Soot is a natural product that forms during incomplete combustion when organic compounds and fossil fuels are pyrolysed [1
]. Due to a lack of oxygen, burning is often incomplete, especially in fireplaces, which leads to the formation of particulate matter (PM) and soot. In addition, polyaromatic hydrocarbons formed during the combustion process are linked to soot particles [2
], and soot PM is harmful to health, with exposure increasing the risk of cardiovascular disease [1
Wood is a main household heating source in Europe, especially in winter, with fireplace emissions approaching dangerously high levels [5
]. In particular, domestic indoor air quality can suffer dramatically if wood is used as the heating source [7
]. A standard and systematic emissions control system, such as those for vehicles, does not exist in Europe; however, the EU plans to restrict domestic fireplace and stove emissions, such as PM2.5, in the near future [6
]. PM emission standards for new fireplaces already exist in some states of the US, which creates more pressure for fireplace manufacturers in Europe to develop efficient working systems that reduce the formation of soot and particulate matter; however, technology for the prevention of soot formation in wood-combustion processes is still in its infancy.
Oxidation processes occur in the top layers of soot particles in which the carbon content is converted into CO and CO2
, which simultaneously reduces soot mass [9
]. Soot oxidises naturally at a sufficiently high temperature and oxygen level; however, these parameters are not well-controlled in a fireplace. Soot formation in diesel engines has been widely studied and is a well-known health risk; this problem has been solved using a catalyst that oxidises soot particles via the oxidation of NO to NO2
]. Noble metals, such as Pt, are normally effective in catalysts designed for use in vehicles and have been used in diesel vehicles to reduce soot accumulation in diesel particulate filters [11
]. Pt is also used in fireplace soot-oxidation catalysts [13
]; for instance Pt-, Pt/Pd- and Ce-supported materials have been studied as catalysts for the reduction of CO and volatile organic compounds (VOC) emissions in fireplaces [15
]; however, Pt- and Pd-containing catalytic converters for residential wood combustion have been shown to increase the emissions of harmful chlorophenols, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans [18
]. With this in mind, as well as the high costs of platinum-group metals, other solutions have been explored, with Ag, among non-platinum-group metal catalysts, exhibiting promising catalytic soot-oxidation performance in diesel vehicles [19
]. In fact, according to previous studies, Ag has been shown to exhibit superior performance to Pt at oxidising soot in studies that tailor catalysts to fireplace conditions [24
In systems designed for cars, the catalyst is attached to a honeycomb structure by dipping, painting or spraying it with ceramic- or metal-substrate materials. Special materials are used to bind the catalyst to the substrate surface to ensure that it remains attached and tolerates stress [27
]. These binders increase the adhesion between the substrate and catalyst materials, and act as joining glues. In this study, we aimed to prepare fireplace soot-oxidation catalysts in which the binder is added to the catalyst during preparation. These painted catalyst substrates were placed in the smoke chambers of the fireplace at a temperature suitable for the catalyst to operate. We prepared a series of alumina-supported Ag materials and studied their performance during soot-oxidation catalysis. The effects of various binders, namely sodium silicate (water glass), acidic aluminium phosphate and aluminium hydroxide, on the performance of the catalyst were investigated. In addition, a Pt catalyst bound with Al(OH)3
was prepared as a reference. Al(OH)3
has been used as a binder in CaO-based pellets that capture CO2
has often been used as a precursor for aluminium phosphate binders, and water glass is versatile since the adhesive and binding properties of sodium silicate have been used to prepare thermally resistant ceramic paints and casting moulds [30
]. Aluminium phosphate has also been used as a binding and coating material in the ceramics industry [32
]. Both water glass and acidic aluminium phosphate transform into ceramics upon heating; however, more deformations are observed with water glass as the binder during severe heating [34
]; we assume that the addition of the binder alters the catalyst’s structure and its chemical composition, thereby influencing its catalytic performance.
