程序升温分析技术.ppt

1、Chapter 6 Temperature Programmed Techniques (Part 2),Temperature Programmed Reduction (TPR),Temperature-programmed reduction (TPR) is a technique for the characterization of solid materials and is often used in the field of heterogeneous catalysis to find the most efficient reduction conditions, det

2、ermine the reducibility of a catalyst and the degree of oxidaton of the active phase.,H2-TPR,characterisation of oxidic catalysts and other reducible catalysts qualitative information on oxidation state quantitative kinetic data optimization of catalyst pretreatment,During a TPR experiment, the cata

3、lyst under investigation is placed in a fixed-bed reactor and exposed to a reducing mixture that continuously perfuses the catalyst bed, while the temperature is increased according to a linear temperature programme. The difference between the inlet and outlet concentration of the gas mixture is mea

4、sured as function of time using a thermal conductivity detector.,The resulting TPR profile contains qualitative information on the oxidation state of the reducible species present and, in that sense, it is a fingerprint. The technique is intrinsically quantitative and the information obtained is of

5、a kinetic nature and, as a consequence, directly correlated with catalytic behavior. On the other hand, information on the structure of the species present is less straightforward than for instance that obtained by spectroscopic methods.,See next slide for Detail Process,Typical process description,

6、A simple container (U-tube) is filled with a solid or catalyst. This sample vessel is positioned in a furnace with temperature control equipment. A thermocouple is placed in the solid for temperature measurement. To remove the present air the container is filled with an inert gas (nitrogen, argon).

7、Flow controllers are used to add hydrogen (for example, 5-10Vol% hydrogen in nitrogen). The composition of the gaseous mixture is measured at the exit of the sample container with appropriate detectors (thermal conductivity detector, mass spectrometer). Now, the sample in the oven is heated up on pr

8、edefined values. Heating values are usually between 1 K/min and 20K/min. If a reduction takes place at a certain temperature, hydrogen is consumed which is recorded by the detector. In practice the production of water is a more accurate way of measuring the reduction. This is due to the potential fo

9、r varying hydrogen concentrations at the inlet, so the decrease in this number may not be precise, however as the starting concentration of water will be zero, any increase can be measured more accurately.,H,2,/,Ar,reactor,molsieve,TCD,FID,Theory of TPR,Thermodynamics of Reduction,The reduction of a

10、 metal oxide MOn by H2 is described by the equation,Thermodynamics predicts under which conditions a catalyst can be reduced: As with every reaction, the reduction will proceed when the change in Gibbs free energy,G, has a negative value. The following Equation shows how G depends on pressures and t

11、emperature:,Thus, G is negative when the ratio p(H2O)/p(H2) is smaller than the equilibrium value and the efficiency with which water is removed from the reactor becomes the decisive factor.,Reduction Mechanisms,In TPR one follows the degree of reduction of the catalyst as a function of time, while

12、the temperature increases at a linear rate. According to the theoretical treatment of the TPR process as given by Hurst and by Wimmers, the rate expression for the reduction reaction:,The way to use Eq. (2-7) is to record a series of TPR patterns at different heating rates, and plot the left-hand si

13、de of (2-7) against 1/Tmax. The result should give a straight line with slope Ered/R.,Applications,The temperatures needed for the complete reduction of a catalyst,TPR and TPO patterns of silica-supported iron and rhodium catalysts,The TPR patterns of the freshly prepared catalysts contain two peaks

14、, the first peak in the TPR pattern of the fresh Rh/SiO2 catalyst is associated with the reduction of Rh-O bonds and the second with Rh-Cl.,The area under a TPR or TPO curve represents the total hydrogen consumption, and is commonly expressed in moles of H2 consumed per mole of metal atoms (H2/M). T

15、he ratios of almost 1.5 for rhodium in the right panel of Figure indicate that rhodium was present as Rh2O3. For iron, the H2/M ratios are significantly lower, indicating that this metal is only partially reduced.,The TPR of supported bimetallic catalysts often reveals whether the two metals are in

16、contact, or not.,The TPR pattern of the 1:1 FeRh/SiO2 catalyst in the above Figure shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less-noble iron. This provides evidence that rho

17、dium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows: As soon as rhodium becomes metallic it dissociates hydrogen; atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously.,Calculate the activation energies of

18、reduction,Attempts to calculate the activation energies of reduction by means of Eq. (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process.,In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affect

19、ed by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts, but are more easily met in unsupported particles.,all catalyst particles are equivalent,Activation energies fo

20、r the reduction can be derived by measuring a series of TPR patterns at different heating rates and using Eq. (2-7). A plot of ln(/Tmax2) versus 1/Tmax yields a straight line corresponding to an activation energy of 111 kJ mol-1 for the reduction of Fe3O4 to Fe in dry H2. The addition of water incre

21、ases the activation energy for this step to 172 kJ mol-1.,Two TPR patterns for the reduction of the Fe2O3 particles: one measured with dry hydrogen as the reductant; and the other with a few percent of water added to the hydrogen,Metal support interaction,Temperature Programmed Oxidation (TPO),Tempe

22、rature Programmed Oxidation (TPO) is performed in an entirely analogous manner to TPR wherein diluted oxygen (strong oxidizing agent)/ carbon dioxide (mild oxidizing agent) is substituted for the reducing gas. TPO can be particularly useful for investigating different forms of carbons, such as graph

