Study on Sulfur and Water Resistance of Marine Ruthenium Catalyst

Research Article

Austin Environ Sci. 2022; 7(2): 1075.

# Study on Sulfur and Water Resistance of Marine Ruthenium Catalyst

Yan H, Li J*, Fan X and Li S

Beijing Key Laboratory of Regional Combined Air Pollution Control, Beijing University of Technology, Beijing, China

*Corresponding author: Jian Li, Beijing Key Laboratory of Regional Combined Air Pollution Control, Beijing University of Technology, Beijing 100124, China

Received: March 23, 2022; Accepted: April 01, 2022; Published: April 08, 2022

## Abstract

A series of ruthenium-based catalysts were prepared by impregnation method and co-precipitation method, and applied to the catalytic oxidation of NO in the fast SCR for NOx removal in marine diesel engines. The effects of different supports and preparation methods on their catalytic activity and anti-sulfur and water-resistant properties were investigated. The Brunanuer-Emmett-Teller (BET), temperature programmed reduction (TPR) method, oxygen temperature programmed desorption (O2-TPD) method, and X-ray diffraction (XRD) were used to analyze and determine the specific surface area, redox performance and characterization of the samples and the dispersion of ruthenium on the surface. The modified Ru/Al-CeZr-2 catalyst had significantly improved its low temperature activity and sulfur resistance and water resistance. The activity remained around 40% with SO2 and H2O existed for 120min. After sulfur and water cut off, the active energy recovered to 52%, which could meet the conditions of fast SCR. It provided an idea for the industrialization process of ship SCR technology.

Keywords: Component: NO catalytic oxidation; sulfur and water resistance; Ruthenium-based catalyst; Cerium zirconium solid solution

## Introduction

In the mode of transportation of goods in the world, sea transportation accounts for a large proportion and is the main mode of transportation in international trade, undertaking more than 95% of the world trade transportation volume [1,2]. Diesel engines are the main power plant of ships, and NOx is one of the main components of harmful emissions from marine diesel engines [3]. Nitrogen oxides can cause three global environmental problems, acid rain, greenhouse effect and ozone layer destruction [4]. Currently, the SCR system is one of the most popular and matures NOx treatment devices. However, the ship space is limited, which makes it difficult for traditional SCR technology to be applied in practice [5]. The study had found that at the front end of the denigration system, about 50% NO was oxidized into NO2 by catalytic oxidation of flue gas, which was also known as fast SCR. The reaction rate was ten times that of standard SCR [6], which could increase the reaction space velocity and reduce the volume of catalyst reaction equipment, which was of great significance to solve the practical problem of difficult layout of SCR systems such as ships.

Currently NO oxidation catalyst mainly included molecular sieves, activated carbon, transition metal and precious metal oxide catalysts. Among them, transition metal oxidation catalysts had been widely studied due to their low price, but they were easily poisoned by SO2 and H2O, while noble metal oxidation catalysts had good antipoisoning effect, but their high price and lack of resources made them suffer in industrial applications. Yao Rui et al. [7] prepared a series of Mn-Co/TiO2 catalysts by impregnation method, and investigated the effect of Ce doping on the catalytic oxidation activity of the catalyst. The oxidation efficiency could reach over 85% at 250°C, but after 300ppm SO2 and 5% H2O were introduced, the catalyst was rapidly inactivated within 3h. Peng Lili et al. [8] pointed out that cerium zirconium solid solution could improve the dispersion of active component Co on the catalyst surface and increase the specific surface area. The prepared CoOx-CeOx/ZrO2 catalyst could achieve 80.9% oxidation rate of NO at 250°C. When 180ppm SO2 was introduced, the catalyst activity could be stable for 1h, but it dropped rapidly after 60min, and it was basically completely deactivated after 3h, however, its anti-sulfur and anti-water performance had not been explored. Li et al. [9] studied the catalytic oxidation of NO by Ru catalysts under different carriers, and the results showed that the optimal load of Ru was 2%, and Ru/TiO2 could maintain a certain NO oxidation activity after 40ppm SO2 and 2.5% H2O were added.

