Next Article in Journal
A Sustainable Supply Chain Integrated with Autonomated Inspection, Flexible Eco-Production, and Smart Transportation
Next Article in Special Issue
Co-Precipitated Mn0.15Ce0.85O2−δ Catalysts for NO Oxidation: Manganese Precursors and Mn-Ce Interactions
Previous Article in Journal
Numerical Investigation of Natural Convection in an Open-Ended Square Channel with Two Suspending Heat Sources
Previous Article in Special Issue
Biomass-Derived Activated Carbon as a Catalyst for the Effective Degradation of Rhodamine B dye
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation

1
National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China
2
Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Processes 2022, 10(9), 1773; https://doi.org/10.3390/pr10091773
Submission received: 16 August 2022 / Revised: 28 August 2022 / Accepted: 1 September 2022 / Published: 4 September 2022
(This article belongs to the Special Issue Synthesis and Application of Novel Nanocatalysts)

Abstract

:
Up to now, a great number of catalysts have been reported that are active in the oxidation of volatile organic compounds (VOCs). However, supported noble-metal catalysts (especially Pt-based catalysts) are still the most excellent ones for this reaction. In this study, Pt species supported on γ-Al2O3 and ranging from single-atom sites to clusters (less than 1 nm) and 1–2 nm nanoparticles were prepared and investigated for oxidizing C2H4. The Pt-loaded γ-Al2O3 nanoflakes (PtAl-NF) and Pt-loaded γ-Al2O3 nanowires (PtAl-NW) were successfully prepared. The samples were characterized using XRD, TEM, XPS, HAADF-STEM, and in situ DRIFTS. Based on in situ DRIFTS, a simple surface reaction mechanism was developed. The stable intermediates CO on single-atom Pt, subnanometer Pt particles, and fully exposed Pt clusters could be explained by the strong binding of CO molecule poisoning Pt sites. Moreover, the oxidation of C2H4 was best achieved by Pt particles that were 1–2 nm in size and the catalytic activity of PtAl-NF was better when it had less Pt. Lastly, the most exposed (110) facets of γ-Al2O3 nanoflakes were more resistant to water than the majorly exposed (100) facets of γ-Al2O3 nanowires.

1. Introduction

Volatile organic compounds (VOCs) are a major cause of air pollution because more and more factories and cities have created many places where VOCs can be released [1,2]. For example, petroleum refining [3], the pharmaceutical industry [4], the production of organic chemical materials [5], spraying, and printing [6] would all make alkanes, alkenes, alkynes, aromatics, halocarbons, and compounds with oxygen, sulfur, and nitrogen. Since a stricter clean air policy has been put into place recently, there has been a lot of focus on reducing VOCs [7]. Catalytic decomposition of VOCs is a good way to reduce air pollution that is also affordable [8,9]. Most of the time, noble metal catalysts on supports and transition metal oxides were used to get rid of VOCs [10,11,12,13,14]. Because they work better as catalysts, supported noble metal catalysts have been used a lot to get rid of VOC streams [15,16,17]. For example, Pt-based catalysts are very good at getting rid of lower hydrocarbons like C2H4, which could lead to full oxidation at temperatures below 200 °C [13].
Noble metals are expensive in trade markets and have a very limited available quantity, which thereby limits their use [18]. As active phases with a higher number of exposed noble metal atoms frequently exhibit a higher activity [19,20], noble metal particle sizes typically determine the ratio of the number of exposed noble metal atoms to the overall number. A lot of focus has been placed recently on single-atom catalysts (SACs) and fully exposed clusters (FECs), in which all of the noble metal atoms are exposed on the support surface. The isolated noble metal structure of SACs catalysts, such as Pt1/Al2O3 [21], Pt1/FeOx [22], Pt1/CeO2 [23], and Pt1/TiO2 [24], show high activity for CO oxidation. It should be kept in mind that ensembled noble metal sites with complete dispersion allow for a higher activity than SACs since isolated noble metal atoms without ensemble sites are unable to perform essential surface reactions that demand metal proximity [25]. This indicates that not all catalytic reactions can be efficiently catalyzed by a noble metal atom that has been dispersed 100% on support. On the one hand, the sensitivity of the reaction’s molecular structure to single atoms, clusters, and particles would vary [25,26]. Contrarily, chemical valence would be controlled by size (single-atom and cluster states display cationic states, and particles generally show metallic states), which are necessary for dissociation and/or creation of target chemical bonds [27].
In this study, the valence states and structural sensitivity of supported Pt species—from single-atom sites to clusters (sub 1 nm) and 1–2 nm nanoparticles—were examined for the C2H4 oxidation. First, nanowire γ-alumina (γ-Al2O3) and nanoflake γ-alumina (γ-Al2O3) were fabricated; the respective exposed surfaces of these two types of alumina were primarily (100) and (110) facets. Second, various Pt species were seen when the same Pt loading (1 weight %) was applied to nanowire and nanoflake γ-Al2O3. The catalytic activity of single-atom, cluster, and nanoparticle Pt was confirmed by combining electron microscopy, in-situ DRIFTS, and X-ray photoelectron spectroscopy. The lower CO desorption and catalyst inactivation rates of single-atom Pt or Pt clusters revealed them ineffective for C2H4 oxidation. Instead, the 1–2 nm nanoparticles reflected the lower binding energy of Pt, permitting easy CO desorption, and were the most active Pt species for C2H4 oxidation.

2. Material and Methods

2.1. Materials

All chemical reagents were used as received without purification. Pt(NO3)2 solution (15 wt.% Pt) was supplied by (Shanghai Jiuling Chemical Co., Ltd., Shanghai, China) Al(NO3)3·9H2O (Chemically Pure), NaOH (Chemically Pure), NaAlO2 (Chemically Pure), and AlCl3·6H2O (Analytical Reagent) were purchased from (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).

2.2. γ-Al2O3 Nanoflake (Al-NF) Preparation [28]

Dropwise additions of 1 M NaOH solution were added to the agitated 0.25 M Al(NO3)3 solution until the pH of the mixture reached 10. Al(OH)3 gel was produced after 1 h of room temperature ageing. Then, a 200 mL Teflon container with the mixture (160 mL) inside was sealed within a steel vessel. The container was heated on a Muffle furnace for 48 h at 200 °C. The solid white result underwent four cycles of centrifugation and deionized water washing. For 12 h, the solid sample was dried at 120 °C. The final products are ground to a particle size of less than 100 µm and then calcined in air for five hours at 550 °C with a 5 °C per minute heating ramp. The sample was named Al-NF.

