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Mechanistic study of the effect of chlorine on selective CO methanation over Ni alumina-based catalysts
Applied Catalysis A: General 486 (2014) 187–192
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Mechanistic study of the effect of chlorine on selective CO methanation over Ni alumina-based catalysts Toshihiro Miyao, Weihua Shen, Aihua Chen, Kazutoshi Higashiyama ∗ , Masahiro Watanabe Fuel Cell Nanomaterials Center, University of Yamanashi, Kofu 400-0021, Japan
a r t i c l e
i n f o
Article history: Received 7 April 2014 Received in revised form 5 August 2014 Accepted 23 August 2014 Available online 30 August 2014 Keywords: Selective CO methanation Ni/alumina catalyst Chlorine residue CO2 methanation Reverse water-gas shift reaction Selectivity controlling additives
a b s t r a c t The effect of residual chlorine of alumina-supported Ni catalysts on the selective CO methanation reaction was investigated. By chloride addition to the catalyst, CO2 methanation and the reverse water gas-shift reaction were significantly suppressed in the course of the reaction. FTIR measurements for CO2 adsorption on a series of modified catalysts with various chlorine-containing additives revealed that chlorine suppressed the dissociation of CO2 on the catalyst surface, which led to the formation of adsorbed CO species. The FTIR results combined with TPD measurements for CO2 desorption indicates that chlorine adsorbed on Ni metal diminished the formation of carboxylate species on the same surface under flowing CO2 conditions. Through the present study, the controlling factor for the selective methanation of CO by chlorine was clarified in high CO2 concentration-containing hydrogen rich gas. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, polymer electrolyte fuel cells (PEFCs) have been attracting a great attention as residential co-generation systems of electricity and heat energy because of their higher total efficiencies compared to those of conventional power plants and of their lower risk to a failure of major power plants at an unpredictable accident or disaster, although, at present, there are still obstacles for the widespread usages. The first obstacle is the high manufacturing cost, and the second is the large size of the system. In most systems, hydrogen is produced via the steam reforming and subsequent water-gas shift reaction of city gas. The reformate gas from the water-gas shift reaction contains around 0.5% CO. In a conventional system, the CO is removed by the preferential oxidation (PROX) process, in which CO is oxidized with air additionally introduced. In order to maintain the selectivity and avoid an extra hydrogen consumption by the side reaction, the reformate gas should be cooled down below 150 ◦ C before the addition of air to the gas. This process, however, leads to increases in the cost and the volume of the fuel processor, e.g., by the additional air-blower and the chamber space for mixing the gases. As a lower cost CO removal process, a selective CO methanation (SCM) process has been proposed as
∗ Corresponding author. Tel.: +81 55 2547096; fax: +81 55 2547096. E-mail address: [email protected] (K. Higashiyama). http://dx.doi.org/10.1016/j.apcata.2014.08.025 0926-860X/© 2014 Elsevier B.V. All rights reserved.
a promising candidate because it requires no pre-cooling of the reformate gas from the water-gas shift reactor, no air supply and no mixing of fuel and air [1]. In order to develop a realistic SCM catalyst, the most important requirements are the suppression of CO2 methanation (CO2 + 3H2 → CH4 + H2 O) and the suppression of the reverse water-gas shift reaction (CO2 + H2 → CO + H2 O) for the outlet gas from the CO shift reaction, which contains 20% CO2 . If significant CO2 methanation proceeds, the process becomes out of control because the reaction is an exothermic reaction. Suppression of the reverse water-gas shift is also important in order to keep the lowered CO concentration. Recently, we reported remarkable improvement of the catalytic selectivity for CO methanation in reformate gas, which contained 20% CO2 , over an Ni-alumina-based catalyst by addition of a small amount of Ru chloride [2,3]. We found that both the reverse watergas shift reaction and CO2 methanation reaction were remarkably suppressed by this addition. At the beginning stage of our investigation, the reason for the suppression of CO2 hydrogenation by Ru chloride addition was unclear. Through the recent study by our group, the true mechanism for the improvement of the selectivity has been recognized, i.e., the residual chlorine on the catalyst surface has a remarkable effect on the improvement of the selectivity for the SCM. Indeed, in many cases, chlorine exhibits a significant inhibition for hydrogenation reactions over various metal catalysts [4]. In most of the previous reports concerning the SCM over Ru or Ni-based catalysts, Cl-free precursors, e.g., Ru nitrosyl nitrate,
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Ru nitrate and Ni nitrate were usually employed, and the additive effect of Ru chloride has rarely been considered [5–7], because chlorine is commonly recognized as an inhibitor for CO hydrogenation. On the other hand, Djinovic et al. reported the effect of chlorine on the catalytic activity of an Ru/Al2 O3 catalyst for the SCM, even though the mechanistic role of chlorine in the change of the catalytic properties was not clarified [8]. In the present study, the effect of residual chlorine on the selectivity of an Ni-alumina-based catalyst is discussed in detail on the basis of in situ spectroscopic measurements. 2. Experimental The Ni–Al mixed oxide powder was prepared by a solutionspray plasma technique [9,10]. A water solution of Ni nitrate and Al nitrate was mixed and sprayed into an evacuated chamber through an argon plasma flame with 10% oxygen in the argon carrier gas. The nitrates decomposed immediately in the flame to a fine oxide powder, which was then collected by a suction filter placed at the outlet of the chamber. For the addition of chloride compounds, the oxide powder was impregnated with aqueous solutions of chloride and then evaporated and dried at 110 ◦ C for 12 h, followed by calcination at 500 ◦ C for 3 h in air. In each case, the amounts of chloride additives (RuCl3 ·3H2 O, NH4 Cl, HCl) were adjusted by Cl amount: 1% Cl by weight vs. that of the Ni–Al oxide powder. In the case of Ru(NO)(NO3 )3 , the amount of the additive was the same as that in the case of RuCl3 addition on an Ru-metal basis. In this study, the plasma-generated powder will be denoted as Ni–Al oxide, because the powder is composed of multiple components, i.e., NiAl2 O4 , Ni oxides and a small amount of Al2 O3 . Each Ni–Al oxide powder sample was wash-coated on a ceramic monolith (400 cells per square inch) with cross-sectional dimensions of 10 mm × 10 mm and 20 mm length, followed by calcination in air at 500 ◦ C, prior to the reaction measurements. The wash-coated monolith was mounted tightly in a quartz microreactor with an inner diameter of 10 mm and then reduced in hydrogen at 500 ◦ C for 1 h before the activity test at atmospheric pressure. The reaction feed involved 1% CO, 20% CO2 , 79% H2 (dry basis), with H2 O corresponding to a steam/CO ratio of 15. Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy (spectrometer, FTIR-4200; DRIFT chamber, DR-500, JASCO) was employed to analyze the CO2 and CO adsorption behavior of the catalysts. Temperature-programmed desorption (TPD) profiles after CO2 adsorption were measured by use of an atmospheric flow system (BELCAT, BEL Japan) with a quadrupole mass spectrometer. Chlorine contents in the series of the samples were analyzed by a combustion-ion chromatography technique (Thermo Fisher Scientific, ICS-1500-AQF-100). Before the measurement, the sample powders were dried at 110 ◦ C for 30 min. 3. Results and discussion Fig. 1 shows the temperature dependence of the SCM over a series of Ni–Al oxide catalysts with different amounts of chloride additives. As seen in Fig. 1(a), at low temperature (190 ◦ C), the catalyst without additive (open triangles) showed the lowest CO concentration, while it increased with increasing amounts of added Ru chloride (1 wt%, open circles; 2 wt%, solid triangles; 3 wt%, solid circles). Because the catalyst without additives showed the highest activity for CO conversion in the lower temperature region, it is considered that Ru has a minor contribution with respect to the CO hydrogenation activity and that the reaction takes place predominantly over Ni sites. Fig. 1(b), which is an expanded view of Fig. 1(a), shows that, in the case of the catalyst without Ru chloride (open triangles), the CO concentration decreased until a temperature of 210 ◦ C was reached and then increased again significantly, due to
Table 1 Amount of chlorine contained in the series of samples. Samples
Amount of chlorine (wt%)
NiAlx Oy 1%RuCl3 /NiAlx Oy 3%RuCl3 /NiAlx Oy HCl/NiAlx Oy NH4 Cl/NiAlx Oy
n.d. 1.10 1.52 1.63 1.22
The quantitative analysis of chlorine was carried out by using a combustion-ion chromatograph technique. Before the analysis, sample powders were dried at 110 ◦ C for 30 min.
the reverse water-gas shift (RWGS) reaction, with increasing reaction temperature. In contrast, the catalysts with chloride additives did not exhibit a significant reverse water gas-shift reactioninduced increase of CO. In order to clarify the effect of the residual chlorine more precisely, quantitative analysis of chlorine was carried out by using a combustion-ion chromatograph technique. The amounts of chlorine contained in the series of the samples were summarized in Table 1. In the case of the unmodified sample, no chlorine was detected. The amount of chlorine contained in the RuCl3 added samples increased with increasing of the addition amount of RuCl3 . In the case of HCl and NH4 Cl added samples, the amounts of the residual chlorine were larger than that for the 1%RuCl3 added one despite the same amount of chlorides in the chlorine-basis were added to the samples. These results mean that the amount of residual chlorine after the heat treatment depends on the kind of the chloride. It is noteworthy that, as seen in Fig. 1(c), methane formation from CO2 methanation was also suppressed by Ru chloride additions, in particular, over the 3 wt%Ru chlorideadded catalyst. As the result, the selectivity for CO methanation was almost completely suppressed up to 250 ◦ C on the catalyst, where selectivity = (COinlet − COoutlet )/CH4 outlet , shown in Fig. 1(d). In the case of the NH4 Cl added catalyst (*), the selectivity was maintained at 100% until 270 ◦ C because of the significant suppression of the methane formation. On the other hand, in the case of unmodified and 1%RuCl3 added catalysts, the selectivity showed quite small values when reaction temperature was higher than 240 ◦ C due to the conversion of the large amount of CO2 into CH4 . Fig. 2 shows diffuse reflectance FTIR spectra for CO2 adsorption and reaction with hydrogen on an unmodified Ni–Al oxide catalyst (A), and a 1% Ru chloride-added catalyst (B) at 230 ◦ C. As seen in Fig. 2(A-a), in the case of the unmodified sample, several bands observed below 1700 cm−1 assigned to carbonate appeared immediately after CO2 introduction [11,12]. In addition to the bands due to carbonate formation, those due to adsorbed CO appeared at 2014 and 1844 cm−1 ; these are assigned to linearly adsorbed CO (denoted as “l-CO”) and bridge-type adsorbed CO (denoted as “bCO”), respectively [13]. When hydrogen was added to the flowing CO2 (b), the bands due to adsorbed CO were intensified significantly, and new bands appeared at 2055 cm−1 and 1936 cm−1 ; these are assigned to weakly adsorbed l-CO and l-CO on edge sites of Ni metal particles [14,15]. It is considered that after CO2 introduction, CO2 adsorbed on Ni sites was dissociated to CO(a) and O(a) to a large extent, and when hydrogen was introduced to the surface, adsorbed oxygen was immediately removed from the Ni surface due to the reaction with hydrogen. Additional CO, which was formed via the reverse water-gas shift reaction, then adsorbed on the vacant sites. In contrast, the Ru chloride-added catalyst shown in Fig. 2(B-a) exhibited almost no peak in the region corresponding to adsorbed CO. These results mean that Ru chloride addition inhibits the adsorption or dissociation of CO2 to a remarkable degree. After the hydrogen addition, the carbonate bands converted to formate bands, 1601 cm−1 and 1390 cm−1 , for both catalysts (Ab, B-b). When the CO2 -hydrogen gas mixture was replaced with pure hydrogen (A-c, A-d), the adsorbed CO bands disappeared
T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192
7000
10
6000
100
(c)
(a)
189
8
80
3000
Selectivity, %
4000
CH4, %
CO , ppm
5000 6 4
2000 1000 0 190
240 Temperature / ºC
290
60 40
2
20
0
0 190
210
230
250
270
290
(e) 190
210
Temperature / ºC 500
230 250 270 Temperature / ºC
290
25
400
20
300
15
CO2, %
CO , ppm
(b)
200
10
100
5
0
0
190
240 Temperature / ºC
290
(d) 190
210
230 250 270 Temperature / ºC
290
Fig. 1. Temperature dependence of selective CO methanation over unmodified Ni-Al oxide (), 1% Ru chloride-added ()), 2% Ru chloride-added () 3% Ru chloride-added (䊉) catalysts and NH4 Cl-added (*) catalysts. (a) Outlet CO concentration, (b) expanded view of (a), (c) outlet CH4 concentration, (d) outlet CO2 concentration, (e) selectivity for CO methanation: selectivity = (COinlet − COoutlet )/CH4 outlet .
