Elimination of NO pollutant in semi-enclosed spaces over sodium-promoted cobalt oxyhydroxide (CoOOH) by oxidation and adsorption mechanism
Graphical abstract
Introduction
Nitrogen oxides (NOx) continue to be one of the most severe air pollutants to the environment and human health [[1], [2], [3], [4], [5], [6], [7], [8]]. In some semi-enclosed spaces such as underground road tunnels and indoor parking areas, NOx emitted from automobiles is accumulated because of the poor ventilation of air in these spaces. As a result, tens of ppm of NOx (predominately NO) can be present in the air, which greatly exceeds NOx limits for ambient air quality standards (for example 40 μg/m3 of Chinese standard, GB3095-2012). Long-term exposure can cause severe harm to human health. Commercial technologies for NOx elimination, such as three-way catalysis (TWC), selective catalytic reduction (SCR) and NOx storage reduction (NSR) [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]] cannot be adopted to eliminate low-concentration NOx in air at ambient temperature. These techniques are not only costly, but also only efficient in treating higher concentrations of NOx and at operational temperatures typically higher than 200 ℃. The elimination of low-concentration NOx from semi-enclosed spaces in an economically practical way, remains a grand challenge.
Two possible approaches are described as follows: Passing air that contains NOx through basic solutions or solid absorbers, in which NOx is either reacted or adsorbed, appears to be a viable solution. However, since NO2 is in most cases much more reactive than NO, it is essential to firstly (catalytically) oxidize NO to NO2, preferably at ambient temperature and high space velocity prior to the adsorption stage. Nevertheless, most early studies are mainly aimed at high-concentration (hundreds of ppm) NO based on actual working condition [[23], [24], [25]]. Until recently a few groups discovered that some early transition metal oxides catalyzed low-concentration NO oxidation at ambient temperature quite efficiently. For example, Shi and coworkers systematically investigated manganese oxide and MnOx-containing binary oxide catalysts for ambient temperature NO oxidation [26,27]. Because of a few restraints, e.g., quick nitrate poisoning and low thermal stability, practical applications of these catalysts have not yet been realized. Recently, Liu [28] and our group discovered that CrOx and Cr-X (X = Zr, Co, Fe, Ni) bimetallic oxides were more efficient in catalytic oxidation of low-concentration NO at ambient temperature [29,30]. Unfortunately, the high toxicity of chromium oxides may preclude their practical applications. In addition, (doped) activated carbon materials have also been found to oxidize NO to NO2 at ambient temperature. For example, Wang et al. [31] discovered that activated carbon nanofibers displayed good efficiency in both oxidizing and trapping low-concentration NO (20 ppm). Particularly, when such materials were modified with transition metal oxides (e.g., MnO2), NO oxidation capacity could be further enhanced [32].
An alternative approach is to use solid materials to adsorb (trap) NO directly. In this case, NO oxidation to NO2 is no longer a prerequisite, even though some of such materials may still catalyze NO oxidation to NO2 as part of the trapping process. In the past few years, researchers have discovered that Pd-exchanged zeolite materials were efficient passive NOx adsorbers (PNA) which could be used to trap NOx from diesel engine exhausts during the cold-start [[33], [34], [35]]. However, these materials are likely too costly for use in removing low-concentration NOx from air. For this latter application, desirable materials should have the following features: low cost, low toxicity, high stability and reusability. Recently, we studied Co-containing NO adsorbers generated by pyrolysis of Co-containing MOF materials [36]. This study demonstrates that cobalt nanoparticles embedded in a carbon matrix are particularly efficient, much more than Ni or Zn particles, in ambient temperature NO abatement.
In this work, we continue our effort to develop Co-containing materials for ambient temperature NO abatement. Herein we report the synthesis and application of CoOOH. This material has found applications as an alternative material for CO sensor [37,38], for developing electrodes of supercapacitors [39], or in photoelectrochemical water oxidation [40]. As a potential ambient temperature NO trapping material, it owns the advantages of low cost and environmentally benign (i.e., low toxicity). Moreover, the high valence state of cobalt in CoOOH (+3 charge) catches our eyes for the strong oxidizing property. Even though this material lacks high-temperature thermal stability, as will be shown below, a simple washing treatment of Na2S2O8 aqueous solution effectively regenerates it after NO removal. Therefore, reusability is not an issue for this material.
Section snippets
Material preparation
CoOOH was prepared by oxidizing a Co(OH)2 precursor with three oxidants: Na2S2O8, O2 and H2O2. The Co(OH)2 precursor was synthesized using NaOH and CoSO4 solutions. Following which, the oxidants were introduced to oxidize Co(OH)2 to CoOOH. The chemical reactions involved in these processes are described as follows:
In a typical synthesis using Na2S2O8 as the oxidant,
Performance for NO removal
Fig. 1a displays ambient temperature NO removal performance of Co-NS, Co-HO and Co-OG. Co-NS maintains 100 % NO removal ratio for ∼6 h, and in the next ∼3 h it gradually decreases to ∼50 %. The removal ratio of this material is comparable to that of the nitrogen-doped Co/C materials developed by us previously [36]. The influence of drying temperature on NO elimination performance is shown in Fig. S1. As the drying temperature increases, NO elimination performance of Co-NS dramatically
Discussion
It is reported that Co-based materials show outstanding performance for NO adsorption and oxidation [64,65]. Unfortunately, cobalt oxide (mostly Co3O4) is not suitable for the low-concentration NO (tens of ppm) elimination at ambient temperature. Herein, picked oxyhydroxide (CoOOH) is synthesized and used as an adsorbent/catalyst under this special working condition. Three CoOOH materials, prepared using Na2S2O8, H2O2 and O2 as oxidant respectively, show a tremendous difference in the
Conclusions
Different CoOOH materials can be prepared using different oxidants to oxidize a Co(OH)2 precipitate. Particularly when Na2S2O8 is used, the CoOOH material generated contains a higher concentration of residual Na and lower crystallinity. This material shows much higher capacity of NO elimination at ambient temperature than the CoOOH materials synthesized using H2O2 and O2 as oxidants. NO stores in CoOOH in the form of surface nitrate (to a much less extent nitrite). Upon storage saturation, the
CRediT authorship contribution statement
Bo Lin: Conceptualization, Methodology, Investigation, Writing - original draft. Aiyong Wang: Visualization, Writing - original draft. Yanglong Guo: Supervision, Writing - review & editing. Yuanqing Ding: Validation. Wangcheng Zhan: Writing - review & editing. Li Wang: Formal analysis. Yun Guo: Resources, Funding acquisition. Feng Gao: 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.
Acknowledgements
This work was supported by the National Basic Research Program of China (2013CB933200), the National Natural Science Foundation of China (21922602, 21577034), 111 Project (B08021), and the Fundamental Research Funds for the Central Universities. Aiyong Wang acknowledges China Scholarship Council for the Joint-Training Scholarship Program with the Pacific Northwest National Laboratory (PNNL). The author from PNNL (FG) is supported by the U.S. DOE/Office of Energy Efficiency and Renewable Energy,
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These authors contributed equally to this work.