Preparation and performance evaluation of hydrogen-producing catalysts for diesel reforming

Ru/Al2O3 catalyst was prepared by standard impregnation method. The catalytic reforming performance of Ru/Al2O3 and commercial high nickel/low nickel catalysts on commercial No.0 diesel oil was studied. The regeneration method of carbon-deposited catalyst was also discussed. The results show that commercial low nickel catalyst has poor catalytic activity and stability for diesel, and increasing the water-carbon ratio can slightly improve the conversion rate of diesel. Increasing the reforming reaction temperature and adding methanol additives can effectively improve the catalytic activity of commercial high nickel catalysts. Ru/Al2O3 is a potential catalyst for diesel reforming, reducing the reforming reaction temperature can effectively prevent the catalyst from high temperature hydrolysis deactivation. Hydrogen peroxide has a good regeneration effect on Ru/Al2O3 catalyst.


Introduction
In the future for a long period of time, fossil energy will still be the main energy in the world [1][2] .In order to achieve the relevant goals of the Paris Agreement on schedule, clean and low-carbon applications of fossil fuels are imperative.As the most important fossil energy, the clean application of diesel has become an important research topic [3][4][5] .The "diesel reforming+SOFC" technology can directly convert the chemical energy of diesel into electrical energy,which is considered to be an important trend of efficient and clean utilization of diesel.At the same time,diesel reforming hydrogen production has obvious advantages in economic terms, it is estimated that the cost of diesel reforming hydrogen production is about 20-30 yuan /kg, lower than electrolytic water hydrogen production 30 yuan /kg, wind power photovoltaic hydrogen production 40 yuan /kg [6][7] .Catalysts are the main factor determining the effectiveness of diesel reforming.With unique advantages such as low cost, easy availability, and higher catalytic activity, non-noble metal catalysts such as Ni base have been widely used in reforming reactions of hydrocarbon fuels such as diesel.Although Ru based catalysts are rare in nature and have significantly higher usage costs than Ni based catalysts, their catalytic activity and anti carbon deposition performance are significantly superior to Ni based catalysts, and they have excellent long-term operational stability.They are considered one of the most promising catalysts for diesel reforming to hydrogen production.Guggilla [8] studied the reforming performance of N-dodecane on Ru-Ni-CeO2-Al2O3 catalyst.Krumpelt [9] compared the selfreforming performance of Fe, Co, Ni, Cu, Pd, Ag, Ru and Pt catalysts for isooctane.In this paper, the catalytic reforming performance of Ru/Al2O3 catalyst, commercial low nickel catalyst and commercial high nickel catalyst materials on commercial No.0 diesel oil was studied.The regeneration method of carbon deposition catalyst was also discussed.

Working principle of hydrogen production by diesel reforming
Taking the steam reforming of n-hexadecane as an example, the reaction equation is as equations (1.1) [10] ,Other side effects are shown in equations (1.2) to (1.9 (1.9)

Experiment
Low nickel catalyst (10% Ni/MgO-SiO2) and high nickel catalyst (40% Ni/MgO-SiO2) are commercial products.The Ru/Al2O3 catalyst is self prepared using a conventional impregnation method.Specifically, weigh a certain amount of analytical pure ruthenium trichloride (RuCl3•3H2O), add it to a certain amount of deionized water, stir well, and configure it to become a standard immersion solution with a content of ruthenium trichloride of 25g/L.Using Al2O3 as the carrier and using an equal volume impregnation method, the Ru/Al2O3 catalyst is obtained by soaking at room temperature for 48 hours, aging and drying at 110 ℃ for 8 hours in an oven, and then roasting at 650 ℃ for no less than 4 hours in a muffle furnace.After roasting, the prepared catalyst was pressed to form, and 40 ~60 mesh particles were selected after grinding, and finally Ru/Al2O3 catalyst was obtained.

