The effects of Pd-Pt ratio and reaction condition on CH4 oxidation over Pd-Pt/Al2O3

Abstract

As a catalyst for methane oxidation, palladium (Pd) supported on alumina (Al2O3) is mainly used. This is because the Pd catalyst exhibits the highest methane oxidation performance under the exhaust conditions of a typical lean burn NGV, although the performance is significantly degraded in the presence of water. In order to overcome the low resistance of Pd to water, alloys with platinum (Pt), the same platinum group (PGM) element, and changes in the support have been studied, and some studies have reported positive results such as improving the stability of palladium metal. However, most of the studies were carried out only under specific exhaust conditions concerning lean burn NGV exhaust environment. This is against the need to utilize the catalyst in every methane exhaust conditions as well as the exhaust environment of lean burn NGV. Optimized catalyst development covering all of the above reaction conditions the need for was raised. Therefore, in this study, the Pd-Pt bimetal catalyst was used to find the optimal reaction conditions and optimal Pd-Pt ratio of bimetal catalyst. The Pd-Pt bimetal catalyst was supported on γ-alumina in an incipient wet impregnation method by controlling the Pd-Pt ratio, and a total of 7 types were prepared. The total amount of the supported metal was maintained at the same number of moles based on 1 wt.% of Pd. The reactor system was configured as a fixed-bed reactor. Each catalyst was manufactured as a powder type, but it was reprocessed into pellets of 20/30 mesh size and fixed to a quartz tube. Gas and water simulating the exhaust environment were injected from the top, methane oxidation reaction occurred in the catalyst, and it entered the FTIR through the bottom of the reactor to analyze the composition of the gas after the reaction. In order to confirm the analysis of the reaction characteristics of the catalyst, several steps of experiments were carried out sequentially. First, an experiment was conducted to find out the tendency of the methane oxidation reaction according to the oxygen concentration in a single metal. The Pd/Al2O3 showed a methane oxidation performance of 90% or more in a CH4 lean condition of O2/CH4 = 4 or more in a dry condition, showing a positive correlation between O2/CH4 and methane oxidation performance. When the oxygen concentration increases, Pd is oxidized to PdO, and this result is consistent with the previous study that PdO is the main active site. In the Pt/Al2O3 catalyst, the methane oxidation performance was greatly reduced to less than 20% when the oxygen concentration was rather increased in the dry condition, and the same result was confirmed even in the wet condition. This is also consistent with the results of previous studies that metal Pt active sites have higher methane oxidation activity than PtO2. In a wet environment, a significant decrease in activity occurred before and after stoichiometric conditions of Pd/Al2O3, which is believed to be due to the generation of Pd(OH)2 with low activity or competition between OH and oxygen for adsorption. Afterwards, as the oxygen concentration increased, it was analyzed that Pd(OH)2 was converted to PdO, or that the methane oxidation activity increased due to the superiority of the oxygen reaction adsorption competition. The 7 types of prepared catalysts with different Pd-Pt ratios were divided into CH4 rich, stoichiometric, and CH4 lean conditions according to the O2/CH4 concentration in dry and wet environments, respectively, and the methane oxidation performance was confirmed through a 200~500 ℃ temperature increase experiment. In all conditions, the performance of the Pt/Al2O3 catalyst was the most inferior. This seems to be because the PtO2 active site maintained after pretreatment was maintained. Pd/Al2O3 showed excellent methane oxidation performance under most conditions, but showed a significant decrease in activity in the presence of low concentration of oxygen in a wet environment, as in the single metal tendency experiment. However, although the Pd-Pt bimetal catalyst having a high Pd ratio showed excellent methane oxidation activity like Pd/Al2O3 under other conditions, the activity did not decrease significantly. This appears to be closely related to the fact that Pd-Pt exists in the form of Pd-Pt oxide by sharing oxygen, and the adsorption capacity for methane is rapidly increased when Pd is adsorbed with oxygen. To support this, catalytic characterizations were done. The possibility of bimetal was confirmed by confirming that Pd and Pt particles existed in the same position in the bimetal catalyst through TEM image. Through H2-TPR analysis, it was also confirmed that the reduction tendency according to the temperature of each catalyst was different as the bimetal was achieved, thereby increasing the possibility of bimetal. In particular, through XPS analysis, it was confirmed that metallic Pd is oxidized to Pd2+ form when Pt is added from the Pd catalyst, and Pt4+ is reduced to Pt2+ when Pd is added from the Pt catalyst, so that Pd-Pt shares oxygen to form a bimetal. DFT calculations were also used. It was confirmed that the adsorption energy for methane at the PdO active site was up to two times higher than that of a single Pd or Pt without oxygen. Crucially, it was confirmed that the adsorption energy for oxygen increases up to 2.5 times when Pt is bonded to PdO, indicating high bonding strength of Pd-Pt oxide to oxygen, and high resistance to the inhibitory effect in the presence of water. In this study, a catalyst with an optimized ratio under various reaction conditions was searched using a Pd-Pt bimetal catalyst. Through the experimental results, it was verified that Pd3Pt1/Al2O3 exhibited the best methane oxidation performance under all reaction conditions. This is thought to be due to the high oxygen adsorption capacity of the Pd-Pt bimetal and the formation of oxide by oxygen sharing.