Sulfur poisoning is a challenge for most nanoparticle metal catalysts. A trace amount of sulfur contaminants could result in dramatic catalytic activity reduction or even irreversible deactivation1-5. Therefore, new approaches to enhance the catalyst sulfur-resistance have continuously attracted attention from academia and industry. Herein, a role reversal of sulfur from poison to promotor is presented for an Rh-based heterogeneous catalyst from supported Rh nanoparticles (NPs) to its single-site catalysts (Rh1/AC, AC: activated carbon) in methanol carbonylation, ethylene and acetylene hydrocarboxylic reaction with a feed containing 1000 ppm H2S (S-feed). In situ free-electron laser/time of flight mass spectrometry (In situ FEL/TOF MS) indicated that H2S could be quickly transformed into catalyst-friendly CH3SCH3 and/or CH3SH on the Rh1/AC, which coordinated with the Rh ions and promoted its methanol carbonylation reaction, possessing a lower energy barrier based on DFT calculations. On the contrary, strong adsorption of H2S on the surface of Rh NPs inhibited the reaction of reactants.
Physical Sciences - Article
Sulfur Poisoning on Rh Nanoparticles but Sulfur Promotion on Its Single-Site Catalyst for Carbonylation
https://doi.org/10.21203/rs.3.rs-401859/v1
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As is well-known, sulfur species bring about great challenges for almost all metal catalysts. Even ppm or ppb levels of sulfur species in a raw stock-feed can result in dramatic activity reduction of NP metal catalysts, often leading to irreversible deactivation. For example, it has been shown that 13 ppb of H2S could deactivate Ni/Al2O3 NPs catalysts in the methanation reaction6 and 7 ppm H2S was reported to lead to 90% activity loss for Ni-Rh alloy NPs7 during methane reforming. The strong adsorption of sulfur on metal NPs was deemed to block the active sites and lead to deactivation1-5,8. Accordingly, large efforts have been paid every year to the desulfurization of the raw stock-feeds in order to ensure the long-term activity of the metal catalyst. Therefore, removing sulfur species from the raw feed through expensive and energy intensive technologies or developing a high sulfur tolerance catalyst has been a significant research topic for industry.
Sulfur is not only detrimental to catalytic reactions, sometimes it can also act as a promoter. Recently, thiophene promoted Au9, sulfur promoted Pd10, thiol protected Au11, as well as sulfur promoted Cu12 have been reported. In addition, adjusting the metal NPs size could regulate the effect of sulfur poisoning or S-bonding13,14. Small Pt NPs are easy to be poisoned by thiophene in ethylene hydrogenation due to the increased coordinatively saturated surfaces13. A strong damped oscillatory size dependence of the adsorption energy for sulfur on facets of Cu nanoclusters from 3 up to sizes of ∼300 atoms is also demonstrated14. The limit of NP dispersion is single metal site catalyst, which has drawn much attention due to versatile coordinative ability and efficient catalytic activity15-20. Herein, impacts of H2S on Rh NPs and its single-site catalysts during methanol carbonylation will be addressed, respectively. And the process of non-toxic transformation of H2S into CH3SCH3 and/or CH3SH species will be disclosed on the single-Rh-site catalysts.
