Surface Area and Porosity Measurements of Steam Reforming and Methanation Catalysts

1. King Saud Univ ., Vo!. 7, Eng. Sci. (Special Issue) , pp. 257-270 (A.H. 141511995) J. King Saud Univ., Vol. 7, Eng. Sei. (2), pp. 175-184 (A.H. 1415/1995)

TECHNICAL NOTES CIVIL ENGINEERING Surface Area and Porosity Measurements of Steam Reforming and Methanation Catalysts Accuracy of Stadia Tacheometry with Optical Theodolites and Levels Yaw D. Yeboah, AIi G. Ma'adhah and Syed A. AIi Petroleum & Gas Technology Division, The Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Abdalla Elsadig Ali

Civil Engineering Department, College for of Engineering, King Saud University, (Received 29/9/1992; Accepted publication 17/3/1993) P.D. Box 800, Riyadh 11421, Saudi Arabia 29/9/1993~ for publication 2R/2/1994) Abstract. Supported nickel(Received catalysts are used forAccepted steam reforming of natural gas and methanation of carbon oxides in hydrogen-rich streams in the production of ammonia, methanol and hydrogen . These catalysts are loaded into reactors in the oxide form and are then activated into metallic nickel state by Abstract. Five optical theodolites, Wild T16, T1, T2, Kern DKM-1, of DKM-2 and one automatic in-:itu reduction . The activity of the catalyst depends on the properties the reduced catalysts . Of level, par- a Wildimportance NA2, wereistested for horizontal distance height accuracy Two cases were consithe nickel (active) surface areaand of the catalyst , sincemeasurement. the catalytic reactions takes place ticular In the first, on dered. the available nickelnominal surface.values of scale factor and additive constants (K and C) were used (typically tOO and 0). In the second, these two parameters were determined in a least squares solution and applied. In the firstpaper case,reports the horizontal accuracy obtained from ± 28surface mm with the, pore NA2 volume, to ± 38 mm the experimental results onranged the BET, nickel areas andwith porethe This mmmethanation with the NA2 to ± 34used mm in with the Wild T16. In the The equivalent figures in height are ± 20and sizeT16. distribution of commercial steam reforming catalysts Saudi Arabia. Variin horizontal distance accuracy obtained with Tt and the, duration DKM-2 (± second case, the catalyst best results ables studied in the reduction were temperature , heatingwas rate , reducing gasthe composition mm),velocity. followedExperiments by the T2 (±were 25 mm), theout NA2 26 mm),Pulse the DKM-1 (± 30 mm) and finallyis the and24space carried in a(± modified Chemisorb 2700 unit , which cap-T16 mm).reduction The height also improved, thewas biggest improvement being obtainedwhile by the Wild T16 in-situ andaccuracy chemisorption . BET area obtained by nitrogen adsorption porosity able(±of36 from ± 34bymm to ± 30 mm (i.e. 12% improvement). cases,are however, the obtained wasi.e. determined mercury penetration method . The results In of both this study discussed in results the light of phys-are better than what istaking generally to be attainable withofstadia and chemical changes placebelieved during the in-situ reduction steamtacheometry. reforming and methanation icalmuch catalysts.

Introduction Introduction the procedure by which horizontalincluding distancessteam and difConventionally, tacheometry Supported nickel catalysts have is a variety of industrial applications ferences and of elevation are determined using processes the opticalare properties of the on telescope methanation reactions. These being applied a large of reforming, for the manufacture of ammonia, and hydrogen. The scale Saudi Arabia or tacheometer). The method theinmeasuring instrument (transit, theodolite, levelmethanol massive expansion plan of petrochemical industries and growing of hydrogen has long been acknowledged as a simple and inexpensive tool needs for mapping areas of at refineries clearly predicts substantial increase the use of(and steam reforming and limited extent. Depending on the distance beinginmeasured some other factors), methanation processes in the Kingdom the horizontal accuracy obtained with[1;2]. tacheometric surveys ranges from 1/1,000 with stadia and self-reduction instruments to perhaps 1/10,000 with subtense bar 257

