alumina catalysts with low alumina content

alumina catalysts with low alumina content

Applied Catalysis A: General, 94 (1993) Elsevier Science APCAT A2418 Publishers 107-115 107 B.V., Amsterdam Morphology of coprecipitated nicke...

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Applied Catalysis A: General, 94 (1993) Elsevier

Science

APCAT

A2418

Publishers

107-115

107

B.V., Amsterdam

Morphology of coprecipitated nickel/alumina with low alumina content

catalysts

Jerzy Zielinski Institute

of Physical Chemistry,

Polish Academy

of Sciences,

ul. Kasprzaka

44152,

01-224 Warsaw (Poland) (Received

28 November

1991, revised

manuscript

received

30 October

1992)

Abstract A series of low-alumina Ni/Al,O, catalysts, designed for carbon monoxide hydrogenation, was examined by temperature-programmed reduction, the BET method, oxygen chemisorption, and X-ray diffraction. Pre-reduced catalysts consisted of nickel crystallites and amorphous non-stoichiometric NiO.Al,O, residues. According to the proposed model of the catalysts the residues appeared in two forms: as clusters situated between/on the nickel crystallites and as inclusions encapsulated within large nickel crystallites. The size of the nickel crystallites, the size and composition of the clusters, and the amount of inclusions were estimated. A decrease of alumina content in the catalysts resulted in an increase in the size of the nickel crystallites, and did not affect the size, composition, and abundance of the clusters. Keywords: decoration

of nickel crystallites;

morphology;

nickel; nickel/alumina.

INTRODUCTION

The activity and selectivity of Ni/A1,03 catalysts in CO hydrogenation differ considerably from those of unsupported nickel. This fact has found widespread support, and a large number of studies [l-16] has been performed to elucidate the role of alumina in the catalysts. However, despite these efforts, the structure of the catalysts and the way in which alumina affects the catalytic properties of nickel remains obscure. It is possible that significant information about these questions may be obtained by examining low-alumina Ni/Al,O, catalysts. Low-alumina Ni/Al,O, catalysts are usually obtained by the coprecipitation method. X-ray diffraction (XRD) examinations of the calcined specimens, NiO/Al,O, precursors, revealed NiO phase and traces of y-Al,O, and NiAl,O, Correspondence to: Dr. J. Zielinski, Institute ul. Kasprzaka 44/52,01-224 Warsaw, Poland. e-mail: JERZY @ALFA.ICHF.EDU.PL.

0926~860)3/‘93/$06.00

0 1993 El sevier

of Physical Chemistry, Polish Academy of Sciences, Tel. ( +48-22)323221 ext. 353, fax. ( +48-22)325276,

Science

Publishers

B.V.

All rights

reserved.

J. Zieliriski/Appl.

Catal. A 94 (1993) 107-115

while the examinations of reduced specimens, Ni/Al,O, phases [l&14-16], catalysts, showed only nickel phase [ 11-161. Considerable efforts have been devoted to establishing the structure of COprecipitated Ni/Al,O, catalysts [ 11-161. Two principal models of the catalysts have been formulated. According to Alzamora et al. [ 111 alumina crystallizes on the surface of growing nickel crystallites, and thereby prevents their coalescence. On the other hand Wright et al. [ 121 suggested that some of the alumina, probably in the form of (AlO, ) - groups, occurs within the nickel crystallites, bringing about their para-crystallinity and thereby high thermal stability. The above models [ 11,121 provided the basis for the formulation of further proposals for coprecipitated Ni/Al,O, catalysts, Thus, Puxley et al. [ 131 suggested that all the alumina of the catalysts is enclosed within nickel, forming a para-crystalline nickel phase, and that there was no alumina phase. In contrast to that opinion, Doesburg et al. [ 141 found that catalysts with a higher alumina content still had an alumina skeleton and thus proposed a model in which para-crystalline nickel particles reside in pores of the support. Finally, Lansik Rotgerink et al. [16] concluded that the catalysts consisted of paracrystalline nickel and alumina-rich phases, but they did not estimate the amount and compositions of these phases. This paper presents the study of morphology of low-alumina coprecipitated Ni/AIZOB catalysts, and is a part of a wider research project aimed to elucidate the role of alumina in carbon monoxide hydrogenation over these catalysts. The following methods were used in this work: temperature-programmed reduction (TPR) , measurements of total and nickel surface area, and X-ray diffraction (XRD ) . The obtained results were used to formulate a model of the catalyst’s structure. According to the model the catalysts consist of nickel crystallites and amorphous non-stoichiometric NiO.Al,O, residues. The residues appeared in two forms, as clusters situated between/on nickel crystallites and as inclusions encapsulated within large nickel crystallites. The size of the nickel crystallites, the size and composition of the clusters, and the amount of the inclusions were estimated. EXPERIMENTAL

