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MgO catalyst
Applied Catalysis A: General 196 (2000) 209–215
Temperature-programmed desorption study of water-gas shift and methane steam-reforming reactions over Li/MgO catalyst Ioan Balint a,∗ , Ken-ichi Aika b a
b
Romanian Academy, Institute of Physical Chemistry, Splaiul Independentei 202, 77208 Bucharest, Romania Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received 15 July 1999; received in revised form 15 October 1999; accepted 18 October 1999
Abstract The water-gas shift (WGS) and methane steam-reforming (MSR) reactions over 1% Li/MgO catalyst were investigated using thermal-programmed desorption (TPD) technique with isotope labelled compounds (C16 O, H2 18 O, C16 O and CD4 ). The experimental results show that WGS reaction over 1% Li/MgO catalyst takes place in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. The highest reaction rate was observed at 617 K and the estimated activation energy, from the desorption peak of reaction product, is 158 kJ/mol. The experimental results suggest that the contribution of WGS side reaction to the total amount of CO2 and H2 , formed in the oxidative coupling of methane (OCM), is important. The MSR reaction between CD4 and water over Li/MgO catalyst could not be evidenced by TPD method due to the weak adsorption of CD4 . However, a room-temperature isotope exchange between CD4 and the hydroxyl groups of 1% Li/MgO could be observed. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Water-gas shift; Temperature-programmed desorption; Methane steam-reforming reaction; Li/MgO catalyst
1. Introduction Lithium doped magnesium oxide catalyst is the subject of many investigations due to the high performances showed in oxidative coupling of methane (OCM) reaction [1–5]. Besides ethane and ethylene, the main products of interest, carbon oxides, are formed as non-selective reaction products. Although extensive kinetic studies have been carried out for OCM reaction [6,7], mainly based on the analysis of products distribution, there are only few studies dealing separately with water-gas shift (WGS) and methane-steam reforming (MSR) side reactions over Li/MgO catalyst. ∗
Corresponding author.
From practical point of view it is difficult to separate the main reaction (OCM) from side reactions (WGS and MSR). The WGS (CO + H2 O → CO2 + H2 ) and MSR (CH4 + H2 O → CO + 3H2 ) side reactions are favoured by the high concentration of water vapor as well as by the high temperature at which OCM reaction takes place. It is well known that WGS reaction can occur efficiently over suitable catalysts at a temperature much lower than that of OCM reaction [3,8]. Since the occurrence of WGS and MSR reactions would change the distribution of OCM reaction products, it is useful to asses the importance of side reactions over Li/MgO catalyst. Transient techniques, such as thermal-programmed desorption (TPD), are well-suited vehicles for investigating surface reactions. When coupled with iso-
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tope labelling of adsorbates, the results become more definitive and speculations can be minimised. The objective of the present study is to investigate WGS and MSR side reactions, over Li/MgO catalyst, using the TPD technique with isotope labelled compounds.
2. Experimental The TPD experiments were carried out on lithium-doped magnesium oxide. A slurry was prepared from high-purity grade magnesium oxide obtained from Soekawa Chemicals (purity min 99.96%) and LiNO3 obtained from Canto Chemicals (CICA reagent, purity min 99.90%). The slurry was dried at 453 K for 48 h and then powdered in an agate mortar. The content of lithium in MgO was 1 mol%. The powder was pressed in pellets at 19 600 kPa and then calcined at 1273 K for 24 h. The sintered pellets were crushed and sieved. A sieve fraction between 1–3 mm was selected for TPD experiment. The TPD experiments were carried out in a conventional, glass made, vacuum system (10−6 Torr). The analysis of desorbed gases was performed with an on-line mass spectrometer (NEVA NAG 110). Prior to every TPD run, the catalyst was heated under vac-
uum to 1173 K for 1 h to remove any impurity from the surface and then cooled to room temperature. The first step of TPD experiments consisted of the adsorption, at room temperature, of one gas (H2 18 O or C16 O) at a constant pressure (at 21 Torr), for 1 min. Then, the sample was evacuated at 10−6 Torr for 30 min. In coadsorption experiments, the catalyst was exposed successively, typically at 21 Torr for 1 min, to H2 18 O and C16 O. After every adsorption step, the catalyst was evacuated for 30 min. Finally, the desorption step was performed at a heating rate of 10 K/min. The temperature programmer used was Chino type model KP. The isotope labelled compounds, with a 99.96% purity for D and 18 O in D2 O and H2 18 O, respectively, and 99% D in CD4 , were supplied by Isotec.