23、ite, amorphous carbon and carbon nanotubes. The most common application of TPO is useful for characterising the nature of deposits containing carbon (coke) in the case of a deactivated catalyst.,Similarities and differences between TPO and TPR,Reducing gas mixture for TPR experiment was 5% H2/N2,Oxi

24、dizing gas mixture for TPO experiment was 5% O2/ He,apparatus,operational procedure,Similar,Figure shows the TPO profile for a coked Pt/alumina catalyst. The volume of oxygen consumed is used to determine the quantity of carbon initially present on the catalyst; the position of the signal shows us w

25、hether or not this concerns a coke that can be easily eliminated, in the case of consumption at low temperature, or a more refractory coke, if the combustion peak occurs at high temperature. In conjunction with a mass spectrometer, it is possible to determine the nature of the compounds in the efflu

26、ents, which may provide information relating to the mechanisms of catalyst deactivation by coking.,Temperature Programmed Sulfidation (TPS),Temperature Programmed Sulfidation (TPS) similar to TPR,TPS investigate sulfidation behavior of the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) ca

27、talysts,TPS of MoO3/Al2O3 catalysts in a mixture of H2S and H2,Catalysts used for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2, TPS

28、 may investigate sulfidation mechanism of these catalyst.,Molecular Scheme of Sulphiding,Temperature Programmed Reaction Spectroscopy (TPRS),Temperature Programmed Surface Reaction (TPSR),TPRS, the species which desorb are the result of chemical conversions of the initially adsorbed species on the s

29、urface of catalysts, the usual detection method applied in TPRS is mass spectrometry.,The typical experimental system will be evacuated to total pressures below 10-5 Pa, with a relatively high pumping rate used in conjunction with a temperature rise rate of the order of 10Ks-1. under these condition

30、s the record of the mass spectrometer signal corresponding to a particular product as a function of time (temperature) will show a maximum or peak. Product evolution begins generally at a temperature which is just sufficient to allow the relevant surface reactions to proceed with a significant rate.

31、 Thereafter, with the temperature rising steadily, the rate of evolution passes through a maximum value, before it becomes deleted substantially. The rate of evolution of a particular product from a surface, when this rate is controlled by rate of the generating reaction on the surface, is usually e

32、xpressed by first- or second order kinetic equations.,n is the number of molecules of reactant adsorbed per area of surface V1 and v2 are kinetic frequency factors Ed and Ed are the respective activation energies of the reaction steps controlling the rates on the surface.,When a chemical conversion

33、takes place on a surface prior to easy desorption of the products, basic points can be drawn as follows.,The values of TM indicate relative rates of reaction on the surface, with higher values of TM corresponding to higher activation energies. Integrated areas under the recorded profile for a specie

34、s are proportional to the amount of that product formed. Different species which evolve with the same value of TM and matching peak profiles originate from a common rate determining reaction on the surface. There is an empirical proportionality between TM and Ed which transaltes in a reasonable appr

35、oximation as TM/K3.8Ed/kJ mol-1.,Temperature-Programmed Reaction Spectroscopy in UHV,TPSR is a very useful tool for investigating which reactions can take place when several species are present on a surface. If desorption follows instantaneously, its peak can be used to derive an activation energy f

36、or the rate-determining step that precedes it.,Figure shows the record obtained from an experiment in which formic acid (HCOOH) is adsorbed initially at a temperature of 200K on to a copper (110) surface.,The experiments are performed by dosing small amounts of CO and NO on the surface at temperatur

37、es below 200 K, after which the surface is heated linearly in time, and all relevant gases are monitored with a mass spectrometer.,Temperature-programmed reaction between CO and NO on the (100) (left) and (111) (right) surfaces of rhodium.,The CO+NO reaction is obviously structure-sensitive. On Rh(1

38、00), the first signal that appears is that of CO2 at 300 K. This implies that at this temperature the NO has already dissociated into N and O atoms. When most of the oxygen is consumed, CO desorption competes with CO2 formation in the temperature range between 400 and 525 K. Above 650 K the N-atoms

39、recombine and desorb as N2, leaving a Rh(100) surface that is partly covered by O-atoms. Hence, Rh(100) is quite effective in dissociating NO and oxidizing CO to CO2, but desorption of N2 is slow, as it occurs at high temperature. Rh(111) is much less reactive than Rh(100). Although it dissociates a

40、ll NO, most of the CO desorbs unreacted and only a small part is oxidized to CO2. A favorable point here is that N2 desorbs at significantly lower temperature than on the Rh(100) surface.,Important information on reaction mechanisms and on the influence of promoters,Figure illustrates how the reacti

41、vity of adsorbed surface species on a real catalyst can be measured with TPRS.,The surface reaction between adsorbed carbon monoxide and hydrogen to methane over rhodium catalysts occurs at lower temperatures in the presence of a vanadium oxide promoter, which is known to enhance the rate of CO diss

42、ociation.,In this figure, the reactivity of adsorbed CO towards H2 on a reduced Rh catalyst is compared with that of CO on a vanadium-promoted Rh catalyst. The reaction sequence, in a simplified form, is thought to be as follows: where denotes an empty site at the surface. Due to the promoting effect of vanadium, the chemisorbed CO molecules react at a somewhat lower te