H2 reaction

Applied Catalysis A: General 413–414 (2012) 132–139

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The effect of potassium on Ni/Al2 O3 catalyst in relation to CO/H2 reaction a,b,∗ ´ Leszek Znak a , Jerzy Zielinski a b

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Warsaw University of Technology, Institute of Chemistry, Łukasiewicza 17, 09-400 Płock, Poland

a r t i c l e

i n f o

Article history: Received 31 July 2011 Received in revised form 27 October 2011 Accepted 31 October 2011 Available online 6 November 2011 Keywords: Nickel/alumina catalyst Nickel dispersion Effect of potassium CO hydrogenation

a b s t r a c t The effect of potassium on nickel/alumina catalysts in relation to CO/H2 reaction was studied. The examinations were performed on the samples containing 90 wt.% of nickel and up to 6.0 mol% of potassium. XRD studies and adsorption measurements (H2 , O2 and CO) showed a small effect of potassium on nickel dispersion; at the same time potassium enhanced heat of CO adsorption, which implies that the promoter locates on Ni surface. Temperature-programmed (TP) studies showed that potassium significantly retards the hydrogenation of pre-adsorbed CO and these results were confirmed by steady state tests of CO/H2 reaction. TP desorption of pre-adsorbed CO provided ambiguous view on the effect of potassium on CO dissociation. Instead, it was revealed that potassium strongly retards hydrogenation of carbon species adsorbed on nickel, which shed a new light on the effect of potassium on CO/H2 reaction. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Potassium is a well-known promoter of nickel catalysts in CO/H2 reaction. A small amount of potassium added to Ni/Al2 O3 catalyst increases production of methane, while a large amount of the promoter decreases production of methane and increases formation of high hydrocarbons. The effect of potassium on chemical/catalytic properties of supported nickel catalysts was the subject of a number of studies [1–9], however, in spite of the efforts, the role of potassium in this reaction appears not fully explained. Generally, potassium is introduced to supported nickel catalysts in the form of aqueous solution of KOH, K2 CO3 or KNO3 . Therefore, after standard calcinations and reduction, potassium appears in the catalysts in the form of K1+ ion. On the contrary, the basic studies of the effect of potassium on nickel are usually performed on well-defined Ni planes with metallic potassium evaporated on them so the deposited potassium appears in the form of Kı+ ions. The difference between the state of potassium in supported catalysts and model objects implies that the effect of potassium on chemical/catalytic properties of supported nickel may be different from the effect observed for the basic Ni planes. The common view on the nature of the effect of potassium on CO hydrogenation on supported nickel was developed on the basis of surface science studies [10–13]. The crucial data was

∗ Corresponding author at: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. Tel.: +48 22 3433333; fax: +48 22 3433353. ´ E-mail address: [email protected] (J. Zielinski). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.048

obtained from the studies of CO/H2 reaction on Ni(1 0 0) plane covered with submolecular quantity of evaporated potassium [13]. The examinations demonstrated that this entity exhibits the same steady-state activity and selectivity in CO/H2 reaction as supported nickel catalysts [3,4], which implied that it might be used as a model of these catalysts in advanced studies of the reaction. Using that possibility, the examinations performed on this model showed that: (i) added potassium enlarges steady-state concentration of carbon formed on Ni surface during CO/H2 reaction and (ii) evaporated potassium increases the rate of CO decomposition/disproportionation on Ni surface [13]. Consequently, accepting the carbide model of CO/H2 reaction on nickel [10–13], these results led to formulation of a simple mechanism of the reaction [13]. It was postulated that potassium enhances CO dissociation on nickel, which enlarges concentration of carbon atoms on Ni surface and thereby decreases formation of methane and increases production of higher hydrocarbons. This view was presented in review publications [14,15]. The work mentioned above [13] provided interesting observations, however, some doubts are raised that the data obtained for CO dissociation on nickel covered with K adatoms was used without any experimental evidence to explain the mechanism of CO/H2 reaction in which potassium appears in the oxidized form. These doubts set the foundations for present work. This paper presents basic physicochemical studies of the potassium promoted nickel/alumina catalysts in relation to CO hydrogenation. It is well known that even thorough examination of all the separate steps of a catalytic reaction cannot definitely determine the reaction mechanism, but these studies may help to find the most probable mechanism of the reaction.