3. Results Separate TPD runs were performed, in the first stage of experiments, for each reactant participating in WGS reaction (C16 O and H2 18 O). A TPD plot obtained after adsorption of C16 O at 21 Torr on 1% Li/MgO is given in Fig. 1. The weakly adsorbed C16 O gives two small desorption peaks at 347 and 455 K. Two other
Fig. 1. TPD spectra of C16 O and C16 O2 from 1% Li/MgO after exposure to C16 O (Pads = 21 Torr, 298 K, 1 min).
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Fig. 2. TPD spectra of H2 18 O and H2 from 1% Li/MgO after exposure to H2 18 O (Pads = 21 Torr, 298 K, 1 min).
C16 O desorption peaks were observed at higher temperature, 617 and 838 K. As can be observed from Fig. 1, most of adsorbed C16 O desorbs in the temperature range 500–700 K, with a maximum at 617 K. Besides C16 O, a small amount of C16 O2 was observed to desorb at high temperature (the maximum is located at 868 K). Fig. 2 shows the TPD spectrum of H2 18 O taken after room-temperature adsorption at 21 Torr for 1 min. The H2 18 O desorption profile exhibits a shoulder at 475 K and a pronounced maximum at 604 K. As can be observed from Fig. 2, the amount of desorbed H2 18 O is around two orders higher than that of C16 O. A small amount of H2 is evolved, parallel to H2 18 O, from the catalyst above 500 K. The occurrence of WGS reaction over Li/MgO was investigated by coadsorbing isotope labelled water (H2 18 O) and C16 O. The conditions of OCM reaction were reproduced approximately by exposing the catalyst to high H2 18 O and C16 O pressure (21 Torr). The adsorption pressure of the reactants (21 Torr), H2 18 O vapor and C16 O, corresponds to a concentration of ≈3% of each compound in gas phase. Fig. 3 shows the TPD profile of C16 O obtained after the coadsorption with H2 18 O. It can be observed that, after
the coadsorption with H2 18 O, the small C16 O peaks at 347 and 455 K (observed in Fig. 1) vanished and the peak at 617 K was reduced to a small shoulder at 662 K. The high temperature C16 O peak located at 838 K (Fig. 1) was shifted to 864 K (Fig. 3) after coadsorption with H2 18 O. When C16 O and H2 18 O were coadsorbed on Li/MgO, a pronounced C16 O18 O maximum was observed at 617 K (Fig. 3). Interestingly, the peak of C16 O18 O is located at the same temperature with the desorption peak of C16 O (see Fig. 1). Besides the main reaction product (C16 O18 O), the desorption of C18 O2 and C16 O2 was observed at 649 and 801 K, respectively (Fig. 4). It should be evidenced that the amount of desorbed C16 O18 O (Fig. 3) is higher by around one order of magnitude than that of C16 O (Fig. 1), C18 O2 and C16 O2 (Fig. 4). An attempt to investigate the MSR reaction over Li/MgO catalyst was made by using the same pattern as described previously for WGS reaction. The isotope labelled reactants (H2 16 O and CD4 ) were coadsorbed separately (each reactant at 21 Torr) at room temperature on Li/MgO. Then, the temperature was raised and the desorbed species were monitored by mass spectrometer. The formation of MSR reaction
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Fig. 3. TPD spectrum of C16 O18 O and C16 O from 1% Li/MgO after coadsorption of H2 18 O and C16 O (Pads = 21 Torr, 298 K, 1 min).
products (C16 O and D2 ) could not be evidenced during TPD runs. The presence of remnant water (which cannot be removed completely by thermal treatment in vacuum at 1173 K) in the structure of Li/MgO catalyst may
have an essential role in WGS and MSR reactions at high temperature. From Fig. 5 can be observed that some amount of water, parallel to traces of H2 , is evolved from the catalyst even after the sample has undergone a standard cleaning procedure (heating to
Fig. 4. TPD spectrum of C18 O2 and C16 O2 from 1% Li/MgO after coadsorption of H2 18 O and C16 O (Pads = 21 Torr, 298 K, 1 min).
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Fig. 5. TPD spectrum of residual water (H2 16 O) and H2 from 1% Li/MgO after the sample was heated to 1173 K and then cooled to room temperature under vacuum (10−6 Torr); D2 O and D2 from 1% Li/MgO after exposure to CD4 (Pads = 21 Torr, 298 K, 1 min).
