Understanding the role of supported Rh atoms and clusters during hydroformylation and CO hydrogenation reactions with in situ/operando XAS and DRIFT spectroscopy

Abstract

Supported Rh single-atoms and clusters on CeO₂, MgO, and ZrO₂ were investigated as catalysts for hydroformylation of ethylene to propionaldehyde and CO hydrogenation to methanol/ethanol with in situ/operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). Under hydroformylation reaction conditions, operando spectroscopic investigations unravel the presence of both single atoms and clusters and detected at first propanal and then methanol. We find that the formation of methanol is associated with CO hydrogenation over Rh clusters which was further confirmed under CO hydrogenation conditions at elevated pressure. The activity of catalysts synthesized via a precipitation (PP) method over various supports towards the hydroformylation reaction follows the order: Rh/ZrO₂ > Rh/CeO₂ > Rh/MgO. Comparing Rh/CeO₂ catalysts synthesized via different methods, catalysts prepared by flame spray pyrolysis (FSP) showed catalytic activity for the hydroformylation reaction at lower temperatures (413 K), whereas catalysts prepared by wet impregnation (WI) showed the highest stability. These results not only provide fundamental insights into the atomistic level of industrially relevant reactions but also pave the way for a rational design of new catalysts in the future.

---

Introduction

The hydroformylation (HF) reaction, an industrially demanding reaction for producing aldehydes via the reaction of olefin and synthesis gas, is predominantly catalyzed by Rh molecular catalysts.^1,2 On the other hand, higher alcohol synthesis (HAS)^3–5 proceeds only in the presence of synthesis gas and is largely catalyzed by Rh solid catalysts.^5–7 Both reactions follow rather similar pathways via C–CO coupling reactions and yet the products are different. Recent studies show that Rh single-atom catalysts (SACs)^8–10 can be promising alternatives for heterogeneously catalyzed hydroformylation reactions^11,12 where the C–CO coupling reaction occurs like in conventional homogeneous Rh-phosphine complexes. Therefore, SACs are considered analogous to molecular catalysts.^13,14 The tunable local environment around the supported single atom makes it particularly attractive to investigate how the C–CO coupling reaction takes place. A recent example shows that Rh-WO_x pair site catalysts can boost the activity for gas phase hydroformylation reactions.¹⁵ However, SACs are also prone to agglomeration due to their high surface energy particularly in a highly reductive environment.^16,17 Several reports claimed that Rh SACs stabilized by ionic liquids,¹⁸ functionalized with an alkyl chain over a solid support,¹⁹ synthesized by tuning surface deficiencies,²⁰ and even with strong metal–support cooperativity^21,22 show superior stability. Zhao et al. observed redispersion of Rh⁰ clusters inside zeolite MFI to Rh single atoms and, ultimately, the formation of Rh⁺(CO)₂ during hydroformylation of ethylene at 50 °C, which shows that the dynamic structural change plays a vital role under operating conditions.²³

In the absence of an olefin, CO hydrogenation to higher alcohols (C₂₊) over Rh catalysts^7,24,25 is a promising route since the synthesis gas mixture can be obtained from various processes based on renewable resources such as biomass conversion,²⁶ CO₂ valorization,²⁷ dry-reforming of CO₂ with CH₄,²⁸ electrochemical conversion of CO₂,²⁹ photochemical conversion of CO₂,³⁰ conversion of plastic waste,³¹ and many others.³² However, until today, there has been no suitable catalyst that can achieve the desired activity and selectivity which are required for industrial scale production.³³ Therefore, it is extremely important to know the effect of Rh nuclearity,^24,34 support,³⁵ adsorbate–adsorbate interaction,³⁶ promoters,^32,33 morphology,²⁵ and particle size.³⁷ Cu-based catalysts are also promising alternatives for CO hydrogenation to higher alcohols; however, they compete with methanol synthesis and require promoters of alkali, transition, and rare-earth metals to produce higher alcohols as selective products.^38–40 The limitation of using other transition metals such as Co is that it catalyzes the competing Fischer–Tropsch process.⁴¹

A typical homogeneous HF reaction follows the classical Heck–Breslow mechanism^42,43 and usually proceeds via several steps such as (a) ligand dissociation from the complex, (b) olefin coordination, (c) alkyl formation via hydride insertion, (d) CO insertion, (e) H₂ oxidative addition and (f) reductive elimination. On the other hand, HAS involves (a) CO dissociation, (b) carbon–carbon bond formation, (c) CO insertion and (d) stepwise hydrogenation to the alcohol.^33,44 While in the HF reaction, all the steps can potentially occur over monometallic Rh, in HAS, synergetic effects between two neighbouring sites are likely required for C–C bond formation and CO insertion steps. For monometallic catalysts, the M⁰ state is assumed to promote the C–C chain growth during HAS whereas M^δ+ is responsible for CO insertion like in the classical HF reaction.^33,45

This brings us to the question of whether Rh SACs can be active for both types of reactions as Rh exhibits a positive charge, and in a reductive environment, Rh⁰ has been observed.⁵ There have been many reports where the co-existence of SACs and small clusters was detected simultaneously under in situ conditions such as propane dehydrogenation,⁴⁶ dry-reforming of methane,^47,48 or CO₂ hydrogenation.^49,50 On the other hand, it is well known that Rh forms Rh(CO)₂⁺-carbonyls in the presence of CO due to the strong interaction of CO with Rh.⁵¹ In many of these catalytic processes, the presence of two species led to two different reaction pathways and hence two different products were observed. A recent study by Wu et al. claimed that the dynamic evolution of Rh single atoms to clusters over CeO₂ led to good activity for CO hydrogenation to ethanol, where the presence of Rh single atoms and clusters was found during the catalytic activity test.³⁴

