## 3 Results
On the contrary, owing to the smallest pore diameter increase in the (H\({}_{2}\)C\({}_{2}\)O\({}_{4}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al compared to the (H\({}_{2}\)NO\({}_{3}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al and (H\({}_{2}\)SO\({}_{4}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al, the surface of Pd/(H\({}_{2}\)C\({}_{2}\)O\({}_{4}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al still showed metal particle agglomeration on the catalyst surface. The BET analysis was carried out to clarify the pore physical properties of the catalysts after active metal Pd loading. Figure 7 shows the N\({}_{2}\) adsorption-desorption isotherms of different Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts. Firstly, the Type IV isotherms of acid pore-widening treatment \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts had a hysteresis loop, indicating that the active metal Pd loading did not change their mesoporous nature. Moreover, the hysteresis loop type of all Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts changed from H1 to H3, indicating that the pore structure of all Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts became significantly irregular after the active metal Pd loading. Table 2 summarizes the chemical composition and textural properties of the different catalysts. The Pd loading of acid pore-widening treatment Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts increased compared to the untreated Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al, owing to the increase in the specific surface area of the acid pore-widening treatment Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al supports. Strikingly, the Pd/(HNO\({}_{3}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al had the largest Pd loading, owing to the largest specific surface area of (HNO\({}_{3}\))\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al. The pore diameters of different catalysts increased after loading compared to the untreated Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalyst, which could be attributed to the fact that the acidic impregnating solution, which produces a certain pore-expanding effect, corrodes the pores during impregnation. The main reason for the pore diameter becoming bigger when the size properties became smaller is that the small pores were blocked (1-2 nm) and the number of pores decreased [29; 30; 31; 32]. Based on XRD, we further investigated the crystalline phases of the catalysts with different acid pore-widening treatments. Figure 8 demonstrates that the XRD diagrams of the catalysts prepared with different acid pore-widening treatments all exhibited three characteristic diffraction peaks, representing the (111) crystal planes of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) and metal Pd\({}^{0}\)[32]. Those traces also indicate (large linewidths and background structure) that much of the material is sub-crystalline transition alumina. These results also verify the fact that the acid pore-widening treatment did not disrupt the \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) structure of the supports as well as the Pd(111) crystal surface of the active metal Pd particles in the catalysts. Figure 8: XRD diagrams of different Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts. Figure 7: N\({}_{2}\) adsorption–desorption isotherms of different Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts. It is well-known that charge properties of catalysts are significant in the performance of catalysts [31]. The surface chemistry properties of Pd on catalysts on different Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts were identified by XPS. Figure 9 shows the Pd 3d XPS spectra of different catalysts. The XPS spectra of Pd 3d can be divided into Pd\({}^{0}\) peak (orange section) and Pd\({}^{2+}\) peak (green section). The sample was corrected for the charge energy shift by the C 1s peak (284.8 eV) [33].