Electronic modulation of metal-support interactions improves polypropylene hydrogenolysis over ruthenium catalysts

Ru/TiO₂ catalyst characterization

We changed the pH in the Ru deposition step, using aqueous NH₃, to modify the catalyst (catalysts are labeled as Ru/TiO₂-x, where x indicates the synthesis pH) while ensuring similar Ru loadings (~3 wt%), surface areas (100 m²/g) (Table 1), and pore volume. According to XPS quantitative analysis, all samples have similar surface Ru/Ti atomic ratios consistent with the same Ru loadings. XRD patterns (Supplementary Fig. 1) show relatively broad reflections of the TiO₂ anatase support with no sign of crystalline Ru. Ex situ UV-Vis spectra reveal comparable bandgap energies, typical of pure anatase TiO₂ (Table 1). All samples contain predominantly 1.4–1.6 nm Ru nanoparticles evenly distributed on TiO₂ (STEM images in Fig. 1a, b and particle size distribution in Supplementary Fig. 2). TiO₂ consists of elongated cylindrical, highly crystalline particles of 35–40 nm length and ~10 nm diameter. In situ Ru K-edge XANES spectra (Supplementary Fig. 3) after pre-reduction with H₂ at 250 °C indicate metallic Ru⁰ and EXAFS spectral analysis (Table 1, Fig. 1c, Supplementary Fig. 3, Supplementary Table 1) show a coordination number of ~8.5, consistent with the STEM images.

TGA results (Supplementary Fig. 4) show that ~10 mmol//\({{{{{{\rm{g}}}}}}}_{{{{{{{\rm{TiO}}}}}}}_{2}}\) of NH₃ is retained after impregnation, an order of magnitude higher than Ti⁴⁺ on the surface¹⁸. NH₃ desorbs in H₂/He at ~250 °C, indicating strong interaction with the catalyst. NH₃ can reduce and modify the TiO₂ surface after high-temperature treatment^19,20. Based on DRIFTS spectra (Supplementary Fig. 5) of Ru/TiO₂-8 directly after impregnation and drying and before the reduction in H₂, ammonia is adsorbed in several modes. Two possible mechanisms are proposed to explain the NH₃ role during sample pre-reduction (Supplementary Figs. 6–8). XPS analysis shows no signs of N-doping of TiO₂ after reduction.

To investigate the modification of the Lewis acidity, we used FTIR of adsorbed CO at −196 °С (Fig. 1d, e). At high CO pressure, Lewis acid site bands appear at 2206 and 2179 cm⁻¹ due to highly electrophilic four-coordinated Ti⁴⁺(O₄) sites on the particle edges and the less acidic five-coordinated Ti⁴⁺(O₅) sites on the 101 crystal planes, respectively²¹. The concentration of Ti⁴⁺(O₄) sites is similar in both samples. The intensity of both bands decreases with evacuation time, but the 2206 cm⁻¹ band is reduced more slowly. Ru/TiO₂-8 retains some CO on both sites even after prolonged evacuation, whereas Ru/TiO₂-1 only marginally on the stronger Ti⁴⁺(O₄) sites (Fig. 1e), indicating stronger binding and Lewis acidity on the former (Supplementary Fig. 9). DRIFTS demonstrates a smaller density of surface Ti-OH groups on the Ru/TiO₂-8 and Ru/TiO₂-12 than on Ru/TiO₂-1 (Supplementary Fig. 10). Partial dehydroxylation enhances the Lewis acidity of Ti⁴⁺ sites due to the redistribution of the electron density on the surface²².

In addition to Ti⁴⁺–CO adducts, FTIR spectra show that Ru particles get partially oxidized by CO forming multiple Ruⁿ⁺(CO)_m (m = 1–4) carbonyls with ν(CO) vibration at 2136, 2106, 2084, and 2055 cm⁻¹ (Supplementary Fig. 11)²³. Due to the low-temperature CO dissociation, clusters of Ruⁿ⁺ and Ru⁰ species form. The stronger interaction of TiO₂ and Ru and the TiO_x peripheral layer shifts the broad ν(CO) band to lower wavenumbers²⁴ (from 2077 to 2059 cm⁻¹, Fig. 1d), epitomizing a proximal MSI in the Ru/TiO₂-8 catalyst.

