The Role of In Situ/ Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

Abstract

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Figure 1

Relationship between the strain of the active chromium sites and reactivity, as determined by the computational work in ref (103). (a) Simplified illustration of the four-, six-, and eight-membered chromasiloxane ring (4CR, 6CR, and 8CR, respectively) models bearing Cr(II) species, with a qualitative indication of their relative abundance and strain, and the correlation with the Cr–O distance and O–Cr–O angle. By moving from yellow to red, the strain increases. (b) Most relevant energetic values as extrapolated from the Gibbs free energy profiles for ethylene binding on Cr(II) and for ethylene insertion and β-hydrogen transfer on the corresponding Cr(III)-ethyl models. (c) Predicted average MW of the PE obtained from the three models, determined by the kinetic competition between chain propagation and chain transfer. Data reproduced with permission from ref (103). Copyright 2022 American Chemical Society.

Figure 2

IR spectroscopy allows the discrimination of Cr(II) sites as a function of their coordination environment. The figure shows the IR spectra, in the ν(CO) region, of CO adsorbed at room temperature as a function of the CO coverage on two Cr(II)/SiO₂ samples subjected to a different thermal history. (a and b) Spectra of a Cr-doped glass monolith (Cr loading of 0.1 wt %) calcined either at (a) 550 or (b) 650 °C and then reduced in CO at 350 °C. Adapted with permission from ref (119). Copyright 2019 Elsevier. The two sequences of spectra have been normalized to the optical thickness of the sample, hence the absolute intensities are comparable. (c and d) Spectra of a Cr/Aerosil300 sample (Cr loading of 1.0 wt %) calcined at 650 °C and (c) reduced in CO at 350 °C or (d) successively treated under vacuum at 650 °C. Data reproduced with permission from ref (59). Copyright 2005 American Chemical Society. The two sequences of spectra have been normalized to the optical thickness of the pellet, hence the absolute intensities are comparable. In all parts, the dotted vertical line indicates ν(CO) of gaseous CO.

Figure 3

IR spectroscopy reveals the CO-induced mobility of the Cr(II) sites at the silica surface, at the expense of the weaker siloxane ligands. The figure shows the IR spectra, in the ν(CO) region, of CO adsorbed at 100 K as a function of the CO coverage (light pink corresponds to maximum coverage) on a Cr(II)/SiO₂ sample (Cr loading of 1.0 wt %) calcined at 650 °C and reduced in CO at 350 °C. The spectra are reported after subtraction of that collected prior to CO dosing. Data reproduced with permission from ref (59). Copyright 2005 American Chemical Society. The spectroscopic regions characteristic for classical and nonclassical carbonyls are indicated, with reference to the position of ν(CO) for gaseous CO (dotted line).

Figure 4

Computational works predict a certain flexibility of the Cr(II) sites at the silica surface. The figure shows several proposed structures for different Cr(II) sites and related carbonyl species as obtained from DFT calculations by (a) Scott and co-workers and (b) Damin and co-workers. Species 1–3 are embedded in a 6CR, while species 4 belongs to an 8CR (in blue). Additional weaker oxygen ligands are shown in red. For species 4, the Cr(II) is stabilized by an extra oxygen ligand belonging to its own 8CR. Relevant bond distances and angles are reported, as well as the computed Δν(CO) values. (a) Adapted with permission from ref (120). Copyright 2012 Elsevier. (b) Adapted with permission from ref (107). Copyright 2015 Elsevier.

Figure 5

IR spectroscopy indirectly detects the adsorbate-induced mobility of the Cr(II) sites at the silica surface. (a) IR spectra of Cr(II)/SiO₂ and of the bare silica support in a spectral region dominated by the overtones and combinations of the fundamental (SiO₄) vibrations of bulk silica. (b) Spectrum of Cr(II)/SiO₂ after subtraction of that of silica and that of the same sample interacting with CO either at room temperature or at 100 K. (c) The same as (b) for another Cr(II)/SiO₂ sample, before and after a short interaction with ethylene at room temperature. Unpublished data.

Figure 6

IR spectroscopy reveals that the adsorbate-induced structural rearrangements of the Cr(II) sites at the silica surface is a function of the adsorbate. The figure shows the evolution of the IR spectra of Cr(II)/SiO₂ interacting with CO at room temperature (bold gray spectra) after contact with a second, stronger, ligand: (a) ethylene, (b) cyclohexene, and (c) NO. All the spectra are reported after subtraction of that collected prior interaction with any probe and in the ν(CO) region. (a) Reproduced with permission from ref (59). Copyright 2005 American Chemical Society. (b) Reproduced with permission from ref (127). Copyright 2016 American Chemical Society. (c) Reproduced with permission from ref (128). Copyright 2010 John Wiley and Sons.

