Characterization of coprecipitated nickel catalysts

Applied Catalysis, 47 (1989) 155-163 Elsevier Science Publishers B.V., Amsterdam

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in The Netherlands

Characterization of Coprecipitated Nickel Catalysts Comparison of NiO/Si02 and NiO/TiO% Catalysts

SOUICHI UCHIYAMA*, KAWATA

YASUO OBAYASHI,

TOSHIAKI

HAYASAKA

and NOBORU

Central Research Laboratories, Idemitsu Kosan Co., Ltd.“, 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba, 299- 02 (Japan) (Received

22 April 1988, revised manuscript

received 17 October 1988)

ABSTRACT The characterization of coprecipitated nickel catalysts was investigated using X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, infrared spectrometry, X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Interaction of nickel species and silica on the NiO/SiOg catalyst evidently occurred through Ni-0-Si bonds formed on precipitation, which resulted in the suppression of crystallization in air and reduction in hydrogen of the nickel species. In contrast, interaction of nickel species and titania on the NiO/TiO, catalyst occurred only after reduction in hydrogen; coverage with TiO, (x < 2) on the nickel species occurred with a decrease in the nickel concentration on the surface. This phenomenon probably allowed the coprecipitated NiO/TiO, catalysts to show characteristic reactivities.

INTRODUCTION

We have recently found that alcohols such as methanol, ethanol and propanol were produced from synthesis gas on coprecipitated NiO/Ti02 catalysts, whereas coprecipitated NiO/Si02 catalysts only produced methane [ 1,2]. The characteristic behaviour of nickel catalysts supported on titania have been discussed on the basis of a strong metal-support interaction (SMSI) [3-71. Few studies of SMSI on coprecipitated catalysts, however, have been reported. In this paper we report in detail the results of the characterization of coprecipitated nickel catalysts and discuss the differences in the characteristic reactivities of coprecipitated NiO/TiO, and NiO/Si02 catalysts. “Participant

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0 1989 Elsevier Science Publishers

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156 EXPERIMENTAL

CatuZyst prepurutio~ The NiO/SiOz and NiO/TiO, catalysts were prepared by coprecipitation as described elsewhere [ 21. The reagents employed were nickel nitrate, sodium silicate and titanium sulphate (Wako). The precipitates were aged at 353 K for 2 h, with the pH adjusted to be over 9.0, followed by washing with distilled water and drying at 393 K for 5 h, and were then calcined at 623 K for 2 h in air. The catalysts obtained in this way had nickel-to-silicon and nickel-totitanium molar ratios of 1.08 and 0.92, respectively, as determined by X-ray fluorescence (XRF) spectrometry.

The structural changes in the catalysts during preparation were investigated by using X-ray diffraction (XRD ) and infrared (IR) spectrometry. XRD patterns were obtained on a Rigaku Rotaflex diffractometer with Cu Kcx radiation. A scan of 2 min-l was used for the 20 range of 4-76”. IR spectra were obtained using a JEOL JIR-100 spectrometer with 2 cm-’ resolution. For the measurement of the IR spectra the sample powder was mounted on a cell without pretreatment and then measured. Measurements by transmission electron microscopy (TEM) were made on a JEOL JEM-1~ system in order to study the size of nickel particles when the catalysts were reduced in hydrogen at a certain temperature. Samples were suspended in ethanol with a supersonic wave. Part of the suspension was pipetted onto a microgrid. These manipulations were carried out in air. Thermogravimetric analyses (TGA) in hydrogen were conducted using a Rigaku Denki Thermoflex CN8078Bl. Catalysts fca. 20 mg) were loaded into an alumina cell. The flow-rate of hydrogen was 80 ml/min. The temperature was raised at 5 K/min from room temperature to 773 K. In situ X-ray photoelectron and secondary ion mass spectroscopy (XPS and SIMS ) were applied to characterize the surface of the catalysts. Measurements were made using a VG ESCALAB spectrometer. Samples loaded on a holder were evacuated or reduced in hydrogen at desired temperatures for 0.5 h in a preparation chamber. After pretreatment, the samples were transferred into an analysis chamber without exposure to air. The spectra were recorded at room temperature under vacuum in the range of 1O-8-1O-g Torr (1 Torr = 133.322 Pa). Al Ka, radiation (1487 eV) was used as an X-ray source. The binding energies of Si, (103.4 eV) and Ti,, (458.5 eV) were used as internal standards in the XPS analyses. Photoionization cross-sections obtained by Scofield [8] were used for the calculation of the surface concentration of each element. The intensities of the peaks correspond with the area of the

