Al2O3 and

Applied Catalysis A: General 172 (1998) 351±358

Catalytic properties of sprayed Ru/Al2O3 and promoter effects of alkali metals in CO2 hydrogenation Di Li, Nobuyuki Ichikuni, Shogo Shimazu, Takayoshi Uematsu* Department of Applied Chemistry, Faculty of Engineering, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba 263-8522, Japan Received 28 March 1998; received in revised form 21 April 1998; accepted 24 April 1998

Abstract Three series of Ru/Al2O3 catalysts were prepared by spray reaction (SPR), impregnation (IMP), and these two methods combined. The characteristics of multi-component SPR particles, as CO2 hydrogenation catalysts, were elucidated. The catalytic activity (TOF) of SPR ®ne particles was higher by one order of magnitude than that of IMP catalyst without any promoters. This is attributed to the formation of more active species at the perimeters between Ru metal and Al2O3 over SPR particles than over IMP catalysts. The addition of alkaline salts promoted the catalytic activity of Ru/Al2O3, irrespective of preparation methods employed. The promotion of alkali metals to Ru/Al2O3 catalysts is probably due to a synergetic effects including the modi®cation of local electron density on Ru metal by the electron donation of alkali metals, the neutralization of residual chlorine ions by the formation of alkaline chloride, and the removal of depositional inactive carbon by alkaline carbonate catalysis. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ru/Al2O3; Spray reaction method; Alkali metal promoters; CO2 hydrogenation

1. Introduction Hydrogenation of CO2 is of great signi®cance in the aspects of global environmental protection and utilization of carbon sources for the syntheses of oxygenates and hydrocarbons. For hydrogenation of CO2, Ru has been regarded as the most effective noble metal and has the highest speci®c methanation activity [1± 3]. As catalyst precursors, the following Ru sources were usually used: RuCl3, K2RuCl6, Ru(NH3)6Cl3, Ru3(CO)12, Ru(acac)3, Ru(NO)(NO3)3 [4,5]. In general, the catalysts prepared from chlorine-free Ru complexes, e.g. Ru3(CO)12 or Ru(acac)3, have high *Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00139-2

activities because of high Ru dispersion and chlorinefree surfaces, while the catalysts prepared from chlorine-containing Ru compounds have low activities due to low Ru dispersion or a residual chlorine poisoning effect [4]. The selection of chlorine-free Ru complexes as catalyst precursors is also very important to decipher the reaction mechanism, because of the absence of chlorine in¯uence. On the other hand, we must understand the fact that Ru catalysts have not found widespread application in industries due to the cost of the metal [2]. The high cost of the catalyst prepared from chlorine-free Ru complexes, such as Ru3(CO)12, will make the utilization of Ru catalysts more dif®cult. Therefore, to select one of the cheap Ru precursors, such as RuCl3, and to develop high activity

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catalysts by a effective preparation method will become extremely valuable. Alkali metals are widely used as promoters for various heterogeneous catalysis. One of the classical examples is the synthesis of ammonia, where the potassium promoter in the fused-iron catalyst is essential for a high catalytic performance [6]. Although the location and the chemical state of potassium promoter have been studied extensively during the last 60 years, the exact role of this promoter has not yet been explained explicitly and the mechanism of the potassium promotion seems to be particularly unclear and still controversial [6,7]. For CO2 hydrogenation, further studies about the mechanism of the alkali metal promotion are thus worthwhile. In the present study, the spray reaction (SPR) method (which has also been called spray decomposition, mist decomposition, or spray pyrolysis) [8,9] was applied to prepare Ru/Al2O3 catalysts. The conventional impregnation (IMP) and the hybrid (HYB) methods were also employed in order to evaluate the characteristics of the SPR catalysts. Ten kinds of Ru/Al2O3 catalysts were prepared by changing the preparation methods and the preparation sequence of the different ingredients during the addition of alkaline salts. The performances, surface characterization, and alkali metal promoter effects for these catalysts are discussed in detail. 2. Experimental 2.1. Catalyst preparation 2.1.1. Spray reaction (SPR) method The required amounts of RuCl3
3H2O (Kanto) and Al(NO3)3
9H2O (Wako) were dissolved in deionized water to obtain a 0.01 M RuCl3 spray solution. A spray solution was atomized by a ultrasonic device and then passed through a quartz tube reactor under the suction of an aspirator, where the reactor was externally heated by two furnaces connected in series. The temperatures of the two furnaces were controlled at 873 and 1273 K, respectively, by two temperature programmers. The pyrolysis proceeded quickly as droplets passed through the high temperature quartz tube within a second. The formed ®ne particles were trapped with a glass ®lter. The catalysts prepared by