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Both the steam reforming and methan~tion catalysts are nickel catalysts but are supported on different types of material. For steam reforming catalysts the support is a low surface area alpha-alumina while gamma-alumina is used for methanation catalyst [3]. They are prepared by either impregnation or co-precipitation of nickel salts on a support. The salts are converted to nickel oxide by calcination in air at 400600°C. This followed by reduction of nickel oxide to metallic nickel [4]. Thus , reduction is the last step in the preparation of supported nickel catalysts and is generally carried out in-situ in industrial reformers and methanators. In-situ reduction is necessary due to the pyrophoric nature of metallic nickel. It is very troublesome for the manufacturer to prepare catalysts where the oxidation state of the metallic element is lower than those existing in air atmosphere [5]. The activity and selectivity of supported nickel catalysts are strongly dependent on the chemical and physical properties of the catalyst. These properties include the chemical composition (bulk and surface) , thermal behavior, pore size distribution , BET and active surface area , dispersion and crystallite size of nickel. Although , it is expected that the properties and characteristics of the reduced catalyst will be different from the oxidized fresh catalyst, often little is known about the reduced catalyst properties. This is mainly due to the fact that the reduction is generally carried out in-situ followed, almost immediately, by the catalytic reaction. The morphology of the catalyst and its associated catalytic properties depend on the manner in which the reduction process is carred out [3 ;6] . Obviously , the goal is to maximize the surface area of nickel and hence the activity of the catalyst. In order to achieve that goal , it is important to have an in-depth knowledge regarding the various factors affecting the reduction process. This paper describes the techniques used for measurement of the BET and nickel surface area as well as for pore volume and pore size distribution determination of supported nickel catalysts. The experimental results from commercial steam reforming and methanation catalysts used in Saudi Arabia are reported. The results are discussed in light of the physical and chemical changes taking place during the reduction of these catalysts. Experimental Techniques Total surface area: B.E. T. method Since the catalytic reactions take place on the surface of the solid catalysts , the activity of catalyst depends, to a large extent , on the availability of the surface. This fact makes it imperative to kow the surface area of catalysts. The principal method

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of measuring the surface area of catalysts is by the adsorption of an appropriate gas (usually nitrogen) on the solid surface. For this purpose, the amount of nitrogen adsorbed at equilibrium at its normal boiling point (-195.8°C) is measured over a range of nitrogen pressures below 1 atm [7 ;8]. The most common method of calculating surface area is that developed by Brunauer, Emmett and Teller [9;10]. It is commonly called B.E.T. method after its inventors and the area obtained by this method is called BET area. In this method, the Langmuir adsorption isotherm is extended to apply to multilayer adsorption, arriving at the following equation: (1)

where va is the volume adsorbed at pressure P , V m is the volume of monolayer of adsorbed gas per gram of catalyst, Po is the saturation or vapor pressure and c is constant for the particular temperature and gas-solid system. A pot of P/[ va(Po-P)] versus P/P o should give a straight line, which can be extrapolated to PlP o = O. From the intercept I and slope S of this plot, the volume of adsorbed gas corresponding to a monolayer vm can be obtained using the relation: vrn

= 1 I (I + S)

(2)

The volume vrn can be readily converted to surface area. For nitrogen, the surface area per gram of catalyst is 4.35 x 104 vrn . Many adsorption data show very good agreement with the B.E.T. equation over values of relative pressure P/Po between approximately 0.05 and 0.3 and this range is usually used for surface area measurement [11;12]. The BET areas reported in this paper were determined using Carlo Erba Sorptomatic 1800 unit. Active surface area: Chemisorption methods It is a known fact that in supported catalysts, only the part of the BET area covered by catalytically active metal atoms is capable of catalysis. The determination of specific metal area, in distinction to the BET area can be accomplished by measuring the amount of gas (such as hydrogen) that is chemisorbed onto the metal surface. Chemisorption is a surface phenomenon. It is specific, and involves valence forces of the same type as those occurring between atoms in a molecule. In contrast , physical adsorption is non-specific and is caused by inteimolecular Van der Waals forces , which are much weaker than the forces created by chemisorption. An important feature of chemisorption is that the surface coverage will not exceed that corresponding to one monolayer of adsorption. This limitation is due to the fact that valence forces