2.3. γ-Al2O3 Nanowire (Al-NW) Preparation [29]

The stirred 0.25 M Al(NO3)3 solution received 0.4 M NaAlO2 solution (72 mL). Al(OH)3 gel was produced after 1 h of room temperature ageing. Then, the mixture was poured into a 200 mL Teflon container and placed within a steel vessel before being sealed. The container was heated on a Muffle furnace for 24 h at 150 °C. The solid white result underwent four cycles of centrifugation and deionized water washing. For 12 h, the solid sample was dried at 120 °C. The final products are ground to a particle size of less than 100 µm and then calcined in air for five hours at 550 °C with a 5 °C per minute heating ramp. The sample was named Al-NW.

2.4. 1 wt.% Pt/Al2O3 Preparation

In 5 mL of distilled water, 200 mg of Pt(NO3)2 solution (15 wt.% as Pt) was added (solution A). 2.97 g of support (Al-NF and Al-NW) from Solution A were combined to make a viscous slurry, which was then evaporated on a hot plate to almost dryness (while stirring continually) and dried for 12 h at 120 °C. The finished products were ground to a particle size of less than 100 µm and calcined in air for five hours at 550 °C with a 5 °C per minute heating ramp. All Pt-loaded samples were named PtAl-NF and PtAl-NW.

2.5. Characterization Techniques

JEM-ARM200F (JEOL, Tokyo, Japan) operated at a 200 kV accelerating voltage to produce the images for transmission electron microscopy (TEM, Tokyo, Japan) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Tokyo, Japan). A tiny sample was equally disseminated in ethanol, put on a Cu grid covered with carbon, and dried for characterization. X-ray photoelectron spectroscopy (XPS) was used to investigate the electrical characteristics of Pt (Al K-alpha, Thermo Scientific, ESCALAB 250Xi, Waltham, MA, USA). The reference point for estimating the binding energies of the Pt element was the maximal strength of the beneficial C 1s signal at 284.8 eV. Powder X-ray diffraction (PXRD, Bruker D8 ADVANCE, Billerica, MA, USA) with monochromatic CuKa radiation (λCuKa = 1.54056 Å) was used to reveal structural analyses. The gas absorption technique Brunauer-Emmett-Teller (BET, Micromeritics®, Norcross, GA, USA) was employed for the determination of the surface area of samples. Surface area measurements were performed using N2 gas at a temperature of 77 K. To remove physisorbed species from the surface, samples were degassed at 150 °C under a vacuum for 24 h. Pore size distribution and pore volume were determined by BJH analysis.
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed on a BRUKER VERTEX 70 FTIR spectrometer equipped with a liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detector. Samples (17 mg) were put into the reaction chamber. The loaded samples were then warmed for 30 min at 400 °C with pure Ar (50 mL min−1). Catalyst background spectra were obtained using Fourier transform infrared spectroscopy (FTIR) when the chamber was cooled to 180 °C, 160 °C, 140 °C, and 120 °C. The chamber was first brought to 120 °C, then allowed to cool to ambient temperature. 50 mL min−1 of mixed gas (10 mL min−1 O2, 35 mL min−1 N2, and 5 mL min−1 0.7% C2H4/N2) was added to the system to replace the pure Ar. At a resolution of 4 cm−1, an average of 128 scans were used to gather all the spectra. The chamber was heated to 140 °C in the Ar environment after 10 min. Following a 30-min Ar purge, the chamber was filled with the mixed gas, and spectra were once more obtained. The same method was used to obtain data at 160 °C and 180 °C.
Then, in a PIKE® infrared cell provided with a regulated environment and temperature, in situ DRIFTS was carried out on PtAl-NF and PtAl-NW to examine the evolution of surface species on catalysts under C2H4 oxidation conditions (Figure 1). The less specular and more diffuse the reflection component is, the smaller the sample particle is, and the more sensitive the measurement is. Particle sizes in the diffuse reflection sample ranged from 2 to 5 μm. As shown in Figure 1, after inserting the sample into the crucible, gently flatten the powder with a metal rod to increase the compact sample’s specular reflection component. The sample was also placed in an in-situ cell that has been warmed to 400 °C. After cooling to a specific temperature, the sample was purged with argon gas for one hour, and the background spectrum was measured.

2.6. Catalytic Activity Tests

All the tests were conducted at 0.1 atm. C2H4 oxidation experiments were carried out in a tube micro-reactor with an internal diameter of 13 mm. The catalyst (100 mg Catalyst + 400 mg quartz sand) was sieved between 40–60 grain size mesh and shifted into the reactor with a bed volume of around 0.5 mL, between two layers of quartz wool, which functions to avoid catalyst powder displacement while also pre-heating the reagents. The reactor was heated using the oven, and the temperature of the catalyst was monitored using a K-type thermocouple put within the reactor, close to the catalytic bed.
The composition of the reactant mixture was adjusted by adjusting the flow rates of C2H4, O2, and N2, meanwhile keeping the overall flow rate constant at 1000 mLmin−1. The conditions were consistent with a gas hourly space velocity (GHSV) of 600,000 mL gcat.−1 h−1. The composition of the effluent gases was monitored online using a Fourier-transform infrared (FT-IR) gas analyzer (MKS, MultiGasTM analyzer, model 6030) equipped with a liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detector. The cut-off wavelength for the detector was 16 μm, the window type was ZnSe, the detector size was 0.25 mm, and the path length was 5.11 m.