SPR were designated as S series: S-1, spr-Ru/Al2O3, prepared from a binary spray solution containing RuCl3 and Al(NO3)3; S-2, co-spr-Ru-K/Al2O3, prepared from a ternary spray solution containing RuCl3, Al(NO3)3 and KNO3. The supported amount of Ru was expressed in molar percentage (mol%). 2.1.2. Impregnation (IMP) method The appropriate amounts of RuCl3
3H2O and/or KNO3 were dissolved in deionized water suf®cient to completely wet the support Al2O3 (Aerosil Al2O3C, Japan Aerosil, g-Al2O3, 100 m2/g). The resultant slurry was aged at room temperature for 24 h and stirred at regular intervals to retain uniformity. After evaporation at 373 K in water bath and drying at 378 K for 12 h, the samples were calcinated at 673 K for 4 h. The catalysts prepared by IMP were symbolized as I series: I-1, imp-Ru/Al2O3; I-2, co-imp-Ru-K/Al2O3, prepared by cooperative impregnation; I-3, seq-impK-Ru/Al2O3, prepared by sequential impregnation of Al2O3, ®rst with a RuCl3 solution and then with a KNO3 solution; I-4, seq-imp-Ru-K/Al2O3, the sequence of impregnation was the reverse of I-3. 2.1.3. Hybird (HYB) method of spray and impregnation One of spr-Al2O3, spr-K/Al2O3 or spr-Ru/Al2O3 (S-1) was impregnated with the other ingredient of the catalyst. The catalysts prepared by HYB was designated as H series: H-1, hyb-Ru/Al2O3, impregnation of spr-Al2O3 with a RuCl3 solution; H-2, hyb(K-Ru)/Al2O3, co-impregnation of spr-Al2O3 with a mixed solution of RuCl3 and KNO3; H-3, hyb-Z-(Ru/ Al2O3) (ZˆLi, Na, K, Rb or Cs), impregnation of sprRu/Al2O3 with a alkaline salt solution; H-4, hyb-Ru(K/Al2O3), impregnation of spr-K/Al2O3 with a RuCl3 solution. 2.2. CO2 hydrogenation CO2 hydrogenation was carried out in a closed circulation system interfaced to a gas chromatograph (SHIMADZU GC-8A) with a TCD detector. The main product was CH4 and the formation of by-products (CO or other hydrocarbons) was small in amount and will be neglected if not otherwise noted. The initial rate r0 (mol/min Ru-mol) of CH4 formation was used to stand for the catalytic activity. The catalyst

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(0.100 g) was pretreated with O2 (32.5 kPa) for 1 h at 673 K, H2 (32.5 kPa) for 2 h at 673 K and evacuated for 1 h at reduction temperature prior to use. The initial partial pressures of H2 and CO2 reactants were 65 and 13 kPa, respectively. 2.3. Characterization 2.3.1. Chemisorption of H2 The chemisorption measurements were undertaken in a static system at 273 K. Desorption was conducted by pumping the catalyst sample (0.500 g) for 15 min, followed by an introduction of H2 (33.3 kPa). The amount of irreversibly chemisorbed hydrogen was calculated from the difference between the ®rst and second adsorption. The catalyst was pretreated under the same conditions as in the catalytic tests. 2.3.2. XRD and TPR XRD patterns were recorded by a X-ray diffractometer (MXP03, MAC Science, Japan) with monochromatized CuK radiation operated at 35 kV and 30 mA. Temperature-programmed reduction (TPR) experiments were implemented by measuring H2 consumption during the reduction of a catalyst with a TCD detector. A 10% H2 in N2 gas mixture was used as the reducing gas at a ¯ow rate of 60 ml/min. The sample was heated from ambient temperature to 973 K at 10 K/min.