157

peaks including the satellite peaks for the NiZp level. The intensity ratios of 58Ni+ to 4*Ti+ and to “Si+ were obtained from the SIMS spectra. The source energy of the Ar+ was 4.9 kV. RESULTS

Structural change of catalysts The results of XRD measurements for the NiO/SiOp catalyst are shown in Fig. 1. The XRD patterns of the samples dried at 393 K or calcined at 623 K showed a poorly crystallized nickel hydrosilicate phase [ 91; Montes et al. [ 91 reported similar XRD patterns for Ni/Si02 catalysts prepared by depositionprecipitation. This compound was converted into poorly crystallized nickel oxide after calcination at 973 K. In the IR spectrum of the sample dried at 393 K, bands at 450,680 and 1010 cm-’ were observed, which were different from those observed in the spectra of nickel oxide and silica as references. No significant change was observed in the spectrum when the sample was calcined at 623 K. Even after calcination at 973 K, the NiO band was not definitive,

10

30 50 70 28 (deg)

Fig. 1. XRD patterns for the NiO/SiO, catalyst: (a) after drying at 393 K; (b) after calcination at 623 K; (c) after calcination at 973 K. o, Ni-silicate layer; l , NiO.

I

I

IO

30

50 29Cdeg)

70

I

Fig. 2. XRD patterns for the NiO/TiO, catalyst: (a) after drying at 393 K; (b) after calcination at 623 K; (c) after calcination at 973 K. a, Ni (OH )?; l , NiO; A, TiO,; X, NiTiO:,.

158 TABLE 1 Nickel particle size after reduction in hydrogen as determined by electron microscopy Catalyst

Reduction temperature/K

Particle size/ nm

NiO/TiO,

573 673 773 573 673 773

13 20 20 Not detected Not detected <5

NiO/SiO*

and the bands at 450,680 and 1010 cm-’ became broad and weak with slight shifts. The XRD pattern of the NiO/TiO, catalyst dried at 393 K showed a poorly crystallized nickel hydroxide phase, 3Ni(OH)2*2Hz0 [lo] (Fig. 2a). This compound was converted into poorly crystallized nickel oxide after calcination at 623 K. Crystallization of the phases identified with NiO, TiOz and NiTiO, occurred when the sample was calcined at 973 K. The band at 465 cm-‘, which is characteristic of the spectrum of nickel oxide, was observed in the IR spectrum of the sample calcined at 623 K, and became strong with increase in the calcination temperature. The IR spectrum of the sample calcined at 973 K was similar to that of nickel oxide. TEM measurements The TEM results are given in Table 1. On the NiO/TiO, catalyst, the average diameter of nickel particles was 13 nm after reduction at 573 K. The nickel particles increased to 20 nm in diameter when the reduction temperature was increased to 673 K, but no further growth was observed after reduction at 773 K. The particles observed after reduction were uniform in shape and size. Nickel particles on the NiO/SiOP catalyst after reduction at 673 K or below were not visible because of their very small size. Although nickel particles were observed after reduction at 773 K, they were still too small (less than 5 nm). TGA measurements Fig. 3 shows the TGA spectra obtained on the two types of catalysts and a standard sample of nickel oxide. The rate of weight loss (dW/dT) was calculated from the spectra and plotted as a function of temperature. In comparison with the results for nickel oxide, the reduction temperature of nickel species in the catalysts was shifted to higher values, although the reduction of nickel

159

T(K)