1173 K under vacuum for 1 h and then cooling to room temperature). After exposure to CD4 (at 21 Torr for 1 min), instead of H2 O, a small amount of D2 O is evolved from Li/MgO (Fig. 5). The shape of D2 O spectrum is identical to that of H2 O. Besides D2 O, traces of D2 were detected at high temperature.
comprises hydrogen defects of MgO. The hydrogen defects represent OH− groups located in the structure of oxide, in other words, protons located on O2− lattice anions. In the bulk of MgO, at low temperature, the charges of hydroxyl groups are compensated by cation vacancies. This type of defects decompose at high temperature with the formation of mobile holes (O− ) and elimination of H2 [10,11]:
4. Discussion
2OH− → H2 + 2O−
Most of the chemisorbed C16 O desorbs in the temperature range 500–800 K. The C16 O adsorbed on higher energy sites (having the desorption peak at 838 K) reacts, partially, with the lattice oxygen of Li/MgO, to give a small amount of C16 O2 (peak at 868 K). The formation C16 O2 could not be detected when C16 O was not preadsorbed on Li/MgO. It is well known that water is strongly adsorbed, in molecular or dissociated form, on Li/MgO [9]. The TPD results show that the amount of desorbed water is at least two orders of magnitude higher (Fig. 2) than that of C16 O (Fig. 1) although each gas was adsorbed at the same pressure (21 Torr for 1 min). Fig. 2 shows that water desorption above 773 K is accompanied by hydrogen elimination. King et al. [10] reported that the source of hydrogen formed at high temperature
The H2 desorption, originating from hydrogen defects of Li/MgO [12], starts slowly at around 700 K (Fig. 5), overlapping the hydrogen formed in WGS reaction. The two hydrogen sources (from hydrogen defects and WGS reaction) make it difficult to identify the contribution of each process to the total amount of hydrogen desorbed. The amount of desorbed C16 O decreases significantly when it is coadsorbed with H2 18 O (Fig. 3). It is obvious that some of adsorbed C16 O is consumed in a surface reaction. Taking into account that the most abundant species on the surface of Li/MgO are the adsorbed C16 O (C16 Oads ) and H2 18 O (H2 18 Oads ), it is rational to consider that the depletion of desorbed C16 O, observed in Fig. 3, is due to the WGS reaction. Therefore, the formation of labelled reaction product
(1)
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(C16 O18 O) in WGS reaction should be expected to explain the decrease of C16 O desorbed. Indeed, in Fig. 2, we can observe the desorption of the most abundant reaction product, C16 O18 O. Accordingly, the main surface reaction is (C16 O)ads + (H218 O)ads → C16 O18 O + H2
(2)
Interestingly, the peak position of C16 O18 O at 617 K (Fig. 3) coincides with the desorption peak of C16 O (Fig. 1). From TPD results, it is obvious that the WGS reaction over 1% Li/MgO catalyst takes place mainly in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. This assertion is in agreement with other studies [13] indicating that the Langmuir–Hinshelwood mechanism is typical for WGS reaction over various types of catalysts. From the desorption spectra of C16 O (Fig. 1) and 16 C O18 O (Fig. 3), it can be assumed that the rate determining step of WGS reaction over Li/MgO is C16 O activation [14]. The formation and release of the reaction product C16 O18 O follows the trend of C16 O desorption. According to experimental results, the C16 O molecules are activated by the weakening of the bonds with the surface during desorption stage. The activated C16 O react readily with dissociatively adsorbed H2 18 O (18 OH− groups) [15,16] to give C16 O18 O. The reaction product (C16 O18 O) is then quickly desorbed from the surface in gas phase. The activation energy of WGS reaction was determined from the position of C16 O18 O peak (TM represents the temperature at which the rate of reaction product formation is maximum). Considering that the desorption of C16 O18 O is a first-order process, the equation utilised for the calculation of WGS activation energy (Ed ) is [17] νCO TM Ed ≈ ln + ln (3) RTM β ln(νCO /β) where ν CO is the normal frequency factor and β is the linear heating rate. Assuming a pre-exponential factor of ν CO = 1013 Hz, β = 10 K/min, TM = 617 K, the calculated activation energy of WGS reaction is ≈158 kJ/mol. This value of the activation energy is typical for catalysed WGS reaction. The activation energy, determined for different types of catalysts in various reaction conditions, for WGS reaction over different catalysts ranges between 116 and 170 kJ/mol [13,18].