3. Materials and Methods
Soot-oxidation catalysts and reference samples were prepared using a variety of mixing orders and preparation techniques, as detailed in Table 1
. All catalysts were prepared using the wet-impregnation method on La-Al2
as the support material. The support material weight was 10 g and the active metal amount was calculated based on that. Water was added in the same ratio as the catalyst support material, 10 mL, such that the suspension was neither dry nor too wet. Catalysts were prepared in a beaker and mixed with a magnetic stirrer 2 h. Sample R1 contained only the support and was used as a reference material in the study. Catalyst R2, which consisted of active Ag metal on the La-Al2
support, was another reference. The following binders were selected for this study: aluminium hydroxide (Sigma-Aldrich, South Korea, reagent grade Al(OH)3
, solid); sodium silicate (also known as water glass, Sigma-Aldrich solution of Na2
, reagent grade, liquid); and acidic aluminium phosphate (l), which was prepared by mixing 85% H3
(Sigma-Aldrich, USA, reagent grade Al(OH)3
) at 150 °C for 2h, such that the P/Al ratio was 23 [34
]. Silver nitrate (Alfa Aesar, Kandel, Germany, 99+% AgNO3
) was used as the active metal precursor and was first diluted with a small amount of deionised water before mixing with the other catalyst components. Except for reference sample R1 and catalyst R2, samples were prepared by adding the binder into aqueous solutions of catalysts; the Ag loading on the catalyst was 7 wt%. Binder concentrations were 5 wt% based on the dry weight of the support material. Catalysts Ag-AH1, Ag-WG1, Ag-AlP and R3 were prepared such that the support material and the aqueous active metal solutions were first mixed together, after which, the binder was added. Ag-WG2 was prepared by first mixing the binder and the active metal, followed by addition of the support material. After the mixing catalysts were dried in an evaporating dish in a fume cupboard for one day, all catalysts were finally calcined under oxidising conditions in a funnel oven at 500 °C for 3 h. R3 was the third reference catalyst and was prepared in the same manner as Ag-AH1, with platinum nitrate (Alfa Aesar 15 wt% Pt(NO3
) instead of AgNO3
, such that the molar amounts of active metal were the same in both catalysts. Different preparation techniques were used for catalysts Ag-AH3 and Ag-WG3: the support material and the binder were mixed together, after which, the mixture was dried in an oven at 500 °C for 3 h. The dried mixture was then added to deionised water and diluted aqueous silver nitrate was added into the mixture. Finally, these two-step catalyst mixtures were calcined at 500 °C for 3 h.
The soot used in this study was collected from the chimney of a real domestic wood-burning fireplace. The carbon content of the soot was determined using elemental analysis (vario Micro cube elemental analyser,Elementar, Langenselbold, Germany) and was found to be ≈78%; the remaining residues were inorganic materials. In addition, the soot texture was characterised using Raman spectroscopy (Figure 7
) using a Renishaw inVia Raman Microscope (Renishaw, Gloucestershire, United Kingdom) with an Ar-ion laser at 514 nm. Three characteristic Raman peaks were observed for soot, namely the D-peak at 1370 cm−1
, the G-peak at 1600 cm−1
and a strong broad peak in the 2500–2800 cm−1
region that is common to all sp2
materials. The first peak corresponds to distortions of the sp2
crystal structure, while the latter corresponds to the stretching of C–C bonds in graphitic materials and it is common to sp2
materials. The soot used in this study was related to the industrial and commercial soot that was studied by Sadezky et al. [42
The catalyst surface areas were measured with a Quantachrome Autosorb-iQ gas-sorption analyser (Quantachrome Instruments, Boynton Beach, Florida) using 150-mg samples. All samples were degassed under vacuum at 350 °C for 150 min prior to any experiment to remove moisture and residual air. These experiments were carried out at liquid nitrogen temperature (−196 °C) and the surface areas were calculated according to Brunauer–Emmett–Teller theory.
The active-metal dispersions of Ag and Pt were examined as described above. All catalysts were reduced in 10% H2
/Ar for 1 h. Catalyst R3 was reduced at 500 °C, whereas the silver catalysts were reduced at 280 °C. The reducing temperatures were determined from temperature-programmed reduction (TPR) data; the Pt dispersion in catalyst R3 was determined using CO pulse titration at room temperature, assuming that the CO-to-surface-Pt ratio was 1:1 [43
]. Silver dispersions were determined using the O2
pulse titration method, assuming that the O2
-to-surface-Ag ratio was 1:2 [44
]; the oxygen was pulse-injected onto the catalyst at 100 °C.