3. Results and Discussion

3.1. Characterizations of Pt/Al2O3 Catalysts

3.1.1. Characterizations of Al2O3 Supports

XRD characterizations were carried out for analysis of the crystalline structures of Al2O3 obtained following the calcination of boehmite (Figure 2). It is evident from the XRD profiles that crystallized γ-Al2O3 was successfully prepared and the obtained data matches well with the JCPDS powder diffraction pattern 10-0425. The diffraction peaks appeared at 2θ = 19.5°, 37.6°, 39.5°, 45.9°, 60.8°, and 67.1° indexed to (111), (220), (311), (222), (400), (511), and (440), which indicated that there were no diffraction peaks attributed to Al(OH)3 or γ-AlOOH phases [30,31]. The crystal size of γ-Al2O3 samples can be determined by Scherrer’s equation from the XRD (440) peaks (Equation (1)).
D = k λ 57.3 / [ L F W H M c o s θ ]
where D is crystal size in the direction perpendicular to the lattice planes, k is a constant with a value of 0.9, λ refers to the wavelength of the X-rays, LFWHM is the width at half the maximum of peaks, and θ stands for the Bragg angle. The crystalline sizes were calculated using equation 1, and the crystal size of Al-NF and Al-NW were respectively determined as 10.8 nm and 8.4 nm. The thickness of Al-NF was about 30–50 nm (Figure S1), which was not consistent with the crystal size derived from XRD (440) peak. The differences may be attributed to the existence of nanoflakes in sample Al-NF as aggregated crystalline grains with sizes of around 10 nm, which fused to form nanoflakes. Furthermore, Figure S2 clearly showed that the uneven zigzag edges of nanoflakes were comprised of small and uniform grains (10 nm). As shown in Figure S3, the diameter of Al-NW was almost identical to the crystal size deduced from XRD (440) peaks, which suggested an uninterrupted growth of the crystal size in the (110) direction.
HRTEM observations were subsequently carried out to confirm the microstructural characteristics of the γ-Al2O3 samples which were earlier inferred from the XRD patterns. The sample prepared in part 2.2 using Al3+/AlO2- exhibited highly distributed γ-AlOOH-based nanoflakes. After calcination at 550 °C, γ-AlOOH transformed into γ-Al2O3 [32,33,34], and the TEM image of γ-Al2O3 nanoflake (Al-NF) has been depicted in Figure 3a. The lengths and thickness of the as-prepared Al-NF were about 200 nm and 40 nm, respectively. Besides, the developed nanoflakes manifested straight edges with angles measuring 76° and 104°, which corresponded to the angle between the (101) and (−101) faces of γ-AlOOH [28,29,35], indicating γ-AlOOH nanoflakes grew with basal planes and lateral faces of (010) and (101). The Al-NF therefore directly inherited the morphology of γ-AlOOH nanoflakes and the exposed γ-AlOOH faces (010) and (101) were converted to basal planes and lateral faces of (110) and (111) of γ-Al2O3 by topological rules [36,37]. The highly magnified TEM image of the γ-Al2O3 nanoflake has been shown in Figure 3c. The lattice fringes of Al-NF with a d-spacing of 0.225 nm, corresponding to the (110) plane of γ-Al2O3, can be observed clearly [38]. The protocol described in part 2.3 produced γ-Al2O3 nanowires (Al-NW) as shown in Figure 3b. The lengths and diameters of the as-prepared Al-NW were observed to be 100 nm and 10 nm, respectively. Figure 3d is a representation of HRTEM images where lattice fringes attributed to (100) planes of the γ-Al2O3 phase, can be observed.

3.1.2. Characterizations of Pt Supported on γ-Al2O3

Pt-loaded γ-Al2O3 samples were synthesized with different γ-Al2O3 supports (PtAl-NF and PtAl-NW) as shown in Table 1. The final concentration of Pt on the support was 1 wt.% for all syntheses. Elemental analysis was carried out to investigate the Pt content. A similar specific surface area, as shown in Table 1, was standard for indifferent effects on Pt dispersion. Moreover, the similar pore volume and average pore size eliminated the variation of mass transfer rate. Thus, the difference between PtAl-NF and PtAl-NW exposed the crystal planes, thus leading to different Pt structures and dispersions.

3.2. Catalytic Activity of PtAl-NF and PtAl-NW

Catalytic conversion of C2H4 using PtAl-NF was accomplished at around 240 °C, using a gas stream containing 0.07% C2H4 and 10% O2 in N2 at a space of 600,000 mL gcat.−1 h−1 (Figure 4). By comparison, the reference sample (PtAl-NW) achieved 80% C2H4 conversion, which was 20% lower than the PtAl-NF catalyst at 240 °C. Pt has been accepted as the active phase in this reaction, in which γ-Al2O3 only served as a carrier and had no involvement in catalysis [13]. In the range of reaction temperature, C2H4 oxidation and O2 activation on different Pt sites at Pt/Al2O3 interface occur through a redox or Mars-Van Krevelen (MvK) mechanism where the lattice O was consumed during the catalyst reduction step accompanied by the oxidation of the reactant, and this was followed by the catalyst re-oxidation step by its reaction with molecular O2 [13]. On the one hand, the C2H4 molecule was absorbed on two adjacent Pt sites and formed a di-σ configuration (C2H4(2S)) [13,44]. Pt, on the other hand, activated O2 to form reactive O species (Pt-O*) [39]. The formation of C2H4(2S) and Pt-O* resulted in effective C2H4 oxidation reaction pathways. It is noteworthy that when compared with PtAl-NF, PtAl-NW having a higher Pt dispersion demonstrated a lower catalytic activity (Figure 4), thereby indicating that Pt atom aggregation on the Pt/γ-Al2O3 catalysts was certainly responsible for the increased reactivity of PtAl-NF.

3.3. Development of Surface Reaction Model

Figure 5 depicts the experimental results of surface species measurement obtained by in situ DRIFTS, displaying the adsorption intensities of each surface species over the 1200–4000 cm−1 range. Broad IR bands in the 3390–3670 cm−1 range represent the adsorption due to the H2O molecule [40,41], with the peak at 3700 cm−1 corresponding to Pt-OH species which showed a higher wavenumber than Al-OH. When the sample was switched from PtAl-NW to PtAl-NF, the IR bands of adsorbed H2O began to decline under the same reaction conditions, whereas the Pt-OH peak vanished completely. This suggested that the γ-Al2O3 nanowires were more hydrophilic inducing the formation of Pt-OH species. As shown in Table 2, the R10 (H2O(S) ↔ H2O + Pt(S)) suggested that adsorption water on the support surface would decrease Pt active sites. The =C-H, -CH3, and -CH2 species were represented by the weak IR bands at 3130 cm−1, 2950 cm−1, and 2910 cm−1 respectively [13,42]. The dehydrogenation of C2H4 (2S) by the surface oxygen adsorbed during the reaction (Pt-O) is thought to be the origin of the creation of the -CH3 and -CH2 species. However, the =C-H species were not seen on the surface of PtAl-NF, indicating that the PtAl-NF catalyst might change =C-H species too quickly for them to be practically observable. The bands about 2350 cm−1 are most likely caused by CO2 [13]. The detected peaks at around 2040 cm−1 and 2110 cm−1 were caused by CO species adsorbed linearly on the Pt [43,44,45,46]. While the peak at 2110 cm−1 indicated linearly adsorbed CO on small Pt particles or single-atom Pt, the peak at 2040 cm−1 indicated the existence of linearly adsorbed CO on Pt particles [44]. The PtAl-NW catalyst’s CO absorption peak intensity was much higher than that of the PtAl-NF catalyst’s, demonstrating the PtAl-superior NW’s CO adsorption capability. Additionally, a bigger Pt dispersion in PtAl-NW was indicated by the peak’s higher intensity at 2110 cm−1, which is consistent with TEM data. The oxidation of the CHX species generated during the reaction may result in the formation of -COOH or -COO (1600–1700 cm−1) [47].
γ-Al2O3 was inert and not reducible. It was not anticipated that the reaction, which only affected Pt, would take place at the surface of such support surfaces. The C2H4 oxidation process (Table 2) had been suggested for these Pt/γ-Al2O3 catalysts’ non-reducible support. The MvK-type reaction on supported-Pt catalysts was demonstrated by FT-IR observations, which showed the synthesis of intermediates after supported-Pt was exposed to C2H4 gas at 180 °C (Figure 5). This observation validated the reaction mechanism. An indication that C2H4 and O2 were adsorbed on Pt and underwent the redox reaction, for instance, was the existence of Pt-OH, =C-H, -CH3, and -CH2 species (Figure 5).