Apparatus The experiments were performed in a glass flow system [ 41 equipped with a gradientless microreactor [ 171. A temperature controller maintained the reactor temperature to within 1 ‘C over the range of - 200-1000” C and provided linear temperature programming. The composition of the stream leaving the reactor was measured by means of a thermal conductivity detector (TCD ) cell.

J. Zieliriski/Appl.

Catal. A 94 (1993) 107-115

109

Materials

The series of Ni/Al,O, catalysts of A1203/Ni mole ratio within 0.019-0.183 was prepared by coprecipitation from Ni(NOB)2 and NaAlO, aqueous solutions. The precipitate was thoroughly washed, spray-dried, calcined in air (400” C, 2 h) , and pre-reduced in a hydrogen stream. Nickel powder, obtained by reduction of NiO, was included in the series. The oxide was prepared by precipitation from aqueous solution of ultrapure Ni(N03)2 and NH,HCO, reactants. The precipitate was washed, dried, calcined, and pre-reduced in a similar way as Ni/A1203 catalysts. The catalyst’s grain size was 0.01-0.04 mm, and the porosity (pore size 4-400 nm) was 0.3-0.7 cm3/g. Measurements

NiO and NiO/Al,O, precursors were characterized by the TPR method. Before a test a sample, typically 25 mg, was dried in a helium stream (40 cm”/ min, 350” C, 0.5 h). The reduction was carried out in an 80% H,+ Ar stream (40 cm3/min) under linearly increasing temperature (8.3 ’ C/min) from 20 to lOOO”C, whereupon the sample was reduced for a further hour. The stream leaving the reactor was dried in a - 78” C trap, and the consumption of hydrogen from the stream was measured. The samples tested by TPR were not used for further examination. Ni and Ni/A1203 catalysts used for further examination were pre-reduced in a hydrogen stream (40 cm3/min, 1 h). The required temperature of reduction was reached by programmed heating (8.3” C/min). Produced water was collected in a - 78” C trap, and after evaporation its amount was measured by means of a TCD cell. The measured values were used to determine o! - the reduction degree of obtained catalysts, and X - mole fraction of alumina in the non-reduced residues. In the calculations it was assumed that: (i) the reduction was complete at 1000’ C, (ii) NiO species were reduced to nickel while Al,O, species were irreducible, and (iii ) all the alumina of the catalysts existed in the form of Ni0.A1203 clusters. The nickel surface area, SNi, and the size of nickel crystallites, D(S,i), in reduced catalysts were determined from measurements of oxygen adsorption. The examined sample was flushed with a helium stream (40 cm”/min, 400 ‘C, 0.5 h) , and then oxygen was adsorbed at 0’ C by introducing 28.5 mm” oxygen pulses into the helium stream. In the calculations it was assumed that the O/ Ni, ratio attained 1.7 [18], the number of metal atoms per square metre of nickel was 1.55 x 101’ [ 191, and nickel particles were spherical. Total surface area, Stat and the size of particles, D (Sot) for both calcined and reduced samples were determined from measurements of argon adsorption at - 195’ C. In the calculations it was assumed that one argon atom occupied 0.138 nm2 [ 201.

110

J. Zieliriski/Appl.

Catal. A 94 (1993) 107-115

The XRD examinations were carried out with a Rigaku-Denki diffractometer, using nickel-filtered Cu Ka radiation, in a specially designed X-ray camera [ 211 allowing in situ pretreatment. Prior to the measurements, NiO/ Al,O, precursors were dried in a helium stream (40 cm3/min, 350°C 0.5 h), and Ni/A1203 catalysts were pre-reduced or depassivated in a hydrogen stream (40 cm3/min, 500°C 1 h). XRD spectra were recorded after the sample temperature was lowered to 150” C. The following reflections were recorded: for precursors (200) and (220), characteristic of NiO phase; and for catalysts (ill), (200), (220), (311), (222), (400) and (331), characteristic of nickel phase. The mean size of NiO and nickel crystallites, D (XRD,io) and D (XRDNi) , was calculated from all the line broadening measurements using Scherrer’s equation with a correction for instrumental broadening, and the lattice parameter was determined from the position of the reflections. RESULTS

