CO2 Conversion with Ni-based Catalysts

• The interface between active metallic phases and oxides significantly impacts catalytic properties.
• Two reverse interfaces are created, Ni/MgO and MgO/Ni on Al2O3, in a Ni/Al2O3 CO2 methanation catalyst.
• Both interface structures show enhanced performance with different turnover frequencies denoting distinct mechanisms.
• With MgO between Ni and Al2O3, the formation of inactive NiAl2O4 spinel diminishes which makes more NiO available for reduction.
• The MgO/Ni interface has high reactivity in CO2 methanation when MgO is on top of Ni.
• Ni-NiO redox mechanism enhances CO2 activation at the MgO/Ni interface.
• Hydrogenation of adsorbed carbon monoxide and carbonate species occurs preferably at interface sites.

Introduction

• Supported metal catalysts, like dispersed metal species on oxides, carbons, and zeolites, are used in catalytic applications.
• Supports facilitate metal dispersion, create stable metal nanoparticles, and prevent sintering.
• Metal-support interactions influence the geometric structures and electronic properties of metals impacting catalytic performance.
• Metal-oxide interfaces are important in catalysis, providing additional sites for chemical reactions.
• The metal-oxide interface structure varies, with the oxide acting as a mechanical support, metal dispersion, or activity modifier.
• Traditional catalysts have metal supported on oxide, while inverse catalysts have oxide supported on metal.

Catalysis Configurations

• Support phase identification can be ambiguous where phases consist of equally sized or smaller particles than the active phase.
• Interactions between metal nanoparticles and oxides induce charge transfer and nanoparticle reshaping.
• They also can form new phases, and create highly active adsorption sites at the interfacial parameter.
• Metal-Support Interactions (MSI) include Strong Metal-Support Interaction (SMSI),
  • SMSI involves the support covering part of the active metal, reducing its chemisorption capacity.
• Electronic Metal-Support Interaction (EMSI) highlights chemical bonding and charge transfer roles at the interface.
  • EMSI modifies electronic and chemical properties.
• Interface characteristics influence catalytic activity and selectivity; tailoring the interface optimizes catalysts.
• Ni is a good CO2 methanation catalyst, but can catalyze the reverse water-gas shift reaction.
• Experimental works report high CO selectivity with Ni on SiO2, while Ni interacting with MgO, Al2O3, or CeO2 leads to high CH4 selectivity.

Managing Metal-Oxide Interactions

• Researchers explore ways to control surface/interface structures for performance.
• Managing metal-oxide interactions uses adjusting support composition, morphology, or traits along with metals' size and composition.
• New interfaces can be created or modified by adding an extra oxide to the catalytic system.
• Different elements create interfaces influencing CO and CH4 formation, which is affected by varying CO adsorption strengths.
• Spatial arrangement of these interfaces and the link between different spatial structures and performances is vital.
• Spatial arrangement effect is examined with CO2 methanation over Ni/Al2O3, enhanced via MgO.
• MgO/Ni interface's spatial structure is engineered to fabricate two interfaces with reverse spatial structure, resulting in differing interactions.

Catalyst Synthesis

• Catalysts are made by sequential incipient wetness impregnation using γ-Al2O3, Ni(NO3)2.6 H2O, and Mg(NO3)2.9 H2O.
• Two impregnation sequences are used:
  • Ni is impregnated first on y-Al2O3, followed by MgO.
  • MgO is impregnated first on y-Al2O3, followed by NiO.
• MgAl2O4 as support for Ni, is prepared via co-precipitation from an aqueous solution of Mg(NO3)2.6 H2O and Al(NO3)3.9 H2O.
  • Ni is then impregnated on MgAl2O4 following the above-mentioned method.

Catalyst Characterization

• X-ray diffraction (XRD) analyses are performed.
  • The Scherrer equation helps extract crystallite size information from the XRD pattern.
• N2 physisorption-desorption experiments are done.
  • The Brunauer-Emmett-Teller (BET) method helps calculate the surface area.
• Scanning Electron Microscopy equipped with Energy Dispersive X-ray spectrometry (SEM-EDX) analyzes element composition.
• X-ray photoelectron spectroscopy (XPS) is performed.
  • Binding energies are calibrated using the C 1 s peak of adventitious carbon.
• Temperature-programmed H2 reduction (H2-TPR), CO2-temperature programmed desorption (CO2-TPD) and H2 chemisorption are carried out.
• Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is performed using a Bruker Tensor 27 spectrometer.
• In situ Ni K edge quick X-ray absorption spectroscopy (QXAS) experiments are performed at the ROCK beamline of the French synchrotron SOLEIL.
• Catalytic performance tests are run.
• Turnover frequencies (TOF, 1/s) are determined from the result of the H2-chemisorption experiments.