Fig. 1. XRD patterns of spr-Ru/Al2O3 (S-1, 5.0 mol%) and impRu/Al2O3 (I-1, 5.0 mol%). BR, before reduction; AR, after reduction. Reduction conditions: 32.5 kPa H2, 673 K, 2 h. The unlabelled peaks were assigned to g-Al2O3 in I-1 patterns and to a-Al2O3 in S-1 patterns.

3. Results and discussion 3.1. Characterization 3.1.1. XRD and TPR The XRD patterns of Ru/Al2O3 catalysts shown in Fig. 1 indicate that the crystal peaks of RuO2 were observed on both SPR (S-1) and IMP catalysts (I-1) before hydrogen reduction. Ru peaks appeared after hydrogen treatment, but a noticeable crystal growth of RuO2 or Ru on SPR catalyst was found compared with that on IMP catalyst because a high temperature was applied for the spray reaction. Moreover, the support precursor Al(NO3)3 was transformed into a-Al2O3 by spray reaction. Such a spray reaction can improve the interaction between metal and support [9,10]; this conclusion was

Fig. 2. TPR profiles of S-1, I-1 and H-1 catalysts.

proved by the TPR results given in Fig. 2. The single signal assigned to the reduction peak of RuO2 was observed at 463 K for I-1 and at 462 K for H-1. For S-1, a obvious shoulder peak appeared at 465 K, along with the main peak at 524 K. This indicates that two kinds of RuO2 existed on SPR catalyst: one is surface

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Fig. 3. The effect of Cs molar loading on the reduction temperature of RuO2 on hyb-Cs-(Ru/Al2O3)(Ru: 5.0 mol%) in TPR profiles.

Fig. 4. Effect of Ru molar loading on r0 in CO2 hydrogenation at 573 K and hydrogen uptakes over spr-Ru/Al2O3 catalysts.

RuO2 on S-1 particles such as that on I-1 or H-1 catalyst, and the other is called interface RuO2 in this study, in which RuO2 is probably interacting with support Al2O3 by sharing the common oxygen atoms. About 60 K difference of RuO2 reduction temperature between the S-1 (main peak: 524 K) and I-1 (463 K) catalysts indicates that the interaction between metal Ru and support Al2O3 was really strengthened on SPR particles. 3.1.2. Effect of alkali metal dopant on TPR profiles CsNO3 was used for the preparation of hyb-Cs-(Ru/ Al2O3) catalysts with different Cs contents in the preparation of H-3 catalysts. The effect of Cs molar loading on the reduction temperature of RuO2 in TPR pro®les can be seen in Fig. 3. The addition of alkaline salt facilitated the reduction of RuO2 and thus the reduction temperatures shifted to lower values. 3.2. Effect of the preparation method of catalysts on activities 3.2.1. Ru loading amount For the SPR catalysts, the effect of molar loading of Ru on activities is shown in Fig. 4. The initial catalytic rate r0 and hydrogen uptake varied with the Ru molar loading, and both reached a maximum at 5.0 mol%. This indicates that catalytic activity per site depended on the Ru concentration; the optimum condition was attained at 5.0 mol% spr-Ru/Al2O3.

Fig. 5. Effect of preparation methods on activities (r0 and TOF) of 5.0 mol% Ru /Al2O3 catalysts in CO2 hydrogenation at 573 K.

