Introduction

Temperature programmed reduction (TPR) is a thermoanalytical technique widely used to characterise both bulk and supported heterogenous catalysts containing metal oxides [1, 2]. Typically, TPR is performed under an atmosphere of hydrogen, carbon monoxide or ammonia, usually blended with an inert gas such as nitrogen, helium, or argon [3,4,5]. In most cases, the TPR process is followed by evolved gas analysis (EGA) to monitor changes in the reductive gas concentration using a range of detectors such as thermal conductivity (TCD) [6], flame ionisation (FID) [7], photoionisation [8], and mass spectrometry (MS) [9]. The identification of reduction kinetic models is an area of particular interest [10,11,12].

Thermomicroscopy (sometimes referred to as hot-stage microscopy) studies the morphology and structural properties of materials optically as a function of temperature [13]. Thermomicroscopy can be performed in both reflected and transmitted modes. The latter technique is predominantly used for studying organic materials such as pharmaceuticals [14, 15], waxes [16, 17], and polymers [18, 19]. Reflected light techniques have been applied to a range of materials including inorganics [20, 21], glasses and ceramics [22, 23], high temperature sintering studies [24, 25], and crystal polymorphism [26, 27]. However, the use of thermomicroscopy with TPR remains largely undeveloped.

In this paper, we report the application of a newly developed thermomicroscopy instrument to the reflected light study of TPR on various metal oxide systems using colour profiling and micrographs.

Experimental

Instrumentation

Schematic 1 gives an overview of the instrument developed for this study. Briefly, the main body is constructed from ground glassware (F, 150 mm, 12 mm ID, 14/23 straight adapter) sealed with silicone gaskets (G) ensuring a low-volume gas-tight system. An induction circuit (C) provides non-contact heating of a stage (B), which comprises an iron rod (15 mm h, 10 mm d) with a cavity to hold a 5 mm ceramic crucible (D). Through the upper gasket, an endoscope style USB microscope (A, Oumij 2 MP) with an in-built focussing wheel is held securely in place focussing down on the sample crucible. The sample is illuminated with the microscopes in-built halo of LEDs. The temperature of the stage is measured using a type K thermocouple (E, 0.25 mm OD wire) situated directly under the sample crucible within a twin bore alumina rod (2 mm OD).

Gas is passed into the top of the unit, via a mass flow controller (Brooks 5878) and exits through a port in the silicone gasket in the base of the instrument and passed directly into a Dreschel bottle containing water. Throughout this study, a flow rate of 30 mL min−1 of inert (N2) or reductive gas (5% H2 in N2) was used.

Software

Software for instrument control and data acquisition was developed in house using Microsoft Visual Studio 2022. The software allows the control of the temperature, real-time image capture, and extraction of red, green, blue (RGB) colour values from regions of interest (ROI). Data are collected and stored as a comma separated variable (.csv) file, typically at a rate of once per second. The software supports a range of experiment types including the linear temperature programmes used in this study. Numerical data were processed using Microsoft Excel (365 edition) and plotted as a total RGB colour value as a function of temperature.

Scheme 1
scheme 1

TPR instrumentation developed and used for this study. A USB microscope, B iron stage, C induction coil, D sample crucible, E type K thermocouple, F ground glassware and G silicone gaskets

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Materials

Nitrogen (99.998%, BOC) was used as an inert gas, and hydrogen in nitrogen (5% H2 95% N2, BOC) was used for reduction experiments.

Temperature calibration of the instrument was achieved using high purity metals gallium (99.99%), indium (99.999%), tin (99.995%), lead (99.999%), zinc (99.99%) and aluminium (99.99%) all purchased from Goodfellow.

Samples of silver (I) oxide (Ag2O, 99 + %), silver (II) oxide (AgO, 99.9%), copper (II) oxide (CuO, 99.7%), copper hydroxycarbonate (abbreviated to CuHC, CuCO3.Cu(OH)2, Cu 55% minimum), lead (IV) oxide (PbO2, 97%) and cobalt (II, III) oxide (Co3O4, 99.7%) were all obtained from Alfa Aesar.

Methods

Temperature calibration

The temperature calibration was achieved through monitoring the optical changes in six metal samples (Ga, In, Sn, Pb, Zn, and Al) using a heating rate of 10 K min−1 to a temperature above the melting transition. All calibration experiments were performed under an inert atmosphere. During melting, the metallic surface relaxes and this is detected through a change in the RGB colour value, usually seen as a step (as shown in Fig. 1). The temperature of the transition is taken from the extrapolated onset of each step and used to generate a calibration graph using the values given in the International Temperature Scale of 1990 [29].

Fig. 1
figure 1

Temperature calibration of the hot stage used in this study. The melting transitions of the metals; gallium (Ga), indium (In), tin (Sn), lead (Pb), zinc (Zn) and aluminium (Al) are observed as a total colour change (RGB) plotted as a function of temperature. Example micrographs are shown before (upper) and after (lower) the melt transition. (Sample mass: 5–35 mg; Heating rate: 10 K min−1; Atmosphere: N2, 30 mL min−1)

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Silver (I) and (II) oxide

The effect of atmosphere on the decomposition/reduction in 10 mg samples of silver (I) oxide and silver (II) oxide was investigated using a heating rate of 10 K min−1 between 30 and 500 °C.

Copper (II) oxide and CuHC

A comparison of the reduction and simultaneous decomposition/reduction in 10 mg samples of copper (II) oxide and CuHC was investigated using a heating rate of 10 K min−1 between 30 and 500 °C.

Lead (IV) oxide

The effect of atmosphere on the decomposition/reduction in 10 mg samples of lead (IV) oxide was investigated using a heating rate of 10 K min−1 between 30 and 600 °C.

Cobalt (II, III) oxide

The effect of heating rate on the reduction in cobalt (II, III) oxide was studied using heating rates of 2.5, 5.0, 7.5 and 10.0 K min−1 between 30 and 600 °C.

Results and discussion

Silver (I) and (II) oxide

The decomposition of silver (I) oxide under nitrogen is shown as a sharp step in the colour profile (Tonset = 377 °C) as the black oxide rapidly converts to silver metal (Fig. 2, upper, dashed line). The reduction process under 5% hydrogen is again shown as a step in the RGB colour profile (Fig. 2, upper, solid line) but at a much lower temperature (Tonset = 158 °C). Comparable results were obtained by Charsley et al. who used reflected light intensity measurement to follow the decomposition of silver (I) oxide [28]. Jelić et al. used thermogravimetry to study both the reduction and decomposition of silver (I) oxide. They also observed single step processes with the reduction occurring over a much lower temperature range of ~ 100 to 150 °C, notably lower than the decomposition which occurred at 380 °C [30].

Fig. 2
figure 2

Temperature programmed reduction (H2, solid line) and decomposition (N2, dashed line) of silver oxides (Ag2O and AgO). Colour change profiles (RGB) are plotted as a function of temperature, the Ag2O RGB colour profiles have been offset by 500 a.u. for clarity. Micrographs of the reduction in Ag2O and AgO have been included along with their corresponding temperatures. (Sample mass: 10 mg, Heating rate: 10 K min−1, Atmosphere: 30 mL min−1 of 5% H2 in N2, or N2 only)

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The decomposition of silver (II) oxide under nitrogen is again shown as a step in the colour profile (Tonset = 440 °C) but at a higher temperature than for silver (I) oxide (Fig. 2, lower, dashed line). The reduction process under 5% hydrogen has a clear onset (Tonset = 135 °C) but occurs over a wider temperature range of ~ 150 °C (Fig. 2, lower, solid line). The low temperature and single stage nature of the reduction in silver (II) oxide has meant, and it has been used as a TPR calibration standard by some other workers [31, 32].

Selected micrographs of the two oxides under 5% hydrogen at 100, 200 and 300 °C are shown to demonstrate the overall change in colour.

Copper (II) oxide and CuHC

The RGB profiles for copper (II) oxide under inert and reducing atmospheres are shown in Fig. 3. The lack of change in the RGB colour profile is consistent with the stability of copper (II) oxide under nitrogen to 500 °C. Under 5% hydrogen the reduction is revealed as a step up in the colour profile (Tonset = 348 °C) as the black oxide changes to the red copper metal as seen in the associated micrographs. Barnes et al. used a hygrometer to monitor the water produced during the reduction in copper (II) oxide under a 5% hydrogen in helium atmosphere. They also observed a similar single step profile under linear heating conditions of 5 K min−1 [33].

Fig. 3
figure 3

Temperature programmed reduction (H2, solid line) and decomposition (N2, dashed line) of copper hydroxycarbonate (CuHC, CuCO3.Cu(OH)2) and copper oxide (CuO). Colour change profiles (RGB) are plotted as a function of temperature, and the CuHC RGB colour profiles have been offset by 400 a.u. for clarity. Micrographs of the reduction in CuHC and CuO, and the decomposition of CuHC have been included along with their corresponding temperatures. (Sample mass: 10 mg, Heating rate: 10 K min−1, Atmosphere: 30 mL min−1 of 5% H2 in N2, or N2 only)

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The RGB colour profile for CuHC under nitrogen is shown by a step down (Tonset = 195 °C) as the green salt decomposes to the black copper (II) oxide. Comparable results were obtained by Koga et al. who used TG–DTA to study the decomposition of a synthetic CuHC (malachite) over a range of different heating rates [34].

The simultaneous decomposition/reduction experiment under 5% hydrogen has a more complex colour profile. Initially, the colour profile falls (Tonset = 185 °C) as the salt decomposes, but then rises (Tonset = 280 °C) as any oxides formed are reduced to the copper metal. These colour changes can also be seen in the associated micrographs. Simultaneous decomposition/reduction experiments are less commonly performed but have been used in the preparation of supported catalysts [35].

Lead (IV) oxide

The behaviour of lead (IV) oxide under inert and reducing atmospheres over the temperature range 300–600 °C is shown in Fig. 4. The decomposition of the black lead (IV) oxide to yellow lead (II) oxide is shown as a step in the colour profile (Tonset = 477 °C) and a peak in the differential colour profile. Karski and Paryjczak used a katharometer to study the TPR of lead and palladium oxides supported on alumina using 8% hydrogen in argon. They found that the pure lead oxide reduction was completed by 600 °C but was at a lower temperature when supported on alumina [36].

Fig. 4
figure 4

Temperature programmed reduction (H2, solid line) and decomposition (N2, dashed line) of lead dioxide (PbO2). Colour change profiles (RGB) and first derivatives (dRGB/dT) are plotted as a function of temperature. Selected micrographs and their corresponding temperatures have been included showing the reduction in lead dioxide. (Sample mass: 10 mg, Heating rate: 10 K min−1, Atmosphere: 30 mL min−1 of 5% H2 in N2, or N2 only)

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The reduction in lead (IV) oxide under 5% hydrogen gives rise to a more complex colour profile with an earlier onset temperature (Tonset = 432 °C). The differential colour profile indicates a two-stage process, and this is supported by the micrographs with a red–orange intermediate (tentatively assigned to lead (II, IV) oxide) being formed around 440 °C. This quickly converts to the yellow oxide similar to that observed under nitrogen. The reduction in this oxide proceeds to liquid elemental lead, as seen in the micrographs as a series of beads forming above 500 °C. This experiment shows the potential benefit of optical monitoring of reduction processes, in that transient species can be observed that may be difficult to detect using other thermal techniques.

Cobalt (II, III) oxide

The effect of four heating rates on the reduction in cobalt (II, III) oxide is shown in Fig. 5A as normalised colour profiles. All profiles are similar having an initial decrease starting around 300 °C followed by a two-stage increase up to 600 °C. The resolution between steps (1 and 2) increases with heating rate. The associated micrographs from the 10 K min−1 experiment show that the colour changes throughout the reduction are quite subtle (black to grey); however, the RGB colour profile is still able to distinguish the two steps. In addition, from the micrographs at 425 °C and 450 °C, a red-brown phase is apparent between steps 1 and 2, a colour not normally associated with cobalt oxides.

Fig. 5
figure 5

Temperature programmed reduction in cobalt oxide (Co3O4). Normalised colour change profile plotted as a function of temperature at various heating rates (A). Heating rate dependency profile (B) for the onset of reduction in step one (1) and step (2). Selected micrographs and their corresponding temperatures have been included for the reduction in cobalt oxide at 10 K min−1. (Sample mass: 10 mg, Heating rate: 2.5, 5.0, 7.5 and 10 K min−1, Atmosphere: 30 mL min−1 of 5% H2 in N2)

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The onsets of steps 1 and 2 are plotted against heating rate (Fig. 5B). This allows the thermodynamic transition temperatures at zero heating rate to be determined via extrapolation in a similar manner to that widely used in DSC studies [37]. The extrapolated value of step 1 was found to be 335 °C, and step 2 was found to be 369 °C at a 0 K min−1 heating rate.

Mishra et al. studied the reduction in cobalt (II, III) oxide using low temperature hydrogen plasma and reported that the process occurs via two stages: Co3O4 → CoO → Co [38]. Bulvachenko et al. studied the reduction in pure and supported cobalt (II, III) in a 10% hydrogen atmosphere and observed a similar two-stage process [39]. Zhang and co-workers looked at the effect of water vapour on the reduction in supported cobalt oxide catalysts in 5% hydrogen in argon. Their results also suggested that the reduction occurred in two stages, but found that water vapour significantly affects the process [40].

The reduction process occurs faster at higher heating rates thus leading to a decrease in hydrogen concentration around the sample and a corresponding increase in the resolution between steps 1 and 2. The experimental results suggest that a three-stage process may occur, in contrast to previous authors, but further work is required to confirm and identify the intermediate oxide.

Conclusions

A thermomicroscope was constructed to allow the study of temperature programmed reduction processes, through micrographs and colour profiles of metal oxides under different atmospheres up to 600 °C.

Comparisons were made of the behaviour of a range of metal oxides heated under nitrogen and 5% hydrogen atmospheres which demonstrated that the colour profile is a viable method for monitoring reaction processes. As many metal oxides are highly coloured, thermomicroscopy may offer an advantage in detecting the presence of transient intermediates, which could be missed by other thermal methods.

Further work will incorporate a mass spectrometer to the thermomicroscope system, tracking the hydrogen concentration and this will be correlated with the colour profiles observed. In addition, the mass spectrometer will allow the study of complex decomposition processes.

Most TPR experiments use linear heating programmes; however, this can lead to issues in non-uniformity of conditions, particularly as a result in the change of the concentration of the reductive gas. This effect was noted in the cobalt (II, III) oxide system where a two-step process was more pronounced at higher heating rates. A follow-up study plans to explore the use of sample controlled thermal analysis (SCTA) where the temperature profile is adjusted to ensure processes occur at a constant low rate [29, 41].