To understand these two reactions in detail, which can occur in parallel, we have synthesized Rh SACs and cluster catalysts over various supports (CeO₂, MgO, and ZrO₂) via precipitation,¹⁶ atom trapping,^52,53 pyrolysis of molecular complexes⁵⁴ and flame spray pyrolysis (FSP).⁵⁵ These supports are chosen due to their variable acidity, reducibility, and, most importantly, variation in reactivity during the catalytic HF reaction as reported in the literature.⁹ The as-synthesized catalysts were characterized ex situ by various state-of-the-art characterization methods and the dynamic behaviour⁵⁶ of Rh catalysts was tracked by in situ X-ray absorption spectroscopy (XAS)⁵⁷ and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS).¹⁶ The products of the two reactions, (a) hydroformylation of ethylene to propanal and (b) CO hydrogenation to methanol/ethanol were monitored using an online mass spectrometer (MS).

Results and discussion

Synthesis and characterization of catalysts

The Rh catalysts synthesized via different routes have different metal contents in the range between 0.8 and 1.5 wt% as confirmed by the inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. A plot of BET surface area as a function of metal content in the catalysts is shown in Fig. S1 of the ESI.† The surface area varies greatly depending on the support and synthesis procedure. For example, Rh/CeO₂ catalysts synthesized via FSP showed the highest specific surface area (133 m² g⁻¹) in comparison to the one synthesized by wet impregnation (56 m² g⁻¹) or precipitation methods (93 m² g⁻¹). This is due to the fast quenching of the resulting solid synthesized by the FSP method which minimizes the particle growth, as explained elsewhere.⁵⁵ Among the catalysts prepared over different supports (CeO₂, ZrO₂ and MgO), ZrO₂ (synthesized by the precipitation method) showed the highest specific surface area of 201 m² g⁻¹, which makes it attractive for application in catalysis.

The powder X-ray diffraction (XRD) patterns of the as-synthesized catalysts showed highly crystalline structures (Fig. S2–S7 in ESI†) in all cases. There is no Rh reflection (at 2θ = 40°) observed, confirming the high dispersion of Rh over different supports. Even when the content of Rh is increased from 0.6 wt% to 5 wt% over the CeO₂ support, we did not observe any reflection associated with Rh metal or RhO_x clusters as shown in Fig. S2.† The FSP method was also successful in preventing the formation of Rh nano-particles with long-range order (Fig. S3†), as also reported in the literature for other metals.^58–60 Aberration-corrected (C_s) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was used to visualize the nuclearity of Rh in the as-synthesized state of the catalysts over different supports. The images and the corresponding Energy Dispersive X-ray (EDX) maps are shown in Fig. 1 and in Fig. S8–S10.† The EDX maps confirm that Rh is uniformly distributed in all the catalysts except the Rh catalyst supported over CeO₂ synthesized by the pyrolysis method. In this case, RhP was formed, which showed nano-particle formation. The presence of some clusters (<2 nm) in the FSP synthesized catalyst cannot be entirely ignored, as observed in the EDX maps (Fig. S8†).

Fig. 1 HAADF-STEM images of Rh catalysts and the corresponding EDX maps. Rh/CeO₂, Rh/ZrO₂, Rh/MgO are synthesized via the precipitation method, and RhP/CeO₂ is synthesized via the pyrolysis route.

We further conducted X-ray absorption spectroscopy (XAS) to elucidate the local structure of the Rh catalysts as shown in Fig. 2. The extended X-ray absorption fine structure analysis (EXAFS) showed that Rh species over CeO₂, ZrO₂, and MgO are highly dispersed with Rh–O as the first coordination shell (without any Rh–Rh scattering) whereas the RhP/CeO₂ catalyst exhibits predominantly the Rh–Rh shell as shown in Fig. 2(a). The X-ray absorption near edge structure (XANES) spectra at the Rh K-edge showed that Rh is in an oxidation state of +3 over CeO₂, ZrO₂ and MgO and in a partially reduced state in the RhP/CeO₂ catalyst as shown in Fig. 2(b). The spectra of Rh catalysts synthesized over ceria with various methods are shown in ESI Fig. S11,† and the corresponding free parameters derived from the fit are shown in Table S2.† The X-ray photoelectron spectroscopy (XPS) investigation further supported the results obtained from XAS. All the catalysts except RhP/CeO₂ contain Rh^III as the predominant species on the surface as shown in Fig. S12 of the ESI.† The components present and their percentages are shown in Table S2.†

Fig. 2 (a) Fourier transform of the k³-weighted EXAFS spectra and (b) XANES spectra at the Rh K-edge of the Rh catalysts supported over CeO₂, ZrO₂, and MgO. Rh₂O₃ and Rh foil are shown as references for comparison.

