Preparation-Properties Relation of Mn-Cu Hopcalite Catalyst

Problem statement: The Mn-Cu hopcalite catalyst was used for the conv ersion of CO to CO2 at low temperatures. It was the catalyst of choice in the gas masks for respiratory protection in mines, aircrafts, military, spatial laboratories. Approach: The efficiency of hopcalite catalyst depends on its surface parameters. Its surface characterist ics can be influenced from the chosen way of the MnO2 and CuO precipitation and from the pressure of pel letizing. Results: The hopcalite samples has been prepared by precipitation of MnO 2 and by adding CuSO 4 further in the solution the adsorption of Cu ions on MnO2 particles surface is achieved. After acidification f the solution up to pH = 3 the copper is precipitated in form of Cu (OH) 2CuCO3 by adding NaHCO3. Precipitate was washed, dried, pressed, crushed, sieved (1-2 mm) and calcined at 1 80°C for 3 h. MnO2 and hopcalite samples were characterized by XRD. The activity was evaluated by determination of its protection time and it was 610 min, better than activity of a commercial catal yst. Specific surface area, pore volume and density were measured by nitrogen adsorption and mercury in t usion porosimetry. The X-Ray diffractograms shows that the only crystallinity of hopcalite come s from MnO2, which is present mainly in amorphous form. By increasing the pressure in the pelletizing step, a significant decrease in the specific surfa ce area (247.64-147.77 m 2 g) and in the total pore volume (446-278 mm 3 g) is observed in the hopcalite samples. Conclusion: The obtained hopcalite catalyst by the two step pr eci itation method shows high catalytic activity. The increasing press ure increases the strength and reduces the specific surface area and pore volume. A pressure of 500 kg cm is recommended for the hopcalite production procedure.


INTRODUCTION
Carbon monoxide is highly toxic and causes human fatalities at concentrations more than 600 ppm in the air. The removal of CO is done by its conversion to CO 2 by using different type of catalysts. Among others, the low temperature oxidation of CO occurs on mixed oxides of manganese and copper, known as hopcalite catalyst (Hutchings et al., 1998). In comparison with other transition metal compounds, manganese oxides are more effective for the low temperature oxidation of CO. On the other hand they are friendly towards environment (Alvarez-Galvan et al., 2003;Sharma et al., 1995;Spivey, 1987).
Hopcalite catalyst is often used for respiratory protection in the gas masks of mining industry, aircrafts, space labs, closed room burning activities (Trimble, 1996;Jagow et al., 1977).
Combination of MnO x with other metal oxides in specific conditions, exhibit significant higher catalytic activity for CO to CO 2 conversion in comparison to the MnO x oxides alone (Jagow et al., 1977).
The preparation of hopcalite catalyst is effectuated in different ways like the coprecipitation of manganese and copper oxides (Hutchings et al., 1998;Li et al., 2007;Morales et al., 2006) controlled oxidation of Cu -Mn (30-50% Mn) alloys at temperatures 400-500°C (Yoon and Cocke, 1998), sol-gel synthesis (Kramer et al., 2006) and the two step precipitation method where the manganese and copper are precipitated in two separate steps.
In the preparation of Mn-Cu hopcalite catalyst by precipitation the procedure includes also washing, drying, pelletizing and calcination of the samples. The parameters used in the process of calcination are proved to influence the efficiency of the hopcalite catalyst (Hutchings et al., 1998), mainly due to the changes in its specific surface area. A special attention is given in this study to the pelletizing process since it influences the specific surface area of the prepared hopcalite catalyst as well.
The pelletizing step in the procedure of hopcalite preparation is necessary to give the needed strength to the catalyst pellets, in order to increase the resistance against possible pulverization inside the gas mask boxes, which works under mechanical stress. Different preparation steps have an influence on the strength of the hopcalite catalyst like the precipitation conditions, adding of binders, pelletizing pressure, drying and calcination conditions.
In this study the effect of changing the pelletizing pressure from 0-1700 kg cm −2 on the surface parameters of hopcalite catalysts is measured. The catalytic efficiency of the produced hopcalite catalyst is compared to a commercial one from SIDACO and the changes in its crystalline structure after copper precipitation and after calcination are determined. The two step precipitation method similar to that of, with slight changes is used to produce the hopcalite catalyst. The hopcalite preparation procedure is described schematically in the Fig. 1.