As can be seen from Fig. 4, the desorption of small amounts of C18 O2 and C16 O2 can be observed at high temperature. This result may be explained by a partial isotope exchange between the 18 O of the adsorbed H2 18 O and lattice oxygen (16 O2− ) of 1% Li/Mg. On the other hand, at low temperature, the ionic oxide surface is hydroxylated by dissociative adsorption of water [15]. An isotope exchange between the hydroxyl groups (16 OH− ) of Li/MgO and 16 OH− formed by the dissociative adsorption of H2 18 O is very likely. The WGS reaction mechanism has been proven to involve the oxygen adatoms, produced via water dissociation [16]. Taking into account the desorption peaks of carbon dioxide (Fig. 4), it is likely that, at lower temperature (649 K), the 18 OH− groups have the main contribution to the formation of C18 O2 , while at higher temperature (801 K), the lattice oxygen participation to the formation of C16 O2 becomes more important. The catalyst exposed to C16 O exhibits a small C16 O2 desorption peak at 868 K (Fig. 1). This result suggests that the lattice oxygen may react, at high temperature, with C16 O to give C16 O2 [19,20]. To explain the formation mechanism of C16 O2 and 18 C O2 , the fast isotope exchange between the reaction product (C16 O18 O) and the lattice oxygen or hydroxyl groups [21] can be taken into consideration too. The former assumption is, in our view, limited by the fast desorption observed for reaction product (C16 O18 O) (Fig. 3). From TPD results, is clear that the C16 O18 O desorbs rapidly after formation. The occurrence of MSR reaction over Li/MgO could not be evidenced by TPD technique. One of the reason is the weak adsorption of CD4 on Li/MgO [22]. It is likely that deuterated methane is completely removed from the catalyst at room temperature, by pumping at 10−6 Torr. However, TPD spectra of the catalyst exposed to CD4 at room temperature (Fig. 5) revealed the formation of small amounts of D2 O and D2 . The isotope exchange process which takes place, at room temperature, between the hydroxyl groups and CD4 is [23] CD4 + OH− → OD− + CD3 H
(4)
The elimination of D2 O at high temperature is accompanied by the desorption of small amount of D2 (Fig. 5). The TPD result indicates that the deuterium of CD4 is exchanged, at room temperature, with two types of hydroxyl groups: surface and bulk hydroxyl groups.
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The resulting surface (OD)s − groups are eliminated as D2 O when the temperature is raised. The bulk (OD)b − groups desorb mainly, at high temperature, by an associative mechanism to give D2 (Eq. (1)) [12]. The exchange rate of the bulk hydrogen defects with the deuterium of CD4 is significantly lower than the exchange rate with surface OH− groups due to the higher coordination degree. This explains the higher amount of D2 O observed comparatively with that of D2 .
5. Conclusions TPD experiments proved that WGS reaction occurs over 1% Li/MgO catalyst in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. The rate determining step under our conditions is the activation of the adsorbed C16 O. The experimental results prove that side reactions, such as WGS reaction, may have an important contribution in establishing the products final distribution in OCM reaction. The source of CO2 and H2 in OCM reaction is, at least partially, WGS side reaction. However, it is difficult to assess qualitatively only from TPD results the contribution of WGS reaction to the total CO2 production. MSR reaction over 1% Li/MgO could not be evidenced by the TPD method. Isotope exchange processes at room temperature, between the surface and bulk hydroxyl groups of Li/MgO and the deuterium of CD4 , were evidenced. Among other sources of hydrogen (i.e. thermal dehydrogenation), the decomposition of the hydrogen defects of Li/MgO at high temperature can be another source of H2 in OCM reaction.
