirculation (REGR), consisting in reforming the fuel with exhaust gas to produce hydrogen in the EGR loop, was developed in the 1980's.^4–9 It improves the overall engine efficiency, decreases the emissions of NO_x and hydrocarbons and reduces the variation of cylinder pressure.^10–13 The REGR reaction involves a complex system of chemical reactions, in which primary reactions lead to the decomposition of the hydrocarbon, while many secondary reactions may also take place between the different products and the reactants. The main catalytic reactions are steam reforming (1), water gas shift (3), dry reforming (4), partial oxidation (5) and total oxidation (6). Steam reforming (1) is normally written with CO as product, but the hydrocarbon can be also steam reformed directly to CO₂ and H₂(2).^14–16

Methane is a typical product associated with the reforming reactions, and may result from the secondary reaction between H₂ and CO, (7) and/or (8), or H₂ and CO₂(9) or directly from hydrocarbon decomposition (10) or hydrogenolysis (11).⁴

In a previous study,¹⁷ we have shown that a Rh(1 wt%) catalyst supported on doped zirconia is very active for hydrogen production in EGR conditions with isooctane as model molecule. It was demonstrated that, in the reaction conditions used, a slight deactivation occurred, attributed to metal sintering and to the poisoning of the metal-support interface. The carbon deposition was negligible. The H₂ yield was very close to that predicted by the thermodynamic equilibrium.

The aim of the present paper is to evaluate the impact of steam and dry reforming reactions on the product distribution and more especially on the hydrogen yield obtained in REGR conditions. For that purpose, the steam and dry reforming as well as the reforming of the exhaust gas recirculation reactions will be carried out by varying the contact time in order to establish the reaction pathways. As methane is a key product to optimize the hydrogen yield, the reactions responsible for methane formation as well as those leading to its transformation will be also studied. Finally, a thermodynamic study will be carried out in order to separate the kinetic limitations from the thermodynamic ones. This work will help adjusting the experimental conditions in order to optimize the hydrogen yield in the REGR conditions.

2. Experimental

2.1 Catalyst

The catalyst used in the present study is a Rh(1 wt%) supported on zirconia doped with La₂O₃, Nd₂O₃ and Y₂O₃. Its preparation, as well as its characteristics, were described in detail in ref. 17. This catalyst presents a BET surface area of 62 m²/g and a rhodium dispersion of 60%.

2.2 Catalytic test

The model feed composition of exhaust gas recirculation (with gasoline) was given by PSA Peugeot Citroën Group. It contains 2.2 vol.% of isooctane, 13 vol.% of CO₂, 12 vol.% of H₂O, 1 vol.% of O₂ and 71.3 vol.% of N₂ (REGR conditions). A blank test in REGR conditions showed that without catalyst all the oxygen is consumed, and it was demonstrated that C₈H₁₈ oxidation reactions occur mainly in the homogeneous gas phase with a C₈H₁₈ conversion of around 6% (≈ 3% for partial oxidation (5) and ≈ 3% for total oxidation (6)). The total gas flow rate was 250 Ncm³ min⁻¹. Anhydrous isooctane (C₈H₁₈) was supplied by Carlo Erba (99.5%); CO₂, O₂ and N₂ were provided by Air Liquide. Ultra pure water was used.

The reforming of exhaust gas recirculation (REGR) reaction was carried out in a quartz tubular continuous flow reactor. The experimental procedure is given in detail in ref. 17. The catalyst was activated under the REGR conditions, i.e. in the presence of all the reactants, by heating with a ramp of 10 °C min⁻¹, from 130 °C to the reaction temperature.

Steam and dry reforming reactions were studied at different temperatures in order to identify the individual contributions of H₂O and CO₂ in the reforming of exhaust gas recirculation, using various catalyst masses. Three reaction conditions were used:

-steam reforming (SR): in the presence of all the reactants of REGR conditions except CO₂ and O₂, replaced by nitrogen gas. The inlet partial pressures of C₈H₁₈ (2.2%) and H₂O (12%) were maintained constant;

-dry reforming (DR): in the presence of all the reactants of REGR conditions except H₂O and O₂, replaced by nitrogen gas. The inlet partial pressure of C₈H₁₈ (2.2%) and CO₂ (13%) were maintained constant;

-reforming of exhaust gas reforming (REGR): corresponding to the REGR conditions given at the beginning of this paragraph.

