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The Application and Characterisation of Nickel Nanoparticles Inside the Mesopores of MCM-41
Dennis J. Lensveld, J. Gerbrand Mesu, A. Jos van Dillen and Krijn P. de Jong
Introduction
The recent development of several structured mesoporous materials has received much attention from the field of catalysis. The textural and structural properties of these materials make them suited for use as support for the catalytically active phase(s). However, stability limitations of MCM-41 and related materials call for dedicated catalyst synthesis procedures. Here we present a study on the influence of the precursor of the catalytically active phase on the ultimate properties of supported nickel (oxide) catalysts. To arrive at a high loading of well-dispersed nickel nanoparticles inside the mesopores a chelating citric acid precursor was applied. This nickel precursor leaves a strongly adhering thin film on the surface of the pore walls, which upon drying, calcination and reduction breaks up in small nickel nanoparticles. As a reference precursor the commonly used nickel nitrate was used.

Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA, Utrecht. http://www.anorg.chem.uu.nl e-mail: k.p.dejong@chem.uu.nl

Experimental
MCM-41 All-silica MCM-41 was obtained from a synthesis gel with composition 1 SiO2 (Aerosil 380) : 0.27 CTABr : 0.19 TEAOH : 40 H2O. After ageing for 24 hours (RT) this gel was transferred to an autoclave and aged hydrothermally for 2-3 days at 150°C. Template was removed by calcination in air (6 hours 550°C). Catalysts Powdered MCM-41 material was impregnated to incipient wetness with solutions of Ni(NO3)2 and Ni3(C6H5O7)2 (citrate-precursor) respectively, dried (12 hours 120°C) and calcined (4 hours 450°C, air). Nickel loading = 10% by weight.

Results
Catalyst ex NO
100 ml N 2 /g
23

Volume adsorbed

Intensity

Catalyst ex citrate

Catalyst ex NO

23

Catalyst ex citrate

Parent MCM-41
Parent MCM-41

TEM micrograph showing the nickel nanoparticles inside the mesopores of the catalyst ex citrate after reduction.
7

1
BET area (m2 g-1) pore volume (ml g-1) pore (nm)

2

3

4

5

6

° 2

catalyst ex NO32catalyst ex citrate parent MCM-41

870 860 1,000

0.92 0.84 0.98

2.9 2.9 3.0

0

0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

1

X-ray diffraction patterns at low angles of the parent MCM-41 material and calcined catalysts. Upon application of nickel the hexagonal order of the support, reflected by the (100), (110) and (200) diffractions, is retained. TEM micrograph showing some nickel nanoparticles inside the mesopores of the catalyst ex nitrate after reduction.

p/p

0

Nitrogen physisorption data of the parent MCM-41 material and calcined catalysts.

Catalyst ex NO

23

Hydrogen consumption

bulk NiO

Catalyst ex citrate

Catalyst ex NO

23

40

50

60

70

80

90

100

° 2

Catalyst ex citrate

X-ray diffraction patterns showing NiO reflections in the calcined catalysts.
700 800 9 0 0 1000 1100

300

400

500

600

Temperature (K)

TPR patterns of the calcined catalysts and bulk NiO.

With XPS a notably higher Ni:Si ratio was measured for the catalyst ex citrate (0.09) vs. nitrate (0.05), indicating a higher dispersion of nickel resulting from the use of a film-forming precursor.

TEM micrograph very large nickel on the MCM-41 after reduction catalyst ex nitrate

showing particles support of the .

Conclusions
Upon application of nickel the favourable structural and textural properties of MCM-41 have been retained, independent of the nickel precursor used. With a non-adhering, common nickel nitrate precursor the mobility during drying and calcination results in the formation of large nickel (oxide) particles at the external support surface. The use of a chelating nickel citrate precursor results in the deposition of a thin film strongly fixed to the mesopore surface. Upon drying, calcination and reduction this thin film breaks up, leaving a high dispersion of nickel nanoparticles, exclusively situated inside the mesopores of the MCM-41 support.

Future work
SAXS will be used to study in more detail the processes occurring during the various stages of catalyst synthesis. Catalytic performance will be tested in the hydrodesulphurisation of thiophene.



Acknowledgements
Leon Coulier (TU Eindhoven, The Netherlands), John Raaymakers, Cor van der Spek and Fred Broersma are kindly thanked for their help during catalyst characterisation.

AVDC, Department of Chemistry, U.U.

August 2000