Development of novel nano-hydroxyapatite doped with silver as effective catalysts for carbon monoxide oxidation

Highlights

• Ag-hydroxyapatite nanorods catalysts were synthetized by microwave hydrothermal method.

• Ag addition did not change the crystalline structure and morphology of hydroxyapatite phase.

• Catalytic activity of CO oxidation increases with low Ag additions at high temperature.

• Catalysts showed thermal stability at 700 °C without pre-treatment in the cyclic tests.

Abstract

A series of novel silver-containing catalysts were synthesized, characterized by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy, N₂ adsorption-desorption isotherms, thermogravimetric analysis and then tested for the oxidation of carbon monoxide (CO). The microwave-hydrothermal method was a useful synthesis pathway to structurally incorporate low amounts of silver (2.5–5.0 atomic percentage) to hydroxyapatite (HA) lattice, preserving the crystalline structure of the HA. Specific surface area and particle size (diameter) of the obtained materials were about 52–55 m²/g and 19–47 nm, respectively. Silver-catalysts showed higher catalytic activities than the unmodified HA sample at temperatures between 600 and 850 °C. The best catalytic results were achieved at around 700–800 °C with silver-containing HA samples. However, due to the significant decrease in the crystallinity of the materials exposed to 800 °C; then, 700 °C was established as the best thermal condition for producing CO₂ and preserving the HA crystalline structure.

Furthermore, cyclic tests demonstrated that silver-containing HA catalysts perform CO oxidation for 3 h through several consecutive catalytic tests at 700 °C without losing neither their activity nor their structural properties, evidencing their high thermal stability under the CO-O₂ atmosphere. Thus, the highest reaction rate values (r_CO2) were obtained with HA containing 2.5% of silver at 700 °C during the five cycles performed, positioning it as a promissory catalyst with high activity during the CO oxidation at high temperatures. The proposed reaction mechanism was established using these silver-doped materials. This work constitutes the first assessment to add low amounts of silver to increase the activity of this kind of catalytic materials for the CO oxidation reaction.

Introduction

Solid materials used in heterogeneous catalysis must have four well known physicochemical characteristics: a high catalytic activity, selectivity through desired products, high thermal stability, and excellent accessibility to active sites [1]. Therefore, the development of solid catalysts is a subject of growing interest in the manufacture of chemical products, refineries, energy, and environmental protection [2]. For example, two greenhouse gases, methane (CH₄) and carbon dioxide (CO₂) can be transformed into more valuable products such as syngas, a mixture of hydrogen (H₂) and carbon monoxide (CO) by using various reforming techniques and heterogeneous catalysts with different chemical compositions and structures [3]. However, in that case, the CO is a non-desired compound highly harmful to human health because of its toxicity. It has been described the occurrence of encephalopathy dysfunction and parkinsonian syndrome, among others, in people exposed to CO due to the formation of carboxyhemoglobin in the blood [4]. The values of ΔH = −230 kJ/mol and ΔG = −257 kJ/mol for the CO oxidation reaction i.e. CO + ¹/₂O₂ = CO₂ at 25 °C, suggest that the exothermic reaction can occurs in the terms of Gibbs free energy and the criteria for spontaneity [5]. However, from a kinetic point of view these gases do not react significantly even if the process takes place at high temperature without a catalyst. One of the functions of the catalyst is to concentrate the CO on its surface, which allows the oxidation reaction to proceed by lowering the activation energy, thus, the rate constant is greatly increased relative to the uncatalyzed reaction [6]. Therefore the catalytic oxidation of CO proves to be one of the most effective techniques for removing this pollutant more efficiently. In this sense, many studies have been reported wherein a wide variety of catalysts for this reaction has been extensively studied under different experimental conditions [7]. Thus, CO can be oxidized in CO₂ and stored to reuse it in a further industrial process, such as dry methane reforming (DMR) as a strategy to reduce the emission of this hazardous gas to the environment.

For this purpose, the design of new catalytic materials with a particular chemical composition, nanometric size, porosity, morphology, crystallographic structure, and high surface area has aroused great interest due to its diverse and extensive catalytic applications [8]. In this line, well-characterized and reproducible solids are preferred. Furthermore, these materials must not lose their crystalline structure nor chemical composition when they are exposed to operational conditions [9]. Looking forward to achieve the CO oxidation in a gas-solid system, some active catalysts doped with different metal oxides have been reported (see Table 1). High specific surface area values were achieved with the materials derived from Ce-BTC and Cu-BTC synthesized by Zhang et al. [10] as well as Cui et al. [11], whose α-Fe₂O₃ based materials derived from MIL-100 achieved 100% of CO oxidation at low temperatures. Furthermore, UiO-66 derived materials with Cu and Pd were obtained by Wang et al. [12] and Bi et al. [13]; these materials showed specific surface areas between 14 and 21 m²/g. The reported MOFs derived materials achieved the complete CO oxidation in a range of 100–300 °C as well as the amorphous Mn-MIL-100 [14], synthesized by Zhang et al. These results showed the advantages of using materials exhibiting high specific surface areas, and therefore a high number of available active sites to promote the desired reaction. On the other hand, several alkaline materials and metal-doped ceramics have been used in a high-temperature range. In these cases, excellent conversion values were reached despite the low specific surface area values of the catalysts (between 1.0 and 1.9 m²/g). Calcium and nickel-containing materials were used by Cruz-Hernandez et al. [15] to reduce the oxidation temperature of CO from 750 to 350 °C. Also, Na₂ZrO₃ studied by Alcántar-Vázquez et al. [16], achieved the 100% CO oxidation above 450 °C. Similar results were obtained for the case of NaFeO₂ and LiFeO₂ synthesized by Gómez-García [17]. Results with lithium zirconate [18] and lithium cuprate [19] also showed that CO oxidation and CO₂ capture processes occur simultaneously in those materials. Domínguez et al. [20] and Guo et al. studied Au-hydroxyapatite in the nanometric range, a complete CO oxidation was obtained at room temperature and above 200 °C.

