This thesis focuses on the understanding of the surface properties of alumina and their role in the interaction with the active phases (Pt) and the corresponding catalytic behavior of Al2O3-based catalysts. Based on understanding of surface properties of Al2O3, we study how these surface properties of alumina affect the active phases (Pt) and the catalytic behavior of Pt/Al2O3.
Alumina
Introduction to Alumina
3-4 In order for oxygen anion to become hexagonally packed (ABAB), high temperature (oC) calcination is necessary for θ-Al2O3 to become α-Al2O3.1, 3-4. χ-Al2O3 and κ-Al2O3, whose oxygen packing is the same with α-Al2O3 as HCP, also require high temperature because structure and textural properties also affect the phase transformation process.3 Only diaspores (α-AlOOH) can phase transform directly into α -Al2O3 at low temperature (500 oC).
Application as catalyst
For the oxygen anion to become hexagonal close-packed (ABAB), calcination at a high temperature oC is required for θ-Al2O3 to become α-Al2O3.1, 3-4 However, χ-Al2O3 and κ-Al2O3, whose oxygen packing is the same as α-Al2O3 as HCP also needs high temperature because the structure and textural properties also affect the phase transformation process.3 Only diaspore (α-AlOOH) can phase transform directly into α-Al2O3 at low temperature (500 oC). ).
Structures and Characterization of γ-Al 2 O 3
Bulk structures of γ-Al 2 O 3
39, 41 However, since γ-Al2O3 are metastable phases rather than thermodynamically stable, electron beam irradiation can cause phase transformation of γ-Al2O3. Various preparation protocols (temperature, precursor and textural properties) affect the formation of γ-Al2O3, leading to different crystallinity, facets, etc.1-3 Many structural studies for γ-Al2O3 have investigated γ-Al2O3 derived from different preparation protocols, which should be carefully considered.
Surface chemistry of γ-Al 2 O 3
In addition, OH groups derived from dissociative adsorption of water act as Brønsted acid sites (proton donating).1, 3 Here, Brønsted acid sites and the Lewis basic sites are usually not simply Lewis acid of γ-Al2O3, I mainly focus on the Lewis acid sites of γ-Al2O3. Various types of Lewis acid sites exist on γ-Al2O3 surfaces, leading to the complexity of understanding the surface chemistry of γ-Al2O3.
![Table 1.2. Suggested models for surface OH groups on γ-Al 2 O 3 . [ ] notes the cation vacancy](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10487913.0/24.892.111.787.381.588/table-suggested-models-surface-groups-notes-cation-vacancy.webp)
Factors to affect the surface properties of γ-Al 2 O 3
Morphology
The critical role of morphologies for various catalytic properties suggests that controlling the morphology of γ-Al2O3 may be a promising way to tailor the catalytic performance of Al2O3-based catalysts. Since anion (chloride, sulfate) and cation (alkali metal) affect the surface properties of γ-Al2O3 (will be discussed in detail in section 1.3.3), residual additives must be clearly washed away after the synthesis of morphology-controlled Al2O3 .
Crystalline phase
Impurities and additives
High thermal temperature induces the phase evolution of γ-Al2O3 with loss of surface area, which can be significantly suppressed by additives such as alkali metal, alkaline earth metal and other elements (Figure These stabilized aluminum oxides can improve the performance of the combustion catalyst.94 However, these additives also modify acid -the base properties of γ-Al2O3.
Motivation
Outline
In this work, the effect of crystal facets on the catalytic behavior of γ-Al2O3 was investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed desorption of ethanol, solid state 27Al NMR, infrared spectroscopy and ethanol dehydration reaction. The formation of ethylene increased with increasing (100) facets, clearly demonstrating the crucial role of these facets as active sites for ethanol dehydration on γ-Al2O3.
Introduction
In this work, a series of platelet γ-Al2O3 samples were synthesized where the number of (100) facets was systematically increased and then correlated with the catalytic behavior of γ-Al2O3 in ethanol dehydration. The catalytic activity of γ-Al2O3 in ethylene formation increased with an increase in the relative ratio of (100) facets.