XAS⁵⁷ is an averaging technique over all species present and has certain limitations when used on heterogeneous samples. For example, over the oxide-supported catalyst, EXAFS cannot distinguish between single site and clusters of a few atoms as explained by Finzel et al.⁶¹ Therefore, as a complementary technique, we carried out diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) investigations with CO as a probe molecule to elucidate the nuclearity of the Rh catalysts synthesized over different supports as shown in Fig. 3. CO adsorbed on Rh shows characteristic symmetric and anti-symmetric CO vibrational frequencies of single site Rh^I(CO)₂ in the range of 2070–2090 cm⁻¹ and 2000–2020 cm⁻¹ respectively.^16,49,62,63 Rh/CeO₂ synthesized via precipitation (PP), flame spray pyrolysis (FSP) and wet impregnation (WI) exhibit Rh single sites as the predominant species as shown in Fig. 3(a), (d) and (e) respectively. Similarly, Rh/ZrO₂ (PP) also showed the presence of symmetrical and asymmetrical CO vibrational frequencies at 2078 and 2007 cm⁻¹ respectively.⁶⁴ On the other hand, the RhP/CeO₂ catalyst synthesized via the pyrolysis method showed the presence of CO adsorption over the Rh clusters in the lower frequency region (1850–1950 cm⁻¹)¹⁶ as shown in Fig. 3(c). The spectra are also shown on the logarithmic scale in Fig. S13.† These results are in agreement with the TEM observations (Fig. 1), which indicate the presence of Rh clusters as RhP/CeO₂. The DRIFT spectra of Rh/CeO₂ (5 wt%) with adsorbed CO showed various features (2126, 2035 cm⁻¹), which can be attributed to the weakly adsorbed CO over Rh³⁺ or Rh²⁺ species present in the form of RhO_x clusters.²³ Upon flowing Ar, two distinct CO absorption frequencies at 2023 and 2092 cm⁻¹ were observed, as shown in Fig. S14,† which we believe arise from the fraction of Rh single atoms present in this high loading catalyst. Moreover, the presence of a broad CO adsorption peak between 1850 and 1900 cm⁻¹ (Fig. S14†) infers the presence of Rh clusters in this catalyst. The Rh/MgO catalyst did not show any CO vibrational frequencies in the range 1800–2100 cm⁻¹, which indicates that Rh supported on MgO is coordinatively saturated as also found previously.^9,65 The CO absorption frequencies of Rh catalysts over various supports and their assignments are shown in Fig. 3(g) as well as in Table S4 of the ESI.†

Fig. 3 CO-DRIFTS spectra at room temperature over various Rh catalysts. (a) Rh/CeO₂ (PP), (b) Rh/ZrO₂ (PP), (c) RhP/CeO₂, (d) Rh/CeO₂ (FSP), (e) Rh/CeO₂ (WI), and (f) 5 wt% Rh/CeO₂ (PP). (g) Table of CO absorption frequency with assignments of Rh species. PP: precipitation, FSP: flame spray pyrolysis, and WI: wet impregnation. ν_s and ν_as stands for symmetric and anti-symmetric CO vibrational frequencies. The green to red color bar represents the experiment over 1 hour at room temperature.

In situ XAS investigations

In order to understand the electronic state of Rh under operating conditions, we further carried out in situ XAS studies at the Rh K-edge for hydroformylation of ethylene (experimental setup is shown in S15 of the ESI†). The catalysts were exposed to a mixture of a 1 : 1 : 1 ratio of ethylene : CO : H₂ while heating from room temperature to 573 K (2 K min⁻¹ and 30 min dwell time) at atmospheric pressure with a total flow of 40 mL min⁻¹. The gas mixture at the outlet was monitored with an online mass spectrometer. The temperature-dependent in situ XANES spectra were collected continuously at the Rh K-edge from room temperature to 573 K in the presence of the reaction mixture as shown in Fig. 4(a)–(d). All Rh catalysts were reduced under HF conditions with a change in the oxidation state of Rh from Rh³⁺ to Rh⁰. Linear combination analysis (LCA) was used to derive the oxidized and reduced components during the reaction as shown in Fig. 4 (e)–(h). The trend of the reduction temperature from the highest to the lowest follows the order of Rh/MgO > Rh/ZrO₂ > Rh/CeO₂ > RhP/CeO₂. However, the catalytic activity does not follow the same trend. Both in Rh/CeO₂ and Rh/ZrO₂, we observed propanal at around a temperature of 500 K, which is the product formed via the HF reaction as shown in Fig. 4(i) and (j). At this stage, almost 80–90% of the Rh species are already reduced which indicates the possible involvement of clusters during catalysis. Interestingly, when all the Rh is reduced, methanol is detected in the MS. This indicates that when Rh forms clusters, the pathway for CO hydrogenation is enhanced as also shown by Wu et al.³⁴ This is supported by our catalytic test over the RhP/CeO₂ catalyst which contains a significant fraction of Rh clusters, for which formation of methanol was detected almost parallel to the formation of propanal (Fig. 4(l)). Rh/MgO did not show any activity even in the presence of reduced rhodium, which indicates that the basic support does not promote HF or methanol formation. Among the Rh/CeO₂ catalysts, Rh synthesized by FSP showed catalytic activity at a lower temperature (ca. 410 K) compared to the catalysts synthesized by precipitation or wet impregnation methods, as shown in Fig. S16 of the ESI.† This might be due to better accessibility of the Rh atoms deposited on high surface area CeO₂ when synthesized via the FSP method.⁵⁵

Fig. 4 In situ XANES spectra at the Rh K-edge of catalysts prepared over different supports ((a)–(d)), the fraction of components (oxidized and reduced) derived from linear combination fitting ((e)–(h)) during the hydroformylation reaction of ethylene, and selectivity of different products monitored using an online mass spectrometer (i)–(l). The dotted lines are a guide to the eye. PP stands for precipitation. Reaction conditions: 25 mg catalysts, 1 : 1 : 1 mixture of ethylene : CO : H₂, T = RT-573 K (ramp rate 2 K min⁻¹), P = 1 bar, flow rate: 40 mL min⁻¹.

