Dissociative adsorption of H2O and CO2 on the clean and O-pre-covered high index Ru surfaces: Corrugated Ru(11−21) and stepped Ru(20−21) surfaces
Graphical abstract
Introduction
The interaction of H2O and CO2 with metal surfaces is of great interest in a number of relevant heterogeneous catalysis, such as water-splitting reaction [H2O → H2 + O2] [1,2], fuel cell process [H2 + O2 → H2O] [3], water-gas shift reaction [H2O + CO → CO2 + H2] [4], CO2 reduction reaction [5], [6], [7], automotive exhaust catalysts [CO + O → CO2] [8,9] and Fischer-Tropsch synthesis (FTS) [10,11], where surface oxygen removal via H2O or CO2 formation is a key elementary step in the overall reaction mechanism [12], [13], [14], [15], [16]. Among all FTS active catalysts, Ru-based catalysts are most active as well as tolerant to water or other oxygen-containing species [17,18].
Extensive experimental and theoretical studies have been carried out to investigate H2O and CO2 dissociative adsorption on the Ru(0001) surface. At low H2O coverage, H2O monomer prefers adsorption at the top site with the molecular plane parallel to the Ru(0001) surface [19,20], and temperature programmed desorption (TPD) shows a single desorption at about 190 K [21]. The investigation of the extreme ultraviolet induced H2O dissociation on Ru(0001) has shown that the main reaction products left on the surface are partially dissociated water (OH+H), and the primary dissociation pathway is identified to be H2O → OH + H [22]. This agrees with the theoretical observations [23], i.e., for H2O dissociation on clean Ru(0001) [H2O → OH+H → O+2H], the first step has lower barrier than the second step (0.75 vs. 0.80 eV). In addition, low coverage pre-adsorbed O can promote H2O adsorption [24] and dissociation [25,26]. On O pre-covered Ru(0001), the calculated barrier for H2O dissociation [H2O + O → 2OH] is 0.62 eV [23]. At 2 × 10−12 mbar, the computed desorption temperature of H2O formed from surface OH disproportionation is 230 K, close to the experimentally detected 210 K [25]. CO2 adsorption and dissociation on the Ru(0001) surface was investigated by reflection-absorption infrared spectroscopy (RAIRS) and TPD at 85 K [27]. RAIRS measurements show that CO2 dissociates into CO over time on Ru(0001), and the dissociation of CO2 appears to be irreversible, in agreement with the calculated results [23,28], i.e., the dissociation of adsorbed CO2 [CO2 → CO + O] needs moderate barrier (0.55 eV) and is highly exothermic (1.47 eV). In turn, CO2 formation from surface O removal by CO is difficult on Ru(0001), and metallic Ru is the least active platinum group metal in CO oxidation [29], [30], [31]. By using TPD, low energy electron diffraction (LEED) and auger electron spectroscopy (AES), Wu et al. [32], did not find CO2 formation and desorption from CO adsorbed on 0.5 ML O-pre-covered Ru(0001) in the temperature range of 100–600 K. Bonn et al. [8], showed that heating of CO and O co-adsorbed Ru(0001) leads only to CO desorption, indicating that CO desorption (or adsorption) energy is lower than CO oxidation barrier. Theoretical studies showed that surface O removal by CO on Ru(0001) has barrier of 1.70 eV [23], in close agreement with the experimentally detected 1.80±0.15 eV [8], and higher than CO desorption energy (1.55 eV). Furthermore, theoretical studies on the Co(0001) [7] and Ru(0001) [23] surfaces have shown that high O and OH pre-coverage do not significantly affect the energetics of H2O and CO2 dissociation.
To our knowledge, however, H2O and CO2 dissociative adsorption on high index Ru surfaces is not well understood. Based on the Wulff construction and surface energies, the optimized morphology of hexagonal close-packed (hcp) Ru has a dihedral-like shape populated mainly by open facets, including Ru(11−21) and Ru(20−21) facets [33,34]. The corrugated Ru(11−21) surface is most open among the Ru surfaces and the high index Ru(20−21) facet is stepped with terrace-step structure. Thus, the interaction of H2O and CO2 with Ru open surfaces is of importance in understanding relevant Ru particle catalytic processes on an atomic scale. In turn, studying surface oxygen removal via H2O or CO2 formation on stepped Ru surfaces is also significant, due to the importance of stepped surfaces for CO direct dissociation [35], [36], [37], [38]. The micro-kinetics simulation data on the corrugated Ru(11−21) surface showed that under the conditions of the optimum C20+ yield, the FTS reaction rate is limited by O removal [16,39]. Accordingly, investigating surface oxygen removal on stepped Ru surfaces with the respect of the planar Ru(0001) surface is appealing and crucial for understanding the specific and general mechanisms of Ru-based FTS.
Here, we investigated H2O and CO2 dissociative adsorption on the clean and O-pre-covered Ru(11−21) and Ru(20−21) surfaces by means of density functional theory (DFT) computations. Compared with the previous prediction on the planar Ru(0001) surface, the adsorption energies of bent CO2, H2O dissociation capability and surface oxygen removal via H2O or CO2 formation capability on these Ru surfaces follow a sequence of Ru(11−21) > (20−21) > (0001).
Section snippets
Method
Spin-polarized density functional theory calculations were carried out using the revised Perdew-Burke-Ernzerhof (RPBE) [40,41] functional and projector augmented wave (PAW) potential [42,43], as implemented in the Vienna Ab Initio Simulation Package (VASP) [44,45]. An energy cut-off of 400 eV and a second-order Methfessel-Paxton [46] electron smearing with σ=0.2 eV were used to ensure accurate energy calculations with errors less than 1 meV per atom. Transition state structures were estimated
Adsorption of surface species
At first we computed the adsorption and co-adsorption of the related surface species on the kinked Ru(11−21) and stepped Ru(20−21) surfaces. The adsorption energies and configurations of the surface species are given in the supplementary information (Figs. S1 and S2), and the most stable adsorption configurations with the corresponding adsorption energies are shown in Fig. 2. On the kinked Ru(11−21) surface, H, O, OH and CO prefer the bridge site. On the stepped Ru(20−21) surface, H and O
Conclusion
Systematic RPBE DFT computation has been performed on the corrugated Ru(11−21) and stepped Ru(20−21) surfaces for understanding mechanisms of H2O and CO2 dissociative adsorption. Along with the previous computations on the planar Ru(0001) surface, we have a comprehensive understanding into the surface structure dependent mechanisms of H2O and CO2 dissociation, as well as surface oxygen removal via H and CO on Ru catalysts, which is important for understanding the overall FTS mechanism,
CRediT authorship contribution statement
Peng Zhao: Investigation, Writing – original draft. Yurong He: Investigation, Writing – original draft. Xiaodong Wen: Conceptualization, Supervision, Writing – review & editing. Haijun Jiao: Conceptualization, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA 21000000 and No. XDA21020100) and Synfuels China, Co. Ltd. We acknowledge general financial support from the BMBF and the state of Mecklenburg-Vorpommern, Germany.
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