Maximized Ir atom utilization via downsizing active sites to single-atom scale for highly stable dry reforming of methane
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Abstract
Noble metals such as iridium with high Tammann temperature are inclined to sintering resistance and may be promising in the high-temperature dry reforming of methane (DRM) process, yet the low atom utilization remains intractable. Herein, we synthesized Ir/TiO2 catalysts via the conventional incipient wetness impregnation method and further downsized the Ir species from a nanoparticulate to a single-atom scale by gradually decreasing Ir loadings from 1.0 wt.% to 0.01 wt.%. With the advantage of single atoms for maximized atom utilization, Ir single atoms were employed to enhance atom utilization in the DRM process. Various characterizations, such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, CO adsorbed in situ diffuse reflectance infrared Fourier transform spectra and X-ray absorption spectra demonstrated the existence of Ir single atoms in 0.01% and 0.05% Ir/TiO2. During the DRM process, Ir single-atom catalysts exhibited a better specific reaction rate of as high as 697.71 molCH4·gIr-1·h-1 at 750 °C compared with that over Ir nanoparticles of mere 447.12 molCH4·gIr-1·h-1, which unambiguously showed the remarkable Ir atom utilization over Ir single atoms. Besides, the Ir single-atom catalysts also exhibited excellent stability during the DRM process for 50 h and revealed outstanding anti-coking and good sintering-resistance properties examined by the thermal gravimetric analysis-mass spectrometer and Raman spectroscopy. The strategy of employing Ir single atoms for the maximum atom utilization in the high-temperature reaction process can pave the way for better exploitation of noble metals in other industrial reaction processes.
Keywords
INTRODUCTION
Carbon dioxide and methane are major parts of greenhouse gases, which are notoriously responsible for the year-on-year global warming and unfavorable climate change[1-4]. Therefore, to retard or even reverse those unsettling environmental issues, greenhouse gases should be scientifically stored[5,6] or converted into value-added chemicals[7-9]. To this end, dry reforming of methane (DRM) process, during which carbon dioxide and methane are simultaneously converted into a mixture of monoxide and hydrogen, known as syngas for the industrially downstream hydroformylation[10], carbonylation[11], and Fischer-Tropsch processes[12-14], is widely investigated in both academic and industrial fields[15]. Therein, one main research point is the selection of active metals. Generally, transition metals, especially Ni-based catalysts[2,16-18] and noble metals (Rh[19], Ru[20], Pt[21], Pd[22], Ir) are both hotspots for the DRM process. Although transition metals have abundant reserves on Earth and show high catalytic activity during the DRM process, they also severely suffer from sintering and the consequent carbon deposition at high temperatures[23], which inevitably causes high operational costs for coke elimination of facilities and regeneration of catalysts. Alternatively, noble metals with excellent activity and stability are promising candidates for the DRM process. Meanwhile, their long-term anti-coking properties[24,25] can efficiently prevent the shutdown of facilities and profitably offset the expense of noble metals in return.
Compared with the other noble metals suitable for the DRM process, iridium has the highest Tammann temperature[13,26], probably inferring the best thermal stability even at harsh reaction conditions. Overall, iridium species have been mainly investigated as either promoters or active sites in the DRM process. As for iridium species as promotors, especially over dual Ni-Ir systems[27-30], iridium species can efficiently transform the carbon depositions generated from CH4 activation over the adjacent Ni nanoparticles and the carbonate species formed from CO2 adsorbed on the support into monoxide, thus fostering the elimination of carbon depositions and consequently rendering the improvement of long-term stability. However, with regard to iridium species as active sites, few researches focus on the correlation of intrinsic properties of iridium with catalytic performances while most reports primarily pay attention to how the physiochemical properties of supports, for example, oxygen vacancies[31], acidity[32] and metal-support interaction[33], affect the CO2 activation and stability during DRM process. Considering the scarcity of iridium, the intrinsic properties of Ir atoms should be exploited as far as possible to maximize atom utilization.
