Copper zinc oxide catalysts for ambient temperature carbon monoxide

oxidation

S. H. Taylor,a G. J. Hutchingsa and A. A. Mirzaeib

a Cardiff University, Department of Chemistry, PO Box 912, Cardiff, UK CF10 3TB. E-mail: taylorsh@cf.ac.uk
b Leverhulme Centre for Innovative Catalysis, University of Liverpool, Department of Chemistry, Liverpool, UK
L69 3BX

Received (in Cambridge, UK) 29th April 1999, Accepted 16th June 1999

Copper zinc oxide catalysts are effective for the ambient 7 °C and consequently the reactor temperature could readily be
temperature carbon monoxide oxidation and display higher maintained isothermally at 20 °C.
specific activity than the current commercial hopcalite All the copper zinc oxide catalysts, irrespective of the
catalyst. preparation atmosphere, showed appreciable activity for the
oxidation of carbon monoxide at 20 °C. Blank reactions under
The catalytic oxidation of carbon monoxide to carbon dioxide at the same conditions demonstrated no conversion. All the copper
ambient temperature and pressure is an important process for zinc oxide catalysts showed an initial decrease in activity over
respiratory protection. In particular, the process is widely the first 30 min on line, but after this initial period steady state
adopted by mining industries and has also found applications in activity was maintained over the 500 min test period. Repre-
deep sea diving and space exploration. Furthermore, new sentative data for the catalysts prepared in an air atmosphere are
applications for the process such as reducing the deactivation of shown in Fig. 1. The steady state activity of the aged copper zinc
carbon dioxide lasers and applications for new sensors have oxide catalysts derived from different atmospheres, and com-
been explored. In the last 10 years low temperature carbon parison with a copper manganese oxide catalyst are shown in
monoxide oxidation has received renewed attention since Table 1.
Haruta et al. demonstrated that gold, highly dispersed on
various oxides, forms catalysts active at sub-ambient tem-
peratures.1 However, the most widely used catalyst is the mixed
copper manganese oxide hopcalite catalyst, CuMn2O4, first
examined in 1921.2,3 Both the gold based and the copper
manganese oxide catalysts are important in terms of their high
activity at ambient temperatures. It is the observation of high
activity at low temperature which has stimulated significant
recent interest in these types of catalysts.4,5 It is interesting to
consider whether other catalysts are capable of sustaining
carbon monoxide oxidation at ambient temperature. Here, we
present the first results showing that copper zinc oxide catalysts,
prepared by co-precipitation, can display much higher activity
for reaction than the current commercial hopcalite catalysts.
Catalysts were prepared using a co-precipitation technique
under different atmospheres, including air, nitrogen, hydrogen
and carbon dioxide. Aqueous solutions of Cu(NO3)2·3H2O
(0.25 mol l21 Aldrich 99.999%) and Zn(NO3)2·6H2O (0.25 mol Fig. 1 Carbon monoxide conversion at 20 °C for aged catalysts prepared
l21 Aldrich 99.999%) were pre-mixed in a 2+1 ratio. The using air vs. time on line (GHSV = 33 000 h21, 100 mg catalyst, 0.45 vol%
resulting solution was stirred and heated to 80 °C in a round CO: Ageing time: (5) 60, (-) 120, (:) 180 (2) 300 min.
bottomed flask fitted with a condenser and equilibrated under a
gas flow of 20 ml min21 for 5 min. An aqueous solution of The atmosphere used during co-precipitation has a marked
Na2CO3 (0.25 mol l21 Aldrich 99.999%) was added to the influence on the activity of the copper zinc oxide catalysts.
continuously stirred flask until a pH in the range 6.8–7.0 was Regardless of ageing time, the general trend in terms of carbon
attained. At this stage the gas flow was passed through the monoxide conversion is air > hydrogen ≈ nitrogen > carbon
solution and the precipitate allowed to age between 60 and 300 dioxide. Increasing the catalyst ageing time up to 180 min
min. After ageing the precipitate was recovered by filtration, increased the carbon monoxide conversion over all the copper
washed several times with hot distilled water and dried in air zinc oxide catalysts. Increasing the ageing time to 300 min
(120 °C for 16 h) and subsequently calcined in static air (550 °C resulted in a slight decrease in conversion for the catalyst
for 6 h) to produce the catalyst. The hopcalite catalyst was also prepared in an air atmosphere, however, the catalysts prepared
prepared in a similar manner using co-precipitation. The under the other atmospheres continued to show an increase in
catalysts were characterised by powder X-ray diffraction. and conversion. The catalyst surface areas were of similar magni-
nitrogen adsorption to determine the BET surface area. tude regardless of the preparation atmosphere, however, there is
The catalysts were tested for CO oxidation using a fixed bed a general increase in surface area as a result of increased ageing
laboratory microreactor. Typically CO (5% CO in He, 5 ml time. Although the overall conversion increases with ageing
min21) and O2 (50 ml min21) were fed to the reactor at time the specific activity shows a gradual decrease.
controlled rates using mass flow controllers and passed over the Under our reaction conditions comparison of the copper zinc
catalyst (100 mg) at 20 °C. The products were analysed using oxide catalyst has been made with the highly active copper
on-line gas chromatography with a 3 m packed Carbosieve manganese oxide catalyst.6 The copper zinc oxide catalysts
column. These conditions are equivalent to a total gas hourly aged for < 180 min are all considerably more active than the
space velocity of 33 000 h21 and CO concentration of 0.45 copper manganese oxide catalysts. For example, the specific
vol%. Under these conditions the adiabatic temperature rise is < activity of copper zinc oxide aged for 60 min is greater than the