MATERIALS AND METHODS
All the catalysts used in this study were prepared after the two step preparation method. In 200 cm 3 distilled water at 50°C were added 30 g KMnO 4 and dissolved under continuous stirring. Than 24 g NH 4 HCO 3 were added and the solution was heated up to 65°C. Than the precipitation of MnO 2 began, accompanied by a heat release which brought the solution up to the temperature 93 °C. Simultaneously the characteristic color of permanganate disappeared. The solution was cooled down to room temperature and added there H 2 SO 4 until pH = 3. In this mixture of precipitated MnO 2 in contact with the solution, we added 26 g of finely grinded CuSO 4 • 5 H 2 O and dissolved it under stirring. Afterwards 28 g of NaHCO 3 was added to precipitate the copper in the form of basic copper carbonate Cu(OH) 2 CuCO 3 . The precipitate was washed with warm distilled water until no further SO 4 2was observed in the washings (BaCl 2 test). The precipitate was dried at 70°C until 14% humidity to give a material denoted as the catalyst precursor which is a fine powder. It was pressed to obtain pellets under 500, 800, 1400 and 1700 kg cm −2 . The pellets were crushed and their fraction of 1-2 mm was selected for further treatment by sieving. The grains were afterwards calcined for 3 h at 180°C. After calcination the hopcalite catalyst samples were kept in exicator under vacuum to protect from humidity. Characterization methods: Samples were characterized by X-Ray diffraction in a Model D 5000 powder diffractometer from Siemens AG Muenchen (CuK α source of wavelength 1.5406 Å). The measurements were carried out in the range of 2θ = 10-90°. The specific surface area, the mesopore volume and the density of the hopcalite catalysts were determined by nitrogen adsorption-desorption isotherm at 77 K and Helium filling in a SORPTOMATIC 1900 apparatus of the producer Carlo Erba using the Volumetric Method (VM). The specific surface area was calculated in accordance with the BET method, the mesopore volume in accordance with the Calvin equation for pore diameters from 2-60 nm (Gregg and Sing, 1991), the cumulative mesopore volume after the Gurvich rule and the density by the helium pycnometer principle (Lowell, 2004). The liquid nitrogen (> 99%), nitrogen gas (99,996%) and helium gas (99.996 %) were products of Messer-Albagas. The macropore volume of the hopcalite samples were measured by means of Mercury Intrusion Porosimetry (MIP) in an apparatus Poremaster 60 of the producer Quantachrome Instruments,with the Software Poremaster Version 8. With the pressures applied from 0.138-24.92 MPa it determines the respective pore diameters of 10.85 µm to 60 nm. Calculation of the macropore diameter for a known pressure exerted on mercury is based on the Washburn's equation (Washburn, 1921). Catalyst testing: The hopcalite sample, prepared using a pelletizing pressure of 500 kg cm −2 and a sample of commercial hopcalite catalyst of the producer SIDACO (Netherland B.V.) were tested for their catalytic efficiency, expressed in term of protection time. The protection time is determined following a standardized method (similar to standard EN 403) and is measured in the laboratory equipment showed in the Fig. 2. The amount of 173.5 g of hopcalite sample is filled in the upper volume (135 cm 3 ) of the gas mask box 10 (cylindrical fixed bed reactor) in which a stream of controlled humidity air and carbon monoxide passes through. The humid air is produced by passing the air firstly through the silicagel bottle 1 in order to dry it after which it is divided in three streams, first one going directly to the psychrometer, second passing through the drying unit 3 (H 2 SO 4 cc) and third one through the humidifying unit 2 (water). By controlling the air flow in the glass cocks R-1 and R-2 the humidity of the air is kept constant at 50%. The humidity is measured at the psychrometer 4. Carbon monoxide was produced separately in laboratory conditions by dropping HCOOH over boiling H 2 SO 4 (cc) and was stored in vessel 6, where it is diluted with dry air 3.3 times. CO is passed through the glass cock R-3 by adding water in the vessel 6 and its constant pressure is kept by the pressostat 8. Wet air (R.H. 50%) and carbon monoxide are mixed in the glass vessel 5. The flow rate of the wet air + CO mixture through the box 10 is measured by the flowmeter 11 and the flow rate of CO by the flowmeter 9. The flow rate of the wet air was kept 1800 L h-1 and the flow rate of CO was kept 30 L h-1 resulting in a concentration of 5000 ppm for CO before entering the box 10. CO concentration was measured after R-5 at the inlet of the box 10 using Draeger tubes of producer Draeger (CH29901) and after R-7 at the outlet of the box using a CO sensor. This sensor is the model MINI-CO of the producer A11 Airtest Technologies, Inc. can detect CO concentrations in the range 0-500 ppm and was calibrated before every measurement for concentration 100 and 300 ppm. The contact time of the hopcalite sample with the flowing gas mixture was 0.267 sec.