Single-atom catalysts (SACs) have gained extensive spotlights in the heterogeneous catalysis field due to their outstanding catalytic activity and maximized metal atom utilization efficiency[34-40]. For example, Zhou et al. have synthesized Ru single atoms supported on nitrogen-doped carbon (Ru1/NC) which were applied in the high-temperature propane dehydrogenation[41]. Compared with the Ru nanoparticulate counterparts, the specific rate of the production of propylene per metal sites increased as the size of the Ru particle decreased and peaked at 428 molC3H6·gRu-1·h-1 over Ru1/NC. In addition, Han et al. have constructed Rh single atoms supported on CeO2 (Rh1/CeO2) for the low-temperature CO oxidation, during which Rh1/CeO2 attained a specific rate of 14.3 molCO·h-1·gRh-1 while only 3.1 molCO·h-1·gRh-1 was yielded over Rh nanoparticles[42]. The abovementioned reports undoubtedly demonstrate the effectiveness of SACs on maximized atom utilization. Therefore, it is plausible that Ir atom utilization may also be greatly improved via the strategy of downsizing Ir active sites to a single-atom scale since our group has previously revealed an excellent specific reaction rate over Ni SACs in the DRM process[43,44].
Herein, we supported Ir species on TiO2 via the conventional incipient wetness impregnation method and acquired Ir nanoparticles, atomically dispersed Ir and Ir SACs by gradually scaling down the Ir loading from 1.0 wt.% to 0.01 wt.%. Combining various characterizations, it was evidenced that Ir SACs were successfully supported on TiO2 and meanwhile had stronger electronic interaction with TiO2 compared with that with bulk Ir nanoparticles. Therefore, Ir SACs showed a better specific reaction rate of 697.71 molCH4·gIr-1·h-1 at 750 °C in the DRM process than that of 447.12 molCH4·gIr-1·h-1 over Ir nanoparticles. Besides, the
EXPERIMENTAL
Synthesis of iridium SACs and nanoparticles supported on TiO2
The conventional incipient wetness impregnation method was employed to prepare Ir SACs and nanoparticles supported on TiO2. In detail, 0.02, 0.10, 0.41, 1.02, and 2.04 mL H2IrCl6 (purchased from Energy Chemical, 98%) aqueous solution (4.9 mgIr/mL) were added into 50 mL deionized water for
Characterizations
The actual Ir loadings of x% Ir/TiO2 catalysts were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES) on an ICPS-8100 instrument (Shimadzu Co., Ltd.). Textural properties were measured by N2 adsorption-desorption isotherms recorded at 77 K using Quantachrome station. Specific surface areas were calculated from the adsorption branch using the Brunauer-Emmett-Teller (BET) method. X-ray diffraction (XRD) patterns were recorded with a PW3040/60 X’Pert PRO PANalytical diffractometer with Cu Kα radiation (λ = 0.15432 nm) operating at 40 kV and 40 mA in the range of 2θ = 10°-80°.
Temperature-programmed reduction (H2-TPR) was performed on a Micromeritics AutoChem II 2920 instrument. For each experiment, 100 mg of each sample was placed in a quartz reactor and pretreated in flowing Ar (30 mL·min-1) at 300 °C for 30 min to remove the physically adsorbed water. After cooling to
The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were conducted on JEOL JEM-2100F operating at an accelerating voltage of 200 kV. The aberration-corrected HAADF-STEM (AC-HAADF-STEM) images were obtained on JEOL JEM-ARM200F operated at 200 kV equipped with a Gatan Quantum 965 image filter system. Before the measurement, the samples were ultrasonically dispersed in ethanol and deposited onto carbon films.
In situ X-ray photoelectron spectroscopy (XPS) analysis was performed using a ThermoFischer ESCALAB 250Xi instrument equipped with an Al Kα excitation source (1,486.8 eV). Adventitious carbon was employed as the internal standard (C 1s = 284.8 eV). Before the measurement, the as-synthesized samples underwent in situ reduction in 10 vol.% H2/N2 at 500 °C for 1 h in an ultra-high vacuum (UHV)-connected high-pressure gas chamber. After this pre-treatment, the samples were transferred into the analyzer chamber under UHV conditions.
In situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) of adsorbed CO were collected on an infrared spectrometer (Vertex 70, Bruker) equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. Spectra were collected in the range 1,000-4,000 cm-1 range at a resolution of
Thermogravimetric analysis (TGA) was performed using a Simultaneous Thermal Analyzer (STA449F5). The catalysts were initially heated to 200 °C in Ar flow with a ramp rate of 5 °C·min-1 and maintained at the temperature for 0.5 h to remove physically adsorbed water. Subsequently, the samples were further heated to 800 °C following the same heating protocol. Raman spectroscopy was conducted using a combined atomic force microscopy (AFM)-Raman system (Bruker NanoWizard Ultra Speed & inVia Raman) with a 532 nm excitation laser.
The X-ray absorption spectra (XAS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at Ir L-edge analysis was performed with Si (111) crystal monochromators at the BL14W1 beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China). Before the analysis at the beamline, samples were pressed into thin sheets 1 cm in diameter and sealed using Kapton tape film. The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040. Ir L-edge EXAFS spectra were recorded in fluorescence mode. Negligible changes in the line shape and peak position of Ir L-edge XANES spectra were observed between two scans taken for a specific sample. The XAFS spectra of these standard samples (Ir, IrO2) were recorded in transmission mode for 0.05% Ir/TiO2, and a solid detector was used to achieve high data quality. For the XAS spectra of reduced catalysts, the samples were reduced in a fixed-bed reactor with 10 vol% H2/Ar at 500 °C. The spectra were processed and analyzed using the Athena and Artemis software code.
Catalytic tests
Methane dry reforming reaction over different samples was evaluated at atmospheric pressure in a fixed bed quartz reactor (inner diameter = 0.76 cm) with 50 mg catalysts. Before each catalytic test, the sample was in situ reduced at 500 °C for 60 min under 10 vol.% H2/Ar and then purged for 30 min with Ar until the temperature increased to 750 °C. The mixture gas of CH4 and CO2 (20 vol.% CH4, 20 vol.% CO2, 1 vol.% N2, Ar balanced) with a total flow rate of 32.3 mL/min [gas hourly space velocity (GHSV) of 38,760 mL·gcat-1·h-1] was fed into the fixed bed reactor. The specific reaction rate and turnover frequency (TOF) for CH4 and CO2 conversion were measured at much higher GHSV (using 2-50 mg of catalyst diluted by 70-80 mesh quartz sand, wherein the CH4 and CO2 conversions were controlled below 25%). The reactor temperature was recorded with a thermocouple inserted close to the catalytic bed of the reactor. The reactants and products were analyzed by a gas chromatograph equipped with a thermal conductivity detector and a
where [CH4] and [CO2] denoted the concentrations (vol.%) detected by gas chromatography (GC), while
where Ft represents the total flow (mL·min-1), whereas ωIr is the iridium metal loading (wt%). TOF (s-1), defined as the number of CH4 or CO2 molecules converted per exposed catalytic site per second, is determined based on:
where the mean particle size (dVA) is obtained from STEM images[45,46], specifically by selecting more than ten images and measuring over one hundred particles. The derivation process of the dispersion formula is presented in the Supplementary Materials. As for Ir single atom/atomically dispersed Ir species, all the Ir atoms are directly bonded with TiO2 support and exposed to the catalytic environment, thus showing 100% dispersion[47,48].
Before carrying out the kinetic experiment, it is necessary to eliminate external diffusion by changing the linear velocity. It is worth mentioning that the CH4 conversion is almost the same when the liner velocity is above 7.89 cm/min [Supplementary Table 1 and Supplementary Figure 2], at which the external diffusion on the reaction is experimentally eliminated.