Hydrogen binding on Ru/TiO₂

H₂ temperature-programmed desorption (TPD) (Supplementary Fig. 12) indicates a similar amount of strongly chemisorbed hydrogen on the Ru (low-temperature peak at 160 °C)²⁵ and TiO₂ (high-temperature shoulder at 250–300 °C)²⁶. Temperature-programmed reduction (TPR) in H₂ flow shows a spike in hydrogen adsorption at 80–90 °C, due to binding on Ru, and a broad peak starting at ca. 250 °C, due to partial TiO₂ reduction (Supplementary Fig. 13). Interestingly, Ru/TiO₂-8 and Ru/TiO₂-12 show more pronounced hydrogen uptakes at 150-200 °C than Ru/TiO₂-1. Hydrogen pulse chemisorption at 35 °C (Table 1) shows that Ru/TiO₂ samples have different hydrogen uptakes. During an experiment, hydrogen can spillover to the TiO₂ support obscuring the real Ru dispersion values. Thus the total H₂ uptake inferred from pulse chemisorption depends on the spillover capacity of TiO₂, rendering accurate quantification of the exposed Ru surface area impossible with the available methods. Also, the pretreatment temperature before pulse chemisorption (300 °C) is insufficient to remove all chemisorbed hydrogen from the sample. Thus, apparent uptakes may be higher for catalysts with weaker chemisorption.

To decouple Ru from support contributions to the hydrogen binding and activation, we used ²H MAS NMR of chemisorbed D₂ (Fig. 2). Chemisorption of ²H on Ru clusters is accompanied by quadrupole interactions with the surrounding electric field gradient (EFG), leading to characteristic sidebands in the NMR spectra. Spectra contours depend on the quadrupole coupling constant (Q_cc), a function of the largest EFG component (V_zz), and the asymmetry parameter η, defined as (V_xx − V_yy)/V_zz. One can obtain the EFG parameters, sensitive to the deuteron local surroundings and the binding type^27,28.

Figure 2a shows a complex line shape after sample reduction followed by saturation with 1 torr of D₂. Deconvolution yields several components (Fig. 2b, c; high-resolution data at 10 kHz shown in Supplementary Fig. 14). The first component (Q_cc~123–130 kHz/η~0.7–1.0) corresponds to Ti-OD formed by deuterium spillover, confirmed by measuring a D₂O-treated pure TiO₂ sample (Fig. 2f, Supplementary Fig. 15, Supplementary Table 2). Also, a simple H/D isotope exchange leads to the formation of Ti-OD groups²⁹, and their presence in the spectra is not a fingerprint of spillover.

The latter peak has a symmetric EFG (η~0.1) and low Q_cc of ca. 70 kHz, distinctly different from the Ti-OD signal (Fig. 2e, Supplementary Table 3), and consistent with Ru_n-D hydrides²⁷. The second component corresponds to deuterons bonded to Ru in atop conformation (see parameters in Supplementary Tables 3 and 4), confirmed using DFT calculations (Supplementary Fig. 16). Figure 2g shows an optimized structure of a Ru cluster on TiO₂ with a D atom, in good agreement with a previous report³⁰, underscoring the preferential formation of atop H instead of (Ru)₂-H bridging binding. The EFG parameters depend on the Ru particle size and the support. For an unsupported Ru₁₂ cluster, EFG is more symmetric, unlike the experimental data. Smaller or larger clusters than Ru₁₂ on TiO₂ provide a less adequate EFG. Thus, ²H NMR, combined with DFT, provides insights into MSI and particle size effects.