RESULTS
The X-Ray diffractograms of the MnO 2 (a), hopcalite sample pressed at 500 kg cm −2 before calcination (b) and after calcination (c) are given in the Fig. 3.
In Table 1 are summarized the results obtained by nitrogen sorption and helium filling of the hopcalite samples prepared using different pelletizing pressures. The BET specific surface area, cumulative mesopore volume and the mesopore average diameter are calculated from the nitrogen sorption data and the grain density of the samples is determined by helium filling at room temperature based on the gas pycnometer principle.
The adsorption-desorption isotherms of N 2 at 77 K on hopcalite samples are given in the Fig. 4.
The influence of the pelletizing pressure on cumulative mesopore volume and on the mesopore average diameter is given on the Fig. 5 and 6 respectively.
By the MIP method are determined the volume of the macropores, their pore size distribution as shown in Fig. 7 and 8 respectively and the average macropore diameter. The total volume of the pores and the sample porosity are calculated considering both nitrogen sorption and mercury intrusion experiments data for the hopcalite catalyst and are presented in Table 2 together with the macropore characterization data.  For the hopcalite catalyst prepared by using a pelletizing pressure of 500 kg cm −2 and for the commercial sample of hopcalite (Hopkalite, Producer SIDACO Netherland B.V.) the protection time was determined in the assembled equipment described in the Fig. 2. The measured protection times were 610 min for the hopcalite sample prepared in this work and 300 min for the commercial sample of SIDACO.

DISCUSSION
A relatively wide peak at 2θ = 13-16° is present in all three diffractograms and it is known to belong to the ε -MnO 2 which is one of the more amorphous phases of the manganese oxide. The peak at 2θ = 42.2° comes from a crystalline glue used to stick the powder samples on the sample holder of the X-Ray diffraction apparatus. Another small peak at 2θ = 21.5° which is present at (a) and (b) but disappear after calcination at (c) comes probably from a hydrated form of MnO 2 . The absence of other peaks in the diffractograms except of these originating from MnO 2 shows that the Cu atoms does not precipitate in a crystalline form and they do not create mixed crystals with MnO 2 either. The presence of MnO 2 in a highly amorphous form is a premise for a high surface contact with the copper atoms, which from the other hand is a prerequisite for the high activity of hopcalite catalyst (Kramer et al., 2006). However this is still a point of discussion since an activity of the highly crystalline hopcalite is observed as well (Schwab and Canungo, 1977).
The increase of pelletizing pressure from 0-1700 kg cm −2 , brings a steady decrease to respective specific surface areas from 247.64-147.77 m 2 g −1 and to respective cumulative mesopore volumes from 0.194-0.130 cm 3 g −1 , while the mesopore average diameter remains the same around 3.2 nm. The density of hopcalite increases steadily with the increasing pelletizing pressure from 3.432 g cm −3 for the pelletizing pressure 500 kg cm −2 to 4.510 g cm −3 for the pelletizing pressure 1700 kg cm −2 .
The decrease in the macropore volume (from 0.275-0.148 cm 3 g −1 ) and in the average macropore diameter (from 470.5 nm to 163.6 nm), by increasing the pelletizing pressure from 500-1700 kg cm −2 is quite significant.
All the hopcalite samples show nearly the same pore size distribution in the range of mesopores. The differences in the specific surface areas and cumulative mesopore volume, may originate from the different positioning of the amorphous basic copper carbonate in within and onto the samples surfaces, caused from the different pressures exerted on them. The mesoporous sample structure is partly created from the thermal decomposition of the basic copper carbonate, which releases CO 2 and H 2 O gases. A superficial basic copper carbonate would less contribute to obtain a porous hopcalite sample with a high specific surface area. The influence of pelletizing pressure on the macropore structure of the hopcalite sample is clearly shown in their cumulative volume and average diameter. The hopcalite macropores are interparticle spaces which get smaller when an increasing pressure is exerted on the tiny, highly amorphous and humid particles.
The better efficiency of our sample shows the advantages of the two step precipitation method of preparation, but comes also from the small and carefully prepared amounts in laboratory conditions while the SIDACO catalyst is produced in industrial scale.

CONCLUSION
The efficiency of Mn-Cu hopcalite catalyst prepared by the two step precipitation method using a pelletizing pressure of 500 kg cm −2 is proved to be better compared to one commercially available catalyst of producer SIDACO.
The hopcalite prepared by the two step precipitation method exhibit a low crystalline form manganese dioxide (ε-MnO 2 ). Copper containing crystals are not present.
The increasing pressure exerted in the pelletizing step of the hopcalite production is proved to decrease the specific surface area and the cumulative pore volume and to increase the density of the samples. The mesopore size distribution remains almost the same, while the macropore volume and average diameter decreases significantly by increasing the pelletizing pressure. Because large specific surface areas of hopcalite samples leads to high activities of the hopcalite catalyst, it is not recommended to exert pelletizing pressures higher than 500 kg cm −2 in the hopcalite catalyst preparation. The protection time of the hopcalite catalyst prepared by two step precipitation method is 610 min and its efficiency better, than the protection time of 300 min shown from a commercial hopcalite catalyst of producer SIDACO.