Investigation of the kinetic properties for the forward and reverse WGS reaction by energetic analysis
Introduction
The high efficiency of the water gas shift (WGS) reaction (CO+H2→CO2+H2) in the production of hydrogen, it has been widely used in the industry [1]. Copper/zinc oxide is frequently used to catalyze this reaction, since this catalyst can be operated at relatively low temperature (473–523 K) [2], [3]. Interestingly, this catalyst is also used for methanol synthesis, where the water gas shift reaction is thought to play an important role in the mechanism [2], [3]. Although numerous studies of the froward water gas shift reaction (named as FWGS) and the reverse water gas shift reaction (CO2+H2→CO+H2O, named as RWGS) over Cu/ZnO catalyst have been published, such as those about the kinetics [4], the mechanism [5], [6], the support effect [7] and the alkali metal promotion effects [8], problems still remain, for example, why copper is the best catalyst for the FWGS and RWGS? Why FWGS and RWGS are the structural sensitivity reactions over the Cu-based catalyst? In addition, it is also unknown the nature of the FWGS and RWGS reactions are easily poisoned by sulphur.
The water gas shift reaction has been studied over the high-surface-area catalyst containing Cu and ZnO, as well as over a Cu single crystal model catalyst, which has a very well controlled surface cleanness and geometric structure. Those studies showed that the kinetics over pure, single-crystal Cu (111) and Cu (110) are very similar to the kinetics over high-surface-area Cu/ZnO, when compared on a ‘per Cu surface atom’ basis, and that the activity increases with increasing Cu-surface-area [9], [10]. This indicates that metallic Cu provides the active site for catalysis. In the present work, we using the single-crystal metal surfaces such as Cu (111), Cu (100), Cu (110) as well as the sulphur modified Cu (111) surface to model the catalysts for FWGS reaction and RWGS reaction, and then calculating the activation energies of each elementary step based on the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) approach [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. At last, by comparing the energetic difference of elementary reactions involved in the FWGS reaction and RWGS reaction over different catalysts to explain the above kinetic properties.
Section snippets
Reaction mechanism for FWGS reaction and RWGS reaction
The mechanism of the water gas shift reaction is not yet completely certain. Some authors support a ‘formate mechanism’, whereby surface hydroxyls (OH(s)) produced from the dissociation of adsorbed H2O combine with adsorbed CO (CO(s)) to produce a surface formate intermediate (HCOO(s)), which then decomposes to H2 and CO2. Other authors favor a ‘surface redox’ mechanism, whereby H2O dissociatively adsorbs to produce oxygen adatoms (O(s)) and H2, following by the well-known reaction of CO(s)
Calculation approach of activation barrier
In general, the activation energy of an elementary step can be obtained by a strictly theoretical method like ab initio quantum chemistry method, but it is almost impossible for the complex surface reaction since the expensive computing time required. To avoid this problem, a general and effective approach for analyzing the mechanism of heterogeneously catalyzed reactions has been developed by Shustorovich [11], [13], and lately reformulated without the assumptions of Morse potential by Sellers
Results and discussion
As we know, the chemisorption heat depends on the surface coverage. In this work, we assume that the chemisorption heat of sulphur varies with the change of surface coverage, that is, set at θ≤0.25 and set when θ=0.50 for fcc (100) (where the Q0 is the atom chemisorption heat of sulphur at zero coverage). According to the scheme of UBI-QEP method, the chemisorption heats of various species involved in the WGS reaction (‘surface redox’) at different sulphur coverage were
Conclusion
By the above analysis, we can gain the following results:
- (1)
For the FWGS reaction, the rate-controlling step is the dissociation of adsorbed H2O, and the dissociation of adsorbed CO2 is the rate-controlling step for the RWGS reaction.
- (2)
Both of the FWGS reaction and the RWGS reaction are structural sensitivity reactions over Cu-based catalyst, and the RWGS reaction is much more structural sensitivity than that of the FWGS reaction.
- (3)
The activity of FWGS and RWGS reaction over Cu (111) is higher than
Acknowledgements
This work was supported by National Natural Science Foundation (No.20273034)), the foundation of State Key Laboratory of Coal conversion and the State Key Laboratory of C1 Chemistry and Technology (Taiyuan University of Technology) of China.
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