ナノ粒子のサイズ・形態制御と構造敏感型触媒プロセスへの応用

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1 Nano-material Processing and Surface Physical Chemistry Atsushi Muramatsu

2 Nanoparticles

3 Nanoparticles 10-9 m = 1 nm Billionth. A particle is composed of several atoms. Characters are expected different from bulk. Number of surface atom becomes equal to that of bulk atom. 3

4 Classification of particles 1m 100μm 10cm 1cm Softball Coin 10μm 1mm 100μm Wheat flour 1μm 10μm 1μm 100nm Pollen Cigarette smoke Virus 100nm 10nm 10nm 1nm Cellophane pore 1A 1nm 4 Electron Microscope Optical Microscope Nanoparticle Submicron particle Cluster Ultrafine particle Colloidal dispersion Fine particle

5 Catalysis

6 6 Catalyst Industrial catalyst activity, selectivity, life, treatment Design surface control bulk control Surface control Metal catalyst species, valence, composition, size

7 Activity Turnover frequency an active site Surface reaction rate at one active site The whole activity of catalyst Dependent on the total surface Moreover, it strongly depends on the surface structure 7

8 Life Catalyst life Maintain the same activity for a long time Life should more than 1 month. Loss of activity as a result of sintering, change in characters, etc. 8

9 9 Selectivity Catalyst changes a specific reaction rate. CO hydrogenation Cu: CO + 2H 2 CH 3 OH Ni: CO + 3H 2 CH 4 + H 2 O Co, Fe: 6CO + 9H 2 C 6 H 6 + 6H 2 O Rh: 2CO + 2H 2 CH 3 COOH Rh: 2CO + 4H 2 C 2 H 5 OH + H 2 O Of course, the reaction conditions affects the rate.

10 Size control The whole catalytic activity is enhanced by increase in the total surface area. TOF (Turnover Frequency) sometimes depends on the size. Quantum effect. 10

11 Catalyst design Detailed characterization of surface Precise control of surface Control of nanostructure of the surface and its evaluation is important. 11

12 Classification of catalysts Homogeneous catalysts Same phase reactant, catalyst, etc. Ex. Acetic acid synthesis: Rh complexes catalyst = liquid phase Heterogeneous catalysts different phase Ex. Solid catalysts supported catalysts unsupported catalysts 12

13 13 Supported catalysts Metal particles are supported on a carrier. The carrier is a porous material. Catalyst metal Support (Carrier)

14 Supported metal catalysts Supports Almost all supports are metal oxides. They have many pores. They have excellent tolerance against mechanical intention. Catalyst metal They are well dispersed on the support. Their size is expected to be 1~2 nm. But, 5~50nm is general for industrial catalysts. 14

15 15 Supports: large surface area Pores

16 16

17 17 Examples of supports: active carbons Yashigara AC Anthracic AC Charcoal AC

18 Active carbons 18

19 Charcoal surface 19

20 Supported metal catalysts Supports Almost all supports are metal oxides. They have many pores. They have excellent tolerance against mechanical intention. Catalyst metal They are well dispersed on the support. Their size is expected to be 1~2 nm. But, 5~50nm is general for industrial catalysts. 20

21 21 Surface structure Step Terrace Kink Atoms of single crystalline metal surface

22 Surface structure 22

23 Adsorption and catalytic reaction

24 Adsorption Physisorption 蠅的吸着 like a fly week: always Chemisorption 蚊的吸着 like a mosquito strong: chemical bonding 24

25 Table Chemisorption and physisorption Features Chemisorption Physisorption Force Chemical bond van der Waals Place Selective nonselective Structure Monolayer Multilayer ΔH 10~100kcal/mol 2~3 kcal/mol Activation energy Large Small Rate Slow Rapid Adsorption and desorption sometimes irreversible Typical type Langmuir BET reversible 25

26 Physisorption 26

27 Physisorption 27

28 Physisorption 28

29 Physisorption 29

30 Physisorption 30

31 Adsorption to reaction

32 32 Catalysis Physisorption Chemisorption To this line, it means ADSORPTION Surface reaction Desorption

33 33 Ex: methanol synthesis Synthesis gas into methanol CO + 2H 2 CH 3 OH C=O: nondissociative adsorption. H-H: dissociative adsorption.

34 34 可逆可逆 Physisorption chemisorption CH 3 OH irreversible Surface reaction

35 Surface reaction Irreversible path in extremely low counteraction rate Surface reaction often = rate determining Surface reaction includes many steps. Arrenius plot is useful to decide the ratedetermining step. 35

36 Ex: methanol synthesis Synthesis gas into methanol CO + 2H 2 CH 3 OH CO gas CO(chemisoption) H 2 gas H 2 (chemisoption) 2H(dissociative adsorption) CO(ad.)+H CHO(ad.) <rate determining> CHO(ad.)+H CH 2 O(ad.) CH 2 O(ad.)+H CH 3 O(ad.) CH 3 O(ad.)+H CH 3 OH(ad.) CH 3 OH(ad.) (desorption)ch 3 OH 36

37 37 Activation energy Arrenius equation k E = Aexp a RT where A is frequency factor, E is activation energy.