The third component gives a very broad single line with no sidebands (Fig. 2d) at a low resonance frequency (−4 to −4.4 kHz, or −53 to −57.3 ppm). These features are not standard for Ru_n-D hydrides, observed previously for free-standing Ru clusters²⁷. The negative chemical shift is similar to chemisorbed hydrogen on Ru particles on SiO₂ and TiO₂, inferred from static ¹H NMR^31,32. The deuteron interaction with the Ru conduction electrons leads to the so-called Knight shift responsible for the −53 ppm peak³³. On Ru/SiO₂, this peak stems from the overlap of different signals due to strongly and weakly chemisorbed hydrogen³¹. We exclude deuteron binding to an oxygen vacancy in TiO₂ because it would lead to resonances close to 0 kHz at −1.07 ppm³⁴ or −0.66 ppm and −0.78 ppm³⁵. Static ¹H NMR studies showed that the exact position and linewidth are highly affected by Na doping of a Ru/TiO₂ catalyst³⁶, i.e., this deuteron is sensitive to interactions with the support. In our case, direct interaction via Fermi contact between Ti³⁺ paramagnetic sites on the partially reduced TiO_x support can affect the position and intensity of this peak.

Sample evacuation at 100 °C leads to a significant reduction in the intensity of the −53.2 ppm peak and shifting to −45.8 ppm (lower Knight shift) (Supplementary Figs. 17, 18, Supplementary Table 5). Interestingly, the ratio of Ti-OD and Ru-D_atop groups remains constant, indicating comparable stability. Thus, the resonance at −53.2 ppm corresponds to the weakly bonded deuteron, not entirely captured in the TPD. Unlike the Ru-D_atop signal, these deuterons are trapped by the Ru’s conduction electrons or some Ti³⁺ charged center of the support. Based on the thermal stability of the adsorbed deuterium studied by ²H MAS NMR and TPD, all three types of surface deuterons are stable at 25 °C and desorb only upon heating.

The ²H MAS NMR spectra reveal significant differences in the deuterons (Fig. 2h,i, Supplementary Fig. 19). The NH₃-treated samples have a much higher content of Ru-bonded deuterium than Ti-OD groups. Higher resolution 10 kHz MAS spectra (Fig. 2h, i) show that Ru-related peaks at 0.18 kHz (2.1 ppm) and ~−4.49 kHz (−58.0 ppm) over Ru/TiO₂-8 are more pronounced than on Ru/TiO₂-1 The data combined (Supplementary Table 6, 7) reveals that Ru/TiO₂-8 has a higher absolute amount of Ru-bonded deuterium. This was further supported by a direct comparison of relative distributions of different deuterons. Thus, NH₃-treated supports promote hydrogen binding to Ru clusters.

Dynamics of hydrogen-Ru/TiO₂ interactions

Hydrogen can partially reduce the TiO₂ support by hydrogen spillover (Fig. 3)²⁹, forming OH groups, and injecting electrons into TiO₂, creating Ti³⁺ sites as a new state in the bandgap (Fig. 3a)³⁷. Thermal excitation could also cause electron delocalization and populate the titania conduction band (CB)³⁸. Specifically, hydrogen spillover creates a Ti³⁺ shallow trap, 0.1–0.2 eV below the CB edge, and a broadband in the FTIR spectra (Fig. 3c), frequently used to study spillover^39,40. Electrons residing in the CB produce a power-law type spectrum distinct from shallow traps.

On Ru/TiO₂-1, hydrogen binding leads primarily to shallow trap electrons (Fig. 3d). Due to the reversible spillover, this partial reduction is sensitive to temperature and hydrogen pressure (Supplementary Figs. 20, 21). A much broader background increase is seen on Ru/TiO₂-8 due to the additional partial CB filling (Fig. 3e, Supplementary Fig. 22). The Ti-OH groups give a slight negative peak of ν(OH) at 3665 cm⁻¹, and a new peak for δ(HOH) at 1614 cm⁻¹ emerges due to the recombinative dehydroxylation of vicinal Ti-OH groups into water. Water formation stimulates the population of CB electrons⁴¹. FTIR shows that Ru/TiO₂-1 is mainly reduced into localized Ti³⁺ states, whereas NH₃-treated samples produce delocalized CB electrons (Fig. 3f). Hydrogen spillover is activated only at sufficiently high H coverage on Ru particles³⁰, i.e., the low H coverage on the Ru/TiO₂-1 sample (detected by ²H NMR) does not promote spillover and TiO₂ reduction to the same extent.