Mesoporous carbon, SiO2 and AC supported Rh NPs catalysts (Rh/CMK-3, Rh/SiO2 and Rh/AC) were separately prepared in the METHODS section. As indicated in Table 1, a severe deactivation from 80 and 100 to 0 moleacetyl/(moleRh×h) was obtained for Rh NPs in the Rh/CMK-3 and Rh/SiO2 catalysts for methanol carbonylation, respectively, when a normal feed was switched into S-feed. In addition, an initial activity of Rh NPs (Rh/AC) was reduced from 163 to 40 moleacetyl/(moleRh×h) in an autoclave reactor and suppressed from 135 to 110 moleacetyl/(moleRh×h) in a fixed-bed reactor, respectively. The activity of Rh/AC increased gradually with time on stream up to ca. 3100 moleacetyl/(moleRh.h) in the normal feed due to the atomic dispersion of Rh NPs under the existence of CH3I/CO only on AC21,22 (Extended Figure 1). The strategy was also used to prepare AC supported single-Rh-site catalysts (Rh1/AC) with the molecular structure of Rh(CO)2I3(O=AC), where O=AC is the carboxyl oxygen groups on the surface of activated carbon21-23 (METHODS). Furthermore, the characterization result of spent Rh/AC in S-feed indicated that H2S could not inhibit the atomic dispersion of Rh NPs (Extended Figure 1b-1h). In contrast, the activity of Rh/AC in S-feed finally reach ca. 3800 moleacetyl/(moleRh.h). Accordingly, it was expectable that the activity increased from 3192 to 3974 moleacetyl/(moleRh×h) was consequentially observed on the Rh1/AC catalyst in a continuous fixed-bed reactor after using S-feed (Table 1 and Extended Figure 2). More interestingly, a sulfur-promoting activity increase from 2021 to 2353 moleacetyl/(moleRh×h) and from 2016 to 2477 moleacetyl/(moleRh×h) was observed, respectively, on both homogeneous single sites [Rh(CO)2I2]- and heterogeneous Rh1/AC catalyst after S-feed being introduced into the autoclave reactor. A similar effect of sulfur-poisoning on the Rh NPs and sulfur-promoting on the Rh1/AC was also observed for ethylene and acetylene hydrocarboxylic reaction in the autoclave reactor (Extended Data Table 1).
The high-resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) showed only isolated Rh atoms on fresh and spent Rh1/AC in both normal feed and S-feed (Figure 1a, Extended Data Figure 3a, 3b and 4). In-situ/ex-situ extended X-Ray absorption fine structure (EXAFS) verified that no bond of Rh-Rh was detected on the Rh1/AC before, during or even after the reaction. These indicated a real process of single-sites catalysis in both normal feed and S-feed (Figure 1b-1c, Extended Data Figure 5-6 and Table 2). Due to the small scattering factors of C, S and O in comparison to that of I abundant in the micropores of AC, longer Rh-ligand distances, and a large structural disorder, the fitting data of in-situ EXAFS characterization cannot reliably identify Rh nearest ligands other than I (or Rh).
X-ray photoelectron spectroscopy (XPS) showed that Rh3+ and Rh4+ existed in spent Rh1/AC in S-feed (Rh1/AC-SR) (Extended Data Figure 7 and Table 3), which exhibited little change comparing with that on spent Rh1/AC in normal feed. Meanwhile, the S 2p3/2 peak ascribed to Rh-S(CH3)2 and Rh-SHCH3 species emerged at 162.7 eV and 164.3 eV respectively in Rh1/AC-SR, implying that H2S was preferentially transformed into CH3-S-CH3 and CH3-SH species24. The formation of Rh-S bonds on Rh1/AC-SR might be indirectly evidenced from the IR experiment (Extended data Figure 8). Except for CO (m/z=28) and I• (m/z=127), a species with m/z=62 (probably CH3SCH3 species), originated from Rh1/AC-SR, was also observed by desorption ionization-time of flight mass spectrometry (LDI-TOF MS) (Extended Figure 9). Based on these combined characterization results, we suggest that the active mononuclear complexes exit as (CH3-S-R)Rh(CO)I2(O=AC) (R=H, CH3) during carnbonylation process on the Rh1/AC-SR. Meanwhile, TEM, EDS-mapping, and the XPS of spent Rh/CMK-3 reacted in S-feed suggested that the strong adsorption of H2S on the surface of Rh NPs blocked the adsorption of other reactants via an isolated Rh-S layer NPs (Figure 1d-f, Extended Figure 10-12).
In-situ mass spectra, obtained by free-electron laser-time of flight mass spectrometry (FEL TOF-MS), showed new species at m/z=33, 34, 47, 48 and 62 from Rh1/AC catalyst in S-feed (Figure 2a-2b), which could be assigned to -SH, H2S, -SCH3, CH3SH, and CH3SCH3 species, respectively. Moreover, the obtained results through changing the wavelength of ionization radiation proved that the species of m/z=62 is CH3SCH3 (Extended Data Figure 13). Interestingly, the ion peak at m/z=62 emerged at 373 K and increased with temperature till 453 K, while that at m/z=34 rapidly decreased from 313 to 433 K (Figure 2c), implying quick transformation of H2S to CH3SCH3. Besides, the reaction of H2S with CH3OH or CH3I could produce CH3SCH3 and CH3SH on the Rh1/AC (Figure 2d and 2e). Moreover, CH3SCH3 from the spontaneous reaction of CH3I with H2S was also observed (Figure 2f).