The examined specimens are characterized in Table 1. TPR tests (Fig. 1) show that NiO/A1203 precursors are difficult to reduce as compared to NiO powder. According to previous studies [4,22] nickel oxide appears in the precursors in a fixed form, that is as non-stoichiometric nickel aluminate and/or pure nickel oxide situated in close vicinity to the alumina. An exception is specimen No. 2 with one of the lowest alumina contents, which additionally contains a separate phase of nickel oxide, distant from alumina species. Fig. 2 characterizes specimen No. 5 (Table 1) reduced at various temperatures. The degree of reduction of the sample is small below 200°C but significantly increases within the temperature range 300-400°C and slowly increases above 400°C. The fraction of alumina in the non-reduced residue increases with the reduction temperature approaching unity at 1000°C. The TABLE

1

Characterization Sample

1 2 3 4 5 6

of examined specimens

Al,O,/Ni mole ratio

0 0.019 0.038 0.081 0.129 0.183

Non-reduced

Red. at 500’ C

Red. at 800’ C

S tot

[email protected],,,)

D(XRD,io)

D(S,i)

DWRDN;)

DfS,,)

DWRD,,)

(m*/g)

(nm)

(nm)

(nm)

(nm)

(nm)

(nm)

103

7.8

10.0

311

125 176 202 224 243

6.4 4.6 4.0 3.6 3.3

5.1 4.7 3.8

37 15.8 7.7 5.5 4.9

34.0 10.1 7.2 5.4 4.3

436 262 101 29 13.8 8.7

46 19.8 10.6 6.8

J. Zieliriski/Appl.

111

Catal. A 94 (1993) 107- 115

0

Fig. 1. TPR profiles of Ni/A1,03 catalysts; 1.

02-

numbers on the graphs denote the catalysts

in Table

50-

0

200

400 temperature,

Fig. 2. Effect of the reduction 1.

600

600

1000

'C

temperature

on the properties

of Ni/Al,O,

catalyst No. 5 in Table

nickel surface area in the specimen attains a maximum at 400°C and above that temperature both the nickel and the total surface areas decline in parallel. Figs. 3 and 4 characterize the catalysts pre-reduced at 500 and 800°C respectively. For both temperatures the degree of reduction equals unity for the alumina-free sample (nickel powder), and steadily decreases with an increase of alumina content. The fraction of alumina in the non-reduced residues is almost independent of alumina content. Increasing the reduction temperature from 500 to 800” C (Figs. 3 and 4) results in a significant decrease of both the total and the nickel surface area, but does not affect the SNi/Stot ratio. Supplementary examinations showed that the SJS,,,, ratio did not change during sintering of the catalysts; the same ratio values were also found for catalysts obtained by pre-reduction of non-calcined samples. The XRD examinations of NiO/Al,O, precursors revealed crystalline NiO phase and an amorphous phase, as found previously [11,14-161. The lattice

112

J. ZieliriskilAppl.

0

0

0.10

0.05 A1203/Ni

Fig. 3. Properties

0.15 rotio

of Ni/A1203 catalysts

reduced at 500” C.

I

0

005

010 A1203/Ni

Fig. 4. Properties

Catal. A 94 (1993) 107-115

015

I 0 20

rotm

of Ni/A1203 catalysts reduced at 800’ C.

parameter of the NiO phase decreased slightly with increasing alumina content, which suggests the presence of alumina and/or nickel aluminate dissolved in the phase. The results of the XRD examinations of Ni/A1203 catalysts showed, consistently with the literature [ 11-161, only crystalline nickel; no oxide phase was recorded. The lattice parameter of the nickel phase was equal to the value for nickel powder. The same size of nickel crystallites was calculated from each reflection, and it was close to the size found from the nickel surface area (Table 1). DISCUSSION

The examined nickel/alumina specimens are highly dispersed, porous systems. Before reduction they consist of crystalline NiO-rich and amorphous