In order to find the origin of methanol formation, we further carried out the reaction in the absence of ethylene (CO : H₂ of 1 : 1) in a quartz capillary micro-reactor at 10 bar pressure in the temperature range of RT-573 K. The temperature dependent XANES spectra and the linear combination analysis of the oxidized and reduced components during the reaction are shown in Fig. 5. The mass spectrometer (MS) detected both methanol and ethanol over Rh/CeO₂ synthesized via the precipitation method (Fig. 5(i)). This further indicates that the nuclearity of Rh plays a crucial role in obtaining methanol and ethanol products as previously reported.^24,34 The Rh/ZrO₂ catalyst showed only methanol in the product stream and no ethanol was observed. Rh/MgO did not show any activity as was also the case for the HF reaction. RhP/CeO₂ showed initially only methanol, and ethanol was only observed at higher temperatures (550 K) when a significant portion of Rh⁰ formed. The Fourier transformed k³-weighted EXAFS spectra after the HF reaction and CO hydrogenation reaction are shown in Fig. S17 of the ESI.† The predominant presence of the Rh–Rh shell indicates that most of the catalysts were reduced completely at the end of the reaction except Rh/MgO and Rh/ZrO₂, which showed a significant proportion of the oxidic species (presence of the Rh–O shell). We further carried out fitting of the Rh–Rh shell to calculate the coordination number of Rh in the spent catalysts. The fits and the free parameters are shown in ESI Fig. S18 and in Table S5.† The coordination numbers obtained from the fits are in the range between 1 and 6. Nevertheless, such low coordination values indicate that the size of the reduced Rh species is extremely small.^66,67 To further confirm whether the concentration of Rh plays a significant role during the reaction, we have conducted the CO hydrogenation reaction over Rh/CeO₂ with three different Rh loadings (1.1, 2.2 and 4.5 wt%). The results are shown in Fig. S19 of the ESI.† The catalytic results indicate that upon increasing Rh content over the CeO₂ support, the formation of methanol and ethanol is enhanced. 4.5 wt% Rh/CeO₂ showed the formation of methanol and ethanol simultaneously like the RhP/CeO₂ catalyst, which indicates that for a highly loaded sample, Rh easily forms nano-particles which can promote the C–CO coupling reaction. These results further signify the role of clusters during HF and CO hydrogenation reactions.

Fig. 5 In situ XANES spectra at the Rh K-edge of catalysts prepared over different supports ((a)–(d)), the fraction of components (oxidized and reduced) derived from linear combination fitting ((e)–(h)) during CO hydrogenation, and selectivity of different products monitored using an online mass spectrometer (i)–(l). The dotted lines are a guide to the eye. PP stands for precipitation. Reaction conditions: 25 mg catalysts, 1 : 1 mixture of CO : H₂, T = RT-573 K (ramp rate 2 K min⁻¹), P = 10 bar, flow rate: 40 mL min⁻¹.

Based on our results, three summary points can be hypothesized for the reaction mechanism. (a) Rh single atom catalysts form the active center upon treatment with syngas and actively take part in the hydroformylation reaction, (b) when Rh clusters are formed, it mainly favors methanol synthesis, and (c) for ethanol formation, both atoms and clusters participate during the reaction as the CO insertion step requires Rh^δ+ species for C–C chain propagation. These points are summarized in Fig. S20 of the ESI.†

In situ DRIFTS investigations

In order to investigate the change in the state of Rh, we carried out DRIFTS experiments under various conditions, as shown in Fig. 6. As discussed before, at room temperature Rh/CeO₂ showed CO vibrational frequencies at 2084 and 2016 cm⁻¹ that correspond to symmetrical and anti-symmetrical vibrational modes of CO of Rh^I (CO)₂ species. The change in the state of Rh is already visible at 453 K under 1% (CO + H₂) as a new CO vibrational frequency appears at 2046 cm⁻¹ (Fig. 6(b)), which corresponds to CO adsorbed on Rh clusters. Upon increasing the temperature to 523 K, clusters of Rh formed and an additional feature at 2036 cm⁻¹ was observed (Fig. 6(c)), which we believe to occur due to the formation of Rh(CO)H species as reported in the literature.³⁷ This trend continued when ethylene was introduced to the reaction mixture. However, even under such a reductive atmosphere, there is still the presence of isolated Rh species (2030 cm⁻¹) as shown in Fig. 6(d). These results correlate with the in situ XAS studies and further hint that Rh atoms and clusters formed under reaction conditions play a crucial role during the HF reaction. Additionally, we carried out DRIFTS experiments on the Rh/CeO₂ catalyst synthesized via the wet impregnation method, which showed higher stability based on our in situ XAS studies (Fig. S16(c)†). The DRIFTS results showed no obvious formation of clusters, unlike for the catalyst synthesized via the precipitation method, as shown in Fig. S21 of the ESI.† A comparison of the H₂-temperature programmed reduction (TPR) profile and the DRIFT spectra collected at 453 K with the catalyst prepared over various supports (CeO₂, ZrO₂, and MgO) is shown in Fig. S23 of the ESI.† The results showed that based on the support, Rh exhibits different reduction temperatures, and it follows the order of Rh/CeO₂ < Rh/ZrO₂ < Rh/MgO. The DRIFT spectra recorded at 453 K under CO + H₂ indicated that Rh/ZrO₂ did not show any Rh cluster formation. On the other hand, Rh/MgO did not show any feature regarding CO adsorption, which infers that Rh is not easily available on the surface of MgO and may be located in the highly coordinated bulk environment.

