With the aim to investigate the effect of TiOx overlayer covered over the Ru nanoparticles (NPs) on the reactivity in FTS, in the present work, we use wet impregnation method to fabricate Ru-based catalysts with small-sized metal nanoparticles by using rutile TiO2 as a support24. In this text, catalysts reduced at specific temperatures were denoted as Ru/TiO2–x, where x refers to reduction temperatures (x = 200–600). The loading of Ru, after calcination in air and the following thorough chlorides removal process, has been determined to be 2.2 wt% by ICP-OES. As indicated by Brunauer–Emmett–Teller (BET) testing (Supplementary Table 1), different Ru/TiO2–x samples are similar in physical textures, with almost the identical surface areas and pore volumes.
Meanwhile, all Ru/TiO2 samples are found to possess analogous morphologies of metal NPs, that is, the Ru NPs are all well dispersed on the support and have a uniform size distribution with an average diameter of ~2 nm (Fig. 1a, b and Supplementary Figs. 1–3). It was attributed to the usage of rutile TiO2 as the support in our research, which possesses the same crystal phase and comparable lattice parameters with that of RuO2 (Supplementary Table 2). As a result, the calcination step by formation of RuO2/TiO2 interphase helps to resist the aggregation of metal nanoparticles even after the high-temperature pretreatment (Supplementary Fig. 4). On the other hand, however, there are significant discrepancies on the micro-structures of Ru NPs on the TiO2 support when pretreated at different reduction temperatures (Fig. 1c and Supplementary Fig. 5). In detail, a distinct morphology of Ru NP can be resolved on the support when sample was reduced at a temperature below 300 °C, while a visible coating on the Ru NPs can be distinguishable after higher temperature pre-reduction. In terms of the SMSI between Ru and rutile TiO2, it was ascribed to the TiOx overlayer over the Ru NPs under high-temperature reduction condition, and the migration of TiOx over Ru NPs was initiated at a reduction temperature higher than 300 °C.
To get a qualitative comparison of the exposure of Ru species after coating by TiOx overlayer, the chemisorption of CO and H2 were measured to estimate the Ru dispersion on these Ru/TiO2–x samples. As seen in Table 1, the values obtained by different probe molecules give the same tendency of the metal dispersions for different Ru/TiO2–x samples, that is, the dispersion of Ru decreases with increasing the reduction temperature from 300 to 600 °C. This can be explained by a gradual encapsulation of the Ru NPs by TiOx overlayer as increasing the reduction temperature from 300 to 600 °C, which was in good agreement with the TEM observations. As compared, the dispersion derived from H2 chemisorption was lower than that from CO chemisorption. It might be caused by the Run+ sites at the Ru–TiO2 interface, which are unavailable for H2 chemisorption due to the SMSI effects25, but it can be readily involved in CO chemisorption as indicated in our further in situ DRIFT spectra experiments. Even so, the H2 uptakes on the Ru/TiO2-200 sample was found to be less than that of the Ru/TiO2-300. This can be explained by the results of H2 temperature-programmed reduction (H2-TPR), in which the predominant reduction of Ru/TiO2 to metallic Ru occurring at a temperature higher than 200 °C (Supplementary Fig. 6). In addition, the decline of the surface metallic Ru exposure by a gradual encapsulation of the Ru NPs by TiOx overlayer as increasing the reduction temperature was also confirmed by the underpotential deposition of copper (Cu upd) experiments, with which the metallic surface area can be semi-quantified in terms of the integral area of current for the reduction deposition of copper on the exposed metal surface26 (Supplementary Fig. 7 and Supplementary Table 3).