3.2.2. Preparation methods The activities of Ru/Al2O3 catalysts prepared by three methods are indicated in Fig. 5. The magnitude order was: S-1> I-1> H-1. The r0 of S-1 was ca. 2 times higher than that of I-1 and the TOF of S-1 was ca. 10 times (Table 1). The r0 of H-1 was the lowest. The higher activity of SPR catalyst is related to the structure of the ®ne particles. One of the characteristics of SPR ®ne particles is the homogeneous mixing of metal and support oxides because both were formed simultaneously from the homogeneous solution [8]; metal bulks or clusters generally observed in IMP catalysts could not be found on SPR ®ne particles, i.e. the metal oxide and support oxide mixed in molecular

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Table 1 The characteristics of 10 kinds of Ru/Al2O3 catalysts Catalysts

r0 (10ÿ2 U)

H/Ru (10ÿ3)

TOF (minÿ1)

BET (m2/g)

S-1 S-2 I-1 I-2 I-3 I-4 H-1 H-2 H-3 H-4

5.2 11.7 2.7 15.3 16.5 14.9 0.80 15.5 28.7 23.8

12.8 6.0 85.9 17.6 13.4 17.3 9.7 7.0 11.3 18.3

4.1 19.5 0.31 8.7 12.3 8.6 0.83 22.3 25.4 13.0

27.5 25.5 71.8 69.2 78.1 67.4 52.0 39.5 33.4 43.7

Ea (kJ/mol) 85.3 103.8 69.2 80.1 124.0 96.5 90.1 81.5

U: mol/min Ru-mol; Reaction temperature: 573 K.

level. Thus, the interfaces between Ru and Al2O3 grow more extensively on SPR catalysts than on IMP catalysts after hydrogen reduction. It is speculated that the formation of more interfaces between Ru and Al2O3 enhanced the activity of SPR catalyst. The following reasons explain the importance of the interfaces for activity. Firstly, Sen and Falconer have reported that two distinct reaction intermediates were present on Ru/Al2O3 catalysts during CO hydrogenation, i.e. one was carbon monoxide on Ru and the other was methoxy on Al2O3 [11]. The methoxy forms on Ru or at the interface between Ru and Al2O3, and then transfers to the Al2O3. If the methoxy is formed at the interface between Ru and Al2O3, the growth of interfaces in SPR catalyst can give rise to active sites. Secondly, it is well known that CO adsorbed on Ru metal is a primary reaction intermediate in CO2 methanation [1,12] and originates from the decomposition of formate acid and/or formate species, and the formation of formate acid and/or formate species is related indirectly to the interfaces between Ru and Al2O3 [13]. Moreover, chlorine ion residue also affects the activity of supported Ru catalysts [4,5]. The contents of chlorine ion residues on S-1 and I-1 catalysts were 0.13 and 0.34 mmol/g, respectively, by X-ray ¯uorescence spectroscopy analysis. Obviously, the higher spray temperature (1273 K) brought about little chlorine ion residue on SPR catalyst, while on IMP, the residual chlorine ion could not be scavenged because of lower calcination temperature (673 K), since the removing temperature of the residual chlorine ion was above 1000 K under hydrogen reduction [14].

Fig. 6. KNO3 effect in different preparation methods and sequences.

3.3. Alkali metal effects 3.3.1. Promoter effects of KNO3 The activities of ten kinds of Ru/Al2O3 catalysts in CO2 hydrogenation are compared in Fig. 6 and the other characteristics are summarized in Table 1. KNO3 promoted the activities of the Ru/Al2O3 catalysts irrespective of the preparation method and processes. The r0 of H-3 catalyst was the highest, since the promoter was directly distributed at the surroundings of surface Ru by the preparation process of H-3 catalyst. The fact that the addition of alkaline salts increased the activities of Ru/Al2O3 catalysts in CO2 hydrogenation can be explained by the following reasons. Firstly, alkali metals modify the local electron density of Ru metal by the electron donation, facilitating the disassociation of CO, which is regarded as the ratedeterminating step of CO2 hydrogenation [1]. Secondly, the promoter itself can take a role of a catalyst [15], e.g. K2CO3 catalytic process is shown as the following reactions: K2 CO3 …s† ‡ C…s† ! K2 O…s† ‡ 2CO…g† K2 O…s† ‡ H2 O…g† ! 2KOH…s† 2KOH…s† ‡ CO…g† ! K2 CO3 …s† ‡ H2 …g† Over all reaction : H2 O…g† ‡ C…s† ! H2 …g† ‡ CO…g† By the catalytic action of K2CO3, the deposited inactive carbon on the surface of catalysts, which is regarded as the primary factor of catalyst deactivation,