3.4. Identification of Active Sites by In-Situ DRIFTS

Supported noble metal-based catalysts, primarily Pt, have undergone substantial research for the reactions in recent years due to their remarkable activity and stability at low reaction temperatures. On the other side, there was an ongoing dispute on the reaction mechanisms, especially regarding active site constructions (single atoms versus small Pt nanoparticles). Understanding the C2H4 oxidation mechanism would advance significantly with direct observation of the catalytic performance of certain sites that offered statistically sufficient information on on-site identification and quantification, as well as activity evaluation. Using infrared (IR) technology, single-atom Pt and Pt nanoparticles could be distinguished and quantified quickly and easily. The use of IR spectra of CO on supported noble metal catalysts is particularly common because of its sensitivity to the atomic and electronic structural architectures of the binding sites [43,44]. The IR peaks in the polycarbonyl region could be attributed to CO molecules that have Pt atoms adsorbed on single atom and nanoparticle surfaces (>2100 cm−1) and surfaces (<2100 cm−1), respectively [43].
Figure 6 and Figure 7 showed the in situ DRIFTS spectra of C2H4 oxidation on PtAl-NF and PtAl-NW and the corresponding IR intensities of CO as a function of the temperature, respectively. It was found that the PtAl-NF remained almost unchanged when the temperature was raised from 120 °C to 180 °C, indicating that the single-atom Pt (2102 cm−1) indeed lacked catalytic activity for the CO intermediates (Figure 6). Moreover, there were no characteristic peaks (<2100 cm−1) that originated from CO molecules adsorbed on Pt nanoparticles, thereby implying that Pt nanoparticles (~1 nm) could oxide CO and then desorb as CO2 at a reaction temperature above 120 °C. However, as C2H4 was oxidized with PtAl-NW catalyst at elevated temperature (Figure 7), the peaks corresponding to Pt nanoparticles (2040 cm−1) appeared but were reduced in intensity. The appearance of the peak could be ascribed to a higher proportion of <1 nm Pt nanoparticles on PtAl-NW as shown in Figure 8c. Because the smaller Pt particles (<1 nm) indicated lower coordination of Pt and led to stronger interactions between Pt and CO [46]. The PtAl-NW showed a redshift of around 10 cm−1 in the region >2100 cm−1, indicating that the CO molecule was adsorbed on Pt species other than a single atom [43]. Compared with 180 °C (Figure 7), the IR peak was red shifted from 2103 cm−1 to 2110 cm−1 at 120 °C because of dipole-dipole coupling between CO molecules. Single-atom Pt and full exposed Pt clusters had similar CO adsorption properties to single-atom Pt because of the similar low coordination [48]. Based on the IR (Figure 7) and HAADF-STEM (Figure 8c) results of PtAl-NW, the IR peaks at 2103 cm−1 to 2110 cm−1 were assigned to CO molecules adsorbed on full exposed Pt clusters.
The IR spectra were collected in a reaction mixture of 0.07 vol. % C2H4 and 20 vol. % O2 from 120 °C to 180 °C on PtAl-NF and PtAl-NW catalysts. The Pt surface could only exist in two stable states: CO-covered (inactive) or O-covered (active), hence the limiting step mostly prevented the C2H4 oxidation (reaction R9 in Table 2). Pt agglomerates with various structural variations, including single-atom Pt, full-exposure Pt clusters, and Pt nanoparticles, displayed varying CO oxidation activities, as shown in Figure 7. According to the Bronsted-Evans-Polanyi relations, Pt nanoparticles (<1 nm), single-atom Pt, and fully exposed Pt had larger CO oxidation barriers, raising the ignition temperature (Figure 4) and lowering CO tolerance (Figure 7). Pt nanoparticles (~1 nm) supplied Pt sites that were simple to set up in a Pt-O active state, allowing CO to be swiftly converted into CO2.

3.5. Structures of Pt Supported on γ-Al2O3

By using high-angle annular dark-field scanning transmission electron microscopy, the morphology and Pt particle size distribution of the samples (PtAl-NF and PtAl-NW) were observed and have been shown in Figure 8. White ellipses show that Pt nanoparticles were evenly dispersed on the alumina surface of PtAl-NF, with an average particle size distribution of about 1.2 nm (Figure 8a and Figure 3b). There were hardly any clusters of single Pt atoms (individual Pt atoms in the agglomerates). On PtAl-NW, however, single-atom Pt, Pt clusters, and Pt nanoparticles were seen in the 0–2 nm range, as shown in Figure 8c. Figure 8d showed that PtAl-NW had a narrower Pt particle size distribution in comparison to PtAl-NW, indicating an improved Pt dispersion and excellent atomic utilization. It may be emphasized that the Pt clusters and single atoms showed virtually perfectly dispersion of Pt atoms across the Al-NW surface. Additionally, the yellow bins in Figure 8d represent the 39% count of single-atom Pt and Pt clusters. As previously explained [49], Pt was evenly dispersed across the surface of the alumina (100 facets). Pt preferred to be anchored on the (100) facets of the γ-Al2O3 surface at the coordinatively unsaturated Al3+ sites, where the formation of full exposed Pt clusters furnished the energetically most stable overlayer structure.

3.6. Electronic Properties of Pt/γ-Al2O3

To investigate the existing valence states of Pt element on Pt/γ-Al2O3 and determine their relationship with their catalytic characteristics, XPS examinations of the catalysts were carried out (Figure 9). Pt 4d experiments were carried out, and the deconvoluted spectra of Pt 4d were obtained. After Gaussian (80%)-Lorentzian (20%) fitting, Pt 4d spectra for both samples were found to comprise two primary doublets reflecting two distinct Pt states. The distinctive peaks of Pt0 were assigned to the binding energy of Pt 4d5/2 at 315.8 eV, whereas the signal at the higher range of 317.8 eV–318.9 eV belonged to Ptδ+ [45]. Furthermore, when the agglomerates decreased, the binding energy of Pt 4d increased, which was in good agreement with the results of HAADF-STEM characterization. The CO band in IR spectra ascribed to Ptδ+-CO with higher binding energy (PtAl-NW), was redshifted compared to the absorption band from lower binding energy Ptδ+ (PtAl-NF), presumably because of the Pt d-electron back-donation to the CO π* anti-bonding orbital [44]. The PtAl-NW catalyst clearly included 46.3% Ptδ+ (318.9 eV) and 53.7% Pt0 (315.8 eV), whereas the PtAl-NF catalyst had 38.1% Ptδ+ (317.8 eV) and 61.9% Pt0 (315.8 eV). It was found that the Pt states of the PtAl-NF catalyst were more electron-rich than those of the PtAl-NW catalyst and maintained the metallic Pt0 state, resulting in a smaller abundance of CO-covered Pt sites (inactive) and a higher abundance of O-covered Pt sites (active) on the surface.

