04 -catalyst preparation

HETEROGENEOUS CATALYSTS

Sergio RojasICP-CSIC

January 2013

1. WHAT IS A CATALYST? RELEVANCE IN TODAYS SOCIETY

2. RELEVANT PARAMETERS DURING THE SYNTHESIS OF

HETEROGENEOUS CATALYTS

1. PREPARATION METHODS

2. SIZE AND STRUCTURE CONTROL

3. CONCLUSIONS

Conversion can not be higher than thermodynamic value

Catalysts is a substance that

accelerates the rate of a chemical

reaction by lowering the

activation energy of the reaction

It is not consumed during the course

of the reaction

Solids (NaOH, Pt/C, V2O5)

Liquids (H2SO4)


Chemical reactions results from

collisions with a certain minimum

energy (Activation Energy)

A catalyst provides an alternative

route for the reaction with a lower

activation energy."

It does not strictly "lower the

activation energy of the reaction0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10

Num

ber o

f par

ticle

sEnergy

Maxwell Boltzman distribution

Molecules that wont react because they lack of energy

Activation Energy


Chemical reactions results from

collisions with a certain minimum

energy (Activation Energy)

A catalyst provides an alternative

route for the reaction with a lower

activation energy."

It does not strictly "lower the

activation energy of the reaction0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10

Num

ber o

f par

ticle

sEnergy

Maxwell Boltzman distribution

Molecules that wont react because they lack of energy

Activation Energy

Activation Energy with catalyst

Extra molecules to react

Catalysts are present in key industrial/society sectorssuch as production of chemicals, energy transformationand environmental processes

Most industrial process rely on the use of solid catalysts(heterogeneous catalysis) Petrochemistry Fine chemicals

85% of all chemical processes have use catalystsduring, at least, one step during their preparation.

Catalysis in industry is dominated by heterogeneous processes World catalysts sales in 2004 amounted to 15 billion US$/a (growth rate

about 5% year) The ratio of product margin divided by catalyst cost is around 100-300

Heterogeneous80%

Homogeneous17%

Biocatalysis3%

Catalysis in Industry

Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley

Heterogeneous catalysis is a surface process

Simplified mechanism heterogeneous catalysis

1. Adsorption of reactants

on to the surface of the

catalyst (active site)

2. Reaction

3. Desorption of products

Two phase process

Solid phase (catalyst)

Gas/Liquid phase

(reactants)

Heterogeneous catalysis is a surface phenomenon The performance of heterogeneous catalysts is determined

by the exposed surface area

Exposed (specific) area increases by decreasing particle size

Stabilization of particles by deposition onto supports

Solid support

Adapted from: G. Mul and J.A. Moulijn. Preparation of supported metal catalysts

What are the components of a heterogeneous catalysts?

Support; stabilize the catalytic particles Catalytic particles; (oxide, metal or sulphide) hold the active

sites Promoters; enhance the catalytic performance or structural

effects

Solid support

Extrudedcylindrical

ceramic

Low manufacturing costsRelatively high pressure dropLarge diffusion length HDS, Methanation

Not common Low-surface-area catalystsAmmonia Synthesis, Fromaldehyde

Regular shape; Most commonGood strength. CO shift Hydrogenation

Low pressure drop; Poor strength HDS

High strength; Low pressure dropSmall diffusion length. Steam Reforming

Low pressure drop; Insensitive to dust; Small diffusion length Exhaust gas cleaning