immediately, even though the formate bands exhibited no changes in their intensities. From these results, we conclude that on the Ni catalyst without chloride additive, the reverse water-gas shift reaction proceeds via dissociation of CO2 on Ni metal sites, while the formation of carbonates and formates is merely a side reaction, and the chloride addition inhibits CO2 dissociation on Ni sites completely. In order to consider the predominant site for CO2 dissociation, e.g., Ni particle surface or interface between Ni particle and alumina support, the surface coverage of dissociated CO2 on the
reduced Ni sites for the unmodified catalyst after CO2 introduction was estimated by CO adsorption on the surface after CO2 dissociation had occurred. As seen in Fig. 3, the integrated area for the adsorbed CO bands (b) increased to 2.1 times larger than the one for the initial surface after CO2 dissociation (a) at 230 ◦ C. It is supposed that when CO was introduced to the CO2 -dissociated surface, the Ni–O(a) might be reduced by the introduced CO. Subsequently, additional CO adsorption proceeded on the vacant sites formed due to this process. The band areas and positions were
(B)
(A)
0.2
0.2
1601 1390 1601
Absorbance
Absorbance
(d)
(c) 2055 1936
1390
(d) (c)
(b) 2014 1844 1649 1447 1230 1533 1360
(a) 3200
2700
2200 Wavenumber
1700 (cm-1)
(b) 1649
(a) 1200
3200
2700
2200
Wavenumber (cm-1)
1700
1447 1230
1200
Fig. 2. FTIR spectra for CO2 adsorption and hydrogenation over Ni–Al oxide catalyst (A) unmodified, (B) RuCl3 added under flowing (A, a) CO2 , (A, b) CO2 and H2 , (A, c) H2 for 5 min, and (A, d) H2 for 30 min, (B, a) CO2 , (B, b-d) CO2 and H2 . These measurements were carried out sequentially at 230 ◦ C.
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T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192 Table 2 Surface composition of the catalysts after reduction treatment.
0.2 2035 1927
Samples (additives)
(d)
Absorbance
2048 1932
(c)
2063 1938
(b) 2021
2200
1865
2000
(a)
1800
1600
Wavenumber
1400
1200
(cm-1)
Fig. 3. FTIR spectra for CO2 and CO adsorption over Ni–Al oxide catalyst without additives at 230 ◦ C under flowing (a) CO2 , (b) CO2 + CO, (c) CO2 , and (d) He. These measurements were carried out sequentially.
not changed by the further introduction of the mixture of CO and CO2 (c) and displacement by flowing He (d) at the same temperature. To clarify the role of residual chlorine on the catalyst for the reverse water-gas shift reaction, we focused on the CO2 dissociation behavior of the catalysts with various types of chloride additives. Fig. 4 shows the results of CO2 adsorption FTIR measurement at 230 ◦ C for the series of modified Ni–Al oxide catalysts with the following additives: (a) RuCl3 , (b) hydrochloric acid, (c) ammonium chloride, (d) Ru nitrosyl nitrate, together with (e) the unmodified catalyst. The spectral feature observed for the (d) Ru nitrosyl nitrate-added sample were qualitatively the same as those for (e) the unmodified sample, as follows. After CO2 introduction, the adsorbed CO bands on the metallic Ni sites at 2017 cm−1 and 1862 cm−1 appeared together with the bands attributed to carbonate and bicarbonate those formed on alumina support at 1648 cm−1 , 1447 cm−1 , 1360 cm−1 , 1230 cm−1 [16]. In contrast, for the lower three spectra for the chloride-added samples, the formation of adsorbed CO species formed via CO2 dissociation was not observed. It is noteworthy that the bands at 1533 cm−1 , which could be assigned to carboxylate on Ni sites, were also diminished
0.1
2017
1862
1648
1533 1447 1360
Non-modified NH4 Cl RuCl3 a
(c) (b) (a)
1600
1400
Cl2p3/2
O1s
Cl/Ni
36.1 37.4 37.5
0.4 1.0 0.9
51.7 53.9 52.8
0.03 0.13 0.11
for the chloride-added samples [12]. These results indicate that the essential factor for the suppression of CO2 dissociation on the Ni metal site by the chloride addition is the existence of chlorine residue on the catalyst surface. Table 2 shows the surface composition estimated by XPS measurements for the samples with RuCl3 , RuNO(NO3 )3 and without additive. As seen in the table, the atomic ratio of Cl to Ni for the samples without additive, NH4 Cl added and RuCl3 added are 0.03, 0.13 and 0.11, respectively. The chlorine detected for the sample without additive should be a contaminant originating from the atmosphere during preparation of the samples or transfer to the XPS chamber. These results indicate that even a relatively small amount of chlorine, as low as 0.1 atomic ratio, has a significant effect for the dissociation and adsorption properties of the Ni catalysts. Fig. 5 shows the changes in the CO2 TPD profiles by chloride addition for the Ni–Al oxide catalysts. As seen in the figure, in the case of the sample without chlorine additive (c), three distinct CO2 desorption peaks were observed. These peaks are denoted as weakly bound (CO2 -w), moderately bound (CO2 -m) and strongly bound (CO2 -s), corresponding to the progression of increasing temperature. The CO2 -w peak is assigned to CO2 adsorbed weakly on the oxide support, because the same desorption peak, i.e., at the same temperature, was observed for the oxidized sample (d). Because the specific surface area of Ni–Al oxide (d) decreased by the reduction treatment, the intensity of the CO2 -w peaks observed for (a-c) were smaller than that for (d). On the other hand, CO2 m and CO2 -h may be attributed to desorption from reduced Ni sites. In contrast, for the chloride-added catalysts (a, b), CO2 -m was completely diminished. Because CO2 -m was not observed for the oxidized state (d), CO2 -m is therefore attributed to CO2 desorption from Ni metal sites, which might be related to CO2 hydrogenation intermediates. CO2 -s was diminished for the NH4 Cl-added sample but remained to a slight degree for the Ru chloride-added one, because the amount of residual chlorine for the NH4 Cl-added sample was much larger than that for the Ru chloride-added one. By the consideration of the FTIR-results, the desorbed CO2 was formed from dissociated CO2 or carboxylate on reduced Ni sites.