The effects of binders and active metals on catalyst texture were studied using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan), at a working distance of 15 mm and an accelerator voltage of 30 kV. Powder X-ray diffraction (PXRD) experiments were conducted using a Bruker AXD D8 Advance diffractometer (Bruker, Karlsruhe, Germany), with a Cu Kα radiation source. Diffraction patterns were collected in the 20–85° 2θ range at a scan rate of 0.03°/min. Plastic and Si-mirror sample holders were used in these experiments.
Catalytic performance was determined using combustion experiments. Figure 8
shows a schematic picture of the combustion test rig. In a real fireplace, soot is solid, but is entrained by gaseous flow and collides with catalyst particles. In our experiment, soot was placed on the catalyst surface in the solid phase. Catalysts were in tight contact with the soot due to vibration-mill mixing. Tight contact has been shown to be the best way of mixing the catalyst and the soot in soot-oxidation processes [45
]. The carbon content of the soot was considered when the soot and the catalysts were mixed together. The catalyst (400 mg) and soot (26 mg) were mixed via ball milling in two parts. La-alumina (200 mg) was then added with a spatula into 150 mg of the catalyst–soot mixture to decrease any possible temperature gradient. The mixture was divided into three separate layers in the reactor with quartz wool to minimise backup pressure. The catalyst samples (catalyst + glass wool) were placed inside the reactor tube in ≈2 cm increments starting from around 18 cm from the top, where the reactor tube inside diameter was 1 cm and the length was 43 cm. The reactor was attached to a gas line heated with a programmed furnace with a heating rate of 7 °C/min. Soot oxidation was carried out using a 10% H2
O, 10% O2
gas mixture (balanced with N2
), with a total gas flow rate of 1180 mL min−1
. Measurement was at normal pressure all the time. Water was added to the heated gas line with a syringe pump after the oven reached 110 °C and the water was vaporised before it reached the catalyst. The water amount was monitored using FTIR spectroscopy (Gasmet FTIR DX-4000 spectrometer, Gasmet technologies, Helsinki, Finland). Gas mass-flow controllers were used to control the composition of the gas mixture. A thermocouple was placed right above the catalyst. The catalyst–soot mixture was finally heated from room temperature to 600 °C; oxidation products, mainly CO2
and CO, were analysed with an online Gasmet FTIR DX-4000 spectrometer.
In this study, three different binder materials, namely aluminium hydroxide, water glass and acidic aluminium phosphate, were added during the preparation of Ag fireplace soot-oxidation catalysts, and the effect of the binder on catalyst performance was studied. A Pt catalyst prepared with the Al(OH)3 binder was used as a reference. In addition, the effect of the preparation method on catalyst performance was also examined. Continuous-flow FTIR was used to study the effect of the binder on the soot-oxidation performance of each catalyst, while textural properties were determined using BET surface area analysis, the dispersiveness of the active metal, SEM and PXRD. The Ag/La-Al2O3 catalyst with the Al(OH)3 binder exhibited the best performed in terms of soot oxidation, and the performance of the Ag/La-Al2O3 catalyst improved notably after addition of the binder. Based on the PXRD data, silver was present in both metallic and oxidised forms when the Al(OH)3 binder was used, including the Ag and Ag/La-Al2O3 catalysts, which explained the superior performance of those catalysts. Al(OH)3 was also shown to improve the porosity of alumina, which makes it an excellent choice for use with the Ag/La-Al2O3 catalyst. The water-glass binder dramatically decreased the performance of the Ag/La-Al2O3 catalyst; very-low levels of dispersion, high Ag crystallinity and blockage of the support material pores by water glass were observed using SEM, while the Ag particles agglomerated on the top of the support material decreased the performance of the catalyst. Acidic aluminium phosphate did not provide performance results as good as those with the Al(OH)3 binder, even though it facilitated the highest levels of dispersion and the best BET surface areas among the Ag-based catalysts, which was ascribable to the absence of oxidised silver. The Ag catalysts exhibited notably better performance under fireplace conditions compared to the Pt catalyst, which was ascribable to the ability of Ag to directly oxidise carbon, whereas Pt-catalysed oxidation proceeded through a NOx-assisted route, and wood burning did not produce high amount of NOx. Overall, Al(OH)3 was found to be a good binder for use with a fireplace soot-oxidation catalyst when alumina was the support material and silver was the active metal.