215
References [1] S.H. Oh, P.J. Mitchell, R.M. Sievert, J. Catal. 132 (1990) 287. [2] E.M. Kennedy, N.W. Cant, Appl. Catal. 75 (1991) 321. [3] Z. Zhang, M. Baerns, Appl. Catal. 75 (1991) 299. [4] S. Ahmed, J.B. Moffat, Appl. Catal. 54 (1989) 321. [5] N.W. Cant, C.A. Lukey, P.F. Nelson, J. Catal. 124 (1990) 336. [6] J.S. Lee, S.T. Oyama, Catal. Rev.-Sci. Eng. 30 (1988) 249. [7] Z. Stansch, L. Mleczko, M. Baerns, Ind. Eng. Chem. Res. 36 (1997) 2568. [8] R. Pitchai, K. Klier, Catal. Rev.-Sci. Eng. 28 (1986) 13. [9] P. Colomban (Ed.), Proton Conductors, Cambridge University Press, Cambridge, 1992, p. 138. [10] B.V. King, F. Freund, Phys. Rev. 29 (1984) 5814. [11] F. Freund, H. Wengler, J. Phys. Chem. Solids 43 (1982) 129. [12] I. Balint, K.-I. Aika, J. Chem. Soc., Faraday Trans. 9 (1995) 1805. [13] R.L. Keiski, T. Salmi, P. Niemistö, J. Ainassaari, V. Pojola, Appl. Catal. A 137 (1996) 349. [14] M. Tinkle, J.A. Dumesic, J. Catal. 103 (1987) 65. [15] E. Knözinger, K.H. Jacob, S. Singh, P. Hofmann, Surf. Sci. 290 (1993) 388. [16] C.T. Campbell, in: D.A. King, D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces, Vol. 6. Coadsorption, Promoters and Poisons, Elsevier, Amsterdam 1993, p. 299. [17] F.C. Tomkins, Chemisorption of Gases on Metals, Academic Press, New York, 1978, p. 63. [18] R.L. Keisky, Acta Universitatis Ouluensis Ser. C Technica 57 (1991) 125–156. [19] K. Coulter, D.W. Goodman, Catal. Lett. 20 (1990) 169. [20] Y.F. Chang, G.A. Somorjai, H. Heinemann, J. Catal. 142 (1993) 697. [21] K.P. Peil, J.S. Goodwin Jr., G. Marcelin, J. Phys. Chem. 93 (1989) 5977. [22] J.M. Deboy, R.F. Hicks, J. Catal. 113 (1988) 517. [23] L. Quanzhi, Y. Amenomia, Appl. Catal. 23 (1986) 173.
Temperature-programmed desorption study of water-gas shift and methane steam-reforming reactions over Li/MgO catalyst Ioan Balint a,∗ , Ken-ichi Aika b a
b
Romanian Academy, Institute of Physical Chemistry, Splaiul Independentei 202, 77208 Bucharest, Romania Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received 15 July 1999; received in revised form 15 October 1999; accepted 18 October 1999
Abstract The water-gas shift (WGS) and methane steam-reforming (MSR) reactions over 1% Li/MgO catalyst were investigated using thermal-programmed desorption (TPD) technique with isotope labelled compounds (C16 O, H2 18 O, C16 O and CD4 ). The experimental results show that WGS reaction over 1% Li/MgO catalyst takes place in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. The highest reaction rate was observed at 617 K and the estimated activation energy, from the desorption peak of reaction product, is 158 kJ/mol. The experimental results suggest that the contribution of WGS side reaction to the total amount of CO2 and H2 , formed in the oxidative coupling of methane (OCM), is important. The MSR reaction between CD4 and water over Li/MgO catalyst could not be evidenced by TPD method due to the weak adsorption of CD4 . However, a room-temperature isotope exchange between CD4 and the hydroxyl groups of 1% Li/MgO could be observed. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Water-gas shift; Temperature-programmed desorption; Methane steam-reforming reaction; Li/MgO catalyst
1. Introduction Lithium doped magnesium oxide catalyst is the subject of many investigations due to the high performances showed in oxidative coupling of methane (OCM) reaction [1–5]. Besides ethane and ethylene, the main products of interest, carbon oxides, are formed as non-selective reaction products. Although extensive kinetic studies have been carried out for OCM reaction [6,7], mainly based on the analysis of products distribution, there are only few studies dealing separately with water-gas shift (WGS) and methane-steam reforming (MSR) side reactions over Li/MgO catalyst. ∗
Corresponding author.
From practical point of view it is difficult to separate the main reaction (OCM) from side reactions (WGS and MSR). The WGS (CO + H2 O → CO2 + H2 ) and MSR (CH4 + H2 O → CO + 3H2 ) side reactions are favoured by the high concentration of water vapor as well as by the high temperature at which OCM reaction takes place. It is well known that WGS reaction can occur efficiently over suitable catalysts at a temperature much lower than that of OCM reaction [3,8]. Since the occurrence of WGS and MSR reactions would change the distribution of OCM reaction products, it is useful to asses the importance of side reactions over Li/MgO catalyst. Transient techniques, such as thermal-programmed desorption (TPD), are well-suited vehicles for investigating surface reactions. When coupled with iso-
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tope labelling of adsorbates, the results become more definitive and speculations can be minimised. The objective of the present study is to investigate WGS and MSR side reactions, over Li/MgO catalyst, using the TPD technique with isotope labelled compounds.