Evaluation of Reforming Performance of Low Nickel Catalysts
The catalytic activity of commercial low nickel catalysts for diesel at different water carbon ratios was studied.The test results are shown in the following figure.The experimental results show that: (1) Low nickel catalysts have lower catalytic activity for diesel, and increasing the water carbon ratio can slightly improve the diesel conversion rate.When the water carbon ratio is 4:1, the conversion rate of diesel is only 15.2%.When the water carbon ratio is increased to 10:1 and 20:1, although the conversion rate slightly increases to 22.9% and 25.7%, it is still at a relatively low level.The reason for the above phenomenon is that,increasing the ratio of water to carbon, that is, increasing the concentration of reactants, will promote the reaction equilibrium to move towards a positive reaction direction.
(2) The increase of water carbon ratio helps to slightly increase the hydrogen concentration in the reforming gas.When the water carbon ratio is 4:1, 10:1, and 20:1, the hydrogen concentration in the reforming gas is 61.4vol%, 63.5vol%, and 65.3vol%, respectively.The reason for the above phenomenon is also that the increase of the water-carbon ratio is conducive to the movement of the reforming reaction in the direction of the positive reaction.In addition, the increase of water-carbon ratio can inhibit the production of CO and CH4 and promote the production of CO2.The effect of temperature on the reforming effect of nickel based catalysts was studied,the results indicate that: (1) An increase in temperature can significantly improve the catalytic activity of high nickel catalysts.When the temperature gradually increases from 400 ℃ to 450 ℃, 500 ℃, and 550 ℃, the conversion rate of diesel gradually increases from 12.5% to 16.2%, 52.3%, and 79.6%.The reason is that the diesel reforming reaction belongs to a strong endothermic reaction, and increasing the reaction temperature is beneficial for the reforming reaction to proceed towards a positive equilibrium direction.
(2) An increase in temperature helps to obtain higher hydrogen content.When the reforming temperature is 400 ℃, 450 ℃, 500 ℃, and 550 ℃, the hydrogen content in the reforming gas is 68.6%, 73.2%, 72.8%, and 72.6%, respectively.This means that an increase in temperature is beneficial for obtaining higher hydrogen content, but the improvement benefit is no longer obvious when the temperature exceeds 450 ℃ .The test results also show that the increase of temperature makes the content of CO slightly increase and the content of CH4 and CO2 slightly decrease.

Research on the Influence of Methanol Additives
The influence of methanol additives on the catalytic performance of high nickel catalysts for diesel was investigated.The experimental results indicate that: (1) methanol additives can significantly improve diesel conversion rate.At the same reforming temperature (550 ℃), the diesel conversion rate after adding methanol is much higher(100%) than that without adding methanol (79.6%, see Chapter IV-B).( 2) Adding methanol additives will cause a slight decrease in the H2 content.After adding methanol additives, the H2 content of the reforming gas was 63.8%, 57.8%, 63.2%, and 66.1% at 400 ℃, 450 ℃, 500 ℃, and 550 ℃, respectively, which was generally lower than the data without methanol additives (see Chapter IV-B).The internal mechanism is that the hydrogen content of methanol (about 12.5%) is slightly lower than that of diesel (about 15.0%).The addition of methanol additives reduces the hydrogen content of the entire reaction mixture, ultimately leading to a decrease in the hydrogen content of the reforming gas.The experimental results show that the addition of methanol additives can lead to a small increase in the content of CH4 in the reforming gas, but the effect on the content of CO and CO2 can be ignored.The experimental results show that: (1) Throughout the entire reaction cycle, the catalyst exhibits good catalytic performance for diesel.In the initial state, the conversion rate of diesel is 76.9%.As the reaction progresses, the conversion rate of diesel gradually increases.By 62 hours, the conversion rate of diesel has increased to 100%.In the subsequent reaction time, the conversion rate remains at 95% to 100%.(2) During the first 285 hours of the reaction cycle, the hydrogen content remained basically constant.Within 0-285 hours, the hydrogen content in the reforming gas fluctuated between 72% and 78%.Since then, the hydrogen concentration at the outlet of the reformed gas has shown a decreasing trend to 289h, which is reduced to 71.67%.From this, it can be inferred that after 285 hours, there may be a certain degree of degradation in catalyst performance, leading to a decrease in hydrogen concentration in the reforming gas.In addition, the test results also show that in the first 208 hours of the reaction cycle, the content of CO and CO2 in the reformed gas is basically constant.In the first 285 hours, the CH4 content of the reformed gas also remained basically constant.In the long time reforming test at high temperature, a small number of catalysts were observed to be damaged during the reforming process, which may be attributed to the structural damage caused by the hydrolysis of alumina support at high temperature.