The evolution of Ru/TiO2 catalysts at different temperature reductions was also investigated with X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure (EXAFS) of Ru K-edge and the fitting results (Fig. 2a and Supplementary Table 4) have demonstrated that the coordination number (CN) associated with Ru–Ru pair (~2.67 Å) is increased gradually from 2.2 to 5.3 as the reduction temperature increased from 200 to 600 °C, while the CN of the Ru–O pair (~1.98 Å) presents an inverse tendency by decreasing from 4.0 to 2.4. It suggests a gradual improvement of the reduction degree of ruthenium oxide to metallic phase. Correspondingly, a shift of edge energy towards Ru foil was observed by the X-ray absorption near-edge structure (XANES) (Supplementary Fig. 8). Nevertheless, the inevitable Ru–O bonding for Ru/TiO2 samples indicates a strong interfacial interaction between Ru and TiO2, further confirming the formation of TiOx coating on the Ru NPs. On the other hand, the soft XANES spectra at the Ti L3,2-edge (Fig. 2b) exhibit a decline in peak intensities with the increase of pretreatment temperature, indicative of an increasing degree of the reduction of TiO2, owing to the formation of TiOx overlayer on Ru NPs. The growth of the reduced TiOx overlayer was also evidenced by the increased concentration of Ti3+ species accompanying by the decreased ratio of surface Ru/Ti estimated from the Ti-2p and Ru-3p in XPS results (Supplementary Fig. 9 and Supplementary Table 5).
On the basis of the above observations, the structure evolution of Ru/TiO2 at different stages of reduction was then suggested in Fig. 2c. Benefiting from the lattice matching of RuO2/TiO2 interphase, a small size of Ru NPs with the improved sintering resistance can be facilely acquired by the following reduction pretreatment. In the initial step of reduction, e.g., Ru/TiO2-300, a dominant metallic Ru will be exposed, and it serves as a typical Ru-based nanocatalysts in FTS. With further increasing the reduction temperature, the SMSI between Ru and TiO2 governs the surface exposure of Ru NPs, and the TiOx thin layer begins to migrate and coating the Ru surface, resulting in a shrinkage of metallic Ru surface by the TiOx overlayer (like that on Ru/TiO2-450). Finally, for the Ru/TiO2-600 sample, an excessive coverage of TiOx on Ru NPs causes a dominant TiOx overlayer on Ru nanocatalysts. Accordingly, a tunable degree of TiOx overlayer on Ru NPs can be easily achieved by varying the pretreatment condition, and it thus provides us an opportunity to explore the effect of metal/support interface of Ru/TiO2 on the activity in FTS.
The catalytic performances of various Ru/TiO2–x catalysts in FTS were then evaluated at 160 °C with a 2 MPa reaction pressure according to our preliminary optimization of experiment conditions for achieving a high C5+ selectivity under a relatively mild condition (Supplementary Fig. 10). Notably, as shown in Fig. 3a and Supplementary Figs. 11, 12, all Ru/TiO2–x catalysts possess an excellent C5+ selectivity with a value up to 90%, indicating the promising application prospect of Ru/TiO2 in FTS for high-carbon products. While the intrinsic reaction rate (reflected as the TOF value) greatly relies on the pretreatment temperature, and shows a volcano-like trend with the increase of reduction temperature (Fig. 3a, Supplementary Fig. 13 and Supplementary Table 6). As compared, samples of Ru/TiO2-200 and Ru/TiO2-300 manifest a much lower intrinsic activity (0.003 s−1). A great promotion of activity was observed for the Ru/TiO2-400 catalyst. Among these catalysts, the Ru/TiO2-450 exhibits the highest activity with an intrinsic TOF value of 0.039 s−1, which was also superior to other Ru-based catalysts reported previously (Supplementary Table 7). The further increasing of pretreatment temperature, however, causes an activity decay of catalysts, with a TOF value of only 0.021 s−1 for Ru/TiO2-600. Correspondingly, a reverse variation of apparent activation energy (Ea) was obtained, i.e., the Ru/TiO2-450 presents the lowest Ea with a calculated value of 62.0 kJ mol−1, which was much lower than the values of other Ru/TiO2 catalysts (Fig. 3b).
The particle size of metal was essential in determining the performance of Ru-based catalysts in FTS. The activity increases as the particle size of Ru nanocatalyst increased, with the small-sized Ru NPs behaving a rather poor activity9,10,11. This explains well the low activity of our small-sized Ru/TiO2 catalysts reduced at low temperatures (Ru/TiO2-200 and Ru/TiO2-300 samples) as well as Ru NPs supported on the irreducible support (Ru/Al2O3-450 in Supplementary Fig. 14 and Supplementary Table 8). While an enhanced activity of Ru/TiO2-450 catalyst suggests that the TiOx overlayer on Ru NPs has a positive contribution on the reactivity of Ru nanocatalysts. However, in terms of the decline of activity for the Ru/TiO2-600 catalyst covered dominantly by TiOx overlayer, the only TiOx overlayer cannot achieve a high activity for the FTS reaction. In this regard, the interface between metal and support of Ru/TiO2 plays a crucial role on the activity promotion, where both the metallic Ru and TiOx overlayer are indispensable. The optimized composition of TiOx overlayer and Ru NPs on Ru/TiO2-450 catalyst makes it possess an enhanced activity.