RESULTS AND DISCUSSION
Iridium species were dispersed onto the rutile TiO2 support via the conventional incipient wetness impregnation method with H2IrCl6 as the precursor. In order to downsize the Ir species from a nanoparticulate scale to an atomically dispersed or even single-atom scale, the nominal Ir loading was gradually decreased from 1.0 wt.% to 0.01 wt.%. The as-obtained samples were denoted as x% Ir/TiO2, where x represented the nominal iridium loading. As shown in Figure 1A, XRD patterns of all the samples exhibit the retention of the rutile phase, while the absence of both iridium and IrO2 can be attributed to the low loading and/or high dispersion of Ir species on TiO2 support. The N2 adsorption-desorption isotherms of the above samples [Figure 1B] also bear few differences and the corresponding BET surface areas
Figure 1. Physical properties of different Ir/TiO2 samples. (A) XRD patterns and (B) N2 adsorption-desorption isotherms. XRD: X-ray diffraction.
The distributions of Ir species were directly observed via the AC-HAADF-STEM. As can be seen in Figure 2A and B, Ir single atoms or atomically dispersed Ir species can be attained when we lower the Ir loadings below 0.05 %. Further examining AC-HAADF-STEM images with a lower magnitude of 0.05% Ir/TiO2 [Supplementary Figure 3] also testifies to high dispersion of Ir single atoms. It is worth noting that a calcination temperature of 500 °C is the prerequisite for acquiring highly dispersed Ir single atoms as a higher calcination temperature, such as 750 °C, leads to Ir agglomeration [Supplementary Figures 4 and 17]. As for the samples with Ir loadings of 0.2%-1.0% [Figure 2C-E], Ir species are mainly in the form of nanoparticles with a similar average size of 1.78 nm regardless of increasing Ir loadings. As mentioned above, the resistance of Ir sintering can result from the inherently high Tammann temperature, which is a promising merit for giving excellent stability at harsh temperatures in the DRM process.
Figure 2. AC-HAADF-STEM images of (A) 0.01% Ir/TiO2; (B) 0.05% Ir/TiO2 and HAADF-STEM images of (C) 0.2% Ir/TiO2; (D) 0.5% Ir/TiO2; (E) 1.0% Ir/TiO2 samples after reduction at 500 °C. AC-HAADF-STEM: Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy.
To further estimate the dispersion and chemical properties of Ir species on Ir/TiO2 catalysts, CO was used as a probe in the in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra to get a structural comparison after in situ reducing the samples at 500 °C. In a typical experiment, after CO reached saturation adsorption, pure He was purged into the reaction chamber to remove the physically adsorbed CO while the chemically adsorbed CO remained on the metal sites. Typically, four characteristic adsorption peaks can be observed in the CO-DRIFT spectra of Ir species. The two features at 2,058 and 1,990 cm-1 can be assigned to the dicarbonyl Ir+(CO)2 species, which signals the existence of Ir single atoms, similar to the results of peaks at around 2,065 and 1,985 cm-1 reported by Lu et al.[49] and Jin et al.[50]. In addition, the peak at 2,036 cm-1 is attributed to the atop binding of CO on Ir nanoclusters while the broad peak centered at 1,820 cm-1 is ascribed to bridged CO species, which are the indicators of Ir clusters or nanoparticles[45,49,50]. For 1.0%, 0.5% and 0.2% Ir/TiO2 samples [Figure 3A-C], all the abovementioned four CO adsorbed peaks can be clearly identified, suggesting that Ir species have relatively wide distributions in those samples. As for 0.05% Ir/TiO2 [Figure 3D], two main dicarbonyl and one weak atop CO adsorption peaks can be ascertained. Combined with the devoid of bridged CO species, it is rational that Ir species are atomically dispersed on TiO2, which is in accordance with the results obtained from the AC-HAADF-STEM images. Although the low Ir loading of 0.01% Ir/TiO2 sets obstacles for evidently monitoring the adsorption peaks
Figure 3. DRIFT spectra of CO adsorption on Ir/TiO2 with different Ir loadings after in-situ reduction at 500 °C. (A) 1.0% Ir/TiO2; (B) 0.5% Ir/TiO2; (C) 0.2% Ir/TiO2; (D) 0.05% Ir/TiO2. DRIFT: Diffuse reflectance infrared Fourier transform spectra.
