Estimating teat canal cross-sectional area to determine the effects of teat-end and mouthpiece chamber vacuum on teat congestion.

The primary objective of this experiment was to assess the effect of mouthpiece chamber vacuum on teat-end congestion. The secondary objective was to assess the interactive effects of mouthpiece chamber vacuum with teat-end vacuum and pulsation setting on teat-end congestion. The influence of system vacuum, pulsation settings, mouthpiece chamber vacuum, and teat-end vacuum on teat-end congestion were tested in a 2×2 factorial design. The low-risk conditions for teat-end congestion (TEL) were 40 kPa system vacuum (Vs) and 400-ms pulsation b-phase. The high-risk conditions for teat-end congestion (TEH) were 49 kPa Vs and 700-ms b-phase. The low-risk condition for teat-barrel congestion (TBL) was created by venting the liner mouthpiece chamber to atmosphere. In the high-risk condition for teat-barrel congestion (TBH) the mouthpiece chamber was connected to short milk tube vacuum. Eight cows (32 quarters) were used in the experiment conducted during 0400 h milkings. All cows received all treatments over the entire experimental period. Teatcups were removed after 150 s for all treatments to standardize the exposure period. Calculated teat canal cross-sectional area (CA) was used to assess congestion of teat tissue. The main effect of the teat-end treatment was a reduction in CA of 9.9% between TEL and TEH conditions, for both levels of teat-barrel congestion risk. The main effect of the teat-barrel treatment was remarkably similar, with a decrease of 9.7% in CA between TBL and TBH conditions for both levels of teat-end congestion risk. No interaction between treatments was detected, hence the main effects are additive. The most aggressive of the 4 treatment combinations (TEH plus TBH) had a CA estimate 20% smaller than for the most gentle treatment combination (TEL plus TBL). The conditions designed to impair circulation in the teat barrel also had a deleterious effect on circulation at the teat end. This experiment highlights the importance of elevated mouthpiece chamber vacuum on teat-end congestion and resultant decreases in CA.

Effect of pulsation rest phase duration on teat end congestion.

The objective of this study was to quantify the effect of d-phase (rest phase) duration of pulsation on the teat canal cross-sectional area during the period of peak milk flow from bovine teats. A secondary objective was to test if the effect of d-phase duration on teat canal cross-sectional area was influenced by milking system vacuum level, milking phase (b-phase) duration, and liner overpressure. During the d-phase of the pulsation cycle, liner compression facilitates venous flow and removal of fluids accumulated in teat-end tissues. It was hypothesized that a short-duration d-phase would result in congestion of teat-end tissue and a corresponding reduction in the cross-sectional area of the teat canal. A quarter milking device, designed and built at the Milking Research and Instruction Laboratory at the University of Wisconsin-Madison, was used to implement an experiment to test this hypothesis. Pulsator rate and ratios were adjusted to achieve 7 levels of d-phase duration: 50, 100, 150, 175, 200, 250, and 300ms. These 7 d-phase durations were applied during one milking session and were repeated for 2 vacuum levels (40 and 50kPa), 2 milking phase durations (575 and 775ms), and 2 levels of liner overpressure (9.8 and 18kPa). We observed a significant reduction in the estimated cross-sectional area of the teat canal with d-phase durations of 50 and 100ms when compared with d-phase durations of 150, 175, 225, 250, and 300ms. No significant difference was found in the estimated cross-sectional area of the teat canal for d-phase durations from 150 to 300ms. No significant interaction was observed between the effect of d-phase and b-phase durations, vacuum level, or liner overpressure.

Short communication: Changes in gait symmetry in healthy and lame dairy cows based on 3-dimensional ground reaction force curves following claw trimming.