CO2 desorption rate (µmol/g-cat.)
Absorbance
(d)
1800
Al2p3/2
11.8 7.7 8.8
1230
(e)
2000
Ni2p3/2
Ratio of Cl2p3/2 (atom%) to Ni2p3/2 (atom%).
20
2200
Ratioa
XPS transitions (atomic%)
1200
Wavenumber (cm-1)
CO2-w CO2-m
16
CO2-s
12
(d) (c) (b)
8 4
(a) 0 25
125
225
325
425
525
Temperature (ºC) Fig. 4. Changes in CO2 adsorption behavior over Ni–Al oxide catalysts with various additives at 230 ◦ C, (a) RuCl3 , (b) HCl, (c) NH4 Cl, (d) RuNONO3 , and (e) without additive.
Fig. 5. Changes in CO2 desorption temperature by chloride addition, (a) RuCl3 , (b) NH4 Cl, (c) without additive, and (d) without additive in the oxidized state.
T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192
191
(A)
0.1
(B) (e)
(e) (d) (c) (b)
Absorbance
Absorbance
(d) (c) (b)
0.1
(a)
(a) 2500
2300
2100
1900
1700
1500
1300
2500
2300
2100
Wavenumber / cm-1
1900
1700
1500
1300
Wavenumber / cm-1
Fig. 6. Changes in CO desorption temperature by residual Cl: (A) without additives, (B) RuCl3 -added catalyst; (a) CO adsorption at 25 ◦ C, (b) He purge at 25 ◦ C, (c) He purge at 100 ◦ C, (d) He purge at 200 ◦ C, and (e) He purge at 300 ◦ C.
(A)
(e)
(B)
(e)
0.1
(d)
0.1
(d)
2500
2300
2100
1900
1700
1500
(c)
(c)
(b)
(b)
(a)
(a)
1300
Wavenumber / cm -1
2500
2300
2100
1900
1700
1500
1300
Wavenumber / cm-1
Fig. 7. Changes in CO desorption temperature by residual Cl: (A) HCl-added catalyst, (B) NH4 Cl-added catalyst; (a) CO adsorption at 25 ◦ C, (b) He purge at 25 ◦ C, (c) He purge at 100 ◦ C, (d) He purge at 200 ◦ C, and (e) He purge at 300 ◦ C.
In order to obtain further information for the adsorption properties of supported Ni metal sites with chlorine, CO adsorption FT-IR measurements were carried out. Fig. 6 shows the results for the samples without chlorine (A) and with chlorine (B). As seen in the figure, for the one without chlorine (A-a), when a 5% CO/He gas mixture was introduced at 25 ◦ C, in addition to linearCO bands appearing at around 2100-1900 cm−1 , large bridged-CO bands appeared around 1900–1700 cm−1 . In contrast, the band in the bridged-CO region for the chloride-added sample (B-a) was significantly weaker than that for (A-a). When the flowing CO/He mixed gas was switched to pure flowing He, the spectral feature for the sample without chlorine (A-b) was almost the same as that obtained under CO/He atmosphere (A-a). In contrast, in the case of the chloride-added catalyst (B-b), the band intensity around 2000 cm−1 decreased significantly. When the sample was heated stepwise in increments of 100 ◦ C, for the one without additive (Ac, d), the adsorbed CO bands were nearly unchanged up to 200 ◦ C. However, for the chloride-added sample, CO desorption took place
at significantly lower temperature (B-c, d). Fig. 7 shows the results for (A) HCl added and (B) NH4 Cl added samples. These results were quite similar with the one shown in Fig. 6 (B) that the most of adsorbed CO bands disappeared after the heat treatment at 200 ◦ C. These results suggest that the adsorption strength of CO on Ni was weakened significantly by chlorine, and the existence of chlorine on the Ni metal particles may affect the chemical state of the Ni metal. From these results, we conclude that residual chlorine existing on Ni metal sites modifies the chemical state of Ni and suppresses CO2 adsorption and dissociation. The remarkable improvement in the selectivity observed for SCM over the chloride-added catalysts is caused by this mechanism. 4. Conclusion CO2 hydrogenation (reverse water-gas shift reaction and CO2 methanation) during SCM in reformate gas containing high
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concentrations of CO2 was significantly suppressed by residual chlorine on the Ni–Al oxide catalysts. CO2 adsorption FTIR measurements for the series of catalysts with various chlorine-containing additives revealed that chlorine suppressed the dissociation of CO2 , which led to the formation of adsorbed CO species on Ni metal sites. Consideration of the results obtained by FTIR measurements combined with the results of CO2 adsorption TPD measurements indicates that chlorine adsorbed on Ni diminished the formation of carboxylate species on Ni-metal sites under flowing CO2 conditions. From these results, the controlling factor for the selectivity of CO methanation should be the presence of chlorine on the Ni catalyst surface. Acknowledgements This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan, the “Low-Carbon Research Network,” funded by MEXT Japan, and JSPS KAKENHI Grant Number 24560944.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Mechanistic study of the effect of chlorine on selective CO methanation over Ni alumina-based catalysts Toshihiro Miyao, Weihua Shen, Aihua Chen, Kazutoshi Higashiyama ∗ , Masahiro Watanabe Fuel Cell Nanomaterials Center, University of Yamanashi, Kofu 400-0021, Japan
a r t i c l e
i n f o
Article history: Received 7 April 2014 Received in revised form 5 August 2014 Accepted 23 August 2014 Available online 30 August 2014 Keywords: Selective CO methanation Ni/alumina catalyst Chlorine residue CO2 methanation Reverse water-gas shift reaction Selectivity controlling additives