Catalytic Performance of Interface Types

• 15Ni/Al catalysts is modified by applying MgO before Ni impregnation (15Ni/xMgAl) or introducing MgO onto 15Ni/Al (xMg15Ni/Al).
• All samples with Al2O3 support have a surface area higher than 100 m²/g.
  • Adding MgO reduces this surface area, more so with increased Mg.
• Ni particle sizes are similar across samples, implying MgO hardly affects it.
• Catalyst CO2 methanation reactivity shows improvement in reaction rate over the 15Ni/xMgAl samples.
• The improvement traces back to the metallic Ni exposed.
• The intrinsic activity or TOF of 15Ni/xMgAl is comparable to that of 15Ni/Al.
• The CO2 reaction rate increases with increasing amount of MgO, but since metal exposed is similar, different mechanism exist.
• There is increased intrinsic activity with 15Ni/5MgAl due to the presence of more Ni contacting with MgO.
• xMg15Ni/Al shows increased activity in addition to pronounced improvement of TOFs.
• Stability test results with no apparent deactivation indicating that the created MgO/Ni interface maintains activity over longer time.

Adding MgO onto the Support

• 15Ni/Al and 15Ni/xMgAl catalyst structure is characterized by XRD.
• The Al2O3(440) diffraction peak shifts to the left compared to the pure γ-Al2O3 support.
  • Lattice expansion is induced by large Ni2+ cations being incorporated into the lattice of y-Al2O3.
• No diffraction peak for MgO means either that it is amorphous, or small.
  • Al2O3(440) peak shows left shift for 1MgAl and 5MgAl, ascribing to lattice expansion induced by incorporation of Mg2+ into Al2O3.
  • MgAl2O4 forms this way.
• NiO diffraction peaks is on 15Ni/1MgAl-ap and 15Ni/5MgAl-ap.
  • Shift of the NiO(200) towards the MgO(200) diffraction can point to NiMgO solid solution.
• H2-TPR helps determine the extent to which MgO modifies the interaction between NiO and Al2O3.
  • Two kinds of Ni2+ reduction is seen- those that are loosely mixed, and those that are strongly mixed and stable
• "Free" NiO comprises only 10% of all Ni species in 15Ni/Al meaning Ni is engaged in strong interaction with Al2O3.
• The formation of NiAl2O4 is suppressed when MgO serves as an intermediate layer between Ni and Al2O3.
• MgO can enter the surface as Ni2+, lessening the amount of reducible "free" Ni.
• NiO interacts with MgO deposited on the y-Al2O3 support as well.
  • Reducibility of Ni2+ is promoted by small amounts of Mg2+.
• Trade-off exists because 1% Mg has been found to yield the best reducibility of NiO through those loadings.

Adding MgO onto NiO

• Structure of xMg15Ni/Al catalysts with MgO/Ni interfaces are characterized using XRD.
• Pattern for 15Ni/Al shows left shift of Al2O3(440) peak.
  • Subsequent deposition of only 5% MgO induces an extra shift within the same way.
  • MgO reacts with NiO and Al2O3 (NiMgO, MgAl2O4).
  • H2-TPR applied to test the reduction behavior of the xMg15Ni/Al catalyst.
  • With the addition of MgO, the "free" NiO fraction shrinks, leading to peak shift.

Properties of MgO Related Interfaces

• The surface concentration confirms reverse structures on these two samples, and MgO resides on the surface of NiO in 5Mg15Ni/Al.
• DRIFTS shows the samples display similar CO2 adorption behavior, as shown by broad peaks between 1700 and 1300.

QXAS

• CO2-TPD tests show a small signal at with *HCO.
• DRIFTS indicates a small range, with only a small number of the species forming methane,
• The white line shows the intensity the start of 1Mg15Ni/Al to be more diminished,
• There are similar observations in in situ XAS during a CO2-TPO process.

CO2 Reactivity and Interface Dynamic's

• Higher rates a lower temperatures suggest better CO2 reactivity within 15NI and I MgINI.
• Bulk oxidation is most rapid for 15NI at 530C.
• Dynamic changes during reaction is verified though comparisons of Ni in data sets, showing stable catalyst structure.