Kinetic analysis of Temperature Programmed Reduction R. Jude vimal Michael National Centre for Catalysis Research 31 January 2009

Heterogeneous Catalysis and Chemical kinetics  Understanding the rates of the chemical steps in catalysis is a challenge to chemists  The main aim of kinetics is to describe the rate of reaction as a function of state variables that define the chemical process.  Chemical kinetics is a. to obtain fundamental insight into reaction mechanisms b. to assist catalyst design and c. to aid reactor design, process development and optimization. Introduction

Schematic Diagram of TPR/TPD and TPO

Before the reaction starts….

When the reaction starts….

What we get from TPR? TPR, Temperature-programmed reduction  Characterization of redox properties of materials, fingerprint of sample  Temperature range of consumption of reducing agent, temperatures of rate maxima  Total consumption of reducing agent, valence states of metal atoms in zeolites and metal oxides  Interaction between metal oxide and support  Indication of alloy formation in bimetallic catalysts  Mechanism and kinetics of reduction

The important Parameters The important parameters that have to be optimized in temperature programmed analysis is  flow rate of carrier gas.  reactant gas/inert gas ratio.  catalyst sample volume/mass.  catalyst sample particle size.  geometry of the reaction vessel (catalyst sample reactor).  heating rate.  signal intensity.  system pressure.

Typical experimental conditions TPDTPRTPO Gas compositionHe or N 2 or Ar 5% H 2 /N 2 5% O 2 /N 2 Flow rate cm/min Heating rate K/min DetectorTCD or MSTCDTCD or MS Measured quantity.H2 evolved H2 consumed CO, CO 2 evolved

Classification of Solid state Processes

Nucleation model Contracting sphere model

Mathematical Models used in Evaluation of kinetic and experimental parameters in Temperature-Programmed Analysis

Kinetic analysis of temperature programmed reactions  The rate equation for the linear temperature rise(  ) is given by where A – pre-exponential factor, E –Energy of activation and f(  ) is the degree of conversion.  Kinetic triplet- Activation energy, Pre-exponential factor, f(  ) or g(  )  Stationary point method  Kissinger’s method  Flynn–Wall–Ozawa method

The kinetic study of temperature-programmed reduction of nickel oxide 1.Metal oxides are widely used in many technological applications, such as coating, catalysis, electrochemistry, optical fibers, sensors, etc. 2.For the preparation of active oxide catalysts, the partial reduction of nickel oxide under hydrogen at elevated temperatures is the effective method. 3.The NiO samples were obtained by gel-combustion method. 4.A green-colored transparent gel was obtained by drying an aqueous solution of nickel nitrate hexahydrate (Fluka, 99.5%) and citric acid (Fluka,99.5%) dissolved in a mole ratio 1.8:1. This gel was made to undergo a self-ignition by heating in air up to 300 ◦C, and an additional heating of up to 500 ◦C produced a very fine nickel oxide powders.

TPR profile of Nickel oxide in H 2 atmosphere

Influence of heating rate on temperatures of reduction

Methods used to evaluate kinetics Stationary point method :  d  /dt = f(T) where d  /dt is the velocity of process and T is the absolute temperature  maximum appears in the so-called stationary point (SP)  Tmax is the temperature at the maximal velocity of process (d  /dt)max  from the slope of the dependence between ln[  (d  /dT) max ] and 1/T max, it is possible to determine the value of the apparent activation energy Ea of the investigated process.  by this method we obviously can obtain only one value of activation energy, the one that can be calculated comparing (d  /dt)max at different temperatures and which corresponds to  max (the degree of conversion at T = T max ).

Kissinger’s Method.. Kissinger’s method:  The apparent activation energy can be determined by Kissinger method without a precise knowledge of the reaction mechanism, using the following equation:  where  is the heating rate, T max and  max are the absolute temperature and the degree of conversion at the maximum mass-loss rate (d  /dt)max, respectively. A is the pre-exponential factor and n is the reaction order.  when n(1 −  max ) n−1 ≈ 1 (for n = 1), kissinger gound the following expression

Kissinger’s method Contd….  from the plot of ln(  /T 2 max ) versus 1/T max and fitting to a straight line the pre- exponential factor A and the apparent activation energy Ea can be calculated from the intercept and slope respectively

Kinetic parameters obtained from Stationary point (SP) and Kissinger’s method(K)

Friedman method.. where the subscript, i, denotes that the corresponding quantity is evaluated at a specific value of the degree of conversion,  i. The apparent activation energy, Ea,  i, at this specific degree of conversion, is evaluated from the slope of the plot ln(  (d  /dT)  I ) versus 1/T  i, known in the literature as the isoconversional line. In this approach, Ea is expressed as a function of degree of conversion (  )

Results and discussion Two different kinetic models have been proposed for the Metal oxides reduction  Nucleation Model  Interface-controlled model The following conclusions can be drawn from results  reduction occurs at the interface between NiO and previously reduced Ni  there is an “autocatalytic” effect  there is an induction (i.e., nucleation) period that depends on the nature of the sample and temperature  added water reduces the reduction rate and increases the induction period

Conclusion  it can be concluded with high reliability that the reduction process occurs at the interface between NiO and previously reduced Ni without any well-ordered intermediate phase.  The constant value of Ea is a consequence of the constant activation energy for crystal growing of Ni crystallites which were induced in nucleation stage of the process whose duration period depends on the heating rate of the system.  The increase in the reduction rate without changing of Ea value is interrelated with existence of “autocatalytic effect”, i.e., it is a consequence of being proportional between the reduction rate and the area size of boundary phase of interaction.  The decrease in reduction rate and increase of Ea values were probably caused by mutually overlapping of Ni growing crystals which leads to the decrease in area size of boundary phase and the rate of process