a b s t r a c t The effect of residual chlorine of alumina-supported Ni catalysts on the selective CO methanation reaction was investigated. By chloride addition to the catalyst, CO2 methanation and the reverse water gas-shift reaction were significantly suppressed in the course of the reaction. FTIR measurements for CO2 adsorption on a series of modified catalysts with various chlorine-containing additives revealed that chlorine suppressed the dissociation of CO2 on the catalyst surface, which led to the formation of adsorbed CO species. The FTIR results combined with TPD measurements for CO2 desorption indicates that chlorine adsorbed on Ni metal diminished the formation of carboxylate species on the same surface under flowing CO2 conditions. Through the present study, the controlling factor for the selective methanation of CO by chlorine was clarified in high CO2 concentration-containing hydrogen rich gas. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, polymer electrolyte fuel cells (PEFCs) have been attracting a great attention as residential co-generation systems of electricity and heat energy because of their higher total efficiencies compared to those of conventional power plants and of their lower risk to a failure of major power plants at an unpredictable accident or disaster, although, at present, there are still obstacles for the widespread usages. The first obstacle is the high manufacturing cost, and the second is the large size of the system. In most systems, hydrogen is produced via the steam reforming and subsequent water-gas shift reaction of city gas. The reformate gas from the water-gas shift reaction contains around 0.5% CO. In a conventional system, the CO is removed by the preferential oxidation (PROX) process, in which CO is oxidized with air additionally introduced. In order to maintain the selectivity and avoid an extra hydrogen consumption by the side reaction, the reformate gas should be cooled down below 150 ◦ C before the addition of air to the gas. This process, however, leads to increases in the cost and the volume of the fuel processor, e.g., by the additional air-blower and the chamber space for mixing the gases. As a lower cost CO removal process, a selective CO methanation (SCM) process has been proposed as
∗ Corresponding author. Tel.: +81 55 2547096; fax: +81 55 2547096. E-mail address: [email protected] (K. Higashiyama). http://dx.doi.org/10.1016/j.apcata.2014.08.025 0926-860X/© 2014 Elsevier B.V. All rights reserved.
a promising candidate because it requires no pre-cooling of the reformate gas from the water-gas shift reactor, no air supply and no mixing of fuel and air [1]. In order to develop a realistic SCM catalyst, the most important requirements are the suppression of CO2 methanation (CO2 + 3H2 → CH4 + H2 O) and the suppression of the reverse water-gas shift reaction (CO2 + H2 → CO + H2 O) for the outlet gas from the CO shift reaction, which contains 20% CO2 . If significant CO2 methanation proceeds, the process becomes out of control because the reaction is an exothermic reaction. Suppression of the reverse water-gas shift is also important in order to keep the lowered CO concentration. Recently, we reported remarkable improvement of the catalytic selectivity for CO methanation in reformate gas, which contained 20% CO2 , over an Ni-alumina-based catalyst by addition of a small amount of Ru chloride [2,3]. We found that both the reverse watergas shift reaction and CO2 methanation reaction were remarkably suppressed by this addition. At the beginning stage of our investigation, the reason for the suppression of CO2 hydrogenation by Ru chloride addition was unclear. Through the recent study by our group, the true mechanism for the improvement of the selectivity has been recognized, i.e., the residual chlorine on the catalyst surface has a remarkable effect on the improvement of the selectivity for the SCM. Indeed, in many cases, chlorine exhibits a significant inhibition for hydrogenation reactions over various metal catalysts [4]. In most of the previous reports concerning the SCM over Ru or Ni-based catalysts, Cl-free precursors, e.g., Ru nitrosyl nitrate,
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Ru nitrate and Ni nitrate were usually employed, and the additive effect of Ru chloride has rarely been considered [5–7], because chlorine is commonly recognized as an inhibitor for CO hydrogenation. On the other hand, Djinovic et al. reported the effect of chlorine on the catalytic activity of an Ru/Al2 O3 catalyst for the SCM, even though the mechanistic role of chlorine in the change of the catalytic properties was not clarified [8]. In the present study, the effect of residual chlorine on the selectivity of an Ni-alumina-based catalyst is discussed in detail on the basis of in situ spectroscopic measurements. 2. Experimental The Ni–Al mixed oxide powder was prepared by a solutionspray plasma technique [9,10]. A water solution of Ni nitrate and Al nitrate was mixed and sprayed into an evacuated chamber through an argon plasma flame with 10% oxygen in the argon carrier gas. The nitrates decomposed immediately in the flame to a fine oxide powder, which was then collected by a suction filter placed at the outlet of the chamber. For the addition of chloride compounds, the oxide powder was impregnated with aqueous solutions of chloride and then evaporated and dried at 110 ◦ C for 12 h, followed by calcination at 500 ◦ C for 3 h in air. In each case, the amounts of chloride additives (RuCl3 ·3H2 O, NH4 Cl, HCl) were adjusted by Cl amount: 1% Cl by weight vs. that of the Ni–Al oxide powder. In the case of Ru(NO)(NO3 )3 , the amount of the additive was the same as that in the case of RuCl3 addition on an Ru-metal basis. In this study, the plasma-generated powder will be denoted as Ni–Al oxide, because the powder is composed of multiple components, i.e., NiAl2 O4 , Ni oxides and a small amount of Al2 O3 . Each Ni–Al oxide powder sample was wash-coated on a ceramic monolith (400 cells per square inch) with cross-sectional dimensions of 10 mm × 10 mm and 20 mm length, followed by calcination in air at 500 ◦ C, prior to the reaction measurements. The wash-coated monolith was mounted tightly in a quartz microreactor with an inner diameter of 10 mm and then reduced in hydrogen at 500 ◦ C for 1 h before the activity test at atmospheric pressure. The reaction feed involved 1% CO, 20% CO2 , 79% H2 (dry basis), with H2 O corresponding to a steam/CO ratio of 15. Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy (spectrometer, FTIR-4200; DRIFT chamber, DR-500, JASCO) was employed to analyze the CO2 and CO adsorption behavior of the catalysts. Temperature-programmed desorption (TPD) profiles after CO2 adsorption were measured by use of an atmospheric flow system (BELCAT, BEL Japan) with a quadrupole mass spectrometer. Chlorine contents in the series of the samples were analyzed by a combustion-ion chromatography technique (Thermo Fisher Scientific, ICS-1500-AQF-100). Before the measurement, the sample powders were dried at 110 ◦ C for 30 min. 3. Results and discussion Fig. 1 shows the temperature dependence of the SCM over a series of Ni–Al oxide catalysts with different amounts of chloride additives. As seen in Fig. 1(a), at low temperature (190 ◦ C), the catalyst without additive (open triangles) showed the lowest CO concentration, while it increased with increasing amounts of added Ru chloride (1 wt%, open circles; 2 wt%, solid triangles; 3 wt%, solid circles). Because the catalyst without additives showed the highest activity for CO conversion in the lower temperature region, it is considered that Ru has a minor contribution with respect to the CO hydrogenation activity and that the reaction takes place predominantly over Ni sites. Fig. 1(b), which is an expanded view of Fig. 1(a), shows that, in the case of the catalyst without Ru chloride (open triangles), the CO concentration decreased until a temperature of 210 ◦ C was reached and then increased again significantly, due to
Table 1 Amount of chlorine contained in the series of samples. Samples
Amount of chlorine (wt%)
NiAlx Oy 1%RuCl3 /NiAlx Oy 3%RuCl3 /NiAlx Oy HCl/NiAlx Oy NH4 Cl/NiAlx Oy
n.d. 1.10 1.52 1.63 1.22
The quantitative analysis of chlorine was carried out by using a combustion-ion chromatograph technique. Before the analysis, sample powders were dried at 110 ◦ C for 30 min.
the reverse water-gas shift (RWGS) reaction, with increasing reaction temperature. In contrast, the catalysts with chloride additives did not exhibit a significant reverse water gas-shift reactioninduced increase of CO. In order to clarify the effect of the residual chlorine more precisely, quantitative analysis of chlorine was carried out by using a combustion-ion chromatograph technique. The amounts of chlorine contained in the series of the samples were summarized in Table 1. In the case of the unmodified sample, no chlorine was detected. The amount of chlorine contained in the RuCl3 added samples increased with increasing of the addition amount of RuCl3 . In the case of HCl and NH4 Cl added samples, the amounts of the residual chlorine were larger than that for the 1%RuCl3 added one despite the same amount of chlorides in the chlorine-basis were added to the samples. These results mean that the amount of residual chlorine after the heat treatment depends on the kind of the chloride. It is noteworthy that, as seen in Fig. 1(c), methane formation from CO2 methanation was also suppressed by Ru chloride additions, in particular, over the 3 wt%Ru chlorideadded catalyst. As the result, the selectivity for CO methanation was almost completely suppressed up to 250 ◦ C on the catalyst, where selectivity = (COinlet − COoutlet )/CH4 outlet , shown in Fig. 1(d). In the case of the NH4 Cl added catalyst (*), the selectivity was maintained at 100% until 270 ◦ C because of the significant suppression of the methane formation. On the other hand, in the case of unmodified and 1%RuCl3 added catalysts, the selectivity showed quite small values when reaction temperature was higher than 240 ◦ C due to the conversion of the large amount of CO2 into CH4 . Fig. 2 shows diffuse reflectance FTIR spectra for CO2 adsorption and reaction with hydrogen on an unmodified Ni–Al oxide catalyst (A), and a 1% Ru chloride-added catalyst (B) at 230 ◦ C. As seen in Fig. 2(A-a), in the case of the unmodified sample, several bands observed below 1700 cm−1 assigned to carbonate appeared immediately after CO2 introduction [11,12]. In addition to the bands due to carbonate formation, those due to adsorbed CO appeared at 2014 and 1844 cm−1 ; these are assigned to linearly adsorbed CO (denoted as “l-CO”) and bridge-type adsorbed CO (denoted as “bCO”), respectively [13]. When hydrogen was added to the flowing CO2 (b), the bands due to adsorbed CO were intensified significantly, and new bands appeared at 2055 cm−1 and 1936 cm−1 ; these are assigned to weakly adsorbed l-CO and l-CO on edge sites of Ni metal particles [14,15]. It is considered that after CO2 introduction, CO2 adsorbed on Ni sites was dissociated to CO(a) and O(a) to a large extent, and when hydrogen was introduced to the surface, adsorbed oxygen was immediately removed from the Ni surface due to the reaction with hydrogen. Additional CO, which was formed via the reverse water-gas shift reaction, then adsorbed on the vacant sites. In contrast, the Ru chloride-added catalyst shown in Fig. 2(B-a) exhibited almost no peak in the region corresponding to adsorbed CO. These results mean that Ru chloride addition inhibits the adsorption or dissociation of CO2 to a remarkable degree. After the hydrogen addition, the carbonate bands converted to formate bands, 1601 cm−1 and 1390 cm−1 , for both catalysts (Ab, B-b). When the CO2 -hydrogen gas mixture was replaced with pure hydrogen (A-c, A-d), the adsorbed CO bands disappeared
T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192
7000
10
6000
100
(c)
(a)
189
8
80
3000
Selectivity, %
4000
CH4, %
CO , ppm
5000 6 4
2000 1000 0 190
240 Temperature / ºC
290
60 40
2
20
0
0 190
210
230
250
270
290
(e) 190
210
Temperature / ºC 500
230 250 270 Temperature / ºC
290
25
400
20
300
15
CO2, %
CO , ppm
(b)
200
10
100
5
0
0
190
240 Temperature / ºC
290
(d) 190
210
230 250 270 Temperature / ºC
290
Fig. 1. Temperature dependence of selective CO methanation over unmodified Ni-Al oxide (), 1% Ru chloride-added ()), 2% Ru chloride-added () 3% Ru chloride-added (䊉) catalysts and NH4 Cl-added (*) catalysts. (a) Outlet CO concentration, (b) expanded view of (a), (c) outlet CH4 concentration, (d) outlet CO2 concentration, (e) selectivity for CO methanation: selectivity = (COinlet − COoutlet )/CH4 outlet .
