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Production of catalysts with an inductive atmospheric plasma torch
Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.
243
Production of catalysts with an inductive atmospheric plasma torch Frederik Cambier CRIF ( Centre of Research for Industries), rue du Bois Saint Jean, 12, B- 4102 Seraing, Belgium Tespint s.a., Avenue Moliere, B- 1190 Brussels, Belgium
Abstract The production of different nanomaterials from oxides to nitrides and carbides is investigated through the control of the process parameters of an inductive atmospheric plasma torch. Introduction Nanopowders can be produced by different kinds of processes: physical processes (laser ablation, evaporation/condensation, etc.), mechanical processes (mechanosynthesis, high deformation at low temperature, etc.), chemical processes (sol-gel, hydrothermal, plasma synthesis). Although the synthesis of nanooxides particles is well known (sol-gel or hydrothermal processes), the synthesis of carbide or nitride nanoceramics with a plasma torch is much more complicated, the main challenges being the control of the process (hydrodynamic flows) and the control of the chemical composition of the end products after the plasma reaction. At the present time, some products such as titanium zinc and cesium nanooxides are already used for health-care applications (dry skin cream, UV-blocking cream, anti-wrinkle cream) or polishing slurries. One of the most interesting characteristics of nanopowders is the specific surface area. Figure 1 shows the total numbers of atoms and surface atoms following the number of shells. In the case of spherical iron nanocrystals, if the dimensions of the clusters are below 3 nm, we have more atoms on the surface than in the bulk of the particles [1].
244 244
Full-shell Clusters
F. F. Cambier Cambier
Toul Number o f Atoms
Surface Ato (%)
o |
Bulk Atoms
i
c o
/
5
•#
1
CD
il E o
\ 1
\
Surface Atoms
1 5
10
15
20
25
30
Particle Size(nm)
Figure 1 : Number of surface atoms according to the total number of atoms for different shells. (Source: Nanoscale Materials in Chemistry, Wiley, 2001 [1])
The aim of this publication is to show various opportunities for nanopowders produced by an atmospheric plasma inductive torch for catalyst applications. Experimental Recently, the CRIF (Centre of Research for Industries) has developed a pilot for producing nanopowders with a plasma torch; this pilot is dedicated entirely to industrial production. The plasma is the fourth state of the matter composed of positive ions, negative particles (electrons and negatives ions) and neutral particles (atoms, molecules, clusters, free radicals); the total electrical charge of the plasma is neutral. In case of finite element calculations and simulations, we can assume that the material in the plasma state has the same behaviour as a perfect gas. To create a plasma, two things are necessary: a high frequency and a high power. In our process, the plasma is produced with a high frequency 60 kW power generator working at 10 MHz. As shown in figure 2, the plasma reactor is composed of two concentric cylinders [2].
Production of of catalysts with an an inductive atmospheric plasma plasma torch
245
Powder
Central g ^ I. p
RF Electrica Supply (MHz
Magnetic oupling
Figure 2 : Plasma reactor (Courtesy of Tekna Plasma Systems).
Three gases are used: • the sheath gas (mainly a diatomic gas such as hydrogen, nitrogen or oxygen) to protect the reactor against the high temperatures, • the central gas (mainly argon) to stabilize the plasma, • the powder gas to create the nanopowders. The powder gas can be a mixture of micropowder, liquid or other precursor gases with reactive gases such as oxygen to create nanooxides, nitride or ammonia to create nanonitrides, methane/ethylene/acetylene + hydrogen to create nanocarbides. We have two possibilities (figure 3) a plasmophysieal and a plasmochemical process. der (or liquid prescursor) + carrier gas
In-flight particle melting and vaporization Nano-powder
j
A *V Plasmo VdaaESr physical process
Plasmo Chemical process _k
Reaction
Figure 3 : Principle of the physical and chemical process (Courtesy of Tekna Plasma Systems).