2. Experimental The TPD experiments were carried out on lithium-doped magnesium oxide. A slurry was prepared from high-purity grade magnesium oxide obtained from Soekawa Chemicals (purity min 99.96%) and LiNO3 obtained from Canto Chemicals (CICA reagent, purity min 99.90%). The slurry was dried at 453 K for 48 h and then powdered in an agate mortar. The content of lithium in MgO was 1 mol%. The powder was pressed in pellets at 19 600 kPa and then calcined at 1273 K for 24 h. The sintered pellets were crushed and sieved. A sieve fraction between 1–3 mm was selected for TPD experiment. The TPD experiments were carried out in a conventional, glass made, vacuum system (10−6 Torr). The analysis of desorbed gases was performed with an on-line mass spectrometer (NEVA NAG 110). Prior to every TPD run, the catalyst was heated under vac-
uum to 1173 K for 1 h to remove any impurity from the surface and then cooled to room temperature. The first step of TPD experiments consisted of the adsorption, at room temperature, of one gas (H2 18 O or C16 O) at a constant pressure (at 21 Torr), for 1 min. Then, the sample was evacuated at 10−6 Torr for 30 min. In coadsorption experiments, the catalyst was exposed successively, typically at 21 Torr for 1 min, to H2 18 O and C16 O. After every adsorption step, the catalyst was evacuated for 30 min. Finally, the desorption step was performed at a heating rate of 10 K/min. The temperature programmer used was Chino type model KP. The isotope labelled compounds, with a 99.96% purity for D and 18 O in D2 O and H2 18 O, respectively, and 99% D in CD4 , were supplied by Isotec.
3. Results Separate TPD runs were performed, in the first stage of experiments, for each reactant participating in WGS reaction (C16 O and H2 18 O). A TPD plot obtained after adsorption of C16 O at 21 Torr on 1% Li/MgO is given in Fig. 1. The weakly adsorbed C16 O gives two small desorption peaks at 347 and 455 K. Two other
Fig. 1. TPD spectra of C16 O and C16 O2 from 1% Li/MgO after exposure to C16 O (Pads = 21 Torr, 298 K, 1 min).
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Fig. 2. TPD spectra of H2 18 O and H2 from 1% Li/MgO after exposure to H2 18 O (Pads = 21 Torr, 298 K, 1 min).
C16 O desorption peaks were observed at higher temperature, 617 and 838 K. As can be observed from Fig. 1, most of adsorbed C16 O desorbs in the temperature range 500–700 K, with a maximum at 617 K. Besides C16 O, a small amount of C16 O2 was observed to desorb at high temperature (the maximum is located at 868 K). Fig. 2 shows the TPD spectrum of H2 18 O taken after room-temperature adsorption at 21 Torr for 1 min. The H2 18 O desorption profile exhibits a shoulder at 475 K and a pronounced maximum at 604 K. As can be observed from Fig. 2, the amount of desorbed H2 18 O is around two orders higher than that of C16 O. A small amount of H2 is evolved, parallel to H2 18 O, from the catalyst above 500 K. The occurrence of WGS reaction over Li/MgO was investigated by coadsorbing isotope labelled water (H2 18 O) and C16 O. The conditions of OCM reaction were reproduced approximately by exposing the catalyst to high H2 18 O and C16 O pressure (21 Torr). The adsorption pressure of the reactants (21 Torr), H2 18 O vapor and C16 O, corresponds to a concentration of ≈3% of each compound in gas phase. Fig. 3 shows the TPD profile of C16 O obtained after the coadsorption with H2 18 O. It can be observed that, after
the coadsorption with H2 18 O, the small C16 O peaks at 347 and 455 K (observed in Fig. 1) vanished and the peak at 617 K was reduced to a small shoulder at 662 K. The high temperature C16 O peak located at 838 K (Fig. 1) was shifted to 864 K (Fig. 3) after coadsorption with H2 18 O. When C16 O and H2 18 O were coadsorbed on Li/MgO, a pronounced C16 O18 O maximum was observed at 617 K (Fig. 3). Interestingly, the peak of C16 O18 O is located at the same temperature with the desorption peak of C16 O (see Fig. 1). Besides the main reaction product (C16 O18 O), the desorption of C18 O2 and C16 O2 was observed at 649 and 801 K, respectively (Fig. 4). It should be evidenced that the amount of desorbed C16 O18 O (Fig. 3) is higher by around one order of magnitude than that of C16 O (Fig. 1), C18 O2 and C16 O2 (Fig. 4). An attempt to investigate the MSR reaction over Li/MgO catalyst was made by using the same pattern as described previously for WGS reaction. The isotope labelled reactants (H2 16 O and CD4 ) were coadsorbed separately (each reactant at 21 Torr) at room temperature on Li/MgO. Then, the temperature was raised and the desorbed species were monitored by mass spectrometer. The formation of MSR reaction
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Fig. 3. TPD spectrum of C16 O18 O and C16 O from 1% Li/MgO after coadsorption of H2 18 O and C16 O (Pads = 21 Torr, 298 K, 1 min).