Figure 7

Operando IR spectroscopy allows the detection of the formation of oxygenated byproducts during the reduction of Cr(VI)/SiO₂ by ethylene. These byproducts are responsible for a structural rearrangement of the reduced chromium sites, which precedes ethylene polymerization. (a) Operando IR spectra collected during reaction of ethylene with Cr(VI)/SiO₂ at 150 °C. Color code: black, spectrum collected prior ethylene dosage; gray, spectra collected in the presence of ethylene as a function of time; and green, spectra collected after 30 min. (b–d) Final spectra after subtraction of those collected prior to ethylene dosing, magnified in (b and d) two spectral regions characteristic of the oxygenated byproducts and (c) in the region containing the first overtone of the ν(Cr=O) vibrational mode. Spectra reproduced with permission from ref (130). Copyright 2017 American Chemical Society.

Figure 8

IR and Raman spectra of TS-1 provide information on the local structure of the Ti(IV) sites and on their adsorbate-induced flexibility. (a and b) IR and Raman spectra, respectively, of pure silicalite (dotted) and TS-1 (full) dehydrated in vacuum at 400 °C. Spectra reproduced with permission from ref (146). Copyright 2006 John Wiley and Sons. (c) Background-subtracted IR spectra of TS-1 activated in vacuum at 400 °C upon increasing coverages of CD₃CN in the ν(C≡N) region (maximum coverage in blue). (d) The same as in (c) in the spectral region characteristic for the zeolite framework modes. Spectra reproduced with permission from ref (147). Copyright 2003 American Chemical Society. (e) Schematic representation of the effect of the adsorption of acetonitrile on the coordination geometry of the Ti(IV) sites in TS-1. (f) Evolution of the IR spectra of TS-1 activated in vacuum at 400 °C upon increasing the coverage of H₂O (from black to blue). (g) Raman spectra of TS-1 activated in vacuum at 400 °C before (black) and after (blue) interaction with H₂O. Spectra reproduced with permission from ref (148). Copyright 2002 American Chemical Society.

Figure 9

IR and Raman spectra of Fe-substituted zeolites permit the speciation of the Fe sites and tracing of their mobility. (a and b) IR and (c) Raman (λ_exc = 1064 nm) spectra of a Fe-silicalite dehydrated in vacuum at 400 °C after calcination at 500 or 700 °C. Reproduced with permission from ref (157). Copyright 1996 Elsevier. (d) Schematic representation of the main Fe species present in Fe-silicalite, depending on the activation conditions, and corresponding IR bands. Fe(III)_IN and Fe(III)_OUT refer to Fe(III) sites inside and outside the framework, respectively. (e) IR spectra of a Fe/ZSM-5 prepared by CVD before (full line) and after (dotted line) interaction with NO₂. Spectra reproduced with permission from ref (161). Copyright 2008 Elsevier.

Figure 10

IR spectroscopy using NO as a probe allows the speciation of Fe sites in a Fe-silicalite as a function of the treatment conditions. (a) IR spectra of NO adsorbed at room temperature as a function of the NO coverage (maximum coverage: full yellow; minimum coverage: dotted yellow) for a Fe-silicalite activated at 500 °C. (b) As in (a) for the same material activated at 700 °C. Spectra reproduced with permission from ref (173). Copyright 2002 Elsevier.

Figure 11

IR spectroscopy reveals the thermal- and adsorbate-induced mobility of boron atoms in B-substituted zeolites. (a) IR spectra of a B-SSZ-13 material as-synthesized (gray) and after calcination at 500 °C (black). Reproduced with permission from ref (178). Copyright 2007 American Chemical Society. (b) Evolution of the background-subtracted IR spectra, in the ν(OH) region, of a B-SSZ-13 activated at 500 °C upon interaction with increasing dosages of NH₃ at room temperature (full coverage in dark green) and after heating at 100 °C in the presence of NH₃ followed by degassing (light green, vertically translated for clarity). (c) The same as in (b) in the 1500–800 cm^–1 region. (d) Schematic representation of the changes in the local structure around the B sites upon template removal, adsorption of NH₃, and further reaction with NH₃ at 100 °C. Reproduced with permission from ref (179). Copyright 2007 American Chemical Society.