These reactions conditions are reported in Table 1(a).

Table 1 Inlet composition (molar %) for the study of (a) isooctane conversion by REGR, SR, DR, (b) methane formation and (c) methane conversion by REGR, SR, DR reactions

		Inlet feed composition (%)
	Reactions	C8H18	H2O	CO2	O2	CH4	CO	H2	N2
(a)	REGR	2.2	12.0	13.5	1.0	—	—	—	71.3
	SR	2.2	12.0	—	—	—	—	—	85.8
	DR	2.2	—	13.5	—	—	—	—	84.3
(b)	MetCO	—	—	—	—	—	8.0	14.0	78.0
	MetCO2	—	—	13.5	—	—	—	14.0	72.5
	HYD	2.2	—	—	—	—	—	14.0	83.8
	DECOMP	2.2	—	—	—	—	—	—	97.8
(c)	CH4 REGR	—	12	13.5	1.0	2.2	—	—	71.3
	CH4 SR	—	12	—	—	2.2	—	—	85.8
	CH4 DR	—	—	13.5	—	2.2	—	—	84.3

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This study was carried out by using a fixed flow rate of the feed and changing the amount of the catalyst in the range of 15 mg – 300 mg at 450 °C, 520 °C and 580 °C. Each experimental run used a fresh catalyst and was carried out for 2 h time-on-stream for each catalyst mass and temperature. The experimental points were collected at 50 min of reaction, which minimizes the effect of catalyst deactivation, which always occurs.¹⁷

The catalyst performances are characterized by the product yield:

Where is the molar flow rate of isooctane at the inlet of the reactor and F^out_x the molar flow rate of the product x at the oulet.

The isooctane, oxygen, carbon dioxide and water conversions, in % (example for isooctane):

The selectivity to H₂, CO₂, CO, CH₄ (example for H₂):

The selectivities are given in mol of product per mol of C₈H₁₈ converted. This avoids a complex normalization procedure to 100%. Reactions (1) to (6) show that the maximum selectivities are 25 for H₂ (eqn (2)), 8 for CO₂ (eqn (2) or (6)) and 16 for CO (eqn (4)).

3. Results and discussion

Fig. 1 shows the evolution of C₈H₁₈ conversion with the ratio of weight of catalyst to C₈H₁₈ flow rate () in the range of 1.0 – 21.0 g_cat/mol C₈H_18 0/h at reaction temperatures of 450 °C, 520 °C and 580 °C.

Fig. 1 Isooctane conversion as a function of contact time () at 450 ºC, 520 ºC and 580 ºC for REGR, SR and DR conditions.

From this figure, it can be seen that the increase in the contact time () results in an increase in C₈H₁₈ conversion. Moreover, the conversion reaches a plateau for the highest W/F values in the majority of reaction conditions: at 450 °C (REGR, SR, DR), 520 °C (REGR, SR, DR) and 580 °C (DR). This indicates that C₈H₁₈ conversion depends on the catalyst mass and approaches the equilibrium conversion for the highest . On the contrary, this phenomenon is not observed at 580 °C for the SR and REGR conditions: a linear correlation between the conversion and the contact time was observed in the whole W/F range, and a C₈H₁₈ conversion of 100% was achieved for a of 21 g_cat/mol C₈H_18 0/h in REGR conditions.

At low contact times, REGR and SR conditions lead to similar C₈H₁₈ conversion whatever the temperature, which means that the presence of CO₂ and O₂ (in REGR conditions) has no impact on the catalyst activity at the beginning of the reaction. Consequently, the reaction between C₈H₁₈ and H₂O (steam reforming) occurs in the same way in the presence or not of CO₂ and O₂. However, when the contact time increases, especially at 580 °C, the C₈H₁₈ conversion increases in a larger extent in the presence of CO₂, O₂ and H₂O together, compared to the conversion obtained by steam reforming. Remembering that C₈H₁₈ oxidation reactions occur mainly in the homogeneous gas phase with a total consumption of oxygen and a C₈H₁₈ conversion of around 6% (see paragraph 2.2), this result indicates that the presence of CO₂ has a positive effect on the isooctane conversion, but only at 580 °C and when the W/F value increases. At lower temperatures, the dry reforming reaction is probably kinetically limited and the steam reforming reaction is the major reaction contributing to isooctane consumption.