L. Znak, J. Zieli´ nski / Applied Catalysis A: General 413–414 (2012) 132–139

2. Experimental 2.1. Apparatus Chemical characterization of the catalysts was carried out in a glass flow system [16], equipped with a gradientless microreactor [17]. A temperature controller maintained the reactor temperature within 1 K and provided linear temperature programming in the range of 100–1100 K. Hydrogen, helium and argon were of 99.999% purity, and carbon monoxide was of 99.99% purity. The gas stream required was fed to the measurement system through a four-way selection valve and, before entering the reactor, it was additionally purified from traces of oxygen and water with an MnO/SiO2 column. In the case of a He stream, the column was maintained at 78 K, which reduced the impurities below 0.1 ppm. The composition of the gas stream leaving the reactor was measured with TCD cell and the results were collected with a computer-controlled system. 2.2. Materials The examinations were carried out on potassium promoted high loaded nickel catalysts, K/90%Ni/Al2 O3 . The previous transient kinetic studies have shown that the activity of 90%Ni/Al2 O3 catalyst in CO/H2 reaction is identical to that of impregnated 20%Ni/Al2 O3 catalyst [18,19]. The use of high nickel samples is attractive in a number of experimental studies. Due to low content of support it increases precision of physicochemical measurements and reduces fraction of potassium adsorbed on alumina, which is advantageous when the potassium effect on catalytic properties of supported nickel is studied. The examined K/90%Ni/Al2 O3 catalysts were obtained by impregnation of the 90%NiO/Al2 O3 precursor [20,21] with chosen quantity of aqueous solutions of KNO3 . The suspension was dried under stirring and than calcined in air at 723 K for 4 h. Content of potassium in the obtained K/90%NiO/Al2 O3 samples, expressed as K/(K + Ni + Al) mole ratio was 0.3, 1.5 and 6.0%. 2.3. Measurement procedure 2.3.1. TP reduction Typically 50 mg of calcined material was used for chemical examination. The sample was in situ dried at 723 K for 0.5 h and then pre-reduced in an 80%H2 + Ar stream of 1 cm3 /s. The reduction was carried out at the temperature increasing linearly by 0.17 K/s from room temperature to 773 K, whereupon it was continued for additional 2.5 h. The stream leaving the reactor was dried in 195 K trap and the hydrogen consumption was measured. After the reduction, the sample was subject to additional treatment consisting of a passivation with O2 pulses (1.6 ␮mol O2 , 0.5 cm3 /s He, 273 K) and subsequent depassivation with H2 stream, which led to stable dispersion of nickel, needed for a series of physicochemical tests. Each of these tests was started with an extra reduction in a H2 stream (0.5 cm3 /s, 753 K, 0.5 h). In case of an adsorption study (H2 , O2 , CO) the sample was purged from adsorbed hydrogen in a He stream (0.5 cm3 /s, 673 K, 0.5 h) whereupon one of the intended test was performed. 2.3.2. H2 adsorption and its TP desorption The adsorption was performed at atmospheric pressure in three stages: 1 – initially at constant temperature of 423 K for 0.25 h, 2 – during gradual cooling from the initial value to 100 K within about 0.5 h, and 3 – at constant temperature of 100 K for 0.25 h. Subsequently, the sample was flushed with a He stream (0.5 cm3 /s, 100 K, 0.25 h) to remove weakly adsorbed hydrogen and the remained, more strongly adsorbed hydrogen, was examined by the temperature programmed desorption (TPD) method. The examination was

