Thermal activation of Pd/CeO₂-SnO₂ catalysts for low-temperature CO oxidation

Highlights

• The counter precipitation method was used to synthesise composite catalysts Pd/CeO₂-SnO₂

• The calcination at 800−1000 °C results in strong enhancement of low-temperature CO oxidation

• Pd/CeO₂-SnO₂ catalysts are thermally stable up 1100 °C due to formation of nanoheterogeneous structure

• SnO₂ nanoparticles serve as stabilisers of Pd_xCe_1-xO_2-δ nanoparticles

• Pd/CeO₂-SnO₂ catalysts are characterized by high water resistance in CO oxidation

Abstract

In this work, the counter precipitation method was used to synthesise Pd/CeO₂-SnO₂ catalysts, which possess excellent low-temperature activity and high thermal stability. It was revealed that calcination of Pd/CeO₂-SnO₂ catalysts at 800−1000 °C induces significant growth of catalytic activity in CO oxidation at T<150 °C. This effect of thermal activation for Pd/CeO₂-SnO₂ catalysts was enhanced when water was admitted to the reaction mixture. In the presence of water the T₅₀ value for the Pd/CeO₂-SnO₂ catalyst calcined at 900 °C becomes 45 °C lower than for the Pd/CeO₂ catalyst. It was found that calcination of the catalysts at T<600 °C leads to the formation of solid solutions based on the fluorite and rutile structures. As the calcination temperature is raised above 600 °C, the solid solutions decompose with formation of catalytically active Pd_xCe_1-xO_2-δdispersed phase on the surface of SnO₂ nanoparticles. The formed nanoheterogeneous structure provides both high thermal stability and high water resistance of Pd/CeO₂-SnO₂ catalysts.

Introduction

Ceria-based catalysts are widely used in various type reactions for practical applications [[1], [2], [3]]. One of the most popular applications of ceria is its use in combination with platinum group metals (PGMs) Pt, Pd and Rh in three way catalysts (TWC) for remediation of exhaust gases from motor vehicles [4]. There are many studies aimed at replacing the expensive PGMs [5]; nevertheless, PGMs are still the irreplaceable components of TWC catalysts because their activity is higher when compared to other known oxide or composite components [4]. At the same time, some researchers think that a more optimal direction of research is to decrease the concentration of PGMs in the catalysts with a simultaneous increase in their efficiency and duration of operation [6]. In this regard, to increase the efficiency of the catalysts, it is necessary to achieve a substantial expansion of the temperature operation range of the TWC catalysts, the specifics of which are associated with varying temperatures over a wide range: from temperatures below room temperature to 1100 °C and above. The ability to oxidize CO at temperatures below room temperature will help to solve the problem of cold start emissions. It is known that most of the harmful emissions occur in the first 30–60 s as the catalyst is warming up to its operating temperature [4]. Another use of catalysts with activity below room temperature is the purification of indoor air, such as dwellings, working areas, vegetable storage facilities, submarines, and orbital stations. According to Machida et al. [7], the best prospect for TWC catalysts is the development of more thermally stable supports with a high dispersion. In this connection, it is important to also retain the highly dispersed state of the active components (PGM-ceria) and their high activity over the entire temperature range, including low temperatures, after thermal treatment [8].

It is known from previous studies that palladium and ceria are the components for the TWC catalysts [[9], [10], [11]]. In the result of intensive studies the last time it was established that the state of Pd in Pd/CeO₂ catalysts depends on Pd content, the preparation method, conditions of thermal treatments in various media and other parameters. For example, in hydrogen reduced catalysts, Pd is present as a Pd⁰ species [[12], [13], [14]]. In catalysts prepared from a homogeneous suspension of colloidal particles and calcined in an oxygen atmosphere, palladium is present as PdO and Ce_1-xPd_xO_2-δ solid solution [15]. In Pd/CeO₂ catalysts, prepared by deposition – precipitation method, Pd induced the formation of three main species: surface PdO_x/Pd–O–Ce clusters, PdO nanoparticles and Pd–O–Ce solid solution [16]. A similar set of Pd forms was found in [17].