immediately, even though the formate bands exhibited no changes in their intensities. From these results, we conclude that on the Ni catalyst without chloride additive, the reverse water-gas shift reaction proceeds via dissociation of CO2 on Ni metal sites, while the formation of carbonates and formates is merely a side reaction, and the chloride addition inhibits CO2 dissociation on Ni sites completely. In order to consider the predominant site for CO2 dissociation, e.g., Ni particle surface or interface between Ni particle and alumina support, the surface coverage of dissociated CO2 on the
reduced Ni sites for the unmodified catalyst after CO2 introduction was estimated by CO adsorption on the surface after CO2 dissociation had occurred. As seen in Fig. 3, the integrated area for the adsorbed CO bands (b) increased to 2.1 times larger than the one for the initial surface after CO2 dissociation (a) at 230 ◦ C. It is supposed that when CO was introduced to the CO2 -dissociated surface, the Ni–O(a) might be reduced by the introduced CO. Subsequently, additional CO adsorption proceeded on the vacant sites formed due to this process. The band areas and positions were
(B)
(A)
0.2
0.2
1601 1390 1601
Absorbance
Absorbance
(d)
(c) 2055 1936
1390
(d) (c)
(b) 2014 1844 1649 1447 1230 1533 1360
(a) 3200
2700
2200 Wavenumber
1700 (cm-1)
(b) 1649
(a) 1200
3200
2700
2200
Wavenumber (cm-1)
1700
1447 1230
1200
Fig. 2. FTIR spectra for CO2 adsorption and hydrogenation over Ni–Al oxide catalyst (A) unmodified, (B) RuCl3 added under flowing (A, a) CO2 , (A, b) CO2 and H2 , (A, c) H2 for 5 min, and (A, d) H2 for 30 min, (B, a) CO2 , (B, b-d) CO2 and H2 . These measurements were carried out sequentially at 230 ◦ C.
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T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192 Table 2 Surface composition of the catalysts after reduction treatment.
0.2 2035 1927
Samples (additives)
(d)
Absorbance
2048 1932
(c)
2063 1938
(b) 2021
2200
1865
2000
(a)
1800
1600
Wavenumber
1400
1200
(cm-1)
Fig. 3. FTIR spectra for CO2 and CO adsorption over Ni–Al oxide catalyst without additives at 230 ◦ C under flowing (a) CO2 , (b) CO2 + CO, (c) CO2 , and (d) He. These measurements were carried out sequentially.
not changed by the further introduction of the mixture of CO and CO2 (c) and displacement by flowing He (d) at the same temperature. To clarify the role of residual chlorine on the catalyst for the reverse water-gas shift reaction, we focused on the CO2 dissociation behavior of the catalysts with various types of chloride additives. Fig. 4 shows the results of CO2 adsorption FTIR measurement at 230 ◦ C for the series of modified Ni–Al oxide catalysts with the following additives: (a) RuCl3 , (b) hydrochloric acid, (c) ammonium chloride, (d) Ru nitrosyl nitrate, together with (e) the unmodified catalyst. The spectral feature observed for the (d) Ru nitrosyl nitrate-added sample were qualitatively the same as those for (e) the unmodified sample, as follows. After CO2 introduction, the adsorbed CO bands on the metallic Ni sites at 2017 cm−1 and 1862 cm−1 appeared together with the bands attributed to carbonate and bicarbonate those formed on alumina support at 1648 cm−1 , 1447 cm−1 , 1360 cm−1 , 1230 cm−1 [16]. In contrast, for the lower three spectra for the chloride-added samples, the formation of adsorbed CO species formed via CO2 dissociation was not observed. It is noteworthy that the bands at 1533 cm−1 , which could be assigned to carboxylate on Ni sites, were also diminished
0.1
2017
1862
1648
1533 1447 1360
Non-modified NH4 Cl RuCl3 a
(c) (b) (a)
1600
1400
Cl2p3/2
O1s
Cl/Ni
36.1 37.4 37.5
0.4 1.0 0.9
51.7 53.9 52.8
0.03 0.13 0.11
for the chloride-added samples [12]. These results indicate that the essential factor for the suppression of CO2 dissociation on the Ni metal site by the chloride addition is the existence of chlorine residue on the catalyst surface. Table 2 shows the surface composition estimated by XPS measurements for the samples with RuCl3 , RuNO(NO3 )3 and without additive. As seen in the table, the atomic ratio of Cl to Ni for the samples without additive, NH4 Cl added and RuCl3 added are 0.03, 0.13 and 0.11, respectively. The chlorine detected for the sample without additive should be a contaminant originating from the atmosphere during preparation of the samples or transfer to the XPS chamber. These results indicate that even a relatively small amount of chlorine, as low as 0.1 atomic ratio, has a significant effect for the dissociation and adsorption properties of the Ni catalysts. Fig. 5 shows the changes in the CO2 TPD profiles by chloride addition for the Ni–Al oxide catalysts. As seen in the figure, in the case of the sample without chlorine additive (c), three distinct CO2 desorption peaks were observed. These peaks are denoted as weakly bound (CO2 -w), moderately bound (CO2 -m) and strongly bound (CO2 -s), corresponding to the progression of increasing temperature. The CO2 -w peak is assigned to CO2 adsorbed weakly on the oxide support, because the same desorption peak, i.e., at the same temperature, was observed for the oxidized sample (d). Because the specific surface area of Ni–Al oxide (d) decreased by the reduction treatment, the intensity of the CO2 -w peaks observed for (a-c) were smaller than that for (d). On the other hand, CO2 m and CO2 -h may be attributed to desorption from reduced Ni sites. In contrast, for the chloride-added catalysts (a, b), CO2 -m was completely diminished. Because CO2 -m was not observed for the oxidized state (d), CO2 -m is therefore attributed to CO2 desorption from Ni metal sites, which might be related to CO2 hydrogenation intermediates. CO2 -s was diminished for the NH4 Cl-added sample but remained to a slight degree for the Ru chloride-added one, because the amount of residual chlorine for the NH4 Cl-added sample was much larger than that for the Ru chloride-added one. By the consideration of the FTIR-results, the desorbed CO2 was formed from dissociated CO2 or carboxylate on reduced Ni sites.