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carried out in an Ar stream (0.5 cm3 /s) at the temperature increasing linearly by 0.17 K/s from 100 K to 753 K. Then, the reactor with the examined sample was cooled down to ambient temperature and the base line was determined. The obtained temperature programmed desorption of pre-adsorbed hydrogen (TPD-Hads ) profiles were used to evaluate dispersion of supported nickel. In the calculations it was assumed that the quantity of hydrogen evolved over 298 K corresponds to the hydrogen adsorbed on nickel with stoichiometry H/Nis = 1 [22]. 2.3.3. O2 adsorption and its TP hydrogenation The adsorption was carried out by pulse method, introducing 1.65 ␮mol of O2 every 60 s into a He stream (0.5 cm3 /s) flowing over the sample maintained at 298 K. The total uptake of O2 was used to calculate dispersion of nickel, assuming the stoichiometry: O/Nis = 1.7 [23]. The TP hydrogenation of pre-adsorbed oxygen (TPH-Oads ) was carried out in an 80%H2 + Ar stream of 0.5 cm3 /s at temperature increasing linearly by 0.17 K/s from 250 K to 750 K. The stream leaving the reactor was dried in a 195 K trap and H2 consumption was measured. 2.3.4. CO adsorption and its TP hydrogenation or desorption The adsorption was carried out by pulse method, introducing 4.5 ␮mol CO portions every 180 s into a He stream (0.5 cm3 /s) flowing over the sample maintained at 298 K. The stream leaving the reactor was passed through a 370 K trap to decompose any Ni(CO)4 which may appear during the adsorption. The total uptake of CO was used to calculate nickel dispersion, assuming that CO/Nis ratio equals 0.5 [24]. The TP desorption of preadsorbed CO (TPD-COads ) was carried in a He stream of 0.5 cm3 /s at temperature ramp of 0.17 K/s from 250 to 700 K. The stream leaving the reactor contained both CO and CO2 . A micro-trap placed between the reactor and the detector was alternatively kept at 78 and 298 K, which allowed for the fairly continuous measurements of both CO and CO2 evolution. The TP hydrogenation of preadsorbed CO (TPH-COads ) was carried out in a H2 stream of 0.5 cm3 /s at temperature ramp of 0.17 K/s from 250 to 700 K. The stream leaving the reactor was dried in a 195 K trap and concentration of the evolved methane was measured. 2.3.5. Carbon deposition and its TP hydrogenation The deposition of carbon was performed using a number of CO pulses (4.5 ␮mol every 180 s) let into a He stream flowing over the sample maintained at 483 K. After that, in order to attain a complete CO decomposition, the sample was heated in He stream at 753 K for 3 h. The obtained carbon deposit was characterized using a TP hydrogenation method. The examinations were carried out in H2 stream of 0.5 cm3 /s at linear temperature rise of 0.17 K/s from 240 to 750 K. The stream leaving the reactor was passed through 195 K trap and concentration of evolved CH4 was measured. After that water condensed in the trap was evaporated and its quantity was measured. 2.3.6. Thermal effect of CO adsorption The measurements were carried out in the flow system, using a specially designed calorimetric cell [25]. The experiments were performed at 298 K for larger samples of 100 mg. The CO pulses of 4.5 ␮mol were introduced every 360 s into a He stream (0.5 cm3 /s) flowing through the cell with examined sample, and both the thermal response and CO uptake were measured. The measurements were calibrated assuming that heat of O2 adsorption on unpromoted Ni/Al2 O3 catalyst equals 418 kJ/mol [26].

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Fig. 1. TP reduction of K/90%NiO/Al2 O3 precursors.