Finally, Fig. 1 shows that, whatever the temperature, the conversion reached in the dry reforming (DR) condition is always lower than those obtained by SR and REGR conditions, and this trend is all the more pronounced as the W/F is important.

In the following, the three reaction conditions (SR, DR and REGR) will be studied in more detail.

3.1 Steam reforming (SR)

Fig. 2 shows the evolution of selectivities to various products (H₂, CO, CO₂ and CH₄) as a function of the C₈H₁₈ conversion obtained for different contact times at 450 °C, 520 °C and 580 °C, for SR conditions.

Fig. 2 Selectivity as a function of isooctane conversion at 450 °C, 520 °C and 580 °C for SR conditions. Data obtained at different contact times.

From this figure it can be seen that the selectivity does not depend directly on the temperature: whatever the temperature and the considered product, the selectivity as a function of the conversion follows the same evolution. H₂ appears to be a primary product, since its selectivity decreases with contact times. CH₄ is at least a secondary product, its selectivity increasing with the contact time with no CH₄ being produced for low values of conversion. This result indicates that CH₄ is probably not produced directly by C₈H₁₈ decomposition (reaction (10)) or C₈H₁₈ hydrogenolysis (reaction (11)). The reactions yielding methane will be discussed in detail in paragraph 3.4.1.

Surprisingly, CO₂ seems to be a primary product and CO a secondary product. The selectivities to H₂ and CO₂ are around 23 and 6 mol/mol, respectively, for a C₈H₁₈ conversion extrapolated at 0%. It means that, for SR conditions, the major reaction is probably the following (2): C₈H₁₈ + 16H₂O → 8CO₂ + 25H₂ which leads to selectivities close to those observed experimentally (25 and 8 for H₂ and CO₂, respectively).

From these results, the following reaction scheme can be proposed, considering that CO is a secondary product and comes from reverse water gas shift reaction (RWGS) and CH₄ from CO₂ or CO:

3.2 Dry reforming (DR)

Fig. 3 shows the evolution of selectivities to various products (H₂, CO and CH₄) as a function of the C₈H₁₈ conversion obtained for different contact times at 450 °C, 520 °C and 580 °C, for DR conditions.

Fig. 3 Selectivity as a function of isooctane conversion at 450 °C, 520 °C and 580 °C for DR conditions. Data obtained at different contact times.

This figure shows that at 450 °C and 520 °C, the selectivity to CO decreases with C₈H₁₈ conversion whereas at 580 °C it is independent of the conversion. Consequently, in these conditions, selectivity to CO seems to depend directly on the temperature. By contrast, the selectivity to H₂ shows to be independent of both the conversion and the temperature. In average, the selectivities to H₂ and to CO are around 7 and 17 mol/mol, respectively, which are close to what is given by the reaction (4): C₈H₁₈ + 8CO₂ → 16CO + 9H₂. The Reverse Water Gas Shift (RWGS) reaction may also occur, slightly increasing the selectivity to CO and decreasing the one to H₂. However, the amount of H₂O, calculated from the O atomic balance, is always close to zero.

The reaction scheme proposed for the dry reforming reaction in the present reaction conditions is the following:

C₈H₁₈ + 8CO₂ → 16CO + 9H₂

H₂ + CO₂ ↔ CO + H₂O

3.3 Reforming of exhaust gas recirculation (REGR)

Fig. 4 shows the evolution of selectivities to various products (H₂, CO and CH₄) as a function of the C₈H₁₈ conversion obtained for different contact times at 450 °C, 520 °C and 580 °C, for REGR conditions.

Fig. 4 Selectivity as a function of isooctane conversion at 450 °C, 520 °C and 580 °C for REGR conditions. Data obtained at different contact times.