246
F. Cambier
In the physical process, we produced the nanopowder from micropowder precursors. We do not introduce any reactive gases (oxygen, nitrogen, methane) into the torch. We can therefore produce nanopowders from any of the metals in Mendeleeff s table starting from the same metal in micropowder form. To avoid any risk of oxidation or self-burning, the collection of the metal nanopowder can be done in a neutral organic liquid. In the chemical process, we introduced the precursors into the inductive plasma torch. These can be micropowders, liquids or gases. At the same time, we can introduce into the torch a reactive gas such as oxygen, nitrogen or ammonia, methane or hydrogen. For example, if we introduce into the torch - Titanium micropowder and oxygen, we can produce titanium oxide nanopowders (TiO2), - Titanium micropowder and methane instead of oxygen, we will produce titanium carbide nanopowders (TiC), - Titanium micropowder and nitrogen instead of methane, we will produce titanium nitride nanopowders (TiN) - Titanium micropowder with argon and hydrogen, we will produce titanium metal nanopowders (Ti). Other materials such as silicon, tantalum, chromium, vanadium, etc. can be produced in the same way. An atmospheric inductive plasma torch offers quite significant advantages for the production of nanopowders for catalysts: - no electrodes (no risk of contamination due to the destruction of electrodes), - high particle residence time (complete reaction), - activation of the surface (possibility of functionalisation of the nanoparticles), - operation under an inert, reducing or oxidizing atmosphere (Ar, Ar/H2, O2, Ar/He, Air, N2, etc.), - high throughput, - easy and low-cost industrialisation. In our process, we have two lines: one dedicated to non-corrosive products, the other dedicated to corrosive materials. For the non-corrosive line, we can recycle the gases; we collect the nonreacted gases, cool them, compress them and send them back into the line at the quench zone. We can thus recycle 90 % of the gases and reduce the cost of the nanopowder end products.
Production catalysts with an inductive inductive atmospheric atmospheric plasma Production of catalysts plasma torch
247
For the corrosive line, the reactor is fully teflon-coated so we can use very aggressive products such as chlorides, ammonia or acetylene as precursors. It is possible to use silicon chloride or titanium chloride as precursor products to produce nanopowders containing silicon or titanium. An internal and an external scrubber neutralise the chloride with water (internal scrubber) and sodium hydroxide (NaOH). If we start with liquids or gases for precursors, the particle size distribution of the end product will be much better controlled. We are extremely careful to avoid environmental pollution through nanopowders: the nanopowder waste water is collected in a container and then flocculated, the waste air is filtered with absolute filters. Table 1 shows the different kinds of nanopowders we can produce with this plasma torch. It is also possible to produce nanoceramic nanopowders such as two oxides, two nitrides, etc. Table 2 shows some nanoceramics produced with a plasma torch. Table 1. Type of nanopowders produced with a plasma torch Oxide powders
Pure metal powders
Carbide powders
Nitride powders
TiO2
Ti
SiC
Si3N4
SiO2
Si
we
TiN
GeO2
Ge
TaC
A1N
B2O3
Al
TiC
BN
SnO2
Sn
CuO
Co
Cr 2 C 3 VC
MnO
Ni
NbC
MoO 3
Cu
B3C
A12O3 Sb2O3
Ag Mo
CO3O4
Fe
Bi 3 O 2 Fe2O3, FeO
248
F. Cambier Table 2. Nanocomposites produced with a plasma torch (3) and (4). Nitride-metal
TiN, TiNxC!_x-Ni, Co, Fe, Mo, AIN-Mo
Nitride-nitride
TiN-AIN, TiN-Si3N4, Si3N4-AlN
Nitride-oxide, carbide Oxide-oxide
AlN-Y2O3(CaO). Si-Al-O-N, Si3N4-Al2O3-Y2O3, Si3N4-SiC, Si3N4-SiC-Al2O3-Y2O3 Al2O3-ZrO2, ZrO2-Y2O3-NiO
Oxide-metal
Al2O3-Ni, ZrO2-Ni, Al2O3-Fe-Cu-Ni
To control the particle size, the quench gas is one of the important parameters. The greater the quench gas flow, the smaller the nanopowders. The idea is to stop the growth of the particles after the plasma reactor. We try to have germination but without any growth of the crystallite. Figure 4 shows some simulations of the influence of the quench gases on the temperature of the particles after the plasma torch [5]. We can see that the temperature drops from 4500°C at the exit of the plasma reactor to 700°C at the end of the chemical reactor. This drop is sharper when the flow of quench gas is greater. We can see in figure 5 that with a plasma quench gas of 225 slpm of argon, the drop in temperature is smoother than the drop in temperature when the quench flow of argon is 375 slpm. Another important parameter of nanopowders, in the case of catalyst applications, is the width of the particle size distribution. This can be narrow, medium or broad. The particle size distribution width depends of the thermal history and residence time of the particles in the plasma which is influenced by the speed of the particles in the reactor. A plasma torch is firstly a hydrodynamic reactor. The flow of the different gases (sheath, central and powder gases) and their method of injection (axial or swirl) have a very significant influence on the speed of the particles and, consequently, on the thermal history and the size of the particles. In the centre of the reactor, the speed of the particles is much greater than the speed of the particles close to the wall of the reactor. To get a narrow particle size distribution, we must design the plasma reactor in such a way that all the particles have the same speed. All the particles will have the same residence time in the reactor. One solution for achieving a homogeneous particle speed is to use a diaphragm to filter particles with the same speed. Figure 5 shows the influence of a diaphragm to control the width of the nanoparticle size distribution.