4. Conclusions

In this study, subnanometer and nanometer scale Pt nanoparticles, single-atom Pt, and full exposed Pt clusters supported on γ-Al2O3 were investigated as the active Pt sites for C2H4 oxidation. Pt-γ-Al2O3 interaction was morphology-dependent, and different states of Pt species could be furnished in this manner. A simple surface reaction mechanism was proposed based on surface species measurements. The 1 nm Pt nanoparticles on γ-Al2O3 exhibited a much higher activity than subnanometer particles and full exposed Pt clusters. The XPS results clearly showed that Pt supported on PtAl-NF had a higher ratio (61.9%) of Pt0 and a lower binding energy Ptδ+ (317.8 eV) than PtAl-NW, which indicated that CO (intermediates) adsorbed on Pt sites had a lower oxidation barrier and are conveniently converted to easily desorbed CO2 (R9). There are, thus, few CO-poisoned Pt sites and high reaction rates. Furthermore, γ-Al2O3 with (110) and (100) basal planes resulted in various Pt morphology and dispersion, and (100) facets preferred to adsorb H2O molecules leading to the occupation of Pt active sites by H2O. Our results demonstrate that Pt with more dispersion (subnanometer particles, fully exposed Pt clusters, and single-atom Pt) has a higher material use efficiency, although there is still room to improve activity per Pt atom. According to the findings, increased atom utilization does not necessarily lead to an ideal catalytic performance when the noble metal loading is constant.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr10091773/s1, Figure S1. TEM images of synthesized γ-Al2O3 (Al−NF); Figure S2. HRTEM images of synthesized γ-Al2O3 (Al-NF). The nanoflakes in sample Al-NF were composed of aggregated crystalline grains with sizes of around 10 nm, which fused to form nanoflakes; Figure S3. TEM images of synthesized γ-Al2O3 (Al-NW); Figure S4. In situ DRIFTS spectra over PtAl-NW at temperatures from 120 to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, 50 mL min−1 flow rate; Figure S5. In situ DRIFTS spectra over PtAl-NF at temperatures from 120 to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, 50 mL min−1 flow rate; Figure S6. In situ DRIFTS spectra over Pt/γ-Al2O3 at temperatures from 120 to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, 50 mL min−1 flow rate.

Author Contributions

S.P. and Z.M. carried out the experiment, analyzed the data, rendered the figures and wrote the manuscript, with the help of J.M., H.W., K.R. and X.W.; B.W. obtained funding for the work and conceived the experiment. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Science and Technology Project of China Energy Investment, which is the development and engineering demonstration of key materials for VOCs oxidation catalysis in the coal chemical industry (ST930021003C).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This research was financially supported by the Science and Technology Project of China Energy Investment (ST930021003C). All individuals included in this section have consented to the acknowledgement.

Conflicts of Interest

The authors declare no conflict of interest.

Novelty Statement

In this work, the structure sensitivity and valence states of supported Pt species, ranging from single-atom sites to cluster (sub 1 nm) and 1–2 nm nanoparticles have been investigated for the C2H4 oxidation by in situ DRIFTS. This study showed that the 1 nm Pt nanoparticles on γ-Al2O3 exhibited a much higher activity than subnanometer particles, full exposed Pt clusters, and single-atom Pt. Our findings show that greater Pt dispersion (subnanometer particles, full exposed Pt clusters, and single-atom Pt) has an increased material use efficiency, but there is still space to enhance activity per Pt atom. The results imply that when the noble metal loading remains constant, increasing atom usage does not always result in optimum catalytic performance.