On the one hand, the selectivity to H₂ decreases from C₈H₁₈ conversions higher than ∼30% at 520 °C and 580 °C, and presents values between those obtained in SR and DR conditions, as expected. Note that, at these temperatures, it is not experimentally possible to obtain conversions lower than roughly 30% by decreasing the mass of catalyst. At 450 °C, conversions as low as 10% can be reached and an optimum in selectivity to H₂ can be seen: it increases up to 30% of C₈H₁₈ conversion, and then decreases. This type of evolution was not observed for SR (C₈H₁₈ + H₂O) and DR (C₈H₁₈ + CO₂) at 450 °C. However, the simultaneous presence of H₂O and CO₂ could be at the origin of this phenomenon. The presence of O₂, or more precisely, the possibility of a total oxidation, could be another possible explanation: for low C₈H₁₈ conversion, if total oxidation predominates the selectivity to H₂ will converge to zero. Although blank experiments reveal that the whole O₂ is consumed in homogeneous gas phase, the hypothesis that a part of O₂ reaches the catalyst surface is not excluded. The presence of an optimal selectivity to H₂ will be discussed more in detail in the next section.

On the other hand, the selectivity to CO increases up to 50% of C₈H₁₈ conversion, and then is constant, as observed for SR and DR conditions. As far as the selectivity to CH₄ is concerned, it increases continuously from 50% of C₈H₁₈ conversion. Thus, for REGR conditions, CO and CH₄ are not primary products, as it was established for the experiments in SR conditions. It can be inferred that the reaction scheme deduced from the experiments performed on SR and DR conditions can be extrapolated for REGR conditions. As the inlet gas contains a large amount of CO₂, it is not possible to determine its net production. CO₂ is very probably a primary product as observed in SR.

A comparison of SR, DR and REGR in terms of H₂ yield, obtained for different contact times at 450 °C, 520 °C and 580 °C, can be made from Fig. 5. It can be seen that the SR presents the most favorable conditions for H₂ production up to 80% of C₈H₁₈ conversion. Above this value the presence of CO₂ and O₂ (REGR conditions) becomes beneficial. For DR conditions, the absence of H₂O induces a drastic decrease in the H₂ yield, and a decrease in C₈H₁₈ conversion in the same W/F range.

Fig. 5 H₂ selectivity as a function of isooctane conversion at 450 °C, 520 °C and 580 °C for SR, DR and REGR conditions. Data obtained at different contact times.

3.4 Reactions involving methane production or consumption

One of the major problems associated with reforming reactions is the formation of methane. H₂ and CH₄ are most of the time thermodynamically and kinetically correlated: if CH₄ formation increases significantly, normally the H₂ production decreases (and vice-versa). Consequently, for this type of systems, it is important to find a catalyst that prevents CH₄ formation or is very active for methane conversion. It has been previously shown that methane is probably not a primary product in REGR conditions. One of the aims of the present study is to find the reactions leading to the formation or consumption of methane in the REGR conditions.

3.4.1. Methane Formation. The formation of CH₄ may result from the reactions between H₂ and CO or CO₂ (CO methanation, MetCO, or CO₂ methanation, MetCO₂),^18–20 H₂ and C₈H₁₈ (Hydrogenolysis, HYD)^21,22 and from C₈H₁₈ (Decomposition, DECOMP).^19,23 These four reactions were studied individually in conditions as close as possible to the REGR conditions (temperature of 580 °C, total pressure of 1.3 bar and WHSV of 137.8 h⁻¹). In REGR standard conditions (with C₈H₁₈ as reactant), the product distribution obtained at the beginning of the catalytic test is the following: H₂ (14.0 vol%), CO (8.0 vol%) and CH₄ (1.4 vol%). The inlet composition for the study of methane formation reaction was based on this product distribution for H₂ and CO and on the inlet composition in REGR conditions for CO₂ and C₈H₁₈, as detailed in Table 1(b). The results are reported in Table 2.

Table 2 Conversion of reactants and yield in products obtained for isooctane REGR and methane formation reactions after 50 min of time-on-stream at 580 °C

		Conversion (%)					Yield (mmol/min/gcat)
		C8H18	H2O	CO2	CO	H2^a	H2O	CO2	CO	H2	CH4
a Negative value of conversion correspond to formation.
REGR	Thermo	100	64	16	—	—			10.2	11.7	4.2
	Exp.	68	54	7	—	—			7.7	12.1	1.2
MetCO	Thermo	—	—	—	45	39	0.7	1.0			1.7
	Exp.	—	—	—	40	33	0.3	1.1			1.4
MetCO2	Thermo	—	—	36	—	42	3.8		3.3		0.2
	Exp.	—	—	35	—	38	3.6		3.4		0.2
HYD	Thermo	91	—	—	—	100					11.7
	Exp.	8	—	—	—	−4					0.2
DECOMP	Thermo	100	—	—	—	—					2.2
	Exp.	2.8	—	—	—	—					0