HA is one of the most common forms of calcium phosphate; considering the stoichiometric formula of Ca₁₀(PO₄)₆(OH)₂, the compound has a Ca/P molar ratio of 1.6667. This material exhibits various biological, mechanical, and catalytic properties, and it has been tailor-made depending on the desired application using different synthesis methods [21], [22]. Furthermore, the ionic radius of their constituent elements allows a considerable degree of transfer or loss of ions within its crystalline structure. Therefore, this chemical property induces to obtain non-stoichiometric compounds with different Ca/P molar ratios between 1.5 and 1.7 [23]. Also, it is essential to mention that the HA structure tolerates a significant number of anionic and cationic substituents that leave the crystallographic structure unchanged [21], [24], [25]. Thus, the general chemical formula for HA structures is Me₁₀(XO₄)₆(Y)₂, wherein Me is a monovalent, divalent or trivalent cation, such as K⁺, Na⁺, Ag⁺, Ca²⁺, Fe²⁺, Mg²⁺, Sr²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Eu³⁺; XO₄ is a trivalent anion (PO₄³⁻, AsO₄³⁻, CrO₄³⁻, SiO₄⁴⁻, SO₄³⁻, MnO₄³⁻, VO₄³⁻, CO₃²⁻, and HPO₄²⁻); and finally, Y can be a monovalent or divalent anion, e.g., OH⁻, F⁻, Cl⁻, Br⁻, I⁻, S²⁻, O²⁻, and CO₃²⁻. Also, calcium-deficient HA compounds (Ca/P < 1.6667) can be obtained by the loss of Ca²⁺ ions and the charge balance would be maintained either by the incorporation of a proton into hydroxyapatite-type materials and loss of one hydroxyl group per missing cation or by the incorporation of two protons [26]. In those cases, the chemical formula for these HA materials is represented by Ca_10-n(HPO₄)_n(PO₄)_6-n(OH)_2-n(H₂O)_n. These modifications allow to improving acidic properties in HA particle surface.

On the other hand, calcium-rich HA materials (Ca/P > 1.6667) can be obtained through the mixture of stoichiometric HA and Ca(OH)₂ compounds or by the partial replacement of PO₄³⁻ ions with other anions, such as CO₃²⁻ ions. This chemical change is a feasible way to obtain HA with higher basic properties [27], [28]. Nowadays, there has been a growing interest in HA-based materials as solids supports and recyclable catalysts due to its physicochemical properties: high thermal stability [29] and good affinity for organic compounds [21]. Thus, HA materials have been used in several applications such as chemical adsorbents [30], drug delivery systems [31], coatings and chromatography [32], imaging applications [33], fuel cells [34], and adsorption for radioactive waste and hazardous metals [35], among others. The study of the catalytic properties of metal-doped HA has been the subject of numerous publications focusing on heterogeneous catalysis for solid-liquid systems (See Table 1, Supplementary material). In contrast, only a few hydroxyapatite materials, doped with Au or Cu, have been used in the oxidation of CO, for example, in the case of the studies by Domínguez [20] and Guo [36] et al. the results showed that

Moreover, silver has been used for many applications as an active phase, e.g. Wang et al. synthesized hollow ZSM-5 zeolite encapsulating Ag nanoparticles for selective catalytic oxidation of ammonia [37]. The results showed a 100% conversion to nitrogen in the temperature range of 100–150 °C. Zhang et al. obtained a complete oxidation of toluene between 300 and 350 °C, by using Ag nanoparticles supported on UiO-66 derivative [38]. Flower-like Ag/ZnO system was studied by Zhang et al. The effects of various Ag content on the photocatalytic properties for the degradation of methylene blue under visible light irradiation were investigated [39]. The results showed that low addition of Ag caused an improvement in the photocatalytic performance. Moreover, Zhou et al. synthesized an Ag-Cu nanoalloy catalyst for the ammonia oxidation, obtaining 100% of NH₃ conversion in the low-temperature range [40]. Therefore, in the present work, silver cation was selected from others cations because the silver-doped materials described above reported favorable results in different catalytic studies. However, the use of Ag-hydroxyapatite for CO oxidation has not been reported. This work aims to analyze the use of HA nanostructures doped with low amounts of silver to obtain an active, thermally stable, and cyclable material (in a solid-gas system) for the oxidation of CO in a high-temperature range.