In situ Raman spectroscopy at 200 °C in He and H₂/He flows (Fig. 3g, h) corroborates this result. The E_g(1) vibrational mode of TiO₂ anatase (~144 cm⁻¹) is sensitive to the concentration of electrons⁴². On Ru/TiO₂-1, the peak position is unaffected by hydrogen (Fig. 3g). In contrast, on Ru/TiO₂-8 it shifts by ~3 cm⁻¹ due to forming CB electrons by hydrogen; shallow traps (Ti³⁺ states) do not contribute.

The extent of reduction and associated charge transfer were monitored using NAP-XPS. Upon introducing hydrogen at 200 °C to Ru/TiO₂-8, the Ti 2p_3/2 peak (458.69 eV; Fig. 3i, Supplementary Fig. 23, Supplementary Table 8) and the main lattice oxygen peak in the O 1 s region⁴³ (Fig. 3j) shift to lower binding energies by 0.15 eV due to modest support reduction. No shifts are evident on Ru/TiO₂-1, indicating low receptivity toward hydrogen. Since the penetration depth of NAP-XPS corresponds to 2–3 nm, the Ru 3d and Ti 2p signals are collected from the whole Ru nanoparticle volume and 1–2 atomic layers of the underlying TiO₂ support⁴⁴. Thus, changes in the XPS spectra reflect charge distribution and MSI, but are not sensitive to the formation of the narrow TiO_x peripheral layer observed in the FTIR of CO experiments (Fig. 1d).

The Ru 3d doublet overlaps with the C 1 s signal caused by carbonaceous deposits on the initial TiO₂ (Fig. 3k). Ru/TiO₂-1 and Ru/TiO₂-8 have slightly different binding energy (BE): 279.70 and 280.12 eV, respectively (Supplementary Table 8). Such values are typical for small Ru particles on TiO₂⁴⁴. Upon exposure of Ru/TiO₂-1 to hydrogen at 200 °C, the BE of the Ru 3d_5/2 peak increases by 0.12 eV; over Ru/TiO₂-8, it shifts by 0.61 eV, indicative of Ru^δ+ species. H₂ activation at the Ru-TiO₂ interface leads to negatively charged H⁻ attached to the metal and H⁺ binding to the nearest oxygen atom (Fig. 3b)⁴⁵. This heterolytic hydrogen activation leads to a partial positive charge on Ru due to forming negatively charged hydrides (H⁻) (Fig. 3b). The direct charge transfer from Ru to TiO₂ CB leads to a partial reduction of TiO₂, as reported in CO₂ reduction⁴⁶. Ru^δ+-H⁻ pairs upon H₂ adsorption were also reported on Ru/carbon nanotubes⁴⁷. Ru on NH₃-treated supports allows spillover of H₂, forming delocalized electrons, reducing the support more extensively, and increasing the positive charge of Ru.

PP hydrogenolysis

PP hydrogenolysis data over the three Ru/TiO₂ samples with two polymer/catalyst weight ratios, 20 and 40, is shown in Fig. 4a–c and Supplementary Figs. 22, 23. The liquid yield increases with time and reaches 74% (Ru/TiO₂-8) and 65–70% (Ru/TiO₂-12) at 6 h, compared to only 63% at much longer times of 20 h (Ru/TiO₂-1) reported earlier⁶. The conversion of the solid residue follows the same trend. The liquid product’s weight-average molecular weight (M_w) (Fig. 4c, Supplementary Tables 9, 10) is lower over Ru/TiO₂-8 and Ru/TiO₂-12 than Ru/TiO₂-1.

PP conversion follows⁶ (i) an initial polymer consumption forming a “heavy” liquid; (ii) a decrease of the liquid M_w; and (iii) consumption of the liquid to light gases (~85% methane). When the M_w reaches a critical value, cascade hydrogenolysis to gas starts. Ru/TiO₂-8 and Ru/TiO₂-12 substantially increase the solid consumption and the liquid C–C bond hydrogenolysis compared to Ru/TiO₂-1, leading to lighter products (Fig. 4c, Supplementary Tables 9, 10).