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holding the molecules on the surface diminish rapidly with distance from the surface [7;13]. Although , the chemisorption technique is simple in concept, it can be complex in application. The optimum measurement conditions vary considerably with the nature of the metal and the adsorbent. In spite of these difficulties, chemisorption is the most frequently employed method for determination of metal surface area of supported metal catalysts [14). The measurement of nickel surface area is most commonly carried out through the chemisorption of hydrogen. The chemisorption of hydrogen on nickel has been extensively studied and reported in detail in the literature [15-17]. It was found that the chemisorption of hydrogen on nickel is well defined at room temperature (about 25°C) and over a pressure range of 100-400 mm Hg. Hydrogen is dissociatively chemisorbed on alumina supported nickel at 25°C with a stoichiometry of one hydrogen atom per nickel surface atom. This stoichiometry is valid over a wide range of dispersion and nickel content [18;19]. Two different techniques are in common use for measuring hydrogen adsorption uptakes: static vaccum technique and dynamic pulse method [15;20]. The former method involves the use of high vacuum facility , while the latter utilizes a flow apparatus. Static vacuum method This method is most commonly used for measuring hydrogen adsorption uptakes on supported nickel catalysts. The apparatus is usually made of glass or metal and equipped with evacuation devices (such as diffusion pump). The general principle involves measuring the amount of gas remaining in the manifold system after contact with the previously evacuated sample. By knowing the amount of gas initially present and subtracting from it the amount remaining after equilibrium with the sample, the extent of adsorption can be determined. Most often a pressure measuring device is used to follow pressure changes caused by gas-sample interactions. Pressure changes in a constant volume system would then be proportional, through the gas laws, to the amount of gas adsorbed [15]. Prior to the chemisorption , the nickel catalysts are reducedipretreated at about 350-400°C. Hence, a furnace is an essential part of the apparatus. The temperature control system of the furnace should be capable to enable variation in heating rate in addition to final temperature. Such a flexibility will allow the reduction process to be carried out in the same sample cell in which chemisorption is performed. It should be

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noted that better results are obtained when reduction and chemisorption are performed in-situ i.e. in the same sample cell, without exposing the sample to air even at ambient condition [13;14;21]. After pretreatment, cooling, and evacuation , the adsorbate (hydrogen) is introduced to the closed (evacuated) system and its pressure is noted. Subsequently , it is expanded and chemisorption commences. The pressure is monitored until no further variation of pressure with time is noted. This may require a time of 30-45 minutes , since slow activated chemisorption continues after the ini~ial non-activated chemisorption. Once the equilibrium is reached, the pressure over the sample can then be increased via a gas burette and readings are again taken. When there is no hydrogen uptake by the sample with increasing pressure , the desirable portion of the isotherm is complete , and the total volume adsorbed can be determined [22]. For the calculation of nickel surface area, it may be assumed that the site density of metallic nickel is 6.77 x 10-2 nm 2/atom, which is based on an equal distribution of the three lowest index planes . It may further be assumed that the metal composition on the surface is identical to the bulk metal composition of the catalyst. A stoichiometry of two nickel atoms per mole of hydrogen is generally assumed for chemisorption. The validity of these assumptions have been reported in the literature [17;18;22]. The specific surface area of the nickel catalyst (SA) in m 2/g catalyst may be represented as: SA = 0.08151 X H

(3)

where X H is the uptake of hydrogen in moles/g catalyst. The chemisorption by static vacuum method was carried out by Carlo Erba Sorptomatic 1800 unit. By using the static vacuum method, the interpretation of chemisorption on practical catalysts can be undertaken with confidence. However, due to their high cost and time consuming procedure, the method is not very convenient for routine use. Flow methods offer a simpler means for measuring chemisorbed volumes [23]. One of tl)e modified techniques of such methods is the dynamic pulse method. Dynamic pulse method

The dynamic pulse method has become a frequently used technique in recent years because of its experimental simplicity and low equipment cost compared to static vacuum method. The method adopts a significantly different approach [20;24;25]. In the dynamic pulse method, defined volumes of hydrogen are injected into the stream of an inert gas (usually helium) which is passed through the sample . Pulses are

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designed to be appreciably less than the anticipated uptake and are fully adsorbed until the monolayer is complete. The analysis of the eluting gas is performed and monitored by a thermal conductivity cell. The addition of pulses is continued until the peaks seen by the detector are identical. When this occurs, the active surface is saturated with the hydrogen. The volume of chemisorbed gas is then calculated from the difference between the gas volume injected as pulses and that measured in the effluent [26]. The major advantage of the method is rapid determination. However, diffusionallimitations are slow but significant chemisorption may not be readily detected. Hence, the major disadvantage of this technique is that weakly chemisorbed gas may not be held by the catalyst and conceivably low results are obtained. These drawbacks , however , do not grossly alter the results of nickel surface area measurements using hydrogen chemisorption by dynamic pulse technique, because nickel adsorbs hydrogen rapidly and with an appreciable binding energy [24-26]. It ~s necessary to emphasize that chemisorption defined by dynamic pulse technique is different from that defined by conventional high vacuum experiments. The simpler procedures described above measure only the chemisorption which occurs rapidly and irreversibly . The static method, in contrast, allows time for equilibrium and includes both reversible and irreversible adsorption. As a consequence , knowledge derived from conventional measurements, in particular the stoichiometry with which gas and metal interact, cannot automatically be transferred from one method to the other [9;26;27].