Production of catalysts with an inductive atmospheric plasma plasma torch
249
0.2 z(m)
Q5=225 slpm (Air)
Q5=375 slpm (Air)
Figure 4: Influence of the quench gases on the temperature of the particles (Courtesy of Tekna Plasma Systems).
Our future research concerning the plasma torch in the field of catalysts involves the production of supported catalysts and the production of bimetallic catalysts.In the case of supported catalysts, the idea is to make a homogeneous coating on micropowder (A12O3, ZrO2, SiO2, etc.) using noble metal (Ag, Au, Pt, Pd, Rh, etc.). The micropowders are introduced after the plasma and the noble metals are injected into the plasma. Figure 6 shows the principle of the process.
250
F. Cambier
Figure 5: Influence of a diaphragm on the width of particles size distribution [5]. SiO2, AI2O3, ZrO2, diamond
Ag, Au, Pt, Pd
Figure 6: supported catalysts produced with a plasma torch.
Conclusions The plasma process can produce a wide range of nanomaterials from oxides, nitrides, carbides and even pure metals. The surface of the particles can be activated, and one of the main challenges will be the functionalisation of these materials by creating nanocomposites. Those new materials offer advantages in the fields of heterogeneous catalysts. References [1] [2] [3] [4] [5]
Nanoscale Materials in Chemistry, Wiley (2001). M. I. Boulos, Pure & Appl. Chem, 57 (1985) 1321. X. L. Jiang, M. I. Boulos, Trans. Nonferrous Met. Soc. China, 11 (2001) 639. M. I. Boulos, Canadian Electrical Association, 717 U 635 (1990) 90. O. Chazot O, D. Vanden Abeele, Annals of the New York Academy of Sciences, 891 (1999)368.
243
Production of catalysts with an inductive atmospheric plasma torch Frederik Cambier CRIF ( Centre of Research for Industries), rue du Bois Saint Jean, 12, B- 4102 Seraing, Belgium Tespint s.a., Avenue Moliere, B- 1190 Brussels, Belgium
Abstract The production of different nanomaterials from oxides to nitrides and carbides is investigated through the control of the process parameters of an inductive atmospheric plasma torch. Introduction Nanopowders can be produced by different kinds of processes: physical processes (laser ablation, evaporation/condensation, etc.), mechanical processes (mechanosynthesis, high deformation at low temperature, etc.), chemical processes (sol-gel, hydrothermal, plasma synthesis). Although the synthesis of nanooxides particles is well known (sol-gel or hydrothermal processes), the synthesis of carbide or nitride nanoceramics with a plasma torch is much more complicated, the main challenges being the control of the process (hydrodynamic flows) and the control of the chemical composition of the end products after the plasma reaction. At the present time, some products such as titanium zinc and cesium nanooxides are already used for health-care applications (dry skin cream, UV-blocking cream, anti-wrinkle cream) or polishing slurries. One of the most interesting characteristics of nanopowders is the specific surface area. Figure 1 shows the total numbers of atoms and surface atoms following the number of shells. In the case of spherical iron nanocrystals, if the dimensions of the clusters are below 3 nm, we have more atoms on the surface than in the bulk of the particles [1].
244 244
Full-shell Clusters
F. F. Cambier Cambier
Toul Number o f Atoms
Surface Ato (%)
o |
Bulk Atoms
i
c o
/
5
•#
1
CD
il E o
\ 1
\
Surface Atoms
1 5
10
15
20
25
30
Particle Size(nm)
Figure 1 : Number of surface atoms according to the total number of atoms for different shells. (Source: Nanoscale Materials in Chemistry, Wiley, 2001 [1])
The aim of this publication is to show various opportunities for nanopowders produced by an atmospheric plasma inductive torch for catalyst applications. Experimental Recently, the CRIF (Centre of Research for Industries) has developed a pilot for producing nanopowders with a plasma torch; this pilot is dedicated entirely to industrial production. The plasma is the fourth state of the matter composed of positive ions, negative particles (electrons and negatives ions) and neutral particles (atoms, molecules, clusters, free radicals); the total electrical charge of the plasma is neutral. In case of finite element calculations and simulations, we can assume that the material in the plasma state has the same behaviour as a perfect gas. To create a plasma, two things are necessary: a high frequency and a high power. In our process, the plasma is produced with a high frequency 60 kW power generator working at 10 MHz. As shown in figure 2, the plasma reactor is composed of two concentric cylinders [2].