2.3.7. Transmission electron microscopy The high-resolution electron microscopy examinations were performed in a Philips CM 20 Ultra Twin electron microscope operated at 200 kV. The examined sample was calcined in air at 673 K for 2 h, reduced in H2 at 773 K for 2 h and then passivated in Ar at 473 K. 2.3.8. XRD studies The measurements were carried out using powder diffractometer D5005 manufactured by Siemens (Bruker AXS), equipped with a position sensitive detector CPS120 manufactured by INEL and the home-made XRD camera(gradientless microreactor [27]. The examined samples were ex situ pre-reduced in a hydrogen stream (1 cm3 /s, 773 K, 2.5 h) whereupon they were passivated with a small O2 portions let into to the helium stream (1.65 ␮mol O2 , 0.5 cm3 /s He, 273 K). The passivated sample of about 20 mg was smeared over a thin, porous quartz glass plate serving as a sample holder. Then the sample was placed into the camera and re-reduced in a hydrogen stream (1 cm3 /s) at temperature increasing linearly by 0.085 K/s up to 753 K. After that, the sample temperature was lowered to 423 K and the additional XRD measurement assigned for determination of the size of Ni crystallites was performed. The size was calculated from Ni(1 1 1) line using Scherer’s equation with a correction for instrumental broadening. The obtained results were used to evaluate dispersion (Fraction Exposed) of nickel using the ´ equation proposed by Borodzinski and Bonarowska [28]. 2.3.9. Activity Ni/Al2 O3 catalysts in CO hydrogenation The activity was measured in a flow system, using a gradientless microreactor [17]. The 5 mg samples of calcined catalysts were mixed with quartz glass powder (200 mg) and then in situ reduced in a H2 stream (1.67 cm3 /s, 773 K, 1 h). After that the reactor temperature was lowered to a chosen value and H2 stream was replaced with 0.5%CO + H2 mixture of 1.5 cm3 /s. The stream leaving the reactor was analysed (CO, CO2 and CH4 ) with an on line gas chromatograph equipped with a methanator and FID detector. 3. Results and discussion 3.1. Preliminary characterization of promoted catalysts 3.1.1. Temperature-programmed reduction The previous studies have shown that the unpromoted 90%NiO/Al2 O3 precursor consists mainly of dispersed nickel oxide decorated with nonstoichiometric NiO·xAl2 O3 species [16,20]. Fig. 1 presents temperature programmed reduction (TPR) profiles of the precursor promoted with various quantity of potassium. The

Fig. 2. Evolution of XRD spectra in the course of TP reduction of passivated 6%K/90%NiO/Al2 O3 sample.

examinations show that potassium slightly lowers the reduction temperature and the effect increases with potassium content. These results are consistent with the reported TPR studies of pre-dried samples (inert gas, 623–673 K, 2–4 h) [29,30], and inconsistent with the TPR studies of non-dried samples [9,31]. The relations suggest that the higher ability of potassium promoted catalysts to sorb water is responsible for the effects [32–34]. In the case of the promoted catalysts, pre-dried samples sorb effectively water produced in the course of TPR test and the reduction temperature is lower [29,30], while non-dried samples release water and the reduction temperature is higher [9,31]. Integration of the profiles in Fig. 1 has shown that potassium has a small effect on degree of NiO reduction and it equals about 0.95. Fig. 2 characterizes TP re-reduction of passivated 6%K/90%NiO/Al2 O3 catalyst as seen by X-ray diffraction. The examinations show large reflexes of Ni phase and small reflexes of NiO phase. In the course of the reduction the former slightly enlarge and the latter diminish to a minimal size. 3.1.2. Nickel dispersion Table 1 depicts the effect of potassium on Ni dispersion in reduced samples, as found from XRD and from H2 , O2 and CO adsorption. The comparison shows that: - The dispersion derived from the XRD measurements is about twice as large as the values obtained from the H2 , O2 and CO adsorption. The discrepancy very likely results from decoration of Ni crystallites with xNiO·Al2 O3 species, implying that about half of Ni surface is blocked for H2 , O2 and CO adsorption. Highresolution electron microscopy of the 90%Ni/Al2 O3 catalyst shows nickel crystallites covered with a layer or aggregates of crystalline nickel aluminate (see Fig. 3).

Table 1 Dispersion of nickel in K/90%Ni/Al2 O3 catalysts. mol.% K

0 0.3 1.5 6.0

Fraction exposed of nickel XRD

TPD-Hads

ads. O2

ads. CO

0.217 – – 0.210

0.090 0.093 0.091 0.104

0.150 0.157 0.132 0.118

0.095 0.102 0.090 0.098

TPD-Hads : determined from TP desorption of pre-adsorbed hydrogen.

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Fig. 3. High resolution transmission electron microscopy image of the 90%Ni/Al2 O3 catalyst.