The chemisorption studies with dynamic pulse method were carried out in Micromeritics Pulse Chemisorbe 2700 unit. The standard unit was modified by incorporating a programmable temperature controller for the furnace to make it capable of carrying out reduction of catalyst samples at preprogrammed heating rates as well as final temperature. This is a multifunctional unit which allows in-situ sample preparation and analysis. The preparation steps may be calcination , reduction and/or passivation while the analysis can be done by chemisorption or physical adsorption. A sequence of sample preparation and chemisorption can be done in-situ, that is, without removing the sample tube from the instrument. This feature eliminates the possibility of contaminating the sample by atmospheric gases during or in-between the above procedures. Pore volume and pore-size distribution: Mercury penetration method The ease of access of reactants to the interior of a catalyst pellet by diffusion is dependent on the pore structure ofthe catalyst. Hence , it is evident that the pore vol-

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ume and pore-size distribution are of great importance. The exact pore structure is very complex , as pores may be of various sizes, shapes , and lengths and are normally interconnected. For quantitative interpretation , pores are generally assumed to be an array of randomly oriented cylindrical capillaries of different radii. A knowledge of the pore structure of fresh catalysts provides an insight into the role of factors affecting the catalytic reactions. Determination of pore size distribution of reduced supported nickel catalysts is particularly important because the active sites are formed during reduction process. If the active sites are largely in very narrow pores or micropores « 2 nm), then the accessibility of the reactive gas to the active metal phase may not be possible. Hence they are not suitable for catalysis. However, if the active sites are along wide channels or pores, catalytic activity may be high. The most common method of measuring the pore size distribution is by high pressure mercury penetration [28;29] . This method is based on the fact that mercury has significant surface tension and does not wet most surfaces. This means that ·the pressure required to force mercury into the pores of a porous material ; such as a catalyst, depends on the pore radii . At equilibrium , the external force and the force opposing entrance caused by the surface tension of mercury to the pore are equal. For a cylindrical pore , this implies the following: r = [- 2 w cos j ]/P

(4)

where r = pore radius, P = applied pressure , j = contact angle between mercury and pore wall , and w = surface tension of mercury. Taking the surface tension of mercury to be 0.48 newtons/meter and the average contact angle to be 1400 , this equation can be reduced to: r = 7500/P

(5)

The smallest pore size that can be detected by mercury penetration is determined by the limit where a macroscopic theory (Kelvin equation) can still be applied. However , the smallest pore size that can be measured by a particular porosimeter depends on the maximum pressure to which mercury can be subjected by the unit [7]. The measurements reported here were carried out using Carlo Erba Porosimeter 2000. In this unit the maximum pressure to which mercury can be subjected to is 2000 atm . Hence pores with less than 3.7 nm (nanometer = 10-9 m) radii cannot be measured by this unit.

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Results and Discussion The catalysts tested in this study are the commercial catalysts manufactured by United Catalysts Inc. (UCI). The steam reforming catalysts are designated as Cll-904 and Cll-9-02 while the methanation catalyst is C13-4-04. The microstructure characteristics of these catalysts as obtained from this experimental study and as given by the manufacturer are shown in Table 1. Conversion of nickel oxide to metallic nickel was performed under different operating conditions. Following the reduction, the nickel surface areas and porosity were determined. The results are presented and discussed in this section. Table I. Microstructural properties of fresh steam reforming and methanation catalysts Property

