ArticleEffect of Pd doping on CH4 reactivity over Co3O4 catalysts from density-functional theory calculations
Graphical Abstract
DFT calculations show that Pd-doped Co3O4 catalysts are more reactive for CH4 dissociation than pure Co3O4 catalysts because of the much lower energy barrier and thus the higher estimated reaction rates.
Introduction
Methane (CH4) is a major component of natural gas and is an important feedstock for the chemical and energy industries. Because of its wide availability, it has been studied extensively for efficient conversion into fuels and chemicals, such as syngas, ethylene, methanol, and aromatics [1, 2, 3, 4, 5]. Because of the significant greenhouse effect of CH4, which is stronger than CO2, there is significant interest in the efficient combustion of CH4 at low concentrations to reduce its emission and to minimize its environmental impact. However, under normal conditions, CH4 is very stable as a result of the high CH3–H bond energy, which makes the splitting of the first C–H bond in CH4 the most difficult step in CH4 activation. The energy barrier to this splitting is often used as an important gauge for the activity of CH4-activation catalysts [6, 7].
Noble metals and transition-metal oxides have been studied extensively to catalyze CH4 combustion [8, 9, 10, 11, 12, 13, 14]. Palladium (Pd) has been shown to have an excellent catalytic activity for CH4 combustion. Wang et al. [15] loaded 0.2% Pd on Si(5.2)–Al–O(1100)–550 and measured temperatures at 10%, 50%, and 90% CH4 conversions (T10, T50, and T90) of 335, 410, and 485 °C, respectively. Jorgensen et al. [16] predicted energy barriers for the first step of CH4 splitting on Pd(001) and (011) surfaces of 0.79 eV and 0.99 eV, where weak CH4 physisorption was ignored. Neurock, Iglesia, and co-workers [8] predicted energy barriers for the first CH4 dissociation step over Pd(111), PdO(101), and PdO(100) surfaces of 0.75, 0.64, and 1.35 eV, respectively. However, the high cost of Pd may prevent its industrial application for CH4 combustion. Cobalt oxide (Co3O4) nanocatalysts have been studied extensively for their excellent catalytic activity and because of the much lower price of Co than Pd, and CH4 combustion was initiated at very low temperatures (Tlight-off) of 200, 175, and 175 °C for the nanoparticle, nanorod, and nanoplate Co3O4 catalysts, respectively [17]. The lowest energy barriers of the first C–H bond splitting of CH4 were calculated to be 0.98 and 0.89 eV over the Co3O4(001) and (011) surfaces [7]. Pd-doped Co3O4 (Pd/Co3O4) catalysts have been prepared to increase the catalytic activity and to control the cost of the catalytic material [19, 20]. Li et al. [21] investigated CH4 combustion over Pd/Co3O4 catalysts, and when doped with Pd by 1% to 10%, Tlight-off, T50, and T100 were reduced significantly from 250 to 170, 360 to 246, and 480 to 300 °C, respectively. Li and co-workers [21] prepared Pd-doped Co3O4 catalysts of different shapes, such as nanocubes, nanobelts, and nanosheets, with 1%, 2%, and 5% Pd. Their reactivities were found to increase as nanocube < nanobelt < nanosheet with the same percentage of Pd dopant, and the reactivities also increased with increasing percentage of Pd dopant. Therefore, the Pd dopant has a positive effect on the catalytic activity of the Co3O4 catalyst for CH4 combustion.
The Pd/Co3O4 catalysts can be expected to have more complex structures than the Pd and Co3O4 catalysts because of the presence of palladium oxide (PdOx) and Co3O4 surfaces and structures in-between. Because CH4 activation has been investigated by first-principles calculations over different Pd, PdO, and Co3O4 surfaces, we have examined the reactivity of the Co3O4(001) surface with an atomic Pd dopant, and this approach has been used to study the effect of dopant on different catalysts [22, 23]. The Co3O4(001) surface was the main nanoparticle- or nanocube-Co3O4-catalyst exposed surface. Two surface terminations were considered, and one was predicted to be more stable under O2-rich conditions [17, 24]. Energy barriers for the first C–H bond dissociation of CH4 were calculated and compared with those calculated previously for the Pd, PdO, and Co3O4 surfaces. Results from the first-principles calculations were used to predict the CH4 reaction rates at different temperatures with different Pd dopant concentrations. These rates were compared with available experimental measurements to determine the effect of Pd dopant on the Co3O4 catalyst reactivity.
Section snippets
Computational
Periodic density-functional theory (DFT) calculations were carried out with the PBE exchange-correlation functional [25] and the PAW pseudopotentials [26, 27], as implemented in the Vienna ab initio program (VASP) [28, 29]. DFT calculations with an exchange-correlation functional such as PBE in the generalized gradient approximation (GGA) treat strongly correlated systems with difficulty [30, 31, 32, 33, 34]. For this reason, we carried out PBE+U calculations with the approach developed by
CH4 dissociation on Pd/Co3O4(001) surfaces
As shown in Fig. 1, the p(1×1) (001)–A surface model is composed of Co8O12, and its surface consists of two penta-coordinated Coo5c, two tri-coordinated O2o,1t, and two tri-coordinated O3o, where o and t in the superscripts and subscripts denote the octahedral and tetrahedral Co ions, respectively. The p(1×1) Pd/(001)–A surface was arrived at by replacing one Coo5c with Pd, which lead to an atomic composition of PdCo7O12, and its surface consists of a penta-coordinated Coo5c, a
Conclusions
First-principles calculations were used to investigate the dissociation of CH4 over Pd-doped Co3O4(001) surfaces using two slab models with different terminations. CH4 physisorption over the Pd-doped Co3O4(001) surfaces was predicted to be very weak, similar to that over pure Co3O4(001) surfaces. CH4 dissociation was predicted to be kinetically preferable at the α Pd–O pair site over the Pd/Co3O4(001)–A surface, which is also consistent with that over the pure Co3O4(001) surface. Pd-doped Co3O4
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This work was supported by the National Natural Science Foundation of China (21473233, 21403277) and the Energy Technologies Institute LLP, UK.
Published 5 May 2017