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will be abstracted and the activities of catalysts are increased relatively. Thirdly, the alkali metal promoter can neutralize residual chlorine ions on Ru metal surfaces by the formation of alkaline chloride which can migrate from Ru surface to the boundaries between Ru metal and Al2O3 support [4,14] and can alleviate the poisoning effect of chlorine ions. Impregnation is the most general method of preparation for supported Ru catalyst. Under the condition of dopant addition, the impregnation processes (cooperation or sequence) of Ru precursor or dopant with support may in¯uence the properties of catalyst, especially when the acidic Al2O3 support and basic dopant are used together [16]. For IMP catalysts, however, the impregnation sequence of KNO3 and RuCl3 had few effects on the activity, i.e. the r0s of I-2, I-3 and I-4 had no obvious changes. For HYB catalysts, on the other hand, the preparation sequences in¯uenced signi®cantly the performances of catalysts. Moreover, Table 1 also shows that alkali metal blocks hydrogen adsorption, this result is consistent with those of other researchers [4,17], and the addition of KNO3 increases the activation energies of CO2 hydrogenation. The preparation process of H-3 catalyst (hyb-Z(Ru/Al2O3)) with the highest activity was used to investigate the mechanism of alkali metal promotion in the following studies. 3.3.2. Additive amounts of alkali metal promoters The hyb-Z-(Ru/Al2O3) (ZˆK or Cs) catalysts were obtained by the H-3 preparation process. The dependence of the catalytic activities on the additive amounts (X) of promoters is shown in Fig. 7. For methanation, the maximum rates were observed at 5.0 mol% for KNO3 promoter and 3.0 mol% for CsNO3 promoter, while for CO product, the rates increased monotonically with the ascending of X values. To explain these experimental results, it is necessary to discuss the location of alkali metal on a catalyst. The following model has been proposed [18±20]: the alkali metal locates on the surface of Al2O3 (®rst at acid site) when the promoter content is very low, and with the increase of the promoter content, the Ru surface or boundaries between Ru and Al2O3 can be covered partly or completely by the alkali metal. We applied this model to explain our

Fig. 7. Effect of additive amounts of alkaline salts on r0 of hyb-Z(Ru/Al2O3) (ZˆK, or Cs, Ru: 5.0 mol%) catalysts prepared by H-3 process in CO2 hydrogenation at 523 K.

Fig. 8. Schematic pictures of catalyst surface over hyb-Cs-(Ru/ Al2O3) (Ru: 5.0 mol%).

results as shown in Fig. 8, where the change of the catalyst surface with the increase of promoter additive amount is illustrated. The Ru particles on the surface of catalyst are almost covered by alkali metal as X>5.0 mol% for KNO3 promoter and X>3.0 mol% for CsNO3 promoter, so that the r0 of CH4 yield decreased sharply. The above results are supported by calculation. The BET surface area of spr-Ru/Al2O3 (S-1) was ca. 27.5 m2/g, which could hold 0.82 mmol/ g of K ions (0.133 nm of ion radii), or 0.51 mmol/g of Cs ions (0.169 nm of ion radii), respectively, on the surface of the catalyst if alkali metal ions were closely

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Fig. 9. Effect of various kinds of alkali metals on r0 of hyb-Z-(Ru/ Al2O3) (Z : 3.0 mol%, Ru : 5.0 mol%) catalysts prepared by H-3 process in CO2 hydrogenation at 523 K.