In-situ diffuse reflectance infrared Fourier transformation spectroscopy on Rh1/AC catalyst in S-feed showed a peak at ~1775 cm-1 ascribed to acetyl species, appearing after reaction for 20 min, earlier than that in the normal feed at 25 min (Extended Data Figure 14a-b), indicating that the existence of sulfur species accelerated the formation of acetyl. Meanwhile, signals of CH3- or CH2- groups at about 2967 and 2850 cm-1 were dramatically attenuated in S-feed, suggesting that a portion of CH3· species were probably quenched by H2S to form CH3SH or CH3SCH3 species (Extended Data Figure 14c-d)25,26. Besides, the in-situ electron paramagnetic resonance spectrometry/electron spin resonance also confirmed that the free radicals species were dramatically quenched during methanol carbonylation on Rh1/AC in S-feed (Extended Data Figure 15).
DFT calculations investigated the coordination effect of CH3SCH3 and CH3SH on the Rh mononuclear complex. The minimum energy of (H2S)Rh(CO)I3(O=AC), (CH3SH)Rh(CO)I3(O=AC), and (CH3SCH3)Rh(CO)I3(O=AC) is 3.5, -9.2 and -14.9 kcal/mol, respectively, when compared with Rh(CO)2I3(O=AC) (Extended Data Figure 16), suggesting that the new formed CH3SH and CH3SCH3 species from H2S favored to coordinate with Rh mononuclear complex to obtain a more stable Rh mononuclear structure. Moreover, the bond length of CH3-I in the chemical adsorption structure of [I--CH3-(SR)Rh(CO)I2(O=AC)] with H2S, CH3SH or CH3SCH3 species, is 2.15 Å, a little longer than that in [I--CH3-Rh(CO)2I2(O=AC)]# (2.13 Å) (Extended Data Figure 17), implying that the coordination of H2S, CH3SH and (CH3)2S could lower the chemical adsorption energy of CH3I. More importantly, the transition state energy barrier of [I--CH3-(SR)Rh(CO)I2(O=AC)]# is 10.0,10.5 and 16.5 kcal/mol for CH3SCH3, CH3SH and H2S coordination, respectively, while that of [I--CH3-Rh(CO)2I2(O=AC)]# is 16.1 kcal/mol, demonstrating that CH3SCH3 or CH3SH coordination as a ligand is beneficial for the Rh mononuclear complex to lower the energy barrier of CH3I oxidative addition step, accelerating the reaction rate of methanol carbonylation on the Rh1/AC catalyst 22,27-30 (Figure 3 and Extended Data Figure 18).
In summary, a reversal sulfur poisoning effect from Rh NPs to its single-sites catalyst was reported. The supported Rh NPs was easily poisoned by H2S, while the Rh1/AC exhibited excellent sulfur-promoting activity for methanol carbonylation. Interestingly, H2S cannot prevent atomic dispersion of Rh NPs on an AC in the presence of CH3I/CO. Similar results were also obtained in ethylene and acetylene hydrocarboxylation. H2S could be rapidly transformed into catalyst-friendly CH3SCH3 and CH3SH ligands on the single-site, while strong adsorption of H2S on Rh NPs formed an isolated Rh-S layer, blocking the adsorption of reactants. It is anticipated that the phenomenon reported here might also be extended to other single-metal-site catalysts in various reactions.