Fig. 6 In situ DRIFTS spectra collected for 1 hour under various conditions of the catalyst Rh/CeO₂ synthesized via precipitation method. (a) 1% CO at room temperature, (b) 1% (CO + H₂) at 453 K, (c) 1% (CO + H₂) at 523 K and (d) 1% (ethylene + CO + H₂) at 523 K.

Conclusions

In conclusion, we have found that highly dispersed Rh catalysts supported over CeO₂, ZrO₂, and MgO showed co-existence of supported single Rh atoms and clusters under conditions of HF of ethylene and CO hydrogenation reactions. The in situ XAS studies infer the formation of reduced Rh particles/clusters while both propanal and methanol were observed during the HF reaction. In addition, the in situ DRIFTS studies showed vibrational frequencies of CO adsorbed over Rh atoms and clusters under reaction conditions, which further supports that such unprecedented reactivity might have occurred due to the presence of both species. The formation of a significant amount of ethanol was observed during the CO hydrogenation reaction over the RhP/CeO₂ catalyst, which also showed the presence of nano-particles (>2 nm) already in the as-synthesized catalyst. Moreover, by increasing the concentration of Rh from 1.1 wt% to 4.5 wt% over the CeO₂ support, we observed the CO hydrogenation reaction at a relatively low temperature. This indicates that a certain size of Rh nano-particles is important to promote the C–C coupling reaction during higher alcohol synthesis. Comparisons of catalytic activity over various supports showed that acidic supports, such as ZrO₂, promote the hydroformylation reaction, whereas basic supports, such as MgO, result in negligible catalytic activity. The varying reactivity towards hydroformylation of ethylene is dependent on the preparation method and support, as summarized in Fig. 7. Among the catalysts synthesized over CeO₂ with various methods, catalysts prepared via flame spray pyrolysis showed activity at low temperatures which indicates that the method of preparation plays a vital role during the reaction. This study will pave the way for further development of catalysts for such an industrially relevant reaction. In the future, we plan to investigate the CO insertion mechanism of the two reactions in more detail by introducing promoters and extending the reactions over further industrially relevant supports.

Fig. 7 Summary of reactivity towards the hydroformylation reaction of ethylene with supported Rh catalysts on CeO₂, ZrO₂, and MgO. PP, FSP, and WI stand for precipitation, flame spray pyrolysis, and wetness impregnation, respectively.

Data availability

Data supporting the article (experimental protocols, XRD, XPS, BET, and TPR results) are available in the ESI.† The raw data can be provided upon request to the corresponding author.

Author contributions

BBS: conceptualization; investigation, data curation & formal analysis (catalyst preparation & catalytic testing, including in situ XAS and DRIFTS); formal analysis (XAS, DRIFT, and XRD); writing the original draft. DN: data curation (XAS and XPS) and formal analysis (XPS); writing-review and editing. DD: data curation and formal analysis (XAS); writing-review and editing. ALN: data curation and formal analysis (TEM); writing-review and editing. LB: data curation (XAS); writing-review and editing. BK: data curation and formal analysis (XPS); writing-review and editing. JDG: overall supervision, funding acquisition, resources, writing–discussion–review and editing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to thank Dr Eduardo Barbosa and Angela Deutsch for nitrogen physisorption measurements and Armin Lautenbach for ICP-OES analysis. Alisa Sinigalia is thanked for her assistance during the XPS measurements. Andrea De Giacinto and Dr Linda Klag are thanked for their assistance with the FSP synthesis. Michael Borchers is thanked for the H₂-TPR measurements. Dr Lu Chen (University of Cambridge, UK) and Prof. Philippe Serp (LCC, CNRS, France) are thanked for the fruitful discussion during the preparation of the manuscript. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of beamtime, allocated to the proposals I-20200891 and I-20211473 and KIT light source (A2020-031-018713). We would like to thank Dr Edmund Welter (P65, DESY) and Dr Anna Zimina (CATACT, KIT Light Source and IKFT) for their assistance in setting up the beamline. HAADF-STEM imaging and EDX imaging were carried out with the support of the Karlsruhe Nano Micro Facility (KNMFi, https://www.knmf.kit.edu), a Helmholtz Research Infrastructure at the Karlsruhe Institute of Technology (KIT, https://www.kit.edu). Support from the Helmholtz Association and Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – SFB 1441 – Project-ID 426888090 is gratefully acknowledged. BBS would like to thank CNRS for startup funding.