Production of of catalysts with an an inductive atmospheric plasma plasma torch
245
Powder
Central g ^ I. p
RF Electrica Supply (MHz
Magnetic oupling
Figure 2 : Plasma reactor (Courtesy of Tekna Plasma Systems).
Three gases are used: • the sheath gas (mainly a diatomic gas such as hydrogen, nitrogen or oxygen) to protect the reactor against the high temperatures, • the central gas (mainly argon) to stabilize the plasma, • the powder gas to create the nanopowders. The powder gas can be a mixture of micropowder, liquid or other precursor gases with reactive gases such as oxygen to create nanooxides, nitride or ammonia to create nanonitrides, methane/ethylene/acetylene + hydrogen to create nanocarbides. We have two possibilities (figure 3) a plasmophysieal and a plasmochemical process. der (or liquid prescursor) + carrier gas
In-flight particle melting and vaporization Nano-powder
j
A *V Plasmo VdaaESr physical process
Plasmo Chemical process _k
Reaction
Figure 3 : Principle of the physical and chemical process (Courtesy of Tekna Plasma Systems).
246
F. Cambier
In the physical process, we produced the nanopowder from micropowder precursors. We do not introduce any reactive gases (oxygen, nitrogen, methane) into the torch. We can therefore produce nanopowders from any of the metals in Mendeleeff s table starting from the same metal in micropowder form. To avoid any risk of oxidation or self-burning, the collection of the metal nanopowder can be done in a neutral organic liquid. In the chemical process, we introduced the precursors into the inductive plasma torch. These can be micropowders, liquids or gases. At the same time, we can introduce into the torch a reactive gas such as oxygen, nitrogen or ammonia, methane or hydrogen. For example, if we introduce into the torch - Titanium micropowder and oxygen, we can produce titanium oxide nanopowders (TiO2), - Titanium micropowder and methane instead of oxygen, we will produce titanium carbide nanopowders (TiC), - Titanium micropowder and nitrogen instead of methane, we will produce titanium nitride nanopowders (TiN) - Titanium micropowder with argon and hydrogen, we will produce titanium metal nanopowders (Ti). Other materials such as silicon, tantalum, chromium, vanadium, etc. can be produced in the same way. An atmospheric inductive plasma torch offers quite significant advantages for the production of nanopowders for catalysts: - no electrodes (no risk of contamination due to the destruction of electrodes), - high particle residence time (complete reaction), - activation of the surface (possibility of functionalisation of the nanoparticles), - operation under an inert, reducing or oxidizing atmosphere (Ar, Ar/H2, O2, Ar/He, Air, N2, etc.), - high throughput, - easy and low-cost industrialisation. In our process, we have two lines: one dedicated to non-corrosive products, the other dedicated to corrosive materials. For the non-corrosive line, we can recycle the gases; we collect the nonreacted gases, cool them, compress them and send them back into the line at the quench zone. We can thus recycle 90 % of the gases and reduce the cost of the nanopowder end products.
