Highly dispersed Pt-based catalysts for selective CO₂ hydrogenation to methanol at atmospheric pressure

Highlights

• A highly dispersed Pt-based catalyst is synthesized for the CO₂ hydrogenation.

• An enhanced CO₂ hydrogenation is achieved at atmospheric pressure and room temperature.

• The methanol selectivity from the CO₂ hydrogenation is as high as 62.6%.

Abstract

Hydrogenation of CO₂ into methanol is promising for achieving the sustainable energy economy, but still has some problems, e.g. low methanol selectivity and high operation pressures (>10 atm). Herein, we prepared a Pt/film hybrid with highly dispersed Pt nanoparticles, and combined Pt/film with In₂O₃ to form a Pt/film/In₂O₃ catalyst. By using a dielectric barrier discharge (DBD) plasma reactor, a CO₂ conversion of 37.0% and a methanol selectivity of 62.6% are achieved in the hydrogenation of CO₂ with H₂ on Pt/film/In₂O₃ at 1 atm and 30 °C. These are higher than those on Pt/In₂O₃ prepared by the conventional high-temperature H₂ reduction (24.9% and 36.5%) and commercial Cu/ZnO/Al₂O₃ (25.6% and 35.1%). The high-energy electrons of the DBD plasma trigger the CO₂ hydrogenation at 1 atm and 30 °C. The higher Pt nanoparticles dispersion, film and In₂O₃ promote the adsorption of CO₂ on Pt/film/In₂O₃, thus enhancing the hydrogenation of CO₂ into methanol. These results are helpful for efficient methanol production from CO₂ hydrogenation under atmospheric pressure.

Introduction

The hydrogenation of CO₂ into methanol, which is a valuable liquid fuel and a key feedstock in chemical industry, is promising for achieving the sustainable energy economy (Perko et al., 2014, Rubert-Nason et al., 2014, Wang et al., 2015, Deerattrakul et al., 2016, Du et al., 2016, Alvarez et al., 2017, Bernskoetter and Hazari, 2017, Kattel et al., 2017, Rui et al., 2017, Yang et al., 2017, Din et al., 2018, Díez-Ramírez et al., 2018, Kar et al., 2018a, Kar et al., 2018b, Leonzio, 2018, Malik et al., 2018, Wang et al., 2018a, Wang et al., 2018b, Yin et al., 2018). Yet, to date, there are still some problems associated with the methanol production from CO₂ hydrogenation. Generally, for improving the efficiency in producing methanol, CO₂ hydrogenation has to be operated at temperatures lower than 400 °C and pressures higher than 10 atm (Deerattrakul et al., 2016, Alvarez et al., 2017). The hydrogenation of CO₂ into methanol is an exothermic process, with a ΔH_298K of −49.5 kJ mol⁻¹ (Wang et al., 2018a, Wang et al., 2018b). Low hydrogenation temperatures are thereby thermodynamically favorable for methanol production (Perko et al., 2014, Rubert-Nason et al., 2014, Alvarez et al., 2017, Kattel et al., 2017, Rui et al., 2017, Din et al., 2018, Kar et al., 2018a, Kar et al., 2018b, Wang et al., 2018a, Wang et al., 2018b). However, from the viewpoint of dynamics, low hydrogenation temperatures make the rates of CO₂ conversion and methanol production slower. It has been demonstrated that higher operation pressures facilitate the formation of methanol from the CO₂ hydrogenation (Perko et al., 2014, Rubert-Nason et al., 2014, Alvarez et al., 2017, Kattel et al., 2017, Rui et al., 2017, Din et al., 2018, Kar et al., 2018a, Kar et al., 2018b, Wang et al., 2018a, Wang et al., 2018b). For example, Rui et al. found that the methanol yield from the CO₂ hydrogenation on a Pd/In₂O₃ catalyst increased linearly from about 0.20 to about 0.90 g g_cat.⁻¹ h⁻¹ with the operation pressures increasing from 10 to 50 atm (Rui et al., 2017). Nonetheless, the high operation pressures make the industrial applications of the hydrogenation difficult.