- Generally, the chemisorption studies indicate that potassium has a small effect on nickel dispersion. At the same time it is worth to mention that potassium only slightly affects the dispersion derived from H2 and CO adsorption, but considerably decreases the dispersion found from the adsorption of O2 that adsorbs most strongly on nickel. It is supposed that potassium diminishes the rate of O2 adsorption which results in lower thickness of NiO layer formed on nickel. 3.2. H2 adsorption Adsorption of H2 on unsupported and alumina supported nickel had already been thoroughly studied [35,36]. The TPD-Hads measurements revealed three forms of hydrogen absorbed at 100 K on Ni powder: ␥-form, located in the subsurface layer and desorbed at 186 K, ␤-form adsorbed in the so called second layer and evolved at about 327 K, and ␣-form, adsorbed directly on the nickel surface and desorbed in 350–670 K range. Fig. 4 characterizes TP desorption of hydrogen pre-adsorbed on the series of potassium promoted catalysts. The profiles show that potassium has only a small effect on the formation of ␣ profile representing the hydrogen strongly adsorbed on nickel, and this fact appears important in relation to these studies. At the same time, the measurements show that potassium cancels ␤ form and strongly

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Fig. 5. TP hydrogenation of oxygen pre-adsorbed on K/90%Ni/Al2 O3 catalysts.

suppresses the formation of ␥ form, which implies that potassium species locate directly on nickel surface. 3.3. O2 adsorption and its hydrogenation Extensive studies of O2 adsorption on basic nickel crystallographic planes have shown that at room temperature the process proceeds in three stages: 1 – dissociative adsorption on top of the nickel surface with formation of various surface structures at O/Nis ratio of about 0.3–0.4, 2 – point nucleation and lateral formation of surface nickel oxide, and 3 – slow thickening of surface nickel oxide layer [37,38]. The high rate and heat of the interaction may considerably increase temperature of examined sample, which enlarges the thickness of the nickel oxide layer. Fig. 5 characterizes TP hydrogenation of oxygen pre-adsorbed on K/90%Ni/Al2 O3 catalysts. The sharp peak at about 365 K represents direct reduction of the surface nickel oxide and the broad profile at higher temperature reflects the reduction of a surface nickel aluminate, which forms in the course of these tests [39]. The examinations in Fig. 5 show that potassium has only negligible effect on the hydrogenation of pre-adsorbed oxygen. These results imply that hydrogenation of oxygen appearing in the course of CO hydrogenation on Ni surface is not responsible for low activity of the catalysts in CO/H2 reaction. 3.4. Carbon monoxide adsorption and its hydrogenation

Fig. 4. TP desorption of hydrogen pre-adsorbed on K/90%Ni/Al2 O3 catalysts.

The examinations in Table 1 show that potassium has insignificant effect on CO adsorption on nickel. However, this view was not confirmed by the measurements of thermal effect of CO adsorption (Fig. 6). The examinations revealed that potassium considerably increased the heat of CO adsorption. Similar results were obtained by Spiewak and Dumesic [40,41] for potassium promoted nickel powder. This effect suggests that potassium: (i) locates directly on nickel surface and (ii) modifies CO adsorption on nickel. It is supposed that nickel surface is covered by –OK groups, and CO adsorption occurs in close vicinity of these groups and subsequently on the potassium free Ni surface. Fig. 7 presents TP hydrogenation of CO pre-adsorbed on K/90%Ni/Al2 O3 catalysts. The profiles consist of a small peak of CO desorption at 275–375 K and a well-formed peak of CH4 production at higher temperature. The position of CH4 peak gradually shifts towards high temperature with growing potassium content, which shows that potassium considerably retards carbon monoxide hydrogenation. Supplementary TPH-COads tests

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Fig. 6. Thermal effect of CO adsorption on K/90%Ni/Al2 O3 catalysts.

performed for K/90%Ni/SiO2 catalysts provided similar results as for K/90%Ni/Al2 O3 catalysts, which indicates that the effect results from the potassium–nickel interaction. Fig. 8 shows advanced TP hydrogenation of CO preadsorbed on K/90%Ni/Al2 O3 catalysts. In these tests, apart from CH4 evolution, H2 O evolution and H2 consumption were also followed. The examinations demonstrate that for both unpromoted and promoted samples the three processes proceed simultaneously; the delay of H2 O evolution is very likely due to H2 O retention on the examined sample. These results show that hydrogenation of preadsorbed CO molecules is nearly one step reaction. However, this view turns out incorrect in case of steady state CO hydrogenations. The transient kinetic studies of CO/H2 reaction on both Ni powder and Ni/Al2 O3 catalyst have demonstrated that nickel surface is covered with substantial amount of carbon species [18,19]. It was demonstrated that the species are slowly hydrogenated to methane as CO appears in gas phase and they are readily hydrogenated as CO is removed from the gas phase. 3.5. Steady state hydrogenation of CO The inhibitory effect of potassium on CO hydrogenation was confirmed by the steady state studies (see Fig. 9). The measurements show that potassium considerably decreases the reaction