References

  1. Huang, R.J.; Zhang, Y.; Bozzetti, C.; Ho, K.F.; Cao, J.J.; Han, Y.; Daellenbach, K.R.; Slowik, J.G.; Platt, S.M.; Canonaco, F.; et al. High secondary aerosol contribution to particulate pollution during haze events in China. Nature 2014, 514, 218–222. [Google Scholar] [CrossRef] [PubMed]
  2. Zhang, Q.; Zheng, Y.; Tong, D.; Shao, M.; Wang, S.; Zhang, Y.; Xu, X.; Wang, J.; He, H.; Liu, W.; et al. Drivers of improved PM2.5 air quality in China from 2013 to 2017. Proc. Natl. Acad. Sci. USA 2019, 116, 24463–24469. [Google Scholar] [CrossRef] [PubMed]
  3. Li, M.; Zhang, Q.; Zheng, B.; Tong, D.; Lei, Y.; Liu, F.; Hong, C.; Kang, S.; Yan, L.; Zhang, Y.; et al. Persistent growth of anthropogenic non-methane volatile organic compound (NMVOC) emissions in China during 1990–2017: Drivers, speciation and ozone formation potential. Atmos. Chem. Phys. 2019, 19, 8897–8913. [Google Scholar] [CrossRef]
  4. Peng, S.; Li, W.; Deng, Y.; Li, W.; Ma, X.; Chen, Y. Removal of low concentration CH3SH with regenerable Cu-doped mesoporous silica. J. Colloid Interface Sci. 2018, 513, 903–910. [Google Scholar] [CrossRef]
  5. Cappelletti, M.; Fedi, S.; Frascari, D.; Ohtake, H.; Turner, R.J.; Zannoni, D. Analyses of both the alkB gene transcriptional start site and alkB promoter-inducing properties of Rhodococcus sp. strain BCP1 grown on n-alkanes. Appl. Environ. Microbiol. 2011, 77, 1619–1627. [Google Scholar] [CrossRef]
  6. Zhang, X.H.; Liu, Q.L.; Xiong, Y.; Zhu, A.M.; Chen, Y.; Zhang, Q.G. Pervaporation dehydration of ethyl acetate/ethanol/water azeotrope using chitosan/poly (vinyl pyrrolidone) blend membranes. J. Membr. Sci. 2009, 327, 274–280. [Google Scholar] [CrossRef]
  7. Sun, W.; Shao, M.; Granier, C.; Liu, Y.; Ye, C.S.; Zheng, J.Y. Long-Term Trends of Anthropogenic SO2, NOx, CO, and NMVOCs Emissions in China. Earth’s Future 2018, 6, 1112–1133. [Google Scholar] [CrossRef]
  8. Huang, H.; Xu, Y.; Feng, Q.; Leung, D.Y.C. Low temperature catalytic oxidation of volatile organic compounds: A review. Catal. Sci. Technol. 2015, 5, 2649–2669. [Google Scholar] [CrossRef]
  9. He, C.; Cheng, J.; Zhang, X.; Douthwaite, M.; Pattisson, S.; Hao, Z. Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources. Chem. Rev. 2019, 119, 4471–4568. [Google Scholar] [CrossRef]
  10. Zhao, J.; Xi, W.J.; Tu, C.S.; Dai, Q.G.; Wang, X.Y. Catalytic oxidation of chlorinated VOCs over Ru/TixSn1-x catalysts. Appl. Catal. B 2020, 263, 18237. [Google Scholar] [CrossRef]
  11. Xue, T.; Li, R.; Gao, Y.; Wang, Q. Iron mesh-supported vertically aligned Co-Fe layered double oxide as a novel monolithic catalyst for catalytic oxidation of toluene. Chem. Eng. J. 2020, 384, 123284. [Google Scholar] [CrossRef]
  12. Singh, S.A.; Madras, G.; Sreedhar, I. Transition Metal (Ni, Cu and Fe) Substituted Co3O4–ZrO2 Catalysts for Lean Methane Combustion. Top. Catal. 2020, 64, 243–255. [Google Scholar] [CrossRef]
  13. Naing, S.; Kristyawan, I.P.A.; Murakami, H.; Hinokuma, S.; Matsumoto, Y.; Omori, T.; Wang, Y.; Yokohata, H.; Kawano, M.; Takebayashi, H.; et al. Development of Detailed Surface Reaction Mechanism of C2H4/C(3)H(6)Oxidation on Pt/Al(2)O(3)Monolith Catalyst Based on Gas Phase and Surface Species Analyses. Combust. Sci. Technol. 2020, 194, 1458–1480. [Google Scholar] [CrossRef]
  14. Khan, H.A.; Hao, J.Y.; Farooq, A. Catalytic performance of Pd catalyst supported on Zr:Ce modified mesoporous silica for methane oxidation. Chem. Eng. J. 2020, 397, 125489. [Google Scholar] [CrossRef]
  15. Yang, A.C.; Zhu, H.; Li, Y.; Cargnello, M. Support Acidity Improves Pt Activity in Propane Combustion in the Presence of Steam by Reducing Water Coverage on the Active Sites. ACS Catal. 2021, 11, 6672–6683. [Google Scholar] [CrossRef]
  16. Gan, T.; Chu, X.F.; Qi, H.; Zhang, W.X.; Zou, Y.C.; Yan, W.F.; Liu, G. Pt/Al2O3 with ultralow Pt-loading catalyze toluene oxidation: Promotional synergistic effect of Pt nanoparticles and Al2O3 support. Appl. Catal. B 2019, 257, 17943. [Google Scholar] [CrossRef]
  17. Kim, J.; Kim, Y.; Wiebenga, M.H.; Oh, S.H.; Kim, D.H. Oxidation of C3H8, iso-C5H12 and C3H6 under near-stoichiometric and fuel-lean conditions over aged Pt-Pd/Al2O3 catalysts with different Pt:Pd ratios. Appl. Catal. B 2019, 251, 283–294. [Google Scholar] [CrossRef]
  18. Weon, S.; Huang, D.; Rigby, K.; Chu, C.; Wu, X.; Kim, J.H. Environmental Materials beyond and below the Nanoscale: Single-Atom Catalysts. ACS EST Eng. 2020, 1, 157–172. [Google Scholar] [CrossRef]
  19. Lang, R.; Du, X.; Huang, Y.; Jiang, X.; Zhang, Q.; Guo, Y.; Liu, K.; Qiao, B.; Wang, A.; Zhang, T. Single-Atom Catalysts Based on the Metal-Oxide Interaction. Chem. Rev. 2020, 120, 11986–12043. [Google Scholar] [CrossRef]
  20. Zhang, T.; Walsh, A.G.; Yu, J.; Zhang, P. Single-atom alloy catalysts: Structural analysis, electronic properties and catalytic activities. Chem. Soc. Rev. 2021, 50, 569–588. [Google Scholar] [CrossRef]
  21. Moses-DeBusk, M.; Yoon, M.; Allard, L.F.; Mullins, D.R.; Wu, Z.; Yang, X.; Veith, G.; Stocks, G.M.; Narula, C.K. CO oxidation on supported single Pt atoms: Experimental and ab initio density functional studies of CO interaction with Pt atom on theta-Al2O3(010) surface. J. Am. Chem. Soc. 2013, 135, 12634–12645. [Google Scholar] [CrossRef] [PubMed]
  22. Qiao, B.; Liang, J.X.; Wang, A.; Xu, C.Q.; Li, J.; Zhang, T.; Liu, J.J. Ultrastable single-atom gold catalysts with strong covalent metal-support interaction (CMSI). Nano Res. 2015, 8, 2913–2924. [Google Scholar] [CrossRef]
