**1. Introduction**

Carbon monoxide (CO) is one of the major air pollutants mainly originating from the incomplete combustion of fossil fuels (e.g., coal and oil) or from industrial production. It not only affects the atmospheric chemistry and climate but also the health of human beings and animals [1]. Different approaches have been developed to reduce CO emissions, including adsorption [2], CO methanation [3,4], and catalytic oxidation [5–9]. Among them, the catalytic oxidation of CO has proven to be one of the most effective techniques for removing this pollutant. In this process, the catalyst preparation is a key step. In the past century, much research has been devoted to the development of CO oxidation catalysts with high activity and high selectivity, including Pt-group-metal (PGMs: Pt, Pd, Ir, Rh, Ru) catalysts [9–11], gold catalysts [12–14], transition metal oxides (Fe2O<sup>3</sup> [15], MnO<sup>x</sup> [16,17], CuO [18], V2O<sup>5</sup> [19], CeO<sup>2</sup> [20], Co3O<sup>4</sup> [21], etc.), metal coordination polymers [2], and metal composite oxides [8] with spinel or perovskite structure catalysts. The gold catalysts and a few transition metal oxide catalysts (such as Co3O4) commonly show a higher performance in CO oxidation at low temperature compared to the PGM ones, but the two former types of catalysts suffer from the easy deactivation in the presence of sulfur and moisture. The PGM catalysts, particularly platinum, have been investigated for nearly a century since Langmuir's pioneering work [22], which showed high activities for CO oxidation in the temperature range of 150 to 250 ◦C with high resistance to sintering and water tolerance [23,24]. In the past several decades, three-way catalysts (TWCs) have been demonstrated as an effective approach to purify vehicle exhaust, which can remove 99% of CO emissions [25,26]. However, the pollutants emitted from separable means of transport are generally low compared to the image of the industrial chimney. Coke oven smoke is a big source of CO emission, in which the CO concentration is even more than 2000 mg/m<sup>3</sup> . However, the coexistence of SO<sup>2</sup> and H2O in the flue gas with CO may reduce the active sites and change the catalyst chemical structure, leading to the diminishing of the catalytic

**Citation:** Zhang, T.; Qiu, W.; Zhu, H.; Ding, X.; Wu, R.; He, H. Promotion Effect of the Keggin Structure on the Sulfur and Water Resistance of Pt/CeTi Catalysts for CO Oxidation. *Catalysts* **2022**, *12*, 4. https://doi.org/10.3390/catal12010004

Academic Editors: Florica Papa, Anca Vasile and Gianina Dobrescu

Received: 25 November 2021 Accepted: 16 December 2021 Published: 22 December 2021

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activity of CO oxidation due to the absorption and transformation of the SO<sup>2</sup> and H2O molecules on the catalyst surface. Therefore, developing a catalyst with high sulfur and water resistance to achieve high-performance CO oxidation, rendering it useful for specific industrial applications, is highly desirable. It was demonstrated that the introduction of a helper component [27] and protective shell [28] in the catalyst, as well as using an optimal catalyst preparation method [29], were beneficial to the improvement of CO oxidation activity and sulfur resistance. Very recently, Jiang [30] investigated the CO oxidation on a Pt single-atom catalyst supported by graphene with a single carbon vacancy (Pt-SG) and double carbon vacancy (Pt-DG) by using first-principles calculations, and they found that Pt-DG possessed a higher sulfur and water resistance due to the fact that carbon divacancy makes Pt less attractive toward SO<sup>2</sup> and H2O molecules compared to Pt-SG, revealing the effect of the support structure to catalyst performance.

Heteropolyacids (HPAs) are useful molecular metal oxide acids, which have served as catalysts for a number of processes due to their good acidity and high-performance oxidation–reduction [31]. Early research results of Golodov [32] and Zhizhina [33,34] showed that HPAs could oxidize CO with H2O in the presence of O<sup>2</sup> and Pd or Pt. The presence of HPAs with a Keggin structure could also significantly improve the CO tolerance of a Pt/C catalyst in PEMFCs [35]. Recently, Yoshida [36] reported that the Keggin-type polyoxometalate-supported gold catalyst exhibited a high catalytic activity for CO oxidation at low temperature and extremely high stability. In previous work, we found that the V2O5-MoO3/TiO<sup>2</sup> catalyst with the Keggin structure had more surface Brönsted and Lewis acid sites, which was beneficial to improve significantly the activity and SO<sup>2</sup> resistance in the NH3-SCR reaction [37]. The above results inspired us to investigate the promotion of the Keggin structure to the sulfur and water resistance of Pt/CeTi catalysts for CO oxidation. Herein, three Pt catalysts using cerium titanium composite oxide (CeTi), ammonium molybdophosphate with Keggin structure-modified CeTi (Keg-CeTi), and molybdophosphate without Keggin structure-modified CeTi (MoP-CeTi) as supports were prepared by an impregnation method under similar conditions. We observed that the Pt/Keg-CeTi catalyst showed a higher SO<sup>2</sup> and H2O tolerance in CO oxidation compared to the Pt/CeTi and Pt/MoP-CeTi catalysts, revealing the remarkable promotion effect of the Keggin structure.

#### **2. Results and Discussion**