Fig. 8. Advanced TP hydrogenation of CO pre-adsorbed on K/90%Ni/Al2 O3 catalysts.

rate and magnitude of the effect is of the same order as in TPH-COads tests. At the same time, it should be noted that the conversion profiles in Fig. 9 are shifted by about 60 K towards higher temperature compared to the TPH-COads profiles in Fig. 8, which arises probably due to the effect of CO present in the gas phase. The steady state CO/H2 reaction is much more intricate process than the hydrogenation of preadsorbed CO. The results in Fig. 9B show that in the course CO/H2 reaction CO2 is produced and its quantity is high in the case of 6%K/90%Ni/Al2 O3 sample. Thermodynamic analysis of the results in Fig. 9 demonstrated that in the case of the promoted sample CO2 concentration is close to the equilibrium value of CO + H2 O = CO2 + H2 reaction. Interesting data on CO2 production in the course of CO/H2 reaction were obtained from in situ XRD studies [42]. The examinations showed that, in contrast to unpromoted sample, in the case of 6%K/90%Ni/Al2 O3 catalyst two processes simultaneously proceed: formation of supersaturated solution of carbon in Ni phase and CO2 evolution. The effects were ascribed to the following reaction: K

Ni · CO + CO−→NiCx + CO2 .

3.6. TP desorption of CO preadsorbed

Fig. 7. TP hydrogenation of CO pre-adsorbed on K/90%Ni/Al2 O3 catalysts.

The TPD-COads method was often used to characterize the interaction of CO with nickel [7,43,44]. The examinations performed in UHV systems for various nickel planes gave a single peak of CO desorption centred at about 450 K [43]. On the other hand, the examinations of supported catalysts, performed at atmospheric pressure in the flow system, always showed the presence of not only CO but also CO2 . The reason for the difference has not been

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Fig. 9. Steady state hydrogenation of CO on K/90%Ni/Al2 O3 catalysts. Reaction conditions: catalyst: 5 mg; pre-prepared reaction mixture: 0.5% CO + H2 ; flow rate: 1.5 cm3 /s.

explained. There are suggestions that surface defects in supported nickel are responsible for the difference. Fig. 10 illustrates TP desorption of CO pre-adsorbed on K/90%Ni/Al2 O3 catalysts. The obtained CO and CO2 profiles are formed as a result of the following processes: 1 – desorption of preadsorbed CO, 2 – dissociation of preadsorbed CO, 3 – associative desorption of CO, and 4 – associative desorption of CO2 [44]. According to the study the low temperature CO profile represents direct desorption of preadsorbed CO and the high temperature CO profile corresponds to associative desorption of CO. Carbon monoxide preadsorbed on Ni/Al2 O3 begins to dissociate at about 400 K [45]. The appearing oxygen reacts readily with adsorbed CO, producing CO2 and creating additional Ni sites, favourable for further CO dissociation. Thus, a large quantity of CO2 and just a minimal quantity of CO evolve in 420–600 K range. The examinations in Fig. 10B show that CO2 evolution is considerably lower for promoted samples. This may suggest that potassium hinders CO2 production and/or potassium adsorbs strongly produced CO2 . At the final stage of the TPD-COads test, over 600 K, CO evolution gradually increases and CO2 evolution decreases. These changes suggest that the proportion of CO and CO2 in the gas phase corresponds to the equilibrium state of 2CO = CO2 + Cgraph reaction. The suggestion was confirmed by the comparison of the reaction quotient, Q = pCO2 /(p2CO · 1), with equilibrium constant of the reaction, Keq (see Fig. 11).

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Fig. 10. TP desorption of CO pre-adsorbed on K/90%Ni/Al2 O3 catalysts.

