03 Catalyst Characterization

8/3/2019 03 Catalyst Characterization

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Catalyst Characterization

Hsin Chu

ProfessorDept. of Environmental Eng.National Cheng Kung University


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1. Introduction

Physical properties: pore size, surface area, andmorphology of the carrier; and the geometry and

strength of the support Chemical properties: composition, structure, and

nature of the carrier and the active catalyticcomponents

Changes during the catalysis process:deactivation


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2. Physical Properties of Catalysts

2.1 Surface Area and Pore Size of the Carrier

Surface areaPore size

Pore size distributionPore structurePore volume

The size and number of pores determine the internalsurface area. It is usually advantageous to have high

surface area (large number of small pores) to maximizethe dispersion of catalytic components.However, if the pore size is too small, diffusionalresistance becomes a problem.


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2.2 surface Area and Pore Size Measurements A standardized procedure for determining the internal surface

area of a porous material with surface area greater than 1 or 2m2/g is based on the adsorption of N

2at liquid N

2temperature

onto the internal surfaces of the carrier. Each adsorbed N2 molecule occupies an area of the surface

comparable to its cross-sectional area (16.22)By measuring the number of N2 molecules adsorbed atmonolayer coverage, one can calculate the internal surface

area. Next slide (Fig. 3.1a)

BET gas adsorptin plotFlatten part: monolayerSecond rise: multiple layers


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The Brunaure, Emmett, and Teller (BET) equation describes therelationship between N2 volume adsorbed at a given partialpressure and the volume adsorbed at monolayer coverage:

where P = partial pressure of N2P0 = saturation pressure at the experimental temperatureV = volume adsorbed at PVm= volume adsorbed at monolayer coverageC = a constant

Next slide (Fig. 3.1b)Linear form of the BET equationThe most reliable results are obtained at relative pressure (P/P0)between 0.05 and 0.3.

0 0

1 ( 1)

( ) m m

P C P

V P P V C V CP


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The same equipment can be used to determine the poresize distribution of porous materials with diameters lessthan 100, except that high relative pressures are used

for condensing N2 in the catalyst pores. Next slide (Fig. 3.1c)

Pore size distribution measurementThe procedure involves measuring the volume adsorbedin either the ascending or descending branch of the BET

plot at relative pressures close to 1.This plot shows the carrier having a significant volumeof mesopores (diameters between 20 and 500)


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Capillary condensation occurs in the pores in accordance with theKelvin equation:

where = surface tension of liquid nitrogen = contact angleV = molar volume of liquid nitrogenr = radius of the poreR = gas constantT = absolute temperatureP = measured pressure

P0 = saturation pressure The form of the Kelvin equation describes the desorption isotherm,

and it is the preferred one for calculation of pore size distribution.(desorption requires a lower pressure compared to adsorption)

0

2 cosln( )

P V

P rRT


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2.3 Pore Size by Mercury Intrusion For materials with pore diameters greater than about 30, the

mecury intrusion method is preferred.The penetration of mercury into the pores of a material is a

function of applied pressure. The Washburn equation:

where d = pore diameter, nmp = applied pressure, atm= wetting or contact angle, between the mercury and the

solid is usually 130o

= the surface tension of the mercury, 0.48 N/m Next slide (Fig. 3.2)

The mercury intrusion method: 30-106 the BET method: < 100

4 cosd

P


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2.4 Particle Size Distribution of the Carrier

Sieves of various mesh sizesReliable only for particles larger than about 40m

(#400=38 m) Light scattering, image analysis, sedimentation,

centrifugation, and volume exclusion (coulter counter)Reliable for finer particles

Laser techniques (more recently)

Next slide (Fig. 3.3)Laser-generated particle size distribution for an Al2O3carrierF: fractionU: cumulation


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2.5 Washcoat ThicknessOptical microscopy is the most commonly used method of obtainingwashcoat thickness.

2.6 Mechanical Strength of a Monolith Monoliths, particularly when used in a stacked mode (e.g., in

stationary pollution abatement) must resist crushing axially. For automobile and truck application, and for ozone abatement in

aircraft, resistance to vibration and radial strength is important.

2.7 AdhesionErosionThermal shocks (start up and shutdown)


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2.8 Location and Analysis of Species within the Catalyst

Scanning electron microscope (SEM) is equipped with anenergy dispersive analyzer (EDX) or wavelength

dispersive analyzer (WDS).The bombardment of a sample with electrons generates Xrays characteristic of the elements present.

Next slides (Fig. 3.4 a ~ Fig. 3.4 d)WDS analysis for catalysts deposited on double-layeredwashcoats(a)(b): topcoat Pt dispersed on bottom coat SiO2(c)(d): topcoat Pd dispersed on bottom coat Al2O3


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Next slide (Fig. 3.5)A electron microscope line profile of a poisoned

automobile exhaust catalystP and Zn, originating from the lubricating oil, areconcentrated on the edge of the washcoatindicating they physically deposited as aerosols.

Gaseous sulfur compounds such as SO2/SO3,penetrate deeply and more uniformly into thewashcoat.


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3. Chemical Properties

3.1 Elemental Analysis

Small amounts of promoter oxides intentionally added(often < 0.1 %) can influence catalyst activity, selectivity,

and life. Carriers are derived from raw materials, which contain

various impurities such as alkali and alkaline-earthcompounds, if used in excess, causing sintering or loss ofsurface area in Al

2O

3. When added in the proper amount,

the same impurities can enhance stability againstsinterting or, in some cases, improve selectivity.

