In order to prove that reduction temperatures shift increasingly to lower temperatures as the reducing gas’ partial pressure increases, materials with a higher reduction temperature were tested.
As a result of its abundance in nature as well as its economic accessibility as a material which is readily available on the market, copper oxide was selected for these experiments. Copper oxide also benefits from having an extremely wide application in the catalysis industry.1
As copper is a transition metal which is easily able to change its oxidation state, it is a good catalyst both for the dehydrogenation and hydrogenation of numerous organic compounds. It is also used in water gas shift processes and the treatment of wastewater. As it has fantastic properties of conductivity, it is also used widely in the motor and wiring industries.2
Further to its uses outlined above, copper is also a good catalyst for the synthesis of ammonia industries, as well as numerous other applications which have been well detailed in scientific papers.3
This article intends to analyze the reduction of copper oxide in order to see how the high pressure and flow of H2 affect this important oxide’s TPR profile. The experiment is also designed to test whether a high pressure of H2 can alter the mechanism by which this oxide reduces.
The reduction of copper oxide is a complicated reaction, as it forms a number of transition states during reduction, some of which are not stable while others are. The following equation summarizes the reduction step:
4CuO + H2 ______ Cu4O3 + H2O
Cu4O3 + H2 ______2Cu2O + H2O
Cu2O + H2 _______Cu + H2O
While Cu2O is a stable species, Cu3O4 is not. This complicates the original oxide’s TPR profile. There are essentially two main peaks presented in the TPR profile. As the Cu4O3 species is not stable, it will reduce very quickly towards the formation of Cu2O. Furthermore, as the temperature continues to increase, the species eventually reduces to Cu.4
Either by increasing the flow of the hydrogen mixture, which increases the partial pressure of H2 over the oxide while being reduced, or by increasing the pressure of H2, the two reduction peaks are significantly affected. The experiments indicate that the reduction profile of the CuO begins shifting towards a lower reduction temperature as the pressure of the reducing mixture increases.
When the reduction temperature is lowered, there is an extremely positive impact upon catalysis, as the sintering of active species in all kinds of catalysts is minimized.
Furthermore, the experiments indicate that a high H2 pressure changes the mechanism by which the oxide is reduced. At atmospheric pressure, the first reduction peak was only a shoulder. At the high H2 pressure, however, there is significantly greater definition in the first peak, while the second peak begins to progressively flatten as the pressure is increased.
A PID EFFI which was modified in order to include the MCCTC option was used for these experiments. Roughly 0.25 g of material was used in each of the experiments. Mallet et al5 outlined the appropriate equation by which this amount was determined. This permits the use of a sufficient level of oxide while preventing the depletion of H2 during reduction.
The reactor’s temperature was increased at a rate of 10 °C per minute all the way up to 550 °C. An MKS Cirrus II mass spectrometer following mass 2 was used in order to record all TPR profiles.
Results and Discussions
50 ml of a 10% H2 balancing Argon at atmospheric pressure was used to obtain the basic TPR profile against which all other samples were compared (Top 1 on Figure 1). This TPR profile indicated one shoulder at 240 °C which corresponded to the transformation of CuO into the unstable species Cu4O3. This very quickly forms a relatively stable species, Cu2O.
At 320 °C, a second well-defined peak appears which corresponds to the reduction of Cu2O into metallic copper, Cu. The pressure was raised either by increasing the flow of the mixture or by increasing the absolute pressure over the sample. As a result, results demonstrated not just a shift in reduction temperature (220 °C versus 240 °C, and 237 °C versus 320 °C), but also a transition on the mechanism of reduction.
When compared to the shoulder illustrated on the basic TPR profile, the first peak is now well-defined. Shown in all the figures (Figures 1 and 2), as the pressure gets towards 20 bar, the first peak undergoes a significant change which makes it better defined.
As shown in Figure 1, CuO’s TPR profiles shift as the pressure raises from atmospheric to 20 bar. Figure 2 corresponds to the same effect as the mixture flow of H2 rises from 50 to 950 ml per minute.
Figure 1: Profiles from a to e represent the shift as function of increasing pressure.
Figure 2: Profiles from a to e represent the shift as function of increasing flow of the reducible mixture.
Both of these cases indicate an almost identical impact upon the original reduction profile; both of the reduction peaks shift towards lower reduction temperatures. An increase in the flow of the reducible mixture displaces the TPR profile (Figure 4) and improves the reduction peak at the reaction’s first step.
The same shift is exhibited by the high pressure (Figure 3), however this improves even further than the first reduction peak, becoming a much more pronounced and defined peak.
Figure 3: This figure represents the temperature trend as function of increasing pressure.
Figure 4: This figure shows the change in the reduction temperature as a function of increasing flow of the reducible mixture.
The addition of the MCCTC option to the PID EFFI transforms this instrument into a crucial tool for catalysis. This option has the ability to demonstrate the behavior of catalysts under high temperature and pressure reaction conditions.
All of the results which were outlined in this article illustrate the positive impact of high pressure and high reducible mixture flow in terms of substantially reducing the reduction temperatures of the varying species of oxide which are present in a catalyst.
Frequently occurring during oxides’ reduction, the sintering of active species can be significantly enhanced by a reduction in the temperature of the reducible species. This leads to a higher dispersion, which in turn produces improved activity and selectivity of the catalyst.
Ultimately, these experiments did not indicate any significant impact on reducing the reduction temperature, regardless of whether high flow of the reducible mixture or high pressure was used.
These results suggest that you should take advantage of the fact that all oxide species on catalysts which are able to be reduced (nearly all of them as a result of the negative free energy of the reduction), should be reduced at the lowest temperature possible.
References and Further Reading
- Chemical Kinetics of Copper Oxide Reduction with Carbon monoxide. Eli A. Goldstein, Reginald E. Mitchel., ScienceDirect, Proceedings of the Combustion Institute (2010)
- Indirect Phase Transformation of CuO to Cu2 O on a Nanowire Surface. Fei Wu, Sriya Banerjee, Huafang Li, Yoon Myung, and Parag Banerjee., Langmuir 2016, 32 (18), 4485-4493
- Reduction of CuO and Cu2 O with H2 : H Embedding and Kinetic Effects in the Formation of Suboxides. Jae Y. Kim, Jose A. Rodriguez, Jonathan C.Hanson, Anatoly I. Frenkel, and Peter L. Lee., JACS 2003
- Formation of Stable Cu2 O from Reduction of CuO Nanoparticles. Jenna Pike, Siu-Wai Chan, Feng Zhang, Xianquin Wang, Jonathan Hanson., Applied Catalysis A: General 303 (2006), 273-277
- The selection of Experimental Conditions in Temperature-Programmed Reduction Experiments. Pilar Mallet and Alfonso Caballero., J. Chem. Soc., Faraday Trans. 1, 1988, 84(7), 2369-2375
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