In paper [18] the catalysts with core-shell structure Pd@CeO₂/Al₂O₃ and conventional Pd/CeO₂/Al₂O₃ catalysts were considered for TWC reactions. The Pd@CeO₂/Al₂O₃ core-shell catalyst had activity exceeding activity of Pd/CeO₂/Al₂O₃ catalyst. Usage of TEM and XPS methods allowed showing the existence of PdO₂ on Pd - ceria interface in Pd@CeO₂ nanoparticles. The core-shell Pd@CeO₂ catalysts were also synthesised by a hydrothermal method [19]. The active Pd species in the catalysts mainly existed as PdO_x oxides. Results showed that the Pd@CeO₂ catalysts exhibited high catalytic activity in methane oxidation.

An improved deposition method was employed to prepare a Pd/CeO₂ catalyst in [20]. The calcination is proposed to facilitate the incorporation of Pd²⁺ ions in ceria lattice. Special flattened clusters of binary Pd-Ce oxides were obtained, which were uniformly fixed on the surface of CeO₂, which led to extremely high activity in the oxidation of CO. It is necessary to note the surface science approach to consider Pd-Ce structures on the surface of well-defined (111) and (100) planes of CeO₂, where Pd₁O and Pd₁O₂ species were proposed [21,22].

Theoretically, it seems interesting to elucidate the nature of the synergistic combination of such properties of Pd-Ce-O catalysts as to their ability to perform room temperature oxidation of CO [23,24] and their thermal stability [8]. However, from a practical standpoint, a combination of satisfactory values of both parameters has not been reached as yet. It is difficult to simultaneously obtain the required levels of activity and thermal stability because high activity is related to a high dispersion of the active component, which is not retained upon temperature elevation due to the sintering of palladium, ceria, and other components of the catalysts [25]. Then, there is additional important factor in the catalyst performance of Pd-ceria based catalysts: namely, influence of the water vapors on catalysts activity [20,26,27].

Evidently, to enhance the thermal stability of the active component in a highly dispersed state, it is necessary to stimulate a strong interaction between palladium and ceria. This can be implemented by doping of ceria with the ions of transition elements.

The most interesting results were obtained via doping of ceria with zirconium. The catalysts based on the Ce-Zr-O composition showed both a higher thermal stability and a greater oxygen storage capacity (OSC) [28,29]. In some works it is noted that doping of ceria with tin also allows a significant stabilisation of the structure and an enhancement of the catalytic properties of ceria [30]. Thus, Hegde [31] reported a significant increase in OSC when Sn was introduced into the composition of the catalyst, with the formation of a solid solution Ce_1-x-ySn_xPd_yO_2-δ which exerted a beneficial effect on the performance of oxidation catalysts. The increase in OSC was attributed to the lattice distortions: part of the oxygen ions with shortened M–O bonds rigidly holds up the lattice, whereas the other part of the oxygen ions, with long M–O bonds, is responsible for bond weakening and provides high activity of the lattice oxygen. In work [32] is underlined that doping of ceria with tin oxide increases the thermal stability and the resistance of nanosized phases to sintering under the action of high temperatures. Overall, tin is promising for doping of CeO₂ because the catalytic performance can be enhanced by increasing the redox power of CeO₂ by incorporation of Sn⁴⁺/Sn²⁺ cations into the fluorite lattice [33]. First, such an incorporation leads to the formation of a greater number of oxygen vacancies. Second, the formation of Ce_xSn_1-xO_2-δ mixed oxides can improve the redox properties of CeO₂ due to the exchange of two electrons between Cе⁴⁺/Ce³⁺ and Sn⁴⁺/Sn²⁺ redox pairs via the 2Ce³⁺ + Sn⁴⁺ ↔ 2Ce⁴⁺ + Sn²⁺ equilibrium [34,35].