Fig. 3. Derivative of weight loss as a function of temperature: (a) NiO; (b) NiO/TiO?; (c) NiO/ SiO?. TABLE 2 Binding energy of Ni2eClPe on coprecipitated nickel catalysts and nickel compounds Sample

Treatment”

Binding energy of Ni 2P,sJl /eV

NiO/SiO,

473 R 573 R 773 R 473 E 573 R 673 R 773 R

858.1 857.5

NiO/TiOP

Ni NiO’ Ni(OH)B’ Ni-silicate layer“

-

856.9 (855.0)b 856.0 856.4 852.6 (855.5)b 852.2 852.6 854.2 856.0 857.0

“R = reduction; E = evacuation. bShoulder peaks. ‘Ref. 11. “Ref. 12.

species in the NiO/SiO* catalyst occurred at a higher temperature than that for the NiO/TiOz catalyst. Even when the temperature reached 773 K, the reduction of nickel species in the NiO/SiOa catalyst was not complete, whereas nickel species in the NiO/TiOp catalyst were almost completely reduced to the metal. XPS and SIMS measurements Table 2 summarizes the results of the XPS and SIMS analyses of the NiO/ SiOz catalyst. Although the Ni2P3,2 binding energy decreased slightly with increase in the reduction temperature, a peak due to nickel metal was not definitive even after reduction at 773 K. A small shoulder peak was observed at 855

160

eV, as shown in Fig. 4c. The peak at high binding energy, i.e., 875.5 eV, suggested the presence of a nickel-silicate phase [ 121. As can be seen in Fig. 5, no significant change in the Ni,,/Si,, intensity ratio after various treatments was observed in the XPS analysis, whereas the same pattern for the 58Ni+/28Si+ intensity ratio was observed in the SIMS analysis. Fig. 6 shows the changes in nickel concentration on the NiO/TiOz catalyst surface with various treatments. In the XPS analysis, the N&,/T&, intensity ratio slightly increased after reduction at 573 K, whereas it drastically decreased as the reduction temperature increased from 573 to 673 K; the ratio obtained after reduction at 673 K declined to 34% of the value obtained after evacuation at 473 K. The results of the SIMS analysis showed more drastic changes than the XPS analysis for the nickel concentration on the NiO/TiOz catalyst surface. As the reduction temperature increased, the 5*Ni+/48Ti+ intensity ratio gradually decreased; the ratio obtained after reduction at 773 K declined to 15% of the value obtained after evacuation at 473 K. The Ni2P3,2 binding energy shifted toward lower values as the reduction temperature increased. A peak identified as nickel metal was observed at 852.6 eV (with a shoulder peak at 855.5 eV) after reduction at 673 K (Fig. 7~). It was obvious from the spectrum after reduction at 773 K that almost all of the nickel species were reduced to the metal; the Ni,,,, binding energy was 852.2 eV.

I

0-o

I

850 BINDING

a75 ENERGY

[eV)

473E 473R 573R 673R PRETREATMENT

773R

Fig. 4. XPS results for the NiO/Si02 catalyst after treatments: (a) reduction at 473 K; (b) reduction at 573 K; (c) reduction at 773 K. Fig. 5. Changes in N&,-to-S&, intensity ratio on XPS analysis and Ni+-to-Sit intensity ratio on SIMS analysis for the NiO/SiO, catalyst as a function of treatment. E, Evacuation; R, reduction.

161

0-o 473E

473R

573R 673R

PRETREATMENT

773R BINDING

ENERGY

CeV)

Fig. 6. Changes in N&,-to-T&, intensity ratio on XPS analysis and Ni+-to-Ti+ intensity ratio on SIMS analysis for the NiO/TiO, catalyst as a function of treatment. E, evacuation; R, reduction. Fig. 7. XPS results for the NiO/TiO, catalyst after treatments: (a) evacuation at 473 K; (b) reduction at 573 K; (c) reduction at 673 K; (d) reduction at 773 K. DISCUSSION