Catalyst preparation
Mesoporous carbon, AC and SiO2 supported Rh NPs catalyst (Rh/CMK-3, Rh/AC and Rh/SiO2) with 1% weight was synthesized by incipient wetness impregnation, followed by dry and calcination, and then reduced by H2 at 573 K for 2 h. The Rh1/AC catalyst (1 wt.%) was prepared through the atomic dispersion strategy of metal NPs. Typically, the coconut activated carbon (AC) was grinded to 40-60 mesh and washed with deionized water until the electrical conductibility of the washings is below 20 μs/cm, dried in static air at 393 K for 12 h. Subsequently, the AC support was impregnated by an aqueous RhCl3∙xH2O solution. After drying under static conditions at 393 K for 12 h, the catalysts were treated in a three-step procedure: 573 K for 2 h in N2 (100 mL/min), 573 K for 2 h in H2 (100 mL/min) (Rh/AC was obtained in this step) and 513 K for 2 h with a mixture of CO/CH3I (CO passed through a bottle filled with CH3I at 30 mL/min), respectively. Finally, the samples were cooled to ambient temperature in a flow of CO. The obtained catalysts were denoted as Rh1/AC.
Catalyst characterization
The high-resolution transmission electron microscope (HR-TEM) images of the samples were obtained with JEM-2100 equipped with an energy dispersive spectrometer (EDS). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were also recorded using a JEM-ARM200F STEM/TEM instrument with a CEOS probe corrector working at 200 kV to guarantee a resolution of 0.08 nm.
X-ray photoelectron spectroscopy (XPS) characterization was performed using a Thermo Fisher ESCALAB 250Xi, equipped with a monochromated Al Ka X-ray source (hυ = 1486.6 eV, 15 kV, 10.8 mA). During data collection, the vacuum and the working pressure were limited to 3 x 10–8 Pa and 7.1 x 10–5 Pa, respectively.
In-situ electron paramagnetic resonance spectrometry/electron spin resonance (In-situ EPR/ESR) (Bruker A200) was conducted to monitor the free electrons in samples to identify the effect of H2S on the radicals produced in the reaction. 5 mg sample was placed in a specific in-situ reactor for in-situ methanol carbonylation reaction, and then cooled to -177 °C with liquid nitrogen. A magnetic field linear sweep was performed with a fixed frequency of microwave irradiation (concrete parameters: center field: 3300 G, sweep width: 6400 G, static field: 100 G, microwave bridge frequency: 9.5 GHz and the power is set as 10 mW). The EPR data was processed and outputted as a 1st derivative of the absorption spectrum. EPR absorption occurs when the irradiation frequency “matches” the energy level separation created by the magnetic field, Boltzmann distribution.
The spent Rh1/AC catalyst in normal and S-feed was soaked in ethanol for 6 h at room temperature to dissolve metal carbonyl on the surface of AC. Ethanol and the scrubbing solution obtained was smeared to a thin slice of KBr. The infrared scanning and signal processing were carried out under normal conditions.
The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were conducted on a VERTEX 80v FTIR spectrometer with a mercury cadmium telluride (MCT) detector cooled by liquid nitrogen. Catalysts were loaded into a PIKE High Temperature Reaction chamber (ZnSe windows). Mass flow controllers were used to control the gas rates across the reactor bed. For each DRIFTS experiments, the catalyst was first calcined in He (40 mL/min) at 573 K for 30 min. Then, the temperature of the catalyst was reduced to 513 K followed by flowing reaction gas. CH3OH, CH3I or liquild CH3I and CH3OH mixtue (30 wt% CH3I) were bubbled by reaction gas (CO, CO/H2, He) at 298 K. During the reaction, spectra were recorded between 800 cm-1 to 4000 cm-1 by averaging 64 scans at a resolution of 4 cm-1 to improve the signal to noise ratio.
The in situ flow-tube free-electron laser time of flight mass spectrometry (In situ FEL/TOF MS) were conduct at Dalian Coherent Light Source31-34 to detect the intermediates and the radicals during methanol gas phase carbonylation reaction. Laser desorption ionization/time of flight mass spectrometry (LDI-TOF MS) was conducted to detect the dissociative fragment of spent Rh1/AC catalyst in normal and S-feed and further identify its single complex structure. And the in-situ formed short-lived intermediate species were also detected with the same TOF-MS instrument. Experiments on a series of ultraviolet wavelengths, i.e., 112 nm, 115 nm, 118 nm, 125 nm, 140 nm, 150 nm, were performed according to the ionization energy of intermediate species. For the LDI-TOF MS experiments, 266 nm lasers were slightly focused on the sample. The skimmed He atom beam passed the sample surface with a distance of ~1mm to carry the dissociated and sputtering species to the detection chamber.
