Effect of Ag doping on Pd/Ag-CeO₂ catalysts for CO and C₃H₆ oxidation

Highlights

• The optimal amounts of Ag doped in Pd/Ag-CeO₂ catalysts improved reducibility and the OSC.

• When excess Ag was doped into CeO₂, Ag particles were formed and the Ce³⁺/Ce⁴⁺ ratio decreased.

• The presence of Ag facilitated the reduction of Pd to Pd⁰ species under oxidative conditions.

• Higher Pd⁰ fraction in Pd/0.3Ag-CeO₂ resulted the higher catalytic activity for the CO and C₃H₆ oxidation.

Abstract

To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO₂ catalysts have been widely used as active catalysts for CO and C₃H₆ oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO₂, which contributes to the oxidation ability of Pd/CeO₂. In this study, Pd/Ag-CeO₂ catalysts were used for CO and C₃H₆ oxidation reactions. When CeO₂ was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce³⁺/Ce⁴⁺ ratio decreased when CeO₂ was doped with excess amounts of Ag. In addition, reduced Pd (Pd⁰), which is an active species for C₃H₆ oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C₃H₆ is important for the oxidation of CO and C₃H₆, the catalyst with the highest Pd⁰ fraction (Pd/0.1Ag-CeO₂ and Pd/0.3Ag-CeO₂) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO₂ catalysts and formed active Pd⁰ species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO₂ for the CO and C₃H₆ oxidation reaction.

Introduction

As concerns about climate change gradually increase, the environmental regulations on greenhouse gas emissions have become stricter. Accordingly, advanced technologies for automobile engines with high fuel efficiency, such as auto start–stop and hybrid engine systems, are required to meet CO₂ emissions regulations. However, “cold start” is a common problem of these technologies (Al Soubaihi et al., 2018, Min et al., 2018, Shen et al., 2013). During cold start, pollutant emission increases because the engine exhaust temperature is lower than the operating temperature of purification catalysts.

In addition to CO₂ regulations, vehicles should meet other guidelines, such as the European emission standards, including nitrogen oxide (NO_x) and particulate matter (PM) emission standards (Aneggi et al., 2009). To meet these regulations, the low-temperature combustion (LTC) technique has been introduced to diesel engines to reduce NO_x and PM emissions. However, the LTC technique lowers the engine exhaust temperature to below the operating temperature of the oxidation catalyst. Consequently, vehicles with LTC engines emit incompletely burned materials, such as carbon monoxide (CO) and hydrocarbons (HCs). Therefore, the oxidation catalyst must be heated to oxidize CO and HCs, which leads to a lower fuel efficiency. Since automobiles must simultaneously achieve high fuel efficiency and low emissions, the development of oxidation catalysts that are active at low temperature is critical.