This paper mainly focused on the research on catalysts that generated NO2 in fast SCR systems. In the above studies, the roasting temperature of catalysts was mostly below 500°C, while marine diesel engines were divided into two-stroke diesel engines and four-stroke diesel engines. The exhaust temperature of four-stroke diesel engines could be higher than 400°C, and the maximum exhaust temperature of Marine diesel engines could not exceed 550°C [10]. SO2 and H2O are important components of gases discharged from ships. Therefore, under the condition that sulfur dioxide and water existed at the same time, it was a major problem to ensure that the oxidation activity of the catalyst could still meet the conditions of fast SCR. Only by overcoming this difficulty could it be widely used in industry. So, in this paper, the activity, sulfur and water resistance of ruthenium based catalysts with different supports under high temperature roasting were studied and CeZr solid solution was selected as the support and further modified with Al. A series of Ru/Al-CeZr-X catalysts were prepared and tested in a fixed-bed microreactor, and the oxidation activity, sulfur resistance and water resistance of the catalysts were discussed.

## Materials

Chemical reagents used in the experiment are: SiO2, γ-Al2O3, ZrO2, CeO2, TiO2 (industrial products). The dispersed metal salts of Ce(NO3)3•6H2O, Zr(NO3)4•5H2O, RuCl3•xH2O, Cr(NO3)3•9H2O serve as a source of support material that requires no further purification.

## Methods

Preparation of carrier: A certain amount of cerium nitrate and zirconium nitrate solid (molar ratio 2:1) was dissolved in deionized water, and stirred to transparent state in ultrasonic water bath at 70°C. 1mol/L sodium hydroxide solution was added to pH value of 12-13, stirred for 3h, and precipitated viscose was obtained. The solid solution carrier of cerium zirconium could be obtained after washing in the filtration device until pH=7, dried in oven at 110°C and roasted in muffle furnace at 600°C for 3h.

The prepared cerium-zirconium solid solution was mixed with γ-Al2O3 (mass ratio 1:1), and physically grounded to obtain an Al-CeZr-0 carrier.

The Al-CeZr-1 composite carrier was prepared by co-precipitation method. A certain amount of solid cerium nitrate and zirconium nitrate (molar ratio 2:1) was dissolved in deionized water, and stirred to transparent state in ultrasonic water bath at 70°C. γ-Al2O3 powder with mass ratio 1:1 of cerium zirconium solid solution was added. Slowly added 1mol/L sodium hydroxide solution to a pH value of 12- 13, stirred for 3h, to get the precipitated viscous substance. Washed in the suction filter device until pH=7, dried in oven at 110°C, roasted in muffle oven at 600°C for 3h.

The Al-CeZr-2 composite carrier was prepared by co-precipitation method. A certain amount of cerium nitrate and zirconium nitrate solid (molar ratio 2:1) were dissolved in deionized water, stirred in an ultrasonic water bathed at 70°C to a transparent state, and slowly added 1mol/L sodium hydroxide solution to pH value of 12-13, added γ-Al2O3 powder with a mass ratio of cerium zirconium solid solution (1:1), stirred for 3h, and obtained a precipitated viscous substance. Washed in a suction filtration device to pH=7, dried in an oven at 110°C, and roasted in a muffle furnace at 600°C for 3h.

Preparation of ruthenium catalyst: A certain amount of RuO2 solution (0.5%wt RuO2) was dissolved in deionized water, then SiO2, γ-Al2O3, ZrO2, CeO2, TiO2 and the prepared composite carrier were added, stirred in ultrasonic water bath, and dried at 110oC. The required catalysts, namely Ru/SiO2, Ru/γ-Al2O3, Ru/ZrO2, Ru/CeO2, Ru/TiO2, Ru/CeZr, Ru/Al-CeZr-0, Ru/Al-CeZr-1, Ru/Al-CeZr-2, could be prepared by roasting at 550°C in muffle furnace. The burned catalyst was ground and crushed, screened to 20-40 mesh, and reserved.