Lameness is a frequent health problem in dairy cows. This preliminary study aimed to detect gait differences between healthy and lame walking cows using 3-dimensional force plates. We examined left-right leg symmetry changes of healthy and lame Holstein dairy cows following claw trimming. Gait scoring (GS) was performed on d -5, 0, 1, and 7 relative to claw trimming. Before the experiment, 5 cows walked normally (initial GS=1) and 4 cows limped moderately on a hind leg (initial GS=3). Gait was measured on d -2, -1, 0, 1, and 7 relative to trimming by obtaining ground reaction forces as cows walked repeatedly across 2 parallel 3-dimensional force plates. From the ground reaction forces, stance phase data were derived using computerized procedures. Left-right leg symmetries of entire curves in the 3 force directions were calculated. Effects of lameness and trimming were analyzed in a mixed model, using a low lameness threshold (GS>1). One week after claw trimming, only one cow was mildly lame. In addition, the symmetries of all 3 dimensions were significantly improved shortly after trimming. Importantly, lameness significantly worsened vertical symmetry. Lame cows walked significantly more slowly than healthy cows. In conclusion, all force symmetries seemed capable of detecting gait responses to claw trimming. Although our results are based on a small number of animals, vertical leg symmetry was affected by lameness.

Udder health of cows changing from tie stalls or free stalls with conventional milking to free stalls with either conventional or automatic milking.

Udder health and milk production were monitored in cows transferred from tie stalls or loose housing with conventional milking to loose housing with either automatic or conventional milking. Data were collected from 182 Finnish farms from September 1999 to February 2006. Data from the first year before and first year after the changes were compared. A total of 88 herds changed from conventional milking (CM herds) to automatic milking (AM herds), 29 of which were housed in tie stalls and 59 of which were housed in a loose housing barn before the change. Additionally, 94 CM herds milked in loose housing barns that had been housed in tie stalls before the change were included. Milk record data consisted of annual herd size, parity, breed, calving dates, test day data [date, milk yield, and cow somatic cell count (SCC)] and records for treatments of clinical mastitis. Calculations were made for energy-corrected milk yield and logarithmic SCC (logSCC), proportion of cows at risk that experienced an SCC >200,000 cells/mL for the first time (highSCC), and number of treatments of clinical mastitis within a herd. Cows in tie stalls had higher milk yield (28.5 +/- 0.29 vs. 26.5 +/- 0.46 kg/d) and a lower logSCC (4.86 +/- 0.01 vs. 4.95 +/- 0.02) than cows in loose housing barns before the change. After the change, CM herds had slightly better udder health than AM herds because the proportion of cows at risk for highSCC was larger in AM herds (3.3 vs. 2.1%). The change in milking and housing systems caused a decline of 0.8 +/- 0.25 kg/d per cow in energy-corrected milk yield, a slight increase in cow logSCC (from 4.88 +/- 0.01 to 4.93 +/- 0.01), and an increase of 0.6% in the proportion of cows having highSCC (from 2.5 to 3.1). The impact was clearer on herds that began automatic milking. Based on the results, the increase in bulk milk SCC of herds milked automatically in Finland was probably due to reduced separation of mastitic milk in AM herds.

Review: Milking machine settings, teat condition and milking efficiency in dairy cows.

Because of technical limitations, an impact of machine milking on the teat tissue cannot be avoided. The continuance of this impact during and after milking depends on a variety of factors related to the physiological regulation of milk ejection, as well as the different production systems and milking machine settings. Milking machine settings aim to achieve a high milking performance, that is, short machine-on time at a maximum of milk harvest. However, a high milking performance level is often related to an impact on the teat tissue caused by vacuum or liner compression that can lead to pathological dimensions of congestion of the tissue or hyperkeratosis as a long-term effect. Toward the end of milking a decrease of milk flow rate causes a raise of mouthpiece and teat end vacuum levels and hence an increase of the impact on the teat tissue and the risk of tissue damage. The mechanical stress by the milking machine activates a cascade of cellular mechanisms that lead to an excessive keratin growth and thickening of the keratin layer. Consequently, a complete closure of the teat canal is disabled and the risk of bacterial invasion and intramammary infection increases. Another consequence of high vacuum impact is fluid accumulation and congestion in the tissue of teat tip and teat basis because of an obstruction in venous return. The present review paper provides an overview of the available scientific information to describe the interaction between different levels and types of system vacuum, mouthpiece chamber vacuum, teat end (claw) vacuum, liner pressure, and the risk of short-term and long-term impacts on the teat tissue.

A model for detection of individual cow mastitis based on an indicator measured in milk.