Furthermore, after downsizing the Ir species from the nanoparticulate to single-atom scale, the interaction between Ir species and TiO2 support can be switched into a stronger state due to the direct contact between Ir single atoms and oxygen atoms of the TiO2 support. In order to give evidence of this inference, XPS experiments were conducted after in situ reducing the Ir/TiO2 samples at 500 °C. As illustrated in Figure 4A, the binding energy of Ir 4d5/2 peak gradually increases from 296.64 eV of 1.0% Ir/TiO2 to 298.11 eV of 0.05% Ir/TiO2, indicating that exposing only Ir single atoms can forge stronger interaction between Ir species and lattice oxygen of TiO2. Moreover, the reduction peaks at 152 and 220 °C attributed to the reduction of Ir species in the H2-TPR profiles [Figure 4B] also shift to higher temperatures after downsizing the scale of Ir species. This resistance to reduction is also an accompanying benefit of Ir single atoms anchored on TiO2, which may be conducive to enhancing stability in the DRM process. Moreover, the reduction peaks of TiO2 supports also shift to lower temperatures due to the occurrence of hydrogen spillover compared with the bare TiO2 support.
Figure 4. Interactions between Ir species and TiO2 support of different samples. (A) In situ Ir 4d XPS spectra and (B) H2-TPR profiles. XPS: X-ray photoelectron spectroscopy; TPR: temperature-programmed reduction.
X-ray absorption spectroscopy was carried out on different Ir/TiO2 samples to further gain insight into the local coordination state of Ir atoms. The Ir L3-edge XANES spectroscopy [Figure 5A] shows the white lines of Ir/TiO2 samples between Ir and IrO2, which reveal the partially positive state of Ir species consistent with the in situ XPS results. In addition, the Fourier-transformed (FT) k3-weighted EXAFS profiles were obtained to further study the fine structure and local environment of Ir species [Figure 5B]. As for the 0.05%Ir/TiO2, the Ir-O scattering at ~2.03 Å is exclusively detected and no Ir-Ir scattering can be observed, thus confirming the single-atom Ir species supported on TiO2. However, for 0.2%, 0.5% and 1.0% Ir/TiO2 samples, the Ir-Ir scattering at 2.73 Å is gradually intensified with the average coordination number of Ir-Ir scattering path increasing to 1.35, 2, 2.79, respectively [Supplementary Table 3]. Therefore, combined with the AC-HAADF-STEM, in situ CO DRIFTs and EXAFS results, we can unambiguously confirm that the Ir species of 0.01% Ir/TiO2 and 0.05% Ir/TiO2 samples are in single-atom state, while those of 0.2%-1.0% Ir/TiO2 samples are mainly composed of nanoparticles.
Figure 5. (A) Ir L3 XANES spectra and (B) Fourier-transformed k3-weighted EXAFS spectra of Ir foil, IrO2, and different Ir/TiO2 samples. XANES: X-ray absorption near-edge structure; EXAFS: extended X-ray absorption fine structure.