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For REGR conditions, the thermodynamic equilibrium predicts a H₂ yield of 11.7 mmol/min/g_cat. Experimentally, this value is easily reached and even exceeded (12.1 mmol/min/g_cat). Furthermore, a maximal conversion of 68% is reached for C₈H₁₈ instead of the 100% predicted by thermodynamics. The high H₂ yield can be explained by the kinetic inhibition of methane formation: it was observed a maximum of 1.2 compared to 4.2 mmol CH₄/min/g_cat predicted by thermodynamics.

For MetCO conditions, one can observe that the amount of CH₄ produced (1.4 mmol/min/g_cat) is almost the same as that obtained in REGR conditions, but lower than the thermodynamic prediction (1.7 mmol/min/g_cat). This result suggests that (i) it is possible that the CH₄ produced during REGR comes from CO and (ii) the MetCO is kinetically limited in the applied conditions.

For MetCO₂ conditions, the product distribution obtained is similar to that predicted at the thermodynamic equilibrium. However, the quantity of methane formed (0.2 mmol/min/g_cat) is low compared to REGR conditions. Hence, probably a small amount of CH₄ is formed from CO₂, but not all the CH₄, because this reaction is thermodynamically limited in the applied conditions.

The HYD conditions show that it is thermodynamically possible to produce a great amount of CH₄ (11.7 mmol/min/gcat) from H₂ and C₈H₁₈. Nevertheless, one can deduce from the results presented in Table 2 that this reaction is strongly kinetically inhibited: the CH₄ formation (0.2 mmol/min/gcat) is extremely low compared to the value calculated at the thermodynamic equilibrium.

For DECOMP conditions, it is thermodynamically possible to completely convert isooctane to produce 2.2 mmol/min/gcat of methane, and also ethylene (2.5 mmol/min/gcat) and propylene (1.2 mmol/min/gcat), as major products. However, only 2.8% of isooctane were converted in the experimental conditions and methane formation was not observed (Table 2), the only products being isobutene and traces of C7–C8 molecules.

In conclusion, the studies on the reactions potentially responsible for methane formation showed that most of the methane probably comes from the reaction between CO and H₂ thus confirming that methane is a secondary product, as proposed in paragraph 3.1. The formation of CH₄ directly from C₈H₁₈ or by methanation of CO₂ is strongly kinetically and thermodynamically limited, respectively.

3.4.2. Methane reactivity. The second objective of the study on methane reactivity is to determine the possibility of consuming CH₄ after its formation in the REGR conditions. Three types of reaction conditions were studied:

-CH₄ REGR conditions, for which C₈H₁₈ was totally replaced by CH₄ (same inlet partial pressure);

-steam reforming (CH₄ SR) conditions, corresponding to the reaction between CH₄ and H₂O only;

-dry reforming (CH₄ DR) conditions, corresponding to the reaction between CH₄ and CO₂ only.

Reactions conditions are summarized in Table 1(c).

The results obtained in these three experimental conditions, reported in Table 3, show that high conversions of CH₄ are obtained whatever the reaction conditions (REGR, SR and DR) without kinetic limitations. The experimental values are close to the thermodynamic predictions. Thus, CH₄ is very reactive at 580 °C confirming that any deviation from the thermodynamic equilibrium is due to CH₄ formation and not to CH₄ reactivity.

Table 3 Conversion of reactants and yield in products obtained for CH₄ REGR, SR and DR reactions after 50 min of time-on-stream

		Conversion (%)				Yield (mmol/min/gcat)
		CH4	H2O	CO2^a	CO	H2O	CO2	H2	CO
a Negative values of conversion correspond to formation.
CH4 REGR	Thermo	100	4	−2	4	—	—	3.5	—
	Exp.	98	6	−2	6	—	—	3.6	—
CH4 SR	Thermo	99	32	—	32	—	1.1	6.0	—
	Exp.	95	30	—	30	—	1.1	6.0	—
CH4 DR	Thermo	98	—	27	—	1.1	—	2.1	1.1
	Exp.	93	—	25	—	1.0	—	2.1	1.0