A dynamic deterministic biological model was developed that generates, for a given cow on a given day, a value for her risk of having mastitis. The model combines real-time information from a mastitis indicator measured in milk with additional factors that are other known risk factors of mastitis but that are not reflected in the indicator. l-Lactate dehydrogenase (LDH), an enzyme whose activity is increased because of mastitis, is used as an example of a mastitis indicator. The additional factors incorporated in the model are days from calving, breed, parity, milk yield, udder characteristics, other disease records, electrical conductivity, and herd characteristics. The model is designed to run each time a new LDH value is recorded and can run in the absence of the additional factors. Electrical conductivity measurements and disease records, where available, also trigger the model to run. As an input, milk LDH activity values (micromol/min per L) are multiplied by milk yield (L) to produce the amount of LDH (micromol/min) and are then smoothed using an extended Kalman filter before being processed by the biological model. The output comprises a risk of acute mastitis and a relative degree of chronic mastitis. The model also produces a days-to-next sample value that allows sampling frequency to be either increased or reduced depending on the risk of mastitis. The days-to-next sample value was designed to make the best use of opportunities afforded by automated, inline sampling technology. The model functionality was investigated using simulated data, and real-farm data of naturally occurring mastitis were then used to validate the model. The results demonstrated that the model is robust to sampling frequency and random noise in the LDH measurements. It was able to detect mastitis reasonably well: Using a threshold mastitis risk of 0.7, sensitivity for detecting clinical mastitis was 82%. Specificity, that is, the ability to avoid misclassifying healthy observations as mastitis, was 99%.

Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.

The main objective of the study was to determine whether the amount of air intake during quarter milking influences the concentration of free fatty acids (FFA) and vacuum fluctuations at the teat end when milking automatically. Air intake in the teat cup was restricted from the normal inlet of 4.5 to 7 L/min to 1.7 and 0 L/min on 2 farms and experiments were carried out as half-udder studies with 40 cows. Blockage of the air inlet reduced FFA from 1.02 to 0.77 mEq/100 g of fat in one herd and from 1.50 to 1.17 mEq/100 g of fat in the other herd. Milk yield per milking was the most significant factor influencing FFA. Air intake accounted for <20% of the variation in FFA concentration. Characteristics of the cow explained the most variation, which could mainly be assigned to the effects of milk yield, fat percentage, fat globule size, and fat globule size distribution. The interval between milkings was not significant when adjusting for milk yields. Blockage of the air inlet caused vacuum fluctuations at the teat end to increase from 15.4 to 21.5 kPa for one model of an automatic milking system (AMS), but from 12.8 to 53.6 kPa for another model. Measurements made with a flow simulator and water revealed that the AMS model and water flow were the most important factors influencing vacuum fluctuations, and that interactions existed between the diameter of the short milk tube and air intake. Free fatty acids in bulk milk from 5,980 herds averaged 0.75 mEq/L of milk for conventional herds and varied from 0.77 to 0.94 mEq/L of milk for the 5 AMS models on the Danish market. Fault detection in 55 herds pointed out that the most frequent faults in conventional herds were air leakages and intake of too much air in the cluster, whereas AMS herds had problems with the cooling and stirring of milk. Correction of the cooling faults caused FFA to decrease by 0.52 mEq/L in the AMS herds. We concluded that air intake during automatic milking is not the most important factor in reducing FFA, whereas milk yield per milking matters the most. More attention should be paid to the cooling and stirring of milk. Reducing the air intake causes vacuum fluctuations during milking to increase significantly.

Synthesis of proteins in Escherichia coli is limited by the concentration of free ribosomes. Expression from reporter genes does not always reflect functional mRNA levels.

Induction of beta-galactosidase from high copy-number plasmids was found to reduce the synthesis of other cellular proteins in Escherichia coli. The reduction depends on the protein in question and on the induction level of the beta-galactosidase. It could be observed transiently within one minute after induction and in some cases also during steady-state induction. Our interpretation is that the concentration of the free ribosomal subunits decreases after induction, leading to an increased competition among the individual ribosome binding sites for ribosomes. The immediate reduction in the synthesis individual proteins after induction of beta-galactosidase was used as an assay to measure in vivo the efficiency of a ribosome binding site. These efficiencies were compared to the calculated affinities between the ribosome binding site of specific mRNA species and the 3' end of 16 S RNA. For several mRNAs with similar Shine-Dalgarno sequences, the sensitivity to competition differed twofold. Our results show, that both transiently during induction of lacZ and also at very high steady-state expression levels, the expression from reporter genes, including the lacZ gene itself, does not reflect the levels of the mRNAs in a simple way.