Furthermore, the Ru/TiO2-450 catalyst also owns an excellent stability in the steady running state of FTS (Supplementary Figs. 10, 11). HAADF-STEM image of the spent Ru/TiO2-450 catalyst suggests that the size of Ru can keep constant after testing (Supplementary Fig. 15). This was also benefited from the SMSI in the Ru/TiO2-450 catalyst, which greatly prohibits the size aggregation of Ru during FTS process.
The role of the TiOx overlayer was then studied by the steady-state isotopic transient kinetic analysis (SSITKA)27,28, by which the evolution of intermediates with the associated coverage and reactivity can be acquired (Supplementary Fig. 16). Limited by the atmospheric pressure condition in this analysis, as shown in Supplementary Fig. 17, CH4 selectivity has an increase because of the preference of hydrogenation over C–C coupling for CHx intermediates. Notably, a good correlation between the intrinsic activity (TOF) of CO consumption and methane generation can be set up. As such, the coverage of CHx (represented as θCH4 in SSITKA) was determined as a function of reduction temperature of Ru/TiO2 (Fig. 4a). From our results, the activity improvement of Ru/TiO2-450 can be attributed to the increased coverage of CHx intermediates on the catalyst surface29. By considering the comparable size of Ru for different Ru/TiO2 samples, a promoted effect toward CO activation to generate CHx intermediates with the aid of TiOx overlayer can be expected on the Ru/TiO2-450 catalyst.
To confirm our proposed mechanism of CO activation, the micro-calorimetry toward CO was measured for Ru/TiO2 samples (Fig. 4b, c). The amount of CO chemisorption follows a trend of Ru/TiO2-300 > Ru/TiO2-450 > Ru/TiO2-600. This can be explained by the decrease exposure of Ru for serving as the adsorbed sites toward CO as increasing reduction temperature. In particular, as compared with other catalysts, Ru/TiO2-450 owns a large portion of CO chemisorption at a relative higher differential heat (>150 kJ mol−1). This was attributed to the CO chemisorption on the interface site, followed by a dissociation of CO with the aid of TiOx overlayer. For comparison, the Ru/TiO2-300 shows a predominant moderate chemisorption toward CO, with a differential heat of 120–150 kJ mol−1 for CO chemisorption on the Ru sites. In contrast, the over-coverage of TiOx on Ru NPs causes a shortage of both the Ru sites and the interfaces for CO chemisorption/dissociation on Ru/TiO2-600.
In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra of CO chemisorption on Ru/TiO2–x indicate that there are three distinct υCO bands located at approximately 2136, 2075, and 2056 cm−1 in the carbonyl region (Fig. 4d). Here, the bands at 2136 and 2075 cm−1 were often observed by performing CO adsorption on well-dispersed, partially oxidized Run+ with a low coordination environment, which therefore were ascribed to multi-carbonyl (Run+(CO)x) and mono-carbonyl (Run+–CO) species adsorbed on partially oxidized Run+ sites on the interface, respectively30,31,32. While the peak at 2056 cm−1 was assigned to linear CO adsorption on metallic Ru (Rux–CO)33,34. After purging H2 into the CO-saturated Ru/TiO2, the gaseous CH4 product with a characterized frequency at 3015 cm−1 was detected35, accompanying by the consumption of CO (Fig. 4e and Supplementary Fig. 18). More importantly, by the complete consumption of CO of Run+(CO)x and Run+–CO, the further conversion of CO was restrained, with Rux–CO as a predominant chemisorption species on Ru surface. In this case, the interface of partially oxidized Run+ sites were supposed to be the active sites for the FTS reaction. As such, the intensity of CO related to FTS on the Ru/TiO2-450 was found to be more remarkable than that of Ru/TiO2-300 and the Ru/TiO2-600 catalysts (Fig. 4d), which was responsible for its higher activity in FTS.
