Production of H2-free CO by decomposition of formic acid over ZrO2 catalysts
Graphical abstract
Introduction
The decomposition of HCOOH is a model reaction that has been studied in heterogeneous catalysis for several decades, as it allows to distinguish between metallic catalysts active for dehydrogenation (HCOOH → H2 + CO2) and acidic catalysts promoting for dehydration (HCOOH → H2O + CO) [1]. Since formic acid is widely considered a viable hydrogen storage molecule, recent works on HCOOH catalytic decomposition have been mainly devoted to the efficient production of H2 via the former reaction [2], [3]. However, HCOOH is also a possible source for the production of CO with extremely high purity due to the facile separation of liquid water from gaseous CO. Although a number of chemicals including methane, olefins, paraffins, and oxygenated compounds such as alcohols, aldehydes, acids, and ketones can be used for CO production [4], [5], [6], selective CO production remain a challenging endeavor. If production of high-purity CO would be reasonably attainable, it could be utilized in many chemical processes since the purity of CO significantly determines the overall process yield and economics [7], [8], [9].
In order to attain the production of H2-free CO from HCOOH (i.e., ca. 100% selective dehydration of HCOOH), we focused our attention on the preparation of a highly active and selective ZrO2 catalyst since ZrO2 has been extensively used as support material as well as catalyst in various applications due to its outstanding mechanical, optical, and electronic properties [10], [11], [12], [13]. Particularly, ZrO2 is the only metal oxide that demonstrates four chemical properties on the surface such as acidic and basic properties as well as oxidizing and reducing properties [14], [15]. The acidity, selectivity, and stability of ZrO2 catalysts can be tuned by varying the preparation method, the zirconium precursor, and the pretreatment conditions [16], [17]. The prepared ZrO2 can exist in three polymorphs, monoclinic, tetragonal, and cubic, whose fraction in the catalyst depends on the heat-treatment temperature [18]. Among these crystalline phases, tetragonal ZrO2 is the most frequently used in heterogeneous catalysis because it has better textural and acid–base properties than the monoclinic one [1].
In this work, the decomposition of HCOOH to high-purity CO over ZrO2 catalysts has been investigated. ZrO2 catalysts were prepared by simple precipitation to hydroxides and subsequent calcination at different temperatures. Their activities were correlated with the surface acid density and the results obtained by temperature-programmed desorption of iso-propanol. Additional experiments varying the operating parameters such as reaction temperature and weight hourly space velocity (WHSV) were conducted in order to produce H2-free CO from HCOOH decomposition over acidic ZrO2 catalyst.
Section snippets
Catalyst preparation
The Zr precursor was prepared using the forward (base-to-acid) precipitation method. An ammonia solution (28 wt.% NH4OH, SK Chemical) was added dropwise to a 0.5 M aqueous solution of zirconyl chloride (ZrOCl2∙8H2O, Kanto Chemicals, 99%) under vigorous stirring at room temperature until the solution pH reached 9.5. The resulting suspension was aged at 373 K for 48 h. The solid product aged was washed with distilled water until no detection of chloride ions. The water filtrated was titrated with the
Characteristics of the prepared ZrO2 samples
The XRD patterns of the ZrO2 sample synthesized at different aging times and calcined at 500 °C for 6 h are shown in Table S1 and Fig. S1. The ZrO2 aged for 48 h showed pure tetragonal phase. The ZrO2 samples aged for 1 and 24 h and subsequently calcined at 500 °C showed a mixture of tetragonal and monoclinic phase and each proportion of tetragonal and monoclinic phases was 0.09:0.91 and 0.87:0.13, respectively. The ZrO2 sample aged over 48 h subsequently calcined at 500 °C showed amorphous phase. The
Conclusions
In this study, ZrO2 catalysts were synthesized by simple precipitation method and calcined at various temperatures. All samples showed pure tetragonal phase, except for ZrO2 (950) and ZrO2 (1000) catalysts that exhibited a mixture of tetragonal and monoclinic ZrO2 phases. The tested catalytic activity based on unit mass of catalyst in the decomposition of formic acid followed the order ZrO2 (900) > ZrO2 (800) > ZrO2 (700) > ZrO2 (600) > ZrO2 (500) > ZrO2 (950) > ZrO2 (1000). Remarkably, this result was in
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The authors contributed equally to this work.