All catalysts show similar methane formation up to ca. 10% at long reaction times due to excessive liquid hydrogenolysis (Supplementary Figs. 24, 25, Supplementary Table 11). To study liquid gasification in more detail, we performed experiments at a higher polymer to catalyst ratio of 20 (Fig. 4b). The liquid stability increases in the order: Ru/TiO₂-12 < Ru/TiO₂-8 < Ru/TiO₂-1 in line with the liquid M_w.

In the PE conversion over Ru/ZrO₂ and Ru/WO_x/ZrO₂, the hydrogen availability on Ru was crucial in controlling the hydrogenolysis selectivity to liquids vs. light gases¹⁵. A high intrinsic H coverage favors liquid products. Conversely, a low H coverage promotes a sequential cascade of C–C rupture to methane.

We hypothesize that the performance differences among catalysts stem from the H coverage on Ru. We perform experiments of varying H₂ pressure (Fig. 4f). An increased hydrogen pressure leads to higher reaction rates over Ru/TiO₂-1; the liquid yield reaches ca. 60% in 6 h (40 bar H₂) and 3 h (50 bar H₂), much faster than the 30 bar experiment. Still, the liquid to gas decomposition is also accelerated (Supplementary Fig. 26, Supplementary Table 12). This competition leads to a maximum liquid yield. The liquid decomposition on the ammonia-treated Ru/TiO₂ samples is less severe, and a maximum is absent at a polymer to catalyst ratio of 20 (Fig. 4a). This maximum is visible at higher catalyst loadings (Fig. 4b, Supplementary Fig. 25) and shifts to shorter reaction times. The increased H coverage at the same H₂ pressure drives the improved catalyst performance, consistent with the ²H MAS NMR data of the higher content of both Ru-H species and spillover (Fig. 2). We propose that the different nature of chemisorbed hydrogen may be partially responsible for the variation in the liquid yields over different samples. Thus, a simple increase in surface coverage of hydrogen is insufficient to get a similarly high liquid yield over Ru/TiO₂-1 and Ru/TiO₂-8.

Alkane hydrogenolysis invokes (Fig. 5) adsorption to the metal, leading to dehydrogenated intermediates, C–C bond breaking of these intermediates, and final hydrogenation and product release. The first step is usually quasi-equilibrated⁴⁸ due to the lower dehydrogenation barrier than the C–C bond breaking⁴⁹. Experiments in D₂ lead to slower dehydrogenation due to a kinetic isotope effect (KIE) in the dehydrogenation step, while the C–C bond breaking is not strongly affected by the H/D change⁵⁰.

PP hydrogenolysis in D₂ is slower⁶, and the M_w of the liquid is larger, consistent with our data on Ru/TiO₂-1 and Ru/TiO₂-8 (Fig. 4d, e). A prominent shift in M_w is evident over Ru/TiO₂-1 (from 2.67 to 14.12 kDa). Quasi-stationary kinetic analysis with standard transition state theory calculations, based on Fig. 5 (see Supplementary discussion I, Supplementary Fig. 27), shows that increasing the H coverage increases the net hydrogenolysis rate, reduces the KIE, and makes the net reaction rate less sensitive to the H/D exchange. Since Ru/TiO₂-8 has a higher H coverage, the reaction rate is less sensitive to deuteration and correlates with the hydrogenolysis data (Fig. 4a). Over the Ru/TiO₂-8, a high H coverage pushes further C–C bond breaking, leading to consumption of the initial polymer with no solid residue. On Ru/TiO₂-1, the lower H coverage makes dehydrogenation more kinetically relevant and the initial polymer deconstruction slower.

In PE conversion on Ru/ZrO₂, higher H coverages accelerate the desorption of reaction intermediates, preventing them from over-cracking, and giving higher liquid selectivity over methane at high H₂ pressures. In PP, an increased H coverage boosts all three reaction stages because the PP initial polymer deconstruction is slower than the product desorption. Thus, altering the H coverage primarily alters this reaction step.