Low-surface-area catalysts Ammonia oxidationHigh Temperature reactions

Spheres

Granules

Pellets

Extrudates

Rings

Monoliths

Gauzes

Sphere Pellet

Ring Minilith Wagon wheel

Monolith

metallic

Irregulargranule

1-2 mm

Porous support body

Metal particles1-10 nm

Support particles20-50 nm

Adapted from: Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley

Macroscopic scale

10 nm

Micros(nanos)copic scale< 2 nm

Microscopic scale involves the structure of the active sites. It determines

the intrinsic activity of the catalyst

The mesoscopic scale the pore system and the sizes of the support

particles as well as catalyst particles of the active phase. It affects

intraparticle mass transfer of the catalyst

The macroscopic length scale involves the size and shape of the catalyst

body. Relevant for properties such as pressure drop, mechanical

strength and attrition resistance

Criteria for a good catalyst

Activity

Selectivity

Thermal and Mechanical Properties

Stability

Morphology

Cost

A catalytic process is the combination of a catalyst a

reactor and reaction conditions (P, T, space velocity)

The shape of the catalyst body (macroscopic scale) is

paramount to determine its performance at the

industrial level

Catalysts preparationRelevant aspects and strategies for the synthesis of solid catalysts for

heterogeneous applications

CATALYST

T, P, pH, templates, size control

agents

SupportOrganic or inorganic

Modified

PrecursorMetal salt or coordination complex

The preparation of supported catalysts aims to attach the

active phase onto the support

Impregnation, co-precipitation (controlled pH or not),

homogeneous deposition, deposition of surfactant (organic

agent) stabilized metal particles

The support is either a powder or a pre-shaped solid the

most common ones being -Al2O3, -Al2O3, SiO2, TiO2 or

carbons

Mixing solutions/solids

Equilibration or aging

Solid liquid separation

Drying Calcination activation

Precipitation is in principle a crystallization process and can occur in the bulk of the liquid or on a relatively inert surface. The support particles act as crystallization nuclei for the active site precursor

Impregnation is related to ion-exchange / adsorption processes and the interaction with the support is dominant

Coprecipitation One or more metals are precipitated

together with the support or precursor

Chemical phases dispersion surface areas, porous structure

and particle size and shape are created in a single steep

It can reach very high metal loading of up to 80%

Low solubility of hydroxides alkaline media

Careful with counter ions

nucleation crystallization

CuOx ZnOx

Zn2+

Cu2+

OH-

CuO//ZnO

Liquid mixing

Nucleation rate growth rate

High supersaturation promotes nucleation Very concentrated solutions of highly soluble precursors

Carbonates or hydroxides are intended due to their low solubility [Ni][OH]2=5.4710-6 moll-1; [Ni][CO3]=5.4710-7 moll-1

Based upon solubility constants the precipitation order of the hydroxides is as follows Fe3+, Cr3+, Cu2+, Zn2+ and Ni2+

Forward precipitation Adding the base solution to

the acid (metal containing) solution (pH increases)

Reverse precipitation Adding the metal solution to

the base (pH decreases) Simultaneous precipitation

Base and acid are added simultaneously to a base solution and pH is carefully controlled

Ce

Fe

NH4OH

343 K

pH=8.0

pH, temperature, stirring, precursors and recovery

and thermal treatments are key features in the final

material morphology, structure and performance.

Mixed Cu/Zn/Al2O3 (methanol synthesis) are usually

prepared by coprecipitation from nitrate precursors

If pH = 7.0 very active catalyst is obtained as compared to

the solid obtained at pH < 6

Impregnation is the simplest method to preparing supported catalysts.

(Water) solution containing the metal precursors is contacted with a porous support Dry impregnation (pore volume impregnation) the exact

amount of liquid to fill the pore volume of the support is used vs Wet impregnation the amount of liquid is only controlled by the solubility of the metal precursor

Electrostatic forces control the adsorption mechanism Depending of the process conditions different profiles

of the active phase are obtained

The Point of Zero Charge is the

pH at which the surface is

electrically neutral

The surface has OH groups PZC is the pH where the surface overall is electrically neutral