Figure 12

IR spectroscopy allows the speciation and quantification of different Cu sites in Cu-exchanged zeolites. (a) IR spectra, in the [SiO₄] vibration region, of different Cu-zeolites treated in O₂ flow at 400 °C. (b) Schematic representation of the two different locations of Cu ions in Cu-SSZ-13 materials (Si, yellow; O, red; Al, light blue; and Cu, green). (c) DRIFT spectra of two Cu-SSZ-13 characterized by different Si/Al ratios. (d) DRIFTS spectra, in the [SiO₄] vibration region, of Cu-SSZ-13 materials with the same Si/Al = 6 ratio and with various Cu loadings. All the samples have been pretreated in an O₂ flow at 500 °C. The spectra in (a) are unpublished, while data reported in (b–d) are reproduced with permission from ref (192). Copyright 2018 American Chemical Society.

Figure 13

IR spectroscopy is sensitive to the oxidation state and local environment of the Cu sites in Cu-SSZ-13 zeolite. (a–c) IR spectroscopy allows the speciation and quantification of the different Cu sites in Cu-exchanged zeolites. (a–c) IR spectra of Cu-SSZ-13 in its hydrated form (hydr), after thermal treatment in O₂ at 400 °C (ox), and after prolonged thermal treatment in inert flow at 400 °C (red) in three different spectral regions. (d–f): IR spectrum of Cu-SSZ-13 after thermal treatment in O₂ at 400 °C (ox), and its time evolution during interaction with NH₃ at 400 °C. The pink spectrum was collected after 30 min. The insets in (d) show the structure of linear diammino and square-planar tetraammino Cu complexes formed in the presence of NH₃ at 400 °C. Unpublished data.

Figure 14

IR spectroscopy reveals the adsorbate-induced mobility of Cu(I) cations in Cu-ZSM-5. (a) IR spectrum in the ν(CO) of a Cu-ZSM-5 preactivated in NH₃ at 500 °C, that of the same sample in the presence of CO at the maximum coverage, and that after prolonged outgassing at room temperature region. (b) The same as in (a) in the [SiO₄] vibrational region. Data reproduced with permission from ref (201). Copyright 2018 American Chemical Society.

Figure 15

In situ IR spectroscopy of adsorbed CO allows the discrimination of adsorption sites at Pt NPs and subnanometric clusters. (a) DRIFT spectra of CO adsorbed at room temperature and at the maximum coverage on four freshly reduced Pt/Al₂O₃ samples differing in Pt loading and particle size (see legend). The spectra are normalized to the intensity of the low-frequency band. The high frequency band corresponds to CO molecules adsorbed on well-coordinated (WC) Pt sites, while the band at lower frequency is assigned to CO molecules interacting with under-coordinated (UC) Pt sites. Adapted with permission from ref (237). Copyright 2016 American Chemical Society. (b) IR spectra of CO adsorbed at room temperature and at the saturation coverage on a 5 wt % Pt/Al₂O₃ sample with an average particle size of 1.4 ± 0.4 nm, with CO dosed either in the gas-phase or in solution (cyclohexane as solvent). The IR spectrum of CO adsorbed from the gas phase shows an additional band at lower frequency, which is ascribed to highly under-coordinated Pt sites (HUC). (c) Sequence of IR spectra collected upon increasing the CO coverage (from brown to red) on the same 5 wt % Pt/Al₂O₃ sample discussed in (b). The data reported in (b and c) are reproduced from ref (242) with permission. Copyright 2022 Royal Society of Chemistry.

Figure 16

Operando IR spectroscopy reveals the occurrence of CO-induced surface reconstruction of Pt NPs as a function of the temperature and under reaction conditions. (a) Evolution of in situ DRIFT spectra of CO adsorbed at the saturation coverage on a Pt/Al₂O₃ catalyst (average particle size 17 nm) as a function of time and temperature. Adapted with permission from ref (33). Copyright 2017 American Chemical Society. (b) Wulff construction of a 9.2 nm Pt NP based on DFT-calculated free energies for bare surfaces (bottom) and CO-saturated surfaces (top). WC Pt atoms are represented in green, and UC Pt sites are in blue. Adapted with permission from ref (33). Copyright 2017 American Chemical Society. (c) DRIFT spectrum of a Pt/Al₂O₃ catalyst (average particle size 19 nm) in the presence of CO at room temperature (saturation coverage, prereaction), and its evolution under CO oxidation reaction conditions (1% CO, 1% O₂) as a function of time and temperature. Adapted with permission from ref (237). Copyright 2016. American Chemical Society.