Long term performance evaluation of low-temperature reforming
To mitigate the risk of catalyst deactivation under higher temperature conditions, a long-term reforming experiment of Ru/Al2O3 catalyst on diesel was conducted under low temperature conditions (740-760 ℃).The experimental results are as follows.
Figure 5. Performance of Ru/Al2O3 catalyst for diesel reforming under lower temperature The experimental results show that,compared with the high-temperature reforming test in Chapter V-A, the impact of reducing the reforming temperature on the diesel reforming performance of Ru/Al2O3 catalyst is as follows:(1) Lowering the reforming reaction temperature effectively avoids the structural damage of the catalyst caused by high-temperature hydrolysis.After reducing the reaction temperature (740-760 ℃), no damage to the catalyst was observed throughout the entire reforming reaction cycle, and the catalyst also exhibited high catalytic activity.(2) The effect of reducing the reforming reaction temperature on diesel conversion rate can be basically ignored.In the high-temperature reforming test, except for the initial state, the conversion efficiency of diesel is basically maintained between 95% and 100%.After reducing the reforming temperature, except for some fluctuations in the initial diesel conversion rate, the diesel conversion rate remained stable between 98% and 100%.(3)Lowering the reforming reaction temperature results in a slight decrease in the hydrogen content of the reforming gas.In the high-temperature reforming test, the hydrogen content in the reforming gas is 73.56% in the initial state.From the beginning of the reforming to 285 hours, the hydrogen content in the reforming gas fluctuates between 72% and 78%.In the low-temperature reforming test, the hydrogen content in the reforming gas remained stable within the range of 67.8% to 73.4%.The main reason is that the diesel steam reforming reaction is a strong endothermic reaction (Equation 1.1), reducing the reaction temperature causes the reaction equilibrium to shift in the opposite direction, resulting in a decrease in the hydrogen content of the reforming gas.

Research on Catalyst Regeneration Methods
The regeneration performance of hydrogen peroxide on Ru/Al2O3 catalyst is shown in the following figure.The horizontal axis represents the regeneration time, the left vertical axis represents the percentage content of O2 in the exhaust gas, and the right vertical axis represents the percentage content of CO2 in the exhaust gas.The test results show that hydrogen peroxide has a good regeneration effect on Ru/Al2O3 carbon deposition catalyst.Only 30 minutes after the start of the regeneration test, the CO2 content in the exhaust gas has reached 30.4%.60 minutes after the start of the regeneration test, the CO2 content in the exhaust gas has reached 86.19%.It can be inferred that the carbon atoms in the carbon in the surface area of Ru/Al2O3 catalyst are easier to combine with the oxygen atoms in hydrogen peroxide, that is, hydrogen peroxide can promote the decomposition and gasification of carbon in the surface area of the catalyst.In addition,the entire catalyst regeneration reaction can be completed within approximately 300 minutes.

Conclusion
The main conclusions of this paper are as follows:(1) Commercial low nickel catalyst has poor catalytic activity and stability for diesel, and increasing the water-carbon ratio can slightly improve the conversion of diesel and increase the hydrogen content.(2)Increasing the reaction temperature and adding methanol additives can effectively enhance the catalytic activity of commercial high nickel catalysts.(3)Ru/Al2O3 is a promising diesel reforming catalyst, and reducing the reforming reaction temperature can effectively avoid catalyst high-temperature hydrolysis deactivation.(4) Hydrogen peroxide has a good regeneration effect on the Ru/Al2O3 catalyst with carbon deposition.

Figure 1 .Figure 2 .
Figure 1.Effect of Water Carbon Ratio on the Reforming Performance of Low Nickel Catalysts

Figure 3 .
Figure 3.The Effect of Methanol Additives on the Catalytic Performance of High Nickel Catalysts for diesel at Different Temperatures 5. Performance Evaluation of Ru/Al2O3 Catalyst for Diesel Reforming 5.1.Long Term Performance Evaluation of High-temperature Reforming A long-term (289 hours) reforming experiment was conducted on diesel using Ru/Al2O3 catalyst at a higher reforming temperature (780-810 ℃), and the results are shown in the following figure.

Figure 4 .
Figure 4. Performance of Ru/Al2O3 catalyst for diesel reforming under higher temperature