Notes and references

1. R. Franke, D. Selent and A. Börner, Applied Hydroformylation, Chem. Rev., 2012, 112, 5675–5732.
2. M. Sparta, K. J. Børve and V. R. Jensen, Activity of Rhodium-Catalyzed Hydroformylation: Added Insight and Predictions from Theory, J. Am. Chem. Soc., 2007, 129, 8487–8499.
3. P. Forzatti, E. Tronconi and I. Pasquon, Higher Alcohol Synthesis, Catal. Rev., 1991, 33, 109–168.
4. X. Xiaoding, E. B. M. Doesburg and J. J. F. Scholten, Synthesis of higher alcohols from syngas – recently patented catalysts and tentative ideas on the mechanism, Catal. Today, 1987, 2, 125–170.
5. H. T. Luk, C. Mondelli, D. C. Ferré, J. A. Stewart and J. Pérez-Ramírez, Status and prospects in higher alcohols synthesis from syngas, Chem. Soc. Rev., 2017, 46, 1358–1426.
6. G. van der Lee, B. Schuller, H. Post, T. L. F. Favre and V. Ponec, On the selectivity of Rh catalysts in the formation of oxygenates, J. Catal., 1986, 98, 522–529.
7. M. Suvarna, P. Preikschas and J. Pérez-Ramírez, Identifying Descriptors for Promoted Rhodium-Based Catalysts for Higher Alcohol Synthesis via Machine Learning, ACS Catal., 2022, 12, 15373–15385.
8. R. Lang, T. Li, D. Matsumura, S. Miao, Y. Ren, Y. T. Cui, Y. Tan, B. Qiao, L. Li, A. Wang, X. Wang and T. Zhang, Hydroformylation of Olefins by a Rhodium Single-Atom Catalyst with Activity Comparable to RhCl(PPh₃)₃, Angew. Chem., Int. Ed., 2016, 55, 16054–16058.
9. J. Amsler, B. B. Sarma, G. Agostini, G. Prieto, P. N. Plessow and F. Studt, Prospects of Heterogeneous Hydroformylation with Supported Single Atom Catalysts, J. Am. Chem. Soc., 2020, 142, 5087–5096.
10. L. Wang, W. Zhang, S. Wang, Z. Gao, Z. Luo, X. Wang, R. Zeng, A. Li, H. Li, M. Wang, X. Zheng, J. Zhu, W. Zhang, C. Ma, R. Si and J. Zeng, Atomic-level insights in optimizing reaction paths for hydroformylation reaction over Rh/CoO single-atom catalyst, Nat. Commun., 2016, 7, 14036.
11. C. Li, W. Wang, L. Yan and Y. Ding, A mini review on strategies for heterogenization of rhodium-based hydroformylation catalysts, Front. Chem. Sci. Eng., 2018, 12, 113–123.
12. S. Hanf, L. A. Rupflin, R. Glaser and S. A. Schunk, Current State of the Art of the Solid Rh-Based Catalyzed Hydroformylation of Short-Chain Olefins, Catalysts, 2020, 10, 1–36.
13. X.-F. Yang, A. Wang, B. Qiao, J. Li, J. Liu and T. Zhang, Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis, Acc. Chem. Res., 2013, 46, 1740–1748.
14. A. Wang, J. Li and T. Zhang, Heterogeneous single-atom catalysis, Nat. Rev. Chem., 2018, 2, 65–81.
15. I. Ro, J. Qi, S. Lee, M. Xu, X. Yan, Z. Xie, G. Zakem, A. Morales, J. G. Chen, X. Pan, D. G. Vlachos, S. Caratzoulas and P. Christopher, Bifunctional hydroformylation on heterogeneous Rh-WO_x pair site catalysts, Nature, 2022, 609, 287–292.
16. B. B. Sarma, J. Jelic, D. Neukum, D. E. Doronkin, X. Huang, S. Bernart, F. Studt and J.-D. Grunwaldt, Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In Situ DRIFT and XAS Spectroscopy Assisted by DFT, J. Phys. Chem. C, 2023, 127, 3032–3046.
17. L. Liu, D. M. Meira, R. Arenal, P. Concepcion, A. V. Puga and A. Corma, Determination of the Evolution of Heterogeneous Single Metal Atoms and Nanoclusters under Reaction Conditions: Which Are the Working Catalytic Sites?, ACS Catal., 2019, 9, 10626–10639.
18. S. Ding, J. Hülsey Max, H. An, Q. He, H. Asakura, M. Gao, J.-y. Hasegawa, T. Tanaka and N. Yan, Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching, CCS Chem., 2021, 3, 1814–1822.
19. G. Zakem and P. Christopher, Active Site Entropy of Atomically Dispersed Rh/Al₂O₃ Catalysts Dictates Activity for Ethylene Hydroformylation, ACS Catal., 2023, 13, 5502–5515.
20. Y. Zheng, Q. Wang, Q. Yang, S. Wang, M. J. Hülsey, S. Ding, S. Furukawa, M. Li, N. Yan and X. Ma, Boosting the Hydroformylation Activity of a Rh/CeO₂ Single-Atom Catalyst by Tuning Surface Deficiencies, ACS Catal., 2023, 13, 7243–7255.
21. L. Qi, S. Das, Y. Zhang, D. Nozik, B. C. Gates and A. T. Bell, Ethene Hydroformylation Catalyzed by Rhodium Dispersed with Zinc or Cobalt in Silanol Nests of Dealuminated Zeolite Beta, J. Am. Chem. Soc., 2023, 145, 2911–2929.
22. I. Ro, M. Xu, G. W. Graham, X. Pan and P. Christopher, Synthesis of Heteroatom Rh–ReO_x Atomically Dispersed Species on Al₂O₃ and Their Tunable Catalytic Reactivity in Ethylene Hydroformylation, ACS Catal., 2019, 9, 10899–10912.