Chem. Commun., 1999, 1373–1374 1373

Table 1 Steady state activity for ambient temperature carbon monoxide oxidation (GHSV = 33 000 h21, 100 mg catalyst, 0.45 vol% CO)

Ageing Preparation Surface area/ CO conversion Rate/molecules

Catalyst time/min atmosphere m2 g21 10216 (%) m22

CuO/ZnO 60 Air 29 38.8 1.50

CuO/ZnO 60 CO2 26 20.5 0.88
CuO/ZnO 60 H2 25 29.8 1.33
CuO/ZnO 60 N2 27 30.4 1.26
Cu/MnxOy 60 Air 28 9.9 0.39
CuO/ZnO 120 Air 36 40.2 1.35
CuO/ZnO 120 CO2 28 21.5 0.86
CuO/ZnO 120 H2 34 34.8 1.14
CuO/ZnO 120 N2 32 33.2 1.16
CuMnxOy 120 Air 26 11.8 0.51
CuO/ZnO 180 Air 46 47.5 1.16
CuO/ZnO 180 CO2 38 25.6 0.76
CuO/ZnO 180 H2 41 40.2 1.10
CuO/ZnO 180 N2 40 38.6 1.08
CuMnx/Oy 180 Air 27 20.4 0.84
CuO/ZnO 300 Air 42 45.0 1.20
CuO/ZnO 300 CO2 40 28.8 0.81
CuO/ZnO 300 H2 45 42.4 1.06
CuO/ZnO 300 N2 43 41.4 1.08
CuMnxOy 300 Air 30 49.5 1.85

corresponding copper manganese oxide catalyst by a factor solution phases formed during the controlled precipitation and
> 3.8. It is only once the copper manganese oxide catalyst has ageing process are important.
been aged for 300 min that it demonstrates higher activity. The At this stage no attempt has been made to optimise the
optimum ageing time for the copper manganese oxide catalyst is activity of the copper zinc oxide catalysts, but it is clear that
720 min showing a specific oxidation rate greater than the best these catalysts show promising performance for the oxidation of
copper zinc oxide by a factor of ca. 2.2. carbon monoxide under ambient conditions. To the best of our
Characterisation of the catalysts by powder X-ray diffraction knowledge this is the first reported study of copper zinc oxide
showed that the preparation atmosphere and ageing process catalyst prepared by co-precipitation under different atmos-
strongly influence the structure of the catalyst precursor. pheres for the oxidation of carbon monoxide at low tem-
Hydrozincite [Zn5(CO3)2(OH)6], gerhardite [Cu2(OH)3NO3], peratures and these systems are now worthy of further
malachite [Cu2CO3(OH)2], aurichalcite [(Cu,Zn)5- investigation.
(CO3)2(OH)6] and rosasite ](Cu,Zn)2CO3(OH)2], were all We thank Chris Kiely and Dave Whittle (University of
determined and the relationship between preparation conditions Liverpool) for electron microscopy and Richard Joyner
and structure is considered to be complex. However, after (Nottingham Trent) for useful discussion.
calcination all the catalysts consisted of CuO and ZnO. The
particle sizes of the oxides determined by X-ray line broadening
decreased with increased ageing time, this effect was most Notes and references
marked with the catalyst prepared under air and carbon dioxide 1 Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 1989, 115,
atmospheres, whilst under nitrogen and hydrogen atmospheres 301.
the effect was relatively minor. Temperature programmed 2 T. H. Rogers, C. S. Piggot, W. H. Bahlke and J. M. Jennings, J. Am.
reduction using hydrogen indicates that there may be some Chem. Soc., 1921, 43, 1973.
mixed oxide formation in the calcined catalyst. The catalysts 3 H. A. Jones and H. S. Taylor, J. Phys. Chem., 1923, 27, 623.
4 G. J. Hutchings, A. A. Mirzaei, R. W. Joyner, M. R. H. Siddiqui and
were all similar by transmission electron microscopy, but subtle
S. H. Taylor, Catal. Lett., 1996, 42, 21.
differences indicate that copper/zinc oxide solid solutions were 5 G. Fierro, S. Morpurgo, M. LoJacono and M. Inversi, Appl. Catal. A,
formed and this is consistent with the evidence from tem- 1988, 166, 407.
perature programmed reduction. The origin of the low tem- 6 E. J. Trimble, Toxicology, 1996, 115, 41.
perature oxidation activity is unclear, but it appears that the
highly dispersed CuO and ZnO, and the presence of solid Communication 9/03426I

1374 Chem. Commun., 1999, 1373–1374