38 Apparent activation energy Plot ln (k)=y axis, 1/T=x axis. Slope = Ea (apparent activation energy) ln (k) Slope = E a 1/T 38

39 Evaluation of catalyst B is excellent because of lower activation energy. ln (k) B catalyst A catalyst 1/T 39

40 Change in activation energy ln (k) When the rate-determining step is changes, the apparent activation energy is changed. 1/T 40

41 41 Reaction path B A D C When the rate-determining step is changes, the apparent activation energy is changed.

42 Catalytic reactions

43 Insensitive Surface area effect Sensitive Structure-sensitive or structure-insensitive Activity depends on size. Higher in smaller size Higher in larger size Maximum in a specific size 43

44 Structure-insensitive reaction 44

45 Structure-sensitive reaction 45

46 Structure-sensitive reaction 46

47 Structure-sensitive reaction 47

48 Catalyst preparation

49 Method Impregnation Including incipient wetness method, etc. Immerse support materials in metal salt solution. Drying it up and calcining it well. Ion exchange Coprecipitation 49

50 Disadvantages Impregnation Easy control for loading. Very difficult to decrease catalyst metal size. Ion exchange Easy control for metal size. Very difficult to increase loading. 50

51 51 Preparation and dispersity Dispersity: ratio of surface metal number to that of bulk. Dispersity depends on the average size of catalyst metal.

52 52

53 53 Preparation and dispersity H, CO uptakes are proportional to number of surface atom. H, CO uptakes are large. Active surface is large.

54 54

55 Control of dispersity (metal size) Disadvantages of conventional method In order to increase dispersity, metal loading should be decreased. To control 1~2 nm in size, loading is limited to be 3~5 wt% for Pt. The whole catalytic activity depends on the metal loading. If possible, larger loading becomes higher activity for higher productivity. 55

56 56 Loading is increased but size should be the same. Our method General Larger loading means larger size.

Support: Single-crystalline anatase-type TiO 2 Monodispersed particles prepared

57 Selective Deposition Method Selective Deposition of Noble Metal Nanoparticles on Well-Defined Oxide Particle and their Application to Hydrogenation Catalyst Support: Single-crystalline anatase-type TiO 2 Monodispersed particles prepared by the Gel-Sol method. Pt/TiO 2 1 nm Tadao SUGIMOTO and Atsushi MURAMATSU, IMRAM 57

58 Selective Deposition Method [Noble metal salt] = mol dm -3 (HAuCl 4 ; RuCl 3, RhCl 3, PdCl 2, H 2 IrCl 6, H 2 PtCl 6 ) NaOH Formation of Storing at room temp. for 24 h ph ~7 precursor complex Support particles = 1.6 g dm -3 Ultrasonic dispersion (30 min) Selective deposition Aging(100, 48 h) Reduction by H 2 if necessary Metal nanoparticles of Au, Ru, Rh, Pd, Ir, and Pt 58

59 Selective Deposition Method Effect of ph on yields of Pt precursor (100, 2days) 100 α-fe 2 O 3 Yield (%) TiO ph 59

60 Selective Deposition Method Effect of ph on adsorption of Pt ions (25, 2days) 100 Adsorption amount of Pt ions (%) TiO 2 α-fe 2 O ph 60

61 61 Selective Deposition Method α-fe 2 O 3 多結晶エリプソイド α-fe 2 O 3 単結晶エリプソイド α-fe 2 O 3 単結晶擬似立方体 α-fe 2 O 3 単結晶平板

62 62 Selective Deposition Method α-feooh β-feooh ZrO 2 (A) Rough surfaces ZrO 2 (B) TiO 2 Smooth surfaces

63 Selective Deposition Method 63 Table Catalytic properties of supported Pt nanoparticles. Support Specific surface area (m 2 Method Pt loading (wt%) Particle size (nm) Dispersity (H/M) 1-octene conversion (%) g -1 ) TiO 2, ellipsoid (anatase) 37.5 This study This study Ion-exchange method Impregnation method α-fe 2 O 3, ellipsoid (A)* 136 This study SiO This study (prepare by Stober method) ZrO 2 (B)** 118 This study Al 2 O This study CSJ-ref. cat ALO6 Ion-exchange method Impregnation method Selective Deposition method Highly dispersed catalysts with high loading

64 Selective Deposition Method 64 Au / hematite

65 Selective Deposition Method 65

66 Selective Deposition Method 66

67 Selective Deposition Method 67

68 Selective Reductive Deposition Method 68 Hematite 20wt% Ni/hematite 5wt%Ni/hematite

69 Liquid-Phase Selective Reductive Deposition Method 69 Ni-Zn/TiO 2 (Zn/Ni=0.1)

70 Liquid-Phase Selective Reductive Deposition Method Zn addition decreased the size. Ni 単独 Ni-Zn (Zn/Ni=0.1) 10 nm 70

71 Liquid-Phase Selective Reductive Deposition Method 71 粒径 4~5nm 10nm 10n m Zn/Ni=0.2 Zn/Ni=1.0

72 Very Hot Topic! Visible light utilization -Preparation of Ti-O-S photocatalyst - Muramatsu Lab. IMRAM, Tohoku Univ.

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ナノ粒子のサイズ・形態制御と 構造敏感型触媒プロセスへの応用

ナノ粒子のサイズ・形態制御と構造敏感型触媒プロセスへの応用