Toluene total oxidation over CuO-CeO 2 /Al 2 O 3 catalyst: nature and role of oxygen species

Transcription

1 Toluene total oxidation over CuO-CeO 2 /Al 2 O 3 catalyst: nature and role of oxygen species Vladimir V. Galvita, Unmesh Menon, Konstantinos Alexopoulos, Guy B. Marin Laboratory for Chemical Technology, Ghent University 1

2 Overview Introduction Experimental methodology Results Conclusions 2

3 Introduction M 2+ From J. Haber et al., J. Catal. 190 (2000) 320 C 7 H 8 +9O 2 7CO 2 +4H 2 O Catalyst used: (11.6wt.%)CuO-(6.34wt.%)CeO 2 /γ-al 2 O 3 3

4 Overview Introduction Experimental methodology Results Conclusions 4

5 TAP reactor system 5

6 Types of experiments Input pulse Single pulse experiment C Output 7 H 8 responses C7 H 8 Size ~ molecules CO 2 - Knudsen diffusion regime - No change in the state of the catalyst t/s Multi pulse experiment t/s Input pulse Size ~ molecules/pulse C 7 H Output 8 CO 2 responses - Molecular diffusion regime - Deliberate change in the state of the catalyst t/s t/s Input pulse Alternating pulse experiment O 2 t C 7 H 8 Output O 2 C 7 H 8 responses CO 2 CO 2 t/s t/s -Two pulses of different gases - By varying the time delay of the pulses, lifetime of intermediates can be determined 6

7 Overview Introduction Experimental methodology Results Conclusions 7

8 Toluene total oxidation C 7 H 8 C 7 H 8 +O 2 (1:9) Oxidation of Toluene occurs by Mars and van Krevelen mechanism Conversion of toluene to CO 2 is more when feed has oxygen In presence of oxygen the catalyst can be oxidized by lattice and/or surface oxygen species 8

9 Effect of partial pressure of di-oxygen Activation energy C 7 H 8 /O 2 ratio (kj/mol) 1:0 68 1:1 59 1:5 51 1: O atoms required for one C 7 H 8 -Catalyst reduces with toluene pulse -Denies easy supply of oxygen -oxygen has to diffuse from the bulk -higher activation energy -Catalyst reduces with toluene pulse -oxygen supplied from the feed -lower activation energy -triggers reaction 9

10 Reaction order of di-oxygen Competition between surface oxygen and lattice oxygen more di-oxygen takes place in reaction mobility of O atoms in the catalyst is higher, lattice oxygen takes place mainly in reaction 10

11 Reaction rate vs. degree of reduction Catalyst reduced by toluene C 7 H 8 /Ar Catalyst reduced by CO C 7 H 8 /O 2 /Ar (1:9) 723K 823K 923K 823K 723K 923K Catalyst cannot be deeply reduced by toluene In the presence of di-oxygen in the feed, catalyst active at higher degrees of reduction Reaction is very sensitive to the degree of reduction of the catalyst 11

12 Role of adsorbed oxygen species First pulse of SPE C 7 H 8 / O 2 =(1:9) 16 O 2 pretreated catalyst T=823 K Only 10% of water formed contained H 2 O Mainly lattice oxygen participates in reaction C 7 H 8 + χ 16 O L χ + χh 2 16 O + C 7 H 8-2 χ C 7 H 8 + χ O s χh 2 O + C 7 H 8-2 χ C 7 H 8 + χ O L χ + χh 2 O + C 7 H 8-2χ + O 2 O L + O s O s + O L + 16 O Lb 16 O L + b 12

13 Role of adsorbed oxygen species First pulse of SPE C 7 H 8 / O 2 /Ar 16 O 2 pretreated catalyst T=823 K O starts occupying the vacant lattice sites O in CO 2 is significantly higher than that in water C 7 H 8-2χ + (-χ) 16 O L (-χ) + 7C 16 O 2 + (4-χ)H 2 16 O C 7 H 8-2χ + (-χ) O L (-χ) + 7C O 2 + (4-χ)H 2 O C 7 H 8-2χ + (-χ) O s 7C O 2 + (4-χ)H 2 O Reaction occurs through Mars and van Krevelen mechanism. O 2 is mainly required for reoxidation of the reduced surface metal centers. Separation of contribution of surface oxygen species is not elucidated. C 7 H 8-2χ + (9-χ/2) 16 O L + (9-χ/2) O L (-χ) + 7C 16 O O + (2-χ/2)H 2 16 O + (2-χ/2)H 2 O C 7 H 8-2χ + (9-χ/2) 16 O L + (9-χ/2) O s (9-χ/2) + 7C 16 O O + (2-χ/2)H 2 16 O + (2-χ/2)H 2 O 13

14 Life time of active surface oxygen Alternating Pulse experiment pump probe C 7 H 8 Time delay of C 7 H 8 /Ar Pulse A = 0.1s B = 1s C = 2s D = 5s E = 10s O 2 CO 2 -Time delay between toluene and oxygen pulse was kept constant -Time delay between oxygen and toluene pulse was varied -CO 2 response collected during the C 7 H 8 pulse varied in size until time delay of C 7 H 8 pulse =5s CO 2 formed during the O 2 pulse constant amount at all time delays of toluene Lifetime of active surface oxygen species ~ 1s 14

15 Overview Introduction Experimental methodology Results Conclusions 15

16 Conclusions 1. Total oxidation of toluene is carried out by the redox, Mars-van Krevelen mechanism. 2. Toluene total oxidation is very sensitive to the degree of reduction of the catalyst. 3. The activity of the copper oxide catalyst in the presence of gas phase oxygen is determined by weakly bound oxygen forms. Life time of this oxygen under reaction conditions is close to 1s. 4. Oxygen isotope exchange experiment shows that surface oxygen could participate in the reaction. 16

17 Acknowledgements GOA (Concerted research action) by Ghent University The Long Term Structural Methusalem Funding by the Flemish Government 17

18 Thank you for your attention!