Experimental Section
- Preparation of facet-oriented γ-Al 2 O 3
- Characterizations
- Catalytic activity measurements
- Calculation of the facet ratio by TEM measurements
The first issue concerns the sensitivity of the technique or instrument used to characterize the crystalline aspects of γ-Al2O3. However, a quantitative measurement of the aspect ratio using bulk techniques such as XRD is very difficult due to the low crystallinity and small domain size of γ-Al2O3.23,32 The second challenge lies in the preparation of a series of samples γ-Al2O3 with systematic variance in morphology.
Results and Discussion
After heat treatment, the SAED pattern for γ-Al2O3 (Figure 2.5b) showed that the main basal planes were perpendicular to the (110) direction, indicating the change of the (010) facet in AlOOH to a (110) facet of γ -Al2O3. BFTEM images were analyzed based on the structural model shown in Figure 2.6a and the results are summarized in Figure 2.6b.
Conclusion
The acid-base properties of Al2O3 with different surface features were studied by XRD, HR-TEM, ethanol TPD and ethanol dehydration reaction rate measurements. This fundamental understanding of the acid-base properties of alumina is useful for the further development of new catalysts with better activity and selectivity.
Introduction
Ethanol TPD showed that the desorption temperature (at a maximum rate of ethylene desorption, Td) of dissociative ethanol was significantly dependent on morphology, crystalline phase and additives. This empirically correlated trend suggests that Td can be used as a descriptor for the acid-base properties of Al2O3 with various modification origins.
Experimental section
Materials
For a general insight into the effect of individual factors on surface modification, we prepared Al2O3 with different morphologies (plate and rod), transition aluminum and Al2O3 supported by metal oxides. Based on a combined TPD/ethanol dehydration study, we were able to demonstrate that the TOF of ethylene is inversely proportional to the Td of dissociative ethanol, independent of the nature of the modification.
Catalyst characterizations
After stabilizing the flame ionization detector (FID) signal from the Agilent 7820A gas chromatograph (GC), TPD experiments were performed under a flow of He (1.0 mL/s) with a heating rate of 10 °C/min. TEM images of the 1% Pt/Al2O3 catalyst were recorded after the ethanol dehydration reaction test was performed for 90 min at 180 °C.
Results and Discussion
- Morphology effects
- Crystalline phase effects
- Effects of additives
- Overall correlations between T d of dissociative ethanol and ethanol dehydration
- Commercial Al 2 O 3
On the other hand, the apparent activation energy barriers for ethylene and ether formation clearly showed different ΔTd dependences (Figure 3.4d). As seen in Figure 3.12, the rates of ethylene and ether formation and the activation barriers were completely stable.
Conclusion
The TOF of diethyl ether formation showed a similar trend, but Al2O3 modified with Pt showed a lower ether formation rate than pure γ-Al2O3. However, ether showed similar apparent activation barrier (84-106 kJ/mol for all modified Al2O3), possibly suggesting different active sites for ethylene and ether formation.
For 1 wt% Pt/Al2O3, the number of specific sites on all the aluminas was sufficient to disperse all the Pt, leading to only highly dispersed Pt clusters (~1 nm). The number of large agglomerated Pt clusters decreased with increased number of sites and interaction strength.
Introduction
Effect of number and properties of specific sites on alumina surfaces for Pt-Al2O3 catalysts. In this work, we investigated how the number and properties of specific sites on an alumina surface influence its interaction with Pt.
Experimental section
Catalyst preparation
The properties of these surface hydroxyls are known to be closely related to the activation temperature and crystalline aspects, as the study by Digne et al. To discern how each aluminum parameter contributes to the Pt-Al2O3 interaction, systematic approaches should include well-defined aluminum model with sensitive and quantitative characterization of aluminum surface properties.19-23 Recently, we have reported that ethanol temperature programmed desorption (TPD) can sensitively characterize the number and properties of specific sites in different types of aluminum with different morphologies, crystal phase and additives.24.
Catalyst characterizations
Catalyst reaction tests: Benzene hydrogenation
Results and discussion
Number of sites
In general, the greater the number of specific sites on alumina, the greater the dispersion of Pt. However, by increasing the Pt loading to 10 wt.%, the number of Pt atoms became larger than the number of anchoring sites.
Properties of sites
The Pt clusters around ~ 1 nm were highly dispersed, consistent with the absence of diffraction peaks for Pt in XRD (Figure 4.17). Again, the Pt size estimate was not accurate due to the overlap of the alumina peak and the underlying width of smaller Pt clusters.
Catalytic reaction: benzene hydrogenation
XRD and TEM results also showed that agglomerated Pt and small Pt aggregates exist at 10 wt. % Pt/Al2O3. It suggests that the intrinsic activities of the Pt clusters were still the same even though the higher conversion by 10 wt. % Pt/T1000 can be attributed to a higher number of Pt surface sites per 10 wt. % Pt/T1000, dominated by highly dispersed Pt clusters.
Conclusion
All results show that the number and properties of sites on aluminum surfaces affect the Pt distribution, leading to different Pt distributions and catalytic properties for benzene hydrogenation. At low Pt loadings (0.5–1 wt% Pt), the number of sites was sufficient to distribute all the Pt, leading to highly dispersed Pt clusters without any agglomerated Pt.
However, T1000 has a new type of surface hydroxyl at 3790 cm-1, which is the origin of the different surface properties compared to T80. All the results clearly show that Pt can be more dispersed with increasing number of sites and interaction strength.
Abstract
Introduction
The reduction of oxidized Pt species in metallic Pt can be implemented in two ways: One is the reduction by reducing agents such as H2 (the most typical one) and the other is autoreduction without reducing agents. When a calcination temperature increases, PtO2 is automatically reduced to metallic Pt despite the oxidizing atmosphere.
Experimental section
Catalyst preparation
Catalyst characterizations
After that, O2 TPD was started with a ramping rate of 10 oC/min to 900 oC under He atmosphere due to sensitivity problem. Also, to observe the surface properties of Pt with different reduction temperatures, the ex situ pretreated sample was loaded and heated under He flow (1 ml/s) at 150 °C for water desorption.
Results and Discussion
The specific interaction between Pt oxide and alumina surfaces was further demonstrated by DRIFTS results. Effect of Pt loading and calcination temperature on the reduction behavior of supported Pt catalysts.
Effect of Pt loading and calcination temperature for the reduction behavior of
Previous reports suggest that the surface hydroxyl at 3766 cm-1 was interpreted as surface hydroxyl at (100) facets that form penta-coordinated Al3+ sites when dehydroxylated.41-42 These results suggest that Pt-O-Al is formed by the anchorage of Pt at the sites of surface hydroxyls on (100) facets. However, after reduction at 500 oC, these hydroxyls reappeared because Pt-O-Al interactions cannot be maintained due to the reduction of Pt oxide, as shown in Figure 5.2b.
Reduction behavior of 3D-like, 2D-like Pt oxide and atomically dispersed Pt
These results also clearly show the easier reduction of 3D-like PtO2 than 2D-like PtO2, consistent with cryogenic H2-TPR and in-situ XRD. That is why O/Pt decreased with increasing Pt loading on RT H2-TPR because 3D-like PtO2 is already reduced at RT.
Auto-reduction behavior of 3D-like, 2D-like Pt oxide and atomically dispersed Pt . 105
The 3D-like PtO2 is automatically reduced to metallic Pt, which becomes less mobile than Pt oxide. Thus, 3D-like PtO2 showed more sinter-resistant behavior than 2D-like PtO2 and atomically dispersed Pt.
This specific interaction between Pt and Al2O3 results in highly dispersed Pt as 1 nm 2D-like PtO2 and atomically dispersed Pt at 1 wt% Pt/Al2O3. I would also like to express my great gratitude to my wife, Eun Jeong Jang, who is also my lab colleague who encouraged me to continue studying until now, while giving me countless encouragement and support during my Ph.D.
![Figure 2.5. BFTEM images and SAED patterns in (a) [010] zone direction of AlOOH and (b) [110]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10487913.0/47.892.170.723.196.613/figure-bftem-images-saed-patterns-zone-direction-alooh.webp)