The catalyst precursor in the

solution becomes fixed to the

support by different means;

reaction, exchange with surface

OH groups or by adsorption

The support has -OH groups

depending on the thermal and

chemical history

Si OOH

Si O SiOHOH

Protonated surface pH< PZC Deprotonated surface pH > PZC

The charge of the surface hydroxyl groups varies with the pH

Electrostatic forces will lead to the preferential adsorption of

anions or cations onto the charged surface

Si OO

Si O SiOO

Si OOH2

+

Si O SiOH2

+OH2

+

pH>pzcpH

OH2+

OH

O-

[PtCl6]2-

[(NH3)4Pt]2+

pHZPC

The maximum loading obtained is a monolayer

In reality it is much lower than a monolayer 1

complex/nm2 vs 8 OH/nm2 for alumina

CAREFUL: Solutions that are mild acid or basic do not

contain sufficient proton cations/hydroxyl anions to

protonate/deprotonate the surface so the pH of the

solution reaches the ZPC of the support

Support PZC ComplexMoO3 < 1 CationsNb2O5 2-2.5 CationsSiO2 4 CationsOxidized carbon 2-4 CationsTiO2 4-6 Cations or anionsCeO2 7 Cations or anionsZrO2 8 Cations or anionsCo3O4 7-9 Cations or anionsAl2O3 8.5 Anions or cationsCarbon black 8-10 Anions


The distribution of the solute is governed by the balance between diffusion of solute into the pores and adsorption onto the support. Concentration, viscosity and

contact time

Drying. Elimination of solvent. Precipitation of ions. Fast drying lead better dispersions by creating higher supersaturation


Uniform distribution. Weakly interacting precursors + mild drying

Egg-shell: Strong adsorption during impregnation. Viscous solution. Slow drying regime.

Egg-yolk. Fast drying regime. Preferential adsorption of other species (citric acid)

Deposition-precipitation (DP). DP is possible by the presence of the support which

provides nucleation sites for the metal precursor after addition of a nucleation agent (base)

It is important to control base addition to avoid concentration gradients during precipitation

Urea is an optimum precipitating agent since it gently varies the pH leading to marginal concentration gradients

Controlling atomic arrangement, size and shape

Size control (metal dispersion) by thermal treatments

Incipient wetness impregnation

Dry at room temperature for 12 h

Ru(NO)(NO3)3

-Al2O3

H2

523 K / 1 h

773 K / 1 h

873 K / 1 h

923 K / 1 h

973 K / 1 h

973 K / 2 h

973 K / 3 h

4Ru

5Ru7Ru

8Ru12Ru16Ru

23Ru

1.5 wt.% Ru

0 5 10 15 20 250

10

20

30

Freq

uenc

y

Rusize(nm)

0

50

100

150

0

100

200

4Ru

8Ru

12Ru

2.00.3 nm

6.01.9 nm

12.04.0 nm0 5 10 15 20 25

0.1

0.2

TOF C

O(s

1)

Rusize(nm)

Size determines catalytic performances

TOF for CO dissociation vs size

microemulsion

0.00 0.25 0.50 0.75 1.00

0.00

0.25

0.50

0.75

1.000.00

0.25

0.50

0.75

1.00

Orgnico

Agu

a

Surfactante

H2O

Orgnico

Particle size

Metal precursor

oil

Surfactant/Oil

Surfactant

Temperature

Ratio0

Rh0

RhCl3

H2O

RhCl3

H2N2

AuHCl4

H2O

HAuCl4H2PtCl6

PtAu

H2PtCl6

H2N2 /NaBH4

H2O

Fe(NO3)3

Fe(NO3)3

NaOH/NH4OH

Metallic particles Bimetallic particles xides

Fe[Oy(OH)x]

Catal.

H2O %

Org.%

Surf.%

oa s/ob nm

RhA 9,6 71,4 19,0 9,18 0,27 n.d.