Therefore, the quantitative procedures used to analyze thecomposition of catalysts are important.


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3.2 Thermal Gravimetric Analysis (TGA) TGA: a useful technique to follow microscopic weight

changesA few milligrams of contaminated catalyst can be loaded intoa quartz pan suspended in the microbalance. A controlled gasflow and temperature ramp is initiated and a profile of weightchange versus temperature is recorded.

Frequently, TGA units are equipped with a mass spectrometerso that the offgases from the catalyst can be measured as a

function of temperature. The weight-temperature profile is helpful in establishingprocedures for regenerating the catalyst and other processes inthe process reactor.


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3.3 Analysis by X-Ray Diffraction (XRD)

Provided a material is sufficiently crystalline to diffractX rays and is present in an amount greater than 1%, XRD

can be used for qualitative and quantitative analysis. The angles of diffraction differ for the various planes

within the crystal. Thus, every compound or element hasits own somewhat unique diffraction pattern.

3.4 Structural Analysis: structure of Al2O3 carriers Next slide (Fig. 3.6)

XRD patterns of amorphous -Al2O3and -Al2O3


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-Al2O3: the high-surface-area, lower -temperature structure-Al2O3: produced at high temperature and haslow surface area

Amorphous: Materials with crystallites smallerthan 50.A well-defined X-ray pattern will not beobtained.Need to be characterized by other techniqueslisted below.


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3.5 Dispersion or Crystallite Size of Catalytic Species

3.5.1 Chemisorption

When a structure has a definite XRD pattern, it usually has lessthan optimum activity. This is because most catalytic reactionsare favored by either amorphous materials or extremely small

crystallites. Frequently, the purpose of the preparation technique is todisperse the catalytic components to maximize theiravailability to reactants:% dispersion =When this is done effectively, only small crystals are presentand the diffraction of X rays is minimized.

Selective chemisorption can be used to measure the accessiblecatalytic component on the surface by noting the amount of gasadsorbed per unit weight of catalyst.

. 100%

.

no of catalytic sites on the surface

theoretical no of sites present


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One assumes that the catalytic surface area is proportional tothe number of active sites. A gas that will selectivelychemisorb only onto the metal and not the support is usedunder predetermined conditions.Hydrogen and CO are most commonly used as selectiveadsorbates for many supported metals.

The measurements are usually carried out in a static vacuumsystem similar to that used for BET surface areameasurements.

Next slide (Fig. 3.7a)Chemisorption isotherm (monolayer coverage, 1 H per metal

site)One can determine the catalytic surface area by multiplyingmolecules adsorbed by cross-sectional area of the site anddividing by the weight of catalyst, e.g., the cross-sectionalarea of Pt is 8.92 and Ni, 6.52.


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The static vacuum technique is time-consuming.Alternatively, a dynamic pulse technique has been usedin which a pulse of adsorbate such as H2 or CO isinjected into a stream of inert gas and passed through abed of catalyst.

Next slide (Fig. 3.7b)Pulse chemisporption profiles

The static method measures only species that are stronglyadsorbed.The dynamic method, performed under equilibriumconditions, measures strong and weakly chemisorbedspecies.Thus, static techniques usually give better dispersionresults.


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3.5.2 Transmission Electronmicroscopy (TEM)

A thin sample is subjected to a beam of electrons. Thedark spots on the positive of the detecting film

correspond to dense areas in the sample that inhibitelectron transmission. These dark spots form the outline of metal particles or

crystallites and, hence, their sizes can be determined.

Next slide (Fig. 3.8)A TEM of sintered Pt, dispersed on TiO2 (500) Following slide (Fig. 3.9)

Crystallites of Pt about 100 in size, dispersed on CeO2


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3.5.3 X-Ray Diffraction

The larger the crystals of a given component, the sharper the peaks on theXRD pattern for each crystal plane.

The scherrer equation:

Where B = the breadth at half-peak height of an XRD lineL = the size of the crystallites = X-ray wavelength = diffraction angle

k = a constant usually equal to 1 As the crystallite size increases, the line breadth B decreases. Next slide (Fig. 3.10)

XRD profile for different crystallite sizes of CeO2

cos

kBL


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3.6 Surface Composition of Catalysts

XRD and TEM measure the structure and/or chemicalcomposition of catalysts extending below the catalyticsurface.The composition of the surface is usually different from

that of the bulk. It is on these surfaces that the active sites exist and where

chemisorption, chemical reaction, and desorption takeplace.

The tools available for surface compositioncharacterization are X-ray photoelectron spectroscopy(XPS), Auger electron spectroscopy (AES), ionscattering spectroscopy (ISS), and secondary-ion massspectroscopy (SIMS).


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XPS is used more widely than the others for studing thesurface composition and oxidation states of industrial catalysts.

XPS refers to the technique of bombarding the surface with X-ray photons to produce the emission of characteristic electrons.

These are measured as a function of electron energy. Because of the low energy of the characteristic electrons, the

depth to which the analysis is made is only ~40 .The composition of this thin layer as a function of depth canbe determined by removing or sputtering away top layers andanalyzing the underlying surfaces.

This technique can provide properties including oxidationstate of the active species, interaction of a metal with an oxidecarrier, and the nature of chemisorbed poisons and otherimpurities.

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03 Catalyst Characterization

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