DRM is a highly endothermic reaction that often requires a high temperature. Prior experiments have shown the ideal reaction temperature of 650 to 800 °C, especially higher than 750 °C, to effectively activate both CH4 and CO2 molecules [Supplementary Figure 7]. However, the subsequent sintering of active sites forming thermodynamical stable agglomerates is detrimental to the catalytic stability due to either a decrease in active metal sites and/or the concomitant formation of coke depositions. Considering the expense of noble metals such as iridium, the maximum utilization of Ir atoms while maintaining catalytic stability is of great importance. To cater to those criteria, Ir loading was decreased to scale down the Ir species from a nanoparticulate to a single-atom scale, and the corresponding DRM performances for 50 h are shown in Figure 6. During the DRM process over different Ir/TiO2 samples, CO2 conversion [Figure 6B] is constantly higher than CH4 conversion [Figure 6A] due to the concomitant occurrence of the reverse water-gas shift reaction. However, with increasing Ir loading, both CH4 and CO2 conversions rise along with the gradually growing H2/CO ratio [Figure 6C], which unequivocally indicates the progressive dominance of the DRM process in the whole reaction[8,51]. Other than CO and H2, water is the only detected by-product due to the simultaneous reverse water-gas shift reaction. During the time-on-stream of 50 h, all the Ir catalysts display excellent stability, which agrees well with our experimental strategy of selecting Ir species with high Tammann temperature as the stable active sites for DRM even at a high temperature of 750 °C. Besides, it is worthwhile to mention that although the CH4 conversions over Ir nanoparticles are higher than that over atomically dispersed Ir or Ir single atoms, increasing the Ir loading from, for example, 0.05% to 0.2%, the CH4 conversion is not proportionally increased, which implies the effective atom utilization over Ir SACs compared with the nanoparticulate counterparts. This reasonable speculation is further tested by purposedly controlling the CH4 conversion below 25% to quantitatively compare the TOF values and specific reaction rates over different Ir/TiO2 catalysts[43,52]. As listed in Table 1 and Supplementary Table 4, considering the exposed Ir atoms as active sites, the TOF values over Ir nanoparticles are slightly higher than that over Ir single atoms, which can be attributed to the existence of step, corner and kink sites on nanoparticles for better activation of reactants. Nevertheless, when all the Ir atoms are considered to acquire specific reaction rates for the assessment of atom utilization, on the contrary, the specific reaction rates over Ir single atoms are higher than those over Ir nanoparticles because of the fully exposing Ir atoms to the reactant molecules[47,48,53]. Particularly, the specific reaction rate over 0.05% Ir/TiO2 is as high as 697.71 molCH4·gIr-1·h-1 at 750 °C while that only ranges from 447.12 to 499.32 molCH4·gIr-1·h-1 over Ir nanoparticles, which corroborates the highest Ir atom utilization from the economic perspective after downsizing the Ir species to single-atom scale while part of the atoms inside Ir nanoparticles do not participate in the catalytic process. Besides, we also compare the DRM performance over 0.05% Ir/TiO2 with reported results over transition and noble metals, which demonstrate the excellent coke-resistance and higher specific reaction rate over the single-atom scale Ir species [Supplementary Table 5]. Other Ir SACs supported on, such as SiO2, MgO and Al2O3, were also tested in the DRM process. The results show that 0.05% Ir/SiO2, 0.05%
Figure 6. DRM performances over different loadings of Ir/TiO2 catalysts. (A) CH4 conversion (B) CO2 conversion and (C) the ratio of
Catalytic reaction rate during DRM process over different loadings of Ir catalysts
| Sample | Dispersion (%) | TOF (s-1)a | Specific reaction ratea (mol·gIr-1·h-1) | Temperature (°C) | ||
| CH4 | CO2 | CH4 | CO2 | |||
| 0.01% Ir/TiO2 | 100 | 12.02 | 20.05 | 225.09 | 375.52 | 700 |
| 32.07 | 60.99 | 600.56 | 1,142.22 | 750 | ||
| 0.05% Ir/TiO2 | 100 | 10.40 | 17.29 | 194.86 | 323.83 | 700 |
| 37.25 | 68.55 | 697.71 | 1,283.78 | 750 | ||
| 0.2% Ir/TiO2 | 62.10 | 17.28 | 28.38 | 200.99 | 330.07 | 700 |
| 42.93 | 72.10 | 499.32 | 838.52 | 750 | ||
