Structure Sensitivity of CO2 Hydrogenation on Ni Revisited

Abstract

Despite the large number of studies on the catalytic hydrogenation of CO₂ to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO₂ hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO₂ to CO are different from those for the subsequent hydrogenation of CO to CH₄. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO₂ hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO₂ hydrogenation and demonstrated that direct dissociation of CO₂ to CO is followed by the conversion of strongly bonded carbonyls to CH₄. These findings provide essential insights into the much debated structure sensitivity of CO₂ hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

Figure 1

HAADF-STEM images of Ni/SiO₂ catalysts. Particle size distributions of (a) Ni2.7, (b) Ni7.2, and (c) Ni12.2 catalyst samples after H₂ pretreatment at 550 °C and passivation by exposure of 2 kPa O₂ in He at room temperature.

Figure 2

Catalytic performance during CO₂ and CO hydrogenation. (a) Ni mass-normalized CO₂ conversion (open triangles), CH₄ formation rates (solid circles), and CO formation rates (crosses) versus particle size during CO₂ hydrogenation at 0.015–0.018 CO₂ conversion (220 °C, 50 mL min^–1 of 5 kPa CO₂ and 20 kPa H₂ in Ar). (b) Ni surface-specific CO₂ conversion (open triangles), CH₄ formation (solid circles), and CO formation rates (crosses) rates versus particle size. Surface-specific rates are calculated with Ni dispersion values derived from H₂ chemisorption. (c) Ni surface-specific CO conversion (open diamonds) and CH₄ formation (closed circles) rates observed during CO hydrogenation (220 °C, 50 mL min^–1 2 kPa CO and 20 kPa H₂ in Ar). Dashed lines are used to guide the eye.

Figure 3

Structure of Ni nanoparticles under reaction conditions. Ni K-edge XANES spectra for Ni3.9 (a), Ni5.9 (b), and Ni12.2 (c) when exposed to H₂ (black line) or CO₂ + H₂ flow (red line) at 220 °C. Δμ XANES results, multiplied by 20, are shown below the spectra. (d) XANES spectra of Ni K-edge for Ni-foil and NiO references with the Δμ XANES spectrum shown at the bottom. (e) MS results and Δμ XANES area during switch from 20 kPa H₂ to 5 kPa CO₂ + 20 kPa H₂ in Ar at 220 °C. Normalized MS response for CH₄ (m/z = 15) and CO (m/z = 28) versus time (top panel). Area of the Δμ XANES features versus time and Ni dispersion (bottom panel). (f) EXAFS R-space plot for Ni3.9 (solid line), Ni5.9 (dotted line), and Ni12.2 (dashed line) after the switch to CO₂ + H₂ and for Ni and NiO references. (g) Operando XRD results of different Ni particle sizes when exposed to 20 kPa H₂ in Ar at 220 °C. (h) Deformation fault probability α versus particle size, as determined from the whole powder pattern modeling.

Figure 4

Surface coverages, catalyst performance, and DRIFTS spectra recorded during CO₂ hydrogenation. (a) Coverage of surface intermediates leading to CH₄ calculated with τ₀ values from SSITKA experiments and Ni surface area from H₂ chemisorption. (b) Reaction orders of H₂ (solid circles) and CO₂ (open circles) for CH₄ formation (16–24 kPa H₂, 4–6 kPa CO₂). (c) CH₄ selectivity (left axis) and CO₂ conversion (right axis) versus particle size. Dashed lines are used to guide the eye. The constant values of the dashed lines for the 5–12 nm range were based on the general trends observed from our kinetic results. (d) DRIFTS spectra of Ni2.7 during steady-state CO₂ hydrogenation at 200 °C. The inset displays the spectra in the 2960–2800 cm^–1 range. (e) DRIFTS spectra of Ni5.9 during steady-state CO₂ hydrogenation at 200 °C. The spectra are corrected for the differences in optical pathlength by using the overtone and combination vibrations of silica in the 2100–1800 cm^–1 region. In addition, the spectra are normalized by Ni surface area as determined from H₂ chemisorption. The same y-axis (absorbance a.u.) is used for the Ni2.7 and Ni5.9 samples. For the insets, the same y-axis is used for both the samples as well.

Figure 5

Surface dynamics of the adsorbed species. (a) Operando DRIFTS–SSITKA result of ¹³CO₂ + H₂ to ¹²CO₂ + H₂ switch during steady-state CO₂ hydrogenation at 200 °C. Ne is added to the ¹³C-containing mixture to account for gas hold-up. The top panel displays the normalized intensities of Ne, ^12/13CO₂, and ^12/13CH₄ obtained by MS as a function of time. The middle panel shows DRIFTS spectra as a function of time. The bottom panel displays the normalized spectral response of the different gas/surface species determined by DRIFTS. (b) Desorption experiment of CO₂ + H₂ to Ar switch.

Figure 6

Response of adsorbed species during switches between CO₂ + H₂ and H₂. Transients of gas and surface species from MS and DRIFTS during switch from CO₂ + H₂ to H₂ (a) and from H₂ to CO₂ + H₂ (b) of the Ni5.9 sample (200 °C, 50 mL min^–1, 5 kPa CO₂, 20 kPa H₂ in Ar).