Steam reforming catalysts Cll-9-04 Cll-9-02

Methanation catalyst

CI3-4-04

Surface area (m~/g) Measured By manufacturer

5.0 3-10

8.7 3-10

79.6 60.105

Pore volume (cm:l/g) Measured

0.18

0. 14

0.50

By manufacturer

0.15-0.30

0 .15-0.30

0.60-0.65

Mean pore radius (nm) Measured

2055

1065

367

By manufacturer

N/A

N/A

N/A

Pore size distribution (% ) 3.7-lOnm 1O-100nm 100-1000 n m

0 6 63

0 26 61

10 66 20

1000-10000 nm

31

13

4

0

0

0

>l0000nm

BET area and porosity of fresh catalysts Steam reforming catalysts BET area and porosity of C11-9-04 and Cll-9-02 catalysts are determined in the fresh oxidized form. The results are given in Table 1. The BET area of steam reforming catalysts C11-9-04 and Cll-9-02 was 5.0 m 2/g and 8.7 m 2/g respectively. According to the manufacturer, the BET area of both of these catalysts should be between

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3-10 m 2/g [30]. Hence the measured areas were within the specified range. Such low areas of steam reforming catalysts are due to the use of alpha alumina support in commercial catalysts which provide a high thermal stability. According to VCl's specifications , the pore volume of Cll-9-04 and Cll-9-02 catalysts should be between 0.15 and 0.30 cm 3/g [30]. The measured pore volume for C11-9-02 (0.18 cm3/g) was within the specified range , but for Cll-9-04 the measured pore volume (0.14 cm3/g) was found to be slightly lower tha~ lower limit. The poresize distributions of these catalysts show that all the pores are in the ragne of 10-10000 nm (Table 1). It should be noted that there are no pores with radii in the ragne of 3.710 nm. The mean pore radius of Cll-9-04 catalyst was 2055 nm and that of Cll-9-02 was 1065 nm. Such large pores are essential for steam reforming catalysts which are likely to have carbon deposition and are subjected to excessive heat load. No pore size distribution measurements from the catalyst manufacturer were available for comparison. Methanation catalyst

The BET area of C13-4-04 methanation catalyst in its fresh form was measured to be 79.6 m 2/g. This value is within the range of 60-105 m 2/g specified by VCI [30]. The BET area of methanation catalyst is larger than steam reforming catalysts because of gamma alumina support of methanation catalyst. The pore volume of a sample of C13-4-04 catalyst was found to be 0.50 cm 3/g. This pore volume was obtained by Carlo Erba 2000 porosimeter in which mercury can be subjected to a maximum of 2000 atm . The measured pore volume therefore represents the volume of pores with radii greater than 3.7 nm. The pore volume specified by VCI is 0.60-0.65 cm 3/g which was measured by mercury penetration method at 4000 atm, corresponding to pore volume of all pores greater than 1.46 nm diameter. Hence , it includes the volume of pores of radii between 1.46 and 3.7 nm. This explains the lesser value of measured pore volume as compared to that specified byVCI [30]. The pore size distribution of C13-4-04 catalyst (Table 1) indicates that more than 80% of the pores are in the region of 10-1000 nm. However , 10% of the pores are in the 3.7-10 nm range. There is strong possibility that pores with less than 3.7 nm radii are also present which cannot be measured . The mean pore diameter from the measured data for C13-4-04 methanation catalyst was found to be 367 nm. There are no published pore size distribution measurements of methanation catalyst for comparison.

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Nickel surface area and porosity of reduced catalysts Steam reforming catalysts The reduction of steam reforming catalysts was performed by flowing hydrogen at a space velocity of 900 h- 1 while heating the sample to 400°C at 10°C/min and then maintaining at 400°C for 16 h. The sample was cooled to room temperature under flowing nitrogen. The reduced sample was analyzed in-situ by chemisorption of hydrogen at room temperature by static vacuum method. The reduction process was repeated and the analysis was performed by pulse chemisorption method. Nickel surface area was determined by both methods (Table 2). For Cll-9-04, the nickel surface area by static vacuum method was found to be 1.48 m2/g as compared to 0.76 m2/g by pulse chemisorption method. The nickel surface area of Cll-9-02 was measured as 1.20 m 2/g by static vacuum method and as 0.69 m2/g by pulse chemisorption method. It should be noted that chemisorption defined by dynamic pulse technique is .different from that defined by static vacuum experiments. Although, the absolute values cannot be compared, the trend is same by both methods. Hence pulse chemisorption technique can be used to distinguish between the samples [8]. Table 2. Nickel surface area and porosity of reduced steam reforming and methanation catalysts Property