The in situ EXAFS (In situ EXAFS) measurements were performed at the SuperXAS beamline at the Swiss Light Source35-38. The ring was operated in top-up mode with 400 mA of 2.4 GeV electrons, which emitted polychromatic radiation in a super bend magnet (2.9 T) with a critical energy of 11.9 keV. The beam was collimated by a Pt-coated mirror and then monochromatized using a channel-cut Si(311) crystal in the QEXAFS monochromator. A rotation speed of 1 Hz was chosen, yielding one EXAFS spectrum each 500 ms. The samples were measured in transmission mode with a 15-cm ionization chamber filled with Ar in front of the sample and another 15-cm ionization chamber filled with Kr behind the powder sample. A third 30- cm ionization chamber filled with Ar was used to measure an Rh metal foil located between the second and a third chamber to obtain an absolute calibration value for the energy scale of each spectrum. This value was related to the relative position of the monochromator crystal, which was measured simultaneously to each data point with a fast optical angular encoder system, yielding an accurate energy scale for the time-resolved spectra. The ionization chambers were operated with a high voltage of 1.5 kV, and the signals were amplified with Keithley 428 current amplifiers and then measured with a national instrument DAC data acquisition board using a fast data acquisition software. The Rh1/AC catalyst was held between 2 quartz wool plugs and placed in a stainless steel flow reactor with quartz flow reactor. The following gases: Ar (N2), CO (50% in Ar), H2 (50% in Ar), and 1000 ppm H2S in Ar were supplied by digital mass-flow controllers. CH3I and methanol vapor were prepared by bubbling CO through liquid CH3I and methanol mixture (30 wt% CH3I). The reaction was started by introducing the CO, CH3I CH4OH, H2, and H2S mixture at 1 bar when the reactor temperature increased to 240 °C in vacuum conditions. We recorded data when the reactants mixture passed through the catalyst.
Ex-situ extended X-ray absorption fine structure (Ex-situ EXAFS) data of Rh K-edge were collected at beamline BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF)39,40. The sample was grinded into 400 meshes and pressed into a small slice with a size of 0.5 cm x 0.5 cm before mounted to the test site. The data were recorded in the fluorescence mode equipped with an Electro-Lytle detector, and the calibration energy is 22117 eV. The original EXAFS data were processed with Athena software and normalized with the energy of 23220 eV, and then analyzed by the Artemis software package. Fourier transformation was applied to process the k3-weighted raw data. The theoretical scattering amplitude and phase-shift functions of all the paths for fitting the EXAFS data were calculated with FEFF6 code. Paths of Rh-CO, Rh-I, Rh-Rh were based on the crystal structure of [C4I2O4Rh2] and Rh Fm-3m. The .cif files could be found in the database of crystallography open database (COD) with the number of 2202025, 2202025, 2100454, respectively. The Reffs of Rh-CO, Rh-I, Rh-Rh were calculated with single scatting at 1.854 Å, 2.621 Å and 2.539 Å, respectively, according to the .cif files mentioned above. Rh-O-AC was set with Reff of 2.2 Å according to the first quick shell path based on DFT calculated models.
Density function theory (DFT) calculation
The activate carbon (AC) model was constructed with carbonyl group based on the molecule geometry of Graphene-like structure. The all-molecular structures were optimized by the density function theory (DFT) calculation using Vienna ab initio simulation package (VASP)41–44. The Perdew-Burke-Ernzerhof (PBE) was used as the exchange correlation function45. The cut off kinetic energy was set to 420 eV. Brillouin zone integration was approximated using the Monkhorst-Pack k-points method46 and set as 1 × 1 × 1. The height of vacuum slab was selected with 12 Å to eliminate the size error. Geometries were optimized until the energy was converged to 1.0 × 10–4 eV/atom and the force was converged to 0.05 eV/Å. Here, it is need to note that the transition states were optimized using the Dimer method. To understand and account for dispersion effect in the entire reaction process, the energy corrections of van der Waals (vdW) were calculated using the semiempirical approach proposed by Grimme47 (DFT-D2 method) in conjunction with the PBE functional. All the stable molecule geometries and the transition state structures of Rh1/AC complexes were checked by the frequency calculations without imaginary frequency and with one imaginary frequency, respectively. In this study, the unit of molecule bond length or distance between atoms is angstrom.