Changes in freezing point of blood and milk during dehydration and rehydration in lactating cows.

We studied the influence of short-term changes in water intake in 4 lactating Holstein cows on diurnal fluctuation of packed cell volume (PCV), freezing point of blood (FPblood), freezing point of milk (FPmilk), and the relationship between changes in FPblood and FPmilk. The experiment lasted 108 h and was divided into 3 periods: 1) control (38 h); 2) dehydration/rehydration with 4 consecutive 12-h sequences: 8 h without water, 0.5-h access to water, 1.5 h without water, and 2-h access to water; and (3) 22 h for reconstitution. Cows were milked at 12-h intervals. Blood was sampled from the jugular vein hourly throughout the experiment, and at 0, 15, 30, 60, 90, 120, 150, 180, 210, and 240 min after initiated rehydration following the 8-h dehydration sequences. Intakes of free water and water in feed were recorded every hour. The PCV was negatively affected by water intake within the hour before sampling. Dehydration lowered FPblood steadily, whereas FPblood increased by 0.024 degrees C within 30 min following a large water intake in the rehydration period. The FPblood was not significantly influenced by actual water intake, but was highly correlated with the available water pool at time of blood sampling. The FPmilk correlated positively with the FPblood collected 1 h before milk sampling, indicating a delay in the transfer of water from plasma to milk. In summary, FPblood and FPmilk decrease during dehydration and increase during rehydration. Rehydration following a long dehydration period caused an increase in FPmilk within 1 h, but not above the initial level for FPmilk of the cow.

Infrared thermography and ultrasonography to indirectly monitor the influence of liner type and overmilking on teat tissue recovery.

Eight Danish Holstein cows were milked with a 1-mm thick specially designed soft liner on their right rear teat and a standard liner mounted under extra high tension on their left rear teat. Four of the animals were overmilked for 5 min. Rear teats were subjected to ultrasound examination on the first day and to infrared thermography on the second day. Teats were submersed in ethanol 20 min post-milking on the second day. Ultrasonography measurements showed that teat canal length increased by 30-41% during milking. Twenty minutes after milking, teats milked with modified standard liners still had elongated teat canals while teats milked with the soft liner were normalized. Overmilking tended to increase teat wall thickness. Approximately 80% of variability in teat canal length, from before teat preparation to after milking, could be explained by changes during teat preparation. Thermography indicated a general drop in teat temperature during teat preparation. Teat temperature increased during milking and continued to increase until the ethanol challenge induced a significant drop. Temperatures approached pre-challenge rather than pre-milking temperatures within 10 minutes after challenge. Teat temperatures were dependent on type of liner. Mid-teat temperatures post-challenge relative to pre-teat preparation were dependent on overmilking. Thermography and ultrasound were considered useful methods to indirectly and non invasively evaluate teat tissue integrity.

Protein engineering of subtilisins to improve stability in detergent formulations.

Microbial proteases are used extensively in a large number of industrial processes and most importantly in detergent formulations facilitating the removal of proteinaceous stains. Site-directed mutagenesis has been employed in the construction of subtilisin variants with improved storage and oxidation stabilities. It is shown that in spite of significant structural homology between subtilisins subjected to protein engineering the effects of specific mutations can be quite different. Mutations that stabilize one subtilisin may destabilize another.

The effect of post milking teat dip and suckling on teat skin condition, bacterial colonisation, and udder health.