Pd is a critical material for the combustion of volatile organic compounds, HCs, and CO because of its excellent catalytic activity for combustion processes (Satsuma et al., 2012, Lang et al., 2017, Hu et al., 2016, Liu et al., 2018, Li et al., 2014, Liu et al., 2016a, Gil et al., 2015, Oh and Hoflund, 2006). Therefore, supported Pd catalysts are used for a wide range of applications, such as three-way catalysts (TWCs), diesel oxidized catalysts (DOCs), and catalytic gas sensors (Shen et al., 2013, Lang et al., 2017, Liu et al., 2018, Jeong et al., 2017, Ciuparu et al., 2000, Jin et al., 2012, Slavinskaya et al., 2015, Shen et al., 2009, Martinez-Arias et al., 2001). Since the activity of Pd catalysts depends on Pd particle size, morphology, and chemical state, these parameters have been closely monitored during experiments (Shen et al., 2013, Liu et al., 2018, Li et al., 2014, Gil et al., 2015, Oh and Hoflund, 2006, Shen et al., 2009, Wang et al., 2012). Cerium oxide (CeO₂) is widely used for oxidation reactions because of its unique redox properties. The state of Ce can be easily shuttled between Ce³⁺ and Ce⁴⁺, which confers CeO₂ excellent oxygen storage capacity (OSC) and numerous surface oxygen vacancies. (Lang et al., 2017, Hu et al., 2016, Jeong et al., 2017, Jin et al., 2012, Martinez-Arias et al., 2001, Shimizu et al., 2010, Grabchenko et al., 2017, Mukherjee et al., 2016, Cargnello et al., 2013, Guimaraes et al., 2003, Park et al., 2019, Kim et al., 2020a, Jeong et al., 2019). In other words, CeO₂ can readily store and release oxygen, and presents high catalytic activity for oxidation reactions. Pd/CeO₂ catalysts have attracted increasing attention because of the strong metal–support interaction (SMSI) between Pd and CeO₂. Gil et al. (2015) demonstrated that CeO₂ stabilized the Pd²⁺ state, resulting in highly dispersed Pd on the CeO₂ surface. Hu et al. (2016) reported that the facet or morphology of CeO₂ affected the structure and oxidation state of supported Pd. The SMSI between Pd and CeO₂ presents a synergistic effect for oxidation reactions (Slavinskaya et al., 2015, Zhu et al., 2005, Tereshchuk et al., 2015). However, the active sites of Pd/CeO₂ during CO and C₃H₆ oxidation are still debatable. Hinokuma et al. (2010) reported that metallic Pd particles released from thermally treated Pd–O–Ce are active for CO oxidation. Conversely, Gulyaev et al. (2011) suggested that Pd_xCe_1−xO₂ solid solutions, in which Pd was oxidized, played an important role in the high activity of Pd_xCe_1−xO₂ for CO oxidation. Additionally, Cargnello et al. (2013) reported that the Pd–CeO₂ interface sites affected CO oxidation rates.

Ag has been added to CeO₂ catalysts to enhance oxidation activity for different reactions, such as CO, soot, ethanol, and HC oxidation (Grabchenko et al., 2017, Qu et al., 2013, Kim et al., 2020b). Additionally, Ag/CeO₂ catalysts are widely used for soot oxidation because Ag transforms gaseous O₂ to active O_x^n- species, which readily combust soot particles (Liu et al., 2016b, Lee et al., 2019, Wang et al., 2018, Zeng et al., 2019). Moreover, experiments and density functional theory calculations have demonstrated that electron transfer from Ag⁰ to Ce⁴⁺ (Ag⁰ + Ce⁴⁺ → Ag⁺ + Ce³⁺) affected the oxidation states of Ag and Ce and resulted in the formation of oxygen vacancies (Aneggi et al., 2009, Shimizu et al., 2010, Grabchenko et al., 2020, Kibis et al., 2019, Preda and Pacchioni, 2011, Luches et al., 2012). The location of Ag⁺ species on the CeO₂ surface has been analyzed using different methods. Some studies have indicated that Ag can be incorporated into the CeO₂ lattice and that Ag species increased the OSC of CeO₂ (Kang et al., 2012, Rana and Parikh, 2017). Conversely, other studies have demonstrated that the increase in OSC was ascribed not to the Ag⁺ ions incorporated in the CeO₂ lattice but to the highly dispersed Ag⁺ species on the CeO₂ surface because the ionic radius of Ag⁺ is too large to be incorporated (Bera et al., 2000, Sarode et al., 2002). In either case, Ag increased the OSC of CeO₂ to improve the oxidizing activity. Additionally, we used Ag-incorporated macroporous CeO₂ catalysts for soot oxidation in a previous study, and reported that the catalyst with a Ag loading of 5 wt.% presented the best performance and formed the most active oxygen species (Lee et al., 2019).

In this study, CeO₂ was doped with low concentrations of Ag (0.1, 0.3, 0.5, and 1 mol.%) using a coprecipitation method, to confirm that Ag doping increased the OSC of CeO₂ (Min et al., 2018, Mukherjee et al., 2016). Thereafter, Pd, an active metal for CO and C₃H₆ oxidation, was impregnated on the Ag-CeO₂ support to elicit a synergistic effect between Pd and Ag-CeO₂ during the reaction. The catalytic activities of the Pd/Ag-CeO₂ catalysts were subsequently compared, and the promoting effect of Ag in Pd/Ag-CeO₂ was analyzed using a series of characterization techniques.