I tried to relate the properties of the Pt-based bimetallic catalyst to the catalytic behavior in each reaction. By mimicking the important catalytic phase of Pd and PtPd bimetallic particles, we can obtain the overall correlation between PdOx and CH4 surface oxidation activity.
Pt-based bimetallic catalysts
Introduction of Pt-based bimetallic catalysts
Application in commercial
Pt is included in Pd catalysts to achieve better catalyst durability by exhibiting steam and sulfur tolerance.32-39 The addition of an appropriate amount of Pt to a Pd catalyst shows significantly improved catalytic performance.40-46. Similarly, it is important to select the right additives to control the catalytic behavior for the desired purpose in the target reaction based on an understanding of the catalyst properties in monometallic and bimetallic catalysts.
Surface analysis for understanding the catalytic behavior of bimetallic catalysts
Conventional tools for characterizing the properties of bimetals and their limits to
12. 4) CH4 oxidation in NGV: Pt is incorporated into Pd catalysts to achieve better catalyst durability by showing tolerance to steam and sulfur.32-39 Addition of appropriate amount of Pt to Pd catalyst shows significantly improved catalytic performance.40 -46. Dependence of (a) Rh and Pd atomic fractions of as-synthesized Rh0.5Pd0.5 NPs and (b) Pt and Pd atomic fractions of as-synthesized Pt0.5Pd0.5 NPs on photoelectron KE and mean free path measured at 25 °C. c) Evolution of Rh and Pd atomic fractions in the Rh0.5Pd0.5 NPs at 300 °C under oxidizing conditions and catalytic conditions.
Advantages of DRIFTS study for surface analysis
The surface properties of support or monometallic particles
Then, the surface restructuring by the transient evolution of WC and UC Pt site fraction was observed through in situ DRIFTS investigation during CO oxidation reaction from 298 K to 413 K (Figure 1.6b). As the reaction temperature increased, the proportion of CO bound to UC Pt sites increased, while the proportion of CO bound to WC Pt sites decreased as shown in Figure 1.6c.
The role of interfacial sites between metal and support
Reprinted with permission from ref. 54. b) Evolution of the IR spectrum during the oxidation reaction of CO on Au/TiO2 at 120 K after dosing O2 gas into saturated CO on Au/TiO2.
The surface composition of bimetallic particles
In contrast, for bimetallic Pt−Pd catalysts, probe molecules can adsorb on both Pt and Pd metal sites. IR spectra of CO adsorption on various Pt and Pd loading of Pt−Pd bimetallic, Pt and Pd catalysts after (a) reduction and (b) oxidation.
It can only explain the catalytic behavior at an initial state, not a steady state of the reaction. Then the fundamental understanding about the role of added secondary metal on the catalytic behavior provides a direction for improving the activity and stability of the conventional catalyst and further a guideline to design the catalyst for a desired reaction.
However, conventional methods are limited to investigate the surface properties of bimetallic catalysts, so it is difficult to understand the crucial properties for determining the catalytic behavior. However, if we carefully control the adsorption procedure and the pretreatment condition for the DRIFTS study, which is very sensitive to the surface area of the loaded metal, the results would be very powerful in that the crucial properties of the catalyst for determining the catalytic behavior. even in bimetallic catalysts.
Herein, we investigated the effect of acid-base properties of alumina on metal-support interaction and coke deposition, which affect the stability of catalysts in propane dehydrogenation (PDH) using PtSn/Al2O3. Furthermore, precise characterization and understanding of alumina's acid-base properties will contribute to developing catalysts with high stability.
Furthermore, we clearly demonstrated that coke precursors were initially induced on the Lewis acid sites on the alumina surface and gradually converted to aromatic compounds with spectroscopic evidence.
All spectra were externally referenced (ie, the 0 ppm position) to the NMR peak of 1 M Al(NO 3 ) 3 . Pulse chemisorption of Pt-only supported Al2O3 samples was performed at 50 °C using the same procedures. Then, the samples were cooled down to -150 °C and CO adsorption was performed with the same procedure as described above.