The teat skin of cows was scored (1: smooth as silk; 2: smooth; 3: slightly rough; 4: rough; 5: cracked; and 6: scores) by trained technicians who moved their fingers down the barrel of the teat with a light touch. Technicians ranked the same population of teats in the same rank order, but their mean values differed by half a score which probably could be related to the skin condition of their own fingers. Half udder experiments were carried out for 6 months at 4 farms with 35 to 52 cows each. A postmilking teat spray with 10% glycerol improved teat skin condition (p < 0.10) compared with no teat spray. A postmilking teat spray with 120 ppm chlorine dioxide did not influence teat skin condition compared with no teat spray. No differences in udder health could be proven between treatments. Control studies revealed that 10% glycerol as an emollient of a postmilking teat spray improved teat skin condition within 3 weeks from being slightly rough to being smooth for lactating cows (p < 0.05) but not for dry cows having smooth teat skin. Neither glycerol nor chlorine dioxide influenced absolute number of bacteria on teat skin after a challenge with Streptococcus uberis and Staphylococcus aureus but half life of S. aureus on unsprayed teats was longest (p = 0.05). Suckling made teat skin more rough than machine milking. Nevertheless, suckling lowered the number of esculin positive bacteria on the teat skin. We concluded that the condition of healthy teat skin (scores 1-4) has no influence on bacterial colonisation in the absence of cracks and sores (scores 5-6).

Influence of switch level of automatic cluster removers on milking performance and udder health.

The effect of a switch level of 200 or 400 g/min for automatic cluster removers on milking performance and udder health was measured with 71 first lactation cows in their first 36 weeks of lactation and with 64 older cows in their first 12 weeks of lactation. Early removal of the milking unit decreased machine-on time by 0.5 min, increased average milk flow rate slightly, improved teat condition significantly and reduced the change in teat end thickness during milking of first lactation cows. Early removal of the milking unit did not affect milk yield or composition, and the incidence and prevalence of subclinical mastitis were the same in the two groups. Fewer older cows developed clinical mastitis in the group switched at 400 g/min, but this was not significant. It is concluded that the milking unit can be detached at a milk flow rate of 400 instead of 200 g/min without having a negative influence on milk yield. Machine-on time is shortened and teat condition improved and udder health does not seem to be affected.

Effects of milkline vacuum, pulsator airline vacuum, and cluster weight on milk yield, teat condition, and udder health.

The influence of milkline vacuum, pulsator airline vacuum, and cluster weight on complete lactation milk yield performance, teat condition, and udder health were studied in 241 cows by using three separate factorial experiments. The first experiment had milkline vacuum set at 38 and 48 kPa, pulsator airline was vacuum was set at 0 and 8 kPa above milkline vacuum, and cluster weight was 1.6 and 2.3 kg. The second experiment had milkline vacuum set at 38 and 48 kPa and cluster weight was 1.6 and 2.3 kg. The third experiment had milkline vacuum set at 32 and 42 kPa and pulsator airline vacuum 0 and 8 kPa above milk line vacuum. The first two experiments were carried out with a high-level milking and the third with a low-level milking system. The average vacuums in the short milk tube during milking for low and high milkline vacuum in the high-level system were 26 and 33 kPa, and 30 and 39 kPa for the low-level milking system, respectively. Milking at low vacuum increased the machine-on time and frequency of liner slip, decreased milk flow rate, but had no influence on teat condition and udder health. The milk yield of high producing cows with long machine-on time was reduced by 5% when milked at a low vacuum. A higher pulsator airline vacuum than milkline vacuum had no influence on milking performance. The use of the light cluster reduced machine-on time. We recommend that mean vacuum in the short milk tube should not be lower than 32 kPa.

Isolation and characterization of mutants with impaired regulation of rpsA, the gene encoding ribosomal protein S1 of Escherichia coli.

In order to select mutants that would help to characterize the post-transcriptional regulation of rpsA, we constructed a strain in which the growth rate on lactose minimal medium is determined by the amount of an rpsA-lacZ' alpha-fragment fusion protein produced, even when this is encoded by a high-copy-number plasmid. In the parental strain, synthesis of the fusion protein is repressed by a wild-type rpsA gene, present on a compatible plasmid. Twenty-eight spontaneous and independent mutants, all of them mapping in the rpsA leader region, were isolated as strains that showed higher growth rates, on lactose medium, due to increased synthesis of the rpsA-lacZ' fusion protein. Among these mutants only three sequence changes were found, mapping 9, 10 and 27 bases upstream of the rpsA start codon. At both the -9 and -10 positions an A to G transition and at -27 a C to G transversion all resulted in a sequence with better complementarity to the 3' end of 16S rRNA. We also isolated two mutations mapping in the plasmid-encoded rpsA structural gene: an ochre nonsense mutation in codon 15 of the rpsA gene and a frameshift mutation, deleting the T residue at position +1186. To facilitate the in vitro assay of alpha-fragment activity we also constructed a strain that overproduces the alpha-acceptor fragment four-fold relative to a strain that is diploid for this lacZ delta M15 allele.