  23. Chen, J.; Wanyan, Y.; Zeng, J.; Fang, H.; Li, Z.; Dong, Y.; Qin, R.; Wu, C.; Liu, D.; Wang, M.; et al. Surface Engineering Protocol To Obtain an Atomically Dispersed Pt/CeO2 Catalyst with High Activity and Stability for CO Oxidation. ACS Sustain. Chem. Eng. 2018, 6, 14054–14062. [Google Scholar] [CrossRef]
  24. DeRita, L.; Dai, S.; Lopez-Zepeda, K.; Pham, N.; Graham, G.W.; Pan, X.; Christopher, P. Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO2. J. Am. Chem. Soc. 2017, 139, 14150–14165. [Google Scholar] [CrossRef]
  25. Dong, C.; Gao, Z.; Li, Y.; Peng, M.; Wang, M.; Xu, Y.; Li, C.; Xu, M.; Deng, Y.; Qin, X.; et al. Fully exposed palladium cluster catalysts enable hydrogen production from nitrogen heterocycles. Nat. Catal. 2022, 5, 485–493. [Google Scholar] [CrossRef]
  26. Li, L.; Jiang, Y.F.; Zhang, T.; Cai, H.; Zhou, Y.; Lin, B.; Lin, X.; Zheng, Y.; Zheng, L.; Wang, X.; et al. Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters. Chem 2021, 8, 749–768. [Google Scholar] [CrossRef]
  27. Chen, S.; Li, S.; You, R.; Guo, Z.; Wang, F.; Li, G.; Yuan, W.; Zhu, B.; Gao, Y.; Zhang, Z.; et al. Elucidation of Active Sites for CH4 Catalytic Oxidation over Pd/CeO2 Via Tailoring Metal–Support Interactions. ACS Catal. 2021, 11, 5666–5677. [Google Scholar] [CrossRef]
  28. Zhang, X.; Cui, W.W.; Page, K.L.; Pearce, C.I.; Bowden, M.E.; Graham, T.R.; Shen, Z.Z.; Li, P.; Wang, Z.M.; Kerisit, S.; et al. Size and Morphology Controlled Synthesis of Boehmite Nanoplates and Crystal Growth Mechanisms. Cryst. Growth Des. 2018, 18, 3596–3606. [Google Scholar] [CrossRef]
  29. Jiao, W.; Wu, X.; Xue, T.; Li, G.; Wang, W.; Wang, Y.; Wang, Y.; Tang, Y.; He, M.Y. Morphological Controlled Growth of Nanosized Boehmite with Enhanced Aspect Ratios in an Organic Additive-Free Cationic-Anionic Double Hydrolysis Method. Cryst. Growth Des. 2016, 16, 5166–5173. [Google Scholar] [CrossRef]
  30. Chen, X.Y.; Huh, H.S.; Lee, S.W. Hydrothermal synthesis of boehmite (gamma-AlOOH) nanoplatelets and nanowires: pH-controlled morphologies. Nanotechnology 2007, 18, 285608. [Google Scholar] [CrossRef]
  31. Kuiry, S.C.; Megen, E.; Patil, S.D.; Deshpande, S.A.; Seal, S. Solution-based chemical synthesis of boehmite nanofibers and alumina nanorods. J. Phys. Chem. B 2005, 109, 3868–3872. [Google Scholar] [CrossRef]
  32. Xu, S.; Jaegers, N.R.; Hu, W.; Kwak, J.H.; Bao, X.; Sun, J.; Wang, Y.; Hu, J.Z. High-Field One-Dimensional and Two-Dimensional 27Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of theta-, delta-, and gamma-Al2O3 Dominated Aluminum Oxides: Toward Understanding the Al Sites in gamma-Al2O3. ACS Omega 2021, 6, 4090–4099. [Google Scholar] [CrossRef]
  33. Liu, Q.; Wang, A.; Wang, X.; Gao, P.; Wang, X.; Zhang, T. Synthesis, characterization and catalytic applications of mesoporous gamma-alumina from boehmite sol. Microporous Mesoporous Mater. 2008, 111, 323–333. [Google Scholar] [CrossRef]
  34. Alphonse, P.; Courty, M. Structure and thermal behavior of nanocrystalline boehmite. Thermochim. Acta 2005, 425, 75–89. [Google Scholar] [CrossRef]
  35. Li, Z.; Liu, G.H.; Li, X.B.; Qi, T.; Peng, Z.H.; Zhou, Q.S. Effects of Cation on the Morphology of Boehmite Precipitated from Alkaline Solutions by Adding Gibbsite as Seed. Cryst. Growth Des. 2019, 19, 1778–1785. [Google Scholar] [CrossRef]
  36. Cai, W.M.; Zhang, S.G.; Lv, J.G.; Chen, J.C.; Yang, J.; Wang, Y.B.; Guo, X.F.; Peng, L.M.; Ding, W.P.; Chen, Y.; et al. Nanotubular Gamma Alumina with High-Energy External Surfaces: Synthesis and High Performance for Catalysis. ACS Catal. 2017, 7, 4083–4092. [Google Scholar] [CrossRef]
  37. Krokidis, X.; Raybaud, P.; Gobichon, A.E.; Rebours, B.; Euzen, P.; Toulhoat, H. Theoretical study of the dehydration process of boehmite to gamma-alumina. J. Phys. Chem. B 2001, 105, 5121–5130. [Google Scholar] [CrossRef]
  38. Chen, X.Y.; Zhang, Z.H.; Li, X.L.; Lee, S.W. Controlled hydrothermal synthesis of colloidal boehmite (gamma-AlOOH) nanorods and nanoflakes and their conversion into gamma-Al2O3 nanocrystals. Solid State Commun. 2008, 145, 368–373. [Google Scholar] [CrossRef]
  39. Kaiser, S.; Maleki, F.; Zhang, K.; Harbich, W.; Heiz, U.; Tosoni, S.; Lechner, B.A.J.; Pacchioni, G.; Esch, F. Cluster Catalysis with Lattice Oxygen: Tracing Oxygen Transport from a Magnetite (001) Support onto Small Pt Clusters. ACS Catal. 2021, 11, 9519–9529. [Google Scholar] [CrossRef]
  40. Ziemba, M.; Schilling, C.; Ganduglia-Pirovano, M.V.; Hess, C. Toward an Atomic-Level Understanding of Ceria-Based Catalysts: When Experiment and Theory Go Hand in Hand. Acc. Chem. Res. 2021, 54, 2884–2893. [Google Scholar] [CrossRef]
  41. Gao, X.; Li, Z.; Chen, C.; Da, C.; Liu, L.; Tian, S.; Ji, G. The Determination of Pore Shape and Interfacial Barrier of Entry for Light Gases Transport in Amorphous TEOS-Derived Silica: A Finite Element Method. ACS Appl. Mater. Interfaces 2021, 13, 4804–4812. [Google Scholar] [CrossRef]
  42. Zaki, M.I.; Hasan, M.A.; Pasupulety, L. Surface Reactions of Acetone on Al2O3, TiO2, ZrO2, and CeO2:  IR Spectroscopic Assessment of Impacts of the Surface Acid−Base Properties. Langmuir 2001, 17, 768–774. [Google Scholar] [CrossRef]
  43. Ding, K.; Gulec, A.; Johnson, A.M.; Schweitzer, N.M.; Stucky, G.D.; Marks, L.D.; Stair, P.C. Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts. Science 2015, 350, 189–192. [Google Scholar] [CrossRef]