Besides the problems in operation conditions, the low selectivity of the CO₂ hydrogenation in producing methanol is also an obstacle for its industrial applications (Perko et al., 2014, Rubert-Nason et al., 2014, Deerattrakul et al., 2016, Alvarez et al., 2017, Kattel et al., 2017, Rui et al., 2017, Din et al., 2018, Díez-Ramírez et al., 2018, Kar et al., 2018a, Kar et al., 2018b, Malik et al., 2018, Wang et al., 2018a, Wang et al., 2018b, Yin et al., 2018). Dispersion of the metal nanoparticles on the catalyst is a key factor affecting the methanol selectivity of the CO₂ hydrogenation (Deerattrakul et al., 2016, Rui et al., 2017, Yin et al., 2018). For example, by increasing the Pd nanoparticle dispersion on a Pd/ZnO catalyst for the CO₂ hydrogenation, methanol yield and selectivity as high as 0.65 g g_cat.⁻¹ h⁻¹ and 63%, respectively, were achieved at 270 °C and 45 atm (Yin et al., 2018). Thermal reduction with H₂ at elevated temperatures and chemical reduction with reducing agents like NaBH₄ or citrate are the two commonly used methods for fabricating metal nanoparticles on the catalysts (Lin et al., 2009, Yan et al., 2016, Sun et al., 2017). For the chemical reduction, due to the use of many reagents (reducing agent or stabilizer), impurities are easily involved in the catalysts. For the thermal reduction, the metal nanoparticles aggregation easily occurs, due to the elevated reduction temperatures. Besides, the amount of the functional groups favorable for CO₂ adsorption and hydrogenation on the catalysts decreases due to the decomposition of the functional groups at high temperatures in the thermal reduction.

Recently, a novel cold-plasma/peptide-assembly (CPPA) method has been developed for preparing noble-metal/film hybrids with highly dispersed noble metal nanoparticles under room temperature (Yan et al., 2013, Pan et al., 2015, Wang et al., 2017, Wang et al., 2018a, Wang et al., 2018b). The cold plasma is an ionized gas mainly containing ions and high-energy electrons. The high-energy electrons exhibit a temperature as high as 10000–100000 K, but the gas temperature is as low as room temperature. The high-energy electrons in the cold plasma can efficiently reduce the noble metal ions into zero-valence state, tune surface properties of materials, promote the self-assembly of biomolecules into two-dimension films, and trigger chemical reactions (Yan et al., 2013, Pan et al., 2015, Wang et al., 2017, Wang et al., 2018a, Wang et al., 2018b). During the CPPA process, the noble metal ions are reduced into zero-valence state by the high-energy electrons of the cold plasma at room temperature. The room-temperature reduction is benefit for keeping the noble metal nanoparticles in more perfect crystal structures, smaller sizes and higher dispersions. During the CPPA process, with the assistance of the high-energy electrons of the cold plasma, the peptides, one of the widely explored biomolecules, assemble into two-dimensional film with a regular network, and the noble metal nanoparticles are embedded into the network of the film. This is also favorable for suppressing the aggregation of the noble metal nanoparticles.

Herein, we combined the CPPA-prepared Pt/film hybrid with In₂O₃ to form a Pt/film/In₂O₃ catalyst with highly dispersed Pt nanoparticles. Selective methanol production from the CO₂ hydrogenation with H₂ is successfully achieved on Pt/film/In₂O₃ in a cold plasma reactor operated at 1 atm and 30 °C. The high-energy electrons of the cold plasma provide the energy required for hydrogenating CO₂ under ambient condition. The CO₂ conversion and methanol selectivity on Pt/film/In₂O₃ are 37.0% and 62.6%, respectively. These are higher than those on the thermal-reduction-prepared Pt/In₂O₃ (24.9% and 36.5%) and commercial Cu/ZnO/Al₂O₃ (25.6% and 35.1%).