| 0.5% Ir/TiO2 | 61.82 | 17.80 | 27.93 | 206.05 | 323.41 | 700 |
| 39.60 | 60.28 | 458.49 | 697.87 | 750 | ||
| 1.0% Ir/TiO2 | 61.47 | 14.58 | 22.19 | 167.80 | 255.43 | 700 |
| 38.84 | 56.26 | 447.12 | 647.71 | 750 | ||
The ability to activate CH4 and the corresponding cracking mechanism were investigated via TPSR experiments using diluted CH4 as the sole reactant because CH4 cracking is of great significance during the DRM process. In CH4-TPSR experiments, two consecutive processes are involved: the CH4 decomposition (CH4 → C + 2H2) and the formation of CO (C + O* → CO) from the above deposited carbon and lattice oxygen atoms on the TiO2. As a result, both the MS signals of H2 and CO can be detected. For the original TiO2 support [Supplementary Figure 12], barely can any CH4 be activated in the temperature-programmed process. After Ir deposition, CH4 cracking occurs, which suggests Ir species are responsible for the CH4 activation. As shown in Figure 7A and B, the activation of CH4 over 1.0% Ir/TiO2 is more inclined to happen than that over 0.05% Ir/TiO2. The temperature point at which CH4 starts cracking is in the order of 0.01% (at 380 °C, Supplementary Figure 13) > 0.05 wt% (at 327 °C) > 0.2 wt% (at 270 °C, Supplementary Figure 14)
Figure 7. Mass spectrometer signals of CH4, CO, CO2, H2 and C2H4 of CH4-TPSR experiments over (A and B) 0.05% Ir/TiO2, (C and D) 1.0% Ir/TiO2 at elevated temperature and constant 750 °C. TPSR: Temperature-programmed surface reaction.
The spent Ir/TiO2 catalysts after DRM reaction for 50 h were then subjected to different characterizations to examine the anti-coking and sintering-resistance properties of Ir species. As shown in Figure 8A, there is nearly no clear weight loss (residual mass above 99.72%) in the high temperature range from 500 to 800 °C by thermal gravimetric analysis (TGA) and the corresponding MS spectra are also absent of the CO2
Figure 8. (A) TGA-MS profiles and (B) Raman spectra of spent Ir/TiO2 catalysts after DRM reaction for 50 h. TGA-MS: Thermogravimetric analysis-mass spectrometry; DRM: dry reforming of methane.
Figure 9. HAADF-STEM images of different Ir/TiO2 samples after DRM reaction for 50 h. (A) 0.01% Ir/TiO2; (B) 0.05% Ir/TiO2; (C) 0.2% Ir/TiO2; (D) 0.5% Ir/TiO2; (E) 1.0% Ir/TiO2. HAADF-STEM: High-angle annular dark-field scanning transmission electron microscopy; DRM: dry reforming of methane.
CONCLUSIONS
In summary, we have synthesized Ir/TiO2 catalysts via the conventional incipient wetness impregnation method and further downsized the Ir species from a nanoparticulate to a single-atom scale by gradually decreasing Ir loadings. Various characterizations, such as AC-HAADF-STEM, CO-DRIFTs and XAFS, demonstrated the existence of Ir single atoms on 0.01% and 0.05% Ir/TiO2 samples. In comparison with the DRM performances over the nanoparticulate Ir/TiO2, the Ir SACs show a better specific reaction rate of as high as 697.71 molCH4·gIr-1·h-1 at 750 °C, which signifies outstanding Ir atom utilization over Ir single atoms. Besides, the Ir SACs also exhibit excellent stability during the DRM process for 50 h and reveal outstanding anti-coking and good sintering-resistance properties. The strategy of employing Ir single atoms for the maximum atom utilization in the high-temperature reaction process can pave the way for better exploitation of noble metals in other industrial reaction processes.
DECLARATIONS
Authors’ contributions
Writing-original draft, visualization, validation, supervision, methodology, investigation, data curation: Li Y
Writing-review and editing, visualization, validation, supervision, methodology, investigation, data curation, conceptualization: Liu C
Investigation, validation: Su Y, Zhao Y
Writing-review and editing, visualization, validation, supervision, resources, investigation, conceptualization: Qiao B
Availability of data and materials
Supplementary Materials is available in the online version of this article.
Financial support and sponsorship
This work was financially supported by the National Key Research and Development Program of China (2021YFA1500503), National Natural Science Foundation of China (21961142006, 22388102, U23A20110), and CAS Project for Young Scientists in Basic Research (YSBR-022).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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