  44. Jeong, H.; Kwon, O.; Kim, B.S.; Bae, J.; Shin, S.; Kim, H.E.; Kim, J.; Lee, H. Highly durable metal ensemble catalysts with full dispersion for automotive applications beyond single-atom catalysts. Nat. Catal. 2020, 3, 368–375. [Google Scholar] [CrossRef]
  45. Jang, E.J.; Lee, J.; Oh, D.G.; Kwak, J.H. CH4 Oxidation Activity in Pd and Pt–Pd Bimetallic Catalysts: Correlation with Surface PdOx Quantified from the DRIFTS Study. ACS Catal. 2021, 11, 5894–5905. [Google Scholar] [CrossRef]
  46. Pramhaas, V.; Roiaz, M.; Bosio, N.; Corva, M.; Rameshan, C.; Vesselli, E.; Grönbeck, H.; Rupprechter, G. Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts. ACS Catal. 2021, 11, 208–214. [Google Scholar] [CrossRef]
  47. Dablemont, C.; Lang, P.; Mangeney, C.; Piquemal, J.Y.; Petkov, V.; Herbst, F.; Viau, G. FTIR and XPS Study of Pt Nanoparticle Functionalization and Interaction with Alumina. Langmuir 2008, 24, 5832–5841. [Google Scholar] [CrossRef]
  48. Peng, M.; Dong, C.; Gao, R.; Xiao, D.; Liu, H.; Ma, D. Fully Exposed Cluster Catalyst (FECC): Toward Rich Surface Sites and Full Atom Utilization Efficiency. ACS Cent. Sci. 2020, 7, 262–273. [Google Scholar] [CrossRef] [PubMed]
  49. Kwak, J.H.; Hu, J.; Mei, D.; Yi, C.W.; Kim, D.H.; Peden, C.H.; Allard, L.F.; Szanyi, J. Coordinatively Unsaturated Al3+ Centers as Binding Sites for Active Catalyst Phases of Platinum on gamma-Al2O3. Science 2009, 325, 1670–1673. [Google Scholar] [CrossRef] [PubMed]
Figure 1. The testing system of in situ diffuse reflectance FT−IR spectroscopy.
Figure 1. The testing system of in situ diffuse reflectance FT−IR spectroscopy.
Processes 10 01773 g001
Figure 2. XRD patterns of synthesized nanoflake (Al-NF) and nanowire (Al-NW) γ-Al2O3.
Figure 2. XRD patterns of synthesized nanoflake (Al-NF) and nanowire (Al-NW) γ-Al2O3.
Processes 10 01773 g002
Figure 3. High-resolution TEM images of synthesized γ-Al2O3. (a,c) Al-NF and (b,d) Al-NW.
Figure 3. High-resolution TEM images of synthesized γ-Al2O3. (a,c) Al-NF and (b,d) Al-NW.
Processes 10 01773 g003
Figure 4. C2H4 combustion over a series of Pt (1 wt.%) catalysts at temperatures from 100 to 250 °C; Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, and GHSV = 600,000 mL gcat.−1 h−1.
Figure 4. C2H4 combustion over a series of Pt (1 wt.%) catalysts at temperatures from 100 to 250 °C; Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, and GHSV = 600,000 mL gcat.−1 h−1.
Processes 10 01773 g004
Figure 5. In situ DRIFT spectra over Pt/γ−Al2O3 at 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, and 50 mL min1 flow rate.
Figure 5. In situ DRIFT spectra over Pt/γ−Al2O3 at 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, and 50 mL min1 flow rate.
Processes 10 01773 g005
Figure 6. In situ DRIFT spectra over PtAl−NF at temperatures from 120 to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, and 50 mL min−1 flow rate.
Figure 6. In situ DRIFT spectra over PtAl−NF at temperatures from 120 to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, and 50 mL min−1 flow rate.
Processes 10 01773 g006
Figure 7. In situ DRIFT spectra over PtAl−NW at temperatures from 120 °C to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, 50 mL min−1 flow rate.
Figure 7. In situ DRIFT spectra over PtAl−NW at temperatures from 120 °C to 180 °C. Reaction conditions: 700 ppm C2H4, 10% O2, N2 balance, 17 mg catalyst, 50 mL min−1 flow rate.
Processes 10 01773 g007
Figure 8. HAADF-STEM images of (a) PtAl-NF and (c) PtAl-NW; Pt particle size distribution of (b) PtAl-NF and (d) PtAl-NW. The count number of Pt particles on each sample was 100.
Figure 8. HAADF-STEM images of (a) PtAl-NF and (c) PtAl-NW; Pt particle size distribution of (b) PtAl-NF and (d) PtAl-NW. The count number of Pt particles on each sample was 100.
Processes 10 01773 g008
Figure 9. XPS Pt 4d spectra of PtAl-NF and PtAl-NW.
Figure 9. XPS Pt 4d spectra of PtAl-NF and PtAl-NW.
Processes 10 01773 g009
Table 1. The concentration of Pt Present on γ-Al2O3 and BET Surface Area.
Table 1. The concentration of Pt Present on γ-Al2O3 and BET Surface Area.
CatalystSurface Area/m2·g−1Pore Volume/mL·g−1Average Pore Diameter/nmPt Loading/wt.%
PtAl-NF66.70.2715.80.98
PtAl-NW76.40.3216.30.97
Table 2. Proposed surface reaction steps of C2H4 for Pt/γ-Al2O3.
Table 2. Proposed surface reaction steps of C2H4 for Pt/γ-Al2O3.
Reaction
R1O2 + 2Pt(S) ↔ 2O(S)Reversible
R2C2H4 + 2Pt(S) ↔ C2H4(2S)Reversible
R3C2H4(2S) + O(S) → CH3CHO(2S) + Pt(S)
R4CH3CHO(2S) + O(S) → CH3COOH(2S) + Pt(S)
R5CH3COOH(2S) + O(S) → CH3(S) + OH(S)+ CO2(S)
R6CH3(S) + O(S) → CH2(S) + OH(S)
R7CH2(S) + O(S) → CH(S) + OH(S)
R8CH(S) + O(S) + OH(S) → CO(S) + Pt(S)+ H2O(S)
R9CO(S) + O(S) → CO2(S) + Pt(S)
R10H2O(S) ↔ H2O + Pt(S)Reversible
R11CO2(S) ↔ CO2 + Pt(S)Reversible
R12CO(S) ↔ CO + Pt(S)Reversible
Note: “(S)” stands for exposed Pt active sites, and X(S) means that the “X” molecule was adsorbed on Pt active sites.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Peng, S.; Ma, Z.; Ma, J.; Wang, H.; Ren, K.; Wu, X.; Wang, B. In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation. Processes 2022, 10, 1773. https://doi.org/10.3390/pr10091773

AMA Style

Peng S, Ma Z, Ma J, Wang H, Ren K, Wu X, Wang B. In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation. Processes. 2022; 10(9):1773. https://doi.org/10.3390/pr10091773

Chicago/Turabian Style

Peng, Shengpan, Ziran Ma, Jing Ma, Hongyan Wang, Kai Ren, Xiaodong Wu, and Baodong Wang. 2022. "In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation" Processes 10, no. 9: 1773. https://doi.org/10.3390/pr10091773

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop