Vibrational spectroscopy of dispersed Re^VIIO_x sites supported on monoclinic zirconia

Abstract

In situ Raman and FTIR spectra complemented by in situ Raman/¹⁸O isotope labelling are exploited for deciphering the structural properties and configurations of the (ReO_x)_n phase dispersed on monoclinic ZrO₂ at temperatures of 120–400 °C under oxidative dehydration conditions and coverages in the range of 0.71–3.7 Re nm⁻². The dispersed (ReO_x)_n phase is heterogeneous, consisting of three distinct structural units: (a) Species-I with mono-oxo termination O=Re(–O–Zr)_m (Re=O mode at 993–1005 cm⁻¹); (b) Species-IIa with di-oxo termination (O=)₂Re(–O–Zr)_m−1 (symmetric stretching mode at 987–998 cm⁻¹); and (c) Species-IIb with di-oxo termination (O=)₂Re(–O–Zr)_u (symmetric stretching mode at 982–991 cm⁻¹); all terminal stretching modes undergo blue shifts with increasing coverage. With increasing temperature, a reversible temperature-dependent Species-IIa ↔ Species-I transformation is evidenced. At low coverages, below 1 Re nm⁻², isolated species prevail; at 400 °C the mono-oxo O=Re(–O–Zr)_m Species-I is the majority species, the di-oxo Species-IIa occurs in significant proportion and di-oxo Species-IIb is in the minority. At coverage ≥1.3 Re nm⁻², at 400 °C the di-oxo Species-IIa prevails clearly over mono-oxo Species-I. Below 80 °C and at a low coverage of 0.71 Re nm⁻², the occurrence of a fourth structural unit, Species-III taking on a tri-oxo configuration (symmetric stretching mode at 974 cm⁻¹) is evidenced. All temperature-dependent structural and configurational transformations are fully reversible and interpreted by mechanisms at the molecular level.

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1. Introduction

Several technologically important catalytic reaction processes benefit from supported rhenium oxide catalysts. Pertinent paradigms of such processes include olefin metathesis over ReO_x/Al₂O₃,^1–3 selective catalytic oxidation of light alcohols over ReO_x supported on TiO₂ and SnO₂,⁴ methanol conversion to dimexothymethane over ReO_x dispersed on TiO₂(anatase), ZrO₂ (monoclinic), SiO₂ and Al₂O₃,^5–8 as well as hydrodesulfurization and hydrodenitrogenation processes over ReO_x/Al₂O₃ catalysts.^9,10

Diverging and often contradictory reports have been published in the literature of supported rhenia catalysts concerning the speciation and the molecular structure of the dispersed dehydrated Re^VIIO_x phase. Combined Raman and IR spectra recorded after cooling at 45 °C under dehydrated conditions were interpreted to suggest the existence of two distinct tri-oxo (O=)₃–Re–O species on Al₂O₃, TiO₂ and ZrO₂ with relative amounts varied by surface coverage,¹¹ whereas the same group proposed the existence of one single tri-oxo (O=)₃–Re–O species based on Raman spectra obtained at room temperature after dehydration at 500 °C for 1 wt% Re₂O₇ on SiO₂, Al₂O₃, TiO₂ and ZrO₂.¹² Consistently with ref. 12, one single isolated species with tri-oxo termination configuration is proposed for ReO_x dispersed at surface densities of 0.3–2.2 Re nm⁻² on TiO₂ and Al₂O₃ based on in situ Raman and FTIR spectroscopy under oxidative dehydrating conditions (at 400 °C and 300 °C, respectively).¹³ Later, the same laboratory, based on an elegant study employing in situ Raman spectroscopy coupled with ¹⁸O/¹⁶O isotope exchange at 450 °C on one single sample (3 wt% Re₂O₇/ZrO₂) suggested the occurrence of a polymeric species with mono-oxo termination configuration.¹⁴ Additionally, a theoretical (DFT and X-ray spectroscopy simulations) study combined with experimental work (XAFS, STEM, TPR, XPS) performed at room temperature after cooling under controlled conditions on one single sample (0.7 wt% Re on Al₂O₃) suggested the existence of isolated tri-oxo ReO_x.¹⁵ Separately, the occurrence of two distinct di-oxo (O=)₂–Re(–O–)₂ species on Al₂O₃ has been convincingly shown following a sound experimental (in situ Raman coupled with ¹⁸O/¹⁶O exchange, in situ IR, UV-Vis, XANES) investigation combined with theoretical analysis (DFT).¹⁶Operando Raman studies have also provided evidence for heterogeneity of the dispersed ReO_x phase on TiO₂.^7,8 An in situ Raman spectroscopic study coupled with ¹⁸O/¹⁶O isotope exchange at 450 °C showed that a tri-oxo (O=)₃–Re–O– configuration prevails on SiO₂.¹⁷ A heterogeneity and an unprecedented reversible temperature-dependent evolution of prevailing structures and configurations (mono-oxo vs. di-oxo) has been established for the dispersed MoO_x, WO_x and ReO_x phases on TiO₂ by means of in situ molecular vibrational (Raman and FTIR) spectroscopy at temperatures of 175–430 °C.¹⁸ Unlimited cycling between the mono-oxo configuration (prevalent at high temperature) and the di-oxo configuration (predominant at intermediate temperature) is feasible by appropriate temperature control and adequate sample exposure.¹⁹ The extent of temperature dependence is connected to the support (TiO₂) surface hydroxylation as demonstrated by complementary Raman spectra in sealed cells under forced dehydrated conditions.¹⁹ The dispersed ReO_x phase on TiO₂(P25) in the submomolayer 0.16–2.5 Re nm⁻² range consists of a mono-oxo O=Re(–O–Ti)_n species and a di-oxo (O=)₂Re(–O–Ti)_m species, in relative amounts that depend on temperature and coverage, as shown by means of in situ molecular vibrational (Raman and FTIR) spectroscopy complemented by ¹⁸O/¹⁶O isotope exchange and Raman spectra in sealed cells under forced dehydrated conditions.²⁰ The occurrence of a mono-oxo and a di-oxo species has also been demonstrated by means of in situ molecular vibrational spectroscopy (Raman and FTIR) coupled with ¹⁸O/¹⁶O exchange for the ReO_x phase dispersed on CeO₂.²¹ An overview of selected reports on the molecular structure of dispersed ReO_x sites is contained in a recent review on vibrational spectroscopy of supported transition metal oxides.²² Molecular vibrational spectroscopy is particularly suited for studying the transition metal oxide phases dispersed on oxide carriers since it can be applied in absence of long range order. Moreover, the combined use of Raman and FTIR spectroscopy, complemented also by isotope exchange provides a comprehensive approach that can differentiate amongst distinct molecular configurations exhibited by (MO_x)_n sites (M = e.g. V, Mo, W, Nb, Re etc.).²² A recently demonstrated neoteric concept of Raman spectroscopy of supported transition metal oxides in sealed cells under forced dehydrated conditions has also contributed in validating results obtained by in situ vibrational (Raman, IR) spectroscopies.^19,20,23,24

In our view, the reasons for the controversy on the reported molecular structures of dispersed transition metal oxides in some pre-2015 reports are to be found in the deficiencies of the research strategies adopted in most of the pertinent investigations. Such deficiencies may include one or more of the following shortcomings: (i) studying only one type of vibrational spectra and/or not complementing the study with isotope exchange measurements; (ii) recording of vibrational spectra at room temperature after cooling; (iii) examining the structural properties of one single sample instead of examining samples in a wide range of loadings; (iv) recording of vibrational spectra at one single temperature etc. Hence, temperature and loading effects have often been overlooked. Indeed, temperature dependent speciations of distinct dispersed oxometallic species are established for MoO_x, WO_x, ReO_x and VO_x sites supported on TiO₂,^{18–20,23,24} whereas the prevalence of particular configurations depending on coverage is also demonstrated.^16,20,23 A holistic approach in the protocol followed for studying the molecular structure of dispersed metal oxide overlayers should include the use of both in situ Raman and in situ FTIR spectroscopies, ideally complemented by ¹⁸O/¹⁶O isotope exchange.

The present work is concerned with unraveling the molecular structure and termination configuration of the species constituting the (ReO_x)_n phase dispersed on monoclinic ZrO₂. The Re=O terminal stretching vibrational wavenumbers of dispersed ReO_x sites with mono-oxo, di-oxo and tri-oxo termination configuration lie close to each other.^16,20,25,26 Hence, locating the band wavenumbers with sufficient accuracy is not adequate for differentiating between mono-oxo, di-oxo and tri-oxo configurations. To this end, in situ Raman and in situ FTIR spectroscopies (the latter in the overtone region) are used in the temperature range of 400–80 °C for a wide range of surface coverage (0.71–3.7 Re nm⁻²) in order to shed light in the issue of heterogeneity (i.e. determine the number of species present and their relative presence depending on temperature and coverage). Moreover, by exploiting the vibrational selection rules of Raman and FTIR spectroscopy as well as the vibrational isotope effects, a differentiation between the various termination configurations is proposed.

2. Experimental section

2.1. Synthesis and textural characterization of ReO_x/ZrO₂ catalysts

The preparation of the ReO_x/ZrO₂ materials took place by incipient wet impregnation of monoclinic ZrO₂ (Alfa Aeasar), which was calcined prior to its use at 600 °C (heating rate, 2 °C min⁻¹) for 4 h. Pre-weighed amounts of precursor NH₄ReO₄ (Alfa Aeasar) corresponding to targeted coverages were dissolved in triply distilled water resulting to Re(VII) concentrations in the range of 1.6 × 10⁻³–8.2 × 10⁻³ M at pH = 7.5 that was adjusted by dropwise adding NH₃ or HNO₃ solutions. The solution speciation of oxo-Re(VII) species was examined by Raman spectroscopy (Fig. S1, ESI†) and it was confirmed that all precursor solutions contained Re exclusively in the form of ReO₄⁻ ions.

For each synthesis, a weighed amount of the pre-calcined support was added in the precursor solution that was equilibrated at 45 °C for 1 h by continuous monitoring and periodic adjustment of the pH (pH = 7.5), and subsequently the solvent was removed by rotary evaporation. The obtained materials were dried for 16 h at 120 °C and were afterwards calcined in a muffle furnace at 450 °C for 3 h under static air. The resulting Re loading (wt% Re) for each sample was determined by icp-analysis and was found -as expected-²⁰ lower than previewed from the weighed-in amounts of NH₄ReO₄ and ZrO₂ due to partial loss of the volatile rhenium oxide by vaporisation.

The specific surface area of the catalysts was determined by the BET method based on N₂ adsorption–desorption isotherms as described previously.²⁰ Hence, the actual Re surface density for each sample, n_s (expressed as Re nm⁻²), could be calculated as

where A_Re is the atomic mass of Re in amu.

The only crystalline phase contained in the prepared samples was, as determined by powder XRD diffraction, monoclinic ZrO₂. Table 1 compiles the principal characteristics for each catalyst sample, denoted as uReZrO₂, where u is the surface density in Re nm⁻².

Table 1 ReO_x/ZrO₂(m) catalysts’ characteristics (initial Re(VII) concentration of precursor solution, wt% Re loading, BET surface area and Re surface density). Precursor solution pH: 7.5. Calcination: 450 °C, 3 h^a

Catalysts	C Re(VII) (M)	Loading (wt% Re)^b	S BET (m^2 g^−1)	n s (Re nm^−2)
a Support ZrO2(m) pre-calcined at 600 °C for 4 h; SBET,ZrO2: 50 m^2 g^−1. b As per icp analysis.
0.71ReZrO2	1.6 × 10^−3	1.09	50	0.71
1.3ReZrO2	3.3 × 10^−3	2.08	52	1.3
1.5ReZrO2	4.1 × 10^−3	2.43	52	1.5
1.9ReZrO2	5.0 × 10^−3	3.08	53	1.9
2.4ReZrO2	5.8 × 10^−3	3.64	49	2.4
3.7ReZrO2	8.2 × 10^−3	5.33	46	3.7

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2.2. Raman and FTIR spectroscopy. Protocols of measurements

2.2.1. In situ steady state Raman spectra under dehydrated feed conditions. A homemade optical Raman cell was used for obtaining the in situ Raman spectra for the ReO_x/ZrO₂(m) materials at steady state under oxidative dehydrated conditions. A detailed description of the in situ Raman cell can be found in previous reports.^27,28 Each sample (130–140 mg), in the form of a pressed wafer made using a hydraulic press at a load of ∼25 kN, was mounted on the in situ Raman cell sample supporter. The cyan 491.5 nm line of a DPSS laser (Cobolt Calypso) was used as the excitation source. The laser power on the wafer was ∼10 mW. The illumination spot on the wafer was ∼2 mm², adjusted by slightly defocusing the incoming beam by a cylindrical lens, thereby reducing sample irradiance. The scattered radiation was collected at right angle on a horizontal scattering plane and analysed by a Spex 1403 0.85 m double monochromator. The signal was detected by a −20 °C – cooled RCA photomultiplier tube interfaced with a Labspec software.

To ascertain that the calcined uReZrO₂ samples do not undergo further gradual ReO_x loss due to ReO_x vaporization, the following procedure was followed for each sample. First, each sample was heated to 400 °C for 1 h in the in situ Raman cell under flowing (30 cm³ min⁻¹) 20% O₂/He gas mixture (He 99.999% and O₂ 99.999% form L′ Air Liquide). Subsequently, temperature was lowered to 250 °C under flowing 20% O₂/He gas and the in situ Raman spectrum was recorded after 1 h and 45 min on stream. Temperature was then raised to 400 °C and an in situ Raman spectrum was obtained under flowing 20% O₂/He gas as reference at 400 °C after 1 h and 45 min on stream. Finally, temperature was lowered again to 250 °C and after 1 h and 45 min exposure to flowing 20% O₂/He gas the in situ Raman spectrum at 250 °C was reproduced. Fig. S2 (ESI†) show the pertinent in situ Raman spectra obtained at 250 °C for three representative samples (0.71ReZrO₂, 1.3ReZrO₂ and 2.4ReZrO₂), before and after the intermediate exposure of the sample at 400 °C under flowing feed gas for ∼3 h and 15 min. It is evident that the spectra are fully reproduced and hence following the initial partial loss of ReO_x due to vaporization during the calcination stage, no further loss of ReO_x had taken place.

Recording of in situ Raman spectra under continuous dehydrated feed conditions (30 cm³ min⁻¹ 20% O₂/He) started at 400 °C after 1 h and 45 min on stream. Temperature was subsequently lowered to 250, 225, 200, 175, 145, 120, 100, 80, 60 and 35 °C using both a sequential and a random order of temperatures, and steady state in situ Raman spectra were collected after 1 h and 45 min sample treatment at each temperature to ensure steady state attainment. After recording the in situ Raman spectra at a certain temperature, the sample was first heated to 400 °C for 1 h and then cooled to the next temperature. After completing each sequence, the sample under study was heated to 400 °C and the full reinstatement of its structural state was checked and confirmed by reproducing the pertinent in situ Raman spectrum at 400 °C. The spectral resolution was set to 7 cm⁻¹ by appropriate adjustments of the entrance and exit monochromator vertical slits’ apertures. A high signal-to-noise ratio could be achieved by performing slow scans with a 1.2 s photon counting per point at 0.25 cm⁻¹ increments. A normalisation procedure of the obtained Raman spectra, previously described in detail,^20,29 was implemented for normalising the spectra and compensate the “path length” effect caused by the coloured samples in the wide 0.71–3.7 Re nm⁻² coverage range studied. Very briefly, first the background of each spectrum was zeroed by subtracting a rectangle, thereby zeroing the counts of the high wavenumber tail of the spectrum and subsequently the whole spectrum was divided by the area under the spectrum trace in the 270–1100 cm⁻¹ wavenumber range that features the main Raman bands of structural relevance.

2.2.2. In situ steady state FTIR spectra under dehydrated feed conditions. The in situ FTIR study was undertaken using an in situ DRIFT cell (Spectra Tech) on a FTIR Nicolet 6700 spectrometer equipped with a KBr beam splitter and an MCTB detector. The procedure followed was identical to the one used for the in situ Raman study. Each catalyst sample was first exposed for 1 h at 400 °C under dry flowing (30 cm³ min⁻¹) 20% O₂/He and the in situ FTIR spectrum at 400 °C was recorded. 64 scans were averaged for each spectrum at a resolution set to 4 cm⁻¹. Subsequently, sequential in situ steady state FTIR spectra were recorded at temperatures of 250, 175, 120 and 400 °C (following 1 h and 45 min exposure under flowing 20% O₂/He at each temperature). The reinstatement of the catalyst molecular structure at 400 °C was verified by reproducing the pertinent spectrum corresponding to each sample. The overtone Re=O stretching spectral range is studied due to strong absorption in the corresponding fundamental stretching region. Reference in situ FTIR spectra under flowing 20% O₂/He were recorded at each temperature also for the pure support ZrO₂ and were subtracted from the in situ FTIR spectra obtained for the samples studied. Hence, difference spectra pertaining to the dispersed (ReO_x)_n phase are reported below.

2.2.3. In situ Raman/¹⁸O isotope exchange study. The procedure for setting up the protocol for undertaking the in situ Raman–¹⁸O/¹⁶O exchange experiments at 400 °C was established by simulating the pertinent reduction/oxidation cycles using a 4.5% H₂/He mixture (H₂ 99.999% from L′ Air Liquide) as the reductant and a 2% ¹⁶O₂/He gas mixture as the oxidant, as described previously.^30,31 It was found that under the specified conditions, a 20 min duration was necessary to attain a sufficient reduction, whilst 10 min were required for an adequate reoxidation by a 2% ¹⁶O₂-containing gas. Accordingly, each ¹⁸O/¹⁶O isotope exchange redox cycle was made up of a 20 min reduction step under flowing (30 cm³ min⁻¹) 4.5% H₂/He and a 10 min oxidation step under flowing (10 cm³ min⁻¹) 2% ¹⁸O₂/He (Linde, certified mixture). Up to 25 redox ¹⁸O/¹⁶O isotope exchange cycles were undertaken for each studied catalyst sample and in situ Raman spectra were periodically recorded at 400 °C after a specified number of ¹⁸O/¹⁶O isotope exchange cycles.

3. Results and discussion

The ReO_x deposition on ZrO₂(m) takes place inside the precursor suspension that contain Re exclusively in the form of ReO₄⁻ moieties (see Fig. S1†) and proceeds via Zr–O–Re(–O)₃ and/or (Zr–O)₂–Re(–O)₃ anchoring (where in the latter case one of the Zr–O–Re anchors involves also a ZrO₂ lattice O). Drying and calcination follows the well-established mechanism of gradual titration of surface hydroxyls by means of successive esterification-like steps and concomitant condensation of water molecules that remain retained by means of H-bonding to the surface layer.^{18–20,23,24,32–38} Hence, after the initial anchoring, bipedal and gradually multipedal ReO_x units are formed at the expense of terminal Re–O sites. Therefore, as established for several supported MO_x transition metal oxides (M = V, Mo, W, Re) for surface coverages below the respective monolayer limit, formation of distinct species takes place in the course of the calcination resulting in heterogeneity of the dispersed MO_x phase.^{16,18–21,23,24,38} Additionally, when coverage exceeds a certain limit, which is characteristic for each system, M–O–M associations take place resulting in (MO_x)_n domain growth.²²

3.1. In situ molecular vibrational (Raman, FTIR) spectra of ReO_x/ZrO₂(m) catalysts. Coverage and temperature effects

3.1.1. Effect of Re coverage. Fig. 1 shows the effect of Re coverage (in the range of 0.71–3.7 Re nm⁻²) on the in situ Raman spectra obtained for all ReO_x/ZrO₂(m) catalysts studied at constant temperature of 400 °C (Fig. 1A), 250 °C (Fig. 1B) and 175 °C (Fig. 1C). Normalised Raman spectra^20,29 are shown in the full spectral region including the contribution of bands due to monoclinic ZrO₂, which are confined up to ca. 750 cm⁻¹.³⁹Fig. 2(A–C) shows the corresponding evolution of the in situ normalized Raman spectra with increasing Re coverage, obtained with higher S/N ratio and focused in the wavenumber range pertaining to stretching Re=O and Re–O– modes. First observations of spectral effects in Fig. 1 and 2 include: (i) the convoluted band envelope due to terminal Re=O stretching is asymmetric with evidence of several overlapping bands in the Re=O stretching region that are known to occur at wavenumbers typically higher than 940 cm⁻¹;^16,25,26 (ii) the peak maximum of the Re=O band envelope undergoes a pronounced blue shift with increasing coverage, e.g. from 991 cm⁻¹ for 0.71 Re nm⁻² to 1002 cm⁻¹ for 3.7 Re nm⁻² at 400 °C; (iii) with increasing coverage, a broad band at ∼885 cm⁻¹ progressively loses intensity relative to the ca. 990–1000 cm⁻¹ Re=O stretching band envelope; and (iv) a broad, however obscured, band at ∼935 cm⁻¹ emerges and progressively gains intensity with increasing coverage, becoming discernible for n_S ≥ 1.3 Re nm⁻². The wavenumber location and broad nature of the latter two bands (i.e. at ∼885 and ∼935 cm⁻¹) are suggestive of Re–O– provenance (i.e. Re–O–Re, O–Re–O and Re–O–Zr).

Fig. 1 Effect of coverage on the in situ Raman spectra obtained for ReO_x/ZrO₂ catalysts under flowing 20% O₂/He at temperatures of: (A) 400 °C; (B) 250 °C; and (C) 175 °C at coverages in the 0.71–3.7 Re nm⁻² range as indicated by each spectrum. The full spectral range of 200–1100 cm⁻¹ is shown. Recording parameters: laser wavelength, λ₀ = 491.5 nm; laser power, w = 10 mW; time constant, τ = 0.3 s; spectral slit width, ssw = 7 cm⁻¹.

Fig. 2 Effect of coverage on the in situ Raman spectra obtained for ReO_x/ZrO₂ catalysts under flowing 20% O₂/He at temperatures of: (A) 400 °C; (B) 250 °C; and (C) 175 °C at coverages in the 0.71–3.7 Re nm⁻² range as indicated by each spectrum. The Re–O stretching region of 800–1050 cm⁻¹ is shown. Recording parameters: laser wavelength, λ₀ = 491.5 nm; laser power, w = 10 mW; time constant, τ = 1.2 s; spectral slit width, ssw = 7 cm⁻¹.

With increasing coverage, the ReO_x domains are expected to grow by means of Re–O–Re associations at the expense of Re–O–Zr anchors. Hence, with increasing coverage the Re–O–Re/Re=O ratio is expected to increase and the Re–O–Zr/Re=O ratio is expected to decrease. In view of the above, the ∼885 cm⁻¹ band that loses intensity relative to the Re=O envelope with increasing coverage is assigned to Re–O–Zr modes and the ∼935 cm⁻¹ band that gains intensity relative to the Re=O envelope with increasing coverage is assigned to Re–O–Re modes. Additional evidence strengthening the assignment of the ∼885 cm⁻¹ band to Re–O–Zr modes will be presented and discussed below, in the context of the temperature dependence of in situ Raman spectra.

A first discussion of the blue-shifting tendency observed with increasing coverage for the band envelope ascribed to terminal Re=O stretching should include a consideration of factors affecting the wavenumber of the terminal Re=O stretching. Such factors may include inter alia: (a) the termination configuration (i.e. mono-oxo, di-oxo etc.); (b) the Re coordination number; (c) vibrational coupling (particularly in case of occurrence of Re–O–Re associations); and (d) variations in basicity of support dative O atoms. In the initial steps of ReO_x deposition, the most basic surface hydroxyls are titrated first, hence with increasing coverage hydroxyls with lower basicity come gradually into play and by that means – in a cascade effect – the terminal Re=O stretching becomes gradually stronger thereby partly justifying the corresponding blue shift with increasing coverage (e.g., from 991 to 1002 cm⁻¹ at 400 °C, Fig. 2(A)).

Fig. 3 shows the evolution of the in situ FTIR spectra obtained at constant temperature of 400 °C (Fig. 3A), 250 °C (Fig. 3B) and 175 °C (Fig. 3C) for ReO_x/ZrO₂ catalysts with coverages of 0.71, 1.3 and 2.4 Re nm⁻² under dehydrating feed conditions of flowing (30 cm³ min⁻¹) 20% O₂/He gas. The spectra are obtained in the overtone region, hence the wavenumber span of the terminal stretching region is approximately doubled. Likewise, the mutual distance between the infrared bands corresponding to the counterpart Raman components comprising the terminal stretching band envelope observed in the Raman spectra (Fig. 2) is also approximately doubled resulting in clearly distinct bands in Fig. 3, thereby ascertaining the multiple character ascribed to the observed “broad” terminal stretching Raman band envelopes (Fig. 2) and excluding the possibility for certain bands as being simply of broad nature.

Fig. 3 Effect of coverage on the in situ FTIR spectra obtained for ReO_x/ZrO₂ catalysts under flowing 20% O₂/He at temperatures of: (A) 400 °C; (B) 250 °C; and (C) 175 °C at coverages of 0.71, 1.3 and 2.4 Re nm⁻² as indicated by each spectrum. The corresponding spectra obtained for ZrO₂ have been subtracted. The dotted lines mark the band wavenumbers for the symmetric stretching modes of Species-I, Species-IIa and Species-IIb deduced by exploiting the respective temperature dependent features portrayed in Fig. 4(B), 5(B) and 6(B). Resolution, 4 cm⁻¹.

3.1.2. Effect of temperature. Fig. 4–6 show the effect of temperature on the in situ Raman and in situ FTIR spectra obtained for ReO_x/ZrO₂ samples under dehydrated feed conditions (flowing 20% O₂/He gas mixture) in the temperature range of 400–120 °C at constant surface coverages of 0.71 Re nm⁻² (Fig. 4), 1.3 Re nm⁻² (Fig. 5) and 2.4 Re nm⁻² (Fig. 6). The spectral regions of Re=O and Re–O– stretching modes are portrayed. Panels 4(A), 5(A) and 6(A) pertain to in situ Raman spectra, whilst panels 4(B), 5(B) and 6(B) pertain to the counterpart in situ FTIR spectra. All spectra are recorded at a sequential order of decreasing temperatures. Notably, as mentioned also in the Experimental section, reheating the samples to 400 °C under flowing 20%O₂/He results in full reinstatement of the corresponding spectrum in each case. Additionally, Fig. S3(A–F)† shows the temperature dependence of the in situ Raman spectra obtained for all six samples studied in the full temperature range of 400–35 °C. We shall, however, confine the discussion mainly to Fig. 4–6 that are adequate for inferring the number of species present and their respective termination configurations. The extent of the temperature-dependent changes observed vary with surface coverage, as discussed below.

Fig. 4 ReO_x/ZrO₂ with surface density of 0.71 Re nm⁻²: (A) sequential (430 → 250 → 175 → 145 → 120 °C) in situ Raman spectra obtained under flowing 20%O₂/He at temperatures as indicated by each spectrum. Spectral recording parameters: see Fig. 2 caption. (B) Sequential (430 → 250 → 175 → 120 °C) in situ FTIR spectra obtained under flowing 20% O₂/He at temperatures as indicated by each spectrum. The corresponding spectra obtained for ZrO₂ have been subtracted. Resolution, 4 cm⁻¹.

Fig. 5 ReO_x/ZrO₂ with surface density of 1.3 Re nm⁻²: (A) sequential (430 → 250 → 175 → 145 → 120 °C) in situ Raman spectra obtained under flowing 20%O₂/He at temperatures as indicated by each spectrum. Spectral recording parameters: see Fig. 2 caption. (B) Sequential (430 → 250 → 175 → 120 °C) in situ FTIR spectra obtained under flowing 20% O₂/He at temperatures as indicated by each spectrum. The corresponding spectra obtained for ZrO₂ have been subtracted. Resolution, 4 cm⁻¹.

Fig. 6 ReO_x/ZrO₂ with surface density of 2.4 Re nm⁻²: (A) Sequential (430 → 250 → 175 → 145 → 120 °C) in situ Raman spectra obtained under flowing 20%O₂/He at temperatures as indicated by each spectrum. Spectral recording parameters: see Fig. 2 caption. (B) Sequential (430 → 250 → 175 → 120 °C) in situ FTIR spectra obtained under flowing 20% O₂/He at temperatures as indicated by each spectrum. The corresponding spectra obtained for ZrO₂ have been subtracted. Resolution, 4 cm⁻¹.

Low coverage ReO_x/ZrO₂ with 0.71 Re nm⁻². The rhenium–oxygen terminal stretching modes are known to occur in the 940–1010 cm⁻¹ region depending on the termination configuration and on the identity of the surrounding ligands.^16,20,25,26Fig. 4(A) shows a pertinent multi-component band envelope exhibiting a strong feature at 995–975 cm⁻¹ (presumably due to symmetric terminal stretching modes)^20,25,26 and a 975–940 cm⁻¹ tail, probably due to antisymmetric modes (of e.g. di-oxo sites) that are known to be weaker and to occur at 10–40 cm⁻¹ lower than their respective symmetric counterpart modes.^20,25,26,28 The temperature-dependent variations of the relative band intensities in Fig. 4(A) imply a corresponding temperature-dependent prevalence of distinct dispersed ReO_x species, thereby corroborating the proposal for the heterogeneity of the dispersed rhenia phase. The counterpart in situ FTIR spectra displayed in Fig. 4(B) show a better separation amongst the respective component bands due to the approximate doubling of their mutual distances in the overtone region. Hence, the main FTIR band envelope due to Re–O terminal symmetric stretching has three clearly discerned components at 1971, 1963 and 1955 cm⁻¹ plus two weaker broad features at ∼1920 and ∼1870 cm⁻¹. The temperature dependent features of the relative intensities observed for the 1971, 1963 and 1955 cm⁻¹ bands in Fig. 4(B) show that their prevalence is due to three distinct ReO_x species. Hence, the asymmetric 975–995 cm⁻¹ Raman band envelope in Fig. 4(A) is reasonably thought to consist of three component bands.

In order to corroborate the evidence supporting the above proposal we performed a peak analysis in the terminal stretching region of the in situ Raman spectra shown in Fig. 4(A) and the results are shown in Fig. 7(A). The criteria for the peak analysis of the Re(=O)₂ terminal stretching region invoke:

(1) The occurrence of five bands due to three Species, as evidenced from the combined observations of the in situ Raman and FTIR spectra as follows: (i) one band at the highest wavenumber due to the terminal Re=O stretching mode of Species-I; (ii) two bands due to the ν_s/ν_as pair due to Species-IIa with ν_s − ν_as ≈ 10–40 cm⁻¹; and (iii) two bands due to the ν_s/ν_as pair due to Species-IIb with ν_s − ν_as ≈ 10–40 cm⁻¹

(2) Fixed band widths and positions (with allowance for the measurement precision, i.e. ±1 cm⁻¹) for each studied sample with surface densities of 0.71, 1.3 and 2.4 Re nm⁻²

(3) Fixed relative I_νs/I_νas intensities for each dioxo species (IIa, IIb), and

(4) Allowance for broader ν_as bands compared to their respective ν_s counterparts.

Fig. 7 Sequential (430 → 250 → 175 → 145 → 120 °C) in situ Raman spectra obtained under flowing 20% O₂/He at temperatures as indicated by each spectrum for ReO_x/ZrO₂ with surface densities of: (A) 0.71 Re nm⁻²; (B) 1.3 Re nm⁻²; and (C) 2.4 Re nm⁻². Peak analysis is shown. The corresponding spectra obtained for ZrO₂ have been subtracted. Spectral recording parameters: see Fig. 2 caption.

The results of the peak analysis show that three peaks are found in the 975–1000 cm⁻¹ symmetric terminal stretching region, at 993, 987 and 982 cm⁻¹, whilst the low wavenumber tail is fitted with two further broader bands at 971 and 955 cm⁻¹, presumably due to antisymmetric terminal stretching modes. Pairing of bands (i.e. assignment of two bands to the same species) can be justified when their mutual relative intensities are maintained in the full temperature range. In view of this principle, the 993 cm⁻¹ band is solely assigned to Species-I, the 987/955 cm⁻¹ pair is assigned to Species-IIa and the 982/971 cm⁻¹ pair to Species-IIb.

The temperature-dependent behavior of the 993 (I), 987 (IIa) and 982 (IIb) cm⁻¹ bands (Fig. 7) shows that with increasing temperature the relative intensity of the 993 cm⁻¹ band due to Species-I is increased at the expense of the 987 cm⁻¹ band (Species-IIa), whilst the 982 cm⁻¹ component (Species-IIb) does not undergo discerned relative intensity changes in the 120–400 °C range. As shown previously for ReO_x/TiO₂ ²⁰ and ReO_x/CeO₂ ²¹ catalysts a O=ReO_n species with mono-oxo termination configuration is favored at low coverage and high temperature, hence Species-I with a single Re=O stretching mode at the highest wavenumber of 993 cm⁻¹ among all bands has a mono-oxo O=ReO_n configuration. Species IIa and IIb, both possess terminal stretching band pairs at 987/955 cm⁻¹ and 982/971 cm⁻¹ conforming to the ν_s/ν_as (i.e. symmetric/antisymmetric) expected pair for sites that take on a di-oxo termination configuration that is expected to give rise to a symmetric stretching mode, ν_s, with higher intensity compared to an antisymmetric stretching mode, ν_as, at wavenumbers conforming to the ν_s − ν_as = ∼10–40 cm⁻¹ rule.²⁵ The above observations in the temperature range of 120–400 °C are interpreted to indicate the following temperature-dependent reversible transformation

A molecular-level mechanism for the above reaction scheme (A) portraying plausible molecular models for Species–I and Species–IIa is shown in Fig. 8. Water molecules retained at the surface layer of oxide carriers by means of H-bonding have previously also been shown to mediate temperature-dependent transformations among distinct MO_x sites in ReO_x/TiO₂, WO_x/TiO₂, MoO_x/TiO₂ and VO_x/TiO₂ catalysts.^{18–20,23,24} Such water molecules retained at the support oxide surface layers by H-bonding are formed during the titration of surface hydroxyls that takes place by the precursor oxometallic species during the deposition steps.^32–37

Fig. 8 Molecular-level mechanism accounting for the reversible temperature-dependent Species-I ↔ Species-IIa transformation mediated by water molecules retained at the surface layer and surface hydroxyls. For simplicity, mononuclear representations are shown. The number of anchoring bonds and the resulting coordination number, CN_Re = 5 are reasonable in view of the oxidation state VII for Re. Gold spheres, Re; red spheres, O; grey spheres, Zr. H atoms (white spheres) are included to account for hydroxylation (see text).

Notably, the reaction scheme shown in Fig. 8 implies that with increasing temperature a gradual increase of Re–O–Zr anchors will take place thereby resulting in an increase of the Re–O–Zr/Re=O ratio. Indeed, as seen in Fig. 4A, a relative strengthening of the ∼885 cm⁻¹ band is observed with increasing temperature, thereby corroborating the initial assignment of the ∼885 cm⁻¹ band (see section 3.1.1 pertaining to the effect of coverage) to Re–O–Zr anchors.

The temperature-dependent features of the in situ FTIR spectra obtained for the low coverage 0.71 Re nm⁻² sample are in full conformity with the above scenario of temperature-dependent interplay involving the mono-oxo Species-I and di-oxo Species-IIa as well as the relative stability of di-oxo Species-IIb in the temperature range of 120–400 °C. On grounds of anharmonicity, which allows the observation of first overtones, the 993, 987 and 982 cm⁻¹ stretching fundamentals due to Species-I (mono-oxo), Species-IIa (dioxo) and Species-IIb (dioxo) are expected to give overtones at wavenumbers slightly below the respective doubled fundamentals,^25,40i.e. the ∼1971, ∼1963 and ∼1955 cm⁻¹ observed wavenumbers conform to the above rule. With increasing temperature, the 1963 cm⁻¹ band due to Species-IIa diminishes and the 1971 cm⁻¹ Species-I band prevails, whilst the 1955 cm⁻¹ Species-IIb band remains unaffected and is better discerned at 400 °C where the 1963 cm⁻¹ Species-IIa band has diminished. The ∼1920 and ∼1870 cm⁻¹ broad bands are assigned to the first overtones of the antisymmetric 971 and 955 cm⁻¹, ν_as, modes of Species-IIb and Species-IIa, respectively.

Intermediate and high coverage ReO_x/ZrO₂ with 1.3 and 2.4 Re nm⁻². Decoding of the temperature-dependent features of the in situ Raman and in situ FTIR spectra obtained for the ReO_x/ZrO₂ catalysts with 1.3 and 2.4 Re nm⁻² shown in Fig. 5(A, B) and 6(A, B) becomes straight-forward in view of the findings discussed for the temperature dependent spectral features for the low coverage 0.71ReZrO₂ sample (vide ante) and the general coverage effect trends presented in section 3.1.1.

The splitting pattern of the FTIR overtones in the symmetric stretching overtone 1960–2000 cm⁻¹ region of the spectra shown in Fig. 5(B) and 6(B) reveals in each case the occurrence of three distinct bands due to Species-I, Species-IIa and Species-IIb. As discussed in section 3.1.1 (Fig. 3) all stretching band wavenumbers exhibit a gradual blue shift with increasing coverage in the range 0.71–3.7 Re nm⁻² studied. Moreover, it is evident that whereas for a low coverage of 0.71 Re nm⁻² the mono-oxo Species-I prevails at e.g. 400 °C, this is no longer the case for higher coverages of 1.3 and 2.4 Re nm⁻² where, as best seen in the in situ FTIR spectra shown in Fig. 5B and 6B, Species-I is a minority species at 400 °C.

The interpretation and the pertinent discussions are facilitated by performing a peak analysis in the terminal stretching region of the in situ Raman spectra shown in Fig. 5A and 6A, obtained for 1.3ReZrO₂ and 2.4ReZrO₂, and the results of the peak analysis are shown in Fig. 7B and C. The band wavenumbers assigned to the different species for 1.3ReZrO₂ are at 1003 cm⁻¹ (Species-I, Re=O, mono-oxo), 995/962 cm⁻¹ (ν_s/ν_as, Species-IIa, di-oxo) and 987/976 cm⁻¹ (ν_s/ν_as, Species-IIb, di-oxo). The respective wavenumbers for 2.4ReZrO₂ are at 1005 cm⁻¹ (Species-I, Re=O, mono-oxo), 998/962 cm⁻¹ (ν_s/ν_as, Species-IIa, di-oxo) and 991/978 cm⁻¹ (ν_s/ν_as, Species-IIb, di-oxo). Table 2 compiles the band wavenumbers and assignments for the various species.

Table 2 Observed band wavenumbers for Raman fundamentals and IR first overtones for Re–O stretching modes for Species-I, Species-IIa and Species-IIb and tentative assignments

Re nm^−2	Species-I, mono-oxo		Species-IIa, di-oxo				Species-IIb, di-oxo				O–Re–O	Re–O–Zr
	O=Re(–O–Zr)n		(O=)2Re(–O–Zr)n				(O=)2Re(–O–Zr)n
	ν 1←0,R	ν 2←0,IR	ν s,1←0,R	ν as,1←0,R	ν s,2←0,IR	ν as,2←0,IR	ν s,1←0,R	ν as,1←0,R	ν s,2←0,IR	ν as,2←0,IR
0.71	993	1971	987	955	1963	∼1870	982	971	1955	∼1920		885
1.3	1003	1994	995	962	1977	∼1910	987	976	1962	∼1940	935–940	890
2.4	1005	1998	998	962	1981	∼1915	991	978	1964	∼1940	935–940	890

---

As seen in Fig. 7B and C, with increasing temperature band (I) due to Species-I (mono-oxo) gains intensity at the expense of the Species-IIa ν_s mode for both 1.3ReZrO₂ and 2.4ReZrO₂. Contrary, however, to the case of the low coverage 0.71ReZrO₂ (Fig. 8A), Species-IIa remains clearly predominant up to 400 °C. Reasonably, the reason lowering the progress of reaction scheme (A) to the left is the increased coverage with its consequent shortage of surface hydroxyls adjacent to Species-IIa caused by their extensive titration during the deposition process.

3.2. In situ Raman spectra and ¹⁸O/¹⁶O isotopic substitution. Vibrational isotope effects

Complementing in situ Raman spectroscopy with ¹⁸O/¹⁶O isotopic labelling sheds additional light into the issue of termination configuration in supported transition metal oxide catalysts.^14,28,41 The vibrational isotope effects, i.e. the isotopic splitting pattern of terminal stretching bands and the isotopic shift allow for valid discussions on band assignments to (M=¹⁸O)_n modes (n = 1, 2, 3) and on the issue of discrimination between e.g. mono-oxo, di-oxo and tri-oxo configurations.^14,41

The theory of vibrational isotope effects based on the harmonic oscillator approximation, i.e. based on a strictly quadratic potential function, results in justified theoretical predictions for the fundamental stretching ν(Re=¹⁸O)_n (n = 1, 2) modes’ wavenumbers of ¹⁸O/¹⁶O-substituted terminal Re=¹⁸O and/or ¹⁸O=Re=¹⁸O sites.^40,42 As a paradigm, the diatomic harmonic approximation for a M=O (M: transition metal) terminal site results in the following formula for the isotopic ratio

Isotopic ratio:

Indicative values for the isotopic ratio for several M=O terminations have been reported.¹⁴ For the Re=O diatomic harmonic oscillator, the isotopic ratio equals 1.0555. The theory and pertinent equations for triatomic M(=O)₂ “molecules” become more involved and elaborate,⁴² however leading to an approximately equal isotopic ratio for the symmetric stretching terminal mode. In accordance, DFT calculations for the effect of ¹⁸O/¹⁶O substitution on the symmetric stretching wavenumber for the di-oxo (O=)₂Mo(–O–Si)₂ structure result in an isotopic ratio of 1.0507,⁴³ effectively identical to the 1.0513 isotopic ratio calculated for a diatomic mono-oxo Mo=O configuration based on eq. (2). Hence, whereas calculations for theoretically predicted wavenumbers for ν(Re=¹⁸O)_n modes can be used for verifying the validity of experimentally observed isotopic shifts, such calculations are not adequate for discriminating between e.g. mono-oxo and di-oxo termination configurations. Additionally, experimental results pertaining to well-documented di-oxo Re(=O)₂ terminations on TiO₂ and CeO₂ have shown that the use of the isotopic ratio to calculate the wavenumber of the ¹⁸O/¹⁶O substituted site (e.g. Re=¹⁸O or Re(=¹⁸O)₂) cannot be used for differentiating between mono-oxo and di-oxo termination configurations.^20,21

Fig. 9(A–C) shows the evolution of the in situ Raman spectra obtained at 400 °C under flowing (10 cm³ min⁻¹) 2% ¹⁸O₂/He upon successive reduction/oxidation cycles using flowing 5% H₂/He and 2% ¹⁸O₂/He (see Experimental section) for samples with coverage of 0.71 Re nm⁻² (Fig. 9A), 1.3 Re nm⁻² (Fig. 9B) and 2.4 Re nm⁻² (Fig. 9C). The number of redox ¹⁸O/¹⁶O substitution cycles is indicated by each spectrum. The gradual progress of the ¹⁸O/¹⁶O substitution and the consequent evolution of the in situ Raman spectra corroborate the aforementioned interpretations and partial conclusions for the occurrence of the three main structural units, namely mono-oxo Species-I, di-oxo Species-IIa and di-oxo Species IIb. Moreover, the observed effects are adequate for further discussions on the reducibility and the relative ease of ¹⁸O/¹⁶O substitution for each individual species.

Fig. 9 Sequential in situ Raman spectra obtained at 400 °C under flowing 2% ¹⁸O₂/He after subsequent H₂/¹⁸O₂ reduction/oxidation cycles as indicated by each spectrum. (A) ReO_x/ZrO₂ with surface density of 0.71 Re nm⁻²; (B) ReO_x/ZrO₂ with surface density of 1.3 V nm⁻²; and (C) ReO_x/ZrO₂ with surface density of 2.4 Re nm⁻²; spectral recording parameters: see Fig. 2 caption.

At 400 °C the ReO_x phase dispersed on the low coverage 0.71ReZrO₂ sample consists of Species-I (mono-oxo, prevalent species), Species-IIa (di-oxo, in significant amount) and Species-IIb (di-oxo, minority species). The following observations are made in Fig. 9A:

(i) After 21 ¹⁸O/¹⁶O substitution redox cycles the band envelope of the convoluted (Re=¹⁶O)_n modes at ∼991 cm⁻¹ exhibits a discerned shoulder at ∼982 cm⁻¹. Such a band is assigned to partially ¹⁸O/¹⁶O substituted di-oxo sites (¹⁸O=Re=¹⁶O).^16,20,21 Theoretical calculations for the wavenumber of a partially substituted di-oxo ¹⁸O=Mo=¹⁶O site justify further such an assignment.⁴³

(ii) During the initial steps of the ¹⁸O/¹⁶O substitution, i.e. after 1, 5 and 8 cycles, the band emerging due to the isotopic shift (eventually at ∼940 cm⁻¹ after 21 cycles) exhibits a low wavenumber shoulder at ∼934 cm⁻¹ indicating a relative ease for the ¹⁸O/¹⁶O substitution of the di-oxo terminal sites of the minority Species-IIb.

(iii) A facile Re–¹⁶O–Zr → Re–¹⁸O–Zr substitution is evidenced for the anchoring site, justified by the isotopic shift undergone by the ∼885 cm⁻¹ Re–O–Zr band already from the 1^{st 18}O/¹⁶O cycle.

For both 1.3ReZrO₂ and 2.4ReZrO₂ samples with coverages of 1.3 and 2.4 Re nm⁻², the di-oxo Species-IIa prevails at 400 °C, whilst the mono-oxo Species-I and the di-oxo Species-IIb also exist in minority. The pertinent isotopic effects observed are similar for the two samples and differ from the ones observed for 0.71ReZrO₂ only due to the different speciation of the dispersed ReO_x phase. More specifically:

(i) A facile Re–¹⁶O–Zr → Re–¹⁸O–Zr substitution is evidenced already after the 1^{st 18}O/¹⁶O cycle (and largely after the fifth cycle), justified by the isotopic shift undergone by the ∼888 cm⁻¹ Re–O–Zr mode.

(ii) The bands emerging due to (Re=¹⁸O)_n at 942 cm⁻¹ for 1.3ReZrO₂ (Fig. 9B) and 946 cm⁻¹ for 2.4ReZrO₂ (Fig. 9C) exhibit in each case a shoulder on their low wavenumber wing in the initial stages of the isotopic exchange clearly seen after the 1^st cycle at 936 cm⁻¹ for 1.3ReZrO₂ (Fig. 9B) and 940 cm⁻¹ for 2.4ReZrO₂ (Fig. 9C), thereby indicating a facile ¹⁶O=Re=¹⁶O → ¹⁸O=Re=¹⁸O substitution for the minority di-oxo Species–IIb. With further progress in the ¹⁸O/¹⁶O substitution the main bands’ masses due to the prevailing Species-IIa at 942 and 946 cm⁻¹ obscure the shoulders.

(iii) After 25 ¹⁸O/¹⁶O substitution cycles the band envelope due to the non-substituted (Re=¹⁶O)_n modes appears red-shifted (995 → 992 cm⁻¹ for 1.3 ReZrO₂, Fig. 9B and 999 → 995 cm⁻¹ for 2.4 ReZrO₂, Fig. 9C) due to the earlier described^28,30 next-nearest-neighbor vibrational isotope effect caused by the facile Re–¹⁶O–Zr → Re–¹⁸O–Zr substitution on the anchoring sites. Moreover, the convolutions of the 992 and 995 cm⁻¹ bands due to the non-substituted (Re=¹⁶O)_n modes after 25 cycles possess each a low wavenumber shoulder (at 985 and 988 cm⁻¹, respectively for 1.3ReZrO₂ and 2.4ReZrO₂), assigned to partially substituted ¹⁸O=Re=¹⁶O sites.^16,20,21,43

(iv) The Re=¹⁶O modes due to the mono-oxo Species-I at 1003 cm⁻¹ (1.3ReZrO₂, Fig. 9B) and at 1005 cm⁻¹ (2.4ReZrO₂, Fig. 9C) are entirely diminished after 8 and 9 ¹⁸O/¹⁶O cycles, confirming a facile Re=¹⁶O → Re=¹⁸O substitution for the mono-oxo Species-I.

3.3. Synopsis of structural interpretations

In the temperature range of 120–400 °C, the (ReO_x)_n phase dispersed on ZrO₂ is built of three main structural units, namely mono-oxo Species-I, di-oxo Species-IIa and di-oxo Species-IIb. At low coverage of n_s = 0.71 Re nm⁻² most ReO_x sites are isolated on ZrO₂. At higher coverages of n_s ≥ 1.3 Re nm⁻², the occurrence of ReO_x in vicinal sites may favor the formation of associated species by means of Re–O–Re linkages. Hence, with increasing loading, a lowering of the Raman band intensity due to Re–O–Zr at ∼885–890 cm⁻¹ relative to Re(=O)_n (n = 1, 2 for mono-oxo and di-oxo species, respectively) at 991–1003 cm⁻¹ is observed (Fig. 2A–C) and a concomitant increase of a partly obscured broad feature at ∼935–940 cm⁻¹ (Fig. 2A–C) that has previously also been assigned as due to bridging modes (Re–O–Re or O–Re–O).⁴⁴

A reversible temperature-dependent interconversion is evidenced between mono-oxo Species–I and di-oxo Species–IIa mediated by water molecules retained at the surface layer according to reaction scheme (A), portrayed also in Fig. 8. The Species-IIa → Species-I transformation taking place with increasing temperature requires the occurrence of two OH sites adjacent to a Species-IIa. Such a mechanism is limited at coverages of n_s ≥ 1.3 Re nm⁻² as seen in Fig. 7B and C due to larger extent of preceded titration of surface hydroxyls, thereby limiting the availability of surface hydroxyls and lowering of the possibility for occurrence of two surface hydroxyls adjacent to each Species-IIa.

As discussed, the Species–IIb di-oxo structural unit does not undergo transformations in the temperature range of 120–400 °C and its presence remains stable, as seen in Fig. 7(A–C). However, as seen e.g. in Fig. S3(A)† pertaining to the low coverage 0.71ReZrO₂ sample, there is evidence for a temperature-dependent reversible Species-IIb ↔ Species-III interconversion. Species-III is represented by a main Re(=O)_n stretching band at ∼974 cm⁻¹ (see spectrum (a) in Fig. S3(A)† pertaining to 35 °C), of which the appearance is observed with decreasing temperature together with a concomitant diminishing of the ∼885 cm⁻¹ Re–O–Zr band. Hence, the ∼974 cm⁻¹ band is assigned to an isolated tri-oxo site possessing less Re–O–Zr anchors, formed according to the scheme:

Fig. 10 shows a molecular-level mechanism accounting for the above reaction scheme (B). The occurrence of tri-oxo Species-III units is not favored at coverages of n_s ≥ 1.3 Re nm⁻² where associated species possessing Re–O–Re linkages prevail.

Fig. 10 Molecular-level mechanism accounting for the reversible temperature-dependent Species-IIb ↔ Species-III transformation mediated by water molecules retained at the surface layer and surface hydroxyls. For simplicity, mononuclear representations are shown. The number of anchoring bonds and the resulting coordination number, CN_Re = 4 are reasonable in view of the oxidation state VII for Re and the tri-oxo termination configuration of Species-III. Gold spheres, Re; red spheres, O; grey spheres, Zr. H atoms (white spheres) are included to account for hydroxylation (see text).

Finally, apart from the temperature and coverage effects, it is noteworthy to also address in synopsis the most recently reported and well-documented effects of metal oxide carrier type on the speciation of the ReO_x dispersed phase. Significantly, the heterogeneity of the dispersed phase is hitherto established also for ReO_x deposited on Al₂O₃,¹⁶ TiO₂ ²⁰ and CeO₂.²¹ In particular, the existence of two di-oxo species has been proposed for ReO_x/Al₂O₃ catalysts,¹⁶ whilst the occurrence of a mono-oxo and a di-oxo species has been evidenced for ReO_x/TiO₂ ²⁰ and ReO_x/CeO₂ ²¹ catalysts. To the contrary, one single ReO_x species taking on a tri-oxo configuration was found for ReO_x/SiO₂.¹⁷

4. Conclusions

In situ molecular spectroscopy is used for studying the vibrational properties and unravelling the structural and configurational characteristics of the (ReO_x)_n phase dispersed on monoclinic ZrO₂. In situ Raman and in situ FTIR spectra (the former also coupled with ¹⁸O/¹⁶O exchange) are exploited under dehydrated feed conditions, covering a surface density span of 0.71–3.7 Re nm⁻².

(a) The (ReO_x)_n phase dispersed on ZrO₂ is heterogeneous, consisting of distinct structural units. In the temperature range of 120–400 °C, three main structural units are identified: (i) Species–I with mono-oxo termination configuration, O=(Re–O–Zr)_m; (ii) Species–IIa with di-oxo termination configuration, (O=)₂(Re–O–Zr)_m−1; and (iii) Species–IIb with di-oxo termination configuration, (O=)₂(Re–O–Zr)_u.

(b) At low coverage of 0.71 Re nm⁻², isolated (monomeric) ReO_x species prevail. At 400 °C, the mono-oxo Species-I predominates, Species-IIa is significantly present, whilst Species-IIb occurs in minority.

(c) A reversible temperature-dependent Species-I ↔ Species-IIa transformation is evidenced, shifting to the right with decreasing temperature.

(d) At coverages of n_s ≥ 1.3 Re nm⁻², Species-IIa clearly prevails over Species-I, in the full temperature range of 120–400 °C.

(e) The terminal stretching wavenumbers of all Species undergo a blue shift with increasing coverage, presumably due to variations in the basicity of dative O support atoms and/or vibrational coupling in associated (ReO_x)_n moieties. As an example, at 400 °C, the terminal Re=O stretching mode for Species-I is found at 993–1005 cm⁻¹, the symmetric stretching mode for Species-IIa is found at 987–998 cm⁻¹ and the counterpart symmetric stretching mode for Species-IIb at 982–991 cm⁻¹.

(f) At temperature below 80 °C and for a low coverage of 0.71 Re nm⁻², the occurrence of a fourth structural unit Species-III is evidenced, taking on a tri-oxo termination configuration with a symmetric stretching mode at 974 cm⁻¹.

(g) The isotopic ¹⁸O/¹⁶O substitution experiments show a facile Re–¹⁶O–Zr → Re–¹⁸O–Zr substitution and a relative ease for (¹⁶O=)₂(Re–O–Zr)_u → (¹⁸O=)₂(Re–O–Zr)_u substitution for the minority Species-IIb.

(h) All temperature-dependent changes and structural transformations are fully reversible under dehydrated feed conditions.

(i) It is feasible to tune the speciation of the (ReO_x)_n phase dispersed on ZrO₂ by suitably controlling the temperature and the coverage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Professor Dimitrios Tsiplakidis (Department of Chemistry, Aristotle University of Thessaloniki) is thanked for assisting the authors with the icp-analysis of the calcined samples. This study was supported by the project “Materials and Processes for Energy and Environment Applications” (MIS 5002556), which was implemented under the “Action for the Strategic Development on the Research and Technological Sector”, funded by the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3dt04270g

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