It was found in the IR spectrum that the band at 1010 cm-’ was observed on the NiO/Si02 catalyst after precipitation. Ueno et al. [ 131 showed that the bands at 810 and 1100 cm-l were due to the symmetric and asymmetric vibrations of Si-0-Si bonds in silica, whereas the band at 975 cm-‘, ascribed to the Si-0 vibration in Ni-0-Si bonds, was observed. Their results indicate that in spite of the high content of nickel in our catalysts, the nickel species in the NiO/SiO, catalyst were present in the form of Ni-0-Si bonds, suggesting that the nickel species were highly dispersed in the silica matrix. The fact that no significant crystallization of nickel species occurred on the NiO/Si02 catalyst after calcination at 973 K may indicate that the Ni-0-Si bonding greatly suppressed the mobility of nickel species. The IR spectrum of the NiO/Ti02 catalyst calcined at 973 K was similar to that of nickel oxide. From the XRD pattern, one can see that a nickel oxide phase definitely appeared on calcination at 973 K, indicating that the nickel species in the NiO/TiO, catalyst crystallized to bulk nickel oxide more easily than those in the NiO/Si02 catalyst. The reducibility of nickel species seems to be due partly to the strength of interaction between nickel species and silica or titania. The nickel species in the NiO/Ti02 catalyst were easily reduced to the metal, suggesting that there would be only a weak interaction between the nickel species and titania, whereas the reduction of nickel species was greatly suppressed in the NiO/Si02 cata-

162

lyst, probably because of a strong interaction between the nickel species and silica. The structural changes and the reduction behaviours of the catalyst were investigated by XPS and SIMS analyses. The Ni,,,, binding energy on the NiO/SiOz catalyst was high in comparison with that of nickel metal after reduction at 773 K. This indicates that it was difficult to reduce the nickel species on the NiO/SiOz catalyst, in good agreement with the TGA results. The binding energy on the NiO/TiO, catalyst drastically decreased to 852.6 Ni,,,, eV after reduction at 573 K. Almost all of the nickel was reduced to the metal after reduction at 773 K, also in good agreement with the TGA results. No significant change in the nickel concentration was observed on the NiO/ Si02 catalyst surface by either XPS or SIMS analysis when the reduction temperature increased. The nickel concentration, in contrast, decreased considerably on the NiO/TiO, catalyst surface with an increase in the reduction temperature. This phenomenon may be due to migration of TiO, onto nickel species [5,14]. It is noteworthy from the TEM results that this phenomenon was not due to the growth of nickel particles, which was suppressed even at high reduction temperatures. The considerable decrease in the XPS and SIMS intensity ratios with respect to the nickel species on the NiO/Ti02 catalyst reduced at 773 K seems to show that the TiO, layer on the nickel surface was fairly thick. It has been reported that the TiO, layer assists the rupture of CO bonds leading to a high activity of nickel catalysts supported on titania in carbon monoxide hydrogenation [ 6,151. Therefore, this study suggested that under reaction conditions such as those in carbon monoxide hydrogenation the nickel species on the NiO/Ti02 catalyst were partly exposed to reactants by the covering of TiO, which results in the characteristic reactivities, whereas those on the NiO/Si02 catalyst were directly exposed to reactants, showing conventional reactivities of nickel catalysts. Comparison of the XPS and SIMS analyses on the NiO/TiO, catalyst provides interesting information: the SIMS analysis showed that the covering of TiO, on nickel species began after reduction at 473 K and gradually proceeded as the reduction temperature increased, whereas the XPS analysis showed that the covering of TiO, on nickel species was initially observed after reduction at 673 K. This difference might be due to the difference in the surface sensitivities of XPS and SIMS analyses. There have been many reports [ 7,161 that a SMSI does not appear unless the reduction temperature attains a certain value, viz., 773 K or more. However, the SIMS analysis evidently showed that SMSI effects appeared even at low reduction temperatures. Spencer [ 171 has shown that Pt/Ti02 catalysts reduced at low temperatures should show SMSI effects on the diffusion of titanium species onto platinum particles, although the rates of diffusion of titanium species are very different at low and high reduction temperatures. It is obvious from this study that the very surface-sensitive SIMS

163

method could be used to detect the slight changes in the catalyst surface even at low reduction temperatures.

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