Production catalysts with an inductive inductive atmospheric atmospheric plasma Production of catalysts plasma torch
247
For the corrosive line, the reactor is fully teflon-coated so we can use very aggressive products such as chlorides, ammonia or acetylene as precursors. It is possible to use silicon chloride or titanium chloride as precursor products to produce nanopowders containing silicon or titanium. An internal and an external scrubber neutralise the chloride with water (internal scrubber) and sodium hydroxide (NaOH). If we start with liquids or gases for precursors, the particle size distribution of the end product will be much better controlled. We are extremely careful to avoid environmental pollution through nanopowders: the nanopowder waste water is collected in a container and then flocculated, the waste air is filtered with absolute filters. Table 1 shows the different kinds of nanopowders we can produce with this plasma torch. It is also possible to produce nanoceramic nanopowders such as two oxides, two nitrides, etc. Table 2 shows some nanoceramics produced with a plasma torch. Table 1. Type of nanopowders produced with a plasma torch Oxide powders
Pure metal powders
Carbide powders
Nitride powders
TiO2
Ti
SiC
Si3N4
SiO2
Si
we
TiN
GeO2
Ge
TaC
A1N
B2O3
Al
TiC
BN
SnO2
Sn
CuO
Co
Cr 2 C 3 VC
MnO
Ni
NbC
MoO 3
Cu
B3C
A12O3 Sb2O3
Ag Mo
CO3O4
Fe
Bi 3 O 2 Fe2O3, FeO
248
F. Cambier Table 2. Nanocomposites produced with a plasma torch (3) and (4). Nitride-metal
TiN, TiNxC!_x-Ni, Co, Fe, Mo, AIN-Mo
Nitride-nitride
TiN-AIN, TiN-Si3N4, Si3N4-AlN
Nitride-oxide, carbide Oxide-oxide
AlN-Y2O3(CaO). Si-Al-O-N, Si3N4-Al2O3-Y2O3, Si3N4-SiC, Si3N4-SiC-Al2O3-Y2O3 Al2O3-ZrO2, ZrO2-Y2O3-NiO
Oxide-metal
Al2O3-Ni, ZrO2-Ni, Al2O3-Fe-Cu-Ni
To control the particle size, the quench gas is one of the important parameters. The greater the quench gas flow, the smaller the nanopowders. The idea is to stop the growth of the particles after the plasma reactor. We try to have germination but without any growth of the crystallite. Figure 4 shows some simulations of the influence of the quench gases on the temperature of the particles after the plasma torch [5]. We can see that the temperature drops from 4500°C at the exit of the plasma reactor to 700°C at the end of the chemical reactor. This drop is sharper when the flow of quench gas is greater. We can see in figure 5 that with a plasma quench gas of 225 slpm of argon, the drop in temperature is smoother than the drop in temperature when the quench flow of argon is 375 slpm. Another important parameter of nanopowders, in the case of catalyst applications, is the width of the particle size distribution. This can be narrow, medium or broad. The particle size distribution width depends of the thermal history and residence time of the particles in the plasma which is influenced by the speed of the particles in the reactor. A plasma torch is firstly a hydrodynamic reactor. The flow of the different gases (sheath, central and powder gases) and their method of injection (axial or swirl) have a very significant influence on the speed of the particles and, consequently, on the thermal history and the size of the particles. In the centre of the reactor, the speed of the particles is much greater than the speed of the particles close to the wall of the reactor. To get a narrow particle size distribution, we must design the plasma reactor in such a way that all the particles have the same speed. All the particles will have the same residence time in the reactor. One solution for achieving a homogeneous particle speed is to use a diaphragm to filter particles with the same speed. Figure 5 shows the influence of a diaphragm to control the width of the nanoparticle size distribution.
Production of catalysts with an inductive atmospheric plasma plasma torch
249
0.2 z(m)
Q5=225 slpm (Air)
Q5=375 slpm (Air)
Figure 4: Influence of the quench gases on the temperature of the particles (Courtesy of Tekna Plasma Systems).
Our future research concerning the plasma torch in the field of catalysts involves the production of supported catalysts and the production of bimetallic catalysts.In the case of supported catalysts, the idea is to make a homogeneous coating on micropowder (A12O3, ZrO2, SiO2, etc.) using noble metal (Ag, Au, Pt, Pd, Rh, etc.). The micropowders are introduced after the plasma and the noble metals are injected into the plasma. Figure 6 shows the principle of the process.
250
F. Cambier
Figure 5: Influence of a diaphragm on the width of particles size distribution [5]. SiO2, AI2O3, ZrO2, diamond
Ag, Au, Pt, Pd
Figure 6: supported catalysts produced with a plasma torch.
Conclusions The plasma process can produce a wide range of nanomaterials from oxides, nitrides, carbides and even pure metals. The surface of the particles can be activated, and one of the main challenges will be the functionalisation of these materials by creating nanocomposites. Those new materials offer advantages in the fields of heterogeneous catalysts. References [1] [2] [3] [4] [5]
Nanoscale Materials in Chemistry, Wiley (2001). M. I. Boulos, Pure & Appl. Chem, 57 (1985) 1321. X. L. Jiang, M. I. Boulos, Trans. Nonferrous Met. Soc. China, 11 (2001) 639. M. I. Boulos, Canadian Electrical Association, 717 U 635 (1990) 90. O. Chazot O, D. Vanden Abeele, Annals of the New York Academy of Sciences, 891 (1999)368.