The effect of potassium on CO dissociation appears to be the crucial question in the study of CO/H2 reaction on nickel catalysts. The TPD-COads tests in Fig. 10 give only indirect suggestions on this subject. The above discussion implies that (i) CO dissociation probably begins as the low temperature CO profile declines and (ii) the dissociation obviously proceeds as CO2 appears in the gas phase. Accordingly, an inspection of the results in Fig. 10 gives no clear view on the effect, and additional experimental studies are

Fig. 11. Thermodynamic analysis of TPD-COads test in Fig. 10.

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Table 2 Quantitative characterization of TP desorption of CO preadsorbed on K/90%Ni/Al2 O3 catalysts. Weight of examined samples: 50 mg. 1 Catalyst, K mol.%

2 K, ␮mol

3 CO adsorb, ␮mol

4 CO desorb, ␮mol

5 CO2 desorb, ␮mol

6 Deposited C, ␮mol

7 Deposited O, ␮mol

0 0.3 1.5 6.0

0 2.1 10.6 42.4

27.3 30.0 25.8 26.5

9.9 11.4 9.8 9.4

9.5 9.9 6.8 4.9

8.0 8.7 9.2 12.2

−1.5 −1.2 2.3 7.3

3 – CO preadsorbed; 4 – total CO desorbed in the course of TPD test; 5 – total CO2 desorbed in the course of TPD test; 6 – C remaining on the sample after TPD test; 7 – O remaining on the sample after TPD test.

necessary to solve that problem. Preliminary examinations of that question suggest that potassium hardly affects CO dissociation on Ni/Al2 O3 catalyst. Table 2 provides numerical characteristics of the TPD-COads tests in Fig. 10. The data in columns 6 and 7 demonstrate that potassium increases quantity of both carbon and oxygen remaining on these samples; the negative values of oxygen deposit suggest additional reduction of the sample. Besides, a comparison of the data in columns 6 and 7 suggests that elemental carbon remains on the unpromoted catalyst, but some unknown carbon species remain on potassium promoted samples.

3.7. TP hydrogenation of C deposit Fig. 12 shows TP hydrogenation of carbon deposited on unpromoted and potassium promoted samples. The examinations show that the deposit formed on unpromoted sample (curve a) is hydrogenated at low temperature, similarly as carbon diffused on nickel surface of the 6.23%Ni/C catalyst (curve b). Instead, the deposit formed on potassium promoted sample is hydrogenated at high temperature (curve c) and the final stage of this profile is nearly identical to the profile recorded for TP hydrogenation of CO preadsorbed on 6%K90%Ni/Al2 O3 (curve d taken from Fig. 7). The similarity suggests that, difficult to reduce species, containing potassium and carbon elements, lower catalytic activity of nickel in CO hydrogenation. The similar conclusion was recently reported for CO hydrogenation on potassium promoted iron catalyst [46]. It was proposed that strengthening of Fe–C bond during molecular as well as during dissociative CO adsorption is responsible for lower activity of iron in CO hydrogenation.

4. Conclusions The effect of potassium on Ni/Al2 O3 catalysts in relation to CO/H2 reaction was studied. The XRD and chemisorption (H2 , CO, O2 ) studies have shown a small effect of potassium on Ni dispersion. At the same time potassium increases considerably heat of CO adsorption. TPH-COads tests showed that potassium strongly retards the hydrogenation of preadsorbed CO. The similar effect was observed in the case of steady state studies of CO/H2 reaction. Two side processes take place in the course of CO/H2 reaction on potassium promoted nickel: carbon dissolves in Ni phase and CO2 appears in the gas phase. The TPD-COads tests did not explicitly show the promoting effect of potassium on CO dissociation/disproportionation on nickel. Instead, it was revealed that potassium strongly retards the reduction of carbon deposits formed on Ni/Al2 O3 catalyst. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

Fig. 12. TP hydrogenation of carbon deposited on nickel catalysts: a – 90%Ni/Al2 O3 ; b – 6.23%Ni/C; c – 6%K90%Ni/Al2 O3 ; d – TPH-COads on 6%K90%Ni/Al2 O3 . The scale of lines b and d was adjusted to the profiles a and c, respectively. The inserted numbers are quantities of CH4 and H2 O ␮mol evolved in these tests.

[32] [33] [34] [35] [36]

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