Elsevier

Catalysis Today

Volume 143, Issues 1–2, 15 May 2009, Pages 80-85
Catalysis Today

Basic evidences for methanol-synthesis catalyst design

https://doi.org/10.1016/j.cattod.2008.11.022Get rights and content

Abstract

A series of Cu–ZnO/ZrO2 catalysts (Znat/Cuat, 0–3; ZrO2, 42–44 wt.%) with markedly improved total (SA, 120–180 m2/g) and metal surface area (MSA, 9–63 m2/g) has been synthesized via the reverse co-precipitation under ultrasound field. Structure, adsorption properties and surface reactivity were probed by BET, XRD, TPR, N2O-titration and TPD measurements of H2, CO and CO2, while the activity pattern in the CO2 to CH3OH hydrogenation reaction (TR, 433–473 K; PR, 1.0 MPa) has been addressed. An optimum ZnO loading (Znat/Cuat, 0.3–0.8) ensures higher process productivity, while the apparent structure sensitive character of the title reaction relies on the synergism of metal hydrogenation and oxide basic sites (dual-site). The extent of the oxide/metal interface, probed by Cuδ+/Cu0 and oxide-to-metal surface area (OSA/MSA) ratios, is the key-parameter allowing the normalization of the catalytic specific activity in a wide range (3–60%) of metal dispersion and the consequent structure insensitive character of the CO2 hydrogenation reaction.

Introduction

Among the various available strategies to reduce carbon dioxide emissions, novel process technologies for CO2-recycling are currently attracting a great scientific and technological concern [1], [2], [3], [4]. In particular, the use of CO2 as “reagent” for producing bulk chemicals and fuels like methanol (MeOH) and dimethyl ether (DME) [1], [2], [3], [4], [5] looks very attractive since, as alternatives to gasoline and gasoil, they ensure superior combustion efficiency and much lower air pollution than oil-based fuels [1], [2]. Nowadays the industrial synthesis of methanol is carried out at 493–573 K and 5–10 MPa pressure by feeding a CO(5%)–CO2(5%)–H2 syngas stream on Cu–ZnO/Al2O3 catalysts [6], [7], [8], [9]. Under such conditions methanol mostly forms from CO2 with CO acting as “tuner” of the WGS equilibrium reaction and scavenger of oxygen atoms from the catalyst surface [6], [7], [8], [9]. However, an effective CO2 conversion is at present limited by a poor specific functionality of current catalysts [10], [11], likely due to the strong hydrophilic character of alumina carrier, and perhaps ZnO, involving a marked negative effect of water on active sites stability [17], [18], [19], [20], [21], [22]. Then, Cu/ZrO2 and Cu–ZnO/ZrO2 systems ensure a superior CO2 hydrogenation functionality in comparison to conventional catalysts [12], [13], [14], [15], [16], [17], though solid-state interactions and reaction atmosphere influence the extent of the metal/oxide interface, playing a crucial influence on the catalytic functionality of Cu systems [17], [18], [19], [20], [21].

Therefore, this paper deals with a systematic investigation of the effects of the Zn/Cu ratio on the structure and adsorption properties of the Cu–ZnO/ZrO2 system to shed light into its reactivity pattern in the CO2-to-methanol hydrogenation reaction [22], [23].

Section snippets

Experimental

A series of Cu–ZnO/ZrO2 catalysts with a constant zirconia loading (41–44 wt.%) and Znat/Cuat atomic ratio between 0 and 2.8 was prepared via the reverse co-precipitation under ultrasound field [22], [23]. After washing the catalysts were dried at 373 K and further calcined in air at 623 K (4 h). The list of catalysts with the relative notation and main physico-chemical properties is given in Table 1.

Surface area (SA) and pore volume (PV) values of “as prepared” catalysts were determined from

Influence of carrier and promoter on the structure and reactivity of the metal Cu phase

Despite of the large amount of studies devoted during last decades to reaction mechanism, kinetics and catalyst optimization, the role of the various surface sites on methanol synthesis functionality is still undefined ([22], [23] and references therein). This depends upon the great sensitivity of the surface composition and morphology to reaction atmosphere [24] hindering a full understanding of the Cu–Zn(O) functionality [22], [23]. This prompted us to carry out a systematic investigation of

Conclusions

The solid-state interaction pattern and the nature of surface adsorption sites of Cu–ZnO/ZrO2 catalysts have been addressed, leading to the following main conclusions:

  • A strong Cu–ZnO interaction effectively promotes the metal dispersion, determining also the redox properties and the reactivity of the Cu–ZnO/ZrO2 system.

  • The interaction of metal Cu particles with both ZnO and ZrO2 leads to the stabilization of Cuδ+ sites at the metal/oxides interface.

  • A “mixture” of Cu0, Cuδ+ and Lewis basic sites

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