## Catalytic activity measurement

The activity of NO catalyst was evaluated in a continuous flow fixed-bed catalytic reactor, which was composed of gas distribution, reaction and flue gas test. The air distribution part was composed of high pressure gas cylinder, mass flow controller and display, mixing tank. The simulated flue gas came from the high-pressure gas cylinder and entered the reactor after being mixed evenly by the mixing tank. The reaction part was carried out in a quartz tube, the catalyst to be evaluated was fixed with high temperature resistant quartz wool, the top of the tube was plugged with a rubber stopper, a type thermocouple is placed vertically above the catalyst bed, and a temperature controller is used to control the temperature conditions required for the evaluation. The reaction test temperature range was 160-420°C. The simulated flue gas consists of inlet concentration NO: 700ppm, O2: 5%, N2 as equilibrium gas, SO2: 200ppm, H2O: 5vol%. The reaction was carried out at atmospheric pressure with a space velocity (GHSV) of 27000h-1.

Measurement of NO catalytic oxidation activity. The NOx concentration was detected by a 42i-HL (NO-NO2-NOx) flue gas analyzer from Thermo Fisher Scientific, USA.

NO conversion rate was calculated by the concentration of NO before and after the reaction (assuming the same inlet and outlet gas volume):

$n=\frac{C_{0}-C_{1}}{C_{0}}\times&space;100%$

η: No conversion rate, %;

C0: NO concentration at reactor inlet, ppm;

C1: NO concentration at the outlet of the reactor, ppm.

## Catalyst characterization

The specific surface area and pore structure of the catalyst were tested on Gemini V specific surface area and porosity analyzer. The sample was degassed at 110°C for 1h before testing. BET and BJH methods were used to measure the specific surface area, pore volume and pore size of the samples.

H2-TPR was carried out on Auto Chem II 2920 chemisorption apparatus. The catalyst was pretreated at 300°C in O2 atmosphere for 1h, cooled by He gas to room temperature, and then infused with a mixture of 10% H2-90% Ar. The temperature was heated to 850°C at a rate of 10°C •min-1, and the TCD detector was used for analysis.

O2-TPD is an important method to study the oxygen storage performance of catalysts. The experiment was conducted on Tp-5080 automatic multi-purpose adsorber. Weigh 100 mg of the sample into a quartz micro-reaction tube, raised it to 300°C in an air atmosphere and keep it for 30h. Then purged from He to room temperature, switched the atmosphere to O2 for 1h to achieve adsorption saturation, switched He purged for 30min, Desorption was then performed at a rate of 10°C/min to 800°C, and the TCD signal was detected.

The phase composition and crystal structure of the samples were characterized by XRD and D8 advance X-ray diffractometer. Cu Ka was used as the radiation source, the tube voltage was 35kV, the tube current was 35mA, and the scanning was carried out in the range of 10~80° at a speed of 5°/min.

## Effect of support on catalytic oxidation of NO and resistance to sulfur and water by Ruthenium

The NO oxidation activity of the Ru-based catalyst was clearly dependent on the carrier. The comparison of the oxidation activity of the Ru-based catalysts of the different carriers was shown in Figure 2a, and the NO conversion rate first rose and then decreased with the increase of temperature, reached the maximum value at around 300°C. Among them, Ru/CeZr, Ru/CeO2 and Ru/ZrO2 obviously increased their oxidation efficiency at a low temperature of 220°C. It could be concluded from Figure 2a that the activity order of different carrier catalysts was loaded: Ru/CeZr> Ru/ZrO2> Ru/SiO2> Ru/γ- Al2O3> Ru/CeO2> Ru/TiO2. It could be seen that the Ru/CeZr catalyst had 79.51% of the highest oxidation activity at 280°C, which was lower than the best active temperature of the ruthenium catalyst with different carriers, indicated that the Ru/CeZr catalyst had the optimal overall activity.