Catalytic activity evaluation: Propane dehydrogenation
IR spectrum collected from alumina support was normalized with the intensity of Brønsted acid sites (cm-1). Finally, to compare the relative ratio of alumina surface area, the samples were pretreated with 20% O2/He (1.0 mL/s) at 400 °C for 1 h and then rinsed with He. To investigate the acid–base properties on the surface of alumina, pyridine adsorption was also performed with 1% pyridine/He flow at 100 °C with the same pretreatment procedure above.
Results and Discussion
Overall, we clearly demonstrated that coke deposition was initiated from Lewis acid sites on the alumina surface. This was consistent with the pyridine adsorption spectra, which are commonly used to characterize the acidic sites on the alumina surface as shown in Figure 2.14. Therefore, strong acid sites on the alumina surface and fewer residual Lewis acid sites are desirable for high activity and superior stability during the PDH reaction.
Although A600 had many penta-coordinated Al3+ sites, as confirmed by solid-state 27Al MAS-NMR analysis (Figure 2.1b), most of these existed in the bulk state, leading to strong acidic sites on the alumina surface and low bulk crystallinity. . Furthermore, the combined results of DRIFTS and Raman spectroscopy indicated that the coke precursor was initially induced on the Lewis acid sites on the alumina surface. Therefore, it is important to control the Lewis acid sites on the alumina surface for superior stability.
Subsequently, the Lewis acid sites gradually opened as the deposited coke species were converted to aromatic compounds by aliphatic hydrocarbons. Consequently, the strong Lewis acid sites formed after dehydroxylation and reducing the number of acid sites in A750 led to the suppression of metal melting and coke deposition, which is beneficial for reducing the number of catalyst regeneration steps. Moreover, a large number of five-coordinated Al3+ sites of A600, as evidenced by solid-state 27Al MAS-NMR, were mainly present in the bulk, rather than on the surface.
The addition of Ce promotes the oxidation activity and thermal stability of Pt through the strong Pt-O-Ce bond.16–18. Here, we shed light on why Mn modification significantly promotes the activities of Pt/Al2O3-based catalysts in the oxidation of HC, CO, and NO, which were tested under practically relevant test conditions. We observed the reduction of Pt particle size after Mn modification via X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis.
Another strategy to utilize Pt efficiently is to change the physico-chemical properties of Pt particles by the secondary metal addition such as Mn.19-25. Previous H2 temperature-programmed reduction (H2-TPR) studies on MnOx-promoted Pt-based catalysts showed the enhanced redox capabilities thanks to close intimate contact between Pt and manganese oxides.19-21 Furthermore, the synergistic effect between Pt and Mn improved the catalytic activity in various oxidation reactions such as n-hexane oxidation and combustion of diesel vehicle emission.22-25 It was also reported that the degree of activity improvement was influenced by the catalyst preparation protocols (Pt precursor/washcoat) and the active component (manganese oxide phase) Although the advantage of using Mn in Pt-based catalysts have been reported for oxidation reaction, there was still a lack of understanding of how the synergistic effect between Pt and Mn affects the catalytic behavior of Mn-doped Pt catalysts.We investigated the details of the altered redox ability and surface properties of Pt- particles near Mn through various characterization techniques, such as H2-TPR, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and CO temperature-programmed desorption (CO-TPD). Oxidation activity tests were performed for the prepared catalysts with monolith samples which were prepared by slurry coating and loading with 70 g/L washcoat.
CO adsorption was performed on the samples using 0.2% CO/He (flow rate = 1 mL/s) at room temperature. The sample was then cooled to room temperature, flushed with He for 0.5 h, and exposed to 1% CO/He flow for 0.5 h. Then the sample was purged with He for 0.5 h to remove the weakly bound CO molecules.
Catalytic activity evaluation: HC/CO/NO oxidation
CO adsorption was performed on the samples using 0.2% CO/He (flow rate = 1 mL/s) at the same temperature. Each spectrum was acquired before and after a 5-min He purge to remove weakly bound CO and gas-phase CO. The desorbed CO or CO2 gases were converted to methane using Ni/Al2O3 catalysts with H2 at 400 °C and then fed into a flame ionization detector (FID) to determine the amount of desorbed CO/CO2.
Results and discussion
Similarly, the size of Pt particles in 2Pt/A-Ce decreased significantly after Mn modification, from 80–130 nm to 30–60 nm, which is larger than that of the series 1Pt/A-Ce and 1Pt/Mn -A -Ce. These reduction peaks shifted to lower temperatures compared to those of Mn-A-Ce (Figure 3.8a). The changes in CO adsorption strength on Pt with Mn modification after oxidative pretreatment were also investigated in a series of Pt/A-Ce and Pt/Mn-A-Ce using DRIFTS, as shown in Figure 3.11.
We report here that the amount of partially oxidized palladium (PdOx) on the catalyst surface shows a linear correlation with CH4 oxidation activity in a series of Pd/Al2O3 and Pt-Pd/Al2O3 catalysts hydrothermally aged under commercially relevant conditions. The normalized PdOx peak areas were quantified and compared with the steady-state CH4 oxidation activities at 300 °C and the activity was found to increase proportionally with the amount of surface PdOx. Overall, we report a general correlation between the amount of surface PdOx and steady-state CH4 oxidation activities in various Pd-based catalysts.
Thus, much effort has been devoted to understanding the nature of the active sites of Pt-Pd bimetallic catalysts for CH4 oxidation. After oxidative pretreatment, the CH4 oxidation activities of the Pd-based catalysts were examined at 300 °C. The obtained results show that the amount of surface PdOx in Pd-based catalysts is critical for the steady-state activity in CH4 oxidation.
The physiochemical properties of the loaded metal particles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and CO chemisorption analyses. Furthermore, after re-oxidation pretreatment of the Pd catalysts, using diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and CO as a probe molecule, we quantified the catalytically relevant sites: partially oxidized palladium (PdOx). Then we found that the amount of surface PdOx is linearly correlated with the CH4 oxidation activity regardless of Pt and Pd composition, synthesis method and support properties.
Catalytic activity evaluation: CH 4 oxidation
Results and Discussion
Methane oxidation activity versus surface PdO x on Pd/Al 2 O 3 catalysts
Deconvolution of the IR spectra of CO adsorbed on hydrothermally aged Pd/alumina catalysts (quantitative absorption band area for PdOx after reoxidation colored in blue). Again, using the IR spectra of CO adsorbed on the xPtyPd/A-H catalysts after reoxidation (Figure 4.17b), the amount of PdOx surface was measured (IR spectra of CO adsorption after oxidation pretreatment were shown in Figure 4.18a). This indicates that the addition of Pt mainly affects the surface composition of Pt-Pd bimetallic catalysts.
It is clear that there is a linear relationship between steady-state CH4 oxidation activities and the amount of surface PdOx quantified from IR spectra regardless of composition, catalyst preparation method and support. Crucially, we found that the addition of Pt to Pd affects the surface composition of Pt-Pd bimetallic particles.
General correlation of steady-state activities and surface PdO x in various Pt-Pd
Surface reconstruction of Pt-Pd catalytsts during CH 4 oxidation
From the simplified IR spectra of CO adsorption on reoxidized Pd catalysts, we were able to quantify the amount of surface PdOx, and this amount was found to have a linear correlation with CH4 oxidation activity independent of Pd loading and support. By extending this characterization method to Pt-Pd bimetallic catalysts, we established a linear correlation between surface PdOx and CH4 oxidation activity at steady state regardless of composition, preparation method, and support. In particular, highlighting the result that the surface PdOx plays a decisive role in the oxidation activity of CH4 rather than the electronic properties of the Pd particles and potentially indicates an active site for CH4.
Using in situ DRIFTS study, we further elucidated the surface reconstruction of Pt-Pd bimetallic particles during the reaction. Jihyeon Lee, Ji Hui Seo, Chinh Nguyen–Huy, Euiseob Yang, Jun Gyeong Lee, Hojeong Lee, Eun Jeong Jang, Ja Hun Kwak, Jun Hee Lee*, Hosik Lee* and Kwangjin An*. Eunkyung Cho, Yong–Hee Lee, Hyunjoung Kim, Eun Jeong Jang, Ja Hun Kwak*, Kyubock Lee*, Chang Hyun Ko* and Wang Lai Yoon.