Unfortunately, it is sometimes difficult to compare results obtained in different laboratories or reported in the literature. There exist no optimum experimental parameters for conducting TPR experiments and parameters such as the rate of heating, the composition of the reducing mixture, gas flow rate, and particle size can all greatly affect the rate of reduction. This Altamira Note examines the effect of some experimental parameters in the resulting TPR signal.
Monti and Baiker1 derived an equation relating the temperature at maximum, Tm, to linear heating rate and hydrogen concentration for a first order process, i.e.: POPRAWIC ULAMKI!!!
[H2] Ea Ea ln(Tm2) = + ln Eq. 1 rT RTm RA
where:
Tm is the temperature at maximum signal;
[H2] is the average hydrogen concentration;
rT is the linear heating rate;
Ea is the activation energy of reduction;
R is the gas constant; and
A is a pre-exponential factor.
This equation predicts a decrease in Tm with increasing hydrogen concentration and with decreasing heating rate. It further predicts that the observed temperature maximum is independent of flow rate, a prediction which is not borne by experiments.
The usefulness of this equation is in comparing data obtained under various conditions. For example, Gentry and coworkers2 in a study of CuO determined Ea to be approximately equal to 67 KJ/mole. Using a flow rate of 20 ml/min, a heating rate of 6.5 K/min, and H2 partial pressure of 0.1, they observed Tm = 280ºC. Using their results and equation (1) it is thus possible to predict Tm for other experimental conditions. Figure 2 shows how the predicted Tm would vary for CuO for various hydrogen concentration and linear heating rates according to equation (1).
The effects of flow rate are not as easy to predict. Intuitively, one would expect a lowering in Tm with increasing flow rate. This is indeed in agreement with literature reports. Monti and Baiker found a lowering in Tm for supported NiO of 15°C as the total flow rate was changed from 30 ml/min to 60 ml/min1. In the TPR of CuO, using 5% H2 and a heating rate of 20 K/min, a change in flow rate from 30 ml/min to 80 ml/min resulted in a lowering of Tm by 15°C. A good rule of thumb is to expect a lowering in Tm of about 10-20'C with a doubling of flow rates.
Since TPR is a bulk process, not all the particle is exposed to the reducing gas at the same time and thus a dependence of Tm on the size of the particle is expected. The prediction of this dependence is complicated by the mechanism by which reduction occurs. Lemaitre3 has examined this dependence for various types of reduction mechanisms, of which perhaps the most important ones from a catalytic viewpoint are: phase-boundary-controlled reduction, typical of bulk oxides; and nucleation-controlled reduction, typical of supported metals.
Interestingly, the predicted behavior of Tm with particle size is different depending on the reduction mechanism. For bulk oxides, an increase in Tm is predicted with increasing particle size. The opposite is predicted for supported metals. These various factors and their effect on the ultimate TPR profile are summarized in Figure 3. They should all be taken into consideration when attempting to compare data taken in different laboratories or under different conditions.
References
1) D.A.M. Monti and A. Baiker, J. Catal. 83, 323 (1983)
2) S.J. Gentry, N.S. Hurst, and A. Jons, J. Chem. Soc; Faraday I, 75, 1688 (1979)
3) J.L. Lemaitre, in "Characterization of Heterogeneous Catalysts", (F. Delannay, ed.) Marcel-Dekker, Inc.