Catalyst evaluation
The methanol carbonylation activity test of Rh/AC, [Rh(CO)2I2]- and Rh1/AC were conducted in high-pressure zirconium reactor (25 ml). The specific reaction conditions were noted below in Table 1. The 1wt% Rh/AC, Rh1/AC, Rh/SiO2 and Rh/CMK-3 were also tested in a fixed-bed reactor. 0.3 g catalyst was placed in the center of the Hastelloy C-276 reactor. The upper and lower space was filled with quartz sand. Upstream from the reactor, the liquid was vaporized and mixed with CO/H2 or CO/H2/H2S in a Hastalloy tube wrapped with heat tape maintained at 513 K. The tailed gas was chilled at 278 K to collect the liquid product. Incondensable gas was analyzed using on-line gas chromatography Agilent 7890B equipped with TCD (chromatographic column: Plot Q), and the collected liquid product was off-line analyzed by FID (chromatographic column: HP-FFAP). The space-time-yield (STY) was calculated based on the total moles of CH3COOH and CH3COOCH3 divided by total moles of Rh per hour.
Extended Data
Extended data for this paper at XXXX. The data that support the findings in this study are in the published article and/or its extended data files. The data sources are deposited on the generalist repository of figshare, and the accession code is xxxxxxx. The whole datasets are available from the corresponding author on reasonable request as well.
Acknowledgements
The financial support of this work by the National Key R&D Program of China (2017YFB0602203), the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDA21020300 and Grant No XDB17020400, and the National Natural Science Foundation of China (22002156), the international partnership program of the Chinese Academy of Sciences (No. 121421KYSB20170012), the Key Technology Team of the Chinese Academy of Sciences (Grant No.GJJSTD20190002), Chemical Dynamics Research Center (Grant No. 21688102), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17000000) and the Natural Science Foundation of Zhejiang Province (LY18B060007) and key Laboratory of Yarn Materials Forming and Composite Processing Technology, Zhejiang Province, China. This research used resources of the Dalian Coherent Light Source.
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Table 1. Effect of H2S on the catalytic activity of Rh NPs (Rh/AC, Rh/CMK-3 and Rh/SiO2) and mononuclear complexes ([Rh(CO)2I2]-, Rh1/AC) catalysts.
Reaction conditions: S-feed: S content 1000 ppm H2S, Rh content was 1.0 wt.% for Rh/AC, Rh/CMK-3，Rh/SiO2 and Rh1/AC. a 0.2g Rh/AC and 0.1g Rh1/AC; 3.0 MPa CO, CH3OH (218.75 mmol), CH3I (20.98 mmol) for Rh1/AC and 3.73mmol for Rh/AC, ultra-pure water (277.8 mmol), 433 K, 20 min in a zirconium autoclave reactor. b 0.3g catalysts, 513 K, 1.7 MPa, CO/CH3OH=1.0 (mole ratio), CO/H2=10 (mole ratio), CH3I/CH3OH =1/24 (mass ratio) for Rh/AC, CH3I/CH3OH =3/7(mass ratio) for Rh/CMK-3, Rh/SiO2 and Rh1/AC in Hastelloy fixed-bed reactor. c 0.1g RhI3, 3.0 MPa CO, CH3OH (81.25 mmol), CH3I (7.82 mmol), ultra-pure water (277.8 mmol), 433 K, 20 min in a zirconium autoclave reactor. The dispersive rate of Rh NP on Rh/AC was in proportion to the concentration of CH3I and CO.* Initial data obtained after 2 h on stream in methanol carbonylation. ** Data obtained after 40 h on stream in methanol carbonylation.
There is NO Competing Interest.
- ExtendedXXdatafinal.docx
Extended Data (Supplementary Figure 1-18, Table 1-3)