products (C16 O and D2 ) could not be evidenced during TPD runs. The presence of remnant water (which cannot be removed completely by thermal treatment in vacuum at 1173 K) in the structure of Li/MgO catalyst may
have an essential role in WGS and MSR reactions at high temperature. From Fig. 5 can be observed that some amount of water, parallel to traces of H2 , is evolved from the catalyst even after the sample has undergone a standard cleaning procedure (heating to
Fig. 4. TPD spectrum of C18 O2 and C16 O2 from 1% Li/MgO after coadsorption of H2 18 O and C16 O (Pads = 21 Torr, 298 K, 1 min).
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Fig. 5. TPD spectrum of residual water (H2 16 O) and H2 from 1% Li/MgO after the sample was heated to 1173 K and then cooled to room temperature under vacuum (10−6 Torr); D2 O and D2 from 1% Li/MgO after exposure to CD4 (Pads = 21 Torr, 298 K, 1 min).
1173 K under vacuum for 1 h and then cooling to room temperature). After exposure to CD4 (at 21 Torr for 1 min), instead of H2 O, a small amount of D2 O is evolved from Li/MgO (Fig. 5). The shape of D2 O spectrum is identical to that of H2 O. Besides D2 O, traces of D2 were detected at high temperature.
comprises hydrogen defects of MgO. The hydrogen defects represent OH− groups located in the structure of oxide, in other words, protons located on O2− lattice anions. In the bulk of MgO, at low temperature, the charges of hydroxyl groups are compensated by cation vacancies. This type of defects decompose at high temperature with the formation of mobile holes (O− ) and elimination of H2 [10,11]:
4. Discussion
2OH− → H2 + 2O−
Most of the chemisorbed C16 O desorbs in the temperature range 500–800 K. The C16 O adsorbed on higher energy sites (having the desorption peak at 838 K) reacts, partially, with the lattice oxygen of Li/MgO, to give a small amount of C16 O2 (peak at 868 K). The formation C16 O2 could not be detected when C16 O was not preadsorbed on Li/MgO. It is well known that water is strongly adsorbed, in molecular or dissociated form, on Li/MgO [9]. The TPD results show that the amount of desorbed water is at least two orders of magnitude higher (Fig. 2) than that of C16 O (Fig. 1) although each gas was adsorbed at the same pressure (21 Torr for 1 min). Fig. 2 shows that water desorption above 773 K is accompanied by hydrogen elimination. King et al. [10] reported that the source of hydrogen formed at high temperature
The H2 desorption, originating from hydrogen defects of Li/MgO [12], starts slowly at around 700 K (Fig. 5), overlapping the hydrogen formed in WGS reaction. The two hydrogen sources (from hydrogen defects and WGS reaction) make it difficult to identify the contribution of each process to the total amount of hydrogen desorbed. The amount of desorbed C16 O decreases significantly when it is coadsorbed with H2 18 O (Fig. 3). It is obvious that some of adsorbed C16 O is consumed in a surface reaction. Taking into account that the most abundant species on the surface of Li/MgO are the adsorbed C16 O (C16 Oads ) and H2 18 O (H2 18 Oads ), it is rational to consider that the depletion of desorbed C16 O, observed in Fig. 3, is due to the WGS reaction. Therefore, the formation of labelled reaction product
(1)
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(C16 O18 O) in WGS reaction should be expected to explain the decrease of C16 O desorbed. Indeed, in Fig. 2, we can observe the desorption of the most abundant reaction product, C16 O18 O. Accordingly, the main surface reaction is (C16 O)ads + (H218 O)ads → C16 O18 O + H2
(2)
Interestingly, the peak position of C16 O18 O at 617 K (Fig. 3) coincides with the desorption peak of C16 O (Fig. 1). From TPD results, it is obvious that the WGS reaction over 1% Li/MgO catalyst takes place mainly in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. This assertion is in agreement with other studies [13] indicating that the Langmuir–Hinshelwood mechanism is typical for WGS reaction over various types of catalysts. From the desorption spectra of C16 O (Fig. 1) and 16 C O18 O (Fig. 3), it can be assumed that the rate determining step of WGS reaction over Li/MgO is C16 O activation [14]. The formation and release of the reaction product C16 O18 O follows the trend of C16 O desorption. According to experimental results, the C16 O molecules are activated by the weakening of the bonds with the surface during desorption stage. The activated C16 O react readily with dissociatively adsorbed H2 18 O (18 OH− groups) [15,16] to give C16 O18 O. The reaction product (C16 O18 O) is then quickly desorbed from the surface in gas phase. The activation energy of WGS reaction was determined from the position of C16 O18 O peak (TM represents the temperature at which the rate of reaction product formation is maximum). Considering that the desorption of C16 O18 O is a first-order process, the equation utilised for the calculation of WGS activation energy (Ed ) is [17] νCO TM Ed ≈ ln + ln (3) RTM β ln(νCO /β) where ν CO is the normal frequency factor and β is the linear heating rate. Assuming a pre-exponential factor of ν CO = 1013 Hz, β = 10 K/min, TM = 617 K, the calculated activation energy of WGS reaction is ≈158 kJ/mol. This value of the activation energy is typical for catalysed WGS reaction. The activation energy, determined for different types of catalysts in various reaction conditions, for WGS reaction over different catalysts ranges between 116 and 170 kJ/mol [13,18].