Ayastuy and Iglesias-González et al. [36,37] revealed that the addition of tin ions enhanced the redox properties and OSC of the mixed oxides, as compared to pure ceria and tin dioxide, and prevented the growth of crystallites of the mixed oxide phase. This observation supports the data of Maciel [32] indicating their increased thermal stability.

Thus, analysis of the literature data demonstrates that Ce-Sn-O is quite a promising composition to be used as the catalyst with palladium deposition for CO oxidation reactions in a wide temperature range. As was noted above, strong metal-support interaction in Pd/CeO₂ catalysts is of primary importance for both the thermal stability and the low-temperature activity in CO oxidation (LTO CO) due to formation of the solid solution Pd_xCe_1-xO_2-δ the structure of which has been reliably established in [[38], [39], [40]]. Our studies show that the catalysts for LTO CO based on palladium and ceria can be thermally activated by calcination in air [41,42]. A required feature for the implementation of the palladium-ceria interaction is a high dispersion of ceria (d < 10 nm) because the crystal surface of large ceria particles does not interact with palladium to form the mixed phase [43]. On the one hand, to synthesise the Pd/CeO₂ catalyst with high activity in LTO CO, it is necessary to use thermal activation for the formation of the catalytically active Pd_xCe_1-xO_2-δ phase, whereas, on the other hand, the thermal stability of the catalyst is determined by the stability of the Pd_xCe_1-xO_2-δ phase [41,42]. Thus, further extension of the thermal stability range of the catalyst active state requires the presence of the third component that serves as a stabiliser of the highly dispersed state of ceria.

As was noted above, tin oxide can additionally enhance the thermal stability of ceria and contribute to an increase in the OSC, therefore, cerium and tin can form mixed oxide phases at certain Ce/Sn ratios [44]. The crystal lattice of ceria is able to dissolve up to 25 % of tin oxide whilst maintaining a fluorite structure, whereas up to 10 % of ceria dissolves in the SnO₂ crystal lattice (rutile type). This suggests that the use of equimolar amounts of tin and cerium will make it possible to obtain, upon thermal activation in oxygen (calcination), mixed oxide phases of fluorite and rutile types, which will possess a higher thermal stability, thus determining the thermal stability of the catalytically active phases Pd_xCe_1-xO_2-δ or Pd_xCe_1-x-ySn_yO_2-δ with the retained high catalytic activity towards LTO CO.

The great variety of approaches to obtaining mixed oxide catalytic supports exists for the moment. It can be gelation [45], complexation [46], combustion [47] etc. The coprecipitation technique has become widespread due to its simplicity and availability. However, precipitation from solution with mixed components can rarely result in high homogeneity of the composite product because of different solubilities and rates of precipitation. As a somewhat overcoming this obstacle a counter precipitation can be used. In this case two distinct single-component solutions are mixed, and precursor species neutralize and precipitate each other. The third, palladium precursor can also be added into one of them.

In this paper, we report that using the method of counter precipitation developed by us for CeO₂–SnO₂ system, it is possible to synthesise composite Pd/CeO₂-SnO₂ catalysts showing an excellent low-temperature activity toward CO oxidation with the ignition temperature about 0 °C. It was also revealed that synthesised mixed Pd/CeO₂-SnO₂ catalysts were characterized by excellent resistance to water vapors under the action of reaction mixture. The enhancement of catalytic activity was provided by preliminary thermal activation in air in the range of 800−1000 °C. The use of XRD, XPS, HRTEM and TPR−CO methods made it possible to investigate the structure and electronic state of the catalyst components, elucidate the role of cerium and tin oxides in the activation and stabilisation of these catalysts, and consider the mechanism of thermal activation in the Pd/CeO₂-SnO₂ system.