Catalytic FTIR studies using the HTHP Transmission Cell or the Praying Mantis High Temperature Reaction Chamber

Using FTIR it is possible to monitor catalysed reactions in-situ, under conditions as close to the real world as possible. For these applications, Specac offer two different accessories that can reach temperatures of up to 910 °C to study the heterogenous reactions between solid and gaseous samples.

---

The High Temperature/High Pressure (HTHP) cell is primarily designed for the analysis of solid pellets in transmission mode. The cell can also be used to analyse gases produced by the thermal decomposition of compounds and can also be adapted to collect reflection spectra from solid pellets.

Harrick’s High Temperature Reaction Chamber is used with the Praying Mantis™ accessory for diffuse reflection studies. Gases introduced into the Environmental Chamber can be pressurised up to 500 psi, whilst the HTHP can be pressurised up to 1000 psi. As with any heatable accessory the maximum achievable temperature is dependent on the pressure and thermal conductivity of any gases introduced within the cell.

Diffuse Reflectance vs. Transmission FTIR

Transmission IR is one of the most widely used techniques for collecting an IR spectrum owing to the simplicity of the analysis. Spectra recorded in transmission generally follow the Beer Lambert law, which states that the absorbance of a sample is directly proportional to the concentration of the chemical species. [1] As a result of this, quantitative analysis of a transmission IR spectrum is straightforward.

Diffuse reflectance studies offer the analyst the ability to study powdered catalysts with high surface areas, whilst transmission studies require the sample to be highly compressed limiting the surface area for solid-gas interactions. Furthermore, the pellet must be sufficiently thin that it is transparent to IR light in the region of interest, unless reflectance studies are performed.

The analysis of DRIFTS spectra is trickier than for transmission. In diffuse reflectance spectra, the relationship between pseudo-Absorbance [-log(%R)] and the concentration of the chemical species is non-linear: for intense bands, most of the reflected light is absorbed as it passes through the sample, whilst in bands of lower intensity most of the reflected light is transmitted to the detector. To correct for this most commercial FTIR spectrometers allow the operator to perform a Kubelka-Munk transformation [2] which attempts to correct the data achieving a spectrum closer to that observed in transmission. Whilst this correction performs well for the quantitative analysis of solid powders, the correction does a poorer job for the analysis of gas sorbates at low concentration on the solid catalyst. Sirta et al. have shown that for high concentrations of gas sorbates on the catalyst surface the Kubelka-Munk transformation gives a near linear relationship to surface concentration. [3] However, at lower surface coverages this linearity breaks down and it is more appropriate to revert to pseudo-absorbance. In the literature a mixture of both forms of y-axis units are reported. [3]

Transmission and DRIFTS studies each offer the analyst a different perspective on the system they are examining, ensuring that both techniques will continue to co-exist alongside one another. The following sections will highlight two studies by Specac customers, highlighting how their research has benefit from each technique.

Supported Ni catalysts in action:  operando infrared spectroscopy unravels carbon dioxide activation over nickel in Specac’s High Temperature High Pressure cell

The increasing CO₂ concentration in Earth’s atmosphere is leading to adverse effect on climate. Power-to-methane strategies, in which green electrons are used to produce methane from CO₂, can be applied to provide electric grid stability and mitigate CO₂ emissions.[4]  Nickel is often considered as an active and cost-effective metal catalyst to convert CO₂ into methane using sustainable hydrogen. While the CO₂ hydrogenation reaction is widely studied, the reaction mechanism is still under debate. Operando FTIR is a powerful technique to identify surface intermediates during reaction and can thus provide new mechanistic insights.

Vogt et al. investigated self-supported wafers of a Ni catalyst supported on 5 different metal oxides in the HTHP cell to establish trends in catalytic activity and to find spectroscopic activity descriptors.[5] After mounting the HTHP cell, the wafers were then exposed to high pressures of a mixture of CO₂ and H₂ at varying temperatures up to 400 °C and FTIR spectra recorded. Adsorbed carbonyl species were found on all catalysts in the 2100–1800 cm^-1 region, with the peak position red shifting as the temperature was increased. Ni nanoparticle size was demonstrated to impact the CO bond strength [6], however the support material was found to have an order of magnitude greater effect. An array of surface species were detected and assigned to carbonates, formates and formyl species. At 400 °C gaseous methane products are detected with the turnover frequency (TOF, i.e. the intrinsic catalytic activity per Ni atom) being greatest for the Ni/TiO₂ catalyst and the lowest for Ni/SiO₂catalyst.