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3.5 Thermodynamic study

A thermodynamic study was performed in order to help to understand two particular behaviors observed by varying the C₈H₁₈ conversion:

-the SR conditions lead to the highest hydrogen yields up to C₈H₁₈ conversions of 80% at 580 °C, but for higher conversions, the best results are obtained in REGR conditions,

-in REGR conditions, for C₈H₁₈ conversions higher than roughly 30%, the selectivity to H₂ decreases with the conversion. At lower conversions, only obtained at 450 °C, the selectivity to H₂ increases with the conversion (up to 27% of conversion): an optimal selectivity to H₂ is evidenced at a medium conversion.

The calculations were based on the minimization of the Gibbs free energy, by taking into account the possibility of obtaining various C₈H₁₈ conversions, as it was observed experimentally, whereas the “classical” calculations give 100% of C₈H₁₈ conversion. For that purpose, the calculations were made considering 100%, 80%, 60%, 40%, 20% and 5% of C₈H₁₈ conversions, simulated by 1.0, 0.8, 0.6, 0.4, 0.2 and 0.05 mol of C₈H₁₈ at the inlet flow, respectively, as described in ref. 17. The inlet partial pressure of H₂O, O₂ and CO₂ and the total molar flow inlet were maintained constant. Consequently, the decrease in the C₈H₁₈ molar flow was compensated by changing nitrogen proportion. In thermodynamic calculations, the only way to take into account kinetic limitations is to exclude one product from the distribution. Since it was experimentally shown that the CH₄ yield is much lower than that given by calculation at the thermodynamic equilibrium, the distribution of products at the equilibrium was calculated considering the absence or the presence of CH₄ as reaction product.

3.5.1. Comparison of SR and REGR at 580 °C. The experimental and calculated H₂ yields are presented in Fig. 6 as a function of the C₈H₁₈ conversion in REGR and SR conditions. This figure shows that, according to the thermodynamics, SR conditions without CH₄ as equilibrium product lead to the highest hydrogen yield i.e. 11.6 mol/mol C₈H₁₈ⁱⁿ up to 68% of C₈H₁₈ conversion. In REGR conditions, 86% of C₈H₁₈ conversion is needed to reach the same H₂ yield at the thermodynamic equilibrium. When CH₄ is included in the calculation as product, the highest H₂ yield is also obtained in the SR conditions up to 74% of C₈H₁₈ conversion. Above this value, REGR conditions are the most favorable conditions to produce the highest amount of H₂.

Fig. 6 Evolution of the H₂ yield at the thermodynamic equilibrium (—) and comparison with experimental evolution (- - -) of H₂ yields for REGR (in black) and SR (in grey) at 580 °C as a function of C₈H₁₈ conversion. Data obtained at different contact times.

Looking at the curves obtained taking or not into account CH₄ in the thermodynamic calculations, one can see that whatever the reaction conditions, SR or REGR, the highest hydrogen yields are obtained, as expected, without production of methane. The experimental H₂ yields obtained at various values of hydrocarbon conversion by varying the contact time (same Fig. 6) are between the limits predicted by thermodynamic calculation with and without methane, for both REGR and SR conditions. For example, in REGR conditions, 100% of isooctane conversion is required to obtain the maximum of H₂ yield (7.25 mol/mol C₈H₁₈ⁱⁿ) by calculation taking into account the formation of methane, whereas the calculation excluding CH₄ formation shows that a C₈H₁₈ conversion of 51% is sufficient to produce the same amount of H₂. Experimentally, 7.25 mol H₂/mol C₈H_18 0 are detected with ∼60% of C₈H₁₈ conversion and CH₄ is formed in low amount. There is no doubt that the presence of methane as equilibrium product has a considerable influence on thermodynamic H₂ yield limit. In order to reasonably use this type of calculation (without methane as product), the experimental CH₄ yield has to be much lower than that calculated at the thermodynamic equilibrium taking into account methane formation. Fig. 7 shows the evolution of the CH₄ yield calculated at the thermodynamic equilibrium with CH₄ as product compared to the experimental values collected during REGR and SR conditions, as a function of C₈H₁₈ conversion. As expected, CH₄ yields are always lower than those calculated at the thermodynamic equilibrium.

