Catalyst characterization

Characterization of heterogeneous catalyst refers to the determination of its physical and chemical characteristics, which are responsible for its performance in a reaction. Characteristics of catalysts include:

• Chemical composition of the bulk and surface of the solids
• Surface  area and porosity (micro, meso and macro)
• Bulk solid structure, phase composition, crystallite size
• Surface morphology
• Surface chemical properties such as:
• location and oxidation state of active metals
• acid-base property
• reducible – oxidizable property
• Aggregate properties such as aggregate or particle size, density, mechanical strength and attrition resistance
• Catalytic properties: activity, selectivity, stability

Objectives of characterizations

The primary objective of catalyst characterization is to understand the relationship among physical, chemical and catalytic properties. For this purpose, the physical and chemical properties are determined by various characterization techniques and related to its activity and selectivity. This is essential for design and process optimization. The characterization is also done to monitor the changes in physical and chemical properties of the catalyst during preparation, activation and reaction stages for better understanding and quality control. Determination of the extent of deactivation of catalysts during the reaction process is also important. Characterization of used catalysts can help to determine the causes of deactivation and minimize it. It also helps to design procedures for catalysts regeneration.

Characterization Techniques

In this section some of the characterization techniques that are most commonly used will be discussed. These techniques are summarized below.

1. Structural analysis
1. Surface area
• widely accepted BET ( Brunauer, Emmet and Teller) method used for analyzing multilayer physisorption isotherms of inert gases to determine the surface area
2. pore analysis by
• BJH method
• mercury intrusion method
3. X-Ray Diffraction (XRD):
• can detect crystalline materials having crystal domains greater than 3-5 nm.
• characterization of bulk crystal structure and chemical phase composition.
2. Chemisorption technique
• determines dispersion of metal in catalysts
• determination  of surface metal area
3. Thermal analysis
1. temperature programmed reduction (TPR):
• measures the rate of reduction of active metals as  function of temperature.
•  can be correlated with activity of catalysts
2. temperature programmed desorption (TPD):
• measurement of rate of desorption of adsorbed molecules as function of temperature
•  mainly used to of study acid – base property of catalysts
3.  Thermo Gravimetric Analysis (TGA):
• measurement of weight loss (or gain) as a function of temperature in a controlled gaseous atmosphere;
•  process associated with mass change can be detected and analyzed
4. Differential Thermal Analysis (DTA):
• monitoring the temperature difference between sample and reference
• process associated with latent heat of transition can be detected and analyzed
4. Spectroscopic techniques
1. Infrared spectroscopy
• identify compounds and investigate sample composition
• Study of structure and  bonds
2. Raman spectroscopy:
• study of oxidation state and interaction of metal oxides
5. Microscopic technique
1. Scanning electron microscopy (SEM):
• image the topography of solid surface
• resolution better than 5 nm.
2. transmission electron microscopy (TEM):
• determines the micro –texture and micro structure
• resolution better than 0.2 nm