When comparing the two catalysts an immediate difference was observed in the nature of the surface species, with peaks assigned to formate (HCOO^–*) detected on the Ni/TiO₂ catalyst and formyl (HCO*) detected in the experiment with the Ni/SiO₂ catalyst. The authors proposed that the reducible support can delocalize the electrons around adsorbed CO by providing adsorbed hydrogen from the support to the *CO or *CO₂, forming formyl and formate species. DFT calculations were used to demonstrate that the formation of formate lowers the energy barrier and hence increasing the TOF for the Ni/TiO₂ catalyst.

These experiments show that the use of operando spectroscopy is a powerful tool to investigate industrially relevant catalysts at work and to find vibrational descriptors for catalytic activity. The knowledge gained will be used to further guide rational catalysts design, in this case looking for catalysts formulations which favour hydrogen transfer between the active phase and the support. The HTHP cell offers a facile way to study a wide variety of catalysts under different reaction conditions, varying gas composition, temperatures and pressures.

Resolving Transient Responses of Catalyst Systems to Reactant Stream Modulation Using the Harrick Low Volume HVC DRIFTS Reactor

A detailed understanding of the reaction kinetics for solid-gas heterogenous catalysis can contribute to the design of novel catalyst materials for efficient chemical production processes. There is a rapidly growing application of modulation excitation spectroscopy (MES), such as Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), to provide unprecedented insight into the mechanism and kinetics of heterogeneous catalyst systems [7]. However, the application of MES DRIFTS is limited due to instrument design challenges, including the rapid switching of the reactor gas environment. Here, we investigate the feasibility of rapid reactor gas switching in the Harrick Low volume High Temperature Reaction Chamber for DRIFTS measurements. We demonstrate that it is possible to rapidly exchange gas phase species to a Harrick Low Volume Reaction Chamber and utilize DRIFTS measurements to observe the time evolution of reactant, surface-bound intermediates and products to provide mechanistic insight to a catalyst reaction system.

The capability of the Low Volume HVC cell for using DRIFTS to observe the transient response of a catalytic system is demonstrated by modulation from a pure O₂ gas stream to a CO-rich reactant mixture for CO oxidation over Pt/SiO₂. The Pt/SiO₂ catalyst was activated with an oxidation treatment by an O₂ treatment (5 % O2, 95 % Ar) for 30 mins followed by a reduction treatment using H₂ (5 % H₂, 95 % Ar) at 350 °C for 30 minutes. The catalyst was then cooled to 160 °C in a pure Ar environment. The catalyst state was equilibrated in an O₂ environment (0.5 % O₂, 99.5 % Ar) for 30 mins after which a background DRIFTS spectra was acquired. The O₂ stream was switched to a CO-rich reactant mixture for CO oxidation (0.5 % O₂, 1.5 % CO, 98 % Ar) and the time evolution of surface and gas phase species was monitored by acquiring rapid scan DRIFT spectra (see figure below):

The Harrick Praying Mantis Low Volume HVC Reaction Chamber enables probing the transient responses from the modulation of the gas environment in the reactants, surface-species and products using DRIFTS. The Harrick accessory provides a new platform for examining transient responses of catalytic activity promoted by catalyst materials for solid-gas heterogenous catalysis.

References

[1] B. M. Weckhuysen, In-Situ Spectroscopy of Catalysts, American Scientific Publishers, Valencia, CA, 2004.
[2] P. Kubelka & F. Munk, Ein Beitrag Zur Optik Der Farbanstriche, Z. Tech. Phys., 1931, 12, 593–601.
[3] J. Sirita, S. Phanichphant & F. C. Meunier, Analytical Chemistry, 2007, 79, 3912–3918. DOI: 10.1021/ac0702802
[4] Schlögl, R., Angew. Chemie – Int. Ed. 54, (2015), 4436–4439. DOI: 10.1002/anie.201405876
[5] Vogt, C., Monai, M., Sterk, E.B., Palle, J., Melcherts, A.E.M., Zijlstra, B., Groeneveld, E., Berben, P.H., Boereboom, J.M., Hensen, E.J. M., Meirer, F., Filot, I.A.W. & Weckhuysen, B.M., Nat. Commun. 10, 5330 (2019). DOI: 10.1038/s41467-019-12858-3
[6] Vogt, C., Groeneveld, E., Kamsma, G., Nachtegaal, M., Lu, L., Kiely, C.J., Berben, P.H., Meirer F. & Weckhuysen B.M., Nat Catal 1, (2018),127–134. DOI: 10.1038/s41929-017-0016-y
[7] P. Müller, I. Hermans, Applications of Modulation Excitation Spectroscopy in Heterogeneous Catalysis. Ind. Eng. Chem. Res. 56, 1123–1136 (2017).

Share

• g
• k
• o
• 4