Fig. 7 CH₄ yield at the thermodynamic equilibrium (—) and experimental CH₄ (- - -) yield as a function of C₈H₁₈ conversion. CH₄ experimental data were obtained for different contact times in REGR (in black) and SR (in grey) conditions.

Finally, the curves obtained by calculation at the thermodynamic equilibrium by varying the C₈H₁₈ conversion (Fig. 6) are relevant to predict that, at low conversion, the SR conditions give higher H₂ yields than REGR, whereas at conversions higher than roughly 75%, the opposite is observed, in agreement with the experimental results.

3.5.2. Presence of a maximum of selectivity to H₂ at medium C₈H₁₈ conversions. The selectivity to H₂ as a function of C₈H₁₈ for REGR conditions at 450 °C (as presented in Fig. 4) reveals the presence of an optimal selectivity at roughly 30% of C₈H₁₈ conversion. At higher conversions, the selectivity decreases. This effect was not observed for SR (C₈H₁₈ + H₂O) and DR (C₈H₁₈ + CO₂), for which low conversions were impossible to reach by decreasing the contact time. With the aim of verifying if it is a thermodynamic effect, thermodynamic calculations were also made by simulating variable C₈H₁₈ conversions for these reaction conditions (Fig. 8).

Fig. 8 Evolution of the selectivity to H₂ (in black) and CH₄ (in grey) at the thermodynamic equilibrium (—) and in experimental conditions (- - -) during REGR at 450 °C as a function of C₈H₁₈ conversion. Data obtained at different contact times.

The results show that the selectivity to H₂ predicted by the thermodynamic calculation (with or without methane), presents also a maximum. Experimentally, the formation of methane is negligible. Thus, if one compares the experimental results with the calculated curve without CH₄, one can observe an optimum at the same C₈H₁₈ conversion (30%). Consequently the maximum of selectivity to H₂ observed at medium conversion can be attributed to a thermodynamic effect.

4. Conclusions

It was previously demonstrated that Rh(1 wt%) supported on ZrO₂(73.8 wt%)-La₂O₃-Nd₂O₃-Y₂O₃(26.2 wt%) is a catalyst that exhibits high activity and stability for H₂ production in REGR conditions.¹⁷ The aim of the present work was to study the reaction scheme of non-elementary steps in the presence of this catalyst. It can be concluded that the REGR reaction occurs in 3 steps:

Step 1:

C₈H₁₈ + 16H₂O → 8CO₂ + 25H₂

C₈H₁₈ + 8CO₂ → 16CO + 9H₂

Oxidation reaction is negligible, except for low isooctane conversions:

Step 2:

CO₂ + H₂ ↔ CO + H₂O

Step 3:

2CO + 2H₂ ↔ CH₄ + CO₂

Step 1 corresponds to the direct reaction between C₈H₁₈ and the co-reactants, i.e. water (SR reaction) or carbon dioxide (DR). However, for low C₈H₁₈ conversions, i.e. at low contact time, the DR reaction is negligible. For the SR reaction, H₂ and CO₂ are primary products. Step 2 corresponds to the RWGS that gives CO, a secondary product. Methane is formed only in step 3 as a tertiary product. Different reactions were studied with the purpose of determining those responsible for methane formation. It was shown that a great amount of CH₄ comes from the methanation reaction between CO and H₂. The formation of CH₄ directly from C₈H₁₈ or by the reaction between H₂ and CO₂ is strongly kinetically and thermodynamically limited, respectively. It was also demonstrated that CH₄ is extremely reactive, which proves the possibility of transforming the methane produced in REGR conditions.

The highest H₂ yield is obtained in SR conditions up to 80% of C₈H₁₈ conversions; but, for higher conversions, the presence of CO₂ and O₂ in the feed (REGR conditions) has a beneficial effect on hydrogen production. Thermodynamic calculations taking into account the experimental conversions and the low amount of methane produced enabled us to demonstrate that this phenomenon is not a kinetic effect but a thermodynamic one. The thermodynamic calculation explains the presence of a maximum of selectivity to H₂ reached at medium conversion at 450 °C.

Acknowledgements

ADEME is gratefully acknowledged for financial support (PREDIT Program n° 06 06 C0137).

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