Catal. A 94 (1993) 107-115

J. Zieliriski/Appl.

113

Al,O,-rich domains, while after reduction they consist of Ni crystallites and amorphous non-stoichiometric Ni0*A1203 residues. Fig. 5 presents a model proposed for Ni/A1203 catalysts. According to this model the residues appear in the form of clusters situated between/on the nickel crystallites, or as inclusions encapsulated within large nickel crystallites. It is supposed that the large clusters originate from Al,O,-rich domains, and that they play an essential role in the formation of the catalyst’s structure, whereas the small clusters originate from NiO-rich domains, and decorate specific sites of the nickel crystallites. The proposed model which is consistent with previous studies [ 11-161, is speculative in a number of its features and is based largely on indirect evidence. The following discussion supports the proposed model. The clusters prevent close contact and therefore coalescence of nickel crystallites, and vice versa, the crystallites prevent agglomeration of the clusters. Nevertheless, at high temperature both coalescence and agglomeration occur, favouring each other. This explains why the SNi/Stotratio observed in the catalysts is constant (Figs. 2-4). The composition of the NiO - A&O3 clusters is considered to be an important factor affecting the structure of the Ni/A1203 catalysts. It determines the cluster-nickel adhesion and the wetting angle, which affect the shape, size, mobility, and distribution of the clusters between/on the nickel crystallites. Therefore, the relations describing the composition of Ni0.A1203 residues (Figs. 3 and 4) suggest that under the chosen reduction conditions the shape, size, and abundance of the clusters on the nickel are independent of alumina content. Fig. 6 shows the mean size of Ni0*A1,03 clusters in the studied catalysts. The size was estimated from the equation: d=

6*m P’

(I)

(stot,-sNi)

where: m, p, and (S,,,- SNi) are mass, specific density, and surface area of the clusters, respectively. In the calculations it was assumed that m equals the total mass of Ni0*A1203 residues (i.e. that the amount of inclusions is negligible), p equals the weighted average for NiO and A1203 oxides, and the clusters form three-dimensional units [6,23-251 of the shape of sphere segments located clusters

inclusion

I\

crystallite

Fig. 5. Model of the Ni/A1,OB catalyst.

J. Zieliriski/Appl.

0

I

1

0.05

0.10 ratio

AI,O,/Ni

Catal. A 94 (1993) 107-l 15

I

0.15

Fig. 6. Size of the NiO-A1203 clusters.

between/on the nickel crystallites. The results presented in Fig. 6 show that at the reduction temperatures selected, the calcu!ated cluster size is constant within a wide range of alumina content. This finding supports the proposed model of the catalyst’s structure. However, at very low alumina content the cluster size increases, which is physically impossible, and is thereafter ascribed to the occurrence of Ni0*A1203 inclusions within large nickel crystallites. Therefore, the mass m assumed in the calculation of the cluster size, is overestimated and the calculated size surpasses the actual size. Comparing the data in Fig. 6 with that in Figs. 3 and 4 suggests that NiO.A1,OB inclusions occur in the nickel crystallites when the surface area of nickel falls below 15 m’/g, i.e. when the nickel crystallites grow above ca. 30 nm. If one assumes, in accordance with the above considerations, that the actual cluster size is constant within the full range of alumina content (Fig. 6) then these relations can be used to estimate the quantity of inclusions. This amount, expressed as the A1203/Ni mole ratio, is about 0.01. The inclusions encapsulated within the nickel crystallites are hardly reduced in comparison with the clusters situated on the nickel crystallites and exposed to gas phase. Therefore the occurrence of the inclusions brings about a decrease of the alumina fraction in the non-reduced residues (see Figs. 3 and 4). Ni0.A1,03 inclusions in the nickel crystallites probably perturb their crystalline lattice and may induce a para-crystallinity of nickel. The experimental results of this work, successfully interpreted with the proposed model, indicate that the inclusions do not directly affect the catalyst’s structure. A large change of alumina content in coprecipitated Ni/Al,O, catalysts probably brings about a quantitative change in the catalyst’s structure. In highalumina catalysts the Ni0.A1203 residues probably form a solid skeleton that supports nickel crystallites [ 141, and in high-nickel catalysts the nickel may

J. Zieliriski/Appl.

Catal. A 94 (1993) 107-115

115

form metal skeleton that supports NiO-A&O3 clusters. Assuming that the chemical/catalytic properties of the nickel catalysts depend not only on the chemical properties of the nickel crystallites and NiO-Al,O, clusters, but also on the size, shape, and arrangement of the constituents it seems that chemical examinations of the catalysts may provide further insight into their structure. This problem is the subject of our current studies.

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