RhB 1,8 85,4 12,8 2,46 0,15 5

RhC 2,6 84,7 12,7 3,69 0,15 10

RhD 0,7 78,4 20,9 0,53 0,27 22

RhE 2,6 76,9 20,5 2,26 0,27 30

3wt% Rh/Al2O3

Size control of Rh particles by using MEMCareful control of water/surfacant ratio determines the size of the

micelles

0 10 20 30 40 50 60 70 80 90 100

110hcp

101hcp 102hcp002hcp

100hcp

222fcc220

fcc

200fcc331fcc

PR6

PR4

PR2

PR1

2 ()

Pt1

111fcc PtRu/C from MEM Positive effect on methanol

electrooxidation

Alloyed Pt-Ru (DRX)

Preferential growht directions (HRTEM)

H2PtCl6 + Ru(NO)(NO3)3Berol O50; isooctano

Polyol method. Easy method to prepare reduced

particles by taking advantage of the reducing power

of the polyol which acts both as solvent and reducing

agent.

CH2OH-CH2OH(l) + 14 OH- 2 CO32- + 10 H2O + 10 e- 0 = 1,65 V

OHF

RTEE log9212.0

Narrow distribution of nanosized Pt particles Does it work with PtSn?

Pt particles in the range 1-5 nm can be obtained

The presence of protecting agents or the support in the reaction medium impedes agglomeration of primary particles

pH of 11 and higher are needed to reduce Ru, Mo or other metals

SnCl2H2O

H2PtCl6EG/NaOH

Carbon

The polyol method does not work for all metallic combinations

Temperature Cell constant TXRFPt/Sn

190C a = 3,937 14

140C a = 3,916 42

140C /5%H2O

a = 3,916 42

aPt3Sn=4,01 Pt/Sn=3

Sn is loss during the synthesis !

45

H2PtCl6SnCl2H2OH2O/HClCarbon

2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0

SnCl

Inte

nsid

ad /

u.a.

Energa / keV

Pt EDS

1

46

Thermal treatments

EDSEDS

2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0

SnCl

Inte

nsid

ad /

u.a.

Energa / keV

Pt

EDS32

1

Sn??

SnClx is volatile during thermal treatments Sn(OH)Cl remains in the solid

SnClx species are volatile at ca. 160-200 C

They can be hydrolized in water to Sn(OH)Cl

47

Cl2HClH2O Cl2

SnClx

PtImp

SnCl3- + H2O Sn(OH)Cl+ HCl + Cl-

48

EDS

20 30 40 50 60 70 80 90fc

c 220

fcc 2

20

fcc 3

11

fcc 2

00

fcc 1

11

u.a.

2 Grados

C00

2

Fresch 1

DRXPt3Sn PtSnx

Pt/Sn=3

Tratamiento con agua

Repeated wasing in water until pH = 7

S. Garca-Rodrguez y col. J. Power Sources 195 (2010) 5564

Encapsulating metal precursors with in dendrimers renders precursors for the formation of very small particles (few atoms)

It is possible to produce alloys and core@shellstructures

Simultaneous reduction of Pt&Ru

precursors

0123456789

3.53.02.52.01.51.00.50.0012345678

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Position / nm

EDX

cou

nts

(Ru

L

1) / a

.u.

/

EDX

cou

nts

(Pt M

1)

/ a.u

.

1.-Reduction of Pt2.-Addition and Ru of Ru

It is possible to control the structure, shape and metal

content when preparing heterogeneous catalysts

Reaction conditions strongly affect the final catalyst

and as a consequence its final performance

There are many aspect yet to be rationalized

Nucleation by addition of adequate agents

Above critical supersaturation nucleation commences

Below critical supersaturation aggregation of particles occurs

The larger the area the more nuclei, the smaller the particle

In microemulsion growing is constrained within the micelles

H. B nemman et al and electrocatalysis at nanoparticles surfaces; A.Wieckowski (Ed)

Dekker, 2003

Nucleacin

Ncleo estable irreversible

Interaccintomos

metlicosInteraccinMetal-ion

M. Lade et al, Colloids and Surf. A 163 (2000) 3

04 -catalyst preparation

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