Figure 17

In situ IR spectroscopy of adsorbed CO allows the discrimination of isolated Pt sites from oxidized clusters. (a) IR spectra collected at different temperatures during a CO-TPD experiment in He performed on a 1 wt % Pt/TiO₂ catalyst obtained via incipient wetness impregnation that was preoxidized for 2 h in air at 300 °C. CO desorbs from Pt_ox sites at higher temperature than from Pt metal sites, with significant desorption only above 200 °C and incomplete desorption even by 350 °C. Adapted with permission from ref (290). Copyright 2017 American Chemical Society. (b) IR spectra collected at different temperatures during a CO-TPD experiment in Ar performed on a 0.5 wt % Pt/HZSM-5 sample prepared by solution grafting of a Pt organometallic complex. Cationic Pt^δ+-polycarbonyl species are easily converted into Pt^δ+-monocarbonyls already at room temperature, the latter being stable at 100 °C. Adapted with permission from ref (207). Copyright 2015 American Association for the Advancement of Science. (c) IR spectrum of CO adsorbed at room temperature on a prereduced 0.05 wt % Pt/TiO₂ catalyst prepared according to a synthetic protocol, which allows the deposition of less than 1 Pt atom per TiO₂ particle, and its evolution upon desorption in inert atmosphere at room temperature. Adapted with permission from ref (290). Copyright 2017 American Chemical Society.

Figure 18

Operando IR spectroscopy allows the identification of the Pt active sites in the CO oxidation reaction. (a) Operando DRIFT spectra collected during the CO oxidation reaction at 50 °C on a Pt/CeO₂–Al₂O₃ sample. The band at 2100 cm^–1 is ascribed to CO adsorbed on Pt_iso sites, while the bands at lower frequencies are assigned to CO interacting with WC, UC, and HUC Pt sites. (b) Consumption rate of adsorbed CO based on the numbers of different Pt sites in the same Pt/CeO₂–Al₂O₃ catalyst, at 50 °C in the first 5 min after the introduction of O₂. Adapted with permission from ref (305). Copyright 2022 Elsevier.

Figure 19

In situ IR spectroscopy of adsorbed CO reveals a certain mobility of the Pt_iso sites as a function of the activation conditions. (a) In situ DRIFTS spectra of CO adsorbed at room temperature on a Pt/CeO₂–Al₂O₃ catalyst as-prepared (black), and on the same catalyst aged at 800 °C for 12 h in air (red) or reduced in 10% H₂ at 400 °C for 1 h (blue). Adapted with permission from ref (305). Copyright 2022 Elsevier. (b) In situ DRIFTS spectra of CO adsorbed at room temperature on a 0.05 wt % Pt/TiO₂ catalyst that was reduced (red), and oxidized (black), sequentially. The intensity of the latter spectrum is multiplied by 100 to allow comparison. Adapted with permission from ref (290). Copyright 2017 American Chemical Society.

Figure 20

In situ IR spectroscopy and INS allow the identification of several types of Pt–H species on Pt NPs. (a) In situ IR spectrum of a freshly reduced Pt/Al₂O₃ catalyst in the presence of H₂ at 70 °C either in the gas phase (10% H₂ in N₂, 20 mL/min) or in the liquid phase (cyclohexane as solvent). The latter spectrum has been collected in ATR-IR mode. The data reported in (a) are reproduced from ref (242) with permission from the Royal Society of Chemistry. (b) INS spectrum of H₂ chemisorbed on the same reduced Pt/Al₂O₃ catalyst. H₂ was dosed at room temperature at an equilibrium pressure of 420 mbar; the INS spectrum was measured at 20 K in the presence of H₂. Reproduced with permission from ref (319). Copyright 2019 American Chemical Society.

Figure 21

Operando IR spectroscopy reveals the occurrence of H₂-induced restructuring of Pt NPs on an industrial Pt/Al₂O₃ catalyst. (a) Evolution of the IR spectra as a function of time (from top to bottom) for a freshy reduced Pt/Al₂O₃ catalyst during dehydrogenation in N₂ flow (20 mL/min) at 120 °C. Adapted with permission from ref (319). Copyright 2019 American Chemical Society. (b) Evolution of the intensity of the bands I–IV as a function of time for the spectra shown in part a). (c) Number of linear (top, empty circles) and multicoordinated (bridge, full squares, and hollow, triangle) occupied sites for Pt₁₃H_n models on γ-Al₂O₃(100). The gray background identifies the stoichiometries accessible experimentally with the Pt/Al₂O₃ catalyst reported in parts (a and b). Adapted with permission from ref (280). Copyright 2011 John Wiley and Sons.