**1. Introduction**

The emissions from ships can be a significant source of air pollution in coastal areas and port cities and can have negative impact on human health and climate [1–3]. Therefore, the International Maritime Organization (IMO) has implemented regulations to reduce emissions from ships. So far, these regulations concentrate mainly on emissions of nitrogen oxides (NOx) and sulfur oxides (SOx). To answer these requirements emission reduction technologies are needed, namely fuel technologies, combustion technologies and/or exhaust gas after-treatment technologies (see e.g., [4]). One solution is to use natural gas (NG) as a fuel.

Liquefied natural gas (LNG) use as marine fuel is increasing and more and more gas engines, mainly dual fuel, are being installed in ships. With LNG both SOx and NOx regulations of IMO can be achieved without any need for after-treatment, since NG is nearly sulfur free resulting in very minor/no SOx emissions while lower NOx levels (compared to diesel) can be achieved due to low combustion temperature of natural gas (in lean-burn conditions). In addition, particle emissions from natural gas combustion are low and only minor black carbon is formed from NG combustion [5–7]. Moreover, CO2 emission is lower with NG use compared to diesel fuels, which is because NG is mainly composed of methane with a higher H/C ratio compared to diesel. The hydrocarbon emissions, on the other hand, are higher with NG compared to diesel fuels [8–11]. Because natural gas is mainly methane, most of the hydrocarbon emissions is also methane. Since methane is a strong greenhouse gas, its emissions should be minimized.

Three different gas engine groups are used for marine applications, namely leanburn spark-ignited engines, low pressure dual fuel engines and high pressure dual fuel engines [12]. For dual fuel engines, the natural gas and air mix is ignited with a small diesel

pilot injection. In addition, the diesel can be used as the main fuel (back-up fuel) if LNG is not available, making this dual fuel concept the most popular one in marine applications.

According to Sharafian et al. [13] LNG use in high pressure dual fuel (HPDF) engines, which are used only for large low-speed oceangoing vessels, can reduce greenhouse gas (GHG) emissions by 10% compared to their heavy fuel oil (HFO)-fueled counterparts. However, the current deployment of medium speed low pressure dual fuel (LPDF) cannot reliably reduce GHG emissions. This is primarily due to the high levels of methane slip from these engines. The methane slip from HPDF engines is reported to be significantly lower compared to LPDF engines [13] but the LPDF is the most popular LNG engine technology with at least 350 ships (e.g., LNG carriers, car/passenger ferries, cruise ships) while HPDF is used in less than 100 ships (e.g., LNG carriers, container ships) [14].

Peng et al. [6] studied the impacts of switching a marine vessel from diesel fuel to natural gas. They showed that the GHG impact of NG compared to diesel is higher especially on lower engine load cases while at higher loads (>75%) the GHG impact is comparable to diesel. The test vessel in their study operated with medium speed dual fuel engines.

ICCT's (The International Council on Clean Transportation) working paper on "The climate implications of using LNG as a marine fuel" also concludes that there is no climate benefit from using LNG, when using a 20-year global warming potential, including upstream emissions, combustion emissions and unburned methane [14]. However, over the 100-year time frame, a life cycle GHG benefit of LNG is reported to be 15% compared to diesel, but this is only for ships with HPDF engines. A life cycle GHG emission study on the use of LNG prepared by Sphera reports GHG emission reductions with LNG operation (compared to HFO fueled ships) 14–21% for 2-stroke slow speed and also 7–15% for 4-stroke medium speed [15].

One key issue in the current and future LNG ships is the control of methane slip. Methane emissions from engines are being reduced, e.g., by better fuel mixing conditions, improvements in combustion chamber design, and by reducing crevices [16,17]. One option is the use of oxidation catalyst. To oxidize methane, a highly efficient catalyst is needed. Although catalysts based on platinum are commonly used for non-methane hydrocarbon and CO oxidation, palladium catalysts have shown good activity for methane oxidation (e.g., [18,19]). Challenge in the development of MOC is the catalyst deactivation since palladium-based catalysts are very sensitive to sulfur poisoning and as little as 1 ppm SO2 present in the exhaust has already been found to inhibit the oxidation of methane [20,21].

Simplified, when palladium-based MOC is to be used the SO2 in the exhaust should be minimized and/or a regeneration procedure to recover the catalyst activity is needed.

There are few scientific studies published about the regeneration of sulfur-poisoned methane oxidation catalysts. Arosio et al. [22] studied the regeneration by short CH4 pulses. Honkanen et al. [23] also used CH4 conditions to regenerate sulfur-poisoned Pd-based catalyst. According to Kinnunen et al. [24] a sulfur-poisoned catalyst can be regenerated under low-oxygen conditions and Lott et al. [25] also favor rich conditions to achieve efficient regeneration. In addition, H2 usage has been studied to recover the catalyst activity [26,27]. Most of these regeneration studies are conducted in laboratories with synthetic gases.

To minimize SOx in the exhaust, the sulfur in the fuel and lubricating oil is to be minimized. However, in the case of natural gas, the sulfur level is already very low (a few ppm). In dual fuel engines, obviously, also the pilot diesel fuel sulfur level is to be minimized, although the pilot fuel use is only one to a few percent of the natural gas use. In this study, we also add a sulfur trap as one choice. A sulfur trap, also called the SOx trap, is an adsorber catalyst, specifically designed to store sulfur. They have earlier been studied and developed to protect NOx adsorbers that can adsorb and store NOx under lean conditions and release it under rich operation but are however poisoned by SOx present in the exhaust gas [28]. In the present study, the SOx traps are connected to MOC for the first time (to authors' knowledge).

In this study, the performance and regeneration of one methane oxidation catalyst (MOC) is studied in engine exhaust. Catalyst performance studies with an engine under lean conditions are done by emission measurements upstream and downstream of the catalyst while the regeneration is done by switching on engine-driving mode to stoichiometric for couple of minutes time. In addition to MOC only, we study the effect of a SOx trap, using it upstream the MOC, to see how it protects the MOC against sulfur poisoning.