According to our results, a catalytic mechanism for CO transformation on the Ru/TiO2–x catalysts was then proposed. Due to the SMSI over Ru/TiO2, the TiOx overlayer on Ru NPs provides oxygen vacancies for anchoring the oxygen atoms from the dissociation of carbonyl group; it thus greatly facilitates the dissociation of CO on the Ru/TiOx interface of catalysts as also suggested by Bell and coworkers36, by which the hydrogenation and C–C coupling can be realized on the Ru sites to produce carbon chain products. As for Ru/TiO2-450, the optimized TiOx overlayer on Ru NP offers it a maximized activity in FTS, while the shortage of interface on both the Ru/TiO2-300 and Ru/TiO2-600 samples leads to a lower activity in FTS. As such, the participation of the TiOx overlayer in the C–O bond dissociation process was responsible for the superior reactivity of the Ru/TiO2-450 catalyst.
Theoretically, we have performed a density functional theory (DFT) study on the CO activation on the model catalyst on a TiOx cluster decorated Ru(001) surface. In Fig. 5a and Supplementary Table 9, we have first studied the thermodynamic stability of different TiOx clusters on the Ru(001) surface at different oxygen chemical potential. As compared with the TiO6 unit in bulk phase of rutile TiO2, the TiOx was reduced facilely on the Ru(001) surface, and the TiO4 cluster was found to be dominant under the oxygen-rich condition. By decreasing the oxygen chemical potential under reduction condition, the reduction of TiO4 occurred readily on the Ru surface through a sequential reduction to TiO3/Ru(001) and TiO2/Ru(001), respectively. This was consistent with the experimental observation of a reduction of the TiOx overlayer under the reduction condition. On the TiO3/Ru(001) surface, the activation of CO by C–O bond cleavage was then estimated, with that occurring on parent Ru(001) surface as a comparison. As we can see in Fig. 5b, the CO bond cleavage on the Ru (001) surface has a much high barrier (2.15 eV), and the laying down of the atop *CO adspecies adsorbed on the surface is the main obstacle during its dissociation, in good agreement with the previous results37,38. In contrast, with a reduced TiO3 cluster decorating on the Ru(001) surface, the bond cleavage of CO adsorbed on the Ru site of interface can be greatly promoted by experiencing a calculated barrier of 1.62 eV, with the aid of TiO3 as the O seizer of carbonyl group to transform to TiO4. Taking into account that our experiments of FTS were conducted at a reaction temperature of 160–200 °C and a reaction pressure of 2 MPa, such a barrier is facile to overcome on the Ru/TiO2 catalysts at the reaction condition of FTS. The as-dissociated C* adspecies can then be conveniently diffused from the TiOx/Ru(001) interface to Ru(001) (0.73 eV) for the further hydrogenation. Meanwhile, the reduction of TiO4/Ru(001) to TiO3/Ru(001) was even thermodynamically more favorable than the surface reduction of O adspecies on the parent Ru(001) surface (Fig. 5a), which can then facilitate the catalytic cycle of CO activation on the interface. As indicated by the previous reports, such C* species is promising to be hydrogenated to CHx species and realize the C–C coupling on the Ru surface to produce the C2+ products39. Accordingly, the Ru/TiO2-450 catalyst, owing to its optimized Ru/TiOx interface for CO activation, shows a superior activity to other Ru/TiO2–x catalysts in FTS.
In conclusion, we have successfully fabricated a highly active Ru/TiO2 catalyst for FTS by fine-tuning the catalyst pre-reduction condition. With increasing the reduction temperature, the TiOx overlayer is gradually enveloping the Ru NPs. The catalyst reduced at 450 °C exhibits a high intrinsic activity under mild conditions with a low apparent activation energy. The participation of TiOx overlayer in promoting CO dissociation plays a vital role in activity enhancement during FTS. An optimized TiOx overlayer on the Ru NPs formed during reduction at 450 °C is evidently able to capture oxygen from carbonyl group adsorbed on the interface of Ru/TiOx. This, in turn, facilitates the cleavage of C–O bonds. This work not only provides an understanding of the mechanism of CO activation on Ru/TiO2 catalysts, but also suggests an effective approach to tailoring the catalytic properties of metal nanocatalysts supported on reducible oxides.