23. M. Zhao, C. Li, D. Gómez, F. Gonell, V. M. Diaconescu, L. Simonelli, M. L. Haro, J. J. Calvino, D. M. Meira, P. Concepción and A. Corma, Low-temperature hydroformylation of ethylene by phosphorous stabilized Rh sites in a one-pot synthesized Rh-(O)-P-MFI zeolite, Nat. Commun., 2023, 14, 7174.
24. M. Schumann, M. R. Nielsen, T. E. L. Smitshuysen, T. W. Hansen, C. D. Damsgaard, A.-C. A. Yang, M. Cargnello, J.-D. Grunwaldt, A. D. Jensen and J. M. Christensen, Rationalizing an Unexpected Structure Sensitivity in Heterogeneous Catalysis—CO Hydrogenation over Rh as a Case Study, ACS Catal., 2021, 11, 5189–5201.
25. N. Yang, A. J. Medford, X. Liu, F. Studt, T. Bligaard, S. F. Bent and J. K. Nørskov, Intrinsic Selectivity and Structure Sensitivity of Rhodium Catalysts for C₂₊ Oxygenate Production, J. Am. Chem. Soc., 2016, 138, 3705–3714.
26. J. J. Spivey and A. Egbebi, Heterogeneous catalytic synthesis of ethanol from biomass-derived syngas, Chem. Soc. Rev., 2007, 36, 1514–1528.
27. S. R. Foit, I. C. Vinke, L. G. J. de Haart and R.-A. Eichel, Power-to-Syngas: An Enabling Technology for the Transition of the Energy System?, Angew. Chem., Int. Ed., 2017, 56, 5402–5411.
28. D. Pakhare and J. Spivey, A review of dry (CO₂) reforming of methane over noble metal catalysts, Chem. Soc. Rev., 2014, 43, 7813–7837.
29. W. Sheng, S. Kattel, S. Yao, B. Yan, Z. Liang, C. J. Hawxhurst, Q. Wu and J. G. Chen, Electrochemical reduction of CO₂ to synthesis gas with controlled CO/H₂ ratios, Energy Environ. Sci., 2017, 10, 1180–1185.
30. E. Lam and E. Reisner, A TiO₂-Co(terpyridine)₂ Photocatalyst for the Selective Oxidation of Cellulose to Formate Coupled to the Reduction of CO₂ to Syngas, Angew. Chem., Int. Ed., 2021, 60, 23306–23312.
31. M. He, B. Xiao, Z. Hu, S. Liu, X. Guo and S. Luo, Syngas production from catalytic gasification of waste polyethylene: Influence of temperature on gas yield and composition, Int. J. Hydrogen Energy, 2009, 34, 1342–1348.
32. V. R. Surisetty, A. K. Dalai and J. Kozinski, Alcohols as alternative fuels: an overview, Appl. Catal., A, 2011, 404, 1–11.
33. M. Ao, G. H. Pham, J. Sunarso, M. O. Tade and S. Liu, Active Centers of Catalysts for Higher Alcohol Synthesis from Syngas: A Review, ACS Catal., 2018, 8, 7025–7050.
34. D. Wu, S. Liu, M. Zhong, J. Zhao, C. Du, Y. Yang, Y. Sun, J. Lin, S. Wan, S. Wang, J. Huang, Y. Yao, Z. Li and H. Xiong, Nature and Dynamic Evolution of Rh Single Atoms Trapped by CeO₂ in CO Hydrogenation, ACS Catal., 2022, 12, 12253–12267.
35. M. Schumann, J.-D. Grunwaldt, A. D. Jensen and J. M. Christensen, Investigations of mechanism, surface species and support effects in CO hydrogenation over Rh, J. Catal., 2022, 414, 90–100.
36. M. Deimel, H. Prats, M. Seibt, K. Reuter and M. Andersen, Selectivity Trends and Role of Adsorbate–Adsorbate Interactions in CO Hydrogenation on Rhodium Catalysts, ACS Catal., 2022, 12, 7907–7917.
37. S. M. McClure, M. J. Lundwall and D. W. Goodman, Planar oxide supported rhodium nanoparticles as model catalysts, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 931–936.
38. M. Gupta, M. L. Smith and J. J. Spivey, Heterogeneous Catalytic Conversion of Dry Syngas to Ethanol and Higher Alcohols on Cu-Based Catalysts, ACS Catal., 2011, 1, 641–656.
39. K. Fang, D. Li, M. Lin, M. Xiang, W. Wei and Y. Sun, A short review of heterogeneous catalytic process for mixed alcohols synthesis via syngas, Catal. Today, 2009, 147, 133–138.
40. R. G. Herman, Advances in catalytic synthesis and utilization of higher alcohols, Catal. Today, 2000, 55, 233–245.
41. K. Xiao, Z. Bao, X. Qi, X. Wang, L. Zhong, K. Fang, M. Lin and Y. Sun, Advances in bifunctional catalysis for higher alcohol synthesis from syngas, Chin. J. Catal., 2013, 34, 116–129.
42. D. S. Breslow and R. F. Heck, Mechanism of the Hydroformylation of Olefins, Chem. Ind., 1960, 467.
43. R. F. Heck and D. S. Breslow, The Reaction of Cobalt Hydrotetracarbonyl with Olefins, J. Am. Chem. Soc., 1961, 83, 4023–4027.
44. S. S. C. Chuang, R. W. Stevens and R. Khatri, Mechanism of C₂₊ oxygenate synthesis on Rh catalysts, Top. Catal., 2005, 32, 225–232.
45. S. S. C. Chuang and S. I. Pien, Infrared study of the CO insertion reaction on reduced, oxidized, and sulfided Rh/SiO₂ catalysts, J. Catal., 1992, 135, 618–634.
46. H. Xiong, S. Lin, J. Goetze, P. Pletcher, H. Guo, L. Kovarik, K. Artyushkova, B. M. Weckhuysen and A. K. Datye, Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria, Angew. Chem., Int. Ed., 2017, 56, 8986–8991.