Steam reforming catalysts Cll-9-02 Cll-9-04

Methanation catalyst C13-4-04

Nickel surface area (m2/g) Static vacuum method

1.48

1.20

8.45

Pulse chemisorb method

0.76

0.69

3.92

Pore volume (cm 3/g)

0.26

0 .16

0.28

Pore size distribution (%) 3.7-lOnm lO-lOOnm

0 18

0 33

64

100-1000 nm

48

58

26

1000-10000nm

34

lO

1

0

0

0

>10000nm

9

The variation of nickel surface area due to temperature and heating rate during reduction was studied. It was observed that increasing the temperature in the range of 250-600°C resulted in an increase of active surface area of up to 400°C and

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decreases thereafter. Above 400°C, a substantial increase in the sint<:!ring rate causes lower surface area. Increasing the heating rate over the range 1 to 20°C/min causes significant decrease in nickel surface area. This decrease also reflect the effect of heating rate on sintering of nickel crystallites . The pore volume of the catalysts was found to increase after reduction (Table 2). The increase was more significant in case of Cll-9-04 than for Cll-9-02. The pore size distribution indicated that percentage of smaller pores increased to some extent due to reduction of nickel oxide to nickel. Methanation catalyst

The reduction of C13-4-04 methanation catalyst was carried out by flowing hydrogen while heating the sample to 250°C at lO°C/min and then maintaining at 250°C for 16 h. A lower temperature was used because the operating temperature of industrial methanators is about 250°C. Chemisorption results of C13-4-04 indicates that the nickel surface area by static vacuum method was 8.45 m2/g and by pulse chemisorption method was 3.92 m 2/g . A significant decrease in pore volume due to reduction was observed (Table 2). However , the pore-size distribution shows only a slight difference when compared to the fresh. The variation of nickel surface area due to temperature and heating rate during reduction was studied. It was observed that increasing the temperature in the range of 250-500°C resulted in monotonic increase of active surface area. This effect was drastically different from that observed with steam reforming catalysts. Increasing the heating rate , however , had little effect on nickel surface area of methanation catalyst. Conclusions

Commercial steam reforming and methanation catalysts were characterized in terms of their BET area , nickel surface area and porosity. BET area was obtained by nitrogen adsorption method and porosity by mercury penetration technique. The results were generally in agreement with that specified by the manufacturer. For the determination of nickel surface area , static vacuum and dynamic pulse methods were used. It was found that nickel area obtained by static vacuum method is higher than that obtained by the pulse chemisorption method. However, due to its ease and speed, pulse chemisorption method can be used for comparison between catalysts. It is also shown that porous structure of the catalyst is not significantly altered due to reduction .

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Acknowledgement. The authors wish to acknowledge the support of the Research Intsitute, King Fahd University of Petroleum and Minerals, Dhahran. References [1]

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[22] Richardson , J.Y . and Cale , T .S. "Interpretation of Hydrogen Chemisorption on Nickel Catalysts. " J. of CataI., 102, (1986),419-432 . [23] Benesi, H.A.; Atkins , L.T. and Mosely , R.B. "Rapid Measurement of Hydrogen Chemisorption by Supported Catalytic Metals." 1. of Catal .. 23, (1971),2 11. [24] Freel , J . "Chemisorption on Supported Platinum : I. Evaluation of a Pulse Method." J. of Catal. , 25 , (1972), 139-148. [25] Ko, E.I. et al. "Preparation , Reduction and Chemisorption Behavior of Niobia-Supported Nickel Catalysts." J. of Caral .. 84 . (1983), 85-94. [26] Jones , R.D. and Bartholomew , e.H. "Improved Flow Technique for Measurement of Hydrogen Chemisorption on Metal Catalysts." Applied Catalysts, 39 , (1988) , 77-88. [27] Carballo. L.. er al. " Hydrogen Chemisorption Studies on Supported Platinum Using the Flow Technique ." J. of Caral. , 52, (1978). 507-514. [28] Drake , L.e. "Pore-Size Distribution in Porous Materials." Ind. Eng. Chem., 41, (1949),780 . [29] ASTM "Standard Test Method for Determination of Pore Volume Distribution of Catalysts by Mercury Porosimetry." ASTM Method No . D-4284. Annual Book of ASTM Standards , Part 31, 1988. [30] United Catalysts Inc. , Product Bulletins ofCll-9-04 , Cll-9-02 and C13-4-04 Catalysts, Louisiville , Kentucky . USA .