CO2 desorption rate (µmol/g-cat.)
Absorbance
(d)
1800
Al2p3/2
11.8 7.7 8.8
1230
(e)
2000
Ni2p3/2
Ratio of Cl2p3/2 (atom%) to Ni2p3/2 (atom%).
20
2200
Ratioa
XPS transitions (atomic%)
1200
Wavenumber (cm-1)
CO2-w CO2-m
16
CO2-s
12
(d) (c) (b)
8 4
(a) 0 25
125
225
325
425
525
Temperature (ºC) Fig. 4. Changes in CO2 adsorption behavior over Ni–Al oxide catalysts with various additives at 230 ◦ C, (a) RuCl3 , (b) HCl, (c) NH4 Cl, (d) RuNONO3 , and (e) without additive.
Fig. 5. Changes in CO2 desorption temperature by chloride addition, (a) RuCl3 , (b) NH4 Cl, (c) without additive, and (d) without additive in the oxidized state.
T. Miyao et al. / Applied Catalysis A: General 486 (2014) 187–192
191
(A)
0.1
(B) (e)
(e) (d) (c) (b)
Absorbance
Absorbance
(d) (c) (b)
0.1
(a)
(a) 2500
2300
2100
1900
1700
1500
1300
2500
2300
2100
Wavenumber / cm-1
1900
1700
1500
1300
Wavenumber / cm-1
Fig. 6. Changes in CO desorption temperature by residual Cl: (A) without additives, (B) RuCl3 -added catalyst; (a) CO adsorption at 25 ◦ C, (b) He purge at 25 ◦ C, (c) He purge at 100 ◦ C, (d) He purge at 200 ◦ C, and (e) He purge at 300 ◦ C.
(A)
(e)
(B)
(e)
0.1
(d)
0.1
(d)
2500
2300
2100
1900
1700
1500
(c)
(c)
(b)
(b)
(a)
(a)
1300
Wavenumber / cm -1
2500
2300
2100
1900
1700
1500
1300
Wavenumber / cm-1
Fig. 7. Changes in CO desorption temperature by residual Cl: (A) HCl-added catalyst, (B) NH4 Cl-added catalyst; (a) CO adsorption at 25 ◦ C, (b) He purge at 25 ◦ C, (c) He purge at 100 ◦ C, (d) He purge at 200 ◦ C, and (e) He purge at 300 ◦ C.
In order to obtain further information for the adsorption properties of supported Ni metal sites with chlorine, CO adsorption FT-IR measurements were carried out. Fig. 6 shows the results for the samples without chlorine (A) and with chlorine (B). As seen in the figure, for the one without chlorine (A-a), when a 5% CO/He gas mixture was introduced at 25 ◦ C, in addition to linearCO bands appearing at around 2100-1900 cm−1 , large bridged-CO bands appeared around 1900–1700 cm−1 . In contrast, the band in the bridged-CO region for the chloride-added sample (B-a) was significantly weaker than that for (A-a). When the flowing CO/He mixed gas was switched to pure flowing He, the spectral feature for the sample without chlorine (A-b) was almost the same as that obtained under CO/He atmosphere (A-a). In contrast, in the case of the chloride-added catalyst (B-b), the band intensity around 2000 cm−1 decreased significantly. When the sample was heated stepwise in increments of 100 ◦ C, for the one without additive (Ac, d), the adsorbed CO bands were nearly unchanged up to 200 ◦ C. However, for the chloride-added sample, CO desorption took place
at significantly lower temperature (B-c, d). Fig. 7 shows the results for (A) HCl added and (B) NH4 Cl added samples. These results were quite similar with the one shown in Fig. 6 (B) that the most of adsorbed CO bands disappeared after the heat treatment at 200 ◦ C. These results suggest that the adsorption strength of CO on Ni was weakened significantly by chlorine, and the existence of chlorine on the Ni metal particles may affect the chemical state of the Ni metal. From these results, we conclude that residual chlorine existing on Ni metal sites modifies the chemical state of Ni and suppresses CO2 adsorption and dissociation. The remarkable improvement in the selectivity observed for SCM over the chloride-added catalysts is caused by this mechanism. 4. Conclusion CO2 hydrogenation (reverse water-gas shift reaction and CO2 methanation) during SCM in reformate gas containing high
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concentrations of CO2 was significantly suppressed by residual chlorine on the Ni–Al oxide catalysts. CO2 adsorption FTIR measurements for the series of catalysts with various chlorine-containing additives revealed that chlorine suppressed the dissociation of CO2 , which led to the formation of adsorbed CO species on Ni metal sites. Consideration of the results obtained by FTIR measurements combined with the results of CO2 adsorption TPD measurements indicates that chlorine adsorbed on Ni diminished the formation of carboxylate species on Ni-metal sites under flowing CO2 conditions. From these results, the controlling factor for the selectivity of CO methanation should be the presence of chlorine on the Ni catalyst surface. Acknowledgements This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan, the “Low-Carbon Research Network,” funded by MEXT Japan, and JSPS KAKENHI Grant Number 24560944.
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