As can be seen from Fig. 4, the desorption of small amounts of C18 O2 and C16 O2 can be observed at high temperature. This result may be explained by a partial isotope exchange between the 18 O of the adsorbed H2 18 O and lattice oxygen (16 O2− ) of 1% Li/Mg. On the other hand, at low temperature, the ionic oxide surface is hydroxylated by dissociative adsorption of water [15]. An isotope exchange between the hydroxyl groups (16 OH− ) of Li/MgO and 16 OH− formed by the dissociative adsorption of H2 18 O is very likely. The WGS reaction mechanism has been proven to involve the oxygen adatoms, produced via water dissociation [16]. Taking into account the desorption peaks of carbon dioxide (Fig. 4), it is likely that, at lower temperature (649 K), the 18 OH− groups have the main contribution to the formation of C18 O2 , while at higher temperature (801 K), the lattice oxygen participation to the formation of C16 O2 becomes more important. The catalyst exposed to C16 O exhibits a small C16 O2 desorption peak at 868 K (Fig. 1). This result suggests that the lattice oxygen may react, at high temperature, with C16 O to give C16 O2 [19,20]. To explain the formation mechanism of C16 O2 and 18 C O2 , the fast isotope exchange between the reaction product (C16 O18 O) and the lattice oxygen or hydroxyl groups [21] can be taken into consideration too. The former assumption is, in our view, limited by the fast desorption observed for reaction product (C16 O18 O) (Fig. 3). From TPD results, is clear that the C16 O18 O desorbs rapidly after formation. The occurrence of MSR reaction over Li/MgO could not be evidenced by TPD technique. One of the reason is the weak adsorption of CD4 on Li/MgO [22]. It is likely that deuterated methane is completely removed from the catalyst at room temperature, by pumping at 10−6 Torr. However, TPD spectra of the catalyst exposed to CD4 at room temperature (Fig. 5) revealed the formation of small amounts of D2 O and D2 . The isotope exchange process which takes place, at room temperature, between the hydroxyl groups and CD4 is [23] CD4 + OH− → OD− + CD3 H
(4)
The elimination of D2 O at high temperature is accompanied by the desorption of small amount of D2 (Fig. 5). The TPD result indicates that the deuterium of CD4 is exchanged, at room temperature, with two types of hydroxyl groups: surface and bulk hydroxyl groups.
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The resulting surface (OD)s − groups are eliminated as D2 O when the temperature is raised. The bulk (OD)b − groups desorb mainly, at high temperature, by an associative mechanism to give D2 (Eq. (1)) [12]. The exchange rate of the bulk hydrogen defects with the deuterium of CD4 is significantly lower than the exchange rate with surface OH− groups due to the higher coordination degree. This explains the higher amount of D2 O observed comparatively with that of D2 .
5. Conclusions TPD experiments proved that WGS reaction occurs over 1% Li/MgO catalyst in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. The rate determining step under our conditions is the activation of the adsorbed C16 O. The experimental results prove that side reactions, such as WGS reaction, may have an important contribution in establishing the products final distribution in OCM reaction. The source of CO2 and H2 in OCM reaction is, at least partially, WGS side reaction. However, it is difficult to assess qualitatively only from TPD results the contribution of WGS reaction to the total CO2 production. MSR reaction over 1% Li/MgO could not be evidenced by the TPD method. Isotope exchange processes at room temperature, between the surface and bulk hydroxyl groups of Li/MgO and the deuterium of CD4 , were evidenced. Among other sources of hydrogen (i.e. thermal dehydrogenation), the decomposition of the hydrogen defects of Li/MgO at high temperature can be another source of H2 in OCM reaction.
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