47. M. Akri, S. Zhao, X. Li, K. Zang, A. F. Lee, M. A. Isaacs, W. Xi, Y. Gangarajula, J. Luo, Y. Ren, Y.-T. Cui, L. Li, Y. Su, X. Pan, W. Wen, Y. Pan, K. Wilson, L. Li, B. Qiao, H. Ishii, Y.-F. Liao, A. Wang, X. Wang and T. Zhang, Atomically dispersed nickel as coke-resistant active sites for methane dry reforming, Nat. Commun., 2019, 10, 5181.
48. B. B. Sarma, G. Agostini, M. G. Farpón, C. Marini, N. Pfänder and G. Prieto, Bottom-up assembly of bimetallic nanocluster catalysts from oxide-supported single-atom precursors, J. Mater. Chem. A, 2021, 9, 8401–8415.
49. J. C. Matsubu, V. N. Yang and P. Christopher, Isolated Metal Active Site Concentration and Stability Control Catalytic CO₂ Reduction Selectivity, J. Am. Chem. Soc., 2015, 137, 3076–3084.
50. A. Aitbekova, L. Wu, C. J. Wrasman, A. Boubnov, A. S. Hoffman, E. D. Goodman, S. R. Bare and M. Cargnello, Low-Temperature Restructuring of CeO₂-Supported Ru Nanoparticles Determines Selectivity in CO₂ Catalytic Reduction, J. Am. Chem. Soc., 2018, 140, 13736–13745.
51. J.-D. Grunwaldt, L. Basini and B. S. Clausen, In Situ EXAFS Study of Rh/Al₂O₃ Catalysts for Catalytic Partial Oxidation of Methane, J. Catal., 2001, 200, 321–329.
52. J. Jones, H. Xiong, A. T. DeLaRiva, E. J. Peterson, H. Pham, S. R. Challa, G. Qi, S. Oh, M. H. Wiebenga, X. I. Pereira Hernández, Y. Wang and A. K. Datye, Thermally stable single-atom platinum-on-ceria catalysts via atom trapping, Science, 2016, 353, 150.
53. B. B. Sarma, J. Kim, J. Amsler, G. Agostini, C. Weidenthaler, N. Pfänder, R. Arenal, P. Concepción, P. Plessow, F. Studt and G. Prieto, One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization–Hydrosilylation Process, Angew. Chem., Int. Ed., 2020, 59, 5806–5815.
54. C. Galdeano-Ruano, C. W. Lopes, D. Motta Meira, A. Corma and P. Oña-Burgos, Rh2P Nanoparticles Stabilized by Carbon Patches for Hydroformylation of Olefins, ACS Appl. Nano Mater., 2021, 4, 10743–10753.
55. R. Koirala, S. E. Pratsinis and A. Baiker, Synthesis of catalytic materials in flames: opportunities and challenges, Chem. Soc. Rev., 2016, 45, 3053–3068.
56. B. B. Sarma and J.-D. Grunwaldt, Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts, Chimia, 2024, 78, 288–296.
57. B. B. Sarma, F. Maurer, D. E. Doronkin and J.-D. Grunwaldt, Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools, Chem. Rev., 2023, 123, 379–444.
58. G. Cavusoglu, D. Miao, H. Lichtenberg, H. W. P. Carvalho, H. Xu, A. Goldbach and J.-D. Grunwaldt, Structure and activity of flame made ceria supported Rh and Pt water gas shift catalysts, Appl. Catal., A, 2015, 504, 381–390.
59. V. Muravev, G. Spezzati, Y.-Q. Su, A. Parastaev, F.-K. Chiang, A. Longo, C. Escudero, N. Kosinov and E. J. M. Hensen, Interface dynamics of Pd–CeO₂ single-atom catalysts during CO oxidation, Nat. Catal., 2021, 4, 469–478.
60. W. J. Stark, J.-D. Grunwaldt, M. Maciejewski, S. E. Pratsinis and A. Baiker, Flame-Made Pt/Ceria/Zirconia for Low-Temperature Oxygen Exchange, Chem. Mater., 2005, 17, 3352–3358.
61. J. Finzel, K. M. Sanroman Gutierrez, A. S. Hoffman, J. Resasco, P. Christopher and S. R. Bare, Limits of Detection for EXAFS Characterization of Heterogeneous Single-Atom Catalysts, ACS Catal., 2023, 13, 6462–6473.
62. L. Basini, M. Marchionna and A. Aragno, Drift and mass spectroscopic studies on the reactivity of rhodium clusters at the surface of polycrystalline oxides, J. Phys. Chem., 1992, 96, 9431–9441.
63. F. C. Meunier, Relevance of IR Spectroscopy of Adsorbed CO for the Characterization of Heterogeneous Catalysts Containing Isolated Atoms, J. Phys. Chem. C, 2021, 125, 21810–21823.
64. Y. Kwon, T. Y. Kim, G. Kwon, J. Yi and H. Lee, Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion, J. Am. Chem. Soc., 2017, 139, 17694–17699.
65. B. B. Sarma, P. N. Plessow, G. Agostini, P. Concepción, N. Pfänder, L. Kang, F. R. Wang, F. Studt and G. Prieto, Metal-Specific Reactivity in Single-Atom Catalysts: CO Oxidation on 4d and 5d Transition Metals Atomically Dispersed on MgO, J. Am. Chem. Soc., 2020, 142, 14890–14902.
66. A. M. Beale and B. M. Weckhuysen, EXAFS as a tool to interrogate the size and shape of mono and bimetallic catalyst nanoparticles, Phys. Chem. Chem. Phys., 2010, 12, 5562–5574.
67. D. A. J. M. Ligthart, R. A. van Santen and E. J. M. Hensen, Influence of particle size on the activity and stability in steam methane reforming of supported Rh nanoparticles, J. Catal., 2011, 280, 206–220.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc02907k

---