Effects of premilking teat preparation on spores of anaerobes, bacteria, and iodine residues in milk.

Premilking teat preparations using individual paper or cotton towels for either 6 or 20 s to reduce bacteria and iodine residues from teat surfaces were determined through Latin square designs applied to 50 cows. A cotton towel used for 20 s was most effective in cleaning teats, probably because of the physical structure of the towel, physical action on teat surface, and scrubbing of the teat ends. Premilking teat preparation of 6 s was inadequate to clean teats and to avoid iodine residues in milk. Teat end erosions increased iodine residue in milk. Two days after a treatment period, iodine content in milk from iodophor-dipped groups was similar to that of the undipped control group. Against our expectation, teat dipping with a .25% iodophor teat dip caused higher iodine residue in milk than a .50% iodophor teat dip. Differences in formulations and inert ingredients of iodophor teat dips indicate a need for further studies.

Milk quality on Danish farms with automatic milking systems.

The bulk-milk quality of 98 Danish farms with automatic milking systems was analyzed from 1 yr before introduction of automatic milking until 1 yr after. Bulk-milk total bacterial count, spores of anaerobes, somatic cell count (SCC), and freezing point increased when automatic milking was introduced and the frequency of milk-quality failures almost doubled. Milk-quality failures were most frequent in the first 3 mo after the start of automatic milking. The increase in spores of anaerobes indicated that the increase in total bacterial count originated partly from contamination of milk from the teat surface and partly from lack of cleaning of the milking equipment or cooling of the milk. The increase in bulk-milk SCC indicated that milk from clinically infected cows and cows with high cell counts was not diverted to the same degree, milking automatically rather than milking conventionally. A self-monitoring program including survey of the bulk-milk quality was established to help farmers in the transition period going from conventional to automatic milking. The program was introduced on 84 farms. Farms on the self-monitoring program reduced bulk-milk cell count. Application of the program did not reduce the frequency of high total bacterial counts and freezing points of the bulk milk to the level of conventional milking. However, the program reduced the overall frequency of milk-quality failures.

Reverse pressure gradients across the teat canal related to machine milking.

Miniature pressure transducers were inserted through the udder cistern wall of 10 cows and placed into the teat cistern and, in addition, beneath the teat end. Data were sampled every millisecond and collected during 59 sequences of manual teat handling pre- and postmilking, 575 attachments, 384 sequences of 30-s milking, and 623 sequences of detachment. Attachment and detachment were mainly done during overmilking in short sequences lasting 8 to 20 s. Reverse pressure gradients across the teat canal occurred during manual teat handling (54%), attachment of the milking unit (29%), milking (1%), and detachment (26%). Overall risk included empty teats. Risk factors at pre- and postmilking teat handling were the compression of teats and the following movement back to normal shape. When the diameter of the mouthpiece orifice of the liner was larger than the teat diameter, the frequency of reverse pressure gradients at attachment was halved compared with attachment of more narrow liners. The method of attaching the milking unit on empty teats without the risk of creating reverse pressure gradients was not identified. Reverse pressure gradients in empty teats may be avoided during detachment of liners if the mouthpiece orifice diameter is greater than the teat diameter. Detachment with the liner in open position reduced the risk of reverse pressure gradients compared with that from the closed position.

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110).

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (approximately 0.15 eV) energy barriers.

Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110): a density functional theory study.

The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissociation of molecular oxygen are investigated by means of density functional theory. The O2 molecule is found to bind strongly to bridging oxygen vacancies, attaining a molecular state with an expanded O-O bond of 1.44 A. The molecular oxygen also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron density and electronic density of state plots that charging of the adsorbed molecular oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissociation at a vacancy are both hindered by relative large barriers. However, we find that the presence of neighboring vacancies can strongly affect the ability of O2 to dissociate. The implications of this in connection with diffusion of the bridging oxygen vacancies are discussed.