Fig. 10. Effect of different alkaline salts on r0 of hyb-K-(Ru/ Al2O3) (K : 3.0 mol%, Ru : 5.0 mol%) catalysts prepared by H-3 process in CO2 hydrogenation at 523 K.

packed on the surface. An aliquot of 5.0 mol% of KNO3 promoter corresponds to 0.83 mmol/g, which is close to the value 0.82 mmol/g; 3.0 mol% of CsNO3 promoter corresponds to 0.49 mmol/g, close to the value 0.51 mmol/g, suggesting that alkali metal promoter covers the catalyst surface with an approximate monolayer state. The r0 of CO product increased with the increase of X values, which may be caused by the catalytic action of alkaline carbonate [15].

ities of the promoted catalysts should be determined by the two different factors of geometric and electronic effect of alkali metals. Moreover, the polarization of Li ion is the strongest among alkali metals [21]; this action may result in an enrichment in the density at the alkali metal adlayer±Ru interface [16]. This characteristic of Li ion may be related to the higher activity of Li-promoted catalysts.

3.3.3. Various kinds of alkali metals The activities of spr-Ru/Al2O3 catalysts doped with LiNO3, NaNO3, KNO3, RbNO3 or CsNO3 (hyb-Z(Ru/Al2O3)) are shown in Fig. 9. The activity of Cspromoted catalyst was the highest, and the activity of K-promoted catalyst was the lowest. From the viewpoint of electron donor, the activity order of the promoted catalysts should be LiNO3Rb‡>Cs‡. But according to the former model, the space effectiveness of the various kinds of alkali metals must be considered. The ion radii of Li , Na, K, Rb, Cs are 68, 97, 133, 147, 169 nm, respectively, the ion radius of Cs is two times larger than that of Li. The larger alkali metal ions may block the adsorption of reactants on surface of catalysts, especially to the larger reactant molecules, and therefore, affect the activity of catalysts. According to this reason, the activity order of the promoted catalysts should be LiNO3>NaNO3> KNO3>RbNO3>CsNO3. Fig. 9 shows that the activ-

3.3.4. Different kinds of alkaline salts The activities of spr-Ru/Al2O3 catalysts doped with KNO3, K2CO3, KOH or KCl (hyb-K-(Ru/Al2O3)) are shown in Fig. 10. The r0 of KCl-added catalyst was the lowest because of the residual chloride; the other alkaline salt-promoted catalysts had the same activities, indicating that the existing states of three kinds of promoters were the same during the catalytic hydrogenation of CO2. KOH promoter was converted to K2CO3 during catalyst preparation because of the reaction with CO2 in the air. On the contrary, KNO3 and K2CO3 promoters were reduced into KOH by the following reactions (1) and (2) during H2 pretreatment [14,22]; then, the produced KOH was changed into K2CO3 on contact with reactant CO2 by reaction (3). KNO3 ‡ 4H2 ! KOH ‡ NH3 " ‡2H2 O "

(1)

K2 CO3 ‡ 4H2 ! 2KOH ‡ CH4 " ‡H2 O "

(2)

2KOH ‡ CO2 ! K2 CO3 ‡ H2 O "

(3)

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In fact, the three kinds of promoters (KNO3, K2CO3, KOH) have the same existing state K2CO3 in the catalytic hydrogenation of CO2, so the catalysts promoted by them have the same activities. 4. Conclusions The catalytic activities of Ru/Al2O3 ®ne particles prepared by SPR were much higher than those obtained by conventional IMP under the same conditions, which indicates that the SPR is a plausible and excellent method to prepare high activity catalysts. The addition of alkaline salts promoted the activities of Ru/Al2O3 catalysts for CO2 hydrogenation. The optimal promotion effect was observed on hyb-Z(Ru/Al2O3) catalyst prepared by impregnation of sprRu/Al2O3 with alkaline salt solution. The optimal additive amount for alkaline salt varied with the kind of alkali metals and was obtained when alkali metal atoms cover the catalyst surface with an approximate monolayer state. The alkaline carbonate is suggested to be the working state for Ru/Al2O3 catalysts promoted by the different kinds of alkaline salts, except for chlorides, in hydrogenation of CO2. The promotion of alkaline salts to Ru/Al2O3 catalysts is probably due to a synergetic effect including the modi®cation of Ru metal local electron density by the electron donation of alkaline promoter, the neutralization of residual chlorine ions by the formation of alkaline chlorides and the removal of depositional inactive carbon, formed on catalyst surface during CO2 hydrogenation, by alkaline carbonate catalysis.

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